In this study we analyzed sediment trap time series from five tropical sites
to assess seasonal variations in concentrations and fluxes of long-chain
diols (LCDs) and associated proxies with emphasis on the long-chain diol
index (LDI) temperature proxy. For the tropical Atlantic, we observe that
generally less than 2 % of LCDs settling from the water column are
preserved in the sediment. The Atlantic and Mozambique Channel traps reveal
minimal seasonal variations in the LDI, similar to the two other lipid-based
temperature proxies TEX86 and U37K′. In addition,
annual mean LDI-derived temperatures are in good agreement with the annual
mean satellite-derived sea surface temperatures (SSTs). In contrast, the LDI
in the Cariaco Basin shows larger seasonal variation, as do the TEX86
and U37K′. Here, the LDI underestimates SST during the
warmest months, which is possibly due to summer stratification and the
habitat depth of the diol producers deepening to around 20–30 m. Surface
sediment LDI temperatures in the Atlantic and Mozambique Channel compare well
with the average LDI-derived temperatures from the overlying sediment traps,
as well as with decadal annual mean SST. Lastly, we observed large seasonal
variations in the diol index, as an indicator of upwelling conditions, at
three sites: in the eastern Atlantic, potentially linked to Guinea Dome
upwelling; in the Cariaco Basin, likely caused by seasonal upwelling; and in
the Mozambique Channel, where diol index variations may be driven by
upwelling from favorable winds and/or eddy migration.
Introduction
Several proxies exist for the reconstruction of past sea surface temperature
(SST) based on lipids. The U37K′ is one of the most
commonly applied proxies and is based on the unsaturation of long-chain alkenones
(LCAs), which are produced by phototrophic haptophyte algae, mainly the
cosmopolitan Emiliania huxleyi (Volkman et al., 1980; Brassell et
al., 1986; Prahl and Wakeham, 1987; Conte et al., 1994). This index exhibits
a strong positive correlation with SST (Müller et al., 1998; Conte et
al., 2006). Another widely used organic paleotemperature proxy is the
TEX86, as originally proposed by Schouten et al. (2002), based on the
relative distribution of archaeal membrane lipids, i.e., glycerol dialkyl
glycerol tetraethers (GDGTs), which in the marine realm are mainly thought to
be derived from the phylum Thaumarchaeota. Schouten et al. (2002) showed that
the TEX86 index measured in marine surface sediments is correlated with
SST, and since then its application in paleoenvironmental studies has
increased (see, e.g., review by Tierney, 2014). However, research has shown that
despite their highest abundance being recorded in the upper 100 m of the
water column, Thaumarchaeota can be present down to a depth of 5000 m (Karner et
al., 2001; Herndl et al., 2005). Accordingly, GDGTs may be found in high
concentrations below a depth of 100 m (e.g., Sinninghe Damsté et al., 2002;
Wuchter et al., 2005), and several studies have indicated that TEX86
might be more reflective of subsurface temperatures in some regions (e.g.,
Huguet et al., 2007; Lopes dos Santos et al., 2010; Kim et al., 2012, 2015;
Schouten et al., 2013; Chen et al., 2014; Tierney et al., 2017; see Zhang and
Liu, 2018 for review).
Most recently a SST proxy based on the distribution of long-chain diols
(LCDs), called the long-chain diol index, or LDI has been proposed (Rampen et al.,
2012). This index is a ratio of 1,13- and 1,15-diols (i.e., alcohol groups at
position C-1 and C-13 or C-15), and the analysis of globally distributed
surface sediments has revealed that this index strongly correlates with SST.
Since then, the index has been applied in several paleoenvironmental studies
(e.g., Naafs et al., 2012; Lopes dos Santos et al., 2013; Jonas et al., 2017;
Warnock et al., 2018).
However, large gaps still remain in the understanding of this proxy. The
largest uncertainty is that the main marine producer of LCDs is unknown.
Although these diols have been observed in cultures of certain marine
eustigmatophyte algae (e.g., Volkman et al., 1992, 1999; Méjanelle et al.,
2003; Rampen et al., 2014b), the LCD distributions in cultures are different
from those observed in marine sediments. Furthermore, Balzano et al. (2018)
combined lipid analyses with 18S rRNA gene amplicon sequencing on suspended
particulate matter (SPM) and did not find a significant direct correlation
between LCD concentrations and sequences of known LCD-producers. Rampen et
al. (2012) observed the strongest empirical relation between surface sediment-derived
LDI values and SSTs for autumn and summer, suggesting that these are
the main growth seasons of the source organisms. Moreover, the strongest
correlation was also observed for the upper 20 m of the water column,
suggesting that the LCDs are likely produced by phototrophic algae which
thrive in the euphotic zone. Nevertheless, LDI temperatures based on surface
sediments reflect an integrated signal of many years, which complicates the
interpretation of the LDI in terms of seasonal production and depth of export
production.
One way of resolving seasonality in the LCD flux and the LDI is to analyze time
series samples from sediment traps that continuously collect sinking
particles in successive time intervals over periods of a year or more. Such
studies have been carried out for the U37K′ as well as
for the TEX86 and associated lipids (e.g., Müller and Fischer, 2001;
Wuchter et al., 2006; Huguet et al., 2007; Fallet et al., 2011; Yamamoto et
al., 2012; Rosell-Melé and Prahl, 2013; Turich et al.,
2013).
However, very few studies have been undertaken for LCDs. Villanueva et al. (2014)
carried out a sediment trap study in Lake Challa (eastern Africa) and Rampen et
al. (2008) in the upwelling region off Somalia. The latter study showed that
1,14-diols, produced by Proboscia diatoms strongly increased early
in the upwelling season in contrast to 1,13- and 1,15-diols; thus they can be
used to trace upwelling. However, neither of these sediment trap studies
evaluated the LDI.
In this study, we assess seasonal patterns of the LDI for sediment trap
series at five sites: in the Cariaco Basin, in the Mozambique Channel and
three sites in the tropical North Atlantic. During this assessment, we compare the LDI values to
satellite-derived SST, as well as results obtained for other temperature
proxies, i.e., the TEX86H and U37K′.
Moreover, for the Atlantic and Mozambique Channel, we compare the sediment
trap proxy signals with those preserved in the underlying sediments, after
settling and burial. Finally, we assess the applicability of the diol index,
based on 1,14-diols produced by Proboscia diatoms (Sinninghe
Damsté et al., 2003), as a tracer of upwelling and/or productivity in these
regions.
Materials and methodsStudy sites and sample collectionTropical North Atlantic
The ocean current and wind patterns of the tropical Atlantic are mostly
determined by the seasonal latitudinal shift of the intertropical convergence
zone (ITCZ; Fig. 1). The ITCZ migrates southward during boreal winter, and
northward during boreal summer. During summer, the southeast trade winds
prevail, whereas during winter the northeast trade winds intensify. The
northeast trade winds drive the North Equatorial Current (NEC) which flows
westward. South of the NEC, the North Equatorial Countercurrent (NECC) flows
towards the east (Stramma and Schott, 1999). The South Equatorial Current
(SEC) flows westward and branches off in the North Brazil Current (NBC;
Stramma and Schott, 1999). When the ITCZ is in the north, the NBC retroflects
off the South American coast and is carried eastward into the NECC, and thus
into the western tropical Atlantic (e.g., Richardson and Reverdin, 1987).
North of the NBC, the Guiana Current (GC) disperses the outflow from the
Amazon River towards the Caribbean Sea. (Müller-Karger et al., 1988,
1995). However, during boreal summer the NBC may retroflect, carrying the
Amazon River plume far into the western Atlantic (e.g., Lefèvre et al.,
1998; Coles et al., 2013). In fact, every late summer/autumn, the Amazon
River outflow covers around 2×106 km2 of the western North
Atlantic, and the river delivers approximately half of all freshwater input
into the tropical Atlantic (see Araujo et al., 2017, and references therein).
(a) Location map showing the five sediment trap mooring
sites in the Cariaco Basin, the tropical North Atlantic (M1, M2 and M4) and
the Mozambique Channel. Two of the moorings in the tropical North Atlantic
(M2 and M4) contain an upper (“U”) and a lower (“L”) trap, shown in the
bathymetric section below (b) with traps depicted as red triangles
and surface sediments shown as black crosses. A similar section profile is
shown for the Mozambique Channel (c), where the sediment trap and
the surface sediments are also indicated. All maps/sections were generated
using Ocean Data View (Schlitzer, 2015). The approximate seasonal positions
of the ITCZ are indicated, in addition to the North Equatorial Current (NEC),
the North Equatorial Countercurrent (NECC), the South Equatorial Current
(SEC), the Mauritania Current (MC), the Guinea Dome (GD), the North Brazil
Current (NBC) and the Guiana Current (GC).
