Star Formation in the Outskirts of Disk Galaxies

The far outer regions of galactic disks allow an important probe of both star formation and galaxy formation. I discuss how observations of HII regions in these low gas density, low metallicity environments can shed light on the physical processes which drive galactic star formation. The history of past star formation at large radii, as traced by observations of old and intermediate-age stars, constrains the epoch at which the highest angular momentum regions of disks were in place; first results for the M31 disk suggest this occured a significant (≳ 8 Gyr) time ago. This revised version was published online in September 2006 with corrections to the Cover Date.     

Uncertainty in 1D Heat‐Flow Analysis to Estimate Groundwater Discharge to a Stream

Temperature measurements have been used by a variety of researchers to gain insight into groundwater discharge patterns. However, much of this research has reduced the problem to heat and fluid flow in one dimension for ease of analysis. This approach is seemingly at odds with the goal of determining spatial variability in specific discharge, which implies that the temperature field will vary in more than one dimension. However, it is unclear how important the resulting discrepancies are in the context of determining groundwater discharge to surface water bodies. In this study, the importance of these variations is examined by testing two popular one‐dimensional analytical solutions with stochastic models of heat and fluid flow in a two‐dimensional porous medium. For cases with low degrees of heterogeneity in hydraulic conductivity, acceptable results are possible for specific discharges between 10^?7 and 10^?5 m/s. However, conduction into areas with specific discharges less than 10^?7 m/s from adjacent areas can lead to significant errors. In some of these cases, the one‐dimensional solutions produced estimates of specific discharge of nearly 10^?6 m/s. This phenomenon is more likely in situations with greater degrees of heterogeneity.     

The Zamu dolerite: A lower Proterozoic preorogenic continental tholeiitic suite from the Northern Territory,Australia∗

The term ‘Zamu Dolerite’ is proposed here to replace ‘Zamu Complex’, the name originally applied to the predominantly mafic intrusives of the South Alligator River area, and to include the other mafic supracrustal intrusive rocks that, with minor felsic differentiates, were emplaced into the strata of the Pine Creek Geosyncline before an 1800 m.y. phase of regional metamorphism. This suite of lower Proterozoic rocks generally forms conformable intrusive tabular bodies which are folded and, in most places, metamorphosed with the enclosing strata. The intrusions are mostly now amphibolite, but in the type area of Zamu Creek, folded but unaltered dolerite rocks are found. The Zamu Dolerite occurs in two broad zones of regional meta‐morphic grade: medium and/or high grades in the northeastern part of the region, and low grade elsewhere. Late‐orogenic Carpentarian granite diapirs have, however, locally superimposed a contact metamorphism on the low‐grade regional metamor‐phic event. Low‐temperature retrograde metamorphism is locally present throughout the Pine Creek Geosyncline, being particularly prevalent within areas of uranium mineralization.

Compared with the dolerites, the amphibolites have the following chemical characteristics: total Fe and Ti enrichment; Mg, K, and Sr depletion; similar values for Zr, Y, Nb, and P₂O₅; lower K:Rb and higher K:Ba ratios. The dolerites and amphibolites are orthopyroxene normative and their major‐ and trace‐element chemistry demonstrates that they closely parallel the trends of continental tholeiitic suites. Compositions of the mafic suite of the Zamu Dolerite recast into the 1‐atm phase diagrams for simplified anhydrous tholeiitic magmas suggest that the more evolved members of this suite were produced by olivine and, possibly, plagioclase fractionation.     

Application of an Analytical Solution as a Screening Tool for Sea Water Intrusion

