Date of Award
Doctor of Philosophy
The overall objective of this dissertation was to investigate the benthic foraminiferal assemblage dynamics of the northwestern part of the Weddell Sea and to utilize them as oceanographic proxies. Spatial and temporal patterns of foraminiferal assemblages are correlated with other proxies in order to reconstruct and better understand Antarctic Peninsula (AP) paleoenvironmental changes, particularly those pertaining to bottom-water circulation, glacial fluctuations, and biological productivity on the eastern margin of the AP (EAP). This area in the northeast Antarctic Peninsula has experienced significant environmental change evidenced by the collapse of both Larsen-A and Larsen -B ice shelves, involving various preconditioned and sequential transitions in cryosphere and ecosystem dynamics. The first project (Chapter 3) presents an expanded foraminiferal and diatom record from Perseverance Drift, northeastern AP. Using benthic foraminifera and diatoms as paleoenvironmental proxies for paleoproductivity and sea ice conditions in the area, a record extending back to 3400 yr BP, constrained via radiocarbon dating of biogenic calcite was produced. The variability of the recor was compared with existing paleoenvironmental marine records of the AP, which recognize Holocene hypsithermal and cooling events, such as the Mid-Holocene Climatic Optimum and the Neoglacial period. Overall, Holocene climatic variability is complex, with warm and cool phasing occurring at different rates and at different times across the AP (Bentley et al., 2009). Based on the foraminiferal and diatom record of Perseverance Drift, this study recognizes two main intervals of climatic variability. The basal unit of the composite core, characterized by high abundances of Globocassidulina spp. Foraminiferal Assemblage (FA), indicates incursions of Weddell Sea Transitional Water over the drift site. This interval implies a period of ‘freshening’ of the water column, coinciding with an open-marine or seasonally open-marine environment during the middle-to-late Holocene Climatic Optimum. The upper unit of the composite core displays characteristics of slightly colder climatic conditions as indicated by the absence of calcareous foraminifera Globocassiulina spp. FA and the pronounced presence of agglutinated FAs that are indicative of the presence of heavy sea ice. Therefore, this interval is interpreted to represent the onset of Neoglaciation at the northeastern tip of the AP. The main objective of the second project (Chapter 4) was to understand modern trends in the foraminiferal-environmental relationship in order to be able to accurately interpret the environmental conditions represented in paleo-records from the eastern part of the AP (EAP), and more specifically the formerly ice shelf-covered Larsen A embayment. This project compared already published data reporting on modern foraminiferal assemblage distributions from the Larsen A embayment (collected in March 2000) with new data reporting on modern foraminiferal assemblage distributions (collected in March 2012) from approximately the same sampling locations that follow an inshore to offshore transect designed to serve as a spatial approximation for temporal differences in ice shelf coverage over the past several decades. Additionally, down-core (2012) samples were analyzed and compared with the modern datasets in an attempt to obtain longer-term information regarding the foraminiferal communities’ status pre-collapse. The main difference within the span of 12 years separating the sampling efforts is the removal of the calcareous component from the foraminiferal dataset in stations from the inner part of the embayment, situated closer to the ice shelf edge. The down-core foraminiferal data, seem to be following the same pattern as observed in the modern datasets, therefore, providing proof of the concept that distance from the ice shelf edge, in combination with the effects of taphonomical dissolution and locally increased food availability, is a factor that can structure the benthic foraminiferal community composition in environments characterized by ice shelf collapse, such as the Larsen A embayment. The third project (Chapter 5) provides an analysis of foraminiferal assemblages collected from sediment core GC16B near the northern margin of Larsen C ice shelf. Foraminiferal analysis of samples collected from this core revealed a diverse assemblage of calcareous benthic foraminiferal taxa in the upper 20 cm of the core, with very low contribution from agglutinated forms. Owing to the very low absolute diatom abundance values that characterize the core, the high abundance of planktonic foraminiferal taxa relative to benthic foraminiferal taxa, and the dominance patterns of the benthic foraminiferal taxa, different hypothesis pertaining to the source of productivity in this sub-ice shelf environment have been employed involving the transport of advected food supply from the open Weddell Sea and the response of the foraminiferal fauna to less productive (than labile) organic matter as their food source.
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