Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Rebecca S. Robinson

Abstract

Nitrate is the most common limiting nutrient in the ocean and plays a critical role in the extent and intensity of marine primary production, and therefore the global ocean’s biological pump. Characterization of the supply and demand of nitrate constrains how ocean biology may regulate climate, so understanding the degree of nitrate consumption in the past is a fundamental step towards understanding controls on past climate. The nitrogen isotopic composition (as δ15N) of phytoplankton biomass can be used to infer the degree of nitrate consumption in nitrate-replete surface waters such as the Southern Ocean. This signal is recorded in the underlying sediment and can be used to construct a history of nitrate utilization. However, δ15N values of phytoplankton biomass are subject to alteration during sinking and sedimentation, leading to uncertainty in estimations. The nitrogen isotopic composition of nitrogen within the shells of diatoms (δ15NDB), a photosynthetic microorganism, is protected from alteration and potentially a more robust tracer of past nitrate dynamics. However, this assumption may be complicated by species-specific isotope effects and the high variation in Southern Ocean diatom assemblages through climate transitions. The goals of this dissertation are twofold: first, to investigate the impact of different Southern Ocean diatom communities (Chapter 1) and individual species (Chapter 2) on the δ15NDB proxy and second, to use δ15NDB to examine paleo-nutrient utilization and oceanographic conditions of the coastal West Antarctic Peninsula (WAP), a region of high seasonal productivity and carbon drawdown (Chapter 3).

Two distinct Southern Ocean surface ocean diatom communities were grown in triplicate cultures to determine the impact of diatom community composition on δ15NDB. We found that although the community growouts had distinct diatom assemblages, the εDB (= biomass δ15N - δ15NDB) was indistinguishable between the two growouts at -4.8 ± 0.8‰. This suggests that species composition is not the primary control on δ15NDB in the Southern Ocean. Furthermore, our measured average εDB was more than 10‰ different from the average value of previous single-species measurements, but consistent with Southern Ocean and North Pacific surface ocean observations. Therefore, if δ15NDB is not altered during sinking and sedimentation, then sedimentary δ15NDB is a robust tool for examining changes to nitrate supply and demand over time.

Single-species cultures of diatoms isolated from the Southern Ocean were grown in triplicate to assess individual species εDB. We show that the average εDB is -2.2 ± 1.0‰, consistent with the Southern Ocean community data and surface ocean observations. We observe a positive linear relationship between εDB and the Si:N of diatom uptake, implying that silicification plays a role in setting δ15NDB. This relationship suggests that heavily silicified diatoms could bias sedimentary δ15NDB toward lower values. However, five of the six diatoms species had indistinguishable εDB from one another, indicating that the impact may be minimal in many cases.

The coastal WAP hosts intense productivity fueled by the delivery of warm, nutrient rich Circumpolar Deep Water (CDW) and seasonal stratification related to sea-ice melting. Stronger Southern Hemisphere westerlies are thought to enhance CDW intrusion onto the WAP shelf and have increased in strength over at least the last four decades. Therefore, it is key to determine the historical variations in CDW, productivity, and westerly winds. These variations are especially relevant in the mid- Holocene, when CDW intrusion likely varied substantially, yet there is disagreement about the timing of CDW influence in paleoproxy records. We use δ15NDB to study the WAP by unambiguously linking past proxy descriptions of warm, sea-ice free intervals with the nutrient characteristics of CDW. We find that contrary to prior hypotheses, variations in the relationship between δ15NDB and bulk sedimentary δ15N cannot be explained by changes to diatom assemblages. We also find a dynamic CDW in the mid-Holocene that is synchronous with atmospheric warming associated with stronger westerlies. As westerly strength increases in the future, we predict that CDW intrusion will play an increasingly dominant role in WAP oceanography, supplying enhanced nutrients and potentially contributing to glacial melting. Altogether, the results presented in this dissertation enhance our ability to quantify past fluctuations in nitrate utilization, which plays a vital role in assessing the biological pump.

Available for download on Monday, August 02, 2021

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