Climate and human impact on a meromictic lake during the last 6,000 years (Montcortes Lake, Central Pyrenees, Spain)

Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic MontcortSs Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area similar to 0.1 km(2); z (max) = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS C-14 dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000-3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690-1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770-1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.