Early winter barium excess in the southern Indian Ocean as an annual remineralisation proxy (GEOTRACES GIPr07 cruise)

Abstract

Abstract. The Southern Ocean (SO) is of global importance to the carbon cycle, and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region need to be better constrained. During this study early austral winter barium excess (Baxs) concentrations were measured for the first time, along 30∘ E in the southern Indian Ocean. Winter Baxs concentrations of 59 to 684 pmol L−1 were comparable to those observed throughout other seasons. The expected decline of the mesopelagic Baxs signal to background values during winter was not observed, supporting the hypothesis that this remineralisation proxy likely has a longer timescale than previously reported. A compilation of available SO mesopelagic Baxs data, including data from this study, shows an accumulation rate of ∼0.9 µmol m−2 d−1 from September to July that correlates with temporally integrated remotely sensed primary productivity (PP) throughout the SO from data spanning ∼20 years, advocating for a possible annual timescale of this proxy. The percentage of mesopelagic particulate organic carbon (POC) remineralisation as calculated from estimated POC remineralisation fluxes over integrated remotely sensed PP was ∼2-fold higher south of the polar front (19 ± 15 %, n=39) than north of the polar front (10 ± 10 %, n=29), revealing the higher surface carbon export efficiency further south. By linking integrated remotely sensed PP to mesopelagic Baxs stock, we could obtain better estimates of carbon export and remineralisation signals within the SO on annual and basin scales.

Biogenic-to-lithogenic handoff of particulate Zn affects the Zn cycle in the Southern Ocean

“…The absence of a widespread, enriched Zn-silica pool, specifically in surface waters south of the polar front where diatoms dominate, is surprising, and is likely caused by differences in phytoplankton community structure (changes in the proportion of silicifying plankton) ( 22 , 23 , 38 ), low abundance of Zn in frustules ( 41 , 44 ), and insufficient remineralization of predominantly phosphoryl-bound cellular Zn (which masks weak signals from the Zn-silica associations in frustules). In the studied regions, the remineralization depths of particulate organic matter are below the mixed-layer depth and reach up to 1000 m ( 45 ). Extensive remineralization of biogenic Zn-phosphoryl and additional scavenging of Zn onto biogenic silica particle surfaces explain the occurrence of Zn-silica particles in deep water, where elevated Zn abundances were found in some particles (normalized Zn fluorescence on the order of 10 −2 , the upper end in the analyzed particles).…”