Nitrate (NO3-) is the dominant form of biological available N in the ocean, a key nutrient required in large quantities by all organisms. The availability of nitrate to phytoplankton growing in the surface waters is a key determinant of marine productivity, which feeds fisheries and takes up carbon dioxide from the atmosphere. The two elements in nitrate, nitrogen (N) and oxygen (O), each naturally occur in more than one stable "isotope," which refers to the number of neutrons in the nucleus. The two isotopes of nitrogen are nitrogen-14 (14N) and nitrogen-15 (15N), and the two dominant isotopes of oxygen are oxygen-16 (16O) and oxygen-18 (18O). The 15N/14N and 18O/16O ratios in a given sample of nitrate are its "isotopic composition," which is quantified in the terms d15N and d18O, respectively. The isotopic composition nitrate helps to identify where and how ocean nitrate is added, removed, transported, and biologically cycled. In addition, nitrogen is trapped in fossils, and its isotopic composition records changes in the ocean nitrogen cycle back through time; measurements in the modern ocean are required to ground-truth and calibrate this use of fossils to reconstruct of past changes. The GEOTRACES Atlantic cruise was the US contribution to an international effort to to develop a comprehensive picture of trace elements and natural isotopes across the North Atlantic Ocean. As part of this effort, we measured the isotopic composition of nitrate, generating a picture of these properties that runs from the surface to the bottom of the ocean and from North America to Europe (Figures 1 and 2 for the nitrogen isotopes and oxygen isotopes, respectively). Several key observations are readily apparent in these "depth sections." First, there is low d15N nitrate in the shallow waters of the western North Atlantic. This is very likely the result of "nitrogen fixation," in which certain marine phytoplankton add new biologically available nitrogen to the ocean. It will be possible to characterize the rate and spatial distribution of nitrogen fixation when these data are compiled with other data and simulated with a computer model. Second, there is a relatively high d15N at roughly 1000 meters depth and strongest to the East. This feature is in water that is flowing northward from the Southern Ocean, the ocean surrounding Antarctica. The high d15N of this water is due to incomplete uptake of surface nitrate by phytoplankton in the high latitude waters of the Southern Ocean, with the remaining (high d15N nitrate) being forced into the oceanÆs interior and flowing northward as "Antarctic Intermediate Water." The measured d15N signal is actually much weaker than was originally emplaced the Southern Ocean, and its d18O signal has been erased (Figure 2). This observation can be used to quantify the cycling within the Atlantic that works to erase the Southern Ocean-derived isotopic signal. Third, below ~2500 meters depth, there is a weak but measurable east-to-west d15N change, with slightly higher d15N on the western side of the basin (Figure 1). This higher d15N signal is in the water known as North Atlantic Deep Water, which is surface water that sinks into the deep ocean in high latitude North Atlantic and then flows southward. Paradoxically, this high d15N water may well derive from the Southern Ocean. The high d15N water at 1000 m described above is carried northward in Antarctic Intermediate Water all the way to Greenland, where its high d15N signal is incorporated into North Atlantic Deep Water and then flows back south. If this is correct, it provides a dramatic example of how the chemical signals of polar ocean processes can be communicated through the deep ocean, even going from one pole to the other and then back again! Having completed this project, the next step will be to merge these data with other data sets to estimate rates of important processes by comparing the data to output from an ocean model. The...
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