Geochemical evidence for African dust inputs to soils of western Atlantic islands: Barbados, the Bahamas, and Florida

Abstract

We studied soils on high‐purity limestones of Quaternary age on the western Atlantic Ocean islands of Barbados, the Florida Keys, and the Bahamas. Potential soil parent materials in this region, external to the carbonate substrate, include volcanic ash from the island of St. Vincent (near Barbados), volcanic ash from the islands of Dominica and St. Lucia (somewhat farther from Barbados), the fine‐grained component of distal loess from the lower Mississippi River Valley, and wind‐transported dust from Africa. These four parent materials can be differentiated using trace elements (Sc, Cr, Th, and Zr) and rare earth elements that have minimal mobility in the soil‐forming environment. Barbados soils have compositions that indicate a complex derivation. Volcanic ash from the island of St. Vincent appears to have been the most important influence, but African dust is a significant contributor, and even Mississippi River valley loess may be a very minor contributor to Barbados soils. Soils on the Florida Keys and islands in the Bahamas appear to have developed mostly from African dust, but Mississippi River valley loess may be a significant contributor. Our results indicate that inputs of African dust are more important to the genesis of soils on islands in the western Atlantic Ocean than previously supposed. We hypothesize that African dust may also be a major contributor to soils on other islands of the Caribbean and to soils in northern South America, central America, Mexico, and the southeastern United States. Dust inputs to subtropical and tropical soils in this region increase both nutrient‐holding capacity and nutrient status and thus may be critical in sustaining vegetation.

How well can we quantify dust deposition to the ocean?

“…As for the previous case, we assume 13 ppm 232 Th in the lithogenic phases (D Th , [72]). This approach is not independent of that used in the preceding section.…”

“…Concentrations of 230 Th and 232 Th have been measured in surface sediments at a number of locations throughout the ETNA (figure 8a), allowing 230 Th-normalized lithogenic fluxes to be calculated (equation (3.7)) using 232 Th as a tracer of lithogenic material with an assumed 232 Th content of 13 ppm [72]. For the most part we take results from the literature [82][83][84][85][86], to which we add previously unpublished results from the senior author's laboratory for four sites, two on the margin and two on the MAR (table 1).…”

“…where C P ( 232 Th) is the particulate 232 Th concentration (µg m −3 ) in the 0.8-51 µm particle size fraction averaged over a depth interval of 235-2100 m (the actual depths of the shallowest and deepest sample vary slightly from station to station), S P is the average particle settling velocity for each station (figure 5f ) and D Th is the 232 Th content of Saharan dust (13 ppm) [72]. Dust fluxes derived using this approach are lower than those from the model of Mahowald et al [31].…”

“…The preserved flux of constituent 'i' in the sediment is evaluated as 232 Th concentrations in the 0.8-51 µm size fraction, averaged over a depth range of 235-2100 m, multiplied by the average particle settling rate shown in figure 5f. In making these calculations it was assumed that all lithogenic material in these samples is derived from dust, and that dust has a 232 Th content of 13 ppm [72]. (b) Dust deposition in the Eastern Tropical North Atlantic calculated using the 230 Th-normalization strategy (equation (3.7)) for the 0.8-51 µm particle size class (red circles, range 0.8 to 3.8 g m −2 yr −1 ) and the more than 51 µm particle size class (open diamonds, range 2.0-3.6 g m −2 yr −1 ) of particles collected by in situ filtration.…”

Trace elements and metal pollution in aerosols at an alpine site, New Zealand: Sources, concentrations and implications

“…Further from source, dust inputs have been shown to contribute to soil formation. For example Saharan dust has been found to contribute to soil formation in the Caribbean Islands and Florida (Muhs et al, 2007). Similarly, Asian dust contributes to Hawaiian soils (Kurtz et al, 2001;Rex et al, 1969), while Australian dust has been shown to contribute to soil formation in New Zealand (Marx et al, 2009).…”