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.

Understanding growth rings in geoduck clams and their historical environmental significance

North Pacific climate recorded in growth rings of geoduck clams: A new tool for paleoenvironmental reconstruction

Robert C. Francis, Nathan J. Mantua, Edward L. Miles, David L. Peterson

Summarized by Baron Hoffmeister

What data were used? Growth chronology (i.e growth patterns that accumulate over years in the shell of the organism, similar to tree rings) of geoduck clams (see figure 1) collected in Washington, USA were used to reconstruct sea-surface temperatures (SST) in the Strait of Juan de Fuca.  

This is an image of a geoduck. These are known to have life spans lasting over 165 years. From How Stuff Works.

Methods: This study used growth ring data in geoduck clams to determine how sea surface temperatures affected the shell growth (something called “accretion”) within these organisms over their life span. 

Results: Geoduck clams are a part of the class Bivalvia (i.e., a marine or freshwater mollusk that has its soft body compressed by a shell; this includes other organisms like snails and squids). These organisms produce their own shells, and the shells continue to grow as these organisms age (unlike organisms like mammals, who stop growing at a certain age). The shell accretion of these organisms can be observed under a microscope from samples of the shells. These are called growth lines and the spacing in between lines indicates how much new shell material the organism produced during a certain period of time (see figure 2). The growth lines of the geoduck clams found within Strait of Juan de Fuca correlated strongly with sea-surface temperatures. Researchers found that when the water was warmer, more growth was observed. This is common for a number of marine bivalves, and these proxy methods help construct a better understanding of sea surface temperatures from the past. 

The top panel is an SEM micrograph of the ring structure in a 163-year-old geoduck clam. An SEM is a scanning electron microscope that uses a focused beam of electrons that interact with the sample and produce signals that can be used to collect data about the surface composition and surface structures. The bottom panel shows the growth index (solid black line) with local air temperatures (dotted line) from 1896 to 1933. From 1900 to 1910, shell accretion correlated with warmer air temperatures.


Why is this study important? This study helps reconstruct environmental conditions and researchers can use this data in conjunction with other climate proxies to better understand how current climate patterns and ocean temperatures can affect marine ecosystems in the North Pacific basin.

The big picture: This study is important, not only for creating a more cohesive climate proxy database, but also indicating that shell accretion in specific marine organisms can provide important climatic data. Bivalves have a large geographic range and the data collected from these organisms through shell accretion studies can allow us to have a better understanding of historic climate conditions worldwide. 


Francis, R. C., Mantua, N. J., Miles, E. L., & Peterson, D. L. (2004). North Pacific climate recorded in growth rings of geoduck clams: A new tool for paleoenvironmental reconstruction. Geophysical Research Letters, 31(6).

Baron Hoffmeister, Environmental Scientist & Geologist

Baron in the Calhan Paint Mines in Calhan, CO.

Hey there! My name is Baron Hoffmeister and I am a graduating senior at the University of South Florida. I am pursuing a Bachelor’s degree in  Environmental science with a minor in geology. I have always been drawn to the outdoors, and extremely curious about nature and how things work. When I decided to attend college I knew that I wanted to study something related to science. I decided to pursue environmental science as I became extremely interested in climate change and resource management.  In my junior semester at USF, I went on my first geology field trip to Fort de Soto Park in St. Petersburg, Florida. This was for USF’s Sedimentary Environments course and the goal of the trip was to study common sedimentary structures associated with barrier island formations. On this field-trip, we explored the barrier islands that make up Fort de Soto park and in several locations took pound core samples and dug trenches. In figure 1 you can observe some of the pound core samples taken from various parts of Fort De Soto Park. This is one of many useful methods that sedimentologists use to understand depositional history within a small region. This hands-on field experience left an impact on me and I immediately fell in love with geology. I was so far along in my environmental science program that it didn’t make sense to switch majors, so I chose to pick up a minor in geology instead. Fortunately, the majority of the geology courses I have taken all allowed me to take trips and participate in fieldwork relating to the courses. Most importantly, each of my professors expresses such a profound passion for geology that it is infectious and this has been instrumental in my admiration for geology. 

Pound core samples from Fort De Soto Park in St. Petersburg, FL.

