Project: US GEOTRACES PMT: the geochemistry of size-fractionated suspended particles collected by in-situ filtration

NSF Award Abstract:
A researcher at the University of California, Santa Cruz will analyze the chemical composition of marine particles collected from the ocean. Marine particles will be collected in two size classes, 1-51um and >51um along the U.S. GEOTRACES Pacific Meridional Transect (PMT), which will sample from Alaska to Tahiti along 152°W. The smaller particles generally remain suspended in seawater, where they can interact with dissolved elements in seawater, whereas the larger particles tend to sink which means these elements can be transported to the bottom of the ocean. We will measure a suite of trace elements (e.g. iron, aluminum, zinc, manganese, cadmium, copper, cobalt, titanium, and barium), some of which are required for life, and others that act as tracers of oceanographic processes, as well as the major biologically-produced carrier phases (e.g. particulate organic carbon and the biominerals, calcium carbonate and biogenic silica) in the size-fractionated particles collected from the US PMT section. The data will be used to assess the relative importance of particle concentration, composition, and size distribution on the cycling and removal of trace elements and their isotopes (TEIs). Results will not only contribute to the overall goal of the GEOTRACES program, but also provide critical information to other science communities interested in the role of particles in sequestering carbon from the atmosphere through a process known as the biological carbon pump. A graduate student and a postdoc will be supported and trained on this project, and undergraduate students will be involved in several aspects of this work. The results from this study will be integrated into class curricula taught by the researcher. The scientist will also work with a science journalist who will sail on the expedition to translate the process and initial results of this research to the public.

Particles play essential roles in the cycling and distribution of trace elements and isotopes (TEI) in the ocean by being sources and sinks for many TEIs. The scavenging of dissolved TEIs by particles is a major removal term for many particle-associated TEIs, but is poorly understood and represents a major uncertainty in our understanding of the biogeochemical cycling of many TEIs. The scavenging of TEIs is affected by particle size, concentration, and composition, whereas the subsequent removal of particles by sinking is influenced by particle dynamics processes such as particle aggregation and disaggregation. We will measure the major (particulate organic matter, calcium carbonate, opal, lithogenic particles) and minor (particulate trace metals) phase compositions of size-fractionated particles (1-51um; >51um) collected by in-situ filtration on the the US GEOTRACES Pacific Meridional Transect (PMT). The PMT will be the first meridional section of the US GEOTRACES program and will cross many gradients in surface productivity, biological community composition, subsurface silica concentrations, and depth of the calcite saturation horizon. Therefore, the US PMT section is expected to vary dramatically in particle concentration, composition, and size distribution, allowing us to examine the factors controlling scavenging efficiency in the ocean.