40Ar/39Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal

Deciphering records of geomagnetic reversals

Polarity reversals of the geomagnetic field are a major feature of the Earth's dynamo. Questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition. A large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field. In addition, over the past two decades, numerical dynamo simulations have also provided insights into the reversal mechanism. Yet despite the large paleomagnetic database, controversial interpretations of records of the transitional field persist; they result from two characteristics inherent to all reversals, both of which are detrimental to an ambiguous analysis. On the one hand, the reversal process is rapid and requires adequate temporal resolution. On the other hand, weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records. This paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations. We discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process. Records from marine sediments dominate the database. They give rise to transitional field models that often lead to overinterpret the data. Consequently, we attempt to separate robust results (and their subsequent interpretations) from those that do not stand on a strong observational footing. Finally, we discuss new avenues that should favor progress to better characterize and understand transitional field behavior.

Geologist at sea: aspects of ocean history

Ocean history is largely read from deep-sea sediments, using microscopic fossils, notably foraminifers. Ice age fluctuations in the ocean's sediments provided for a new geologic understanding of climate change. The discovery of rapid decay of ice masses at the end of glacial periods was especially important, yielding rates of sea level rise reaching values of 1 to 2 m per century for millennia. Thanks to deep-ocean drilling, the overall planetary cooling trend in the Cenozoic was recognized as occurring in three large steps. The first step is at the Eocene-Oligocene boundary and is marked by a great change in sedimentation patterns; the second is in the middle Miocene, associated with a major pulse in the buildup of Antarctic ice masses and the intensification of upwelling regimes; and the third is within the late Pliocene and led into the northern ice ages. Evolution in the sea is linked to these various steps.

Tephrostratigraphy and the Acheulian to middle stone age transition in the Kapthurin Formation, Kenya

Sites containing Acheulian, Sangoan, Fauresmith, and Middle Stone Age artefacts occur within and below the Bedded Tuff, a widespread volcaniclastic member of the Kapthurin Formation, Kenya. The Bedded Tuff eruptive complex consists of up to twelve tephra beds, intercalated sediments, and paleosols. Two pumiceous units, high in the Bedded Tuff sequence, have been dated by(40)Ar/(39)Ar, one to 235+/-2 ka (Deino & McBrearty, 2002, Journal of Human Evolution, 42, 185-210, cf. Tallon, 1978, Geological Background to Fossil Man, pp. 361-373, Scottish Academic Press), the other to 284+/-12 ka (Deino & McBrearty, 2001), the latter now providing a minimum age estimate for all underlying archaeological sites. Bedded Tuff outcrops are correlated through field stratigraphic and electron microprobe geochemical analyses of individual beds. Bedded Tuff units show increasingly evolved composition through the stratigraphic succession, indicating that the beds are the product of intermittent eruption of a single differentiating magma system, and the chemical signatures of these beds permit the chronological ordering of archaeological sites. Our results indicate that the transition to Middle Stone Age technology occurred prior to 285 ka in this region of East Africa. The interstratification of sites containing Acheulian, Sangoan, Fauresmith, and Middle Stone Age artefacts suggests that these technologies were contemporary in a single depositional basin over the duration of the transition.

The revolution that wasn't: a new interpretation of the origin of modern human behavior

Proponents of the model known as the "human revolution" claim that modern human behaviors arose suddenly, and nearly simultaneously, throughout the Old World ca. 40-50 ka. This fundamental behavioral shift is purported to signal a cognitive advance, a possible reorganization of the brain, and the origin of language. Because the earliest modern human fossils, Homo sapiens sensu stricto, are found in Africa and the adjacent region of the Levant at >100 ka, the "human revolution" model creates a time lag between the appearance of anatomical modernity and perceived behavioral modernity, and creates the impression that the earliest modern Africans were behaviorally primitive. This view of events stems from a profound Eurocentric bias and a failure to appreciate the depth and breadth of the African archaeological record. In fact, many of the components of the "human revolution" claimed to appear at 40-50 ka are found in the African Middle Stone Age tens of thousands of years earlier. These features include blade and microlithic technology, bone tools, increased geographic range, specialized hunting, the use of aquatic resources, long distance trade, systematic processing and use of pigment, and art and decoration. These items do not occur suddenly together as predicted by the "human revolution" model, but at sites that are widely separated in space and time. This suggests a gradual assembling of the package of modern human behaviors in Africa, and its later export to other regions of the Old World. The African Middle and early Late Pleistocene hominid fossil record is fairly continuous and in it can be recognized a number of probably distinct species that provide plausible ancestors for H. sapiens. The appearance of Middle Stone Age technology and the first signs of modern behavior coincide with the appearance of fossils that have been attributed to H. helmei, suggesting the behavior of H. helmei is distinct from that of earlier hominid species and quite similar to that of modern people. If on anatomical and behavioral grounds H. helmei is sunk into H. sapiens, the origin of our species is linked with the appearance of Middle Stone Age technology at 250-300 ka.