The magnitude of particulate carbon export from the upper ocean and efficiency of its transfer into the interior remains one of the least predictable processes influencing the global carbon cycle. The overarching goal of the EXPORTS program is to develop mechanistic models predicting the strength and efficiency of this exported carbon. A central requirement – one might argue the central requirement – of the ambitious set of field measurements necessary to develop satellite-driven models is accurate measurement of sinking particle fluxes and their biological and chemical compositions. These measurements must be embedded in a broader suite of physical, biological, and optical observations, so a second requirement is that the team quantifying particle flux cooperates and works closely with the broader EXPORTS science team. Here, we propose a hypothesis-driven study of particle fluxes that both advances our understanding of the ocean biological carbon pump and meets the broader program goals above.
The broad hypothesis of the EXPORTS program is that the strength and efficiency of the biological carbon pump can be predicted from satellite ocean color observations. Implicit in this hypothesis are a number of assumptions which must be tested. The specific hypotheses we will address are 1) that the biological origin of the particles sinking out of the euphotic zone exerts significant control on both the magnitude of the sinking carbon flux and its rate of attenuation with depth; and 2) that temporal variability in the magnitude and attenuation of sinking particle flux is caused by biological processes.
Our proposed study will employ a mature, field-tested technology – quasi-Lagrangian, Neutrally Buoyant Sediment Traps (NBSTs) – to sample sinking particles at 5 depths in the upper 500 m of the ocean for each ecosystem state characterized during the EXPORTS field program. To test the hypothesis that biology drives temporal variability in flux, we must also characterize water column physical structure and particle properties at very high vertical and temporal resolution using a WireWalker deployed alongside the NBST array. We will determine fluxes of particle mass and major bioelements (organic and inorganic C, N, biogenic Si, and 234Th), estimate the time-resolved flux variability using Optical Sediment Traps, quantify sinking particle identities and size distribution by microscopy (gel traps), and identify the organismal contents within the sinking particles by DNA sequencing. By linking the subsurface NBST drift trajectories to measurements and models of the physical particle field, we will be able to connect our subsurface particle flux observations to simultaneous measurements in the euphotic zone and upper twilight zone by the broader EXPORTS team.
The principal investigators in this project have experience with the proposed field sampling techniques, with integration of sediment trap observations into a multidisciplinary analysis, and with the goals and proposed execution of the EXPORTS program. Buesseler and Estapa are currently carrying out a comprehensive field intercomparison of sediment traps and carbon flux measurement techniques, and will apply their experience directly to the quantification of sources of uncertainty in the sediment trap measurements. Estapa, Durkin and Omand have been refining the integration of chemical, optical, and genetic field techniques for the last two years. Buesseler is leading a parallel proposal to estimate sinking particle fluxes using 234Th deficits, which will also inform estimated trap uncertainties. Omand is a co-PI on a parallel proposal to carry out a suite of AUV measurements; if funded we expect to coordinate operations of our drifting assets in the field. Beyond the major bioelement fluxes, sinking particle size distribution, microscopy, and genetic analyses proposed here, we expect to liberally share trap samples with other EXPORTS teams and for long-term archival.