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Map of the East Siberian Arctic Shelf (ESAS) showing the location of the ISSS-08 sampling stations. Key regions referred to in the text are highlighted. The lower courses and outflows of four great Russian Arctic rivers are labelled.
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Siberian permafrost contains a globally significant pool of organic carbon
(OC) that is vulnerable to enhanced warming and subsequent release into the
contemporary carbon cycle. OC release by both fluvial and coastal erosion has
been reported in the region, but the behaviour of this material in the Arctic
Ocean is insufficiently understood. The bal...
Contexts in source publication
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... marine organic matter ( Schouten et al., 2013;Sinninghe Damsté et al., 2002). Sourced from the cell membranes of bacte- ria and thaumarchaeota, they have been found in a range of terrestrial and marine sediments dating back millions of years ( Schouten et al., 2013). Branched GDGTs (brGDGTs) contain 4-6 methyl branches along two C 28 alkyl chains (Fig. S1 in the Supplement) and are produced by terres- trial bacteria in peats and soils (Weijers et al., 2006(Weijers et al., , 2007. They have also been found to be abundant in other terres- trial settings, including lakes and rivers (Blaga et al., 2009;De Jonge et al., 2014). Isoprenoidal GDGTs contain two C 40 isoprenoid chains with ...
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... terres- trial settings, including lakes and rivers (Blaga et al., 2009;De Jonge et al., 2014). Isoprenoidal GDGTs contain two C 40 isoprenoid chains with varying number of cyclopentane rings. One of these, crenarchaeol (cren), which is dominantly produced by marine thaumarchaeota, contains a cyclohexane unit in addition to four cyclopentane rings (Fig. S1). The ra- tio of brGDGTs to cren forms the basis of the branched and isoprenoidal tetraether (BIT) index (Hopmans et al., 2004), a proxy for tracing terrestrial material in marine sediments. The BIT index has been used to infer terrestrial to ma- rine transitions along river-ocean transects in (sub)-Arctic and non-Arctic regions (Kim et ...
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... used in present study were collected from across the ESAS (130 to 175 • E; Fig. 1). This area, including the Laptev and East Siberian seas, spans the outflows of the Lena, Yana, Indigirka and Kolyma rivers, with a combined drainage area of 3.7 km × 106 km and a discharge of 7.3 × 10 11 m 3 yr −1 (Gordeev, 2006; see Table 1). Annual organic carbon ex- port into the Laptev and East Siberian seas is estimated as 10.22 ...
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... for up to 9 months per year, with the majority of the sediment and water discharge during the early summer (Gordeev, 2006). Offshore there is a narrow channel between the coastline at ∼ 140 • E and the New Siberian Islands, known as the Dmitry Laptev Strait (DLS), splitting the ESAS up into two distinct areas -the Laptev Sea and East Siberian Sea (Fig. 1). The New Siberian Islands themselves are remnants of the Great Arctic Plain, which once covered 1.6 km × 106 km between the modern coastline and the shelf edge, and was inundated during the early-middle Holocene, and now exists as sub- stantial subsea permafrost ( Kienast et al., 2005;De Jonge et al., 2014). Samples in this study have ...
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... et al. (2005). The ESAS samples have also been classified latitudinally, into the nearshore ESAS (< 150 km from river outflows) and offshore ESAS (> 150 km from river outflows). In total, 92 sediment samples were collected in September 2008 dur- ing the International Siberian Shelf Study expedition (ISSS- 08; Semiletov and Gustafsson, 2009; Fig. 1), along with six samples from terrestrial ice complexes. Briefly, sedi- ment cores and surface sediments were collected with a dual gravity corer (GEMAX) and a van Veen grab sampler. The sediment cores were sliced into 1 cm sections and trans- ferred to pre-cleaned polyethylene containers with stainless steel spatulas. Similarly, ...
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Planktonic archaea include predominantly Marine Group I Thaumarchaeota (MG I) and Marine Group II Euryarchaeota (MG II), which play important roles in the oceanic carbon cycle. MG I produce specific lipids called isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), which are being used in the sea surface temperature proxy named TEX86. Alt...