The eastern tropical North Atlantic is characterized by upwelling caused by
the interaction between the trade winds and the movement of the ITCZ. Cropper
et al. (2014) measured upwelling intensity along the northwest African coastline
between 1981 and 2012, in terms of wind speed, SST and other meteorological
data. They recognized three latitudinal zones: weak permanent annual
upwelling north of 26∘ N, strong permanent upwelling between 21 and
26∘ N, and seasonal upwelling between 12 and 19∘ N related
to the seasonal migration of the trade winds. Southeast of Cape Verde,
large-scale cyclonic circulation forms the Guinea Dome (GD; Fig. 1), which
centers around 10∘ N, 22∘ W (Mazeika, 1967), i.e., close to
mooring site M1. The GD is a thermal upwelling dome, formed by near-surface
flow fields associated with the westward NEC, the eastward NECC and the
westward North Equatorial Undercurrent (NEUC) (Siedler et al., 1992). It
forms a cyclonic circulation as a result of the eastward flowing NECC and the
westward flowing NEC (Rossignol and Meyrueis, 1964; Mazeika, 1967). The GD
develops from late spring to late fall due to the northern ITCZ position and
the resulting Ekman upwelling, but shows significant interannual variability
(Siedler et al., 1992; Yamagata and Iizuka, 1995; Doi et al., 2009) judging
from general ocean circulation models. According to Siedler et al. (1992),
upwelling is most intense between July and October when the ITCZ is in the GD
region and the NECC is strongest.
At three sites, we analyzed five sediment trap series along a longitudinal
transect in the North Atlantic (∼12∘ N) to determine seasonal
variations in the LDI. This transect has been previously studied for Saharan
dust deposition in terms of grain sizes (van der Does et al., 2016), as the
tropical North Atlantic receives approximately one-third of the wind-blown
Saharan dust (e.g., Duce et al., 1991; Stuut et al., 2005), which might
potentially act as fertilizer because of the high iron levels (e.g., Martin
and Fitzwater, 1988; Korte et al., 2017; Guerreiro et al., 2017; Goudie and
Middleton, 2001, and references therein). Furthermore, Korte et al. (2017)
assessed mass fluxes and mineralogical composition, Guerreiro et al. (2017)
measured coccolith fluxes for two of the time series, and Schreuder et
al. (2018a, b) measured long-chain n-alkanes, long-chain n-alkanols and
fatty acids, and levoglucosan for the same sediment trap samples and surface
sediments as analyzed in this study.
At site M1 (12.00∘ N, 23.00∘ W), the sediment trap,
referred to as M1U, was moored at a water depth of 1150 m (Fig. 1). This
mooring is located in the proximity of the Guinea Dome; therefore, it might
potentially be influenced by seasonal upwelling. At station M2
(13.81∘ N, 37.82∘ W), two sediment traps were recovered,
i.e., an “upper” (M2U) trap at a water depth of 1235 m, and a “lower”
(M2L) trap at a depth of 3490 m. Lastly, at mooring station M4
(12.06∘ N, 49.19∘ W), an upper and lower trap series
were also recovered and analyzed (M4U and M4L), at depths of 1130 and 3370 m,
respectively. This mooring site may be seasonally affected by Amazon River
discharge (van der Does et al., 2016; Korte et al., 2017; Guerreiro et al.,
2017; Schreuder et al., 2018a). All sediment traps were equipped with
24 sampling cups, which sampled synchronously over 16-day intervals from
October 2012 to November 2013, using HgCl2 as a biocide and borax
as a pH buffer to prevent in situ decomposition of the collected material.
Mozambique Channel
The Mozambique Channel is located between Madagascar and Mozambique and is
part of the Agulhas Current system hugging the coast of South Africa
(Lutjeharms, 2006). The Agulhas Current system is an important conveyor in
the transport of warm and salty waters from the Indian Ocean to the Atlantic Ocean
(Gordon, 1986; Weijer et al., 1999; Peeters et al., 2004). The northern part
of the channel is also influenced by the east African monsoon winds (Biastoch
and Krauss, 1999; Sætre and da Silva, 1984; Malauene et al., 2014). Between September and March, these
winds blow from the northeast, parallel to the Mozambique coastline, favoring
coastal upwelling. Additionally, the Mozambique Channel is largely influenced
by fast-rotating, mesoscale eddies that migrate southward towards the
Agulhas region. Using satellite altimetry, Schouten et al. (2003)
observed four to six eddies on average, ca. 300 km in diameter, propagating yearly from the
central Mozambique Channel (15∘ S) toward the Agulhas area
(35∘ S) between 1995 and 2000. Seasonal upwelling occurs off
northern Mozambique (between ca. 15 and 18∘ S) (Nehring et al.,
1984; Malauene et
al., 2014), from August to March with a dominant period of about 2 months
although periods of 1–4 weeks have also been observed (Malauene et
al., 2014).
The sediment trap was moored at 16.8∘ S and 40.8∘ E, at a
water depth of 2250 m (Fig. 1; Fallet et al., 2010, 2011) and was the same
type as that used for the North Atlantic transect. We analyzed the LCD proxies for
two respective time intervals: the first interval covered ca. 3.5 years, from
November 2003 to September 2007, with a sampling interval of 21 days. The
second interval covered another year, between February 2008 and February 2009,
with a sampling interval of 17 days. Previously, Fallet et al. (2011)
published foraminiferal, U37K′ and TEX86 records
for the first time interval, and the organic carbon content for the follow-up
time series. For further details on the deployments and sample treatments, we
refer to Fallet et al. (2011, 2012). The two surface sediments are located
across the narrowest transect between Mozambique and Madagascar, and were
analyzed for U37K′ and TEX86 by Fallet et
al. (2012) and for LCDs by Lattaud et al. (2017b).
Cariaco Basin
The Cariaco Basin is one of the largest marine anoxic basins (Richards,
1975), located on the continental shelf of Venezuela. The basin is
characterized by permanent stratification and is strongly influenced by the
migration of the intertropical convergence zone (ITCZ). During late autumn
and winter, the ITCZ migrates to the south which results in decreased
precipitation and trade wind intensification that in turn induces upwelling
and surface water cooling. This seasonal upwelling is a major source of
nutrients that leads to strong phytoplankton growth along the Venezuelan
coast (e.g., Müller-Karger et al., 2001; Thunell et al., 2007). Between
August and October, the ITCZ moves northward again, resulting in a rainy
season and diminishing the trade winds which inhibits upwelling. During this
wet season the contribution of terrestrially derived nutrients is higher. Due
to the prevalent anoxic conditions in the basin, there is no bioturbation;
this has resulted in the accumulation of laminated sediments that provide
excellent annually to decadally resolved climate records (e.g., Peterson et
al., 1991; Hughen et al., 1996, 1998). Moreover, in November 1995, a time
series experiment started to facilitate research on the link between
biogeochemistry and the downward flux of particulate material under anoxic
and upwelling conditions (Thunell et al., 2000). This project (CARIACO;
http://imars.marine.usf.edu/cariaco, last access: November 2018)
involved hydrographic cruises (monthly), water column chemistry measurements
and sediment trap sampling (every 14 days). One mooring containing four
automated sediment traps (Honjo and Doherty, 1988) was deployed at
10.50∘ N and 64.67∘ W, at a bottom depth of around 1400 m.
These traps were moored at a depth of 275 m, just above the oxic/anoxic
interface (Trap A), at 455 m (Trap B), at 930 m (Trap C) and at 1255 m
(Trap D). All traps contained a 13-cup carousel which collected sinking
particles over 2 weeks, and were serviced every 6 months. For further details on trap deployment and recovery, and sample
collection, storage and processing we refer the reader to Thunell et
al. (2000) and Goñi et al. (2004). In addition to the sediment trap
sampling, the primary productivity of the surface waters was measured every
month using 14C incubations (Müller-Karger et al., 2001, 2004).
For this study, we investigated two periods, i.e., May 1999–May 2000 and
July 2002–July 2003 for traps A and B. These years include upwelling and
non-upwelling periods, as well as a disastrous flooding event in
December 1999 (Turich et al., 2013). Turich et al. (2013) identified the
upwelling periods, linked to the migration of the ITCZ, as indicated by
decreasing SST in the CTD (temperature at -1 m water depth) and
satellite-based measurements (indicated by grey boxes in Figs. 8 and 10), and
shoaling of the average depths of primary production and increased primary
production. Moreover, Turich et al. (2013) evaluated the
U37K′ and TEX86 proxies for the same two time
series for which we analyzed the LCD proxies.
