RECENT ATMOSPHERIC FALLOUT OF NONCARBONATE MINERAL GRAINS ON BERMUDA AND ITS RELATIONSHIP TO NAO

Atmospheric fallout traps were placed at eight environmentally diverse localities on Bermuda to document influx of noncarbonate mineral grains (10-50 µm). Seven sites collected for one year, while the eighth, Paynter's Vale, collected data for three years. Trap collecting heads at all sites were changed on an approximately quarterly basis to evaluate seasonal variation in noncarbonate mineral grain influx. Residues collected in the traps allowed assessment of the source, transport, and depositional mechanisms of the airborne particles as a function of seasonal rainfall, wind data, and other climatic parameters.

During the collection intervals, noncarbonate mineral grain influx was greatest in the winter months, with values ranging from a low of 1.60 to a high of 11.95 grains/cm²/yr. Summer values ranged from 0.00 to 7.75 grains/cm²/yr. Cross-island analysis along a west-east transect showed noncarbonate mineral influx was higher on the western end of Bermuda during the winter intervals and higher at the eastern sites during summer intervals.

Seasonal variations in noncarbonate mineral grain influx recorded at opposite ends of Bermuda suggest two sources for atmospheric dust: a Saharan source in summer, and a North American source in winter. Additionally, local climatic parameters such as rainfall and average wind speed during the sampling intervals weakly correlate with noncarbonate mineral grain influx. Regionally, over the three-year collection period at Paynter's Vale, influx values show strong similarity to Normalized North Atlantic Oscillation (NAO) Index for the same period. Increases in noncarbonate mineral grain influx correspond to a weakening of the NAO in the vicinity of Bermuda.