CONSTRAINTS ON OUR ABILITY TO DETECT SEDIMENT SUPPLY SIGNALS IN THE STRATIGRAPHIC RECORD (Invited Presentation)

The flux of sediment across landscapes is sensitive to environmental forcings, including source terrain uplift and precipitation rates. Unfortunately, detecting paleo-changes in sediment supply from stratigraphy is challenging. In this presentation, we explore some of the reasons for this difficulty. We start by examining existing deterministic theory that links topographic attributes and morphodynamic rates to the magnitude of sediment in flux, including channel slopes, depths, widths, and shoreline migration rates. While this theory adequately predicts mean morphological responses, stochastic autogenic processes add spatiotemporal variability around the mean state. We compare the theory for predicting mean states to results from a physical delta experiment, while also quantifying the variability around the mean state. This is done for an experiment forced with constant input rates of sediment and water and a constant base level rise rate, set to balance accommodation with the input flux of sediment. We then explore the morphological response of deltaic systems to cycles in the delivery rate of sediment. Theory derived from our experimental results suggests that detectable morphological responses, which fall outside the stochastic variability, require the magnitude and time scale of supply signals to fall outside the autogenic process variability. Finally, we compare the autogenic morphological variability present in Earth-surface systems to our ability to estimate geomorphological attributes from stratigraphic data. As an example, autogenic processes produce variability in transport slopes out to approximately ± 30% the mean state. In contrast, existing methods to estimate paleo-slopes from strata carry order of magnitude uncertainties. These results suggest that while autogenic processes impart noise on paleo-slope (and therefor paleo-sediment flux) estimates, other assumptions and simplifications present in our mean field theory are larger culprits. However, estimates of other parameters linked to sediment flux, for example channel depths and widths from preserved bars and channel bodies, carry smaller uncertainties and might therefor be more useful for paleo-environmental interpretations.