In his Comment, Molenaar argues that foreland sedimentation is determined by accommodation space, not sediment yield and therefore our methodology of measuring sediment volume does not actually measure erosional fluxes from the Alps. Using sediment volumes to estimate erosion is always a risky proposition as few orogenic systems are closed to sediment loss and mass balance is only possible in a closed system. Following (Hay et al., 1992) and (Kuhlemann, 2000), we (Willett et al., 2006) argue that the Alpine depositional system is largely closed if we consider distal basins, including the Rhine and Rhone deltas and the Adriatic Sea. If this is correct, we are, in fact, measuring erosional yield from the Alps.
Molenaar's second point, that the Po Basin and the Adriatic Sea serve as the foreland of the Apennines as well as the Alps is true. However, we have accounted for the relative contribution of the Apennines. We were perhaps not clear in our original paper, as we simply cited Bartolini et al. (1996), but following their analysis, we assume that only 25% of the Pliocene and Quaternary sediment in the Adriatic and 33% of the sediment in the Po is from an Alpine source. This is a conservative estimate; Kuhlemann (2000) assumed that 50% of the sediment was Alpine derived. Unless more than 90% of the Pliocene sediment in the Adriatic derives from Apenninic sources, our conclusion that the Alpine yield increased from the Miocene to the Pliocene in the southern drainage basins is robust. Furthermore, the Apennines are not relevant to the northern drainage basins, which also exhibit an increase in sediment yield at the onset of the Pliocene.
Molenaar also takes issue with the prevailing theory for the Messinian salinity crisis (MSC). We find the chronology and sequence of events developed over the last decade (Krijgsman et al., 1999; Rouchy and Caruso, 2006; Roveri and Manzi, 2006)) quite compelling and the many meticulous studies of the late Messinian stratigraphy provide exceptional resolution of changes in depositional and climatic environment. No reasonable tectonic model can explain the rapid changes and short dura tion of the MSC (650 k.y.) or, particularly, the basin-wide synchronicity of the MSC depositional events, leaving us to investigate hydrologic and climatic forcing (Pierre et al., 2006). The final 180 k.y. of the MSC is characterized by the Lago Mare depositional conditions. The deposits of the Lago Mare are typically high-energy, fluvio-deltaic clastic sediments, brackish water lagoonal deposits and reworked evaporites, all of which suggest an increase of fresh water supplied to the basins and an increase in fluvial transport, and the simplest explanation for these two phenomenon is higher precipitation in proximal drainage basins. In addition, the extensive evidence of submarine erosion indicates a lowered base level for some fraction of the MSC. This would result in reworking of shelf sediments and thus provides an explanation for the extreme post-evaporitic Messinian sedimentation rates we find. For this reason, we do not interpret these sediments as being diagnostic of increased Alpine erosion, although the alternative interpretation lends more support to our thesis that the climate became more erosive during Lago Mare time.
Finally, we have not ignored independent data on climatic change at this time. The evidence for a global change in ice volume at precisely the onset of the Lago Mare conditions is one of the most compelling points that this is a climatically-driven event, and we cited two of the papers documenting this global change (Hodell et al., 2001; Vidal et al., 2002). Regional data are more contentious. There is not palynological support for extensive vegetation change at the onset or termination of the MSC. However, there is a clear increase in the relative frequency of Pinus pollen at the onset of Lago Mare conditions (Bertini, 2006; Fauquette et al., 2006). This is often interpreted as representative of a change in pollen transport conditions, but this could include increased fluvial transport in response to increased precipitation in source areas. This would also explain the apparent increase in continentality in the near shore marine record as observed by Warny et al. (2003).