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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 22, GB1011, doi:10.1029/2007GB002974, 2008

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Contrasting lipid biomarker composition of terrestrial organic matter exported from across the Eurasian Arctic by the five great Russian Arctic rivers ¨ rjan Gustafsson1 Bart E. van Dongen,1,2 Igor Semiletov,3,4 Johan W. H. Weijers,5,6 and O Received 15 March 2007; revised 3 October 2007; accepted 18 October 2007; published 8 February 2008.

[1] Surface sediments outside the great Russian Arctic rivers (GRARs; Ob, Yenisey,

Lena, Indigirka, and Kolyma) were investigated for their lipid biomarker composition to elucidate compositional distinctions of the exported organic matter (OM) across this continent-scale climosequence of the Siberian Arctic. The lipid biomarker composition is dominantly terrestrial (high molecular weight (HMW) n-alkanols, n-alkanoic acids, n-alkanes; branched glycerol dialkyl glycerol tetraethers (GDGTs), b-sitosterol, other sterols, and triterpenoids) with only minor marine contributions (e.g., the ratio of terrigenous-to-aquatic n-alkanes was 17–80, the TOC/TN ratio was 10–16, and the branched and isoprenoid tetraether (BIT) index >0.88). There is a large contribution of C23 and C25 homologues relative to other long-chained n-alkanes, suggesting substantial contribution of probably Sphagnum-derived OM. The C23 and C25 contribution decreases eastward, signaling either a decrease in the potential contribution of Sphagnum or a shift within the n-alkane distribution of Sphagnum species or increased aeolian input, due to more arid conditions in the east. Other distinctions in molecular OM composition across the climosequence include increased concentrations of both HMW n-alkanoic acids and b-sitosterol relative to HMW n-alkanes in the eastern sediments. This suggests that the OM exported by the eastern GRARs is, despite their higher bulk radiocarbon ages, less degraded, which is consistent with increasing permafrost and a shorter annual thaw period in eastern Siberia. Taken together, this benchmark study of the current composition of terrestrially exported OM suggests distinguishing continent-scale trends in molecular composition of the OM across the west-east set of GRARs, which reflects both differences in vegetation and climate. If the climate in the eastern Russian Arctic region becomes more like the current state in the western part, these results would predict a greater degree of decomposition of the old terrestrial OM released by the eastern GRARs and thus greater remineralization and release as CO2 and methane. ¨ . Gustafsson (2008), Contrasting lipid biomarker composition of Citation: van Dongen, B. E., I. Semiletov, J. W. H. Weijers, and O terrestrial organic matter exported from across the Eurasian Arctic by the five great Russian Arctic rivers, Global Biogeochem. Cycles, 22, GB1011, doi:10.1029/2007GB002974.

1. Introduction

1 Department of Applied Environmental Science (ITM), Stockholm University, Stockholm, Sweden. 2 Now at School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK. 3 International Arctic Research Center, University of Alaska, Fairbanks, Alaska, USA. 4 Pacific Oceanological Institute, Russian Academy of Sciences, Vladivostok, Russia. 5 Department of Marine Biogeochemistry, Royal Netherlands Institute for Sea Research, Texel, Netherlands. 6 Now at Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, Bristol University, Bristol, UK.

Copyright 2008 by the American Geophysical Union. 0886-6236/08/2007GB002974$12.00

[2] Estuarine surface sediments of major rivers provide an integrated archive of terrestrial organic matter (OM) exported from the heterogeneous mosaic of the upstream drainage basin. River discharge is particularly important for the Arctic Ocean since, although it contains only 1% of the world ocean volume [Menard and Smith, 1966], it receives 11% of the global input of freshwater and terrestrial dissolved organic matter (DOM) [Aagaard and Carmack, 1989; Gordeev et al., 1996; Anderson et al., 1998; Macdonald et al., 1998]. The drainage basins of the Arctic rivers store large amounts of OM in their taiga and tundra soils, which account for 25– 33% of the entire global soil reservoir of OM [Waelbroeck et al., 1997; Oechel et al., 2000].

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Table 1. Sample Location, Hydrological Data, and Bulk Geochemical Data of the Surface Sediments Sample ID Geological and physiographic regionsb Latitude; longitude Basin area, 106 km2c Water runoff, km3/ac TSM, 106t/ac,d TOC, mg/ge d 13CTOC, % TNf, mg/g TOC/TN mass ratio D14C, %g

Ob

Yenisey

Lena

Indigirka

Kolyma

1 west Siberian lowlands 72°650N;73°440E 2.54 – 2.99 402 – 429 13.4 – 16.5 9.2 ± 0.2 27.4 ± 0.1 0.92 ± 0.01 10.0 ± 0.1 314 ± 3

2 west Siberian lowlands 72°610N;79°860E 2.44 – 2.59 580 – 620 13.2 – 14.5 19.4 ± 0.3 26.5 ± 0.1 1.85 ± 0.03 10.5 ± 0.1 175 ± 3

N-8 central Siberian plateau 71°960N;129°540E 2.43 – 2.49 528 – 530 11.7 – 17.6 4.8 ± 0.2 25.0 ± 0.1 0.39 ± 0.03 12.3 ± 0.8 608 ± 3

