The ratio of mechanical to chemical denudation in alluvial systems, derived from geochemical mass balance

Abstract

ABSTRACTMass balance equations are derived which link the ratiosTs/(suspended load/dissolved load from chemical weathering) andTb/Ts(bed load/suspended load), with any two geochemical components present in the source rock and the alluvial system. If the dissolved load is unknown the ratios can be estimated from the relatively insoluble silica and alumina. The ratioTs/, which for large river basins depends on climate and relief, can thus potentially be determined from ancient alluvial sequences.The equations help define the source composition of a group of 13 modern rivers for whichTs,and alluvial geochemistry are known. These rivers together drain 27% of the continental surface. For a source area with the average continental sandstone to shale ratio of 0·6 the observed average value ofTs/is obtained when limestone, sandstone and shale are present in the proportions 6·7:21·6:35·7. The figure of 64% sediment in the source area is very similar to the 66% determined by Blatt and Jones (1975) from geological maps of the continents. The equations also show that average bed load transport rate into these 13 basins is about 27% of total transport, and into the Amazon basin about 37%. Bed load transport rates out of the basins, into the sea, are relatively very small.