EESS talk on "Biomineral recrystallization: Are pristine biominerals reliable paleotemperature proxies?"

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Event details

Date and time 06.10.2020 12:1513:00  
Place and room
ZOOM
Speaker Dr Arthur Adams, postdoctoral researcher, Laboratory for Biological Chemistry (LGB), EPFL
Category Conferences - Seminars
Abstract:
Texturally unaltered i.e. “pristine” minerals are generally considered as robust paleotemperature proxies. This axiom is mostly applicable to large (>1 µm diameter), organic-matter poor, and chemically stable abiogenic minerals. In contrast, most biogenic minerals used in paleotemperature reconstructions consist of reactive biocarbonates with nanocrystalline ultrastructures enveloped in an organic-matter matrix. These properties are conducive to a texturally imperceptible but ultrastructurally pervasive low-temperature isotopic reequilibration process termed stable mineral recrystallization. This implies that even the analysis of pristine biominerals can result in inaccurate paleotemperatures. 
To study this process, we analyzed the stable isotope compositions of biogenic carbonates (foraminifera tests) before and after short-term (hours–weeks) low temperature (90 °C) reactions in 18O-enriched fluids at calcite saturation (Ωcalcite = 1). In all cases, foraminifera after reactions were indistinguishable from pristine foraminifera. These experiments allowed us to determine where isotope exchange occurs in biogenic ultrastructures and the rate of isotopic reequilibration. We observe that (1) recrystallization occurs pervasively, continuously and rapidly, even at low temperatures and short timescales and (2) the rate of recrystallization is a factor of the ultrastructural composition of the biogenic carbonate framework. Extrapolating the isotopic exchange rate over >10 Myr results in a reequilibrated calcite, that if used for paleotemperature calculations, would lead to a calculated paleotemperature >3 °C higher than its original precipitation temperature.
These results are applicable to all biogenic carbonate paleoclimate proxies, even in the absence of any evidence for recrystallization. Carbonates precipitated at low temperatures e.g. deep water organisms, are particularly vulnerable to this process, which may explain some anomalously-high calculated precipitation temperatures in the paleoclimatic record.

Short biography:
Arthur Adams is a postdoc working on carbonate isotope geochemistry at the Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland. He completed his BSc at Brandon University, Canada, and continued his studies at the University of Bern, Switzerland, where he received his Masters and PhD on the study of low temperature dolomitization and carbonate recrystallization. His research focuses on stable mineral recrystallization, a process capable of rapidly modifying the isotopic composition of a mineral at low temperatures without any textural evidence for recrystallization. This process challenges the presumption that “texturally pristine” biominerals e.g. foraminifera tests, bivalve shells, and corals are robust paleoclimate proxies.
 

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Practical information

  • General public
  • Free
  • This event is internal

Organizer

  • EESS - IIE

Contact

  • Prof. Anders Meibom, LGB

Tags

Biomineralization foraminifera isotope paleothermometer paleoclimatology recrystallization

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