UNIVERSITY OF BROWN
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PICTURE: A compound known for shedding reconstructions of sea surface temperature is proving to be a great solution for past sea ice reconstruction, a new study. show more CREDIT: KAREN WANG
PROVIDENCE, RI [Brown University] – Sea ice is a critical indicator of changes in the earth’s climate. A new discovery from Brown University researchers could provide scientists with a new way to reconstruct information about the abundance and distribution of sea ice from the ancient past that could help understand the man-made climate change that is happening now.
In a study published in Nature Communications, researchers show that an organic molecule, commonly found in sediments of high latitude oceans, known as tetra-unsaturated alkenone (C37: 4), is produced by one or more previously unknown species of ice algae becomes . As the sea ice concentration subsides and flows, so do the algae associated with it, as well as the molecules that they leave behind.
“We have shown that this molecule is a powerful indicator of sea ice concentration,” said Karen Wang, Ph.D. Brown student and lead researcher. “If we look at the concentration of this molecule in sediments of different ages, we can reconstruct the sea ice concentration over time.”
Other types of alkenone molecules have been used as proxy for sea surface temperature for years. Algae that live on the ocean’s surface produce different amounts of alkenones known as C37: 2 and C37: 3 at different temperatures. Scientists can use the ratios between these two molecules in lake sediments to estimate past temperature. C37: 4 – the focus of this new study – has long been seen as a problem for temperature measurements. It appears in sediments that come from near the Arctic and affect the ratios C37: 2 / C37: 3.
“This is what the C37: 4 alkenone was best known for – it lowered temperatures,” said Yongsong Huang, principal researcher on the National Science Foundation-funded project and professor at the Brown Department of Earth, Environmental and Planetary Science. “Nobody knew where it came from or if it was useful for anything. People had some theories, but no one really knew. “
To find out, the researchers examined sediment and seawater samples with C37: 4, which were taken from icy places in the Arctic. They used advanced DNA sequencing techniques to identify the organisms present in the samples. This work resulted in previously unknown species of algae from the order Isochrysidales. The researchers then cultivated these new species in the laboratory and showed that they were, in fact, the ones producing an exceptionally high frequency of C37: 4.
The next step was to see if the molecules left behind by these ice-dwelling algae could be used as a reliable sea ice proxy. To this end, the researchers examined the concentrations of C37: 4 in sediment cores at several locations in the Arctic Ocean near today’s sea ice edges. In the recent past, the sea ice in these places was known to be very sensitive to regional temperature fluctuations. This work found that the highest concentrations of C37: 4 occurred when the climate was coldest and the ice was at its peak. The highest concentrations were due to the Younger Dryas, a period of very cold and icy conditions that occurred about 12,000 years ago. When the climate was warmest and the ice ebbed, C37: 4 was sparse.
“The correlations we found with this new proxy were far stronger than with other markers,” said Huang, a research fellow at Brown’s Institute for Environment and Society. “No correlation will be perfect because modeling sea ice is a messy process, but this is probably as strong as it gets.”
And this new proxy has some additional advantages over others, say the researchers. Another method of reconstructing the sea ice is to look for fossil remains of a different type of algae called diatoms. However, this method has become less reliable in the past as fossil molecules can decompose. Molecules like C37: 4 tend to be more robustly conserved, so they may be better suited for reconstruction over a longer period of time than other methods.
The researchers plan to continue researching these new species of algae to better understand how they are embedded in sea ice and how they produce this alkenone compound. The algae appear to live in salt bubbles and channels in the sea ice, but can also bloom shortly after the ice has melted. Understanding these dynamics will help researchers better calibrate C37: 4 as a sea ice proxy.
Ultimately, the researchers hope that the new proxy will enable a better understanding of sea ice dynamics over time. This information would improve models of past climates and enable better predictions of future climate change.
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Other co-authors of the study were Markus Majaneva, Simon Belt, Sian Liao, Joseph Novak, Tyler R. Kartzinel, Tim Herbert, Nora Richter and Patricia Cabedo-Sanz. The work was supported by the National Science Foundation (EAR-1762431).
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