Contrasting Rare Earth Element Signatures Between Stromatolitic Carbonates and Lagoon Waters at Shark Bay, Western Australia: Implications for Paleo‐Environmental Reconstructions of Microbial Habitats

Rare earth elements and yttrium (REY) signatures in stromatolitic carbonates have emerged as powerful geochemical proxies for reconstructing paleo‐depositional environments of microbial habitats. The applicability of such a proxy relies on the assumption that REY substitutes for calcium into crystal lattices, directly reflecting the composition of the fluid from which the carbonate precipitated. The REY signatures of stromatolites are often similar to those of open ocean seawater. In restricted environments, however, REY fractionation can occur between waters and stromatolites, questioning their reliability to reconstruct microbial habitats through Earth's history. In this contribution, we determined the REY concentrations and partition coefficients (Kd (stromatolite‐fluid) ) of sub‐recent stromatolitic carbonates and ambient waters from the hypersaline Hamelin Pool in the Shark Bay lagoon, Australia. Shale‐normalized REY patterns of stromatolite morphologies show (except colloform morphologies) a middle REY SN enrichment relative to the light and heavy REY SN . These signatures differ from those of seawater and ambient waters, suggesting that today's waters cannot be directly compared with stromatolitic carbonates from Shark Bay, which formed over thousands of years in a complex microbial mat system. Stromatolite morphologies such as colloform, smooth, or pustular structures formed in supratidal and intertidal environments exhibit the most variable Kd (stromatolite‐fluid) values for the REY. The most dominant process affecting REY geochemistry in the Shark Bay stromatolites is most likely organic matter degradation and subsequent REY release into porewaters from which the carbonates formed in a (semi)closed microbial mat system. Cerebroid structures of the deepest lagoonal environment in the subtidal zone show the most constant Kd (stromatolite‐fluid) values throughout the REY series, reflecting the least closed microbial mat system and a direct relationship between stromatolite morphology and water chemistry.