Rare earth elements in Holocene reefal microbialites: a new shallow seawater proxy
Rare earth elements in Holocene reefal microbialites: a new shallow seawater proxy
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DOI：
10.1016/s0016-7037(99)00400-7
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发表时间：
2000-05
影响因子：
5
通讯作者：
G. Webb;B. Kamber
中科院分区：
文献类型：
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作者：
G. Webb;B. Kamber
测定了大堡礁苍鹭礁浅层礁架空腔的全新世镁方解石微生物岩中的稀土元素和钇 (REE + Y) 浓度。 52 个微生物岩样品的页岩归一化 REE + Y 模式显示：（1）均匀重 REE 富集（NdSN/YbSN= 0.236，SD = 0.026）； (2) 一致的负Ce和正La异常； (3) 海洋 Y/Ho 比值（56.17，SD = 2.66）； (4) Gd 轻微正异常。所有这些特征都与氧合良好的浅层环境海水的地球化学一致。稀土元素分配系数是相对于浅珊瑚海海水计算的。它们在整个质量范围内是均匀的（相对 SD = 10.2%），几乎比珊瑚和海水之间的质量高两个数量级。因此，与骨架碳酸盐相比，陆源无碎屑的现代微生物岩是海水稀土化学更可靠的代表。由于成岩作用存在问题，古代石灰岩作为稀土元素替代物的来源被认为在很大程度上存在问题，部分原因是与现代骨架碳酸盐相比，某些石灰岩中稀土元素浓度相对较高。然而，现代微生物岩中的高稀土元素浓度表明，具有相对较高稀土元素浓度的古代石灰岩，如果没有受到陆源碎屑的污染，可能反映了原始海水的化学性质。如果存在陆源污染，则可以根据同时出现的微量元素浓度（Sc、Hf、Th）和 Y/Ho 比率轻松检测到。因此，古老的石灰岩，特别是珊瑚礁石灰岩，可以提供可靠的海水稀土元素代表。微生物岩在距今 3.5 Ga 的干净石灰岩中出现，表明在地球历史的大部分时间里重建浅海 REE 化学的可能性，对古地理学和古氧化还原研究具有重要意义。
The concentration of rare earth elements and yttrium (REE + Y) was determined in Holocene Mg-calcite microbialites from shallow reef framework cavities at Heron Reef, Great Barrier Reef. Shale-normalized REE + Y patterns of 52 microbialite samples show: (1) uniform heavy REE enrichment (NdSN/YbSN= 0.236, SD = 0.026); (2) consistent negative Ce and positive La anomalies; (3) marine Y/Ho ratios (56.17, SD = 2.66); and (4) slightly positive Gd anomalies. All of these features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater. REE partition coefficients were calculated relative to shallow Coral Sea seawater. They are uniform (relative SD = 10.2%) across the entire mass range and almost two orders of magnitude higher than those between coral and seawater. Hence, terrigenous detritus-free, modern microbialites are a more reliable proxy for seawater REE chemistry than are skeletal carbonates. Ancient limestones have been considered largely problematic as sources for REE proxies owing to perceived problems with diagenesis, partly on the basis of relatively high REE concentrations in some limestones compared to modern skeletal carbonates. However, high REE concentrations in modern microbialites suggest that ancient limestones with relatively high REE concentrations, if not contaminated by terrigenous detritus, may reflect original seawater chemistry. Terrigenous contamination, if present, is readily detectable on the basis of co-occurring trace element concentrations (Sc, Hf, Th) and Y/Ho ratio. Hence, ancient, particularly reefal, limestones may provide reliable seawater REE proxies. The occurrence of microbialites in clean limestones as old as 3.5 Ga suggests the possibility of reconstructing shallow marine REE chemistry over most of Earth history with important implications for paleogeography and paleoredox studies.