孟氏螺蛳壳体氧同位素组成记录的季节性温度变化

SEASONAL TEMPERATURE CHANGES RECORDED BY OXYGEN ISOTOPIC COMPOSITION IN THE SHELLS OF ANULARYA MANSUYI

软体动物壳体的碳酸盐氧同位素组成与周围水体环境密切相关,是重建古气候的重要载体。孟氏螺蛳个体较大,是滇中地区特有的螺蛳属,在滇中地区第四纪湖岸阶地广泛分布,然而其壳体的氧同位素能否用于重建区域季节性气候信号仍不清晰。本研究通过收集杞麓湖的现生孟氏螺蛳壳体,沿着壳体螺旋生长方向连续取样(选取两个壳体,分别采集45个和46个样),测试其壳体的碳酸盐δ^(18)O值,分析并讨论了壳体δ^(18)O与湖水温度和湖水δ^(18)O的耦合关系,研究结果表明,1)孟氏螺蛳壳体内部沿生长方向的氧同位素具有显著的季节性的波动特征;2)壳体碳酸盐氧同位素符合平衡沉降条件下的氧同位素分馏特征,其δ^(18)O值与湖泊温度和湖水δ^(18)O密切相关;3)杞麓湖现代孟氏螺蛳壳体δ^(18)O值的波动主要受温度的季节性变化控制,在具有湖水δ^(18)O值的情况下,壳体δ^(18)O值可靠记录了区域季节性湖水温度变化,其中基于夏季湖水δ^(18)O值重建的温度与区域大气温度更为吻合。

The oxygen isotopic composition(δ^(18)O)of aquatic mollusk shells, which is sensitive to climatic and environmental changes, has a great potential to record environmental conditions. Previous studies have shown that seasonal variations in water temperature and water δ^(18)Ocontrol the oxygen isotopic signatures of the shells. Anularya mansuyi is endemic to lakes in Central Yunnan, China, with a life span of 3~4 years. Their shells may be excellent archives of seasonal lake environment conditions and suitable for paleo-environmental studies. But it is unclear whether the oxygen isotopic composition of Anularya mansuyi shell can reliably reconstruct the regional seasonal climate variability. Here, we present oxygen isotope records from two shells of newly dead modern Anularya mansuyi(QLH-A:length is 4.5 cm, width is 2.3 cm and weight is 4.84 g, QLH-B:length is 4.8 cm, width is 2.6 cm, weight is 8.40 g)collected from Qilu Lake(24° 04′36″~24° 14′21″N, 102° 33′48″~102° 52′36″E) in Central Yunnan Province, in April 2014. The annual mean air temperature around the lake area is ca. 16 ℃, and annual precipitation is ca. 899 mm. The shells were subsampled from the aperture to apex along growth direction at an interval of about 3 mm, 45 and 46 δ^(18)O data were obtained from QLH-A and QLH-B respectively. Raman spectroscopy results on modern Anularya mansuyi shell samples suggest that shell carbonate is composed of aragonite. The results are as follows: First, the δ^(18)O records of two Anularya mansuyi shells show remarkable similarities during their same growth period, ranging from ca.-6.8‰~-3.2‰. According to aragonite-temperature equation, the predicted δ^(18)O value of Anularya mansuyi shell is-5.0‰, which is calculated using the annual average air temperature(16 ℃)and the measured δ^(18)Ovalue(-4.5‰)of Qilu Lake during the growth of the shell. It is consistent with the average of measured δ^(18)O values of two shells(-5.1‰ and-5.2‰), indicating that Anularya mansuyi precipitates its shell under isotopic equilibrium.Second, the intra-shell δ^(18)O values of Anularya mansuyi QLH-A and QLH-B show similar fluctuations, which is consistent with seasonal variation of lake water temperature and lake water δ^(18)O, and temperature is the dominated effecting factor. With measured seasonal lake water δ^(18)O(summer:-5.3‰, winter:-4.2‰)and intra-shell δ^(18)O values, we reconstructed the seasonal temperature of the lake water(summer: 24.7 ℃, winter: 13.0 ℃). Comparison of the calculated temperature with the measured lake water temperatures(summer: 23.5 ℃, winter: 13.8 ℃)suggests that Anularya mansuyi shells faithfully record seasonal variations in water temperature.Third, based on the summer lake water δ^(18)O, the reconstructed temperature(8.0~24.7 ℃)is consistent with measured atmospheric temperature throughout the year. While based on the lake water δ^(18)Oof winter, the reconstructed temperature overestimates the atmospheric temperature, which is related to the fact that the water temperature of Qilu Lake is much higher than the atmospheric temperature in winter.The intra-shell oxygen isotopes of Anularya mansuyi shells can capture the seasonal variations of lake water temperature. Whether Anularya mansuyi shells can be used as a paleo-temperature proxy, it needs further studies.