Abstract:

Mineral dust is a key component in the global iron cycle with the atmosphere as the conveyor belt transporting emitted particles from the continents to the oceans or elsewhere on land. Mineral dust represents two thirds of the total aerosol mass in the atmosphere and ranges from particle sizes less than a micron to one hundred microns. While in the atmosphere and once deposited, mineral dust impacts significantly the Earth system, its climate and biogeochemical cycles. Historically, earth system models only considered mineral dust finer than 10 microns for their greater shortwave radiation absorption and greater distance transport potential. Current observations point to a dramatic underestimation of coarser atmospheric dust, which is likely to bias earth system model results. Loess is mineral dust deposited on continental surfaces encompassing short to long-range transported particles dependent on environmental conditions. Loess is one of the few archives capable of recording past atmospheric circulation and together with its intercalated paleosols, archive locally past climate changes.
Loess and paleosol sequences (LPS) of Eastern Europe are excellent archives of past climate change. Millennial-scale climate variations have been demonstrated in loess units from the last (MIS 4-2) and penultimate (MIS 6) glacial periods. Such observations in even older loess units are extremely rare, mainly due to sampling strategies and/or geological constraints (e.g. sedimentation rates, weathering).
The 27 m long Suhia Kladenetz (SK) loess and paleosol sequence was sampled continuously at a 2 cm resolution, resulting in 1336 sampling depths. Magnetic, colorimetric and granulometric properties were studied. These highlighted the Mid-Brunhes climatic transition, as well as millennial-scale climatic variations during MIS 4-2, MIS 6, and 12 stages. The SK LPS will be used to demonstrate the development and quantification of a paleoprecipitation record for the last 800 thousand years. Two approaches were undertaken. For the first approach, variations in goethite concentration, determined via the diffuse reflectance spectroscopy (DRS), are demonstrated to be a tracer of water availability. The pedogenic nature of the goethite in the MIS 12 loess unit is demonstrated by granulometric and magnetic data and shown to accompany decalcification of the horizon in question and re-precipitation of calcite in the underlying horizon. Variations in goethite concentration are thereafter converted into absolute values of mean annual precipitation (MAP) via a transfer function relating the sample’s maghemite content to MAP. For the second approach, a climofunction relating the pedogenic enhancement of modern topsoils developing within loess across northern Bulgaria to modern-day precipitation was developed. The climofunction is then applied to the full SK LPS. The quantified paleoprecipitation record obtained through both approaches are compared and evaluated against modeled MAP data and other regional paleoclimate record.