Abstract
The geochemical composition of foraminiferal tests is a valuable archive for the reconstruction of paleo-climatic, -oceanographic and -ecological changes. However, dissolution of biogenic calcite and precipitation of inorganic calcite (overgrowth and recrystallization) at the seafloor and in the sediment column can potentially alter the original geochemical composition of the foraminiferal test, biasing any resulting paleoenvironmental reconstruction. The δ11B of planktic foraminiferal calcite is a promising ocean pH-proxy but the effect of diagenesis is still poorly known. Here we present new δ11B, δ13C, δ18O, Sr/Ca and B/Ca data from multiple species of planktic foraminifera from time-equivalent samples for two low latitude sites: clay-rich Tanzanian Drilling Project (TDP) Site 18 from the Indian Ocean containing well-preserved (‘glassy’) foraminifera and carbonate-rich Ocean Drilling Program (ODP) Site 865 from the central Pacific Ocean hosting recrystallized (‘frosty’) foraminifera. Our approach makes the assumption that environmental conditions were initially similar at both sites so most chemical differences are attributable to diagenesis. Planktic foraminiferal δ18O and δ13C records show offsets in both relative and absolute values between the two sites consistent with earlier findings that these isotopic ratios are strongly influenced by diagenetic alteration. Sr/Ca and B/Ca ratios in planktic foraminiferal calcite are also offset between the two sites but there is little change in the relative difference between surface and deep dwelling taxa. In contrast, δ11B values indicate no large differences between well-preserved and recrystallized foraminifera suggesting that despite extensive diagenetic alteration the δ11B of biogenic calcite appears robust, potentially indicative of a lack of free exchange of boron between pore fluids and the recrystallizing CaCO3. Our finding may remove one potential source of uncertainty in δ11B based pH reconstructions and provide us with greater confidence in our ability to reconstruct pH in the ancient oceans from at least some recrystallized foraminiferal calcite. However, further investigations should extend this approach to test the robustness of our findings across a range of taphonomies, ages and burial settings.
Highlights
Foraminifera precipitate their calcium carbonate tests from the seawater in which they live, providing a record⇑ Corresponding author at: School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
The delicate biogenic muricae that are characteristic of the dominant Eocene surface dwelling groups Acarinina and Morozovelloides have been overgrown by large, euhedral, blade-like crystals presumably having acted as a locus for crystallisation (Fig. 2g vs. o)
In cross-section it is evident that the structural integrity of the test wall has been compromised with dissolution exploiting existing lines of weakness such as the position of the former primary organic membrane leading to delamination of the inner test wall and there are pervasive small inorganic crystals projecting from the internal test surface
Summary
Foraminifera precipitate their calcium carbonate tests from the seawater in which they live, providing a record. ⇑ Corresponding author at: School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK. There are many chemical proxies (elemental and isotopic) that can be measured in foraminiferal tests to reconstruct both past environments and organism ecology. The classic d18O and d13C proxies provide insights into changes in past ocean temperatures, salinity and/or global ice volume, and ocean dissolved inorganic carbon, respectively (Lea, 2014; Zeebe and Wolf-Gladrow, 2001).
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