Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Rebecca S. Robinson

Abstract

The Southern Ocean plays a key role in modulating global climate via physical and biogeochemical processes, both today and in the past. Wind-driven upwelling brings nutrient-rich deep water to the surface, enabling carbon exchange with the atmosphere and fueling primary production that facilitates long-term storage of carbon in the deep ocean. The nitrogen isotopic composition of organic matter within diatom microfossils (δ15NDB) has been used to reconstruct changes in nutrient utilization related to upwelling-driven supply and biological demand and consequent impact on climate across Pleistocene glacial-interglacial cycles (2.58 Ma - 11.7 ka) and over the Holocene (11.7 ka - present). However, there remain outstanding questions regarding the impact of diatom community assemblage on δ15NDB records in addition to spatial gaps, particularly in the most polar reaches of the Southern Ocean proximal to the Antarctic ice sheet. The goals of this dissertation are to quantify the effects of the shifting relative abundance of Chaetoceros resting spores on δ15NDB records spanning glacial-interglacial cycles (Chapter 1), apply these findings to a western Antarctic Peninsula δ15NDB record generated from a resting spore-rich sediment core to reconstruct Holocene oceanographic conditions (Chapter 2), and to investigate changes in biological pump efficiency in a region variably influenced by sea ice with another coastal Antarctic δ15NDB record (Chapter 3).

Resting spores of the diatom genus Chaetoceros are potentially problematic for the δ15NDB proxy because they form under nutrient-depleted conditions, preserve well in sediment due to heavy silicification, and become more abundant in sediment underlying the present-day open ocean during glacial periods due to an equatorward shift of the sea ice edge. Genus-specific δ15NDB measurements from a seasonally laminated sediment core and laboratory culture experiments capturing how Chaetoceros resting spores record nutrient conditions in their nitrogen isotopic composition reveal that resting spores record lower δ15NDB values than other diatoms. Shifting relative abundance throughout glacial-interglacial records does not significantly bias δ15NDB, but the effect should be considered when interpreting sedimentary records with greater variability in resting spore relative abundance.

The Antarctic Peninsula is a region experiencing rapid warming and environmental change, with mounting concern over glacial melt driven by intrusion of relatively warm Circumpolar Deep Water onto the continental shelf. Past variability in oceanographic conditions on the western Antarctic Peninsula is reconstructed with a δ15NDB record spanning the end of deglaciation and throughout the Holocene. After correcting for variable Chaetoceros resting spore relative abundance and comparing our δ15NDB record with other proxies, we find enhanced nutrient utilization in the last two thousand years, consistent with surface stratification and deep Circumpolar Deep Water intrusion driven by a poleward shift in the Southern Westerly Winds.

Continuing to fill in the coastal Antarctic gap in δ15NDB records, we measured δ15NDB and quantified the diatom community assemblage from a high-resolution Holocene-aged sediment core collected offshore of the Wilkes subglacial basin in East Antarctica. Nutrient utilization and biological pump efficiency are sensitive to sea ice extent and nutrient supply from modified Circumpolar Deep Water, illuminating critical ocean-cryosphere-atmospheric feedbacks. Enhanced nutrient supply and decreased utilization in the late Holocene implies both increased glacial melt and decreased CO2 sequestration in the future.

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