Palaeoenvironment Studies
DEVELOPING ANALYTICAL METHODS: First Studies in Ethiopia
A common problem when analyzing a proxy for palaeoenvironment is that a result may have been primarily caused by climate, land cover or both, and thus be easy to misinterpret. During Eshetu and Terwilliger’s first research collaboration in Ethiopia, we unexpectedly found a means of using two proxies to address this problem. We analyzed stable nitrogen isotopic compositions (δ15N) and stable carbon isotopic compositions of soil organic matter (δ13Csom) from soils we sampled along two transects spanning more than 2500 m in elevation each.
Studies had suggested that the δ15N value of a soil was directly linked to nitrogen-cycle chemistry of its ecosystem. This chemistry, in turn, was influenced by environment. Evidences most often supported degree of aridity---especially rainfall---as the most discernible influence on nitrogen cycle soil chemistry and hence soil δ15N values. Nonetheless, soil δ15N values were not used for inferring palaeoclimate because little was known about the time interval over which an indirect relationship between soil δ15N values and climate could last and the extent to which land cover could also affect δ15N values through local influences on the N cycle.
δ13Csom values reflect the proportional inputs of their plant sources with C3 or C4 photosynthetic pathways. Environmental conditions, in turn, influence whether C3 or C4 plants can best grow in an area. Like soil δ15N values, degree of aridity can certainly influence whether C3 or C4 plants grow best in an area and δ13Csom values had long been used as a coarse resolution proxy for past conditions of water availability to plants.
Many field calibration studies suffer from the fact that both temperature and rainfall differ with elevation and in opposite directions, making it impossible to tell which factor is related to variations in a proxy and how. Terwilliger and Eshetu’s transects had the fortuitous distinction that average annual rainfall increased and temperature decreased with increase in elevation along one transect (Dry), but although temperature also decreased with increase in elevation, rainfall was not related to elevation along the other transect (Wetter).
Because δ15N values were only related to elevation in their “Dry transect”, Eshetu and Terwilliger’s results showed soil δ15N values to decrease with increase in rainfall but to not change with other environmental factors examined (Figure 2). Furthermore, in this and later more specific studies, δ15N values did not differ as a function of whether vegetation cover was planted by humans or not.
As expected from existing studies, δ13Csom values decreased as climate became cooler and wetter but---unexpectedly---only from middle to highest elevations. The opposite trend occurred from the lowest (hottest and driest) to mid-elevations (Figure 3). The mid-elevations had the most intense and longest history of land use for agriculture, which may have been even more favorable for C4 plants than aridity. History of human activity may have contributed to our results but this factor could not be inferred from δ13Csom values alone. By putting the δ13Csom values together with their corresponding δ15N values in a simple model, however Terwilliger and Eshetu and were able to decipher whether a trend in δ13Csom best corresponded with rainfall or some other factor, such as effects of human activities on land cover.
Although the palaeoenvironmental group still uses combined δ13Csom and soil δ15N analyses as a first pass for inferring rainfall versus land use history, the soil δ15N values are only sensitive to rather large changes in rainfall. Where greater sensitivity is needed, we now perform more time-consuming but accurate analyses of stable hydrogen isotopic compositions of n-alkanoic acid compounds (for more information, see Development and Application section).