In situ measurement of mesopelagic particle sinking rates and the control of carbon transfer to the ocean interior during the Vertical Flux in the Global Ocean (VERTIGO) voyages in the North Pacific
Trull, T and Bray, SG and Buesseler, KO and Lamborg, CH and Manganini, S and Moy, C and Valdes, J, In situ measurement of mesopelagic particle sinking rates and the control of carbon transfer to the ocean interior during the Vertical Flux in the Global Ocean (VERTIGO) voyages in the North Pacific, Deep Sea Research Part II: Topical Studies in Oceanography, 55, (14-15) pp. 1684-1695. ISSN 0967-0645 (2008) [Refereed Article]
Among the parameters affecting carbon transfer to the ocean interior, particle sinking rates vary three
orders of magnitude and thus more than primary production, f-ratios, or particle carbon contents [e.g.,
Boyd, P.W., Trull, T.W., 2006. Understanding the export of marine biogenic particles: is there consensus?
Progress in Oceanography 4, 276–312, doi:10.1016/j.pocean.2006.10.007]. Very few data have been
obtained from the mesopelagic zone where the majority of carbon remineralization occurs and the
attenuation of the sinking flux is determined. Here, we report sinking rates from 300m depth for the
subtropical (station ALOHA, June 2004) and subarctic (station K2, July 2005) North Pacific Ocean,
obtained from short (6.5 day) deployments of an indented rotating sphere (IRS) sediment trap operating
as an in situ settling column [Peterson, M.L.,Wakeham, S.G., Lee, C., Askea, M.A., Miquel, J.C., 2005. Novel
techniques for collection of sinking particles in the ocean and determining their settling rates.
Limnology and Oceanography Methods 3, 520–532] to separate the flux into 11 sinking-rate fractions
ranging from 4820 to 42md 1 that are collected by a carousel for further analysis.
Functioning of the IRS trap was tested using a novel programming sequence to check that all
particles have cleared the settling column prior to the next delivery of particles by the 6-hourly rotation
cycle of the IRS. There was some evidence (from the flux distribution among the cups and
photomicroscopy of the collected particles) that very slow-sinking particles may have been under--
collected because they were unable to penetrate the brine-filled collection cups, but good evidence for
appropriate collection of fast-settling fractions.
Approximately 50% of the particulate organic carbon (POC) flux was sinking at greater than
100md 1 at both stations. At ALOHA, more than 15% of the POC flux sank at 4820md 1, but low fluxes
make this uncertain, and precluded resolution of particles sinking slower than 137md 1. At K2, less
than 1% of the POC flux sank at 4820md 1, but a large fraction ( 15–45%) of the flux was contributed
by other fast-sinking classes (410 and 205md 1). PIC and BSi minerals were not present in higher
proportions in the faster sinking fractions, but the observations were too limited to rule out a ballasting
contribution to the control of sinking rates. Photographic evidence for a wide range of particle types
within individual sinking-rate fractions suggests that biological processes that set the porosity and
shape of particles are also important and may mask the role of minerals.
Comparing the spectrum of sinking rates observed at K2 with the power-law profile of flux
attenuation with depth obtained from other VERTIGO sediment traps deployed at multiple depths
[Buesseler, K.O., Lamborg, C.H., Boyd, P.W., Lam, P.J., Trull, T.W., Bidigare, R.R., Bishop, J.K.B., Casciotti,
K.L., Dehairs, F., Elskens, M., Honda, M., Karl, D.M., Siegel, D., Silver, M., Steinberg, D., Valdes, J., Van
Mooy, B., Wilson, S.E., 2007b. Revisiting carbon flux through the Ocean’s twilight zone. Science
316(5824), 567–570, doi: 10.1126/science.1137959] emphasizes the importance of particle transformations
within the mesopelagic zone in the control of carbon transport to the ocean interior.