Elevated uranium concentrations in Lake Baikal sediments: Burial and early diagenesis

Keywords

• Uranium diagenesis;
• Lake Baikal;
• Trace elements;
• Fe- and Mn-oxides;
• Biogeochemical cycling in lacustrine surface sediments and pore water

1. Introduction

On Earth's surface environment, uranium (U), a radioactive metal from the actinide series, primarily occurs in two redox states with distinctly different physico-chemical characteristics. Under oxic conditions, such as in modern seawater, U is predominantly present as soluble and stable U(VI) carbonate complex (UO₂(CO₃)₃^4 −) with a concentration of around 3.3 μg l^− 1 (14 nM) and exhibits conservative behaviour with a high oceanic residence time of 0.3 to 0.6 million years (Ku et al., 1977 and Dunk et al., 2002). U concentrations in freshwater lakes are usually about one or two orders of magnitude lower than in the ocean (e.g. Falkner et al., 1997, Nagao et al., 2002, Chappaz et al., 2010 and Li et al., 2011), depending on the lithology of the lake's watershed (Palmer and Edmond, 1993, Windom et al., 2000, Dunk et al., 2002 and Andersen et al., 2016). The largest single sink for U in the ocean is by diffusion across the sediment-water interface into organic-rich, oxygen-depleted sediments (Anderson et al., 1989, Klinkhammer and Palmer, 1991, Calvert and Pedersen, 1993 and Morford and Emerson, 1999). Here it can be reduced to U(IV) at sedimentary redox conditions near those for the conversion of Fe(III) to Fe(II) (Ginder-Vogel et al., 2006), and precipitate either as uraninite (UO₂), U₃O₇ or U₃O₈ (e.g. Klinkhammer and Palmer, 1991 and Crusius et al., 1996) or as a monomeric U(IV) species that readily associates with phosphate or carbonate functional groups (e.g. Bargar et al., 2013 and Morin et al., 2016). Thereby, microbially mediated reduction of aqueous U(VI) to sparingly soluble U(IV) is considered to be the most important process controlling the biogeochemical cycling of U in the subsurface (Williams et al., 2013 and Bargar et al., 2013). In Phanerozoic times, where significantly higher atmospheric and oceanic O₂ concentrations allowed for a more pronounced role of oxygen in the U cycle, this may have been responsible for U contents in black shale of well above 100 μg g^− 1 (Partin et al., 2013). Redox transitions influence the ²³⁸U/²³⁵U isotope ratio, which makes sedimentary U an important redox tracer due to the distinctly different geochemical behaviour of the reduced and oxidized species, and the long residence time in the ocean (Brennecka et al., 2011 and Andersen et al., 2016). A detailed understanding of the processes involved in the accumulation of U in the sediment and during early diagenesis is thus required.

In addition, uranium is associated with particulate matter in the water column, termed particulate non-lithogenic U (PNU; Anderson, 1982), either in the form of organic complexes (Knauss and Ku, 1983 and Hirose and Sugimura, 1991) or co-precipitated with Mn- and Fe-oxides (Duff et al., 2002, Dunk et al., 2002 and Cumberland et al., 2016). In well-oxygenated marine and lacustrine sediments, U concentrations typically remain in the range found within the Earth's crust, i.e. around 2.8 μg g^− 1 (Taylor and McLennan, 1995). However, U concentrations of up to 250 μg g^− 1 may be present in phosphatic rocks, and fine-grained sedimentary rocks typically contain more U than coarser-grained rocks due to the presence of clays and organic matter to which U adsorbs (e.g. Bird, 2012).

Oligotrophic (nutrient poor) and pervasively oxic Lake Baikal (Siberia, Russia) is the largest lake on Earth by volume and harbours about 20% of the world's available freshwater resources. Its watershed is dominated by granitoid rocks (Fagel et al., 2007), which, compared to other contemporary lacustrine environments, contribute to a comparatively elevated average U concentration of 420 ng l^− 1 in the water column (Falkner et al., 1991 and Falkner et al., 1997). However, despite high O₂ concentrations down to the lake bottom and into the top sediment layer, it has been shown that sedimentary U contents can reach > 20 μg g^− 1 (Petrov et al., 1999, Zhmodik et al., 2003, Zhmodik et al., 2005, Chebykin et al., 2004, Chebykin et al., 2007 and Goldberg et al., 2005), i.e. about 5 times higher than the rocks of Lake Baikal's watershed (Jahn et al., 2009 and Litvinovsky et al., 2011). These U concentrations are also far higher than observed in other freshwater lake sediments where they typically range from 0.5 to 5 μg g^− 1 (e.g. Markich, 2002). Mass balance calculations result in a U residence time of about 160 years in the water column, compared to 400 years for Lake Baikal's water mass (Falkner et al., 1997), suggesting that a significant portion of U is removed into, and permanently buried within the sediment. This is striking when considering that the residence time of U in the ocean exceeds the one for its water by about two magnitudes.

In this manuscript, we investigate and discuss the transport and settling of U in the sediment, and processes of remobilization and final sequestration during early sediment diagenesis leading to the high sediment content of U. The present study represents the third project on the diagenetic evolution and elemental composition of Lake Baikal sediments and pore water based on the same sediment cores (Och et al., 2012 and Och et al., 2014).