Citations
... 2l,5c) indicate that most of the OC in these sediments indeed has a marine origin. The limited contribution of terrestrial brGDGTs to the core location fits the observations of a rapid reduction in fluvially discharged brGDGTs offshore at, for example, the Siberian (Sparkes et al., 2015), Iberian (Zell et al., 2015;Warden et al., 2016), and East China continental shelf (Zhu et al., 2011). ...
Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of this OC in these rivers remains poorly understood. We investigate soil-to-sea transport of soil OC by the Godavari River in India using glycerol dialkyl glycerol tetraether (GDGT) lipids in soils, river suspended particulate matter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bay of Bengal. The abundance and composition of GDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, SPM and riverbed sediments from the whole basin contain more 6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, where mobilisation and transport of soils is limited due to deficient rainfall and damming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments are relatively high year-round, suggesting that they have an aquatic source. Aquatic brGDGT production coincides with elevated values of the isoprenoid GDGT-0 / crenarchaeol ratio in SPM and riverbed sediments from the upper basin, indicating low-oxygen conditions. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the north and east tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs, suggesting that they are transported together. However, river depth profiles in the downstream Godavari reveal no hydrodynamic sorting effect on brGDGTs in either season, indicating that brGDGTs are not closely associated with mineral particles. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤ 63 µm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The large change in brGDGT distributions at the river–sea transition implies that this zone is key in the transfer of soil OC, as well as that of the environmental signal carried by brGDGTs from the river basin.
... Delivery of TerrOC into the marine realm is consistent with the high abundance of terrestrial palynomorphs (i.e., pollen and spores; (Contreras et al., 2013;Pross et al., 2012) and other terrestrial biomarkers (including high BIT indices; Bijl et al., 2013;Pross et al., 2012;Figure 3a) in the same sediments. These findings are analogous to those observed in modern (high-latitude) environments, where large river systems in northern Eurasian and North American (Guo et al., 2004;Hilton et al., 2015;Semiletov et al., 2011;Sparkes et al., 2015) are capable of transporting large quantities of TerrOC from soils into the marine realm. ...
Terrestrial organic carbon (TerrOC) acts as an important CO2 sink when transported via rivers to the ocean and sequestered in coastal marine sediments. This mechanism might help to modulate atmospheric CO2 levels over short‐ and long‐ timescales (10³–10⁶ years), but its importance during past warm climates remains unknown. Here we use terrestrial biomarkers preserved in coastal marine sediment samples from Wilkes Land, East Antarctica (∼67°S) to quantify TerrOC burial during the early Eocene (∼54.4–51.5 Ma). Terrestrial biomarker distributions indicate the delivery of plant‐, soil‐, and peat‐derived organic carbon (OC) into the marine realm. Mass accumulation rates of plant‐ (long‐chain n‐alkane) and soil‐derived (hopane) biomarkers dramatically increase between the earliest Eocene (∼54 Ma) and the early Eocene Climatic Optimum (EECO; ∼53 Ma). This coincides with increased OC mass accumulation rates and indicates enhanced TerrOC burial during the EECO. Leaf wax δ²H values indicate that the EECO was characterized by wetter conditions relative to the earliest Eocene, suggesting that hydroclimate exerts a first‐order control on TerrOC export. Our results indicate that TerrOC burial in coastal marine sediments could have acted as an important negative feedback mechanism during the early Eocene, but also during other warm climate intervals.
... During previous reports, brGDGTs in marine sediments were considered to be particular tracers for terrestrial soil organic matter (Hopmans et al., 2004). Recent research has reported that the distributions of brGDGTs in marine sediments and continental soils are distinct (Hopmans et al., 2004;Peterse et al., 2009;Sparkes et al., 2015). Sinninghe Damsté (2016) established the average number of cyclopentane moieties in tetramethylated brGDGTs (#Ringstetra), and reported that #Ringstetra value of marine-derived brGDGTs (>0.7) is higher than that of the soil-derived brGDGTs (<0.7). ...
