Change in the North Atlantic circulation associated with the mid-Pleistocene transition

Abstract

Abstract. The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20–MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37∘34.285′ N, 10∘7.562′ W; 2585 m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55∘ N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south–north, and east–west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.

Oceanic Impact on European Climate Changes during the Quaternary

“…Strong sinking rates, although relatively superficial, would allow the recirculation of warm surface water along the European margin and prevent severe thermal drops. This is what happened during T VII, VI, and I [76,77,84,87].…”

“…The migration of the AF blocked the northward flow of the NAC and, in consequence, the arrival of warm water to the high-latitude European margin. At the same time, the southernmost European margin received warm water via the AzC [76] (Figure 6b). The extremely southern position of the AF during some peak glacial conditions produced the maximal diversion of the NAC, which acquired an almost pure west-east direction [55,65,77].…”

Sea-Air CO2 Exchange in the SW Iberian Upwelling System during Two Contrasting Climate Cycles: 860–780 ka and 630–520 ka

“…It is known that during this cycle the NADW formation was very weak [32], which favoured the presence of AABW in site U1385 [31], as the low δ 13 C indicates (Figure 3b). Episodes of sharp and severe weakening of the AMOC would have driven lowered production, and carbon export, in the Iberian upwelling system during MIS 21-20.…”

“…This interpretation is corroborated by the coincidence of drops in PAR with rapid rises of AABW in the area (Figure 3b,d). The most prolonged interval of reduced production and, thus, of reduced biological CO 2 absorption, occurred during MIS 20 glacial maximum and T IX (Figure 3d), when surface circulation in the North Atlantic almost collapsed [31].…”

“…These events were related to major reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) that occurred as a consequence of deglaciations [30]. Nevertheless, no research has been conducted to explore a potential variation of the sea-air CO 2 exchange in upwelling areas, related to the major reorganizations in both surface [31] and deep-water [32] circulation that occurred in the North Atlantic during the Mid-Pleistocene Transition (MPT;~800-650 ka from present). The objectives of the present work are to explore the potential change in the CO 2 flux in the western Iberian margin (Figure 1), and the behaviour of this region as an uptake/release area during climate cycles before/after the MPT.…”