Award: OCE-0928409

GEOTRACES is an international study that will revolutionize our knowledge of the oceans. Its guiding mission is “to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions” (GEOTRACES Science Plan). In a warming world, the science plan notes that “understanding the processes that control the transport and fate of these contaminants is an important aspect of protecting the ocean environment. Such understanding requires accurate knowledge of the natural biogeochemical cycling of these elements so that changes due to human activity can be put into context …  and their impact on the carbon cycle and climate understood.” Its model is the early 1970’s GEOSECS Program, which systematically obtained geochemical data from the global oceans, and which has formed a basis of our knowledge of the oceans. Since then, there have been tremendous technical developments and new approaches to studying the oceans. GEOTRACES is a 21^st century response to these developments.

This project is part of the North Atlantic Zonal Transect cruise, whose cruise track is Lisbon-Mauritania-Bermuda-Woods Hole, and is a collaboration of scientists from three universities (Columbia, U Hawaii, U South Carolina). Our project was to measure the isotope ratios and concentrations of neodymium (Nd), which is part of the suite of key ‘trace elements and isotopes, or ‘TEIs’ in the GEOTRACES Program. This obscure sounding TEI is part of the plan because it acts as a ‘dye’ on water masses and can be used to trace where they come from and where they go. Moreover, it can be used in conjunction with other ‘TEIs’ to help to understand what is happening to them. Nd is supplied to the oceans mainly from the continents, in solution and on particles (suspended sediment and wind-born dust). The ‘Nd isotope ratio’ (which acts as the ‘dye’) in particles or in water masses reflects the age of the continents that they come from. Because these ages are different in different places (e.g. very old continent around the North Atlantic, younger around the Pacific), the Nd isotope ratios vary between different water masses. As a ‘dye’, it will keep its ‘color’ until new Nd is added. If water masses of two ‘colors’ mix, the new ‘color’ reflects the mixture. If new Nd is supplied at a specific location, it will affect its ‘color’.

Nd is one of the TEIs whose use had not been developed yet during the GEOSECS program, and as a result there is a dearth of seawater Nd isotope data and hence major gaps in our knowledge of its sources, sinks, and cycling in the ocean. In fact, in this project, our new measurements are equivalent to more than 20% of the published measurements at the time of this report. Here are some of our primary results thus far.

 (1) A primary objective is to determine if the Nd in the deep oceans reflects the original water masses, which formed in the high latitude North Atlantic and the Antarctic, or if they are significantly changed by addition of extraneous Nd in mid-latitudes. We found that away from the continental margins, the Nd in the deep oceans by and large reflects the mixture of water masses derived from high latitudes. This means that Nd isotopes are an effective water mass tracer. (2) Near the African margin the sampling took place in a zone of high biological productivity, in the middle of the belt highly impacted by Saharan wind-blown dust. Here, the ‘dye’ aspect of Nd isotopes provides new information compared to other TEIs, and indicates that there are different biologically active and inactive components of Nd in the same water par...