西北太平洋钡生物地球化学及其对弱光层有机碳再矿化的指示

The formation and variation of particulate barium (PBa) in the ocean are largely influenced by organic carbon export and remineralization. PBa concentrations are therefore used to quantify the organic carbon remineralization in the twilight zone, and its fluxes to the deep ocean and accumulation rates in sediments are applied to reconstruct export productivity. Enabled by the recent success in the determination of stable Ba isotopic compositions in seawater, Ba isotopes have been suggested helpful to elucidate the biogeochemistry of Ba in the ocean and thus to extend and improve its applicability as a geochemical proxy. .In the framework of the NSFC Major Research Plan “Inter-Sphere Interactions of the Earth System in the West Pacific Ocean”, this project selects the western North Pacific as the study site to examine biogeochemical behavior of Ba isotopes in the twilight zone and its linkage and implication to the organic carbon remineralization and paleoproductivity. The main hypothesis is as follows. Particle adsorption mainly induces a distinct isotope fractionation with heavier dissolved and lighter particulate Ba isotopic compositions in the euphotic zone. During particle sinking and its subsequent decomposition in the twilight zone, the regenerated Ba with low Ba isotopic signatures largely returns to the dissolved pool. Meanwhile, a fraction precipitates as barite in oversaturated microenvironments generating another isotope fractionation, which potentially incorporates productivity signals since higher export and remineralization of organic carbon generally results in elevated barite formation..The major objectives of this proposal are thus (1) to determine the spatial distribution, seasonal variability, and fractionation effect of Ba isotopes in the western North Pacific, (2) to evaluate the organic carbon remineralization flux in the twilight zone of the western North Pacific and its influence on the carbon fixation by biological pump, and (3) to testify whether Ba isotopes can be a useful proxy for carbon export and remineralization in the present and past ocean. Via a new avenue of stable Ba isotopes to link carbon transport in the water column and its burial in sediments, this project can improve our understandings to the second key question of the Major Research Plan that is “the material exchange and energy transfer across a fluid-solid interface in the West Pacific Ocean”.

海洋颗粒钡的形成和变化与有机碳的输出和再矿化有关，因此，钡元素被广泛应用于示踪有机碳再矿化和反演生物生产力，但其原理和机制仍不清晰。近期钡稳定同位素的成功开发有望厘清海洋钡循环中的一些未解问题，如重晶石沉淀是否主导弱光层颗粒钡极大值，并且可为地球化学研究提供新指标。本项目针对重大研究计划第五重点资助方向“西太平洋海水元素循环、沉积过程和古气候记录”，核心科学问题是：海洋弱光层中颗粒钡同位素的分布特征和分馏机制是什么，其变化能否指示有机碳再矿化？主要研究内容包括：（1）分析西北太平洋钡同位素的空间分布、季节变化与分馏效应；（2）评估西北太平洋弱光层有机碳再矿化通量及其重要性；（3）检验颗粒钡同位素指示现代海洋和古海洋碳过程的科学依据。本项目基于钡稳定同位素新视角，有机联系水体碳传输与沉积物碳埋藏，可为解答重大研究计划第二关键科学问题“西太平洋流固界面跨圈层物质与能量交换过程”提供思路和佐证。

本项目计划采集溶解态和颗粒态钡浓度和同位素数据，探究西北太平洋和南海钡元素分布特征和控制过程，解析钡稳定同位素分馏机制和指标作用，特别关注其在海洋弱光层中对有机碳再矿化的指示。通过三年的工作开展，项目组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。发现南海北部外陆架至陆坡真光层中溶解钡同位素较之颗粒钡同位素重0.4-0.5‰，且颗粒钡浓度与叶绿素、颗粒有机碳/氮、生源硅等生源颗粒浓度不相关，从而推断海洋真光层中钡同位素分馏的主要原因是颗粒物被动吸附轻质量钡同位素、而非浮游植物主动吸收。揭示西北太平洋1000米以上水体的颗粒钡同位素垂直分布趋势与其浓度大致相反，弱光层500米附近存在微弱的极小值，对应颗粒钡浓度极大值，反映了重晶石沉淀过程中的分馏效应；颗粒态与溶解态钡同位素之间的表观分馏系数在弱光层顶部200-300米处呈现极大值，对应于真光层下颗粒钡浓度的迅速升高，指示了弱光层有机碳再矿化的显著增加。建立西北太平洋弱光层中颗粒钡浓度与耗氧速率关系的经验公式，据此计算150-600米范围内有机碳再矿化通量为4-6 mmol C m-2 d-1，占真光层净群落生产力的50-90%。上述系列研究厘清了海洋不同水层中钡同位素不同的分馏效应和控制机制，为验证其能否成为有机碳输出和再矿化的新指标奠定了基础；且首次利用颗粒钡定量西北太平洋副热带海域的弱光层有机碳再矿化通量，可深化对该寡营养海域生物泵固碳效率的认识。其中，南海真光层钡同位素工作已发表于地学主流期刊Geochimica et Cosmochimia Acta。