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

Abstract

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 (~ 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 δ13C and δ18O values. This is related to the isotopically depleted riverine input. The δ13CDIC in riverine water is reduced by the contribution of 12C from riverside mangroves. Deep atmospheric convection and intensive rains contribute 18O-depleted freshwater in the river catchments, finally reducing salinity in the Java Sea. The depleted δ13C signature in carbonates is further enhanced by the lack of green algae and inorganic carbonates and abundance of coral debris. Low δ18O 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.

Microplastics as a sedimentary component in reef systems: A case study from the Java Sea

“…4). The sediment composition is nearly identical to previous facies descriptions (Utami et al., 2018, 2021) with corals and molluscs as the dominant skeletal components. Based on their grain size and composition all samples can be attributed to the sedimentary facies defined by Utami et al .…”

“…Clay minerals (smectite) and fine‐grained quartz are thought to be derived from neighbouring larger islands, such as Sumatra. Subsequently, they are transported into the study area by oceanic currents (Utami et al., 2021). In the reef platforms they are concentrated below the local wave base, indicated by bulk fine fraction contents of >50% (Utami et al., 2021).…”

“…Subsequently, they are transported into the study area by oceanic currents (Utami et al., 2021). In the reef platforms they are concentrated below the local wave base, indicated by bulk fine fraction contents of >50% (Utami et al., 2021). A similar hydrodynamic control has been shown before for microplastic distribution in siliciclastic sediments in the lagoon of Venice (Vianello et al., 2013).…”

“…The highest concentrations of microplastic and siliciclastic minerals are found in sediment with >50% fine fraction content, indicating similar transport behaviour. Mineralogy and associated bulk grain sizes are from Utami et al (2021).…”

“…Aragonite is the dominant mineral phase, while low‐magnesium calcite and high‐magnesium calcite occur in fairly similar amounts in all sedimentary facies. Small amounts of siliciclastic minerals (quartz and smectite) are preferentially deposited in deeper waters below the wave base (Utami et al., 2021).…”