Assessing the growth rate of the South Atlantic coral species Mussismilia hispida (Verrill, 1902) using carbon and oxygen stable isotopes

Abstract

Determining a coral's growth rate is one of the first steps for establishing if a species can serve a suitable natural archive for assessing and investigating past environments and climates in tropical oceans. Carbon and oxygen stable isotope ratios (δ13C and δ18O) are commonly used as proxies for cloud cover and sea surface temperatures (SST) respectively. Once those proxies are shown to have annual seasonality, the lengths and numbers of δ13C and δ18O cycles can be used to estimate growth rates and to date coral records. We investigated the growth rate of Mussismilia hispida (Verrill, 1902), a coral species endemic to the western South Atlantic, using a high-resolution sampling technique (0.5 mm) to retrieve carbonate powder from the corallite theca walls of three colonies (13MH-1, 13MH-2, and 13MH-3) for δ13C and δ18O analyses. High-resolution sampling allowed us to fully track annual isotopic cycles through continuous sections of coral skeletons using isotopic cycle lengths (i.e., the distance between two peaks) to estimate growth rates. To validate that isotopic cycles were truly annual, two samples of colony 13MH-1 were dated by U–Th series. The colonies, 13MH-1, 13MH-2, and 13MH-3 presented mean growth rates of 2.83 ± 0.51, 3.21 ± 0.86, and 3.71 ± 0.82 mm/year respectively. U–Th dating of two samples from colony 13MH-1 indicated a time interval of 13 years (1944–1955), during which the mean growth rate was 3.57 ± 1.03 mm/year – similar to the stable isotope results. Our results showed that the δ18O and δ13C records of M. hispida colonies at the Rocas Atoll are dominated by short and medium-term variations. The short-term variations appear to be governed by annual seasonality (i.e., SST and solar radiation levels) and were used to determine the extension rates of the three colonies, whereas, the medium-term variation has a decadal signal and seems to be governed by different climate patterns that affect rainfall variability and interannual SST in the South Atlantic Ocean. The well-defined cycles of M. hispida, together with its widespread geographical distribution, make it a promising natural archive for environmental and climate reconstructions.