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(Past records of) environmental change: some bio- and isotope geochemical applications


Alexandra Rodler
AMGC, Vrije Universiteit Brussel

ABSTRACT: I will give a brief introduction to my pre-VUB research and plans for the time at VUB as part of the EOS project ET-HOME. This will focus on a range of topics, including tracing environmental change (microbial diversity) in the geological past (Snowball Earth, paleo-redox conditions) and experimental validation (trace metal sorption, incorporation) of an emerging paleo-redox tracer (the Cr-isotope system), to trace metal incorporation in modern microbialite systems in order to better understand ancient sedimentary archives for discussing habitability on early Earth (ET-HOME).
The Snowball Earth hypothesis proposes that the Neoproterozoic was punctuated by several global low latitude glaciations. This era further witnessed a significant rise in environmental oxygenation; however, debates about the termination of these Neoproterozoic glaciations and how it might have influenced (catalysed?) the evolution of complex multicellular life are ongoing. To add to the discussion of these post-glacial environmental conditions, we apply a multiproxy approach in an ongoing study1 of carbonate sediments of the Witvlei Group, Namibia, that were deposited following the two major Neoproterozoic glaciations (Sturtian, ca. 717-643 Ma; Marinoan, ca. 635 Ma).
Microbially-mediated sedimentary carbonates (microbialites) pervasively precipitated in coastal environments throughout much of the Precambrian and are generally considered among the oldest evidence for life on Earth. They form due to an interplay of microbial growth, decomposition, mineral precipitation and sediment deposition. However, their chemical composition may preserve key information characteristic of their depositional environments: ongoing work using the Cr-isotope system2 suggest this as promising tracer for constraining paleo-environmental conditions recorded in ancient microbialites. However, incorporation mechanisms of Cr and other metals/metalloids into microbialite carbonate material, and accompanying isotopic fractionations associated with those processes, remain poorly understood.

1 Rodler A.S., Hohl S.V., Gaucher C., Germs G.J.B., Hegenberger W., Goderis S., Claeys Ph., Frei R., Neoproterozoic environmental conditions: a novel metal stable isotope perspective. Goldschmidt Abstracts 2019.
2 Bruggmann S., Rodler A.S., Klaebe R., Frei R., The utility of modern microbialites for recording environmental conditions. Goldschmidt Abstracts 2019.