Carbon and strontium isotope fluctuations and paleoceanographic changes in the late Neoproterozoic Araras carbonate platform, southern Amazon craton, Brazil

Abstract

The late Neoproterozoic Araras Group is exposed along the southern border of the Amazon craton and at the northern Paraguay belt, central Brazil. It comprises carbonate rocks over 600 m thick overlying Marinoan glaciogenic diamictites, and includes the first well-documented cap carbonate sequence in South America. δ 13C values for Araras carbonates, interpreted as representative of the original seawater ( δ 18O > − 10‰), exhibit a trend from strongly negative δ 13C values around − 9‰ at the base to enriched 13C compositions towards the top of the succession reaching values above 0‰. The cap carbonate succession is composed of deep platform deposits of dolomite (Mirassol d'Oeste Formation) covered by limestone rich in crusts and cements (Lower Guia Formation). These rocks show δ 13C values around − 5‰ and anomalous sedimentary structures such as vertical tubes, megaripple bedding (tepee-like structure), and crystal fans of aragonite pseudomorphs. The anoxic deep-platform limestone succession (Guia Formation) that overlies the cap succession presents a homogeneous C-isotope curve, with δ 13C values around − 2.5 to − 1‰. Upsection, values switch to positive (+ 0.1 to + 0.3‰) in platform dolomites and shoreface to shallow-platform deposits (Serra do Quilombo Formation), but exhibit oscillations to slightly negative values in the overlying peritidal facies (Nobres Formation). 87Sr/ 86Sr values follow the general C-isotope trend, with values increasing upsection from 0.7074 to 0.7088. Striking shifts in δ 13C and 87Sr/ 86Sr values are observed close to stratigraphic surfaces. The correlation between the isotope record and the relative sea-level curves supports the close association between paleoceanographic changes and paleoenvironmental evolution during the terminal Neoproterozoic on the Araras carbonate platform of Amazon craton.