Growth mechanisms and environmental implications of carbonate concretions from the ~ 1.4 Ga Xiamaling Formation, North China

Abstract

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments, and have the potential to reflect seawater chemistry indirectly. In fine-siliciclastic settings, they preferentially form in organicrich mudstones, owing to a significant fraction of the bicarbonate required for carbonate precipitation resulted from the decomposition of organic matter in sediments. In the Member IV of the Xiamaling Formation (ca. 1.40-1. 35 Ga), North China, however, carbonate concretions occur in organic-poor green silty shales (avg. TOC =~0.1 wt%). In order to elucidate the mechanism of the concretion formation and their environmental implications, a thorough study on the petrographic and geochemical compositions of the concretions and their host rocks was conducted. Macro-to microscopic fabrics, including deformed shale laminae surrounding the concretions, "cardhouse" structures of clay minerals and calcite geodes in the concretions, indicate that these concretions are of early diagenetic origin prior to the significant compaction of clay minerals. The carbon isotope compositions of the concretions (− 1.7‰ to + 1.5‰) are stable and close to or slightly lower than that of the contemporaneous seawater, indicating that the bicarbonates required for the concretion formation were mainly sourced from seawater by diffusion rather than produced by methanogenesis or anoxic oxidation of methane (AOM); the rare occurrence of authigenic pyrite grains in the concretions likely indicates that bacterial sulfate reduction (BSR) did not play a significant role in their formation either. Almost all the calcite in the concretions has low Mn-Fe in nuclei but high Mn-Fe in rims with average Mn/Fe ratio close to 3.3. The calcite shows positive Ce anomalies (avg. 1.43) and low Y/Ho ratios (avg. 31). This evidence suggests that Mn reduction is the dominant process responsible for the formation of calcite rims while nitrate reduction probably triggered the precipitation of calcite nuclei. Prominence of Mn reduction in the porewater likely indicates that there was sufficient oxygen to support active Mn-redox cycling in the overlying seawater.

Full-Scale Pore Structure Characteristics and the Main Controlling Factors of Mesoproterozoic Xiamaling Shale in Zhangjiakou, Hebei, China

“…It should be noted that although many scholars have analysed the influence of mineral composition on pore structure, most of the research results have not removed the concealment of TOC content on pore structure, and the correlation between each clay mineral composition and pore structure of different scales is rarely discussed under the removal of TOC concealment [ 23 , 29 , 46 ]. After removing the concealing effect of TOC content on the micro-nano pore structure, this study comprehensively and deeply reveals the influence of brittle minerals such as quartz and calcite, clay minerals such as smectite, I/S and illite, on the formation and development of the PV and SA of micropores, mesopores, and macropore.…”

“…The thermal maturity reflects the dual effects of compaction and time experienced by shale. According to Liu et al [ 29 ], the Xiamaling shale in Zhangjiakou, Hebei has been in a slow subsidence and deep burying state for a long time since its deposition to the end of the Carboniferous; then, this shale started lifting at the beginning of the Permian and subsided rapidly in the middle of the Cretaceous. The negative relationship between EqRo and the TOC-normalized PV and SA of different nanoscale pores of the Ximaling shale is predominantly due to the deep progressive burial and compaction of the shale.…”

“…The thick of the third section is up to 350 m and contains black carbonaceous, silty and grey-black siliceous shale, as well as oil shale accumulated by red algae and brown algae, indicating that the Xiamaling shale in this region has superior hydrocarbon generation potential, with favourable shale gas exploration prospects. Additionally, according to the latest isotopic timing results, the Ximaling shale formation was deposited 1368 Ma years ago, and formed at Jixian system in Precambrian, indicating that the Ximaling shale is the oldest marine shale found in China, and it is one of the oldest ancient marine shales in the world [ 28 , 29 ].…”

In siturare earth element analysis of a lower Cambrian phosphate nodule by LA-ICP-MS

“…Kim et al 2012;Chen et al 2015b;Ye et al 2020). For example, MREE enrichments of the Mesoproterozoic Xiamaling carbonate concretions (Liu et al 2019) and the Ediacaran Doushantuo cap carbonates (Wu et al 2019) are both attributed to metal oxide reduction. Since carbonate samples generally have low TOC, it is reasonable to exclude organic matter as a candidate.…”

A Pulsed Oxygenation in Terminal Paleoproterozoic Ocean: Evidence From the Transition Between the Chuanlinggou and Tuanshanzi Formations, North China

“…Some other studies, however, suggested a highly dynamic redox landscape for the mid‐Proterozoic ocean, with spatio‐temporally heterogeneous seawaters (e.g., Diamond & Lyons, 2018; Gilleaudeau & Kah, 2015; Hardisty et al., 2017; Planavsky et al., 2018; Poulton et al., 2010; Sperling et al., 2014; Tang et al., 2016). Multi‐proxy evidence from several sedimentary basins seems to support an episodic or spatially limited oxygenation model in the mid‐Proterozoic ocean that is at least identifiable regionally (Chen et al., 2020; Diamond & Lyons, 2018; Liu et al., 2020, A. Q. Liu et al., 2019; Planavsky et al., 2018; Shang et al., 2019; Sperling et al., 2014; Tang et al., 2020; Wang et al., 2018; Zhang et al., 2018; Zhang et al., 2016). Some paleoredox proxy data used for arguing an elevated p O 2 in the interval of 1.4–0.9 Ga, however, are apparently contradictory.…”