Solar activity recorded by the early Paleogene lacustrine varve in the Nanxiong Basin

The Paleogene Period, recognized as a quintessential greenhouse climate interval in Earth’s history characterized by elevated global temperatures and heightened atmospheric CO2 concentrations, has become a focal area in deep-time paleoclimate research. The current generation of paleoclimatic reconstructions for this interval remains largely constrained to orbital forcing regimes and tectonic-scale frameworks, resulting in a temporal resolution that is inadequate for effectively predicting future global climate change. The deep-time lacustrine varved sediments can record seasonal-to-annual scale paleoclimate information, demonstrating distinct advantages among various interannual paleoclimate proxies, making them ideal research materials for predicting future global climate change. The Nanxiong Basin (25°03ʹ–25°16ʹN, 114°08ʹ–114°40ʹE), located in southeastern China, has complete preservation of Late Cretaceous to Early Paleogene terrestrial strata. This basin has emerged as a hotspot for high-resolution paleoclimatic reconstruction due to its lithostratigraphic completeness and continuous cyclostratigraphic archives spanning the Cretaceous-Paleogene (K-Pg) transition. The terrestrial stratum is dominated lithologically by red silty mudstone and muddy siltstone. The Paleocene Guchengcun Formation in its northwestern sector hosts multiple layers of grayish-black mudstones with varying thicknesses, one of which preserves clear lamination structures. Current academic research demonstrates a paucity of investigations into the paleoclimatic conditions governing the genesis of these mudstones and associated laminations. In this study, high-resolution climate proxies were performed for this laminated sequence (optical microscope observation, EDS, and μXRF), combined with spectral analysis to explore whether it is an annual varve and to reveal the paleoclimate information it records, particularly focusing on solar activity. The results show that: 1) the laminar structure exhibits distinct differences in color, mineral, and chemical composition. Through microscopic observation, it is found that the thickness of the layer pairs varies greatly. One type displays relatively thin couplets (0.3–0.5 mm) within the common thickness range. In contrast, the other type consists of a relatively thick light-colored layer and a dark-colored layer, with individual couplets reaching 3–4 mm thickness. Specifically, the content of endogenous lacustrine components such as carbonates and calcium (Ca) elements is higher in the light layer compared to the dark layer. Conversely, the concentration of extrinsic lacustrine components—including clay minerals, titanium (Ti) elements, and organic matter—is significantly greater in the dark layer than in the light layer; 2) Integrating paleoclimatic records, we propose that the differences between the light and dark layers primarily result from seasonal climate variations throughout the year, indicating that this laminated sequence represents a typical varve. The formation process is as follows: during the summer with more rainfall, a dark layer rich in exogenous debris and organic matter forms; conversely, in the dry and hot winter, intense evaporation leads to substantial carbonate precipitation, resulting in a light-colored layer; 3) The spectral analysis of Ti, Ca and a* value reveals four stable cycles, ranging from 0.20 to 0.46 mm,0.83 to 1.46 mm,2.00 to 4.60 mm, and 6.12 to 7.28 mm, of which 0.20 to 0.46 mm is the thickness of the typical varve, and the latter three cycles may be associated with El Niño-Southern Oscillation (ENSO), the 11-year Schwabe sunspot cycle and the 22-year Hale cycle. Solar activity regulates the climate of the study area through both “top-down” and “bottom-up” mechanisms. During years of high solar activity, precipitation increases along with increased exogenous inputs to the lakes; conversely, evaporation intensifies during low solar activity years while enhanced authigenic carbonate deposits are formed in the lake.