海洋钡稳定同位素组成、分馏效应及其古生产力替代指标可能性研究

The nutrient-like distribution of dissolved barium (DBa) in seawater, with low surface and elevated deep water concentrations, is commonly attributed to the biologically mediated incorporation of DBa in or adsorption onto biogenic particles in the upper ocean and regeneration via particle dissolution and/or degradation at depth. Importantly, within this cycle, a maximum of excess (or biogenic) particulate Ba (Baxs), most likely resulting from barite (BaSO4) precipitation in oversaturated microenvironments generated by degradation of sinking organic aggregates, is frequently observed in the mesopelagic zone between 100 and 600 m water depth. Therefore, Baxs or barite fluxes to the deep ocean and their accumulation rates in sediments have been widely applied to reconstruct export productivity. However, the exact mechanisms controlling the Ba behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully understood. .Recently, the first measurements of stable Ba isotopes in seawater have been reported, which are potentially helpful to better understand the biogeochemistry of Ba in the ocean and thus to extend and improve its applicability as a paleoproxy. My latest work showed that dissolved Ba isotopic compositions (δ137Ba_DBa) in the upper water column are significantly heavier than those in the deeper waters documenting an apparent nutrient-like fractionation. Moreover, δ137Ba_DBa signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and δ137Ba_DBa from the concentrations of nitrate and phosphate this implies that direct effects of surface biology on Ba dynamics are quantitatively unimportant..Nevertheless, the information on productivity could still be recorded by particulate Ba isotopic compositions in Baxs or barite. Through collecting Ba isotope data in both dissolved and particulate phases, this proposal attempts to investigate the oceanic Ba cycle and relevant controlling processes via the new avenue of stable isotopes. The Ba isotopic compositions in particles of different size fractions and in different Ba-carrier phases will be analyzed. In particular, the isotope fractionation during the extensive barite precipitation in the intermediate waters and its potential as a proxy for paleoproductivity will be explored.

溶解钡在海洋中的分布具有上层浓度低、深层浓度高的特征，与营养盐类似；而颗粒钡的形成和输出与有机碳的生产、输出和矿化有关。因此，钡是反演古生物生产力的重要替代指标，但其原理和机制仍不清晰。钡的稳定同位素有可能解析这一重要科学问题。申请人的最新研究发现，上层海水溶解钡同位素组成显著重于深层水，表观上呈现“营养盐型”的分馏模式；但又有别于硅酸盐这一典型生源要素，其硅同位素比值因生物吸收在真光层内随深度递减，而溶解钡同位素组成在上层100米水体没有变化，因此其直接的生物控制效应并不显著。但是，生物生产力信息仍有可能记录在颗粒钡同位素组成中。为验证这一科学假设，本项目拟耦合研究溶解和颗粒钡同位素组成，从稳定同位素新视角探讨海洋钡元素的内部循环过程和控制机制；分析不同粒级颗粒物和不同颗粒相所结合的钡同位素信息，尤其关注中层水大量重晶石沉淀过程中的钡同位素分馏效应，检验其作为古生产力替代指标的科学依据。

本项目计划采集南海溶解态和颗粒态钡同位素组成数据，利用稳定同位素新手段探讨海洋钡元素的循环过程、控制机制与指标作用。通过三年的工作开展，项目组基本完成了计划书的拟定任务，回答了项目书所列的主要科学问题。建立和完善了双稀释剂多接收器电感耦合等离子体质谱仪钡稳定同位素分析方法，测定多种国际标准参考物质的钡同位素组成（δ138/134Ba）均与文献报导值吻合，分析精度优于0.06‰（2SD）。在此基础上，系统测定了南海北部外陆架至陆坡真光层中溶解钡和总悬浮颗粒钡的同位素组成，发现150 m以浅水体中的颗粒钡同位素较之对应溶解钡轻~0.5‰，且颗粒钡浓度与叶绿素、颗粒有机碳、生源硅等生源颗粒浓度不相关，从而提出了颗粒吸附，而非生物吸收，导致真光层钡同位素发生显著分馏的新假说。进一步，测定了南海海盆由表至弱光层底（1000 m之上）的总悬浮颗粒钡浓度和同位素组成，发现不同季节、不同站位均在150-200 m处呈现颗粒钡浓度极大值，主要缘于弱光层颗粒有机碳矿化所产生的微环境中重晶石的大量沉淀；同时，北部海盆SEATS站150-200 m处的颗粒钡同位素组成较之上下水层偏轻，指示了重晶石沉淀过程中发生了钡同位素分馏，且该分馏效应可联系颗粒有机碳输出和再矿化通量。上述系列研究首次耦合分析海洋水体溶解态和颗粒态钡同位素组成，揭示了不同水层钡同位素不同的分馏效应和控制机制，为阐释海洋钡循环提供了新视角，初步验证了利用钡同位素示踪和反演海洋环流、营养盐循环和生物生产力的可能性。其中，南海真光层钡同位素工作已撰写论文投稿至地学主流期刊Geochimica et Cosmochimia Acta。