Early diagenesis of skeletal remains in marine sediments : a 10 years study

A 10 years long expereimental approach of weathering processes affecting "fresh" organoclastic material at various depths (0-4800 meters) in different regions of the world allowed us to characterize, from an ecological point of view, the endolithic populations and associated microorganisms and to evaluate their role in the maturation of organoclastic sediments. We defined the rules governing the settling, growth and succession patterns of the microborers populations depend on environmental conditions (depth, t°, pO2, ...) in a way that will enable future modelisation work. From a general point of view, the prime alteration affecting skeletal remains is of biological origin. It is due to microorganisms growing onto and inside the skeletons, at the precise level of the organic matrices, that weather the organic compounds of these matrices by discharging specific hydrolases at their contact. A fast step of extraction of the most labile compounds (high speed constant) precedes a much slower biodegradation step of refractory compounds. Anaerobic biodegradation takes place after aeroibic biodegradation. Other kinds of microorganisms appear implicated but anaerobic biodegradation processes do not intrinsically differ and do not develop slowlier. Its apparent slowness mainly proceeds from the fact that only refractory compounds of the skeletal matrices usually reach the anoxic layers of the sediments. Anaerobic microbiocenoses associated with skeletal carbonates appear adapted to the biodegradation of such less labile compounds of the organic matrices. Early biological diagenesis events have important repercussions on other diagenetic, physical as well as chemical processes. From a mechanical standpoint, the biological extraction of the organic "cement" between skeletal crystallites fasten abrasion processes by reducing the general cohesion of the skeleton. Moreover, grazing and browsing organisms seeking for endolithic microflora contribute to such abrasion and disruption phenomenons. This leads to an appeciable increase of the CaCO3 content of the fine grain size fraction of sediments. From a chemical point of view, the disparition of the organic sheaths protecting CaCO3 joined to crushing and abrasion effects greatly increase the carbonate dissolution phenomenons and, paradoxally, even in thermodynamically supersaturated media like shallow tropical environment. From that point of view, the importance of high magnesian calcites, their microstructures and organic content, have to be stressed in connection with the control of the buffering capacity of marine waters.