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

摘要

This paper presents dating results of AMS C-14, Pb-210 and Cs-137 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 C-14 dating are most reliable, because the mobility of Pb-210 and Cs-137 in peat profiles causes overestimation of depositional rate with the larger errors in the deeper layers. The (FC)-C-14 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 C-14 curve" from a peat profile is necessary for determining post bomb ages. For the pre-nuclear bomb ages, acidbase-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 C-14 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.