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      Reconstructing Neoproterozoic seawater chemistry from early diagenetic dolomite

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          Abstract

          The pairing of calcium and magnesium isotopes (δ44/40Ca, δ26Mg) has recently emerged as a useful tracer to understand the environmental information preserved in shallow-marine carbonates. Here, we applied a Ca and Mg isotopic framework, along with analyses of carbon and lithium isotopes, to late Tonian dolostones, to infer seawater chemistry across this critical interval of Earth history. We investigated the ca. 735 Ma Coppercap Formation in northwestern Canada, a unit that preserves large shifts in carbonate δ13C values that have been utilized in global correlations and have canonically been explained through large shifts in organic carbon burial. Under the backdrop of these δ13C shifts, we observed positive excursions in δ44/40Ca and δ7Li values that are mirrored by a negative excursion in δ26Mg values. We argue that this covariation is due to early diagenetic dolomitization of aragonite through interaction with contemporaneous seawater under a continuum of fluid- to sediment-buffered conditions. We then used this framework to show that Tonian seawater was likely characterized by a low δ7Li value of ∼13‰ (∼18‰ lower than modern seawater), as a consequence of a different Li cycle than today. In contrast, δ13C values across our identified fluid-buffered interval are similar to modern seawater. These observations suggest that factors other than shifts in global seawater chemistry are likely responsible for such isotopic variation.

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          Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis

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            Diagenetic Models and Their Implementation

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              Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering.

              Weathering of uplifted continental rocks consumes carbon dioxide and transports cations to the oceans, thereby playing a critical role in controlling both seawater chemistry and climate. However, there are few archives of seawater chemical change that reveal shifts in global tectonic forces connecting Earth ocean-climate processes. We present a 68-million-year record of lithium isotopes in seawater (δ(7)Li(SW)) reconstructed from planktonic foraminifera. From the Paleocene (60 million years ago) to the present, δ(7)Li(SW) rose by 9 per mil (‰), requiring large changes in continental weathering and seafloor reverse weathering that are consistent with increased tectonic uplift, more rapid continental denudation, increasingly incongruent continental weathering (lower chemical weathering intensity), and more rapid CO(2) drawdown. A 5‰ drop in δ(7)Li(SW) across the Cretaceous-Paleogene boundary cannot be produced by an impactor or by Deccan trap volcanism, suggesting large-scale continental denudation.
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                Author and article information

                Journal
                Geology
                Geological Society of America
                0091-7613
                1943-2682
                December 10 2020
                April 01 2021
                December 10 2020
                April 01 2021
                : 49
                : 4
                : 442-446
                Affiliations
                [1 ]Weizmann Institute of Science, Rehovot 76100, Israel
                [2 ]Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
                [3 ]Commonwealth Scientific and Industrial Research Organisation (CSIRO), Kensington, WA 6151, Australia
                [4 ]Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA
                [5 ]Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
                [6 ]Department of Earth and Planetary Sciences, McGill University, Montreal, QC H3A 0E8, Canada
                Article
                10.1130/G48213.1
                649bf4fa-a26d-4d3b-a29d-681a65c2746c
                © 2021
                History

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