T87 New Voices in Geobiology

Monday, 10 October 2022
8:00 AM - 12:00 PM
507

INTERPRETING SINGLE-FOSSIL AND BULK ORGANIC CARBON ISOTOPES FROM THE MESOPROTEROZOIC VELKERRI FORMATION OF AUSTRALIA TO BETTER UNDERSTAND THE ENVIRONMENTAL CONTEXT OF EARLY EUKARYOTIC EVOLUTION

Paper number: T87-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

The Mesoproterozoic Roper Group of Australia contains some of the oldest definitive eukaryotic fossils and thus is an important window into the early evolution of eukaryotes. In this study, we look at δ13Corg of 100 individual microfossils as well as bulk δ13Corg through the Roper Group Velkerri Formation to obtain a short-term view of the conditions of the water column during the deposition of sediments as well as information on fossil paleoecology. We analyzed δ13Corg from individual microfossils via nano-EA mass spectrometry, and found a range of average δ13Cfossil per horizon from -31.7‰ to -24.03‰ compared to the bulk rock δ13Corg, which ranges from -35.22‰ to -27.74‰. In the standard model of eukaryotic evolution, Mesoproterozoic eukaryotes were limited to oxygenated surface waters in a stratified ocean with strong δ13C and oxygen gradients. Such conditions would leave the microfossils’ δ13Corg values partially determined by their location in the water column. Because the biological pump would have left the surface 13C-enriched, the δ13C of organisms living near the surface would be greater than the δ13C of the bulk organic matter.

Our results reinforce this standard model with the exception of 2 horizons where some δ13Cfossil are less than the bulk rock values. Results from these two horizons potentially indicate that the fossils’ habitats were located out of the surface ocean, perhaps in anoxic portions of the water column. However, the majority of our data supports the hypothesis that eukaryotic organisms were living in surficial and partially oxygenated waters. Though our data could potentially result from such conditions as N-limitation or partial mixing of the water column, we rely heavily on the veracity of bulk redox proxies in our interpretation of our results. Specifically, trends in our mean bulk and fossil δ13C appear to correlate with changing redox conditions as documented by Nguyen et al. (2019), and lighter δ13Corg values associated with periods of euxinia may represent relatively higher contributions by methanogenic carbon metabolisms relative to sulfate reduction. If the Velkerri Formation was not a consistently stratified, anoxic-at-depth basin, then our data may instead be tracking changes in the water column δ13C gradient recorded by organisms living in the surface ocean.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-383318

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