Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China, and its paleoenvironmental and paleomagnetic implications

Abstract

Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climaticallysensitive. An almost continuous 626 m long sediment core was drilled in an infilled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine-grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X-ray diffraction analysis, scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe 3 S 4 ). The greigite is of early diagenetic origin and formed in an inerval of high lake level and inferred relatively warm, humid climate. The greigite-enriched zones are separated by an interval of relatively high total sulfur and organic carbon content, and we infer that in the adjacent greigite-bearing zones, the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a time scale of several million years. The greigite zones contain narrow intervals of normally magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix.

Magnetic mineral diagenesis

“…Nevertheless, sulphate values are not zero and magnetite dissolution is a widely reported process in organic-rich lake sediments (e.g., Anderson and Rippey, 1988;Snowball, 1993a, b;Thouveny et al, 1994;Rosenbaum et al, 1996;Snowball, 1996;Stockhausen and Zolitschka, 1999;Demory et al, 2005;Ao et al, 2010;Nowaczyk, 2011;Su et al, 2013;Fu et al, 2015). Much iron oxide dissolution in lake sediments could occur via iron reduction; nevertheless, widespread iron sulphide formation indicates that sulphidic diagenesis is not a negligible process in lake sediments.…”

Palaeoclimate evolution across the Cretaceous–Palaeogene boundary in the Nanxiong Basin (SE China) recorded by red strata and its correlation with marine records

“…Environmental magnetism as a proxy has been widely used in studies of palaeoclimatic changes in Quaternary loess-palaeosol successions (Evans and Heller, 2001;Hao and Guo, 2005;Maher and Possolo, 2013;Maher, 2016), Tertiary red clay successions (Liu et al, 2003;Nie et al, 2008;Zhao et al, 2016), and other older aeolian deposits (Hao et al, 2008;Zhan et al, 2011), as well as in studies of lake sediments (Snowball et al, 1999;Fu et al, 2015;Hu et al, 2015) and marine sediments (Larrasoaña et al, 2008;Peters et al, 2010). In this paper, we review previous work (mainly in terms of geochronology and palaeoclimatology) and report some defects in the established chronological framework and palaeoclimatic record.…”

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

“…Although pyrrhotite and greigite may form as early diagenetic metastable phases during the chemical reaction pathway to pyrite, studies have shown that they may be preserved if the concentration of organic matter, and consequently organic-bound sulfur, is low and pyritization is hampered by Fe excess (Blanchet et al, 2009;Kao et al, 2004;Roberts et al, 1996;Skinner et al, 1964). Cyclic preservation of greigite in a Pliocene lacustrine sequence from Lake Qinghai (China) was recently linked to periods of high lake levels and humid climate that resulted in oxygen depletion of the bottom water (Fu et al, 2015).…”