Measured and modeled particle export in equatorial and coastal upwelling regions

Abstract

The focus of this work was to improve understanding of particle cycling and export from the surface ocean in three areas: (1) methods validation, (2) description of regional and temporal variability and (3) evaluation of particle export mechanisms and implications. To explore the 234 Th method of estimating particle export, I first studied the sensitivity of 234Th export to circulation by imposing a hypothetical advection and mixing field on a simple dissolved and particulate 234Th cycle for the central equatorial Pacific. This work confirmed previous studies which estimated particle sinking fluxes using 234Th and implicated vertical advection as a dominant component of the 234Th budget. However, it also suggested that these studies over-estimated sinking particle fluxes in not incorporating horizontal mixing of 234Th. In a second study, I evaluated the role of diatoms in new and export production in the equatorial Pacific by synthesizing data from a suite of cruises. Differential nutrient utilization was shown to be a dominant feature of equatorial biogeochemistry such that large, rapidly sinking and slowly-dissolving diatoms were only dominant in new and export production under high nutrient (iron) conditions. In a third study, I characterized particle export in the western and central equatorial Pacific on two zonal transects and compared these results with data from the JGOFS EqPac program, concluding that the presence of nutrients and the propagation of Kelvin Waves and Tropical Instability Waves were the major determining factors for production variability. In the Santa Barbara Channel, I collected data across the continuum of particle size classes to verify the mechanism by which 234Th traces the particle cycle. Finally, I synthesized data from divergent oceanic regimes to describe the biogeochemical implications of the Two-State ecosystem hypothesis.