Molybdenum isotope signatures of mid-Cretaceous black shales: Implications on the spatial dimension of anoxic events?
Final Report Abstract
The overall goal of this study was to develop, apply and test Mo- (original proposal) and additionally U isotope systematic of black shales from Mesozoic oceanic anoxic events (OAEs) as potential paleo-redox tracer to quantify the expansion of seafloor anoxia during these events compared to today. Both isotope systems display isotope fractionation between oxic and anoxic sinks and have such the potential to be used as paleo-redox proxy. Initial results from the first funding period showed that both, Mo and U isotopes systematics of black shales from two important OAEs (i.e. the mid-Cretaceous OAE 2 and the lower Jurassic T-OAE), with long duration and large spatial distribution, show a significant offset towards lighter isotope compositions relative to that from black shales from the modern Black Sea; the latter were used as modern equivalent for the Mesozoic black shales. These results indicate a larger expansion of seafloor anoxia during both OAEs relative to today. The most important goal of the second funding period was to investigate the relationship between Mo- and U isotope systematics. This is particular important, as both isotope systems can also be affected by local redox variations which may result in misleading interpretations of Mo- and U isotope compositions of black shales. The investigation of further modern organic-rich sediments from the Baltic Sea and the Cariaco basin showed that 1) alternating anoxic and oxic conditions result in isotope records (particular for Mo) that are difficult to interpret, likely due to pore water Mo and U exchange; 2) there seems to be a significant difference, particular for U isotope fractionation, between relatively open (Cariaco basin) and very isolated basins, such as the Black Sea. The latter appears to be by far the best equivalent for depositional environments during OAE 2 and T-OAE (at the investigated sites). In constantly anoxic to strongly euxinic basins, Mo- and U isotopes seem to be similar affected by local redox variations, resulting in a correlation. Thus a combination of both isotopes systems, seems to provide the most reliable paleo-redox proxy. Applying this new tool to the investigated Mesozoic samples indicates 1) prolonged expansion of seafloor anoxia (by 1-2 orders of magnitude) during large parts of the mid-Cretaceous around OAE 2, but no significant further enhancement of seafloor anoxia during OAE 2 compared to the periods before and after it. 2) Seafloor anoxia during the T-OAE were expanded to a similar degree than during OAE 2, but ocean anoxia recovered after the T-OAE to almost modern levels. As indicated by additional S isotope analyses on carbonate associated sulfate, sea level changes and basin isolation also played an important role in the basin evolution, however, were not the driver for the evolution of oceanic anoxia during the T-OAE within the European Epicontinental Seaway.
Publications
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2008. Fractionation of 238U/235U in modern and ancient anoxic marine environments: A paleoredox tracer? Goldschmidt conference meeting, Vancouver, 2008. Geochimica et Cosmochimica Acta, Volume 72.2008, Issue 12, p. A646.
Montoya Pino, C., Weyer, S., van de Schootbrugge, B., Oschmann, W.
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2008. Mo- and U-isotope variations in black shales: Potential tracers for the quantification of oceanic anoxia. IODP/ICDP Kolloquium Hannover, 12.-14.03.2008, pp. 139.
Weyer, S., Montoya Pino, C., Pross, J., Oschmann, W.
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2009. Global versus regional anoxia during the OAE-2 and the T-OAE: constraints from Mo and U isotopes. Meeting of the Geologische Vereinigung, Göttingen, Goldschmidt Conference Abstracts 2009, pp. A897.
Montoya Pino, C., Weyer, S., van de Schootbrugge, B., Anbar, A.D., Arz, H.W., Oschmann, W., Pross, J.
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2010. Global enhancement of ocean anoxia during the OAE-2: A quantitative approach using U isotopes. Geology, Vol. 38. 2010, Issue 4, 315-318.
Montoya-Pino, C., Weyer, S., van de Schootbrugge, B. Anbar, A.D., Oschmann, W., Arz, H.W., Pross, J.
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2010. Quantification of the spatial enhancement of ocean anoxia during the OAE2 and the T-OAE using Mo and U isotope signatures. IODP-Kolloquium, Frankfurt, 2010, Geophysical Research Abstracts
Vol. 12, EGU2010-4252-2.
Montoya Pino, C., Weyer, S., Anbar, A.D., van de Schootbrugge, B., Oschmann, W., Pross, J., Arz, H.W.
