image: Fig. 1 from Parton et al.: Map showing location of the study site and extent of bajada system in southeast Arabia, including other identified sections of the Al Ain fan (UAE--United Arab Emirates). view more
Credit: Geology and Ash Parton et al.
Boulder, Colo., USA - Considerable debate surrounds the migration of human populations out of Africa. Two predominant hypotheses concerning the timing contrast in their emphasis on the role of the Arabian interior and its changing climate. In one scenario, human populations expanded rapidly from Africa to southern Asia via the coastlines of Arabia approx. 50,000 to 60,000 years ago. Another model suggests that dispersal into the Arabian interior began much earlier (approx. 75,000 to 130,000 years ago) during multiple phases, when increased rainfall provided sufficient freshwater to support expanding populations.
Ash Parton and colleagues fall into the second camp, writing, "The dispersal of early human populations out of Africa is dynamically linked with the changing climate and environmental conditions of Arabia. Although now arid, at times the vast Arabian deserts were transformed into landscapes littered with freshwater lakes and active river systems. Such episodes of dramatically increased rainfall were the result of the intensification and northward displacement of the Indian Ocean Monsoon, which caused rainfall to reach across much of the Arabian Peninsula."
Parton and colleagues present a unique alluvial fan aggradation record from southeast Arabia spanning the past approx. 160,000 years. Situated along the proposed southern dispersal route, the Al Sibetah alluvial fan sequence provides a unique and sensitive record of landscape change in southeast Arabia. This record is to date the most comprehensive terrestrial archive from the Arabian Peninsula, and provides evidence for multiple humid episodes during both glacial and interglacial periods.
Evidence from the Al Sibetah alluvial fan sequence indicates that during insolation maxima, increased monsoon rainfall led to the widespread activation of drainage systems and grassland development throughout regions that were important for the dispersal of early human populations.
Previously, the timing of episodes of increased humidity was largely linked to global interglacials, with the climate of Arabia during the intervening glacial periods believed to be too arid to support human populations. Parton and colleagues suggest, however, that periods of increased rainfall were not driven by mid-high latitude deglaciations every ~100,000 years, but by periods of maximum incoming solar radiation every ~23,000 years.
They write, "The occurrence of humid periods previously identified in lacustrine or speleothem records highlights the complexity and heterogeneity of the Arabian paleoclimate, and suggests that interior migration pathways through the Arabian Peninsula may have been viable approximately every 23,000 years since at least marine isotope state (MIS) 6," about 191 thousand years ago.
FEATURED ARTICLE
Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal
Ash Parton et al., University of Oxford, Oxford, UK. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36401.1.
Other recently posted Geology articles published online ahead of print on 18 Feb. 2015 are highlighted below.
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Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body
C. Alwmark et al., Lund University, Lund, Sweden. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36429.1.
Work by C. Alwmark, L. Ferrière, and co-authors provides definite evidence for a meteorite impact origin of the Hummeln structure (located in the Småland province, Sweden), closing an about 200 years old debate. The discovery and characterization of shocked quartz grains in rocks collected at proximity of Lake Hummeln allows them to add Hummeln to the list of the 186 (now 187!) confirmed meteorite impact craters on Earth. Interestingly, due to its age of formation, ~467 million years ago, their study, published in Geology, also further the theory of intense bombardment of Earth during the Middle Ordovician. Despite its relatively small size, ~1.2 km in diameter (and 160 m deep), similar to the young and famous Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved. According to the authors of this study, this finding somewhat contradict the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years. This study shows that the search for impact craters on Earth should continue in order to better understand Earth's bombardment history.
Discovery of extraterrestrial component carrier phases in Archean spherule layers: Implications for estimation of Archean bolide sizes
Tanja Mohr-Westheide et al., Museum für Naturkunde-Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36548.1.
The solid planetary bodies in the Solar System experienced major bombardment by asteroids and comets in the first billion years of evolution. The dynamic nature of planet Earth has all but obliterated the record of this bombardment. Only traces of this cataclysmic period are so-called spherule layers in some of the oldest rocks, including those of the >3.2 billion-year-old Barberton Mountain Land, South Africa. Spherule beds are remnants of impact ejecta from those large impact events or direct fallout from the impact plumes. They carry unique enrichments of projectile-derived elements. Spherule layers in a new drill-core from Barberton have been investigated for traces of extraterrestrial projectiles and their carrier phases, which have been elusive since recognition of these early impact deposits in the late 1980s. Detailed microanalysis established that nickel-chromium-spinel carries tiny particles of platinum group metals responsible for the spherule layer enrichment in extraterrestrial component. This discovery sets the scene for further analysis of whether this component represents original projectile matter or condensation product from the impact plume, and how it was deposited at diverse abundances into the spherule deposits. Once this is understood, the magnitude of these early impact events can be assessed.
The response of the southern Greenland ice sheet to the Holocene thermal maximum
Nicolaj K. Larsen et al., Aarhus University, Aarhus, Denmark. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36476.1.
