Europeanglacial dust deposits: Geochemical constraints on atmospheric dust cycle modeling

For a long time global paleodust numerical simulations have greatly underestimated dust sources other than modern deserts. Recent modeling experiments incorporating glaciogenic sources of dust have positively improved the agreement between model and paleodust data. This highlights the importance of accurately representing all areas potentially subjected to deflation during an investigated interval. Geochemical results, obtained from European loess sequences collected along a 50°N transect, combined with dust emission simulations reveal the geographical distribution of the most important European dust sources between 34 ka and 18 ka. We demonstrate that most European dust traveled only a few hundred kilometers or less within the boundary layer from its source before deposition. We conclude that our results encourage acquisition of similar geochemical data for other relevant areas in the world. Further, they could provide critical constraints to benchmark atmospheric models, contributing to improve their performance in simulating dust cycle and associated climate feedbacks. 1. Present and Past Dust Cycle Modeling In modern climate, uncertainties concerning dust emissions are high [Huneeus et al., 2011], but the geographical distribution of the main sources is well known. For past or future climates the uncertainties are even higher, due to potential changes in dust sources which up-to-date models are not able to simulate accurately. This may be a consequence of model biases in simulating precipitation or vegetation cover (resulting in a soil that is too wet or covered with too much snow or vegetation) [Mahowald et al., 2006; Werner et al., 2002], or of an inadequate representation of the inhibiting effect of these variables on dust emissions. The limited spatial resolution of numerical experiments, which in the case of Europe, does not incorporate the complexity of the topography, may also play a role. To correct for these model features, a general circulation model (GCM) study [Mahowald et al., 2006] was proposed introducing glaciogenic sources in six regions of the world (five of which are in the Northern Hemisphere). The glaciogenic source areas correspond to the extensive moraine systems bordering the Northern Hemisphere ice sheets and ice caps, which contributed to global dust emissions in addition to present-day deserts and other barren midlatitude regions. In Mahowald et al. [2006], large areas of alluvial plains where rivers drained meltwater from ice sheets (outwash plains) and mountain glaciers were also included as sources containing dust of glaciogenic origin. Introducing glaciogenic sources increased the simulated dust deposition fluxes for the Last Glacial Maximum (at about 21 ka, during sea level lowstand) from 2 to more than 3 times the current values, thus improving the agreement between model and field data. In particular, continental records show huge loess accumulation rates, especially between approximately 40 ka and 18 ka. Studies with an Earth System model of Intermediate Complexity (EMIC), at a lower spatial resolution than a GCM, but with the capacity to run over much longer time intervals, have also incorporated the contribution of dust from glaciogenic areas. One EMIC study has suggested the significant impact of dust on the dynamics of ROUSSEAU ET AL. ©2014. American Geophysical Union. All Rights Reserved. 7666 PUBLICATIONS Geophysical Research Letters