Instrumental data
Satellite SST, precipitation and wind speed time series of the M1, M2 and M4
moorings in the Atlantic derive from Guerreiro et al. (2017, 2019), who
retrieved these data from the Ocean Biology Processing Group (OBPG, 2014;
Frouin et al., 2003), the Goddard Earth Sciences Data and Information
Services Center (2016) (Huffman et al., 2007; Xie and Arkin, 1997) and the
NASA Aquarius project (2015a, b) (Lee et al., 2012) (see Supplement of
Guerreiro et al., 2017 for detailed references). The SST and chlorophyll a
time series data for the Mozambique Channel were adapted from Fallet et
al. (2011), who retrieved these data from the Giovanni database (for details
see Fallet et al., 2011). Surface sediment proxy temperatures were compared
to annual mean SST estimates derived from the World Ocean Atlas (2013)
(decadal averages from 1955 to 2012; Locarnini et al., 2013). Sea surface
temperature data for the Cariaco Basin were adopted from Turich et al. (2013)
and combined with additional CTD temperatures from the CARIACO time series
database for depths of 2, 5, 10, 15 and 20 m
(http://www.imars.usf.edu/CAR/index.html, last access: November 2018;
CARIACO time series composite CTD profiles; lead principal investigator:
Frank Müller-Karger).
Lipid extractionTropical North Atlantic
The 120 sediment trap samples were sieved through a 1 mm mesh wet-split into
five aliquots (van der Does et al., 2016), of which one was washed with
Milli-Q water, freeze-dried and homogenized for chemical analysis (Korte et
al., 2017). For organic geochemistry, sub-aliquots (by weight) were extracted
as described by Schreuder et al. (2018a). Briefly, ca. 100 mg dry weight of
sediment trap residue and between 1.5 and 10 g dry weight of surface
sediment were extracted by ultrasonication using a mixture of
dichloromethane : methanol (DCM/MeOH) (2:1; v/v), and
were dried over a Na2SO4 column. For quantification of LCDs,
LCAs and GDGTs, we added the following internal standards to the total lipid
extracts (TLEs): 2.04 µg C22 7,16-diol (Rodrigo-Gamiz et
al., 2015), 1.50 µg 10-nonadecanone (C19:0 ketone)
and 0.1 µg C46 GDGT (Huguet et al., 2006), respectively.
Subsequently, the TLEs were separated into apolar (containing n-alkanes),
ketone (containing LCAs) and polar (containing LCDs and GDGTs) fractions over
an activated (2 h at 150 ∘C) Al2O3 column by eluting
with hexane/DCM (9:1; v/v), hexane/DCM (1:1; v/v) and
DCM/MeOH (1:1; v/v), respectively. The apolar fractions were
analyzed by Schreuder et al. (2018a) for n-alkanes. Polar fractions were
split for GDGT (25 %) and LCD (75 %) analysis. The LCD fraction was
silylated by the addition of BSTFA
(N,O-bis(trimethylsilyl)trifluoroacetamide) and pyridine, and were heated
at 60 ∘C for 20 min, after which ethyl acetate was added prior to
analysis. The ketone fraction was also dissolved in ethyl acetate, and
analyzed by GC (gas chromatography) and GC/MS (gas chromatography mass spectrometry). The GDGT
fraction was dissolved in hexane/isopropanol (99:1, v/v),
filtered through a 0.45 µm polytetrafluoroethylene (PTFE) filter
and analyzed by HPLC-MS (high-performance liquid chromatography – mass spectrometry).
Mozambique Channel
Aliquots of the sediment trap samples from the Mozambique Channel were
previously extracted and analyzed by Fallet et al. (2011) and Fallet et al. (2012),
respectively. The sediment trap material was extracted by ultrasonication
using a mixture of DCM/MeOH (2:1; v/v), dried over
Na2SO4, and separated into apolar, ketone and polar fractions
via alumina pipette column chromatography, by eluting with hexane/DCM
(9:1; v/v), hexane/DCM (1:1; v/v) and DCM/MeOH (1:1;
v/v), respectively. These existing polar fractions of the sediment trap
material were silylated (as described above), dissolved in ethyl acetate and
reanalyzed for LCDs by GC-MS. As no record was kept of the division of the
extracts and polar fractions into aliquots, we report the results in relative abundance
rather than concentrations and fluxes of diols.
Cariaco Basin
Sediment trap material was extracted as described by Turich et al. (2013). Briefly, 1/16 aliquots of
the trap samples were extracted by means of Bligh–Dyer extraction with
sonication using a phosphate buffer and a trichloroacetic acid (TCA) buffer.
The extracts were then separated by adding 5 % NaCl in solvent-extracted
distilled deionized water, the organic phase was collected, and the aqueous
phase was extracted twice more. The extracts were pooled and dried over
Na2SO4 and separated by means of Al2O3 column
chromatography, eluting with hexane/DCM (9:1; v/v),
DCM/MeOH (1:1; v/v) and MeOH. For this study, the DCM/MeOH
(1:1; v/v) fraction was silylated (as described above), dissolved in
ethyl acetate, and analyzed for LCDs using GC-MS. Similar to the Mozambique
Channel samples, no record was kept of the division of extracts and polar
fractions into aliquots; thus, we report the results in relative abundance.
Instrumental analysisGDGTs
The GDGT fractions of the surface sediments and sediment traps SPM samples of
the tropical North Atlantic were analyzed for GDGTs using
ultra-high-performance liquid chromatography mass spectrometry (UHPLC-MS). We
used an Agilent 1260 HPLC, which was equipped with an automatic injector,
interfaced with a 6130 Agilent MSD and HP ChemStation software according to
Hopmans et al. (2016). Compound separation was achieved by two silica BEH
HILIC columns in tandem (150 mm × 2.1 mm; 1.7 µm; Waters
ACQUITY) in normal phase, at 25 ∘C. GDGTs were eluted isocratically
for 25 min with 82 % A and 18 % B, followed by a linear gradient to 35 % B in 25 min and
finally a linear gradient to 100 % B in the last 30 min. “A” denotes
hexane; “B” denotes hexane/isopropanol (9:1; v/v). The flow rate was constant at
0.2 mL min-1, and the injection volume was 10 µL. The
APCI-MS (atmospheric pressure chemical ionization – mass spectrometry) conditions are
described by Hopmans et al. (2016). Detection and quantification of GDGTs was
achieved in selected ion monitoring mode (SIM) mode of the protonated molecules ([M + H]+) of the GDGTs. We used a
mixture of crenarchaeol and C46 GDGT (internal standard) to assess
the relative response factor, which was used for quantification of the GDGTs
in the samples (cf. Huguet et al., 2006).
Sea surface temperatures were calculated by means of the TEX86H
as defined by Kim et al. (2010), which is a logarithmic function of the
original TEX86 index (Schouten et al., 2002):
TEX86H=logGDGT-2+GDGT-3+[Cren′]GDGT-1+GDGT-2+GDGT-3+[Cren′],
where the numbers indicate the number of cyclopentane moieties of the
isoprenoid GDGTs, and Cren′ reflects an isomer of crenarchaeol,
i.e., containing a cyclopentane moiety with a cis stereochemistry
(Sinninghe Damsté et al., 2018). The TEX86H values were
translated to SSTs using the core-top calibration of Kim et al. (2010):
SST=68.4×TEX86H+38.6
The branched isoprenoid tetraether (BIT) index is a proxy for the relative
contribution of terrestrially derived organic carbon (Hopmans et al., 2004). We calculated the modified version as reported by de Jonge et al. (2014,
2015) which is based on the original index as proposed by Hopmans et
al. (2004), but includes the 6-methyl brGDGTs:
BIT=brGDGTIa+brGDGTIIa+IIa′+[brGDGTIIIa+IIIa′]brGDGTIa+[brGDGTIIa+IIa′]+brGDGTIIIa+IIIa′+[Cren],
where the numbers reflect different branched GDGTs (see Hopmans et al., 2004)
and Cren reflects crenarchaeol. The branched GDGTs were always around the
detection limit in the Atlantic samples, implying a BIT index of around zero
and thus minimal influence of soil organic carbon (Hopmans et al., 2004);
therefore, the BIT index is not discussed any further.