IK43-46a east Siberian highlands 72°060N;150°460E – 71°020N;152°600E 0.36 54 – 61 12.9 14.6 ± 0.2 26.6 ± 0.1 0.99 ± 0.02 14.7 ± 0.2 527 ± 3

IK-71 east Siberian highlands 70°000N;163°700E 0.65 – 0.66 103 – 132 16.1 17.3 ± 1.1 26.7 ± 0.1 1.09 ± 0.04 15.9 ± 1.2 503 ± 2

a

Combined surface sediments from stations IK43, 44, 45, and 46. According to AMAP [1998]. c Data from Holmes et al. [2002], Gordeev et al. [1996], and Rachold et al. [2004]. d TSM, Total suspended matter. e TOC, total organic carbon. f TN, total nitrogen. g Calculated according to equation given by Stuiver and Polach [1977]. b

[3] Numerical climate models forecast an amplified warming in the continental Arctic region [e.g., Zwiers, 2002]. This makes it reasonable to expect that the Arctic is one of the first areas where climate warming effects on large-scale biogeochemical cycles will be observed [Macdonald, 1996; Dickson et al., 2000; Serreze et al., 2000] most pronounced in the east Siberian region [ACIA, 2004]. For instance, the huge carbon inventory in Siberian soils is predicted to be affected by global warming through permafrost destabilization [Stendel and Christensen, 2002] with wide-reaching consequences such as increasing river discharge [e.g., Savelieva et al., 2000; Peterson et al., 2002; 2006; Yang et al., 2002], and changing plant cover. This will in turn change the input fluxes and composition of terrigenous carbon to coastal seas [Dittmar and Kattner, 2003; Benner et al., 2004; Guo et al., 2004]. The provenance and molecular composition of OM are key characteristics that affect the large-scale biogeochemical cycling of carbon. However, it is still largely unclear how system responses to climate change will impact the delivery of terrestrial OM to the Arctic Ocean. While the OM entering the Mackenzie – Beaufort Sea system has been well characterized at a molecular compositional level [e.g., Yunker et al., 1995, 2002; Gon˜i et al., 2000, 2005; Belicka et al., 2004; Drenzek et al., 2007], such important baseline data is much less detailed for the Eurasian Arctic and then only for single rivers [e.g., Peulve et al., 1996; Fahl and Stein, 1997; Zegouagh et al., 1998; Fernandes and Sicre, 2000; Fahl et al., 2003; Stein and Macdonald, 2004]. The major objective of the present study is to provide a detailed and coherent benchmark of the lipid biomarker composition of the terrestrial OM exported by the great Russian Arctic

rivers (GRARs; the Ob, Yenisey, Lena, Indigirka, and Kolyma), spanning a continent-scale (140° longitude) variation in climate, permafrost, hydrology and vegetation coverage. A simultaneous investigation of all five GRARs offers the possibility to evaluate links between specific features of this climosequence and the molecular-level OM composition, which may prove beneficial in predicting the effects on OM composition and thus large-scale carbon cycling under a changing climate.

2. Materials and Methods 2.1. Study Area and Sampling [4] The five GRARs have different drainage basin areas, hydrology and annual discharge (Table 1) and the locations of their river mouths span over 4000 km along the Eurasian rim of the Arctic Ocean (Figure 1). The watersheds of the two west Siberian rivers (Ob and Yenisey) are mainly located in the region of nonpermafrost and/or permafrost islands and flow into the Kara Sea. The east Siberian rivers, on the other hand, are located in the permafrost region that covers approximately half of the territory of Russia and flow into the Laptev Sea (Lena) and the East Siberian Sea (Indigirka and Kolyma; Figure 1a). In addition, both the Kolyma and Indigirka rivers have their headwater almost entirely within the Arctic. In contrast, the other three rivers begin their flow far south of the Arctic region [Woo, 1992], with the Arctic defined as the area north of the 10°C July isotherm [AMAP, 1998]. [5] The climate of northeastern Eurasia (central Siberian plateau and the east Siberian highland), containing the drainage basins of the three eastern GRARs (Figure 1), is

Figure 1. (a) Map of the Siberian Arctic region, based on a map obtained from the National Geophysical Data Center, National Oceanic and Atmospheric Administration, U.S. Department of Commerce [Jakobsson et al., 2000], with colorcoded bathymetry and topography, flow paths of the great Russian Arctic rivers (GRARs) as thick black lines, sampling locations as open circles, and sample ID as mentioned in Table 1. (b) Land cover map of Russia based on the SPOT4vegetation satellite data (http://terranorte.iki.rssi.ru) [Bartalev et al., 2003; Bartholome´ and Belward, 2005; Groisman and Bartalev, 2006] with flow paths of the GRARs as black lines. 2 of 14

