Internal Architecture of a carbonate ramp exposed to high amplitude sea-level fluctuations: Evidence from the NW shelf of Australia
1Geological Institute, RWTH Aachen University; 2CAU Kiel, Institute of Geosciences, Germany; 3School of Earth Sciences, University of Melbourne, Australia; 4Division of Earth Science, The Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Japan
The North West Shelf of Australia represents an extensive tropical carbonate ramp and forms an important template for the interpretation of similar systems found within the sedimentary record. Yet, little is known about the development of the distally steepened ramp during the mid-to-late Quaternary, a period during which NW-Australia was subject to high-frequency glacioeustatic changes in sea level and climate. This research presents core and seismic-reflection data from a mid to outer ramp transect at the Northwest Shelf. The investigated interval displays a distinct pattern with alternating changes in core color from dark to light. Dark intervals formed during humid interglacials and are composed of calcitic skeletal carbonates and elevated amounts of fluvial-derived siliciclastic input. Light intervals are predominantly comprised of inorganic precipitated aragonitic carbonates, which formed during arid glacial lowstands. Humid interglacials are characterized by limited sedimentation across the Northwest Shelf of Australia. Yet, substantial amounts of skeletal carbonates were deposited during the Holocene and Marine Isotope Stage 11.
The presented results are consequential for the interpretation of seismic and outcrop data found within the rock record, as they emphasize the strong variability of carbonate production in response to glacioeustatic changes in climate, sea level, and antecedent topography. It further highlights a system, which predominantly produces and exports inorganic-derived aragonite during glacial low stands, thereby offering an alternative to the commonly applied platform model of “highstand-shedding”.
Contrasting intensity of aragonite dissolution in glacial vs. interglacial intervals of a sea-level controlled subtropical carbonate succession
1CAU Kiel, Institute of Geosciences, Germany; 2Geological Institute, RWTH Aachen University, Germany; 3Institute of Geology and Paleontology, Tohoku University, Japan; 4Centre de Formation et de Recherche sur les Environnements Méditerranéens, Université de Perpignan, France
Aragonite and high-Mg calcite are abundant in modern, neritic temperate water systems but are nearly absent from their fossil counterparts. Dissolution of these metastable mineral phases will often leave no visible trace in the sedimentary record. Furthermore, it has been proposed that dolomitization is driven by reflux of mesohaline, aragonite undersaturated waters and that dolomite crystal growth is tightly coupled to aragonite dissolution in a temperate carbonate slope system. This study aims to clarify the processes responsible for this aragonite loss and associated dolomite formation in temperate carbonates. Biomarkers and microscopic techniques in combination with pore water analysis are used to investigate sediment cores from IODP Site U1460 on the outer ramp of the western Australian Shelf. It is shown that synsedimentary aragonite dissolution is negligible but increases significantly in a burial depth of ~ 5 m. This increase is controlled by the onset of incipient sulfate reduction, which is also interpreted to lower the kinetic inhibition for dolomite formation. However, the intensity of aragonite dissolution does not increase linearly but shows clear variations based on the availability of reactive organic matter, which is higher in interglacial compared to glacial intervals. Aragonite dissolution and Mg2+ loss from high-Mg calcite contribute to the precipitation of dolomite preferentially in interglacial sediments. This mechanism provides an indirect link between dolomite formation, aragonite dissolution, and orbital cycles. The outcome of this study contributes to a better understanding of the timing and mechanism of aragonite dissolution.
Microplastics as a sedimentary component in reefs systems: A case study from the Java Sea
1CAU Kiel, Institute of Geosciences, Germany; 2Geological Institute, RWTH Aachen University, Germany; 3Geotechnology Research Center, Indonesian Institute of Sciences, Indonesia; 4Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Germany
Microplastic pollution has been reported from coral reef systems all over the tropics. Exposure to microplastics has several negative impacts on coral health. Despite this potential risk for reef systems, the controlling processes for microplastics dispersion and accumulation in reef sediments are still understudied. Presented here is a study of microplastics (125 µm – 5 mm) distribution in two tropic atoll reef platforms in Kepulauan Seribu, Indonesia. Sediment samples were collected in different facies zones within the reef platform. 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 (µ-FT-IR) spectroscopy. All recovered microplastics were classified as secondary microplastics, derived from marine and local sources, with fibers as the most abundant type. Microplastics are showing similar transport and accumulation behavior as fine siliciclastic grains. The 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 environments but also 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 from below 3.5 cm are unlikely to be remobilized under modal weather conditions in the studied equatorial reefs. Subtidal reef sediment therefore can be regarded as a permanent sink for microplastics.
