Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue5.2000:
Organic Geochemistry 31 (2000) 363±374
www.elsevier.nl/locate/orggeochem
The importance of terrestrial organic carbon inputs on Kara
Sea shelves as revealed by n-alkanes, OC and d13C values
M.B. Fernandes a,1, M.-A. Sicre b,*
a
Instituto de QuõÂmica, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco A, Sala 607, 21949-900, Rio de Janeiro/RJ, Brazil
Laboratoire des Sciences du Climat et de l'Environnement, Domaine du CNRS, Avenue de la Terrasse, 91191 Gif-sur-Yvette Cedex, France
b
Abstract
Organic carbon (OC) and n-alkanes were investigated in the suspended matter (SM) and sur®cial sediments of the
Arctic Ob and Yenisei Rivers and the adjacent Kara Sea. Organic nitrogen and d13C values were determined as additional parameters in the sediments. OC and n-alkane concentrations were highly variable in space. This variability was
interpreted as the result of a complex sedimentation pattern involving the discharges of these two rivers and the seasonal occurrence of a shore-ice front along the coast. Using the constant ratio of high molecular weight odd n-alkanes
to OC in both rivers, we estimated that more than 70% of the OC preserved in shelf sediments is terrestrially-derived.
This result was substantiated by the overall dominance of land-derived n-alkanes. A second approach using d13C values
and assuming binary dilution of riverine and marine OC led to comparable estimates. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Organic carbon; d13C; n-Alkanes; Ob River; Yenisei River; Kara Sea
1. Introduction
Rivers are the prime sources of fresh water and sediments to the ocean, representing 70% of the world's
solid inputs to the marine realm (Milliman, 1991).
However, river discharges can vary substantially with
changes in climate and human land-use. In the twentieth
century, the worldwide development of industrial and
urban centers has motivated the construction of dams
for hydroelectric plants and irrigation conduits inducing
major alterations of river ¯ow and discharge of suspended solids reaching coastal margins (e.g. River Nile).
It is estimated that 90% of riverine particulate inputs
remain trapped in coastal areas (Mantoura et al., 1991).
The extent to which these areas can retain or export ¯uvial material can aect the global carbon mass balance.
The understanding of the transfer of ¯uvial organic
carbon (OC) to the marine environment on a global
* Corresponding author.
E-mail addresses: Marie-Alexandrine.Sicre@lsce.cnrs-gif.fr
(M.-A. Sicre), milena@ensp.®ocruz.br (M.B. Fernandes).
1
Present address: CESTEH/ENSP, FundacËaÄo Oswaldo
Cruz, Rua Leopoldo BulhoÄes 1480, Manguinhos, 21041-210,
Rio de Janeiro/RJ, Brazil.
scale requires an extended geochemical database. Apart
from rivers discharging into temperate North-American
and European shelves, regions such as the Eurasian
shelves of the Arctic Ocean are poorly documented (e.g.
Dmitriev and Pivovarov, 1983; Fernandes, 1996; Yunker et al., 1996; Broyelle, 1997). The present study aims
to partly ®ll these gaps. Samples were collected in September 1993 during the SPASIBA 3 cruise of the R/V
Dimitriy Mendeleev, as part of the international Joint
Global Oceanic Flux Studies (JGOFS) and the Scienti®c
Program on the Arctic and Siberian Aquatorium (SPASIBA) (Lisitsyn and Vinogradov, 1995). Elemental
organic carbon and nitrogen (OC and N), carbon stable
isotope (d13C), and n-alkanes (ALKs) were determined
in the suspended matter (SM) and surface sediments of
the Ob and Yenisei estuaries and Kara Sea to estimate
the amount of terrestrially-derived OC buried in shelf
sediments.
2. Study area
The Ob and Yenisei rivers are the main source of
fresh water into the Arctic Ocean. Their water discharges
reach 400 and 630 km3/year, respectively (Khublarian,
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00006-1
364
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
1994), as a result of a higher runo in the Yenisei basin
(218 mm/year) than in the Ob (164 mm/year) (Spitzy and
Ittekkot, 1991). Water and suspended sediment discharges show a strong seasonality. Up to 90% of the Ob
and Yenisei's solid suspended in¯ux is delivered during
spring ¯oods (P®rman et al., 1995). The Ob mean solid
discharge, around 504 kg/s, is almost 4 times higher
than that of the Yenisei, averaging 130 kg/s (Dai, 1995).