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Figure 1

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severe, with average summer temperatures between +7°C and +9°C, winter temperatures below 40°C and semiarid – arid conditions. The most arid conditions can be found in the basins of the Kolyma and Indigirka rivers [FAO, 2001; Naumov, 2004]. In contrast, the west Siberian lowland, the drainage basin of the two western GRARs Ob and Yenisey, holds a transitional place between this extreme continental climate and the less extreme Atlantic climate of Europe, with average summer temperatures comparable to northeastern Eurasia but much higher winter temperatures (around 20°C) [FAO, 2001; Kremenetski et al., 2003; Karavaeva, 2004]. Consequently, the drainage areas of these two western rivers, and to a lower degree the Lena River, have a higher annual precipitation and higher precipitation to evaporation ratios compared to the drainage areas of the Kolyma and Indigirka rivers. [6] The GRARs have a combined freshwater discharge around 1700 km3/a, by far the greatest portion of the total freshwater discharge to the Arctic Ocean [Opsahl et al., 1999]. The majority of this annual discharge (>80%) is concentrated in the time period from April to June. The hydraulic discharge of Indigirka and Kolyma are combined just about 10% of the GRAR total (Table 1). In contrast, the total suspended matter (TSM) discharge is comparable for all five GRARs (Table 1). This relatively lower hydraulic discharge but higher TSM yield of the eastern rivers is comparable to the North American Mackenzie River [Gordeev et al., 1996], although the amount of solids that the Mackenzie River delivers is far greater than the Siberian rivers [Yunker et al., 1995; Macdonald et al., 1998; Stein, 2000; Yunker et al., 2002]. This likely reflects the greater relief and thus a stronger weathering of the east Siberian highland drainage basins of Indigirka and Kolyma. It should be noted that besides river transport also coastal erosion forms a significant flux of TSM to the Arctic, especially in near shore zones [Semiletov, 1999; Semiletov et al., 2005]. [7] The vegetation cover for all drainage basins varies from north to south (Figure 1b). The northern areas are characterized by different tundra and wetland vegetation. The west Siberian lowland is the world’s largest highlatitude wetland, and possesses over 900,000 km2 of peatlands [Kremenetski et al., 2003]. The dominant vegetation types are mosses (mainly Sphagnum), sedges, shrubs and trees. Southward, particularly in the drainage areas of the Ob, Yenisey, and Lena rivers, the vegetation becomes more dominated by different types of forests (Figure 1b) [Bordovskiy, 1965; FAO, 2001; Kremenetski et al., 2003; Bartholome´ and Belward, 2005; Groisman and Bartalev, 2006]. [8] Surface sediment samples of the Lena, Indigirka and Kolyma estuaries were collected using a van Veen grab sampler (dimensions 20 30 cm), a light weight sampler designed to take large samples in soft bottoms [Riddle, 1989; Somerfield and Clarke, 1997], during the second Russia-United States cruise 2004 in the east Siberian and Laptev seas. Typically, between 2 and 4 kg of sediments were collected to have enough material for subsequent compound-specific radiocarbon analysis. For this reason sediment from 4 locations (IK43-IK46) were combined for the Indigirka estuarine sample to provide a sufficiently

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large sample. In addition, surface sediment samples of the Ob and Yenisey estuaries were obtained during the third Russia-United States cruise 2005 in the Kara Sea, using the same van Veen grab sampler, used by the same operator and using the same protocol. Detailed sampling locations and other parameters are listed in Table 1. In all cases sediment integrity was first visually inspected before approximately 0 –2 cm was measured manually and subsampled from the van Veen sampler; the rest was discarded. The samples were kept frozen at 20°C until processed in the laboratory. 2.2. Extraction and Fractionation [9] The bulk sediments were thoroughly mixed prior to sub sampling and small amounts of material were used for elemental analyses (performed by the UC Davis Stable Isotope Facility; http://stableisotopefacility.ucdavis.edu) and radiocarbon analyses (at the National Ocean Sciences Accelerator Mass Spectrometry Facility at Woods Hole Oceanographic Institution). For biomarker analysis, sub samples, typically between 10 and 20 g, were freeze dried, a mixture of three standards (tetracosane-2H50, tricontane-2H62 and hexadecan-2-ol) was added, and the sediments were extracted using a Soxhlet apparatus with dichloromethane/methanol (DCM/MeOH, 2:1, v/v) for 24 h. The total lipid extracts (TLEs) were concentrated using rotary evaporation and treated with activated (2N HCl) copper curls to remove elemental sulfur. Subsequently, aliquots of the TLEs were separated into three fractions using BondElut1 column chromatography (Strata NH2; 5 mm, 70 A; Varian, Middelburg, Netherlands) [e.g., Kim and Salem, 1990] and elution with DCM/isopropanol (2:1 v/v; 12 ml; ‘‘neutral lipid fraction’’), an acetic acid solution (a 2% solution in diethyl ether; 12 ml; ‘‘acid fraction’’) and MeOH (12 ml; ‘‘phospholipid fraction’’). For the present study, further analyses were only performed on the neutral lipid and acid fractions. The neutral lipid fraction was further separated into two fractions using a column packed with 100% activated alumina by elution with hexane/DCM (9:1 v/v; 3 ml; ‘‘hydrocarbon fraction’’) and DCM/MeOH (1:1 v/ v; 3 ml; ‘‘polar fraction’’). The hydrocarbon, polar and acid fractions were analyzed using gas chromatography/mass spectrometry (GC/MS). Blanks were run to ensure that no contamination was introduced during the extraction and separation procedure and were found to be 4 we were still able to detect them clearly. We estimate that the total amounts of hopanes and hopenes in the Lena, Indigirka and Kolyma samples are below 0.02, 0.09 and 0.06 mg/g OC, respectively. 3.2.2. Acid Fraction [15] All acid fractions are dominated by a homologous series of C14 – C30 n-alkanoic acids, with the C24 member being the most abundant, showing an even-over-odd carbon number predominance (Figure 2b and Table 2). The nalkanoic acid CPI (defined in Table 3) is similar for all samples, with values between 4.1 and 4.5 (Table 3). In contrast, the n-alkanoic acid ACL (defined in Table 3) varies from 24.3 to 26.0 (Table 3). 3.2.3. Polar Fraction [16] All polar fractions contain a homologous series of C16 – C32 n-alkanols, dominated by the C28 homologue (in the Kolyma estuary sediment the C26 homologue) and a predominance of the even numbered homologues (Figure 2c and Table 2). The n-alkanol ACL, CPI and TARn-alkanol ratios (all defined in Table 3) span ranges of 25.9– 26.6, 5.6– 13.9 and 48– 145, respectively (Table 3). Substantial amounts of steroids, in particular b-sitosterol (24-ethylcholest-5-en-3b-ol; 3 to 15 times more abundant than the other sterols), are also present in all samples (Figure 2c and Table 4). b-sitosterol is in fact the most abundant compound present in the polar fraction of the Lena, Indigirka and Kolyma estuary sediments (Figure 2c and Table 4). Other steroids that are also present in substantial amounts include cholest-5-en-3b-ol, 24-methylcholest-5-en-3b-ol (campersterol), 24-ethylcholest-5,22E-dien-3b-ol (stigmasterol), 24-ethyl-5a-cholestan-3b-ol (Figure 2c and Table 2). In contrast, the marine-derived 4,23,24-trimethyl-5a-cholest22-en-3b-ol (dinosterol) [Boon et al., 1979; de Leeuw et al., 1983] could not be detected in any of the sediments. In contrast to the sterols, triterpenoids, represented by friedelan-3-one, a-amyrin (urs-12-en-3b-ol) and b-amyrin (olean12-en-3b-ol) are only present in minor amounts (Figure 2c). 3.2.4. GDGTs [17] The east Siberia River estuary sediments contain substantial amounts of GDGTs, including both nonisoprenoidal branched GDGTs and isoprenoidal GDGTs (Figure 2d). All sediments contain much higher relative abundances of branched GDGTs compared to the isoprenoidal GDGTs (Table 5). This difference is clearly reflected in the BIT