Si isotope thermometry in silicified carbonate
1Universität Göttingen; 2Deutsches Geoforschungszentrum GFZ, Potsdam; 3Freie Universität Berlin; 4Bundesanstalt für Materialforschung und -prüfung, Berlin
Cherts, including silicified carbonates, are one of the most detailed and alteration resistant archives of near-surface environments. Yet, the information disclosed in form of stable isotope ratios of Si and O cannot be confidently translated into conditions prevailing at the Earth surface in deep time. Thermometry based on δ18O is compromised by the lack of knowledge about the fluid’s δ18O value and attempts to determine Si sources or temperatures from δ30Si remain unsatisfying.
We investigated carbonate silicification in a Lower Cambrian silicified zebra dolomite that we analyzed for δ30Si by laser ablation MC-ICP-MS and δ18O using SIMS. Successively replaced carbonate layers show systematically decreasing δ18O values from 14.4 to 13.4 ‰ and systematically decreasing δ30Si values from 0.9 ‰ to ca. -2.0 ‰. We show that quantitative Si precipitation in a closed system best explains these data, requiring positive ε30Si values, which has long been proposed for thermodynamic equilibrium using ab-initio models. We exploit the modal abundances of the successively formed silica phases to quantify the fractional Si depletion from the fluid and to infer the Ɛ30Si values. Using a temperature calibration based on an ab-initio model (Dupuis et al., 2015), we determine the temperatures of carbonate replacement to be approx. 60°C and calculate the fluid δ18O to have been approx. -11 ‰, which is consistent with a meteoric water source. This approach opens a new avenue for determining initial fluid δ18O values in deep time and could thus solve long-standing disputes about hot vs. temperate Precambrian oceans.
Geochemical screening of Eocene bivalves: disentangling environmental signals from diagenetic overprint
1Goethe-Universität, Frankfurt a.M., Germany; 2The Natural History Museum, London, United Kingdom; 3Senckenberg Institute and Natural History Museum, Frankfurt a.M., Germany; 4Geozentrum Nordbayern, University Erlangen-Nürnberg, Erlangen, Germany
The Eocene ´greenhouse´ climate represents the warmest period within the Cenozoic and has therefore become especially interesting as an analogue for estimated future climate scenarios. For paleo-climate reconstructions, bivalves represent valuable proxy archives with a high temporal resolution, due to their distinct, periodic layering (growth increments). However, interpreting environmental signals from fossil bivalves can be challenging, because of the species-specific mineralogy of the shells (calcite or aragonite) and its associated geochemical behaviour, as well as its resilience against diagenetic alteration.
For the current study several middle Eocene (Lutetian) aragonitic valves of the species Venericor planicosta from different localities of the Anglo-Paris-Basin were analysed. To disentangle the environmental signals from possible diagenetic or biological influences, EPMA mapping, Cathodoluminescence (CL), as well as SEM imaging were employed. The CL and the SEM analyses revealed no increased incorporation of Mn or an extensive recrystallization respectively, suggesting a pristine preservation of the original shell material. These findings are further underlined by results of EPMA mapping of Mg, Na, Sr and S, which display oscillating element distribution patterns with increased element ratios (Me/Ca) along growth increment boundaries. Moreover, an increasing trend of the Sr/Ca ratio along the growth axis of each shell, points to potential ontogenetic effects. The Me/Ca ratios pattern are similar in all specimens, independent from their geological age or sample location and are not expected for diagenetically influenced material. Accordingly, the aragonitic shells are considered as excellent preserved archives with a high potential to resolve Eocene seasonality, e.g. by using δ18O and ∆47.