Modern, equatorial carbonates from Kepulauan Seribu, Indonesia : facies, isotope geochemistry, and microplastic as a sedimentary component

Utami, Dwi Amanda; Kukla, Peter (Thesis advisor); Pfeiffer, Miriam (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Carbonates in the humid equatorial zone have characteristics that distinguishes them from their sub-tropical counterparts and therefore should be re-examined as a distinct depositional system. Equatorial carbonates are also known to share some similarities with cool water systems that can complicate the distinction between non-tropical and equatorial carbonates. 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. Microplastic pollution recently has been reported from coral reef systems all over the tropics, including Indonesia which is known as the second-largest contributor of mismanaged plastic waste to the ocean. Exposure to microplastics has negative impacts on coral health that in the long run threaten its ability to act as framework builders in coral reef systems. This study aims to investigate the processes that control the distribution of sedimentary and environmental facies of modern carbonate in Kepulauan Seribu. The study further evaluates the differences between humid equatorial carbonates and other carbonate systems, using isotope geochemistry in combination with XRD and SEM analysis. Additionally, to analyzes the controlling processes for microplastic dispersion and accumulation in reefs sediments. Surface sediment from the reef platforms of Pramuka, Panggang, and Semak Daun in the Kepulauan Seribu complex serve as the basis of this study. Statistical analysis based on texture and composition reveal that there are four sedimentary facies; coral grainstone, coral packstone/grainstone, coral-mollusc packstone, and mollusc wackestone. The occurrence of mollusc wackestone in the lagoon is controlled by water depth, while sand apron and reef front do not show significant facies separation with water depth. The cooccurrence of these different facies in the same depth window is contrary to the common thought that changes in bathymetry should be reflected in facies changes. A satellite derived environmental facies map generated by an image analysis algorithm indicates that environmental facies distribution is mainly controlled by water depth, density of seagrass cover and coral abundance. The sand apron can be subdivided into three environmental facies with no, sparse and dense seagrass cover. The deeper water zone can be separated into shallow and deep subtidal parts of lagoons and platform margins. In the lagoon, satellite derived environmental facies directly correlates with sedimentary facies. No direct correlation of environmental facies to sedimentary facies was possible in the sand apron due to the heterogeneity and complexity of the environment. However, the mean sediment grain size is significantly smaller in areas of the sand apron colonized by dense seagrass. A characteristic property of shallow water (<20 m) sediments from Kepulauan Seribu is their increased low-Mg calcite (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. The depleted δ13C signature in carbonates is further enhanced by the lack of green algae and inorganic carbonates and the abundance of coral debris. Low δ18O values in carbonates are favored by the high water-temperatures in the equatorial setting. Deep atmospheric convection and intensive rains further contribute 18O depleted freshwater in the river catchments, finally reducing salinity in the Java Sea. 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 are present as a sedimentary component in Kepulauan Seribu. Microplastics were concentrated using density floatation and characterized by light and scanning electron microscopy. Some particles were identified as polypropylene using micro Fourier transform infrared (µFT-IR) spectroscopy. Phthalates, a common plastic additive, was detected on the surface of microplastic particles which demonstrates that contaminants associated with microplastics could become bioavailable to corals after ingestion. All recovered microplastics were classified as secondary microplastics, likely derived from marine and local sources, with fibers as the most abundant type. Microplastics are showing similar transport and accumulation behaviour as fine siliciclastic grains. Abundance of microplastic is controlled by the proximity to the source area of larger plastic debris and hydrodynamic processes. Microplastics are not only present in low energy environment but also in high energy settings such as e.g. the reef crest. Processes that contribute to accumulation in reef sediments are biofouling, interlocking, and the creation of compound grains. Microplastics are present in sediment close to the seafloor (0-3.5 cm) but also in a depth between 3.5 and 7 cm. Microplastic particles below 3.5 cm are unlikely to be remobilized under modal weather conditions in the studied equatorial reefs. Therefore, microplastics in subtidal reef environments from the equatorial zone can be a good practical indicator of Anthropocene strata.

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