Other language confidence: 0.7970869632804093
The vertical distribution of organisms in the sediment indicates that animals can be present as deep as 15 cm although at very low abundance at such depths (Fig. 4, Fig. 5 and Fig. 6). Oligochaetes and nematods are the only groups able to deeply penetrate into the sediment at significant densities (Fig. 4) in contrast to all other groups, which stay closer to the sediment surface. Maximal densities however seem to shift to the sediment surface with increasing bathymetric depth, as suggested in Fig. 5 and Fig. 6, so that all animal groups are more concentrated near the surface in the deepest parts of Lake Baikal. In such case, the depth of sediment mixing due to bioturbation appears to decrease with increasing bathymetric depth (Fig. 2b).
The vertical distribution of organisms in the sediment indicates that animals can be present as deep as 15 cm although at very low abundance at such depths (Fig. 4, Fig. 5 and Fig. 6). Oligochaetes and nematods are the only groups able to deeply penetrate into the sediment at significant densities (Fig. 4) in contrast to all other groups, which stay closer to the sediment surface. Maximal densities however seem to shift to the sediment surface with increasing bathymetric depth, as suggested in Fig. 5 and Fig. 6, so that all animal groups are more concentrated near the surface in the deepest parts of Lake Baikal. In such case, the depth of sediment mixing due to bioturbation appears to decrease with increasing bathymetric depth (Fig. 2b).
The vertical distribution of organisms in the sediment indicates that animals can be present as deep as 15 cm although at very low abundance at such depths (Fig. 4, Fig. 5 and Fig. 6). Oligochaetes and nematods are the only groups able to deeply penetrate into the sediment at significant densities (Fig. 4) in contrast to all other groups, which stay closer to the sediment surface. Maximal densities however seem to shift to the sediment surface with increasing bathymetric depth, as suggested in Fig. 5 and Fig. 6, so that all animal groups are more concentrated near the surface in the deepest parts of Lake Baikal. In such case, the depth of sediment mixing due to bioturbation appears to decrease with increasing bathymetric depth (Fig. 2b).
In all abyssal stations, densities are never over an average of c. 3100 individuals m−2 (Fig. 3, Table 1). In contrast, the shallow station (CON01-427, Posolskoe Bank) harbours the highest observed densities (oligochaetes reach densities as high as 13573 individuals m−2 on average). Gammarids are present in this latter station at 128 m deep, while they are absent from all deep stations. The presence of some groups is anecdotal, such as Hydrachnidia (one specimen in a core at 388 m and two specimens in a core at 625 m) and chironomid larvae (two larvae in a core at 625 m). Interestingly, the two deepest Vydrino cores (CON01-105-7, 600 m, and CON01-106-3, 700 m) are virtually free from animals, suggesting that these stations are perhaps the best choice for the study of stratigraphy and climate proxies.
Water content and dry bulk density of pilot core to CON01-603-2
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.
Values of measured chlorophyll (HPLC=High Pressure Liquid Chromatography) are the mean concentrations of each sampling point from 5 to 30 m depth. For the OC2 chl-a calculations, the least clouded acquisitions in 2001 (2001/07/19) and 2002 (2002/07/20) were chosen. Note the considerable chl-a overestimation caused by the influences of terrigenous input in case 2 waters.