LCAs
The ketone fractions of the surface sediments and sediment traps samples of
the tropical North Atlantic were analyzed for LCAs on an Agilent 6890N gas
chromatograph (GC) with flame ionization detection (FID) after being dissolved in
ethyl acetate. The GC was equipped with a fused silica column with a length
of 50 m, a diameter of 0.32 mm and a coating of CP Sil-5 (film
thickness = 0.12 µm). Helium was used as the carrier gas, and the
flow mode was a constant pressure of 100 kPa. The ketone fractions were
injected on-column at a starting temperature of 70 ∘C, which was
increased by 20 ∘C min-1 to 200 ∘C followed by
3 ∘C min-1 until the final temperature of 320 ∘C was
reached. This end temperature was held for 25 min.
The U37K′ index was calculated according to Prahl and
Wakeham (1987):
U37K′=[C37:2][C37:2]+[C37:3]
The U37K′ values were translated to SST following the
calibration of Müller et al. (1998):
SST=U37K′-0.0440.033
We also applied the recently proposed BAYSPLINE Bayesian calibration of
Tierney and Tingley (2018). They and others have shown that the
U37K′ estimates substantially attenuate above
temperatures of 24 ∘C (e.g., Conte et al., 2001; Goñi et al.,
2001; Sicre et al., 2002). The Bayesian calibration moves the upper limit of
the U37K′ calibration from approximately 28 to
29.6 ∘C at unity. As our traps are located in tropical regions with
SSTs > 24 ∘C, we applied this calibration as well.
LCDs
The silylated polar fractions were injected on-column on an Agilent 7890B GC
coupled to an Agilent 5977A MS. The starting temperature was 70 ∘C,
and was increased to 130 ∘C by 20 ∘C min-1, followed by a
linear gradient of 4 ∘C min-1 to an end temperature of
320 ∘C, which was held for 25 min. A total of 1 µL was injected, and
separation was achieved on a fused silica column (25×0.32 mm)
coated with CP Sil-5 (film thickness 0.12 µm). Helium was used as
the carrier gas with a constant flow of 2 mL min-1. The MS operated with
an ionization energy of 70 eV. Identification of LCDs was carried out in full scan
mode, scanning between m/z 50 and 850, based on characteristic fragmentation
patterns (Volkman et al., 1992; Versteegh et al., 1997). Proxy calculations
and LCD quantifications were performed via analysis (in SIM mode) of the
characteristic fragments (m/z 299, 313, 327 and 341; Rampen et al., 2012;
m/z 187 for internal diol standard). For quantification of LCDs in the
sediment traps and seafloor sediments of the tropical Atlantic, the peak
areas of the LCDs were corrected for the average relative contribution of the
selected SIM fragments to the total ion counts, i.e., 16 % for the
saturated LCDs, 9 % for unsaturated LCDs and 25 % for the
C22 7,16-diol internal standard.
Sea surface temperatures were calculated using the LDI, according to Rampen
et al. (2012):
LDI=[C301,15-diol]C281,13-diol+C301,13-diol+[C301,15-diol]
These LDI values were converted into SSTs using the following equation
(Rampen et al., 2012):
SST=LDI-0.0950.033
Upwelling conditions were reconstructed using the diol index as proposed by
Rampen et al. (2008):
Diol index=C281,14-diol+[C301,14-diol]C281,14-diol+[C301,14-diol]+C301,15-diol
In 2010, Willmott et al. introduced an alternative diol index, which is
defined as the ratio of 1,14-diols over 1,13-diols. As the index of Rampen
et al. (2008) includes the C30 1,15-diol, it can be affected by
temperature variation; therefore, we would normally prefer to use the
index of Willmott et al. (2010). However, we often did not detect the
C28 1,13-diol, or it co-eluted with cholest-5-en-7-one-3β-ol, compromising the calculation of the diol index of Willmott et
al. (2010). Moreover, the temperature variations in all three sediment traps
are minimal as recorded by the LDI. Accordingly, we chose to apply the diol
index according to Rampen et al. (2008).
Potential fluvial input of organic carbon was determined by the fractional
abundance of the C32 1,15-diol (de Bar et al., 2016; Lattaud et
al., 2017a):
FC321,15-diol=C321,15-diolC281,13-diol+[C301,13-diol]+C301,15-diol+[C321,15-diol]
The fractional abundance of the C32 1,15-diol was always lower than
0.23, suggesting low input of river-derived organic carbon (Lattaud et al.,
2017a).
ResultsTropical North Atlantic
We analyzed sediment trap samples from a longitudinal transect (∼12∘ N) in the tropical North Atlantic (two upper traps at a depth of ca.
1200 m, and three lower traps at ca. 3500 m; Fig. 1), covering
November 2012–November 2013, as well as seven underlying surface sediments,
for LCDs, LCAs and GDGTs. Below we present the results for these lipid
biomarkers and associated proxies.
LCDs
The LCDs detected in the sediment trap samples and surface sediments from the
tropical North Atlantic (Fig. 2) are the C28, C30 and
C30:1 1,14- (not in surface sediments), C28 and
C30 1,13-, and the C30 1,15-, and C32 1,15-diols.
We detected the C28 1,14-diol and C29-OH fatty acid in
the traps from M1 and M4, in a few samples of the M2 traps and in all surface
sediments. For most samples from M2U and M2L, the C28 1,14-diol was
often part of a high background signal, making identification and
quantification problematic. In these cases, 1,14-diol fluxes and the diol index
were solely based on the (saturated and monounsaturated) C30
1,14-diol.
Relative concentrations of biomarker lipids for the M1, M2 and M4 mooring sites
in the tropical North Atlantic. Upper panels show the percentages of
lipid biomarkers in the lower traps (“L”; 3500 m) and the surface
sediments (“Sed.”) relative to the annual flux-weighted concentrations in
the upper traps (“U”; 1200 m; set at 100 %). The lower panel shows the
preservation of the individual LCDs (sediments versus upper trap
flux-weighted concentration) for the three sediment trap sites. For M1 and M2
the sedimentary LCD concentrations were based on the average of the two
nearby underlying surface sediments (Fig. 1). When no bar is shown the
LCD was not detected in the surface sediments.
The average [1,13+1,15]-diol flux is 2.6 (±1.0) µg m-2 d-1 at M1U, 1.4 (±1.2) and 1.2 (±1.1) µg m-2 d-1 for M2U and M2L, respectively, and 7.0
(±7.8) and 2.2 (±3.3) µg m-2 d-1 for M4U and
M4L, respectively (Fig. 3). The [1,13+1,15]-diol and 1,14-diol
concentrations in the underlying sediments vary between 0.05 and
0.50 µg g-1 and between 3 ng g-1 and
0.06 µg g-1, respectively. The 1,14-diol flux for M1U
averages 0.5 (±0.8) µg m-2 d-1 with a pronounced
maximum of 3.5 µg m-2 d-1 in late April (Fig. 5a),
irrespective of the total mass flux. The average 1,14-diol flux at M2 is much
lower and is similar for the upper and lower traps, being around 0.01–0.02
(±0.01) µg m-2 d-1. At M4, the average 1,14-diol
fluxes are 0.3 (±0.5) and 0.1 (±0.2) µg m-2 d-1 for the upper and lower trap,
respectively. There are two evident maxima in the [1,13+1,15]-diols and
1,14-diol fluxes in late April and during October/November, concomitant with
maxima in the total mass flux (Fig. 3d, e). However, in the lower trap
this flux maximum is distributed over two successive trap cups, corresponding
to late April/early May (Fig. 3e, j).
Lipid biomarker fluxes for the tropical North Atlantic sediment
traps, i.e., M1, upper and lower M2, and upper and lower M4 in
panels (a) to (e). Lipid biomarker fluxes (iGDGTs in
purple; C37 alkenones in orange; 1,13- and 1,15-diols in black;
1,14-diols in red) are indicated on the left y axis, and the total mass
flux (grey stack; Korte et al., 2017) is shown on the right y axis. Lipid biomarker
concentrations are plotted in panels (f) to (j), with
biomarker concentrations on the left y axis, and the total mass flux on the
right y axis. Note that the y axes are different per sediment trap site,
but identical for upper (U) and lower (L) traps.
The LDI ranged between 0.95 and 0.99 in all traps, corresponding to
temperatures of 26.0 to 27.3 ∘C with no particular trends (Fig. 4).