My favorite part about being a scientist is that it allows me to spend time outdoors learning about the environment and the process that takes place that shapes the world we live in. This has always driven my passion for science and has carried over into my personal life. Any opportunity that I can find to go and explore nature I jump at. Figure 2 is a photo from my last trip to Colorado where I had the chance to explore the Calhan Paint Mines and study the large clay deposits in this region. It was very cold and windy that day. I believe with the windchill the temperature that day was in single digits. There was also a brief snow shower that rolled through and covered the entire park in a fresh layer of snow while we were there. After living in Florida for the past five years it was nice to finally see some snow again! 

Currently, I am interning for a contract management group before I apply to graduate school for sedimentary geology to start in the Fall of 2021.  I am interested in studying sedimentary geology and its relation to paleoclimate. Specifically, I am interested in how past climates have affected the rates of sedimentation and carbon cycling. I want to use this information to understand how current climate change patterns affect carbon cycling and sedimentation throughout the world. Science communication is critical for sharing ideas, research, and for education, but it is also crucial for being a great scientist. That’s why I have decided to write for Time Scavengers. I am excited about this learning process and the opportunity to educate others about geology, and understanding climate change!

I would tell any aspiring scientist to work hard and pursue an education, even if it is through your own efforts and experience.

Understanding the Permian-Triassic transition with fossil data before and after the mass extinction event

Environmental instability prior to end-Permian mass extinction reflected in biotic and facies changes on shallow carbonate platforms of the Nanpanjiang Basin (South China) 

Li Tian, Jinnan Tong, Yifan Xiao, Michael J. Benton, Huyue Song, Haijun Song, Lei Liang, Kui Wu, Daoliang Chu, Thomas J. Algeo

Summarized by: Baron C. Hoffmeister. Baron Hoffmeister is currently an undergraduate senior at the University of South Florida pursuing a degree in Environmental Science and Policy with a minor in Geology. After graduation, he plans to attend a graduate program for environmental management. When graduate school is complete, he plans on working for the National Parks services. Baron is apart of the geology club as well as the fishing club and spends his free time hiking, fishing, and socializing with friends.

What data were used? Fossil taxa found in Southern China carbonate platforms that date back to the Permian-Triassic extinction event, ~251 million years ago. This data sheds light on the mass extinction event that spans the Paleozoic and Mesozoic Eras. 

Methods: Analytical evaluation of fossils present from three separate stratigraphic areas across South China, from before, during, and after the Permo-Triassic extinction event. 

Results: In this study, the fossil data evaluated at each site led to the discovery of common trends. Each formation had similar fossil accumulations, even though the formation would have been located a far distance apart. This means that each location was affected similarly  by the same event for the accumulation of similar fossils to appear in the corresponding strata. This is hypothesized to be the late Permian mass extinction. Another similarity between the three areas was that each section had a foraminifera gap between strata boundaries. At the same time, each boundary represented a different aspect of a shallow marine environment. For example, the Wennga Formation had strata before the extinction boundary that was littered with Permian fauna fossils that occurred in shallow marine environments. Post-extinction boundary strata didn’t possess these fossils. This is another indication of the severity of the mass extinction event. The Taiping section had different types of rock formations with different compositions; the transition of the rock from before the extinction to after showed a rapid die-off of organisms living in this area. Finally, in the Lung Cam section, there were fewer fossils than the other two (most likely due to poor fossil preservation conditions); however, the fossils that were found resembled those in the other sections studied. Further, the Lung Cam section had foram gaps consistent with the other sections.

Skeletal composition within strata at each study section. The three sections had similar organisms preserved in each and even showed similar gaps in fossil occurrences, indicating where the extinction event happened.

Why is this study important? This study strengthens what we know about the Permian-Triassic transition. These fossils, across multiple areas, were present in a shallow marine environment and were greatly affected by environmental instability during this time. The strata at each location, Wengna, Taiping, and Lung Cam, are remnants from the fatal conditions in the marine environment at this time. This can better help us understand and conceive how shallow marine organisms could be affected today during climate change. 

The big picture: This study shows the significant changes in fossils from  before and after the largest extinction event in Earth’s history. There is consistent evidence within and between each section studied, indicating a widespread event that negatively affected shallow marine life during this time.

Citation: Tian, Li, Jinnan Tong, Yifan Xiao, Michael J. Benton, Huyue Song, Haijun Song, Lei Liang, Kui Wu, Daoliang Chu, and Thomas J. Algeo. “Environmental instability prior to end-Permian mass extinction reflected in biotic and facies changes on shallow carbonate platforms of the Nanpanjiang Basin (South China).” Palaeogeography, Palaeoclimatology, Palaeoecology 519 (2019): 23-36. DOI: 10.1016/j.palaeo.2018.05.011