Sea water intrusion into aquifers is problematic in many coastal areas. The physics and chemistry of this issue are complex, and sea water intrusion remains challenging to quantify. Simple assessment tools like analytical models offer advantages of rapid application, but their applicability to field situations is unclear. This study examines the reliability of a popular sharp‐interface analytical approach for estimating the extent of sea water in a homogeneous coastal aquifer subjected to pumping and regional flow effects and under steady‐state conditions. The analytical model is tested against observations from Canada, the United States, and Australia to assess its utility as an initial approximation of sea water extent for the purposes of rapid groundwater management decision making. The occurrence of sea water intrusion resulting in increased salinity at pumping wells was correctly predicted in approximately 60% of cases. Application of a correction to account for dispersion did not markedly improve the results. Failure of the analytical model to provide correct predictions can be attributed to mismatches between its simplifying assumptions and more complex field settings. The best results occurred where the toe of the salt water wedge is expected to be the closest to the coast under predevelopment conditions. Predictions were the poorest for aquifers where the salt water wedge was expected to extend further inland under predevelopment conditions and was therefore more dispersive prior to pumping. Sharp‐interface solutions remain useful tools to screen for the vulnerability of coastal aquifers to sea water intrusion, although the significant sources of uncertainty identified in this study require careful consideration to avoid misinterpreting sharp‐interface results.     

Screening for Heat Transport by Groundwater in Closed Geothermal Systems

Heat transfer due to groundwater flow can significantly affect closed geothermal systems. Here, a screening method is developed, based on Peclet numbers for these systems and Darcy's law. Conduction‐only conditions should not be expected where specific discharges exceed 10^?8 m/s. Constraints on hydraulic gradients allow for preliminary screening for advection based on rock or soil types. Identification of materials with very low hydraulic conductivity, such as shale and intact igneous and metamorphic rock, allow for analysis with considering conduction only. Variability in known hydraulic conductivity allows for the possibility of advection in most other rocks and soil types. Further screening relies on refinement of estimates of hydraulic gradients and hydraulic conductivity through site investigations and modeling until the presence or absence of conduction can be confirmed.     

Early Paleocene vegetation and climate in Jiayin, NE China

The pollen and spores of the Wuyun Formation (Danian, Early Paleocene) from Jiayin County, Heilongjiang Province, Northeast China, are studied in this paper. The Danian vegetation at Wuyun was composed of mixed temperate and subtropical broad-leaved forest, with an admixture of conifers. The climatic parameters, obtained by the Coexistence Approach based on the palynological and megafossil data, are Mean Annual Temperature of 14.8–16.8°C, Mean Annual Precipitation of 815.8–1,571.8 mm. Combining other climatic parameters in the Tertiary, we obtained the latitudinal temperature gradients: 0.24 in Paleocene, 0.1 in Eocene, 0.45 in Miocene and 0.55 in Pliocene, in the area of East Asia. All these values and modern latitudinal gradient of 0.7 suggest a climatic transition from Paleocene to today: the temperature at higher latitudes first increased then decreased gradually during last 65 Ma, while it changed slightly in lower latitudes.     

Generation and emplacement of fine-grained ejecta in planetary impacts

We report here on a survey of distal fine-grained ejecta deposits on the Moon, Mars, and Venus. On all three planets, fine-grained ejecta form circular haloes that extend beyond the continuous ejecta and other types of distal deposits such as run-out lobes or ramparts. Using Earth-based radar images, we find that lunar fine-grained ejecta haloes represent meters-thick deposits with abrupt margins, and are depleted in rocks ?1 cm in diameter. Martian haloes show low nighttime thermal IR temperatures and thermal inertia, indicating the presence of fine particles estimated to range from ∼10 μm to 10 mm. Using the large sample sizes afforded by global datasets for Venus and Mars, and a complete nearside radar map for the Moon, we establish statistically robust scaling relationships between crater radius R and fine-grained ejecta run-out r^* for all three planets. On the Moon, r^* ∼ R^−0.18 for craters 5-640 km in diameter. For Venus, radar-dark haloes are larger than those on the Moon, but scale as r^* ∼ R^−0.49, consistent with ejecta entrainment in Venus’ dense atmosphere. On Mars, fine-ejecta haloes are larger than lunar haloes for a given crater size, indicating entrainment of ejecta by the atmosphere or vaporized subsurface volatiles, but scale as R^−0.13, similar to the ballistic lunar scaling. Ejecta suspension in vortices generated by passage of the ejecta curtain is predicted to result in ejecta run-out that scales with crater size as R^1/2, and the wind speeds so generated may be insufficient to transport particles at the larger end of the calculated range. The observed scaling and morphology of the low-temperature haloes leads us rather to favor winds generated by early-stage vapor plume expansion as the emplacement mechanism for low-temperature halo materials.