Evidence for periods of wetter and cooler climate in the Sahel between 6 and 40 kyr BP derived from groundwater

Abstract

Concentrations of noble gases, stable isotopes and 14C in samples from the Continental Terminal groundwaters of Niger provide evidence for more humid and cooler climate phases in West Africa in the Holocene and the late Pleistocene. During humid phases, even within the Holocene, the soil temperature was up to 5.5°C cooler than today, which is partly attributed to atmospheric cooling, but also to a change in the relationship between air and soil temperature due to increased vegetation. Intense rainfall events and increased groundwater recharge are consistently indicated by stable isotope data and excess air concentrations, i.e., the component of dissolved atmospheric gases in excess of solubility equilibrium. This finding encourages the use of excess air as an additional, humidity‐related climate indicator.

North African vegetation–precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM

“…Cook, 1999). The negative surface-temperature anomalies in the early and mid-Holocene experiments are substantial (more than 6K in the OAV experiments despite larger incoming shortwave radiation at the top of the atmosphere), in line with temperature reconstructions from groundwater samples (Beyerle et al, 2003), and are associated with increased cloudiness due to enhanced convection, reflecting solar radiation back to space (cf. Braconnot et al, 2007;Patricola and Cook, 2007;Bosmans et al, 2012).…”

“…New parameterizations for canopy interception and soil evaporation have been implemented into the land component in order to improve the simulation of the land hydrology and vegetation cover (Oleson et al, 2008) as in Handiani et al (2013). CCSM3's dynamic-vegetation model DGVM is based on the Lund-Potsdam-Jena (LPJ) model Levis et al, 2004b;Bonan and Levis, 2006) and simulates the spatiotemporal distribution of 10 plant functional types (PFTs; 7 tree PFTs and 3 grass PFTs) which are differentiated by physiological, morphological, phenological, bioclimatic, and fire-response attributes (Levis et al, 2004b). The land and atmosphere components are integrated with a 30 min time step, while vegetation structure and PFT population densities are updated annually (Levis et al, 2004b).…”

Orbital forcing and role of the latitudinal insolation/temperature gradient

“…Yet despite increased summer insolation across all latitudes of the Northern Hemisphere during the midHolocene, there is little or no evidence that low latitudes underwent warming in the same way as high latitudes. Rather, evidence from both marine sst's (Rimbu et al 2003;Rimbu et al 2004) and terrestrial proxies Sawada et al 2004;Peyron et al 2000;Beyerle et al 2003) indicates widespread low latitude cooling that is contrary to the increase in summer insolation. A compilation of mid-Holocene temperature reconstructions by latitude at 6 ka BP illustrates the contrast between high latitudes where temperatures were warmer than present and low latitudes where temperatures were largely cooler than present (Fig.…”

Applications of Radiocarbon Dating Method

“…Studies of the paleo-aquifer around the world show that during the LGM temperatures were lower by 3-5 °C even at low latitudes (Stute et al 1995a,b). Figure 4 shows the noble gas temperature reconstructions in the Sahel region (Beyerle et al 2003). The gap in the record observed between 23 and 15 kyr BP indicates reduced groundwater recharge during the arid LGM.…”