ИСТИНА

Rapid inundation of the Laptev Sea shelf due to the global sea level rise led to drastic changes in water mass properties and sedimentation environments (Bauch et al., 1999, 2001; Taldenkova et al., 2008, 2009; Pogodina et al. 2009). We investigate foraminiferal assemblages from three AMS 14 C dated sediment cores from mid-outer shelf aging back to 12.3 cal.ka. Reconstructions of past events are based on established downcore variability in the relative abundance of species and ecological groups distinguished in the Arctic marginal seas in dependence to offshore changes in water depth and river runoff influence (Polyak et al., 2002). In order to verify the distribution of river-proximal, river-intermediate, river-distal groups on the Laptev Sea shelf and to improve the reconstructions we apply the newly obtained data on the occurrence of modern foraminifers in surface sediments. Fossil assemblages from paleoriver valleys on the mid-outer shelf, PS 51/159 in the west (Khatanga paleovalley) and PS 51/135 and PS 51/138 in the east (Yana and Lena paleovalleys, respectively) evidence a transition from fluvially affected to dominantly marine settings. Predominance of opportunistic (Elphidium clavatum) and riverproximal species illustrates freshened nearshore environments, during initial stages of river estuaries flooding. Subsequent changes from inner-shelf to outer-shelf environments are shown by the increase in diversity and abundance of foraminifera, appearance of river-distal species and decreasing representation of E. clavatum. Timing of the observed changes in assemblages related to sea-level rise and shelf flooding differs in dependence to the actual water depths of the studied sites which vary from 40 to 60 m. Interestingly, the oldest brackishwater assemblages, especially in the western Laptev Sea, contain abundant planktic foraminifers including subpolar species, which are rare or absent in the younger more marine assemblages. This probably indicates enhanced water circulation in the former river estuaries and advection of the open sea waters with reversed bottom currents (Dmitrenko et al., 2001).