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.