Constraining the Timespan of Fluvial Activity From the Intermittency of Sediment Transport on Earth and Mars.

The timespan recorded in deposits from fluvial activity is a key gap in our understanding of ancient environments on Earth and Mars. Because riverine sediment transport occurs under time‐variable water discharge, common models that represent sediment transport with a single bankfull discharge require an intermittency factor. For this reason, the ability to predict intermittency factor values based on environmental factors would improve estimates of time from fluvial deposits. To address this knowledge gap, we calculated intermittency factors from 201 modern rivers and six fans and deltas with depositional timespans of months to millions of years. Intermittency factors range from 0.0064–0.73; they are uncorrelated with averaging timescale, bed‐material grainsize, or climate aridity, but are larger in river catchments with greater rates of denudation relative to precipitation. Application to ancient fluvial systems on Mars indicates long‐lived depositional river systems for up to 104–106 years. Plain Language Summary: To estimate the lifespan of ancient river courses, geologists estimate the time it took to transport and deposit all of the sediment within a landform, such as an alluvial fan or delta. This method relies on sediment‐transport calculations, which often assume a single reoccurring flood event that fills the river channel, even though river discharge varies in time from low flows to large floods. While the discharge of the channel‐filling flood event can be constrained from channel geometry or deposit characteristics, flow variability is unknown, and an intermittency factor is needed to adjust the sediment transport calculation. In this work, we calculated intermittency factors for 201 rivers and six river deposits on Earth. We also examined correlations between intermittency factor and environmental variables, and found no correlation with averaging timescale, bed‐material grainsize, or climate aridity. However, intermittency factors are larger in catchments that receive a large supply of sediment. Applying the new intermittency factor values to river deposits on Mars suggests that some of those rivers persisted for millions of years. Key Points: Rivers transport sediment intermittently; intermittency factor relates river duration to deposit volume and bankfull river characteristicsIntermittency factors we calculated have a 5–95 percentile range of 0.0064–0.73, independent of timescale, grainsize, and climateApplication to Mars implies fluvial activity over thousands to millions of years [ABSTRACT FROM AUTHOR]