Paleosol Isotopic Evidence for Environmental Change at Hadar

今天，埃塞俄比亚的哈达尔地区是一个炎热、干燥、沙漠地带，毗邻地势更高、更凉爽、更潮湿的埃塞俄比亚高原。哈达尔组堆积在340万至230万年前，蕴藏着这些地区之间构造边界历史的许多证据，并产生了丰富的上新世化石，包括南方古猿。这一组的下部80%是在沉积盆地中沉积的，也是阿法斯化石的来源，沉积盆地的环境比今天看到的要潮湿和凉爽得多，类似于现代高原。地层的上部反映了今天更干燥、更温暖的条件，与以前的时期不同，因为最近发现了一个地质不整合。不整合之上和之下的地质地层含有碳酸盐，可以用碳和氧的稳定同位素来测量气候地层学。古土壤中的碳同位素提供了早期湿适应植物与干适应植物的相对丰度的量度，而同一地层中的氧同位素可用于推断古温度、海拔高度和相对降水量。将哈达尔组分层古土壤的同位素信号与该地区的当代标准进行比较，将能够更准确和详细地重建这些古代环境。在SBER和INT司的共同支持下，该项目将更好地了解1)阿法尔山的环境背景，2)不整合处的地形、气候和环境转变的性质，以及3)它们与哈达尔的生物变化和物种形成的关系。

The Hadar region of Ethiopia is today a hot, dry, desert zone that lies adjacent to the higher, cooler, and more moist Ethiopian Plateau. The Hadar formation, accumulated between 3.4 and 2.3 million years ago, harbors much evidence of the history of the tectonic boundary between these regions and has yielded abundant fossils of Pliocene age, including Australopithecus afarensis. The lower 80% of this formation, and the source of A. afarensis fossils, was deposited in a sedimentary basin characterized by an environment much wetter and cooler than that seen today, similar to the modern plateau. The upper portion of the formation reflects the dryer, warmer conditions of today, and is distinct from the preceding periods by a recently discovered geological disconformity. Geological strata above and below the disconformity contain carbonates from which a climatic stratigraphy can be measured using stable isotopes of carbon and oxygen. Carbon isotopes in paleosols provide a measure of the relative abundance of wet-adapted versus dry-adapted plants in earlier time periods, while oxygen isotopes in the same strata may be used to infer paleo-temperature, -elevation, and relative precipitation. Comparison of the isotopic signals from the stratified paleosols in the Hadar formation to contemporary standards in the region will permit a more precise and detailed reconstruction of these ancient environments. Jointly supported by the SBER and INT divisions, this project will provide a better understanding of 1) the environmental context of A. afarensis, 2) the nature of the physiographic, climatic, and environmental transitions at the disconformity, and 3) their relationship to biological change and speciation at Hadar.