Tracking trace elements and isotopes in the oceans

by Jeremy Jacquot
Thursday, January 5, 2012

A report from the GEOTRACES cruise across the Atlantic

For decades, scientists have been trying to piece together the enormously complicated puzzle that is the ocean. They have collected many different kinds of information, from trace elements like iron to tritium isotopes, from many different parts of the ocean. Bringing together these disparate pieces to form a more complete picture is crucial to understanding how human activities, the marine food web, the global carbon cycle and the circulation of seawater are all interconnected.

Recognizing the need for such a broad study, a group of geochemists launched the international GEOTRACES program a few years ago. The first U.S.-led cruise kicked off last October in Lisbon, Portugal. The cruise was supposed to be a 52-day exploration of the North Atlantic, heading south from Lisbon to the western coast of North Africa, across the North Atlantic and ending at Woods Hole, Mass. The overarching goal was to measure trace element distributions in a broad swath of the ocean and the near-ocean atmosphere, and to learn more about the biological, chemical and physical processes that influence them.

I embarked from Lisbon with a crew of 32 researchers in October, hoping to measure the concentrations of dissolved copper and of free, or ionic, copper in seawater. I described the expedition’s plans in the October 2010 EARTH, and followed up with several dispatches from the ship, the R/V Knorr, at EARTH online. Unfortunately (as I noted in the last of those dispatches), technical difficulties with the Knorr shortened the cruise to only about three weeks, and we were only able to complete a third of our work. Nevertheless, we still managed to collect a slew of samples and obtained some intriguing results that we hope to flesh out further during a follow-up cruise this fall.

The Cruise

Our plans included a number of preset stops — or stations — where we intended to stop and collect samples. Our first station was just a few hours south of Lisbon, where the Mediterranean flows into the Atlantic. Aside from a few hiccups, including some minor sample contamination issues and a small fire caused by an equipment malfunction, the sampling at the first batch of stations went well. Though many of us quickly began to lose sleep, we managed to stay on task and keep things operating smoothly. It wasn’t long before we began to produce interesting data.

That would all change several days later.

We continued south along the coast of northwest Africa, where large algal blooms form year-round. The convergence of freshly upwelled nutrient-dense waters and wind-blown dust from the Sahara makes this area a hotbed of globally significant biogeochemical processes, such as the export of organic matter from the surface waters to the deep ocean, and thus worth investigating.

While en route to a station near the Mauritanian shore — close to some of the most productive waters along our journey — the ship suddenly stuttered and began to weave back and forth. We heard an ominous “thunk” — the sound of the port thruster shaft rupturing, as we soon learned during an emergency meeting called to decide the fate of our voyage.

The failure meant two things: The ship’s speed was now almost halved and our ability to remain stationary while sampling would be precarious at best. After a few days of discussion between the crew and the lead scientists, we settled on ending the cruise after the Mauritanian leg of our journey, which would allow us to gather samples from four more stations and complete a third of our proposed work. The captain did allow a few of us to remain onboard for the journey back to Charleston, S.C., where the ship would be unloaded and repaired. The scientists who remained were able to make measurements and collect samples without having the ship come to a full stop.

Our ill-fated cruise ended at Mindelo, a city on São Vicente in the Cape Verde Islands. By then we had collected more than 4,600 water, aerosol and particle samples, from which we would measure more than 100 unique markers of biological activity and chemistry, including the concentrations of about 30 trace elements and isotopes. Though we were all disappointed that we had not been able to get all of the samples and data that we had wanted, we felt encouraged by our preliminary findings and looked forward to applying the lessons that we had learned during this trip to the next cruises.

What follows is a brief and necessarily incomplete account of some of the preliminary results from the waters off Portugal and Mauritania. Most of the researchers, including me, have a long way to go before we are ready to present our data for peer review — so consider this a mere snapshot of what will emerge over the coming months.

Nutrients, Arsenic and Mercury

Different research teams on the cruise looked at different issues. Gregory Cutter, a geochemist at Old Dominion University in Virginia, and his colleagues focused on surface levels of nitrates and phosphates — nutrients that are essential for growth — which they measured all the way from Lisbon to Charleston. Using seawater supplied by a pump towed alongside the ship that let the team monitor these nutrients continuously, the team found relatively low levels of phosphates, ranging from 2 to 7 nanomoles per liter (see sidebar) along most of the cruise track, although the levels were significantly higher, up to 120 nanomoles per liter, in the upwelling waters off northwest Africa. The low levels along most of the track are low enough to stress the phytoplankton and likely limit their growth.

