Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia

Abstract

A 110-m-thick sequence of Late Neogene lacustrine silts and clays is sealed under Quaternary cover beds in a Cretaceous meteor crater at Yallalie in semiarid to subhumid southwest Western Australia. Paleomagnetic dating confirms that these sediments accumulated between 3.6 and ∼2.5 Ma. This Middle Pliocene (Piacenzian) interval coincides with a period of pronounced global warming recorded at middle to high latitudes in both hemispheres and plant microfossils preserved at Yallalie include species now confined to temperate and subtropical–tropical rainforest in Australasia as well as taxa that dominate dry sclerophyll forest, woodland, scrub–heath and semiarid vegetation types in northern Southwest Australia today. The region at the present time supports highly diverse xeromorphic heathland, shrubland and woodland, and the data suggests species diversity was richer in the Middle to Late Pliocene. Transient upsurges in the relative abundance of semiarid zone taxa, in particular Chenopodiaceae–Amaranthaceae, and halophytic diatoms demonstrate that warm, seasonally wet conditions within and beyond the crater were interrupted by three distinct episodes of aridification around 2.90, 2.59 and 2.56 Ma. How closely the Yallalie sequence mirrors trends in continental climates elsewhere in the middle latitudes of the southern hemisphere is as yet unclear. However, we note that the period of record overlaps with initiation of continental glaciation and an increase in the frequency and intensity of climatic fluctuations between 2.8 and 2.6 Ma in the middle latitudes of the northern hemisphere. Since the Yallalie arid vegetation response appears to be similar to that seen in southeastern Australia during Late Pleistocene glacial maxima, the vegetation patterns suggest alternating cycles of high humidity and aridity analogous to Late Pleistocene glacial–interglacial conditions that were taking place by 2.5 Ma, an observation that ties in well with δ 18O variations seen in marine cores. The data hint that the frequency of phases of relative aridity increased around 2.6 Ma. When fully analyzed, the high-resolution microfossil record will offer an exceptional opportunity to compare variability in Middle Pliocene terrestrial climates in the middle latitudes of the southern hemisphere with analogous records in the northern hemisphere. Examples are the Chinese loess, Lake Baikal in Russia and Pula Maar in Hungary, which are providing important data on orbit-related long-term climatic fluctuations during the Late Neogene and Early Quaternary.