Global weathering variations inferred from marine radiogenic isotope records

Determining the past record of chemical weathering is essential for understanding changes in climate and atmospheric CO₂, such as those that occur throughout the Cenozoic (the last 65 my). Many natural radiogenic isotopes in seawater are sensitive to variations in chemical weathering, but taken alone cannot distinguish such changes from those caused by variations in erosional source (such as composition, geographical location or ocean circulation). However, comparison of isotope systems with different sources and different behaviour in seawater can resolve such effects, and the relationship between weathering and climate change can be examined on both long and short timescales.     

Episodic shear zone associated fluid flow in the Curnamona Province, South Australia

Two distinct generations of fluid flow associated with shear zone activity have been identified in Willyama Supergroup rocks of the southern Curnamona Province in northeastern South Australia. Fluids in the first event are inferred to have been sourced from the devolatilisation of Willyama Supergroup metasedimentary rocks during prograde metamorphism associated with the (1.61–1.58 Ga) Mesoproterozoic Olarian Orogeny. The second episode of fluid flow occurred during the (c. 500 Ma) Cambrian Delamerian Orogeny and resulted in localised rehydration of the Willyama Supergroup. Fluids were isotopically light and most likely sourced from prograde Delamerian metamorphism and dehydration of fault rocks and entrained meteoric waters that originally were involved in (c. 700 Ma) Neoproterozoic Adelaidean rifting. A key outcome of this study is the identification of this previously unrecognised fluid flow system that was active during the Delamerian Orogeny.     

Response of the St. Joseph River to lake level changes during the last 12,000 years in the Lake Michigan basin

The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 ¹⁴C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline. As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to 30 m of infill sediment below the modern floodplain. About 3 × 10⁸ m³ of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan. The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m) of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend. It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic feature rather than a classic unsorted moraine that would drape over pre-glacial topography. This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.     

Predicting gully rejuvenation after wildfire using remotely sensed burn severity data

The loss of surface vegetation and reduced infiltration caused by wildfires can trigger gully rejuvenation, resulting in damage to downstream aquatic resources and risk to human life and property. We developed a spatially explicit metric of burn severity — the Burn Severity Distribution Index (BSDI) — and tested its ability to predict post-fire gully rejuvenation in 1st and 2nd order basins burned in the 2000 Valley Complex fires in the Sapphire Mountains of western Montana. The BSDI was derived from burn severity data interpreted from Landsat 7 satellite imagery using the Normalized Burn Ratio (NBR) method, and ranged from 0.0 for completely unburned basins to 4.0 for basins burned entirely at high severity. In July 2001 rainstorms with peak 30-minute intensities of up to 17 mm h^− 1 triggered gully rejuvenation in 66 of the 171 basins examined. The frequency of gully rejuvenation was higher in basins with higher BSDI values, increasing from zero for basins with a BSDI less than 1.3 to 67% for basins with a BSDI greater than 3.0. Binary logistic regression indicated that BSDI was a more significant predictor of gully rejuvenation than basin morphometric variables. The absence of gully rejuvenation in several basins with a high BSDI was attributed to low gradient, dense riparian vegetation, or concentration of high burn severity at lower elevations in the basin. The presence of gully rejuvenation in several basins with a low BSDI was associated with false negative NBR classification errors in northwest aspects, and concentration of severe burn impacts in the drainage headslopes. BSDI is a useful metric for predicting gully rejuvenation after wildfire. The use of the BSDI in Burned Area Emergency Response team assessments could improve the planning, implementation, and monitoring of burned area recovery treatments.     

Microalgal productivity, community composition, and pelagic food web dynamics in a subtropical, turbid salt marsh isolated from freshwater inflow

Carbon entering the food web originating from microalgal productivity may be as important to salt marsh consumers as carbon originating from vascular plant production. The objective of this study was to further our understanding of the role played by microalgae in salt marshes. We focused on microalgal productivity, community dynamics, and pelagic food web linkages. Across three consecutive springs (2001–2003), we sampled the upper Nueces Delta in southeast Texas, United States; a shallow, turbid system of ponds and elevated vegetated areas stressed by low freshwater inflow and salinities ranging from brackish (11) to hypersaline (300). Despite high turbidity and low external nutrient loadings, microalgal productivity was on the order of that reported for vascular plants. Primary productivity in surface waters ranged from 0 to 2.02 g C m⁻² d⁻¹ and was usually higher than primary productivity associated with the benthos, which ranged from 0 to 1.14 g C m⁻² d⁻¹. This was likely due to high amounts of wind-driven resuspended sediment limiting production at greater depths. Most of the water column microalgal biovolume seemed to originate from the benthos and was comprised mostly of pennate diatoms. But true phytoplankton taxa were also observed, which included cryptomonads, chlorophyhtes dinoflagellates, and cyanobacteria. Succession from r-selected to K-selected taxa with the progression of spring, a common phenomena in aquatic systems, was not observed. Codominance by both potentially edible and less edible taxa was found. This was likely due to decreased grazing pressure on r-selected taxa as salinity conditions became unfavorable for grazers. In addition to a decoupled food web, reduced primary and net productivity, community respiration, and microalgal and zooplankton population densities were all observed at extreme salinities. Our findings suggest that a more accurate paradigm of salt marsh functioning within the landscape must account for microalgal productivity as well as production by vascular plants. Because the value of microalgal productivity to higher trophic levels is taxa specific, the factors that govern microalgal community structure and dynamics must also be accounted for. In the case for the Nueces Delta, these factors included wind mixing and increasing salinities.     

