Towards a mechanistic understanding of the decoupling between oceanic iron and carbon remineralization length-scales

Dissolved iron (DFe) supply controls half of ocean primary productivity and has a major influence on many aspects of the global carbon cycle. To date, studies of iron biogeochemistry have largely focused on the diverse modes of external DFe supply and their regional influences. Hence, they have largely ignored iron cycling in the ocean’s interior - a key vector in the annual resupply of upper ocean DFe stocks and a major unknown hindering model development. Trace metal-clean sediment trap studies provided initial evidence of decoupling between the remineralization length-scales of iron and the major elements (P, C and N), however, the drivers of this decoupling remain largely unexplored. Here, we combined in situ and in vitro approaches to explore the underlying biotic and abiotic mechanisms setting iron remineralization length-scales. Innovative particle interceptors / incubators (RESPIRE and TM-RESPIRE) were deployed at three different depths within the upper mesopelagic of the Subantarctic Southern Ocean to measure in situ the bacterial remineralization of sinking particles and the corresponding DFe release rates. These deployments enabled concurrent quantification of the vertical attenuation in particulate Fe and C fluxes. In addition, they permitted the investigation of the different processes driving Fe remineralization and their complex interplay. Further insights into Fe and C decoupling came from shipboard incubations of resuspended mesopelagic particle assemblages which quantified the influence of scavenging on dissolved iron replenishment from bacterially-mediated iron remineralization. Together, these approaches help us improve or mechanistic understanding of the C and Fe cycles by: quantifying the subsurface decoupling of the iron remineralization relative to major elements, and through teasing apart the biotic and abiotic transformations leading to this decoupling.