Unravelling the drivers of shallow CaCO3 sediment dissolution in an acidifying ocean
Project Team: Prof. Bradley Eyre, Assoc. Prof. Kai Schulz, Laura Stoltenberg, Coulson Lantz
Coral reefs provide a myriad of ecosystem services associated with their high biological diversity (Hoegh-Guldberg, 1999; Harrison and Booth, 2007). Critical to this biodiversity is the calcium carbonate (CaCO3) coral reef structure, which provides habitats for a large number of species. Reef structures are formed through the growth and build-up of coral aragonite skeletons, red and green calcareous macroalgae and other calcareous organisms such as bryozoans, echinoderms, and foraminifera. However, for a coral reef structure to be in a state of net accretion, the production of CaCO3 must exceed its loss through physical erosion and dissolution as follows (eyre et al., 2014);
CaCO3 accretion = CaCO3 production - physical loss of CaCO3 - CaCO3 dissolution
Ocean acidification refers to the lowering of the ocean pH due to the uptake of anthropogenic CO2 from the atmosphere. When CO2 dissolves in seawater it forms H2CO3 (carbonic acid), which rapidly dissociates into a HCO3- (bicarbonate ion) and a H+ (hydrogen ion). Some of the excess H+ combines with CO32- (carbonate ion) to form HCO3- and the remaining H+ lowers the seawater pH (pH = -log [H+]). The effect of ocean acidification on the decreased production of coral reef CaCO3 (calcification) is well documented (e.g. Gattuso et al., 1999; Marubini et al., 2003; Langdon et al., 2005; Dove et al., 2013).
However, CaCO3 production is only part of the equation determining coral reef accretion (see above), and much less is known about the effects of ocean acidification on the other terms (i.e. physical erosion and dissolution) (eyre et al., 2014). In particular, the dissolution of CaCO3 in coral reef sediments has largely been neglected with dissolution often excluded from coral reef carbonate budgets developed by geologists (e.g. Mallela and Perry, 2007), while biologists are mainly focused on calcification rates (e.g. Schneider and Erez, 2006). These sediments are critical to the formation of the modern, shallow reef environments such as lagoons, reef flats and coral sand cays. In addition, permeable carbonate sediments cover around 40% of the continental shelves (Milliman and Droxler, 1996).
The aim of this study is to use in situ advective benthic chamber incubations over diel cycles of photosynthesis and respiration with, and without, lowered pH (i.e. future ocean acidification scenarios) across different reefs, sediment types and seasons to determine the controls on coral reef CaCO3 sediment dissolution. We will also do in situ manipulative experiments to better elucidate the controls on CaCO3 sediment dissolution.
Cyronak, T. and B. D. Eyre. 2016. The synergistic effects of ocean acidification and organic metabolism on calcium carbonate (CaCO3) dissolution in coral reef sediments. Marine Chemistry 183, 1-12.
Eyre, B. D., Andersson, A. J. and Cyronak, T. 2014. Benthic coral reef calcium carbonate dissolution in an acidifying ocean. Nature Climate Change 4, 969-976.
Cyronak, T., A. Santos, I. R. and Eyre, B. D. 2013. Permeable coral reef sediment dissolution driven by elevated pCO2 and porewater advection. Geophysical Research Letters 40, 4876-4881.
Cyronak, T., Santos, I. R, McMahon, A. and Eyre, B.D. 2013. Carbon cycling hysteresis in permeable carbonate sands over a diel cycle. Limnology and Oceanography 58, 131-143.