Microprecipitation and δ18O analysis of phosphate for paleoclimate and biogeochemistry research
Microprecipitation and δ18O analysis of phosphate for paleoclimate and biogeochemistry research
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DOI：
10.1016/j.chemgeo.2017.03.032
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发表时间：
2017-06-05
期刊：
影响因子：
3.9
通讯作者：
Colman, A. S.
中科院分区：
文献类型：
--
作者：
Mine, A. H.;Waldeck, A.;Colman, A. S.
磷酸盐氧同位素比值为古气候重建和水体磷地球化学研究提供了有力的工具。生物矿物和溶液中磷酸盐和水的氧同位素组成之间的温度依赖性偏移已被广泛用于古气候研究以及水和土壤环境中磷循环的研究。在许多应用中，样本量很小，例如，哺乳动物牙釉质的连续取样用于古季节性研究，牙形石元素用于气候和海洋三角洲O-18重建，或在海洋地球化学研究中分离和分析海洋和淡水环境中溶解的磷酸盐。目前测定磷酸盐氧同位素组成（δ O-18（p））的方法需要将磷酸盐纯化为磷酸银（Ag 3 PO 4）。最终沉淀Ag 3 PO 4的技术分为两类：慢速微沉淀和快速微沉淀。我们已经测试了这两种方法，并确定了影响测量的Δ O-18（p）值的工件，这些值是由常用的快速Ag 3 PO 4沉淀方法产生的。特别地，通常使用的方法倾向于（a）磷酸盐的不完全沉淀，伴随相关的同位素分馏，和（B）产生氧化银污染物，其氧同位素组成对用于制备沉淀试剂溶液的水的δ O-18敏感。这些人工产物通常在千分之0.5至2的量级上，这在许多研究中足够细微而未被检测到，但足够大而导致温度估计在2-8摄氏度的量级上的误差。我们开发了一种快速沉淀技术，避免了上述问题。缓慢的微沉淀和快速的微沉淀在此开发导致Δ O-18（p）的测量是无法区分的，从传统的大批量沉淀的Ag 3 PO 4。
Phosphate oxygen isotope ratios provide a powerful tool for paleoclimate reconstruction and the study of phosphorus biogeochemistry in aquatic systems. The temperature dependent offset between the oxygen isotope composition of phosphate and water in biogenic minerals and in solutions has been used extensively in paleoclimate research and the study of phosphorus cycling in aquatic and soil environments. Sample sizes are small in many applications, e.g., serial sampling of mammalian tooth enamel for paleoseasonality studies, the use of conodont elements for climate and ocean delta O-18 reconstruction, or the isolation and analysis of dissolved phosphate from marine and freshwater environments in biogeochemistry studies. This has pushed the development of techniques that allow for processing and analysis of a few micromoles of phosphate.Current approaches to phosphate oxygen isotopic composition (delta O-18(p)) determination require purification of phosphate as silver phosphate (Ag3PO4). The techniques for the final precipitation of Ag3PO4 fall into two categories: slow vs. rapid microprecipitations. We have tested both and identify artifacts that impact measured delta O-18(p) values resulting from commonly used methods for rapid Ag3PO4 precipitation. In particular, commonly used methods are prone to (a) incomplete precipitation of phosphate, with associated isotopic fractionation, and (b) the production of silver oxide contaminants whose oxygen isotope composition is sensitive to the delta O-18 of water used to prepare the precipitation reagent solution. These artifacts are commonly on the order of 0.5 to 2 parts per thousand, which is subtle enough to go undetected in many studies, but large enough to cause errors in temperature estimates on the order of 2-8 degrees C.Slow microprecipitations lead to complete precipitation of phosphate without contaminating O-bearing phases. We develop a rapid precipitation technique that avoids the problems noted above. Both the slow microprecipitation and the rapid microprecipitation developed herein lead to delta O-18(p) measurements that are indistinguishable from those made on conventional large-batch precipitations of Ag3PO4.