The Lena River in central Siberia is one of the major pathways translocating
terrestrial organic matter (OM) from its vast catchment area to the coastal
zone of the Laptev Sea and the Arctic Ocean. The permafrost soils of its far south-stretching catchment, which store huge amounts of OM, will most likely
respond differently to climate warming and remobilize previously frozen OM
with distinct properties specific for the source vegetation and soil. To
characterize the material discharged by the Lena River, we analyzed the
lignin phenol composition in total suspended matter (TSM) from surface water
collected in spring and summer, surface sediments from Buor Khaya Bay
along with soils from the Lena Delta's first (Holocene) and third terraces
(Pleistocene ice complex), and plant samples. Our results show that
lignin-derived cinnamyl : vanillyl (C
Within the permafrost-affected soils of the high northern latitudes lies a huge organic carbon (OC) reservoir, estimated to be as big as 1400–1850 Pg carbon, representing about 50 % of the global soil OC (Tarnocai et al., 2009). Currently most of this OC pool remains frozen and is therefore excluded from biogeochemical cycles. Over the last decades mean annual air temperatures in the Arctic have increased more strongly than the global mean, and this trend is projected to continue (IPCC, 2013). As a result, annual permafrost thaw depths and Arctic river runoff have increased (McClelland et al., 2012; Peterson et al., 2002), likely leading to enhanced mobilization and export of old, previously frozen soil-derived OC (e.g., Guo et al., 2004; Schuur et al., 2008; Vonk et al., 2010). Consequently, the great Arctic rivers play an important role in global biogeochemical cycles by connecting the large permafrost carbon pool of their hinterlands with the Arctic shelf seas and the Arctic Ocean.
Terrigenous sediments reaching the nearshore zone and shelves serve as archives recording changes in material derived from river catchments and from erosion of permafrost coasts. The particulate organic matter associated with these sediments consists of a complex mixture of compounds from different aquatic and terrigenous sources with different chemical/physical recalcitrance towards decomposition and mineralization. Determining the sources (e.g., phytoplankton, vegetation, surface soil, mineral-associated soil, peat) and quality of OC transported by Arctic rivers is therefore important to understand the effects of climate change on the river watersheds as well as on the Arctic coastal zone.
Recent studies using molecular organic compounds and their carbon isotopes have shown that there are great differences in the age, quality, and source of OM exported by individual rivers (Dickens et al., 2011; Drenzek et al., 2007; Feng et al., 2013; Goñi et al., 2013, 2000; Gustafsson et al., 2011; Karlsson et al., 2011; Kuzyk et al., 2008; Unger et al., 2005; Vonk et al., 2010). The catchments of the great Arctic rivers in North America and Siberia cover several climate zones. Their response to climate change will most likely vary strongly between the temperate and high-latitude regions, affecting river biogeochemical carbon cycling in different ways. Knowing where the OM derives from (southern vs. northern part of the catchment), as well as if and how the relative contributions of climatic zones to riverine POC may change with climate warming, is important for understanding and evaluating different permafrost thawing scenarios and their projected effect on the global climate.
Research efforts on studying Arctic rivers has increased in the last decades, and the spatial and temporal data resolution on dissolved and particulate organic matter has improved. Nonetheless, the resolution is still relatively low, especially for riverine POC. The main reasons for this are the great logistical difficulties of conducting fieldwork in these remote Arctic regions under mainly severe climate conditions, especially for winter and spring campaigns.
