On the preservation of climate variations in the stratigraphic record

Abstract

In stratigraphic studies, basin deposits are often used as a proxy for climatic variations. Understanding which climatic variations are likely to be recorded in these deposits is vital to their interpretation. Previous studies showed that proximal sediment input variations are unlikely to be recognisably preserved in the stratigraphic record at the end of the alluvial system. However, the role of a simultaneously varying sediment transport capability of the sedimentary system is unknown. An open source numerical model is developed and used in this study to investigate how various climatic variations are treated by, and recorded in, an alluvial fan. A focus lies on varying the general sediment redistribution capability of the system in sync with the sediment input into the system.

The results presented in this paper indicate that input signals form the catchment area are indeed buffered by an alluvial fan. Sediment leaving the fan at its distal end does however directly reflect both rapid (tens to hundreds of thousands of years) and long (millions of years) changes in climatic forcing. This is facilitated by changes in the sediment transport capability of the system. The distal end of the system therefore immediately experiences increases and decreases in the amount of sediment leaving the system. A single climate signal can therefore create a two step change in the output signal of the alluvial fan. The first step is then related to a direct increase of the sediment transport rate and the second step is related to the delayed arrival of sediment derived from the catchment area.

It was also found that, within the alluvial fan, gravel front propagations generally occur in two scenarios. The first scenario involves a prolonged scarcity of new sediment input, which allows coarse material to travel downstream for it is not being covered by or diluted with newly arriving sediment. The second scenario involves a rapidly increasing sediment transport capability over input ratio, which can transport coarse material downstream before it is covered by newly arriving sediment.