Large paleoenvironmental insights from tiny molecules that "don't do anything": lessons from French and Ethiopian deposits.

International audiencePaleoenvironmental records provide insights into fundamental questions about structural changes in past human populations. Three problems with proxies lower the resolution of such records: 1) spatio-temporal discontinuity, 2) reflecting multiple factors at once, and 3) decomposition beyond recognition. Analyses of specific lipid molecules that "don't do anything" in soils and sediments are overcoming these obstacles. These are products of decomposition that resist further chemical change. Stable hydrogen isotopic compositions (δD) of molecules with long carbon chains in fatty acids and alkanes are providing records of paleoprecipitation. The records' accuracy may be undermined by changes in cover of their plant sources. Correcting for changes in C3 and C4 plant cover did not alter δD trends from fatty acids in Ethiopian soil sections and reflected shifts in precipitation amounts. In contrast, δD values of specific alkanes in soils along an elevation transect of Ethiopian agricultural stations did not reflect precipitation trends unless differences in C3 and C4 plant cover were accounted for. Pentacyclic triterpene methyl ethers (PTMEs) include potential biomarkers for specific plant types. The PTME miliacin is specific to broomcorn millet in the French Lake Bourget area. In addition to providing precipitation inferences that are not affected by changes in plant sources, its contents in sediments provide insights into millet agriculture by Bronze Age peoples where artifacts such as seeds have decomposed. Another PTME, crusgallin, is found only in grasses and its δD values are showing greater sensitivity to changes in precipitation than fatty acids from soils near ancient Ethiopian civilizations

Palaeoenvironmental Change and the Rise and Fall D'MT and Aksum in Northern Ethiopia: How an unambiguous proxy for rainfall can improve interpretations of micromorphological and botanical data

International audienceConsidered the fourth most powerful commercial empire in the Old World during its peak, Aksum (ca. 150 BCE - 700 CE) was by far sub-Saharan Africa's most complex ancient polity. It was preceded in the Tigray Plateau of Ethiopia and Eritrea by a less studied entity, D'MT (ca. 800 - 400 CE) that had many of the same material accoutrements connoting complexity. As the region is subject to significant spatio-temporal changes in climate and has relief that makes it highly vulnerable to erosion, it has been assumed that environmental changes played significant roles in the rise and fall of these polities. The exposed soils of the region's many ravines, promise its most ubiquitous archives of palaeoenvironmental inferences. In the absence of lakes and their archives of pollen, a combination of δ13C analyses of soil organic matter (SOM) with identification of charcoalised wood provided information about past percent biomasses of C3 and C4 plants as well as tree species for palaeovegetation records. A combination of micromorphological and elemental analyses provided insights about historical fire activity. Nonetheless, changes in these records could have been caused by climate and/or land clearing trends and thus they are an ambiguous suite of proxies. Analyses of stable hydrogen isotopic compositions (δD) of specific lipid molecules in organic matter are increasingly being used to provide unambiguous records of palaeoprecipitation for regions at tropical latitudes. The resolution of precipitation records δD values can be affected by the types of their vegetation sources, however. We analysed δD values of land plant derived fatty acids and used our carbon isotopic and elemental analyses to correct for possible effects of changes in C3 and C4 vegetation sources. The correction did not lead to any changes in trends of the δD fatty acid records. The fatty acids analysed are present in most land plants. We then analysed δD values of crusgallin, a pentacyclic triterpene methyl ether (PTME) known only to be produced by grasses and probably only C4 grasses in our study sites. Narrowing the range of possible plant sources to this extent proved the most sensitive means of detecting trends in precipitation over time. The δD records made it more possible to decipher the relative influences of precipitation and land clearing on changes in micromorphological and vegetation data, in some cases sharpening insights about the influences of human activity. Results thus far support hypotheses that the rise of both D'MT and Aksum were favored by slight increases in rainfall during an overall drying trend. At least in the hinterlands, much more marked indications of land clearing can be inferred during D'MT's emergence than Aksum's