Orbitally forced organic matter accumulation recorded in an Early Permian mid-latitude palaeolake

Abstract

Organic matter (OM) sequestration acts as a significant sink of the CO2 in Earth's carbon cycle and is tightly associated with the climate and depositional environments. However, the dominant factors controlling OM burial in different depositional settings and the orbitally forced variations in the lacustrine OM accumulation remain poorly constrained. Here, we investigate the depositional environment of the Lucaogou Formation in southern Junggar Basin, northwestern China and further decipher the astronomically forced OM accumulation in different depositional environments. Sedimentological and cyclostratigraphic studies show that both shallow lake and semi-deep to deep lake deposits recorded significant Milankovitch signals among of which ∼405 kyr, ∼91 kyr, 34.5–37.4 kyr, and 17.3–22.8 kyr are interpreted to be long eccentricity, short eccentricity, obliquity, and precession cycles, respectively. A linkage among the lithofacies, natural gamma ray (NGR), and total organic carbon (TOC) content suggests that the depositional environment and OM accumulation were in pace with the hydrological cycle and lake-level changes, in response to the astronomically forced changes in the ice volume and monsoonal intensity. In the semi-deep to deep lake, TOC content tightly tracks the lithofacies and shows an in-phase relationship with NGR. However, this phase relationship has changed with sedimentary facies. The gypsiferous shale deposited in the restricted environment during lowstand intervals has higher TOC content than highstand dolomitic mudstone, demonstrating an anti-phase relationship with NGR data. Our study indicates that the OM sequestration is a dynamically cumulative process of supply and degradation. The different responses of the OM sequestration to the climate change in different environments suggest that detailed sedimentological studies are essential for the cyclostratigraphic analysis.