A quantitative diatom analysis was carried out on a sediment core from the small infilled lake basin of Marcacocha and was compared with other proxy data (e.g., pollen and plant macroremains) in order to reconstruct environmental changes between 800 and 1850 AD in the Cuzco region of SE Peru. Five stratigraphical zones were distinguished by means of constrained cluster analysis. Very few diatoms were present between 790 and 1070 AD, possibly reflecting dry and cool conditions, at a time when anthropogenic impact was limited around the basin. These conditions were probably interrupted by several flooding events, causing short-term macrophyte-dominated stages in the lake. The transition at ca. 1070 AD represented the most prominent change in the diatom record and corresponded with a marked shift towards higher temperatures, as deduced from plant macroremains and the pollen record. This transition further coincided with a sudden climate shift recorded elsewhere in tropical South America. Between 1070 and 1650 AD, our proxy data indicate the existence of a stable, shallow lake as evidenced by increased diatom production and high concentrations of charophyte oospores. The transition in the diatom data at ca. 1650 AD, with peaks in diatom abundance centered on 1700 AD, lags the start of the "Little Ice Age" (LIA; around 1490–1530 AD), though matches a major cool and dry phase of the LIA at the end of the 17th and early 18th centuries as recorded in Peruvian ice cores. This could be caused by a threshold that was passed after the lake level had lowered sufficiently, due to a cooling climate causing a drier lake environment, combined with infilling processes. Further transitions in the diatom community may be interpreted less in terms of climatic change, and more as increasing sensitivity to local environmental changes, such as lake level decrease due to infilling processes and increased anthropogenic activity in the catchment area. The final stage of infilling occurred after ca. 1845 AD, with complete colonization of the remaining lake surface by Juncaceae and the ongoing accumulation of peat. |
