Effects of seawater-pH and biomineralization on the boron isotopic composition of deep-sea bamboo corals

The ocean is currently absorbing excess carbon from anthropogenic emissions, leading to reduced seawater-pH (termed ‘ocean acidification’). Instrumental records of ocean acidification are unavailable from well-ventilated areas of the deep ocean, necessitating proxy records to improve spatio-temporal understanding on the rate and magnitude of deep ocean acidification. Here we investigate boron, carbon, and oxygen isotopes on live-collected deep-sea bamboo corals (genus Keratoisis) from a pHtot range of 7.5–8.1. These analyses are used to explore the potential for using bamboo coral skeletons as archives of past deep-sea pH and to trace anthropogenic acidification in the subsurface North Atlantic Ocean (850–2000 m water depth). Boron isotope ratios of the most recently secreted calcite of bamboo coral skeletons are close to the calculated isotopic composition of borate anion in seawater (dBborate) for North Atlantic corals, and 1–2& higher than d Bborate for Pacific corals. Within individual coral skeletons, carbon and oxygen isotopes correlate positively and linearly, a feature associated with vital effects during coral calcification. dB variability of 0.5–2& is observed within single specimens, which exceeds the expected anthropogenic trend in modern North Atlantic corals. dB values are generally elevated in Pacific corals relative to dBborate, which may reflect pH-driven physiological processes aiding coral calcification in environments unfavorable for calcite precipitation. Elevated dB values are also observed proximal to the central axis in multiple Atlantic and Pacific specimens, relative to dBborate, which might reflect ontogenetic variability in calcification rates. Although the observed boron isotope variability is too large to resolve the present anthropogenic ocean acidification signal at the studied depths in the North Atlantic ( 0.03–0.07 pH units), pH changes P0.1 units might still be reconstructed using dB measurements in bamboo corals. 2015 Elsevier Ltd. All rights reserved.