Density banding pattern of the south western Atlantic coral Mussismilia braziliensis

Abstract

The South Atlantic Ocean is a region that lacks data regarding the long-term monitoring of oceanographic parameters. In addition, proxy indicators in this region are not yet well established. We evaluate the density-banding pattern of the coral Mussismilia braziliensis to calibrate it for use as a proxy of paleoceanographic parameters, such as temperature and light availability. Colonies of the coral were stained with alizarin red in the field and in an aquarium system to determine the periodicity and cyclicity of the skeletal density banding and to determine whether this coral species presents banding pattern characteristics useful as an environmental proxy. The results show that M. braziliensis forms annual density bands in nature that comprise a pair of high- and low-density bands (light and dark bands, respectively, in negative X-radiography). Furthermore, M. braziliensis forms high-density bands starting from October to November, when the average monthly temperature is between 25°C and 25.5°C, and low-density bands starting from May, when temperature is around 26.5°C and 26°C. In a time series (1997 to 2005) of M. braziliensis growth observed in core slabs and collected in Abrolhos, no significant difference in the widths of the low and high density bands was observed, although clear asymmetry was observed in the temperature seasonality. Thus, the resultant linear extension rate is lower during the warming period comparing with the cooling period. However, colonies of this species that were maintained for one year in a closed aquarium system at a constant temperature of 26°C±0.2°C and 12h of illumination also formed a new and complete density cycle despite the absence of seasonality in temperature and photoperiod during the experiment. This result implies that the primary cue for density banding is endogenous, possibly the reproductive cycle, and that temperature plays a secondary role in fine-tuning the banding cycle. Thus, we conclude that this species can be used as an archive for environmental parameters. On the other hand, it can be used to evaluate environmental impacts induced by human action and/or by global climatic changes on the growth of corals in recent decades and centuries along the coast and the continental shelf of the tropical western Atlantic Ocean.