Environmental conditions in the preglacial late Ordovician subtropical Iapetus Ocean: geochemistry of the type section of the Lower Hartfell Shales (Katian) at Hartfell, Dumfries & Galloway, Scotland.

The pre-Hirnantian glaciation Katian Lower Hartfell siltstones were deposited in deep quiet water, comparable to those of the Ulleung basin of the Japan Sea, which suggests, together with the likely paleogeographic position, that Hartfell was on the landward side of a back arc basin. The major element/Al ratios do not vary much up the section and indicate a dominantly continental source, with predominantly physical weathering, but with chemical weathering during transport to the final site of deposition. The abundant quartz silt indicates significant eolian transport. Climate may have changed from hot and arid to hot and humid from the lower to the upper part of the Hartfell siltstones, with possibly an increase in surface water temperature from 25 ^o C to 50 ^o C. Salinity is assumed to be normal marine based on the fauna, despite the freshwater indicated by trace element ratios. Relative paleoproductivity changes suggest a slight fluctuating decrease in productivity from the wilsoni to the linearis zones. The actual productivity seems low compared with the Ulleung basin and very low compared with younger Hirnantian black shales. Overall, the redox sensitive trace elements and ratios suggest that the Hartfell sediments started oxic, became dysoxic to anoxic, and then returned to oxic.