Density banding in corals: barcodes of past and current climate change

The predicted sea surface temperature (SST) rise over the next decades is likely hazardous to coral health because precipitation of the calcareous skeleton depends primarily on SST. Temperature modulates vertical growth and density of the skeleton with seasonal SST changes resulting in an alternation of high-density and low-density bands (HDB and LDB). Notably, growth rates and the timing of the HDBs and LDBs relative to the seasons differ on a global scale within geographic regions. In this contribution, we use combined information of skeletal density and seasonally resolved oxygen isotope SST estimates from massive Porites from a Late Miocene (9 Ma) reef in the eastern Mediterranean Sea (Crete, Greece) to understand reef vulnerability over short and geological periods of time. Three types of HDB-SST relationships have been found: (1) coincidence of HDB with summer, (2) winter or (3) autumn and spring. The latter doubles HDBs in a year and implies maximum calcification is coupled to the taxon-specific optimum SST during the transitional seasons and reduced at its respective critical winter and summer SSTs. Modeling with a nonlinear temperature-calcification relationship reproduces the climate barcode of density bands. The model should be relevant for other poikilothermic carbonate producers in reefs and platforms and has implications for judging geographic distributions and causes of extinctions of corals, benthic carbonate communities and entire carbonate systems. With regard to the causes underlying expansion and demise of carbonate platforms and reefs in geological history, we expect the model predictions to help for a deeper understanding of biotic responses during hyperthermals or coolings and possibly also for identifying regions in the modern ocean where corals are endangered or taking advantage of global warming.