Phanerozoic trends in deep water rejuvenation: Is there a relation between global temperature and ocean mixing?

The concept of a warm, sluggish ocean recurs in the palaeoceanographic literature, yet over the last few years, both observation and model studies have challenged this concept repeatedly. Nevertheless, observations in the modern do link the ongoing anthropogenic warming to the slowing down of oceanic circulation. This mismatch between the different scales of observations presents a critical problem to our understanding of the past ocean. Here, we present a critical evaluation of this concept through an extensive series of intermediate complexity Earth system model experiments. Multiple paleogeographic scenarios across the Phanerozoic, CO2 concentration, and orbital configuration have been simulated to evaluate the relations between planetary surface temperatures and deep-water rejuvenation rate. Combined, the results of these simulations present a very limited contribution of warm climates to the global ocean circulation slowdown. For most experiments, warmer conditions enhanced overall oceanic turnover due to an increase in vertical density gradient, supporting more efficient downwelling. However, this state is only achieved in the long term, with some slowdown after the initial warming. The overall range of turnover time, even during the slowest period of deep-water rejuvenation, remains within the same order of magnitude as the modern. In light of these findings, it is unlikely that at any point through the Phanerozoic did oceanic turnover rate changed in a magnitude that would impact the mixing state of most marine dissolved chemical elements, at least at current flux state.