Effects of Extreme Monsoon Precipitation on River Systems Form And Function, an Early Eocene Perspective

Here we document effects of extreme monsoon precipitation on river systems with mountainous drainage basin. We discuss the effects of individual extreme monsoon seasons, as well as long-term changes in Earth surface system's form and function. The dataset spans across 1000 m of stratigraphy across ca 200 km of Paleocene and Early Eocene river deposits. The excessive 3-dimensional outcrops, combined with our new Carbon isotope, ichnological and paleosols record allow reconstruction of long-term river system's evolution during the Paleocene-Eocene Thermal Maximum (PETM) ca 56 million years ago, the transient global warming events during Early Eocene Climate Optimum (EECO) ca 53 to 51.5 million years ago, as well as the effects of highly peaked precipitation events during single monsoon seasons. On the single season scale, the increase in precipitation peakedness causes high discharge flooding events that remove large quantities of sediment from the drainage basin, due to stream erosion and landslide initiation. The initiation of landslides is especially significant, as the drainage basin is of high gradient, the monsoon intensification is accompanied by significant vegetation decline, as the monsoon cycle changes to multi-year droughts interrupted by extreme monsoon precipitation. These large discharge floods laden with sediment cause rapid deposition from high-velocity currents that resemble megaflood deposits in that they are dominated by high-velocity and high deposition rate sedimentary structures and thick simple depositional packages (unit bars). Such high deposition rates cause locally rapid channel bed aggradation and thus increase frequency of channel avulsions and cause catastrophic high-discharge terrestrial flooding events across the river basin. On long time scales, fluvial megafan systems, similar to those, e.g. in the Himalayan foreland, developed across the ca 200 km wide river basin, causing significant sediment aggradation and a landscape with high frequency river avulsions. In the river systems apex, close to where the river exited the mountainous drainage basin, the return period of river avulsions is extremely short, as seen by the high degree of channel amalgamation. Laterally and distally the channel return period diminishes as the avulsions occur across larger area and the channel size is smaller. Our dataset demonstrates expansion of these fluvial megafan systems in direct response to the PETM and the six consequent EECO global warming events on time scales of 104-105 yrs across 100s of km. We suggest that monsoonal river systems similar to those described here are particularly sensitive to climate forcing, and more likely to respond with minimal signal modulation effects, if the delivery of the extreme precipitation is contemporaneous drainage basin-wide. Then the highly peaked high volume monsoonal precipitation is able to initiate concurrent system-wide large-discharge flooding events and cause system-wide 'flushing' (splash effect). Our results suggest that such monsoonal river systems in particular, and perhaps Earth surface systems in general, are more likely to be sensitive to extreme climate events rather than to changes in average conditions.