Early seawater-derived dolomite altered under distinct geochemical-buffering conditions: Insights into early diagenetic remobilization of rare earth elements

Early-formed seawater-derived dolomite is often used to reconstruct seawater chemistry and fluid-rock interaction, yet the fidelity of its geochemical proxies remains uncertain. Rare earth elements (REE) are commonly regarded as immobile during diagenesis. However, growing evidence suggests that REE can be remobilized even under low water-rock interactions. Here, we present petrographic, isotopic, and REE data from early-formed dolomite (RD1) of the Musayr Formation, northwest Saudi Arabia, to evaluate geochemical proxy preservation, inheritance, and remobilization during early diagenesis. The host limestone shows evidence of pervasive meteoric alteration, pre-dating dolomitization, and exhibits a near-flat to MREE-bulged pattern, consistent with diagenetic remobilization of REE. The analyzed Oyster shells display vitally fractionated trace and REE signatures but meteorically modified 613C-618O values, questioning the reliability of their isotopic fingerprints for paleo-seawater reconstructions. Two distinct RD1 dolomite types are defined. The first (RD1-TUs) has 613C, 618O, REE patterns, and Y/Ho ratios indicative of formation from near-normal seawater under suboxic, restricted conditions, with indication of strong rock buffering of REE. The second type (RD1-2MWB) exhibits an MREEbulged pattern and elevated Fe-Mn-Sr concentrations, reflecting localized recrystallization under more reducing conditions or analytical contamination artifact. This suggests that REE deviations reflect coupled remobilization within host carbonates and marine restriction during RD1-TUs dolomitization, rather than open-system diagenesis. These findings demonstrate that early seawater-derived dolomite can retain marine isotopic signatures while recording non-seawater REE patterns governed by rock-buffered diagenesis. Crucially, different geochemical proxies capture distinct diagenetic processes, underscoring the need for multi-proxy approaches in paleoenvironmental reconstructions.