Particulate matter chemistry and dynamics in the twilight zone at VERTIGO ALOHA and K2 sites

J. K. B. Bishop and T. J. Wood

Deep-Sea Research I 55, 1684-1706.

Abstract. Understanding particle dynamics in the `Twilight Zone' is critical to prediction of the ocean's carbon cycle. As part of the VERtical Transport In the Global Ocean (VERTIGO) project, this rarely sampled regime extending from the base of the euphotic layer to 1000m, was characterized by double-paired day/night Multiple Unit Large Volume in-situ Filtration System (MULVFS) deployments and by100 high-frequency CTD/transmissometer/ turbidity sensor profiles. VERTIGO studies lasting 3 weeks, contrasted oligotrophic station ALOHA (22.751N 1581W), sampled in June-July 2004, with a biologically productive location (471N 1611E) near station K2 in the Oyashio, occupied July-August 2005. Profiles of major and minor particulate components (Corg, N, P, Ca, Si, Sr, Ba, Mn) in <1, 1-51, and >51um size fractions, in-water optics, neutrally buoyant sediment trap (NBST) fluxes, and zooplankton data were intercompared. MULVFS total Corg and C-Star particle beam attenuation coefficient (CP) were consistently related at both sites with a 27uM per m-1 conversion factor. At K2, CP profiles further showed a multitude of transient spikes throughout the water column and spike abundance profiles closely paralleled the double peaked abundance profiles of zooplankton. Also at K2, copepods contributed 40% and 10%, night and day, respectively to >51mm Corg of MULVFS samples in the mixed layer, but few copepods were collected in deeper waters; however, non-swimming radiolarians were quantitatively sampled. A recent hypothesis regarding POC differences between pumps and bottles is examined in light of these results. Particulate >51mmCorg, N, and P at both ALOHA and K2 showed strong attenuation with depth at both sites. Notable at ALOHA were unusually high levels of >51mm Sr (up to 4nM) in the mixed layer, a reflection of high abundances of SrSO4 precipitating Acantharia. Notable at K2 were major changes in water column inventories of many particulate components to 700m over 10 days. Carbon mass balance, with the consideration of particle inventory changes included, indicated that over 98% and 96% of primary produced Corg was remineralized shallower than 500m at ALOHA and K2, respectively. Production of CaCO3 was estimated to be 0.06, 0.89, and 0.02 mmol/m2/d at ALOHA and at K2 during two separate week long study periods, respectively. Similarly, Si production was estimated to be 0.08, 10.7, and 4.2 mmol/m2/d. An estimated 50% and 65% of produced Si was remineralized by 500m at ALOHA and K2, respectively. Little carbonate dissolutionwas seen in the upper 500m at ALOHA, a reflection of 400% super saturation of surface waters and the 700m deep saturation horizon. Over 92% of produced CaCO3 was dissolved shallower than 500m at K2 and biological enhancement of dissolution was readily apparent in waters above the 200m calcite saturation horizon.

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