Flocculation and phytoplankton cell size can alter 234Th-based estimates of the vertical flux of particulate organic carbon in the sea

Anya Waite, P.S. Hill

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    20 Citations (Scopus)


    The deficit of Th-234 relative to its radioactive parent U-238 in the surface ocean can yield reliable estimates of vertical Particulate Organic Carbon (POC) fluxes to deeper waters, but only when coupled with an accurate ratio of POC concentration to activity of Th-234 on sinking matter. Assuming a simple partitioning of suspended phytoplankton mass between single cells and flocs, we calculate the ratio of the POC flux estimated from Th-234 deficit to the actual POC flux (p ratio, Smith, J.N., Moran, S.B., Speicher, E.A., in press). The p ratios are calculated under the assumption that particle surface area is correlated with Th-234 activity and particle volume is correlated with POC concentration. The value of the p ratio depends on the relative contributions of single cells and flocs to the vertical flux. When large single cells make up a significant fraction of the vertical flux, p ratios are less than one, meaning POC fluxes estimated from Th-234 deficits underestimate actual POC fluxes. When large single cells are abundant but do not sink fast enough to contribute to vertical POC flux, p ratios are greater than one (up to 3 x overestimate). Factor analysis of the model indicates that altering the extent of flocculation in suspension and changing the density and maximum size of phytoplankton cells have the greatest effects on the p ratio. Failure to measure the properties of flocs when characterizing the ratio of POC to thorium on sinking matter potentially leads to large overestimation of the POC flux (over 20 x). Failure to characterize the POC to thorium ratio of large particles, by, for example, destruction of phytoplankton cells in pumps, can lead to underestimation of POC flux. Estimates of POC flux should be most reliable in highly flocculated suspensions populated by small cells and rapidly sinking flocs. These conditions are often associated with intense phytoplankton blooms. (c) 2005 Elsevier B.V. All rights reserved.
    Original languageEnglish
    Pages (from-to)366-375
    JournalMarine Chemistry
    Issue number3-4
    Publication statusPublished - 2006


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