Free-drifting sediment net traps were deployed 14 times at depths between 80
and 500 m for 1–3 days each during the late monsoon–intermonsoon transition
in the central Arabian Sea. Two locations (19.5 and
15.5
Sinking particles are an important component of the biological pump. Once below the euphotic zone, organic matter and nutrients in sinking particles are removed from the biogeochemical processes of the upper ocean; the timescale of this sequestration depends on the depth to which the material sinks or is remineralized (Buesseler and Boyd, 2009). For sediment trap data, this is typically quantitated using either a power decay function (Martin et al., 1987) or by relating organic matter fluxes to ballast mineral flux (Armstrong et al., 2002). Using either approach, the decay function within oxygen-deficient zones (ODZs, waters with zero or unmeasurable oxygen; Jensen et al., 2011) is different – less decay – from that of the oxic portions of the ocean (Hartnett and Devol, 2003; Roullier et al., 2014; Van Mooy et al., 2002). That is, the relative proportion of organic matter that passes through ODZs is proportionally higher than that for oxic waters. There are many possible reasons for this, and currently they are poorly evaluated.
Potential mechanisms (M) that might cause organic carbon fluxes through ODZ waters to be less attenuated include those that are directly related to ODZs themselves: (M1) the possibility that anoxic degradation is sufficiently slower or less comprehensive relative to oxic carbon remineralization, (M2) the possibility that the microbial consortia in ODZ regions have a higher growth efficiency and incorporate chemoautotrophy, thus adding carbon to the flux, and (M3) zooplankton dynamics in ODZ regions might alter the flux attenuation in a variety of ways.
Mechanisms that are not necessarily specific to ODZ regions, but might simply be coincident, include (M4a) mineral protection where differences in bio- and lithogenic mineral inputs control the remineralization profile of sinking organic matter in the ocean, (M4b) changes in the sinking speed of particles due to differences in ballast, and (M5) inputs of recalcitrant organic matter, which could alter the flux attenuation. An additional potential mechanism (M6) is temperature. Marsay et al. (2015) suggest that temperature plays a role in determining the length scale of sinking particle remineralization, which could result in changes in apparent flux attenuation. Each of these potential mechanisms for enhanced flux through ODZ regions has merit, and they are briefly introduced below.
While the long-term (decade to millennia) importance of (M1) oxygen effects on organic matter preservation is well established (Arnarson and Keil, 2007; Hedges and Keil, 1995), the importance of oxygen over short time periods or during early diagenesis has been an extremely hard issue to address (Cowie and Hedges, 1992b). There seems to be an effect; Hartnett et al. (1998) suggested that the diminished oxygen exposure of ODZ sinking particles increases the efficiency with which they are transferred to deeper waters, and Van Mooy et al. (2002) observed direct oxygen effects in incubations of ODZ water from the eastern tropical North Pacific.
The microbial loop within an ODZ can influence the flux in a number of ways. If the community grows with a higher growth efficiency than oxic communities (M2a), then ODZ communities could produce more biomass per unit respiration. For example, if a dark aerobic community has a growth efficiency of 10 % and a suboxic ODZ community has a growth efficiency of 20 %, then, for every unit of respiration, twice as much new biomass will result, and this will manifest itself as a change in the attenuation coefficient. This concept has been loosely explored by W. Naqvi (2014, personal communication). Additionally, the long-held assumption that heterotrophic respiration is the only significant biogeochemical process in the mesopelagic has been overturned by the discovery of abundant chemoautotrophic microorganisms (Dalsgaard et al., 2003; Kuypers et al., 2003). Thus, a subcomponent of M2 could be the (M2b) chemoautotrophic production of new organic matter in ODZs, which would add to the flux if it sinks (Close et al., 2014; Roland et al., 2008).
Zooplankton dynamics within the ODZ are complex and could alter the sinking flux in a variety of ways. When they migrate into the ODZ, zooplankton (M3a) deliver dissolved nutrients (Wishner et al., 2013) that can support chemoautotrophy and (M3b) create pulses of defecated material that is not subject to remineralization in shallower waters (Cavan et al., 2015). Both of these things would enhance the flux. However, the ODZ is a harsh environment for a eukaryotic obligate aerobe, and if the ODZ hosts fewer zooplankton than an aerobic water column (Levin et al., 2015; Wishner et al., 2013) then it is logical to hypothesize that (M3c) the overall effect of zooplankton respiration will be less, which might in and of itself be enough to result in enhanced fluxes through the upper water column.
