Record of abrupt changes of last climate cycle in European glacial dust deposits

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• Denis-Didier Rousseau - Laboratoire de Meteorologie Dynamique & CERES-ERTI
• Thursday 26 April 2018, 17:30-18:30
• Bawden Room, West Court, Jesus College.

If you have a question about this talk, please contact Heather L. Ford.

This presentation is an overview to the project ACTES , supported by the French ANR , and previous projects I conducted on European loess sequences. The main aim was to study the record of abrupt climate changes, corresponding to the Dansgaard-Oeschger and Heinrich events, in European terrestrial records, especially loess sequences. Loess is an eolian material that can be considered in a first order as “paleodust”. This study was designed as a data-model comparison to investigate how these sequences recorded the DO events in a periglacial environment, how the dust was emitted and deposition occurred, and from which source zones.

Europe has been strongly impacted by the millennial climate changes related to variations in the sea-ice extent and therefore also affected the moisture sources of precipitation on the Greenland ice sheet. These variations in the extent of the sea ice during the last climatic cycle (LCC, about 130-15 kyr) impacted the westerlies and the position of the polar jet stream, and consequently storm track trajectories. Furthermore, the presence of ice sheets and ice caps over Great Britain, Scandinavia and the Alps enhanced the zonal circulation, as recorded by the European paleodust deposits located along the 50°N parallel.

Loess sequences are well developed all over Europe, but especially in the so-called loess belt between 48° and 52°N. Such intensive deposition of paleodust over Europe has been favored by the reduced arboreal cover (even practically absent in NW Europe during both GS and GIs, by sea-level lowering, exposing large areas of the continental shelves to eolian erosion, and by strong increases in fluvial transport and sedimentation by periglacial braided rivers. Extensive investigations of European loess series along a longitudinal transect at 50°N reveal that the millennial-scale climate variations observed in the North-Atlantic marine and Greenland ice-core records are well preserved in loess sequences. Among them, the Nussloch loess site yields an important record of the LCC although its paleosol-loess unit couplet succession is not unique, but observed with a variable thickness and a diverse nature of the paleosols in sequences ranging from Western Europe eastward to Ukraine over more than 1800 km. Recent numerical simulations of the past global dust cycle for the first time included glaciogenic dust sources and, compared to earlier attempts, resulted in an improved performance when confronted to data available for the Last Glacial Maximum. Still, even the improved modeling failed to capture spatial and temporal dynamics of past dust deposition. We achieve recently a step increase in understanding sub-continental scale climate change by identifying dust sources and constraining dust residence time in the atmosphere. Using dust deposition over Europe during the last glacial cycle, geochemical fingerprinting, and numerical dust emission simulations we identify the main aerosol sources for different depositional areas. Dust was transported at low elevation and over regional distances only. The glaciogenic sources considered so far in climate modeling, like frontal moraines and outwash plains of the European ice-sheets, were of considerably less relevance for the global dust budget than proposed earlier. The main contributors were regions between 48°and 52°N, with variable hot spots depending on climate conditions. Loess units are interpreted to correspond to coarse paleodust transported at rather low elevations, in the active layer of the atmosphere (about 300 to maximum 3000 m) at regional to local scales, while finer paleodust deposited at high latitudes seems transported at much higher elevations.

A recent study raised the problem in correctly estimating the sedimentation (SR) and mass accumulation (MAR) rates of the sequences for comparison with model estimates, which cannot be estimated by just taking into account the whole thickness of the considered deposits as classically performed. To solve this issue, Greenland ice and northwestern European eolian deposits are compared in order to establish a link between GI and the soil development in European mid-latitudes, as recorded in loess sequences. For the different types of observed paleosols, the precise correlation with the Greenland records is applied to propose estimates of the maximum time lapses needed to achieve the different degrees of maturation and development. To identify these time lapses more precisely, two independent ice-core records are compared: d180 and dust concentration, indicating variations of temperature and atmospheric dustiness respectively in the Greenland area. This method slightly differs from the definition of a GI event duration applied in other studies where the sharp end of the d18O decrease gives the end of a GI. The same methodology is applied to both records (i.e., the GI last from the beginning of the abrupt d18O increase or dust concentration decrease until when d18O or dust reach again their initial value) determined both visually and algorithmically, and compare them to GI published estimates.

This talk is part of the Quaternary Discussion Group (QDG) series.

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