南沙群岛永暑礁晚全新世软体动物记录与古环境变化

LATE HOLOCENE MICROMOLLUSCAN RECORDS AND PALEOENVIRONMENTAL CHANGES IN YONGSHU REEF, SOUTH CHINA SEA

对南沙群岛永暑礁南永3井软体动物的丰度、营养结构、化学元素和堆积速率的定量分析表明,晚全新世永暑礁潟湖经历了数次相对开放与封闭、与外海交流时强时弱以及盐度多次显著变化的过程。与氧同位素和北半球晚全新世快速气候事件的进一步对比表明,晚全新世快速变化的气候事件也波及到南海,由此造成的潟湖水动力环境、古盐度和沉积速率的变化深刻地影响着软体动物的发育。基于软体动物的替代指标和氧同位素变化的综合分析,初步划分了晚全新世永暑礁潟湖7个变化阶段。

Core Nanyong-3, drilled in the Yongshu reef of the Nansha Islands, the South China Sea, records very abundant micromolluscs （gastropods and bivalves）. Highly dense sampiing of the core allows us to carry out a study of high-resolution environmental changes through quantitative analysis on proxy records of micromolluscs. Based on quantitative analysis on the abundance, trophic structure, chemical elements and accumulating rate of micromollusks, this paper demonstrates that the lagoon of the Yongshu reef underwent several shifts between relatively open and close environments during the late Holocene, resulting in changes of connection degree of the lagoon with outer sea as well as salinity of the lagoon.Additionally, in comparison of the micromollusc proxies with the oxygen isotope and the temperature anomaly occurring in the northern hemisphere during the late Holocene, it is found that the rapid climatic change in the late Holocene might have affected the South China Sea. The changes of hydrodynamics, salinity and sedimentary rate of the lagoon in the late Holocene might have profoundly influenced the development of micromolluscs. All these changes were apparently recorded in micromolluscs. By synthesizing these proxies and the oxygen isotope, seven stages can be preliminarily divided for the Yongshu reef in the last 1 700 years. The first stage （core depth of 476--590cm） is characterized by climatic fluctuation, with oxygen isotope ranging between -2.8‰ and -3.8‰, low micromolluscan abundance, once fewest bivalve suspension-feeders and obvious Mg/Ca ratio fluctuation of gastropod Acteocina excepting high value at the bottom core, all indicating that the invasion of ocean water into the lagoon which resulted in relative closure and opening of the lagoon to outer sea might be changeable. The second stage （core depth of 389-476cm） is characterized by warm climate, with lighter oxygen isotope changing from--4.8‰ to-3.5‰, high micromollusc abundance, dominant bivalve suspension-feeders and two high Mg/Ca ratios of 22.19× 10^-4 and 27.25×10^-4, showing that the increasing invasion of ocean water resulted in further opening of the lagoon to the outer sea. The third stage （core depth of 315-389cm） represented cold climate, in which there are heavier oxygen isotope （-2. 5‰-3.7‰）,very low micromollusc abundance, few bivalve suspension-feeders and low Mg/Ca ratios, indicating that the lagoon might be relatively close. The fourth stage （core depth of 267-315cm） is distinguished by warm climate, corresponding to “Medieval Warm Period” , with the lighter value of oxygen isotope fluctuating between -3.3‰ and -4.8‰, higher micromollusc abundance （the second highest value） and absolutely dominant bivalve suspensionfeeders. But it is interesting that the Mg/Ca ratio of Acteocina is not high. Presumably it is due to diluted salinity of the lagoon by the sharp increase of precipitation. At the fifth stage（core depth of 174-267cm） the climate might be fluctuant, with obvious changes of both the oxygen isotopes （from -2.8‰ to -4.9‰） and micromollusc abundance and low Mg/Ca ratios. Additionally, deposit-feeders were plentiful at this stage. The sixth stage （core depth of 50-174cm） is comparable with “Little Ice Age”, characterized by heavier oxygen isotopes changing between -2.3‰ and -4‰ and very low micromollusc abundance as well as low Mg/Ca ratios. The seventh stage （core depth of 0--50cm） belongs to recent warm climate, with oxygen isotopes rapidly becoming light, micromollusc abundance obviously increasing and high Mg/Ca ratios appearing again, implying strengthening invasion of ocean water into the lagoon, and further opening of the lagoon to outer sea.