Minerals are a strong determinant on organic matter burial in sediments (Keil and Cowie, 1999; Keil and Mayer, 2014). How this impact is felt in the water column remains largely unconstrained. Inorganic material acts as a catalyst for particle formation, altering the flux of organic carbon (Lee et al., 2009). A possible mechanism linking mineral and enhanced flux would be that minerals can provide abundant points of interaction with organic matter, promoting aggregation and sorption (e.g., Passow and De La Rocha, 2006). (M4) Organic–mineral interactions might then protect the organic matter from remineralization, at least on the short timescale of sinking particles (Arnarson and Keil, 2005). Overall, organic carbon export might be enhanced both by increasing sinking rates and slowing degradation. Whether this mechanism could explain ODZ enhanced fluxes has not been explored.
Another potential factor linking carbon fluxes and mineral material is (M4b) sinking speed (Le Moigne et al., 2012). In both the Martin equation and the ballast model, sinking speed and remineralization rate combine to control the flux and attenuation. If, for some reason, ODZ regions have particles with enhanced sinking speeds, then lower attenuation per unit depth would result. Changes in sinking speed could result from changes in species composition or from changes in dust inputs, or the efficiency with which zooplankton and other community members package material into sinking particles (Cavan et al., 2015).
Lithogenic minerals might also impact sinking carbon by (M5) bringing refractory terrestrial organic matter with them (Burdige, 2007; Keil et al., 2004). To date, analyses of terrestrial carbon in sinking fluxes has largely been constrained to oxic water columns very close to land where the inputs are significant (Hedges et al., 1988).
Finally, the effect of temperature (M6) is to lengthen the depth scale of remineralization for colder (slower) situations (Matsumoto, 2007; Marsay et al., 2015). Since many ODZ regions are tropical or subtropical, temperature effects on particle degradation should result in higher attenuation (more degradation) in warm ODZ waters, but this appears not to be the case.
We evaluated these possible mechanisms using a combination of sediment trap
and incubation studies, by measuring a variety of mineral and organic
markers, and by comparing our data to recent literature for the Arabian Sea
that uses complimentary analyses that help us address mechanisms where our
data are insufficient. Direct analyses of our sediment trap materials
included mineral surface area and surface-associated elemental compounds
(using X-ray photoelectron spectroscopy), organic matter quantity and bulk
Surface-tethered, free-drifting sediment net traps (Peterson et al., 2005)
were deployed from the R/V
Spatial contours of water column parameters during the
sediment trap deployments, clockwise from upper left: density, nitrate,
nitrite, chlorophyll fluorescence, a map of the region including CTD
stations (and with the trap deployment stations labeled 1–3), and oxygen.
Squares in the upper left panel mark where sediment traps were deployed. A
2
Three stations were occupied and a total of 14 sinking particle samples
collected at depths from the thermocline to 500 m (Table 1). Two sizes of
nets were used: a 2 m diameter net as originally described by Peterson et al. (2005),
and a smaller 1.25 m diameter net (all dimensions scaled to the new
diameter). The smaller nets were used for shallower sampling (< 200 m)
and the larger ones used at depths greater than 200 m. Both nets used
53
Meta- and elemental data for the samples. Sta denotes the station number, T.D. is the trap diameter in meters, Deploy is the deployment time in hours, and wt % In is the weight percentage of inorganic mass.
Four additional samples were collected at depths > 1000 m by
deploying the net traps as vertical nets and raising them through 100 m of
the water column at 2 m min
The 500 mL to 2 L samples from sediment trap collection cups were
transferred to polypropylene bottles, swimmers (which were very minimal in
these oxygen-poor waters) were picked out, and the sample was frozen and
transported to the lab at the University of Washington. In the lab samples
were thawed and spun at 15 000
Elemental analyses for particulate carbon, nitrogen quantities and
XPS surface characterizations were done at the University of Washington's
Surface Analysis Recharge Center (
Amino acids were liberated via acid digestion (Cowie and Hedges, 1992a) and
quantified by liquid chromatography–triple quadrupole mass spectrometry
using an isotope pairing technique similar to that of Piraud et al. (2005).
