Where did the signal go? Why sediment flux is not always the best place to look for a record of the rock uplift history.

Abstract:

The assumption that the rock record contains details on past climatic and tectonic conditions relies on the fact that these forcings are evident in the sediment flux that feeds a deposit. We use the CHILD landscape evolution model to explore whether step changes in rock uplift rate are recorded in the topography of and sediment flux delivered from a small (64 km²) erosional watershed. We model only the fluvial network using the detachment-limited stream-power equation. We find that the watershed immediately responds to either a change in rock uplift rate, however the signal of the change takes time to propagate through the network. All rock uplift perturbations, regardless of their duration or magnitude, are evident in the morphology of the channels for a period of time. If rock uplift rates continue to change, evidence of past perturbations may be erased from the topography; the time it takes for the landscape to erase previous signals is a function of the response time of the landscape. In contrast, the sediment flux at the outlet of the watershed does not always record every uplift perturbation, and only when rock uplift rates are steady for periods longer than the response time of the watershed does the sediment flux reflect the current rock uplift rate. As a result, the sediment flux at the watershed outlet rarely reflects the current forcing. The response time scales with the size of the source area, so the sediment flux delivered from larger watersheds is even less likely to reflect the current forcing. Our results are generated using the simplest possible fluvial incision model that does not allow for deposition. As such, we illustrate the case that is most likely to directly transmit rock uplift signals into sediment flux signals. This suggests that even if all of the sediment eroded from a small watershed is directly deposited in an adjacent basin, the entire uplift history of the landscape will not be recorded in the rocks.