North Atlantic Ice‐Rafting, Ocean and Atmospheric Circulation During the Holocene: Insights From Western Mediterranean Speleothems

Abstract

In this study, we present a Holocene rainfall index based on three high‐resolution speleothem records from the Western Mediterranean, a region under the influence of the westerly winds belt modulated by the North Atlantic Oscillation (NAO). On centennial to millennial timescales, we show that the North Atlantic ice‐rafting events were likely associated with negative NAO‐like conditions during the Early Holocene and the Late Holocene. However, our data reveal that this is not clearly the case for the mid‐Holocene ice‐rafting events, during which we also show evidence of positive NAO‐like patterns from other paleo‐oceanographic and paleo‐atmospheric data. Hence, contradictory mechanisms involving prolonged periods of both north and south shifts of the westerly winds belt (resembling positive and negative NAO‐like patterns) might at least partially trigger or amplify the ice‐rafting events and the slowdown of the Atlantic Meridional Overturning Circulation.

Hydroclimate variability from western Iberia (Portugal) during the Holocene: Insights from a composite stalagmite isotope record

“…Repschläger et al, 2017), this would have also impacted the state of the NAO (e.g. Ait Brahim et al, 2019; Wassenburg et al, 2016).…”

“…Repschläger et al, 2017), this would have also impacted the state of the NAO (e.g. Ait Brahim et al, 2019;Wassenburg et al, 2016). Over orbital and millennial time scales, hydroclimate in westcentral Portugal has generally tracked North Atlantic SST variability (Sánchez Goñi et al, 2008Goñi et al, , 2013Goñi et al, , 2016, and these changes have been preserved within BG stalagmites (Denniston et al, 2018) with warmer (cooler) Atlantic SSTs corresponding to more humid (drier) conditions at BG.…”

How Far North Did the African Monsoon Fringe Expand During the African Humid Period? Insights From Southwest Moroccan Speleothems

“…The stable isotope subsamples from WIN1 and WIN2 were drilled along the stalagmites growth axes using a hand drill with a spatial resolution of 1 mm. Isotope-Ratio Mass Spectrometry (Wassenburg et al, 2016) and Chaara cave (Ait Brahim, Wassenburg, et al, 2019) (blue dot), and other proxy sites (black circles) are shown in the map: 3 = OCE437-7-GC27 and 5 = OC437-07-GC37 (Tierney et al, 2017); 4 = GeoB 6007 (Kuhlmann et al, 2004) and GeoB 6007-2 (Holz et al, 2007); and 6 = ODP Site 1002 (Haug et al, 2001). Westerly jet stream (light blue arrow), ITCZ (gray dashed line), and typical positions of the Icelandic Low (IL) (black circle) and the Azores High (AH) (brown circle) are also shown in the map.…”

“…However, in contrast to the aforementioned monsoonal signal in SW Moroccan speleothems during the mid-Holocene, a completely different climate pattern was observed in speleothem records from N Morocco (Figure 3). Based on speleothem δ 18 O records from N Morocco (~34°N) and a compilation of climate records, there is evidence that positive NAO-like conditions prevailed during the mid-Holocene, whereas the early Holocene was characterized by relatively negative NAO-like conditions on multimillennial time scales (Ait Brahim, Wassenburg, et al, 2019). However, our data show that the speleothem δ 18 O values dropped to values as low as −8.5‰ in SW Morocco, which together with other hydroclimate records, suggest enhanced rainfall amount in SW Morocco during the AHP.…”

“…In particular, the pattern of modern NAO-precipitation correlation (e.g., revealed that the Wintimdouine cave location is just north of the southern boundary of the NAO-influenced area where less (more) winter rainfall is linked to the positive (negative) NAO state. However, the rainfall variability at the site may not have been solely related to NAO-like conditions in the past, due to, for instance, the nonstationary behavior of the NAO resulting from the gradual shifts of the Azores High and Icelandic Low positions (Ait Brahim, Wassenburg, et al, 2019;Deininger et al, 2016;Wassenburg et al, 2016), as well as possible climate regime changes. Our new records, spanning from 11.5 Kyr BP to the present, allow us to reconstruct the Holocene hydroclimate history of NW Africa at the critical latitude of~31°N, which provides new insights into the timing of the AHP and the northernmost WASM fringe.…”