Palaeoclimate, sequence stratigraphy and diagenesis

Abstract

Past and present climates have been characterised in terms of temperature and aridity/humidity. Global changes of atmospheric temperature occur on a periodicity of hundreds of millions of years (Icehouse and Greenhouse states of the Earth) to 500,000years (glacial and interglacial episodes). Diagenetic conditions are influenced by temperature through the action of thermally controlled kinetic reactions. Meteoric waters influence diagenesis through changing the amounts and chemistry of pore-fluid: in arid climates the volume of groundwater flux is diminished in comparison to humid climates. Relative sea-level change impacts shallow and deep burial diagenesis by changing near-surface chemical conditions and the propagation of heat into the subsurface. Relative sea-level changes can induce sediment surface temperature changes of 20°C when the sediment surface changes from being close to or beneath the thermocline to being sub-aerially exposed. Such sea-level changes typically occur on a timescale of half a million years. In a Greenhouse world, such sea-level changes occur on a similar periodicity to glacial–interglacial cycles. On a longer time scale, Greenhouse to Icehouse conditions occur approximately every 100ma. Burial diagenesis and cementation may thus be episodic on timescales associated with palaeoclimatic and sequence stratigraphic events occurring on the surface, a few metres to thousands of metres above the zone where diagenesis is occurring.