Early diagenetic reactive transport modeling of uranium concentrations and 'stable' isotopes
Abstract
Uranium concentrations ([U]) and isotopic ratios (238U/235U or δ238U) can provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The possibility of constraining changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. However, our current understanding of the controls on the isotopic partitioning is limited, complicating interpretations of [U] and δ238U fluctuations in both the carbonate and organic-rich shale rock record. Because uranium reduction primarily occurs in sediments rather than in the water column, accumulation and isotopic fractionation are modulated by processes at or below the sediment-water interface. As such, variations in productivity-driven upwelling, basin connectivity, and sedimentation rate can influence the accumulation and isotopic fractionation of uranium into organic-rich shales—the largest lever on the δ238U composition of seawater. Within organic-rich shales, profiles of δ238U and [U] are also influenced by redox conditions within the local sedimentary basin, which in turn influence the early diagenetic reactions that determine the final partitioning of uranium and its resulting isotopic signature. To investigate the interplay of these factors on uranium cycling, we constructed an early diagenetic biogeochemical reaction network within a reactive transport model framework to determine how the primary biological and physical controls influence microbially mediated uranium reduction. We test the sensitivity of uranium reduction and isotopic fractionation to depositional factors, such as bottom-water redox conditions and sedimentation rate. Our results demonstrate that these controls play a major role in determining uranium accumulation and isotopic fractionation, and can result in diagnostic patterns in both concentrations and δ238U. Although our findings complicate the use the uranium isotopic records for interpreting global redox patterns, these predictive patterns can provide new information about local sedimentation patterns. More broadly, this work provides important new constraints on the major controls on δ238U in the past oceans, as a critical step in its development as a paleoredox proxy.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP33F1787L
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0489 Trace element cycling;
- BIOGEOSCIENCESDE: 4875 Trace elements;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY