Will human impacts on Alpine geomorphic processes scale up to the depositional record?

The Anthropocene is widely described as producing a rupture in the global stratigraphic signature, attributable to human activities. There is no doubt that human activities have introduced new products into the stratigraphic record; and that humans are modifying the geomorphic processes that produce the sediment which then becomes incorporated into that record. The stratigraphic literature is replete with simplistic generalisations of how sediment flux to the continental shelf is changing, such as increasing due to soil erosion or decreasing due to hydropower related sediment flux disconnection. Here we argue that human impacts on geomorphic processes in the Anthropocene are unlikely to be stationary for long enough for them to be seen consistently across the depositional record of many different environments. Illustrating this for a major inner-Alpine drainage basin, the Swiss Rhône, we show that human-driven global climate-change is indeed dramatically altering the geomorphic process regimes of Alpine environments. However, there are three broad reasons why this is unlikely to be seen in the future geological record. First, the geomorphic response that drives increased sediment delivery is transient because of the significant regime changes associated with global climate change impacts. Second, such increases are countered by other human impacts, notably those on sediment flux, which are tending to reduce the connectivity of sediment sources to downstream sediment sinks. Third, human impacts on both sediment sources and connectivity are nonstationary, driven by both exogenous factors (here illustrated by the worldwide economic shock of 2008) and endogenous ones, notably human response to the perceived problems caused by both sediment starvation and sediment over-supply. In geomorphic terms, then, there is a difference between the pervasive nature of Earth system shifts that we see in the pre-Holocene depositional record and the more ephemeral impacts of the Earth system – human coupling associated with the Anthropocene. The extent to which this is the case is likely to vary geographically and temporally as a function of the degree and nature of human impacts on geomorphic processes. Thus, the primary challenge for future prediction will be as much the prediction of the complex and reflexive nature of human response as it will be geomorphic processes themselves.