Neoarchean deep marine paleotemperature: Evidence from turbidite successions

Numerous studies using various proxies have been conducted to ascertain the temperature of the Archean ocean. If, as some of these investigations indicate, the Archean ocean was relatively hot, with a cooling trend through time to the Phanerozoic, this would have had consequences for sediment erosion, transport and deposition. Substantially higher surface temperatures would cause the viscosity of water, an important variable in defining flow hydrodynamics, to be lowered. In particular, less viscous water would have affected the development of current ripples. These are dependent on the existence of the laminar sublayer, as the sweeps of turbulent eddies impinging on this layer build small mounds that evolve into ripples. The laminar sublayer thins at higher temperatures allowing sand grains of smaller sizes to destroy it. At modern surface temperatures current ripples cannot form in coarse-grained sands, and if a hotter ocean existed in the Archean it would have inhibited current ripple formation in medium-grained sands, and possibly even fine-grained sands. Examination of numerous Neoarchean, deep-water, turbiditic successions in western Superior Province led to the realization that ripple cross-lamination is rare in all these units. As the formation of current ripples is dependent on: the flow depositing sediment of the proper grainsize, velocities in the appropriate range, and existence of a viscous sublayer, and the first two prerequisites were met during deposition from these decelerating turbidity currents, the existence of the viscous sublayer during the majority of flows must be questioned. The occurrence of current ripple cross-lamination in well exposed turbidites of the Neoarchean, McKellar Harbour Formation was quantified and found to be substantially less than the abundance of ripple cross-lamination present in Phanerozoic turbidites, indicating that the Neoarchean deep ocean may have been considerably warmer than the deep ocean of today. (C) 2011 Elsevier B.V. All rights reserved.