Identifying and Constraining Biocalcification Stress from Geologic Ocean Acidification Events

Kitch, Gabriella Dawn

doi:https://doi.org/10.21985/n2-xdv0-cv69

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Identifying and Constraining Biocalcification Stress from Geologic Ocean Acidification Events

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Anthropogenic consumption of fossil fuels releases carbon into the Earth-atmosphere-ocean system at rates unmatched by geologic processes. Yet, some geologic time periods also show evidence of climate forcing related to rapid natural greenhouse gas emissions and can serve as tools to help understand the future trajectory of climate. Geologic events that record ocean acidification (OA) may even inform feedback loops and ecosystem responses to modern OA. Geologic studies that examine carbon cycle perturbations typically focus on a global response rather than effects on local ecosystems and organisms. However, organisms that make up ocean ecosystems have profound effects on climate regulation. This dissertation research attempts to quantify the biotic response to OA events during two distinct time periods in Earth’s history utilizing a multiproxy approach of high-precision Thermal Ionization Mass Spectrometer techniques, namely the calcium (δ44/40Ca), radiogenic (87Sr/88Sr) and stable strontium (δ88/86Sr) isotope systems. OA events are transient disruptions to the dissolved carbonate chemistry of seawater that involve decreases in pH and carbonate mineral saturation states (Ω). Sediment records from the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) and Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2; ~94 Ma) show a decrease in the depth of carbonate burial, or shoaling in the calcite compensation depth, which indicates a decrease in Ω. In addition, both events are associated with biotic turnovers. Geochemical proxies can provide more quantitative information about carbonate precipitation rates and output. Theoretical models, laboratory experiments, and analyses of natural samples all point to precipitation-rate control of carbonate δ44/40Ca values. The 87Sr/86Sr proxy reflects inputs to the marine realm. Relatively high 87Sr/86Sr ratios indicate the dominance of continental silicate weathering inputs, while lower 87Sr/86Sr ratios indicate the dominance of mantle-derived inputs. While 87Sr/86Sr ratios provide information about the inputs, the stable Sr isotope system can constrain output via carbonate burial, where relatively high δ88/86Sr values indicate high carbonate burial rates and vice versa for low carbonate burial rates. Combining δ88/86Sr with δ44/40Ca and 87Sr/86Sr can constrain changes in carbonate production. This dissertation presents high-precision, high-resolution isotope records that identify biocalcification crises that occur in association with major geologic events. More specifically, foraminiferal δ44/40Ca records leading up to, and throughout the PETM (Chapter 2) and OAE2 (Chapter 3) show elevated values consistent with reduced precipitation rates during primary carbonate formation. Relatively heavy δ44/40Ca values across the PETM correspond with decreased boron isotope (δ11B) values across the interval, which are well-understood to reflect decreased seawater pH, as well as decreased carbon isotope (δ13C) values leading into the event. Overall, the comparison of multiple proxies indicates a volcanic driver of the PETM that resulted in carbonate chemistry stress. During OAE2, elevated δ44/40Ca values correlate with an increase in malformed specimens and decreased test diameters, both morphometric signs of organismal stress. The bulk and foraminiferal δ44/40Ca records across OAE2 also show increased values that correspond with perturbations in other isotopic proxies for Large Igneous Province emplacement (Chapter 3). Interestingly, in addition to providing evidence for OA prior to OAE2, the records also show potential evidence of ocean alkalinization and highlight OAE2 as a geologic case study to advance ocean carbon dioxide removal technologies. Complementary measurements of 87Sr/86Sr and δ88/86Sr across OAE2 (Chapter 4) are compared to δ44/40Ca values to test hypotheses surrounding early-diagenetic overprinting of the Western Interior Basin (WIB) sections and indicate that diagenesis is confined to discrete intervals, strengthening the fidelity of these records. In addition, Chapter 4 also presents and analyzes heterogeneous 87Sr/86Sr records that likely result from localized inputs to the Western Interior Basin (WIB) sections. The evidence of OA during OAE2 presented in this dissertation establish the event as a prime target for analysis of δ11B values, using a methodology developed during my PhD research (Appendix 4). Overall, the chapters of this dissertation demonstrate the utility of the high-precision, multi-proxy approach specifically to increase understanding of ecosystem effects during carbon cycle perturbations.

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