Chromium Cycling in Redox‐Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications

• Published in: Geophysical research letters Vol. 49; no. 21; pp. e2022GL099154 - n/a
• Main Authors: Janssen, David J., Rickli, Jörg, Wille, Martin, Sepúlveda Steiner, Oscar, Vogel, Hendrik, Dellwig, Olaf, Berg, Jasmine S., Bouffard, Damien, Lever, Mark A., Hassler, Christel S., Jaccard, Samuel L.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-11-2022; John Wiley and Sons Inc
• Subjects: Anoxia; Anoxic basins; Anoxic sediments; Anoxic waters; Aquatic plants; Atmospheric Processes; Basins; Biogeochemical Cycles, Processes, and Modeling; Biogeochemical Kinetics and Reaction Modeling; Biogeochemistry; Biogeosciences; Chemical composition; Chemocline; Chromium; Continental crust; Cryosphere; Cycles; Deep water; Earth; Earth surface; euxinia; Fractionation; Global Change; History; Hydrology; Isotope composition; Isotope fractionation; Isotopes; Lake sediments; Lakes; Limnology; Marine Systems; Metals; Oceanography: Biological and Chemical; Oceanography: General; Oceans; Oxygen; Paleoceanography; Paleoclimatology; Paleoclimatology and Paleoceanography; paleoproxy; Precambrian; Redox properties; Reduction; Removal; Research Letter; Sediment; Sediment samplers; Sediment samples; Sediments; Stable isotopes; Surface water; Tracers; Water circulation; Water column; Weathering
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL099154

Chromium stable isotope composition (δ53Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ53Cr have not been assessed in modern redox‐stratified basins. We present new chromium (Cr) concentration and δ53Cr data in dissolved, sinking particulate, and sediment samples from the redox‐stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non‐quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox‐stratified basins show analogous patterns. This challenges assumptions from δ53Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non‐quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ53Cr from oxidative continental weathering.
Plain Language Summary
Chromium stable isotope composition in sediments has been used extensively to understand changes in oxygen availability throughout Earth's history. These reconstructions are built on assumptions of chromium (Cr) transfer between oxic and anoxic waters such that sediments directly reflect oxic surface waters, and therefore trace redox cycling at Earth's surface. However, this has not been tested in modern redox‐stratified systems. Here, we show that Cr concentrations and stable isotope compositions deviate from these assumptions. Chromium is neither efficiently removed in anoxic waters, nor do sediments faithfully record water column isotope signatures. A synthesis of new and published observations from euxinic waters indicates these findings appear universal. Previous assumptions of paleo‐studies are not supported by modern systems and should be revised.
Key Points
Non‐quantitative Cr removal at the chemocline drives chromium (Cr) isotope fractionation and the accumulation of light Cr in euxinic deep waters
Authigenic sediment δ53Cr differs from overlying water, contrary to paleoproxy application assumptions, and is similar to continental crust
δ53Cr signals in these settings may therefore reflect internal redox processes and not fractionation due to oxidative subaerial weathering