The bedrock of Mallorca, Spain, consists almost entirely of limestone, making the fundamental base of the island primarily and homogenously carbonate. Yet the sedimentary layer directly above this limestone parent rock has quartz, silicates, metals, and rare earth minerals, none of which can be found in prominent concentrations in the parent rock. This layer is called terra rosa soil, a colloquial name for a variety of reddish, clay-loaded soils found in Mediterranean zones from South Australia to the United States—and, of course, in the European Mediterranean. In Mallorca, the terra rosa soil alternates in the geological stratigraphy with eolianite, a dominantly carbonate rock formation derived from lithified wind-blown sediment. Muhs et al. (2010) analyzed the chemical composition of these geological layers down to the parent rock at five different locations on the island to determine if the terra rosa soil has wind-blown African dust as its primary source. Confirming the source of the paleosols will not only allow scientists to further understand the current patterns of global dust travel, but also begin to document the variations in the rate of dust travel and magnitude throughout different climatic eras. .— Elise Wanger
Muhs, D., Budahn, J., Avila, A., Skipp, G., Freeman, J., Patterson, D., 2010. The role of African dust in the formation of Quaternary soils on Mallorca, Spain and implications for the genesis of Red Mediterranean soils. Quaternary Science Reviews 29, 2518–2543.
The African Sahara or Sahel regions make a probable source of terra rosa soil simply by a matter of deduction. Terra rosa soil—which Muhs et al. call paleosols since the informal “terra rosa” constitutes a number of different soil taxa, and paleosols include sediments deposited by gravity from the mountains—has a different particulate size and mineral composition than the dissolved bedrock, and the acute contrast between parent rock and soil negate the possibility that the paleosols could derive entirely from Mallorca itself. Silt from France, although chemically compatible with the paleosol, has limited capacity for transport to Mallorca given the geographic distribution of the silt and the patterns of global tradewinds. More importantly, wind-blown silt from northern France should deposit at least thin layers along the route, meaning that such sediment would be observed in central and southern France, and isn’t. Also, wind-traveling silt from Europe peaked during glacial periods, and the Mallorca paleosols most likely were formed during interglacial periods. Mallorca paleosols have high levels of marine fossils whose species correspond to interglacial eras, and such fossils in the soil indicate a higher sea level such as observed during interglacial periods (since during interglacial eras less water gets stored as ice). The fossil shells have also been dated by radioactive decay with elements such as uranium, although these methods can be unreliable. However, paleomagnetic dating and optically stimulated luminescence (OSL) agree with the shell dating estimates, as well as each other. Paleomagnetic dating consists of observing which direction iron molecules in igneous rock orient themselves in relation to the Earth’s poles, which switch magnetic directions intermittently every couple of millennia, making North the new South (and vise versa). Whatever the magnetic orientation of the time that material cooled and hardened into rock will be the manner in which the metals are oriented. OSL involves shining a light on a material to excite the unstable electrons outside a valence shell, the amount of which increases at a consistent rate from the time of light exposure, which would be the last time the sediment was at the surface and therefore around when it was deposited.
While dating methods and fossil organisms suggest paleosols develop interglacially, eolianite builds during glaciation. While currently submerged underwater, the carbonate-sand beach sources of eolianite would be exposed during glacial periods, when sea levels are at least 120 m lower. Further evidence comes from eolianite dating in the two most recent layers, which correspond perfectly to the last two glacial periods, about 21,000 and between 200,000–125,000 years ago. Therefore, given the low levels of paleosol composites in eolianite, the dust source that creates the reddish deposits of silicates and metals must be dominant during interglacial periods and fairly inconsequential during glacial ones. African dust has been the most dominant source of wind-blown sediment during our current interglacial state, and Muhs et al. predict that during glaciations, African dust would most likely remain suspended in the atmosphere longer and not deposit so heavily near its source. The low levels of dust that still managed to descend on Mallorca during glacial eras would get diluted in the carbonate sands anyway, reduced to a minor contributor of the dominant composition.
As probable as the hypothesis of African dust as the paleosol source may be, however, it warrants confirmation by analysis of the mineral content. Muhs et al. used two samples of African dust that they assumed to be reflective of the Sahara-Sahel combination of sources that would reach Mallorca. The first sample came from wind-blown dust deposited in Barbados, which had already been confirmed as African-derived in previous studies and likely an accurate indicator of the mix that would reach Mallorca. Yet just in case weathering during the travel or inaccurate dust combinations didn’t truthfully portray sources analogous to the paleosols, Muhs et al. also used samples from the “red rain” (2526) dust events that deposited red dust from the Sahel and Sahara carried in rain clouds to Mallorca repeatedly between 1982–2003. These two sources overlapped in composition almost completely, meaning that Muhs et al. could confidently use their range of elemental components as that of African wind-blown dust reaching Mallocra.
Trace elements—such as scandium (Sc), chromium (Cr), thorium (Th), tantalum (Ta), hafnium (Hf), and zirconium (Zr)—and rare earth elements, some of which include the elements above, create unique identification codes that allow scientists to decipher the origin of a sediment. These elements do not leach into other layers easily, and don’t change or deplete during chemical weathering, making them highly reliable. The proportions of trace elements tended to fall in the range of African dust in the paleosols, while the non-carbonate sections of eolianite (since the more pure carbonate has little else to be measured) rarely fell within even the most extreme variation observed. The ratio between silicon (Si) and aluminum (Al) as well as the proportions of carbonate, manganese oxide, calcite and quartz, were also documented to get a complete picture of the region’s geography. The Si/Al ratio for the paleosols fell just outside the African dust range at one site, but fit well within the range at the others, while the eolianite sample always had a much higher Si/Al. That one site didn’t completely correspond to the African dust ratio suggests that some eolianite (probably the quartz sections) eroded into the paleosol layers.
As expected, Muhs et al. concluded that African dust fits as the most logical source of Mallorca paleosols, which may suggest other terra rosa soils also share this source as a contributor. According to the patterns in the stratigraphy between paleosols and eolianite, the amount of dust from Africa depositing in Mallorca shifts in intensity between interglacial and glacial eras. More dust suspended in the atmosphere absorbs more solar radiation, keeping the Earth’s surface cool in a reverse Greenhouse Effect. As the suspended particulates keep surface cool and prevent rain clouds from forming due to inversion layers, less vegetation grows and more sediment gets exposed, which can add more dust to the atmosphere in a positive feedback cycle. Thus increased dust flux could act as a counter to rising temperatures from global warming.