A correction for sediment load upon the ocean floor: Uniform versus varying sediment density estimations—implications for isostatic correction

Abstract

The isostatic correction for the sediment load on oceanic basement has often been calculated using a uniform sediment density, irrespective of either water depth or total sediment thickness. An alternative method is proposed here in which the sediment density is calculated using the density-depth equations of Hamilton (1976) for calcareous, clay and terrigenous sediment sequences. This alternative method takes into account the water depth and total sediment thickness when estimating the sediment density, for example, deep water (>4000 m) sediment sequences are likely to be clay-rich; thick sediment sequences (>2000 m) usually have a high terrigenous component and shallow sequences are likely to be calcareous-rich. Sediment thicknesses in excess of 1500 m are generally close to a continental margin. These density estimations are compared with measurements from 9 Ocean Drilling Program drill sites, 1 Deep Sea Drilling Project drill site and from densities derived from seismic interval velocities. The most commonly used estimates of uniform sediment density were used to calculate isostatic corrections for a 12 km thick sediment sequence. These results were then compared with the results determined using the proposed alternative method of estimating sediment densities. The proposed method produces isostatic corrections similar to those derived by published methods for sediment sequences < 1 km, but compensates better for thicker sediment sequences. Differences between an isostatic correction using a uniform sediment density and the method proposed here exceed 100 m for thinner sequences (700–1000 m) and range up to 2000 m for thicker sequences (10–12 km). These discrepancies clearly have implications for other studies, for example in palaeoceanography, mantle-plate temperature studies and basinal analyses.