Modern organic carbon deposition in the Laptev Sea and the adjacent continental slope: surface water productivity vs. terrigenous input

P e r g a m o n

Org. Geochem. Vol. 26, No. 5/6, pp. 379-390, 1997

© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain PII: S0146-6380(97)00007-7 0146-6380/97 $17.00 + 0.00

Modern organic carbon deposition in the Laptev Sea and the adjacent continental slope: surface water productivity vs. terrigenous input

K I R S T E N F A H L and R U E D I G E R S T E I N

Alfred-Wegener-lnstitute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany

(Received 22 April 1996; returned to author for revision 25 June 1996; accepted 21 January 1997) Abstract--Sediment samples from the Laptev Sea, taken during the 1993 RV Polarstern expedition ARK IX/4 and the RV Ivan Kireyev expedition TRANSDRIFT I, were investigated for the amount and composition of their organic carbon fractions. Of major interest was the identification of different processes controlling organic carbon deposition (i.e. terrigenous supply vs. surface water productivity).

Long-chain unsaturated alkenones derived from prymnesiophytes, and fatty acids derived from diatoms and dinoflagellates, were analysed by means of gas chromatography and mass spectrometry. First results on the distribution of these biomarkers in surface sediments indicate that the surface water pro- ductivity signal is well preserved in the sediment data. This is shown by the distribution of the 16:1(n-7) and 20:5(n-3) fatty acids indicative for diatoms, and the excellent correlation with the chlorophyll a concentrations in the surface water masses and the biogenic-opal content and increased hydrogen indi- ces of the sediments. The high concentration of these unsaturated fatty acids in shallow water sediments shows the recent deposition of the organic material. In deep-sea sediments, on the other hand, the con- centrations are low. This decreased content is typical for phytoplankton material which has been degraded by microorganisms or autoxidation. In general, the alkenone concentrations are very low, suggesting low production rates by prymnesiophytes. Only at one station from the lower continental margin influenced by the inflow of Atlantic water masses, were some higher amounts of alkenones determined. Long-chain n-alkanes as well as high C/N ratios and low hydrogen indices indicate the im- portance of (fluvial) supply of terrigenous organic matter. © 1997 Elsevier Science Ltd

Key words--fatty acids, alkenones, marine organic matter, terrigenous organic matter, Laptev Sea, Arc- tic Ocean

INTRODUCTION

In relation to the world's ocean, the Arctic Ocean is rather less productive due to the permanent ice- cover (Subba R o a and Platt, 1984); however, re- gional differences occur. In marginal seas (such as the Laptev Sea) characterized by an increased flu- vial nutrient supply, near ice edges, a n d at local/re- gional upwelling cells, significantly raised primary production rates are expected. The m a p p i n g of sedi- mentological, geochemical a n d biological data reflecting the surface water productivity in surface sediments, and the subsequent comparison to recent oceanographic and biological parameters, will allow one to elucidate the most important processes deter- mining primary production in the Arctic Ocean.

Besides nutrients, the major Eurasian rivers also transport large a m o u n t s of dissolved a n d particulate material (i.e. chemical elements, siliciclastic a n d or- ganic matter) onto the shelves where it is accumu- lated, or further transported, toward the open ocean by different mechanisms (sea-ice, icebergs, turbidity currents, etc.). F o r example, the a n n u a l discharge o f suspended sediments by the Lena River is 17.6 x 106 tons, a n d the a m o u n t of dis-

solved organic carbon reaches m a x i m u m values of 11 mg/l during summer floods (Martin et al., 1993a). Thus, river-derived (terrigenous) organic material contributes major proportions to the or- ganic carbon in the sediments of the Laptev Sea and its adjacent continental slope.

The major goal of this study is to determine the a m o u n t and composition of the organic carbon fraction and to characterize the mechanisms con- trolling organic carbon deposition in surface sedi- ments from the Laptev Sea and its adjacent continental slope (i.e. surface water productivity vs.

terrigenous input).

FACTORS DETERMINING PRIMARY PRODUCTIVITY AND TERRIGENOUS ORGANIC CARBON FLUX The most important factors controlling organic carbon enrichment in the marine e n v i r o n m e n t are (1) increased surface water productivity, (2) increased preservation of organic carbon in anoxic a n d / o r high-sedimentation-rate environments, a n d (3) increased supply of terrigenous organic carbon (e.g. Berger et al., 1989; Stein, 1991). To distinguish between these different mechanisms, more detailed 379

380 Kirsten Fahl and Ruediger Stein information about the quantity as well as quality of

the organic matter is required. To get an estimate of the composition of the organic carbon fraction (i.e. to estimate the terrigenous and marine pro- portions) Rock-Eval pyrolysis parameters (HI), el- emental analysis data (C/N ratio), and Ol3Cor~

values are useful indicators in organic carbon-rich ( T O C > 0 . 5 % ) , immature sediments (Tissot and Welte, 1984; Stein, 1991). For a more precise deter- mination of the marine and terrigenous proportions of the organic carbon fraction, other methods such as kerogen/coal petrology, gas chromatography (GC), and mass spectrometry (MS) are required.