Briefly, samples were hydrolyzed in 6 M HCl via programmed microwave
assistance in a MARS 5 microwave system (CEM Corp), brought to dryness in a
CentriVap system (LabConCo), resuspended in Milli-Q water and filtered
through a 13 mm 0.2
Lignin phenols were analyzed by microwave-assisted cupric oxide oxidation
(Goni and Montgomery, 2000) followed by trimethylsilyl derivatization and
capillary gas chromatography–mass spectrometry (Keil et al., 2011). Yields
of cinnamyl “C”, syringyl “S” and vanillyl “V” phenols were used to
evaluate vascular plant sources of the sedimentary organic bulk mixture.
These phenol yields are conventionally represented as lambda (
Peptides specifically diagnostic of proteins of interest were identified
using a targeted peptidomics approach (Bertrand et al., 2013). Briefly,
peptides were extracted as per Bridoux et al. (2015), where samples were
suspended in a lysis buffer (7 M urea, 2 M thiourea, carrier ampholytes (2 %
Toward the end of the cruise, sinking particles collected from 80 m depth at
station 3 were incubated in large trilaminate bags (Ward et al., 2008, 2009).
The bags were vacuum-emptied and then filled with water from a Nisken bottle
as per Ward et al. (2008), during which time a small syringe containing a
slurry of sinking particles collected from the net traps was injected into
the bags through a three-way valve (see also Chang et al., 2014). Injection
volumes were kept constant but the quantities of added organic matter were
not measured. Oxygen was added to each bag using a syringe filled with
O
Based on the CTD casts and nutrient analyses for the three main stations and the
seven additional CTD-only stations, the water column conditions changed along
the transect. The area around station 2 (15
Sediment net traps have the advantage of a large collection area relative to
other sediment trap types, allowing for rapid collection of large amounts of
settling material in short periods of time (Peterson et al., 2005). Total mass
fluxes trapped by the drifting nets ranged from 63.2 g m
Sediment trap fluxes for inorganic material
Organic carbon contents range from 8.9 to 24.6 wt % organic carbon, with
the minimal values generally associated with the deeper samples (Table 1,
Fig. 2). Organic carbon is lost from the sinking particles faster
(shallower) at station 3 than the other two stations (Fig. 2). Similar
trends are observed for the total nitrogen content of the samples (Table 1).
The C
Weight percent organic carbon
Total carbon on the surfaces of the particles, as measured by XPS, is
highest (60–72 at. %) in the 80 m samples and decreases to
Atom percentages of different elements identified by XPS on
the surfaces of the sinking particles:
Using the Redfield ratio for organic matter, as modified by Hedges et al. (2002),
the amount of inorganic mass was calculated as the difference
between the mass flux and the total organic matter flux (Table 1). Settling
particles range from 55 to 84 % inorganic matter. The inorganic matter flux
ranges from 9 to 39 g m
Amino acid abundances and compositions change as a function of depth but show little variation with station (Fig. 5). The mole percent of non-protein amino acids, which is an indicator of the diagenetic state of the amino acid pool (Cowie and Hedges, 1994), is less than 1 at the surface and less than 3 in the deeper samples (Fig. 5a). The amino acid degradation index (DI), as calculated using the formulation of Dauwe et al. (1999), ranges from 0.25 to 1.0 and represents relatively “fresh” organic matter (Table A4). At stations 2 and 3 the DI decreases with depth (Fig. 5b).
For the sediment trap samples, the copper oxide yield of lignin phenols is
< 2 mg gdw
Target peptide quantities were typically below 400 fg mgdw
Target peptide quantifications. The upper row shows peptide
quantifications as a function of depth, and the lower middle panels as a
function of oxygen. HH combines data from 10 peptides specific to three forms
of the anammox-specific enzyme hydrazine hydrolase. HO shows results from
four peptides specific to the anammox enzyme hydroxylamine/hydrazine
oxidoreductase. Three peptides specific to N
Incubation studies evaluating the change in DIC under
different initial oxygen conditions
Incubation experiments conducted using sinking particles from station 3, 80 m depth, showed inhibition of DIC production when either oxygen concentrations were low (Fig. 7a) or when dust was added to the collapsible incubation bags (Fig. 7b).
