Other language confidence: 0.8200584969366485
In order to get a complete geochemical signature, 14 P-rich concretions, chosen among the different cores, were acid digested (Table 3a and Table 3b). In a clean laboratory, 1.7 to 36 mg of concretions were digested overnight in a concentrated mixture of Suprapur acid (3 ml HCl/2 ml HNO3/1 ml HF) at 90 °C in sealed Teflon beakers. After evaporation to dryness, the residue was dissolved in 2.5 ml of 2% HNO3 Suprapur and diluted to 12 ml with Milli-Q water. During the same procedure, we have also dissolved and analysed, for comparison, a pure vivianite from Anlua, Cameroon (tubular crystals, MRAC collection).
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.
The water content was determined from samples retrieved with cylindrical syringe samplers, weighed wet and weighed again after freeze-drying. The clay-rich layers are characterised by a low, the diatomaceous layers are characterised by a high level of water.
The water content was determined from samples retrieved with cylindrical syringe samplers, weighed wet and weighed again after freeze-drying. The clay-rich layers are characterised by a low, the diatomaceous layers are characterised by a high level of water.
The sediment cores were sampled with a resolution of 10 cm, resulting in a total of 290 samples (Table 2). Around 250 mg of dried sediments was mechanically crushed through 100–200-μm mesh then processed by an alkaline digestion (Lithium meta-borate) in a Pt crucible at 1000 °C for 1 h. The residue was dissolved over night in a nitric acid matrix and then major and minor elements (Al, Ba, Ca, Fe, K, Mg, Mn, Na, P, S, Si, Sr, Ti) were analysed by Atomic Emission Spectrometry ICP-AES (Thermo Optek Iris Advantage, Royal Museum for Central Africa, Tervuren, Belgium). Y and Au internal standards were used to correct for instrumental drift. For both trace and major elements analyses, external calibrations were performed using artificial standard solutions and dissolved mineralised natural rock standard (e.g., BHVO-1, DWA, CCH-1 SGR-1, JGB-1).
In selected intervals, we measured titanium and iron contents in parallel to rock magnetic parameters (Fig. 9). Titanium content is a good reflection of detrital input since minerals containing titanium are not very sensitive to dissolution. Iron, however, is rather mobile and involved in the redox history of highly porous sediments: the spike of iron observed on top of the sedimentary column (Fig. 9A) marks the redox front. We observed a strong similarity between the titanium and HIRM curves: the detrital input decreases from the late glacial to the Holocene. In ancient sediments, HIRM and titanium display similar variations with high values in glacials and low values in interglacials (Fig. 9B).
No significant HIRM change is observed at the transition between oxidising and reducing conditions in the sediment (Fig. 9A). This implies that HIRM is not affected by redox conditions and further confirms that the “hard” magnetic mineral content is the best tracer of detrital input (Peck et al., 1994). On the other hand, the S-ratio seems to be related to the redox conditions in the sediment (see Section 7.2). The ARM has also to be considered with caution as it is mainly influenced by the ferrimagnetic contribution, which is itself influenced by post depositional processes. This is seen in Fig. 9 where ARM variations are partly influenced by S-ratio variations.
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.