For most M2 and M4 samples the C28 1,13-diol was below the
quantification limit; hence, LDI was always around unity, corresponding to
26.9 to 27.3 ∘C (Fig. 4), whereas in other samples the C28
1,13-diol co-eluted with cholest-5-en-7-one-3β-ol, prohibiting the
calculation of the LDI and the diol index (Figs. 4, 5). The flux-weighted
annual average LDI-derived SSTs were 26.6 ∘C for M1U, and
27.1 ∘C for M2U, M2L, M4U and M4L. The underlying sediment was very
similar, with LDI values of between 0.95 and 0.98 corresponding to 26.0 and
26.9 ∘C (Fig. 6). The diol index varied from 0.03 to 0.30 in M1U,
showing a pronounced maximum during spring (Fig. 5a). The diol index at M2
ranged between 0.01 and 0.03 without an evident pattern, whereas the diol
index at M4 ranged from 0.01 to 0.10 and showed the same pattern in the lower
and upper trap, with the highest values during spring (ca. 0.1), followed by
a gradual decrease during summer (Fig. 5e, f).
Temperature proxy records for the tropical North Atlantic. The
panels show (a) the upper trap station M1, (b) the upper
trap station M2 and (d) the lower trap M2, respectively, and
(c) the upper trap station M4 and (e) lower trap station
M4, respectively.
Phytoplankton productivity records for the tropical North Atlantic.
Panels (a)–(c) and (e)–(f) show the
1,14-diol fluxes (left y axis; black) and the diol index (right y axis;
grey) for the sediment traps. The y axes are the same for these panels.
Wind speed and precipitation data were adapted from Guerreiro et al. (2019);
for references regarding remote sensing parameters, see Guerreiro et
al. (2017). Panels (d) and (g) show the C37
alkenone fluxes (left y axis; black) and combined fluxes of
Emiliania huxleyi and Gephyrocapsa oceanica (from Guerreiro
et al., 2017; right y axis; grey) for the upper traps of M2 and M4.
Flux-weighted average (annual) proxy results for the sediment traps
compared with the underlying sediments (crosses) and annual mean SST (red
line; specific for the coordinates of the surface sediments; World Ocean
Atlas 2013 1/4∘ grid resolution).
Panels (a), (b) and (c) show the LDI,
U37K′ and TEX86 temperature results, respectively.
Triangles reflect sediment trap results (red represents upper/∼1200 m;
blue represents lower/∼3500 m), and crosses represent surface
sediments. In the case of the U37K′ and TEX86, the
green and purple triangles and grey crosses reflect the temperatures
calculated using the BAYSPLINE and BAYSPAR models (Tierney and Tingley, 2014,
2015, 2018), whereas the other temperatures were calculated using the
Müller et al. (1998) and Kim et al. (2010; TEX86H)
calibrations, respectively. Panel (d) shows the flux-weighted
average diol index values for the sediment traps and the diol index estimates
for the surface sediments.
LCAs
We detected C37, C38 and C39 long-chain
alkenones in the sediment trap and surface sediments. The C37:3
alkenone was generally around the limit of quantification for the M2L and M4L
traps, and below the limit of quantification for four out of the seven
surface sediment samples, whereas the C37:2 alkenone was always
sufficiently abundant. The annual mean fluxes of the C37 LCAs were 4.3
(±3.5) µg m-2 d-1 for M1U, 1.2 (±0.9) µg m-2 d-1 and 0.4 (±0.2) µg m-2 d-1 for M2U and M2L, respectively, and 2.9
(±5.1) µg m-2 d-1 and 1.2 (±2.0) µg m-2 d-1 for M4U and M4L, respectively. The
concentrations of the C37 LCAs in the underlying surface sediments
ranged between 0.02 and 0.41 µg g-1. At M4, the two total
mass flux peaks at the end of April and during October/November were also
clearly pronounced in the C37 alkenone fluxes (Figs. 3d, e, 5g), as
well as the increased signal in the cup reflecting the beginning of May,
which followed the cup which recorded the peak in total mass flux at the end
of April. The U37K′ varied from 0.87 to 0.93,
corresponding to 25.1 to 27.0 ∘C (Fig. 6b) for three out of seven
surface sediments in which the C37:3 was above the
quantification limit. The flux-weighted average SSTs were 26.1 ∘C
for M1U, 25.7 and 26.4 ∘C for M2U and M2L, respectively, and 28.2
and 27.5 ∘C for M4U and M4L, respectively (Fig. 6). SST variations
per sediment trap were generally within a 2–3 ∘C range (Fig. 4)
with no apparent trends.
GDGTs
The main GDGTs detected were the isoprenoidal GDGT-0, -1, -2, -3,
crenarchaeol and the isomer of crenarchaeol. Branched GDGTs were typically
around or below quantification limit. The average iGDGT flux in M1U was 15.5
(±4.6) µg m-2 d-1, 2.4 (±1.1) and 2.6 (±0.3) µg m-2 d-1 in M2U and M2L, respectively, and 4.3
(±1.5) and 2.9 (±1.2) µg m-2 d-1 in M4U and
M4L, respectively (Fig. 3). The surface sediments exhibited iGDGT
concentrations between 0.4 and 1.7 µg g-1. Sediment
TEX86H values varied between 0.62 and 0.69, corresponding to
24.3 to 27.4 ∘C. The TEX86H flux-weighted average SSTs
were 25.2 ∘C for M1U, 27.3 and 26.6 ∘C for M2U and M2L,
respectively, and 27.8 and 26.7 ∘C for M4U and M4L, respectively.
SSTs typically varied within a range of 1–2 ∘C. At M2U, the
TEX86H temperatures decrease slightly (ca. 1–2 ∘C)
between January and July (Fig. 4b).
Mozambique Channel
For two time series (November 2003–September 2007 and
February 2008–February 2009), we analyzed LCDs collected in the sediment
trap at a depth of 2250 m as well as nearby underlying surface sediments
(Fig. 1). The main LCDs observed in the sediment traps and surface sediments
were the C28 1,12-, 1,13- and 1,14-diols, the C30 1,13-,
1,14- and 1,15-diols, and the C32 1,15-diol. We also observed the
C30:1 1,14 diol in some trap
samples, and the C29 12-OH fatty acid in all trap and sediment
samples. In 24 samples, the C28 1,13-diol co-eluted with
cholest-5-en-7-one-3β-ol, and thereafter we did not calculate the LDI
for these samples. The C28 1,14-diol was not affected by this
cholest-5-en-7-one-3β-ol due to its much higher abundance compared
with the C28 1,13-diol; therefore, the diol index was still
calculated. The LDI varied between 0.94 and 0.99, i.e., close to unity,
corresponding to 25.5 to 27.2 ∘C, without an evident trend
(Fig. 7a). The diol index ranged between 0.11 and 0.69, showing substantial
variation, although not with an evident trend (Fig. 7b). The average
LDI-derived temperature of the two underlying surface sediments was
26.0 ∘C.
The LDI-derived temperatures, in addition to the TEX86H
and U37K′-derived temperatures and satellite SST (Fallet
et al., 2011) (a) and the diol index (b) for the Mozambique
Channel sediment trap. The black cross in panel (a) reflects the
average LDI temperature of two underlying surface sediments, with the LDI
calibration error. The chlorophyll a data are from Fallet et al. (2011).
Cariaco Basin
We analyzed LCDs for two time series (May 1999–May 2000 and
July 2002–July 2003) from the upper (Trap A; 275 m) and the lower (Trap B;
455 m) trap in the Cariaco Basin. The main LCDs detected for both time
series are the C28 1,14-, C30 1,14-, C30:1
1,14-, C28 1,13-, C30 1,15- and C32 1,15-diols,
as well as the C29 12-OH fatty acid. For some samples we did not
compute the LDI, as the C28 1,13-diol co-eluted with
cholest-5-en-7-one-3β-ol. In a similar fashion to the Mozambique
Channel, the C28 1,14-diol was not affected by this co-elution due
to its much higher abundance compared with the C28 1,13-diol;
therefore, the diol index was therefore still calculated. The calculated LDI
values ranged between 24.3 and 25.3 ∘C and 22.0 and 27.2 ∘C
for Trap A and B of the 1999–2000 time series, respectively, with the lowest
temperature during winter and the highest during summer. For the 2002–2003
time series, LDI temperatures for Trap A ranged between 23.3 and
26.2 ∘C and between 22.5 and 26.5 ∘C for Trap B.
For the May 1999–May 2000 time series, the diol index varied between 0.05
and 0.97 for Trap A and between 0.05 and 0.91 for Trap B (Fig. 8) with
similar trends, i.e., the lowest values of around 0.1–0.2 just before the
upwelling period during November, rapidly increasing towards values between
ca. 0.8 and 1 during the upwelling season (January and February). For the
time series of July 2002–July 2003, the diol index showed similar trends,
i.e., diol index values around 0.8–0.9 during July, which rapidly decrease
towards summer values of around 0.2–0.3. Similar to the 1999–2000 time
series, the lowest index values (ca. 0.2) are observed just before the
upwelling period (during September), after which they increase towards values
of around 0.8–0.9 between December and March at the start of the upwelling
season. At the end of the upwelling season the diol index increases, followed
by another maximum of around 0.6 during May.
