Uranium Isotopes and Ocean Anoxia during Cretaceous Ocean Anoxic Event 2
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Kulenguski, Joseph
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Abstract
Ocean Anoxic Events (OAEs) are discrete periods in Earth history of decreased marine oxygen concentrations. They are often associated with volcanism, marked by increased organic carbon burial in the oceans, and positive carbon isotope excursions (δ13C) in the marine carbonate record. One such event, OAE 2, occurred roughly ~94 mya and is associated with global deposition of organic rich black shales and an accompanying positive carbon isotope excursion recorded across multiple localities worldwide. Whereas the presence of black shale is indicative of a locally anoxic environment, it is difficult to determine the global extent of anoxia as many seafloor environments are not preserved in the geologic record. Therefore, a global redox proxy such as uranium isotopes must be used to determine the conditions of the global oceans. Here we investigated marine carbonates of two OAE 2 localities in Italy and Mexico, the Umbria-Marche Sequence and the Guerrero-Morelos Platform, respectively. The Umbria-Marche Sequence was a deep-water pelagic carbonate section, and the Guerrero-Morelos Platform was a shallow water platform carbonate section. A full geochemical analysis of both settings was completed to determine the depositional and diagenetic history of each succession. The diagenesis was analyzed using thin sections from Italy and Mn, Sr, Mn/Sr ratio, Fe, and δ18O values from each section. With the exception of an interval at ~30m in Mexico there is minimal diagenetic influence in both sections. Productivity and sediment sources were measured using Ba, Zn, Ni, and P concentrations. These elements have higher concentrations in Italy representing a signature from surface water biogenic materials, as opposed to Mexico which shows a mixed source including skeletal materials, carbonate mud, and early marine cements. The uranium concentrations in Italy were too low to accurately measure the δ238U values due to the pelagic nature of the carbonates. This is due to a few factors including the calcitic mineralogy of the carbonates that incorporate significantly less U than aragonite, as well as the exclusively biogenic nature of the carbonates from Italy, with biogenic carbonates known to incorporate less U than abiotic carbonates. In contrast, the Mexico section has much higher U concentrations and this is likely due to the presence of some abiotic carbonates in the platform section, including syn-sedimentary marine cements which likely have higher U concentrations. An important conclusion made from this study is that platform carbonates provide a better record for U isotopes than pelagic carbonates. δ238U values were measured in Mexico and started close to modern carbonate values at the base of the section indicating well-oxygenated oceans prior to OAE 2. There was an initial negative δ238U excursion that could indicate a brief pulse of anoxia pre-OAE 2, but it is immediately followed by re-oxygenation to modern carbonate values. At the start of OAE 2, as constrained by the beginning of the OAE 2 positive δ13C excursion, we can follow a large negative U isotope excursion from 25-55m before values return towards modern carbonate values at the top of the section. This indicates a large expansion of global marine euxinia during OAE 2 followed by a return to oxygenated oceans. In comparison to the coeval OAE 2 section from Eastbourne, UK., the δ238U record in the Mexico section appears to be less susceptible to diagenesis and offers a much better record of the global oceans across OAE 2. The Mexico U isotope excursion was matched by a positive δ13C excursion with excellent negative covariance. This indicates that the organic carbon burial associated with the positive δ13C excursion is directly linked to the expansion of marine euxinia represented by the negative δ238U excursion. This, in turn, strengthens the link between the carbon cycle and ocean redox across OAEs, with volcanism causing increased CO2 output, global warming, and an enhanced hydrological cycle. This drives increased weathering and nutrient runoff into the oceans that leads to enhanced primary productivity, eutrophication, and an expansion of marine anoxia. This study highlights the potential of using paired isotope geochemistry, such as δ13C and δ238U, to evaluate global processes across OAEs.
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Keywords
Oxygen, Cretaceous, Uranium isotopes, Ocean anoxic events, Euxinia, Carbonates