Date of Award

August 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Zunli Lu

Keywords

Earth, iodine, oxygenation, paleo-oceans, Phanerozoic, Pleistocene

Subject Categories

Physical Sciences and Mathematics

Abstract

Oxygen in the oceans is an important part of the significant and complex evolution of Earth’s climate, with great significance for the evolution of life in the oceans. My PhD research has been primarily driven by three major questions: (1) How can we reliably reconstruct oxygen levels in ancient oceans? (2) How did oceanic oxygen levels evolve throughout Earth’s history? (3) How did oceanic oxygen levels affect habitability of the Earth? Most studies of the Earth’s oxygen history have focused on the atmosphere and deep oceans, but in contrast, I focused on the upper ocean (the top tens to a few hundred meters in the water column) where we see the oxygen minimum zones in the present oceans. This is a critical zone where animals have diversified most dramatically in Phanerozoic (542 million years ago (Ma) to present day) and where geologists have the best fossil record for biology/environment comparison. I used a novel proxy, the iodine-to-calcium ratio (I/Ca) in carbonate rocks and planktic microfossils, which can track oxygen changes over a range of values where most modern marine animals are sensitive, i.e., at higher oxygen levels rather than euxinia (containing H2S) or anoxia (no O2). Four major projects in my dissertation are:

1) Phanerozoic upper ocean oxygenation history and its coevolution with life.

When and how oceanic oxygen had evolved to modern-like levels has remained elusive in Earth’s oxygen history, because few redox proxies can track secular variations in dissolved oxygen concentrations around threshold values for metazoan survival in the upper ocean. To address this question, we measured I/Ca in an extensive Phanerozoic collection of shallow marine carbonates and simulated marine iodine cycle in an Earth system model. We found that (1) I/Ca spiked during the Devonian, supporting a major rise in atmospheric O2 at ~400 Ma. (2) a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma, likely driven by a shift in organic matter remineralization to greater depths, which may be due to increasing size and biomineralization of eukaryotic plankton.

2) Planktic foraminiferal I/Ca proxy in the Southeast Atlantic Ocean.

Planktic foraminiferal I/Ca is a promising tool to reconstruct the extent of past upper ocean oxygenation, but a thorough assessment is necessary to evaluate both its potential and its limitations. We used foraminifers from Holocene core-tops (Southeast Atlantic Ocean) to document planktic I/Ca across a range of oceanographic conditions. We found that low planktic I/Ca can be used empirically to indicate hypoxia (O2 < 70–100 µmol/kg) in the upper water column. At a site located in the Benguela Upwelling System, down-core I/Ca records suggested that only small changes occurred in upper ocean oxygenation during the past 240 ka, probably related to strong upwelling dynamics in this region.

3) Bottom water oxygen changes in the glacial oceans and their driving mechanisms.

Reliable, quantitative paleo-O2 data is needed to test whether climate models can replicate past climate conditions in order to improve the forecast of future oceanic oxygenation changes under possible global warming. To address this question, we developed I/Ca in the benthic foraminiferal Cibicidoides spp. as a novel semi-quantitative bottom water oxygen proxy. We then applied this proxy to five ocean drilling cores to reconstruct the bottom water oxygen levels in the glacial-interglacial oceans. Using a multi-proxy approach, we found that low-O2 water (< 50 µmol/kg) may have been more extensive in the glacial Atlantic and Pacific Oceans compared to modern/Holocene, and the driving mechanisms for glacial deoxygenation may vary by ocean basin.

4) Direct comparison of benthic foraminiferal surface porosity and I/Ca proxies.

Benthic foraminiferal surface porosity (the mean percentage of surface area covered by pores; higher porosity: lower oxygenation) and I/Ca (higher I/Ca: higher oxygenation) are both promising paleoceanographic proxies that need testing in down-core studies. Here we report the first down-core comparison (~45 kyr) of these proxies in a core from a cold seep site on the southern Brazilian margin (26°40′S, 46°26′W, 475 m water depth). The two proxies are overall consistent, with porosity values generally low (< 10%) and I/Ca ranges between ~4 and ~6 µmol/mol throughout the core, suggesting that bottom water oxygen concentrations at the site remained above 50 µmol/kg during the last 45 kyr. Potential seafloor methane release during the last glacial period (40-20 ka), as indicated by anomalously negative δ13C values in foraminifera, apparently had limited impact on bottom water oxygenation, and interactions between competing processes potentially affecting bottom water oxygenation (i.e., water column stratification and productivity) may have limited the magnitude of changes in bottom water oxygen levels at the core site.

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