Intensive chemical weathering in the Arctic during the Miocene Climatic Optimum

The Arctic today has shallow, chemically immature, and frost-dominated weathering regimes. At Sokli, Finland (68 degrees N), a 70 m deep palaeo-weathering profile is developed in a Devonian carbonatite pipe that represents fundamentally different past weathering environments. Formation of the apatite-francolite P-ore likely began under Palaeogene warm, humid climates. Later, cryptomelane (K-Mn oxide) crusts developed within the ore that have yielded peak 40Ar/39Ar ages of 16.20 +/- 0.13 Ma (2 sigma)., The crusts formed at the redox front during the Miocene Climatic Optimum (-16.9-14.7 Ma) under mean annual temperatures -12-14 degrees C warmer than today. The presence of the cryptomelane crust at shallow depth (15 m) indicates very low erosion rates since formation, consistent with its position on a tectonically stable Archaean craton and in the cold-based ice-divide zone of successive Fennoscandian ice sheets. The Miocene Climatic Optimum triggered a pulse of intensive weathering in mid- and low latitudes; the Sokli cryptomelane ages demonstrate that intensive chemical weathering extended into the Arctic.