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.
doi:10.1016/j.dsr.2008.07.012
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Particulate matter chemistry and dynamics in the twilight zone at VERTIGO ALOHA and K2 sitesJ. K. B. Bishop and T. J. Wood
Deep-Sea Research I 55, 1684-1706.
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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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