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Incorporation and preservation of molybdenum and uranium isotope variations in modern marine sediments

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Molybdenum and uranium isotope variations are potentially powerful tools for reconstructing the paleoredox history of seawater. Reliable application and interpretation of these proxies requires not only detailed knowledge about the

Molybdenum and uranium isotope variations are potentially powerful tools for reconstructing the paleoredox history of seawater. Reliable application and interpretation of these proxies requires not only detailed knowledge about the fractionation factors that control the distribution of molybdenum and uranium isotopes in the marine system, but also a thorough understanding of the diagenetic processes that may affect molybdenum and uranium isotopes entering the rock record. Using samples from the Black Sea water column, the first water column profile of 238U/235U variations from a modern euxinic basin has been measured. This profile allows the direct determination of the 238U/235U fractionation factor in a euxinic marine setting. More importantly however, these data demonstrate the extent of Rayleigh fractionation of U isotopes that can occur in euxinic restricted basins. Because of this effect, the offset of 238U/235U between global average seawater and coeval black shales deposited in restricted basins is expected to depend on the degree of local uranium drawdown from the water column, potentially complicating the interpretation 238U/235U paleorecords. As an alternative to the black shales typically used for paleoredox reconstructions, molybdenum and uranium isotope variations in bulk carbonate sediments from the Bahamas are examined. The focus of this work was to determine what processes, if any, fractionate molybdenum and uranium isotopes during incorporation into bulk carbonate sediments and their subsequent diagenesis. The results demonstrate that authigenic accumulation of molybdenum and uranium from anoxic and sulfidic pore waters is a dominant process controlling the concentration and isotopic composition of these sediments during early diagenesis. Examination of ODP drill core samples from the Bahamas reveals similar behavior for sediments during the first ~780ka of burial, but provides important examples where isolated cores and samples occasionally demonstrate additional fractionation, the cause of which remains poorly understood.

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  • 2012

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Investigations into Crustal Composition and Oxidative Weathering in the Archean

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Archean oxidative weathering reactions were likely important O2 sinks that delayed the oxygenation of Earth’s atmosphere, as well as sources of bio-essential trace metals such as Mo to the biosphere.

Archean oxidative weathering reactions were likely important O2 sinks that delayed the oxygenation of Earth’s atmosphere, as well as sources of bio-essential trace metals such as Mo to the biosphere. However, the rates of these reactions are difficult to quantify experimentally at relevantly low concentrations of O2. With newly developed O2 sensors, weathering experiments were conducted to measure the rate of sulfide oxidation at Archean levels of O2, a level three orders of magnitude lower than previous experiments. The rate laws produced, combined with weathering models, indicate that crustal sulfide oxidation by O2 was possible even in a low O2 Archean atmosphere.

Given the experimental results, it is expected that crustal delivery of bio-essential trace metals (such as Mo) from sulfide weathering was active even prior to the oxygenation of Earth’s atmosphere. Mo is a key metal for biological N2 fixation and its ancient use is evidenced by N isotopes in ancient sedimentary rocks. However, it is typically thought that Mo was too low to be effectively bioavailable early in Earth’s history, given the low abundances of Mo found in ancient sediments. To reconcile these observations, a computational model was built that leverages isotopic constraints to calculate the range of seawater concentrations possible in ancient oceans. Under several scenarios, bioavailable concentrations of seawater Mo were attainable and compatible with the geologic record. These results imply that Mo may not have been limiting for early metabolisms.

Titanium (Ti) isotopes were recently proposed to trace the evolution of the ancient continental crust, and have the potential to trace the distribution of other trace metals during magmatic differentiation. However, significant work remains to understand fully Ti isotope fractionation during crust formation. To calibrate this proxy, I carried out the first direct measurement of mineral-melt fractionation factors for Ti isotopes in Kilauea Iki lava lake and built a multi-variate fractionation law for Ti isotopes during magmatic differentiation. This study allows more accurate forward-modeling of isotope fractionation during crust differentiation, which can now be paired with weathering models and ocean mass balance to further reconstruct the composition of Earth’s early continental crust, atmosphere, and oceans.

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  • 2020