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Figure 2. Total ion current gas chromatography (GC) chromatograms of the (a) hydrocarbon fraction of the Indigirka, (b) acid fraction of the Yenisey, and (c) polar fraction of the Indigirka River estuary surface sediment. The n-alkanes are represented by solid circles; internal standard (IS); hopanes are represented by plus symbols; hopenes are represented by asterisks; n-alkanoic acids are represented by stars; nalkanols are represented by open inverted triangles; triterpenoids are represented by solid inverted triangles. Letters in Figure 2c indicate cholesta-5,22E-dien-3b-ol (a); cholest-5-en-3b-ol (b); 5acholestan-3b-ol (c); 24-methylcholest-5,22E-dien-3b-ol (d); 24-methylcholest-5-en-3b-ol (e); 24-methyl5a-cholestan-3b-ol (f); 24-ethylcholest-5,22E-dien-3b-ol (g); 24-ethylcholest-5-en-3b-ol (h); and 24ethyl-5a-cholestan-3b-ol (i). Numbers indicate carbon chain length, and bb indicates stereochemistry of hopanes at C-17 and C-21 positions. (d) High-performance liquid chromatography mass spectrometry (HPLC/MS) base peak chromatogram of tetraether lipids in the surface sediment of the Kolyma River estuary.

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Table 2. Concentrations of Long-Chained n-Alkanes, n-Alkanoic Acids, and n-Alkanols in Surface Sediments of Russian Arctic River Estuaries Ob

Yenisey

HMW n-alkanesb S(C27 + C29 + C31) n-alkanes HMW n-alkanoic acidsc S(C24 + C26 + C28) n-alkanoic acids HMW n-alkanolsd S(C26 + C28 + C30) n-alkanol

17 7.0 2.4 1.4 20 13

Concentrationsa 18 9.4 4.2 2.7 43 29

HMW n-alkanesb S(C27 + C29 + C31) n-alkanes HMW n-alkanoic acidsc S(C24 + C26 + C28) n-alkanoic acids HMW n-alkanolsd S(C26 + C28 + C30) n-alkanol

1.8 0.76 0.26 0.15 2.2 1.4

Concentrationse 0.91 0.48 0.22 0.14 2.2 1.5

Lena

Indigirka

Kolyma

2.6 1.4 1.5 0.8 3.1 2.0

25 13 8.0 4.5 32 19

20 11 17 9.0 35 21

0.55 0.28 0.31 0.16 0.64 0.43

1.7 0.91 0.55 0.31 2.2 1.3

1.2 0.65 0.96 0.52 2.0 1.2

a

Units are mg/g Sed. High molecular weight (HMW), sum of the concentrations of C20 – C34 n-alkanes. c Sum of the concentrations of C20 – C30 n-alkanoic acids. d Sum of the concentrations of C20 – C32 n-alkanols. e Units are mg/g OC. b

index [Hopmans et al., 2004] (defined in Table 5), showing values between 0.88 and 0.92 (Table 5).

