Abstract
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 the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million years (66% CI).
Highlights
IntroductionBoron isotopes (δ11B) in marine carbonates have been exploited as a proxy for the marine carbonate system (i.e. pH, pCO2) on multiple timescales within the Cenozoic (e.g. Sanyal et al, 1995; Palmer et al, 1998; Pearson and Palmer, 2000; Foster et al, 2012; Penman et al, 2013; Anagnostou et al, 2016; Gutjahr et al, 2017; Chalk et al, 2017)
As for the history of calcite compensation depth (CCD) change, which factors into our estimation of Neogene DIC change, comparing equivalent scenarios with contrasting CCD reconstruction we find only small differences in estimated DIC change (Supplementary Fig. S8), with the scenarios based on the Sime et al (2007) record consistently resulting in slightly greater rates of long-term DIC increase because of the tendency of longterm CCD deepening in that record
As with the apparent orbital scale fluctuations in reconstructed Middle Miocene Climate Optimum (MMCO) pCO2, the pH fluctuations implied by the Greenop et al (2014) boron isotope data result in substantial fluctuations in reconstructed saturation state, with the MMCO episodes of low pH corresponding to the lowest saturation levels (1.6–2.1) that we find over the entire Neogene study interval
Summary
Boron isotopes (δ11B) in marine carbonates have been exploited as a proxy for the marine carbonate system (i.e. pH, pCO2) on multiple timescales within the Cenozoic (e.g. Sanyal et al, 1995; Palmer et al, 1998; Pearson and Palmer, 2000; Foster et al, 2012; Penman et al, 2013; Anagnostou et al, 2016; Gutjahr et al, 2017; Chalk et al, 2017). Pleistocene is not straightforward as calculation of pH and pCO2 requires records of seawater chemistry (e.g. δ11Bsw, [Ca]sw) and a second carbonate system parameter (e.g. total alkalinity (ALK), or dissolved inorganic carbon (DIC)) which currently have considerable uncertainties (Horita et al, 2002; Brennan et al, 2013; Pälike et al, 2012; Raitzsch and Hönisch, 2013; Greenop et al, 2017). The uncertainty window on these estimates reflects uncertainty in calcite saturation and δ11Bsw estimates, δ11Bc measurement error, and possible atmosphere–ocean disequilibrium Despite these relatively large uncertainties, it is clear that early Cenozoic pCO2 was much higher than in the Plio-Pleistocene and CO2 change was an important driver of climate change in the early Cenozoic. Our new record of ocean carbonate chemistry allows us to examine the role of CO2 in the climate system over the course of the Neogene and contextualize future carbon emission pathways against this geological perspective
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