The localities of La Bullana 3 and LA Bullana 2B (Valencia, E Spain) have yielded remains of Apod... more The localities of La Bullana 3 and LA Bullana 2B (Valencia, E Spain) have yielded remains of Apodemus gorafensis, Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet. and Asoriculus cf. gibberodon the former, and Apodemus gorafensis, Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet., Asoriculus cf. gibberodon, Castillomys gracilis, Occitanomys brailloni, Occitanomys sp., Paraethomys meini, Ruscinomys sp., Eliomys intermedius, Debruijnimys cf. julii and Atlantoxerus sp. the latter. Based on the study of these micromammal assemblages, we propose an Early Pliocene age (MN14) for both sites. The presence a gerbilid related to Debruijnimys julii in La Bullana 2B open new questions about the phylogenetic relationship between Debruijnimys species from the Miocene and Pliocene of the Iberian Peninsula. Furthermore, preliminary paleomagnetic data indicate an age between 4.997 Ma and 4.896 Ma for La Bullana 2B, and between 5.235 Ma and 4.997 Ma for La Bullana 3.
The fossil quarries of southeastern Spain's Baza Basin have been producing abundant vertebrat... more The fossil quarries of southeastern Spain's Baza Basin have been producing abundant vertebrate remains for the last 30 years. Of specific interest are the lithic artifacts found in some excavations. During the last decade, hundreds of these Oldowan type tools were excavated from the stratigraphically equivalent quarries of Barranco Leon and Fuente Nueva-3, near the town of Orce. Lacking volcanic deposits, the chronology relies on magnetostratigraphy to estimate the age of these early homonins. Previously, the only confirmed results were from short sections around the quarries, where Reverse polarity was found. We have made an initiative to expand the magnetostratigraphy downward until a magnetozonation could be established. In concert, we expanded the lithostratigraphy both vertically and regionally, and expanded the fossil excavations into lower sites. The Reverse magnetozone was extended until 40m (n=70 samples) below the lithic artifact quarries. The lower, Normal magnetozone...
The unusually stable remanent magnetization found in many gabbroic rocks actually comes from clin... more The unusually stable remanent magnetization found in many gabbroic rocks actually comes from clinopyroxene crystals. While not magnetic itself, clinopyroxene acts as a crystallographic host to two arrays of magnetite inclusions, formed during slow cooling by exsolution. In the cone-sheet samples studied (Messum, Namibia), this exsolution takes place at 850° +/-50° C. The typical size of a magnetite inclusion in
ABSTRACT Titanomagnetite inclusions exsolved in silicate minerals are a powerful tool for explori... more ABSTRACT Titanomagnetite inclusions exsolved in silicate minerals are a powerful tool for exploring the Earth's magnetic field behavior because they are protected from chemical alteration by their silicate hosts and are ubiquitous in Precambrian terranes composed of gabbros and anorthosites. Yet the extreme shape anisotropy of these needle-shaped inclusions creates the potential for remanence anisotropy and nonlinear remanence acquisition. To better determine the extent of these processes, we measured the thermal remanence magnetization (TRM) acquired by single-crystal samples of clinopyroxene (Messum Complex, Namibia) and plagioclase (Bushveld Complex, South Africa) at four field strengths (20,50,100, and 140 muT) in 6 orientations (positive and negative X, Y, and Z). Mineral alteration during thermal cycling was negligible and was monitored by measuring saturation isothermal remanent magnetization (SIRM) before and after each TRM step. The 6 three-dimensional magnetization vectors recorded at each field strength were used to calculate a least-squares, best-fit TRM tensor. In general, the clinopyroxene-hosted inclusions showed significantly higher anisotropy than those in plagioclase. Clinopyroxene grains displayed a strongly oblate anisotropy (2 < F < 128, 13 < P < 672), while plagioclase grains showed a comparatively more subdued prolate anisotropy (0.5 < F < 5, 1 < P < 11). The degree of anisotropy associated with each silicate phase is a consequence of the number of inclusion orientations in clinopyroxene (n = 2) and plagioclase (n = 5). The tensors' eigenvalues and eigenvectors for all samples were unaffected by variations in applied field strength. Samples acquired magnetization linearly up to fields of 100 muT, at which point samples approached saturation remanence. Additionally, we note that the slope of linear remanence acquisition was dependent on the crystallographic orientation of the single crystals. These results demonstrate that single crystals of clinopyroxene and plagioclase do exhibit strong remanence anisotropy. Despite this, their linear remanence acquisition up to 100 muT fields ensures that they will be useful recorders of paleointensity for most periods of geomagnetic behavior. Monte Carlo simulations using average anisotropy tensors allowed us to determine how many randomly oriented crystals are needed in a standard bulk sample to overcome the single-crystal anisotropy and accurately record the Earth's magnetic field direction. Thus, so long as a large enough population of grains is contained within a single bulk sample, their extreme remanence anisotropy can be overcome, and accurate paleodirections and paleointensities can be obtained.
