Date of Award

May 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Zunli Lu

Keywords

greenhouse climate, I/Ca, I/TOC, OAE 2, PETM, redox

Subject Categories

Physical Sciences and Mathematics

Abstract

Anthropogenic global warming affects marine ecosystems in complex ways, and declining ocean oxygenation is a growing concern. Forecasting the geographical and bathymetric extent, rate, and intensity of future deoxygenation and its ecological effects, however, remains highly challenging because of the complex feedbacks in the climate-ocean-biota system. Global warming lowers the solubility of oxygen in the ocean and drives an enhanced hydrological cycle with increased nutrient delivery to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to strong marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially at the interface between oxic and anoxic conditions. To address this problem, I use I/Ca in bulk carbonate and I/TOC in black shale to reconstruct redox changes in the upper and deep waters in multiple ocean basins through the Cretaceous OAE 2 at the Cenomanian-Turonian boundary (94 Ma). The iodine proxy records are interpreted in the context of a wide range of other geochemical data and Earth system biogeochemical model simulations.

Iodate and iodide are the most thermodynamically stable iodine ions in seawater. The speciation of iodine depends on the redox condition of the ambient seawater. Iodate is abundant in oxic water, and iodide dominates in anoxic water. Iodate is the only iodine ion that can precipitate with carbonate, and carbonate associated iodine increases with iodate concentration in the matrix solution. Therefore, I/Ca values in carbonate can be used as a redox proxy of seawater.

In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and variability in I/Ca through the OAE interval, reflecting the importance of local controls and potentially suggesting a high spatial variability in redox state. Both I/Ca and cGENIE model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, highlighting important regional differences in redox conditions during OAE 2. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.

I/TOC and Iorg records were generated from six sections in proto-North Atlantic and southern Indian Ocean. I/TOC in modern surface and subsurface sediment decreases with decreasing bottom-water oxygen, a feature potentially useful for reconstructing seafloor paleo-redox. I/TOC decreases into OAE 2, suggesting more reducing bottom-water conditions. Higher I/TOC values (more oxic conditions) are recorded in two high-latitude and possibly in one low-latitude site, where higher oxygen levels are also indicated by other redox indicators and Earth System Modeling by cGENIE. I/TOC and I/Ca records at a near-equatorial coastal site in Morocco (Tarfaya) decreased during OAE 2, likely suggesting reducing water throughout the water column due to strong vertical mixing and shallow paleo-depth, consistent with a wide range of other proxies. These observations indicate that coeval carbonate and shale iodine records may be able to depict oxygenation changes in different parts of the water column, and reflect the change in marine iodine reservoir.

Another rapid global warming event, the Paleocene-Eocene Thermal Maximum (PETM, ~55.5 Ma), may represent a better analog for present and future global warming. I apply the iodine to calcium ratio (I/Ca) in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans, and compared the reconstruction with modeled oxygen levels. The reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the Atlantic, Indian Oceans, and possibly the Pacific Ocean, as well during the PETM, due to vertical and potentially lateral expansion of OMZs.

Overall, this thesis further establishes the use of iodine as a proxy for ocean oxygen levels in the geological history.

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