Evolution of ocean oxygen deficient zones during the Miocene: insights from and for the I/Ca paleo-oxygenation proxy
Creators: name: Hess, Anya Victoria
Publisher: name: No Publisher Supplied
Year: 2024

description: In recent decades, the oceans have been losing oxygen at a rate of &gt;~2% per decade, with the largest effect in already low oxygen shallow-water areas called oxygen deficient zones (ODZs). This threatens marine ecosystems, including fish that billions of people rely on for food and economic stability. Understanding how ocean oxygenation will change with continued climate change is therefore vital. Models agree that deoxygenation will continue to at least the year 2100, but beyond this have not reached a consensus. By reconstructing ocean oxygenation in the past, particularly during warm periods analogous to our future climate, we can help resolve this discrepancy. Chapter 1 of this dissertation (Zhou, Hess et al., 2022) develops a methodology for measuring iodine-to-calcium (I/Ca) in foraminifera along with other trace elements used for paleoceanographic research (e.g., Mg/Ca as a proxy for ocean temperatures). The I/Ca ratio in planktonic foraminifera, micro-organisms that live in the water column and create carbonate hard parts that are preserved in the fossil record, responds to oxygen concentrations in the upper water column. Because iodine is volatile, it has traditionally been measured in solutions with basic pH, where it is more stable. However, this means that it must be measured separately from other trace elements, limiting its utility. This chapter tests the feasibility of measuring I/Ca in acidic solutions using experiments on a standard solution with known elemental composition, in a carbonate coral standard widely measured during I/Ca analyses, and in planktonic foraminifera. Results show that with proper handling iodine is not lost from solution over the course of a typical 8-hour run, that I/Ca values in the coral standard are similar when measured in acidic and basic solutions, and that I/Ca in foraminifera follow trends observed in data measured in basic solutions. Thus, we conclude that I/Ca can be measured alongside other trace elements in acidic solutions, broadening the applicability of this proxy and opening the door for further proxy development via comparison with other paleoceanographic data. Chapter 2 examines relationship between I/Ca ratios in core-top planktonic foraminifera and iodate and oxygen concentrations in the overlying water column. In samples from an east-west transect of sites through the southern lobe of the ETP ODZ (S-ETP ODZ), I/Ca values are higher than expected for sites with relatively low minimum oxygen concentrations (&lt;100 µmol/kg), with some I/Ca values exceeding 4 µmol/mol. This can be explained by higher iodate concentrations in this lobe of the ODZ than in the northern lobe and in the Arabian Sea. We therefore urge caution in applying a global calibration, particularly in the S-ETP. However, lower I/Ca values in the core of the ODZ suggests that the proxy is still a good indicator of relative oxygenation. The relatively short distance (≤~600 n mi) over which iodate, oxygenation, and I/Ca shifts occur in this area suggest that caution should be taken when interpreting paleoceanographic records where sites have moved tectonically towards or away from the heart of an ODZ. We also find that I/Ca values in species from different water depth habitats vary predictably based on patterns in iodate concentration spatially and with depth in the upper water column, possibly providing another line of evidence for identifying the core of an ODZ in paleoceanographic records. Finally, sites from areas with intermediate oxygen concentrations (~100-170 µmol/kg), plotted alongside all existing core top I/Ca data, suggest that I/Ca may correlate linearly with minimum water column oxygenation, rather than by thresholds as previously suggested. Chapter 3 (Hess et al., 2023) examines the response of oxygenation in the eastern tropical Pacific (ETP), today one of the largest and strongest ODZs in the world, to the Miocene Climatic Optimum (MCO; ~17.0-14.8 Ma), when temperatures and atmospheric pCO2 were similar to projections beyond the year 2100. Relatively high I/Ca ratios (&gt;4 µmol/mol) during the MCO at Site 845 suggest that the northern lobe of this ODZ was well oxygenated at that time, with minimum oxygen concentrations in the water column exceeding 100 µmol/kg. During the subsequent cooling, falling I/Ca beginning at ~15.1 Ma and rising nitrogen isotopes in foraminifera beginning at ~14.6 Ma indicate deoxygenation and denitrification, which suggests oxygen concentrations below 5 µmol/kg. We hypothesize that MCO oxygenation was caused by decreased Walker Circulation, the prevailing equatorial zonal wind cell, and resultant decreased upwelling and productivity in the ETP, leading to less drawdown of oxygen by remineralizing organic matter. Sea-surface temperature gradients both zonally and meridionally were lower during the MCO, suggesting weaker wind fields at that time, including Walker Circulation. As Walker Circulation today drives upwelling in the ETP, this likely led to weaker upwelling and ultimately oxygenation of the ETP. To the extent that the MCO can be taken as an analog for future climate change, this suggests that recent deoxygenation trends may ultimately reverse. Chapter 4 examines the oxygenation response of the Arabian Sea to Miocene climate change. Today, this area is another major ODZ, the result of upwelling caused by the Asian monsoon. In contrast to the I/Ca shift observed in the ETP in Chapter 3, Arabian Sea I/Ca ratios at Sites 730 and 714 are relatively low (&lt;2.5 µmol/mol) throughout the ~20-8 Ma study period, suggesting that oxygen concentrations were &lt;100 µmol/kg throughout that time. Based on records of redox-sensitive elements (Mn) and nitrogen isotopes in bulk sediments previous researchers have shown deoxygenation beginning at ~15 Ma. This coincides with increasing monsoonal activity also beginning at ~15 Ma. Here, we present new records from distal-ODZ Site 714 in the Maldives that suggest progressive expansion of the ODZ: at ~13.8 Ma, Mn/Ca falls and U/Ca increases in planktonic foraminiferal authigenic coatings, which indicating decreasing bottom water oxygenation, and nitrogen isotopes in bulk sediment samples begin to rise, indicating increasing denitrification. Considering these lines of evidence, we suggest that a proto-ODZ with oxygen concentrations &lt;100 µmol/kg existed since at least 20 Ma, consistent with reports of a proto-monsoon since 23 Ma, then expanded and deoxygenated beginning at ~15 Ma in response to monsoonal intensification. The persistence of the Arabian Sea ODZ throughout the MCO implies a role for regional processes, such as the Asian monsoon, and the coincident deoxygenation beginning at ~15 Ma in the ETP and Arabian Sea suggests a teleconnection between the two basins and a role for global processes. In total, this dissertation expands the utility of the I/Ca proxy by providing a methodology for measuring it alongside other trace elemental ratios. It reveals nuances of the proxy, suggesting that the relationship between I/Ca and oxygenation may be linear and showing that the relationship between I/Ca in species occupying different water depth habitats can give insights into the location of ODZs. It also identifies areas where caution should be taken in its application, particularly areas with anomalous iodate concentrations relative to oxygenation, areas with highly spatially variable oxygenation at oxygen concentrations near 100 µmol/kg, at sites that may have moved towards or away from an ODZ through the study period, and in certain species of foraminifera (Globoquadrina conglomerata, which has anomalously high I/Ca values for its water depth habitat, and Globorotalia tumida, which has low I/Ca values even outside of ODZs). Finally, the proxy has been used alongside other paleoceanographic proxies to reconstruct the response of two major ODZs to Miocene climate change, showing that both global and regional processes play a role in ODZ response to climatic warming.