ABSTRACT Paleomagnetic data frequently display bipolar data that are not exactly antipodal. One c... more ABSTRACT Paleomagnetic data frequently display bipolar data that are not exactly antipodal. One cause of this biasing is the addition of another later (secondary) magnetization that cannot be removed by laboratory demagnetization experiments. The magnitude of this hidden secondary magnetization can be calculated when only two values are known: the non-antipodal angle (measured), and the angle between the Normal direction (measured) and secondary direction (measured or assumed). Furthermore, a minimum magnitude of the secondary magnetization can be calculated with knowledge only of the non-antipodal angle. Applied to three published data sets (sandstone, limestone and diorite intrusion) with non-antipodal angles of 11°, 28° and 11° respectively; a secondary component is calculated at 25%, 28% and 16% of the characteristic magnetization remaining after laboratory cleaning. Near surface alteration (weathering) appears to be the source of secondary magnetization in two of these cases. A directionally consistent, biasing effect of a few degrees is made on the mean direction as compared to the conventional calculation of averaging the non-antipodal directions.
Australopithecus afarensis is the best-known and most dimorphic species in the early hominin foss... more Australopithecus afarensis is the best-known and most dimorphic species in the early hominin fossil record. Here, we present a comparative description of new fossil specimens of Au. afarensis from Nefuraytu, a 3.330-3.207 million-years-old fossil collection area in the Woranso-Mille study area, central Afar, Ethiopia. These specimens include NFR-VP-1/29, one of the most complete mandibles assigned to the species thus far and among the largest mandibles attributed to Au. afarensis, likely representing a male individual. NFR-VP-1/29 retains almost all of the distinctive archaic features documented for Au. afarensis. These features include a posteriorly sloping symphysis, a low and rounded basally set inferior transverse torus, anterosuperiorly opening mental foramen, a lateral corpus hollow bound anteriorly by the C/P3 jugae and posteriorly by the lateral prominence, and the ascending ramus arising high on the corpus. Dental morphology and metrics of the Nefuraytu specimens also falls...
ABSTRACT Paleoamagnetic data from the Guadelupian (Upper Permian) Nosoni Formation of the Redding... more ABSTRACT Paleoamagnetic data from the Guadelupian (Upper Permian) Nosoni Formation of the Redding Section (RS) consistently exhibit two directional components of magnetization in addition to a late Quaternary normal weathering overprint. Six sites in ash flow tuffs and tuffaceous sedimentary rocks, spanning 200 m of section, yield data that allow precise characterization of all components. Two additional sites in calcareous mudstones were too heavily overprinted to yield stable paleomagnetic directions. Thermal demagnetization of the tuffaceous samples in 16-18 steps between natural remanent magnetization and 660°C provides a detailed data base from which to calculate directional components. Analysis of demagnetization trends identifies a prefolding reversed direction carried by both magnetic and hematite and a postfolding normal direction carried by hematite only. Petrologic considerations suggest paragenesis of these oxides in the Permian (primary magnetite and hematite) and Early-Middle Jurassic (metamorphic hematite).Least square analysis of progressively demagnetized vector endpoint trends between 540° and 600°C yields a formation mean direction in magnetite having D=156.4°, I=-29.7°, k=101, x95=7.6°. This result is in close agreement with the composite prefolding direction (D=160.3°, I=-31.5°), carried by both magnetite and hematite, determined by the intersection of remagnetization circles. The postfolding hematite-carried overprint direction calculated by the latter technique has D=352.3°, I=35.0°. Paleopoles corresponding to these two direction are located at 57.6°N, 103.0°E (magnetite formation mean) and 67.0°N (hematite overprint), coinciding within error to Upper Permian and Early Jurassic (respectively) reference poles from the North America craton. These data indicate no significant rotation or latitudinal translation of the sample sites with respect to cratonic North America since the Permian. This conclusion contrasts with seemingly reliable paleomagnetic results from elsewhere in the RS, which indicate major post-Permian clockwise rotations. These rotations are evidently specific to individual structural domains and are probably manifestations of oroclinal bending about vertical axes.
