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doi:10.2204/iodp.proc.336.110.2012

Conclusions

These data indicate that atmospheric particulate matter moving over the Atlantic from Europe and Africa carries culturable microorganisms and that CFU concentrations in these air masses correlate with particulate concentrations. Further, these data demonstrate a “fertilization effect,” where surface water prokaryote concentrations relate to near-surface atmospheric particulate concentrations. As can be seen in Figures F3 and F4, there were numerous episodes of African dust presence in the near-surface atmosphere over the period of this study. The dark and light orange filters in these two figures are similar in color to desert surface soils of the Sahara and Sahel and to longer period samples that were collected aboard the JOIDES Resolution during Ocean Drilling Program Leg 209, when African dust was present in the atmosphere. These samples will be evaluated for chemical composition at shore-based facilities. A colleague at the University of La Laguna in the Canary Islands notified me of dust episodes impacting air quality there on 30 September and 13 October and forecasted the presence of dust at Expedition 336 drill sites on 1 and 15 October, respectively. Following the dust episode of 1 October, dust that fell onto the roof over the bridge of the JOIDES Resolution was present at concentrations that were clearly visible, and the dust imaged (in a zip-top bag) in Figure F8 was collected from that location. The projected model forecast for the 15 October episode is presented in Figure F9, and Figures F3 and F4 clearly illustrate the arrival of African dust at our research sites for these two episodes.

Particle count data and the strength of correlations with surface water communities and atmospheric communities will be evaluated again after chemical analyses of the high-volume filters. This evaluation is necessary to verify sources and to determine if the light gray to dark gray filters are an artifact of diesel exhaust or particulates from biomass burning in Europe or Africa. These data along with those such as wind speed, wind direction, and ship orientation will be valuable in determining which air masses and their sources influenced these communities. As an example, if the illustrated peak in atmospheric particulate matter that occurred on 8–10 October is removed (this was a period of low wind speed, where ship exhaust was contributing to particulate counts, as illustrated in Figs. F1 [low wind condition] and F3 [graying of the filter material]), then the correlation between total CFU and particle counts in the low-volume B samples strengthens from an r value of 0.360–0.410 and a P value of 0.017–0.008. (shift from a weak to a moderate strength correlation). Obviously, additional shore-based data and a more careful interpretation of the data are needed to accurately evaluate the true extent of the observed relationships outlined in this report and for those yet to be determined.