Coral based proxies of ocean acidification and climate change

Abstract

Ocean acidification is now viewed as one of the main global stressors threatening oceans and seas. It is one of the insidious consequences of the anthropogenic rise in atmospheric CO2, which occurs due to the absorption of this greenhouse gas in seawater. Global surface oceans have already acidified by about 0.1 pH units since preindustrial times, and models project that they will further decrease 0.3 to 0.4 pH units by the end of this century. This change in seawater chemistry towards more corrosive conditions will encompass a 150% increase in the concentration of protons and a 50% decrease in carbonate ions with respect to pre-industrial values. This progressive transition has the potential to affect marine ecosystems in many ways, and it is generally agreed that calcifying organisms will have more difficulties to grow. To better understand the magnitude of this problematic it is important to know the ranges of variability of pH and other related parameters in seawater at different time scales. This knowledge is crucial, for instance, to establish levels of pH tolerance for marine organisms. Because of the scarcity of reliable instrumental records of seawater pH, which span three decades at the most, past variations of pH need to be reconstructed using proxies in suitable archives. Corals are suitable for this purpose, particularly from the tropics, where massive growing species such as Porites allow for multi-centennial reconstructions at temporal resolutions of weeks. In the Mediterranean Sea, where no such massive corals exist, other species of corals are being considered as suitable archives for paleoenvironmental reconstructions. In this presentation, we will make a synthesis on the actual efforts to explore and exploit coral based proxies of ocean acidification and other climate/global change related parameters. We will start with a review on the different reconstructions of paleo-pH that have been produced so far in the global oceans, with a focus on the boron isotope paleo-pH-meter used on massive tropical corals. We will then move to less studied proxies and archives which offer potential for paleoenvironmental reconstructions in the Mediterranean Sea, and we will sum up by remarking the potential and necessity of long-term culture calibrations with the manipulation of multiple parameters to reliably test all proxies, an approach that can be undertaken at the purpose designed experimental aquarium facilities of Institut de Ciències del Mar