Olduvai Chron Research Articles

Absolute paleointensity determinations by using of conventional double-heating and multispecimen approaches on a Pliocene lava flow sequence from the Lesser Caucasus

We report 28 successful Thellier type absolute geomagnetic paleointensity determinations from a Pleistocene lava sequence composed of 39 successive flows in the Djavakheti Highland (Lesser Caucasus, Georgia). Additionally, multispecimen technique provided the estimation of geomagnetic field strength for 12 independent cooling units.Paleointensity studies were performed using both Thellier type double heating and multispecimen techniques. Samples selection was mainly based on uni-vectorial remanent magnetization, thermal stability and domain size of the samples. Flow-mean Thellier paleointensity values range from 16.3±5.2 to 71.0±0.3μT, while intensities obtained using multispecimen approach vary from17.2±2.3 to 69.3±7.9μT.One of the flows is located near a possible discontinuity in the sequence and yields a rather low Thellier absolute intensity (16.3±5.2) suggesting a transitional regime and the onset of the Matuyama-Olduvai polarity transition, which does not appear on the directional record. Multispecimen paleointensities from the same flow, however, yield higher, close to present day values which makes untenable the hypothesis of occurrence of transitional field. Thus the whole sequence was emplaced in a short time between the Olduvai chron and 1.73±0.03Ma, as suggested by available radiometric and paleomagnetic data (Caccavari et al., 2014).

Geophysical investigation of Pleistocene volcanism and tectonics offshore Capo Vaticano (Calabria, southeastern Tyrrhenian Sea)

Magma upwelling forming volcanic plumbing systems in back arc settings is typically controlled by extensional tectonic structures of the upper crust. Here we investigate this process in the area between the volcanic arc of the Aeolian Islands and the Calabrian arc (SE Tyrrhenian Sea) by integrating morpho-bathymetry and reflection seismic data with the outcomes of “Inverse 3D magnetic modeling” of previously gathered aeromagnetic data. Morpho-bathymetric data highlight the presence of a seamount ∼10km offshore Capo Vaticano Promontory (eastern Calabria). This feature, named Capo Vaticano seamount is composed of a series of NE-trending ridges, the greatest of which (R1) is ∼12km long, and 2.4km wide, displays asymmetric flanks with a landward steep slope, oblique morphological steps and elongated NE-trending rims. The position of the R1 ridge summit fits the Reduced-to-the-Pole peak of a high-intensity magnetic anomaly straddling Capo Vaticano Promontory and its offshore prolongation. Seismic and bathymetric data highlight two extensional fault systems affecting the offshore of Capo Vaticano Promontory during the Plio-Pleistocene: (a) a Pliocene NW-trending, SW-dipping normal fault system, and (b) a Pleistocene NE-trending, SE-dipping normal fault system. The younger system is composed of a series of en-echelon branching normal faults bounding the eastern side of the R1 ridge. Aeromagnetic data modeling imaged a complex 3D-magnetized body below the R1 ridge exhibiting a sub-vertical conduit-like structure in the shallow part, and a NE-striking, sheet-like shape inclined by 45° in depth. The location of the sub-vertical conduit coincides with the summit of the R1 ridge. The magma uprising at the root of the volcano was controlled by the Pliocene NW-trending faults whereas its further upwards migration was ostensibly controlled by the Pleistocene NE-trending faults. Both fault systems are responsible for the high level of fracturing that likely favored the upward migration of magma. The younger extensional systems also controls the present-day, mantle derived, fluid escapes observed at the summit of the R1 ridge. Relying on seismic stratigraphic evidence as well as the normal polarity of the magnetic anomaly, the R1 ridge probably started to form during the Olduvai chron (early Pleistocene, 1.81–1.96Ma). Accordingly, the Capo Vaticano volcano may represent the result of magmatic activity that predates the Aeolian volcanic arc.

