Deconstructing Terminations I and II: revisiting the glacioeustatic paradigm based on deep-water temperature estimates

Benthic and planktonic oxygen isotope (δ18Occ) and Mg/Ca analyses in two cores from the Northeast Atlantic have permitted the reconstruction of surface- and deep-water temperature (Tdw) and δ18O (δ18Ow) variations across the last two deglaciations. These records allow the timing of de-glacial melt-water pulses reaching the Northeast Atlantic to be compared with the evolution of local deep-water Tdw–δ18Ow conditions. Although each glacial termination is unique in detail, a similar pattern of hydrographic change is reconstructed for both deglaciations, with the first major decrease in deep-water δ18Ow (due to sea-level and/or purely local deep-water change) occurring in parallel with the onset of intensely cold glacial surface-water temperatures, and prior to a ‘terminal’ ice-rafting and melt-water event. The evolution of deep-water across both de-glaciations involved two transient incursions of cold, low-δ18O water into the deep Northeast Atlantic, the second of which was particularly pronounced each time. These pulses of cold deep-water are interpreted to reflect the incursion of water directly analogous to modern Antarctic Bottom Water (AABW), and containing a significant component of brine rejected during sea-ice formation. The results presented here show that the same type of transient changes in deep-water circulation that occurred across Termination I also occurred across Termination II, and that as a result of these deep-ocean changes, the timing of each benthic δ18O ‘termination’ cannot precisely reflect the timing of de-glacial sea-level change, as many palaeoceanographic interpretations (and some controversies) are prone to assume. Such ‘imprecision’ (in timing especially) may well extend to marine isotope stage (MIS) boundaries in general, as a principle of hydrographic variability and its expression in the geological record.