Seasonal proxy-derived temperature and upwelling/productivity
records for the sediment traps in the Cariaco Basin. Panels (a),
(b) and (c) show the May 1999–May 2000 time series LDI-,
U37K′- and TEX86H-derived temperature
reconstructions for Trap A (depth of 275 m; solid symbols) and Trap B (depth
of 455 m; dashed symbols), respectively. Panels (e), (f)
and (g) show the proxy data for the July 2002–July 2003 time
series, with CTD-temperatures (1 m depth) in red. The
U37K′, TEX86H and CTD temperatures are
adopted from Turich et al. (2013). The horizontal lines reflect the average
proxy-derived temperatures (Trap A is denoted using solid lines; Trap B is
denoted using dashed lines). Panels (d) and (h) show the
1,14-diol based diol index (Rampen et al., 2008) for the 1999–2000 and
2002–2003 time series, respectively, for Trap A (depth of 275 m; solid
symbols) and Trap B (depth of 455 m; dashed symbols). Primary productivity
in mg C m-3 h-1 is plotted in green (data adopted from Turich et
al., 2013). The shaded area reflects the period of upwelling.
DiscussionLCD sources and seasonality
The 1,14 diols can potentially be derived from two sources:
Proboscia diatoms (Sinninghe Damsté et al., 2003; Rampen et al.,
2007) or the dictyochophyte Apedinella radians (Rampen et al.,
2011). The non-detection of the C32 1,14-diol, which is a biomarker
for Apedinella radians (Rampen et al., 2011), and the detection of
the C30:1 1,14 diol and C29 12-OH fatty acid, which are
characteristic of Proboscia diatoms (Sinninghe Damsté et al.,
2003), suggests that Proboscia diatoms are most likely the source of
1,14-diols in the tropical North Atlantic, the Mozambique Channel and the
Cariaco Basin.
In the Cariaco Basin, the diol index shows a strong correlation (visually as
correlation analysis was not possible due to differently spaced data in time)
with primary production rates, suggesting that Proboscia
productivity was synchronous with total productivity (Fig. 8), although for
the 1999–2000 time series there is a disagreement during January/February.
Primary productivity in the Cariaco Basin is largely related to seasonal
upwelling which occurs between November and May when the ITCZ is at its
southern position. Hence, the diol index seems to be an excellent indicator
of upwelling intensity in the Cariaco Basin.
The index also shows considerable variation over time in the Mozambique
Channel (Fig. 7b). Previous studies have shown that upwelling occurs in the
Mozambique Channel between ca. 15 and 18∘ S (Nehring et al., 1984;
Malauene et al., 2014), i.e., at the location of our sediment trap. Upwelling
is reflected by cool water events and slightly enhanced chlorophyll a
levels; Malauene et al. (2014) observed cool water events at ca. 2-month
intervals although periods of 8 to 30 days were also noted. The two
main potential forcing mechanisms for upwelling in the Mozambique Channel are
the east African monsoon winds and the mesoscale eddies migrating through
the channel. Fallet et al. (2011) showed that subsurface temperature, current
velocity and the depth of surface-mixed layer all revealed a dominant
periodicity of four to six cycles per year, which is the same frequency as
that of the southward migration of mesoscale eddies in the channel
(Harlander et al., 2009; Ridderinkhof et al., 2010), implying that eddy
passage strongly influences the water mass properties. Wavelet analysis of
the diol index for the 2003–2007 period (Fig. S1 in the Supplement) revealed
short periods, occurring around January of 2004, 2005 and 2006, of
significant (above the 95 % confidence level) variability at about
bimonthly frequencies (60-day period). Both the frequency (bimonthly) and the
timing (boreal winter) of the observed time periods of the enhanced diol index
variability are similar to those of the cool water events as observed by
Malauene et al. (2014), associated with upwelling (Fig. 7b). The strongest
variability of the diol index at about bimonthly frequencies occurred in the
first half of 2006. During the same period, salinity time series showed the
passage of several eddies that had a particularly strong effect on the upper
layer hydrography (Ullgren et al., 2012). Malauene et al. (2014) showed that
neither upwelling-favorable winds, nor passing eddies, can independently
explain the observed upwelling along the northern Mozambique coast. The two
processes may act together, and both strongly influence the upper water layer
and the organisms living there, potentially including the LCD producers.
The least (seasonal) variation in the diol index is observed at M2 in the
tropical North Atlantic (Fig. 5b, c), which is likely due to its central open
ocean position, associated with relatively stable, oligotrophic conditions
(Guerreiro et al., 2017). In contrast, M4 and M1 are closer to the South
American and west African coast, respectively, and thus are potentially under
the influence of Amazon river runoff and upwelling, respectively, and
specific wind and ocean circulation regimes (see Sect. 2.1.1). However, at
M4, the diol index is also low (max. 0.1), suggesting low Proboscia
productivity (Fig. 5e, f). At M1, in contrast, we observe enhanced values for
the diol index of up to ∼0.3 during spring (Fig. 5a). Most likely, an
upwelling signal at this location is associated with the seasonal upwelling
of the Guinea Dome. This upwelling is generally most intense between July and
October (Siedler et al., 1992), due to the northward movement of the ITCZ and
the resulting intensified Ekman upwelling. Specifically, during this period,
the trade winds are weaker, atmospheric pressure is lower and the regional
wind stress is favorable to upwelling of the North Equatorial Undercurrent
(Voituriez, 1981). Indeed, a decrease in wind speed and increased
precipitation from summer to autumn was observed (Fig. 5a) which confirms
that during these seasons the ITCZ was indeed in a northern position, and
that during 2013 the upwelling associated with the Guinea Dome was most
favored between July and October. The timing of the diol index peak, i.e.,
between March and June, is consistent with previous sediment trap studies
elsewhere which have shown that Proboscia diatoms and 1,14-diols are
typically found during pre-upwelling or early upwelling periods (Koning et
al., 2001; Smith, 2001; Sinninghe Damsté et al., 2003; Rampen et al.,
2007). The surface sediment at 22∘ W just east of M1 also reveals
the highest diol index (0.22), likely due to its closer vicinity to the
Guinea Dome center. Several studies have reported P. alata diatoms
offshore of northwest Africa (Lange et al., 1998; Treppke et al., 1996;
Crosta et al., 2012; Romero et al., 1999), pointing to P. alata as a
plausible source organism. The sedimentary annual diol indices compare well
with the sediment trap indices (Fig. 6d), which is consistent with the
results of Rampen et al. (2008). Our results clearly show that the diol index
reflects different things in different regions. This is due to the ecology of
Proboscia spp. where blooms occur during stratification, during
early upwelling and post-bloom, and from high nutrients to low nutrients (see
Rampen et al., 2014a; references in Table 1). Therefore, the type of
conditions reflected by the diol index is specific for every region.
To assess variations in the seasonal production of 1,13- and 1,15-diols in
the tropical Atlantic, for which we have the most complete data set, we
calculated the flux-weighted 1,13- and 1,15-diol concentrations for the
different traps, and summed these per season (Fig. 9). Highest production is
observed in autumn, followed by spring and summer, with the lowest production
during winter (∼60 % compared with autumn). This is in agreement
with Rampen et al. (2012) who observed, for an extensive set of surface
sediments, the strongest correlation between LDI and SST for autumn,
suggesting that production of the source organisms of the LDI mainly occurs
during autumn. At M4, there are two evident peaks in the 1,13- and 1,15-diol
fluxes at the end of April and October 2013. These maxima correlate with
peaks in other lipid biomarker fluxes (i.e., 1,14-diols, C37
alkenones and iGDGTs), total mass flux, calcium carbonate (CaCO3),
OM (organic matter) and the residual mass flux which includes the deposition flux of Saharan dust (Korte et al., 2017).
