Late Cretaceous Carbon Isotope Chemostratigraphy and Analysis of Perturbations in Global Carbon and Sulfur Biogeochemical Cycles

The Late Cretaceous (∼100-66Ma) is one of the most significant deep-time greenhouse periods of Earth history, characterized by major volcanigenic events, including eruption of Large Igneous Provinces (LIPs) and peak rates of ocean crust production, elevated atmospheric CO2 levels, significant increases in global mean temperature, maximum glacio- and tectonoeustatic sea level rise resulting in widespread development of carbonate platforms and shallow epicontinental seas, and the presence of numerous short-term (< 1 my) carbon cycle perturbations, such as the well-studied Oceanic Anoxic Events (OAE's), and many smaller scale events that are less intensively studied.;This dissertation mainly investigates perturbations in global biogeochemical cycles during the Late Cretaceous (Middle Cenomanian through Early Campanian) by compiling: 1) carbon isotope reference curves from the Western Interior Basin of North America, which contribute signifiicant data to the development of a global chemostratigraphic correlation framework for the Late Cretaceous; 2) an in-depth analysis, using data-model comparison techniques, of how different environmental factors, such as changing volcanism, continental weathering-organic carbon burial feedback, and sea-level change, affected the global carbon cycle during the Middle Cenomanian through Cenomanian-Turonian OAE2; and 3) data pertaining to the sulfur cycle during the Middle Cenomanian through early Turonian based on the sulfur isotope composition of pyrite from the Western Interior Seaway and an ODP core from the proto-Atlantic (Demerara Rise). An additional chapter addresses aspects of the modern biogeochemical cycle of nitrogen.;Chapter 2 reports a new, high-resolution δ13C curve from 3 cores in the central Western Interior basin of North Amerisca. Although the Western Interior has occasionally been interpreted as an isolated marginal basin of the global Cretaceous ocean, new composite carbon isotope data developed in this study provide a sensitive record of Late Cretaceous carbon cycle events, including both major (e.g. Mid-Cenomanian Event (MCE), OAE 2, and Late Turonian Event), and minor carbon isotope excursions that are recognized and correlated in records from Europe to Asia, reflecting changes in the global carbon cycle during Cenomanian to Campanian time. This new chemostratigraphic correlation framework, integrated with a revised Late Cretaceous time scale and trans-Atlantic biostratigraphic correlations, allows export of revised ages for Late Cretaceous events to other localities within and outside the basin,, and contributes to the development of a consistent global chronostratigraphy for the Late Cretaceous.;Chapter 3 focuses on an interesting nuance in the details of carbon isotope chemostratigraphy that has significant implications for the interpretation of Late Cretaceous events. The trend in δ13C compositions of organic carbon in the Western Interior vs. carbonates in Europe, measured during the time spanning the MCE to OAE 2, suggests significant changes in the global carbon cycle. By adopting a simple numerical modeling approach, we are able to argue that release of CO2 from volcanism, which may have started following the termination of the MCE, played a key role in driving observed variations in carbon isotope curves. This view suggests that increased atmospheric pCO2 resulted in a weathering - organic carbon burial feedback and larger carbon isotopic fractionation between δ 13Corg and δ13Ccarb.;In Chapter 4, the sulfur isotope composition of pyrite was studied to assess responses of the sulfur cycle to two different large-scale organic carbon burial events, MCE and OAE2. A significant large positive excursion in δ34S in pyrite, observed in the Western Interior and Demerara Rise during the MCE, is interpreted to reflect low sulfate conditions that prevailed during the Middle Cenomanian. These new sulfur isotope records provide supporting evidence for the importance of lowered oceanic sulfur concentrations for regulating the sulfur and carbon cycle during the Late Cretaceous.;Chapter 5 examines the effects of human perturbations on the pre-Anthropocene global nitrogen cycle, which is built to approximate the steady state mass and isotope composition of reservoirs distributed on land and in the oceans. The impact of human interventions since the 1700's is most significant on land, primarily driven by land-use changes.