Quantifying the effects of subsurface heterogeneity on hillslope runoff using a stochastic approach

The role of heterogeneity and uncertainty in hydraulic conductivity on hillslope runoff production was evaluated using the fully integrated hydrologic model ParFlow. Simulations were generated using idealized high-resolution hillslopes configured both with a deep water table and a water table equal to the outlet to isolate surface and subsurface flow, respectively. Heterogeneous, correlated random fields were used to create spatial variability in the hydraulic conductivity. Ensembles, generated by multiple realizations of hydraulic conductivity, were used to evaluate how this uncertainty propagates to runoff. Ensemble averages were used to determine the effective runoff for a given hillslope as a function of rainfall rate and degree of subsurface heterogeneity. Cases where the water table is initialized at the outlet show runoff behavior with little sensitivity to variance in hydraulic conductivity. A technique is presented that explicitly interrogates individual realizations at every simulation timestep to partition overland and subsurface flow contributions. This hydrograph separation technique shows that the degree of heterogeneity can play a role in determining proportions of surface and subsurface flow, even when effective hillslope outflow is seen. This method is also used to evaluate current hydrograph separation techniques and demonstrates that recursive filters can accurately proportion overland and base-flow for certain cases.     

Carbon and nitrogen isotopic compositions of alkyl porphyrins from the Triassic Serpiano oil shale

The carbon and nitrogen isotopic compositions of seven of the most abundant alkylporphyrins from the Serpiano oil shale (marine, Triassic) were determined. For the C31 and C32 butanoporphyrins, values of delta 13CPDB and delta 15NAIR averaged -24.0% and -3.1%. In contrast, the C31 and C32 methylpropanoporphyrins, DPEP, and a C30 13-nor etioporphyrin had delta 13C and delta 15N values averaging -27.5 and -3.3%, respectively. Carbon and nitrogen isotopic values for kerogen averaged -30.8 and -0.9, whereas those for total extract averaged -31.6, and -4.0%. The butanoporphyrins apparently derive from a biological source different from that giving rise to the other porphyrins, their 13C enrichment not being related to carbon isotopic fractionation accompanying diagenetic reactions. The delta 15N values for all the porphyrins indicate that the depletion of 15N observed in the kerogen is of primary origin. Consistent with the very high abundance of hopanoids and methyl hopanoids in the aliphatic hydrocarbon fraction, it is suggested that cyanobacterial fixation of N2 may have been the main cause of 15N depletion.     

A piston-driven shock in the solar corona

A solar flare that occurred on the west limb at 1981, March 25, 2038 UT generated a massive, rapidly-expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km s^–1. An associated magnetohydrodynamic shock travelled out ahead of the transient with a velocity estimated to be approximately 1000 km s^–1. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.     

X. The Eucla Basin in South Australia

At some time prior to the Ptychagnostus gibbus Zone of the Middle Cambrian the area of deposition of Upper Precambrian (or Lower Cambrian) well‐sorted sands, silts and dolomite was affected by tectonic movements producing uplift of the Tyennan Geanticline and change in the shape of the depositional basin (Spry, Chapter I). Continued tectonic activity and more rapid sinking of the sea floor resulted in a change in sedimentary association from well‐sorted sediments of the orthoquartzite‐limestone suite to poorly sorted sediments of the greywacke suite. Initially siltstone was the main deposit in the Dundas, Huskisson River, Ulverstone, Deloraine and Beaconsfield areas and this has been likened to the initial euxinic phase of geosynclinical development elsewhere (Campana, 1961b).

Silt seems to have been the predominant normal deposit during the Middle and early Upper Cambrian, but siliceous oozes and some limestone were also formed. Carbonaceous, pyritic and calcareous silts were deposited. Inter‐bedded with the silts are poorly‐sorted greywackes and greywacke conglomerates with a disrupted framework and graded bedding. Banks and Jennings interpret these as mostly turbidity current deposits. The proportion of greywacke and conglomerate varies through the successions in a cyclic manner (Carey and Banks, 1954; Banks, 1956) such that a conglomerate‐rich section is followed by a greywacke‐rich section and this by a predominantly lutaceous section. These cycles may be interpreted as due to tectonic instability and variation in height of the source area. Faulting of Upper Middle Cambrian and Lower Dresbachian age has been demonstrated near Ulverstone. Campana and King state: “The proportion of coarse material increases upwards in the Dundas and Huskisson successions at least.”

