AFRICAN DUST TRANSPORT IN THE QUATERNARY TO THE WESTERN ATLANTIC OCEAN AND THE ORIGIN OF BERMUDA'S SOILS AND PALEOSOLS

The visually striking, reddish-brown, clay-rich paleosols overlying high-purity (average of 97% calcium carbonate) eolianites on the island of Bermuda have attracted the attention of geologists for more than a century. Some of the earliest estimates of the duration of Quaternary glacial and interglacial periods were made using Bermuda's paleosols. Theories of origin range from in situ dissolution of carbonates with residual accumulation to airborne dust accumulation from distant sources. We studied soils and paleosols on Quaternary eolianites of five different ages on Bermuda. The non-carbonate fraction of the soils is clay-rich and is dominated by hydroxy-interlayered clay (a mixed-layer chlorite-vermiculite). Many soils also contain a phosphate mineral (possibly woodhouseite), quartz, and goethite; the oldest paleosols contain boehmite. Potential soil parent materials on Bermuda include rare, sand-sized fragments of locally derived, but deeply buried volcanic bedrock (magnetite, diopside, zeolite, phlogopite, chlorite, calcite, and plagioclase), the fine-grained (<20 µm) component of distal loess from the lower Mississippi River Valley (mica, smectite, kaolinite, quartz, K-feldspar, plagioclase, calcite and dolomite), and wind-transported dust (<20 µm) from Africa (quartz, mica, kaolinite, chlorite, K-feldspar, plagioclase, calcite and gypsum). These three parent materials can be differentiated geochemically using trace elements that are generally immobile in the soil-forming environment. Local volcanic bedrock on Bermuda has Eu/Eu*, La/Yb, Gd/Yb, Sc-Th-La, Th/Ta, and Sc/Th that differ from both African dust and lower Mississippi River valley loess. African dust and lower Mississippi River Valley loess overlap only partly in their Eu/Eu* and Th/Ta values. Bermuda soils have Eu/Eu*, La/Yb, Gd/Yb, Sc-Th-La, and Th/Ta that, when considered collectively, indicate derivation from some combination of local volcanic bedrock and African dust. Of these two parent materials, African dust appears to have been the more important influence. Fine-grained Mississippi River Valley loess does not appear to have contributed significantly to Bermuda soils, despite climate models that indicate dust from this source (as well as Africa) could have reached Bermuda during the last glacial period.