Radiocarbon age inversions and progression: source and causes in Late Holocene sediments from Lake Chapala, western Mexico

摘要

The results of eight radiocarbon datings of Lake Chapala sediments (site T46) are presented, the age inversions (AI) observed and their age progression discussed. As deduced from some AIs and the 210Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5–25 cm. The linear sedimentation rates (LSRs) calculated from 14C ages do not match the LSR calculated from unsupported 210Pb activity for the upper sediments. This demonstrates the usefulness of dating sediments with complementary radiometric techniques such as short-lived isotope counting (SLIC), i.e., 210Pb and 137Cs. This approach leads to the following conclusions: (1) The incorporation of detrital particles with ancient carbon into the sedimentary column of the lake occurred by a combination of: (a) the presence of outcrops of hydrothermal petroleum with ages 40 ka (ka = thousands of years) in the lake, and (b) mass transport due to the presence of two elongated gyre circulation patterns integrated by cyclonic circulation (counterclockwise) in the north portion of the lake and anticyclonic circulation in the southern part. (2) Consequently, the 14C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the 210Pb method. (3) A bioturbation mechanism is not necessary to explain the 14C AI in the top 70 cm and from 110 to 150 cm depth of the sediments. (4) According to the biological proxies data for the last 600 years B.P., the paleoclimate at Lake Chapala has changed from sub-humid to dry environmental conditions, and eutrophication has increased over the past 100 years due to local input from ongoing agricultural activities.