The distribution of n-alkanes determined by GC, for example, allows an identification of contri- butions of land-derived vascular plant material (characterized by long-chain C29 and C3j n-alkanes) and of marine phytoplankton (dominated by C~7 and C19 n-alkanes) (e.g. Blumer et al., 1971;

Kolattukudy, 1976; Prahl and Muehlhausen, 1989).

For fatty acids, it is generally accepted that mar- ine compounds are mainly represented by short- chain unsaturated fatty acids up to 22:6(n-3) indi- cating phytoplankton (or zooplankton) pro- ductivity, whereas the long-chain saturated fatty acids indicate a terrigenous input. Nearly the same is true for the wax esters. Based on the composition of the "marine" fatty acids it is possible to obtain more information about the productivity-controlling phytoplankton species. Diatoms mainly synthesize 16:1(n-7) and 20:5(n-3) fatty acids (Kates and Volcani, 1966; Ackman et al., 1968; Kattner et al., 1983; Fahl and Kattner, 1993; Graeve et al., 1994), whereas dinoflagellates synthesize the 18:4(n-3) and 22:6(n-3) compounds (Sargent and Henderson, 1986; Graeve, 1993).

At lower latitudes, the alkenones which were pro- duced by prymnesiophytes (Volkman et al., 1980), are used as (paleo-)temperature and (paleo-)pro- ductivity markers (Brassell and Eglinton, 1984;

Marlowe et al., 1984; Brassell et al., 1986; Prahl and Wakeham, 1987; Prahl et al., 1988).

(HI) and oxygen index (OI) were determined as described by Espitali6 et al. (1977).

For the lipid analyses the sediment samples were stored at -80°C or in dichloromethane/methanol (2:1, by vol.) at -23°C until further treatment. The sediment (2 g) was homogenised, extracted and pur- ified as recommended by Folch et al. (1957) and Bligh and Dyer (1959). An aliquot of the total extract was used for analyzing n-alkanes and alke- nones.

A l k a n e s

The alkanes were separated from the other frac- tions by column chromatography with hexane. The composition was analysed with a Hewlett Packard gas chromatograph (HP 5890, column 30 m x 0.25 ram; film thickness 0.25 #m; liquid phase: HP) using a temperature program as follows:

60°C (I min) to 150°C (rate: 10°C/min), then to 300°C (rate: 4°C/min), then 300°C (45 min isother- mal). A volume of 1 #1 was injected (cold injection system: 60°C (5s) to 300°C (60s, rate: 10°C/s) using helium as carrier gas. Squalane was added as an internal standard to provide quantitative data.

A lkenones

The alkenones were separated from the other fractions by column chromatography with hexane/

ethylacetate (95:5 and 90:10, by vol.). A saponifica- tion step with 1 M potassium hydroxide in 95%

methanol for 2 h at 90°C followed. Fractions were analysed by means of a Hewlett Packard gas chro- matograph (as described for the alkane analysis) using a temperature program as follows: 60°C (1 min) to 270°C (rate: 20°C/rain), then to 320°C (rate: l°C/min), then 320°C (20 rain isothermal). A volume of 1 #1 was injected (cold injection system:

60°C to 105°C (rate: 3°C/s), then to 320°C (rate:

10°C/s), then 320°C (60 s isothermal). Identification of the alkenones was achieved from GC retention times and MS fragmentation patterns. For quantifi- cation, octacosanoic acid methyl ester was used as an internal standard.

METHODS

The surface sediment samples from the Laptev Sea shelf and slope were taken in 1993 during the RV P o l a r s t e r n expedition A R K IX/4 (Fiitterer, 1994; Fig. 1) and the Transdrift I expedition with RV Ivan K i r e y e v (Kassens and Karpiy, 1994; Fig. 1).

The sampling was carried out with a giant box- corer.

Total nitrogen and organic carbon contents were determined by means of a Heraeus CHN-analyzer (for details concerning the method see Stein, 1991).

C/N ratios were calculated as "total organic car- bon/total nitrogen ratios" based on weight percen- tage. The Rock-Eval parameters hydrogen index

F a t t y acids

An aliquot of the total extract was used for pre- paring fatty acid methyl esters and free alcohols by transesterification for 4 h at 80°C with 3% concen- trated sulfuric acid in methanol. After extraction with hexane the product was analysed by GC (as above) but using D B - F F A P as liquid phase: the temperature program was as follows 160°C to 240°C (rate: 4°C/min), 240°C (15min isothermal) (modified according to Kattner and Fricke, 1986).

