Solar Cycles Recorded In The Laminites From The Wuqiangxi Formation Of South China

The Sun is the primary driver in the changes of Earth's climate system.The sunspot cycle is a short-term cycle of solar activity(~11 yr),which can lead to the variation of climate in decadal to centennial time interval and be recorded in the sediments under specific depositional environments(i.e.,as varves).However,due to the rarity of the favorable conditions for the formation and preservation of varves,sunspot cycles recorded in geological records are mainly concentrated in Phanerozoic,with only a few are known in the Precambrian.In addition,whether the periodicity of solar activity remains consistent and stable during the periods of the large climatic events(e.g.,the “Snowball Earth” events)is an intriguing scientific question.In this study,we reported the signals of solar activity recorded in laminites from the top of the Wuqiangxi Formation in South China at the dawn of global Sturtian glaciation(~715 Ma),and provided a relevant explanation for the formation and preservation of the laminites.In the meantime,the laminites of the Wuqiangxi Formation allow a glimpse into the climatic conditions in South China before onset of the glaciation.The discoveries are briefly summarized as follows.1)The Wuqiangxi laminites consist of alternating submillimeter-scaled fine-sand to silt light laminae and organic-and-clay-rich dark laminae.Microfabric and composition analyses show that they are seasonally original siliciclastic biolaminites.The light laminae deposited from winter to early spring,while the dark laminae are formed of microbial mats in warm seasons,from late spring to early fall.Each light lamina and the successive dark lamina constitute a laminar couplet which stands for an annual interval.The presence of the laminites suggested strong seasonality in the climate in South China before onset of the glaciation.2)Spectral analysis and evolutionary fast Fourier transform of the thickness series of 1434 laminar couplets revealed prominent periodicities of ~11 yr and ~22 yr,matching well with the Schwabe and Hale solar cycles,respectively.Variations in the geochemical series of terrigenous clast related elements(e.g.,Al,Th),proxy elements for primary productivity(e.g.,Ba/Ti,Cu/Ti,Zn/Ti),redox sensitive elements(e.g.,V/Ti,U/Ti)and the Chemical Index of Alteration(CIA)of dark laminae show the same periodicities.These likely suggested that the sediment deposition and microbial activity were susceptible to the subtle climatic changes influenced by solar activity.3)The preservation of solar signals in the Wuqiangxi laminites may have been facilitated by quiet depositional environment of lower shoreface,microbial mat protection,and early diagenetic silicification.The research fills in the gap in solar activity study in Neoproterozoic and suggests that the solar periodicity remain stable at the dawn of the snowball Earth.The transformation from relatively dark laminites to relativey light laminites upwards in the Wuqiangxi Formation may imply the gradual shifting of ancient climate in the south China from relatively warm to relatively cold.