Vertical water mass mixing and plankton blooms recorded in skeletal stable carbon isotopes of a Red Sea coral

The environmental interpretation of the ¹³C/¹²C variations in the skeletons of massive corals is still a matter of debate. A 19-year seasonal skeletal ¹³C/¹²C record of a shallow-water Porites coral from the northern Red Sea (Gulf of Aqaba) documents interannual events of extraordinarily large plankton blooms, indicated by anomalous ¹³C depletions in the coral skeleton. These blooms are caused by deep vertical water mass mixing, convectively driven in colder winters, which results in increased supplies of nutrients to the surface waters. The deep vertical mixings can sometimes be driven by the cooling occurring throughout the Middle East after large tropical volcanic eruptions. We therefore have evidence in our coral skeletal ¹³C/¹²C record for an indirect volcanic signal of the eruptions of El Chichón (1982) and Mount Pinatubo (1991). Deep mixing induced ¹³C/¹²C variations of the dissolved inorganic carbon in the surface waters can be neglected at this location. We therefore suggest that the ¹³C skeletal depletions can be best explained by changes in the coral's autotrophy-heterotrophy diet, through increased heterotrophic feeding on zooplankton during the blooms. Increased feeding on ¹³C-depleted zooplankton or increased heterotrophy at the expense of autotrophy can both result in a ¹³C-depleted coral skeleton. However, this suggestion requires more testing. If our conclusions are substantiated, seasonal skeletal ¹³C/¹²C records of corals which change from autotrophy under normal conditions to increased heterotrophy during bloom events may be used as indicators of ocean paleoproductivity at interannual resolution, available from no other source.