30,000+ years of reef survival

A/Prof. Jody Webster & Prof. Yusuke Yokoyama on GS Maya. © ECORD-IODP

Recently-published research confirms the Great Barrier Reef has been resilient to major changes in climate, sea level and water quality on a 30,000 year timescale.

Jody Webster is a marine geoscientist. He participated in our Can We Save the Reef? symposium at the Australian Museum in April 2018. With Yusuke Yokoyama, he led IODP Expedition 325 aboard the Greatship Maya.

Drill core samples of fossil reefs from transects across the shelf edge at Noggin Pass (offshore Cairns) and Hydrographers Passage (offshore Mackay / Whitsunday Islands) show how the reefs responded to major environmental changes. Their findings include:

• There were five widespread reef-death events in this period. The first two (around 30,000 and 22,000 years ago) were caused by falling sea levels. The next two (around 17,000 and 13,000 years ago) were caused by rising sea levels. The last one (around 10,000 years ago) was associated with sediment increase and reduced water quality.

• Reefs along the transects migrated seawards and landwards as the sea level went down and then up again, as depicted in this sketch by J.T. Keane:

Cartoon showing the evolution of the Great Barrier Reef between about 30-10 ka (credit: James Keane/Nature Geoscience; https://doi.org/10.1038/s41561-018-0127-3)

• Around 10,000 years ago the sea started to flood the old coastal plain that is now the Great Barrier Reef lagoon.

• Reef-building corals were continually present along the transects for the entire 30,000 year period of the study, providing local refugia to establish new reefs as old reefs died.

• Some sea level changes evident in the core samples occurred more rapidly than previously understood. This suggests models of glacier dynamics may be too conservative – not good news for coral reefs under projected climate change.

The Hydrographer’s Passage drill transect, showing the surface geomorphic context and drill-hole locations. The red lines represent the penetration depths. © Jody Webster

Reef 5 on the left hand side of this transect chart represents the modern outer barrier reef. The other drill locations were on the upper slope of the continental shelf to the east.

A simplified stratigraphic section showing the distribution of recovered core intervals, coral assemblages, their interpreted palaeowater depths, and selected ages. Chronostratigraphic boundaries of the four main shallow reef sequences (Reefs 1–4) are represented by solid coloured lines and long dashes; short dashes show their corresponding deep-water fore-reef slope deposits. The x axis represents the distance across the shelf and is schematic. © Jody Webster

Coral species identified in the fossilised core samples still live along the GBR today. They include the following groups from the Lizard Island Field Guide: Acroporidae (including Acropora, Montipora) Merulinidae and Poritidae

This research provides a positive long-term perspective on reef survival over millenia. However, it offers no assurance that GBR reefs will continue their recovery over the next few decades, given the current rate of increase in sea surface temperature (0.7º C per 100 years), the much larger dimensions of the modern GBR lagoon and the potential for further year-on-year mass coral bleaching and severe tropical storms.

Reports of these findings are available here and here. The full citations are: Jody M. Webster et al, Response of the Great Barrier Reef to sea-level and environmental changes over the past 30,000 years. Nature Geoscience 11, 426-432 (2018) and Yusuke Yokohama et al, Rapid glaciation and a two-step sea level plunge into the Last Glacial Maximum Nature 559, 603-607 (2018)

Corals have been present in Australian waters for much longer than 30,000 years – see also The last 126,000 years and Precambrian, Devonian, Triassic and Jurassic Reefs.