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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Jun 2018

Research article | 27 Jun 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

A three year time-series of volatile organic iodocarbons in Bedford Basin, Nova Scotia: a Northwestern Atlantic fjord

Qiang Shi and Douglas Wallace Qiang Shi and Douglas Wallace
  • Department Oceanography, Dalhousie University, Halifax, Canada

Abstract. We report weekly observations of volatile organic iodocarbons (CH3I, CH2ClI and CH2I2) over the time-period May 2015 to December 2017 from 4 depths in Bedford Basin, a coastal fjord (70m deep) on the Atlantic coast of Canada. The fjord is subject to winter-time mixing, seasonal stratification and bloom dynamics, subsurface oxygen depletion, local input of freshwater and occasional intrusions of higher density water from the adjacent continental shelf. Near-surface concentrations showed strong seasonal and sub-seasonal variability which is compared with other coastal time-series. Relationships with other properties lead to the hypothesis that near-surface iodocarbon production is linked to reduction of iodate to iodide under conditions of post-bloom phytoplankton senescence, nutrient stress or viral lysis following seasonal disappearance of nutrients. The vertical variation of CH2I2 and CH2ClI within the upper 10m is consistent with rapid photolysis of CH2I2.

Average annual sea-to-air fluxes (62nmolm−2day−1) of total volatile organic iodine were slightly higher than observed in other coastal and shelf time-series and polyiodinated compounds contributed 85% of the total flux. Fluxes were subject to strong interannual variability (2–3X) as a result, mainly, of wind-speed variability. Near-surface net production of CH3I averaged 1.0pmolL−1day−1 and was similar to rates in the English Channel but an order of magnitude higher than in shallow waters of the Kiel Fjord, Germany, possibly due to higher microbial degradation in the latter.

The near-bottom (60m) time-series showed evidence for CH3I production associated with organic matter degradation, and a possible switch from production of CH3I via an alkylation pathway to production of CH2I2 by a haloform-type reaction. Near-bottom CH3I production varied strongly between years but was generally ca. 20 times lower than near-surface production. Post-intrusion decreases of iodocarbons at 60m suggested removal timescales of 14, 65 and 70 days for CH3I, CH2I2 and CH2ClI respectively.

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Qiang Shi and Douglas Wallace
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Qiang Shi and Douglas Wallace
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Publications Copernicus
Short summary
Time-series observation can reveal processes and controlling factors underlying production and loss of iodocarbons in the ocean and provide data for testing hypotheses/models. Here, we report weekly observations of iodocarbons from May 2015 to Dec. 2017 at 4 depths in Bedford Basin, Canada. Iodocarbon in near-surface waters showed strong seasonal variability and similarities and differences in their correlation with temporal variations of potentially-related properties and causal factors.
Time-series observation can reveal processes and controlling factors underlying production and...