This is the second of two papers (see same issue) dealing with particulate organic matter from the Lena River delta and adjacent Buor Khaya Bay. The Lena River is one of the biggest Siberian rivers in terms of water and sediment discharge and an important source of sediment as well as dissolved and particulate organic matter to the Laptev Sea and Arctic Ocean (Holmes et al., 2002, 2012; Rachold, 1999). In recent years, several studies have investigated the input, composition, and transport mechanisms of sediments delivered by the Lena River and by erosion of permafrost coasts (e.g., Charkin et al., 2011; Günther et al., 2013; Karlsson et al., 2011; Rachold and Hubberten, 1999; Semiletov et al., 2011). However, it is still under debate as to how OM from the two main sources (riverine vs. coastal erosion) affects the total carbon budget and cycling in the Laptev Sea. Our samples were taken during field campaigns in the summers of 2009 and 2010 as well as in spring 2011. Here, we present new data on particulate OC composition and quality from riverbank soil profiles of the eastern Holocene first delta terrace and the Pleistocene third terrace of Kurungnakh Island (e.g., Schwamborn et al., 2002), surface water particulate matter along the main delta channels, and surface sediments from Buor Khaya Bay. We used the lignin phenol composition to distinguish the sources of OM transported by the river, namely the taiga forest in the southern catchment versus the tundra covering the northernmost part of the watershed including the delta. The alkaline cupric oxide (CuO) oxidation products are also used to characterize the degree of aerobic degradation of lignin in these samples.
Lignin is a biopolymer produced almost exclusively by terrestrial vascular plants. Through CuO oxidation it is possible to break up the polymer structure and analyze the main building blocks, the lignin-derived phenols, and other CuO oxidation products by gas chromatography–mass spectrometry (GC-MS). This method has been successfully applied in numerous studies to a variety of environments including the Arctic to trace soil-derived OM and differentiate between gymnosperm and angiosperm plants as well as between woody and non-woody tissues as sources (see Bianchi et al., 2007; Goñi et al., 2000; Hedges and Mann, 1979; Kuzyk et al., 2008; Onstad et al., 2000; Opsahl et al., 1999; Prahl et al., 1994; Tesi et al., 2011). Furthermore, lignin is believed to be a rather recalcitrant fraction of soil organic matter, although this model is currently under debate (Feng et al., 2008).
Considering that, our study in the Lena Delta can serve as possible benchmark against which future changes in OM composition and quality associated with a warming Siberian Arctic could be assessed. Because of our sampling location in the delta covered by tundra vegetation, we provide lignin compositional information from the Lena River including the whole catchment and compare these results with data from more southern Lena River sampling locations (e.g., Amon et al., 2012). Further, characterizing the riverine particulate organic matter can improve our understanding of organic matter delivery cycling in the near-coastal zone of Buor Khaya Bay and the Laptev Sea.
The Lena River is one of the largest Russian Arctic rivers, draining an area
of
The Lena River delta is the largest Arctic delta, with an area of
Lena River water and sediment discharge is not equally distributed through the different delta channels (Fig. 1b). Approximately 80–90 % of the total water and up to 85 % of the sediment discharge are delivered through the three main eastern channels to Buor Khaya Bay east of the delta, i.e., through the Sardakhsko–Trofimovskaya channel system (60–75 % water, 70 % sediment) and the Bykovskaya channel (20–25 % water, 15 % sediment). Only a minor portion is discharged to the north and west through the Tumatskaya and Olenyokskaya channels (5–10 % water, 10 % sediment; Ivanov and Piskun, 1999).
All riverbank bluffs sampled here belong to the first terrace, which is elevated (5 to 16 m) over the active floodplains. The bluff profiles vary strongly in sediment composition and organic matter content. Within the profiles, sandy layers derived from extreme flooding events (Schwamborn et al., 2002) and aeolian input (Kutzbach et al., 2004; Sanders, 2011) alternate with buried surface soil layers and peat layers rich in fibrous plant and root detritus in different stages of decomposition. The peat layers are either of autochthonous or allochthonous origin. Allochthonous material is eroded from river banks further upstream and re-deposited in the delta.