The localities of La Bullana 3 and LA Bullana 2B (Valencia, E Spain) have yielded remains of Apod... more The localities of La Bullana 3 and LA Bullana 2B (Valencia, E Spain) have yielded remains of Apodemus gorafensis, Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet. and Asoriculus cf. gibberodon the former, and Apodemus gorafensis, Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet., Asoriculus cf. gibberodon, Castillomys gracilis, Occitanomys brailloni, Occitanomys sp., Paraethomys meini, Ruscinomys sp., Eliomys intermedius, Debruijnimys cf. julii and Atlantoxerus sp. the latter. Based on the study of these micromammal assemblages, we propose an Early Pliocene age (MN14) for both sites. The presence a gerbilid related to Debruijnimys julii in La Bullana 2B open new questions about the phylogenetic relationship between Debruijnimys species from the Miocene and Pliocene of the Iberian Peninsula. Furthermore, preliminary paleomagnetic data indicate an age between 4.997 Ma and 4.896 Ma for La Bullana 2B, and between 5.235 Ma and 4.997 Ma for La Bullana 3.
The fossil quarries of southeastern Spain's Baza Basin have been producing abundant vertebrat... more The fossil quarries of southeastern Spain's Baza Basin have been producing abundant vertebrate remains for the last 30 years. Of specific interest are the lithic artifacts found in some excavations. During the last decade, hundreds of these Oldowan type tools were excavated from the stratigraphically equivalent quarries of Barranco Leon and Fuente Nueva-3, near the town of Orce. Lacking volcanic deposits, the chronology relies on magnetostratigraphy to estimate the age of these early homonins. Previously, the only confirmed results were from short sections around the quarries, where Reverse polarity was found. We have made an initiative to expand the magnetostratigraphy downward until a magnetozonation could be established. In concert, we expanded the lithostratigraphy both vertically and regionally, and expanded the fossil excavations into lower sites. The Reverse magnetozone was extended until 40m (n=70 samples) below the lithic artifact quarries. The lower, Normal magnetozone...
