δ¹¹B Variability in <i>Ammonia parkinsoniana</i> and Implications for Paleo‐pH in Coastal Margins

Abstract Boron isotopes have been used to reconstruct marine pH but less frequently in marginal marine environments (lagoons, coastal shelves), where paleo‐archives are abundant. The dynamic nature of these systems complicates environmental reconstruction, requiring a multi‐proxy approach to constrain changes in conditions. Celestun Lagoon (Yucatan, Mexico), influenced by mixing groundwater and seawater, offers a unique setting to explore such systems. We analyzed salinity ( 87 Sr/ 86 Sr, δ 18 O), temperature (δ 18 O, Mg/Ca), and respiration (δ 13 C, weight% organic C) proxies alongside δ 11 B and B/Ca in calcite of infaunal foraminifera Ammonia parkinsoniana . Results revealed spatial variability in δ 11 B calcite along the lagoon salinity gradient, ranging from 8.06 to 18.53‰, with higher δ 11 B variability near groundwater discharge and seagrass beds. This suggests low‐pH, high‐DIC groundwater and primary production influence the δ 11 B signature. Low concentrations of dissolved [B], [Mg], and [Ca] in groundwater shift the dissociation constant pK B * and possibly the fractionation factor α B . Notably, some δ 11 B calcite values (8.06‰) were lower than expected (11.0‰), suggesting complex influences from source δ 11 B water , pK B *, or dietary carbon with distinct boron signatures. Despite variability, foraminifera δ 11 B calcite (mean ± SD = 13.22 ± 3.41‰, n = 21) aligned with calculated mean δ 11 B borate (13.64 ± 1.55‰, n = 19), supporting potential use of δ 11 B for paleo‐pH reconstructions in marginal marine settings. However, a large sample size may be required to capture average conditions. These findings emphasize the need for more studies to constrain boron isotopes in marginal marine environments which are critical for interpreting paleo‐archives in continental shelves and shallow seas.