The ecology and structure of many tropical coral reefs have altered markedly over the past few decades. Drivers of this degradation range from direct local damage from destructive human practices to global scale climate stressors, and especially those associated with elevated sea‐surface temperature anomalies. A major consequence of these climatic and pervasive local stressors has often been a rapid decrease in the abundance of habitat building corals, which has consequently reduced reef structural complexity and coral carbonate production rates. Equally, many reefs have been impacted by changes (both increases and decreases) in the abundance of bioeroding taxa such as parrotfish, urchins, sponges and microendolithic organisms. The collective effect has been to alter the rates and relative balance of carbonate producing and eroding processes on many reefs. Such changes are of increasing interest because these processes directly regulate net rates of reef carbonate production and sediment generation, and collectively can impact upon multiple geo‐ecological functions on reefs. These functions include reef‐building and the capacity of reefs to accrete vertically in response to sea‐level rise, and the supply of sands necessary to sustain beaches and reef islands. This talk will discuss recent progress in developing methodologies to estimate rates of reef carbonate production, reef growth and sediment generation. It will then use selected recent field examples to highlight changes in these processes in response to ecological disturbance, and highlight the potential to integrate these data into numerical and lab-based modelling approaches than be used to predict coastal wave exposure under future sea level rise scenarios.

The effects of changing climate and environmental conditions on coral reef islands have received a lot of attention, and the findings are discussed broadly. The low elevation of such islands above mean sea level and the largely unconsolidated sediment is exposing them to hydrodynamic processes. Coral reef islands are formed by sediment sourced from the surrounding reef systems and depend on these reef systems as continuous material suppliers. Shifts in these governing conditions may affect these landforms, however the results of such shifts remain controversial as island-response is likely to be regionally specific. The present sedimentological study addresses the formation of a reef island in the Spermonde Archipelago, Sulawesi, and its development through time. Sediment cores of 10 m length taken on the island allowed to reconstruct the sedimentary history of this mid-shelf island. The carbonate facies from these cores reflects the development of the island and thus allows for inferring the evolution of the surrounding ecosystem as well as the hydrodynamic regime that governed sedimentation. While sediment from the maximum depth of the cores mirror parautochthonous accumulation in a lagoonal environment, subsequent sedimentation is thought to be the result of hydrodynamic events with oscillating intensity.

Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, many early-diagenetic cements that developed in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of thin sections, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant Mg calcite (3.9 – 7.2 Mol% Mg), bladed low Mg calcite (0.4 – 1.0 Mol%) and mini-micritic Mg calcite (crystal size < 0.1 µm; 3.2 – 3.3 Mol% Mg). The consecutive development of (1) fibrous aragonite, (2) equant Mg calcite and (3) bladed low Mg calcite is explained by shifts in pore water pH and alkalinity through fluid kinetics and microbial sulfate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Mg calcite, as inferred from the presumable detection of an extracellular polymeric matrix. Radiocarbon analyses of five Halimeda segments furthermore indicate that intragranular aragonite and Mg calcite cementation of microstructurally complex carbonate constituents in the shallow marine phreatic environment is a slower process than intergranular ooid cementation, characterized by relatively smooth surfaces.

Microbial mediation is considered an important process for the formation of primary dolomite at ambient temperature. Yet, no structural, mineralogical, chemical or isotopic means exist to discern this mode of dolomite formation from secondary dolomite. To explore the utility of metal isotopes in allowing this distinction we characterize magnesium and calcium stable isotope ratios in primary (proto)dolomites from a modern hypersaline environment.

Samples from the Khor Al-Adaid sabkhas in Qatar show consistent isotopic differences of Ca isotopes (Δ^44/40Ca) of -1.1 and -1.8 ‰ between solution and (proto-)dolomite and -0.3 to -0.7 ‰ between solution and organic phases, consistent with a previously postulated two-step fractionation process that enriches microbially mediated dolomite in ⁴⁰Ca (Krause et al., 2012). Mg isotopes reveal a more complex picture with varying magnitudes of fractionation across different microbial zones in the shallow subsurface in agreement with a wide range of Δ^26/24Mg-values previously observed in the sabkahs of Abu Dhabi (Geske et al., 2015) and suggested impact of microbial activity on δ²⁶Mg (Riechelmann et al., 2020). The high variability is moreover modulated by authigenic palygorskite formation close to the water-sediment interface that yields consistently ²⁶Mg-enriched signatures.

While clay-free sites suggest simple Mg precipitation from seawater into dolomite (Shalev et al., 2020), our data shows that Mg uptake into clay minerals does not allow a straightforward identification of a characteristic isotopic fingerprint in ancient primary dolomite, but shows potential to obtain a detailed picture of specific microbial involvement.

Here, we present element to Ca ratios (Mg/Ca, Sr/Ca, Na/Ca and Mn/Ca) and stable isotope data (δ¹⁸O, δ¹³C) of the parasitic foraminifer Hyrrokkin sarcophaga, collected from two different host organisms, Desmophyllum pertusum - a cold-water coral commonly found in cold-water coral reefs and Acesta excavata - a bivalve associated with cold-water coral reefs.

Our results reveal that the geochemical signature in H. sarcophaga is influenced by the host organism. Sr/Ca ratios are 1.1 mmol mol^-1 higher in H. sarcophaga that infest D. pertusum, which could be an indication that dissolved host carbonate material is utilised in shell calcification. Similarly, we measured 3.1 ‰ lower δ¹³C and 0.3 ‰ lower δ¹⁸O values in H. sarcophaga that lived on D. pertusum, which might be caused by the direct uptake of the host’s carbonate material with a more negative isotopic composition. Moreover, we observe higher Mn/Ca ratios in foraminifera that lived on A. excavata but did not penetrate the host shell compared to specimen that did.

H. sarcophaga is therefore, unlikely to be a reliable indicator of paleoenvironmental conditions using Sr/Ca, Mn/Ca, δ¹⁸O or δ¹³C unless the host organism is known and its geochemical composition can be accounted for. Still, these results provide interesting insights in the calcification process of these specialized foraminifera.