Solar activity and El Niño−Southern Oscillation−like signals in a lacustrine stromatolite from the Upper Cretaceous Songliao Basin, Northeast China

Interannual to centennial scale climate variability is often associated with solar activity and the El Niño−Southern Oscillation (ENSO). However, due to the limited temporal resolution of most stratigraphic archives and constraints on dating, high-resolution paleoenvironmental records of such signals in deep time remain scarce. A lacustrine stromatolite from the Upper Cretaceous Qingshankou Formation in northeastern China preserves exceptionally well developed micron scale, light-dark laminae couplets. These couplets are interpreted as annual deposits based on their regular alternation of mineral and organic content, analogous to varves in modern lacustrine systems. Light laminae are interpreted to have formed during warm and humid spring-summer seasons, when enhanced algal productivity promoted calcite precipitation and increased rainfall contributed to detrital influx. In contrast, dark laminae are interpreted to reflect cold and dry autumn-winter conditions, during which the die off and subsequent decomposition of organic matter produced during the preceding spring and summer led to the accumulation of organic-rich material. This seasonal rhythmicity provides a foundation for high-resolution time series analysis. Variations in climate sensitive elements (e.g., Fe, Ca) across the laminae display periodicities of ∼11 years and ∼5 years, corresponding to solar sunspot (11-year) and ENSO-like (∼5-year) climate cycles. Carbon and oxygen isotope data, as well as relative element abundance and element ratios obtained across 106 lamina couplets, indicate that the stromatolite growth environment was characterized by brackish conditions with slight fluctuations in humidity and salinity.