Understanding carbon fluxes through ODZs is critical for ascertaining and
predicting how oceanic systems have and will respond to climate change. The
size of ODZs is exceptionally sensitive to climate change and variability;
it only takes a 1 % reduction of the ocean's O
There are three major permanent ODZ regions: the eastern tropical North and
South Pacific (ETNP, ETSP) and the Arabian Sea. Unfortunately, to date there
have been few systematic evaluations of sinking particles within and outside
these ODZ regions. The simplest way to evaluate different regions is to take
the available data and fit it to the Martin curve (Martin et al., 1987).
While this power fit to data does not have a direct ecological relevance for
the “attenuation” term, it does serve the purpose of providing a convenient
single number that can be compared between regions. In the original “Martin
curve” paper, three sets of drifting sediment traps were deployed within an ODZ
region, while the other six locations were all well oxygenated (Martin et al.,
1987). The attenuation coefficients for the ODZ regions averaged 0.59
In the ETNP, Van Mooy et al. (2002) observed low particle attenuation in ODZ at a station north of Martin's and calculated an attenuation coefficient of 0.40. Using benthic lander data, flux attenuation along the Washington and Mexican margins are thought to differ, with the ODZ waters off Mexico having an average attenuation coefficient of 0.36 while the oxic Washington margin has an attenuation coefficient of 0.93 (Devol and Hartnett, 2001; Hartnett and Devol, 2003). In these studies, differences in attenuation were interpreted as oxygen effects on microbial degradation of the sinking material.
In the ETSP, systematic evaluations beyond Martin et
al. (1987) are few. Levin et al. (2002) evaluated benthic system ecology and energy flow at
30
In the Arabian Sea there is a great deal of flux data. Along the western margin, Lee
et al. (1998) combined sediment trap, primary production and
Thus, it appears to be a truism that ODZ regions “allow” proportionally more organic material to transit to the deep sea relative to oxygenated systems. The mechanisms that might cause this remain poorly evaluated and potentially include many different components, such as (M1) the possibility that anoxic degradation is sufficiently slower or less comprehensive relative to oxic carbon remineralization (Van Mooy et al., 2002); (M2) chemoautotrophy and a highly efficient microbial loop within the ODZ might add carbon to the flux (Close et al., 2014; Roullier et al., 2014); (M3) zooplankton migration into ODZ regions might enhance chemoautotrophy by providing dissolved nutrients, might result in defecation and “enhanced” fluxes within the upper ODZ (Wishner et al., 2013) or might reduce remineralization if population sizes are small; and (M4) additions of ballast might sorb and protect organic matter from decay (M4a; Arnarson and Keil, 2005; Howard et al., 2006) or change sinking speeds (M4b; Berelson, 2002; Roullier et al., 2014), (M5) inputs of refractory organic matter (Goni et al., 2009), and (M6) temperature effects (Marsay et al., 2015).
Although our data set cannot directly evaluate each of these potential mechanisms, the multiple proxy approach we employed, combined with our shipboard incubation experiments and recent literature by Rouillier et al. (2014), can be used to develop a heuristic model of the particle flux in the Arabian Sea ODZ.
In sediments there is a well-established “oxygen effect” where organic
carbon remineralization is more extensive under oxic rather than reducing
conditions, especially over timescales of months to millennia (Arnarson and
Keil, 2007; Cowie et al., 1995; Hedges et al., 1999). Little is known,
however, about how effective this mechanism is over short time periods or for
organic material in the water column. Hartnett et al. (1998) invoked an
oxygen effect to explain consistencies between sediments and sinking
particles, but others, including Lee (1992), have found little or no direct
evidence of such an effect over time periods of days. A particle with a
sinking speed between 10 and 300 m d
The deck-board incubation studies (Fig. 7) strongly suggest an oxygen
effect, with a threshold of about 20
Two amino acid indicators are commonly used in sediments to evaluate degradation: the mole percentage of non-protein amino acids (higher being more degraded) and the Dauwe–Middelburg index (Dauwe et al., 1999) (lower being more degraded). When measured on the sinking particles, both of these indicators suggest a difference between the ODZ stations and the more oxygenated station 3, with the sinking particles at station 3 having undergone more extensive degradation (Fig. 5).
