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A re-assessment of elemental proxies for paleoredox analysis
Chemical Geology ( IF 3.9 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.chemgeo.2020.119549
Thomas J. Algeo , Jiangsi Liu

Abstract Paleoredox conditions are commonly evaluated based on elemental proxies but, despite their frequency of use, most of these proxies have received little comparative evaluation or assessment of their range of applicability to paleomarine systems. Here, we evaluate 21 elemental proxies, including six proxies based on the C-S-Fe-P system (TOC, S, TOC/S, DOPT, Fe/Al, Corg/P), nine proxies based on trace-metal enrichment factors (CoEF, CrEF, CuEF, MoEF, NiEF, PbEF, UEF, VEF, ZnEF), and six additional proxies from Jones and Manning (1994) (U/Th, Uauth, V/Cr, Ni/Co, Ni/V, (Cu+Mo)/Zn), in 55 Phanerozoic marine formations. We used principal components analysis (PCA) to determine relationships among these 21 proxies in each formation and then sought to identify patterns across the full database. The first principal component (PC1) accounted for 40.1% of total dataset variance on average, with the highest median loadings on trace-metal enrichment factors (NiEF 0.82, MoEF 0.76, all nine >0.50). The next highest median loadings are on C-S-Fe-P proxies (TOC 0.58, DOPT 0.30, Corg/P 0.28), with bimetal proxies yielding uniformly lower loadings (Ni/Co 0.18, V/Cr 0.13). PCA of the factor loadings for the 55 study formations demonstrated associations among the 21 elemental proxies linked to specific sediment host phases: (1) an organic cluster associated with TOC, Mo, V, and Zn, (2) a uranium cluster associated with all U-based proxies, and (3) a sulfide cluster associated with S and Fe as well as the trace metals Co, Cu, Ni, and Pb (i.e., the major and typical minor constituents of diagenetic pyrite). The findings of the present study have important ramifications for use of elemental proxies for paleoredox analysis. First, all of the proxies examined here are influenced by environmental redox conditions to some degree, although the degree of redox influence on any given proxy can vary considerably from one formation to the next. Second, sedimentary enrichment of most proxies depends on the presence of specific mineral and organic host phases, and evaluation of elemental redox proxy data requires an understanding of how elements are partitioned among those phases. Third, no single proxy is a universally reliable redox indicator, although some are more consistently useful than others—notably, TOC and trace-metal EFs. Fourth, because of this inherent variability in proxy response, adoption of redox proxy thresholds established in earlier published studies is discouraged. Instead, we recommend that future redox studies establish redox thresholds on a formation-specific basis through internal cross-calibration of multiple redox proxies.

中文翻译:

用于古氧化还原分析的元素替代物的重新评估

摘要 古氧化还原条件通常基于元素代理进行评估,但尽管使用频率很高,但这些代理中的大多数几乎没有对其适用于古海洋系统的范围进行比较评估或评估。在这里,我们评估了 21 种元素代理,包括基于 CS-Fe-P 系统的 6 种代理(TOC、S、TOC/S、DOPT、Fe/Al、Corg/P),基于痕量金属富集因子的 9 种代理( CoEF、CrEF、CuEF、MoEF、NiEF、PbEF、UEF、VEF、ZnEF),以及来自 Jones 和 Manning (1994) 的六个附加代理(U/Th、Uauth、V/Cr、Ni/Co、Ni/V,( Cu+Mo)/Zn),在 55 个显生宙海相地层中。我们使用主成分分析 (PCA) 来确定每个地层中这 21 个代理之间的关系,然后试图识别整个数据库中的模式。第一主成分(PC1)占40。平均占总数据集方差的 1%,微量金属富集因子的中值负荷最高(NiEF 0.82,MoEF 0.76,所有九个 >0.50)。次高的中值负载是 CS-Fe-P 代理(TOC 0.58,DOPT 0.30,Corg/P 0.28),双金属代理产生均匀较低的负载(Ni/Co 0.18,V/Cr 0.13)。55 个研究地层的因子载荷的 PCA 证明了与特定沉积物宿主相相关的 21 个元素代理之间的关联:(1) 与 TOC、Mo、V 和 Zn 相关的有机簇,(2) 与所有相关的铀簇基于 U 的替代物,以及 (3) 与 S 和 Fe 以及痕量金属 Co、Cu、Ni 和 Pb(即成岩黄铁矿的主要和典型次要成分)相关的硫化物簇。本研究的结果对使用元素替代物进行古氧化还原分析具有重要意义。首先,这里检查的所有代理都在一定程度上受环境氧化还原条件的影响,尽管氧化还原对任何给定代理的影响程度可能因一个地层到下一个地层而有很大差异。其次,大多数替代物的沉积富集取决于特定矿物和有机宿主相的存在,元素氧化还原替代物数据的评估需要了解元素如何在这些相之间分配。第三,没有单一的替代品是普遍可靠的氧化还原指标,尽管有些指标比其他指标更有用——尤其是 TOC 和痕量金属 EF。第四,由于代理响应的这种固有可变性,不鼓励采用早期发表的研究中建立的氧化还原代理阈值。相反,我们建议未来的氧化还原研究通过多个氧化还原代理的内部交叉校准,在特定地层的基础上建立氧化还原阈值。
更新日期:2020-05-01
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