Steroid biological marker hydrocarbons as indicators of organic matter diagenesis in Deep Sea sediments: geochemical reactions and influence of different heat flow regimes

Abstract

Specific geochemical transformation reactions of steroid hydrocarbons have been chosen to illustrate the molecular evolution of organic matter in deep sea sediments. The progression of isomerisation of (Δ5→Δ4)-sterenes, (20R→ 20S)-diasterenes, (20R→20S)-spirosterenes, and (14α→14β) monoaromatic anthrasteroids with depth has been monitored in core samples from ten DSDP sites. Areas with high geothermal gradients are compared to those with low geothermal heat flow. The formation of regular sterenes and anthrasteroids from functionalized precursors starts close to the sediment/water interface soon after deposition of the organic matter. Due to the high reaction rates these low temperature reactions are independent of the geothermal regime within the limits of sample resolution of this study. In contrast to this, the depth of formation of diasterenes and spirosterenes is dependent on the geothermal gradient. Sedimentation rates indirectly determine, through the effect of the resulting burial depth, the age of the sediments in which these compounds first occur. There is a significant effect of the geothermal gradient on most of the isomerisation reactions studied, and it is most pronounced for the conversion of the 14α(H)-B ring monoaromatic anthrasteroids to the 14β(H) isomers.