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SUMMARY:EESS talk on "Biomineral recrystallization: Are pristine biominera
 ls reliable paleotemperature proxies?"
DTSTART:20201006T121500
DTEND:20201006T130000
DTSTAMP:20260930T195037Z
UID:37621e10ccfd5c2677894522b3b5621b9654a3d0700ec03234d4c412
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr Arthur Adams\, postdoctoral researcher\, Laboratory for Bio
 logical Chemistry (LGB)\, EPFL\nAbstract:\nTexturally unaltered i.e. “pr
 istine” minerals are generally considered as robust paleotemperature pro
 xies. This axiom is mostly applicable to large (>1 µm diameter)\, organic
 -matter poor\, and chemically stable abiogenic minerals. In contrast\, mos
 t biogenic minerals used in paleotemperature reconstructions consist of re
 active biocarbonates with nanocrystalline ultrastructures enveloped in an 
 organic-matter matrix. These properties are conducive to a texturally impe
 rceptible but ultrastructurally pervasive low-temperature isotopic reequil
 ibration process termed stable mineral recrystallization. This implies tha
 t even the analysis of pristine biominerals can result in inaccurate paleo
 temperatures. \nTo study this process\, we analyzed the stable isotope co
 mpositions of biogenic carbonates (foraminifera tests) before and after sh
 ort-term (hours–weeks) low temperature (90 °C) reactions in 18O-enriche
 d fluids at calcite saturation (Ωcalcite = 1). In all cases\, foraminifer
 a after reactions were indistinguishable from pristine foraminifera. These
  experiments allowed us to determine where isotope exchange occurs in biog
 enic 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 recryst
 allization 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 c
 alculations\, would lead to a calculated paleotemperature >3 °C higher th
 an its original precipitation temperature.\nThese results are applicable t
 o all biogenic carbonate paleoclimate proxies\, even in the absence of any
  evidence for recrystallization. Carbonates precipitated at low temperatur
 es e.g. deep water organisms\, are particularly vulnerable to this process
 \, which may explain some anomalously-high calculated precipitation temper
 atures in the paleoclimatic record.\n\nShort biography:\nArthur Adams is a
  postdoc working on carbonate isotope geochemistry at the Swiss Federal In
 stitute of Technology Lausanne (EPFL)\, Switzerland. He completed his BSc 
 at Brandon University\, Canada\, and continued his studies at the Universi
 ty of Bern\, Switzerland\, where he received his Masters and PhD on the st
 udy 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 temperat
 ures without any textural evidence for recrystallization. This process cha
 llenges the presumption that “texturally pristine” biominerals e.g. fo
 raminifera tests\, bivalve shells\, and corals are robust paleoclimate pro
 xies.\n 
LOCATION:ZOOM
STATUS:CONFIRMED
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