The injection volume was 1/A. The fatty acids and alcohols were identified by standard mixtures and quantified using methylnonadecanoate as internal standard.

O e~ O O

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30 °N /'5 °N

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50 °E r40 °E ~ig. 2. Distribution map of the total organic carbon content and hydrogen index values (rag HC,,gC) in surface sediments from the Laptev Sea and the adjacent continental nargin (after Stein and Ntirnberg, 1995). Open arrow indicates Atlantic water inflow. The hatched line symbolizes the ice margin during Sept. 93 (Eicken et al., 1995). Samples were taken during the RV Polarstern expedition ARK IX4 (F~tterer, 1994) and the RV lvan Kireyev expedition Transdrift I (Kassens and Karpiy, 1994).

L -1 _2.

Organic carbon deposition in the Laptev Sea 383 1500

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Fig. 3. Long-chain n-alkane ^{( C 2 7 +} C 2 9 q- C 3 1 ) concentrations (A), calculated CPI index (B) and ratio of short-chain (C17 + Cm) to long-chain (C27 + C29 + C3l) n-alkanes (C).

384 Kirsten Fahl and Ruediger Stein RESULTS AND DISCUSSION

The Lena River run-off is of considerable import- ance to the hydrochemical and depositional struc- ture in the Laptev Sea. The large brackish surface plume extends to about 200 miles northward (L6tolle e t al., 1993; Cauwet and Sidorov, 1996);

approximately 84% of the total outflow is directed to the east and northeast. This is reflected in the total organic carbon (TOC) distribution (Fig. 2).

Maximum TOC values of up to 2% occur in the vicinity of the eastern Lena Delta, off the Kotuy river mouth, southwest of the New Siberian Islands, and the central part of the lower Laptev Sea conti- nental slope. Areas of high TOC concentration commonly correspond to low HI values ( < 100 mg HC/gC) and high C/N ratios ( > 7) indi- cating the dominance of terrigenous organic matter (Stein and Niirnberg, 1995; Stein, 1996). However, in the central part of the Laptev Sea and along the upper continental slope, the hydrogen indices reach values > 100 mg HC/gC suggesting the presence of significant concentrations of marine organic matter.

The distribution of the bulk parameters is sup- ported by the distribution of the terrigenous bio- markers (Fig. 3, Table 1).

It is generally accepted that long-chain n-alkanes and long-chain wax esters indicate terrigenous input (Yunker e t al., 1995; Peulv6 et al., 1996). The high- est concentrations of these markers (Fig. 3A, Table 1) were found in the vicinity of the eastern Lena Delta. Peulv6 et al. (1996) reported the same trends for high molecular weight hydrocarbons (C23-C35). The concentration decreases in the direc- tion of the shelf and the continental slope. The low- est contents were measured in the deep-sea environment, which is presumably caused by a decreasing terrigenous flux toward the open ocean.

The C42 wax ester content decreases from 40 to 3#g/g TOC (Table 1), the long-chain n-alkanes from 1.5 to 0.2 mg/g TOC. In general, high contents of terrigenous organic material are shown by high CPI indices (1.9-5.1, Fig. 3B) (Bray and Evans, 1961). Fresh terrigenous organic matter shows a

CPI index of 3-10 (Brasseli e t aL, 1978; Hollerbach, 1985), whereas fossil material varies around 1 depending on the state of decomposition. Marine organic material shows no predominance of odd- over-even carbon chain lengths of n-alkanes.

The TOC maximum along the lower continental slope characterized by low HI values (Fig. 2) and high C/N ratios (Stein, 1996), and indicative of ter- rigenous sources, may be related to the inflow of Atlantic water masses laterally transporting (organic carbon-enriched) suspended matter.

The accumulation of marine organic carbon is mainly controlled by primary production and sedi- mentation rates (e.g. M/Jller and Suess, 1979;

Berger e t al., 1989; Stein, 1991). Highly productive environments, such as upwelling areas with values of > 250 gC m -2 yr -1, are characterized by accumu- lation rates of I gC cm -2 kyr -1, whereas open-ocean environments with productivity values of about 50 gC m -2 yr -I display accumulation rates of about 0.005 gC cm -2 kyr -1 (Stein, 1991). The Laptev Sea, characterized by sedimentation rates of about 40- 60 cm kyr -1, displays accumulation rates of about 450 gC cm -2 kyr -1 (Stein and Schubert, 1996). The maximum rates occur on the shelf. At the ice edge primary productivity values may be significantly increased (e.g. Nelson et aL, 1989). This is sup- ported by the marine fatty acid distribution (Fig. 4) in the surface sediments of the Laptev Sea. It is well established that phytoplankton contains par- ticular fatty acids. Thus, the fatty acid composition of cultured diatoms is dominated by 16"1 and 20:5 fatty acids (e.g. Kates and Volcani, 1966; Orcutt and Patterson, 1975; Pohl and Zurheide, 1979;

