HOT OFF THE PRESS! New constraints on shallow-water carbonate sediment budgets in the Great Barrier Reef over the last 20,000 years – how much reef is really out there and how has it changed over time?

Hinestrosa, G., Webster, J.M. and Beaman, R.J., 2022. New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef. Scientific Reports, 12(1): 924.

Our new publication in the journal Scientific Reports builds on the discoveries made by the International Ocean Discovery Program (IODP) Expedition 325 (Great Barrier Reef Environmental Changes) and RV Southern Surveyor Expeditions SS09/2008, SS07/2007, with financial support from the Australian Research Council (grant no. DP1094001) and ANZIC IODP (Australian and New Zealand International Ocean Discovery Program Consortium); and of course relied on the help of our international network of scientific collaborators, including a special mention to Dr. Thomas Felis from MARUM, Germany and Professor Yusuke Yokoyama from the University of Tokyo.

Coral reefs, and in general shallow-water carbonates, play a key role in the global carbon cycle, affecting ocean and climate processes. Yet, the extent of their influence in the global carbon cycle, particularly during the postglacial rise in the atmospheric concentration of carbon dioxide of ~100 ppm is poorly constrained. To solve this puzzle, we need to start by understanding how much calcium carbonate associated with coral reefs there is on the planet.  

So we began by looking at the Great Barrier Reef, the largest extant coral reef system in the world. This reef system has experienced numerous episodes of growth and demise of the past 400-500 ky, but we focused our attention on the last 20,000 years of postglacial development since the Last Ice Age. As well as the more recent Holocene reefs (~8,000 ka to present) the Great Barrier Reef has a large system of drowned reefs along its shelf edge that spans the early- and mid-postglacial periods (>20-9 ka).  

Using recently acquired datasets, including the precise temporal constraints provided by the IODP Expedition 325 reef cores, we calculated the shallow‐water CaCO3 volumetrics and mass for the entire Great Barrier Reef region over the past 20,000 years and extrapolated these results globally. In our estimates, we also included deposits that have been neglected in some previous global carbonate budgets: Holocene Halimeda bioherms (see associated work by GRG collaborator Dr Mardi McNeil on the bioherms) located on the shelf, and those postglacial pre‐Holocene (now) drowned coral reefs located on the shelf edge.

Our results show that in the Great Barrier Reef alone, the drowned reef deposits represent about 135 Gt of accumulated CaCO3, equivalent to 16–20 % of the mass of the younger Holocene reef deposits. Applying plausible assumptions, we estimate a global presence of ca. 8100 Gt CaCO3 of Holocene reef deposits, ca. 1500 Gt CaCO3 of drowned reef deposits and ca. 590 Gt CaCO3 of Halimeda shelf bioherms.  

Significantly, we also found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and two periods of sharp atm. CO2 increase (14.9–14.4 ka, 13.0–11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with episodes of known rapid reef accretion inferred from previous global carbon cycle models and confirmed by the precise chronology provided by the Exp. 325 reef cores from the GBR shelf edge.

We hope that further surveys of drowned reefs around the world and geobiochemical modelling work can help us better understand influence of shallow-water carbonates on the global carbon cycle and postglacial climate.  


Gustavo, Jody and Rob


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