EESS talk on "Chromium cycling in euxinic basins: Implications for the δ53Cr paleoredox proxy from a modern system (Lake Cadagno, TI)"
Event details
| Date | 05.04.2022 |
| Hour | 12:15 › 13:15 |
| Speaker | Dr David Janssen, Group Leader Tenure Track, Department Surface Waters Research & Management, EAWAG |
| Location | Online |
| Category | Conferences - Seminars |
| Event Language | English |
Abstract:
Dissolved chromium concentrations ([Cr]) and stable isotope distributions (δ53Cr) are sensitive to redox cycling due to variable solubilities of the oxidized and reduced forms of Cr (Cr(VI) and Cr(III), respectively), and the large isotope fractionations associated with redox interconversions. This redox control of both [Cr] and δ53Cr has driven significant interest in Cr records as a potential paleoredox proxy. In addition, due to the toxicity of Cr(VI), this redox control on Cr mobility has informed strategies for Cr remediation efforts. Interpretations of Cr proxy data in the sediment record and strategies for Cr remediation rely on the general geochemical assumption that reducing conditions should result in enhanced reduction and removal isotopically light Cr into sediments, affecting the dissolved Cr pool accordingly. However, few data from natural modern systems with anoxia are available for a direct assessment of this framework. With this motivation, I present water column and near-surface sediment δ53Cr and [Cr] in the meromictic Lake Cadagno (Ticino, Switzerland), which has been used as a modern analog for low-sulfate, euxinic oceanic conditions throughout Earth’s history. In Lake Cadagno, the euxinic monimolimnion is enriched in [Cr] relative to overlying oxic waters, in contrast to expectations from Cr(VI) and Cr(III) solubility. This general trend of [Cr]euxinic > [Cr]oxic is supported by the limited data available from other anoxic systems (e.g. oceanic inlets and mediterranean seas). While dissolved δ53Cr in euxinic water column samples is isotopically light relative to overlying oxic waters, deep waters are isotopically heavy relative to sediments. Furthermore, sediment δ53Cr is indistinguishable from average upper continental crust δ53Cr, and therefore do not record isotope fractionation due to intense redox cycling. These data have important implications for [Cr] and δ53Cr paleoproxy applications – namely that Cr may not be efficiently sequestered into sediments in euxinic systems, and that reconstructions of water column δ53Cr from euxinic sediments may require additional corrections, which are unconstrained at present. In addition, the accumulation and mobility of Cr(III) in reducing systems has implications for contaminant remediation efforts, indicating that efficient Cr reduction alone may be insufficient for Cr sequestration.
Short biography:
Dr. David Janssen first began researching aqueous geochemistry during his BSc in Chemistry at Humboldt State University (Ca, USA), where he studied contaminant metals in a constructed wetlands treatment system. He conducted his PhD at the University of Victoria in BC, Canada with Dr. Jay Cullen, focusing on the distributions of Zn and Cd in the subarctic northeast Pacific Ocean. After briefly working for Fisheries & Oceans Canada on long-term environmental monitoring projects, David began a postdoc at the University of Bern in fall 2017, working with Dr. Samuel Jaccard, investigating Cr cycling in the ocean and the potential paleoproxy applications of Cr stable isotopes. In October 2021 David started as a tenure track group leader in aqueous geochemistry, where his research group focuses on the distributions, transport and availability of toxin and micronutrient metals, and the emerging potential of metal stable isotopes as paleoproxies.
Dissolved chromium concentrations ([Cr]) and stable isotope distributions (δ53Cr) are sensitive to redox cycling due to variable solubilities of the oxidized and reduced forms of Cr (Cr(VI) and Cr(III), respectively), and the large isotope fractionations associated with redox interconversions. This redox control of both [Cr] and δ53Cr has driven significant interest in Cr records as a potential paleoredox proxy. In addition, due to the toxicity of Cr(VI), this redox control on Cr mobility has informed strategies for Cr remediation efforts. Interpretations of Cr proxy data in the sediment record and strategies for Cr remediation rely on the general geochemical assumption that reducing conditions should result in enhanced reduction and removal isotopically light Cr into sediments, affecting the dissolved Cr pool accordingly. However, few data from natural modern systems with anoxia are available for a direct assessment of this framework. With this motivation, I present water column and near-surface sediment δ53Cr and [Cr] in the meromictic Lake Cadagno (Ticino, Switzerland), which has been used as a modern analog for low-sulfate, euxinic oceanic conditions throughout Earth’s history. In Lake Cadagno, the euxinic monimolimnion is enriched in [Cr] relative to overlying oxic waters, in contrast to expectations from Cr(VI) and Cr(III) solubility. This general trend of [Cr]euxinic > [Cr]oxic is supported by the limited data available from other anoxic systems (e.g. oceanic inlets and mediterranean seas). While dissolved δ53Cr in euxinic water column samples is isotopically light relative to overlying oxic waters, deep waters are isotopically heavy relative to sediments. Furthermore, sediment δ53Cr is indistinguishable from average upper continental crust δ53Cr, and therefore do not record isotope fractionation due to intense redox cycling. These data have important implications for [Cr] and δ53Cr paleoproxy applications – namely that Cr may not be efficiently sequestered into sediments in euxinic systems, and that reconstructions of water column δ53Cr from euxinic sediments may require additional corrections, which are unconstrained at present. In addition, the accumulation and mobility of Cr(III) in reducing systems has implications for contaminant remediation efforts, indicating that efficient Cr reduction alone may be insufficient for Cr sequestration.
Short biography:
Dr. David Janssen first began researching aqueous geochemistry during his BSc in Chemistry at Humboldt State University (Ca, USA), where he studied contaminant metals in a constructed wetlands treatment system. He conducted his PhD at the University of Victoria in BC, Canada with Dr. Jay Cullen, focusing on the distributions of Zn and Cd in the subarctic northeast Pacific Ocean. After briefly working for Fisheries & Oceans Canada on long-term environmental monitoring projects, David began a postdoc at the University of Bern in fall 2017, working with Dr. Samuel Jaccard, investigating Cr cycling in the ocean and the potential paleoproxy applications of Cr stable isotopes. In October 2021 David started as a tenure track group leader in aqueous geochemistry, where his research group focuses on the distributions, transport and availability of toxin and micronutrient metals, and the emerging potential of metal stable isotopes as paleoproxies.
Practical information
- General public
- Free
- This event is internal
Organizer
- EESS - IIE
Contact
- Prof. Janet Hering, UPHERING EPFL and EAWAG Director