Microfossils as proxies for Holocene climate in semi-enclosed basins: the Hudson River estuary, New York, USA and the Marmara Sea, Turkey

• Main Author: Gurung, Damayanti
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: CUNY Academic Works 2015
• Subjects: climate change; Diatom; Foraminifera; paeosalinity; sea level change; Geology; Paleontology; Hudson; Ice Sheet
• Online Access: https://academicworks.cuny.edu/gc_etds/959; https://academicworks.cuny.edu/cgi/viewcontent.cgi?article=1973&context=gc_etds

The transition from the late Pleistocene glacial to the warmer Holocene interglacial occurred within a series of climatic oscillations. The Holocene, a relatively warm and stable period, was also marked by smaller climatic variability of millennial- to-centennial-scale oscillations. Furthermore, during this period global sea level was much lower than at present. My present study will analyze micro-fossils as proxies for their response to these climatic fluctuations, as well as marine influences during the late Pleistocene to Holocene periods. Sediments from marginal basin environments have been selected for the study due to their connection to the open sea and terrestrial environments, providing high-resolution sedimentary records that document the interaction of marine and terrestrial processes. Such sedimentary records are studied in brackish and marine sediments from the Hudson River estuary and Marmara Sea, respectively. The retreat of the Laurentide Ice Sheet during the late Pleistocene to early Holocene periods permitted fluvial and estuarine conditions to develop in the Hudson Valley. As glacial lakes were breached, the global sea-level rose and flooded the Hudson Valley, fresh and marine waters mixed, recording in the sediments the changes in climate. The objectives of this study are to use the microfossil diatom flora as proxies for salinity and changes in freshwater discharge from the mid- to late Holocene. Multibeam bathymetry and five long sediment cores were studied from Newburgh to Peekskill. Detailed sedimentologic and biostratigraphic (diatoms) analyses were conducted together with geochemical scanning. These results were correlated to a calibrated radiocarbon and short-lived radionuclides chronology, permitting the correlation of the Hudson River estuary results to previously documented regional climate studies. The diatom assemblages show that marine influence was strongest, with freshwater inflow reduced, from 6500 to 3500 cal. yr. BP. This interval was interrupted by an increase in freshwater benthic epiphytic diatoms, from 5400 to 5800 cal. yr. BP, suggesting an increase in freshwater discharge. The freshwater diatoms begin to increase in abundance from ~3500 to 1300 cal. yr. BP, indicative of increase in freshwater influences. The gradual increase in the abundance of freshwater diatoms indicates a decrease in salinity in the area. After 1300 cal. yr. BP, the sediment was dominated by freshwater diatoms with decrease in salinity suggesting further increase in freshwater influences. The Diatom Salinity Index (DSI) measured for the last 6500 cal. yr. BP similarly show comparable changes with an overall trend toward a gradual decrease in salinity. During the late glacial, Marine Isotope Stage 2, the Marmara Sea transformed into a brackish lake as global sea level fell below the sill in the Dardanelles Strait. The goal of this study is to develop a high-resolution record of the lacustrine-to-marine transition of Marmara Sea, in order to reconstruct regional and global climatic events at a millennial scale. For this purpose, the paleoshorelines of Marmara Sea were mapped along the northern, eastern, and southern shelves at Çekmece, Prince Islands, and Imrali, using data from multibeam bathymetry, high-resolution subbottom profiling (chirp) and ten sediment cores. Detailed sedimentologic, biostratigraphic (foraminifers, mollusk, diatoms), and geochemical analyses were correlated to a calibrated radiocarbon chronology. The results provide evidence for cold and dry conditions prior to 15,000 cal. yr. BP, warm conditions of the Bölling-Allerod from ~15,000 to 13,000 cal. yr. BP, a rapid marine incursion at 12,000 cal. yr. BP, and a still stand of sea-level and sediment reworking of the paleoshorelines during the Younger Dryas at ~11,500 to 10,500 cal. yr. BP. The development of a strong stratification and influx of nutrients as Black Sea waters spilled into Marmara Sea occurred at 9200 cal. yr. BP. Stable environmental conditions developed in Marmara Sea after 6000 cal. yr. BP as sea level reached its present shoreline and the basin floors filled with sediments, achieving their present configuration.