Changes in climate affect the biodiversity of even the deepest-dwelling animal communities in the ocean, according to a new finding by scientists at MIT and the US Geological Survey.
The research counters the long-held "stability-time" hypothesis that deep-sea life--three kilometers or more beneath the surface of the ocean--is insulated and relatively impervious to large-scale climatic changes at the water's surface. Those changes take place over a few thousand to tens of thousands of years, a period of short time on the evolutionary scale.
"Our evidence suggests that deep-sea environments undergo climatically driven temperature, nutrient and organic carbon flux changes during glacial-interglacial cycles," said Maureen Raymo, assistant professor in the Department of Earth, Atmospheric and Planetary Sciences.
"We discovered that there were changes in dominance in species and that diversity decreased during glacial periods," she added. "This is the first study to systematically put the stability-time hypothesis to the test."
Professor Raymo, a marine geologist, and colleague Thomas Cronin, a paleontologist at the US Geological Survey, published their findings in the February 13 issue of Nature.
Little is known about deep-sea bottom or "benthic" life. It was only the late 1960s when scientists discovered that the deep sea supported high species diversity. Professor Raymo said the deep sea must be viewed as an integral part of the global ecosystem.
"We're learning about how our world and the environment and the plants and animals that live in it are affected by climate. We want to see how dramatic the effects can be and how rapidly communities can adjust to them," she said.
In their research, Drs. Raymo and Cronin studied communities of tiny crustaceans called ostracodes that live many kilometers below the ocean's surface in the North Atlantic Ocean. There are more than 15 species of ostracodes.
The scientists studied the fossilized remains of ostracodes in a 200-meter-long core of sediment drilled from the deep North Atlantic in an area called Site 607. The core was obtained by the Ocean Drilling Program, which is funded by the National Science Foundation and a consortium of 15 member nations.
Looking down the core is like taking an archaeological trek millions of years into the past, Professor Raymo said. She was able to see changes in the diversity of ostracode species during glacial and interglacial periods by making geochemical and faunal measurements on the sediment core.
41,000-YEAR CYCLES
Drs. Raymo and Cronin discovered that ostracode biodiversity increased and decreased in time with 41,000-year climate cycles. Those cycles are related to a periodic wobble in the tilt of the Earth's axis of rotation caused by gravitation.
"We measured trends in biodiversity from 2.3-to-2.8-million-year-old microscopic fossil shells," Professor Raymo said. "We found that the bottom-dwelling communities living in the North Atlantic have been inherently unstable in the face of past global climate changes. The community of ostracodes varied greatly, decreasing in diversity during cold glacial periods and recovering species richness during warmer periods."
Professor Raymo said that the research not only challenges the stability-time hypothesis, but also underscores the importance of studying the past to better understand biodiversity over long time scales and into the future.
"Predicting the impact of human activities on climate and, in turn, on biodiversity will remain difficult until we gain a better understanding of how climate has affected biodiversity in the past," she said. "And this is causing people to rethink their understanding of deep-sea ecology."
The research was funded by the US Geological Survey Global Change Program and an American Chemical Society grant.
A version of this article appeared in MIT Tech Talk on February 26, 1997.
