Hydrochemistry,origin and evolution of sedimentary subsurface fluids: II. Early diagenetic pore-water evolution in high sedimentation-rate offshore basins |
| |
| Authors: | Reinhard Hesse |
| |
| Institution: | 1. Department of Geological Sciences, McGill University Montreal, H3A 2A7, Quebec, Canada
|
| |
| Abstract: | High sedimentation-rate basins are characteristic of the continental margins, where sedimentation rates up to 500m/Ma and more are no exception. The sediments are rich in organic matter and undergo a well-established sequence of bacterial oxidation reactions, starting at the sediment/water interface with: (1) the oxidation zone (absent in euxinic basins), which is followed downwards by (2) the nitrate reduction zone, (3) the sulfate reduction zone, (4) the carbonate reduction zone, (5) the fermentation zone and (6) the decarboxylation zone, which marks the onset of thermocatalytic reactions characteristic of the intermediate stage (and deeper stages) of diagenesis. This sequence of bacterially mediated organic-matter decomposition reactions in rapidly deposited sediments on the continental margins is well reflected in the vertical trends of pore-water chemistry. Carbon isotopic fractionation permits to distinguish between zones (1) to (3) with negative δ13C values for the CO2 generated, and between zones (4) and (5) with increasingly heavier carbon-isotopes in CO2 (up to + 15 to + 25‰ δ13C). The formation of gas-hydrates causes characteristic chemical and isotopic signatures in pore waters retrieved from hydrate-bearing sediments. After hydrate decomposition (in the sampling process or at the base of hydrate zones) marked chlorimity decreases accompanied by increases in heavy oxygen isotopes are observed. These are distinctly different from the effects of meteoric water influx, which are not accompanied by positive δ18O values. The opposite trend of increasing salinity at shallow burial depths, rather than pore-waters freshening, is observed in the vicinity of evaporites. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
