Constraints in using Cerium-anomaly of bulk sediments as an indicator of paleo bottom water redox environment: A case study from the Central Indian Ocean Basin
Constraints in using Cerium-anomaly of bulk sediments as an indicator of paleo bottom water redox environment: A case study from the Central Indian Ocean Basin
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DOI：
10.1016/j.chemgeo.2005.06.009
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发表时间：
2005-10
期刊：
影响因子：
3.9
通讯作者：
J. Pattan;N. Pearce;P. G. Mislankar
中科院分区：
文献类型：
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作者：
J. Pattan;N. Pearce;P. G. Mislankar
中印度洋海盆5 m长沉积岩芯（AAS-05/GC-02）28个剖面的稀土元素丰度沿着主量元素和微量元素组成显示出明显的稀土分馏特征。岩芯顶部101 m的沉积物具有相对较低的∑REE丰度（接近平均页岩），样品/页岩REE比非常接近1，强烈表明主要为陆源。稀土元素丰度随深度增加近2倍（∑REE=167-341 ppm），与Mn、Ti、P和蒙皂石有较强的协变关系。元素间的相关性和页岩标准化的稀土模式表明，轻，中，重稀土选择性携带的陆源，氧化锰和自生磷酸盐相分别表明一个明显的稀土分馏。这些分馏效应随着地核深度的增加而变得更加明显。海洋沉积物中的铈（Ce）异常被用作追踪古底层水氧化还原条件的有前途的工具之一，其在沉积物中的变化范围为0.10至−0.08。Ce异常总体上在顶部4 m为正异常，但在顶部101 m有较高的Ce异常，Mn/Ti和Ce/Ti反映了在富氧底层水条件下沉积物的相对沉积。负铈异常之间的4和5米的深度，这可以解释为指示一个亚氧/缺氧环境。由Ce异常推断的氧化还原环境进行了比较，一些其他氧化还原敏感参数，以测试其可靠性。这些指标包括Mn含量、总有机碳、U/Th、自生铀、Cu/Zn和V/Cr比值，都表明沉积物是在含氧的底层水条件下沉积的。因此，4和5米的岩芯深度之间观察到的负Ce异常并不代表一个亚氧/缺氧的环境，但更可能是由于保留的负Ce异常所造成的自生磷酸盐和自生富铁蒙脱石，这是由铁的羟基氧化物和生物蛋白石之间的反应，表明一个早期的成岩过程，并与海水稀土元素的平衡。早期成岩作用是造成这部分负Ce异常的主要原因。因此，我们建议，铈异常的散装沉积物作为古海洋底层水氧化还原条件的指示需要谨慎使用。
The total rare earth elements (∑REE) abundance along with major and a range of trace element chemistry of twenty-eight sub-sections in a 5 m long sediment core (AAS-05/GC-02) from the Central Indian Ocean Basin shows a distinct REE fractionation. Sediments from the top ∼1 m of the core have comparatively low ∑REE abundance (close to average shale), with a sample/shale REE ratio very close to 1 strongly suggesting a dominantly terrigenous source. The down-core ∑REE abundance shows a nearly two fold increase (∑REE=167–341 ppm) with depth and the REE strongly co-varies with Mn, Ti, P and smectite. Inter-element correlations and the shale-normalized REE patterns suggest that light, middle and heavy REE are selectively carried by terrigenous, Mn oxide and authigenic phosphate phases respectively suggesting a distinct REE fractionation. These fractionation effects become more pronounced with increasing depth in the core. The Cerium (Ce)-anomaly in marine sediments is used as one of the promising tools to trace paleo bottom water redox conditions and this varies in the sediments from 0.10 to −0.08. The Ce-anomaly in general is positive in the top 4 m but the top ∼1 m have higher Ce-anomaly, Mn/Ti and Ce/Ti suggest sediment deposition relatively under more oxic bottom water condition. The negative Ce-anomaly between 4 and 5 m depth, which could be interpreted to indicate a suboxic/anoxic environment. The redox environment inferred by the Ce-anomaly is compared to a number of other redox sensitive parameters to test its reliability. These include Mn content, total organic carbon, U/Th, authigenic Uranium, Cu/Zn and V/Cr ratio that all suggestive of deposition of sediments under oxygenated bottom water condition. Thus, the negative Ce-anomaly observed between 4 and 5 m core depth does not represent a suboxic/anoxic environment, but is more likely due to the retention of a negative Ce-anomaly caused by authigenic phosphate and authigenic Fe-rich smectite which is formed by the reaction between iron oxyhydroxide and biogenic opal suggests an early diagenetic process and is in equilibrium with seawater REE. Early diagenetic process is responsible for the part of negative Ce-anomaly observed here. Therefore, we suggest that the Ce-anomaly of bulk sediments as an indicator of paleo-ocean bottom water redox conditions needs to be used with a caution.