Another possible hint to an oxygen effect is the attenuation of the histone protein marker in the samples. Histones are insoluble alkaline proteins that package DNA into structural units (nucleosomes) (Kornberg, 1974). Histones are found only in eukaryote nuclei, not in bacteria or archaea (Kornberg, 1974). We targeted the general diatom histone H4 because it is a good tracer for the transfer of diatom protein to the ocean interior (Bridoux et al., 2015). If there is an oxygen effect, we would expect histones traveling through the ODZ to be less effectively degraded relative to the oxic station 3, but we do not observe this. Instead, our histone values show no discerning trends with depth or oxygen content, suggesting that there is no clear oxygen effect for these proteins. Thus, the targeted peptidomics data are inconsistent with the overall total amino acid data, suggesting that histones do not degrade in the same way the bulk amino acid pool does. How these different measures relate and whether one is a better marker for a degradative effect remain unknown.
Finally, measuring the amount of carbon on mineral surfaces relative to the
total organic carbon content of a sample can be a useful tool for evaluating
the relative “degradative state” of organic matter in sediments (Arnarson
and Keil 2001, 2007). Fresh particles are typically rich
in organic matter (low C
Thus, the majority of indicators we measured are at least partially consistent with an oxygen effect, but the evidence to date is somewhat circumstantial and certainly not definitive. It is known that addition of fresh organic matter to incubations stimulates denitrification (Babbin et al., 2014; Chang et al., 2014), suggesting that denitrification is responsive to additions of carbon, but what is not known is whether in situ denitrification rates would match those of aerobic respiration under similar conditions.
Because ODZ regions host a wide range of chemoautotrophic organisms, there
is the possibility that chemoautotrophy adds carbon to the sinking flux
(Close et al., 2014). This has been termed “substrate injection”, and if this
is common in the Arabian Sea ODZ, there should be bulk isotopic and biomarker
measurements supporting this added material. Of the things we measured, the
bulk stable
What material might this be? Quantifications of peptides associated with the
chemoautotrophic process anammox suggest that anammox proteins are present
but do not contribute substantially to the sinking flux (Fig. 6). The two
anammox peptide biomarkers were rarely detected (< 50 % of the
samples contained measurable amounts of the markers) and their abundance
per unit mass was only 1–20 % of that N
How do we reconcile two biomarkers that appear to advocate opposite interpretations? The observations are not necessarily at odds. As early as 1994, Azam et al. (1994) proposed a complex microbial loop within the Arabian Sea ODZ comprising of active free-living and particle-attached bacteria working together to effectively transfer material between the dissolved, suspended and sinking pools. Close et al. (2014) suggested a something similar based on their lipid data, and Roullier et al. (2014) suggested that the attenuated flux in the Arabian Sea was partly the result of an efficient microbial loop that effectively reconstituted and utilized the energy of the sinking flux. This is a compelling hypothesis that requires further research. What are the growth efficiencies of heterotrophic communities in oxygenated waters relative to those of the heterotrophic–autotrophic community in an ODZ? If the ODZ community grows with less respiratory loss, this might account for the appearance of chemoautotrophic carbon in the sinking particles and might also be the mechanism leading to the “enhanced flux”.
It is worth noting that the (M1) oxygen effect and (M2) efficiencies and chemoautotrophy mechanisms are not easily dissected and discerned from each other. For example, since the shipboard incubations measured the accumulation of DIC, a change in the growth efficiency of the microbial community as a function of oxygen content would result in the same observation (less DIC accumulation) as a true oxygen effect would. Similarly, since we did not develop a peptide marker for aerobic respiration, we cannot compare the abundance of the denitrification peptide marker against an oxic counterpart. Thus, we cannot say whether there is relatively less protein associated with denitrification in the ODZ relative to aerobic respiratory proteins in the oxygenated waters.
Our bacterial ABC transporter protein data provide clues pertinent to this
issue. ABC transporter proteins are ATP-requiring enzymes used to transport
compounds across membranes. They are among the most commonly detected
proteins in the ocean (Tanoue et al., 1995; Wang et al., 2011) and are
critical for the heterotrophic function of acquiring organic compounds from
the environment. Thus, they are a good marker for heterotrophic processing.