The first terrace is characterized by wet polygonal tundra with depressed
polygon centers and elevated polygon rims. Phytologically, the polygon
centers are dominated by hydrophilic sedges like
The sampling sites presented in this study are located in the eastern part of the Lena Delta and adjacent Buor Khaya Bay (Fig. 1b). Permafrost soil samples, total suspended matter (TSM) from surface waters, and surface sediments were collected during two expeditions in August 2009 and July/August 2010. Additional TSM samples were collected during the Lena River freshet in late May 2011. Four Holocene permafrost peat bluffs of different heights (3 to 8 m above river level in August 2009 and July/August 2010) were sampled along the main channels of the first delta terrace (all sampling sites in Fig. 1b and Table 1). In order to obtain samples that reflect the original state of the frozen permafrost soils, thawed material was removed with a spade for the total height of each bluff. Frozen pieces of peat were excavated at different depths using a hatchet and hammer.
Samples presented in this study and analyzed for lignin phenol composition. Bluff height is given in meters above river level [m a.r.l.] measured in August 2009 and July/August 2010. All total suspended matter samples from 2009 to 2011 were taken from the surface water layer with a sampling depth of ca. 0.5 m. Additional surface water samples used for total suspended matter determination can be found in Table S1 in the Supplement. Not applicable denoted by n/a.
Suspended particulate matter of Lena River surface water was sampled at
different stations in the main river channels of the delta on the Russian
vessel
Surface sediment samples from the Lena riverbed and off Muostakh Island were
taken in 2009 using a grab sampler onboard the
The peat and sediment samples were stored in pre-combusted glass jars (4.5 h
at 450
In addition to the samples taken for this study, we analyzed five samples
(two from the early Holocene, three from the Pleistocene) from a profile on
Kurungnakh Island, which were taken in 2002 and provided by Lutz
Schirrmeister from the AWI Potsdam, Germany. A detailed description of the
study site and the paleoenvironmental interpretation can be found in
Wetterich et al. (2008). Furthermore, vegetation samples collected
further south along the Lena River were provided by Ulrike Herzschuh and
Juliane Klemm from the AWI Potsdam, Germany (for more information on the
sampling sites see Herzschuh et al., 2009; Klemm and Zubrzycki, 2009;
Zubrzycki et al., 2012). Plant species analyzed here were
Peat and sediment samples were freeze-dried, homogenized, and subsampled for elemental and biomarker analysis.
All filters were oven-dried at 40
Weight percent organic carbon (OC) and total nitrogen (TN) content of soil and sediment samples were determined by high-temperature combustion after removal of carbonates as described by Goñi et al. (2003). The particulate organic carbon (POC) and particulate nitrogen (PN) content of TSM were analyzed on Ø 25 mm and Ø 47 mm GF/F obtained from the same water sample as the respective Ø 142 mm filters, which were scraped for lignin phenol analysis.
Alkaline CuO oxidation was performed at Oregon State University based on the
method described by Goñi and Montgomery (2000). Alkaline oxidations
were carried out with nitrogen-purged 2 N NaOH at 150
Quantified reaction products included eight lignin-derived compounds:
vanillyl phenols (V: vanillin, acetovanillone, vanillic acid), syringyl
phenols (S: syringealdehyde, acetosyringone, syringic acid), and cinnamyl
phenols (C:
In addition, non-lignin-derived phenols were also quantified, including
The concentration of different lignin phenol groups of marine sediment
samples and riverine suspended matter samples was used to infer the
contribution of gymnosperms and angiosperms to the total lignin-derived OM.
The end-member (EM) properties from the literature (as shown in Amon et al.,
2012) in the form of C
The surface water TSM concentrations showed strong spatial (within the
delta) and temporal (seasonal/annual) variability (Table 2). The
concentrations varied from 3.1 to 174.9 mg L
Total suspended matter (TSM) concentrations in Lena Delta surface
waters (2009 to 2011) and atomic particulate organic carbon (POC) to
particulate total nitrogen (PN) ratios. Note that for August 2009 there are only
OC and TN contents of first terrace soil samples varied strongly within individual riverbank bluffs and between the bluffs. The OC contents ranged from 1.02 to 17.14 wt % and the TN contents from 0.03 to 0.45 wt % (Table 3, Fig. S6 in the Supplement). The highest values (> 10 wt % OC) were not necessarily only found in the topsoil layers, as they were also found within bluff profiles associated with layers containing plant remains like twigs and leaves. Lower OC and TN contents (< 2 wt % and < 0.1 wt %, respectively) were found in layers with high sand contents. The atomic OC to TN ratios (OC : TN) of these samples show a similar distribution pattern. The ratios varied from 21.7 to 68, with the highest values (> 40) in samples rich in plant remains.
