Isotopic and Geochemical Signatures of Carbonate Rocks and Its Implications for Incipient Weathering Processes

Determining carbon sources and sinks is crucial for understanding the global carbon cycle; yet, the enigma of 'missing' sinks remains unresolved. Recent studies have proposed carbonate weathering as a potential carbon sink, underscoring the necessity to clarify its mechanisms. Previous investigations of carbonate weathering predominantly relied on soil profiles, which were limited by the scarcity of incipient weathering layers. To explore these incipient weathering processes, surface-weathered carbonate rocks were collected from dolomite lenses within the Neoproterozoic Liangjiehe Formation (Nanhua System) in Guizhou, China. The pristine dolomite displays δ13C values ranging from -5.26 to -3.35‰ and δ18O values from -13.79 to -12.83‰. These isotopic signatures suggest that the dolomite formed under high-latitude, cold climatic conditions prevalent during the Nanhua Period. Comprehensive petrographic and geochemical analyses of the surface-weathered dolomite rocks revealed two distinct stages of incipient weathering. In Stage I, there is a decrease in Rare Earth Elements (REEs) content, accompanied by the leaching of nickel (Ni) and cobalt (Co). The δ13C values fluctuate between -7.61 and -2.52‰, while δ18O values range from -12.22 to -8.06‰. These observations indicate a weakly acidic microenvironment. In Stage II, there is an enrichment of manganese (Mn), molybdenum (Mo), and zinc (Zn), with δ13C values extending from -16.56 to -12.43‰ and δ18O values from -8.46 to -7.03‰. These changes suggest a transition to a neutral or alkaline microenvironment, with the isotopic compositions of carbon and oxygen in the dolomite being influenced by atmospheric carbon dioxide (CO2) and atmospheric precipitation. This study represents a pioneering investigation into the mineralogical and geochemical variations associated with the incipient weathering process of carbonates, indicating that surface-weathered carbonate rocks may serve as an underutilized archive for reconstructing the dynamics of incipient weathering.