Oxygen isotope fractionation in marine sediments during respiration
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Atmospheric oxygen is about 23 ‰ higher in 18 O than ocean surface water, whereas oxygen in isotopic equilibrium with ocean water would be only 6 ‰ higher in 18 O. The fractionation of 18 O during respiration has been measured on natural populations in unfiltered marine surface water samples. The decrease of dissolved oxygen concentration and the increase in δ 18 O due to respiration was measured as a function of time. An average enrichment factor of 21 ‰ was calculated for the removal of oxygen in a closed system. The results indicate that the enrichment of 18 O in the atmosphere, and possibly the present oxygen concentration, may be controlled by biogenic processes.
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磷酸盐氧同位素（δ18OP）是一种有效的磷源示踪方式.由于湖泊沉积物的组成十分复杂，必须对样品进行除碳处理和纯化处理.本文对目前应用较为广泛的几种前处理方法进行比较，包括NaClO处理和H2O2处理等除有机碳方法，以及Blake法和McLaughlin法等纯化方法，以期获得适用于湖泊沉积物磷酸盐氧同位素的前处理方法.结果表明：（1）NaClO处理可以保证较高的无机磷提取效率，同时能有效地减少无机磷提取液中有机质含量，且对不同形态磷的破坏较小；H2O2对有机质的去除效果不稳定，且处理后样品磷形态之间发生转化，显著增加了提取的无机磷浓度.（2）Blake法和McLaughlin法分别采用磷钼酸铵（APM）+磷酸铵镁（MAP）沉淀和CePO4沉淀对样品进行纯化.在纯化处理湖泊沉积物过程中，Blake法优于McLaughlin法，主要体现为有机质去除率高，并且磷的回收率较为稳定.（3）经Blake法纯化丹麦Nordborg湖沉积物样品得到的Ag3PO4中C、N含量低于McLaughlin法且重现性好.本文结合NaClO法与Blake法的优点，建立了一种适合湖泊沉积物的磷酸盐氧同位素前处理方法：首先用2.5% NaClO对沉积物样品进行除碳预处理，然后对磷酸盐提取液依次通过氢氧化镁共沉淀（MAGIC）、APM+MAP沉淀、阳离子交换树脂处理，最后生成Ag3PO4沉淀.;Oxygen isotope of phosphate (δ18OP) was a very effective proxy for tracing sources. Due to the complicated composition of lake sediments, samples must be treated by carbon elimination and purified before the δ18OP measurement. In order to establish a suitable pretreatment method for δ18OP of lake sediments, this study compared several widely-used pretreatment methods, including organic carbon elimination by NaClO treatment and H2O2 treatment, purification by Blake method and McLaughlin method. The results showed:(1) NaClO kept high inorganic phosphate extract ratios, and removed organic matter efficiently with less impacts on different phosphate species. The organic removal efficiency by H2O2 was not stable while the phosphate species changed after treatments and inorganic phosphate concentration increased significantly. (2)Ammonium phosphomolybdate(APM)+ ammonium phosphate(MAP)and CePO4 was used by Blake method and McLaughlin method,respectively. Blake method had better performance on the purification of lake sediments samples than McLaughlin method, and a higher organic matter removal rate than McLaughlin method with stable phosphate throughput. (3) The Ag3PO4 produced by the Blake method had lower C, N contents and better repeatability than the McLaughlin method. The advantages of both the NaClO treatment and the Blake method were made full use to establish an improved δ18OP pretreatment method for lake sediments with modified procedures. It is recommended to use 2.5% NaClO to remove organic matter before phosphate extraction, and then purify the phosphate extractions through MAGIC, APM+MAP, and cation resin treatment, in turn.
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