Sedimentation rates in the Yenisei estuary (3±100 cm/
1000 years; Lein et al., 1995) are low in comparison to
other major world estuaries. The vast catchment areas
for these Arctic rivers (5 2.5 106 km2) combined with
low rainfall results in extremely low discharge/drainage
basin area ratios (< 0.2 m/year) (Milliman, 1991). As a
consequence of low-lying terrain and low precipitation,
sediment yields (river load/drainage basin area) are also
correspondingly low (4±9 t/km2/year) (Milliman, 1991).
Signi®cant quantities of dissolved and colloidal organic
matter (OM), in particular humic substances, enter the
waterways during freshet, as soils and peat deposits are
¯ushed by melt waters (Telang et al., 1991). As a result,
dissolved organic carbon (DOC) inputs into Siberian
rivers are more than 3 times higher than particulate
organic carbon (POC) yields, which represent less than
0.5 t/km2/year (Spitzy and Ittekkot, 1991).
Low nutrients and low water temperatures do not
favor primary production in the Kara Sea, which varies
from mesotrophic to oligotrophic (Vedernikov et al.,
1994, 1995; Vinogradov et al., 1994, 1995). At the time
of the SPASIBA 3 cruise, diatoms were the dominant
phytoplankton species (Lisitsyn et al., 1994; Dai and
Martin, 1995).
3. Experimental
3.1. Sampling
Samples were collected in September 1993, well after
the peak river ¯ow (June±July). Collection sites are
located between 68 and 76 N, along the Ob and Yenisei
rivers and in the adjacent Kara Sea. Three longitudinal
sections of the estuary (riverine zone, mixing zone and
marine zone) were distinguished based on the location
of frontal zones and salinity values (Fig. 1b; Fernandes
and Sicre, 1999). Salinity values ranged from 0 to 32
throughout the study area.
Surface water samples were collected above the halocline, between 3 and 9 m. 15 to 80 L of water were
pumped into glass containers and then ®ltered on-board
ship on pre-extracted Whatman GF/F glass ®ber ®lters
(147 or 290 mm diameter, 0.7 mm porosity). Filters and
sur®cial sediments (0±3 cm) were wrapped in aluminum
foil and kept frozen at ÿ18oC until extraction. All surfaces in contact with the samples were of Te¯on or
stainless steel.
3.2. Analytical procedure
Filters for hydrocarbon analysis were extracted three
times by ultrasonication in 200 mL of a mixture of
CH3OH:CH2Cl2 (1:3 v/v) for 15 min. The three extracts
(600 mL) were combined in a round bottom ¯ask and
dried with MgSO4 overnight. The total lipid extract was
®ltered through glass wool, concentrated by rotary evaporation and then transferred to a 4 mL vial. Sediments
were freeze-dried and homogenized. An aliquot was
extracted in a re¯ux system with 200 mL of a mixture of
CH3OH: CH2Cl2 (1:3 v/v) for 2 h followed by ultrasonication for 15 min. The total lipid extract was ®ltered,
concentrated by rotary evaporation and dried with
MgSO4 overnight. These extracts were then treated in the
same way as those of the particulate phase. Particulate and
sedimentary non-aromatic hydrocarbons were isolated
from the lipid mixture by elution with hexane through a
pipette ®lled with silica gel (Fernandes et al., 1997).
3.3. Hydrocarbon identi®cation and quanti®cation
Quanti®cation of individual n-alkanes was based on
comparison of peak areas with those of perdeuterated
tetracosane added prior to extraction. The quanti®cation limit for individual compounds was estimated at
around 1±3 ng (signal/noise 5 10) and the reproducibility of the method was estimated at 10%. Blanks
showed no detectable contamination. Hydrocarbons
were analyzed using a CP-Sil-8 CB Chrompack capillary
column (50 m 0.32 mm i.d. 0.25 mm ®lm thickness)
mounted in a Girdel 3000 gas chromatograph (GC)
equipped with a glass needle injector heated at 250 C
and a Flame Ionization Detector (FID). The GC oven
was programmed from 80 to 300 C at 2 C/min. Helium
was used as carrier gas. Data acquisition and processing
was performed on a Varian Vista CDS 402 integrator.
