RADIOCARBON AGE INVERSIONS: SOURCE AND CAUSES. EXAMPLE LAKE CHAPALA, WESTERN MEXICO, LATE HOLOCENE SEDIMENTS

The results of eight AMS C datings of Lake Chapala core sediments (site T46: 20° 14.45’N; 103°15.43’W; 1515 m asml) are presented and both the age inversions (AI) observed and their age progression are discussed. Lake Chapala is in the Citala Rift (CR) of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction (JTJ), which is characterized by active volcanism (Ceboruco and Volcan de Fuego) and tectonism (1995, earthquake M=8). The JTJ neotectonics are due to the motion of the Cocos Plate relative to the North America Plate. Paleo- and active hydrothermal activity are typical of CR, particularly the presence of hydrothermal petroleum manifestation.

The linear sedimentation rates (LSR) 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 Lake Chapala occurred by a combination of: (a) the existence of in-shore outcrops of hydrothermal petroleum with ages older than 40 ka 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) The ¹⁴C ages of shallow lake sediments are one order of magnitude greater because of this phenomenon. (3) A bioturbation mechanism is not necessary to explain the ¹⁴C age AI in the top 70 cm and from 110-150 cm depth of the sediments.