Possible Late Holocene equatorial palaeoclimate record based upon soils spanning the Medieval Warm Period and Little Ice Age, Loboi Plain, Kenya

Abstract

Wetland and floodplain soils in the East African Rift of Kenya provide a record of changing palaeoclimate and palaeohydrology compatible with climate records for the mid-Holocene through the late Holocene Medieval Warm Period (∼AD 800–1270) and Little Ice Age (∼AD 1270–1850), documented previously in nearby lacustrine sites. Soils forming from volcaniclastic source materials in both Loboi Swamp and laterally adjacent Kesubo Marsh, two wetland systems of latest Holocene age, were investigated using micromorphology, whole-soil geochemical analysis, and stable isotope analysis of soil organic matter (SOM). Wetland formation was abrupt and possibly related to climate shift from drier conditions associated with the mid-Holocene and Medieval Warm Period, to wetter conditions associated with the Little Ice Age. Pre-wetland sediments are floodplain volcanic sandy silts comprising buried Inceptisols (SOM δ 13C=–15‰ PDB) that fine upward to fine silt and clay, which are overlain by clays and organic-rich sediment (peat) (SOM δ 13C=–26‰ PDB). Stable isotopes record an abrupt shift from 20 to 40% C3 vegetation (scrubland mixture of warm-season grasses and Acacia) to 100% C3 (wetland dominated by Typha) that occurred about 680±40 years BP (C-14 date from 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. Soils at Kesubo Marsh, located 2–3 km east of Loboi Swamp, consist of two buried mid-Holocene, 4000–4600 years BP (two C-14 dates from bulk SOM) Inceptisols developed from fluvially derived volcanic sand (SOM δ 13C=–15‰ PDB) and separated from the latest Holocene surface soil (SOM δ 13C=–17.5‰ PDB) by an unconformity and prominent stone line. Both the Loboi Swamp and Kesubo Marsh surface soils show increases in Zr, Fe, and S relative to buried soils, as well as higher leaching indices. Elevated Zr may reflect zircon grain inputs by either aeolian dust (during drier climate conditions) or fluvial sheetflood inputs, whereas higher leaching and elevated Fe and S represent pyritization associated with wetter conditions.