Cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments

Cenozoic records of TI isotope compositions recorded by ferromanganese (Fe-Mn) crusts have been obtained. Such records are of interest because recent growth surfaces of Fe-Mn crusts display a nearly constant TI isotope fractionation relative to seawater. The time-series data are complemented by results for bulk samples and leachates of various marine sediments. Oxic pelagic sediments and anoxic marine deposits can be distinguished by their TI isotope compositions. Both pelagic clays and biogenic oozes are typically characterized by epsilon(205)TI greater than +2.5, whereas anoxic sediments have epsilon(205)TI of less than -1.5 (epsilon(205)TI is the deviation of the (TI)-T-205/(TI)-T-203 isotope ratio of a sample from NIST SRM 997 TI in parts per 104). Leaching experiments indicate that the high epsilon(205)TI values of oxic sediments probably reflect authigenic Fe-Mn oxyhydroxides. Time-resolved TI isotope compositions were obtained from six Fe-Mn crusts from the Atlantic, Indian, and Pacific oceans and a number of observations indicate that these records were not biased by diagenetic alteration. Over the last 25 Myr, the data do not show isotopic variations that significantly exceed the range of TI isotope compositions observed for surface layers of Fe-Mn crusts distributed globally (epsilon(205)TI = +12.8 +/- 1.2). This indicates that variations in deep-ocean temperature were not recorded by TI isotopes. The results most likely reflect a constant TI isotope composition for seawater. The growth layers of three Fe-Mn crusts that are older than 25 Ma show a systematic increase of epsilon(205)TI with decreasing age, from about +6 at 60-50 Ma to about +12 at 25 Ma. These trends are thought to be due to variations in the TI isotope composition of seawater, which requires that the oceans of the early Cenozoic either had smaller output fluxes or received larger input fluxes of TI with low epsilon(205)TI. Larger inputs of isotopically light TI may have been supplied by benthic fluxes from reducing sediments, rivers, and/or volcanic emanations. Alternatively, the TI isotope trends may reflect the increasing importance of TI fluxes to altered ocean crust through time. (C) 2004 Elsevier B.V. All rights reserved.