The characteristics of a region’s soils are largely controlled by two factors: climate and geology. Soils are mostly composed of broken-down minerals from the bedrock (along with, critically, organic matter), and the weathering process is driven by climatic conditions. Soils can also form within the sediments deposited in an area, however, as is the case in the flood plains of river valleys. And there are other things that can transport sediment to a new home—like wind.
This is what makes the soil of the island of Bermuda so interesting. It seems like they don’t belong. The island’s shallow bedrock is composed of gray limestone, mainly in the form of broken-up bits of skeletons from marine organisms—the same stuff that makes up the beach sand. The soil, however, is rusty-red and clay-rich.
The reason for this sharp contrast has long been debated, with two hypotheses gaining the most support. One possibility is that the limestone contains a fair amount of clay, which is left behind as the limestone dissolves. (Limestone weathers quickly in rainwater, as evidenced by hard-to-read gravestones.) In this view, it’s not that the clay soil “doesn’t belong”, it’s just that it’s concentrated by the removal of the other minerals.
The second possibility is that, much like many of the human residents of Bermuda, the clay immigrated to the island. This is actually known to be the case in the Caribbean, where considerable amounts of dust are deposited after blowing clear across the Atlantic Ocean—a gift from the arid regions of northern Africa. (The Large dust storms that carry it are actually visible from space.) While Bermuda receives much less African dust than the Caribbean islands, it is still measureable there. Still, many have wondered if Bermuda was too far north for the dust to add up and explain the soils.
A recent study published in the Journal of Geophysical Research provides new data to test these hypotheses by chemically fingerprinting Bermuda’s soils and comparing them to the signatures of possible source materials.
Examining the minerals in the soil showed that most were consistent with African dust, though some most likely came from the deeper, volcanic bedrock that is the source of the non-carbonate bits in the limestone. That would suggest that the truth lies in a mixture of the two hypotheses.
Careful measurements of trace metal elements like neodymium, chromium, scandium, and thallium told a similar story. The ratios of these elements, a sort of chemical fingerprint, lie between those of the volcanic bedrock and the African dust. Because of this, the researchers conclude that the soils of Bermuda seem to have formed from both local and imported material, perhaps in equal parts.
The majority of the airborne dust is almost certainly from Africa, but some could have come from central North America, as well. During the colder glacial periods, drier conditions in these regions (and glacial erosion in the US) led to greater dust production. Evidence of this can be seen in the “loess” (wind-blown silt) deposits of the midwestern US. Unfortunately, the chemical signature of North American dust is too similar to the African dust to tease apart their relative contributions to Bermuda’s soil. Instead, researchers have to rely on what we know about wind patterns and glacial climates to determine that Africa was probably responsible for the lion’s share in the past as well as the present.
The stories of boats and planes disappearing in the Bermuda Triangle may be mostly nonsense, but there are some things that commonly enter the region but don’t leave. You wouldn’t have to dive to the bottom of the sea to look for the resting place of these drifters, though. You can find them just beneath the grass of Bermuda’s beautiful golf courses—a little earth that caught a ride in the sky.