Organic carbon (OC), total nitrogen (TN), and atomic OC : TN ratios of the Lena Delta soil samples (first and third terrace) and Buor Khaya Bay surface sediments.
Buor Khaya Bay surface sediments showed generally lower OC and TN contents than observed for the first and third delta terraces (Table 3), ranging from 1.67 to 2.47 wt % and from 0.09 to 0.18 wt %, respectively. The highest OC and TN contents (2.47 wt % OC and 0.18 wt % TN) were analyzed for sample L09-34 off Muostakh Island (see Fig. 1b). The island is mainly composed of Pleistocene Yedoma deposits and highly affected by coastal erosion providing a lot of particulate matter throughout the open water season. The highest OC : TN ratio of 20.9 was determined off the Sardakh–Trofimovskaya channel system (sample L10-36; see Fig. 1b, Table 3), where the majority of the Lena River water and sediment discharge occurs.
Tables 4 and 5 summarize the sediment- and OC-normalized CuO product yields of samples presented in this study. Yields of individual samples can be found in the supplementary material (Tables S4 and S5).
Sediment-normalized yields of CuO oxidations products of Lena Delta
soils, total suspended matter (TSM), surface sediments, and vegetation
samples in milligram per gram dry weight sediment (mg g
Carbon-normalized yields of CuO oxidation products of Lena Delta
soils, surface water total suspended matter (TSM), and surface sediments in
milligrams per 100 milligrams of organic carbon [mg 100 mg
On average the plant samples exhibit the highest V, S, C, and P phenol
yields per gram dried sediment/plant tissue (dws), i.e.,
An overview of the CuO yield per 100 mg OC (
Carbon-normalized yields of phenols groups shown as box-and-whisker plots when the number of samples was large enough and as individual samples for smaller numbers.
The bulk samples of the first delta terrace show a broad range of C
The acid to aldehyde ratios of vanillyl and syringyl phenols (Ad
The EM unmixing was performed for the TSM and surface sediment samples. The
EM properties of moss and peat contribution in this model do not represent
the range of values observed in our samples. Fig. 5 shows our Pn
Therefore, we applied an unmixing model distinguishing between the four
major vegetation sources for OM: woody and no-woody gymnosperm and
angiosperm tissues. We used C
End-member ratios taken from the literature used for the unmixing model here and our calculated relative amounts of V, S, and C phenols. For abbreviations see description in Table 4 and 5.
The median values of the unmixing solutions (obtained by Monte Carlo
simulation) of angiosperms (woody
Results of unmixing model including relative abundances of V, S, and C phenols; median mixing coefficients; and gymnosperm to angiosperm ratio. See Tables 4 and 5 for abbreviations.
Surface water suspended particulate matter sampled in highly dynamic systems like a river delta can only provide very local snapshots of the suspended matter properties. The Lena Delta is characterized by a dynamic hydrology and fast changes in local conditions of erosion and accumulation, which are related to changes in water velocity and turbidity that lead to channel migration and branching (Fedorova et al., 2015). Longer time series covering several years and seasons are needed to observe catchment-related changes in these properties independent of the natural variability. Further, it is important to consider the season of TSM sampling: in the summer season in July and August the active layer depth is deepest; riverbank erosion along the delta channels is very pronounced; and streams draining ice complex deposits and thermokarst lakes transport more sediment, providing local delta-derived sediment to the river surface water. During the ice breakup and associated spring flood in late May to early June, the soils in the delta and northern catchment are still frozen. Riverbanks and bluffs are eroded by ice jamming against the riverbank and by thermal abrasion by relatively warmer Lena River water. The eroded material mixes with sediment transported from the south and is exported with the flood to the Laptev Sea coastal zone.
