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Revisiting diagenesis on the Ontong-Java plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

DOI: 10.1002/2014PA002759 DOI Help

Authors: Oscar Branson (University of Cambridge) , Elizabeth Read (University of Cambridge) , Simon Redfern (University of Cambridge) , Christoph Rau (Diamond Light Source) , Henry Elderfield (University of Cambridge)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Paleoceanography

State: Published (Approved)
Published: October 2015
Diamond Proposal Number(s): 8532 , 5846

Abstract: The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analysed to generate trace element and isotope palaeoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their palaeoceanographic interpretation. A previous study of G. tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite, and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca [Brown and Elderfield, 1996]. Here, we revisit these specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe micro-analyses (EPMA) to re-evaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore-water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and re-precipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting palaeoproxy signals.

Journal Keywords: Diagenesis; Palaeoproxy; Synchrotron; Tomography

Subject Areas: Biology and Bio-materials, Earth Science

Instruments: I13-2-Diamond Manchester Imaging