Particulate organic carbon export across the Antarctic Circumpolar Current at 10°E: Differences between north and south of the Antarctic Polar Front (original) (raw)

2017, Deep Sea Research Part II: Topical Studies in Oceanography

The vertical distribution of 234 Th was measured along the 10ºE meridian between 44ºS and 53ºS in 29" the Antarctic Circumpolar Current (ACC) during the austral summer of 2012. The overarching 30" goal of this work was to estimate particulate organic carbon (POC) export across three fronts: The 31" Sub-Antarctic Front (SAF), the Antarctic Polar Front (APF) and the Southern Polar Front (SPF). 32" Steady state export fluxes of 234 Th in the upper 100 m ranged from 1600 to 2600 dpm m-2 d-1 , 33" decreasing with increasing latitude. Using large particle (>53 µm) C/ 234 Th ratios, the 234 Th-derived 34" POC fluxes at 100 m ranged from 25 to 41 mmol C m-2 d-1. Observed C/ 234 Th ratios decreased 35" with increasing depth north of the APF, while south of the APF, ratios remained similar or even 36" increased with depth. These changes in C/ 234 Th ratios are likely due to differences in the food 37" web. Indeed, satellite images, together with macronutrients and dissolved iron concentrations 38" suggest two different planktonic community structures north and south of the APF. Our results 39" indicate that higher ratios of POC flux at 100 m to primary production occurred in 40" nanophytoplankton dominated surface waters, where primary production rates were lower. 41" Satellite images prior to the expedition suggest that the higher export efficiencies obtained in the 42" northern half of the transect may be the result of the decoupling between production and export 43" (Buesseler 1998). Transfer efficiencies to 400 m, i.e. the fraction of exported POC that reached 44" 400 m, were found to be higher in the south of the APF, where diatoms were dominant and salps 45" largely abundant. This suggests different remineralization pathways of sinking particles, 46" influencing the transfer efficiency of exported POC to depth. 47" references therein). However, scarce spatial coverage of sediment traps studies, and the possible 73" biases associated with traps (mainly due to hydrodynamics and solubilization; Buesseler et al., 74" 2007; Usbeck et al., 2003) warrant the use of other complementary approaches to quantify the 75" spatial and temporal variability of the biological pump in the SO. 76" A widely applied approach to estimate particle export is the use of the radionuclide pair 77" 234 Th/ 238 U. Thorium-234 is a naturally occurring, short-lived radionuclide (T 1/2 = 24.1 days) 78" produced by the alpha decay of 238 U (T 1/2 = 4.5 x 10 9 years). Due to its high particle affinity, 79" thorium is rapidly adsorbed onto particle surfaces (Moore and Millward, 1988). In contrast 80" uranium is conservative in oxic systems (Chen et al., 1986). Thus, the deviation of 234 Th/ 238 U from 81" unity can be used as a proxy for particle dynamics (e.g., formation and export) in the ocean 82" surface. Further, the half-life of 234 Th is similar to the time scales of processes that determine 83" particle dynamics in the ocean (such as the development of phytoplankton blooms), allowing for 84" fine-scale observations of particle export and remineralization. 85" The fronts of the ACC have been found to coincide with boundaries between regions of similar 86" phytoplankton biomass (Sokolov and Rintoul, 2007). Moreover, phytoplankton composition and 87" distribution appear to be strongly linked to physical zonation within the SO (Laubscher et al., 88" 1993; Read et al., 2002). Sediment records also reflect such boundaries, with large opal 89" accumulation found south of the APF (Geibert et al., 2005; Tréguer and De La Rocha, 2013), as a 90" consequence of spatial segregation of phytoplankton communities due to difference temperature 91" and nutrient regimes (Falkowski et al., 1998). This in turn also affects zooplankton community 92" composition and distribution (Hunt and Hosie, 2005; Pakhomov and McQuaid, 1996; Pollard et 93" al., 2002a) and their grazing dynamics. Thus, physical controls on biogeochemical zonation are 94" expected to influence the pelagic community structure, which will affect the composition of the 95" sinking particles, and hence the downward flux of organic matter (Korb et al., 2012; Quéguiner, 96" 2013) 97"