Atlantic forcing of Western Mediterranean winter rain minima during the last 12,000 years

Abstract

The limited availability of high-resolution continuous archives, insufficient chronological control, and complex hydro-climatic forcing mechanisms lead to many uncertainties in palaeo-hydrological reconstructions for the Western Mediterranean. In this study we present a newly recovered 19.63��m long core from Lake Sidi Ali in the North African Middle Atlas, a transition zone of Atlantic, Western Mediterranean and Saharan air mass trajectories. With a multi-proxy approach based on magnetic susceptibility, carbonate and total organic C content, core-scanning and quantitative XRF, stable isotopes of ostracod shells, charcoal counts, Cedrus pollen abundance, and a first set of diatom data, we reconstruct Western Mediterranean hydro-climatic variability, seasonality and forcing mechanisms during the last 12,000��yr. A robust chronological model based on AMS 14C dated pollen concentrates supports our high-resolution multi-proxy study. Long-term trends reveal low lake levels at the end of the Younger Dryas, during the mid-Holocene interval 6.6 to 5.4��cal ka BP, and during the last 3000 years. In contrast, lake levels are mostly high during the Early and Mid-Holocene. The record also shows sub-millennial- to centennial-scale decreases in Western Mediterranean winter rain at 11.4, 10.3, 9.2, 8.2, 7.2, 6.6, 6.0, 5.4, 5.0, 4.4, 3.5, 2.9, 2.2, 1.9, 1.7, 1.5, 1.0, 0.7, and 0.2��cal ka BP. Early Holocene winter rain minima are in phase with cooling events and millennial-scale meltwater discharges in the sub-polar North Atlantic. Our proxy parameters do not show so far a clear impact of Saharan air masses on Mediterranean hydro-climate in North Africa. However, a significant hydro-climatic shift at the end of the African Humid Period (���5 ka) indicates a change in climate forcing mechanisms. The Late Holocene climate variability in the Middle Atlas features a multi-centennial-scale NAO-type pattern, with Atlantic cooling and Western Mediterranean winter rain maxima generally associated with solar minima

Environmental Drivers of Holocene Forest Development in the Middle Atlas, Morocco

“…The reported depths represent a corrected depth scale that accounts for core expansion in the liners. Full details of the bathymetric and seismic surveys and coring procedure are given in Zielhofer et al (2017a).…”

“…Tigalmamine experiences a similar temperature regime to Lake Sidi Ali (mean 10 • C, mean temperature of the coldest month (January) of 2.5 • C and mean temperature of the warmest month (July) of 20 • C) and significantly higher rainfall (930 mm/yr) (Cheddadi et al, 1998). The surrounding forest vegetation is characteristic of the FIGURE 2 | Topographic setting and bathymetry of Lake Sidi Ali, showing the drilling position of core sequence ALI 01-10 (Zielhofer et al, 2017a and this study) and core 91-C (Barker et al, 1994;Lamb et al, 1999).…”

“…Reliance on 210 Pb/ 137 Cs analysis and 26 AMS radiocarbon dates on terrestrial pollen concentrates and macrofossils circumvents the problem of hardwater errors for bulk radiocarbon dates and underpins the high-resolution and multiproxy palaeolimnological study (Zielhofer et al, 2017a). Key findings include the detection of pervasive variability in Atlanticderived winter precipitation, and a Mid Holocene shift in prevailing climate pattern reflecting the emergence of centennial dynamics resembling the North Atlantic Oscillation (NAO-like) after 5,000 cal yr BP (Zielhofer et al, 2017a). The main horizon of anthropogenic catchment disturbance is also constrained by geochemical and sedimentological data to 1,400 cal yr BP (Zielhofer et al, 2017a), and is supported by evidence for a relatively old, soil-derived pollen component in some dating samples .…”

“…Key findings include the detection of pervasive variability in Atlanticderived winter precipitation, and a Mid Holocene shift in prevailing climate pattern reflecting the emergence of centennial dynamics resembling the North Atlantic Oscillation (NAO-like) after 5,000 cal yr BP (Zielhofer et al, 2017a). The main horizon of anthropogenic catchment disturbance is also constrained by geochemical and sedimentological data to 1,400 cal yr BP (Zielhofer et al, 2017a), and is supported by evidence for a relatively old, soil-derived pollen component in some dating samples . New research also reveals that Lake Sidi Ali acts as a receptor for remote (Saharan) dust, providing indications of millennial-scale fluctuations in dust mobilization across the end of the African Humid Period (Zielhofer et al, 2017b).…”

“…The lake lies in a structural depression at the contact between Middle Jurassic limestones and Lower Jurassic dolomites. In September 2012, the lake displayed a maximum water depth of 38 m and anaerobic conditions at the hypolimnion (Zielhofer et al, 2017a). A shallow sub-basin to the SW is partly separated from the main basin by early Quaternary basalt flows.…”