Cambrian paleosols and paleoenvironments of Ireland

Abstract

Early Cambrian rocks of Ireland have been recognized by the cosmopolitan trace fossil Oldhamia. Early Cambrian age has been confirmed by discovery of other trace fossils (Manykodes pedum, Arenicolites didymus, A. sparsus, Histioderma hibernicum) and acritarch palynology (Skiagia orbicularis) near Bray, County Wicklow. This study found these fossils in paleosols at its type locality near Bray, along with microbial traces of “old elephant skin” (Rivularites repertus) and drab-haloed filament traces (Prasinema gracile). These red paleosols have distinctive gypsum desert roses forming a subsurface (By) horizon, as evidence of hyperarid precipitation (MAP 130−152 mm), comparable with paleosols associated with vendobiont fossils in Ediacaran rocks of South Australia and Nevada. Cambrian land surfaces were not barren wastes, but supported microbial earth communities with diverse animal burrows, fed in part by aerobic photosynthetic organisms, revealed by organic δ¹³C of −18.2 to −25.4 ‰. Measured rates of chemical weathering in molar depletion per year and per mm of precipitation of 0.3−0.6 μmol/yr/mm cationic bases and of 0.07−0.2 μmol/yr/mm phosphorus, were comparable with Ediacaran paleosols, and with Phanerozoic soils and paleosols. Ediacaran-Early Cambrian biotic enhancement of weathering on land was a prelude to provision of nutrients for the spectacular Cambrian evolutionary explosion of life in the sea.