Carbon, sulphur, and mercury geochemistry in a crater lake in the Siberian Traps

In the Siberian Traps Large igneous province (LIP), the emplacement of magma in sedimentary strata led to explosive release of volatiles and the formation of large pipe structures with explosion craters. Of the hundreds of pipes present, several still have preserved crater lake sediments, representing the only known end-Permian sedimentary archive proximal to the Siberian Traps. Here we present new data from a core from the best studied crater lake section, the October pipe/crater located west of Bratsk in East Siberia. The S26 core contains 505 meters of sediments and in order to investigate the geochemical processes in the lake and the possible role of brine influx from the breccia pipe below, we have analysed carbon isotopes, the mercury content, δ³⁴S and δ⁵⁶Fe in pyrite and δ⁵⁶Fe in magnetite, from the breccia pipe and crater sediments. In addition, reference samples from two other breccia pipes and relevant sedimentary strata from the Tunguska Basin were analysed for comparison. Our previous work has shown that the lake was saline, stratified and anoxic/euxinic, with up to 4 wt.% TOC, and abundant framboidal pyrite. The sediments are dominated by sandstone and siltstone, sourced from the material ejected during the pipe formation, and have not experienced heating above ca. 50 °C. The basal deposits are coarse and contain reworked magnetite-apatite ore that originally precipitated from hydrothermal fluids within the upper parts of the pipe. The sediments are calcite-cemented and the volcanic minerals altered to clays and zeolites, with occasional oxidized zones showing that the water level fluctuated, in accordance with rapid subsidence resulting from dissolution of deeper-seated Cambrian evaporites. New results show a marked shift in the isotope systems from the breccia pipe and into the basal lake deposits. The δ³⁴S shifts from background sedimentary values (+20-30‰) in the pipe breccia, to -7-0‰ in the lower half of the lake sediments followed by an increase to +20 ‰ towards the top of the lake. The trend is evident in the δ⁵⁶Fe in magnetite as well, with a 0.4 ‰ negative shift from the breccia and into the lake sediments, followed by an upward increase in the stratigraphy. We suggest that basinal brines rich in iron and sulphate were partly reduced in the stratified lake, leading to pyrite precipitation and isotope fractionation. The data shows the interplay between lake processes and fluid seepage from a hydrothermal system, making these deposits unique for understanding the consequences of LIP formation.