Geochemical Heterogeneity of Estonian Graptolite Argillite

Abstract

This paper describes vertical fine-scale geochemical heterogeneity of Estonian graptolite argillite (GA). GA samples from Pakri and Saka outcrop sections were collected at 20 cm intervals for chemical analysis of major and trace elements, including rare earth elements. The study indicates GA enrichment in U, V, Mo and Pb with respect to the average black shales and thus confirms the formerly reported data on GA geochemistry in general. However, the content of enriched elements and other trace metals was recorded to vary greatly across the sequences suggesting that trace metal distribution in GA is notably more heterogeneous than previously assumed. The origin of the observed complex distribution of trace elements was likely controlled by the interplay of different primary metal supply-sequestration factors/processes, such as synsedimentary redox-driven sequestration of redox sensitive elements, the provenance of clastic input, the postsedimentary redistribution, etc.

Depositional framework of the East Baltic Tremadocian black shale revisited

“…The encounter of stacked successions of finely parallel and subtle lenticular laminated fabric in the lower part of the Pakri section indicates that the intermittent advective transport-controlled deposition of mud was likely one of the major mechanisms behind laminated mud accumulation. However, the same interval is also characterised by the high content of OM (LOI 5008C) and V, Mo and U (Voolma et al 2013; Fig. 2), which designate that the sedimentary environment should have been favourable for accumulating and preserving biogenic matter and redox-sensitive trace metal enrichment.…”

“…Both studied sections presented a high content of redox-sensitive elements such as V, Mo and U (recorded maximum content: V ¼ 1500 ppm, Mo ¼ 1800 ppm, U ¼ 800 ppm; Voolma et al 2013). The average concentration of those metals was detected to be considerably higher in the Saka section (enrichment factor (2014) V ¼ 1.2, U ¼ 2.95, Mo ¼ 3.9 respective to the average content in the Pakri section; Voolma et al 2013). Widely used geochemical palaeoredox proxies, such as V/(V þ Ni) and V/Cr ( Fig.…”

“…Fig. 2 presents schematic sections of both study locations, sampling intervals together with enriched trace metal profiles, and some commonly used geochemical palaeoredox and environmental indices based on data of Voolma et al (2013). Note that the geochemical data are results of analyses of homogenised whole-rock samples, each representing average chemical composition of 20 cm long interval of studied sections.…”

“…For this study, the Pakri and Saka sections were sampled at 20-cm intervals. From each collected field sample, approximately half was crushed and homogenised for major and trace element analyses (Voolma et al 2013). The rest was used for lithological study.…”

“…Recently, submillimetre-to millimetre-scale vertical lithological heterogeneity has been recognised as a common feature in a wide variety of mudstones, including organic-rich mudstones (Aplin & Macquaker 2011 and references there in). In the case of the Türisalu Fm., the existence of pronounced spatial and subtle decimetre-scale vertical trace metal variability has been documented (Pukkonen & Rammo 1992;Soesoo & Hade 2012;Voolma et al 2013). Whereas the syngenetic enrichment of redox-sensitive trace metals is a widely accepted pathway for metal sequestration in typical black shales (e.g.…”