Climate-driven palaeofloral fluctuations on a volcanic slope from the low latitudes of the Palaeotethys (early Permian, West Sumatra)

Abstract

The present paper focuses on plant diversity fluctuations within three ecosystems from a volcanic slope in the sequence analytical context of three superimposed cyclicities. This analysis was possible thanks to the isotopic age evaluation of the volcanic Merangin section that bracketed the Mengkarang Formation on West Sumatra between 296.77 ± 0.04 and 296.14 ± 0.09 Ma (Asselian, Lower Permian). The Merangin section consists of a 490 m long fining and thinning tuffaceous aggregational sequence composed of eight short-eccentricity fining and thinning upwards parasequences. The palaeoflora along the sequence varies from tropical wet taxa, comprising cordaitalean and ferns, to mesic-xeric taxa, typified by seed ferns and early conifers. Herein the palaeofloral content of 14 consecutive lahars from a single volcanic accretion wedge in the seventh parasequence is described in detail. It displays a gradual increase in the ratios of (seed) ferns (Macralethopteris hallei, Sphenopteris sp. and gigantopterids) and early conifers (Dicranophyllum molle, Tobleria bicuspis), reaching a 100% in the 6th bed, and returning to 100% cordaitalean and ferns. The high ratio of seed ferns and early conifers in the 6th lahar is interpreted as reflecting a temporary lowering of the tree line caused by a glacial maximum from a high order cycle (obliquity or precession), superimposed on a short-eccentricity cycle, which in turn is superimposed on our third-order cycle palaeofloral transition described earlier for the whole sequence. Along this lower Permian section from the low latitudes from the Palaeotethys, glacial-interglacial cycles are seen to affect the fluctuations of the vegetation line in a chiefly similar way as during the Quaternary. The diversity fluctuations for three ecosystems (wetlands, lower and upper slope) are modelled for two of the three or more superimposed cycles. Emphasis is put on the appearance of a glacial palaeoflora on top of the sequence.