Turbidity currents brought fragments of grey, red, black and banded cherts, banded slate, quartzite, basalt and golden mica (this last presumably from breakdown of Precambrian mica schist) to the Dundas area. In view of the known distribution of chert in western Tasmania a westerly or north‐westerly source is likely. Turbidity currents deposited fragments of chert, claystone, quartzite, slate, greywacke, quartz mica schist, chloritised basic lava and spilite in the Deloraine area indicating a source area with Precambrian rocks and earlier Cambrian sediments and lavas. Near Rocky Boat Harbour the source area contained dolomite, ultrabasic rocks, granite, and Precambrian quartzites and schists.

A difference between the fauna in the silts and in the greywackes is evident in the Hodge Slate at Dundas and the Kateena Formation near Ulverstone at least. The “dendroids” in the Hodge Slate are in the siltstone and the fragmentary trilobites and cystoids in the greywacke. This suggests that the fossils in the greywackes are thanatocoenotic as might be expected and introduces the possibility of remanié fossils and of shallow water fauna intercalated with deeper water fauna. The bathymetric conditions suggested by Hills and Thomas (1954) for the Cambrian of Victoria may thus not be applicable to Tasmania.

Deposition was also interrupted from time to time by lava flows, some of them, at least, submarine. The Mt. Read Volcanics may be Lower Cambrian but acid and basic lavas and pyroclastic rocks are interbedded with or overlie Middle and Upper Cambrian sediments at Zeehan, Dundas, Ulverstone, Smithton and Beaconsfield. Acid volcanic rocks are commoner near the Tyennan Geanticline and basic rocks further away. Possibly during the Dresbachian ultrabasic rocks were intruded as sheets and dykes into Precambrian and earlier Cambrian rocks and by Franconian time were exposed to erosion at Adamsfield.

Deposition may have commenced later at Smithton (Upper Middle Cambrian), Beaconsfield (Lower Dresbachian) and Adamsfield (Lower Franconian) than at Dundas (Lower Middle Cambrian).

Campana and King express the thoughts of Bradley (1957, pp. 114–115) and the author when they state: “The Dundas Group reflects a eugeosynclinical cyclic sedimentation under unstable tectonic conditions. The group is no doubt a synorogenic suite comparable with the Flysch as it was deposited in the narrow subsiding Dundas Trough which developed along the Mt. Read Volcanic Arc, and which is similar to the present deeps of archipelago areas. Such a comparison is enhanced by the succeeding Ordovician conglomerates and sandstones, comparable in some respects with the molassic deposits which displaced the Flysch sedimentation in the Pre‐Alpine troughs (Fig. 12).”

The Cambrian rocks were folded or tilted at least along the western and northern margin of the Tyennan Geanticline and near New River Lagoon, the Tyennan Geanticline was rejuvenated, the Asbestos Range Geanticline raised and the highland areas near Ulverstone and Zeehan uplifted late in the Cambrian or very early in the Ordovician.     

A preliminary Ordovician biostratigraphy of Tasmania

In Tasmania sedimentary rocks containing shelly fossils at many horizons, some graptolitic horizons and numerous conodont horizons were deposited, apparently continuously, from the Tremadoc Epoch into and after the Llandovery Epoch. Superposition can be demonstrated over this interval so that the position of preserved faunas or biotas relative to one another is clear in most cases. Twenty successive Ordovician faunas are recognized. During the interval represented by this succession the geographical affinities of the faunas were generally with eastern Asian, North American and South American faunas, but in the later part of the Ordovician and in the Silurian the affinities were wider. Because of the clarity of the succession, the duration of deposition and relationships of the faunas, the Tasmanian succession is likely to be a useful link in world correlation, especially for isolated areas such as parts of South America and southeastern Asia.     

Subregionalization of Low-Frequency Summer Drought Variability in the Southeastern United States

Freshwater resources are particularly limited in the Southeastern United States. In recent years, severe summer droughts have placed substantial strain on municipal and industrial water resources, emphasizing the need for further research on the causes of long-term moisture deficits. Previous research has identified a Southeast drought region and specific low-frequency ocean-atmosphere influences on drought. In this article, we identify three distinct subregions of low-frequency summer drought variability in the Southeastern United States using principal components analysis. Multidecadal drought variability is most pronounced in the Southeastern Atlantic Coastal States (SEACS) subregion. The SEACS drought variability is significantly associated with ocean-atmosphere variability in the Atlantic and Pacific basins. The significance of ocean-atmosphere influences on drought in the Southeast is spatially confined within only the SEACS subregion. The Eastern Gulf States (EGS) subregion exhibits a long-term increase in summer moisture that is significantly associated with Northern Hemisphere surface temperature increases during the last century. The peninsular Florida subregion is characterized by high-frequency drought variability that is not associated with any of the climate indexes included in this study.