Can silicon isotopes be used to assess anthropogenic impacts and nutrient utilisation in Lake Baikal, Siberia?

Abstract:

Silicon isotope geochemistry (²⁸Si, ²⁹Si, ³⁰Si) represents a growing field in Earth Sciences providing information to constrain and understand biogeochemical cycling on land and in oceans. Here we present records of δ³⁰Si (³⁰Si/²⁸Si) from the Lake Baikal drainage basin in central Siberia to understand silicon cycling through the dominant river tributaries and into Lake Baikal itself, the world’s deepest and most voluminous lake containing one fifth of all freshwater not stored in glaciers and ice caps.

Waters were collected along an upstream transect for the five dominant Lake Baikal inflows as well as from the Selenga Delta which account for >50% of the annual riverine flow to the lake. Samples for dissolved silicon (DSi) concentrations and silicon isotopic signatures (δ³⁰Si_DSi) were filtered and acidified in the field with isotopic analyses conducted on a Neptune + Multi-Collector ICP-MS using wet plasma mode with Mg doping of samples and standard-sample-standard bracketing. Analytical reproducibility is 0.11‰ (2σ) and blanks are <1% of signal intensity. The DSi concentration of all river inflow samples varies from c. 2.5-6.3 ppm, which contrasts with concentrations of < 1ppm from the Angara River, the lake's only outflow. Combined with river δ³⁰Si_DSi values of c. + 0.94 to +1.52‰ and lake water and outflow values of c. +2.50‰, the results show significant (up to 60%) biological utilisation of silicon entering the lake.

With the Lake Baikal having been designated a World Heritage Site since 1996, due to its high level of biological endemicity, the work in combination with sediment cores from the lake provides an insight into the transportation and fate of silicon in the basin and an assessment of how climate change and expanding anthropogenic activities in the region including urbanisation, deforestation, agriculture and mining have impacted biogeochemical cycling.