September 11, 2011
Research by College of Wooster Geologist Mark Wilson and three colleagues found that small holes drilled in fossil corals on San Salvador Island in The Bahamas provided evidence of a previously unknown significant drop and rise in sea-level.
WOOSTER, Ohio – The discovery of numerous small holes drilled in fossil corals on San Salvador Island in The Bahamas has led to the documentation of rapid global climate change 123,000 years ago during the Last Interglacial. Geologist Mark Wilson found these excavations carved by marine sponges and clams, and made the suggestion in 1991 that they represented a previously unknown significant drop and rise in sea-level. It was an unpopular idea then, but now Wilson and his colleagues have documented this important event in the Sept. 11 edition of the prestigious international journal Nature Geoscience.
“What we discovered was a fossil coral reef that was eroded down to a flat surface when sea-level dropped and exposed the reef to the biological erosion of clams and sponges, followed by the physical erosion of waves and weathering about 123,000 years ago,” says Wilson, the Lewis M. and Marian Senter Nixon Professor of Natural Sciences and Geology at The College of Wooster. “Then a new coral reef grew again after sea-level rose about 119,000 years ago. These two reefs show that sea-level fell 4-6 meters and then rose again to its previous level within only 4,000 years.”
William G. Thompson of Woods Hole Oceanographic Institution, the senior author of the paper, provided uranium-thorium radioactive dates on the corals above and below the erosion surface using a new method he was instrumental in developing. This chronology gave the critical time context to the sea-level event story. Two other authors, Al Curran and Brian White of Smith College, are experts on coral reefs and stratigraphy.
The significance of this study, according to Wilson, is that “we now know that sea-level can fluctuate dramatically during a time like our own because we also live in an interglacial interval.”
Wilson points out that sea-level changes like this when glacial ice volume changes, meaning there must have been a remarkably short interval of global cooling (which produced greater ice volume, lowering sea-level) and then warming (which melted the ice, raising sea-level).
The most valuable outcome of the research is the fact that the Last Interglacial (LIG) is often cited as an analogue for future changes in sea-level. “Estimates of LIG sea-level change, which took place in a world warmer than that of today, are crucial for estimates of future rates of rise under IPCC (Intergovernmental Panel on Climate Change) warming scenarios,” say the authors in the article.
“With our evidence, we can, for the first time, show a detailed magnitude and timing of sea-level fluctuations due to climate change without human influence,” says Wilson. “This shows that the system can change rapidly naturally.”
Despite the findings, Wilson is quick to affirm the conclusive evidence that current global climate change is, in fact, affected by people. “Humans have had a significant role,” says Wilson, “particularly through human-produced atmospheric carbon dioxide.”
As to why sea-levels fluctuated so rapidly between 123,000 and 119,000 years ago, Wilson says, “We don’t know. There is a very delicate balance that could have been affected by changes in solar radiation, volcanic eruptions, oceanographic shifts, or other factors.” What Wilson does know is that paleontological evidence coupled with radioactive dating revealed a rapid climate fluctuation that will provide vital information about the history of planetary change and valuable clues for the future.
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