The unusually stable remanent magnetization found in many gabbroic rocks actually comes from clin... more The unusually stable remanent magnetization found in many gabbroic rocks actually comes from clinopyroxene crystals. While not magnetic itself, clinopyroxene acts as a crystallographic host to two arrays of magnetite inclusions, formed during slow cooling by exsolution. In the cone-sheet samples studied (Messum, Namibia), this exsolution takes place at 850° +/-50° C. The typical size of a magnetite inclusion in
ABSTRACT Titanomagnetite inclusions exsolved in silicate minerals are a powerful tool for explori... more ABSTRACT Titanomagnetite inclusions exsolved in silicate minerals are a powerful tool for exploring the Earth's magnetic field behavior because they are protected from chemical alteration by their silicate hosts and are ubiquitous in Precambrian terranes composed of gabbros and anorthosites. Yet the extreme shape anisotropy of these needle-shaped inclusions creates the potential for remanence anisotropy and nonlinear remanence acquisition. To better determine the extent of these processes, we measured the thermal remanence magnetization (TRM) acquired by single-crystal samples of clinopyroxene (Messum Complex, Namibia) and plagioclase (Bushveld Complex, South Africa) at four field strengths (20,50,100, and 140 muT) in 6 orientations (positive and negative X, Y, and Z). Mineral alteration during thermal cycling was negligible and was monitored by measuring saturation isothermal remanent magnetization (SIRM) before and after each TRM step. The 6 three-dimensional magnetization vectors recorded at each field strength were used to calculate a least-squares, best-fit TRM tensor. In general, the clinopyroxene-hosted inclusions showed significantly higher anisotropy than those in plagioclase. Clinopyroxene grains displayed a strongly oblate anisotropy (2 < F < 128, 13 < P < 672), while plagioclase grains showed a comparatively more subdued prolate anisotropy (0.5 < F < 5, 1 < P < 11). The degree of anisotropy associated with each silicate phase is a consequence of the number of inclusion orientations in clinopyroxene (n = 2) and plagioclase (n = 5). The tensors' eigenvalues and eigenvectors for all samples were unaffected by variations in applied field strength. Samples acquired magnetization linearly up to fields of 100 muT, at which point samples approached saturation remanence. Additionally, we note that the slope of linear remanence acquisition was dependent on the crystallographic orientation of the single crystals. These results demonstrate that single crystals of clinopyroxene and plagioclase do exhibit strong remanence anisotropy. Despite this, their linear remanence acquisition up to 100 muT fields ensures that they will be useful recorders of paleointensity for most periods of geomagnetic behavior. Monte Carlo simulations using average anisotropy tensors allowed us to determine how many randomly oriented crystals are needed in a standard bulk sample to overcome the single-crystal anisotropy and accurately record the Earth's magnetic field direction. Thus, so long as a large enough population of grains is contained within a single bulk sample, their extreme remanence anisotropy can be overcome, and accurate paleodirections and paleointensities can be obtained.
ABSTRACT Paleomagnetic data frequently display bipolar data that are not exactly antipodal. One c... more ABSTRACT Paleomagnetic data frequently display bipolar data that are not exactly antipodal. One cause of this biasing is the addition of another later (secondary) magnetization that cannot be removed by laboratory demagnetization experiments. The magnitude of this hidden secondary magnetization can be calculated when only two values are known: the non-antipodal angle (measured), and the angle between the Normal direction (measured) and secondary direction (measured or assumed). Furthermore, a minimum magnitude of the secondary magnetization can be calculated with knowledge only of the non-antipodal angle. Applied to three published data sets (sandstone, limestone and diorite intrusion) with non-antipodal angles of 11°, 28° and 11° respectively; a secondary component is calculated at 25%, 28% and 16% of the characteristic magnetization remaining after laboratory cleaning. Near surface alteration (weathering) appears to be the source of secondary magnetization in two of these cases. A directionally consistent, biasing effect of a few degrees is made on the mean direction as compared to the conventional calculation of averaging the non-antipodal directions.
Australopithecus afarensis is the best-known and most dimorphic species in the early hominin foss... more Australopithecus afarensis is the best-known and most dimorphic species in the early hominin fossil record. Here, we present a comparative description of new fossil specimens of Au. afarensis from Nefuraytu, a 3.330-3.207 million-years-old fossil collection area in the Woranso-Mille study area, central Afar, Ethiopia. These specimens include NFR-VP-1/29, one of the most complete mandibles assigned to the species thus far and among the largest mandibles attributed to Au. afarensis, likely representing a male individual. NFR-VP-1/29 retains almost all of the distinctive archaic features documented for Au. afarensis. These features include a posteriorly sloping symphysis, a low and rounded basally set inferior transverse torus, anterosuperiorly opening mental foramen, a lateral corpus hollow bound anteriorly by the C/P3 jugae and posteriorly by the lateral prominence, and the ascending ramus arising high on the corpus. Dental morphology and metrics of the Nefuraytu specimens also falls...
