PALAEOCLIMATES 133 and climate changes These are palaeoclimate proxies The most climatically informative structures are glaciogenic features (striated pavements, tills, and tillites), distinctive soil types (e.g laterites and bauxites), and indicators of aridity (e.g evaporites and aeolianites) Supplementary evidence of climate is provided by a range of other features, such as other types of palaeosol (e.g calcretes, gypcretes, and vertisols), clay mineralogy (reflecting weathering regime), storm deposits, reefs and oolites, glendonites, and fusain (from wildfires) Coals, often considered to require moist tropical climates, are now known to accumulate equally well in temperate mires Critical information on past climates (particularly quantitative information on palaeotemperature) comes from oxygen isotopic (d18O) data derived from carbonate fossils, but such data have to be treated with care because of problems associated with diagenetic resetting Palaeobotanical data, particularly regarding plant appearance (physiognomy), are generally qualitative, and have also been claimed to provide quantitative information The density of leaf stomata may be related to the concentration of atmospheric carbon dioxide, and there is, today, a striking relationship between mean annual temperature and leaf shape (percentage of entire-margined, as opposed to toothedmargined, species) in a biome Entire-margined leaves dominate warm biomes Fossil reptiles such as crocodilians, like their modern descendants, have global distributions that are principally controlled by temperature A coldestmonth mean temperature of 5.5 C marks the minimum thermal limit for the group (corresponding to a modern mean annual temperature of ca 14.2 C) Such evaluations of temperature have been successfully applied globally for the Mesozoic and Cenozoic and compared with GCM output on climate for those times Marine Carbonates Modern shelf seas are dominated by carbonate facies in areas where organic productivity is high, generally in low latitudes (between about 30 N and 30 S) with strong insolation and warm normal-salinity seawaters (>20 C and 35% salinity) Carbonate shelves also occur in temperate waters, particularly where rates of terrigenous run-off are low However, rates of productivity in these areas are much lower than in warmer waters, and the biota is dominated by different species Nonetheless, dominance of carbonates in a particular rock succession may not necessarily indicate warm waters, particularly for more ancient rocks in which the biota have only tenuous links with modern forms (see Sedimentary Environments: Carbonate Shorelines and Shelves) Present-day warm-water shelves unpolluted by fine-grained terrigenous run-off have diverse communities with hermatypic (reef-building) corals and codiacian calcareous algae and may include ooids, aggregates, and pellets; these latter grain types reflect carbonate precipitation and early cementation This is known as the chlorozoan skeletal grain association and can be recognized as far back as the Triassic Modern temperate-water carbonate factories are dominated by benthic foraminiferans, molluscs, bryozoans, barnacles, and calcareous red algae This is termed the foramol skeletal grain association and can be recognized, albeit equivocally, into the Mesozoic Era The polewards expansion of reef and carbonate facies (by about 10 latitude) in the Mesozoic and Early Cenozoic, relative to today (30 today, ca 40 in the Mesozoic and Palaeogene), suggests that the Earth was more equable during these times The principles of palaeotemperature determination from carbonates using oxygen isotope ratios are well established, and for many years oxygen isotopic ratios have been used to quantify sea-surface temperatures and bottom-water temperatures The technique has most successfully been applied using the fossil shells of organisms considered to be robust to diagenetic changes (e.g planktonic forams, coccolithic oozes) Such data have shown that ocean waters have changed their isotopic ratios in concert with the waxing and waning of the ice-caps during Quaternary glacial cycles, the oceans becoming relatively depleted in 16 O during glacial episodes (shells therefore showing 18 O enrichment) by comparison with interglacials In the longer term the d18O trend for ocean waters suggests that seawater has become progressively depleted in 16O (leading to positive d18O values) since about 55 Ma This has occurred in a series of steps, probably reflecting, first, the cooling of deep waters, followed by the onset of permanent ice on Antarctica at around the Eocene–Oligocene boundary Then followed a series of significant more positive excursions, through the Oligocene and Miocene, before a major and permanent swing in the Miocene (15 Ma), which is taken to reflect the formation of the Antarctic ice-sheet The growth of the northern hemisphere ice-sheets is evaporation) Under lignites and coals V, vitrinite; F, fusinite; I, illite; V, vermiculite; Sm, smectite; K, kaolinite; CL, chlorite; ppt, precipitation; evap, evaporation; *, unaltered parent rock with fines under both arid and frigid terrains; y, vermiculite can form under a wide variety of conditions