Insolation, moisture balance and climate change on the South American Altiplano since the Last Glacial Maximum

Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt. %Corg, %CaCO₃, and % biogenic silica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the δ¹³C of modern lacustrine organic sources and the δ¹³C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000-13,500 yr BP) and lake water was fresh under the associated outflow conditions. The Pleistocene/Holocene transition (13,500-7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP. Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (< 12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system.