Past African dust inputs in the western Mediterranean area controlled by the complex interaction between the Intertropical Convergence Zone, the North Atlantic Oscillation, and total solar irradiance

Pierre Sabatier,Didier Swingedouw,Maxime Leblanc,Boris Vannière,Fabien Arnaud,Jérôme Poulenard,Benjamin Chazeau,Marie-Charlotte Bellingery,Yoann Copard,Christophe Colin,Charlotte Skonieczny,Yves Perrette,Maxime Debret,Marie Nicolle,Jean-Louis Reyss,Julien Didier,Emmanuel Malet,Maëlle Kelner,Anne‐Lise Develle ,Isabelle Jouffroy‐Bapicot ,C Piot

doi:10.5194/cp-16-283-2020

Abstract

Abstract. North Africa is the largest source of mineral dust on Earth, which has multiple impacts on the climate system; however, our understanding of decadal to centennial changes in African dust emissions over the last few millenniums is limited. Here, we present a high-resolution multiproxy analysis of sediment core from high-elevation Lake Bastani, on the island of Corsica, to reconstruct past African dust inputs to the western Mediterranean area over the last 3150 cal BP. Clay mineralogy with palygorskite and a clay ratio associated with geochemical data allow us to determine that terrigenous fluxes are almost exclusively related to atmospheric dust deposition from the western Sahara and Sahel areas over this period. High-resolution geochemical contents provide a reliable proxy for Saharan dust inputs with long-term (millennial) to short-term (centennial) variations. Millennial variations have been correlated with the long-term southward migration of the Intertropical Convergence Zone (ITCZ), with an increase in dust input since 1070 cal BP. This correlation suggests a strong link with the ITCZ and could reflect the increased availability of dust sources to be mobilized with an increase in wind and a decrease in precipitation over western and North Africa. For centennial to decadal variations, wavelet analyses show that since 1070 cal BP, the North Atlantic Oscillation (NAO) has been the main climatic forcing, with an increase in Saharan dust input during the positive phase, as suggested by previous studies over the last decades. However, when the ITCZ is in a northern position, before 1070 cal BP, wavelet analyses indicate that total solar irradiance (TSI) is the main forcing factor, with an increase in African dust input during low TSI. With climate reanalysis over the instrumental era, during low TSI we observe a significant negative anomaly in pressure over Africa, which is known to increase the dust transport. These two climatic forcing factors (NAO, TSI) modulate Saharan dust inputs to the Mediterranean area at a centennial timescale through changes in wind and transport pathways.

Highlights

The dust cycle is an important part of the Earth system: each year, an estimated 2000 Mt of dust is emitted into the atmosphere (Shao et al, 2011)
We present a high-resolution African dust record from sediment cores sampled in Lake Bastani, a high-elevation lake (2092 m a.s.l.) located on Corsica in the northwestern Mediterranean Sea (Fig. 1), which records the long-term to short-term variability of African dust inputs over the late Holocene with high temporal resolution, allowing for the discussion of centennial climate forcing and filling the gap between long-term instrumental observations and millennial reconstructions
The non-carbonate ignition residue (NCIR) is relatively constant and up to 80 %, with small variation over the upper 10 cm. This NCIR is mainly composed of biogenic silica and terrigenous input from both the watershed and the eolian flux

Summary

Introduction

The dust cycle is an important part of the Earth system: each year, an estimated 2000 Mt (metric tons) of dust is emitted into the atmosphere (Shao et al, 2011). Mineral dust contributes to the carbon cycle as an external source of nutrients to the oceans and to remote terrestrial ecosystems (Maher et al, 2010; Pabortsava et al, 2017; Shao et al, 2011; Yu et al, 2015). Understanding dust emission, transport, and deposition requires the study of both past and present climatic variations. Advances in remote sensing and modeling have improved understanding of the dust cycle, while improvements in paleo-sciences allow for the reconstruction of both dust record emissions through time and source region fingerprinting (Marx et al, 2018). African dust emissions and transports exhibit variability on diurnal to decadal timescales under different atmospheric patterns (Evan et al, 2016). The neglect of atmospheric dust reduction in the early to mid-Holocene in climate models could partly explain the model–data temperature discrepancy in the Northern Hemisphere (Liu et al, 2018)

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