Cutter’s group also measured the concentration of arsenate, the predominant form of the toxic metal arsenic in the ocean and a close cousin of phosphate. The team found that phosphate is a third less abundant than arsenate at the surface.

This is a problem for phytoplankton: When the concentration of phosphate drops below that of arsenate, the phytoplankton may accidentally incorporate the arsenate into their cells, where it wreaks havoc on their metabolism. To combat this threat, some phytoplankton have developed the ability to convert arsenate into less toxic forms such as monomethylarsenic and dimethylarsenic. As expected, Cutter’s team measured the highest levels of these detoxification products in those areas where the phosphate levels fell significantly below arsenate. Similarly, they discovered that the concentrations of at least one of these products could be used as a way to detect phosphate stress in some phytoplankton species.

On the next cruise, the team hopes to measure the different forms of arsenic, which they think will enable them to map out nutrient deficiencies — such as phosphate distress — of phytoplankton on the ocean surface, Cutter says. When combined with data on iron, cobalt, zinc and other “micronutrients” that organisms only need in small quantities, these analyses could yield valuable insights into how stressed phytoplankton utilize their available resources. Because phytoplankton collectively represent one of the largest sinks of anthropogenic carbon dioxide in the world, this map could help scientists determine which areas of the ocean are responsible for more or less carbon uptake based on their ambient levels of phosphate and other nutrients, Cutter says.

Mercury levels in the ocean are also of concern, as some forms of mercury are toxic. GEOTRACES researchers Carl Lamborg of the Woods Hole Oceanographic Institution (WHOI) and Katlin Bowman of Wright State University in Ohio tracked four different forms of mercury while on the cruise, including elemental mercury, the vapor form through which mercury enters the ocean, and monomethylmercury, which bioaccumulates in fish and can have harmful neurological effects in humans.

Similar to many bioactive trace metals and nutrients like nitrates, the distribution of mercury was lower in the surface waters and gradually increased with depth. Lamborg and Bowman found relatively high concentrations of elemental mercury, about 100 to 200 femtomoles per liter, in all of their samples throughout the water column and particularly in the deepwater, down to about 4,500 meters. The levels were well in excess of what Lamborg and others have observed off the eastern coast of North America, where typical concentrations of elemental mercury are only about 40 femtomoles per liter. Monomethylmercury was present at all stations and all depths; concentrations were highest in the upper 1,000 meters. Nobody quite knows where all of the mercury — especially the elemental mercury and monomethylmercury — in the eastern North Atlantic appears to be coming from, so pinpointing its source, or sources, is one of Lamborg’s goals on the next cruise.

Although the origin of mercury in the ocean is still unknown, Lamborg has a hunch that the source of the monomethylmercury is microbial in nature. How this form of mercury materializes and where it comes from have perplexed researchers for years. What they do know is that monomethylmercury is generated at depth in anoxic sediments by iron- and sulfate-reducing bacteria.

But Bowman says that these microbes do not account for the patterns observed in the eastern North Atlantic. Because the highest concentrations of monomethylmercury did usually coincide with low oxygen levels, however, she says that it may have been produced by other anaerobic bacteria as a side-product of the respiration of organic matter. Some of these bacteria release methyl groups when they produce vitamin B-12, and these groups can react with mercury, forming monomethylmercury. This form is then expelled out of the cell as a waste product. Bowman’s findings demonstrated that the greatest levels of monomethylmercury occurred in areas of high biological productivity such as the upwelling-fueled North African coastline.

Lamborg and Bowman also uncovered one additional mystery on this cruise: Dimethylmercury, a form of mercury that is generally only detected at measurable levels in deepwater and has been found in other areas of the North Atlantic, was missing. On the next cruise, they hope to solve this mystery by determining exactly where in the basin it exists and at what rates it appears and disappears.In another study, Lamborg is measuring the mercury content of corals to help track the changes over time in mercury inputs to the oceans. His research so far, he says, is revealing that two-thirds of the mercury that ends up in the oceans is anthropogenic in origin whereas one-third is natural, primarily from volcanoes. But there are many more data to be counted. Lamborg and Bowman hope that pairing the results of the analyses of samples from the recent cruise with other researchers' data will give them a clearer picture of how the various forms of mercury cycle throughout the North Atlantic.

Elements From on High

Though most of the researchers on the cruise were focused on measuring the elements present in the water column, some took more interest in the elements wafting down from the sky.

