An anthropogenic origin of the “Sirente crater,” Abruzzi, Italy

Abstract

In this paper, we review the recent hypothesis, based mostly on geomorphological features, that a ~130 m-wide sag pond, surrounded by a saddle-shaped rim from the Sirente plain (Abruzzi, Italy), is the first-discovered meteoritic crater of Italy. Sub-circular depressions (hosting ponds), with geomorphological features and size very similar to those exhibited by the main Sirente sag, are exposed in other neighboring intermountain karstic plains from Abruzzi. We have sampled present-day soils from these sag ponds and from the Sirente sags (both the main “crater” and some smaller ones, recently interpreted as a crater field) and various Abruzzi paleosols from excavated trenches with an age range encompassing the estimated age of the “Sirente crater.” For all samples, we measured the magnetic susceptibility and determined the Ni and Cr contents of selected specimens. The results show that the magnetic susceptibility values and the geochemical composition are similar for all samples (from Sirente and other Abruzzi sags) and are both significantly different from the values reported for soils contaminated by meteoritic dust. No solid evidence pointing at an impact origin exists, besides the circular shape and rim of the main sag. The available observations and data suggest that the “Sirente crater,” together with analogous large sags in the Abruzzi intermountain plains, have to be attributed to the historical phenomenon of “transumanza” (seasonal migration of sheep and shepherds), a custom that for centuries characterized the basic social-economical system of the Abruzzi region. Such sags were excavated to provide water for millions of sheep, which spent summers in the Abruzzi karstic high pasture lands, on carbonatic massifs deprived of natural superficial fresh water. Conversely, the distribution of the smaller sags from the Sirente plain correlates with the local pattern of the calcareous bedrock and, together with the characteristics of their internal structure, are best interpreted as natural dolines. In fact, reported radiocarbon ages for the formation of the main sag pond and of the smaller sags differ (significantly) by more than two millennia, thus excluding that they were all contemporaneously formed by a meteoritic impact.