Background composition of pore waters in Lake Baikal bottom sediments
Introduction
At present, many studies are focused on the subaquatic discharge of oil, gas and deep waters occurring at the bottom of Lake Baikal (Matveeva et al., 2003, Klerkx et al., 2003, Granina et al., 2001, Granina et al., 2007, Zemskaya et al., 2010, Zemskaya et al., 2015, Minami et al., 2010, Pogodaeva et al., 2013). However, there are very few data on the background composition of pore waters in the bottom sediments of Lake Baikal.
Lake Baikal, the world's largest freshwater body, is situated in Central Asia. It was formed as a result of intracontinental rifting (Mats et al., 2001). The basin, with a maximal depth of 1642 m, is situated in the center of the tectonically active rift zone and is filled with sediments (up to 7.5 km thick), the most ancient of which are approximately of Oligocene age (Hutchinson et al., 1992). The lake is morphologically divided into three deepwater basins (Southern Baikal, Central Baikal, and Northern Baikal), which are separated from one another by two underwater elevations: the Buguldeika-Selenga Isthmus and the Academichesky Ridge (Atlas of Lake Baikal, 1993).
The water column of Lake Baikal is an inertial system. It takes Baikal tributaries approximately 400 years to completely renew the lake waters. The time necessary for the surface waters to penetrate into the water column center (i.e., to a depth of 300 m from the surface or 100 m from the bottom) is approximately one decade (Weiss et al., 1991). Horizontal water exchange is caused by cyclonic macrocirculations in all layers within each basin. Moreover, compensated water exchange occurs between neighboring basins (Shimaraev et al., 1995, Shimaraev et al., 1996). Compared to the waters of shallow fresh lakes, each basin of Lake Baikal has an ion composition that is stable in time and space (Falkner et al., 1991, Grachev et al., 2004). A large amount of oxygen (9.6–12.8 mg/L) is observed at all depths of the lake, including the bottom water (Shimaraev et al., 1996, Killworth et al., 1996). Oxygen also penetrates into the bottom sediments, and a layer of oxidized sediment covers the entire lake bottom (Atlas of Lake Baikal, 1993).
The sediments in the three deepwater basins of Southern, Central and Northern Baikal have identical chemical composition as a result of the homogenization of fine fractions of incoming terrigenous material carried by steady currents (Gvozdkov, 1998). There are very few data on the composition of the liquid phase (pore water) of bottom sediments of Lake Baikal. Moreover, these data were mostly collected in anomalous areas and the Selenga River delta. According to these data, the pore waters of bottom sediments show heterogeneity in their chemical composition within the lake bottom area and are bicarbonate-sulfate, sulfate and chloride waters (Mizandrontsev, 1975, Granina et al., 2001, Granina et al., 2007).
This paper presents the results of a study on the background chemical composition of pore waters from bottom sediments in Lake Baikal to determine the extent of variability in pore water composition in this great lake. Equations of chemical composition variation as a function of sediment depth are also inferred from the obtained data to examine diagenetic changes with depth.
Section snippets
Sampling
Bottom sediments were sampled from areas with regular sedimentation rates within the pelagic zones of the three basins of Lake Baikal (Fig. 1, Table 1), during expeditions between 2003 and 2006. Four cores ranging from 45 to 200 cm in length were collected in Southern Baikal (up to depth of 1480 m) at sites St2GC 2003, St97GC 2003, St1GC 2005, and St1BC 2005. Cores of 100 and 200 cm length were sampled in Central Baikal (up to 1600 m) at sites St81GC 2003 and StGC16 2006, respectively. In Northern
Lithology of bottom sediments
Bottom sediments sampled from deepwater sites in Southern, Central, and Northern Baikal were gray, homogeneous, diatomaceous silts, with distinct horizontal layering (black hydrotroilite intercalations). Turbidites were either absent in the sediments or had minimal thickness (< 5 cm). The uppermost layer of all cores (5–15 cm) was oxidized and of a reddish-yellow-brown color. Fe-Mn crusts were found on the boundary between the upper oxidized and lower reduced sediments. The position of the
Discussion
Similar chemical compositions were measured in the pore waters of bottom sediments from 10 central sites in the three basins of Lake Baikal and 16 sites from other areas of the lake. The concentration profiles are of the same type at each site in the three basins (Fig. 8).
In deepwater areas with more uniform sedimentation rates of the three basins of Lake Baikal, the pore waters have a similar chemical composition. This is attributed to stable chemical parameters of the water column in both
Conclusions
The pore waters of Lake Baikal have uniform salt composition across the lake floor. Like the lake waters, the background pore waters of Lake Baikal have a calcium bicarbonate composition with low mineralization. The total concentrations of ions in the pore waters increases with depth during diagenetic transformations due to the increase in the concentration of bicarbonate and calcium ions; however, this occurs without changing the class and group of the waters. Equations of background
Acknowledgments
We are very grateful to Irina Tomberg for the nutrients analysis. The research was carried out within the state assignment of FASO of Russia № 0345–2016–0007 and № 0345–2016–0008. The service of the deepwater manned Mir submersible was organized and financially supported by Fund of the Conservation of Lake Baikal, and the Metropol Company.
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