Sea level and climate change from geochemical analysis of corals from the Great Barrier Reef: IODP Expedition 325

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences

Abstract

One of the largest uncertainties in projections of environmental change involves the response of the Greenland and Antarctic ice sheets to 'greenhouse warming'. These ice sheets play an important role in defining climate through affecting wind patterns, through reflecting sunlight back to space and through providing sources of fresh water the can influence ocean circulation. As the planet warms, we expect these ice sheets to respond, but we are not sure in exactly what ways. The Greenland ice sheet is likely to start melting, and indeed there is already evidence to suggest that this might be happening. The Antarctic ice sheet may initially grow slightly (since it is so cold that a slight warming simply makes it easier to snow there). However, evidence from the past combined with modern observations suggests that these ice sheets are capable of extremely rapid decay through catastrophic collapse of large segments, resulting in large and rapid sea level rises. The work proposed here is focussed on the Great Barrier Reef of Australia, which, although far from the ice sheets, is well placed to reveal evidence for past ice sheet growth and collapse. We will use analysis of the chemistry of corals collected in drill cores down through the reef to reconstruct changes in water depth, and hence sea level, in the interval 11,000- 20,000 years ago. This interval coincides with the decay of the ice sheets from their maximum extent during the last ice age, to their present size. At the peak of the last ice age, global sea level was about 120m below its present level, due to the amount of water bound up as ice, so as the ice sheets melted, sea level rose. Our coral records will be used with other evidence from the types of corals and algae in the cores, to identify the rate, timing and magnitude of these sea level changes, as well as to reconstruct changes in local temperature. These data will contribute to our understanding of the triggers of ice sheet collapse and sea level rise.

Publications

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Brenner L (2020) Coral Record of Younger Dryas Chronozone Warmth on the Great Barrier Reef in Paleoceanography and Paleoclimatology

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Felis T (2014) Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum. in Nature communications

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Hathorne E (2013) Interlaboratory study for coral Sr/Ca and other element/Ca ratio measurements in Geochemistry, Geophysics, Geosystems

 
Description Background: The project had two components: a) MODERN CORALS: analysis of modern corals for calibration and benchmarking to aid subsequent palaeoclimate and water depth reconstruction, and, b) FOSSIL CORALS: geochemical analysis of fossil corals from the GBREC cores to reconstruct climate and sealevel change from the last glacial maximum to ~10ka BP.
Summary of main achievements, outcomes and significance:
a) MODERN CORALS:
• We have demonstrated that skeletal del13C of Isopora corals varies as a function of water depth, thereby supporting the hypothesis presented in our proposal. Despite the anticipated scatter, there is a highly significant difference (of ~0.5‰; p<0.01; Mann-Whitney U Test) between the composition of corals from 0-2m and those from 5-12m water depth at the same reef. This supports the potential use of coral del13C as an aid to palaeowater depth reconstruction. Importantly, Isopora is the dominant well-preserved coral in the GBREC cores.
• We, along with Expedition 325 colleagues, have used high resolution sampling to demonstrate that skeletal del18O and Sr/Ca of modern Isopora and Porites corals in the region record seasonality and changes in mean temperature and water composition, thereby supporting their potential for climate reconstruction in the GBREC cores.
b) FOSSIL CORALS:
• Our results, along with those of other Expedition 325 colleagues, are providing reconstructions of sea level and climate of the LGM through the first half of the last deglaciation. Oxygen isotopic composition is recording the combined influences of sea level, temperature and regional water isotopic composition change. Sr/Ca analyses are suggesting the LGM was ~2-4 ° C cooler than present. When this is compared with the skeletal del18O changes (LGM-present ~3‰) we infer a major regional freshening over the deglaciation and Holocene. We have also discovered a significantly steepened meridional temperature gradient along the Great Barrier Reef Region during the deglaciation (published in Nature Communications). This work provides new constraints on our understanding of the ability of corals and coral reefs to respond to rapid climate and sea level change.
• Skeletal del13C records capture changes from the LGM into the early deglaciation and through to the pre-industrial that are similar to those recorded in the EPICA ice core and interpreted as reflecting changes in the global carbon cycle. More data (to come from colleagues) will allow us to assess evidence for rapid sea level changes as per the results from modern corals.
Exploitation Route Our findings on the ability of reefs to cope with previous rapid sea level and climate change may have implications for the management of the Great Barrier Reef in the face of future climate change.
Sectors Environment