A project funded by the Leverhulme Trust aiming to use the boron isotopic composition of tropical coral skeletons to provide new insights into the magnitude and impact of ocean acidification (OA) over the last 100 years.
Global warming is not the only consequence of rising levels of carbon dioxide (CO2) – around half the emissions of CO2 from burning fossil fuel since the industrial revolution has been absorbed by the oceans (Fig. 1). CO2 is an acidic gas and this has caused ocean pH (a measure of acidity & the -log10[H+]) to decline, a process known as ocean acidification (nice summary here).
So far the oceans have acidified by ~0.1 pH unit. This may not sound like much but actually is a 30% increase in the H+ concentration. Due to the nature of the ocean carbonate system this change in pH has shifted the abundance of dissolved inorganic carbon species, decreasing the concentration of carbonate ion by ~16%. Again this may not sound like a big deal, but because this has lowered the saturation state of calcium carbonate (CaCO3) it is making it harder for marine organisms like corals to make their aragonite (a form of CaCO3) skeletons (Fig. 2). As CO2 continues to rise in the future, the oceans will continue to acidify and some recent estimates suggest that by 2050 coral reefs will transition from net accumulation of CaCO3 to net dissolution (see here). The full consequences of this are hard to estimate but clearly OA is only going to make it harder for corals (and other marine calcifiers) to deal with anthropogenic stresses.
Corals and boron isotopes
The boron isotopic composition of coral skeletons varies predominantly as a function of pH of the seawater in which the coral grew (Fig. 4). This proxy has a firm grounding in theory (e.g. here) but because corals increase their internal pH to favour calcification there is a large offset between d11B-derived pH and seawater pH (Fig. 3&4). pH up-regulation (i.e. internal pH increase) is both energetically expensive for the coral and an essential part of the calcification process, but it’s magnitude, and that of the d11B “vital-effect”, appears to be a close function of external pH (Fig. 4).
This leads to two competing hypothesis that we will test in this project:
- Internal pH regulation in corals is not a function of environmental parameters other than external pH; once calibrated, d11B in coral skeletons is a high fidelity recorder of variations in seawater pH.
- The ability of a coral to increase its internal pH is a function of other environmental variables. d11B in coral skeletons is a monitor of the magnitude of pH up-regulation.
Regardless of which hypothesis is correct, extended, high resolution temporal records of d11B-pH in corals represents a novel opportunity to improve our understanding of how calcifying organisms like corals have already responded to historic ocean acidification and potentially how they will fare under future OA.