贵州阿哈湖物质循环过程中的铁同位素地球化学及其指示意义

Iron isotope behavior in geochemical cycle of the Aha Lake,Guizhou Province,and its implications

使用AGMP-1氯化物型阴离子交换树脂(100~200目)对夏季贵州阿哈湖流域水体悬浮颗粒物等样品进行了化学分离,并在多接收电感耦合等离子体质谱仪(MC-ICP-MS)上进行了铁同位素分析。分析结果表明,夏季阿哈湖湖水分层期间湖水悬浮颗粒物及各端员环境样品的铁同位素组成变化较大:湖水悬浮颗粒物的δ56Fe为负值,分布范围为-1.36‰^-0.10‰之间;各支流河水悬浮颗粒物的铁同位素组成在-0.88‰^-0.16‰之间;大气颗粒物的平均铁同位素组成为+0.06‰±0.02‰;而未经化学清洗的浮游藻类的铁同位素组成为+0.08‰。对比研究表明,湖水悬浮颗粒物的铁同位素组成不仅受各输入端员的影响,湖泊内部复杂的生物地球化学过程也对颗粒物的铁同位素组成产生了重要影响。陆源输入的颗粒有机结合态铁使得湖泊表层悬浮颗粒物的铁同位素组成偏低,而大气沉降颗粒物和湖泊表层的浮游藻类整体上对铁同位素组成的影响并不显著。"ferrous wheel"铁循环对于氧化还原界面附近水层中铁同位素的重分配起到了主要的控制和影响作用。δ56Fe值与Fe/Al呈现良好的负相关关系,也显示出活性铁的循环迁移是造成氧化还原界面附近水层中悬浮颗粒物的铁同位素组成变化的重要原因,表明铁同位素与Fe/Al可能可以作为表征水体生物地球化学环境的良好指标。

Suspended particulate materials （SPM） were investigated for their iron isotope composition so as to assess their isotope behaviors during the iron biogeochemical cycle in summer in the Aha Lake of Guizhou Province. All the samples were purified with the anion exchange chromatography （Bio-Rad AGMP-1 resin, 100--200 mesh） and the iron isotope composition was analyzed on Nu Plasma MC-ICP-MS. δ^56Fe values of SPM sampled in the lake and the rivers display statistically a negative shift compared with IR- MM-014. The values of lake samples vary from - 1.36‰ to -0.10‰, while those of river samples range from -0.88‰ to -0.16‰. Besides, the average iron isotope composition of aerosol samples is ＋ 0.06‰±0.02‰. The phytoplankton was also sampled and analyzed for the iron isotope composition of the whole sample including intracellular and extracellular iron, which yielded a value of ＋0.08‰. It is suggested that the complex biogeochemical process plays an important role in changing the δ^56Fe values of SPM in the lake apart from various inputs. During stratification in summer, it is the organically bonded iron particles that make the iron isotope composition of SPM in the epilimnion light, with an average value of -0.29‰. An iron cycle is established near the redox boundary where the upwardly diffusing Fe （Ⅱ） is oxidized and the iron particles formed will continuously sink back to the reducing zone, with which the cycle is completed and the δ^56Fe values of SPM reach the minima - 0.88‰ （DB） and - 1.36‰ （LJK）, just below the redox boundary. Due to random transportation and diffusion, the profiles of δ^56Fe values of the SPM near the redox regions develop to an approximate Gaussian shape. Besides, the formation of ferrous sulfides may be another reason for the light iron isotope composition of the SPM near the water-sediment interface. Moreover, there is a good negative correlation of the δ^56Fe value with the Fe/A1 ratio especially for DB location, which further indicates that the transformation and diffusion of reactive iron play an important role in modifying the δ^56Fe value of the SPM near the redox boundary, and also suggests that the iron isotope and the Fe/A1 ratio can serve as indicators of hydro-biogeochmical conditions.