Coral reef development and sea-level changes over the past 50,000 years: new evidence from the north-west shelf of Australia

Webster, Jody M., Dechnik, Belinda, Sanborn, Kelsey, Yokoyama, Yusuke, Braga, Juan Carlos, Renema, Willem, Humblet, Marc, Beaman, Robin J., Nothdurft, Luke D., Webb, Gregory E., Zhao, Jian-Xin, Murphy, Richard J., Gallagher, Stephen J., O’Leary, Michael, and Paumard, Victorien (2022) Coral reef development and sea-level changes over the past 50,000 years: new evidence from the north-west shelf of Australia. In: Camoin, Gilbert F., and Hallman, Nadine, (eds.) Coral Reefs and Sea-Level Change: Quaternary Records and Modelling. International Association of Sedimentologists, pp. 215-273.

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Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia’s North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps.

In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km south-east of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart.

High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines and tidal-estuarine channel systems on the adjacent shelf between ~ 90 to 110 mbsl.

The authors have completed detailed logging, high-spatial resolution hyperspectral scanning, petrologic, mineralogic and sedimentary facies analysis of these cores, including a precise palaeoenvironmental reconstruction based on coral, algal and larger benthic foraminifera assemblages; and extensive radiometric dating. The authors have observed a complex suite of lithologies including in situ coralgal reef frameworks, well-lithified to friable grainstones, packstones and coralline algal floatstone facies separated by at least two major palaeosol horizons.

Together with thirty 14C-AMS and closed-system U/Th ages spanning 10.7 to > 50 ka, the authors define a complex but consistent record of four distinct chrono-stratigraphic units (Units 1 to 4), representing a repeated succession of shallow reef to deep reef-slope depositional settings as the platforms experienced repeated sea-level oscillations (interstadial/stadial to glacial/deglacial) over the last 75,000 yr.

Two distinct phases of shallow-water, high-energy reef development are defined. The age of the older, diagenetically distinct reef unit (Unit 3) is unknown but interpreted to have developed before the MIS 4 lowstand (~ 65 ka). However, firm chronological constraints on the MIS 3 development of the younger reef unit (Unit 2), place the position of relative sea-level (RSL) between ~ 63 to 75 + 1.8 mbsl by 45.95 to 39.23 + 0.2 ka, consistent with other predictions and observations for the region.

Following its exposure and demise due to sea-level fall to the Last Glacial Maximum (LGM), the platform system was unable to re-establish fully as it was reflooded during the subsequent deglacial sea-level rise. Deeper reef slope (Unit 1) facies dominate the core tops between ~ 13.2 to 10.7 ka, representing a major hiatus in shallow-water reef development on the platforms.

Deglacial sea-level rise was either too fast and/or other environmental conditions inadequate (i.e. massive riverine sediment flux due to the strengthening Australian summer monsoon and/or reworking of shelf sediments) to allow re-establishment of shallow-water coral reef development on the platforms apart from a few isolated and distal locations (i.e. Ashmore, Cartier, Adele and Scott Reefs).

Item ID: 77261
Item Type: Book Chapter (Research - B1)
ISBN: 978-1-7398845-1-2
Copyright Information: © 2022 International Association of Sedimentologists
Funders: Australian Research Council (ARC)
Projects and Grants: ARC grant DP1094001, ARC grant DP120101793
Date Deposited: 31 Jan 2023 03:20
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370504 Marine geoscience @ 100%
SEO Codes: 18 ENVIRONMENTAL MANAGEMENT > 1805 Marine systems and management > 180504 Marine biodiversity @ 100%
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