Signals of change transformed, lost and found in the sediment archive
IPGP - Îlot Cuvier
Séminaires de Potamologie
The volume, locus and characteristics of sedimentary deposits record the response of the Earth’s surface to an ever-changing climate and tectonic history. However, the sediment routing system that links the eroding landscape to down-system depositional sink can transform climatic and tectonic signals. This means that inverting the sedimentary archive for tectonic or climatic signals it is a non-trivial task, since process and product are non-linear. Here we tackle this challenge using an idealised 2-D model of catchment erosion and fan deposition, where we can vary tectonic and climatic parameters to investigate the sensitivity of simple sediment routing systems to external forcing. Finally we apply the insight gained from the model to interpret field examples from the Tremp Basin, Spanish Pyrenees. We calculate the grain size distribution of gravel clasts down-system using a similarity approach, with fan architecture calculated by sediment budget balance. For the simple case of constant catchment rainfall followed by a single stepped increase in precipitation, the response recorded within the basin is the deposition of a coarse conglomerate sheet that extends down the length of the fan. Changes in fault slip rate produce a more complex stratigraphy due to the interplay of catchment release and the change in basin accommodation space. The catchment/fan system then typically returns to topographic steady state with a response time of ~1 Myr, and so typically producing a short-lived change in depositional characteristics. These results suggest that the Earth’s landscape is highly buffered to high amplitude changes in climate and that signals of past change are likely to transform during propagation through the sedimentary routing system. Our results indicate that extensive sheet conglomerates, such as that at the Paleocene-Eocene boundary in the Tremp Basin, Spain, can be explained by a rapid and sustained increase in precipitation, with amplitude significantly larger than background climate oscillations and we argue that insights from this model can be applied widely to invert tectonic and climatic signals in the sedimentary record.