International Geologiical Congress - Oslo 2008


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OSP-01 General contributions to marine geoscience & paleoceanography


Paleoceanographic and paleoclimate proxies from pelagic marl-limestone geochemistry and rock magnetism, Lower Kimmeridgian, Vocontian Basin, France


Slah Boulila, Université Pierre et Marie Curie (France)
Bruno Galbrun, Université Pierre et Marie Curie (France)
Linda A. Hinnov, Johns Hopkins University (United States)
Marc de Rafelis, Université Pierre et Marie Curie (France)
Pierre-Yves Collin, Université Pierre et Marie Curie (France)


Chemical (%CaCO3, Mn, δ13C, and δ18O) and magnetic susceptibility (MS) analysis of Lower Kimmeridgian (Platynota ammonite zone p.p.) at Chteauneuf-d'Oze, SE France, reveals a primary sedimentary cyclicity controlled by Earth's orbital parameters (precession, obliquity, and eccentricity). MS coupled with magnetic hysteresis experiments, and the %CaCO3 record provide strong evidence for combined detrital input (dilution cycles) and marine carbonate production (productivity cycles). These two synchronous processes were driven by the precession index and "fingerprinted" in the basic marl-limestone couplets, modulated by orbital eccentricity. Mn coupled with cathodoluminescence (CL), and δ13C record redox cycles in which the marls were deposed under oxidizing conditions, and the limestones under reducing conditions. The redox process appears to be preferentially controlled by obliquity cycles. δ18O, although affected in part by diagenesis, records temperature controlled by precession index cycles. In light of our previous study of the Chteauneuf-d'Oze section indicating that 405-kyr eccentricity cycles were involved in third-order depositional sequences, we suggest a depositional model that links paleoclimate to sea-level changes in the formation of couplets and supercouplets: insolation maxima induced warm conditions, high sea-level and formation of the limestones; insolation minima induced cold conditions, low sea-level and formation of the marls. The nonlinear responses of the oceanic-climatic system to orbital forcing via the studied proxies, i.e. obliquity expressed itself in redox cycles and detrital input and marine carbonate production were sensitive to precession index, is a major discovery in this Kimmeridgian sedimentary sequence. Here, we interpret these differential responses as follow: insolation intensity by precession dynamics may have been involved in the forcing of detrital flux to the basin operating antagonistly to the oceanic carbonate productivity; whereas total summer energy by obliquity dynamics involved in the delivery of oxygenated water to the sea floor and then leading to redox cycles.


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