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

摘要

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 ⅣV 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.