4. Discussion 4.1. Terrestrial Versus Marine Origin of Organic Matter in GRAR Estuary Sediments [18] Bulk geochemical analysis of all GRAR estuarine sediments indicates terrestrially dominated OM sources. The TOC/TN ratios (10 to 15.9; Table 1) fall between those of C/N of marine OM (around 6) and terrestrially derived OM (>15) [Bordovskiy, 1965; Hedges et al., 1986]. However, TN is here used in this ratio and not total organic nitrogen (TON), which, as seen in other Arctic sediments,

could cause a shift in the C/N ratio to lower values due to inclusion of inorganic N such as NH+4 in clay interstitial layers [Schubert and Calvert, 2001; Fahl et al., 2003; Drenzek et al., 2007]. The depleted d13CTOC values of 25.0 to 27.4% (Table 1) are typical for land plants using the C3 synthesis pathway [Meyers, 1997] and comparable to earlier observations in these regions [e.g., Guo et al., 2004; Semiletov et al., 2005], supporting the predominantly terrestrial origin of these sediments. Besides material of terrestrial biosynthetic origin, recent research showed that 1 –10% of the OC in the GRAR estuaries is present as combustion-derived black carbon [Guo et al., 2004; M. Elmquist et al., Source apportionment of black carbon in

Table 3. Source-Diagnostic Distributions of n-Alkanes, n-Alkanoic Acids and n-Alkanols in Surface Sediments of Russian Arctic River Estuaries Ob

Yenisey

Lena

Indigirka

Kolyma

n-Alkanes CPI25 – 33a ACL25 – 33b C23/(C23 + C29)c TARn-alkaned

3.2 28.0 0.47 80

6.5 28.5 0.38 49

5.5 28.6 0.37 17

5.5 28.5 0.33 50

7.3 28.2 0.35 46

CPI20 – 30a ACL20 – 30b

4.2 25.3

n-Alkanoic Acids 4.5 26.0

4.1 24.8

4.4 25.2

4.2 24.3

CPI20 – 32a ACL20 – 32b TARn-alkanole

5.6 26.6 79

6.6 26.6 145

13.9 26.6 48

8.2 25.9 106

8.9 26.2 115

n-Alkanols

a CPIin, carbon preference index, = 1=2 S(Xi + Xi+2 + . . . + Xn)/S(Xi1 + Xi+1 + . . . + Xn1) + 1=2 S(Xi + Xi+2 + . . . + Xn)/S(Xi+1 + Xi+3 + . . . + Xn+1), where X is concentration. b ACLin, average chain length, = S(iXi + . . . + nXn)/SXi + . . . + Xn), where X is concentration. c Ratio of the C23 n-alkane to the sum of the C23 and C29 n-alkanes. d TARn-alkane, ratio of terrigenous-to-aquatic n-alkanes, = sum of the C27, C29 and C31 n-alkanes to the sum of the C17 and C19 n-alkanes. e TARn-alkanol, ratio of terrigenous-to-aquatic n-alkanols, = sum of the C26 and C28 n-alkanols to the sum of the C16 and C18 n-alkanols.

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Table 4. Concentrations of Sterols in Surface Sediments of Russian Arctic River Estuaries Ob

Yenisey

Cholesta-5-en-3b-olb 24-Methylcholest-5,22E-dien-3b-olc 24-Methylcholest-5-en-3b-old 24-Ethylcholest-5,22E-dien-3b-ole 24-Ethylcholest-5-en-3b-olf

1.0 0.69 0.84 1.2 3.5

Concentrationsa 0.97 1.05 1.2 2.1 7.0

Cholesta-5-en-3b-olb 24-Methylcholest-5,22E-dien-3b-olc 24-Methylcholest-5-en-3b-old 24-Ethylcholest-5,22E-dien-3b-ole 24-ethylcholest-5-en-3b-olf

0.11 0.08 0.09 0.13 0.38

Concentrationsg 0.05 0.05 0.06 0.11 0.36

Lena

Indigirka

Kolyma

0.10 0.07 0.12 0.35 1.9

0.70 0.54 1.3 1.3 25

0.91 0.59 2.05 2.2 39

0.02 0.01 0.03 0.07 0.40

0.04 0.03 0.08 0.08 1.4

0.06 0.04 0.14 0.15 2.7

a

Units are mg/g Sed. Cholesterol. c Brassicasterol. d Campesterol. e Stigmasterol. f b-sitosterol. g Units are mg/g OC. b

surface sediments from the seven largest Pan-Arctic rivers using natural abundance radiocarbon and molecular markers, submitted to Global Biogeochem. Cycles, 2007; hereinafter referred to as Elmquist et al., submitted manuscript, 2007]. [19] The abundant presence of biomarkers such as C16 – C34 n-alkanes (with an odd-over-even predominance), C14 – C30 n-alkanoic acids and C16 – C32 n-alkanols (both with an even-over-odd predominance; Table 2), sterols such as bsitosterol (Table 4 and Figure 3) and smaller amounts of higher plant triterpenoids [Bray and Evans, 1961; Eglinton and Hamilton, 1963, 1967; Pant and Rastogi, 1979], further supports a predominantly terrestrial origin. The n-alkane and n-alkanoic acid concentrations are comparable to those observed in the estuary of the Mackenzie River (e.g., station 5 in the work of Drenzek et al. [2007]), with estimations of 0.50 mg/g OC and 0.45 mg/g OC, respectively (note that the Mackenzie River concentrations are measured after saponification). The CPIs of the n-alkanes range from 3.2 to 7.3