Kattner and Brockmann, 1990). The same result has been found for sea-ice diatoms (Whitaker and Richardson, 1980; Gillan et al., 1981; Nichols e t al., 1986) and natural phytoplankton blooms domi- nated by diatoms (Kattner et al., 1983; Mayzaud et al., 1989). The highest concentrations of the 16:l(n- 7) and 20:5(n-3) compounds (0.9 mg/g TOC) which are mainly synthesized by diatoms (Kates and Volcani, 1966; Ackman e t al., 1968; Kattner et al., 1983; Fahl and Kattner, 1993; Graeve e t al., 1994), Table 1. Wax ester and ketone contents of surface sediment samples from the Laptev Sea and the adjacent continental margin

Location Sample Position Water depth (m) C42 C37:3" C37:2

Latitude Longitude wax ester #g/g ketone/tg/g ketone #g/g

TOC TOC TOC

Lena Delta IK9306 72°00.1 'N 131 °00. I'E 18 40.19 0.05 0.01

IK9316 73°00.1'N 131°50.3'E 28

Shelf IK9365 75°48.3'N I 19°91.1 'E 40 28.95 0.03 0.0 l

IK9368 75°41.8'N 125°86. I'E 41 21.07 0.01 --

IK9370 75°31.3'N 129°56.8'E 44 24.12 0.01 --

Upper cont. PS2458 78 ° 10.0'N 133 °23.9'E 983 3.12 -- --

margin

PS2467 77°05.0'N 126 ° 13.4'E 284 2.47 -- --

PS2474 77°40.2'N 118°34.5'E 1497 4.21 - - --

Lower cont. PS2471 79 °09.3'N 119°46.9'E 3048 2.53 0.50 0.07

margin

Dash indicates concentrations of less than 0.01/~g/g TOC; IK: RV Ivan Kireyev; PS: RV Polarstern.

*C37:4 alkenone could not be identified.

N

q~ m

m

Chlorophyll a and phaepigment maximum in the surface-water (90mg/m 2, Boetius et al., 1996) and biogenic opal maximum (3-5%, Stein and N0rnberg, 1995) ~'11 \

© P~ O ga. C~ O • 135 ~tg/g TOC liJll~l J J l llJ Ice margin during Sept. 93 (Eicken et al., 1995) Fig. 4. Distribution or the sum of 16:1(n-7) and 20:5(n-3) fatty acids in surface sediments from the Laptev Sea and the adjacent continental margin. Samples were taken during the RV PolarslerJ~ expedition ARK IX/4 (FiJtterer, 1994) and the RV Ira, Kir<t'e~, expedition Transdrift i (Kassens and Karpiy, 1994). The hatched line symbolizes the ice margin during Sept. 93 (Eicken et (11., 1995). Arrow shows the position of maximum chlorophyll a concentration in the surface water (Boetius et al., 1996) and bio- genic opal in the surface sediments (Stein and Niirnberg, 1995).

386 Kirsten Fahl and Ruediger Stein w e r e o b t a i n e d n e a r t h e ice e d g e , w h e r e m e l t i n g p r o -

c e s s e s i n d u c e i n c r e a s i n g p h y t o p l a n k t o n g r o w t h . T h e s e v a l u e s a r e well c o r r e l a t e d w i t h h i g h c o n - c e n t r a t i o n s o f c h l o r o p h y l l a a n d p h a e o p i g m e n t s o f a b o u t 9 0 m g m -2 in t h e s u r f a c e w a t e r m a s s e s ( B o e t i u s et al., 1996) a n d b i o g e n i c o p a l o f 3 5 % (Stein a n d N f i r n b e r g , 1995) in t h e s u r f a c e sedi- m e n t s . D e s p i t e t h e d o m i n a n c e o f t h e t e r r i g e n o u s i n p u t in t h e w h o l e a r e a ( u p t o 9 9 % o f t o t a l o r g a n i c m a t e r i a l is t e r r i g e n o u s ) t h e h i g h v a l u e s o f t h e r a t i o s h o r t - to l o n g - c h a i n n - a l k a n e s a t t h e ice e d g e ( s t a t i o n P S 2 4 5 8 ) o f a b o u t 1 (Fig. 3C) i n d i c a t e a sig- n i f i c a n t m a r i n e i n f l u e n c e . S i m i l a r to t h e m a x i m u m f a t t y a c i d v a l u e s o b t a i n e d n e a r t h e ice e d g e , h i g h c o n c e n t r a t i o n s a l s o o c c u r in t h e w e s t e r n a n d n o r t h - e r n p a r t o f t h e L e n a D e l t a . I n t h e s e a r e a s o f i n c r e a s i n g p r o d u c t i v i t y , t h e t o t a l f a t t y a c i d c o n t e n t ( t h e p r i n c i p a l m a r i n e o r g a n i c c o m p o u n d s ) m a y r e a c h m o r e t h a n 0 . 2 % o f t h e T O C (Fig. 4, T a b l e 2).

