The Tagus Prodelta (W Portugal) and the Muros Ría (NW Spain) are areas of high deposition rates registering high-resolution palaeoclimatic records for western Iberia. We compare the climatic conditions of the two areas over the last two millennia based on proxies of temperature (sea surface temperatures and oxygen isotopes), continental input (grain size, iron and magnetic susceptibility) and productivity (inorganic and organic carbon, carbon isotopes, benthic foraminifera and diatoms). Biogeochemical changes in the Tagus Prodelta reflect widely recognized North Atlantic climatic periods encompassing the Roman Period (AD 0-350), the Dark Ages (AD 400-700), the ‘Mediaeval Warm Period’ (MWP; AD 800-1200) and the ‘Little Ice Age’ (LIA; AD 1300-1750). The atmospheric North Atlantic Oscillation (NAO) drives the Tagus Prodelta multidecadal, long-term variability in precipitation-river input during cold periods (negative NAO) and marine upwelling during warmer periods (positive NAO), a scheme that is reversed in the Galician region. The Muros Ría shows only local hydrodynamics until AD 1150, including a ‘suboxic’ event in the inner Ría around AD 500-700. Since AD 1150 Atlantic warm upwelled waters have ventilated the outer Ría but only reach the inner Ría at AD 1750. The twentieth-century records are also interpreted as a reflex of the inverse NAO mode in both areas, resulting in amplification of the LIA biogeochemical water conditions. Centennial-scale solar activity appears to be another important forcing mechanism (or the only one, if solar activity drives the NAO and ‘Bond-cycles’) behind changes in the hydrography of the Tagus Prodelta, and primary production, bottom ventilation and organic carbon degradation in the Muros Ría.

We present planktonic foraminiferal fauna and isotope records from the SE Atlantic that highlight the nature of millennial-scale variability over the last 100 kyr. We derive a hypothesis-driven age model for our records based on the empirical link between variations in Greenland temperature, ocean circulation, and carbonate preservation in the deep SE Atlantic. Our results extend earlier findings of an antiphase (seesaw) relationship between north and south for the largest abrupt events of Marine Isotope Stage (MIS) 3–2 and the last deglaciation. In particular, we find that Heinrich Stadials were paralleled by inferred southward shifts of the thermal Subtropical Front. These were followed by pronounced rebounds of the front with the return to interstadial conditions in the north. Our results also shed light on the mechanism of glaciation. In contrast to the last deglaciation, which was a globally symmetric change superposed by interhemispheric asynchronicity, we find that the descent into full glacial conditions at the onset of MIS 4 (~70 ka) displayed interhemispheric synchrony. We suggest that this globally synchronous descent into glacial MIS 4 was preconditioned by orbital changes, but the timing was ultimately determined by abrupt changes in ocean/atmosphere circulation patterns i.e., the bipolar seesaw.