Late Pliocene sedimentation in Lake Baikal: implications for climatic and tectonic change in SE Siberia

Abstract

Within the framework of the Baikal Drilling Project (BDP), a 192 m long sediment core (BDP-96-1) was recovered from the Academician Ridge, a submerged topographic high between the North and Central Basins of Lake Baikal. Sedimentological, clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth, corresponding to ca. 2.4–3.4 Ma. The aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of Northern Hemisphere glaciation in Late Pliocene time. A major climate change occurred in the Lake Baikal area at about 2.65 Ma. Enhanced physical weathering in the catchment, mirrored in the illite to smectite ratio, and temporarily reduced bioproduction in the lake, reflected by the diatom abundance, evidence a change towards a colder and more arid climate, probably associated with an intensification of the Siberian High. In addition, the coincident onset of distinct fluctuations in these parameters and in the Zr/Al ratio suggests the beginning of the Late Cenozoic high amplitude climate cycles at about 2.65 Ma. Fluctuations in the Zr/Al ratio are traced back to changes in the aeolian input, with high values in warmer, more humid phases due to a weaker Siberian High. Assuming that the sand content in the sediment reflects tectonic pulses, the Lake Baikal area was tectonically active during the entire investigated period, but in particular around 2.65 Ma. Tectonic movements have likely led to a gradual catchment change since about 3.15 Ma from the western towards the eastern lake surroundings, as indicated in the geochemistry and clay mineralogy of the sediments. The strong coincidence between tectonic and climatic changes in the Baikal area hints at the Himalayan uplift being one of the triggers for the Northern Hemisphere Glaciation.

Introduction

The intensification of Northern Hemisphere Glaciation (NHG) in Late Pliocene time is clearly documented in marine sediment cores (e.g. Jansen et al., 1990, Maslin et al., 1995, Haug et al., 1995, Thiede et al., 1998). Major cooling took place between 3.1 and 2.5 Ma. Maslin et al. (1998) suggest that the onset of glaciation in the Eurasian Arctic occurred at about 2.74 Ma, ca. 40 ka prior to that in Alaska (2.70 Ma) and 200 ka prior to that in northeast America (2.54 Ma).

Little information, in contrast, is available concerning the reason for NHG. It may have been a combination of several processes that took place more or less at the same time. The main triggers suggested are the closure of the Panama Isthmus, amplitude changes of the 41 ka orbital cycle (obliquity) and uplift of the Himalaya Mountains to the southwest of Lake Baikal (e.g. Hodell et al., 1990, Raymo and Ruddiman, 1992, Driscoll and Haug, 1998, Haug and Tiedemann, 1998, Maslin et al., 1998). Uncertainty for the role of the Himalaya uplift, which is supposed to have led to increased chemical weathering and an associated CO₂ decrease in the atmosphere (Hodell et al., 1990, Raymo and Ruddiman, 1992), arises from the poor age control of this process. Based on tectonic investigations, Harrison et al., 1992, Molnar et al., 1993 assume that the Himalayan Belt including the Tibetan Plateau already reached its maximum elevation about 8 Ma ago. However, Hsü, 1976, Han et al., 1997, based on palaeobotanical observations suggest that by Early Pliocene, the Tibetan Plateau elevation was considerably lower. Further north, in the Tien Shan Mountains (Sun et al., 1999), Altai Mountains (Delvaux et al., 1999) and Baikal Rift System (Mats, 1993), major tectonic changes are assumed to have started in the Late Pliocene (Kuzmin et al., 2000, Mats et al., 2000).

For a better knowledge about the time when tectonic changes occurred in Asia and about their probable relation to the intensification of NHG, it is thus of great importance to investigate the Pliocene climatic and tectonic history of continental Asia. However, apart from the Chinese loess sequences, there have been no high-resolution continental records of Pliocene age. New opportunities recently became available from Lake Baikal, whose evolution is supposed to have been related to the Himalaya uplift (Molnar and Tapponnier, 1975, Tapponier and Molnar, 1979). Within the scope of the international Baikal Drilling Project (BDP), a long sediment core of 192 m (BDP-96-1) was obtained from Academician Ridge, a submerged topographic high between the North and Central Basin of Lake Baikal (Fig. 1). Core BDP-96-1 has a generally constant sedimentation rate of 4 cm/ka and covers the entire period since ca. 5 Ma (BDP Leg II Members, 1997, BDP-Members, 1998). Preliminary analyses of the diatom distribution (BDP Leg II Members, 1997, Williams et al., 1997, BDP-Members, 1998) and pollen assemblages (Demske et al., 1999, Demske et al., 2000) in core BDP-96-1 indicate drastic climatic changes between 2.8 and 2.6 Ma. In addition to diatoms and pollen, clay minerals in Baikal sediments can be a particularly useful proxy for reconstructing climate changes. This has been shown by clay mineral investigations of the Quaternary sediment cores BDP-93-1 and -2 from the South Basin of Lake Baikal (Melles et al., 1995a, Oberhänsli et al., 1998, Yuretich et al., 1999; Fig. 1). The sensitivity of clay minerals to the weathering conditions in the Baikal catchment was confirmed by initial TEM and EDX analyses of clay minerals of BDP-96-1 (Müller et al., 2000).

In this paper, we present high-resolution sedimentological, clay mineralogical and geochemical data of BDP-96-1 from the time interval between ca. 3.4 and 2.4 Ma. Based on these data, changes in the weathering regime, atmospheric dynamics and clastic sediment sources are reconstructed and discussed in light of both the regional tectonic activity and the global climate change during intensification of NHG.