Assessing the impact of diagenesis on δ 11 B, δ 13 C, δ 18 O, Sr/Ca and B/Ca values in fossil planktic foraminiferal calcite

Kirsty M. Edgar, Eleni Anagnostou, Paul N. Pearson, Gavin L. Foster
2015 Geochimica et Cosmochimica Acta  
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 d 11 B of planktic foraminiferal
more » ... oraminiferal calcite is a promising ocean pH-proxy but the effect of diagenesis is still poorly known. Here we present new d 11 B, d 13 C, d 18 O, 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 d 18 O and d 13 C 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, d 11 B values indicate no large differences between well-preserved and recrystallized foraminifera suggesting that despite extensive diagenetic alteration the d 11 B of biogenic calcite appears robust, potentially indicative of a lack of free exchange of boron between pore fluids and the recrystallizing CaCO 3 . Our finding may remove one potential source of uncertainty in d 11 B 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. Long term precision of standards gives an uncertainty of 2% for Sr/Ca and 5% for B/Ca ratios. * % loss of X/Ca from samples = Glassy or 'primary' TDP Site 18 Sr/Ca -Frosty ODP Site 865 Sr/Ca ratios. Cibicidoides data are not directly comparable with one another because of different water depths of two sites. 196 K.M. Edgar et al. / Geochimica et Cosmochimica Acta 166 (2015) 189-209
doi:10.1016/j.gca.2015.06.018 fatcat:txls5ropwvempf3bpl77bhoxbu