648
Land-Ocean Systems in the Siberian Arctic: Dvnamics and History
of freshwater (riverine) diatom valves occurring in the turbidites of Core PS2471, also supports
this explanation (Y. Polyakova, unpubl. data 1995). Thus, the organic matter in the turbidites
displays a strong terrigenous signal indicated by low hydrogen index values (<100 mg HC / g
TOC) and high CIN ratios (> 10) (Figure 8). The terrigenous origin is also indicated by a
maximum occurrence of lignin phenols in the three turbidite intervals of Core PS2471 (J.
Lobbes, unpubl. data, 1997). Surprisingly, the long-chain n-alkanes (C27, C29 and C31)
indicative for higher land plants and commonly used in sediments as a biomarker for
terrigenous organic matter supply (e.g., Simoneit et aI., 1977; Prahl and Muehlhausen, 1989),
do not correlate with the hydrogen index and CIN records, i.e., they do not show any clear
maximum in these turbidites (Figure 8). The same phenomenon has also been described by
Schubert and Stein (1997) in sediments from the central Arctic Ocean. That means, the longchain n-alkane concentrations cannot be used to estimate the terrigenous organic carbon supply
into Arctic Ocean sediments in general. Other terrigenous components, such as lignin or
carbohydrates, may contribute significantly to the terrigenous organic carbon proportion (cf.,
Eggers, 1994).
The intervals inbetween the turbidites of Unit C are rather organic-carbon-poor (0.3%) and
display higher hydrogen index values, lower CIN ratios, and higher abundances of marine
biomarkers (short-chain n-alkanes and, although less pronounced, alkenones) (Figure 8). This
relative increase in the marine signal is probably caused by a reduced supply of terrigenous
organic matter.
The most prominent change in the organic carbon and biomarker composition is observed in
Unit B and Unit A (Figure 8). Maximum abundances of long-chain n-alkanes, the clear
dominance of long-chain over short-chain n-alkanes, and hydrogen index values of 100-200
mg HC / g TOC still suggest a dominance of organic matter of terrigenous origin. Maximum
abundances of short-chain n-alkanes, short-chain fatty acids, and alkenones, however, were
also observed in the same interval suggesting significant amounts of marine organic matter were
preserved in the sediments, especially in Unit B. In general, elevated amounts of marine
organic matter in deep-sea sediments can either be explained by high primary productivity or
increased preservation rates (e.g., Berger et aI., 1989; Stein, 1991 and further references
therein). At least from a comparison with measured low modern primary productivity values of
about 25 mg C/m 2 /d (Gleitz, pers. comm. 1996) of the presently seasonally ice-covered study
area, distinctly enhanced primary productivity seems to be less probable. A low productivity is
also suggested from the almost complete absence of marine planktonic diatoms in Core PS2471
(Y. Polyakova, unpubl. data 1995). Instead, a reduced supply of terrigenous organic matter
and better preservation of marine organic matter due to higher sedimentation rates is a more
reasonable explanation.
It cannot be excluded, however, that part of the short-chain fatty acids are of freshwater
origin due to a high supply of lacustrinelfluviatile organic matter by the rivers draining into the
Laptev Sea. This possibility is supported by the distribution pattern of freshwater
palynomorphs in Laptev Sea surface sediments (Kunz-Pirrung, 1997) and the frequent
occurrence of the freshwater algae Botryococcus spp. in the sedimentary record of Core
PS2471 (B. Boucsein, pers. comm., 1997). Further detailed kerogen and palynomorph
microscopy work will allow a more definite interpretation of the biomarker signals.
The maximum abundances of alkenones may be related to the influence of Atlantic water
masses (cf. Figure 1; Fahl and Stein, 1997a). Since coccolithophorides as the main producers
of alkenones, are absent in the fossil record of this interval of Core PS2471 (Ntirnberg et aI.,
1995), carbonate dissolution is assumed to have affected the sediments in this time interval.
Carbonate dissolution may have been caused by increased C02 production during the
decomposition of marine organic matter (de Vernal et aI., 1992; Steinsund and Hald, 1994).
