Residence time and chemical weathering of a dark clayey (‘old blue’) till in east-central Sweden

Dark clayey (‘old blue’) tills occur at multiple locations throughout central Sweden. They have drawn scientific interest due to their over-consolidated state from ice sheet over-riding and high clay contents. These tills are overlain by younger glacial sediments and therefore predate the last deglaciation, but their absolute ages remain uncertain. This study evaluates the residence time and chemical weathering of a dark clayey till at Forsmark, east-central Sweden, using meteoric 10 Be and major oxide and trace element geochemistry, in the context of assessments of post closure safety of a planned repository for long-lived high-level spent nuclear fuel. The Forsmark landscape in the late Quaternary has periodically been a subglacial, subaquatic, or subaerial environment. Because 10 Be has been supplied to sediments in each of these environments, we also measured meteoric 10 Be in younger tills at Forsmark and tills from three sites above the highest postglacial shoreline, including dark clayey till at Bograngen. We then evaluated the 10 Be inventory of the Forsmark dark clayey till in the context of two end-member till-deposition scenarios; early last ice sheet advance (MIS 2-3) or before MIS 3. The most likely scenario is that the residence period for the dark clayey till at Forsmark starts in early MIS 2-late MIS 3. In this scenario, the advancing last ice sheet sourced fine-grained till components from sedimentary rocks and clay-rich sediments deposited in the Bothnian Sea during the MIS 3 interstadial. These sediments were then compacted as ice thickened to its Last Glacial Maximum configuration and subsequently overlain by a deglacial sandy-silty till. Dark clayey tills may not represent one specific date prior to the last deglaciation. Rather, they are a distinct sediment facies sourced from water-lain deposits, which accumulated during interstadial or interglacial conditions, and fine-grained sedimentary rocks, and were emplaced and compacted during ice sheet advances. All sediments studied at Forsmark and Bograngen display minimal chemical weathering. This conclusion is best elucidated from ACNK diagrams, where the sediment compositions approximate those of parent rocks, and is supported by a lack of indications of leaching or enrichment of elements contained in major oxides or of REE-Y along vertical sediment profiles. Minimal chemical weathering in these sediments is consistent with young ages (MIS 1 – late MIS 3) but is not in itself age diagnostic. Sediments at the remaining two sites from above the highest shoreline have the most 10 Be and show slight illitization of plagioclase, which together provide evidence that these samples have a longer (pre-Holocene) history of subaerial exposure. Our interpretation that the dark clayey till at Forsmark dates from early in the last period of ice cover indicates active erosion of bedrock and sediments by the ice sheet. In the context of performing assessments of post-closure repository safety over future glacial cycles, the Forsmark dark clayey till is therefore an inappropriate analogue for soil development through multiple ice-free interglacial/interstadial periods and for the long-term evolution of groundwater buffering capacity, which is governed by bicarbonate generated through chemical weathering.