ABSTRACT Paleoamagnetic data from the Guadelupian (Upper Permian) Nosoni Formation of the Redding... more ABSTRACT Paleoamagnetic data from the Guadelupian (Upper Permian) Nosoni Formation of the Redding Section (RS) consistently exhibit two directional components of magnetization in addition to a late Quaternary normal weathering overprint. Six sites in ash flow tuffs and tuffaceous sedimentary rocks, spanning 200 m of section, yield data that allow precise characterization of all components. Two additional sites in calcareous mudstones were too heavily overprinted to yield stable paleomagnetic directions. Thermal demagnetization of the tuffaceous samples in 16-18 steps between natural remanent magnetization and 660°C provides a detailed data base from which to calculate directional components. Analysis of demagnetization trends identifies a prefolding reversed direction carried by both magnetic and hematite and a postfolding normal direction carried by hematite only. Petrologic considerations suggest paragenesis of these oxides in the Permian (primary magnetite and hematite) and Early-Middle Jurassic (metamorphic hematite).Least square analysis of progressively demagnetized vector endpoint trends between 540° and 600°C yields a formation mean direction in magnetite having D=156.4°, I=-29.7°, k=101, x95=7.6°. This result is in close agreement with the composite prefolding direction (D=160.3°, I=-31.5°), carried by both magnetite and hematite, determined by the intersection of remagnetization circles. The postfolding hematite-carried overprint direction calculated by the latter technique has D=352.3°, I=35.0°. Paleopoles corresponding to these two direction are located at 57.6°N, 103.0°E (magnetite formation mean) and 67.0°N (hematite overprint), coinciding within error to Upper Permian and Early Jurassic (respectively) reference poles from the North America craton. These data indicate no significant rotation or latitudinal translation of the sample sites with respect to cratonic North America since the Permian. This conclusion contrasts with seemingly reliable paleomagnetic results from elsewhere in the RS, which indicate major post-Permian clockwise rotations. These rotations are evidently specific to individual structural domains and are probably manifestations of oroclinal bending about vertical axes.
... of Texas, Galveston, Texas 77550 Robert С Belcher* Geophysics Branch, NASA-Goddard Space Flig... more ... of Texas, Galveston, Texas 77550 Robert С Belcher* Geophysics Branch, NASA-Goddard Space Flight Center, Greenbelt, Maryland 20771 Gary R. Scott ... are two pole posi-tions from northeastern Mexico by Nairn (1976) and one from southern Mexico by Guerrero (1975), all of ...
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abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet. and Asoriculus cf. gibberodon the former, and Apodemus gorafensis,
Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet., Asoriculus cf. gibberodon, Castillomys gracilis,
Occitanomys brailloni, Occitanomys sp., Paraethomys meini, Ruscinomys sp., Eliomys intermedius, Debruijnimys cf. julii and Atlantoxerus
sp. the latter. Based on the study of these micromammal assemblages, we propose an Early Pliocene age (MN14) for both sites. The presence a gerbilid related to Debruijnimys julii in La Bullana 2B open new questions about the phylogenetic relationship between Debruijnimys species from the Miocene and Pliocene of the Iberian Peninsula. Furthermore, preliminary paleomagnetic data indicate an age between
4.997 Ma and 4.896 Ma for La Bullana 2B, and between 5.235 Ma and 4.997 Ma for La Bullana 3.
abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet. and Asoriculus cf. gibberodon the former, and Apodemus gorafensis,
Paraethomys aff. abaigari, Stephanomys dubari, Apocricetus barrierei, Sciuridae indet., Asoriculus cf. gibberodon, Castillomys gracilis,
Occitanomys brailloni, Occitanomys sp., Paraethomys meini, Ruscinomys sp., Eliomys intermedius, Debruijnimys cf. julii and Atlantoxerus
sp. the latter. Based on the study of these micromammal assemblages, we propose an Early Pliocene age (MN14) for both sites. The presence a gerbilid related to Debruijnimys julii in La Bullana 2B open new questions about the phylogenetic relationship between Debruijnimys species from the Miocene and Pliocene of the Iberian Peninsula. Furthermore, preliminary paleomagnetic data indicate an age between
4.997 Ma and 4.896 Ma for La Bullana 2B, and between 5.235 Ma and 4.997 Ma for La Bullana 3.