(Table 3), with the exception of the Ob sample, comparable to those typically observed for extant plants (>5) [Rieley et al., 1991; Hedges and Prahl, 1993] and those reported earlier for Russian Arctic sediments [Fahl and Stein, 1997; Fernandes and Sicre, 2000; Fahl et al., 2003]. Significantly lower n-alkane CPIs are reported for the Mackenzie River estuary sediments (1.8 to 2.7) [Yunker et al., 1993, 1995, 2002; Drenzek et al., 2007], most likely reflecting a large petrogenic input to this North American river. The GRAR CPIs of the n-alkanoic acids (4.1 to 4.5) and the n-alkanols (5.5 to 13.3; Table 3) are similarly in agreement with a predominantly higher plant origin. [20] The slightly lower n-alkane CPI value for this Ob sample (CPI = 3.2) is comparable to n-alkane CPIs of 0.8–

Table 5. Concentrations of GDGTs and BIT Indices in Surface Sediments of East Siberian Arctic River Estuaries Indigirka

Kolyma

Isprenoidal GDGTsb Branched GDGTsc

Concentrationsa 0.4 10

Lena

2.8 37

2.8 49

Isprenoidal GDGTsb Branched GDGTsc BIT indexe

Concentrationsd 0.09 2.1 0.92

0.19 2.5 0.88

0.16 2.8 0.91

a

Units are mg/g Sed. Sum of glycerol dialkyl glycerol tetraether (GDGT) I and IV (Figure 2d). Sum of GDGT V, VI, and VII (Figure 2d). d Units are mg/g OC. e Branched and isoprenoid tetraether (BIT) index = index. calculated following the equation of Hopmans et al. [2004]; (BIT index = S(V + VI + VII)/S(V + VI + VII + IV)] (Figure 2d)). b c

Figure 3. Abundances of n-alkanes, n-alkanoic acids, nalkanols, and b-sitosterol along the 140° longitude Eurasian-Arctic climosequence. Ob (O); Yenisey (Y); Lena (L); Indigirka (I); and Kolyma (K).

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Figure 4. Ratios of the (a) C23 n-alkane to the sum of the C23 and C29 n-alkanes, (b) high molecular weight (HMW) n-alkanoic acids to the HMW n-alkanes, (c) b-sitosterol to HMW n-alkanes, and (d) the n-alkanoic acid carbon preference index (CPI) to n-alkane CPI along the 140° longitude Eurasian-Arctic climosequence. Ob (O); Yenisey (Y); Lena (L); Indigirka (I); Kolyma (K).

3.5 for other Ob estuary sediments [Fahl et al., 2003]. The absence of both a detectable unresolved complex mixture (UCM), typical of heavily biodegraded OM [Gough and Rowland, 1990, 1991; Gough et al., 1992; van Dongen et al., 2003] and abiogenic but thermally more stable 17a(H),21b-hopanes [Mackenzie, 1984; Brassell, 1985] makes an input of OM with a thermally mature origin, such as natural oils seeps, weathering of petroleum source rocks or even petroleum contamination, less likely. Furthermore, recent PAH analyses of this sediment sample indicates a methylphenanthrenes to phenanthrene ratio that also supports a very low petrogenic input in the Ob estuary sample (Elmquist et al., submitted manuscript, 2007). In addition, values as low as the Ob CPI are commonly observed in sediments and this value is still consistent with a predominantly higher plant origin [Brassell et al., 1978]. The large variability in the CPI value for the terrestrial end-member prevents CPI-based quantitative assessment of the fractional contribution of land sources to the total OM. [21] The concentrations of the low molecular weight (LMW) n-alkanes (C17 and C19) and n-alkanols (C16 and C18), originating from aquatic phytoplankton, are much lower than their high molecular weight (HMW) terrestrial counterparts in all five GRAR estuary samples. Thus the associated TARn-alkanes and TARn-alkanol, spanning 17– 80 and 48 – 123 (Table 3), respectively, supports an overwhelmingly terrigenous source of the OM. [22] A predominantly nonterrestrial source for the hydrocarbons observed in the sediments of the Lena estuary has been suggested [Zegouagh et al., 1996, 1998]. That propo-

sition was based on an absence of HMW n-alkanoic acids and hydroxy n-alkanoic acids. In contrast to Zegouagh et al. [1996] substantial amounts of HMW n-alkanoic acids were observed in all estuary sediments of the present study, including the Lena estuary (Table 2 and Figures 2b and 4). Comparable to Zegouagh et al. [1998], no hydroxy nalkanoic acids have been observed, but this does not necessarily indicate an absence of terrestrially derived material. For instance, North Sea sediments, although dominated by terrestrially derived OM, were also characterized by the absence of detectable amounts of hydroxyl n-alkanoic acids [van Dongen et al., 2000]. The overwhelming presence of many other terrestrial biomarkers and complete absence or only minor amounts of marine biomarkers, as also shown in several other, both Russian and Canadian/American, Arctic sediment studies [e.g., Yunker et al., 1995, 2002; Peulve et al., 1996; Fahl et al., 2003; Stein and Macdonald, 2004], suggest a predominantly terrestrial source for the hydrocarbons, including those observed in the Lena River estuary. [23] Biomarker analyses also indicate, comparable to earlier Arctic sediment studies [Yunker et al., 1995; Fahl et al., 2003] the presence of substantial amounts of specific sterols synthesized by higher plants, such as campesterol and foremost b-sitosterol (Table 4). However, these sterols are not exclusively synthesized by higher plants and additional input from phytoplankton and bacteria, although a minor process at best [Volkman, 2005], is possible [Gagosian et al., 1983; Volkman, 1986; Volkman et al., 1998]. Brassicasterol, generally synthesized by both marine and freshwater phytoplankton, is also abundantly present (Figure 2c).