The ATP transporter peptide we evaluated was quite abundant at all stations
(150–250 fg mgdw
One of the more intriguing mechanisms that might influence the flux is the presence and activity of zooplankton. Their actions may alter the flux in a variety of ways. If they feed in surface waters and then migrate into the ODZ for protection from fish, and they defecate in the ODZ, this would result in deliver of packaged particles that sink without having to pass the upper portions of the ODZ where remineralization is fastest (Cavan et al., 2015; Wishner et al., 2013). An argument against this is that if it occurs within the ODZ it should also occur within oxic water columns (e.g., Cavan et al., 2015). Alternatively, if zooplankton feed and defecate in surface waters and are inactive in the ODZ, then sinking particles would simply pass them by and not be re-ingested or repacked.
In addition to providing fecal material, zooplankton deliver dissolved materials (ammonium) that can promote chemoautotrophy (anammox), thus indirectly leading to “enhanced” fluxes. Our data set is very limited in its ability to address the role of zooplankton. Even though many sediment traps were deployed, our vertical resolution is too low to evaluate whether there is a “spike” in the flux at the depth that the zooplankton migrate to. We also have no zooplankton-specific biomarker to help evaluate whether their fecal material is a more important component of the sinking flux within the ODZ.
In a recent study, Roullier et al. (2014) used an underwater vision profiler (UVP) to collect high-resolution data of the particle flux within the Arabian Sea ODZ. They observed increases in the sinking flux that were associated with zooplankton assemblages living at the deeper boundaries of the ODZ, and suggest that zooplankton-derived injections of sinking particles are an important component controlling the deep flux. These data were collected deeper than our sediment traps and, thus, while their data clearly support a zooplankton contribution to the issue, they do not help explain why we (and others) observe attenuated fluxes above the zone where the zooplankton aggregated, within the heart of the ODZ.
Similar to an oxygen effect, there is abundant evidence that mineral
protection is an important mechanism promoting organic matter survival and
burial in long-term sediment records (M4a) (Keil and Mayer, 2014; Kennedy
and Wagner, 2011; Mayer, 1994). A variety of experimental approaches have
also shown mineral protection occurs for some fresh material sorbed to
mineral surfaces (for example; Arnarson and Keil, 2005; Le Moigne et al.,
2013). In the water column, a mineral protection mechanism would rely
on either biominerals created by plankton or the delivery of lithogenic minerals
from land. Aeolian dusts from Africa deliver minerals (and pollutants; Dachs
et al., 1999) to the Arabian Sea, where they are thought to sensitively
influence the dynamics of sinking particles (Barkmann et al., 2010). We
evaluated the potential effects of dust inputs by adding dust to shipboard
incubations at a variety of oxygen contents (Fig. 7) and observed
immediate reduction in DIC accumulation rates. This suggests that aeolian
sources of mineral to the Arabian Sea ODZ may directly influence the rate at
which organic matter is remineralized, thus enhancing the flux. Our
experiments were conducted by adding large quantities of dust
(
To seek corroboration between the incubations and the sediment trap samples, we analyzed the sediment trap material by XPS. Interestingly, no aluminum was detected in any sample, suggesting that terrestrial inputs of lithogenic material were minimal during the time of our sampling. Silica and calcium were both detected at 5–10 wt % of the surface material. The only differences observed for the biogenic markers was that the calcium content at station 2 (ODZ) was lower than at stations 1 or 3. This suggests that the sinking particles at station 2 were slightly richer in Si-rich diatom material relative carbonate compared to the other two stations.
Overall, our data are inconclusive about the effect of mineral on the remineralization rate of the organic matter; incubations suggest a strong effect, but direct measurements of the sinking particles suggest relatively minor contributions of biogenic mineral and no detectable aeolian dust. These results do not exclude this as an important mechanism, and other modeling and data analysis efforts have suggested that it must be important in the Arabian Sea (Armstrong et al., 2002; Barkmann et al., 2010; Howard et al., 2006; Klaas and Archer, 2002). It may simply be that our sampling period (transition to the intermonsoon) was the incorrect period to observe a strong effect.
