The mineralogic and isotopic fingerprint of equatorial carbonates: Kepulauan Seribu, Indonesia

摘要

Kepulauan Seribu is an isolated patch reef complex situated in the Java Sea (Indonesia) and is a typical example for a humid, equatorial carbonate system. We investigate the mineralogical and isotopic fingerprint of Panggang, one of the reef platforms of Kepulauan Seribu, to evaluate differences to other carbonate systems, using isotope in combination with XRD and SEM analysis. A characteristic property of shallow water ( 20 m) sediments from Kepulauan Seribu is their increased LMC content (similar to 10%) derived from some genera of rotaliid foraminifers and bivalves. The relative abundance of these faunal elements in shallow waters might be related to at least temporary turbid conditions caused by sediment-laden river runoff. This influence is also evidenced by the presence of low amounts of siliciclastic minerals below the regional wave base. Kepulauan Seribu carbonates are characterized by very low delta C-13 and delta O-18 values. This is related to the isotopically depleted riverine input. The delta C-13(DIC) in riverine water is reduced by the contribution of C-12 from riverside mangroves. Deep atmospheric convection and intensive rains contribute O-18-depleted freshwater in the river catchments, finally reducing salinity in the Java Sea. The depleted delta C-13 signature in carbonates is further enhanced by the lack of green algae and inorganic carbonates and abundance of coral debris. Low delta O-18 values in carbonates are favored by the high water temperatures in the equatorial setting. Since equatorial carbonates in SE Asia, including the Java Sea, are typically influenced by high turbidity and/or river runoff, the observed distinctively low isotope values likely are characteristic for equatorial carbonate systems in the region.