The identi®cation of individual n-alkanes was performed by comparison of their retention times with
those of authentic standards. Structural assignment was
con®rmed by gas chromatography/mass spectrometry
(GC/MS) on selected samples. GC/MS analyses were
carried out on a Varian 3400 GC coupled to an Ion
Trap Varian Saturn mass spectrometer. Samples were
injected using an on-column, Septum, Programmable
Injector (SPI) on a DB5 J & W Scienti®c capillary column (30 m 0.25 mm i.d. 0.25 mm ®lm thickness)
programmed from 80 to 300 C at 3 C/min. Operating
conditions were: ion source temperature 140 C, electron
energy 70 eV, scanning from 40 to 600 amu, 0.6 scan/s.
3.4. Bulk analyses
POC and SM data were obtained by Dai (1995) during
the same cruise. The analytical reproducibility of POC
measurements was estimated at 2%. Sedimentary
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
365
Fig. 1. Study area (a) and sampling locations along the riverine, mixing and marine zones of the Ob and Yenisei River estuaries and
Kara Sea shelf (b).
366
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
organic carbon (OC), organic nitrogen (N) and d13C
were analyzed following the experimental procedure by
Girardin and Mariotti (1991). Brie¯y, samples were
decarbonated in 25 mL of HCl (0.001 M), followed by
the addition of HCl (0.5 M) till a constant pH around 3
was reached. They were centrifuged, washed with deio
nized water and dried for 12 h at 50 C. After being
homogenized and weighed, aliquots were combusted in
an elemental analyzer (Carlo Erba, NA 1500 NC,
Fisons). The CO2 and N2 produced were chromatographically separated. OC and N were then measured by
thermal conductivity. The analytical reproducibility of
these measurements was estimated at 3%. Combustion gases were then cryogenically puri®ed through several temperature traps and introduced into the isotopic
ratio mass spectrometer ®tted for rapid switching (VG
Sira 10 Spectrometer). Samples were compared to Pee
Dee Belemnite (PDB) as reference to determine their
d13C values. The experimental error was estimated to be
better than 0.1%o based on successive injections of
the tyrosine standard.
4. Results and discussion
4.1. Suspended matter
SM concentrations (Dai, 1995) are in the lower range
of concentrations reported for world rivers (Meybeck,
1982; Ittekkott, 1988): they range from 4 to 143 mg/L
(average of 30 mg/L, n=7) in the Ob River, and from 1
to 23 mg/L (average of 5 mg/L, n=11) in the Yenisei
River, while < 1 mg/L in the Kara Sea. OC percentages
(Dai, 1995) vary from 3 to 5% in the Ob River (average
of 4%, n=7), and from 4 to 11% in the Yenisei (average
of 7%, n=11). They are higher in the Kara Sea, ranging
from 3 to 19% (average of 12%, n=5).
All concentration data for the particulate n-alkanes
are given in Table 1. Total particulate
P odd carbon
number n-alkanes from C25 to C31 ( ALK25±31) and
OC concentrations (in ng/L and mg/L, respectively) are
higher in the Ob River than in the Yenisei. Total nalkanes (in mg/L) decrease seawards in both estuaries
(Fig. 2). Except for some stations of the northern Kara
Sea, n-alkane particle loads (in mg/g) are relatively constant, generally around 10 mg/g (Fig. 2), indicating the
rather homogeneous composition of riverine SM. This
translates into a high correlation between n-alkane concentrations and OC or SM values (r>0.95, P 20mg/g).
