LATE HOLOCENE EQUATORIAL PALEOCLIMATE RECORD BASED UPON SOILS AND PALEOSOLS SPANNING THE MEDIEVAL WARM PERIOD AND LITTLE ICE AGE, LOBOI PLAIN, KENYA

Wetland and floodplain soils in the East African Rift of Kenya provide a record of changing paleoclimate and paleohydrology compatible with climate records for the Medieval Warm Period (~AD 800-1270) and Little Ice Age (~AD 1270-1850) documented previously in nearby lacustrine sites. Soils forming from volcaniclastic parent materials in both Loboi Swamp and laterally adjacent Kesubo Marsh, two wetland systems of mid- to late Holocene age, were investigated using micromorphology, whole-soil chemical analysis, and stable isotope analysis of soil organic matter (SOM). Loboi Swamp is a small freshwater wetland ~ 2 km² in area, groundwater-fed by several warm springs and seeps (associated with the rift border-fault system), and is dominated by Typha domingensis (“cattail”) and Cyperus papyrus (“papyrus”). Wetland formation was abrupt and probably related to climate shift, based on micromorphological, geochemical and stable isotopic (soil organic matter, SOM) study of a 1 m deep soil pit and 8 piston cores (1.5-4 m long). Basal sediments are floodplain volcanic sandy silts comprising buried Inceptisols (SOM δ¹³C = –15 ‰ PDB) that fine upward to fine silt and clay, which are overlain by clays and organic-rich sediment (peat) (SOM δ¹³C = –26 ‰ PDB); stable isotopes record an abrupt shift from 20-40% C3 vegetation (scrubland mixture of warm-season grasses and Acacia) to 100% C3 (wetland of cattail and papyrus) that occurred about 500-700 years ago (conventional radiocarbon dates of seeds). Soils developed on the periphery of the wetland show evidence for fluctuations in hydrologic budget, including siderite and redoximorphic features formed during wetter phases, and vertic (shrink-swell) and clay illuviation features developed during drier phases. Kesubo Marsh, located 2-3 km east of Loboi Swamp, consists of two buried mid-Holocene 4000-4600 ¹⁴C yr BP (bulk humate) Inceptisols developed from volcanic sand (SOM δ¹³C = –15 ‰ PDB) and a modern surface soil (SOM δ¹³C = –17.5 ‰ PDB). Both the Loboi Swamp and Kesubo Marsh surface soils show net Zr additions of 100-200% (assuming immobile Ti), as well as 50% additions of Na, K, Al, and Si, which together indicate a significant eolian dust flux to the wetland in the past few hundred years, perhaps associated with late Holocene (3,000 years to present) aridification documented across sub-Saharan Africa.