Export and mesopelagic particle flux during a North Atlantic spring diatom bloom

Spring diatom blooms are important for sequestering atmospheric CO₂ below the permanent thermocline in the form of particulate organic carbon (POC). We measured downward POC flux during a sub-polar North Atlantic spring bloom at 100m using thorium-234 (²³⁴Th) disequilibria, and below 100m using neutrally buoyant drifting sediment traps. The cruise followed a Lagrangian float, and a pronounced diatom bloom occurred in a 600km² area around the float. Particle flux was low during the first three weeks of the bloom, between 10 and 30mgPOCm^-2d^-1. Then, nearly 20 days after the bloom had started, export as diagnosed from ²³⁴Th rose to 360-620mgPOCm^-2d^-1, co-incident with silicate depletion in the surface mixed layer. Sediment traps at 600 and 750m depth collected 160 and 150mgPOCm^-2d^-1, with a settled volume of particles of 1000-1500mLm^-2d^-1. This implies that 25-43% of the 100m POC export sank below 750m. The sinking particles were ungrazed diatom aggregates that contained transparent exopolymer particles (TEP). We conclude that diatom blooms can lead to substantial particle export that is transferred efficiently through the mesopelagic. We also present an improved method of calibrating the Alcian Blue solution against Gum Xanthan for TEP measurements.