P®rman et al. (1995) reported elevated DDT levels in
this area of the Kara Sea that could be attributed to
transport by drifting-ice. The Yamal Current ¯owing
towards the northeast at the time of sampling can also
be envisaged as a potential vector of additional SM
inputs (Fig. 1b; Burenkov and Vasil'kov, 1995). However, this hypothesis cannot be substantiated without
samples collected within this current ¯ow. Prevailing
odd and high molecular weight (5 C25) n-alkanes indicate major vascular plant wax inputs. n-C27 is dominant
and carbon preference index values (CPI24±32) are generally > 3 (Table 1). The occurrence of alkylated polycyclic aromatic hydrocarbons, such as octahydrochrysenes and tetrahydropicenes, generated from the
aromatization of 3-hydroxy triterpenoids and other constituents of higher plants, further supports the hypothesis
of a dominant terrestrial origin (Fernandes, 1996).
Total n-alkane particle loads in the eastern Eurasian
Lena Delta and Laptev Sea are substantially higher than
values found in this work (Table 2). Compositional features indicate signi®cant planktonic inputs (Broyelle,
1997) that are not observed in the Kara Sea, at least not
during the SPASIBA 3 cruise. Such inputs may account
partially for the observed dierences in concentrations
with respect to our data. At the freshet of the Mackenzie
River, with a similar basin cover (vegetation and soil),
values are lower than ours (Table 2), but n-alkane and
SM levels are also signi®cantly correlated (Yunker et al.,
1991).
4.2. Sediments
Only a small portion of the OC transported by the Ob
and Yenisei SM is preserved in the sediments. OC percentages in both rivers are relatively constant at around
1.5% (n=17) (Fig. 3). OC percentages in sediments of
the Kara Sea are signi®cantly lower (average of 0.7%,
n=6), but close to the average OC concentrations of
®ne-grained shelf and delta sediments (0.75%) calculated for world rivers by Berner (1982). Besides direct
inputs from soil erosion, the OC content of shelf sediments may also re¯ect better preservation of OM in ®ne
¯uvial particles which bear the highest mineral surface
areas (Prahl and Carpenter, 1983; Prahl, 1985; Mayer,
1994a,1994b; Keil et al., 1998).
d13C values determined from sediments of both rivers
(Table 3) are close to the isotopic signature of continental C3 vegetation (e.g. GonÄi et al., 1997, 1998;
Ruttenberg and GonÄi, 1997). High C/N ratios suggest
that the contribution of plankton (C/N
www.elsevier.nl/locate/orggeochem
The importance of terrestrial organic carbon inputs on Kara
Sea shelves as revealed by n-alkanes, OC and d13C values
M.B. Fernandes a,1, M.-A. Sicre b,*
a
Instituto de QuõÂmica, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco A, Sala 607, 21949-900, Rio de Janeiro/RJ, Brazil
Laboratoire des Sciences du Climat et de l'Environnement, Domaine du CNRS, Avenue de la Terrasse, 91191 Gif-sur-Yvette Cedex, France
b
Abstract
Organic carbon (OC) and n-alkanes were investigated in the suspended matter (SM) and sur®cial sediments of the
Arctic Ob and Yenisei Rivers and the adjacent Kara Sea. Organic nitrogen and d13C values were determined as additional parameters in the sediments. OC and n-alkane concentrations were highly variable in space. This variability was
interpreted as the result of a complex sedimentation pattern involving the discharges of these two rivers and the seasonal occurrence of a shore-ice front along the coast. Using the constant ratio of high molecular weight odd n-alkanes
to OC in both rivers, we estimated that more than 70% of the OC preserved in shelf sediments is terrestrially-derived.
This result was substantiated by the overall dominance of land-derived n-alkanes. A second approach using d13C values
and assuming binary dilution of riverine and marine OC led to comparable estimates. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Organic carbon; d13C; n-Alkanes; Ob River; Yenisei River; Kara Sea
1. Introduction
Rivers are the prime sources of fresh water and sediments to the ocean, representing 70% of the world's
solid inputs to the marine realm (Milliman, 1991).
However, river discharges can vary substantially with
changes in climate and human land-use. In the twentieth
century, the worldwide development of industrial and
urban centers has motivated the construction of dams
for hydroelectric plants and irrigation conduits inducing
major alterations of river ¯ow and discharge of suspended solids reaching coastal margins (e.g. River Nile).
It is estimated that 90% of riverine particulate inputs
remain trapped in coastal areas (Mantoura et al., 1991).
The extent to which these areas can retain or export ¯uvial material can aect the global carbon mass balance.
