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、干燥的沙漠地带，毗邻更高、更冷、更潮湿的埃塞俄比亚高原。 哈达尔组是在340万至230万年前积累起来的，它包含了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，包括南方古猿阿法种。 下80%的地层，以及A.阿法人化石沉积在一个沉积盆地，其特点是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，并且通过最近发现的地质不整合与之前的时期不同。 在不整合面之上和之下的地质地层含有碳酸盐，可以使用碳和氧的稳定同位素来测量气候地层。 古土壤中的碳同位素提供了一个衡量较早时期湿适应与干适应植物相对丰度的指标，而同一地层中的氧同位素可用于推断古温度、海拔和相对降水量。 将哈达尔组分层古土壤的同位素信号与该地区当代标准进行比较，将使这些古老环境的重建更加精确和详细。 在SBER和INT部门的共同支持下，该项目将提供对1）A的环境背景的更好理解。afarensis，2）自然地理，气候和环境的过渡在不整合的性质，和3）它们的关系，生物变化和哈达尔物种形成。

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.