According to Guerreiro et al. (2017), the maximum in total mass flux at the
end of April 2013 is likely caused by enhanced export production due to
nutrient enrichment as a result of wind-forced vertical mixing. The peak at
the end of October 2013, is likely associated with discharge from the Amazon
River. Moreover, both peaks are concomitant with prominent dust flux maxima,
suggesting that Saharan dust also acted as nutrient fertilizer (Korte et al.,
2017; Guerreiro et al., 2017). Guirreirro et al. (2017) suggested that during
the October–November event the Amazon River may not only have acted as
nutrient supplier, but also as buoyant surface density retainer of
dust-derived nutrients in the surface waters, resulting in the development of
algal blooms within just a few days, potentially explaining the peak 1,13-
and 1,15-diol fluxes, as well as the peak fluxes of the other lipid
biomarkers. However, they might also partially result from enhanced particle
settling, caused by factors such as dust ballasting or faecal pellets of
zooplankton (see Guerreiro et al., 2017, and references therein). This agrees
with the results of Schreuder et al. (2018a) that show that the n-alkane
flux also peaks concomitantly with the peaks in total mass flux and
biomarkers, whereas n-alkanes are terrestrially derived (predominantly
transported by dust); therefore, increased deposition can not result from
increased primary productivity in the surface waters.
Seasonal summed flux-weighted average of 1,13-/1,15-diol
concentrations in all sediment traps (station M1 upper trap, station M2 upper
and lower trap, and station M4 upper and lower trap) of the tropical North
Atlantic.
The C37 alkenone flux at M4U also reveals these two distinct maxima
at the end of April and October during 2013 (Fig. 5g). Interestingly, this
flux, as well as the alkenone flux at M2U, is consistent with coccolith
export fluxes of the species Emiliania huxleyi and
Gephyrocapsa oceanica (Guerreiro et al., 2017). In fact, when we
combine the coccolith fluxes of both species, we observe strong correlations
with the C37 alkenone fluxes for both M2U and M4U (Fig. 5d and g,
respectively; r=0.77 and 0.92 for M2U and M4U, respectively;
p-values < 0.001). This implies that these two species are the main LCA
producers in the tropical North Atlantic, which agrees with previous findings
(e.g., Marlowe et al., 1984; Brassell, 2014; Conte et al., 1994; Volkman et
al., 1995).
Preservation of LCDs
The sediment trap data from the North Atlantic can be used to assess the
relative preservation of LCDs, as well as other proxy lipid biomarkers, by
comparing the flux-weighted concentration in the traps with the
concentrations in the surface sediments. For all four biomarker groups, i.e.,
C37 alkenones, iGDGTs, 1,14-diols and 1,13- and 1,15-diols, we
observe that the flux-weighted concentrations are generally higher in the
upper traps (ca. 1200 m) compared with the lower traps (ca. 3500 m;
Fig. 2) by a factor of between 1.2 and 4.4, implying degradation during
settling down the water column. The concentrations in the surface sediments
are 2 to 3 orders of magnitude lower (i.e., between
0.1 %–1.5 % of upper trap signal), implying that degradation of
lipids mainly takes place at the water–sediment surface rather than in the
water column. A similar observation was made for levoglucosan in these
sediment traps (Schreuder et al., 2018b). Both are functionalized polar
lipids with alcohol groups and thus are chemically relatively similar when
compared to species such as fatty acids (carboxyl group) or n-alkanes (no functional
groups). These degradation rates are likely linked to the extent of the
oxygen exposure time (Hartnett et al., 1998; Hedges et al., 1999) at the
seafloor (Hartnett et al., 1998; Sinninghe Damsté et al., 2002), as
during settling the lipids are exposed to oxygen for weeks, whereas for
surface sediments this is typically decades to centuries. Our results compare
well with several other sediment trap studies which showed that LCDs, LCAs
and iGDGTs generally have a preservation factor of around 1 % (surface
sediment versus trap) (e.g., Prahl et al., 2000; Wakeham et al., 2002; Rampen et
al., 2007; Yamamoto et al., 2012).
We also identified the C30 and C32 1,15-keto-ol in
the Atlantic as well as in the Mozambique and Cariaco sediment traps and surface
sediments. These lipids are structurally related to LCDs, occur
ubiquitously in marine sediments (e.g., Versteegh et al., 1997, 2000; Bogus
et al., 2012; Rampen et al., 2007; Sinninghe Damsté et al., 2003; Wakeham
et al., 2002; Jiang et al., 1994) and were inferred to be oxidation products
of LCDs (Ferreira et al., 2001; Bogus et al., 2012; Sinninghe Damsté et
al., 2003). We did not detect 1,14-keto-ols, which supports the hypothesis
of Ferreira et al. (2001) and Sinninghe Damsté et al. (2003) that the
silica frustules of Proboscia diatoms sink relatively fast; thus,
they are exposed to oxygen for a shorter period than the producers of 1,13- and
1,15-diols, and are therefore less affected by oxidation. Alternatively, the keto-ols
are not oxidation products but are produced by unknown organisms in the water
column. In fact, Méjanelle et al. (2003) observed trace amounts of
C30 1,13- and C32 1,15-keto-ols in cultures of the marine
eustigmatophyte Nannochloropsis gaditana. Thus, an alternative
explanation for the non-detection of 1,14-keto-ols is that, in contrast to
the 1,15-keto-ols, they were not produced in the water column.
For both the tropical Atlantic and the Cariaco Basin, we observe highly
similar LDI values for the upper and the lower traps. In the Atlantic there
is no statistical difference between the upper and lower traps that are
2200 m apart (two-tailed p>0.8), but we have insufficient data for the
Cariaco Basin for statistical comparison (Figs. 6a, 8a, e). This suggests
that degradation in the water column does not affect the LDI proxy. This is
in agreement with Reiche et al. (2018) who performed a short-term degradation
experiment (<1 year) and found that the LDI index was not affected by oxic
exposure on short timescales. However, the oxygen exposure time on the
seafloor is much longer; Rodrigo-Gámiz et al. (2016) showed for sediments
in the Arabian Sea (deposited under a range of bottom water oxygen
conditions) that different LCDs had different degradation rates, which
compromised the LDI ratio. For the three sites in the tropical North
Atlantic, we calculated the flux-weighted average proxy values for every
sediment trap and compare these with the underlying surface sediments
(Fig. 6a–c). For all indices, i.e., diol index, LDI,
U37K′ and TEX86, we observe very good
correspondence between the sediment trap and surface sediment values,
implying minimal alteration of the proxies after settling and during burial.
Similarly, for the Mozambique Channel, the mean diol index and LDI from the
sediment trap (i.e., 0.41 and 0.97, respectively) are very similar to the
surface sediment values (i.e., 0.42 and 0.95, respectively). In agreement
with the consistent diol indices, we observe that all individual LCDs are
also preserved relatively equally in the tropical Atlantic
(1.2 %–4.3 % at station M1, 0.1 %–2.9 % at station M2 and
0.03 %–0.16 % at station M4). This contrasts with Rodrigo-Gámiz
et al. (2016) who found that the 1,15-diols have the highest degradation
rate, followed by the 1,14- and 1,13-diols. Only the C32 1,15-diol
seems relatively better preserved than the other LCDs at all three North
Atlantic mooring sites (Fig. 2), suggesting that the C32 1,15-diol
is less impacted by degradation. The C32 1,15-diol likely partially
derives from the same source as the other 1,13- and 1,15-diols, but is also
produced in fresh water systems (e.g., Versteegh et al., 1997, 2000; Rampen
et al., 2014b; de Bar et al., 2016; Lattaud et al., 2017a, b). Hence, the
different preservation characteristics might be the result of a different
source for this LCD.
Relationship between LDI and SST
In the tropical Atlantic and the Mozambique Channel, the LDI-derived SSTs
show minimal variability (<2∘C), whereas in the Cariaco Basin we
observe much larger changes that range from 22.0 to 27.2 ∘C
(Fig. 8). Both time series in the Cariaco Basin show low temperatures between
November and May, associated with the seasonal upwelling and surface water
cooling, and significantly higher temperatures during the rainy summer.
However, during the warmest periods, the LDI temperatures are generally lower
than those measured at the surface by CTD, whereas during the colder phases,
the LDI agrees well with the measurements. The LDI calibration reaches unity
at 27.4 ∘C; therefore, is not possible to resolve the highest
temperatures which are between ca. 28 and 30 ∘C. However, the
LDI-derived temperatures are sometimes well below 27.4 ∘C where the
CTD data suggest SSTs > 28 ∘C. Consequently, the LDI-based
temperatures agree with CTD-based SSTs within calibration error for most of
the record, but during summer when the SST is highest, they are offset
outside the calibration error (ΔT∼2.5∘C).
