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Chromium Cycling in Redox-Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications
David J Janssen1,2,3, Jörg Rickli1,2,4, Martin Wille1
1Institute of Geological Sciences University of Bern Bern Switzerland.
Geophysical Research Letters
|January 2, 2023
Summary
Chromium stable isotope composition (δ⁵³Cr) reveals how redox conditions change. In stratified basins, incomplete chromium reduction causes light isotopes to accumulate in deep waters, impacting paleoredox interpretations.
Area of Science:
- Geochemistry
- Isotope Geochemistry
- Paleoceanography
Background:
- Chromium stable isotope composition (δ⁵³Cr) is a key tracer for Earth's redox history.
- Geochemical controls on δ⁵³Cr in modern redox-stratified basins remain understudied.
- Redox-stratified basins serve as analogs for ancient ocean conditions.
Purpose of the Study:
- To assess geochemical controls on δ⁵³Cr in a modern redox-stratified basin.
- To investigate chromium isotope fractionation during reduction and removal processes.
- To evaluate the implications for interpreting δ⁵³Cr records in paleoredox studies.
Main Methods:
- Analysis of chromium (Cr) concentration and δ⁵³Cr in water, particulate, and sediment samples.
- Study conducted in Lake Cadagno, Switzerland, a redox-stratified modern analog.
- Comparison with existing data from other redox-stratified basins.
Main Results:
- Incomplete Cr reduction and removal above the chemocline drives δ⁵³Cr fractionation.
- Isotopically light Cr accumulates in euxinic deep waters of Lake Cadagno.
- Authigenic Cr in sediments is isotopically distinct from overlying waters but matches continental crust.
Conclusions:
- Non-quantitative Cr reduction and removal, not just quantitative, cause significant isotope fractionation.
- Sedimentary δ⁵³Cr records may be offset from overlying water compositions.
- Paleoredox interpretations based on δ⁵³Cr require re-evaluation of fractionation assumptions.
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