... 0.96), while the IIIa/IIa values ranged from 0.92 to 1.16 (avg. 1.04), as shown in Figure 4A ( Zhou et al., 2014;Sparkes et al., 2015;Xiao et al., 2016;Crampton-Flood et al., 2018;Cao et al., 2020;Zhang et al., 2020;Liu et al., 2021). In Yangtze River Basin soil, the #Rings tetra values ranged from 0.2 to 0.32 (avg. ...
... where A C represents the contribution of brGDGTs in the Yangtze River soil, A marine represents the contribution of marine in situ production of brGDGTs, #R C represents the mean value of #Rings tetra in the Yangtze River catchment soil (a mean of 0.29), and ( IIIa IIa ) C represents the mean of IIIa/IIa in the Yangtze River catchment soil (a mean of 0.35). Moreover, #R marine and ( IIIa IIa ) marine are considered as the marine in situ production of brGDGTs in the marine environment, with averages of 0.96 and 1.04, respectively Sparkes et al., 2015;Xiao et al., 2016;Crampton-Flood et al., 2018;Cao et al., 2020;Zhang et al., 2020, Liu et al., 2021. As a result, the contributions of brGDGTs derived from Yangtze River catchment ranged from 34 to 91% ( Figure 4B), while marine in situ production of brGDGTs ranged from 9 to 66% ( Figure 4C). ...
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are components of bacterial membranes in terrestrial soils, which are widely used in paleoenvironmental reconstruction in global terrestrial soils and marine sediments. In marine sediments, the mixed sources of brGDGTs complicate the applications of brGDGT-related indicators in reconstructing terrestrial environments. In this study, we reported the spatial distribution of brGDGT-related indicators (MBTʹ 5ME, CBTʹ 5ME, #Ringstetra, and IIIa/IIa) in surface sediments from the East China Sea (ECS). MBTʹ 5ME and CBTʹ 5ME showed a stepped trend from the inner shelf to the outer shelf, and #Ringstetra and ∑IIIa/∑IIa values in sediments of the ECS are distinct compared with those in the catchment soils, suggesting marine in situ production of brGDGTs. We also examined the existence of marine in situ brGDGTs and quantitatively determined the contributions of terrestrial and in situ production of brGDGTs. This study reported mixed sources of soil-derived brGDGTs were dominant, and marine in situ brGDGTs were overprinted. Our results indicate that there were predominantly marine in situ brGDGTs (avg. 60.5 ± 5.5%) in the outer shelf due to the weak riverine transportation and were characterized by high #Ringstetra and IIIa/IIa.
... This has improved source apportionment of OC in bulk sediments across Arctic regions and timescales (e.g., Vonk et al., 2012;Goñi et al., 2013;Martens et al., 2020) and in sediment density fractions (Tesi et al., 2016b), in suspended particulate organic matter (e.g., Vonk et al., 2010Vonk et al., , 2014Karlsson et al., 2016), and at the molecular level (e.g., Drenzek et al., 2007;Gustafsson et al., 2011;Feng et al., 2013). Extensive studies of a wide set of molecular biomarkers (e.g., Fahl and Stein, 1997;Goñi et al., 2000;Belicka et al., 2004;Yunker et al., 2005;van Dongen et al., 2008;Tesi et al., 2014;Sparkes et al., 2015;Bröder et al., 2016) have provided growing insights into OC distribution and fate, particularly for terrigenous organic matter. Access to this growing number of observational data in a readily accessible interactive format would be greatly beneficial to wider system assessments and interpretations of organic matter in the Arctic Ocean. ...
Biogeochemical cycling in the semi-enclosed Arctic Ocean is strongly influenced by land–ocean transport of carbon and other elements and is vulnerable to environmental and climate changes. Sediments of the Arctic Ocean are an important part of biogeochemical cycling in the Arctic and provide the opportunity to study present and historical input and the fate of organic matter (e.g., through permafrost thawing).
Comprehensive sedimentary records are required to compare differences between the Arctic regions and to study Arctic biogeochemical budgets. To this end, the Circum-Arctic Sediment CArbon DatabasE (CASCADE) was established to curate data primarily on concentrations of organic carbon (OC) and OC isotopes (δ13C, Δ14C) yet also on total N (TN) as well as terrigenous biomarkers and other sediment geochemical and physical properties. This new database builds on the published literature and earlier unpublished records through an extensive international community collaboration.
This paper describes the establishment, structure and current status of CASCADE. The first public version includes OC concentrations in surface sediments at 4244 oceanographic stations including 2317 with TN concentrations, 1555 with δ13C-OC values and 268 with Δ14C-OC values and 653 records with quantified terrigenous biomarkers (high-molecular-weight n-alkanes, n-alkanoic acids and lignin phenols). CASCADE also includes data from 326 sediment cores, retrieved by shallow box or multi-coring, deep gravity/piston coring, or sea-bottom drilling. The comprehensive dataset reveals large-scale features of both OC content and OC sources between the shelf sea recipients. This offers insight into release of pre-aged terrigenous OC to the East Siberian Arctic shelf and younger terrigenous OC to the Kara Sea. Circum-Arctic sediments thereby reveal patterns of terrestrial OC remobilization and provide clues about thawing of permafrost.
CASCADE enables synoptic analysis of OC in Arctic Ocean sediments and facilitates a wide array of future empirical and modeling studies of the Arctic carbon cycle. The database is openly and freely available online (https://doi.org/10.17043/cascade; Martens et al., 2021), is provided in various machine-readable data formats (data tables, GIS shapefile, GIS raster), and also provides ways for contributing data for future CASCADE versions. We will continuously update CASCADE with newly published and contributed data over the foreseeable future as part of the database management of the Bolin Centre for Climate Research at Stockholm University.
... Sediment samples were extracted using a modified Bligh-Dyer method Dogrul Selver et al., 2015;Sparkes et al., 2015). Five grams of sediment was ultrasonically extracted using a monophasic mixture of methanol/dichloromethane/de-ionised water (2:1:0.8 ...
Methanotrophic bacteria (MB) are an important group of microorganisms, involved in the greenhouse gas (GHG) cycles. They are responsible for the utilization of methane, one of the main GHGs, which is released in large amounts (via biogenic and abiogenic processes) during coal formation. This study aimed to determine the main factors affecting the distribution of the MB in two lignite-bearing series of the Turów and Bełchatów coal basins. Distribution of MB in the lignite profiles was studied using methanotroph-specific lipid biomarkers such as amino-bacteriohopanepolyols (NH-BHPs) and C-3 methylated BHPs. BHP results were combined with physical and chemical properties of the studied sediments. In general, lignites were richer in BHPs than the mineral samples, which points to the important role of the intrinsic methane cycling. NH-BHP speciation confirmed that the methanotrophic community of the studied sediments was a combination of both type I and, especially, type II methanotrophs. Based on geological data, it was suggested that elevated temperature during diagenesis intensifies decomposition of methanotroph-specific biomarkers (aminopentol and 3-Me BHT). It was found that the tested BHPs can derive from both fossil and living MB. The presence of metabolically active methanotrophs should therefore be accounted for during studies aimed at using lignite deposits as a source of methane.
... Thus, brGDGTs have received substantial interest over the last few decades due to their potential in reconstructing MAT and soil pH in terrestrial environments (Schouten et al., 2013). In marine environments, sedimentary brGDGTs were initially assumed to originate predominantly from continental soil erosion and river input (e.g., Hopmans et al., 2004;Kim et al., 2006;Sparkes et al., 2015). However, some observations showing distinct brGDGT distributions in marine environments compared to adjacent terrestrial soils suggest in situ marine production (Hu et al., 2012;Peterse et al., 2009;Zhu et al., 2011). ...