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Fahl et al. [2003] observed a decreasing trend in brassicasterol concentrations in surface sediments from Ob and Yenisey estuaries northward to the southern Kara Sea, making the freshwater origin the most likely source in these systems. In addition, no other specific marine steroids, including dinosterol, were observed, making the higher plant/freshwater phytoplankton origin the most likely source for the majority of the steroids observed in these sediments. The absence of dinosterol in surface sediments of Arctic River estuaries has been noted before and could, for instance, not be positively identified in Mackenzie River sediment samples [Yunker et al., 1995]. [24] Branched glycerol dialkyl glycerol tetratetraethers (GDGTs; V, VI, VII in Figure 2d) are ostensibly synthesized by anaerobic soil bacteria and thus most likely of pure terrestrial origin [Weijers et al., 2006b], whereas the isoprenoid GDGTs in the Arctic estuarine sediments likely have a mixed terrestrial and marine origin [Powers et al., 2004; Weijers et al., 2004, 2006a, 2006b; Herfort et al., 2006b; Kim et al., 2006]. Methanogenic archaea in peats produce predominantly GDGT I and to a lesser extent GDGTs II and III [e.g., Kates et al., 1993; Pancost et al., 2000] and marine Crenarchaeota predominantly synthesize GDGT I and crenarchaeol (GDGT IV in Figure 2d) [Sinninghe Damste´ et al., 2002]. Although crenarchaeol has also been detected in peats and soils [Weijers et al., 2004, 2006a], its amounts are considerably lower than the branched GDGTs resulting in BIT values of soils typically >0.90 [Weijers et al., 2006a]. The BIT index values of 0.88 to 0.92 obtained for the eastern GRARs are typical for sediments receiving high amounts of fluvial terrestrial organic matter [Hopmans et al., 2004; Herfort et al., 2006a; van Dongen et al., 2006; Weijers et al., 2006a], and again in support of a predominantly terrestrial origin of OM in these sediments. [25] To summarize, both bulk geochemical and a broad suite of molecular biomarker analyses of this first comprehensive study of the five GRAR estuaries indicate that the OM across the continent-scale climosequence of the coastal Siberian Arctic are overwhelmingly dominated by terrestrial sources. The majority of this land-derived OM is transported to the estuaries by fluvial processes, by riverine runoff and coastal erosion, as aeolian transport provides only a very minor input [e.g., Stein and Macdonald, 2004]. 4.2. Distinguishing Among Different Terrestrial Sources of the Exported Organic Matter [26] The terrestrial OM transported to all GRAR estuarine sediments, as illustrated above, are originating from diverse sources. For instance, n-alkanes, n-alkanoic acids, n-alkanols, b-sitosterol, campesterol and triterpenoids are all indicative of a higher plant origin, brassicasterol for freshwater phytoplankton origin, while branched GDGTs traces an anaerobic soil bacteria origin. [27] The C23 to (C23 and C29) n-alkane with values around 0.4 (Table 3) further suggests an influx of OM rich in midchain (besides C23 also predominantly C25) n-alkanes, following Nichols et al. [2006] probably Sphagnum-derived OM, to the estuarine sediments. Indeed in the Russian Arctic, large areas are dominated by tundra, bogs and

