Late Holocene variations in Arctic shelf hydrology and sea-ice regime: evidence from north of the Lena Delta

Abstract

Diatom assemblage studies are used to interpret past changes in river runoff (salinity) and sea-ice regime in the vicinity of the vast Lena River delta, southern Laptev Sea shelf. On the basis of their distribution in surface sediments, the shelf region outside the strong influence of riverine waters is characterized by a dominance in sea-ice diatoms and other marine species. Their numbers increase steeply (>20%) within the area of drifting pack ice. In contrast, the marginal zone of the delta, where exceedingly low salinities prevail, is marked by freshwater diatoms showing values higher than 70%. Using the environmental information from the surface sediments, the downcore distribution patterns of the main ecological groups of diatoms were investigated on a sediment core that covers the past 2800 cal. years BP. Although the freshwater group indicates some temporal variations in salinities, the study site north of the Lena River delta remained under a dominantly riverine influence for most of the three recognized phases. In contrast, the relative abundance of sea-ice species gives evidence that pack-ice conditions were more severe during the oldest phase (older than~2700 cal. years BP). The most significant changes are observed in the uppermost core section (younger thañ 300 cal. years BP) when the relative abundance of freshwater diatoms decreases from 80% down to below 20%. This dramatic decrease is interpreted as a major shift from a more northward-directed to the modern, dominantly eastern outflow pattern. Because the dispersal and fate of riverine waters and its role on the ice regime as well as on water mass properties is a central issue in understanding short-and longerterm climatic changes in the Arctic and beyond, it needs to be tested using more cores if this most recent change in outflow pattern from the delta is connected to climate change or simply a result of channel migration within the delta.

Early to middle Holocene changes in Laptev Sea water masses deduced from diatom and aquatic palynomorph assemblages

“…In addition to previously published data of diatom assemblage records in surface sediments from the Siberian arctic seas (Polyakova, 1994(Polyakova, , 1997Cremer, 1999a,b), recent studies indicate that diatoms are a valuable proxy for reconstructing major hydrological parameters in the shallow arctic seas (e.g., summer surface-water salinity and sea-ice conditions), as well as sedimentation processes in the river-proximal shelf areas (Bauch and Polyakova, 2000;. Likewise, the prevalence of arctic water masses and riverine freshwater in the Kara and Laptev seas as well as the rather similar species composition of diatom assemblages in both seas (Cremer, 1999a,b; allow us to apply the relation between surface salinity and freshwater diatoms in coretop sediments from the Kara Sea to downcore diatom records from the southeastern Laptev Sea (Bauch and Polyakova, 2003).…”

Late Holocene dinoflagellate cysts as indicators for short‐term climate variability in the eastern Laptev Sea (Arctic Ocean)

“…Furthermore, a correlation between T. clarkei and Cyst of P. dalei is not observable. Bauch and Polyakova (2000) reconstructed freshwater discharge and sea-ice cover from a diatom record of the inner Laptev Shelf off the Lena delta. Diatom assemblages show some variability in the past 2800 cal.…”

“…Variable reservoir age corrections have been applied previously to radiocarbon datings from the Laptev Sea. Bauch (1999) corrected all datings for a 14 C reservoir effect of 400 yr, whereas Bauch and Polyakova (2000) used a reservoir age correction of 313 yr but did not provide data used for the calculation. Dating by AMS 14 C of five bivalve shells, which were collected in the early twentieth century before the nuclear trials had started, allowed calculation of a reservoir age of 370 ± 49 yr (Bauch et al, 2001, in press).…”

“…In general, microfossil and stable oxygen isotope records from shelf sediments in the Eurasian Arctic indicate palaeoenvironmental variability on millennial to centennial time-scales (e.g. Kunz-Pirrung, 1998;Polyakova, 1999;Bauch and Polyakova, 2000;Duplessy et al, 2001;Voronina et al, this issue). In the Laptev Sea and the Barents Sea sedimentation rates exceed 100 to 200 cm kyr −1 during certain time intervals (e.g.…”

Arctic Siberian shelf environments—an introduction

“…Diatom assemblage studies from surface sediments have shown (Bauch and Polyakova, 2000) that the pack ice area is typically associated with enhanced abundance of sea-ice species, whereas the landfast ice zone is characteristical for abundant freshwater species. The two microwave images (top left) reveal quite contrasting distribution of sea-ice concentration (source: http:// iup.physik.uni-bremen.de:8084/amsr/amsre.html).…”