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are ubiquitous in marine environments. Nevertheless, it is unclear whether these brGDGTs are derived from terrestrial soils or produced in situ. Recently, it was proposed that the weighted average number of cyclopentane moieties of tetramethylated brGDGTs, #Ringstetra, could be used to identify sources of sedimentary brGDGTs in marine environments. However, little is known about #Ringstetra in seawater. In this study, we report #Ringstetra data in suspended particulate materials (SPM) in the surface shelf waters of the East China Sea (ECS) and coastal South China Sea (SCS), as well as in core-top sediments of the deep SCS. According to a two-end member model, #Ringstetra values in SPM increase offshore from 0.48 to 0.86 in the ECS and from 0.39 to 0.83 in the SCS, suggesting increasing marine-produced brGDGTs. An offshore increasing trend of #Ringstetra is also observed in the shelf surface sediments of the ECS and SCS, suggesting increased contributions of marine-produced brGDGTs. However, the offshore increasing trend in core-top sediments of the SCS ceases at a water depth ca. 100 m, with constantly low #Ringstetra values (0.3 ± 0.1) at sites deeper than 780 m and a transition value of 0.58 at 329 m. Instead of interpreting the low #Ringstetra values in deep-sea sediments as the result of increased contributions of terrestrial-derived brGDGTs, we propose that brGDGTs in deep-sea sediments are produced in situ. The lower pH in the deep-sea bottom water and sediment porewater than in the seawater column could be the reason for the low #Ringstetra values. We further propose that pH reconstructions of deep-sea sediment cores (e.g., >1000 m water depth) with the least terrestrial influence should reflect pH changes in marine bottom environments.
... Earlier investigations of terrOC in Arctic margin surface sediments reported a strong decrease of terrigenous biomarker concentrations with increasing water depth/distance from the shore for the East Siberian Arctic Shelf (Bischoff et al., 2016;Bröder, Tesi, Salvadó, et al., 2016;Doğrul Selver et al., 2015;Sparkes et al., 2015Sparkes et al., , 2016Tesi et al., 2014;Vonk et al., 2010) and parts of the North American Arctic margin (Goni et al., 2013). The extent of terrOC degradation appears to be depending on its exposure to oxygen, which in turn is a function of the sediment transport time (e.g., Keil et al., 2004;Mollenhauer et al., 2007). ...
... They were highest close to the Lena River delta and northward (roughly along 130°E). This also agrees with the patterns reported for terrestrial biomarkers such as lignin phenols (e.g., Bröder, Tesi, Andersson, et al., 2016;Karlsson et al., 2014;Tesi et al., 2014) and solvent-extractable lipids (e.g., Bischoff et al., 2016;Bröder, Tesi, Andersson, et al., 2016;Doğrul Selver et al., 2015;Karlsson et al., 2011Karlsson et al., , 2014Sparkes et al., 2015). The comparison of the distribution of bulk OC with terrOC revealed that the high OC concentrations on the outer eastern East Siberian Sea shelf are probably to a large part caused by marine primary production, since terrOC concentrations in that area are rather low. ...
Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off‐shelf, or buried in the sediments. While our understanding of the fate of permafrost‐derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes (n = 109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these ~2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first‐order degradation rate constant of 1.5 kyr⁻¹ is derived by combining a previously established relationship between water depth and cross‐shelf sediment‐terrOC transport time with mineral‐associated terrOC loadings. This yields a terrOC degradation flux of ~1.7 Gg/year from surficial sediments during cross‐shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC‐mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost‐released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon‐climate feedback.
... Deepening hydrologi- cal flow paths and thermokarst erosion events are mobilis- ing "old" carbon that has been stored in deep permafrost for thousands of years ( Gustafsson et al., 2011;Feng et al., 2013Feng et al., , 2015Vonk et al., 2012;Vonk and Gustafsson, 2013;IPCC, 2013). OC delivered by fluvial erosion and transport has been identified offshore using molecular biomarkers and has been shown to deposit and/or degrade rapidly on the shelf (Bröder et al., 2016;Do˘ grul Selver et al., 2015;Karlsson et al., 2015;Sparkes et al., 2015Sparkes et al., , 2016Tesi et al., 2014Tesi et al., , 2016. In ad- dition to fluvial erosion, coastal erosion delivers a significant amount of sediment and OC (44±10 Tg OC yr −1 ) to the Arc- tic Ocean ( Vonk et al., 2012). ...