Palaeomagnetism and 40Ar/39Ar age of a Pliocene lava flow sequence in the Lesser Caucasus: record of a clockwise rotation and analysis of palaeosecular variation

SUMMARY A palaeomagnetic and rock-magnetic investigation has been carried out on a Pliocene lava flow sequence in the Djavakheti Highland in the central Lesser Caucasus in the Republic of Georgia. In addition, a 40 Ar/ 39 Ar dating and electronic microscopic studies were performed on samples of this sequence, named the Saro section, which consists of 39 successive lava flows of doleritic basalts. A characteristic magnetization could be isolated in all studied 39 flows,yielding reverse-polarity directions inall cases,amean directionD =202.2 ◦ ,I =–60.6 ◦ (N = 39, α95 = 2.0 ◦ , k = 138) being obtained. Thermomagnetic experiments (strong-field versus temperature curves) suggested low-Ti titanomagnetites and low Curie-temperature titanomagnetites with a rather high titanium content (x ≈ 0.5–0.7) as the main carriers of remanence. Their domain structure is characterized by a mixture of single- and multidomain grains. 40 Ar/ 39 Ar dating yielded an age of 1.73 ± 0.03 Ma, interpreted as the eruption age of the uppermost lava flow of the sequence. Analysis of palaeomagnetic results and radiometric data from the present and a previous study allows two different explanations about the time of emplacement of the section: (i) The lower 36 flows of the sequence might have been emitted between the normal-polarity Reunion and Olduvai chrons, and the upper three flows after the Olduvai chron, with a long hiatus in volcanic activity of more than 150 kyr or (ii) The whole sequence has been emitted between 1.778 and 1.73 ± 0.03 Ma, after the Olduvai chron. Comparison of the palaeomagnetic results obtained in this study with the expected direction shows that while inclination values agree well, declination shows an eastward deviation of 19.2 ◦ ± 5.8 ◦ . This discrepancy can be explained with a clockwise vertical-axis rotation of the sequence, which might have been produced by extensional structures with strike-slip component, which can be found in the study area. Virtual geomagnetic pole (VGP) scatter recorded in the sequence is somewhat lower than expected. To analyse this behaviour, VGP scatter of the sequence was first calculated with respect to the palaeomagnetic pole obtained from the Fisher mean of all VGPs. Then it was calculated with respect to the appropriate Eurasian palaeomagnetic pole from VGPs obtained after applying a 19.2 ◦ counter-clockwise rotation of the sequence (i.e. undoing the observed vertical axis rotation). The observed scatter with both independent methods appeared to be very similar, allowing the deduction that

Evidence of late Gelasian dispersal of African fauna at Coste San Giacomo (Anagni Basin, central Italy): Early Pleistocene environments and the background of early human occupation in Europe

Since the late 70s, the Early Pleistocene (Gelasian) site of Coste San Giacomo (Anagni Basin, central Italy) has been known amongst palaeontologists for its diverse vertebrate fauna. During the last 5 years, new excavations and the drilling of a 46-m-deep core have provided novel pieces of information. Palaeomagnetic data, pollen and small vertebrates analyses are presented here for the first time and combined with the updated list of the large vertebrates and ostracod analysis in a multidisciplinary perspective. Large and small mammals, pollen and ostracod analyses have allowed an integrated palaeoenvironmental reconstruction of the sedimentary sequence, depicting the evolution of the alluvial plain in the surrounding landscape. Moreover, magnetostratigraphy, pollen and small mammal biochronological data have confirmed the position of the Coste San Giacomo Faunal Unit, focusing the possible age of the mammal assemblage around 2.1 Ma, in a reversed phase before the base of the Olduvai chron. In particular, the occurrence of the large vole Mimomys pliocaenicus has important biochronological significance. The Coste San Giacomo site offers a unique opportunity to investigate the faunal and environmental changes that occurred in Mediterranean Europe during the Early Pleistocene, coinciding with major climatic changes at a global scale. The occurrence of taxa such as Hippopotamus sp. in the assemblage provides evidence of early dispersal events of African taxa prior to the early Homo diffusion into Europe.

An astronomically-tuned climate framework for hominins in the Turkana Basin

Understanding the influence of orbital climate cycles on hominin evolution remains a key challenge in paleoanthropology. The two major unresolved issues are: the absence of a climate proxy yielding high-resolution (< 20 kyr) terrestrial climate records, and the lack of age control on hominin fossil occurrences at sufficiently high resolution. Here we present a novel climate proxy, strontium isotope ratios ( 87Sr/ 86Sr) of lacustrine fish fossils from the Turkana Basin, that solves these issues by recording orbitally forced variation in summer monsoon intensity over the Ethiopian Highlands. We successfully applied the climate proxy to a ~ 150 kyr time interval of ~ 2 million year old paleolake deposits containing hominin fossils. Existing age control of the studied interval was improved by a new magnetostratigraphic record precisely locating the base of the Olduvai chron (C2n) near the bottom of the sequence. Spectral analysis demonstrates that 87Sr/ 86Sr variability is primarily determined by precession, which enables us to place hominin fossils in an astronomically-tuned climate framework. The Sr climate proxy is potentially applicable to all hominin-bearing lake deposits in the Turkana Basin, ranging in age from ~ 4.2 to 0.8 million years ago (Ma). Our results demonstrate that between ~ 2 and 1.85 Ma the Turkana Basin remained well-watered and inhabited by hominins even during periods of precession maxima when summer monsoon intensity was lowest. This is in contrast to other basins in the East African Rift System (EARS) that were impacted heavily by precession-forced droughts. We hypothesize that during lake phases, the Turkana Basin was an aridity refugium for permanent-water dependent fauna – including hominins – over the precessional climate cycles.

