Origin of holocene beachrock cements in northeastern Brazil: Evidence from carbon and oxygen isotopes

Abstract

This study has utilized carbon and oxygen isotopic data to unravel the origin of Holocene beachrock cements of Rio Grande do Norte State coast, northeastern Brazil. The cements are exclusively constituted of Mg-calcite, with isopachous prismatic rim, equant spar, cryptocrystalline coating or pore-filling, pseudo-peloidal, radial and scalenohedral habits. The δ18OVPDB values of most (93%) of the samples range from −1.8‰ to +0.5‰, which are compatible with precipitation from marine waters. The δ13CVPDB values of these cements are +1.7 to +3.6‰ (average + 3.05‰) suggesting derivation from marine dissolved carbon. In a few cases, the Mg-calcite cement shows low δ13CVPDB (−7.3‰ and −7.8‰) and δ18OVPDB (−4.4‰ and −4.3) values, which may indicate precipitation or recrystallization of marine cements under the influence of meteoric waters.