Rare earth elements and yttrium (REY) composition of paleo-seawater and redox conditions of localized water masses during deposition are documented by the REY preserved in modern pristine chemical sedimentary phosphorites. However, contrary to modern phosphorites, some older ones (e.g., Cambrian and Proterozoic phosphorites) display anomalous REY patterns that deviate from modern seawater, and limit the use of their parameters (e.g., Ce anomalies) as a reliable proxy for redox conditions in this case. In this work, elements of bulk-rocks and apatite grains, SrNd isotopes, and phosphate oxygen isotopes (δO) from Jingxiang (JX), Yichang (YC), and Xingshen (XS) phosphorites of the Ediacaran Doushantuo Formation in South China were investigated to address this quandary. The REY indexes and patterns of bulk-rocks and apatite, as well as Nd isotopes of bulk-rocks, suggest that the phosphorites deposited in the lower, middle, and upper members of the Ediacaran Doushantuo Formation record the sources of REY as being of terrigenous, mixed, and seawater origins, respectively. During the deposition of the Ediacaran Doushantuo Formation, high δO values (18.61 ± 1.04) of all bulk-rocks, close to contemporary marine authigenic apatite, indicate that phosphorus has been involved in strong marine bio-cycling, assuming a final terrestrial source. However, REY composition of the seawater column recorded by authigenic apatite in the lower member of Doushantuo Formation exhibited a right-leaning REY pattern, representing a terrestrial REY end-member. The Sr/Sr ratios of bulk-rocks (0.71031 ± 0.00719) and apatite grains (0.71098 ± 0.00099) also remained consistent throughout the Ediacaran Doushantuo Formation, except for the high Sr/Sr values found in bulk-rocks (0.72200 ± 0.00085) and apatite (0.72271 ± 0.00064) in the lower member of JX, which are presumed to result from source rocks with high radioactive Sr. Combined with REY indexes (Ce anomalies, Y/Ho ratios, and the differentiation of REY) and constant Sr/Sr values of apatite, varying REY patterns from the lower to upper members of the Doushantuo Formation indicate that the scavenging rate of terrestrial REY inputs to the ocean by suspended particles (e.g., FeMn oxyhydroxides) in the water column is controlled by the degree of seawater oxidation. Our study highlights that REY inputs to seawater could undergo seawater cycling (i.e., the removal of REY by particulates of seawater under oxygen-containing conditions), and only REY that undergo sufficient seawater cycling can reflect the true redox conditions of the water column. Therefore, the sources and cycling of REY in seawater should be carefully identified when applying Ce anomalies and Y/Ho ratios of phosphorites to trace the redox conditions of ancient seawater.

中文翻译：

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扬子地台（华南）埃迪卡拉系磷矿记录的REY-P循环


现代原始化学沉积磷矿中保存的 REY 记录了古海水的稀土元素和钇 (REY) 成分以及沉积过程中局部水团的氧化还原条件。然而，与现代磷矿相反，一些较古老的磷矿（例如寒武纪和元古代磷矿）表现出偏离现代海水的异常 REY 模式，并限制了其参数（例如 Ce 异常）作为氧化还原条件的可靠代理的使用。案件。本文对华南埃迪卡拉系陡山沱组荆乡（JX）、宜昌（YC）和兴神（XS）磷矿的块岩和磷灰石颗粒元素、SrNd同位素和磷酸盐氧同位素（δ18O）进行了研究来解决这个困境。块岩和磷灰石的 REY 指数和模式以及块岩的 Nd 同位素表明，埃迪卡拉系陡山沱组下、中、上段沉积的磷矿记录了 REY 的来源为陆源。 、混合源和海水源。在埃迪卡拉系陡山沱组沉积过程中，所有块体岩石的δ18O值较高（18.61±1.04），接近当代海洋自生磷灰石，表明磷参与了强烈的海洋生物循环，假设最终来自陆地。然而，陡山沱组下段自生磷灰石记录的海水柱REY组成呈现右倾REY模式，代表陆地REY端元。除了在块体岩石中发现较高的 Sr/Sr 值（0.72200 ± 0.71）外，整个埃迪卡拉系陡山沱组块体岩石的 Sr/Sr 比值（0.71031 ± 0.00719）和磷灰石颗粒（0.71098 ± 0.00099）也保持一致。锦西下段发育磷灰石(00085)和磷灰石(0.72271±0.00064),推测其为高放射性Sr烃源岩的产物。结合REY指数(Ce异常、Y/Ho比值、REY分异)和常数磷灰石的 Sr/Sr 值，从陡山沱组下部到上部不同的 REY 模式表明，水柱中悬浮颗粒（例如 FeMn 羟基氧化物）输入海洋的陆地 REY 的清除率受程度控制。海水氧化。我们的研究强调，海水中的REY输入可以经历海水循环（即在含氧条件下通过海水颗粒去除REY），并且只有经过充分海水循环的REY才能反映水柱的真实氧化还原条件。因此，利用磷矿的Ce异常和Y/Ho比值追踪古代海水的氧化还原条件时，应仔细识别海水中REY的来源和循环。