Secret to Earth’s ‘Big Chill’ Found in Underground Water

Scientists studying the oceans depend on data from rivers to estimate how
much fresh water and natural elements the continents are dumping into the oceans.
But a new study in the Aug. 24 issue of Science finds that water quietly
trickling along underground may double the amount of debris making its way into
the seas. This study changes the equation for everything from global climate to
understanding the ocean’s basic chemistry.

Since the late 1990s, Asish Basu, professor of earth and environmental sciences
at the University of Rochester, has been sampling water and sediments from two
of the world’s largest rivers, the Ganges and the Brahmaputra of the Indian subcontinent,
to understand a period in Earth’s history called the Great Cool-Down. Forty million
years ago, the global climate changed from the steamy world of the dinosaurs to
the cooler world of today, largely because the amount of carbon dioxide, a greenhouse
gas in the atmosphere, dropped significantly. Scientists have speculated that
the cause of this cooling and the decline in atmospheric carbon dioxide was the
result of the rise of the Himalayan mountains as the Indian and Asian continental
plates pushed into one another. They believe the erosion of the new mountains
increased the rate of removal of carbon dioxide from the atmosphere since the
process of weathering silicate rocks such as those in the Himalayas absorbs carbon
dioxide. This erosion may have depleted the atmosphere of a potent greenhouse
gas and triggered the Great Cool-Down.

Coinciding with the cooling period and Himalayan uplift 40 million years ago was
a consistent change in the ratio of two isotopes of the element strontium in the
oceans’ water-a change that continues to this day. Since strontium often comes
from eroding silicates, it seemed obvious to scientists that the Ganges and Brahmaputra
rivers were simply eroding the Himalayas into the ocean, but when they measured
the amount of strontium in those rivers, they found it was far too low to account
for the mysterious ratio change in the oceans, and thus too low to account for
triggering the cool-down. To determine if enough silicate had eroded to spark
the climate change, Basu and his colleagues analyzed both ground water and river
water samples from the Bengal delta where the Ganges and Brahmaputra rivers empty.
They found the missing strontium and confirmed the culprit that nudged down the
thermostat.

"Deep underground in the Bengal Basin, strontium concentration levels in
the ground water are approximately 10 times higher than in the Ganges and Brahmaputra
river waters," Basu explains. Knowing the speed the water is moving underground,
Basu and his team calculated how much strontium could be leached out of the Bengal
Basin and into the Indian Ocean. They calculated that about 1.4 times more strontium
flows into the ocean through the groundwater than through the rivers above-easily
enough to account for the 40 million-year rise.

This study has other impacts in understanding ocean chemistry. "This means
that we have to re-evaluate the residence times, the time a particular element
remains in the ocean water before settling out, of various chemical elements and
species," says Basu. "Most current studies on the ocean’s chemistry
are based on the supposition that the global rivers are the only carriers responsible
for bringing in dissolved materials to the oceans. Our study changes that perception
permanently."

In addition, since the oceans are the biggest factor driving global weather, doubling
the influx of fresh water will demand that global climate models must be restructured
as well. Fresh water is lighter than salt water and so tends to float to the surface
in the sea. This difference in density could move volumes of warm and cold water
in ways that scientists gauging only the water’s temperature would not normally
predict.

Working with Basu on the project were Stein Jacobsen of Harvard University, Robert
Poreda and Carolyn Dowling of the University of Rochester, and Pradeep Agarwal
of the International Atomic Energy Agency in Vienna, Austria. The research was
partially supported by grants from the National Science Foundation.