Palaeoclimate across the Late Pennsylvanian–Early Permian tropical palaeolatitudes: A review of climate indicators, their distribution,and relation to palaeophysiographic climate factors

摘要

Global-scale compilations of palaeoclimate indicators include records of the temporal and spatial occurrence of coal, laterite, bauxite, Vertisols, calcrete, eolianite, and evaporite at the scale of geological stage. These palaeoclimate indicators provide the primary evidence for palaeoclimate change during the Late Palaeozoic,and have been used to infer a long-term climatic transition from humid to arid conditions on equatorial Pangaea from Late Pennsylvanian through Early Permian time. The cause(s) of Late Pennsylvanian–Early Permian climate trends are unknown but must have resulted from climate factors operating on timescales of tectonic change (10~6–10~7 yr), such as tectonic drift, assembly of Pangaea, orogenesis, and long-term carbon cycling.Although higher-resolution, local- to regional-scale palaeoclimate reconstructions for the Late Pennsylvanian– Early Permian exist, they generally lack the time control necessary for accurate correlation among sites. Nevertheless, these high-resolution palaeoclimate reconstructions provide details about Permo-Pennsylvanian palaeoclimate that are not perceptible in lower-resolution global datasets. These studies indicate that (1) although the Late Pennsylvanian equatorial latitudes were more humid than the Early Permian tropics, there was also considerable variability in the amount and seasonality of rainfall, and (2) there were several short (1–3 Ma) excursions toward relatively more humid climate during the long-term Early Permian transition to aridity in western and central equatorial Pangaea. These higher-resolution climate changes were controlled by climate factors which operated on relatively short timescales (10~4–10~6 yr) such as continental ice-sheet dynamics, sea-level change and associated changes in land–sea distribution, and variations in palaeoatmospheric PCO_2.Although lithological indicators and geochemical proxies provide the basis for reconstructing past climate,they seldom provide diagnostic evidence to determine which of the possible climate factors were important. To narrow the possible causes of Late Pennsylvanian–Early Permian climate change, we review and evaluate both conceptual and numerical models that have been previously used to explain Late Palaeozoic climate change in light of the detailed spatial and temporal proxy records from across near-equatorial Pangaea. Our ability to test these models is currently limited by our inability to make accurate correlations among proxy sites due to uncertain dating. Nonetheless, we suggest that on tectonic timescales continental drift, increasing atmospheric PCO_2,and deglaciation could explain much of the low-latitude climate record, while changing atmospheric PCO_2 and orbitally-driven glacial–interglacial cycles could account for higherresolution climate variability on Pangaea.