Geology and chronology of the continental Pleistocene archeological and paleontological sites of the Orce area (Baza basin, Spain)

The Guadix-Baza Basin hosts a continuous continental record of paleoenvironmental, paleobiological and geological changes from around 8 Ma to 205 ka. A remarkable succession of Pleistocene mammal sites is found in the Lower and Upper members of the Baza Formation in the Orce area. The Upper Member contains the Early Pleistocene paleontological and archeological sites of Barranco León-5 and Fuente Nueva 3. Both sites belong to the early Pleistocene zone defined by Allophaiomys aff. lavocati and a small Mimomys savini. This zone can be correlated with that of central Europe containing Mimomys savini and Microtus praehintoni. These sites are located in the marginal lacustrine deposits of the Baza lake system and originated in expanded phases of the main lake when meteoric surface and shallow groundwater inputs were predominant. The Barranco León-5 site is related to a sudden event of high-energy currents entering a lacustrine - palustrine domain. These currents transported gravels, bones and lithic industries from a short distance (maximum, hundreds of meters). In contrast, the diverse levels in Fuente Nueva 3 do not show evidence of noticeable transport by traction currents. The age of the mammal succession and sites is based on magnetostratigraphy. In the Middle and Upper members of the Baza Formation, only reversed polarities are found (Matuyama epoch). The location of the Olduvai chron is still unclear and could have not been recorded due to the common hiatuses observed in the sedimentary successions. In the Lower Member, normal and reverse polarities (Gauss epoch) are found in the Galera and Vélez-Salar sections. The later contains the site of Fuente Nueva 1, which is pre-Olduvai chron in age according to paleontological data and biostratigraphic correlation.

Correction to “A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge” by Gaud Pouliquen, Yves Gallet, Jérôme Dyment, Philippe Patriat, and Chiori Tamura

High‐resolution records of the geomagnetic field intensity over the last 3.5 Myr provided by paleomagnetic analyses of marine sediments and volcanics have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. ln subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.

THE FOSSIL VERTEBRATE DATABASE OF THE NATURAL HISTORY MUSEUM OF FLORENCE AND HIGH-RESOLUTION MAGNETIC STRATIGRAPHY IN THE UPPER VALDARNO BASINS, AS A CLUE TO DATE OLD COLLECTIONS

The well established biochronologic sequence of the Villafranchian Stage in Italy is mainly based on faunal associations from the Upper Valdarno (UV) mostly collected since the late 18 hundreds, and housed in the Natural History Museum of Florence. The old collections were assembled from mostly unidentified stratigraphic levels, and their position possibly reconstructed from the surrounding geologic features. The recent magnetostratigraphic assessment of the sequence marked the earliest finds about 3.0 Ma in themid Pliocene. The end of the Pliocene was recorded by the Olduvai magnetochron in the Matassino and Poggio Rosso sites, and by other sparse assemblages. The Tasso Faunal Unit, assembled in the UV, is assigned to the Pleistocene, yet to be clarified by magnetostratigraphic data. The calibration of old, poorly timed faunas was greatly facilitatedby the computer automated catalogue of the Museum. The possibility of numerous feed-back controls enhanced any contradictory information in fossil collections and made them most fruitful for paleomagnetic calibrations: the Faella fauna and the Faella main outcrop are the ones that will be first re-examined. It is in fact now evidenced the potential role of the catalogue for accomplishing the calibration of old findings timed around the Olduvai chron, with an accuracy depending on the available record of their inferred stratigraphic levels. Numerical dates will make the old collections comparable to the new ones and both fitting into a comprehensive framework of faunal, chronologic, sedimentary evolution of the UV basin, and development in the Plio-Pleistocene climate changes.

A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge

High‐resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A.1n subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.

