The spatiotemporal extent of the Green Sahara during the last glacial period.

The Sahara Desert, one of today's most inhospitable environments, has known periods of enhanced precipitation that supported pre-historic humans. However, the Green Sahara timing and moisture sources are not well known due to limited paleoclimate information. Here, we present a multi-proxy (δ18O, δ13C, Δ17O, and trace elements) speleothem-based climate record from Northwest (NW) Africa. Our data document two Green Sahara periods during Marine Isotope Stage (MIS) 5a and the Early to Mid-Holocene. Consistency with paleoclimate records across North Africa highlights the east-west geographical extent of the Green Sahara, whereas millennial-scale North Atlantic cooling (Heinrich) events consistently resulted in drier conditions. We demonstrate that an increase in westerly-originating winter precipitation during MIS5a resulted in favorable environmental conditions. The comparison of paleoclimate data with local archaeological sequences highlights the abrupt climate deterioration and the decline in human density in NW Africa during the MIS5-4 transition, which suggests climate-forced dispersals of populations, with possible implications for pathways into Eurasia.

Hydroclimate variability in the Caribbean during North Atlantic Heinrich cooling events (H8 and H9).

We present a speleothem record from western Cuba, spanning the period 98.7-84.9 ka BP. Our record shows two distinctive periods of high δ18O corresponding to dry and/or cold periods during 85-87.6 and 90.2-93.1 ka BP, synchronous with Heinrich events 8 and 9 (H8 and H9). Hence, we provide the first proxy evidence of the local Caribbean climate response to H8 and H9. Interestingly, H8 is more pronounced compared to H9, which may be a local response to lower temperatures in the North Atlantic resulting in a weak AMOC and reduced deep water formation, therefore a stronger south shift of the ITCZ. Our data complement existing speleothem records from western Cuba which, collectively, provide a nearly continuous paleoclimate time-series spanning the last 100 ka BP, indicating a consistent response to millennial-scale events as dry and/or cooler conditions. The comparison with regional paleoclimate records reveals an anti-phased relationship with South America, caused by the southern movements of the ITCZ during millennial-scale events which lead to dry conditions in the Caribbean and a stronger South American Monsoon System.

Timing and structure of the Younger Dryas event and its underlying climate dynamics.

The Younger Dryas (YD), arguably the most widely studied millennial-scale extreme climate event, was characterized by diverse hydroclimate shifts globally and severe cooling at high northern latitudes that abruptly punctuated the warming trend from the last glacial to the present interglacial. To date, a precise understanding of its trigger, propagation, and termination remains elusive. Here, we present speleothem oxygen-isotope data that, in concert with other proxy records, allow us to quantify the timing of the YD onset and termination at an unprecedented subcentennial temporal precision across the North Atlantic, Asian Monsoon-Westerlies, and South American Monsoon regions. Our analysis suggests that the onsets of YD in the North Atlantic (12,870 ± 30 B.P.) and the Asian Monsoon-Westerlies region are essentially synchronous within a few decades and lead the onset in Antarctica, implying a north-to-south climate signal propagation via both atmospheric (decadal-time scale) and oceanic (centennial-time scale) processes, similar to the Dansgaard-Oeschger events during the last glacial period. In contrast, the YD termination may have started first in Antarctica at ∼11,900 B.P., or perhaps even earlier in the western tropical Pacific, followed by the North Atlantic between ∼11,700 ± 40 and 11,610 ± 40 B.P. These observations suggest that the initial YD termination might have originated in the Southern Hemisphere and/or the tropical Pacific, indicating a Southern Hemisphere/tropics to North Atlantic-Asian Monsoon-Westerlies directionality of climatic recovery.

Multi-decadal to centennial hydro-climate variability and linkage to solar forcing in the Western Mediterranean during the last 1000 years.

Here we present a new composite record from two well-dated speleothem records from two caves in Northern Morocco. The high-resolution record covers the last millennium allowing to detect multi-decadal to centennial periodicities. Over the industrial period, δ18O values of our speleothems are shown to be dominated by the main mode of decadal variability in the North Atlantic region: the North Atlantic Oscillation (NAO). Statistical analyses confirm the previously reported multi-decadal variability related to the influence of the Atlantic Multidecadal Oscillation (AMO) in the region. High power and persistent centennial-scale periodicities, similar to the Vries-Suess 200-year solar cycle, are observed as well. Indeed, comparison between solar activity reconstructions and our record confirms the in-phase relationship on centennial time-scales. Low δ18O values, and hence negative phases of NAO that bring precipitation towards the Western Mediterranean, are observed during well-known solar minima periods. The results are consistent with previous models which describe low irradiance as a  trigger for southward shifts of precipitation-bearing westerlies during winter.

Impact of agricultural practices on groundwater quality in intensive irrigated area of Chtouka-Massa, Morocco.

The Plio-Quaternary aquifer of Chtouka is located in Southwestern of Morocco. The intensive agricultural activity in Chtouka basin requires the mobilization of 94% of fresh water resources for irrigation. This overexploitation, along with the succession of drought years, sea water intrusion and various sources of pollution, affected the quality and availability of groundwater resources. Several sampling campaigns were carried out in different sites of the study area in order to investigate the spatial variation of groundwater quality. The temporal evolution of groundwater level shows that the water table was subjected to a gradual decline during the last decade, indicating an intensive exploitation mainly in irrigated areas. In the Southern part around Belfaa and the irrigated area along Massa River, nitrate concentrations exceed 50mg/L, which is the threshold set by the World Health Organization, while in the northern part around Biougra and Ait Amira, the nitrate concentration is mostly below 50mg/L indicating a relative good groundwater quality. This finding can be explained by the improvement of agricultural practices, particularly the conversion of flood and sprinkler irrigation to drip irrigation (80% of the total irrigated area) in most of the developed farms in this part of the study area. Moreover, the exploitation of groundwater from the deep aquifer, due to the increasing water demand in the region, can also explain the low chemical concentrations since the deep aquifer is not affected by anthropogenic pollutants or marine intrusion. Stable isotopes (18O and 2H) highlight the different origins of groundwater, indicating the complexity of the aquifer system path flows, which is attributable to the intensive exploitation and irrigation water return.