The U.S. GEOTRACES North Atlantic zonal section cruise was scheduled for 15 October through 5 December 2010. Because of a major irresolvable mechanical failure of the ship's propulsion system, the expedition had to be terminated in the Cape Verde Islands on 4 November after completing about one third of the planned track. This required on-the-spot changes in return travel for all sea-going investigators as well as in shipping arrangements for all their samples and equipment. The funds requested in this proposal address the PI's need to cover a graduate student and purchase supplies associated with his participation in the continuation of the North Atlantic section cruise scheduled for autumn 2011. The additional funds for the graduate student would cover the effort to prepare for a second cruise whereas the supply request is to replace those lost during the first cruise. The proposed research would significantly improve our knowledge of cross margin exchange of trace elements, as well as the impact of atmospheric inputs on ocean chemistry.
"Trace elements and isotopes [TEIs] play important roles in the ocean as nutrients, as tracers of processes now and in the past, and as contaminants. Their biogeochemical cycling has direct implications for research in such diverse areas as the carbon cycle, climate change, ocean ecosystems, and environmental contamination......Improved understanding of the biogeochemical cycles and large-scale distributions of TEIs will inform many areas of environmental research, from climate science to planning for future global change" (GEOTRACES Science Plan, 2005). This NSF-funded project was a small part of the US GEOTRACES effort (see www.usgeotraces.org/). Specifically, in another NSF project (OCE 0927951), we were funded to participate in the US GEOTRACES North Atlantic Zonal Section. This oceanic survey across the North Atlantic from Portugal to Massachusetts was carefully constructed to allow researchers to investigate trace elements in various oceanic processes/phenomena including: a) the meridional overturning circulation, b) carbon cycle, c) ocean margin exchange, d) atmospheric inputs, and e) hydrothermal sources and sinks. The elements we are studying will most especially allow us to investigate cross margin exchanges, redox processes and atmospheric inputs. This cruise was scheduled for late fall 2010 aboard the R/V Knorr. Unfortunately, propulsion problems with the Knorr caused the cruise to be cancelled in the Cape Verde Islands after only one-third of the section was completed. The original chief scientists of the cruise (W. Jenkins, Woods Hole; E. Boyle, MIT; G. Cutter; Old Dominion) were then funded for a continuation cruise scheduled that took place in late fall 2011. This RAPID proposal allowed us to quickly obtain the additional supplies needed to participate in the continuation cruise. It is difficult to describe outcomes of this RAPID project separately from our original main project (OCE 0927951), with the exception that we were successful in participating in the continuation cruise and thereby obtained a full suite of clean trace element samples from across the North Atlantic. This has, for example, allowed us to put together the first oceanic section of the element gallium (Ga). Gallium is a fairly obscure element (best known as a component of some advanced electronics). However, knowledge of its oceanic distribution is quite useful. To understand the utility of the oceanic gallium distribution, we need to draw connections between it and two other important elements in the ocean: iron and aluminum. Iron (Fe) is important because there are some areas of the surface ocean where iron input is so low that its lack of availability is what limits biological productivity. Iron comes to much of the surface ocean via dust input. Because iron is rapidly taken up by the biota, its distribution in surface ocean waters reflects both the dust input and the biological removal. To study dust input to the ocean, we have a bit of a Goldilocks problem. If an element is rapidly removed from the surface ocean, then its distribution reflects both input (e.g., dust) and removal processes. If an element is very slowly removed from the surface ocean, the its distribution may get averaged out by ocean circulation processes. So, to effectively use elemental distributions to study a process like dust input, we may need to look at elements with differing removal rates. That is where aluminum (Al) and gallium come into play. Both aluminum and gallium get into the surface ocean via dust input. Aluminum is fairly rapidly removed, though less so than iron; gallium is even less reactive than aluminum. So, by comparing the iron, aluminum, and gallium distributions, we can get a better picture of the iron input to the surface ocean. Again, that is a vital importance because low iron input limits the production of the microscopic plants at the base of the oceanic food chain in some parts of the ocean. Our work with this GEOTRACES North Atlantic section is helping resolve some key issues in North Atlantic Ocean dust input. For instance, it’s known that the Sahara desert (North Africa) is an important dust source. But, waters close to the African coast have lower aluminum and gallium contents than waters closer to North America leading to speculation that the Saharan dust gets wafted high into the atmosphere and doesn’t get deposited near the African coast. But changes in the gallium/aluminum ratio suggest an alternative explanation: the high levels of these elements near North America simply reflect progressive deposition of dust as the water circulates westward. This project also supported the training of a graduate student and two undergraduates.
