Accelerated phosphorous leaching during abrupt climate transitions in a temperate Atlantic ecosystem in Northwest Spain recorded by stalagmite P/Ca variations

Abstract. In natural ecosystems, phosphorus cycling regulates terrestrial productivity and may respond to climate variations. Seasonal to several year monitoring studies capture the short-term controls on P release but may miss longer term feedbacks. There is an important observational gap of the centennial to millennial scale response of the P cycle to climate oscillations. Cave carbonates such as stalagmites and flowstones, which precipitate from infiltrating groundwater, may record past changes in P loss on these timescales. Here, we examine trends in P/Ca ratios in four coeval stalagmites from coastal caves in NW Iberia during two climate transitions, the Penultimate Glacial Maximum through the Last Interglacial (145 to 118 ky BP) and an intermediate glacial climate state interrupted by an abrupt cooling event of the Greenland Stadial 22 (92 to 80 ky BP). We conduct sensitivity tests with a model to assess the degree to which drip water pH and in-cave drip water chemical evolution could affect the stalagmite P/Ca record. Both during the last deglaciation and during Greenland Stadial 22, we find large (3–10-fold) transient increases in stalagmite P/Ca at the onset of abrupt cooling events and during the rapid recovery from some events. These increases are much larger than can be explained by variations in P incorporation due to drip water pH or in-cave chemical evolution and likely reflect significantly increased drip water P/Ca ratios at the onset and end of abrupt stadial events. Two climatic factors may contribute to this increased leaching. First, soil temperatures may reach minimum values during these transition states, if the temperature minimum leads to increased thickness and duration of snow cover which raises soil temperatures. Minimum winter soil temperature suppresses microbial recycling of P. Second, the transitions into and out of stadial events may feature the highest frequency of freeze-thaw events which change the physical soil structure and lead to stronger spring flushing of P.