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marshes [Bordovskiy, 1965; FAO, 2001; Kremenetski et al., 2003; Kimble, 2004; Bartholome´ and Belward, 2005; Groisman and Bartalev, 2006]. All these vegetation types are rich in Sphagnum, indicating that Sphagnum-rich peats are likely to be a major source of OM to the GRARs. Unfortunately, the n-alkane distributions in Sphagnum species have not been specifically determined for Russian Arctic ecosystems. We are therefore using published analyses of Sphagnum species in non-Arctic peat bogs showing large amounts of C23 and C25 n-alkanes [Corrigan et al., 1973; Baas et al., 2000; Nott et al., 2000; Pancost et al., 2002; Nichols et al., 2006] as an available reference point. [28] The C23 to (C23 and C29) n-alkane ratio shows a clear trend from west (0.47) to east (0.33; Table 3 and Figure 4a). The west Siberian lowland, covering the drainage areas of both the Ob and Yenisey, is indeed predominantly covered with Sphagnum-rich peatland (>900,000 km2) [Kremenetski et al., 2003]. In contrast, and also in agreement with the observed trend, the extent of peatland and other Sphagnum rich land cover is much lower in the eastern areas (Figure 1b). The basins of the Kolyma and Indigirka rivers are characterized by relatively arid conditions, with summer precipitation below 100 to 150 mm, a droughty period that lasts for 30 to 100 d and a precipitation-to-evaporation ratio between 0.3 and 0.5 [Naumov, 2004]. In contrast, the summer precipitation in the basin of the more western Siberian rivers (Ob, Yenisey and Lena) is much higher (up to 400 mm) with a precipitation-to-evaporation ratio above 1.3 to 1.5, indicating that the habitats in these areas are relative more wet [FAO, 2001; Karavaeva, 2004; Naumov, 2004]. These arid conditions can also influence the observed trend. Besides an increased Aeolian plant wax transport and potential for preferential transport of n-alkanes at the lower end of the vascular plant range these arid conditions may possibly also cause a shift in the n-alkane distribution within the Sphagnum species. Recent research indicates that Sphagnum species growing under drier conditions, in contrast to species growing in relatively wet habitats, such as S. magellanicum and S. capillifolium [Andrus, 1980; Ha´jkova´ and Ha´jek, 2004a, 2004b], are dominated by the higher n-alkanes (C27 to C33 n-alkanes) [Nott et al., 2000]. This suggests that a shift in the n-alkanes distribution may reflect not only a variation in relative amounts of Sphagnum-derived OM transported to the estuaries but also a shift in drainage basin aridity. [29] Differences in vegetation and/or environmental conditions could also be the cause of other molecular-compositional shifts observed along the Eurasian-Arctic climosequence. For instance, the ratios of both HMW nalkanoic acids and b-sitosterol to HMW n-alkanes show clearly increasing trends from west to east of 0.14 – 0.83 and 0.21– 2.3, respectively (Figures 4b and 4c). These trends may signal increased preservation due a lesser extent of microbial remineralization, as will be discussed below. However, the relative amounts and proportions of all biomarkers are dependent on the plant species [e.g., Gunstone et al., 1994; Christie, 2003, and references therein] indicating that shifts in biomarker ratios may also indicate shifts in vegetation cover. As espoused above, the detailed molecular composition may also vary for similar plant species growing

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Figure 5. Correlation plot of the summed concentrations of campesterol and b-sitosterol versus the summed concentrations of the three most predominant high molecular weight n-alkanes. at different sites or under different environmental conditions [Maffei, 1996; Sachse et al., 2006]. This is mainly due to the different functions these compounds have within the plants. For instance, sterols act as rigidifiers in cell membranes while HMW n-alkanes and n-alkanoic acids act as a protective coating in leaf waxes. This indicates that, although the observed ratios are largely influenced by degradation processes, a response to different environmental conditions, such as the more arid conditions in the basins of the Kolyma and Indigirka rivers, cannot be excluded. 4.3. Trends in Degradation of Exported Terrestrial Organic Matter [30] Decay of OM occurs initially in the water column and then in the microbially active upper zones of the sediment, and involves the loss of diagnostic characteristics (functional groups) of the original biological compounds [Mackenzie et al., 1982; Maxwell and Wardroper, 1982; Brassell et al., 1984; de Leeuw et al., 1989]. Thus the presence of substantial amounts of compounds such as nalkanoic acids, n-alkanols and sterols relative to n-alkanes (Table 2 and 4) suggest a low proportion of OM decay. Furthermore, the n-alkanol distributions are characterized by a maxima at n-C28 (or n-C26), comparable to particulate matter samples of the Mackenzie River [Yunker et al., 1995]. This indicates the presence of ‘‘free’’ n-alkanols rather than more refractory ‘‘bound’’ n-alkanols [Cranwell, 1981; Yunker et al., 1995], suggesting the presence of fresh material in the river sediments. Bacterial biomarkers, such as hopanes and hopenes, which are typically produced by bacteria during the early decay of OM, are also only present in relatively small amounts in the GRAR sediments. The relatively small contribution of these bacterial biomarkers thus also suggests a low degree of OM decay. In addition, recent research indicates substantial amounts of bacteriohopanepolyols (BHP), the precursors of these hopenes/ hopanes, in the estuary sediments of the east Siberian rivers (B. E. van Dongen et al., Bacteriohopanepolyols in surface

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sediments of the east Siberian River estuaries, manuscript in preparation, 2007), further supporting the low degree of OM decay. It should be noted that although degradation of OM seems to be at a low degree, recent research indicates that degradation of OM transported by both coastal erosion and riverine run-off causes strong carbon dioxide and methane out gassing in the Arctic Siberian coastal zones [Semiletov, 1999; Shakhova et al., 2005; Semiletov et al., 2007; Shakhova and Semiletov, 2007]. [31] Relatively larger amounts of HMW n-alkanoic acids or sterols, in particular b-sitosterol, compared to HMW nalkanes (Figures 4b and 4c), are observed in the estuary sediments of the eastern Siberian rivers compared to the western estuaries, indicating a lower degree of decay toward the east. Following Fahl et al. [2003], this is further illustrated by the summed concentrations of campesterol and b-sitosterol as a function of the HMW n-alkanes (Figure 5). Similar to Fahl et al. [2003], a linear correlation for the western GRAR estuaries is observed. However, both samples from the Kolyma and Indigirka estuaries fall outside this correlation, with both showing an excess of b-sitosterol and campesterol, indicating a lower degree of OM decay in the eastern GRARs. The ratio of n-alkanoic acid CPI to n-alkane CPI also show a trend from west (>1) to east (