Constraining the processes that control the distribution of carbon in the ocean is critical for understanding how the ocean’s role in the carbon cycle will evolve as global temperatures rise. However, these processes are difficult to measure directly. In this dissertation, computational methods are used to estimate the rates at which particulate organic carbon (POC) aggregates, disaggregates, remineralizes, and is exported down the water column through gravitational settling and zooplankton mediated transport. First, an inverse model is built to estimate rates of particle cycling from POC concentration data collected near Station P in the North Pacific as part of the EXPORTS program. While the model is able to obtain posterior estimates of particle cycling rates that are consistent with the POC concentration data, these estimates were sensitive to the choice of prior estimates. A revised version of this model is then applied to data from the GEOTRACES Pacific meridional transect GP15, which spans a variety of biogeochemical regimes from the south Alaskan gyre to the south Pacific subtropical gyres. Inverse estimates suggest that along GP15, settling speeds of large particles increased with depth below the euphotic zone, but there was no dependence of settling speed on seawater viscosity. The magnitude of POC settling flux out of the euphotic zone was directly related to primary production such that the highest flux occurred in the mesotrophic subarctic gyre and the lowest flux occurred in the oligotrophic subtropical gyres, but the export efficiency was generally low (< 5%) regardless of trophic status. POC appeared to be more cohesive in oligotrophic regions dominated by picoplankton, and, contrary to previous studies, no effect of temperature on remineralization was observed. Finally, a computer vision-based method is demonstrated for estimating carbon settling fluxes from particles visible in camera images with high spatiotemporal resolution while greatly decreasing the amount of human labor required for producing these flux estimates. Overall, this work contributes to a better understanding of how carbon is cycled and exported to depth in the present ocean, and provides insights into how these processes may change in the future.

The concentration and the major phase composition (particulate organic matter, CaCO3, opal, lithogenic matter, and iron and manganese oxyhydroxides) of marine particles is thought to determine the scavenging removal of particle-reactive TEIs. Particles are also the vector for transferring carbon from the atmosphere to the deep ocean via the biological carbon pump, and their composition may determine the efficiency and strength of this transfer. Here, we present the first full ocean depth section of size-fractionated (1–51 µm, >51 µm) suspended particulate matter (SPM) concentration and major phase composition from the US GEOTRACES North Atlantic Zonal Transect between Woods Hole, MA and Lisbon, Portugal conducted in 2010 and 2011. Several major particle features are notable in the section: intense benthic nepheloid layers were observed in the western North American margin with concentrations of SPM of up to 1648 µg/L, two to three orders of magnitude higher than surrounding waters, that were dominated by lithogenic material. A more moderate benthic nepheloid layer was also observed in the eastern Mauritanian margin (44 µg/L) that had a lower lithogenic content and, notably, significant concentrations of iron and manganese oxyhydroxides (2.5% each). An intermediate nepheloid layer reaching 102 µg/L, an order of magnitude above surrounding waters, was observed associated with the Mediterranean Outflow. Finally, there was a factor of two enhancement in SPM at the TAG hydrothermal plume due almost entirely to the addition of iron oxyhydroxides from the hydrothermal vent. We observe correlations between POC and CaCO3 in large (>51 µm) particles in the upper 2000 m, but not deeper than 2000 m, and no correlations between POC and CaCO3 at any depth in small (<51 µm) particles. There were also no correlations between POC and lithogenic material in large particles. Overall, there were very large uncertainties associated with all regression coefficients for mineral ballast (“carrying coefficients”), suggesting that mineral ballast was not a strong predictor for POC in this section.

Ferromanganese (FeMn) crusts are of interest and clear importance both as a unique paleoceanographic archive of seawater chemistry and as a potential resource for valuable and critical metals. Presently, there is a need to shift FeMn crust research from purely exploration-based studies to more targeted expeditions that can precisely locate FeMn crusts with characteristics of interest for scientific research and resource evaluation. Understanding the influence of general oceanographic conditions on FeMn crust formation is key to defining permissive regions. This thesis investigates variations in the geochemistry of FeMn crusts from the west-central Pacific that are associated with regional oceanographic conditions, including proximity to the oxygen minimum zone and high biological productivity maintained at the equator, as well as water depth controls and the effects of diagenetic phosphatization. Chemical analysis of over 60 FeMn crusts reveals that the oxygen minimum zones in the northern and southern tropics in the west Pacific exhibit strong controls on the composition of recent FeMn crust growth as well bulk FeMn crusts. Manganese contents are increased near the oxygen minimum zones, which is shown quantitatively for a broad region in this thesis for the first time. Iron is instead enriched in deeper waters where oxidation of iron is increased, which may be caused by higher carbonate ion concentrations and pH at depth and the availability of deep sources of iron. High biological productivity in the equatorial upwelling zone enriches iron, magnesium, and barium in FeMn crusts that are forming below this zone due to increased efficiency of the biological pump. Diagenetic phosphatization is commonly observed in crusts from depths above 2000 meters, but does not exceedingly dilute manganese oxides or elements of economic interest. Synchrotron analyses of phosphatized FeMn crust layers indicate that phosphatization does not change the speciation of lead as hypothesized based on previous sequential leaching studies. Overall, is it shown that the general oceanographic parameters studied here influence FeMn crust composition based on their geographic location and water depth, and these parameters can help create criteria for future crust exploration.

Heightened biological activity was observed in February 1996 in the high-nutrient low-chlorophyll (HNLC) subarctic North Pacific Ocean, a region that is thought to be iron-limited. Here we provide evidence supporting the hypothesis that Ocean Station Papa (OSP) in the subarctic Pacific received a lateral supply of particulate iron from the continental margin off the Aleutian Islands in the winter, coincident with the observed biological bloom. Synchrotron X-ray analysis was used to describe the physical form, chemistry, and depth distributions of iron in size fractionated particulate matter samples. The analysis reveals that discrete micron-sized iron-rich hotspots are ubiquitous in the upper 200m at OSP, more than 900km from the closest coast. The specifics of the chemistry and depth profiles of the Fe hot spots trace them to the continental margins. We thus hypothesize that iron hotspots are a marker for the delivery of iron from the continental margin. We confirm the delivery of continental margin iron to the open ocean using an ocean general circulation model with an iron-like tracer source at the continental margin. We suggest that iron from the continental margin stimulated a wintertime phytoplankton bloom, partially relieving the HNLC condition.