Microanálise e geoquímica em testemunhos de corais como elementos de interpretação dos impactos das forçantes naturais e antrópicas sobre a costa do Brasil

Abstract

Stony corals are forming reefs. By secreting calcium carbonate from the bottom of their polyps, these zooxanthellae corals form an exoskeleton, usually composed of aragonite crystals. The coral growth patterns vary from seasonal to centennial scale and can be characterized by measuring the growth rate, the variability of the stable isotopes of oxygen and carbon, and the elemental ratios Sr/Ca, Mg/Ca, U/Ca, Cd/Ca, Ra/Ca (among others) in its skeleton. In a global context, reefs play a key role as atmospheric carbon sinks. Faced with the evidence of a warmer ocean in the modern era, the sea surface temperature (SST) has been considered an important control factor in calcification and coral growth. Generally, calcification tends to increase with increasing SST within a narrow acceptable range for the full functioning of coral metabolism. In this work, a reanalysis of the growth rates of sampled corals cores on the brazilian coast has developed (Salvador-Ba - Bahia de Todos os Santos, Abrolhos Marine National Park-Ba and Armação dos Búzios-RJ) employing a combination of growth bands (high and low density) aided by the luminescence method, and radioisotope dating of U and Th. The differences in time lines for the two methods varied from 1 year to the case Abrolhos to 7.4 years to Búzios (in specific sections of the core). Calcification variations were analyzed in the coral skeleton and interpreted by using ratios Sr/Ca and U/Ca (both proxies of TSM), climate series of AMO and PDO, and pelagic oceanic pH. We have identified a decrease in calcification rate exoskeleton time studied in Salvador sample of 0.4 g/cm2 and an increase in Abrolhos 0.4 g/cm2 and Búzios 0.3 g/cm2 except in the 1950s to the late 1980s and 1910 to the late 1930s, respectively. A X-ray microtomography was used for the determination of coralline micro-structures, wich are the most relevant parameters to the microporosity and anisotropy. To Abrolhos and Búzios, it was identified an increase in total porosity of the exoskeleton, especially at the beginning of 1940 until the end of the 1980s and between 1890-1930 respectively. It was noted a strong association between reduced calcification pattern with increasing porosity. The cores of Siderastrea stellata species collected in Abrolhos and Búzios showed high association of the reasons Sr/Ca and U/Ca with calcification rate, featuring a similar response to other authors to the Great Barrier in Australia (DE'ATH et al., 2009) and the central region of the Red Sea (CANTIN et al., 2010). Regarding the reasons Ba/Ca, Salvador and Abrolhos showed the variables that contributed to this increase as oil production and population growth forcings (economy) and SST (ocean). At Buzios, SST (ocean), oil production, population growth and NDVI forcings (economy). After the 1990s, the impact of economic factors, in addition to oceanic variables respond more significantly the increase on the ratio Ba/Ca in all places almost simultaneously on the Brazilian coast.