W e e x p e c t e d s u c h h i g h c o n c e n t r a t i o n s in t h e vicin- ity o f t h e e a s t e r n b r a n c h o f t h e L e n a D e l t a , w h i c h p r e s e n t s t h e m a i n o u t f l o w o f t h e river, w h e r e a n e n h a n c e d fluvial n u t r i e n t s u p p l y m a y c a u s e e n h a n c e d p h y t o p l a n k t o n p r o d u c t i v i t y . B u t , b e c a u s e o f t h e l o w f a t t y a c i d c o n c e n t r a t i o n s in t h i s a r e a c o m p a r e d to t h e h i g h c o n t e n t s in t h e n o r t h - w e s t e r n p a r t , t h i s r e s u l t l e a d s u s to a s s u m e t h a t t h e r e a r e p r o b a b l y o t h e r p r o c e s s e s w h i c h i n f l u e n c e t h e p h y t o - p l a n k t o n g r o w t h .

It m a y be t h a t t h e h i g h c o n c e n t r a t i o n s o f s u s - p e n d e d m a t e r i a l a c t a s s o m e k i n d o f " s h a d o w "

w h i c h d e c r e a s e s t h e l i g h t p e n e t r a t i o n a n d r e d u c e s p h o t o s y n t h e s i s . A s i m i l a r m e c h a n i s m w a s d e s c r i b e d b y P a l m i s a n o et al. (1988) w h o s u g g e s t e d t h a t c o r n -

m u n i t i e s o f a l g a e m a y e n t e r a " s t a t i o n a r y p h a s e " o f z e r o g r o w t h d u e to l i g h t l i m i t a t i o n b y s e l f - s h a d i n g . T h e l o w c o n t e n t o f m a r i n e b i o m a r k e r s in t h i s a r e a is well c o r r e l a t e d w i t h p i g m e n t d a t a a c c o r d i n g to H e i s k a n e n a n d K e c k (1996). T h e l o w c h l o r o p h y l l a a n d h i g h p h a e o p i g m e n t c o n c e n t r a t i o n s i n d i c a t e t h a t t h e p i g m e n t s h a v e a l r e a d y u n d e r g o n e a l t e r a t i o n . I n a d d i t i o n to t h e s h a d o w h y p o t h e s i s m e n t i o n e d a b o v e t h e r e a r e s e v e r a l p o s s i b l e m e c h a n i s m s c a u s i n g t h e l o w c o n t e n t s o f m o s t o f t h e m a r i n e b i o m a r k e r s . R e c y c l i n g b y m i c r o o r g a n i s m s m a y b e r e s p o n s i b l e (Saliot et al., 1996) a s well a s g r a z i n g b y z o o p l a n k - t o n ( V o l k m a n a n d M a x w e l l , 1986). O u r r e s u l t s , a n d t h e d a t a o b t a i n e d w i t h i n t h e f r a m e w o r k o f t h e in- t e r n a t i o n a l p r o g r a m S P A S I B A ( M a r t i n et al., 1993b; H e i s k a n e n a n d K e c k , 1996; P e u l v 6 et al., 1996; Saliot et al., 1996), s h o w a n i n s i g n i f i c a n t in- f l u e n c e o f p r i m a r y p r o d u c t i v i t y in t h e L e n a R i v e r a n d t h e e a s t e r n p a r t o f t h e delta. T h e h i g h f a t t y a c i d c o n t e n t in t h e n o r t h - w e s t e r n p a r t o f t h e L e n a D e l t a is c o r r e l a t e d w i t h t h e p o s i t i o n o f t h e L a p t e v Sea p o l y n y a . T h e w i n t e r ice c o v e r o f t h e L a p t e v Sea is c h a r a c t e r i z e d b y t h e o c c u r r e n c e o f a n a p p r o x i - m a t e l y 1800 k m l o n g , n a r r o w z o n e o f o p e n w a t e r o n t h e m i d - s h e l f ( D e t h l e f f et al., 1993, 1994;

