Controls Over Sinking Particle Remineralisation Depth - Initial Results From the COMICS Programme

Numerical models suggest that the depth at which organic matter sinking from the ocean surface is remineralised back to CO₂ exerts a profound control atmosphere/ ocean CO₂ partitioning and hence climate. Understanding the dominant control over this depth is crucial for producing models that credibly represent remineralisation - currently there is a wide range of representations of remineralisation and the resultant nutrient fields in CMIP5 models reflecting a lack of consensus regarding the processes responsible for regulating remineralisation depth. Potential controls over this depth, all of which have some support in the published literature, include the structure of the ecosystem in overlying waters, interior temperature and interior dissolved oxygen levels. We have already shown a strong statistically robust relationship between temperature and mineralisation depth which we believe may be causative however when this is implemented in a numerical model it leads to deeper mineralisation than can be supported by the climatological interior nutrients field. We hypothesise that this occurs because our existing suite of field studies focusses on low nutrient post bloom oligotrophic regions and is biased away from high production regions such as blooms, upwelling regions and the equator with these regions being characterised by a breakdown of temperature control over mineralisation depth and instead a dominance of shallow mineralisation. Determining the validity of this hypothesis is key to producing ecologically realistic and computationally efficient models of the ocean carbon cycle. To this end we have been exploring controls over remineralisation depth via a carefully targeted programme of observations in selected ocean ecosystems. and have recently completed field studies focussed on the role of community structure and oxygen in the highly productive region of the Southern Ocean downstream of the subantarctic island of South Georgia where iron limitation is lifted and production levels are amongst the highest on the planet and in the Benguela upwelling. The Southern Ocean study was characterised by shallow mineralisation despite the cold temperatures. Accounting for this behaviour may have the potential to accurately reproduce the interior nutrient field and avoid relaxation to climatology.