Reef core insights into mid-Holocene water temperatures of the southern Great Barrier Reef

Abstract

The tropical and subtropical oceans of the Southern Hemisphere are poorly represented in present‐day climate models, necessitating an increased number of paleoclimate records from this key region to both understand the Earth's climate system and help constrain model simulations. Here we present a site‐specific calibration of live collected massive Porites Sr/Ca records against concomitant in situ instrumental water temperature data from the fore‐reef slope of Heron Reef, southern Great Barrier Reef (GBR). The resultant calibration, and a previously published Acropora calibration from the same site, was applied to subfossil coral material to investigate Holocene water temperatures at Heron Reef. U‐Th‐dated samples of massive Porites suggest cooler water temperatures with reduced seasonal amplitude at ~5.2 ka (2.76–1.31°C cooler than present) and ~7 ka (1.26°C cooler than present) at Heron Reef. These results contrast the previous suggestion of a mid‐Holocene Thermal Maximum in the central GBR around 5.35 ka and 4.48 ka, yet may be explained by differences in temperature of the shallow ponded reef flat (central GBR) and the deeper reef slope waters (this study) and potential large reservoir correction errors associated with early radiocarbon dates. Combining coral‐based water temperature anomaly reconstructions from the tropical and subtropical western Pacific indicates a coherent temperature response across the meridional gradient from Indonesia and Papua New Guinea down to the southern GBR. This similarity in reconstructed temperature anomalies suggests a high probability of an earlier expression of a mid‐Holocene Thermal Maximum on the GBR between ~6.8 and 6.0 ka.

A U‐Th Dating Approach to Understanding Past Coral Reef Dynamics and Geomorphological Constraints on Future Reef Growth Potential; Mazie Bay, Southern Great Barrier Reef

“…Relative sea level (RSL) was ~1 m higher than present on the inner‐shelf GBR by ~6,800 yr. BP (Chappell, ; Leonard et al, ; Lewis et al, ), and at least 0.75 m higher than present at Mazie Bay at ~5,800 yr. BP (Leonard et al, ). Coral‐derived Sr/Ca sea surface temperature (SST) reconstructions from the Central GBR also suggests SSTs were ~1.2 °C warmer at ~6,200 yr. BP (recalibrated age—Gagan et al, ; Sadler et al, ) compared to average SSTs of the early 1990s.…”

Calibration of Sr/Ca, Li/Mg and Sr‐U Paleothermometry in Branching and Foliose Corals

“…Secondary infilling may confound reconstructions at seasonal resolution but would likely have a smaller influence on reconstructions over longer (>annual) timescales. We expect that the relationships presented in this study developed using many replicate colonies of branching Acropora and Pocillopora will be especially valuable for capturing the interannual mean temperature using fossil samples of branching Acropora and Pocillopora genera that are common in reef cores (Sadler et al, ; Sadler, Webb et al, ; Sadler, Nguyen, et al, ) and have attempted to be utilized in millennial‐scale reconstructions (Guilderson et al, ; Sadler, Webb, et al, ; Toth et al, ). While further work is required to inform the use of Sr‐U to reconstruct temperature on subannual timescales, Sr‐U performs well for reconstructing mean temperature per colony especially when data from multiple years are averaged.…”

“…The use of different coral genera as temperature archives, particularly the relatively common and easily identified branching fossil corals (e.g., Pocilloporidae and Acroporidae ), has expanded with the wider availability of U‐Th dated specimens and reef drilling, thus providing an opportunity to address key questions (Sadler et al, ; Sadler, Webb, et al, ), such as temperature gradients during glacial‐interglacial transitions (Brenner et al, ). The branching coral genera Pocillopora and Acropora are especially widely distributed across both modern as well as fossil reefs and live within a relatively well‐defined and generally shallow marine environment.…”

Influence of marine biochemical cycles on seasonal variation of Ba/Ca in the near-shore coral Cyphastrea, Rat Island, southern Great Barrier Reef

“…2). These visually developed chronologies were tested and refined by tuning the Sr/Ca data to monthly averaged SST since the inverse dependency of the coral Sr/Ca on ambient SST has been validated by many independent studies (Beck et al, 1992;Gagan et al, 1998;DeLong et al, 2011;DeLong et al, 2013;Bolton et al, 2014;Sadler et al, 2016). Thus, low winter SST corresponds to peak Sr/Ca and vice versa.…”