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 shift s in the isotopic composition of organic carbon, C/N, wt. %C-org, %CaCO3, a nd % biogenic silica illustrate the dynamic changes in lake level that occu rred 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 dev elop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer f unctions based on the delta C-13 of modern lacustrine organic sources and t he delta C-13 of modern sedimented organic matter from core-tops. Lake leve l was slightly higher than modern during much of the post-LGM (20,000-13,50 0 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 gr adual regression, punctuated by minor trangressions. Following a brief high stand at about 7250 yr BP, lake level dropped rapidly to 85 m below the mod ern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level i ncreased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly betwee n 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 N ino-Southern Oscillation variability, are the most likely driving forces be hind millennial-scale shifts in lake level that reflect regional-scale chan ges in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic sys tem.