Study of Jinchuan Mire in NE China I: AMS 14C, 210Pb and 137Cs dating on peat cores

Abstract

This paper presents dating results of AMS 14C, 210Pb and 137Cs methods on two young peat profiles from Jinchuan Mire in southeast Jilin of China. JC1 (50-cm long) from a hummock of the mire covers 1962–2007 CE with an accumulation rate of 1.12 cm/y, whereas JCA (92-cm long) from a lawn site involves a 1000-year record with a mean sedimentation of 0.09 cm/y. Comparison of the dating results of the last 100-y part of the peat cores, chronologies based on the AMS 14C dating are most reliable, because the mobility of 210Pb and 137Cs in peat profiles causes overestimation of depositional rate with the larger errors in the deeper layers. The F14C of peat samples were significantly lower than that of the atmospheric CO2 for the post nuclear bomb period because of three reasons: (1) uptake of old CO2 dissolved in the water by aquatic plants during photosynthesis; (2) long-term mixing of the atmospheric CO2 in multiple year growth of the plants; and (3) perhaps dilution effect caused by strong surface runoff. Among these factors, Factor (1) accounts for the major influence. Hence, a “bomb 14C curve” from a peat profile is necessary for determining post bomb ages. For the pre-nuclear bomb ages, acid-base-acid (ABA) treatment of a peat sample may cause an older-than-true age due to removal of organic acids produced during photosynthesis. This observation calls for detailed investigation of mechanism study and proper pretreatment of peat samples for 14C dating. The large difference of the depositional rates in the same peatland between JC1 and JCA indicates that estimation of carbon storage in a peatland should be based on the database of multiple sites rather than a couple of cores. For the same reason, one should not use a depositional rate either from a core with distance in the same peatland or from an adjacent peatland.