Variations of Density and Porosity with Depth in Deep-sea Sediments

TLDR

Sediment porosity decreases and density increases under overburden pressure on the seafloor, a key issue in earth sciences. The study uses Deep Sea Drilling Project data to establish laboratory density and porosity profiles versus depth in the seafloor. Consolidation tests were used to estimate elastic rebound and construct in situ density and porosity profiles for key sediment types, and methods were developed to predict gradients and calculate original sediment thicknesses. Porosity decreases only 8–9 % in the upper 100 m of pelagic clay, calcareous, and terrigenous sediments and 4–5 % in siliceous oozes, with rebound peaking at ~7 % in pelagic clay from 300 m depth, while terrigenous sediments may rebound up to ~9 % over 1 000 m, implying >2 000 m of original material was compressed to the present 1 000 m thickness.

Abstract

ABSTRACT Reduction of sediment porosity and increase in density under overburden pressure in the sea floor are important subjects in earth sciences. Data and samples from the Deep Sea Drilling Project allow a new look at these subjects, and are used to establish profiles of laboratory values of density and porosity versus depth in the sea floor. To construct in situ profiles, the results of consolidation tests are used to estimate the amount of elastic rebound (increase in volume) which has occurred after removal of the samples from overburden pressure in the boreholes. In situ profiles of porosity and density versus depth are constructed for some important sediment types: calcareous ooze, siliceous oozes (diatomaceous and radiolarian oozes), pelagic clay, and terrigenous sediments. There is l ss reduction of porosity with depth in the first 100 m in these deep-water sediments than previously supposed: 8 to 9% in pelagic clay, calcareous and terrigenous sediments, and only 4 to 5% in the siliceous sediments. From depths of 300 m the most rebound is in pelagic clay (about 7%), and the least in diatomaceous ooze (about 2%); calcareous ooze and terrigenous sediment should rebound from 300 m about 4 to 5%. Terrigenous sediment, from the surface to 1,000 m depth, probably rebounds a maximum of about 9%. Methods are described and illustrated to predict density and porosity gradients in the sea floor, and to compute the amounts of original sediments necessary to have been compressed to present thicknesses. Slightly over 2,000 m of original sediments would have been required for compr ssion to a present-day thickness of 1,000 m of terrigenous sediments.