Lake Qinghai, in North China, is the largest interior plateau lake in Central Asia, and is sensitive to climate change and the environmental effects of Tibetan Plateau uplift. We have obtained an almost continuous 626 m long sediment core from an in-filled part of the southern lake basin, which documents both the age of the origin of the lake and the evolution of the East Asian monsoon during the Late Cenozoic. High-resolution magnetostratigraphy provides a chronology back to ca. 5.1 Ma. Analysis of lithofacies and depositional environments reveal that the change from eolian to lacustrine facies occurred at ca. 4.63 Ma, corresponding to a shift from an arid or semi-arid to a more humid climate, which resulted in the origin of Lake Qinghai. Changes in sediment lithology and mean grain size indicate that the lake level fluctuated considerably, superimposed on a long-term trend from higher to lower levels in response to variations in the East Asian Monsoon. This archive is a significant additional source of information on regional and global environmental change, complementing the existing records from north China, which are mainly based on analysis of loess deposits.

Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. 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 (Fe₃S₄). 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.