134 PALAEOCLIMATES marked by a further more positive excursion in the Quaternary equatorial zones appear to have been more arid than they are today Evaporites Wildfires and Palaeosols Evaporite salts may form anywhere on Earth where evaporation exceeds rainfall (see Sedimentary Rocks: Evaporites) Their preservation is controlled by the nature of the post-depositional climate, the ingress of groundwaters, and subsequent burial history Modern coastal evaporites accumulate between 15 and 35 latitude, but intracontinental evaporites may occur beyond 50 , sometimes being frozen for several months in the year So salts not necessarily imply permanent warmth The formation and preservation of the more soluble salts of potassium require low atmospheric humidity (less than 50%) Salts may form very rapidly under ideal conditions (100 m in Ka), so short-lived episodes of evaporation may lead to great thickness of salts Fusain, almost pure carbon in sedimentary rocks, is a product of wildfires Natural fires, mostly triggered by lightning strikes, are favoured in strongly seasonal climates where vigorous plant growth is promoted by a distinct wet season, followed by a dry season creating tinder that becomes fire-prone during thunder storms at the onset of the next wet season Fusain-rich sediments are frequently associated with other evidence of strong seasonality (distinct wet–dry seasons) such as vertisols (and associated smectite-rich clay mineralogies) and calcretes Laterites (or ferricretes) reflect the relative accumulation of haematite, goethite, aluminium hydroxides (e.g gibbsite), and kaolinite within soil profiles (see Soils: Palaeosols) Pedogenic and groundwater laterites form best in humid tropical climates with a distinct dry season Bauxites (aluminium-rich laterites) often occur in association with laterites Some, like those of the Cretaceous of the Tethyan region, occur on karstic surfaces cut into carbonate platforms (see Sedimentary Processes: Karst and Palaeokarst) In such cases the detrital materials are likely to have been brought into the system, probably by aeolian activity Clay minerals in oceanic sediments reflect the weathering processes in the adjacent continental areas and the climatically controlled distribution of soils Differential settling causes some depositional segregation of clay species (e.g near-shore accumulation of kaolinite); nonetheless, recognizable patterns in the ancient can be preserved (see Clay Minerals) Glaciogenic and Cold Water Sediments About 10% of the Earth is covered by ice today, but this increased to 30% during the Quaternary glacials Ice and permafrost provide a series of erosional and depositional fingerprints (e.g striated pavements and boulders, dropstones, ice wedges, and glacial tills (diamicton)) (see Sedimentary Processes: Glaciers) as well as a range of geomorphological features, such as eskers, kames, etc., some of which have been recognized in association with ancient glacial systems (e.g in the Lower Palaeozoic of the Sahara and the Late Palaeozoic of Gondwana) However, boulders may be dropped into deep water through mechanisms other than ice rafting (e.g from the roots of floating trees) More cryptic evidence of cold-water precipitation comes from glendonites – carbonate nodules pseudomorphing the mineral ikaite, the low-temperature polymorph of calcium carbonate (CaCO3 Á 6H2O) It is today associated with clathrates in cold deep waters and in shallow polar waters Glendonites are reported from Cretaceous localities in Australia and Canada and from the Jurassic of Siberia Coals and Lignites Coals and lignites are indicative of an excess of precipitation over evaporation: vegetation accumulates in wetland mires, which, if rain occurs on most days of the year, will grow upwards above the general level of the regional water table Preservation of peat requires rapid burial and/or anoxic conditions Frigid conditions favour preservation, as environments in which rainfall is evenly spread throughout the year Today such conditions occur in equatorial zones and in high mid-latitudes In the Mesozoic Aeolianites Although coastal dune complexes may form across a range of latitudes, major sand seas occur predominantly in arid environments Such aeolianites provide both local and regional palaeowind data, which may be used to test palaeoclimate models generated by GCMs (see below) (see Sedimentary Processes: Aeolian Processes) Palaeoclimate Models A recent approach to understanding past climate regimes on Earth has been through the application of complex computer models, specifically atmospheric general circulation models (AGCMs), ocean general circulation models (OGCMs), and recently even more complex coupled ocean–atmosphere general circulation models (OAGCMs) There are now many contributions in this field, the palaeoclimatic