R e i m n i t z et al., 1994). D u r i n g w i n t e r t h e p o l y n y a s a r e a r e a s o f i n t e n s i v e sea-ice f o r m a t i o n , s a l i n i t y i n c r e a s e , c o n v e c t i o n , a n d l a r g e h e a t loss i n t o t h e at- m o s p h e r e ; s p r i n g t i m e is c h a r a c t e r i z e d b y a n ac- c u m u l a t i o n o f h e a t a n d r a p i d m e l t i n g o f sea-ice ( Z a k h a r o v , 1966). T h e s e f a c t o r s m a y i n d u c e i n c r e a s - i n g p r i m a r y p r o d u c t i o n e s p e c i a l l y in t h i s a r e a . T h e c o n c e n t r a t i o n s o f t h e f a t t y a c i d s in t h e L a p t e v Sea Table 2. Fatty acid composition (weight%) of surface sediments taken during the 1993 RV Kireyev cruise

Fatty acids 9307 9309 9315 9316 9318 9320 9 3 2 3 9 3 2 7 9340 9349 9 3 5 3 9 3 6 8 9370 9384 9373 A 93Z2

14:0 3.3 4.8 5.7 10.2 7.4 9.1 8.2 7.6 8.5 5.8 10.6 13.6 22.2 7.3 11.1 4.3

15:0 16.7 l.l 12.6 2.1 1.5 .- - - 2.2 3.2 2.7 2.3 4.7 1.6 1.4 4.4

16:0 24.0 36.5 28.1 37.3 35.6 25.1 32.8 29.6 2 7 . 3 3 2 . 9 28.8 38.3 49.0 29.3 34.1 20.2 16:1(n-7) 39.9 38.8 33.8 23.2 35.8 42.3 39.9 44.8 37.9 37.3 38.4 22.2 16.1 24.6 3 4 . 8 48.4

1 6 : 1 ( n - 5 ) . . . 0.7 - 3.8 1.7 6.9 -~ 0.6 2.3

1 6 : 2 ( n - 6 ) . . . 1.3 -

1 6 : 3 ( n - 3 ) . . . 1.6 - - - 0.7

16:4(n-?) . . . 2.1 --- - 0.7

18:0 - - 6.0 11.1 9.6 6.5 5.1 9.2 2.9 -- 9.3 5.7 3.8 6.1 10.8 - 5.8

18:1(n-9) 5,6 6.9 - - 8.2 6.4 17.2 5.2 4.2 5.1 5.4 4.7 6.7 - - 14.0 3.2 8.6

18:1(n-7) 4.1 1.7 5.8 2.6 0.6 - - 3.6 6.0 4.2 3.8 - 10.1 8.9 1.7

18:2(n-6) 3,5 3.3 - - 5.5 3,8 - 1.6 -- - - 1,6 - - 2,2 1.0 3.2

1 8 : 3 ( n - 3 ) . . . .

18:4(n-3) 1.1 - - 1.5 1.3 - - - - 0.5 2.4 . . . .

20:1(n-9) . . . 1.3

20:1(n-7) . . . 3.6

20:4(n-6) . . . .

20:5(n-3) 5.3 4.2 2.9 5.3 4.9 1.2 4.7 3.9 7.5 1,3 3.6 6.2 1.9 2.3 1.6 2.3

22:1(n-11) . . . .

22:1(n-9) . . . .

22:5(n-3) . . . .

22:6(n-3) . . . 0.7

% Total sats, 44.0 48.4 57.5 59.2 51.0 39.3 5 0 . 2 42.3 39,0 48.0 47.8 58.0 8 2 , 0 49.0 46.6 34.7

% Total monos, 49.6 47.4 39.6 34.0 42.8 5 9 . 5 45.1 53.3 49.0 5 0 . 7 48.6 35.8 16.1 48.7 51.1 62,3

% Total PUFA 6,4 4.2 2.9 6,8 6.2 1.2 4,7 4.4 12.0 1.3 3.6 6.2 1,9 2.3 2,3 3.0

Dash indicates not detected or trace amounts (< 0.1%). Total sats.: Total saturated; Total monos.: Total monounsaturated; Total PUFA:

Total polyunsaturated fatty acids.

Organic carbon deposition in the Laptev Sea 387

sat. fatty a c i d - - C - - O - " - CH

/

I[ 1

u n s a t , fatty a C i d - - - - C - - O - - C H ~ 0

• - - 0

Fluvial sediment and nutrient supply Polynya

f - - ~ .

,.Deep ^Sea )

Fig. 5. Scheme indicating the most important processes controlling the organic carbon flux in the Laptev Sea and the adjacent continental margin and deep-sea areas (fluvial sediment and nutrient supply, gravitational sediment downslope transport, primary production and vertical organic carbon flux, and transport by bottom currents). Additionally, the chemical structures of typical terrigenous (long-chain wax esters, long-chain n-alkanes, and lignin phenols) and marine (triaclyglycerols, alke-

nones, phospholipids, and chlorophyll a) biomarkers are shown.

polynya are comparable with the contents of the fatty acids near the ice edge.

