The studies presented in this dissertation integrate stable calcium (Ca) and radiogenic and stable strontium (Sr) isotope geochemistry to understand how the Earth system responds to significant perturbations in the global carbon cycle. The primary focus is on the effects of massive releases of volcanic CO2 for ocean carbonate geochemistry and the incorporation of Ca and Sr isotopes into carbonate-bearing minerals. The main tools used in these studies are δ44/40Ca and δ88/86Sr, which collectively represent recent advances in non-traditional stable isotope measurements. Analysis of δ88/86Sr necessarily includes measurement of traditional radiogenic Sr isotope ratios (87Sr/86Sr). Large and rapid releases of CO2 are expected to cause ocean acidification, which encompasses short-lived decreases in seawater pH, [CO32-], saturation states with respect to carbonate minerals, and carbonate burial. These changes exert fundamental controls on the fractionation of Ca and Sr isotopes between seawater and carbonate minerals, and thus, can be recorded by variations in the δ44/40Ca and δ88/86Sr signatures of carbonate sediments and rocks. Moreover, carbonate 87Sr/86Sr ratios are sensitive to changes in continental weathering driven by elevated atmospheric CO2 levels, and transient mass-flux imbalances between weathering inputs and carbonate burial outputs can alter seawater δ44/40Ca and δ88/86Sr values. I investigate these mechanisms during three times in Earth history proposed as ocean acidification events. First, I present δ44/40Ca, 87Sr/86Sr, and δ88/86Sr records across the end-Permian mass extinction and argue that CO2 release from the Siberian Traps Large Igneous Province (LIP) eruption, as well as contemporaneous sea-level fall, enhanced carbonate weathering and lowered seawater δ44/40Ca and δ88/86Sr values. Next, I apply the same tools to shallow-water carbonates that formed in the mid-Pacific Ocean during the early Cretaceous Oceanic Anoxic Event (OAE) 1a, which is associated with eruption of the Ontong Java Plateau LIP. Precipitation rate-dependent changes in fractionation best explain the strong correlation between δ44/40Ca and δ88/86Sr. The data suggest that biocalcifiers actively responded to changes in seawater carbonate chemistry and further reveal that shallow-water carbonates can preserve primary geochemical signals. Finally, I use the δ44/40Ca-87Sr/86Sr-δ88/86Sr multi-proxy to decipher the “cap carbonate precipitation mystery” of the Neoproterozoic Marinoan deglaciation. to decipher the “cap carbonate precipitation mystery” of the Neoproterozoic Marinoan deglaciation. I argue that the basal cap dolostone rapidly precipitated in a freshwater-dominated environment and that the overlying limestone formed in a meltwater-seawater mixed environment. In conclusion, these studies provide critical new insights into how the Earth system responds to rapid CO2 additions and demonstrate that the δ44/40Ca, 87Sr/86Sr, and δ88/86Sr proxies powerfully trace global carbon cycle perturbations throughout Earth history.

Creator

• Wang, Jiuyuan

DOI

• https://doi.org/10.21985/n2-cp8a-yy78

Subject

• Geochemistry

Language

• en

Alternate Identifier

• etdadmin_upload_793208
• http://dissertations.umi.com/northwestern:15455

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright