Thiede et at.: Dynamics and History ofthe Laptev Sea and its Continental Hinterland
705
volume), investigations were performed. These analyses concentrated on river suspended
particulate matter as well as on surface sediments from all major river systems draining into the
Laptev Sea and the adjacent Arctic Ocean. In addition, ice samples from the Laptev Sea and the
Arctic Ocean were studied. Accordingly, the Laptev Sea can be divided into two different
sedimentary provinces. While the eastern part is controlled by Lena River discharge, the
western part is controlled by sediments supplied by the Khatanga River. Sea ice and sea-floor
sediments from the western Laptev Sea can be distinguished from those from the eastern Laptev
Sea by (i) the dominance of pyroxene in contrast to the amphibole-rich sediments of the eastern
Laptev Sea, (ii) high amounts of the clay mineral smectite, which are lower in the eastern
Laptev Sea, and (iii) significantly lower 87Srl 86 Sr isotope ratios. In addition, sea-floor
sediments of the Arctic Ocean indicate that Sr isotope ratios and heavy mineral assemblages can
be used to identify sediment transportation pathways from Central Siberia to the Arctic Ocean.
The Lena River is considered to be the main sediment source for the Laptev Sea. About ISO
km upstream from the Lena Delta measurements of suspended sediment transport were made
during the past decades. But the quantitative evaluations of suspended sediment discharge made
by different investigators using the same data sets range from 11.8 to 21 million tons per year.
Hence, the exact amounts of sediment actually accumulating in the Lena Delta and finally
reaching the sea is difficult to estimate. However, data on suspended sediments along a transect
from the southeastern Lena Delta to the Laptev Sea have shown no concentration gradient
during the onset of the river break-up in 1996.
Another important sediment source for the Laptev Sea is coastal erosion. Field investigations
as well as air and satellite images indicate that coastal retreat varies between 2.5 and 6 m per
year. Rough estimations of the amount of sediments released to the sea due to erosion are of the
same order compared to data on sediment discharge of the rivers feeding the Laptev Sea (Are,
1996; Are, this volume). However, reliable estimates for land-ocean sediment transport will be
the focus of further investigations in the Lena Delta.
The modern organic-carbon cycle
To understand the processes controlling organic-carbon flux and composition in the Laptev Sea,
biological, micropaleontological, organochemical, and sedimentological investigations were
performed on water samples and surface sediments of the Laptev Sea (Fahl and Stein, 1997;
Stein et a!., this volume). Data from the water column indicate a higher biological activity in the
eastern Laptev Sea. This is also reflected in surface sediments, with increased contents of
organic matter in this area. Micropaleontological parameters and specific biomarkers can be
used to distinguish between marine and terrigenous sources of organic matter of surface
sediments. Organic carbon maxima off the river mouths correlate with increased abundances of
freshwater algae, plant debris, and terrigenous biomarkers (e.g., long-chain n-alkanes and wax
esters, light a 13 corg) as well as high clay content. This points to the terrigenous source of this
organic matter. Concentrations of terrigenous markers decrease with increasing distance from
the source. The distribution of the marine biomarkers (e.g., short-chain fatty acids) correlates
with sea ice distribution. The lowest concentrations of short-chain fatty acids are in ice-covered
areas, whereas the highest amounts are located near the ice edge. Fatty-acid distribution
correlates with the distribution of chlorophyll a and biogenic opal content, indicating increased
surface-water productivity near the ice edge (Fahl and Stein, 1997).
To understand the modern organic carbon cycle, variations in modern organic carbon
accumulation (flux) rates must also be considered. Accumulation rates of total organic carbon
may reach 0.2 to 2 gCcm- 2 ky-l, decreasing to 0.02 gCcm- 2 ky-l at the lower slope. A decrease
in accumulation rates from the Laptev Sea towards the lower slope reflects a decrease in
terrigenous organic matter supply in offshore direction.
705
volume), investigations were performed. These analyses concentrated on river suspended
particulate matter as well as on surface sediments from all major river systems draining into the
Laptev Sea and the adjacent Arctic Ocean. In addition, ice samples from the Laptev Sea and the
Arctic Ocean were studied. Accordingly, the Laptev Sea can be divided into two different
sedimentary provinces. While the eastern part is controlled by Lena River discharge, the
western part is controlled by sediments supplied by the Khatanga River. Sea ice and sea-floor
sediments from the western Laptev Sea can be distinguished from those from the eastern Laptev
Sea by (i) the dominance of pyroxene in contrast to the amphibole-rich sediments of the eastern
Laptev Sea, (ii) high amounts of the clay mineral smectite, which are lower in the eastern
Laptev Sea, and (iii) significantly lower 87Srl 86 Sr isotope ratios. In addition, sea-floor
sediments of the Arctic Ocean indicate that Sr isotope ratios and heavy mineral assemblages can
be used to identify sediment transportation pathways from Central Siberia to the Arctic Ocean.
The Lena River is considered to be the main sediment source for the Laptev Sea. About ISO
km upstream from the Lena Delta measurements of suspended sediment transport were made
during the past decades. But the quantitative evaluations of suspended sediment discharge made
by different investigators using the same data sets range from 11.8 to 21 million tons per year.
Hence, the exact amounts of sediment actually accumulating in the Lena Delta and finally
reaching the sea is difficult to estimate. However, data on suspended sediments along a transect
from the southeastern Lena Delta to the Laptev Sea have shown no concentration gradient
during the onset of the river break-up in 1996.
Another important sediment source for the Laptev Sea is coastal erosion. Field investigations
as well as air and satellite images indicate that coastal retreat varies between 2.5 and 6 m per
year. Rough estimations of the amount of sediments released to the sea due to erosion are of the
same order compared to data on sediment discharge of the rivers feeding the Laptev Sea (Are,
1996; Are, this volume). However, reliable estimates for land-ocean sediment transport will be
the focus of further investigations in the Lena Delta.
The modern organic-carbon cycle
To understand the processes controlling organic-carbon flux and composition in the Laptev Sea,
biological, micropaleontological, organochemical, and sedimentological investigations were
performed on water samples and surface sediments of the Laptev Sea (Fahl and Stein, 1997;
Stein et a!., this volume). Data from the water column indicate a higher biological activity in the
eastern Laptev Sea. This is also reflected in surface sediments, with increased contents of
organic matter in this area. Micropaleontological parameters and specific biomarkers can be
used to distinguish between marine and terrigenous sources of organic matter of surface
sediments. Organic carbon maxima off the river mouths correlate with increased abundances of
freshwater algae, plant debris, and terrigenous biomarkers (e.g., long-chain n-alkanes and wax
esters, light a 13 corg) as well as high clay content. This points to the terrigenous source of this
organic matter. Concentrations of terrigenous markers decrease with increasing distance from
the source. The distribution of the marine biomarkers (e.g., short-chain fatty acids) correlates
with sea ice distribution. The lowest concentrations of short-chain fatty acids are in ice-covered
areas, whereas the highest amounts are located near the ice edge. Fatty-acid distribution
correlates with the distribution of chlorophyll a and biogenic opal content, indicating increased
surface-water productivity near the ice edge (Fahl and Stein, 1997).
To understand the modern organic carbon cycle, variations in modern organic carbon
accumulation (flux) rates must also be considered. Accumulation rates of total organic carbon
may reach 0.2 to 2 gCcm- 2 ky-l, decreasing to 0.02 gCcm- 2 ky-l at the lower slope. A decrease
in accumulation rates from the Laptev Sea towards the lower slope reflects a decrease in
terrigenous organic matter supply in offshore direction.