Coral reefs are more than just a vital part of the ocean. They can also reveal clues about the past. Analysis of coral skeletons can paint a rich picture of the ecological history of an ecosystem, from temperature variability to land use changes.
On the US Virgin Island of St. Croix, the ruins of a Danish sugar plantation built of harvested coral bricks could hold the key to understanding how and why the region was devastated by the 18th-century transatlantic slave trade.
Researchers at Georgia Institute of Technology and University of California, Los Angeles (UCLA) will travel to St. Croix to analyze this coral. They hope to determine how mining, dredging, and coral erosion affect nearshore biodiversity, contemporary coral populations, and to measure bathymetry or depth underwater. The project uniquely combines archeology and oceanography.
said Isaiah Bolden, co-principal investigator and assistant professor in the College of Earth and Atmospheric Sciences at Georgia Tech. “This can give us an interesting and climatically informed perspective on the timing of the transatlantic slave trade and why and how St. Croix became such a part of history.”
The first stages of the project involve collecting coral. Justin Dunnavant, PI associate, National Geographic Explorer, and assistant professor of archeology at UCLA, will use 3D photogrammetry, the process of combining images at different angles to create a 3D view, to identify the coral species that were used in building the farms. . Then the researchers will collect live and historical coral samples using minimally invasive techniques so that Bolden can analyze their composition.
Coral skeletons are a rich source of historical data. Living corals grow like trees in annual rings and can be dated by counting these rings. For the dating of ancient samples, the team is also applying uranium-thorium dating, a type of radiometric dating that relies on a natural “clock” that forms when radioactive uranium inside coral skeletons naturally decays into thorium.
Determining the ages of farm structures will help reveal whether corals were harvested from the sea floor alive or whether these structures were constructed from pre-existing coral rubble. Further analyzes can reveal clues about how the ecosystem responded to direct human impacts from the eighteenth century to the present. To this end, the project will also sequence proteins trapped in the skeletons of fossilized and contemporary corals on St. Croix to investigate genetic differences.
Coral skeletons can reveal more than age and genetic differences. The mineral “impurities” that are substituted in the growth bands of the limestone-like calcium carbonate skeleton of corals can be measured and used to infer sea surface temperature, salinity, pH, runoff, and many other environmental conditions during the life of the coral.
“Corals are trying to build an original skeleton made of calcium and carbonate ions,” Bolden said. “The problem is that seawater isn’t just a pure mixture of these two components, so some of these other things get in the way. For example, the element strontium, which has similar chemical behavior to calcium, is incorporated into the skeleton at a faster rate during cooler temperatures.” from warmer temperatures. This means that we can use the ratio of strontium to calcium across growth ranges in the coral skeleton as an indicator of past temperatures.”
Making these measurements involves drilling and removing powders from the coral growth bands and then using a mass spectrometer to analyze the chemical composition of the powders. The data, in turn, can be combined with automated records from modern times to develop equations that translate chemical changes into environmental changes.
“This is a really amazing opportunity to study how local coral reefs register and respond to climatic and human changes during a dark period of human colonization and civilization,” Bolden said. “How can we relate these new environmental and climatic records to the written historical record to give more detail about the story of colonization and the slave trade on St. Croix?”
St. Croix Ecosystem, Past and Present
The researchers will also collect contemporary data to build a better understanding of the modern coral reef ecosystems found on St. Croix. They will collect and analyze seawater samples and conduct surveys on coral species and coverage to capture current seasonal conditions and trends in coral health.
Throughout the project, researchers will collaborate with St. Croix Universities and local high schools to ensure that research not only benefits the community, but also benefits it by giving students research opportunities.
“I’m really interested in this opportunity to provide a climatic context for the history that’s often taught in schools to discover things we haven’t been taught,” Bolden said. “We’re talking about decolonizing Earth science and uncovering untold stories.”
the quoteCroix: Researchers Plan to Use Coral to Unravel History of Slave Trade on St. Croix (2023, April 18), Retrieved April 18, 2023 from https://phys.org/news/2023-04-coral-unravel-history-slave-st. html
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