... The seabed consists of permafrost that devel- oped sub-aerially, was flooded during Holocene sea-level rise and is now being buried by sediment sourced from fluvial and coastal erosion (Kienast et al., 2005). Geochemical stud- ies investigating the sources and offshore distribution of or- ganic matter have noted differences between east and west, nearshore and offshore sections of the shelf ( Bischoff et al., 2016;Karlsson et al., 2015;Semiletov et al., 2005;Sparkes et al., 2015Sparkes et al., , 2016Tesi et al., 2014). ...
... A minority of sediments used in this study were sol- vent extracted for biomarker analysis prior to collecting Raman spectroscopy measurements ( Bischoff et al., 2016;Do˘ grul Selver et al., 2015;Sparkes et al., 2015). An ul- trasonic extraction process used methanol, dichloromethane and pH-buffered distilled water in order to remove ex- tractable material. ...
Warming-induced erosion of permafrost from Eastern Siberia mobilises large amounts of organic carbon and delivers it to the East Siberian Arctic Shelf (ESAS). In this study Raman spectroscopy of carbonaceous material (CM) was used to characterise, identify and track the most recalcitrant fraction of the organic load: 1463 spectra were obtained from surface sediments collected across the ESAS and automatically analysed for their Raman peaks. Spectra were classified by their peak areas and widths into disordered, intermediate, mildly graphitised and highly graphitised groups and the distribution of these classes was investigated across the shelf. Disordered CM was most prevalent in a permafrost core from Kurungnakh Island and from areas known to have high rates of coastal erosion. Sediments from outflows of the Indigirka and Kolyma rivers were generally enriched in intermediate CM. These different sediment sources were identified and distinguished along an E–W transect using their Raman spectra, showing that sediment is not homogenised on the ESAS. Distal samples, from the ESAS slope, contained greater amounts of highly graphitised CM compared to the rest of the shelf, attributable to degradation or, more likely, winnowing processes offshore. The presence of all four spectral classes in distal sediments demonstrates that CM degrades much more slowly than lipid biomarkers and other traditional tracers of terrestrial organic matter and shows that alongside degradation of the more labile organic matter component there is also conservative transport of carbon across the shelf toward the deep ocean. Thus, carbon cycle calculations must consider the nature as well as the amount of carbon liberated from thawing permafrost and other erosional settings.
... The seabed consists of permafrost that developed 10 sub-aerially, was flooded during Holocene sea-level rise and is now being buried by sediment sourced from fluvial and coastal erosion (Kienast et al., 2005). Geochemical studies investigating the sources and offshore distribution of organic matter have noted differences between east and west, nearshore and offshore sections of the shelf (Bischoff et al., 2016;Karlsson et al., 2015;Semiletov et al., 2005;Sparkes et al., 2015Sparkes et al., , 2016Tesi et al., 2014). ...
... A minority of sediments used in this study were solvent extracted for biomarker analysis prior to collecting Raman spectroscopy measurements (Bischoff et al., 2016;Dogrul Selver et al., 2015;Sparkes et al., 2015). An ultrasonic extraction process used methanol, dichloromethane and pH-buffered distilled water in order to remove extractable material. ...
... There could also be a sorting effect across the shelf, rather than degradation. Whereas solvent extractable biomarkers of 25 terrestrial processes have been shown to reduce rapidly across the ESAS (Bröder et al., 2016;Bischoff et al., 2016;Sparkes et al., 2015), most of the Raman-amenable CM present in these samples is likely to be resistant to degradation (Galy et al., 2008). Therefore, an alternative reason for Highly Graphitic CM to be present in high concentrations on the Distal ESAS is if it is preferentially delivered and deposited there. ...