The understanding of the transfer of ¯uvial organic
carbon (OC) to the marine environment on a global
* Corresponding author.
E-mail addresses: Marie-Alexandrine.Sicre@lsce.cnrs-gif.fr
(M.-A. Sicre), milena@ensp.®ocruz.br (M.B. Fernandes).
1
Present address: CESTEH/ENSP, FundacËaÄo Oswaldo
Cruz, Rua Leopoldo BulhoÄes 1480, Manguinhos, 21041-210,
Rio de Janeiro/RJ, Brazil.
scale requires an extended geochemical database. Apart
from rivers discharging into temperate North-American
and European shelves, regions such as the Eurasian
shelves of the Arctic Ocean are poorly documented (e.g.
Dmitriev and Pivovarov, 1983; Fernandes, 1996; Yunker et al., 1996; Broyelle, 1997). The present study aims
to partly ®ll these gaps. Samples were collected in September 1993 during the SPASIBA 3 cruise of the R/V
Dimitriy Mendeleev, as part of the international Joint
Global Oceanic Flux Studies (JGOFS) and the Scienti®c
Program on the Arctic and Siberian Aquatorium (SPASIBA) (Lisitsyn and Vinogradov, 1995). Elemental
organic carbon and nitrogen (OC and N), carbon stable
isotope (d13C), and n-alkanes (ALKs) were determined
in the suspended matter (SM) and surface sediments of
the Ob and Yenisei estuaries and Kara Sea to estimate
the amount of terrestrially-derived OC buried in shelf
sediments.
2. Study area
The Ob and Yenisei rivers are the main source of
fresh water into the Arctic Ocean. Their water discharges
reach 400 and 630 km3/year, respectively (Khublarian,
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00006-1
364
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
1994), as a result of a higher runo in the Yenisei basin
(218 mm/year) than in the Ob (164 mm/year) (Spitzy and
Ittekkot, 1991). Water and suspended sediment discharges show a strong seasonality. Up to 90% of the Ob
and Yenisei's solid suspended in¯ux is delivered during
spring ¯oods (P®rman et al., 1995). The Ob mean solid
discharge, around 504 kg/s, is almost 4 times higher
than that of the Yenisei, averaging 130 kg/s (Dai, 1995).
Sedimentation rates in the Yenisei estuary (3±100 cm/
1000 years; Lein et al., 1995) are low in comparison to
other major world estuaries. The vast catchment areas
for these Arctic rivers (5 2.5 106 km2) combined with
low rainfall results in extremely low discharge/drainage
basin area ratios (< 0.2 m/year) (Milliman, 1991). As a
consequence of low-lying terrain and low precipitation,
sediment yields (river load/drainage basin area) are also
correspondingly low (4±9 t/km2/year) (Milliman, 1991).
Signi®cant quantities of dissolved and colloidal organic
matter (OM), in particular humic substances, enter the
waterways during freshet, as soils and peat deposits are
¯ushed by melt waters (Telang et al., 1991). As a result,
dissolved organic carbon (DOC) inputs into Siberian
rivers are more than 3 times higher than particulate
organic carbon (POC) yields, which represent less than
0.5 t/km2/year (Spitzy and Ittekkot, 1991).
Low nutrients and low water temperatures do not
favor primary production in the Kara Sea, which varies
from mesotrophic to oligotrophic (Vedernikov et al.,
1994, 1995; Vinogradov et al., 1994, 1995). At the time
of the SPASIBA 3 cruise, diatoms were the dominant
phytoplankton species (Lisitsyn et al., 1994; Dai and
Martin, 1995).
3. Experimental
3.1. Sampling
Samples were collected in September 1993, well after
the peak river ¯ow (June±July). Collection sites are
located between 68 and 76 N, along the Ob and Yenisei
rivers and in the adjacent Kara Sea. Three longitudinal
sections of the estuary (riverine zone, mixing zone and
marine zone) were distinguished based on the location
of frontal zones and salinity values (Fig. 1b; Fernandes
and Sicre, 1999). Salinity values ranged from 0 to 32
throughout the study area.
