Project: Sub-Arctic Ecosystem Response to Iron Enrichment Study

Iron fertilization in the NE subarctic Pacific
Objective: To determine the influence of iron on the production of the trace gases and their impact on the atmospheric chemical and physical properties and on climate flux of climatically active gases.

Experiment: The iron fertilization expedition involved three ships that sampled both the surrounding and the iron-enriched waters. The addition of iron prompted an explosive growth of phytoplankton that turned blue waters of the NE Pacific green. The bloom was recorded by a NASA satellite as it spread into a 500 km2 patch.

In July, 2002 the SERIES was carried out in the NE Pacific near Ocean Station Papa (50N, 145W). Three ships, the CCGS John P. Tully, the M/V El Puma (Mexico) and M/V Kaiyo Maru from Japan, along with 45 researchers from 20 institutions across Canada as well as international collaborators participated in the experiment. The objectives were to study the response of phytoplankton, bacteria and zooplankton to the addition of Fe, the effect on carbon flux to the deep ocean, the influence of Fe on the production and cycling of climatically active trace gases and its influence on the atmospheric sulfur budget, sulfate aerosols and cloud microphysics. The decline and fate of the iron stimulated diatom bloom as been reported (Boyd et al. (2004) Nature, doi:10.1038).

These data report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

Prior to commencing the joint iron and SF6 addition, 48-h oceanographic survey (7-8 July 2002) was conducted in the vicinity of Ocean Station Papa (OSP, 508N, 1458W) to identify an appropriate site with HNLC characteristics typical of this region (LaRoche et al. 1996; Nishioka et al. 2001). A suitable site was located 50 km northeast of OSP, and the surface waters were enriched on 10 July 2002 (denoted as day 0 of SERIES) with dissolved iron to .1 nmol L-1, along with the concurrent addition of the tracer SF6 (>400 fmol L-1) following procedures reported in Law et al. (1998). Throughout SERIES, mixed-layer SF6 concentrations were always significantly higher than background levels, and thus we did not add any more SF6 to the patch. However, on day 6, a second iron infusion was required that raised dissolved iron by around 0.6 nmol L-1.

Related files

SERIES Report

SERIES El Puma Cruise Report

SERIES Tully Cruise Report

SERIES Stations Sampling Timeline

SERIES Project Data Report by Doug Mackie

SeaWIFS Image of the SERIES Bloom