Modern and late Pleistocene B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi determined with laser ablation ICP-MS

Incorporation of boron into foraminiferal shells is thought to be primarily governed by the carbonate chemistry of the ambient seawater, suggesting that it can be reconstructed from B/Ca ratios. To this end, B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi from South Atlantic core top samples have been analyzed using laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) to provide additional information on intratest trace element heterogeneity. Results show that boron is heterogeneously distributed within and between shells, with content variations of approximately +/- 43% displayed within a single shell. B/Ca is higher in the youngest chambers, opposite to the observed between-chamber variability of Mg/Ca. This may be explained by ontogenetic changes of physiological processes that increase the pH of the calcifying fluid and thus the borate concentration while decreasing Mg/Ca to promote calcification. Despite this heterogeneity, mean B/Ca ratios are positively correlated with the deepwater calcite saturation state (Delta[CO32-]), in line with previous studies. We apply this empirical relationship to reconstruct Delta[CO32-] for the late Pleistocene to Holocene using samples from a depth transect in the equatorial Atlantic. Reconstructed Delta[CO32-] values confirm previous studies suggesting that CaCO3-oversaturated North Atlantic Deep Water was reduced during glacial periods, whereas CaCO3-undersaturated Antarctic Bottom Water expanded vertically and propagated northwards. In summary, our data demonstrate that bulk B/Ca in P. wuellerstorfi reliably reflects variations in Delta[CO32-], despite the strong physiological control of boron incorporation.