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Proposal Title: Gulf of Aden Drilling: Testing African Climate-Human Evolution Hypotheses
Proponents: Peter B. deMenocal, Kensaku Tamaki, Gerald Ganssen, Francis H. Brown, Phillipe Huchon, Gen Suwa, Shigehiro Katoh, Tim White, Warren Prell, Jan Backman
View PDF of full Proposal (575-Full3)
Project Summary:
Leading paleonvironmental hypotheses of African hominin evolution state that critical Plio-Pleistocene junctures in early hominin evolution, such as the extinction of A. afarensis ("Lucy") and first appearances of Homo and robust australopithecines between 3.0-2.6 Ma, were the result of dramatic shifts in East African climate and vegetation which presented new adaptive and speciation pressures. These hypotheses make specific, testable predictions about faunal responses to different modes of environmental change and variability, but terrestrial records of East African climate variability are typically too rare, incomplete, and geographically dispersed to quantitatively test this fundamental question.
Deep-sea sediments accumulating in the Gulf of Aden and along the Somali margin are continuous, well-dated, and quantitative archives of African paleoclimate variability during this same time period. Furthermore, the late Neogene history of East African volcanic ash deposition from Kenyan and Ethiopian volcanoes is fully preserved in these more-proximal sediments. Since the chronology of hominin evolution is largely constrained by radiometric dating of these tephra layers (n=100-150 over the last 5 Ma), we propose to establish land-sea tephra correlations linking the terrestrial record of East African faunal evolution into the complete record of African paleoclimatic and paleoceanographic variability preserved in the Gulf of Aden sediments. By establishing a network of land-sea tephra correlations, ODP now has a unique opportunity to directly place the fossil record of African faunal evolution within the context of regional and global environmental change over the last 5 Ma.
Drilling Objectives
We propose a set of eight drill sites in the Gulf of Aden and along the Somali margin which collectively address the following three scientific objectives:
1) Reconstruct the Late Neogene history of northeast African climate and vegetation change. We exploit the proximity of the Gulf of Aden sites to northeast Africa and favorable monsoon wind transport vectors to reconstruct changes in northeast African aridity (dust flux and provenance) and vegetation cover (fossil pollen) spanning the last ca. 10 Ma. Many drill sites are less than 500 km away from Ethiopian and Kenya fossil hominin localities. Several of the proposed drill sites (GOA 1, 2, 3, 7, 8) are very proximal to northeast Africa and thus should provide a faithful record of changes in regional surface climate and vegetation cover during the Pliocene-Pleistocene. GOA-3, -4, -5 have lower sedimentation rates, allowing target drilling depths to extend to ca. 10 Ma.
2) Reconstruct variations in the Indian monsoon intensity and its paleoceanographic signatures off northeast Africa at tectonic, orbital, and millennial timescales. The surface and subsurface oceanographic temperature and nutrient fields at several drill sites are strongly influenced by the seasonally-reversing wind field associated with the Indian monsoon. Near Sites GOA 6, 7, 8 off Somalia, the strong summer southwest winds of the Findlater Jet parallel the Somali coast and promote upwelling of cold, nutrient-rich waters and high surface productivity. These sites offer excellent archives of past changes in monsoonal upwelling and productivity spanning the last ca. 5 Ma. Proposed sites GOA 1, 2, 6, 7, 8 have very high accumulation rates (ca. 15-40 cm/kyr) which will permit very detailed (century-scale) investigations into monsoonal variability throughout the Pliocene-Pleistocene.
3) Reconstruct the late Neogene history of explosive volcanism in East Africa and establish precise tephrostratigraphic correlations with East African hominin localities. We aim to further exploit the proximity of these drill sites to develop a comprehensive and well-dated catalogue of northeast African explosive volcanic eruption events. Past eruptions of Kenyan and Ethiopian volcanoes have produced tephra airfall deposits which can be geochemically correlated (using major and trace element, REE, and/or isotopic analyses of glass compositions) throughout northeast Africa and, importantly, into the Gulf of Aden sediments. Tephra layers are abundant and macroscopic in Gulf of Aden sediments. There have been ca. 100-150 large eruptions of northeast African volcanoes over the last ca. 5 Ma. Ages of fossil hominid specimens are largely constrained by radiometric dating of these same ash beds. Thus, this objective establishes the critical stratigraphic framework for linking the detailed regional paleoclimate and paleoceanographic histories resulting from objectives (1) and (2) with fossil hominin (and other vertebrate) localities from terrestrial sequences in Ethiopia, Kenya, and Tanzania.
4) Use the Gulf of Aden paleoclimate, paleovegetation, paleoceanographic, and tephrostratigraphic data to directly place the fossil record of East African faunal evolution within the context of regional and global environmental change to formally test African climate-evolution hypotheses (all sites). Collectively, this Gulf of Aden drilling would establish the framework for quantitatively assessing models of vertebrate evolution in Africa. This final objective reflects the overall scientific mission of this proposed leg - to directly place the African fossil record of hominin (and other mammalian) evolution within the context of the detailed, complete, and continuous marine sediment record of regional (and global) paleoenvironmental change. Such a test is not possible from terrestrial sequences alone due to their incomplete nature. ODP can contribute materially to one of the largest scientific problems concerning the origin of our species.