Surface water samples were collected above the halocline, between 3 and 9 m. 15 to 80 L of water were
pumped into glass containers and then ®ltered on-board
ship on pre-extracted Whatman GF/F glass ®ber ®lters
(147 or 290 mm diameter, 0.7 mm porosity). Filters and
sur®cial sediments (0±3 cm) were wrapped in aluminum
foil and kept frozen at ÿ18oC until extraction. All surfaces in contact with the samples were of Te¯on or
stainless steel.
3.2. Analytical procedure
Filters for hydrocarbon analysis were extracted three
times by ultrasonication in 200 mL of a mixture of
CH3OH:CH2Cl2 (1:3 v/v) for 15 min. The three extracts
(600 mL) were combined in a round bottom ¯ask and
dried with MgSO4 overnight. The total lipid extract was
®ltered through glass wool, concentrated by rotary evaporation and then transferred to a 4 mL vial. Sediments
were freeze-dried and homogenized. An aliquot was
extracted in a re¯ux system with 200 mL of a mixture of
CH3OH: CH2Cl2 (1:3 v/v) for 2 h followed by ultrasonication for 15 min. The total lipid extract was ®ltered,
concentrated by rotary evaporation and dried with
MgSO4 overnight. These extracts were then treated in the
same way as those of the particulate phase. Particulate and
sedimentary non-aromatic hydrocarbons were isolated
from the lipid mixture by elution with hexane through a
pipette ®lled with silica gel (Fernandes et al., 1997).
3.3. Hydrocarbon identi®cation and quanti®cation
Quanti®cation of individual n-alkanes was based on
comparison of peak areas with those of perdeuterated
tetracosane added prior to extraction. The quanti®cation limit for individual compounds was estimated at
around 1±3 ng (signal/noise 5 10) and the reproducibility of the method was estimated at 10%. Blanks
showed no detectable contamination. Hydrocarbons
were analyzed using a CP-Sil-8 CB Chrompack capillary
column (50 m 0.32 mm i.d. 0.25 mm ®lm thickness)
mounted in a Girdel 3000 gas chromatograph (GC)
equipped with a glass needle injector heated at 250 C
and a Flame Ionization Detector (FID). The GC oven
was programmed from 80 to 300 C at 2 C/min. Helium
was used as carrier gas. Data acquisition and processing
was performed on a Varian Vista CDS 402 integrator.
The identi®cation of individual n-alkanes was performed by comparison of their retention times with
those of authentic standards. Structural assignment was
con®rmed by gas chromatography/mass spectrometry
(GC/MS) on selected samples. GC/MS analyses were
carried out on a Varian 3400 GC coupled to an Ion
Trap Varian Saturn mass spectrometer. Samples were
injected using an on-column, Septum, Programmable
Injector (SPI) on a DB5 J & W Scienti®c capillary column (30 m 0.25 mm i.d. 0.25 mm ®lm thickness)
programmed from 80 to 300 C at 3 C/min. Operating
conditions were: ion source temperature 140 C, electron
energy 70 eV, scanning from 40 to 600 amu, 0.6 scan/s.
3.4. Bulk analyses
POC and SM data were obtained by Dai (1995) during
the same cruise. The analytical reproducibility of POC
measurements was estimated at 2%. Sedimentary
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
365
Fig. 1. Study area (a) and sampling locations along the riverine, mixing and marine zones of the Ob and Yenisei River estuaries and
Kara Sea shelf (b).
366
M.B. Fernandes, M.-A. Sicre / Organic Geochemistry 31 (2000) 363±374
organic carbon (OC), organic nitrogen (N) and d13C
were analyzed following the experimental procedure by
Girardin and Mariotti (1991). Brie¯y, samples were
decarbonated in 25 mL of HCl (0.001 M), followed by
the addition of HCl (0.5 M) till a constant pH around 3
was reached. They were centrifuged, washed with deio
nized water and dried for 12 h at 50 C. After being
homogenized and weighed, aliquots were combusted in
an elemental analyzer (Carlo Erba, NA 1500 NC,
Fisons). The CO2 and N2 produced were chromatographically separated. OC and N were then measured by
thermal conductivity. The analytical reproducibility of
these measurements was estimated at 3%. Combustion gases were then cryogenically puri®ed through several temperature traps and introduced into the isotopic
ratio mass spectrometer ®tted for rapid switching (VG
Sira 10 Spectrometer). Samples were compared to Pee
Dee Belemnite (PDB) as reference to determine their
d13C values. The experimental error was estimated to be
better than 0.1%o based on successive injections of
the tyrosine standard.
