Chronology of dust archives

This chapter reviews the dating methods used on loess. It also takes time to review the principles behind these methods and, above all, the events they date. One section presents the differences between organic chronology (that of the vegetation that enabled accumulation by trapping the dust), the sedimentation chronology and the chronology of the setting of soils, gleys and pseudo-gleys.

This chapter deals with the dating of dust archives, and mainly focuses on loess archives that are the most common terrestrial dust archives. It explores the various methods used for loess dating, including cyclostratigraphy based on observed patterns of variations of physical, chemical, and (or) biological parameters along loess-paleosol sequences, magnetostratigraphy, tephrostratigraphy, aminostratigraphy, luminescence dating (OSL/IRSL), radiocarbon dating, and others. It provides fundamental concepts and practical aspects and examines the challenges and limitations of these techniques. The discussion is based on examples that illustrate each of the approaches. The chapter ends with applications of these methods to the elaboration of robust chronostratigraphic frameworks and paleoclimatological and paleoenvironmental reconstructions.

Figure 1: Schematic illustration of the discrepancy between mineral accumulation rate derived chronology and organic chronology—in orange: the mineral chronology, in green the organic chronology, in black when organic and mineral chronology match, dotted line to remember the previous state—(a and b): high and medium mineral accumulation rate, the organic chronology matches the mineral accumulation rate—(c): mineral accumulation rate slows down, the organic matter is still incorporated in the loess sediment, moves downward in the loess previously accumulated and mixes with organic matter accumulated during the phase “b”. In the upper part, the organic chronology no longer follows the mineral accumulation rate and inflects.—(d): high mineral accumulation rate resumes and organic chronology matches the mineral accumulation rate derived chronology—(e): rate of soil-formation (pedogenesis) outpaces mineral accumulation rates reduced to null and organic carbon continues to be incorporated at depth. The carbon age profile of upper part of deposit, gradually takes the form of a (modern) soil, i.e., smoothed S shape. The carbon stock, at each depth interval of this soil, is a mixture of carbon of different ages, covering the entire period of pedogenesis and of the
carbon accumulated during phase “d”. Because of this mixture, the average age of the carbon in the upper level is not zero, the 14C content is not that of the atmosphere but it’s lower. We face a strong divergence between the organic chronology and the mineral accumulation rate (which remains null all along the “e” period).—(f ): The period of climatic improvement stops and a new phase of climatic deterioration follows. This is most likely to result in an erosive phase that will eliminate the A horizon of the e phase soil and carry away some of the previously accumulated sediment. Both mineral (orange) and organic (green) chronologies are impacted.—(g): high mineral accumulation rate, the organic (green) chronology matches the mineral (orange) accumulation chronology. The last panel shows all the phases previously described. The solid lines are for the chronology as it was captured in the loess deposit and as it will be established today, the dotted line is for the organic (orange) and mineral (green) chronologies as it evolved during the deposit. The last sub-panel put together the organic and the mineral chronologies to highlight the differences.

Reference: Hatté C., Antoine P., Fuchs M., Guyodo Y., Lagroix F., Moine O., Piotrowska N., Rousseau D.-D., 2024. Chronology of dust archives. Encyclopedia of Quaternary Sciences, 3rd edition – doi: 10.1016/B978-0-323-99931-1.00166-5