Abstract
AbstractModern-day ocean circulation behaves as a complex forced convective system that is characterized by the decrease in water temperature but increase in water density with depth. The dissolved oxygen content – which initially decreases due to biological oxygen demand – also increases with depth. In contrast to the present-day scenario, we propose that during the Archaean and Proterozoic eons inverted profiles could have developed such that, with depth, ocean water temperature increased and density and dissolved oxygen decreased. These inverted temperature and density profiles resulted in palaeo-ocean circulation behaving as a free convective system. It is proposed that this free convection, which may have been stable, or chaotic and subject to secondary instabilities, hindered the deep oxygenation of the palaeo-ocean. It may not be coincidental that the great oxygenation event (GOE) and Huronian glaciations are contemporaneous, in a similar way that the Neoproterozoic oxygenation event (NOE) is known to have been associated with glaciations. The global-scale external forcing required to switch the natural convective system to its present-day configuration is suggested to have been associated with Neoproterozoic glaciations and the subsequent lowering of ocean water salinity that accompanied them. We propose that this inverted the ocean water density gradient, allowing the oxygenation of the oceans for the first time. It is beyond the scope of this work to model the complex natural convection system, but we hope that geophysicists and numerical modellers will quantitatively evaluate the hypothesis proposed here to validate or refute our proposition.
Highlights
The oxygenation mechanism of modern oceans is a well-defined physical process where atmospheric oxygen – absorbed into the water at the ocean–air interface – is transported to the depths by (1) complex convective mixing aided by eddy currents and turbulence, and (2) high-latitude externally forced subduction of cold, saline and oxygenated dense waters; such transportation has probably been underway since the Baykonurian glaciation during early Cambrian time
The natural convection of the ancient ocean will probably have operated for c. 2000 Ma until powerful external forces and/or perturbations forced the system to switch and start to behave like a forced-convection system
The required perturbation is suggested to have been the lowering of ocean water salinity together with major cooling associated with the Neoproterozoic glaciations, along with the gradual cooling of the Earth
Summary
The oxygenation mechanism of modern oceans is a well-defined physical process where atmospheric oxygen – absorbed into the water at the ocean–air interface – is transported to the depths by (1) complex convective mixing aided by eddy currents and turbulence, and (2) high-latitude externally forced subduction of cold, saline and oxygenated dense waters; such transportation has probably been underway since the Baykonurian glaciation during early Cambrian time (at 547 Ma; Germs & Gaucher, 2012). Scott et al (2008) proposed that the deep oceans were oxygenated by 551 Ma, and that this was accompanied by a decrease in euxinic conditions within the water column. Scott et al (2008) proposed that the deep oceans were oxygenated by 551 Ma, and that this was accompanied by a decrease in euxinic conditions within the water column. The present-day oceanic circulation is forced by wind stress, heat differentials, freshwater fluxes at the surface, tidal forcing, topography of the ocean floors and geothermal heat fluxes (Munk & Wunsch, 1998; Adcroft et al 2001). The oxygenation of the ocean by absorption and convective mixing is strongly dependent on the thermal gradient of water and is expected to decrease with increasing temperature and ocean stratification. An increase in ocean stratification may inhibit convective mixing of oxygen-rich surface waters into the deeper ocean (Keeling et al 2010) and may be important in limiting the oxygenation of deep waters due to the dominant role of stratification in polar convection at the present day
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