Relationship between Lithogenics and Primary Production in the Mid Latitude Central Pacific

Based on surface-layer water properties, phosphorus (P) and nitrate do not appear to be significant limiting nutrients to phytoplankton growth in the transition zone. The injection of aerosol-derived iron from Asian dust storms (Martin et al., 1987), which is known to cause significant short-term increases in subtropical productivity in central regions of the North Pacific Central Gyre (Ditullio and Laws, 1991) is probably of greater relevance.

To estimate the flux of iron in association with mineral aerosol particles, we used the value of 1.6 wt.% for the abundance of Fe in mineral aerosols (Maeda et al., 2002). The solubility (in seawater) of elements attached to mineral aerosols is important in determining their impact on oceanic biogeochemical cycles. Solubility is dependent primarily on pH, sources, and particle size. Although it is impossible to give definite solubility values, the solubility in seawater of P attached to aerosol particles is estimated to be <1%-50% (Wollast and Chou, 1985; Duce et al., 1991).

The fixation rate of OC (FixationOrganic) can be estimated as follows:

_ . - 1.6 X 10"2 X FLUXAerosol Fixationorganic — a x-^-

where a is the solubility in seawater of an element attached to aerosol particles and b is the Fe:C ratio of particulate OM. If it is assumed that FLUXAerosol is equal to the lithogenic flux, than the minimum value of a is 1%, and that of b — 7.9 x 10-5 (Boyd et al., 2004), respectively. FixationOrganic fluctuates between 1 and 106 (mean 21) mg m-2 day-1 at Site 5, between 8 and 193 (mean 48) mg m-2 day-1 at Site 7, and between 2 and 65 (mean 18) mg m-2 day-1 at Site 8, respectively. Since we adopted a solubility of 1% here, these values should be minima. The higher FixationOrganic values suggest that iron input may play an important role in enhancing primary and export production in this area, unless other nutrients become limiting.

Dust-storm activity, which is responsible for releasing mineral dust to the atmosphere in eastern Asia (e.g., the Gobi and Takla Makan deserts), is greatest in spring (March-May) as a result of the combined effects of low rainfall, increased occurrence of high winds associated with cold fronts, and freshly ploughed soil for spring planting (e.g., Watts, 1969; Uematsu et al., 1983) (Fig. 9). There is no meteorological station on mid-latitude central

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Figure 9: (a) Frequency of dust storm reports in the eastern part of Asia in 1981 and early 1982 (Uematsu et al., 1983). (b) Temporal variation of the atmospheric dust concentration records observed at Tagawa and Karita-wakahisa cities in the Fukuoka (33°N, 131°E), the western part of Japan in 1998 (Environmental section of Fukuoka local government, personal communication). (c) Lithogenic flux observed at the shallow trap of Site 7.

North Pacific oceanic islands for monitoring eolian dust. Two stations (at Tagawa and Kanda in Fukuoka, 33°N, 131°E, western Japan) have accumulated dust concentration records for the last 5 years. The dust flux is low during summer, autumn, and winter, with the highest levels recorded from February through May (Environmental section of Fukuoka local government, personal communication). Another data set was provided by the University of Nagoya, which has analyzed seasonal variation of vertically integrated aerosol backscatter at 532 nm at Nagoya City (35°N, 137°E) since March 1994. No data were available covering the period of the sediment-trap experiments, but 3-year measurements at Nagoya present general seasonal trends regarding the amount of dust in the upper atmosphere (4-8 km); the dust was transported more extensively over the Pacific in this layer than in the lower layer. Aerosol concentrations were very high each year from April to May in spite of large, sporadic short-term (< 1 week) fluctuations from September to early October and December to January. However, lithogenic fluxes at Site 7 showed relatively low values during these periods. In contrast, low aerosol concentration was always observed from June to August, late October to November, and February to early March, when large lithogenic fluxes were observed at Site 7. Therefore, it is evident that the maximum lithogenic flux of the settling particles into the ocean's interior lagged behind the maximum aerosol concentration in the atmospheric column.

The time lag was partly due to transport time from the Japanese islands to Site 7 and settling time from the surface ocean to the sediment trap. Although Site 7 is approximately 2,500 km downwind from the Japanese islands, the high speed of the jet stream (often >500 km day-1) quickly transports dust to the site (Duce et al., 1980). Biogenic aggregates (such as diatom and coccolithophores) cause accelerated sinking of small non-biogenic particles with settling speeds of 160-200 m day-1; thus, it takes < 10 days for particles to sink from the upper water column to the shallow trap depth (e.g., Takahashi, 1986). Because the actual time lag could be between 15 days and 2 months, it is unlikely that eolian dust was transported directly from the source area to the trap site through the atmosphere.

For a single floating particle of dust, the small particle size (< 8 mm) prevents it from being removed rapidly from the ocean surface. This suggests that the eolian dust was transported from the source region to the ocean by the wind, and that it then remained suspended in the upper water layer of the Kuroshio Current and Kuroshio Extension for up to 2 months; the dust was then removed by incorporation into biogenic pellets or amorphous aggregates when primary production was activated. In other words, transport of lithogenic matter in the transition zone was by both westerly winds and the Kuroshio Extension. Although phytoplankton primary production has now been demonstrated to be iron-limited in some areas of the oceans (e.g., Boyd et al., 2000, 2004; Tsuda et al., 2003), recent studies emphasize that there are more complex interactions within the ocean than simple iron limitation or sufficiency, with evidence of co-limitation by iron along with light and other macro- and micronutrients (e.g., Si, N, P, Co, and Zn). Aerosol particles are so enriched in heavy metals that these micronu-trients provide more constraints for primary and export production in the transition zone in the North Pacific (Fig. 9).

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