Publication Date:
2022-05-25
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): 875-891, doi:10.1016/j.dsr.2009.01.008.
Description:
Direct measurements of new production and carbon export in the subtropical
North Atlantic Ocean appear to be too low when compared to geochemical based
estimates. It has been hypothesized that episodic inputs of new nutrients into surface
water via the passage of mesoscale eddies or winter storms may resolve at least some of
this discrepancy. Here, we investigated particulate organic carbon (POC), particulate
organic nitrogen (PON), and biogenic silica (BSiO2) export using a combination of water
column 234Th:238U disequilibria and free-floating sediment traps during and immediately
following two weather systems encountered in February and March 2004. While these
storms resulted in a 2-4 fold increase in mixed layer NO3 inventories, total chlorophyll a
and an increase in diatom biomass, the systems was dominated by generally low
234Th:238U disequilibria, suggesting limited particle export. Several 234Th models were
tested, with only those including non-steady state and vertical upwelling processes able to
describe the observed 234Th activities. Although upwelling velocities were not measured
directly in this study, the 234Th model suggests reasonable rates of 2.2 to 3.7 m d-1.
Given the uncertainties associated with 234Th derived particle export rates and
sediment traps, both were used to provide a range in sinking particle fluxes from the
upper ocean during the study. 234Th particle fluxes were determined applying the more
commonly used steady state, 1-dimensional model with element/234Th ratios measured in
sediment traps. Export fluxes at 200 m ranged from 1.91 ± 0.20 to 4.92 ± 1.22 mmol C
m-2 d-1, 0.25 ± 0.08 to 0.54 ± 0.09 mmol N m-2 d-1, and 0.22 ± 0.04 to 0.50 ± 0.06 mmol
Si m-2 d-1. POC export efficiencies (Primary Production/Export) were not significantly
different from the annual average or from time periods without storms, although absolute
POC fluxes were elevated by 1-11%. This increase was not sufficient, however, to
resolve the discrepancy between our observations and geochemical based estimates of
particle export. Comparison of PON export rates with simultaneous measurements of
NO3
- uptake derived new production rates, suggested that only a fraction, 〈 35%, of new
production was exported as particles to deep waters during these events. Measured bSiO2
export rates were more than a factor of two higher (p 〈 0.01) than the annual average,
with storm events contributing as much as 50% of annual bSiO2 export in the Sargasso
Sea. Furthermore it appears that 65 - 95% (average 86 ± 14%) of the total POC export
measured in this study was due to diatoms.
Combined these results suggest that winter storms do not significantly increase
POC and PON export to depth. Rather, these storms may play a role in the export of
bSiO2 to deep waters. Given the slower remineralization rates of bSiO2 relative to POC
and PON, this transport may, over time, slowly decrease water column silicate
inventories, and further drive the Sargasso Sea towards increasing silica limitation.
These storm events may further affect the quality of the POC and PON exported given
the large association of this material with diatoms during these periods.
Description:
This study was funded by the National Science Foundation (Chemical
Oceanography Grants OCE-0244612 and OCE-0241645).
Keywords:
Episodic events
;
Storms
;
Particle Export
;
Th-234:U-238 disequilibria
;
Biogenic silica
;
Organic carbon
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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