646
Land-Ocean Systems in the Siberian Arctic: Dvnamics and History
the seasonally ice-free shelf areas of the Laptev Sea, significant amounts of marine organic
matter were determined in surface sediments, with a maximum near the ice edge as indicated by
increased hydrogen index and biogenic opal values (Stein and Ntirnberg, 1995; Stein, 1996)
and high concentrations of unsaturated fatty acids (Fahl and Stein, 1997a).
The surface sediment data of cores PS2471, PS2474, and PS2476 (Table 1; Fahl and Stein,
1997a) fall into the general trends described in the previous paragraph. Fresh terrigenous
organic matter seems to be dominant as indicated by hydrogen index values < 100 mg HC / g
TOC, the dominance of long-chain n-alkanes in the total n-alkane fraction, the occurrence of
long-chain wax esters, and CPI values between 2.6 and 4.1. Marine (and freshwater ?) organic
matter is present as shown by the occurrence of short-chain n-alkanes and fatty acids, but only
occurs in minor amounts. The significant occurrences of alkenones (as an indicator for
prymnesiophytes; Volkman et a!., 1980; Marlowe et aI., 1984; Prahl et aI., 1988) at the location
of Core PS247 I (0.57 /lg / g TOC) is surprising because in all other surface sediments from the
Laptev Sea - probably caused by the very low abundance of prymnesiophytes in the Laptev Sea
- only very small amounts of alkenones were determined « 0.07 /lg/g TOC; Fahl and Stein,
1997a). Fahl and Stein (1997a) interpreted the occurrence of alkenones in the surface sediment
of Core PS2471 as beeing derived from coccolithophorides or other prymnesiophytes
transported by Atlantic water masses along the continental slope (cf., Figures 1 and 2).
Late Quaternary changes in organic carbon composition at the Laptev Sea continental margin
and their paleoenvironmental significance
The records of cores PS2474 and PS247 1 also comprising pre-Holocene time intervals display
significant variations in total organic carbon content, whereas the Holocene time interval
represented in all three cores, is characterized by less variable TOC values (Figure 5). Based on
the C/N ratios (Figure 6), the organic matter in the non-turbidite sections is generally a mixture
between marine and terrigenous material. The TOC contents of the two cores from the upper
slope are generally higher than those in the non-turbidite sequences of the deepest, distal Core
PS2471, which may reflect a higher supply of terrigenous organic matter in nearshore areas
(decreasing towards the open ocean) and/or better preservation of organic matter because of
distinctly higher sedimentation rates in the shallower cores (Table 2). In order to understand the
processes controlling the organic carbon deposition and their change through late Quaternary
times, i.e., to distinguish between (autochthonous) marine organic carbon produced in the
water column and allochthonous terrigenous organic matter supplied by rivers, currents, and/or
down-slope transport (Figure 2), the organic carbon and biomarker records of Core PS2471 are
discussed in more detail.
The oldest time interval (Unit C) is dominated by turbidites characterized by relatively high
organic carbon contents. The exact age of these turbidites is still under discussion as already
mentioned. It is, however, likely that the turbidites were deposited during glacial intervals
(Oxygen Isotope Stage 2 or 4) when the sea level was low and high amounts of terrigenous
material were transported close to the shelf edge resulting in an instability of the upper slope
and gravitational down-slope transport of material. With this process huge amounts of organic
matter may have been transported down-slope and deposited in the deep sea environment where
normally much less organic matter is preserved. Such a process has also been described for
several other deep-sea regions in the world's ocean, e.g., in the Canadian Basin (Grantz et a!.,
1996) and in the Northeast Atlantic (Stein et aI., 1989; Stein, 1991). The composition of the
organic matter buried in the turbidites depends on the origin of the turbidite. While the turbidites
in the Northeast Atlantic originated from a shelf edge influenced by coastal upwelling and
increased primary productivity where the sediments contain large amounts of marine organic
matter (Stein et aI., 1989; Stein, 1991), the turbidites in Core PS2471 originate from the Laptev
Land-Ocean Systems in the Siberian Arctic: Dvnamics and History
the seasonally ice-free shelf areas of the Laptev Sea, significant amounts of marine organic
matter were determined in surface sediments, with a maximum near the ice edge as indicated by
increased hydrogen index and biogenic opal values (Stein and Ntirnberg, 1995; Stein, 1996)
and high concentrations of unsaturated fatty acids (Fahl and Stein, 1997a).
