Fluvial cut-and-fill terraces record climate oscillations

Fill terraces cut into fluvially deposited sediment are ubiquitous features along rivers, and are hypothesized to reflect natural, periodic changes in climate driven by Milankovitch cycles. However, little is known about the physical links between climate and the formation of fluvial cut-and-fill terraces. Quantifying climatic controls on fill terrace formation will improve understanding of (i) how climate oscillations are preserved in fluvial landscapes over thousands of years to tens of millions of years, (ii) the complex interactions between river discharge and sediment supply that control shifts between fluvial aggradation and incision, and (iii) the utility of fill terrace records for quantitatively inferring climate oscillations.

Using a model of long-term fluvial erosion and sediment transport (Yuan et al., 2019, JGR-SE), we demonstrate and quantify a direct, physical explanation for the relationship between climate oscillations and the formation of fluvial cut-and-fill terraces. In contrast to autogenic sediment signals observed far from the source area (Yuan et al., 2019, JGR-SE), our model shows a clear link between climate oscillations and sediment signals in locations proximal to the source area. In particular, we show that the thickness of fill terraces in a foreland basin adjacent to a mountain belt is proportional to the amplitude and period of rainfall (climate) oscillations but is modulated by the rate of uplift of the mountain belt. For typical orogenic uplift rates of a few mm/yr, climatic oscillations at Milankovitch periods lead to aggradation/incision cycles of a few tens to hundreds of meters as observed in nature. We also explain the time lag (on the order of 20-25% of the forcing period) that is commonly observed between the timing of maximum (minimum) rainfall and erosion (deposition) in the basin. Our model predictions for fill terrace thickness and time lag, as well as their relationship to climatic drivers, are consistent with field observations. Resolving the complexity of sediment transport, deposition and erosion processes and how each of them is affected by climate, our new results provide a method for quantitatively extracting information about past climate variations from the fill terrace record.