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

Abstract

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 210 Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5-25 cm. The linear sedimentation rates (LSRs) calculated from 14 C ages do not match the LSR calculated from unsupported 210 Pb 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., 210 Pb and 137 Cs. 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 14 C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the 210 Pb method. (3) A bioturbation mechanism is not necessary to explain the 14 C 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.

Understanding the dynamics and contamination of an urban aquifer system using groundwater age (14C, 3H, CFCs) and chemistry

“…In contrast, the δ 13 C values of these samples (−9.0 and −8.6‰, respectively) indicate that the δ 13 C soilCO2 is approximately −18‰ under the given pH conditions (7.1 and 6.6 pH units). This value represents a combination of C 3 and C 4 organic material, which is consistent with the mixed vegetation appearing in recharge areas, namely, grassland and shrubland with Quercus and Pinus as their major taxa, according to Zárate‐del‐Valle, Ramírez‐Sánchez, Fernex, Simoneit, and Israde‐Alcántara ().…”