Palaeoclimate, sequence stratigraphy and diagenesis

Past and present climates have been characterised in terms of temperature a nd aridity/humidity. Global changes of atmospheric temperature occur on a p eriodicity of hundreds of millions of years (Icehouse and Greenhouse states of the Earth) to 500,000 years (glacial and interglacial episodes). Diagen etic conditions are influenced by temperature through the action of thermal ly controlled kinetic reactions. Meteoric waters influence diagenesis throu gh changing the amounts and chemistry of pore-fluid: in arid climates the v olume of groundwater flux is diminished in comparison to humid climates. Re lative sea-level change impacts shallow and deep burial diagenesis by chang ing near-surface chemical conditions and the propagation of heat into the s ubsurface. Relative sea-level changes can induce sediment surface temperatu re changes of 20 degrees C when the sediment surface changes from being clo se to or beneath the thermocline to being sub-aerially exposed. Such sea-le vel changes typically occur on a timescale of half a million years. In a Gr eenhouse world, such sea-level changes occur on a similar periodicity to gl acial-interglacial cycles. On a longer time scale, Greenhouse to Icehouse c onditions occur approximately every 100 ma. Burial diagenesis and cementati on may thus be episodic on timescales associated with palaeoclimatic and se quence stratigraphic events occurring on the surface, a few metres to thous ands of metres above the zone where diagenesis is occurring. (C) 2000 Elsev ier Science B.V. All rights reserved.