北部湾暖池水域表层沉积特征及其环境指示*

海洋暖池的发展、变化及其效应与沉积环境有密切关系。文章对北部湾暖池主要水域进行表层沉积物取样, 测试鉴定其粒度、化学元素、微体古生物化石, 并对数据进行分析比较。结果表明: 研究区物源来自陆区上陆壳风化, 在水动力作用下粒度组成分异明显, 平均粒径为1.1Φ~6.9Φ (Φ=-log₂d, d为直径, 单位: mm)。常量元素以SiO₂占优, 其平均质量分数最高达到66.61%, 其次为Al₂O₃; 微量元素的质量分数平均为9.05~297.00μg?g^-1, 稀土元素的平均质量分数为0.34~69.43μg?g^-1, 化学蚀变指数(CIA)平均为47.33, 轻重稀土元素分馏比为7.46~10.07。古生物多样性指数(H°)为0~2.52。在海湾尺度下化学要素分布的空间异质性显著。在研究区西部, 大部分化学元素质量分数高, 沉积物颗粒最细, 稀土元素分馏显著, CIA较高, 物质来源多样, 古生产力较高; 而东部则几乎表现相反, 以砂质沉积物为主, 除了CaO、MnO、Sr较高之外, 大部分元素质量分数低且稀土元素分馏弱, CIA低下, 古生产力偏低, 显示出物源有限和海洋自生碳酸盐沉积的可能性。浮游类有孔虫以表层暖水种占优, 硅藻为热带和亚热带种, 属种内在结构显示出暖水、高盐等特征。该暖池兼具一定的独立性和开放性, 对生物活动、生物多样性乃至碳酸盐沉积等具有维护作用。

The primary sedimentary characteristics of Warm Pool in Beibu Gulf and its environmental indication*

The development and change of a warm pool have close relationships with sedimentary environment. For the lack of integrated modern sedimentary environmental studies on the Warm Pool of Beibu Gulf, the samples of bottom sediments from the study area within the main part of the Warm Pool were collected and laboratory analysis and identification in granularity, geochemical elements and micropaleontology was carried out. The study and comparison using the above data showed that the provenance came from surrounding up continent crust. Median grain size ranged 1.07Φ~6.88Φ; SiO₂ dominated in macro-geochemical elements with maximum 66.61% and followed by Al₂O₃; micro-geochemical elements and REEs ranged 9.05~297.00μg?g^-1, 0.34~69.43μg?g^-1, respectively, and an averaged chemical index of alteration (CIA) 47.33 and (La/Yb)_N 7.46~10.07 and paleo-biodiversity index of foraminifera and diatom 0~2.52 meaning relatively significant spatial variation under gulf scale. There was primary difference between western and eastern parts of the study area: the former with highest concentration of large part of geo-chemical elements, fine grain, significant fractural in REEs, high CIA and relatively rich provenance and the latter being almost opposite with low elements, low CIA, and so on but with high CaO, MnO and Sr revealing limited provenance supply and probably authigenic carbonate sedimentation. Warm-water foraminifera and subtropical-tropical diatom species took advantage in the study area and ratios between the abundance of species also indicated warm and high saline seawater characteristics especially in the eastern part of the study area. This Warm Pool is either a dependent or an open system, and has a positive effect on maintaining the biological activity, biodiversity and sedimentary environment, especially carbonate sedimentation.