Abstract
Abstract. We report on the sub-basin variability in particulate organic carbon (POC) remineralization in the western and central Mediterranean Sea in late spring during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean sea) cruise. POC remineralization rates were estimated using the excess biogenic particulate barium (Baxs) inventories in the mesopelagic layers (100–1000 m depth) and compared with prokaryotic heterotrophic production (PHP). Baxs-based mesopelagic remineralization rates (MRs) ranged from 25±2 to 306±70 mgCm-2d-1. MRs were larger in the Algero-Provençal (ALG) Basin than in the Tyrrhenian (TYR) and Ionian (ION) basins. Our Baxs inventories and integrated PHP data also indicated that significant mesopelagic remineralization occurred down to 1000 m depth in the ALG Basin in contrast to the ION and TYR basins, where remineralization was mainly located above 500 m depth. We propose that the higher and deeper MRs in the ALG Basin were sustained by an additional particle export event driven by deep convection. The TYR Basin (in contrast to the ALG and ION basins) presented the impact of a previous dust event, as reflected by our particulate Al water column concentrations. The ION and TYR basins showed small-scale heterogeneity in remineralization processes, reflected by our Baxs inventories and integrated PHP data at the Tyrr long-duration station. This heterogeneity was linked to the mosaic of blooming and non-blooming patches reported in this area during the cruise. In contrast to the western Mediterranean Sea (ALG Basin), the central Mediterranean Sea (ION and TYR basins) showed lower remineralization rates restricted to the upper mesopelagic layer during the late spring PEACETIME cruise.
Highlights
In the ocean, the remineralization rate associated with sinking particles is a crucial variable for the air–sea CO2 balance (Kwon et al, 2009)
The main water masses are as follows: (1) from west to east, Atlantic Surface Water (SW) is gradually replaced by Ionian Surface Water (ISW) and Levantine Surface Water (LSW); (2) Winter Intermediate Water (WIW); (3) Levantine Intermediate Water (LIW), which is present at intermediate depths and is characterized by a local maximum of salinity and a local minimum of dissolved oxygen concentration; and (4) Mediterranean Deep Water (MDW)
Particulate biogenic Baxs, biogenic Ba fraction (%), and particulate Al, Sr and Ca concentrations are reported in Fig. 2 in the upper 2000 m of the water column along a zonal transect crossing the three main sub-basins
Summary
The remineralization rate associated with sinking particles is a crucial variable for the air–sea CO2 balance (Kwon et al, 2009). Most of the sinking particulate organic carbon (POC) conversion (i.e. remineralization) into CO2 by heterotrophic organisms (i.e. respiration) occurs within the mesopelagic zone (100–1000 m) (Martin et al, 1987; Buesseler et al, 2007; Buesseler and Boyd, 2009). Particulate biogenic barium (Baxs ) is a geochemical tracer of POC remineralization in the mesopelagic layer. Baxs occurs in the form of barite (BaSO4 crystals) in the dark ocean as a by-product of prokaryotic remineralization. In a global ocean undersaturated with respect to barite (Monnin and Cividini, 2006), Baxs precipitates inside oversaturated biogenic micro-environments during POC degradation by heterotrophic prokaryotes, through sulfate and/or barium enrichment (Dehairs et al, 1980; Stroobant et al, 1991; Bertram and Cowen, 1997; Ganeshram et al, 2003). By applying a transfer function relating Baxs to O2 consumption (Dehairs et al, 1997) Baxs has been widely used since the
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