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Two boreholes, one to a depth of 300 m, and the other to a depth of 100 m below sediment surface were drilled at 53°41´48´´N-108°21´06´´E. The core was collected down to 200m in the first borehole, and totally in the second one. Yield of the cores was more than 95%. Sediments consist of terrigenic and biogenic silts. All along the section, clay layers alternate with layers of diatomaceous silt. Studies of the section revealed that sediments accumulated under deep water during all the period of their deposition; no hiatuses or unconformities were found. Studies done in Russia, USA, Japan and Germany gave results that are consistent with each other. Correlation of magnetic properties with the palaeomagnetic scale revealed that the age of sediment at 200 m is 5 My. Mean sediment accumulation rate was constant and equaled 4 cm per 1 ky, Rhythmic structure of the sediments consisting of layers of diatom-barren clays and diatomaceous silts is due to cyclic changes of cold and warm climates.
In order to characterise Lake Baikal sedimentary responses to global climatic changes that may be recorded in marine sediments, we compared our paleomagnetically dated climate-proxy record from Lake Baikal with benthic and plankontic δ18O curves of ODP Site 983, a site close to ODP Site 984. The neighbouring site was chosen for comparison because although the quality of the ODP Site 984 paleomagnetic record is high, its δ18O records are of lower quality than those of ODP Site 983. Synchronous paleomagnetic variations observed in ODP Sites 983 and 984 sediments (Fig. 10) show that the premise of our age model based on paleomagnetic correlation is identical, if the reference curve used for correlation is from ODP Site 983. We can, therefore, compare climatic records from ODP site 983 and Lake Baikal. The climatic proxy used for Lake Baikal sediment is the HIRM record since it displays the detrital input variations (Peck et al., 1994).
Paleointensity versus age of all the sedimentary sequences of the present study, of the synthetic curve resulting from its compilation from other curves, and of the reference curve from ODP Site 984 (Channell, 1999). For the compilation, data have been averaged using a sliding window of 2 ka (the variance is marked by the grey shadow). Dashed lines show some of the correlations. The grey lines show the location of the low paleointensities related to geomagnetic excursions. Note that the lowest paleointensities in the time span of Blake are at c. 129 ka. (see Fig.11)
Palaeomagnetism was the method used for dating sediments older than the time span covered by AMS 14C dating. Geomagnetic palaeointensities recorded in Lake Baikal sediments were tuned to a reference curve (the record from ODP Site 984, Channell, 1999) whose chronology is well constrained (Demory et al., 2005a-this volume and Demory et al., 2005b-this volume). The palaeointensity record from ODP Site 984 is of high quality, is well dated and covers the time span of the present study. Anchored by a geomagnetic excursion (the Iceland basin event, dated at 186–189 ka according to Channell et al. (1997)), this age model is constrained by 55 correlation points for a time span of ca. 200 ky. The age models for both core sections in the interval 100–150 ky are shown in Fig. 2.
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.
At both sites, the lowest dry bulk density values (ca. 0.40 g cm−3) correspond to intervals with high diatom concentrations and high sediment accumulation rate. By contrast, the top and bottom of the sections analysed, rich in clay minerals, have high dry bulk density but low diatom concentration and sedimentation rate (Fig. 3).
The diatom succession at Academician Ridge is similar to the one from Continent Ridge and the two records, despite having very different sampling resolution, can be easily compared on the basis of their main floristic changes (Fig. 6). BVAR at Academician Ridge is about half that of Continent Ridge. There are also marked differences in the relative abundance of some taxa. At Academician Ridge, S. grandis and A. baicalensis are more abundant and S. formosus and C. sp. cf. operculata are less abundant than at Continent Ridge. The large peak in vegetative cells of A. skvortzowii found at Continent Ridge (DAZ 2) is absent at Academician Ridge, and is the most striking difference between the two records.
C/N mass ratios remain constant throughout MIS 3 and into MIS 2, with values between 6.3 and 8.9, indicating no significant terrestrial input of organic matter (Fig. 3). Low %TOC values during the interstadial increase from 0.4 to 0.7 between 57.8 and 43.7 kyr BP with a concurrent gradual increase in δ13C(organic) amid oscillations between −23.2‰ and −26.1‰ (Fig. 3). %TOC falls to 0.4 between 40.9 and 39.4 kyr BP whereas δ13C(organic) remains high at c. 24‰ with a peak value of −23.6‰ at 39.4 kyr BP. The subsequent two-stage increase in %TOC from 39 to 37.9 kyr BP and between 37.3 and 36.9 kyr BP is marked by a period of δ13C(organic) lowering to c. −26.6‰ before δ13C(organic) increases after 37.9 kyr BP to −24.8‰, values comparable to those prior to the %TOC decline at 40.9 kyr BP.
We established a mastercurve “Baikal 200” of relative paleointensity, which represents a new synthetic paleomagnetic archive for Central Eurasia. The synthetic record is composed of mean values of the six records with respect to a sliding time window of 2 ka. This compilation has been restricted to the last 200 ka in order to maintain a representative population of points (between 2 and 68). However, we present this synthetic record together with individual records and the reference paleointensity curve (Fig. 11) for the following reasons: – Each relative paleointensity record has a different resolution (e.g., sedimentation rates in CON 01-605-3 are five times higher than in VER 98-1-14). During stack procedure, smoothing of the data had the effect of lowering the resolution of the paleomagnetic information. – This stack does not provide more information on timing of the geodynamo changes since the records are tuned to ODP Site 984.
S. grandis is by far the species that contributes the most to the total biovolume accumulation rate with a peak value just above 4×106 μm3 cm−2 year−1. Its relative contribution is over 50% in most samples (Fig. 5). Only taxa such as S. carconeiformis, C. ornata and the vegetative cells of A. skvortzowii contribute significantly (with maximum around 0.6×106 μm3 cm−2 year−1). Other taxa, despite having large relative percentages in parts of the interglacial contribute little to the total BVAR. Biovolumes for benthic taxa were not calculated but their contribution to the total biovolume accumulation rate can be considered negligible as most of these taxa are of small size and the intervals in which they dominate in relative percentages correspond to low diatom valve accumulation rate.