Resultados paleomagnéticos de los depósitos pliopleistocenos de la cuenca de Palma (Mallorca, España)

The results from the paleomagnetic analysis of the plio-pleistocene deposits located in the eastern (Bahia Azul Section) and western (Banc d'Eivissa) borders of the Palma Basin are presented in this paper. The Bahia Azul section is composite by a marine terrace at +70-74 m a.s.1., overlapped by nine littoral dune systems separated by erosional surfaces. The Banc d'Eivissa section is constituted of a lower marine terrace at +24-25 m a.s.l., buried by two dune systems, although there are more numerous in the vicinity. The lithological spectrum at both sections holds a main calcarenitic nature, but the available outcrops display a different degree of weathering making difficult the establishment of a continuous magnetoestratigraphic sequence. The paleontological content (Rosello y Cuerda, 1973) and paleomagnetic results of the Banc d'Eivissa deposits are no sufficient to establish a precise chronological sequence, and only is possible to locate them within the reverse polarity Matuyama epoch. On the contrary, the lower half of the Bahia Azul sequence (marine terrace plus the 3 first dune systems) presents a consistent reverse polarity. The paleontological content of these materials indicates an early Pleistocene age (Emilian cycle: Cuerda y Sacares, 1992) which agree with the obtained paleomagnetic results. Therefore, the whole sequence is interpreted to be located between the normal polarity chrons of Jaramillo and Olduvay (Clr. 1n and C2n of Cande and Kent, 1995) within the Matuyama epoch, but can not be totally rejected the possibility that them could be placed after the Olduvai Chron.

Match between late Pleistocene Great Bahama Bank and deep-sea oxygen isotope records of sea level

Throughout the Pliocene and Pleistocene, a hierarchy of sea-level fluctuations alternately flooded and exposed the Great Bahama Bank, controlling its sedimentary evolution. The persistent rhythm of sea level is best shown in the upper Pleistocene to Holocene section in which 10 cycles of sea-level rise and fall are identified in core borings on the lee side of the bank. The same number of sea-level oscillations is indicated within the Brunhes chron of the deep-sea oxygen isotope record. In the Bahamas, sea-level oscillations are inferred from meter-scale parasequences of shallow-marine carbonates, interpreted largely as transgressive and highstand deposits, sandwiched between discontinuity horizons interpreted as subaerial-exposure horizons that developed during lower sea level. Subaerial exposure shifted from being largely restricted to the platform interior during the late Pliocene (Olduvai chron), to occurring across the entire lee side of the platform at the beginning of the Brunhes chron. This change on the platform is highly suggestive of the deep-sea oxygen isotope proxy of sea level which indicates a fundamental paleoclimatic shift at approximately the same time. Despite dating limitations, results from the Bahamian cores can be used to push the calibration of the sea-level record back to the late Pliocene.

Late Pliocene continental magnetochronology in the Guadix-Baza Basin (Betic Ranges, Spain)

The late Pliocene shallow lacustrine sequences in the Guadix-Baza basin (Betic Ranges, southeast Spain) are a suitable setting for mammal chronology because of the abundance of rich, mammal-bearing sites. Stratigraphic superposition of late Ruscinian and Villanyian vertebrate fossils makes the Galera Section very appropriate for magnetostratigraphic dating. Sediments are characterized by a weak but measurable magnetization predominantly carried by magnetite. Other remanence carriers, such as iron sulphides and hematite, may be significant only in some particular lithologies, such as dark palustrine organic silts and red alluvial beds, respectively. Direct correlation of the Galera Section to the geomagnetic polarity time scale provides a reliable dating of the late Ruscinian and Villanyian mammal units. The Ruscinian/Villanyian boundary (MN15/MN16) is found to occur in the lower Gauss chron, and approximately correlates to the 3.3 Ma cooling event observed in stable-isotope and planktonic foraminifera deep sea records. The earliest occurrence of late Villanyian assemblages (MN17) in the lower part of the Matuyama chron allows dating of the early/late Villanyian boundary (MN6/MN17) to approximately 2.4–2.6 Ma. Based on the presence of late Villanyian assemblages underlying the base of the Olduvai chron, a maximum age of 1.95 Ma (top of C2r) is attributed to the Villanyian/Biharian (MN17/MmQ) boundary. A 300 kyr sedimentary gap in the Galera Section is suggested to correlate with a probable early Villanyian hiatus, an hypothesis that agrees with the absence of fossils of this age elsewherein the Baza Formation.