Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy

• Published in: Earth and Planetary Science Letters
• Main Authors: Sosdian, S. M., Greenop, R., Hain, M. P., Foster, G. L., Pearson, P. N., Lear, C. H.
• Other Authors: University of St Andrews. School of Earth & Environmental Sciences, University of St Andrews. St Andrews Isotope Geochemistry
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Boron isotopes; Foraminifera; Carbonate system; Neogene; Miocene; GE Environmental Sciences; GC Oceanography; QD Chemistry; DAS; SDG 13 - Climate Action; SDG 14 - Life Below Water; GE; GC; QD; Ocean acidification
• Online Access: http://hdl.handle.net/10023/15632; https://doi.org/10.1016/j.epsl.2018.06.017

This work was funded by NERC Grant NE/I006427/1 (CHL and GLF). PNP and GLF were supported by NERC/DEFRA/DECC Grant NE/H017581/1 (U.K. Ocean Acidification Research Programme). MPH was supported by NERC fellowship (NE/K00901X/1). SMS acknowledges the financial support provided by the Welsh Government and Higher Education Funding Council for Wales through the Sêr Cymru National Research Network for Low Carbon, Energy and Environment Returning Fellowship. Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO2, carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO2, and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (e.g. δ11Bsw, [Ca]sw, CCD). We show that the choice of δ11Bsw influences both seawater pH and CO2 while [Ca]sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO2. Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching ∼7.6 ± 0.1 units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO2 record and climate, all CO2 scenarios show a peak at ∼9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (i.e. significant CO2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of ...