Warming-induced erosion of permafrost from Eastern Siberia mobilises large amounts of organic carbon and delivers it to the East Siberian Arctic Shelf (ESAS). In this study Raman spectroscopy of Carbonaceous Material (CM) was used to characterise, identify and track the most recalcitrant fraction of the organic load. 1463 spectra were obtained from surface sediments collected across the ESAS and automatically analysed for their Raman peaks. Spectra were classified by their peak areas and widths into Disordered, Intermediate, Mildly Graphitised and Highly Graphitised groups, and the distribution of these classes was investigated across the shelf. Disordered CM was most prevalent in a permafrost core from Kurungnakh Island, and from areas known to have high rates of coastal erosion. Sediments from outflows of the Indigirka and Kolyma rivers were generally enriched in Intermediate CM. These different sediment sources were identified and distinguished along an E-W transect using their Raman spectra, showing that sediment is not homogenised on the ESAS. Distal samples, from the ESAS slope, contained greater amounts of Highly Graphitised CM compared to the rest of the shelf, attributable to degradation or, more likely, winnowing processes offshore. The presence of all four spectral classes in distal sediments demonstrates that CM degrades much slower than lipid biomarkers and other traditional tracers of terrestrial organic matter, and shows that alongside degradation of the more labile organic matter component there is also conservative transport of carbon across the shelf toward the deep ocean. Thus, carbon cycle calculations must consider the nature as well as the amount of carbon liberated from thawing permafrost and other erosional settings.
... The source difference between brGDGTs and iGDGTs led researchers to develop a branched and isoprenoid tetraether (BIT) index, expressed as the relative abundance of terrestrial-derived brGDGTs to aquatic-derived Thaumarchaeotal (Hopmans et al., 2004). Subsequent studies found that the BIT index is specific to soil organic carbon because GDGTs are absent in vegetation (e.g., Walsh et al., 2008;Sparkes et al., 2015). The BIT index is generally higher than 0.9 in soils, but close to 0 in marine sediments devoid of terrestrial inputs (Weijers et al., , 2014. ...
... the East Siberian Arctic Shelf (Sparkes et al., 2015). Similar to Bohai Sea in this study, the compounds brGDGT IIa and IIIa are also ubiquitously present in these environments. ...
... Such disparity supports the hypothesis that brGDGTs produced in marine environments have higher IIIa / IIa values because labile intact polar brGDGTs are mainly produced in situ, whereas recalcitrant core brGDGTs are composed of more allochthonous terres- (Fig. 4c). A strong linear correlation was observed between the IIIa / IIa ratio and the distance from the river mouth (R 2 = 0.58; p < 0.05; Fig. 4d), in accordance with the data of the BIT index and δ 13 C org (Sparkes et al., 2015). All lines of evidence support the concept that marine-derived brGDGTs have higher IIIa / IIa values than terrestrial-derived brGDGTs. ...
Presumed source specificity of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from bacteria thriving in soil/peat and isoprenoid GDGTs (iGDGTs) from aquatic organisms led to the development of several biomarker proxies for biogeochemical cycle and paleoenvironmental reconstructions. However, recent studies reveal that brGDGTs are also produced in aquatic environments besides soils and peat. Here we examined three cores from the Bohai Sea, and found distinct difference in brGDGT compositions varying with the distance from the Yellow River mouth. We thus propose an abundance ratio of hexamethylated to pentamethylated brGDGT (IIIag / IIa) to evaluate brGDGT sources. The compilation of globally distributed 1354 marine sediments and 589 soils shows that the IIIag / IIa ratio is generally <ĝ€0.59 in soils and 0.59-0.92 and >ĝ€0.92 in marine sediments with and without significant terrestrial inputs, respectively. Such disparity confirms the existence of two sources for brGDGTs, a terrestrial origin with lower IIIag / IIa and a marine origin with higher IIIag / IIa, which is likely attributed to a generally higher pH and the production of brGDGTs in cold deep water in marine waters. The application of the IIIag / IIa ratio to the East Siberian Arctic Shelf proves it to be a sensitive source indicator for brGDGTs, which is helpful for accurate estimation of organic carbon source and paleoclimates in marine settings.
