Carbonate-associated fluorescent dissolved organic matter (CAFDOM): Establishing a new proxy for dissolved organic matter in ancient oceans

Dissolved organic matter (DOM) is the largest reservoir of reduced carbon in the modern ocean, playing a crucial role in the global carbon cycle and climate dynamics. However, there remains a lack of a proxy to directly track the composition and abundance of DOM in ancient oceans. In this study, we present a new approach for extracting and detecting carbonate-associated fluorescent dissolved organic matter (CAFDOM), aiming to establish it as a proxy for tracking DOM in ancient oceans. Our approach relies on three types of materials: (i) experimental samples with specifically defined properties, (ii) natural carbonate samples with known diagenetic histories (IODP core material, Marion Plateau), and (iii) well-preserved Ediacaran carbonate rocks from China. Following the protocol developed herein, the CAFDOM analysis of simulated carbonate samples shows good reproducibility for identifiable components, with relative standard deviations of less than 11% and average extraction rates exceeding 80%. Using fulvic acid as a natural model for DOM, carbonate precipitation experiments show that CAFDOM positively responds to solution DOM concentration in three distinct modes: (i) an exponential increase from zero with fulvic acid concentrations of 0–0.4 g/L; (ii) another exponential increase from zero with fulvic acid concentrations of 0.4–1.4 g/L; and (iii) a weak linear increase from a particular threshold with fulvic acid concentrations greater than 1.4 g/L. These modes are likely linked to critical shifts in the intermolecular forces of the DOM and its aggregation states during incorporation into the carbonate lattice. We propose that the abundance of DOM in ancient oceans can be constrained by combining the abundances with pairwise ratios of the CAFDOM components, and further evaluation and verification based on additional information of the investigated samples. Analysis of IODP core samples suggests that CAFDOM levels are likely influenced by the recrystallization from aragonite to calcite, authigenic carbonate formation, and replacement dolomitization. Based on the criteria established in this study, we analyzed Ediacaran carbonate samples (635–551 Myr) for CAFDOM. These data reveal significant variability in DOM in the Ediacaran marginal ocean, both in terms of concentration and fluorescent components. These findings are promising and suggest that, after conducting mineralogical characterization and screening for secondary alterations of sample material, CAFDOM can serve as an effective proxy for tracking DOM in ancient oceans.