With a warmer climate in store for the Arctic, knowledge on the Greenland ice sheet's response to previous warm periods is extremely valuable. From 8 to 5 thousand years ago the local temperatures in southern Greenland were about 2 to 4 degrees Celsius warmer than present -- this period is known as the Holocene thermal maximum. This temperature increase is also what the Arctic is projected to reach by the year 2100 A.D. Thus, it is ever more interesting to study the Greenland ice sheet's response to previous warm periods. A Danish-led research team is now able to piece together periods of widespread glacier retreat during the Holocene thermal maximum using a combination of field evidence from glacial lakes and ice sheet modeling. The lakes act as excellent archives of glacial extent as they store sediment from the glaciers when the ice is advanced. The sediment showed that during the warm period the ice sheet was retreated -- and by comparing available models for ice sheet retreat with the actual intervals from sediment cores -- the best models were scrutinized. Results show that in the peak warmth during the Holocene thermal maximum the Greenland ice sheet was losing mass at an annual rate of 100 gigatonnes for several millennia, and delivered an equivalent of 16 centimeters of global sea-level rise.
A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: Implications for sequence stratigraphy
Luca Colombera et al., University of Leeds, Leeds, UK. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36385.1.
Fluvial successions form an important record of past Earth surface processes and may host natural resources such as water and hydrocarbons. Interpretations of fluvial strata in terms of changes in external controls on deposition and predictions of the subsurface distribution of aquifer- or reservoir-quality rocks have often relied on expected hypothetical relationships between the proportion of sedimentary bodies deposited in fluvial channels versus overbank settings, and the rate of floodplain aggradation -- a measure of the rate at which sediment accumulates through deposition by a river system. Current paradigms are founded on the intuitive idea that slower rates of aggradation facilitate reworking of overbank sediment via lateral channel cutting and migration, thereby increasing the proportion and lateral extent of bodies representing the products of channel infill. This study tests these assumptions against a number of ancient fluvial successions. No significant relationships are observed between aggradation rates and the relative proportions of channel-fill and overbank deposits. These results suggest that the interplay of factors that control deposition of fluvial successions in sedimentary basins is too complex to produce the simple relationships expected. Approaches for interpreting the rock record or for predicting subsurface architecture based on this simple paradigm should therefore be revised.
Melts of sediments in the mantle wedge of the Oman ophiolite
Karsten M. Haase et al., GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36451.1.
ABSTRACT: The rocks in the crustal section of the Oman ophiolite show an increasing input of a subduction component with time, most likely reflecting the generation of the ophiolite above a subducting slab. Field relations, new geochemical data, and Nd-Hf isotope data for felsic to mafic intrusive rocks in the mantle harzburgite from the Haylayn block in the Oman ophiolite suggest late magmatic events in a mantle wedge shortly before obduction of the ophiolite. Incompatible element contents and low delta-Nd and delta-Hf of the felsic rocks exclude differentiation from mafic magmas, but are consistent with an origin by partial melting of pelagic sediments similar to leucogranites in continental collision zones. These melts apparently mixed with mafic magmas resembling enriched late-stage lavas from the ophiolite. The leucogranitic intrusions into the mantle wedge confirm the transfer of melts of sediments from the subducted plate into the mantle at subduction zones. We suggest that the enrichment of Rb, K, and Pb observed in the Oman boninites is caused by addition of melts of sediments similar to those from the Haylayn block to the boninite source in the mantle wedge.
Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous
Yuan Gao et al., China University of Geosciences, Beijing, China, and Stanford University, Stanford, California, USA. Published online ahead of print on 18 Feb. 2015; http://dx.doi.org/10.1130/G36427.1.
ABSTRACT: The Late Cretaceous (late Campanian to Maastrichtian) was characterized by a variable greenhouse climate, with evidence for cooling and/or glaciation and warming events. Most of these climatic signals are derived from marine records, and knowledge of the terrestrial climate, especially in the mid-latitudes, is limited due to fragmentary geological records on continents. Here we report mid-latitude terrestrial stable oxygen and carbon isotope data from pedogenic carbonates in the nearly continuous Late Cretaceous age SK-1 core drilled in the Songliao Basin, northeastern China. Our data indicate a punctuated, mid-latitude terrestrial climate in the Late Cretaceous. We interpret the negative excursion of pedogenic carbonate δ18O in the early Maastrichtian to be the result of decreasing temperature and/or strengthened westerlies during global cooling and possible glaciation, providing valuable mid-latitude terrestrial evidence for this event. The negative δ13C isotopic excursion ca. 66 Ma is modeled as higher primary productivity caused by increasing temperature and precipitation in response to a warming climate in the latest Cretaceous. Our continuous stable isotopic records in the Songliao Basin are in accordance with previously published global Late Cretaceous records of climate variability from marine and terrestrial regions, and demonstrate the sensitivity of mid-latitude terrestrial climate in a greenhouse world.
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Journal
Geology