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 IndoPacific 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 interbranch 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 ii 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 te...

Evidence of reduced mid‐Holocene ENSO variance on the Great Barrier Reef, Australia

“…Cooler than present SSTs of −2.0°C at 5200 yBP have recently been inferred from Sr/Ca records from a Porites coral core from Heron Island, southern GBR, which corroborates with the evidence presented here for GPI [ Sadler , ]. This recent paleo‐SST record from Heron Island differs from an earlier Sr/Ca resolved SST reconstruction that suggested 1°C warmer conditions at 5300 yBP in the central GBR [ Gagan et al ., ]; however, recalibration of the C 14 date of this coral places it at ~6200 yBP [ Sadler , ], supporting evidence of similar extension rates to modern corals at 6000 yBP at Magnetic Island [ Lough et al ., ]. As linear extension has also been shown to be lower at offshore than inshore sites in the modern record [ Lough et al ., ], an alternate interpretation of lower linear extension rates at GPI is that the waters surrounding these reefs were more oligotrophic in the mid‐Holocene.…”

“…Using this as a first order assumption for the fossil GPI corals this suggests SST may have been up to ~2.5°C cooler by 5200 yBP than at 6000 yBP [ Lough et al ., ]. Cooler than present SSTs of −2.0°C at 5200 yBP have recently been inferred from Sr/Ca records from a Porites coral core from Heron Island, southern GBR, which corroborates with the evidence presented here for GPI [ Sadler , ]. This recent paleo‐SST record from Heron Island differs from an earlier Sr/Ca resolved SST reconstruction that suggested 1°C warmer conditions at 5300 yBP in the central GBR [ Gagan et al ., ]; however, recalibration of the C 14 date of this coral places it at ~6200 yBP [ Sadler , ], supporting evidence of similar extension rates to modern corals at 6000 yBP at Magnetic Island [ Lough et al ., ].…”

“…Results from wavelet transforms of coral luminescence indices between ~5200 and 4300 yBP at GPI suggest less frequent and less intense ENSO events during the mid‐Holocene, which agrees well with previous observations from elsewhere on the GBR [ Lough et al ., ] and in the wider Pacific [ Cobb et al ., ; McGregor et al ., ; Zhang et al ., ]. The reduction in ENSO frequency in the 2–7 year band after 5200 yBP is also coincident with lower linear extension rates in the fossil corals compared to modern values, suggestive of cooler SSTs, and is in agreement with a recent Sr/Ca reconstruction from the southern GBR [ Sadler , ]. This record encompasses the period when significant and widespread reef flat declines and reef turnoff events occurred on the GBR at 5500 yBP and 4600 yBP, respectively [ Smithers et al ., ; Perry and Smithers , ].…”