New paper: Soil mineral depletion drives early Holocene lake acidification

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By Richard Chiverrell

Every year, about May, student examination season begins, teaching winds down and the thoughts of Physical Geographers turn to fieldwork, travel, seeing the world, all preferably with sediment corer in hand. In 2007 an intrepid team of John Boyle, Rich Chiverrell, Andy Plater and Oliver Boyle (John’s brother) set sail, more or less literally for Norway, on the Newcastle to Bergen ferry. Our travels were to test a numerical model developed by John Boyle that suggested that changes in lake and terrestrial ecosystems, the acidification of surface waters during the early postglacial period, could be explained by the weathering and depletion of the mineral apatite from soils. The modelling was published in a brilliantly titled paper ‘Loss of apatite caused irreversible early-Holocene lake acidification’ in the journal Holocene. We docked in Bergen around 6pm and then drove for 12 hours to north of Nordfjord to the island of Vågsøy, avoiding reindeer, sleep, horrendous sheeting rain and sadly all the Nordfjord ferries (closed for the night), to sample the sediments of Kråkenes Lake. We arrived around 6am and grabbed 4-5 hours sleep in a delightful sea front cottage, before sampling a 8.96-m-long core from the terrestrialized peat bog that forms the southwestern arm of the lake using a 1-m-long, 70-mmdiameter Russian corer in 10 overlapping drives. The coring was completed in 4 hours, a pretty remarkable 2 days. After surviving on meagre rations of vacuum packed gammon and Uncle Ben’s stir fry sauce (note other brands exist) prompted by the lack of fishing prowess displayed by members of the team (they did catch seaweed) and Sunday shopping hours in Norway (i.e. closed), we had two days to explore the catchment and the seaboard of western Norway on our journey south. Thankfully the shops opened Monday and more fitting food stocks were procured.

Twelve months later we won a Natural Environment Research Council Grant to carry out the first validation of John’s numerical model, principally testing whether the changes in soil primary mineral concentrations after the end of the last Ice Age deplete at the same rate and extent as the acidification observed in many postglacial and formerly glaciated lakes. Easily weathered minerals in this case apatite (source of mineral phosphorus) appear to be quickly released to soils and water courses producing a base-rich phase seen in lake sediment records with an associated higher ecological productivity early after deglaciation, and this phase ends as the supply of base declines. The acidification that follows had previously been attributed to climate change and plant-soil community succession. The findings of this research, with the laboratory work completed by research assistant Dr Ian Thrasher, have just been published in the journal Geology in a paper called ‘Soil mineral depletion drives early Holocene lake acidification’. Hopefully you will be convinced by our case! The explanation in this paper has some wide ranging implications, mainly arising from that this acidification process is difficult to reverse, unless you completely refresh land surfaces e.g. a new glaciation, producing new terrain by volcanic eruption or really really large-scale turn-over of soils by erosion. We will be following these implications up with further papers on phosphorus dynamics and other implications in the coming months.

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