Reef cores as records of Holocene water temperatures of the southern Great Barrier Reef: Geochemical analysis of traditional (Porites) and non-traditional (Acropora) reef building corals

Abstract

In order to accurately predict future climate variations induced by natural and anthropogenic forcings, we require a detailed understanding of the Earth’s climate system. With such temporally limited instrumental records, proxy-based reconstructions are an invaluable source of climate information. Current paleoclimate databases indicate a lack of records representing the southern hemisphere and tropics; key regions that can be represented by high resolution, coral geochemistry-based climate reconstructions. Reef coring techniques allow the recovery of significant proportions of coral material from throughout the Holocene, Pleistocene and beyond. Given the high species diversity of Indo-Pacific coral reefs, the presence of massive Porites commonly used for climate reconstruction is likely to be limited in reef cores. Corymbose Acropora, however, displays a high abundance in modern and fossil reef records and thus could act as a major source of new climate information from the Earth’s recent history. Previous investigations of Acropora geochemistry have focused on branches, with significant levels of secondary thickening smoothing environmental signals otherwise recorded by the Sr/Ca ratio paleothermometer. This project investigates the potential of Acropora inter-branch skeleton as a climate archive, following the identification of annual density banding patterns in computed tomography scans. Scanning electron microscopy based documentation of structure and growth patterns reveal that inter-branch skeleton is atypical for Acropora. Low linear extension rates, a simplified skeletal structure with limited secondary aragonite deposition, approximately horizontal density bands and seasonal cyclicity in Sr/Ca ratios bear a greater resemblance to massive corals such as Porites. These observations, combined with a high abundance in past and present Indo-Pacific reefs, suggest that inter-branch skeletal growth in Acropora may prove to be a valuable source of untapped paleoclimate information. A site-specific calibration for Acropora inter-branch skeleton Sr/Ca ratios to water temperature was conducted using modern corymbose Acropora colonies and concomitant instrumental water temperature records from the lagoon and fore-reef slope of Heron Reef, southern Great Barrier Reef (GBR). Large inter-colony differences in Sr/Ca ratios were observed in the modern cores, suggesting that inter-branch skeleton may not be a suitable archive for absolute temperature reconstruction. Normalisation of each Sr/Ca record to the corresponding core mean does, however, remove these inter-colony differences and allows the reconstruction of the seasonal water temperature amplitude with a sensitivity of −0.05 mmol/mol/°C. Application of the resulting transfer equation to the modern inter-branch skeleton Sr/Ca record successfully identified the greater seasonal temperature range of the lagoon compared to the adjacent reef slope. A site-specific calibration also was completed for southern GBR Porites at Heron Reef using the reef slope water temperature record. Recent reef coring expeditions on Heron Reef have recovered Porites (U-Th dated at 5,216±0.041 years before present [BP] and 6,962±0.044 years BP) and corymbose Acropora (U-Th dated at 6,128±0.027 years BP) samples from the mid-Holocene. These cores represent the suggested period of ‘mid-Holocene Thermal Maximum’ and thus can provide significant insights into previous conditions in the southern GBR over a period somewhat analogous to present day warming. Temperature reconstructions from the two Porites cores suggest cooler summertime and mean annual temperatures compared to modern conditions. These results contrast previous reconstructions for the inshore central GBR, yet most likely represent differences in the environmental reconstruction (reef flat vs reef slope) and/or radiocarbon dating uncertainties, rather than a meridional difference in climate response to forcing. Combining our southern GBR reconstructions with coral studies from across the tropical and subtropical western Pacific suggests a large scale spatial consistency in relative temperature variability over the Holocene. The similarities between our higher latitude reconstructions and those from Indonesia, Papua New Guinea and New Caledonia suggest that the southern GBR experienced a mid-Holocene Thermal Maximum sometime between 6,800–6,000 years BP. However, additional reef coring will be required to acquire coral material that dates over this suggested warming interval. Following changes in the precession cycle, mid-Holocene seasonal temperature amplitudes suggest a decrease in mean temperature seasonality at 6,962 and 5,216 years BP. The Acropora based reconstruction indicates a slight increase in the seasonal cycle at 6,128 years BP, during the presumed warmer interval, although this reconstruction remains within error of the modern record. This PhD project has provided a novel assessment and application of Sr/Ca ratios incorporated within Acropora inter-branch skeleton as an archive for high resolution paleothermometry. While the reconstructed seasonal amplitude in water temperature is associated with large errors, this new archive may prove useful over longer reconstruction periods, with greater temperature variations than experienced over the Holocene. Following an updated Porites calibration specific to the southern GBR, the project successfully utilised reef core material to provide the first mid-Holocene water temperatures for the southern GBR and provided an updated estimate of the timing of mid-Holocene warming in the GBR.