Stratigraphic and Geochemical Investigations of Late Cretaceous Oceanic Anoxic Events and Greenhouse Sea Level Trends

Jones, Matthew Madden

doi:https://doi.org/10.21985/n2-g408-vn51

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Stratigraphic and Geochemical Investigations of Late Cretaceous Oceanic Anoxic Events and Greenhouse Sea Level Trends

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This dissertation researches the Cenomanian, Turonian, and Coniacian Stages (100.5 - 86.3 Ma) of the Cretaceous Period, a geologic interval of interest due to its extreme global warmth, iconic faunas, and oceanic anoxic events (OAEs). Five globally dispersed geochemical and stratigraphic case studies reconstruct interactions of the mid-Cretaceous carbon cycle with large igneous province (LIP) volcanism, astronomical cycles (i.e., Milankovitch cycles), sea level oscillations, and paleoclimatic and paleoceanographic conditions. Chapters 2 and 3 of the dissertation investigate deep sea and continental drill cores to characterize paleoclimatic conditions and geochemical cycling over multi-million year spans of time in the mid-Cretaceous. From rhythmically bedded cores of Demerara Rise in the tropical North Atlantic, Chapter 2 develops an astronomically-tuned time scale for the Turonian Stage, along with an interpretation for Turonian carbon burial dynamics driving globally-correlative carbon isotope signals. The history of terrestrial environmental conditions in the Cretaceous stratigraphic record is much sparser, including responses to OAEs in the continental environment. Chapter 3 presents a relatively rare and continuous ~5 million-year sedimentary record of Turonian-Coniacian geochemical data and paleoclimate insights from lacustrine shales of the Songliao Basin in northeastern China. Chapters 4 through 6 have a higher temporal resolution, focusing on a narrower 600-800 kiloyear time interval across the Cenomanian-Turonian Stage boundary (93.9±0.15 Ma), to investigate the causes, timing, and biotic consequences of a major Mesozoic oceanic anoxic event – OAE2. Chapter 4 presents the most expanded stratigraphic record of OAE2 in the central Western Interior Basin (U.S.) from the SH#1 core recovered during fieldwork in 2014. In addition to generating a finely resolved chemostratigraphic record of OAE2, the proximal SH#1 core preserves distinct geochemical signals of carbonate diagenesis, which trace to flooding surfaces in shoreface deposits nearby. Accordingly, these geochemical signatures record relative sea level through OAE2; they exhibit short eccentricity pacing of parasequences and, therefore, a climatic control on sea level oscillations. Chapter 5 further utilizes stratigraphically expanded and conformable records from the Western Interior to quantify the timing of large igneous province volcanism that triggered OAE2, along with local environmental responses and global phenomenon, such as expanded anoxia. Using this refined chronostratigraphy of OAE2, Chapter 6 assesses the ramifications of rapid rates of LIP volcanism and CO2 degassing on the marine ecosystem and geochemistry, in particular the effects on marine carbonate chemistry. A global compilation of carbonate contents through OAE2 detects a shoaling of the ocean’s carbonate compensation depth (CCD) at pelagic sites while carbonate sedimentation continued at shallower epicontinental sites. Consistent findings of a shoaling CCD, model reconstructions of volcanic fluxes, and morphological alterations in calcifying marine taxa, suggest that ocean acidification, in addition to namesake anoxia, occurred during OAE2, and may have been a fundamental paleoceanographic response to episodes of LIP volcanism through the Mesozoic.

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