Geochemical signatures of beachrock: archives of coastal pollution, paleoenvironmental change, and Holocene sea-level fluctuations

Abstract

Beachrock offers a unique geochemical record of palaeoenvironmental change, relative sea-level (RSL) oscillations, and anthropogenic impact, etc. This paper synthesises major, trace, rare earth element (REE), and stable isotope (δ13C, δ18O) data across three frameworks: (i) palaeoclimatic reconstruction, (ii) Holocene RSL history, and (iii) coastal pollution monitoring. Relative to the upper crust, beachrock is enriched in CaO (20–50 wt%) and MgO (∼2 to >10 wt%) and depleted in SiO2, Al2O3, Na2O, K2O, and TiO2, indicating carbonate dominance. Trace-element proxies (Sr/Ba, V/Cr, V/(V+Ni), Rb/K) constrain palaeosalinity and palaeoredox, placing formation in suboxic-oxic transitional settings. REE patterns (Lan/Ybn=1.2–3.5; Ce/Ce*=0.75–0.95; Eu/Eu*≈0.60–0.75; Y/Ho≈45) provide seawater-like signatures, with peloidal microbially mediated cements as the most robust archives. Cement δ13C and δ18O yield palaeotemperatures (21–32°C in tropics), fluid-mixing histories, and diagenetic overprint. Beachrock sequesters metals (loid) (Cu, Zn, Pb, Cd, As, Cr), organic pollutants, and microplastics at the land-sea interface, establishing a stratigraphic sentinel for Anthropocene contamination. MICP pathways also suggest nature-based coastal resilience potential. We advocate standardised protocols, global databases, and multi-proxy integration to unlock beachrock’s full potential.