GEOTRACES Atlantic Workshop Importance of mesopelagic fluxes and recycling to trace element biogeochemistry Peter J Statham and Richard Lampitt- NOC Southampton UK Can regard the mesopelagic later as a valve- if material passes likely to be locked away in deep ocean for decades to centuries, if recycled to dissolved forms may be re-entrained into upper ocean Existing focus on C (inorganic and organic) Si N & P However potentially critical for other elements e.g. Fe, Mn Ba- but little known about detailed processes As identified in IMBER and GEOTRACES science plans, understanding recycling of elements in the mesopelagic crucial to fate of many key bio-reactive elements GEOTRACES Atlantic Workshop

GEOTRACES Atlantic Workshop Some of the challenges How to improve collection of particles and estimate vertical fluxes? How to estimate recycling from water column particles? Relevance of episodic large events vs background processes (time-scales and measurement locations?) Bacterial processes appear to dominate recycling- BUT how much do we know about these processes? Role of aggregate processes in recycling? GEOTRACES Atlantic Workshop

Possible approaches and where in Atlantic Direct measurements of flux/processes Improvements to shallow traps - use of neutrally buoyant traps to reduce artefacts In situ sampling of real settling particles, and incubations PELAGRA Examples of indirect measurement of fluxes Th-234- (T½ 24.1d) potentially time-integrating (See recent review in Mar Chem.) C and other TEI fluxes Other Th isotopes e.g. Th-228 (T½ ~1.9yr) ANIMATE mooring network Where in the Atlantic ?? Contrasting productivity locations Link to existing long term stations e.g. ANIMATE programme, BATS GEOTRACES Atlantic Workshop

GEOTRACES Atlantic Workshop