Microfossil and stable-isotope evidence for changes in Late Holocene palaeoproductivity and palaeoceanographic conditions in the Prydz Bay region of Antarctica

Abstract

Microfaunal and microfloral data from two gravity cores, and stable-isotope results from one of these cores taken on Fram Bank near Prydz Bay, Antarctica indicate changes in sea-ice patterns and oceanographic conditions which may have been linked to regional and global climate changes that occurred over the past 8000 yr. Modern diatom assemblages indicative of ice free conditions 1–2 months of the year, and dominated by Nitzschia species became prevalent some time around 2000–2700 yr B.P. The occurrence of Chaetoceros spp. diatom spore-dominated assemblages, and increased abundances of benthic and planktonic foraminifera, ostracods, and diatoms, coupled with carbon- and oxygen-isotope changes around 2700–3400 yr B.P. strongly suggests that this area of the shelf experienced conditions conducive to increased productivity during this time period. Based on diatom assemblage associations recognized in modern environments, the upper water column of Fram Bank shelf waters was probably stratified during that time due to the presence of low-salinity melt water and a very shallow mixed layer, protected from storms, with sea-ice cover for less than 10 months per year. The close correspondence of δ13C and δ18O values from N. pachyderma (r2 = 0.74) throughout one core suggests that whatever oceanographic conditions influenced δ18O also influenced δ13C. Changes in Prydz Bay microfossil abundances and isotopes may result from alterations in circulation patterns, upwelling conditions, or sea-ice patterns which significantly affected benthic and planktonic productivity in the area. The environmental conditions indicated by older sediments in the cores are less clear, but an ice tongue may have been in place 3200–3800 yr B.P., and minor fluctuations in productivity are indicated around 6000 and 7000–7500 yr B.P. The lower 100 cm of both cores may not represent in situ deposition, but if the corrected AMS dates reflect the actual age of the sediments analyzed, microfossil and isotope data indicate that productivity, and oceanographic conditions were very variable around 8500-8600 yr B.P. The chronology adopted herein must be regarded as tentative until further study of the area has been undertaken.