The contribution of Willett et al. (2006) is interesting because understanding the effects of climate change on erosion rates may help ascertain the causality of human activity. However, their evidence for climate influence on Alpine erosion rates is indirect and not robust. It is not erosion or exhumation rates that are dealt with by Willett et al. (2006), but merely accommodation, which is a function of rates of creation of sedimentation space, sediment supply, and deposition; these are quite different issues.
The amounts of sediment per time unit show similar patterns north and south of the Alps, in the Molasse and Po basins, respectively. The upper most Messinian is aberrant only in the Po Basin (Willett et al., 2006). Because this would argue against climate influence on Alpine erosion, Willett et al. explain it by assuming desiccated Mediterranean basins and, consequently, a significantly lowered base level during the aftermath of the Messinian salinity crises. The only argument left for climate influence is a decrease in amounts of sediment in pre-Messinian time and an increase in post-Messinian time. This could reflect slow establishment of evaporative climate conditions before and during formation of evaporites, and the return to normal climate after the salinity crisis. However, Willett et al. use the ‘desiccation model’ (Hsü et al., 1973) that is commonly explained by tectonic closure of the Atlantic–Mediterranean connection, and climate implicitly does not play a role in this model. Even if this model is correct, Alpine erosion serves as a very insensitive climate proxy at best.
A second serious problem with Willett et al.'s argument is that the Po Basin is not only a Molasse basin for the Alps but also a synsedimentary foreland basin of the northern Apennines. The Po Basin and other Northern Apennine foreland basins received clastic sediment both from the Alps and from the Apennines themselves. Foreland basins successively developed from the Oligocene until the Pleistocene as a result of loading upon advancement of Apennine thrust sheets. During advancement of the deformation front, the foreland, as well as thrust-top basins, were successively uplifted and eroded, yielding clastic sediment. The Miocene, and in particular the Messinian, was a time of intense tectonic reorganization of the Mediterranean, which consisted of a number of basins in a complex array of differentially moving plates and microplates. Tectonic activity is well documented in the Apennines. Angular unconformities between Lago Mare deposits and underlying evaporites in the northern Apennines mark Messinian tectonic phases (Artoni et al., 2004). Similar Tortonian and Messinian syntectonic activity took place in the Eastern Betic basins (Krijgsman et al., 2006) and other parts of the Mediterranean (e.g., Aksu et al., 2005). Several phases of synsedimentary tectonic activity deforming individual Messinian evaporitic successions have been reported from the Levantine basin (Netzeband et al., 2006). This forms compelling evidence for a major change in the Messinian plate kinematic framework.
Although the clastic nature of part of the Messinian evaporite deposits was recognized early, e.g., in the Sicilian basins (Schreiber et al., 1976), it has long been ignored. Recent studies reveal that the Mediterranean never dried up (Roveri et al., 2001; Matano et al., 2005) and the evaporite deposits in the deep basins are cannibalized shallow-marine evaporites (Manzi et al., 2005; Roveri and Manzi, 2006). Deposition in deeper basins by mass-flow mechanisms such as turbidity current flows, submarine debris flow, and large-scale slides-slumps is in full agreement with the well-documented Messinian tectonic activity.
Accommodation of the Po Basin thus cannot be used to quantify Alpine erosion rates. Neither can a dramatic lowering in base level be evoked for explaining increased rates of net deposition. A desiccated Mediterranean, merely based upon wrongly interpreted core material with nodular calcium sulfates (Hardie and Lowenstein, 2004), seems unlikely.
The significant climate changes during the salinity crisis (e.g., Andersen et al., 2001) and afterwards in the Pliocene (e.g., Chandler, et al., 1994) are not reflected at all in the data presented by Willet et al. It thus seems that tectonic activity in the source and depositional areas was the main factor determining the amounts of sediment per stratigraphic time unit.