Mid-Holocene Coral Mortality in the South China Sea

In an intriguing paper published in the Journal of Quaternary Science, Yu et al. (2010) detected and dated growth hiatuses -- periods of time between coral demise and subsequent recruitment-initiated new growth -- in massive Porites colonies in the northern South China Sea dating back in time to the mid-Holocene, which was a period of greater-than-current warmth that is often referred to as the Holocene Climatic Optimum. This work -- which was conducted on a fringing reef in a small bay near Sanya City on the southern coast of Hainan Island -- revealed six hiatuses in coral growth of a few decades each that occurred between about 6900 and 6000 years before present, when they say that analyses of Sr/Ca and Mg/Ca ratios indicate sea surface temperatures (SSTs) were "generally warmer than present," and which the studies of Yu et al. (2005) and Wei et al. (2007) suggest were "1-2°C warmer than present."

Although these results might be interpreted to imply that it was the warmer temperatures of the mid-Holocene that killed the corals, such is not the case; for Yu et al. write that the coral deaths in one part of the reef "could not be due to high-SST-induced bleaching as the Sr/Ca-based summer SST in the final year (~27.5°C) was actually lower than in the previous summer (~30.3°C)." And they add that the deaths of the other corals "were also unlikely to be related to SST anomalies, as the Sr/Ca-based terminal monthly SST in [those] samples appear to be well within the normal range of monthly SST in the area." In addition, they write that the Sr/Ca and Mg/Ca profiles of the corals suggest that "mortality occurred in different seasons, possibly related to different causes (e.g. flooding, disease), but unrelated to anomalous SST-induced bleaching."

In light of these several unique and important findings, it is beginning to look ever more probable, as Idso et al. (2000) contended long ago, that many recent episodes of coral bleaching may also have been caused by something other than unduly high SSTs, or that many of the cases of coral bleaching experienced over the past few decades that do indeed appear to have been precipitated by unseasonably high SSTs may have been "aided and abetted" by other more localized anthropogenic-induced factors that could have weakened the corals and made them more susceptible to heat-induced bleaching. And a number of such "accomplices" come to mind.

There is, for example, the live reef-fish trade, fishermen using dynamite and cyanide, sea life depleted to the point of exhaustion by over-fishing, physical damage caused by barbed hooks and scything nets, prawn trawlers stirring up sediments, the "bycatch" associated with prawn harvesting that is left to die, tourists and the developers who build resorts and marinas for them, and catamarans and dive boats that take visitors to reefs and dump their sewage in the sea on the way home, as well as other physical and chemical insults to once-pristine reef environments, including the rising levels of nutrients and toxins in coastal waters caused by runoff from agricultural activities on land and associated increases in sediment delivery to reefs that are augmented by deforestation, coastal development, construction, mining, drilling and dredging.

Clearly, there are many unfortunate things that mankind does that lead to the weakening of corals and make them an easier mark for both disease and thermal stress. And those things are likely the primary cause of most coral bleaching experienced in our day and age.

Additional References
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO₂, global warming and coral reefs: Prospects for the future. Technology 75S: 71-93.

Wei, G.J., Deng, W.F., Yu, K.F., Li, X.H., Sun, W.D. and Zhao, J.X. 2007. Sea surface temperature records in the northern South China Sea from mid-Holocene coral Sr/Ca ratios. Paleoceanography 22: 10.1029/2006PA001270.

Yu, K.F., Zhao, J.X., Wei, G.J., Chang, X.R. and Wang, P.X. 2005. Mid-late Holocene monsoon climate retrieved from seasonal Sr/Ca and delta O-18 records of Porites lueta corals at Leizhou Peninsula, northern coast of South China Sea. Global and Planetary Change 47: 301-316.