Mineral inputs can potentially control more than just the reactivity of whatever organic material is sorbed to it, the abundance of mineral in the sinking particles plays a large role controlling the sinking speed (M4b) (Burd et al., 2010; Jackson and Burd, 2002). Given a single and constant degradation rate, any change in sinking speed will change how long the particle spends in the ODZ and thus influence how much it is remineralized there. Complicating matters is the fact that the ocean contains many different types of particles that aggregate and disaggregate and sink at different speeds. Riley et al. (2012) has suggested that fast sinking particles can typically account for the carbon demand of the deep sea, and that more slowly sinking particles are effectually remineralized within the water column. Use of the Martin attenuation coefficient does not help in this regard because it combined the two terms, but the ballast model allows some insight (Armstrong et al., 2002; Klaas and Archer, 2002; Le Moigne et al., 2012). If ballasting is an important control on sinking speed, and sinking speed controls the flux attenuation in the ODZ, then samples from the ODZ should have proportionally more minerals compared to station 3. Our data show the opposite trend; samples from station 3 (more oxygenated) have higher percentages of inorganic material (Fig. 3). This is inconsistent with ballast-driven sinking speeds being the primary driver of the reduced attenuation in the ODZ. However, as with everything else we measured, the data have the caveat that we did not measure sinking speeds directly, nor did we separate the material into pools with different sinking rates. In other studies where that has been done, the deep flux has been attributed to ballasted organic matter with fast sinking speeds or to fast-sinking materials derived from zooplankton (Cavan et al., 2015).
Conversely, organic material has the potential to minimize any sinking speed effect of mineral ballast (Azetsu-Scott and Passow, 2004). As particles sink they interact with other components in the water such as exopolymers, and if these materials become associated, sinking speeds can slow and even become negative (buoyant). Thus, the impact of ballast on organic matter reactivity and sinking speed remains an area ripe for further research. To summarize, direct evidence of a ballasting effect on the collected sinking particles is inconclusive, but lab studies with added lithogenic material show an inhibition in degradation rates, suggesting that further research on this topic is warranted.
One poorly evaluated possibility is that ODZ regions, for whatever reason, are prone to having refractory material within them, thus providing sinking particles with material that is naturally resistant to degradation. We assessed this by measuring terrestrial lignin phenols in the trap material. Our assumption was that the lignin would be highly refractory and that if this mechanism was important we would detect more lignin at the ODZ stations relative to station 3 further south. This is somewhat logical, as (a) the lignin found in the open in the ocean is generally thought to be refractory (Goni et al., 2009; Meyersschulte and Hedges, 1986), (b) station 1 is closest to the Indus River – a known and important source of lignin to the Arabian Sea (Cowie, 2005; Cowie et al., 1999; Vandewiele et al., 2009), and (c) station 2 is in a region where the dust deposition is roughly 2–3 times that of station 3 (Grand et al., 2015). However, the lignin yield was quite low, comparable to that found in the dissolved pool of the equatorial Pacific (Meyersschulte and Hedges, 1986) and we did not observe any differences in the quantities or compositions of the minimal lignin contribution to the sinking particles at the three stations and the contribution of lignin to the total organic matter was far less than 0.1 %, implying that refractory organic material from land was not an important component controlling the flux.
An additional component that we did not evaluate is the possibility that the phytoplankton community above the ODZ waters natively create “selectively preserved” organic matter that is strongly resistant to remineralization such as algeanans (Gelin et al., 1996, 1999). However, visualizations of the sinking material using a dissecting scope aboard ship did not result in any suggestion of differences between the various stations in the physical structure or composition of the sinking material.
