TN397 SCOPE-Gradients-4 Particle Flux

Particle Interceptor Traps (PITs; Knauer et al., 1979) were prepared, deployed, and processed according to the following protocol to quantify vertical particulate matter fluxes during the GRADIENTS IV expedition. To isolate sinking particles and prevent flushing, a high-density brine solution was prepared by dissolving 4250 g of potassium chloride (KCl) in 50 L of Milli-Q water, which was then pre-filtered to eliminate background contamination. This filtered KCl solution was added to individual PIT tubes with a mouth opening of 0.0039 m², which were subsequently overlaid with filtered seawater (FSW) passed through a 0.2/0.8 µm cartridge filter to establish a distinct density interface. Ten PITs were deployed at each of three distinct depths for a collection period of approximately 24 hours. The PITs at each depth were affixed to a PVC cross attached to a single free-floating array. Upon recovery, the density interfaces were visually identified and marked, and the overlying seawater layer was siphoned off to 5 cm above the brine interface. The remaining volumes containing the captured particulate matter were pre-screened through a 335 µm NITEX mesh, pooled into depth-specific carboys, and homogenized by gentle rolling. Finally, precise volumetric aliquots were drawn from the mixture for discrete biogeochemical analyses, including Particulate Adenosine 5-Triphosphate (PATP, 0.5 L), Particulate Phosphorus (PPO4, 0.5 L), Total Particulate Carbon and Total Particulate Nitrogen (TCTN, 2.0 L), Particulate Inorganic Carbon (PIC, 1.5 L), Particulate Silica (PSi, 0.25 L), and dissolved nutrients from the filtrate (Nuts, 0.125 L). Equivalent volumes of the filtered KCl solution were filtered and analyzed concurrently for blank corrections.