Biogeochemical proxy evidence of gradual and muted geolimnological response of Lake Nkunga, Mt. Kenya to climate changes and human influence during the past millennium

Abstract

Lake Nkunga is a crater Lake on the north eastern slopes of Mount Kenya that provides a record of catchment changes covering the last millennium. A multi proxy study was carried out on 89 cm of sediment core retrieved from 20 m near the lake shoreline. The mineralogy, magnetic mineralogy, organic and elemental geochemical proxies indicate a rejuvenating lake during the Medieval Climatic Anomaly with limited sediment supply and sustained by ground water. A progressive response to wetter conditions commences ca. 810 cal yr. BP with an increase in sediment influx that peaked at 500 cal yr. BP, a period that encompasses the start of the Little Ice Age (LIA). The establishment of the present-day maar conditions may have occurred during this period of the LIA. The lake has been relatively stable with declining terrestrial input from 290 cal yr. BP to present. The inferred changes in Lake Nkunga levels from deep to shallow phases are characterized by slow and muted response to both abrupt (e.g. Medieval Climate Anomaly) and sustained and prolonged climate shifts (e.g. Little Ice Age), reflecting the resilience of geolimnological and catchment processes in this lake and its watershed to climatic changes and human influence. This study provides new insights into the utility of biogeochemical proxies from nearshore lake cores in the equatorial east Africa highlands whose responses to extreme weather events are not well understood over the last 1000 years.