The lowest concentrations of marine fatty acids occur in the shallow ice-covered shelf areas as well as in the ice-covered deep-sea environment. On the shelf, the low concentrations can be attributed to low primary productivity due to the sea-ice cover whereas, in the deep sea, the low concentrations may be explained by a combination of low primary productivity and other mechanisms. That is, autoxi- dation, alteration by grazers and degradation by microorganisms (Saliot et al., 1996) due to the long residence time in the water column, as well as in the surface sediments, may have caused the low concen- trations. On the other hand down-slope transport (turbidity currents) might be important (Fig. 5).

The fatty acid composition of various surface sediments are presented in Table 2 and Table 3. In general, the surface sediments are characterized by high proportions of saturated (34 85%) and short- chain monounsaturated fatty acids (14-60%) due to the effect of degradation and autoxidation of the unstable polyunsaturated compounds. The highest proportions of the polyunsaturated fatty acids (around 5 % of total fatty acids) occur in the sur- face sediments from the shallow shelf due to the short residence time in the water column, whereas the surface sediments of the continental slope, and in the deep sea, are characterized by the absence of

the polyunsaturated compounds with the exception of station PS2458 (ice edge). The rather high pro- portions of the 18:1 fatty acid were found in nearly all surface sediment samples. It is well established that particulate matter in waters of low productivity is deficient in polyunsaturated fatty acids, but it contains high levels of saturated fatty acids and fatty acids with 18 carbon atoms (Goutx and Saliot, 1980; Kattner et al., 1983; Mayzaud et al., 1989; Henderson et al., 1991; Graeve, 1992). In all surface sediment samples no marine fatty alcohols occur, which would indicate zooplankton growth. It is generally accepted that polar copepods accumu- late large amounts of lipids in the form of wax esters, or energy-rich long-chain triacylglycerols (Hagen, 1988; Hagen et al., 1993). Moderate amounts of the long-chain marine 22:1(n-11) fatty acid occur in one surface sediment sample from the eastern part of the Laptev Sea (PS2484; 3% of total fatty acid).

It is generally accepted that alkenones are only synthesized by prymnesiophytes (Volkman et al.,

1980). At lower latitudes, characterized by tempera- tures of > 10°C, these lipids are used as paleotem- perature markers (Brassell and Eglinton, 1984;

Marlowe et al., 1984; Brassell et al., 1986; Prahl and Wakeham, 1987; Prahl et al., 1988). In the Arctic Ocean and its marginal seas, where very low surface water temperatures of <<5°C are typical,

388 Kirsten Fahl and Ruediger Stein

Table 3. Fatty acid composition (weight%) of surface sediments taken during the 1993 RV Polarstern cruise

Fatty acids 2458 2465 2469 2471 2472 2473 2474 2483 2484

14:0 7.1 8.0 14.0 10.5 3.9 7.5 11.6 5.9 3.9

15:0 2.1 2.5 5.2 8.2 7.5 6.2 - 0.9

16:0 28.5 34.0 69.2 29.5 62.2 34.1 33.3 51.3 29.0

16: l(n-7) 32.6 l 7.7 16.8 20.1 14.6 32.7 24.5 37.7 25.0

16:1(n-5) 5.8 - - 5.8 - - - - - - 1.1

16:2(n-6) 0.5 2.1 . . . .

1 6 : 3 ( n - 3 ) . . . 3.9

1 6 : 4 ( n - ? ) . . . .

18:0 6.5 10.2 8.5 11.1 10.9 10.5 - - 10.6

18:1(n-9) 6.8 18.1 15.0 7.3 12.3 7.9

18:1 ( n - 7 ) . . . . 3 . 7 - - - - - 1 . 6 - - 2 . 2

1 8 : 2 ( n - 6 ) 1.5 . . . 2.3 -- 1.6

1 8 : 3 ( n - 3 ) . . . .

18:4(n-3) - - 3.8 - 1.7 • - - - - 5.1

20:1(n-9) . . . 6.6

20:1(n-7) - 5.7 . . . . 9.7

20:4(n-6) . . . .

20:5(n-3) 7.5 . . . .

22:1(n-11) 3.1 . . . 3.1

22:1(n-9) . . . .

22:5(n-3) . . . .

22:6(n-3) . . . .

