Main controlling factors of coral skeletal carbon isotopic composition and skeletal extension rate: High-resolution study at Hainan Island, South China Sea

Abstract

To understand the main controlling factors of coral skeletal carbon isotope ratio (delta(13)Cc), we undertook high-temporal- resolution (similar to 1.5 d) measurements of delta(13)Cc for Porites lutea collected from the east coast of Hainan Island, China. The results were compared with factors that have previously been proposed to control coral delta(13)Cc: skeletal extension rate, carbon isotope composition of dissolved inorganic carbon in seawater (delta(13)Cw), and light availability to the coral colony. Hainan Island is characterized by strong summer and winter monsoons that result in distinctive seasonal variations in environmental and coral growth conditions. Compared with other tropical/ subtropical areas, this climate is advantageous in evaluating the effects of different environmental factors on delta(13)Cc. We found that delta(13)Cc varied in phase with solar radiation. Increased total suspended matter (TSM) in the water column was found to reduce delta(13)Cc. These data suggest that light availability, the interplay between solar intensity and attenuation of light by TSM in the water column, is the main controlling factor of delta(13)Cc in the studied coral. In addition, slower skeletal extension was found during periods of lower solar radiation and high turbidity than other periods of similar temperature conditions, suggesting that both factors influence skeletal growth. Seasonal variations in delta(13)Cw were not only too narrow in range to explain the observed delta(13)Cc, but also showed an opposite trend to that of delta(13)Cc. We also failed to detect delta(13)Cc variations due to the kinetic isotope effect (i.e., the discrimination of heavier isotopes during CO2 hydration and hydroxylation and the resulting simultaneous negative shifts of delta(13)Cc and delta(18)Oc from equilibrium values), despite large variations in the measured skeletal extension rate. This outcome probably reflects the high skeletal extension rates of the studied coral (average, 15 mm/a; minimum, 4 mm/a).