Resolving the era of river-forming climates on Mars using stratigraphic logs of river-deposit dimensions

Abstract

River deposits are one of the main lines of evidence that tell us that Mars once had a climate different from today, and so changes in river deposits with time tell us something about how Mars climate changed with time. In this study, we focus in on one sedimentary basin – Aeolis Dorsa – which contains an exceptionally high number of exceptionally well-preserved river deposits that appear to have formed over an interval of >0.5 Myr. We use changes in the river deposits' scale with stratigraphic elevation as a proxy for changes in river paleodischarge. Meander wavelengths tighten upwards and channel widths narrow upwards, and there is some evidence for a return to wide large-wavelength channels higher in the stratigraphy. Meander wavelength and channel width covary with stratigraphic elevation. The factor of 1.5–2 variations in paleochannel dimensions with stratigraphic elevation correspond to ∼2.6-fold variability in bank-forming discharge (using standard wavelength-discharge scalings and width-discharge scalings). Taken together with evidence from a marker bed for discharge variability at ∼10 m stratigraphic distances, the variation in the scale of river deposits indicates that bank-forming discharge varied at both 10 m stratigraphic (102–106 yr) and ∼100 m stratigraphic (103–109 yr) scales. Because these variations are correlated across the basin, they record a change in basin-scale forcing, rather than smaller-scale internal feedbacks. Changing sediment input leading to a change in characteristic slopes and/or drainage area could be responsible, and another possibility is changing climate (±50 W/m2 in peak energy available for snow/ice melt).