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

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 1000 m, was characterized by double-paired day/night Multiple Unit Large Volume in-situ Filtration System (MULVFS) deployments and by ∼100 high-frequency CTD/transmissometer/turbidity sensor profiles. VERTIGO studies lasting 3 weeks, contrasted oligotrophic station ALOHA (22.75°N 158°W), sampled in June–July 2004, with a biologically productive location (47 °N 161°E) 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 >51 μm 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 27 μM 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 >51 μm 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 >51 μm Corg, N, and P at both ALOHA and K2 showed strong attenuation with depth at both sites. Notable at ALOHA were unusually high levels of >51 μm Sr (up to 4 nM) 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 700 m 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 500 m at ALOHA and K2, respectively. Production of CaCO3 was estimated to be ∼0.06, 0.89, and 0.02 mmol m−2 d−1 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 mol m−2 d−1. An estimated 50% and 65% of produced Si was remineralized by 500 m at ALOHA and K2, respectively. Little carbonate dissolution was seen in the upper 500 m at ALOHA, a reflection of 400% super saturation of surface waters and the 700 m deep saturation horizon. Over 92% of produced CaCO3 was dissolved shallower than 500 m at K2 and biological enhancement of dissolution was readily apparent in waters above the 200 m calcite saturation horizon.