Sediment geochemistry of ODP Holes 206-1256B and 206-1256C

We measured the chemical composition of 100 samples from the 250-m sediment sequence retrieved from Ocean Drilling Program Site 1256 in the Guatemala Basin using a newly developed microwave-assisted acid digestion protocol followed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis. We compared these data gathered onshore to the results from the flux fusion prepared samples analyzed by shipboard ICP-AES during the leg and published in the Leg 206 Initial Reports volume, as well as to 35 randomly selected samples that were prepared by flux fusion at Boston University and analyzed by ICP-AES. Comparison of the newly developed acid digestion protocol to shore-based flux fusion demonstrates that the microwave-assisted acid technique yields a complete digestion, and because this procedure includes boric acid, it is safe for use with HF acid as boric acid neutralizes excess HF. The precision for nearly all elements in shore-based acid digestions is better than 3% of the measured values, including for elements such as Ni, Cr, and V, which are typically difficult to measure in biogenic-rich sediments. The shore-based flux fusions, while better than shipboard reported precision values (as expected), has precision better than 3% of their respective measured values for all major elements (Si, Al, Ti, Fe, Mn, Ca, Mg, Na, and K) and several trace elements (Ba and Sr). Results for P, Cr, Ni, V, Sc, and Zr are better than 5% of their measured values. Not only does the newly developed acid digestion provide better analytical results than the typical flux fusion method, the shore-based acid procedure also exhibits downhole lithologic and chemical characteristics similar to the shipboard flux fusion prepared results. These results confirm that the current shipboard methods are adequate for first-order geochemical interpretations and that the microwave-assisted acid digestion holds great potential to be the primary technique of preparing sediments on future Integrated Ocean Drilling Program expeditions.

本研究采用全新开发的微波辅助酸消解方案，结合电感耦合等离子体原子发射光谱（inductively coupled plasma–atomic emission spectroscopy，ICP-AES）分析技术，对取自危地马拉盆地大洋钻探计划（Ocean Drilling Program）1256站位的250米沉积岩芯序列中的100份样品完成了化学成分测定。本研究将陆上实验室获取的上述测试数据，与本航次期间通过船载ICP-AES分析经熔剂熔融法制备的样品所得结果（已发表于《第206航次初始报告》卷册），以及波士顿大学采用熔剂熔融法制备、经ICP-AES分析的35份随机选取样品的测试结果进行了对比。将本研究新开发的酸消解方案与陆上熔剂熔融法开展对比后发现，微波辅助酸消解技术可实现完全消解；且由于该实验流程添加了硼酸，可中和过量氢氟酸（HF），因此可安全使用氢氟酸进行消解。陆上酸消解实验中，几乎所有元素的分析精度均优于对应测量值的3%，其中包括镍（Ni）、铬（Cr）和钒（V）这类在富生物成因沉积物中通常难以精准测定的元素。陆上熔剂熔融法的分析精度虽优于航次报告中船载分析的精度（符合预期），其所有主量元素（硅（Si）、铝（Al）、钛（Ti）、铁（Fe）、锰（Mn）、钙（Ca）、镁（Mg）、钠（Na）、钾（K））及部分微量元素（钡（Ba）、锶（Sr））的分析精度均优于对应测量值的3%。磷（P）、铬（Cr）、镍（Ni）、钒（V）、钪（Sc）及锆（Zr）的测试结果精度则优于对应测量值的5%。新开发的酸消解方案不仅较常规熔剂熔融法获得了更优的分析结果，陆上酸消解流程所呈现的井下岩性与化学特征，也与船载熔剂熔融法制备样品的分析结果高度一致。上述研究结果证实，当前船载分析方法足以满足一级地球化学解释的需求，且微波辅助酸消解技术具备成为未来综合大洋钻探计划（Integrated Ocean Drilling Program）航次中沉积物制备核心技术的巨大潜力。