(Table 1) Strontium, Neodymium, Hafnium and Lead isotopic compositions from ODP Hole 146-888B, 168-1027B and 168-1027C@en

Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposed continental areas. Detrital input usually dominates their geochemical budget, and the composition of these sediments should record potential changes in their continental sources. However, along margins, mineral sorting associated with transport and sedimentary processes induces significant chemical and isotopic fractionation over a few tens of kilometers. The study of margin sediments should help to quantify the extent of modification of the continental terrigenous supply when it reaches deep oceans. Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of the Cascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominant end-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only 10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5 Myr and they all appear unaffected by glacial-interglacial climate cycles. The average isotopic compositions of the two sites are slightly different, but are both dominated by continental signature; these values can be used in future studies to identify any subducted sediment contribution to the Cascades Arc. On a finer scale, there are differences in the isotopic signature between samples dominated by clay minerals and those with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr and Hf isotope ratios than sands, and we interpret the difference as resulting from mineral sorting during transport of the particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, while coarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase-epidote and zircon. ODP Site 1027 is located 100 km further away from continent than ODP Site 888 and contains more clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888. Similar differences in isotopic signatures related to the distance to continent certainly occur in other areas in the word, and will account for a large part of differences known between continental sources and deep-sea sediments.沉积于大陆边缘的大洋沉积物，主要源自附近裸露陆区的侵蚀产物。碎屑输入通常主导其地球化学收支，此类沉积物的组成理应记录其陆源物质的潜在变化。然而，沿大陆边缘，与搬运及沉积过程相关的矿物分异作用，会在数十公里尺度上引发显著的化学与同位素分馏。对边缘海沉积物的研究，有助于量化陆源碎屑供给抵达深海时的改造程度。已报道的卡斯卡德前弧最北部区域（大洋钻探计划（Ocean Drilling Program, ODP）888与1027站位）的57件沉积物的Sr、Nd、Hf及Pb同位素组成显示，其主要受来自附近加拿大科迪勒拉造山带的两个端元贡献。科迪勒拉造山带亏损型西部区域的侵蚀产物主导了碎屑输入，而其东部富集地体的贡献仅占输入总量的10%~28%。过去3.5百万年以来，沉积物的物源未发生显著变化，且均未受冰期-间冰期气候旋回的影响。两处站位的平均同位素组成存在小幅差异，但均以陆源特征为主；这些数值可用于后续研究，以识别卡斯卡德弧中是否存在俯冲沉积物的贡献。在更精细的尺度上，以黏土矿物为主的样品与岩性较粗的样品之间，同位素特征存在差异。对于给定的Nd同位素组成，细粒沉积物的Sr与Hf同位素比值较砂质沉积物更具放射成因特征，我们将该差异解释为颗粒搬运过程中矿物分异作用所致。细粒沉积物富集具有放射成因Sr与Hf的矿物（如黏土与云母），而粗粒碎屑则通过斜长石-绿帘石及锆石携带源岩的非放射成因矿物组分。ODP 1027站位距离大陆较ODP 888站位远100公里，且黏土含量更高。因此，其整体Sr与Hf同位素组成的放射成因特征显著强于ODP 888站位。类似的、与距大陆距离相关的同位素特征差异，无疑也存在于全球其他区域，并将在很大程度上解释已知的陆源物质与深海沉积物之间的差异。