Cruising the Atlantic to trace elemental movements

by Jeremy Jacquot
Thursday, January 5, 2012

When it comes to the science of climate change, one of the least understood issues is the oceans' future in a changing global environment. Measurements over the past two decades show that the oceans' surface waters have been warming since the 1950s, and that large influxes of carbon dioxide have already made the oceans more acidic.

Furthermore, although scientists don’t yet fully grasp how the oceans' chemistry and biology are altering as a result of climate change, they do know that they are changing. A more thorough understanding of how these chemical and biological cycles function — what processes control their distributions, what their sources and sinks are and, crucially, how human perturbations are impacting them — would therefore help scientists more accurately predict how the concentrations of vital nutrients and elements like phosphates and manganese will fluctuate over time.

It was with this in mind that a small group of geochemists assembled in late 2000 to lay out the initial plans for a broad research initiative that will focus on the study of trace element and isotope cycles. The culmination of their efforts is GEOTRACES, an international program that is undertaking a decade-long study of the oceans.

The mission

GEOTRACES is modeled on the hugely successful GEOSECS (Geochemical Ocean Sections Study) program of the 1970s, a U.S.-led global initiative that measured the global distribution of a number of geochemical “tracers,” radioactive isotopes that decay on predictable timescales and can help scientists track the movement of water masses in the oceans. GEOSECS measured ocean water properties — including concentrations of naturally occurring carbon-14, radon-222 and radium-226, as well as bomb-derived carbon-14 and tritium — at multiple depths in the ocean along transects from the Arctic to the Antarctic. This created the first global, three-dimensional picture of these isotopic tracers' distributions in the oceans — and kick-started a decades-long boom in marine geochemical research, establishing many of the techniques and methods used in modern tracer geochemistry.

Armed with much more sophisticated sampling protocols and advanced analytical techniques than GEOSECS, GEOTRACES — an international coalition of researchers from more than 30 countries — hopes to do for marine geochemistry what its predecessor did for the field decades ago.

The bread and butter of the GEOTRACES program will be more than a dozen research cruises that cut across large regions of the ocean. The first of the U.S.-led transoceanic section cruises will depart from Lisbon, Portugal, on Oct. 15, taking a group of researchers (including me) from where the Mediterranean flows into the Atlantic south of Portugal to a region along the coast of northwest Africa where the combination of dust blown from the Sahara and upwelling of nutrient-rich deepwater fuels huge algal blooms. The cruise will pass through the nutrient-depleted central gyres — large systems of wind-driven rotating ocean currents — near the center of the North Atlantic and end in Woods Hole, Mass., 52 days later.

Along the way, we will collect samples of ocean water and aerosols from the atmosphere to measure a wide range of trace elements (elements present in trace amounts on Earth), including cobalt, iron, zinc, copper, aluminum and manganese. In addition, we will also measure a number of isotopes, including isotopes of nitrogen, carbon and several trace metals, to improve our understanding of ocean currents and past climate conditions. With these data in hand, we hope to quantify the physical pathways and element concentrations that give the Atlantic Ocean its unique properties.

We on the GEOTRACES mission plan to look at the distributions and concentrations of certain key trace elements and isotopes (TEIs) that occur in fractional amounts throughout the water column. TEIs include isotopes of elements like nitrogen and carbon, including nitrogen-15 and carbon-13, metals like zinc and copper, and radioactive isotopes like thorium and carbon-14. We want to relate these TEIs to geochemical and biological processes in the oceans, such as carbon cycling and nitrogen fixation. One of the main rationales for the program is to provide a robust baseline of measurements so that scientists can more effectively track and project changes over the coming decades.

The program will also seek to elucidate the various oceanic processes that regulate distributions of TEIs throughout the water column and to find out how fluctuating climate conditions in the past have affected them through time. That in turn should help researchers determine how future climate conditions will continue to alter the geochemistry of the oceans.

How TEIs shape the ocean environment

Though some areas of the ocean are richer in TEIs than others due to a variety of physical processes or a surfeit of anthropogenic inputs, their concentrations rarely exceed a nanomole — a billionth of a mole — per liter of seawater. Yet these trace elements play crucial roles in many of the oceans' — and thus the planet’s — biogeochemical cycles, sometimes single-handedly shaping the structure and composition of entire ecosystems.