Our travels south of Portugal and along the North African coastline placed us squarely in the path of distinct air masses — and therefore traveling aerosol particles — from Europe and the Sahara Desert. Mineral dust carried by winds is thought to be one of the largest inputs of iron and other trace metals to remote oceanic areas; thus it represents a potentially significant source of nutrients for phytoplankton. At the same time, however, it can also be hazardous because one of the trace metals in dust, copper, is known to be toxic at even moderate concentrations.

The dust, which mostly consists of soil particles that are swept up into the lower atmosphere by strong winds, can be transported long distances over the course of several days or weeks before finally alighting on the ocean surface. During their migration, the dust particles are often exposed to changing atmospheric conditions and may encounter particles from anthropogenic emissions or other continental regions; as a result, the elemental composition and chemical nature of particles originating from the same area can be variable.

Despite their mixed heritages, aerosol particles can still be characterized to a high degree of accuracy by their color, provenance and solubility in seawater. Saharan dust, for instance, has a red-to-orange hue that reveals its desert origins, and it does not dissolve very well. Maritime dust — dust transported by air masses that haven’t made contact with major landmasses in a few days — is harder to discern; the filters used to collect it often appear clean, and it has low trace metal concentrations.

European or North American dust particles, meanwhile, tend to be gray and, perhaps because of their small size (and thus higher surface to volume ratio), are much more soluble — up to an order of magnitude higher than desert particles. And whereas the composition of Saharan dust closely matches that of the North African upper continental crust, the composition of European and North American dust has a much higher proportion of anthropogenic particles that derive from fossil fuel emissions. By some estimates, the North Atlantic receives roughly eight to 40 times more dust than the Pacific, most of which is from the Sahara. In fact, the Bodélé region, located on the southern outskirts of the Sahara Desert, is the single largest source of dust in the world.

On the cruise, Rachel Shelley of Florida State University and Ana Aguilar-Islas of the University of Alaska at Fairbanks collected aerosol and rainfall samples using large samplers equipped with different-sized filters to catch a broad range of elements and isotopes. Using a model that simulates air mass movements, Shelley determined that we came into contact with several dust events as we made our way south of Portugal and down the North African coast.

Other researchers found similar results that also suggest the cruise went through at least one (if not more) large dust event. Christopher Measures and Mariko Hatta, both of the University of Hawaii at Mānoa, measured dissolved concentrations of iron, aluminum and manganese throughout the cruise; they found that levels of iron in the surface waters at the four stations off Mauritania were well over twice the norm. These elevated concentrations likely contributed to the already robust levels of primary productivity that we witnessed in that area. Aluminum, also a good indicator of dust because the metal is abundant in continental crust, was also very high. Based on these levels, Measures says, the region seems to receive on average several grams of mineral dust per square meter every year — more than most of the ocean.

The ongoing analyses of another team, led by Daniel Ohnemus of WHOI, should also help to quantify how much dust is dissolved in the surface waters. Shelley’s and Aguilar-Islas' work, once analyses are completed later this year, should also shed more light on how different the composition of particles from different areas can be and how soluble they are in seawater. Several modeling and observational studies have suggested that the patterns of air mass movements may already be changing in response to climate change. Through their research, Shelley and Aguilar-Islas hope to improve scientists' understanding of how dust impacts the ocean and how this may change over the coming decades.

The GEOTRACES North Atlantic Cruise: Part 2

The next cruise, slated for this October, will pick up where we left off last year: the Cape Verde Islands. Our new ship, the R/V Atlantis — perhaps best known as the support vessel for the deep-sea submersible Alvin — will be able to support up to 34 researchers. The cruise, following the original plans, will take us from the Cape Verde Islands northward through the nutrient-depleted central gyres — large systems of wind-driven rotating ocean currents — near the center of the North Atlantic and finally to Woods Hole.

Robert F. Anderson, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory and one of GEOTRACES' leaders, is confident that the cruise’s original goals will be achieved this fall. There will be some gaps, however, as some of last year’s participants have conflicting cruises and will be unable to attend, and others will be working on new projects.

Doing a second cruise this fall — which was not originally planned — may push back the next U.S.-led GEOTRACES cruise, a transect of the Pacific Ocean tentatively scheduled for 2012, by up to a year. This would give the researchers, many of whom are engaged in both cruises, sufficient time to adjust their plans accordingly.

Anderson says he is optimistic about GEOTRACES' future. “Thanks to superb pre-cruise planning, precise choreography of at-sea activities and heroic efforts of everyone aboard the Knorr, the epic sampling effort was carried out successfully until mechanical problems with the ship terminated the cruise,” he says. “Simply knowing that we can do it affords hope and inspiration for the remaining cruises of the GEOTRACES program.”

For now, stay tuned for more results as we get them.

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