It is useful to determine the distributions of dissolved trace elements in the ocean because of the information they can provide on a wide variety of processes including biological productivity. Our project involved obtaining and analyzing selected trace elements in approximately 900 samples on an oceanographic cruise from the Peru margin to Tahiti. This section was designed to pass through a number of interesting and critical oceanic regimes including the highly productive Peru margin/upwelling zone, the associated Peru margin oxygen minimum and gradient in intermediate water oxygen concentrations, the gradient in biological productivity going from the Peru margin to Tahiti, hydrothermal sources/sinks along the East Pacific Rise, and a gradient in atmospheric inputs. While a wide variety of trace elements and isotopes were determined on cruise samples by various labs, our samples were analyzed for the elements barium (Ba), gallium (Ga), molybdenum (Mo), nickel (Ni), and vanadium (V). This suite includes elements that play a role in the cycling the critical nutrient, nitrogen (Mo, Ni, V); an element associated with input of materials to the surface ocean with dust (Ga), and an element whose incorporation into solid phases is related to biological productivity (Ba).

Key findings of our work can be summarized briefly. For Mo and V, ours are the first oceanic sections of these elements. Previous work suggested that these two elements showed little variability in their distributions, with nearly constant (i.e., conservative) concentrations of Mo and V showing only about a 5% decrease in shallow water compared to the deep ocean. While our work generally confirms these patterns, we also observed depletions of these elements associated with areas of intense nitrogen cycling and oxygen depletion off the Peru margin and with removal onto hydrothermal particles. We also observed changes in the Ni distribution correlated with indicators of intense nitrogen cycling. Thus, our work adds to the understanding of the role of these elements in the cycle of a key nutrient.

Our Ga section provides the first data on that element in the South Pacific. The section shows the expected comparatively low values in surface and intermediate waters due to low dust input. Deep water values are similar to deep water concentrations in the Atlantic, suggesting little influence of sediment sources on the Ga distribution. Comparisons with the geochemically-similar, but more reactive element aluminum (Al), suggest bottom input of Al but removal of that element in the mid-water column. Interestingly, near the Peru margin, we observe slightly increased dissolved Al but slightly lowed Ga, opposite of what we would expect based on their reactivities.

The Ba section is important because Ba incorporated into the sediments (as barite) is an indicator of productivity in the overlying waters. But the mechanisms linking barite formation, productivity, and the dissolved barium distribution are unclear. Our section can be interpreted as indicating barium removal in intermediate waters in the productive region off Peru and regeneration in deep waters in the same region.

This project also resulted in the training of one masters student and one doctoral student.

Last Modified: 04/05/2017
Modified by: Alan M Shiller