Take iron, which is widely considered to be the most “limiting” nutrient in the ocean. A limiting nutrient is one that, because of its scarcity, keeps vital processes like photosynthesis and nitrogen fixation in check. Iron depletion in the oceans can have a significant knock-on effect on climate change: Phytoplankton that depend on iron can collectively draw down huge quantities of carbon dioxide. Any obstacle to their capacity to photosynthesize — like a lack of iron — will restrict their ability to absorb carbon dioxide.

A lack of iron also has implications for the larger marine ecosystem. Because there is no adequate substitute for iron, even an area brimming with other nutrients will only be able to support small communities of photosynthesizers and, in turn, small populations of zooplankton, fish and other species higher up the food chain. For these reasons, measuring levels of iron — which tend to be low in the North Atlantic — and several other key micronutrients is an emphasis of this cruise.

We also hope to measure TEIs such as copper that can become toxic to many forms of marine life at even mildly elevated concentrations. Studies have shown that populations of Prochlorococcus, a tiny cyanobacterium that packs a major photosynthetic punch (accounting for well over half of all primary production in some regions of the ocean), languish in direct relation to the amount of copper in the surrounding waters.

Mercury is another contaminant we will study. The toxic metal’s rising presence in surface waters worldwide is a direct consequence of rising industrial emissions and has implications for not only marine organisms, but humans as well. Through bioaccumulation, mercury becomes more concentrated with each step up the food chain. As a result, the fish that humans eat tend to have the highest accumulations of mercury in their bodies. Excessive levels of mercury in humans can trigger a wide range of debilitating diseases and cause organ damage.

As befits their name, trace elements can also be used as signals to “trace” the biological, physical and chemical routes of the ocean. Radioactive trace isotopes, in particular, are valuable paleoceanographic proxies: As they travel through the ocean, decaying at known rates, they offer a window into the impact of climate change on the ocean through time.

The chemicals added to the oceans by human activities can be used to study ocean ventilation, the process by which the well-oxygenated surface layer waters are transported to the deep ocean. If these water masses can be traced as they sink and get entrained in the global oceanic conveyor belt over a period of millennia, they become useful tools with which to follow and quantify large-scale processes such as ocean mixing. Because these chemicals have been present in the ocean for only a few decades — a much shorter timescale than the ocean’s mixing time, which lasts well over a millennium — these elements are known as transient tracers.

Tritium, a rare radioactive isotope of hydrogen, is one such radioactive trace isotope. Tritium became artificially enhanced in the natural world due to nuclear weapon testing in the second half of the last century — offering oceanographers the unusual opportunity to follow a transient tracer as it slowly made its way through the ocean. From the tests, tritium was deposited across the ocean surface in the Northern Hemisphere, from which researchers carefully traced its movements. By comparing its properties to water masses above and below the layer that contains tritium over many years, the timescales of ventilation and circulation were better understood.

The wider benefits of GEOTRACES

When paired with sophisticated environmental models, TEI distributions could enable researchers to pinpoint the locations of important features — such as low-oxygen (dead) zones or areas of high primary productivity — with greater ease and accuracy. These data, when incorporated into advanced climate models, could furnish more robust estimates of how much carbon the oceans hold and illuminate the oceans' complex role in the world’s past and future climate. Our current understanding of trace element cycling in the oceans is still fairly rudimentary, so we can’t properly predict how future changes in TEI distributions will impact ocean ecosystems, let alone how the cycles themselves will fluctuate.

The oceans remain an understudied system; large stretches of the oceans are still virtually unsampled. Even the known distributions are being revisited in light of recent vast improvements in sampling and analytical methodologies.

My job on the cruise is to measure the speciation of copper — specifically the relative abundance of the free cupric ion form (Cu2+) in different parts of the ocean — and relate it to the diversity and structure of cyanobacterial communities. As copper levels will vary widely from one region of the Atlantic Ocean to the next, I expect to find distinct communities of Prochlorococcus and other cyanobacteria at different sampling stations. The broad range of geochemical regimes that we will encounter along the way, from the fertile waters off Africa to the nutrient-poor central gyres, will also yield an unprecedented look at the copper cycle, its sources and sinks.

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