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Isotopically anomalous organic carbon in the aftermath of the Marinoan snowball Earth
Author(s) -
Liljestrand Frasier L.,
Laakso Thomas A.,
Macdonald Francis A.,
Schrag Daniel P.,
Johnston David T.
Publication year - 2020
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12383
Subject(s) - snowball earth , earth (classical element) , astrobiology , carbon fibers , early earth , earth science , geochemistry , chemistry , geology , materials science , paleontology , physics , astronomy , glacial period , composite material , composite number
Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed ε o r g - c a r b ). Sedimentary deposits immediately overlying the Marinoan snowball Earth diamictites, however, stray from this prediction. In stratigraphic sections from the Ol Formation (Mongolia) and Sheepbed Formation (Canada), we observe a temporary excursion where the organic matter has anomalously heavy δ 13 C and is grossly decoupled from the carbonate δ 13 C. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of snowball Earth. For example, physical oceanographic modeling suggests that a strong density gradient caused the ocean to remain stratified for about 50,000 years after termination of the Marinoan snowball event, during which time the surface ocean and continental weathering consumed the large atmospheric CO 2 reservoir. Further, we now better understand how δ 13 C records of carbonate can be post‐depostionally altered and thus be misleading. In an attempt to explain the observed carbon isotope record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. By comparing the model output to the sedimentary data, stratification alone cannot generate the anomalous observed isotopic signal. Reproducing the heavy δ 13 C in organic matter requires the progressively diminishing contribution of an additional anomalous source of organic matter. The exact source of this organic matter is unclear.