Dados da Submissão

Título

PETROLOGICAL AND GEOCHEMICAL CHARACTERIZATION OF HOLOCENE CARBONATE BANKS IN THE COASTAL REGION OF ESPÍRITO SANTO, BRAZIL

Texto do resumo

Carbonate banks on the western shelf of the South Atlantic Ocean are an essential element in the marine landscape, formed by a complex ecosystem of diverse organisms. Despite the many studies, such as in Abrolhos and Foz do Amazonas, the role of each identified species and the interaction between the formative processes are still not fully known. Furthermore, many are formed only by paleobanks under conditions different from the current ones, possibly associated with climate changes and relative sea-level oscillations throughout the Holocene. In the coastal region of the Espírito Santo state, southeastern Brazil, there is an extensive carbonate paleobank in the intertidal area. This is formed by finely laminated carbonate layers with intergranular porosity and constituted by the intercalation of white and pinkish-brown laminae. The bank outcrops have a flat exposure, are oriented NE-SW, and are formed by small natural pools with shelter-rich biodiversity. A survey of vertical profiles was carried out at the site, with samples collected for mineralogical and geochemical studies in carbonate layers. The objective of the study was to characterize the laminae and use geochemical proxies to identify the main processes that formed this atypical occurrence. The techniques applied to characterize the mineralogy were XRD and SEM-EDS, and the elemental composition was XRF. Radiocarbon dating (¹⁴C) was obtained through AMS at the Poznań Radiocarbon laboratory of six samples, three white laminae, and three pinkish-brown. Structurally and texturally, the white laminae are irregular, curved, concave-convex, compact, massive, with a very fine texture, and composed of microcrystalline carbonate minerals. The pinkish-brown laminae follow the sinuosity and irregularity of the white laminae but with less hardness than the white ones. The mineralogical composition is mainly composed of magnesian calcite (81 to 85%), secondarily quartz (~11%), and aragonite (4 to 8%). The quartz may be related to the presence of carbonate mud in the pores, while the aragonite is to bioclasts. The magnesian calcite consists of organized anhedral microcrystalline ridges (2 to 5µm) and thin laminae. The SEM also observed extracellular polymeric substances (EPS) with mineral precipitation, composed mainly of Ca, Mg, C, and O (EDS). The ages obtained from the white laminae ranged from 4,101 to 3,896 years cal. BP, while the pinkish-brown ones ranged from 3,625 to 3,333 years cal. BP. Initially, both laminae indicate formation by biotic process due to the relationship between the concentrations of Sr (1,353 to 1,860 ppm) and Mg (27,498 to 28,282ppm), possibly of microbial origin related to the microcrystalline texture, with strong marine influence based on the Mg/Ca ratios (0.015mol.mol-1). The results indicate depositional conditions different from the current ones, possibly with greater water depths, which at this time would be +3 meters, according to previous studies. This discovery will be used as a geochemical proxy in paleoenvironmental reconstruction and in future research on the role of these microbial communities in the carbon cycle. This atypical carbonate formation provides excellent evidence of the climatic oscillations and marine sedimentary dynamics of the South Atlantic shelf during the Late Holocene.

Palavras Chave

carbonate paleobanks; geochemical proxies; marine sedimentary dynamics; climate change.