Interestingly, the U37K′- and
TEX86H-derived temperature trends show the same phenomenon
(Turich et al., 2013; Fig. 8), where the proxy temperatures are cooler than
the measured temperatures during the warmer months. However, in contrast to
the U37K′ and LDI, the TEX86H also
overestimates the SST during the cold months. For U37K′,
Turich et al. (2013) pointed out that a time lag between synthesis, export
and deposition could potentially explain the difference between the proxy and
CTD temperatures. However, previous analysis of plankton biomass, primary
productivity, bio-optical properties and particulate organic carbon fluxes
for the same time period (Müller-Karger et al., 2004), as well as the
total mass and terrigenous fluxes assessed by Turich et al. (2013) showed the
best correlation at zero-time lag on the basis of their 14-day sample
interval. We compared our LDI temperature estimates with monthly CTD
measurements between a depth of 0 and 50 m, the temperature at the depth of
maximum primary productivity and the temperature at the chlorophyll maximum
(Turich et al., 2013; http://www.imars.usf.edu/cariaco, last access:
November 2018) (Fig. 10). During the upwelling season, temperatures are
significantly lower due to the upward migration of isotherms, whereas during
the non-upwelling period, temperatures are higher, particularly in the upper
20 m, and the water column is more stratified (Fig. 10). LDI underestimates
the SST during stratification, which suggests that the LCD producers may
thrive at depths of ca. 20–30 m. During upwelling, LDI temperatures agree
better with SST, implying that the habitat of the LCD producers is
potentially closer to the surface, coincident with the shoaling of the
nutricline and thermocline (Fig. 10). However, these absolute differences in
LDI temperatures are generally within the calibration error (2 ∘C);
thus, these seasonal variations in LDI temperatures should be interpreted
with caution. Turich et al. (2003) found that the
U37K′-derived temperatures agreed reasonably well with
the measured temperatures at the chlorophyll maximum, which is generally
found below a depth of 20 m (average depth of 30–34 m; ranging between 1
and 55 m) in the Cariaco Basin. The LDI temperatures are almost always
higher than the temperatures at the chlorophyll maximum (Fig. 10), and higher
than the temperatures at a depth of 30 m, implying that the LDI producers
may reside in the upper 30 m of the water column; this is consistent with
the results of Rampen et al. (2012) which showed that LDI-derived
temperatures have the strongest correlation with the temperatures in the
upper 20 m of the water column. This also agrees with Balzano et al. (2018)
who observed the highest LCD abundances within the upper 20 m of the water
column in the tropical Atlantic.
LDI temperature records for the Cariaco Basin time series
May 1991–May 2000 and July 2002–July 2003 for Trap A (depth of 275 m;
solid symbols) and Trap B (depth of 455 m; dashed symbols), with CTD-derived
temperatures at depths of 2, 10, 20, 30 and 50 m (in red;
http://www.imars.usf.edu/CAR/index.html, last access: November 2018;
CARIACO time series composite CTD profiles), the temperature at the depth of
maximum primary production (PP maximum; green) and the temperature at the
depth of the chlorophyll maximum (yellow; data adapted from Turich et al.,
2013). The shaded area represents the upwelling season.
In the Mozambique Channel, the LDI temperature variations are much smaller
(<2∘C; Fig. 7a) than the seasonal SST variation – ranging
between ca. 24.5 and 30.5 ∘C. Accordingly, during the warmest months
of the year, the difference between LDI-derived and satellite-derived SST is
outside the calibration error (i.e., >2∘C). However, this is
similar to the U37K′ and TEX86H which also
did not reveal seasonal variations. This lack of seasonality was explained by
the lateral advection and resuspension of fine sediment material by migrating
mesoscale eddies which ending up in the deeply moored sediment trap (Fallet
et al., 2011, 2012). Most likely, this also explains the lack of seasonal variation
in our LDI record (Fig. 7a). Nevertheless, the average LDI temperature for
the sediment trap of 26.4 ∘C agrees reasonably well with the annual
mean satellite-derived SST of 27.6 ∘C for the sampled years.
Additionally, there is a good agreement with the average LDI temperature of
26.0 ∘C for the two underlying surface sediments, as well as with
the decadal average SST of 26.7 ∘C for 1955–2012 (Locarnini et al.,
2013) given by the World Ocean Atlas (2013). For the North Atlantic, we also
observe rather constant LDI temperatures during the year (Fig. 4) which
contrasts with seasonal variations in satellite SSTs of ca. 3 to
5 ∘C. Nevertheless, differences are mostly within the calibration
error, except at M1 and M2 where LDI-derived temperatures are between 0.5 and
2.8 ∘C higher than satellite SSTs during winter and spring. Similar
to the LDI, the TEX86H and U37K′-derived
SSTs for the tropical Atlantic sediment traps also do not reveal clear
seasonal variation. As all three proxies show minimal seasonal variability,
this might indicate that the lipids are potentially allochtonous and
partially derive from distant regions, resulting in an integrated average
temperature signal, similar to the Mozambique Channel. Nevertheless, the
flux-weighted annual LDI temperatures of the tropical Atlantic sediment traps
(26.6 for M1 and 27.1 ∘C for M2 and M4) agree well with the annual
mean satellite-derived SSTs of 26.1, 26.0 and 27.5 ∘C for M1, M2 and
M4, respectively. Moreover, the LDI-derived temperatures in the underlying
sediments (26.5, 26.6 and 26.7 ∘C, respectively) do not only agree
well with those found in a single year in the sediment traps, but they also
agree with the decadal average SSTs for 1955 to 2012 (26.2, 27.1 and
26.3 ∘C, respectively; Locarnini et al., 2013; Fig. 6a).
Conclusions
In this study we evaluated LCD-based proxies, particularly the LDI, in
sediment trap time series from five sites in the tropical North Atlantic, the
Cariaco Basin and the Mozambique Channel. For the North Atlantic we found
that ca. 25 %–85 % of the export of these lipid
biomarkers was preserved during settling from 1200 to 3500 m in the water column, and that
generally less than 2 % was preserved in the surface sediments. Despite
substantial degradation at the seafloor, likely linked to the prolonged
oxygen exposure time, LCD-derived temperatures from the sediments are
generally very similar to the annual mean LCD-derived temperatures in both
the deep and shallow traps as well as to the annual mean SST for the specific
sampling year and on decadal timescales for the specific sites. In the
Cariaco Basin we observe a seasonal signal in the LDI linked to the upwelling
season reflecting temperatures of the upper ca. 30 m of the water column.
The LDI temperatures in the Mozambique Channel and the tropical Atlantic
reveal minimal seasonal change although the seasonal SST contrasts amount to
3–5 ∘C. For the Mozambique Channel this is likely caused by the lateral
advection of resuspended sediment by mesoscale eddy migration, a signal not
substantially altered by diagenesis. Seasonal variations in the diol index
are minimal in the central and western North Atlantic and 1,14-diol
concentrations are rather low, implying little Proboscia diatom
productivity. However, in the eastern Atlantic, closest to the African
continent, the diol index attains a clear spring maximum that is likely
associated with upwelling in the Guinea Dome during summer to autumn,
suggesting the diol index reflects a pre-upwelling signal, consistent with
the current knowledge on Proboscia ecology. In the Cariaco Basin,
controlled by seasonal upwelling, the diol index reveals the same clear
seasonal trend observed in primary productivity, arguing that
the diol index is an excellent indicator of upwelling intensity for this
location.
Data availability
The data reported in this paper are archived using PANGAEA
(https://doi.pangaea.de/10.1594/PANGAEA.898278; de Bar et al., 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-16-1705-2019-supplement.
Author contributions
MWdB, JSSD and SS designed the experiments, and MWdB
carried them out. JU carried out the time series analysis. JBWS, GJAB and
RCT deployed sediment traps and collected sediment trap materials. MWdB
prepared the paper with contributions from all coauthors.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
We are grateful to Laura Schreuder and Denise Dorhout for analytical support,
Wim Boer for help with MATLAB calculations (BAYSPLINE), Laura Korte and
Catarina Guerreiro for constructive discussions, and Isla Castañeda,
Ulrike Fallet and Courtney Turich for providing and working up samples. This
research was funded by the European Research Council (ERC) under the
European Union's Seventh Framework Program (FP7/2007-2013) ERC grant
agreement no. 339206 to Stefan Schouten and ERC grant agreement no. 311152 as
well as NWO project no. 822.01.008 to Jan-Berend W. Stuut. Stefan Schouten and
Jaap S. Sinninghe Damsté receive financial support from the Netherlands
Earth System Science Centre (NESSC) through a gravitation grant from the
Dutch ministry for Education, Culture and Science (grant number 024.002.001).
Review statement
This paper was edited by Markus Kienast and reviewed by two
anonymous referees.
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