Sediment Residence Times in Large Rivers Quantified Using a Cosmogenic Nuclides Based Transport Model and Implications for Buffering of Continental Erosion Signals.

The weathering of continental surfaces and the transport of sediments via rivers into the oceans is an integral part of the dynamic processes that shape the Earth's surface. To understand how tectonic and climatic forcings control regional rates of weathering, we must be able to identify their effects on sedimentary archives over geologic timescales. Cosmogenic nuclides are a valuable tool to study rates of surface processes and have long been applied in fluvial systems to quantify basin‐wide erosion rates. However, in large rivers, continual processes of erosion and deposition during sediment transport make it difficult to constrain how long sediments spend within the fluvial system. In this study, we examine the role of rivers in buffering erosional signals by constraining the timescales of fluvial transport in large rivers across the world. We apply a stochastic numerical model based on measurements of cosmogenic nuclides concentrations and calculate sediment residence times of 104–105 years in large rivers. These timescales are equal to or longer than climatic cycles, implying that changes to rates of erosion brought on by climatic variations are buffered during transport in large rivers and may not be recognizable in the sedimentary record. Plain Language Summary: Large rivers are the most effective agent for transporting sediment from the weathering continents into the oceans, with the world's biggest rivers draining nearly half of the continental surface. In this work, we calculate the time sediment spends in large rivers between weathering and deposition in four large rivers across the world. We do this by simulating the processes of sediment erosion and deposition in rivers and applying this model to new and existing data. The results of this model show that the time it takes for sand to be eroded from the source rock and transported down the river is tens to hundreds of thousands of years. These extended timescales mean that sediment transport in large rivers buffers the effect of climatic fluctuations on weathering rates. This finding can explain how seemingly contradictory evidence of climatic variations impact erosion rates, while products of erosion measured at the oceans show no significant changes during these times. Key Points: We constructed a numerical model simulating sediment transport dynamics in large‐scale fluvial systems constrained by cosmogenic nuclidesExamining data from four large rivers across the world, we constrain sediment residence time in large riversEstimated residence times of sediments in four large rivers across the world range 104–105 yr [ABSTRACT FROM AUTHOR]