The primary goal of the Exports Processes in the Ocean from RemoTe Sensing (EXPORTS) Program is to develop a predictive mechanistic understanding of the export and fate of fixed carbon (C) from global ocean primary production. Inherent in these predictions is the need to reduce uncertainties in C export and to develop models that facilitate future C export predications across a variety of spatial and temporal scales. The first phase includes large-scale field campaigns using remote sensing, autonomous platforms and process-level studies to build upon current knowledge. As such, critical components to the EXPORTS Program are quantitative measurements of sinking particle fluxes and their attenuation at scales similar to the physical and biological processes that influence fixed C export.
We propose using the particle-reactive tracer, thorium-234 (half-life = 24. 1 d) to quantify the spatio-temporal variability in particle flux from the well-lit surface layer, and its attenuation with depth below. The disequilibrium in 234Th from its soluble parent, uranium-238, provides quantitative information on where particle export and remineralization occurs. We will collect an unprecedented set of 234Th profiles (≈ 60) to obtain a 3D time-series of particle export and remineralization rates over meso- (~ 10 km) and submesoscales (~ 1 km).
The link from 234Th to C flux is based upon determining the ratio of particulate 234Th to C (will work for nitrogen, biogenic silica and particulate inorganic C) measured on depth resolved profiles of size-fractionated particles (>1 to 50-100 µm range) collected using in-situ pumps (subsamples will be shared). We will combine 234Th data with satellite products and results from a 3D high-resolution coupled physical-biogeochemical model with 234Th dynamics to produce synthesized flux data products.
The underlying hypothesis of EXPORTS is that the export and fate of net primary production (NPP) can be quantified by knowing the characteristics of surface ocean ecosystems, thereby linking biotic processes with remotely sensed properties.. Our study is directly related to the first 2 EXPORTS Science Questions (SQ’s): How small spatio-temporal variations in ecosystems characteristics and biophysical processes are reflected in localized variations in export (SQ1); and the degree to which upper ocean and mid-water food webs impact C transfer efficiencies (SQ2). Combined with other EXPORTS measurements, our 234Th approach will allow us to link controlling mechanisms of export with remotely sensed properties and reduce uncertainties on the fate of ocean primary production (SQ3). Furthermore, these questions can be approached via more specific 234Th sub-hypotheses:
H1. In the absence of grazing, diatoms increase the absolute magnitude of particulate C flux below the euphotic zone (EZ) (relative to smaller cells) and this export occurs during senescence and aggregation.
H2. Under conditions of low grazing, rapid sinking of diatoms (or other mineral containing organisms, such as coccoliths) results in a higher efficiency of transfer versus smaller cells.
H3. With increased zooplankton grazing, the magnitude of particulate C export will decrease, but the extent of particulate C flux attenuation and transfer efficiency of material below the EZ will be controlled by sub-surface foodwebs.
H4. During non-bloom periods, or when zooplankton grazing and phytoplankton growth are in near balance, vertical mixing of dissolved organic carbon (DOC) will be a significant pathway for C export and a potential for export of labile DOC.
234Th is needed to address these hypotheses and is fundamental for developing mechanistic global ocean models of the fate of fixed C in marine systems needed in the EXPORTS synthesis and integration Phase II. Only 234Th provides rates of C flux over the smaller spatial and vertical scales required to understand processes that regulate water column particle export and C attenuation.