Our TSM concentrations from July/August 2009 and 2010 (mean values are 28.5 and
19.85 mg L
Spatial distribution of carbon-normalized lignin concentrations
(
We chose to only discuss the carbon-normalized (
The C
Our C
Although
The ratios of
Lignin phenol composition has been widely used to identify sources of
terrigenous OM in aquatic and soil systems and characterize the degree of
aerobic degradation (e.g., Benner et al., 1990;
Goñi and Hedges, 1992; Hedges and Mann, 1979; Hernes and Benner, 2002;
Tesi et al., 2007). The acid to aldehyde ratios of vanillyl and syringyl
(Ad
The TSM Ad
Additionally, sorption of dissolved lignin to mineral surfaces could have an
effect on the Ad
In contrast to the surface water TSM snapshots, the surface sediments from Buor Khaya Bay integrate the sedimentary OM and associated lignin phenol signal over a certain period of time depending on the local accumulation rates and the sediment re-working by waves and land-fast ice affecting the shallow coastal zone. The surface sediments therefore reflect an average of the OM transported to the coastal zone and smooth the seasonal and interannual differences in OM properties as well as the differences between OM sources. Buor Khaya Bay sedimentary OM is mainly derived from three sources, i.e., terrigenous OM transported by the Lena River, terrigenous OM derived from coastal erosion of the Buor Khaya coast predominantly consisting of Pleistocene ice complex deposits, and aquatic (riverine and marine) primary production. The latter source is negligible when discussing lignin phenols.
The sediment-normalized (
High lignin phenol concentrations are generally associated with the coarse
particulate OM fraction in soils and suspended material and they decrease
with decreasing grain size (Carrington et al., 2012;
Guggenberger et al., 1994; Hedges et al., 1986). An offshore gradient of
decreasing grain size off the delta coast and towards greater water depths
has been reported for Buor Khaya Bay (Charkin et al., 2011). The
spring flood could play a major role in transporting coarser lignin-bearing
OM to the coastal zone, which is in agreement with the high spring flood
We observed a generally high contribution of terrestrial organic matter to the Buor Khaya Bay sediments based on the OC : TN ratios (Table 3). An offshore trend of decreasing OC : TN ratios likely reflects the increasing marine contributions by plankton as well as decreasing amounts of terrigenous material reaching offshore locations, which supports similar findings on OM sources in Buor Khaya Bay surface sediments (e.g., Karlsson et al., 2011; Tesi et al., 2014).
The contribution of woody and non-woody gymnosperm and angiosperm tissues
based on C
Further, sedimentary P
The relatively high Ad
It is difficult to assess where the lignin degradation occurred. Oxidative
degradation of the lignin macromolecule in soils by fungi is known to
increase Ad
Notably, the highest Ad
As, in a first approximation, gymnosperm vegetation is restricted exclusively
to the taiga part of the Lena River catchment, we use the gymnosperm to
angiosperm ratio as an estimate of the relative contributions of taiga and
tundra. Therefore, we combined the model solutions for woody and non-woody
contributions of gymnosperms and angiosperms, respectively. According to the
model presented here, the fractions of gymnosperm and angiosperm-derived OM
varied strongly in the summer TSM samples. However, the mean gymnosperm
contributions for spring 2011 and the summers 2009 and 2010 were very
similar (Table 7), i.e., 0.4 (
The comparison of C
We compared the gymnosperm fractions in our samples with the results from Amon et al. (2012), who estimated a total gymnosperm contribution of 70 % to Lena River dissolved lignin. Despite a broad range of ratios in our summer TSM and surface sediments, we infer a substantially lower gymnosperm contribution to particulate OM in the delta surface water and Buor Khaya Bay surface sediments than further upstream at Zhigansk. This finding clearly indicates the overprint of TSM signatures by higher contributions of angiosperm OM contributing to the total TSM load between Zhigansk, located at the taiga–tundra transition zone, and our sampling sites in the delta. As mentioned above, this might be due to the inefficient transport of POM from distal catchment areas to the delta and its intermediate storage on floodplains (e.g., Aufdenkampe et al., 2007; 2011; Moreira-Turcq et al., 2013; Zocatelli et al., 2013). In contrast, dissolved organic matter including dissolved lignin is transported with the flow of the water, which might lead to a more efficient transport of taiga-derived DOM to the delta, thus explaining the difference in modeled gymnosperm contributions in Amon et al. (2012) and our study. The resulting considerable impact of the northern part of the catchment area to the POM composition is disproportional to its small spatial extent within the Lena River drainage area. It further implies environmental changes associated with above average climate warming expected for the high northern latitudes will most likely increase the disproportional OM input by enhanced permafrost thawing in the north compared to the southern catchment.
