60th Annual Report on Research 2015

In the first year of this award, research accomplishments included (i) establishing the methodology for Li isotope analysis of carbonate samples at the University of Southern California, (ii) assembling samples of biogenic carbonates for studying vital effects, (iii) testing for variations in seawater δ⁷Li during across the Triassic-Jurassic transition, and (iv) completing a modeling analysis of Cenozoic changes in δ⁷Li, and publishing a related paper.

The current report focuses on activities during the second year of the award period. The major achievement during this time involved completing the δ⁷Li measurements and complementary analyses of the biogenic carbonates that comprise a major component of the planned research, as described in greater detail below. Postdoctoral fellow Mathieu Dellinger led the analytical effort, under supervision of PI West, and presented the results at the 2015 Southern California Geobiology Symposium at UC Riverside, at the 2015 Goldschmidt Meeting in Prague, Czech Republic, and at the 2015 AGU Annual Meeting in San Francisco. A related manuscript is on the cusp of submission for peer-reviewed publication.

Specifically, samples from over 25 biogenic carbonate specimens were micro-drilled, prepared through a cleaning and ion chromatography protocol, and analyzed via mass spectrometry (MC-ICP-MS at the California Institute of Technology) for δ⁷Li. These samples included 9 aragonitic mollusks, 8 calcitic mollusks, 5 brachiopods, and 3 echinoderms. In addition to the bulk analysis of each individual specimen, several shells were sampled at multiple sites in order to identify intra-shell variability in δ⁷Li. Sample solutions were also analyzed for trace element concentrations and ratios (e.g., Li/Ca, Mg/Ca, Sr/Ca) via HR-ICP-MS at the University of Southern California. The mineralogy (i.e., relative proportion of calcite vs. aragonite) of each sample was determined on aliquots of sample powders by X-ray Diffraction (XRD) at the Los Angeles County Natural History Museum.

Results (Figure 1) show that mineralogy exerts a first order control on the δ⁷Li of biogenic carbonates, consistent with prior data from inorganic carbonates precipitated in laboratory experiments. For calcitic shells, measured δ⁷Li of bivalve species ranges from +32 to +40‰ and is systematically enriched in the heavy isotope relative to seawater and to inorganic calcite (Δ⁷Li_{calcite–seawater} = −2 to −5‰). The Li isotope composition of aragonitic bivalves, ranging from +16 to +22‰, is slightly fractionated to both high and low δ⁷Li relative to inorganic aragonite. These results suggest a strong influence of vital effects on Li isotopes during bio-calcification of bivalve shells. The largest intra-shell Li isotope variability is observed for mixed calcite-aragonite shells (more than 20‰) whereas in single mineralogy shells, intra-shell δ⁷Li variability is generally less than 3‰. In contrast to bivalves, measured brachiopods exhibit little fractionation compared to inorganic calcite with a mean δ⁷Li of 27.0±1.5‰, and echinoderms also show limited variability between species. The wide range of bivalve compositions suggests that these organisms may not reliably record seawater isotopic composition, while brachiopods and echinoderms may be more promising candidates – though more work on a wider diversity of samples from these groups will be warranted in the future to confirm these observations. Our results thus have important implications for using Li isotopes in carbonates in order to reconstruct the composition of seawater in the geological past.

Building on the experience gained while supported by this PRF award, postdoctoral fellow Dellinger successfully secured an independent research fellowship at Durham University, UK, to continue to pursue research in isotope geochemistry and its application to the geologic carbon cycle. Though he begins his fellowship tenure in late 2015, he plans to remain involved in this project. During the second year of this award, in addition to completing the biogenic portion of the research, he assembled a sample set of carbonate sediment cores and altered coral samples from the Bahamas to test for diagenetic effects on carbonate δ⁷Li. Dellinger prepared 82 samples and analyzed a subset for δ⁷Li. During the award extension over the coming year, he will complete these analyses, present the results at an international conference, and prepare a related manuscript. Together with the biogenic carbonate data collected over the past year, these results will allow us to complete the main goals of the proposed research, namely identifying the vital and diagenetic effects that are likely to determine the fidelity of Li isotope reconstructions from marine carbonates.