Matsumoto (2007) suggested that colder oceans more effectively transfer
carbon to the interior because degradation is temperature-dependent and
Marsay et al. (2015) showed evidence of this in the North Atlantic. Could
temperature impact fluxes through an ODZ? At first glance, it seems
unlikely given that the three major ODZ regions of the ocean are in tropical
waters and thus should have higher attenuation rates. The median temperature
of the upper 500 m in the Arabian Sea during our sampling was uniform
at 18.5
No single parameter we measured or mechanism explored seems to singularly control the flux through the Arabian Sea ODZ, but the mechanisms (M1) oxygen effect and (M2) growth efficiency and chemoautotrophy have the strongest support within our data set and based on the literature. The effect of (M3) zooplankton and of mineral ballast (which occurs as part of several mechanisms) appears to be most important deeper within the ODZ (deeper than our sediment traps were deployed; Roullier et al., 2014), and the role of (M4b) sinking speed remains unclear. The role of (M4a) mineral protection seems plausible and is supported by model efforts (Barkmann et al., 2010; Howard et al., 2006) but since no lithogenic minerals were detected by XPS in our samples, mineral protection remains to be further evaluated and might likely only be important under certain circumstances. The only mechanisms that we can rule out appears to be that of (M5) refractory terrestrial inputs and (M6) temperature, but we note that we did not evaluate algeanans.
A conceptual model for how an ODZ system might funnel carbon to the deep sea
thus starts with the upper ODZ being a region of intense microbial activity
where respiratory reactions are either slowed by the lack of oxygen (M1,
oxygen effect) or are highly efficient at retaining energy within the system
(high growth efficiency, M2). Disentangling these two possibilities is of
importance if we are to better understand the forces controlling carbon
fluxes in a changing ocean, and also important for understanding the role of
the changing oxygen content of the oceans (Stramma et al., 2010). Bacterial
growth efficiencies are known to vary widely from < 5 to
Assuming a higher growth efficiency relative to oxic waters, the ODZ microbial community could then funnel energy to its chemoautotrophic community. The major chemoautotrophs in ODZ regions are thought to be slow-growing anammox bacteria (Lam and Kuypers, 2011); although these might grow slowly, they also keep biomass within the ODZ until they are grazed or otherwise transferred into the sinking pool. This combination of reduced respiration, increased efficiency and chemoautotrophy probably explains most observations of upper ODZ regions, including the presence of relatively high amounts of non-sinking loosely aggregated material (Roullier et al., 2014). The factors controlling how this material is transferred (or not) to the sinking pool remain to be explored. The inputs of dust are likely to enhance the flux by sorbing and protecting dissolved organic matter (Burdige, 2007) and could also change sinking speeds by providing ballast, but again, this remains to be directly demonstrated.
Deeper within the ODZ, two processes appear likely to be important; the presence of a deep zooplankton population (Roullier et al., 2014; Wishner et al., 2008) likely repackages suspended material and adds to the sinking flux, and the remineralization of slowly sinking organic matter results in sinking rates that increase as a function of depth (Berelson, 2002). The overall observation of “enhanced” fluxes is thus the result of a complex series of processes, each of which may dominate at any given time or depth.
To evaluate this conceptual model, and to begin to place values on each of the potential mechanisms, it is necessary to change our approach to evaluating sinking particles in ODZ regions. Particles need to be sampled and/or evaluated with higher vertical resolution than is currently done using sediment traps (e.g., Roullier et al., 2014), and new and existing methods for evaluating sinking speeds and particle dynamics (McDonnell et al., 2015) need to be applied to ODZ regions. The sinking, suspended and free-living/dissolved components of the carbon and microbial community should be evaluated together, and additional biomarkers for evaluating different sources and reactions need to be developed and applied. Examinations of zooplankton dynamics must to be incorporated into these microbial and geochemical studies of carbon flow. Finally, it would be useful to simultaneously evaluate both the remineralization rates and the sinking speeds of different types of particles in order to better understand how each factor influences carbon export.
Richard G. Keil and Allan H. Devol managed the project and Richard G. Keil wrote the manuscript with the assistance of Allan H. Devol. Jacquelyn A. Neibauer oversaw all laboratory analyses, processed samples on all the mass spectrometers, and helped with manuscript writing and proof reading. Christina Biladeau measured the amino acids and conducted the mineral SA and XPS analyses as part of her under-graduate degree in oceanography and interpreted early data sets as a part of her educational experience. Kelsey van der Elst collected the sediment trap samples, helped with incubation experiments and made substantial improvements to the sampling plan while at sea.
This research was funded by US-NSF grants 0926395 and 1153935 to R. G. Keil. We
thank Heidi Berkenbosch and Chloe Anderson for help at sea and in the lab,
and the captain and crew of the R/V