Discerning biotic from abiotic carbonate cementations in Holocene beachrock: mineralogical and geochemical fingerprints for biosignatures

Abstract

Beachrock cementation, mediated by physicochemical and biological processes, plays a pivotal role in the rapid lithification of coastal sediments. Distinguishing between biotic and abiotic carbonate cements is critical for quantitatively reconstructing diagenetic pathways and accurately evaluating paleoenvironmental conditions. We analyzed Holocene beachrocks from Hainan and the Xisha Islands to compare their cement types. Beachrocks from Hainan are characterized by isopachous high-Mg calcite (HMC) rinds, HMC-rich peloidal fabrics, and blocky–drusy low-Mg calcite (LMC), whereas those from the Xisha Islands are dominated by acicular aragonite. HMC rinds composed of equilong fibrous crystals represent marine phreatic cements with minimal diagenetic alteration, whereas LMC forms through meteoric vadose processes, as indicated by red cathodoluminescent rings. Acicular aragonite nucleates within organic-rich micrite envelopes, with its rapid growth governed primarily by inorganic mechanisms. Peloidal fabrics exhibit distinct fluorescence patterns, dull cathodoluminescence, and elevated Mn/Fe ratios, consistent with their interpretation as microbially mediated precipitates formed during organic matter degradation under intermittently reducing conditions. Notably, peloidal fabrics and aragonite are enriched in bioactive metals (V, Fe, Zn, Pb) relative to coral Porites, consistent with active microbial metabolism. Unlike HMC rinds and acicular aragonite, peloidal fabrics display high total rare earth element (REE) contents, uniform REE distributions, LREE depletion (low LREE/HREE), negative Ce anomalies, positive La anomalies, Y/Ho ratios (∼45), and REE partitioning patterns closely matching modern seawater. These distinctive geochemical signatures establish peloidal fabrics as high-fidelity, high-resolution archives of paleo-seawater chemistry, providing a robust tool for deciphering ancient marine environments.