Abstract
Abstract. Boron isotope systematics of planktonic foraminifera from core-top sediments and culture experiments have been studied to investigate the sensitivity of δ11B of calcite tests to seawater pH. However, our knowledge of the relationship between δ11B and pH remains incomplete for many taxa. Thus, to expand the potential scope of application of this proxy, we report δ11B data for seven different species of planktonic foraminifera from sediment core tops. We utilize a method for the measurement of small samples of foraminifera and calculate the δ11B-calcite sensitivity to pH for Globigerinoides ruber, Trilobus sacculifer (sacc or without sacc), Orbulina universa, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Globorotalia menardii, and Globorotalia tumida, including for unstudied core tops and species. These taxa have diverse ecological preferences and are from sites that span a range of oceanographic regimes, including some that are in regions of air–sea equilibrium and others that are out of equilibrium with the atmosphere. The sensitivity of δ11Bcarbonate to δ11Bborate (e.g., Δδ11Bcarbonate∕Δδ11Bborate) in core tops is consistent with previous studies for T. sacculifer and G. ruber and close to unity for N. dutertrei, O. universa, and combined deep-dwelling species. Deep-dwelling species closely follow the core-top calibration for O. universa, which is attributed to respiration-driven microenvironments likely caused by light limitation and/or symbiont–host interactions. Our data support the premise that utilizing boron isotope measurements of multiple species within a sediment core can be utilized to constrain vertical profiles of pH and pCO2 at sites spanning different oceanic regimes, thereby constraining changes in vertical pH gradients and yielding insights into the past behavior of the oceanic carbon pumps.
Highlights
The oceans are absorbing a substantial fraction of anthropogenic carbon emissions, resulting in declining surface ocean pH (IPCC, 2014)
Specimens of O. universa and P. obliquiloculata are living in the upper thermocline; G. menardii is found in the upper thermocline until the thermocline depth specific to the location; N. dutertrei is living near thermocline depths and G. tumida is found in the lower thermocline
Data from the multiple approaches for calculating calcification depth (CD1, CD2, and CD3) imply that some species inhabit deeper environments in the western equatorial Pacific (WEP) relative to the Arabian Sea, which in turn are deeperdwelling than the same morphospecies occurring in the Indian Ocean
Summary
The oceans are absorbing a substantial fraction of anthropogenic carbon emissions, resulting in declining surface ocean pH (IPCC, 2014). Because planktonic foraminifera species complete their life cycle in a particular depth habitat due to their ecological preference (Ravelo and Fairbanks, 1992; Farmer et al, 2007), it is theoretically possible to reconstruct water column profiles of pH using boron isotope ratio data from multiple taxa (Palmer and Pearson, 1998; Anagnostou et al, 2016). The potential use of an analogous approach to reconstruct past profiles of seawater pH was first highlighted by Palmer and Pearson (1998) on Eocene samples to constrain water depth pH gradients In these boron isotope-based studies, it was assumed that boron isotope offset from seawater and foraminiferal carbonate was constant, which is an assumption not supported by subsequent studies (e.g., Hönisch et al, 2003; Foster et al, 2008; Henehan et al, 2013, 2016; Raitszch et al, 2018; Rae, 2018). Boron isotope ratio differences between foraminifera species inhabiting waters of the same pH make the acquisition of more core-top and culture data essential for applications of the proxy
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