Authors: Hinestrosa, G., Webster, J.M., Beaman, R.J.
Publication: 31st IAS Meeting of Sedimentology, 22-25 June 2015 International Association of Sedimentologists, Krakow, Poland
Coral reefs constitute one of the most extensive carbonate factories and, over geological timescales, can be a significant oceanic and atmospheric CO2 source. Yet their role in postglacial climate change remains elusive. This understanding is hindered by the difficult access to the Pleistocene coral reefs drowned during the last transgression. Using a unique dataset of two shelf-edge sites of the central Great Barrier Reef (GBR), Australia, we provide new constraints on local- and shelf-scale shallow-water carbonate budgets and also extend these estimations globally. Pre-Holocene, shallow-water carbonate accumulation has been previously unaccounted for in global models, and our first-order estimates provide important new constraints on postglacial atmospheric and climate change.
The datasets were composed of a dense array of 2D seismic lines, high-resolution (5 x 5 m) multibeam bathymetry, seafloor samples, and data from 34 boreholes in 17 drilling locations (IODP Expedition 325) with downhole logs and cores, radiometric ages and lithological interpretations. A digital elevation model (DEM) of the entire GBR (100 x 100 m) and a recently available GIS dataset of the Holocene reef features were used for regional estimations of carbonate accumulation.
Locally, we reconstructed the 3D sub-surface architecture of the two study sites (Hydrographers and Noggin Passages) using the seismic profiles, which, together with the bathymetry data, permitted the calculation of bulk volumetrics of the postglacial reef. Core interpretations confirmed the shallow (0-30 m), reefal nature of the shelf-edge formations, and radiometric ages confirmed their pre-Holocene, postglacial ages (11 to >20 ka BP). The cores provided the petrophysical parameters (density, porosity) needed to assess the CaCO3 accumulations at these two sites.
Regionally, the DEM of the GBR was used to reconstruct the postglacial transgression, and to quantify the marine-flooded area through time. Applying geomorphic and volumetric parameters calculated at the study sites, it was possible to obtain a reasonable estimate of carbonate accumulation in the GBR from the Last Glacial Maximum through to ca. 10 ka BP. The GIS dataset and published GBR data were used to estimate the Holocene reefal accumulation.
We found that, despite occupying only between 1.2 to 2.4% of the total GBR shelf, the shelf-edge reefs constitute an important portion (10 to 20%, 135 Gt) of the total postglacial shallow-reef CaCO3 accumulation in the GBR. The majority (ca. 750 Gt) is attributed to the Holocene reefs. Applying the GBR parameters to published estimates of global Holocene reef areas and assuming similar Holocene-to-Pleistocene accumulation ratio to the GBR, we estimate that some 1500 Gt of CaCO3 may have accumulated globally in shallow waters during pre-Holocene, postglacial times. These results suggest a stronger influence of coral reefs in postglacial climate change than previously acknowledged. Interestingly, the timing of the drowned reefs accumulation in the GBR matches episodes of postglacial sharp increase in atmospheric CO2 and a period of decrease in atmospheric δ13C.