Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000 Ma Closure age for the Purana Basins?

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including ⁸⁷Sr/⁸⁶Sr and δ ¹³C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α₉₅ = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α₉₅ = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.     

Little River Experimental Watershed,a keystone in understanding of coastal plain watersheds

The US Department of Agriculture-Agricultural Research Service Southeast Watershed Research Laboratory (SEWRL) initiated a hydrologic research program on the Little River Experimental Watershed (LREW) in 1967. Long-term (52 years) streamflow data are available for nine sites, including rainfall-runoff relationships and hydrograph characteristics regularly used in research on interactive effects of climate, vegetation, soils, and land-use in low-gradient streams of the US EPA Level III Southeastern Plains ecoregion. A summary of prior research on the LREW illustrates the impact of the watershed on building a regional understanding of hydrology and water quality. Climatic and streamflow data were used to make comparisons of scale across the nine nested LREW watersheds (LRB, LRF, LRI, LRJ, LRK, LRO, LRN, LRM, and LRO3) and two regional watersheds (Alapaha and Little River at Adel). Annual rainfall for the largest LREW, LRB, was 1200 mm while average annual streamflow was 320 mm. Annual rainfall, streamflow, and the ratio between annual streamflow and rainfall (Sratio) were similar (α = 0.05) across LREWs LRB, LRF, LRI, LRJ, LRK, and LRO. While annual rainfall within the 275 ha LRO3 was found to be similar to LRO and LRM (α = 0.05), annual streamflow and Sratio were significantly different (α = 0.05). Comparisons of annual rainfall, streamflow, and Sratio between LRB and the regional watersheds indicated no differences (α = 0.05). Based upon this analysis, most regional watersheds shared similar hydrologic characteristics. LRO3 was an exception, where increases in row crops and decreases in forest coverage resulted in increased streamflow. LREW data have been instrumental in building considerable scientific understanding of flow and transport processes for these stream systems. Continued operation of the LREW hydrologic network will support hydrologic research as well as environmental quality and riparian research programs that address emerging and high priority natural resource and environmental issues.     

LONG TERM SEDIMENT YIELD AND MITIGATION IN A SMALL SOUTHERN PIEDMONT WATERSHED

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Effects of Prosopis juliflora Invasion on Native Species Diversity and Woody Species Regenerations in Rangelands of Afar National Regional State,Northeast Ethiopia

Investigation of the invasion of Prosopis juliflora and its effects on indigenous plant species are important for the control of the species. The study aimed to assess: (1) the effects of Prosopis juliflora invasion on the diversity of plant species in Awash Fentale and Amibara Woredas; (2) the effects of Prosopis juliflora invasion on the regeneration potential of native woody species. Sample collection was performed in habitats of Prosopis juliflora thicket, Prosopis juliflora mixed with native species stands, non-invaded woodlands, and open grazing lands. The vegetation was stratified into invasion levels of Prosopis juliflora and then a random sampling technique for data collection. Among species of plants, the highest proportion of species, 75 (47.8%), was recorded under non-invaded woodlands, but the lowest proportion of species, 22 (14%), was recorded under open grazing lands. The invasion of Prosopis juliflora reduced the Shannon diversity index. The mean values of the Shannon diversity index and species richness under Prosopis juliflora mixed with native species (H′=2.22, R=14) and non-invaded woodlands (H′=2.23, R=13) were significantly higher than Prosopis juliflora thicket (H′=1.96, R=12) and open grazing lands (H′=1.84, R=10). The highest total density (358 stems ha^?1) of seedlings was recorded under Prosopis juliflora mixed with native species. But, the lowest total density (153 stems ha^?1) of seedlings was recorded under Prosopis juliflora thickets. Moreover, 102 trees ha^?1 native woody species were recorded under Prosopis juliflora thicket, but 1252 trees ha^?1 native species were recorded under non-invaded woodlands. If the invasion of Prosopis juliflora and its effects on native species diversity continue coupled with a drier climate, plant diversity of the Afar flora region will be highly affected and its ecosystem services will be under question. Thus, the participation of all stakeholders and multidisciplinary research approaches should be designed for the management of the species and rehabilitation of the rangelands in the region.     

Kinetic control of oxidation state at thermodynamically buffered potentials in subsurface waters

Dissolved oxygen (DO) and organic carbon (C_org) are among the highest- and lowest-potential reactants, respectively, of redox couples in natural waters. When DO and C_org are present in subsurface settings, other couples are drawn toward potentials imposed by them, generating a bimodal clustering of calculated redox potentials. Which cluster a couple is drawn toward is determined by whether the couple’s oxidant or reductant is more concentrated. Generally, reactants >10^-6M are near equilibrium with their dominant complementary reactant and in a cluster, whereas reactants <10^-6M are relatively slow to react and diverge from the clusters. These observations suggest that reactions of higher-potential oxidants with lower-potential reductants commonly proceed simultaneously, regardless of the presence of other potential reactants, with the rates of reaction being determined more by concentration than relative potentials. As DO or C_org decreases, the potential gap separating couples diminishes. In waters having quantifiable concentrations of higher potential oxidants O₂ and NO₃^-, [H₂] was not diagnostic of their presence. In the water we analyzed having no quantifiable O₂ or NO₃^-, redox potential calculated with [H₂] was similar to potentials calculated for SO₄^2- reduction and methanogenesis. Composite reactions, NO₃^-→N₂ and O₂→H₂O, are best characterized in multiple steps due to slow reaction of low-concentration intermediates. The [CO] data we report, among the first for groundwater, are high compared to water equilibrated with the atmosphere.     

Paleomagnetic evidence for a stationary Marion hotspot: Additional paleomagnetic data from Madagascar

The island of Madagascar experienced widespread magmatism at ca. 90 Ma due to its interaction with the Marion hotspot. Previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the Marion hotspot has remained fixed for the past 90 Ma. We report paleomagnetic data from northeastern Madagascar (d'Analava Complex). Samples were collected from basalts, rhyolites, gabbros and a dolerite dyke. Sixty samples from 5 sites yield a paleomagnetic pole at 66.7°S, 43.5°E (A95 = 10.7°) and a grand mean pole (GMP) calculated from 10 different studies covering the entire island of Madagascar falls at 68.9°S, 49.0°E (A95 = 4.4°). This pole translates to a paleolatitude for the Volcan de l'Androy (focal point of the hotspot) at 45.2° + 6°/− 5°S compared to the current location of the Marion hotspot at  46°S. Our results confirm, and expand upon, previous studies that argue for the fixity of the Marion hotspot for the past 90 Ma.     

Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron

Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α₉₅=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing⁴⁰Ar/³⁹Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.