yr−1 of fossil-fuel carbon now passing through the atmosphere.
(^) Third, a remarkable natural iron-ex study initiated on 7–8 August 2008 has now
been evaluated. It was the dusting of the oceanic Gulf of Alaska, an HNLC area, by
volcanic ash from an eruption of Mt. Kasatochi in the Aleutian Islands. Fortunately,
some oceanographic information (Hamme et al. 2010) could be developed from
satellite records, moorings, and a research cruise in the area. The ash was carried east
in an expanding plume by the then prevailing winds (Plate 16.3a) generating a bloom,
primarily of diatoms, of about twice the usual late-summer standing stocks (Plate
16.3b). The extent of the bloom was documented by both SeaWIFS and MODIS
satellite sensors (Plate 16.3c), and satellite chlorophyll levels were confirmed by ship-
borne observations. Ash particles were identified in the Gulf surface from a sample
taken at 50°N, 145°W (Station P) on 21 August. Experiments with fresh ash added to
seawater do induce both strong iron release and diatom growth. Both pCO 2 and pH
recorded at a Station P mooring shifted sharply down and up, respectively, over a
week starting about 12 August (Plate 16.3d), and mixed-layer fluorescence from a
seaglider operating nearby roughly doubled (Plate 16.3e). Late August fucoxanthin
levels were unusually high, indicating that the bloom included diatoms. Alternative
hypotheses for this bloom have been plausibly rejected (Hamme et al. 2010).
(^) Hamme et al. calculated that the Kasatochi-induced drop in pCO 2 (∼25 μatm)
implies a drawdown of 0.3–0.7 moles C m−2, given the mixed-layer depths.
Extrapolating to the area of the bloom (maximally 2 million km^2 ) suggests a
maximum likely drawdown of 0.017 GtC of carbon. Based on that, Hamme et al.,
applying an unlikely high export rate of 50%, suggest that the entire ash-plume effect
might have sequestered <0.01 GtC. The SERIES iron-addition study in the same
region estimated a 7% export rate from strongly enhanced production. Thus, a
realistic sequestration estimate would be <0.001 GtC. The Kasatochi plume was
larger than any intentional iron-addition experiment that could be done, and it would
be very costly to exceed the modest likely levels of iron it supplied over such a large
region. Thus, “Kasatochi” shows that potential for OIF to reduce atmospheric CO 2 is
trivial. Projecting the 0.001 GtC sequestered to the portion of the Antarctic seas with
substantial silicate, the sequestration might be 0.01 GtC per treatment (calculation by
Phillip Boyd). The oceans are already absorbing ∼200 times that much annually. OIF
is not going to help us. Unfortunately, volcanoes are not going to help much, either.
Decadal-scale Changes in Ocean Conditions and
Biota
(^) Climate is changing, and it will change in some direction, probably warming, even if