Holocene microbialite records of terrigenous influence on water quality for the offshore southern Great Barrier Reef

Salas-Saavedra, Dr Marcos1, Webb, Professor Gregory1, Sanborn, Dr Kelsey2, Zhao, Professor Jian-xin1, Webster, Professor Jody 2, Nothdurft, Dr Luke3, Nguyen, Dr Ai1

1School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia, 2Geocoastal Research Group, School of Geoscience, University of Sydney, Sydney, Australia, 3School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reef ecology. Projected trends for the Great Barrier Reef (GBR) suggest continuing declines in reef health. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the GBR, longer-term patterns of water quality are poorly understood. Without historical data, it is difficult to disentangle natural and anthropogenic reef behaviour to better model anthropogenic effects. Here we present the first proxy-based long-term Holocene water quality reconstruction for any reef. The geochronological framework provided by rotary coring on Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8,200 to 1,800 years before present (BP) using centennial resolution microbialite-based geochemical proxies. Microbialites, which form part of growing reef framework, contain a robust proxy record of water quality through incorporation of trace elements (e.g., rare earth elements-REEs, Zr, Th, etc.) from ambient seawater. Trace elements associated with terrigenous flux were measured in dated microbialites as well as poorly consolidated Pleistocene limestone and palaeosol formed at the Pleistocene-Holocene unconformity at Heron and One Tree reefs. Paleosol samples have REE patterns similar to the Queensland based shale proxy Mud of Queensland (MuQ) consistent with local soil formed on exposed reefal limestone. Framboidal pyrite within the palaeosol suggests anoxic soil conditions during initial inundation. Younger microbialite-hosted REE and yttrium (REY) normalised to MuQ (subscript SN) have seawater-like patterns (e.g., light REE – LREE – depletion and high Y/Ho ratios) but with a well-defined, non-linear trend of changing water quality through the ensuing Holocene.

Immediately following reef initiation (>8,300 yrs BP) data suggest increasing terrigenous influence to 8,000 yrs BP on the basis of coordinated, more mud-like microbialite proxies, including reduced LREE depletion (e.g., (Nd/Yb)SN > 0.4) and lower Y/Ho ratios (< 53) with higher concentrations of lithofile elements. Proximal seawater became ‘cleaner’ from ~7,000 years ago, with opposing REY trends reflecting seawater with less terrigenous influence but showed marked mid-Holocene variability related to changing regional climatic factors. The strong fluctuation between intervals of high and low relative terrigenous sediment influence correlates well with particular regional and more global climate records, such as, the Indian-Australian Summer Monsoon (IASM) strength, high turbidity periods at 7.0, 5.4, and 2.7 ka BP with dampened El Niño Southern Oscillation (ENSO) frequency and fluctuations in local relative sea level. Water quality improved significantly after 3,200 yrs BP.

The new microbialite geochemical record provides a fully independent new water quality proxy and a means to interpret reef growth dynamics in relation to changing water quality associated with climate evolution at centennial to millennial scales. Such records provide valuable context for predictions of modern reef behavior in a changing world where coastal water quality is more likely to decline than to improve.


Biography

Dr. Marcos Salas Saavedra obtained his BSC in Biological Sciences at the Austral University of Chile in 2010 and he finished his PhD in Geochemistry and Geochronology at the University of Queensland in 2019. His research interest are in Biogeochemistry, Isotope Geochemistry, Carbonate Reefs Geology, U-series dating and Palaeoenvironment.

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