The surface sediment data of cores PS2471, PS2474, and PS2476 (Table 1; Fahl and Stein,
1997a) fall into the general trends described in the previous paragraph. Fresh terrigenous
organic matter seems to be dominant as indicated by hydrogen index values < 100 mg HC / g
TOC, the dominance of long-chain n-alkanes in the total n-alkane fraction, the occurrence of
long-chain wax esters, and CPI values between 2.6 and 4.1. Marine (and freshwater ?) organic
matter is present as shown by the occurrence of short-chain n-alkanes and fatty acids, but only
occurs in minor amounts. The significant occurrences of alkenones (as an indicator for
prymnesiophytes; Volkman et a!., 1980; Marlowe et aI., 1984; Prahl et aI., 1988) at the location
of Core PS247 I (0.57 /lg / g TOC) is surprising because in all other surface sediments from the
Laptev Sea - probably caused by the very low abundance of prymnesiophytes in the Laptev Sea
- only very small amounts of alkenones were determined « 0.07 /lg/g TOC; Fahl and Stein,
1997a). Fahl and Stein (1997a) interpreted the occurrence of alkenones in the surface sediment
of Core PS2471 as beeing derived from coccolithophorides or other prymnesiophytes
transported by Atlantic water masses along the continental slope (cf., Figures 1 and 2).
Late Quaternary changes in organic carbon composition at the Laptev Sea continental margin
and their paleoenvironmental significance
The records of cores PS2474 and PS247 1 also comprising pre-Holocene time intervals display
significant variations in total organic carbon content, whereas the Holocene time interval
represented in all three cores, is characterized by less variable TOC values (Figure 5). Based on
the C/N ratios (Figure 6), the organic matter in the non-turbidite sections is generally a mixture
between marine and terrigenous material. The TOC contents of the two cores from the upper
slope are generally higher than those in the non-turbidite sequences of the deepest, distal Core
PS2471, which may reflect a higher supply of terrigenous organic matter in nearshore areas
(decreasing towards the open ocean) and/or better preservation of organic matter because of
distinctly higher sedimentation rates in the shallower cores (Table 2). In order to understand the
processes controlling the organic carbon deposition and their change through late Quaternary
times, i.e., to distinguish between (autochthonous) marine organic carbon produced in the
water column and allochthonous terrigenous organic matter supplied by rivers, currents, and/or
down-slope transport (Figure 2), the organic carbon and biomarker records of Core PS2471 are
discussed in more detail.
The oldest time interval (Unit C) is dominated by turbidites characterized by relatively high
organic carbon contents. The exact age of these turbidites is still under discussion as already
mentioned. It is, however, likely that the turbidites were deposited during glacial intervals
(Oxygen Isotope Stage 2 or 4) when the sea level was low and high amounts of terrigenous
material were transported close to the shelf edge resulting in an instability of the upper slope
and gravitational down-slope transport of material. With this process huge amounts of organic
matter may have been transported down-slope and deposited in the deep sea environment where
normally much less organic matter is preserved. Such a process has also been described for
several other deep-sea regions in the world's ocean, e.g., in the Canadian Basin (Grantz et a!.,
1996) and in the Northeast Atlantic (Stein et aI., 1989; Stein, 1991). The composition of the
organic matter buried in the turbidites depends on the origin of the turbidite. While the turbidites
in the Northeast Atlantic originated from a shelf edge influenced by coastal upwelling and
increased primary productivity where the sediments contain large amounts of marine organic
matter (Stein et aI., 1989; Stein, 1991), the turbidites in Core PS2471 originate from the Laptev