Variable sequence of events during the past seven terminations in two deep-sea cores from the Southern Ocean

The relationships among internally consistent records of summer sea-surface temperature (SSST), winter sea ice (WSI), and diatomaceous stable isotopes were studied across seven terminations over the last 660 ka in sedimentary cores from ODP sites 1093 and 1094. The sequence of events at both sites indicates that SSST and WSI changes led the carbon and nitrogen isotopic changes in three Terminations (TI, TII and TVI) and followed them in the other four Terminations (TIII, TIV, TV and TVII). In both TIII and TIV, the leads and lags between the proxies were related to weak glacial mode, while in TV and TVII they were due to the influence of the mid-Pleistocene transition. We show that the sequence of events is not unique and does not follow the same pattern across terminations, implying that the processes that initiated climate change in the Southern Ocean has varied through time

Rekonstruktion der Oberflächenwassermassen der östlichen Laptevsee im Holozän anhand von aquatischen Palynomorphen

Aquatic palynomorphs (dinoflagellate cysts, chlorococcaJ algae, acritarchs, several groups of zoomorphs) in recent and Holocene sediments of the Laptev Sea sheJf and the adjacent continental slope were studied microscopically. The data were processed by means of multivariate statistical analysis in order to define ecological preferences and to reconstruct the history of surface water masses in the eastem Laptev Sea during the Holocene. The occurrence and distribution pattern of some palynomorphs in recent sediments are clearly related to the hydrographic conditions of surface water masses. The polar surface water masses of the Laptev Sea shelf are documented in the sediments by dinoflagellate cyst assembJages with Brigantedinium spp., Algidasphaeridium? minutum and related morphotypes. lmpagidinium? pallidum, Nematosphaeropsis labyrinthus and Operculodinium centrocarpum characterize the assemblage on the continental slope. Nematosphaeropsis labyrinthus and Operculodinium centrocarpum indicate the influence of warmer Atlantic water masses. The chlorococcal algae assemblages are composed of Pediastrum spp. and Botryococcus cf. braunii which usually live in freshwater. Their occurrence in the marine environment indicates that the Laptev Sea shelf is strongly influenced by the freshwater input of the rivers OJenek, Lena and J ana. The development of the surface water masses during the last 7400 years was reconstructed in two sediment cores from the eastern Laptev Sea by qualitative and quantitative analyses of dinoflagellate cysts, chlorococcal algae and other groups of aquatic palynomorphs. Between 7400 and 7000 B.P. the sea level was 20 m beJow present level and the Lena and Yana drained through an estuary. Therefore, the influence of higher saline water masses was very low due to the freshwater inpul. Around 6400 B.P. the freshwater input into the Laptev Sea was probabely twice as high as today, indicated by the high concentrations of chlorococcal algae. At approximately the same time influence of marine water masses increased with rising sea level. The period between 5200 and 2500 B.P. was characterized by a stepwise decrease of freshwater input. Since 2500 B.P. the fluvial input is similar to the present situation. During summer, the present average sea-ice margin is located at 77°N. The sea-ice margin has migrated landwards between 2500 and 1050 B.P. The productivity in surface water masses increased near the ice margin. At that time period some dinoflagellate cyst species documented, that Pacific surface water masses entered the eastern Laptev Sea sporadically. Since 1050 B.P. the average sea-ice margin in summer moved northwards and reached the present position since 550 B.P. During he last 550 years a decrease of warm adapted dinoflagellate cyst species indicates a development of present day polar conditions