T11 New Developments in Geochemical Proxies for Paleoceanographic Research

Sunday, 4 November 2018
8:00 AM - 12:00 PM
Room 140

LITHIUM ISOTOPE COMPOSITIONS OF MODERN AND FOSSIL CENOZOIC BRACHIOPODS

Paper number: T11-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

The stable lithium isotope (δ7Li) composition of past seawater is a promising proxy for changes in silicate weathering, which are responsible for modulating the global carbon cycle over geologic time. Foraminiferal tests record a large increase (~9‰) in seawater δ7Li over the past ~50 Myr, potentially supporting hypotheses for changes in global silicate weathering occurring in tandem with Cenozoic cooling (Hathorne & James, 2006, Earth Planet. Sci. Lett.; Misra & Froelich, 2012, Science). However, it remains unclear whether forams are faithful recorders of seawater Li isotope composition and particularly whether they are subject to vital effects that could bias interpretation (Vigier et al., 2015, C. R. Geosci.; Roberts et al., 2018, Geochim. Cosmochim. Acta). To date, it has not been possible to test the reliability of the Cenozoic foram Li isotope records as there are no reconstructions using different archives. To fill this gap, we measured δ7Li compositions of 35 calcareous specimens of brachiopods from throughout the Cenozoic. Recent work suggests that brachiopods may be reliable recorders of seawater δ7Li composition (Dellinger et al., 2018, Geochim. Cosmochim. Acta); to confirm that this is the case, we also examined in detail the δ7Li systematics in modern calcareous and phosphatic brachiopod specimens from 17 and 10 genera, respectively. The δ7Li compositions of recent calcareous brachiopods agree with previous findings (mean = +27‰), similar to that expected for inorganic calcite precipitated from seawater (Marriott, et al., 2004, Chem. Geol.). Phosphatic brachiopod δ7Li compositions are more variable, ranging from 20‰ to 46‰ (mean = +31‰). The δ7Li compositions we measured from fossil calcitic brachiopods broadly agree with Cenozoic δ7Li foram records, and in the few cases of divergence from the foram record, we examine trace element ratios and preservation of shell textures. We conclude that large changes in seawater composition during the Cenozoic are likely to be robust, having been recorded in two different carbonate archives.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-321650

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