Quantifying OAE2 marine oxygen levels from coupled Mo and U isotopes: A Tethyan perspective.

Abstract

U and Mo isotopes are promising geochemical proxies for globally averaged ocean redox conditions in deep time. Because the Mo and U isotope compositions of seawater are controlled primarily by variation in ocean redox conditions, coupled Mo and U isotope records can be used to reconstruct variation in the relative sizes of oxic/suboxic, anoxic and euxinic sinks over geological time. The different sensitivities of Mo and U to anoxic and euxinic conditions creates a situation in which joint evaluation of the Mo and U isotope records provide greater constraints on variation in ocean redox conditions than either proxy can in isolation. Here we report new and published Mo- and U-isotope data from shallow and deep marine black shales spanning the OAE2 along the subtropical Western Tethys. These data are used to infer the evolution of the seawater Mo and U isotope compositions and to investigate changes in marine oxygen levels during this interval. Because the incorporation of Mo and U isotopes into organic rich sediments is influenced by local bottom water redox conditions, we also report Fe speciation data from which oxygen levels in the marine substrate were determined in a sample-bysample basis. Our Fe speciation data suggest expansion of the Oxygen Minimum Zones (OMZs) into the shallow western Tethys ocean waters; as well as the presence of an oxygenated deep Tethys ocean. These differences in bottom water redox conditions explain observed heterogeneities on black shale Moisotope compositions within the Western Tethys. Box modeling of the seawater coupled Mo- and U- isotope system suggest that anoxic and euxinic bottom water redox conditions occupied approximately 40 % and 15 % of the marine substrate respectively. These estimates are in accordance with previous estimates based on U-isotopes. The similar evolution of the seawater Mo, U and C isotope compositions suggest enhanced shallow marine anoxia as an important mechanism controlling the C-cycle during the OAE2. The observed heterogeneities in ocean bottom water redox conditions may explain the differences on the degree of affection of marine life during the OAE2.