Soil nutritional gradients as long-term proxy for hominin subsistence strategies in geologically dynamic settings: the case of Acheulean Rodafnidia on Lesbos island, NE Aegean

The environmental and living conditions of a region are shaped by its relief, geology and climate, and control factors like hydrology and soil formation. While long-term climatic fluctuations and associated changes in environmental conditions are commonly viewed as the dominant natural factor in human evolution, the role of geological and pedological processes has so far received little attention. However, it makes a big difference to consider the effects of large-scale environmental changes in a homogeneous "static" landscape or to include the multitude of dynamic landscape factors that can lead to strong local effects with regard to the structure of the landscape and the availability of water and food.

The NE Aegean is a key region of Pleistocene hominin presence at the crossroad between Africa, Asia and Europe situated in a geologically highly unstable region. Rodafnidia, an open-air Lower Paleolithic site on Lesbos, has revealed a unique Acheulean assemblage from excavated fluvio-lacustrine deposits dated between 476 and 164 ka BP. This site and its surrounding region represent a key location to study hominin subsistence and mobility and to investigate potential trans-Aegean migration corridors during Pleistocene sea-level lowstands. Geologically, Rodafnidia is situated on middle Pleistocene fluvial sediments consisting mainly of reworked early Miocene ignimbrites, and Pliocene marly limestones and marls, whereas the wider region is characterized by strong geochemical and pedochemical contrasts including nutritionally depleted soils on ophiolitic rocks, highly productive soils on marshy coastal deposits along the Kalloni Gulf, and a series of fault-controlled thermal sulfur springs at Lisvori and Polichnitos. We hypothesize that the attractiveness of Rodafnidia site for hominin presence was influenced by the local geology and tectonic activity controlling the long-term soil nutritional status of the region. We employ a combined geological-pedological study to unravel the paleoenvironmental conditions of the wider region. Our approach offers, in return, valuable insights into hominin-landscape interaction, relevant to landuse, resource exploitation and dispersal potential.

Our systematic sampling and analysis of rocks, soil and water offers clues to the soil nutritional characteristics of the main lithological units exposed in the wider Rodafnidia area. Results reveal distinct differences in the nutritional status of soils developed on different geological substrates. While volcanic soils in the immediate Rodafnidia region and marshy soils along the coast comprise well-balanced nutritional levels, serpentinite soils dominating the ophiolitic highlands display highly problematic properties such as low Ca/Mg ratios and enhanced heavy metal concentrations. Soils on hot spring deposits are puzzling as they display both beneficial characteristics (high soil organic carbon, high calcium) and potentially harmful enhanced heavy metal levels.

In a pedological context, Rodafinidia is located in a narrow zone of highly productive, nutrient rich soils in a wider region of geologically induced nutrient deficiencies. During sea-level lowstands, hominins along with other continental fauna could have crossed terrestrial passages between western Asia and the Eastern Aegean, and sites like Rodafnidia emerge as likely beneficial locations for hominin subsistence strategies.