18.5kaB.P.以来东北四海龙湾玛珥湖全岩有机碳同位素记录及其古气候环境意义

STABLE CARBON ISOTOPE RECORD OF BULK ORGANIC MATTER FROM THE SIHAILONGWAN MAAR LAKE, NORTHEAST CHINA DURING THE PAST 18.5ka

结合含水量、TOC含量和TOC/TN比值变化曲线,18·5kaB·P·以来的四海龙湾玛珥湖沉积物全岩有机碳同位素组成(δ13CTOC)记录可划分为3个阶段:1)末次冰期晚期(18·5~14·7kaB·P·),δ13CTOC值偏正,变化范围为-29·50‰^-26·18‰,平均值约为-28·10‰;2)末次冰消期(14·7~11·7kaB·P·),δ13CTOC值显著偏负,变化范围为-33·92‰^-28·40‰,平均值约为-31·75‰,在δ13CTOC值变化曲线上表现为一个低谷,但在类似YoungerDryas的冷干事件期间(12·7~11·7kaB·P·),δ13CTOC值再次显著偏正,最高可达-28·4‰;3)全新世以来(11·7kaB·P.至今),δ13CTOC值变化幅度不大(-30·85‰^-27·37‰),基本上都在平均值-29·1‰左右。研究表明,大气CO2浓度变化是影响18·5kaB·P·以来四海龙湾玛珥湖δ13CTOC值变化的主导因素。

Sihailongwan Maar Lake is one of the eight Maar lakes in Longgang Volcanic Field, Northeast China. Three Sino-Germany cooperational drilling campaigns were conducted from August 1998 to July 2001, and three composite profiles of lake sediments have been constructed. Correlated with analytic results of water contents, TOC concentrations and TOC/TN ratios of composite profile Ⅱ , the stable carbon isotope record of bulk organic matter （δ^13 C TOC） can be divided into three stages during the past 18.5ka： （1） the late stage of the Last Glacial （18.5 - 14.7kaB. P. ）. The 8＇3Croc values were less negative, within the range of -29.50‰ ～ -26.18‰ and with the average of -28.10‰. （2） the last deglaciation （ 14.7 ～ 11.7kaB. P. ）. The δ^13 C TOC values became much more negative, which changed from -33.92‰ to -28.40‰, with the average of -31.75‰. There generally appears a valley on the δ^13 C TOC curve, with the exception that during the period of a Younger Dryas-like cold event （12.7 ～ 11.7kaB. P. ） , the δ^13 C TOC values reach a peak of -28.4‰. （3） the Holocene （11.7kaB, P. ～ present）. The 8＇3 CToc values remained relatively stable （ - 30. 85‰ ～ - 27.37‰） around the average of - 29. 1‰. The organic matter of lake sediment deposited during the late stage of the Last Glacial （ 18.5 ～ 14.7kaB. P. ） consists mainly of aquatic phytoplankton with low primary productivity, which reflects a cold and dry paleoenvironment. Because of the lower atmospheric CO2 concentration during this period, the dissolved CO2 concentration in lake water was also lower. Consequently, the photosynthesis of phytoplankton using HCO3^- as part of its carbon source resulted in less negative δ^13 C TOC values. During the last deglaciation （ 14.7 ～ 11.7kaB. P. ） , phytoplankton was still the main source for the organic matter in lake sediment despite an increasing contribution from land-derived organic matter during the BФlling-Allercd warm interval. Because of the largely increased atmospheric CO2 concentration during the last deglaciation, the dissolved CO2 concentration in lake water increased to an enough level to meet the need of the dissolved CO2 as the main carbon source for photosynthesis of phytoplankton, which resulted in a negative shift of δ^13 C TOC values. In the record of Sihailongwan Maar Lake, the Holocene started from 11.7kaB. P. Corresponding to climatic amelioration, primary productivity of land and aquatic plants increased evidently during the Holocene. Land-derived organic matter became the major contribution to lake sediment, although aquatic phytoplankton still played an important role. Because of the relatively stable atmospheric CO2 concentration during the Holocene, the δ^13C values of aquatic phytoplankton changed little. Similarly, the variation range of δ^13C values of land plants is limited, probably revealing the characteristic δ^13C values of C3 plants. The relatively stable δ^13CTOC values during the Holocene should be the integration of these two factors. Based on comprehensive research, it can be concluded that the change of atmospheric CO2 concentration is the control factor to the variation of δ^13CTOC values of the Sihailongwan Maar Lake during the past 18.5ka.