Unlocking tropical (palaeo)environmental histories: sub-seasonal modern and neogene environmental reconstructions using the sclerochemistry of giant clams (Tridacna) and larger benthic Foraminifera
Authors:   , Fursman Maximilian Holland 
Published: February 2025

Coral reef ecosystems are highly vulnerable to changing environments, both those driven by local anthropogenic activities, such as pollution, and by global impacts, such as climate change-induced warming. Both activities can negatively impact the health and growth of reef-building organisms. However, turbid reef environments, characterised by elevated suspended sediment loads and low light conditions, have been shown to exhibit increased resilience to adverse environment parameters. As such, they are considered as potential refugia for future reef life. 4D-REEF is a multidisciplinary, multi-project research consortium (funded by EU Horizon 2020), aimed at deepening our understanding of the past, present and future of turbid reef ecosystems and this thesis is one part of the greater project. It aims to investigate how diagenetic alteration, seasonality and anthropogenic influence can impact sclerochemical results, as well as to evaluate the accuracy of a newly developed programme in sclerochronological assessments. To achieve these objectives several modern aquarium and natural habitat, as well as Pliocene (4 – 2.7 Ma) and Miocene (∼17 Ma) fossil Tridacna sp. and Larger Benthic Foraminifera (LBF) were analysed as archives. Both organisms are typical reef calcifiers and Tridacna sp. were chosen due to their longevity, rapid shell accretion and excellent fossilisation potential and LBF due to their abundance and rapid response and adaptability to environmental changes.
An assessment of how diagenetic processes can impact sclerochemistry, specially stratigraphic age, was performed using a Miocene aragonitic Tridacna sp. from East Borneo using a combination of XRD, TIMS, LA-(MC-)ICPMS and EMP analysis. The SIS-based age of the sample was found to have been strongly affected by diagenetic alteration relative to its magneto-/bio-stratigraphic age imparting an older SIS-derived age as a result. Through EMPA based elemental mapping we showed that ∼10 𝜇m cracks, infilled with very high [Sr] minerals, were driving these discrepancies, highlighting the importance of pre-analysis screening of a sample for such alterations. This result was later also used to inform the focus area for analysis in a Pliocene Tridacna sp.
The effect of seasonality on Tridacna sclerochemistry was assessed using LA-ICPMS analysis and a counted shell chronology of a modern sample of T. squamosa, which had been removed from natural conditions in the Vietnam Sea and grown for ten years in Burgers’ Zoo Aquarium, Arnhem, The Netherlands, at both several years of constant temperature and, later, of induced seasonality. This long-term, quasiculturing experiment served to highlight the impact that biophysiological processes such as growth rate and stress response have on sclerochemistry, and how they can potentially overshadow seasonal variations in the shell chemistry in reef-dwelling organisms. A decrease in growth rate, potentially as a result of organismal stress (from ∼10 𝜇m/day prior to induced seasonality to ∼3 𝜇m/day post induced seasonality) is shown to have potentially impacted El/Ca ratios such as Mg/Ca and Sr/Ca. Such ratios are known to display seasonal trends, but these are not discernable in our results, potentially as a result of the decreased growth rate in this period overshadowing them. Na/Ca ratios however still displayed clear seasonal variation, suggesting that biophysiological processes do not obfuscate the environmental variability in [Na] incorporation. The results also showed that the micromilling sampling size for stable isotope analysis was impacted by decreased growth rates, which was used to inform sampling methods for our Pliocene Tridacna sp.
LBF community compositions were assessed using modern LBF samples from various islands in the Spermonde Archipelago, South Sulawesi, collected over ∼15 years of fieldwork, to examine spatial and/or temporal trends in community composition and sclerochemistry with regards to proximity to the city of Makassar, known for its anthropogenic pollution output. Elemental analysis of foraminifera genus Amphistegina from the study sites showed spatial and temporal trends in [P], [Mn], [Cu], [Ba] and [Pb], though interpretation of the sources of pollutants is difficult to assess due to complex socio-economic factors and interplay between island communities. Seawater Cu and Pb reconstructions displayed clear zones of pollution up to an order of magnitude higher than unpolluted regions of the ocean, potentially as a result of increased fishing activity by locals and harmful fishing methods such as blast fishing. No statistically significant community composition trends were found across the islands studied, although this analysis cannot rule out the possibility that a more in-depth assessment may reveal some.