Millennial‐scale ocean acidification and late Quaternary decline of cryptic bacterial crusts in tropical reefs

Abstract

Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21,000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14,000 years with largest reduction occurring 12,000-10,000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects.

Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora, Society Islands, French Polynesia)

“…This observation suggests that apart from nutrient input and runoff from a basaltic hinterland, additional environmental factors must be responsible for abundant microbialite formation. In accordance with the data compiled by Riding et al (2014), it could possibly be explained by low alkalinity at the peak of MIS 5e.…”

“…Microbialite crusts usually occur in core sections older than approximately 6 kyr bp . This is in accordance with the findings in the other cores in Bora Bora (Gischler et al ., ) and in Tahiti, and has been explained by changes in environmental parameters such as light and energy, nutrients and alkalinity (Camoin et al ., ; Seard et al ., ; Heindel et al ., ; Riding et al ., ). The youngest, progradational section of the windward Puhia fringing reef is detrital and largely composed of unconsolidated sand and rubble.…”

“…In accordance with the data compiled by Riding et al . (), it could possibly be explained by low alkalinity at the peak of MIS 5e.…”

Coral-reef records of Quaternary changes in climate and sea-level

“…Turbidity may result from microplankton blooms promoted by excess nutrients, or an excess of terrigenous clays in runoff from the adjacent hinterland, and is commonly thought of as inimical to coral growth but, as Brown and Perry (2012) have shown, some corals thrive in such circumstances. Lastly, because glacial cycles are accompanied by changes in the CO 2 content of the atmosphere and oceans it might be expected that increasing acidity would have a deleterious effect (Doney et al 2009;Riding et al, 2014). However, in the present analysis, it ultimately matters little whether these processes are or are not effective in moderating coral growth and development; their influence cannot be reliably identified in the fossil record.…”

Low Florida coral calcification rates in the Plio-Pleistocene

“…During and before the optimum, however, pCO 2 reached values expected for the end of this century through the burning of fossil fuels (IPCC, 2013;Seki et al, 2010). Modelling of the oceanic carbonate systems suggest the long-term pCO 2 changes to have had no effect on the saturation state of seawater with regard to aragonite (Hönisch et al, 2012), but evidence exists that rates of microbial carbonate precipitation and skeletal accretion of planktic foraminifera differed over the last glacial-interglacial cycle (Barker, 1986;Beaufort et al, 2011;Riding et al, 2014). The Plio-Pleistocene Florida carbonate platform represents a stack of shallow marine carbonate sequences formed during sea level highstands which are separated by paleosols or thin freshwater units formed during lowstands.…”