Continental humid and arid zones during the jurassic and cretaceous

Abstract

The global distribution of climatically significant criteria is surveyed for six successive time intervals: Hettangian-Toarcian, Aalenian-Callovian, Oxfordian-Tithonian, Berriasian-Barremian, Aptian-Cenomanian and Turonian-Maastrichtian. The criteria are primarily mineralogical—evaporites, coals, bauxites, ironstones and kaolinite—but include sedimentological and palaeontological aspects of facies, e.g. aeolian sands, fossil ferns and xerophytes, fresh-water invertebrates. In Early and Mid Jurassic times an arid zone can be distinguished in low- to mid-latitudes in western Pangaea, with humid zones elsewhere. In the Late Jurassic this arid zone spread northwards to embrace much of southern Eurasia. There followed in the Early Cretaceous a sharp reversal to more humid conditions over a large area extending from the eastern margin of North America to the Middle East, a major consequence of which was the widespread replacement of carbonates by coarse siliciclastic facies. By Late Cretaceous times humid conditions were well established also in North America and the South Atlantic margins, and the tropical arid zone had contracted to a minimum. The 87 Sr/ 86 Sr ratio of marine carbonates provides an independent monitor of continental runoff, which is related to (a) area and (b) extent of aridity. Changes in the ratio confirm that, taking sea-level stand into account, the Late Cretaceous was appreciably more humid than the Late Jurassic. These climatic changes are related primarily to the progressive breakup of Pangaea in the Late Jurassic and Cretaceous which, together with sea-level rise in the Mid to Late Cretaceous, caused a significant increase in maritime influence. This implies that monsoonal winds probably had a major role in promoting higher precipitation rates. A puzzle remains as to why there was a spread of aridity in the Late Jurassic. Finally, some implications for terrestrial plant evolution are noted.