PALEOCLIMATIC RECONSTRUCTION FROM THE BRYN MAWR FORMATION (MIOCENE) PALEOSOL, U.S. MID-ATLANTIC COASTAL PLAIN

Paleosols are excellent time and process integrators of pedogenesis, but uncertainty remains as to which climatic, hydrologic, and geochemical processes dominate soil morphology, when specific properties get locked into the soil, and if paleosol characteristics are resistant to diagenetic alteration when distant from the surface environment. This study explores the chemical and physical characteristics of the Bryn Mawr Fm (Miocene) paleosol preserved in the inner Coastal Plain of Cecil County, MD, uses it to partly reconstruct the late Cenozoic soil forming environment of the mid-Atlantic region, adds to a growing database of paleosol observations assembled for the mid-Atlantic region, and interprets the results in the context of comparable modern subtropical settings. Rubified colors are a key characteristic of the Bryn Mawr Fm paleosol that contrast sharply with the yellow-brown colors of demonstrably late Pleistocene and Holocene soils shaped by modern mean annual temperature (MAT) and precipitation (MAP) conditions in the mid-Atlantic region. We measure particle size (PSDA), bulk geochemistry, and iron-oxide geochemistry, specifically the oxalate to dithionite extractable (FeO/FeD) ratio and the goethite to hematite (G/H) ratio. The Bryn Mawr Fm paleosol parent, a quartzose pebbly sand, lacks weatherable minerals; nevertheless the paleosol contains silt, authigenic clay, and gibbsite, all of which point to an aeolian source and cumulic soil development. FeO/FeD ratios fall between 0.02-0.08 indicating mostly crystalline iron phases consistent with pedogenesis over 10⁶ yrs. These ratios, among the lowest measured in our lab, are lower than those extracted from a nearby early Pleistocene kame. Major oxide concentrations are input to a recently published data-driven spline model to reconstruct MAT and MAP and compared to a recent compilation of global MAP and G/H ratio data that are shown to strongly covary. Model results and relative high G/H ratios indicate that the paleosol has encoded a paleo MAT and MAP that is both warmer and wetter than the modern soil forming environment in the mid-Atlantic region. These properties probably have been encoded early in the peodgenic development of the Bryn Mawr Fm soil in the late Miocene and Pliocene and have persisted through subsequent Pleistocene pedogenesis.