Award: OCE-1233005

A better understanding of the role of metals on the ocean?s biology is needed. Plankton need certain metals like iron and manganese to photosynthesize. In certain ocean regions the concentration of metals dissolved in seawater limits the rate of photosynthesis which in turn can limit the rate of uptake of carbon dioxide (CO2) from the atmosphere and, by consequence, the earth?s future (and past) surface temperature. We measured the isotopic composition of the CO2 dissolved in seawater (referred to as 13C/12C) on an oceanographic research cruise in the South Pacific Ocean between the coast of Chile and the island of Tahiti as part of the GEOTRACES project. The first goal was to compare the distribution of 13C/12C to the simultaneously measured distributions of concentrations of phosphate (PO4) which is an essential nutrient for photosynthesis and cadmium (Cd) a metal involved in photosynthesis. The second goal was to identify the processes which controlled the distributions of 13C/12C, PO4 and Cd in this region of the South Pacific Ocean. One important characteristic of 13C/12C and Cd is that one can measure these two characteristics on forams (a type of plankton that precipitates a shell of calcium carbonate) preserved in ocean sediments which potentially allows one to reconstruct the distribution of PO4 in ocean during previous times (e.g., during the Last Glacial Maximum (LGM) about 20,000 years ago when the atmospheric CO2 level was about half of today). The distributions of 13C/12C, PO4 and Cd measured during the GEOTRACES cruise indicated that in the eastern portion of the cruise track near the Chilean coast the rates of photosynthesis and respiration by marine organisms was the primary factor controlling the distributions. In contrast, in the western portion of the cruise track near Tahiti physical mixing of water parcels was the primary control on the distributions. These results were not completely surprising as the cruise track was chosen because it crossed ocean regions that have anomalously high rates of photosynthesis (near Chilean coast) and anomalously low rates of photosynthesis (near Tahiti) in the surface ocean. The results have significant implications in order to use 13C/12C and Cd measured on foram shells to reconstruct past distributions of the PO4 in the ocean. The relationships between 13C/12C and PO4 and between Cd and PO4 are significantly different today for forams living near Tahiti and near the coast of Chile. Thus we need to account for these differences when using 13C/12C and Cd on preserved forams shells to reconstruct the PO4 distribution in this ocean regions during the LGM. The ocean?s PO4 distribution is tightly coupled to the ocean?s CO2 distribution which in turn affects the atmosphere?s CO2 concentration. For example, accurate reconstructions of the ocean?s PO4 distribution during the LGM will yield insight into the processes causing the low atmospheric CO2 levels during the LGM. Last Modified: 12/30/2016 Submitted by: Paul D Quay