Brown scientists find a new way to understand climate change
Brown University
Sea ice is a critical indicator of changes in the Earth’s
climate. A new discovery by Brown University researchers could provide
scientists a new way to reconstruct sea ice abundance and distribution
information from the ancient past, which could aid in understanding
human-induced climate change happening now.
In
a study published in Nature
Communications, the researchers show that an organic molecule often
found in high-latitude ocean sediments, known as tetra-unsaturated alkenone (C37:4), is produced by one or more
previously unknown species of ice-dwelling algae. As sea ice concentration ebbs
and flows, so do the algae associated with it, as well as the molecules they
leave behind.
“We’ve shown that this molecule is a strong proxy for sea ice concentration,” said Karen Wang, a Ph.D. student at Brown and lead author of the research.
“Looking
at the concentration of this molecule in sediments of different ages could
allow us to reconstruct sea ice concentration through time.”
Other types of alkenone molecules have been used for years as proxies for sea surface temperature. At different temperatures, algae that live on the sea surface make differing amounts of alkenones known as C37:2 and C37:3. Scientists can use the ratios between those two molecules found in sea sediments to estimate past temperature. C37:4 — the focus of this new study — had been long considered a bit of problem for temperature measurements. It turns up in sediments taken from closer to the Arctic, throwing off the C37:2/C37:3 ratios.
“That
was mostly what the C37:4 alkenone was known for — throwing
off the temperature ratios,” said Yongsong Huang, principal investigator of the
National Science Foundation-funded project and a professor in Brown’s
Department of Earth, Environmental and Planetary Science. “Nobody knew where it
came from, or whether it was useful for anything. People had some theories, but
no one knew for sure.”
To
figure it out, the researchers studied sediment and sea water samples
containing C37:4 taken from icy spots around the
Arctic. They used advanced DNA sequencing techniques to identify the organisms
present in the samples. That work yielded previously unknown species of algae
from the order Isochrysidales. The researchers then cultured those new species
in the lab and showed that they were indeed the ones that produced an
exceptionally high abundance of C37:4.
The next step was to see whether the molecules left behind by these ice-dwelling algae could be used as a reliable sea ice proxy. To do that, the researchers looked at concentrations of C37:4 in sediment cores from several spots in the Arctic Ocean near the present-day sea ice margins. In the recent past, sea ice in these spots is known to have been highly sensitive to regional temperature variation.
That work found that the highest concentrations of C37:4 occurred when climate was coldest
and ice was at its peak. The highest concentrations dated back to the
Younger-Dryas, a period of very cold and icy conditions that occurred around
12,000 years ago. When climate was at its warmest and ice ebbed, C37:4 was sparse, the research found.
“The
correlations we found with this new proxy were far stronger than other markers
people use,” said Huang, a research fellow at the Institute at Brown for
Environment and Society. “No correlation will be perfect because modeling sea
ice is a messy process, but this is probably about as strong as you’re going to
get.”
And
this new proxy has some additional advantages over others, the researchers say.
One other method for reconstructing sea ice involves looking for fossil remains
of another kind of algae called diatoms. But that method becomes less reliable
further back in time because fossil molecules can degrade. Molecules like C37:4 tend to be more robustly
preserved, making them potentially better for reconstructions over deep time
than other methods.
The
researchers plan to further research these new algae species to better
understand how they become embedded in sea ice, and how they produce this
alkenone compound. The algae appear to live in brine bubbles and channels
inside sea ice, but it may also bloom just after the ice melts. Understanding
those dynamics will help the researchers to better calibrate C37:4 as a sea ice proxy.
Ultimately,
the researchers hope that the new proxy will enable better understanding of sea
ice dynamics through time. That information would improve models of past
climate, which would make for better predictions of future climate change.
Other
coauthors on the study were Markus Majaneva, Simon Belt, Sian Liao, Joseph
Novak, Tyler R. Kartzinel, Timothy Herbert, Nora Richter and Patricia
Cabedo-Sanz. The work was supported by the National Science Foundation
(EAR-1762431).