4. Results and discussion
4.1. Suspended matter
SM concentrations (Dai, 1995) are in the lower range
of concentrations reported for world rivers (Meybeck,
1982; Ittekkott, 1988): they range from 4 to 143 mg/L
(average of 30 mg/L, n=7) in the Ob River, and from 1
to 23 mg/L (average of 5 mg/L, n=11) in the Yenisei
River, while < 1 mg/L in the Kara Sea. OC percentages
(Dai, 1995) vary from 3 to 5% in the Ob River (average
of 4%, n=7), and from 4 to 11% in the Yenisei (average
of 7%, n=11). They are higher in the Kara Sea, ranging
from 3 to 19% (average of 12%, n=5).
All concentration data for the particulate n-alkanes
are given in Table 1. Total particulate
P odd carbon
number n-alkanes from C25 to C31 ( ALK25±31) and
OC concentrations (in ng/L and mg/L, respectively) are
higher in the Ob River than in the Yenisei. Total nalkanes (in mg/L) decrease seawards in both estuaries
(Fig. 2). Except for some stations of the northern Kara
Sea, n-alkane particle loads (in mg/g) are relatively constant, generally around 10 mg/g (Fig. 2), indicating the
rather homogeneous composition of riverine SM. This
translates into a high correlation between n-alkane concentrations and OC or SM values (r>0.95, P 20mg/g).
P®rman et al. (1995) reported elevated DDT levels in
this area of the Kara Sea that could be attributed to
transport by drifting-ice. The Yamal Current ¯owing
towards the northeast at the time of sampling can also
be envisaged as a potential vector of additional SM
inputs (Fig. 1b; Burenkov and Vasil'kov, 1995). However, this hypothesis cannot be substantiated without
samples collected within this current ¯ow. Prevailing
odd and high molecular weight (5 C25) n-alkanes indicate major vascular plant wax inputs. n-C27 is dominant
and carbon preference index values (CPI24±32) are generally > 3 (Table 1). The occurrence of alkylated polycyclic aromatic hydrocarbons, such as octahydrochrysenes and tetrahydropicenes, generated from the
aromatization of 3-hydroxy triterpenoids and other constituents of higher plants, further supports the hypothesis
of a dominant terrestrial origin (Fernandes, 1996).
Total n-alkane particle loads in the eastern Eurasian
Lena Delta and Laptev Sea are substantially higher than
values found in this work (Table 2). Compositional features indicate signi®cant planktonic inputs (Broyelle,
1997) that are not observed in the Kara Sea, at least not
during the SPASIBA 3 cruise. Such inputs may account
partially for the observed dierences in concentrations
with respect to our data. At the freshet of the Mackenzie
River, with a similar basin cover (vegetation and soil),
values are lower than ours (Table 2), but n-alkane and
SM levels are also signi®cantly correlated (Yunker et al.,
1991).
4.2. Sediments
Only a small portion of the OC transported by the Ob
and Yenisei SM is preserved in the sediments. OC percentages in both rivers are relatively constant at around
1.5% (n=17) (Fig. 3). OC percentages in sediments of
the Kara Sea are signi®cantly lower (average of 0.7%,
n=6), but close to the average OC concentrations of
®ne-grained shelf and delta sediments (0.75%) calculated for world rivers by Berner (1982). Besides direct
inputs from soil erosion, the OC content of shelf sediments may also re¯ect better preservation of OM in ®ne
¯uvial particles which bear the highest mineral surface
areas (Prahl and Carpenter, 1983; Prahl, 1985; Mayer,
1994a,1994b; Keil et al., 1998).
d13C values determined from sediments of both rivers
(Table 3) are close to the isotopic signature of continental C3 vegetation (e.g. GonÄi et al., 1997, 1998;
Ruttenberg and GonÄi, 1997). High C/N ratios suggest
that the contribution of plankton (C/N