Two-dimensional imaging of porewater chemistry to investigate the heterogeneity of diagenetic processes in Amazonian mangrove sediments

Quantifying biogeochemical processes in coastal sediments requires analytical approaches capable of resolving microscale variability in redox-sensitive solutes. Conventional porewater sampling techniques provide limited spatial resolution and often disturb in situ equilibria, obscuring fine-scale heterogeneity associated with bioturbation, root activity, and microbial processes. These limitations are particularly critical in mangrove sediments, where organic matter remineralization and redox dynamics are highly heterogeneous. In addition, small-scale geomorphological contrasts between erosional and depositional settings can influence sediment structure, permeability, and diagenetic pathways. Here, we applied two-dimensional Diffusive Equilibration in Thin Films (2D-DET) coupled with colorimetric detection to map porewater solutes associated with early diagenesis in mangrove sediments from two sites (MAR1 and MAR2) in the Marapanim River Estuary (Pará State, Brazil). The sites, sampled in winter 2025, represent erosional and depositional zones on opposite sides of a tidal channel. Two-dimensional distributions of dissolved Fe and Mn (Fe_d and Mn_d), PO₄³⁻, H₂S, NO₂⁻, NO₃⁻, and NH₄⁺ were quantified. Hyperspectral imaging enabled the discrimination of Fe_d and PO₄³⁻ distributions within a single gel. In general, Fe_d was broadly distributed throughout the imaged porewaters (to ~17 cm depth) at both sites, with patchy concentrations reaching up to ~500 µmol L^-1. Dissolved H₂S, measured at MAR1, was largely absent across most of the profile, allowing Fe_d to remain mobile. In contrast, PO₄³⁻ was preferentially enriched at greater depths, indicating partial Fe-P decoupling likely related to efficient phosphate retention in shallow sediments and accumulation under more reducing conditions at depth. Mn_d distributions were comparatively more homogeneous than Fe_d, consistent with slower redox kinetics. Near-zero NO₂⁻ and NO₃⁻ concentrations combined with elevated NH₄⁺ indicate dominant ammonification and nitrification that is inhibited or masked by nitrate consumption processes. Clear contrasts emerged between geomorphological settings. At the erosional site (MAR1), Fe_d and Mn_d concentrations were higher, more laterally variable, and NH₄⁺ maxima occurred deeper in the sediment, consistent with enhanced porewater flushing and advective transport. In contrast, the depositional site (MAR2) exhibited more persistent Fe-P decoupling and shallower NH₄⁺ accumulation. Such differences could be attributed to differences in grain size, permeability and mudflat slope and therefore porewater residence time. Two-dimensional imaging further revealed pronounced lateral heterogeneity associated with biogenic structures. At MAR1, a microzone showed elevated sulfide and Fe_d depletion, consistent with localized pyritization and associated phosphate release. In another Fe_d/PO₄³⁻ gel from MAR1, microzones linked to sediment coloration and young Rhizophora plants reflected alternating Fe_d release and removal under contrasting redox conditions. At MAR2, a near-surface zone exhibited Mn_d enrichment coupled with Fe_d depletion beneath a Rhizophora seedling, consistent with a root-influenced redox microenvironment. Overall, results demonstrate the capacity of 2D-DET to resolve geomorphology and biota-driven microscale diagenetic organization in macrotidal Amazonian mangrove sediments that is not accessible using conventional porewater techniques.