FORMATION AND PRESERVATION OF HYDROMORPHIC FEATURES IN COASTAL AND NEAR-COASTAL SOILS AND PALEOSOLS (Invited Presentation)

In coastal successions, paleosols are often stratigraphically below marine flooding surfaces. Pedogenesis occurs during periods of erosion or depositional hiatuses, creating an unconformity between the underlying paleosol and the overlying marine deposit and resulting in marine overprinting of the paleosol. However, changes in hydrology associated with sea level rise, including oscillations in shallow groundwater, shifts in salinity, and increased tidal and storm influence can also impart hydromorphic features on soils that are not submerged. Modern analogs in Texas provide insight into the rates and processes that result in observable marine overprinting in paleosols. By identifying hydromorphic characteristics in soils with varying degrees of marine influence and comparing them against suites of features preserved in paleosols, we can unravel what processes were associated with the unconformity and gain insight into what happened to the paleosol as it was being submerged.

We use descriptive multivariate statistics including hierarchical clustering, principal components analysis (PCA), and non-metric multidimensional scaling (NMDS) to analyze soils experiencing marine inundation and paleosols underlying marine or marginal marine deposits. At each location we compare the presence of hydromorphic characteristics and soil-forming factors, which include: 1) additions of overlying sediment or marine organisms, 2) redoximorphic features (matrix gley, mottling, drab-haloed root traces), 3) changes in mineralogy (pyrite, siderite, carbonates, or evaporites) and clay chemistry (replacement of interlayer cations with Na⁺, loss of smectite), and 4) a marine isotopic signature. Diagenesis restricts which traits in modern soils can be observed in paleosols (e.g., clay chemistry, δO¹⁸). Recognizing how features cluster together provides insight into marine alteration, with uncommon features potentially relating to specific depositional environments. Ordination axes may identify controls on the formation of hydromorphic features such as flooding frequency, energy, or water chemistry. We use five paleoVertisols from the Pennington Fm. (upper Miss., TN) to test how outputs match paleoenvironmental interpretations made from outcrop, micromorphological, and geochemical observations.