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      Precision in Biostratigraphy: Evidence For a Temporary Flow Reversal in the Central American Seaway During Or After the Oligocene-miocene Transition

      1 , 2 , 1 , 3 , 1
      Journal of Foraminiferal Research
      GeoScienceWorld

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          Abstract

          The Oligocene-Miocene Transition (OMT) was a time of significant oceanic, climatic, and biotic change, but there is still a great deal we do not understand about its effects, particularly in terms of ocean circulation. The Central American Seaway (CAS) was an important ocean gateway at this time; recent fully coupled modeling results have suggested a possible temporary reversal of surface flow, from westward to eastward, during the OMT. Such a flow reversal would have altered numerous oceanographic properties and the dispersal of marine taxa. Here, we find a mismatch in the timing of the Atlantic vs. Pacific first appearances of the tropical mixed layer planktic foraminifer Paragloborotalia kugleri, a key zonal marker for the OMT. The first appearance ages for P. kugleri from fourteen ocean drilling sites vary from ∼23.2–23.05 Ma in the Pacific to ∼23.05–22.7 Ma in the Atlantic. Key requirements for including a site in this compilation are: 1) sampling resolution; 2) independent non-biostratigraphic chronology, such as magnetostratigraphy or orbital tuning; and 3) a preference for shore-based biostratigraphic analyses rather than shipboard estimates. Although we explore alternative explanations, we conclude that, given the restricted nature of the CAS gateway, timing of dispersal, and results from previous modeling efforts, CAS flow reversal is the most parsimonious explanation for the delayed first appearance of P. kugleri in the Atlantic relative to the Pacific. We suggest that after originating in the tropical Pacific, P. kugleri was initially blocked from dispersal into the Atlantic by westward surface circulation through the CAS during the latest Oligocene. During the OMT, circulation reversed and Pacific surface water flowed through the CAS into the Atlantic, allowing P. kugleri to disperse into the Atlantic. Previously published ocean-climate simulations suggest that the cause of this reversed flow may be related to the progressive constriction of Tethys and opening of the Drake Passage at the time of the OMT, compounded by a short-lived glaciation event in Antarctica and possible change in meridional temperature gradient and prevailing wind patterns in the tropics.

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          Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean.

          The ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth. At present, this depth is approximately 4,500 m, with some variation between and within ocean basins. The calcite compensation depth is linked to ocean acidity, which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate. Geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the Eocene/Oligocene boundary (approximately 34 million years ago) when significant permanent ice sheets first appeared on Antarctica, but the relationship between these two events is poorly understood. Here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical Pacific Ocean that cover the Eocene/Oligocene boundary. We find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40,000 years each, synchronous with the stepwise onset of Antarctic ice-sheet growth. The glaciation was initiated, after climatic preconditioning, by an interval when the Earth's orbit of the Sun favoured cool summers. The changes in oxygen-isotope composition across the Eocene/Oligocene boundary are too large to be explained by Antarctic ice-sheet growth alone and must therefore also indicate contemporaneous global cooling and/or Northern Hemisphere glaciation.
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            Review and revision of Cenozoic tropical planktonic foraminiferal biostratigraphy and calibration to the geomagnetic polarity and astronomical time scale

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              The heartbeat of the Oligocene climate system.

              A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced "heartbeat" in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2-million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans.
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                Author and article information

                Journal
                Journal of Foraminiferal Research
                GeoScienceWorld
                0096-1191
                October 23 2019
                October 23 2019
                : 49
                : 4
                : 357-366
                Affiliations
                [1 ]Department of Geosciences, University of Massachusetts – Amherst, 627 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003-9297
                [2 ]Current Affiliation: School of Earth Sciences, University of Bristol, Willis Memorial Building, Queens Road, Bristol BS8 1RJ
                [3 ]Institute for Geophysics, Jackson School of Geosciences, University of Texas Austin, 2305 Speedway Stop C1160, Austin, TX 78712-1692
                Article
                10.2113/gsjfr.49.4.357
                7be035ff-753c-48ef-a5fe-f4f3b73f8d28
                © 2019
                History

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