Dominik Weiel

Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence-carrying minerals. Estimations of relative paleointensity variations yielded a well-documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry C-14 ages and an incomplete Be-10 record, still two different interpretations of the paleomagnetic data are possible, with long-term sedimentation rates of either 1.3 or 4 cm kyr(-1) However, both models implicate highly variable sedimentation rates of up to 10 cm kyr(-1), and abrupt changes in rock magnetic parameters might even indicate several hiatuses.

Organic Carbon in Arctic Ocean Sediments: Sources, Variability, Burial, and Paleoenvironmental Significance

In the preceding chapters, organic carbon sources, pathways and transformation processes at the sea floor have been presented and discussed. In the following chapter, particulate organic carbon in Arctic Ocean sediments, its composition, burial and paleoenvironmental significance will be discussed. With respect to the Arctic Ocean’s organic carbon sedimentary records, we concentrate on surface sediment data and sediment cores representing the late Quaternary time interval, because information about organic carbon accumulation during pre-Quaternary (i.e., Jurassic to Tertiary) and its paleoenvironmental implications is very sparse (e.g., Thiede et al. 1990). For completeness, however, a short overview on pre-Quaternary records will be presented here.

Sediment composition of the Central Arctic Ocean

The sediment composition of deep-sea cores from the central Arctic Ocean, the Fram Strait, and the Yermak Plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200,000 years. Layers with high amounts of coarse, terrigenous ice-rafted debris (IRD) and often high contents of smectite were deposited during extensive glaciations in northern Eurasia, when ice sheets reached the northern continental margins of the Barents and Kara seas and discharged icebergs into the Arctic Ocean. Intercalated layers with relatively low IRD and smectite contents, but abundant planktic foraminifers in the coarse fraction were deposited during periods of Atlantic Water inflow to the Arctic Ocean and seasonally open waters (leads) in a sea ice cover with only few icebergs in the Arctic Ocean. High IRD contents in the sediments reflect the presence of ice sheets on the Kara and Barents seas shelves and the hinterland during the entire oxygen isotope stage 6 (ca 190-130 ka), in substage 5b (ca 90-80 ka), at the stage boundary 5/4 (around 75 ka), and in late stage 4/early stage 3 (ca 65-50 ka). These results are in excellent correlation with those from recent field work in northern Scandinavia, European Russia, Siberia, and on the shelves. Relatively low amounts of IRD in central Arctic Ocean sediments from the Late Weichselian glacial maximum (ca 24-18 ka) correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time.Oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the Arctic Ocean. The synchroneity of freshwater events and IRD input suggests a common source. Strongest events were associated with deglaciations of the Barents and Kara seas after the ice sheets had blocked the outflow of large rivers for several millennia. The outflow of freshwater from large ice-dammed lakes occurred at ca 130, 80-75, and 52 ka. Freshwater events in the central Arctic Ocean during the last deglaciation (ca 18 ka) were relatively small compared to the previous events. This indicates that during most of the Late Weichselian glacial maximum a river outflow from northern Siberia to the Arctic Ocean was possible.Atlantic Water inflow to the Arctic Ocean and seasonally open waters in the ice (leads) occurred during the interglacials of oxygen isotope stage 1 and substage 5e, during several interstadials (stage 3, substages 5a and 5c), and to a lesser degree within stadials and glacials (stages 2, 4, and 6). With the exception of the interglacials, these periods were times of strong ice growth on the continents as revealed by terrestrial data. The coincidence suggests that open waters in the Arctic Ocean and the Nordic Seas were an important moisture source (in addition to more southerly sources) which fostered the growth of ice sheets on northern Eurasia.