Despite the annual, seasonal, and spatial variability, the distribution of
lignin phenols in our Lena delta surface water TSM samples clearly reflects
the main vegetation characteristics of the Lena River catchment. The
gymnosperm fraction derived from the taiga covering most of the catchment
and the angiosperm fraction derived predominantly from the northern tundra
zone contribute about equally to the spring and summer TSM samples. However,
because of possible contributions of angiosperm OM from the taiga zone, for
example from elevated treeless areas, the 50 % angiosperm vegetation have
to be interpreted as the maximum contribution from tundra zone. Considering
the relatively small area covered by tundra (
Based on the low acid to aldehyde ratios of vanillyl and syringyl phenols
(Ad
The marginal filter leading to flocculation of dissolved and particulate organic matter and rapid sedimentation seems to be the dominant reason for high lignin contents off the major delta outlets. Similar to the TSM samples, the lignin distribution within the surface sediments of Buor Khaya Bay points to a mixed gymnosperm and angiosperm vegetation source of organic matter, and the modeled contributions are also about equal for both sources. The additional contribution of angiosperm-derived OM to Buor Khaya Bay sediments through coastal erosion makes it difficult to unambiguously distinguish between angiosperm-derived OM from the Lena watershed and from coastal erosion of ice complex deposits. However, as gymnosperm vegetation is not present in the Lena Delta and along the Buor Khaya coast today and their respective Holocene and Pleistocene deposits but covers the southern part of the Lena River catchment, the fact that we find gymnosperm-derived OM in surface sediments suggests that a substantial amount of sedimentary organic matter in Buor Khaya Bay originates from the Lena River catchment. The additional source of angiosperm OM contributed by coastal erosion results in dilution of the gymnosperm signal with distance to the delta.
The surface sediments were strongly degraded, resembling the Lena Delta summer samples and implying that at least some summer TSM is transported from the delta to the coastal zone. However, the strong degradation of sedimentary organic matter close to Muostakh Island consisting of Pleistocene ice complex and being affected by coastal erosion, which most likely happened after thawing on land, makes it complicated to distinguish between degraded ice complex and degraded summer TSM-derived organic matter.
In the future, more severe warming is expected for the high northern latitudes (IPCC, 2013), which will presumably influence the northernmost part of the Lena River catchment, i.e., the tundra zone with the delta, more strongly than the southern part. On the basis of our data it should be possible to trace changes in OM contribution and quality from different parts of the Lena River catchment area. Additionally, more research is needed to investigate the fate of Lena River and ice complex organic matter, particularly their degradability on land, in the water column, and post-depositionally, in order to understand their potential for possible increase in greenhouse gas release from the Arctic.
Additional data on individual CuO oxidation products for the samples
presented here can be found in PANGAEA (
The Helmholtz Young Investigators program of the Helmholtz Society (GM), the
Alfred Wegener Institute, and a fellowship from the German Academic Exchange
Service (DAAD) to M. Winterfeld supported this study. We thank Waldemar Schneider from
the Alfred Wegener Institute, Potsdam, Germany, and our Russian partners from
the Tiksi Hydrobase, Lena Delta Reserve, and the Arctic and Antarctic
Institute, St. Petersburg, Russian Federation, for logistical support in
preparation and during the expeditions. We acknowledge the good
collaboration with the crews of the vessels