Climatic trends for the last 70 ky BP in the SE Pacific: the Rano Aroi and Rano Raraku records from Easter Island

Abstract

This paper discusses a complete record of vegetation history since the Bølling (GI-1e) warming (14,500 cal yr BP) up to the Holocene in Latvia. To date, this is the only complete record of such age in the eastern Baltic area and the northernmost area for which Bølling records are present. Combining pollen evidence, pollen accumulation rates (PAR) and plant macrofossil data, we assess the local and regional vegetation development, and we attempt to separate the true Lateglacial vegetation signal by removing the obviously redeposited thermophilous pollen; however, we remove not only their signal, we discuss the possibilities of separating the redeposition signal of the so-called “local Lateglacial trees”, pine and birch, by looking at their corrosion and degradation. The results show that the Bølling warming in the eastern Baltic area was a treeless tundra community consisting of the shrubs Betula nana, Dryas octopetala and Salix polaris. The Older Dryas cold spell is clearly recognised as a decline in the total concentration of plant macrofossils and PARs at between 14,200 and 13,500 cal yr BP. At 13,460 cal yr BP, the B. nana macrofossils disappear, and tree birch (Betula sect. Albae) appears, marking the start of tree birch forest. The presence of pine forest is confirmed by a variety of macrofossils, including bark, wood, needles and seeds, since 13,400 cal yr BP, at the same time at which pine stomata are found. The first identified pine stomata finds are associated with a Pinus PAR over 3000 grains cm−2 yr−1 and pine macrofossil finds with a Pinus PAR over 4000 grains cm−2 yr−1. During the warmest period of the GI-1a (Allerød) at 13,000–12,700 cal yr BP, a pine forest with deciduous trees (birch – Betula pendula and aspen – Populus tremula) developed in the study area. The Younger Dryas (GS-1) cooling strongly affected the floral composition in eastern Latvia. The PAR of the tree taxa declined abruptly from a maximum value at 12,700 to below 1000 grains cm−2 yr−1 at 12,600 cal yr BP. The response time for the pine forest to collapse was 100 years according to the PAR data. Pine macrofossils disappear simultaneously with the pollen signal at 12,600 cal yr BP, yet occasional Pinus stomata are recorded throughout the Younger Dryas (GS-1). The landscape was treeless shrub tundra again, with D. octopetala, S. polaris, B. nana and Juniperus present. Picea is introduced in the region within the cold Younger Dryas and is represented by stomata (12,400–12,200 cal yr BP), needles, seeds and wood (since 12,050 cal yr BP up to the Holocene). The Pleistocene/Holocene boundary at 11,650 cal yr BP is marked by changes both in vegetation composition and sediment type. The organic rich gyttja accumulated instead of silts and clays, and the start of the Holocene warm period permitted forest re-expansion in eastern Latvia.