Publication Date:
2022-05-26
Description:
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009):1143-1167, doi:10.1016/j.dsr.2009.04.001.
Description:
An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic
site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical
Transport in the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at
ALOHA, while 234Th:238U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial
variability was large at both sites- even greater than seasonal variability as reported in prior studies. This
variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration
purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with
the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux
predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap
calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th below the
deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at
depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of
234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is
similar on sinking particles and mid-sized particles collected by in-situ filtration (〉10-50 μm at ALOHA and
〉5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of
234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column
will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux
and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and
10’s to 100’s km at a resolution that is difficult to obtain with other methods.
Description:
Funding for VERTIGO in the US was provided primarily by research grants from the US
National Science Foundation Programs in Chemical and Biological Oceanography with additional support by
the US Department of Energy (DAS). For TWT, support came from the Australian Cooperative Research
Centres program.
Keywords:
Thorium-234
;
Particle flux
;
Sediment trap
;
Scavenging
;
North Pacific
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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