% Total sats. 44.2 54.7 83.2 53.7 85.4 60.0 61.6 57.2 44.4

% Total monos. 48.3 41.5 16.8 44.6 14.6 40.0 38.4 37.7 55.6

% Total PUFA 7.5 3.8 1.7 - - - - 5.1 -

Dash indicates not detected or trace amounts (< 0.1%). Total sats.: Total saturated; Total monos.: Total monounsaturated; Total PUFA:

Total polyunsaturated fatty acids.

a l k e n o n e s c a n n o t be u s e d to e s t i m a t e t e m p e r a t u r e s . I n t h i s r e g i o n , t h e a l k e n o n e s s h o u l d o n l y be u s e d as a p a l e o p r o d u c t i v i t y i n d i c a t o r . W i t h t h e e x c e p t i o n o f o n e s u r f a c e s e d i m e n t s a m p l e , w h i c h w a s c o l l e c t e d f r o m t h e l o w e r m o s t c o n t i n e n t a l s l o p e ( w a t e r d e p t h o f 3 0 4 7 m , s t a t i o n P S 2 4 7 1 ) , t h e c o n c e n t r a t i o n o f a l k e n o n e s is r a t h e r l o w in t h e e n t i r e L a p t e v Sea (less t h a n 0.007 # g / g T O C ) ( T a b l e 1). T h i s is p r o b - a b l y c a u s e d b y t h e l o w a b u n d a n c e o f p r y m n e s i o - p h y t e s a n d l o w t e m p e r a t u r e s in t h e L a p t e v Sea.

T h e r e f o r e , t h e u s e o f t h e s e lipids f o r r e c o n s t r u c t i n g p a l e o t e m p e r a t u r e o r p a l e o p r o d u c t i v i t y is l i m i t e d . T h e r a t h e r h i g h c o n c e n t r a t i o n s o f a l k e n o n e s in t h e s u r f a c e s e d i m e n t a t s t a t i o n PS2471 m a y be c a u s e d b y c o c c o l i t h o p h o r i d e s o r o t h e r p r y m n e s i o p h y t e s w h i c h w e r e t r a n s p o r t e d b y A t l a n t i c w a t e r m a s s e s a l o n g t h e c o n t i n e n t a l slope. T h i s a s s u m p t i o n m a y b e r e s o l v e d a f t e r o u r i n v e s t i g a t i o n o f t h e c o r r e - s p o n d i n g s e d i m e n t c o r e PS2471 in w h i c h h i g h a l k e - n o n e c o n c e n t r a t i o n s o c c u r .

CONCLUSION

d u c t i v i t y n e a r t h e ice e d g e . T h u s t h e f a t t y acids, a s r e l a t i v e l y u n s t a b l e o r g a n i c m o l e c u l e s , c a n b e u s e d a s b i o m a r k e r s f o r r e f l e c t i n g r e c e n t p r o c e s s e s . B e c a u s e o f t h e i r h i g h c o n c e n t r a t i o n in a r e a s o f e n h a n c e d m a r i n e p r o d u c t i v i t y in t h e L a p t e v S e a t h e y a r e s i g n i f i c a n t f o r t h e m a r i n e o r g a n i c c a r b o n b u d g e t . T h e t e r r i g e n o u s b i o m a r k e r s ( l o n g - c h a i n n- a l k a n e s a n d w a x e s t e r s ) in L a p t e v S e a s u r f a c e sedi- m e n t s w e r e m a i n l y s u p p l i e d b y t h e S i b e r i a n rivers.

T h e i r c o n c e n t r a t i o n s d e c r e a s e w i t h i n c r e a s i n g dis- t a n c e f r o m t h e s o u r c e ; t h e l o w e s t c o n c e n t r a t i o n s w e r e f o u n d in t h e d e e p - s e a e n v i r o n m e n t .

Associate Editor - - J . O . G r i m a l t

Acknowledgements--We thank W. Drebing, M. Siebold, and K. Weber for technical assistance and discussions. We t h a n k N. Andersen and the two a n o n y m o u s reviewers for n u m e r o u s constructive suggestions and comments. The financial support by the Ministry for Education, Science, Research, and Technology (BMBF) is gratefully acknowl- edged. This is contribution No. 1145 of the Alfred- Wegener-Institute for Polar and Marine Research.

Specific b i o m a r k e r s c a n be u s e d to d i s t i n g u i s h b e t w e e n m a r i n e a n d t e r r i g e n o u s s o u r c e s o f t h e or- g a n i c m a t t e r o f t h e L a p t e v S e a s u r f a c e s e d i m e n t s . T h e d i s t r i b u t i o n o f t h e m a r i n e b i o m a r k e r s is well c o r r e l a t e d w i t h t h e sea-ice d i s t r i b u t i o n . T h e l o w e s t c o n c e n t r a t i o n s o f t h e m a r i n e f a t t y a c i d s w e r e f o u n d in t h e i c e - c o v e r e d a r e a s w h e r e a s t h e h i g h e s t a m o u n t s w e r e l o c a t e d in t h e s e d i m e n t s n e a r t h e ice m a r g i n . T h e m a r i n e f a t t y a c i d d i s t r i b u t i o n c o r r e - l a t e s well w i t h t h e c h l o r o p h y l l a a n d b i o g e n i c o p a l c o n t e n t , i n d i c a t i n g a n i n c r e a s e d s u r f a c e w a t e r p r o -

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