Land-Ocean Systems in the Siberian Arctic: Dvnamics and History
of freshwater (riverine) diatom valves occurring in the turbidites of Core PS2471, also supports
this explanation (Y. Polyakova, unpubl. data 1995). Thus, the organic matter in the turbidites
displays a strong terrigenous signal indicated by low hydrogen index values (<100 mg HC / g
TOC) and high CIN ratios (> 10) (Figure 8). The terrigenous origin is also indicated by a
maximum occurrence of lignin phenols in the three turbidite intervals of Core PS2471 (J.
Lobbes, unpubl. data, 1997). Surprisingly, the long-chain n-alkanes (C27, C29 and C31)
indicative for higher land plants and commonly used in sediments as a biomarker for
terrigenous organic matter supply (e.g., Simoneit et aI., 1977; Prahl and Muehlhausen, 1989),
do not correlate with the hydrogen index and CIN records, i.e., they do not show any clear
maximum in these turbidites (Figure 8). The same phenomenon has also been described by
Schubert and Stein (1997) in sediments from the central Arctic Ocean. That means, the longchain n-alkane concentrations cannot be used to estimate the terrigenous organic carbon supply
into Arctic Ocean sediments in general. Other terrigenous components, such as lignin or
carbohydrates, may contribute significantly to the terrigenous organic carbon proportion (cf.,
Eggers, 1994).
The intervals inbetween the turbidites of Unit C are rather organic-carbon-poor (0.3%) and
display higher hydrogen index values, lower CIN ratios, and higher abundances of marine
biomarkers (short-chain n-alkanes and, although less pronounced, alkenones) (Figure 8). This
relative increase in the marine signal is probably caused by a reduced supply of terrigenous
organic matter.
The most prominent change in the organic carbon and biomarker composition is observed in
Unit B and Unit A (Figure 8). Maximum abundances of long-chain n-alkanes, the clear
dominance of long-chain over short-chain n-alkanes, and hydrogen index values of 100-200
mg HC / g TOC still suggest a dominance of organic matter of terrigenous origin. Maximum
abundances of short-chain n-alkanes, short-chain fatty acids, and alkenones, however, were
also observed in the same interval suggesting significant amounts of marine organic matter were
preserved in the sediments, especially in Unit B. In general, elevated amounts of marine
organic matter in deep-sea sediments can either be explained by high primary productivity or
increased preservation rates (e.g., Berger et aI., 1989; Stein, 1991 and further references
therein). At least from a comparison with measured low modern primary productivity values of
about 25 mg C/m 2 /d (Gleitz, pers. comm. 1996) of the presently seasonally ice-covered study
area, distinctly enhanced primary productivity seems to be less probable. A low productivity is
also suggested from the almost complete absence of marine planktonic diatoms in Core PS2471
(Y. Polyakova, unpubl. data 1995). Instead, a reduced supply of terrigenous organic matter
and better preservation of marine organic matter due to higher sedimentation rates is a more
reasonable explanation.
It cannot be excluded, however, that part of the short-chain fatty acids are of freshwater
origin due to a high supply of lacustrinelfluviatile organic matter by the rivers draining into the
Laptev Sea. This possibility is supported by the distribution pattern of freshwater
palynomorphs in Laptev Sea surface sediments (Kunz-Pirrung, 1997) and the frequent
occurrence of the freshwater algae Botryococcus spp. in the sedimentary record of Core
PS2471 (B. Boucsein, pers. comm., 1997). Further detailed kerogen and palynomorph
microscopy work will allow a more definite interpretation of the biomarker signals.
The maximum abundances of alkenones may be related to the influence of Atlantic water
masses (cf. Figure 1; Fahl and Stein, 1997a). Since coccolithophorides as the main producers
of alkenones, are absent in the fossil record of this interval of Core PS2471 (Ntirnberg et aI.,
1995), carbonate dissolution is assumed to have affected the sediments in this time interval.
Carbonate dissolution may have been caused by increased C02 production during the
decomposition of marine organic matter (de Vernal et aI., 1992; Steinsund and Hald, 1994).