Evolution of the Middle Jurassic paleoclimate: Sedimentary evidence from coal-bearing strata in the Santanghu Basin, NW China.

[Display omitted] • The Middle Jurassic coals contain abundant Ca-aluminophosphates as well as elevated concentrations of P and Sr. • The sudden decrease in the chemical weathering index reveals a change from hot-humid to seasonally arid paleoclimate. • Formation of Ca-aluminophosphates was a response to the global abrupt cooling. Aalenian-Bajocian (Middle Jurassic) cooling events have been widely postulated, but the evolution of the paleoclimate in the northeastern part of Tethys, especially that recorded in terrestrial coal-bearing sedimentary successions, has received little attention. Jurassic coals in the Santanghu Basin, NW China, are well preserved and provide an opportunity to reveal the early Middle Jurassic climate. These coals contain kaolinite, illite, quartz, and carbonate minerals. Coal 1# to 3# is especially unusual in having abundant aluminophosphates. Compared to worldwide and Chinese coals, the contents of elements P 2 O 5 (0.21 %) and Sr (757.3 ppm) of Coal 1# to 3# are noticeably elevated. The provenance of the coal-bearing strata is upland areas on both sides of the basin comprising intermediate rocks, such as andesites. Based on chemical weathering indices and paleoclimatic indicators of non-coal samples, the paleoclimate of the early Middle Jurassic was reconstructed. A sudden decrease in the chemical weathering index reveals a change from humid to seasonally arid paleoclimate in the Aalenian-Bajocian. The deposition of the aluminophosphate minerals,as well as enriched phosphorus and strontium, is mainly controlled by the flow of Sr- and/or Al-rich solutions caused by sedimentary environments (low pH and oxidizing conditions). The syngenetic Ca-aluminophosphate formation may have been linked to the climatically induced changes in hydrological cycling, in response to the global abrupt cooling of the early Middle Jurassic. [ABSTRACT FROM AUTHOR]