Loss of apatite caused irreversible early-Holocene lake acidification

Abstract

Lakes created by retreating ice at the end of the last glaciation underwent rapid acidification during the first few thousand years of their existence, a phenomenon attributed in part to progressive leaching of soil bases since it was discovered more than 80 yeas ago. Though a role for leaching is still acknowledged, the most recent studies see this as subordinate to the effects of biological and climatic changes initiated by deglaciation, chiefly primary vegetation succession and species immigration. Until now there has been no quantitative test of the relative importance of these different mechanisms. Here I use geochemical modelling of runoff acidity to show that the extent and timing of early Holocene lake acidification in eight published palaeoecological records can be explained by leaching of the calcium phosphate mineral apatite from the granitic till soils of their catchments, at a rate controlled by simple dissolution kinetic factors. This shows that a single progressive abiotic process is capable of explaining a natural phenomenon that is currently attributed to complex biological and climatic interactions. The finding has important implications for our understanding of long-term lake ecosystem development because the mechanism is inherently irreversible, in contrast to the alternative ecological and climatic mechanisms which are not. Furthermore, the finding reinforces the view that long-term ecosystem modelling cannot safely neglect nutrient limitation, as is currently the practice in widely used global dynamic vegetation models.