Reef growth on the Great Barrier Reef in response to sea-level rise

A new model of Holocene reef initiation and growth in response to sea-level rise on the Southern Great Barrier Reef

by: Sanborn et al. 

Summarized by: Baron Hoffmeister

What data were used?: This study analyzed sediment cores taken from the One Tree Reef of the Southern Great Barrier Reef in Queensland, Australia. Data was collected from the layers and sediment grains found within core samples taken from 12 different locations on the reef.

Methods: This study used biogenetic facies interpretation (i.e. physical, chemical, and biological aspects found within sediment and rock formations) from core samples to reconstruct reef growth and sea-level conditions.

Results: This study concluded that reef growth after a significant sea-level rise in the Pleistocene occurred in three stages. The first stage occurred over eight thousand years ago and was a rapid and shallow coral growth in presumably clear water. The average growth was around 6mm per year. The second stage of reef growth was between seven to eight thousand years ago, and this occurred with either turbid (i.e. cloudy water) or deeper water (i.e. over 5 meters in depth) conditions. The average growth was around 3mm per year. The third stage of growth was composed of shallow branching coral assemblages averaging 5mm of growth per year. This was referred to as a “catch up” in the reef growth sequence and continued until the reef reached the top of the sea level. It is hypothesized that more sediment-tolerant corals continued to slowly build up across the reef during this time. These are the types of corals that are now dominant on the Great Barrier Reef. This study also successfully identified six coral assemblages, and three algae assemblages correlating with specific paleoenvironments, creating a new model (see figure 1) for interpretation of samples containing similar assemblages for future studies. Using geochronology (i.e. dating rock formations) a lag of 700-1000 years of reef growth was confirmed in this experiment. There was a significant gap of growth on the wind-sheltered portion of the reef, which is the opposite of what was hypothesized previously (that corals would grow faster in wind-sheltered areas). Figure 1 shows a new model for reef growth response from the results found in this study.

Figure 1. The new model for reef growth after the flooding of the Pleistocene basement (the bottom most rock layer). This graph describes the One Tree Reef Holocene growth. This includes the three phases of growth and the composition of these three growth stages with its corresponding depth.

Why is this study important?  This study is important for determining how corals and other reef-building organisms respond to environmental change and stress like sea-level change. Understanding past environmental conditions are crucial for understanding how current environmental conditions can affect reef growth today.

The big picture: This study not only provides new and important data of reef growth response to historical climatic changes but can also be used to predict present-day reef response to sea-level change. As sea level continues to occur, a more comprehensive understanding of the way coral and reef-building organisms respond to environmental changes could lead to preserving the reefs as the ocean conditions change. The new model this study found can provide important data for how reefs grow, and provide important paleoenvironmental interpretation data.  

Citation: Sanborn, Kelsey L., Jody M. Webster, Gregory E. Webb, Juan Carlos Braga, Marc Humblet, Luke Nothdurft, Madhavi A. Patterson et al. “A new model of Holocene reef initiation and growth in response to sea-level rise on the Southern Great Barrier Reef.” Sedimentary Geology 397 (2020): 105556.

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