Tracing a Water Journey

In the Southern Ocean, cold surface water sinks to about 1,500 feet (500 meters) and travels in the dark for thousands of miles before resurfacing, some 40 years later, near the equator in the Pacific, Atlantic, and Indian ocean basins. Scientists call this major water mass the Sub-Antarctic Mode Water, or SAMW, and it is regarded as a powerhouse of a mixer in the oceans. It’s also critical to marine life; when it warms and rises into the sunlit subtropical and tropical waters, the nutrients it contains are estimated to fuel up to 75 percent of the microscopic plants growing there.

Before the cool surface water sinks, the chemical properties of the SAMW are modified by the growth and distinct physiology of two common phytoplankton: diatoms built around an internal architecture of silica, and coccolithophores, built upon a calcium carbonate skeleton. Local conditions of the water, including temperature, salinity, and both nutrient and trace element chemistry, influence where and how fast these two groups grow.

Marine chemist Nick Bates, senior scientist and director of research at BIOS, is among oceanographers fascinated by the critical marine processes that take place in the vast and remote Southern Ocean, famously known for ice-choked waters and months of winter darkness. This summer, Bates and colleagues from four U.S. research organizations received a four-year, $4 million grant from the National Science Foundation to create, as they described in their application, “an exceptionally detailed field and modeling effort that will document and quantify the remarkable, interconnected processes that chemically connect two important oceanic ecosystems half a world apart.”

“We hit the jackpot,” Bates said when describing his reaction to the news of the award, which he shares with scientists Barney Balch of the Bigelow Laboratory for Ocean Sciences in Maine, Matt Long from the National Center for Atmospheric Research in Colorado, Dennis McGillicuddy from Woods Hole Oceanographic Institution in Massachusetts, and Pete Morton from Florida State University.

Preparations for the project have already begun, and will swing into full gear in January 2019, when the first of two funded research expeditions travels out of South Africa for research that will range from the subtropics to the Antarctic polar front. A second cruise a year later, in 2020, will also travel out of South Africa.

Researchers, including BIOS research specialist Becky Garley, will identify, track, and study eddies across the Indian Ocean sector of the vast Southern Ocean that support rich populations of either coccolithophores (which have been found to release carbon dioxide into the atmosphere in regions near Antarctica) or diatoms (a common phytoplankton group in the Southern Ocean that act as a carbon dioxide sink, or reservoir, during photosynthesis), plus their associated microbial communities.

The scientists leading the project will study the complexity of the biological and chemical conditioning of the SAMW. Their data, which will focus on some very small marine life, will provide information about the large-scale oceanic controls of the biological carbon pump, which removes atmospheric carbon dioxide to the deep ocean over hundreds of years.

During the first 40-day expedition, scientists will take measurements and make observations by crossing defined areas of the ocean (called transects), develop models, and deploy gliders and floating drifters that follow the movements of the eddies. The drifters provide real-time information about the ocean’s circulation and chemistry over months-long periods, while collecting information on its biology, such as the transport of organisms. The first results from the project are expected in mid-2019, with a full synthesis of data running through 2021, at least.

The research is familiar to Bates, who is a lead investigator of the Bermuda Atlantic Time-series Study (BATS) and Hydrostation S projects—studies of the ocean surrounding Bermuda that provide the longest record of the rate of natural and human-caused change to ocean warming and acidification. “If we care about how we are altering the planet, we have to care about looking at the planet over long periods of time, which provide a meaningful history,” he said.