Lacustrine shallow biogenic gas production linked to climatically forced increase in organic matter burial following the Mid-Pleistocene Transition

The sediments in the Hetao Basin of North China during the Quaternary are dominated by fluvial-lacustrine deposition, with widespread development of biogenic gas. However, the controlling factors of organic matter (OM) accumulation and preservation in biogenic gas source rocks remain unclear, and their potential influence by the Mid-Pleistocene Transition (MPT) requires further investigation. This study analyzed gamma ray (GR), Rubidium strontium ratio (Rb/Sr), mean grain size, total organic carbon (TOC), total nitrogen (TN) and organic carbon stable isotopes (δ¹³C_org), along with optically stimulated luminescence (OSL) and electron spin resonance (ESR) dating, to establish a two million-year astronomical timescale and explore the impact of the MPT on accumulation and preservation of OM. The results indicate that: 1) The MPT at ca. 1.1 to 0.75 Ma is well recorded in the GR, Rb/Sr and mean grain size data of the lake sediments, and is characterized by a shift in the dominant cyclicity from ∼41-kyr obliquity to ∼100-kyr eccentricity cycle; 2) The sedimentary cycles of the Hetao Basin lakes were primarily controlled by astronomical forcing through dynamic feedbacks within the ice sheet–climate system. These cycles represent the indirect climatic effects of orbital variations via their modulation of ice volume and monsoon intensity; 3). After the MPT, increased input of terrestrial organic matter during interglacial periods, favorable depositional environments and the extension of glacial-interglacial cycles created favorable conditions for the accumulation and preservation of organic matter in source rocks, thereby providing the inputs required for biogenic gas formation.