Latitude-Dependent Climatic Responses to Milankovitch Orbital Forcing and its Expression in Cretaceous Rocks of the Northern Hemisphere

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Latitude-Dependent Climatic Response to Milankovitch Orbital Forcing and its Expression in Cretaceous Rocks of the Northern Hemisphere Bradley B. SagemanEAR-0001093 The proposed research seeks to document and constrain changes in the behavior of the hydrologic cycle and atmospheric circulation patterns over North America during the mid-Cretaceous greenhouse. The key to this reconstruction is the preservation of orbital signals in hemipelagic deposits of the Western Interior basin. These deposits offer an opportunity to examine the effect of the same climatic changes on two fundamentally different pathways of sedimentation in the basin that are sensitive to changes in hydrologic and atmospheric conditions: detrital and biogenic. Because orbital forcing places predictable constraints on the behavior of these two pathways, a succession in which orbital parameters are quantified offers the possibility to deconvolve the relative roles of each. Ultimately, by placing processes like biogenic sedimentation within a quantified time series, greatly improves the ability to reconstruct biogeochemically significant events such as the flux of organic matter to sediments resulting from increased primary productivity. This component of the global carbon cycle, which has been inferred to play a major role in many critical events in Earth history, such as "oceanic anoxic events," is notoriously difficult to constrain in the ancient rock record. This project is based on recent quantitative documentation of orbital signals in rhythmic limestone-marlstone facies of Cenomanian-Turonian (C-T) age in the central Western Interior using spectral techniques. With that work as a foundation, PI will apply improved spectral methods to reconstruct detailed sedimentation rates for the C-T interval, and quantify changes in biomarker-type organic compounds based on the spectrally defined time scale. His data set will include samples from core comprising a paleolatitudinal transect of the basin. These two components will allow him to test the following key working hypotheses: (1) Does limestone-marlstone formation reflect climatic forcing of siliciclastic dilution, carbonate productivity, or a combination of both? (2) Does the unique pattern of bedding in the C-T deposits reflect latitude-dependent forcing of different orbital parameters through different depositional pathways? Specifically, does obliquity dominate detrital flux through forcing of the higher latitude hydrologic cycle, and precession dominate biogenic flux (carbonate) through forcing of lower latitude surface water conditions? and (3) How are the Milankovitch scale variations expressed in limestone-marlstone facies influenced by longer-term changes in the ocean-climate system, such as the global carbon burial and ocean anoxic event termed OAE II? Can he detect feedback in the carbon cycle-climate relationship via changes in latitude-dependent orbital forcing (i.e., did obliquity become more dominant due to carbon burial, CO2 drawdown, and climatic cooling)? The application of an integrated quantitative cyclostratigraphic-organic geochemical methodology represents a significant innovation in the study of ancient climate-ocean dynamics and will significantly advance knowledge of the Milankovitch climate-sedimentation linkage during the mid-Cretaceous greenhouse.