Evidence for Triassic salt domes in the Tunisian Atlas from gravity and geological data

Detailed gravity data were analyzed to constrain two controversial geological models of evaporitic structures within the Triassic diapiric zone (Triassic massifs of Jebel Debadib and Ben Gasseur) of the northern Tunisian Atlas. Based on surface observations, two geological models have been used to explain the origin of the Triassic evaporitic bodies: (1) salt dome/diapiric structure or (2) a “salt glacier”. The gravity analysis included the construction of a complete Bouguer gravity anomaly map, horizontal gravity gradient (HGG) map and two and a half-dimensional (2.5D) forward models. The complete Bouguer gravity anomaly map shows a prominent negative anomaly over the Triassic evaporite outcrops. The HGG map showed the location of the lateral density changes along northeast structural trends caused by Triassic/Cretaceous lithological differences. The modeling of the complete Bouguer gravity anomaly data favored the diapiric structure as the origin of the evaporitic bodies. The final gravity model constructed over Jebel Debadib indicates that the Triassic evaporitic bodies are thick and deeply rooted involving a dome/diapiric structure and that the Triassic material has pulled upward the younger sediment cover by halokinesis. Taking in account kinematic models and the regional tectonic events affecting the northern margin of Africa, the above diapirs formed during the reactive to active to passive stages of continental margin evolution with development of sinks. Otherwise, this study shows that modeling of detailed gravity data adds useful constraints on the evolution of salt structures that may have an important impact on petroleum exploration models.     

Tectonic evolution of the Dabieshan orogen: In the view from polyphase deformation of the Beihuaiyang metamorphic zone

The Dabieshan and its geological counterpart in the Sulu area represent the eastern part of the Qinling-Dabie orogenic belt in Eastern China. This Orogen corresponds to the collision zone between the North and South China blocks (denoted as NCB and SCB, respectively) during the Early Mesozoic. Since the discovery of ultra-high-pressure (UHP) metamor- phism[1?3], research of the Dabieshan has made great progress from petrological work (e.g. Cong and Wang, 1999 and enclosed references)[…     

Three-dimensional liquefaction potential analysis using geostatistical interpolation

We applied three-dimensional geostatistical interpolation to evaluate the extent of liquefiable materials at two sites that liquefied during the 1994 Northridge Earthquake. The sites were the Balboa Blvd site and the Wynne Ave. site located in the alluvial San Fernando Valley. The estimated peak ground accelerations at the sites are 0.84 g (Balboa Blvd) and 0.51 g (Wynne Ave.). These sites were chosen because surface effects due to liquefaction were not predicted using available techniques based on thickness and depth of liquefiable layers (Ishihara [Ishihara K. Stability of natural deposits during earthquakes. Proceedings of the 11th international conference on soil mechanics and foundation engineering, vol. 1. Rotterdam, The Netherlands: A.A. Balkema; 1985. p. 321–76.]) and the Liquefaction Potential Index (Iwasaki et al. [Iwasaki T, Tatsuoka F, Tokida K, Yasuda S. A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan. In: Proceedings of the second international conference on microzonation, San Francisco; 1978. p. 885–96.]). During the earthquake, both sites experienced surface effects including ground cracking and extension as a result of liquefaction. Foundations and buried utilities were damaged at both sites. The sites were investigated after the event by researchers with the United States Geologic Survey using standard penetration tests (SPT) and cone penetration tests. In this paper, liquefaction potential was estimated for each soil sample using results from SPTs according to the updated Seed and Idriss simplified procedure. The probability of liquefaction was estimated by applying an indicator transform to the results of the liquefaction potential calculation. We compared our results to detailed geologic mapping of the sites performed by other researchers. Using geostatistical interpolation to estimate the probability of liquefaction is a useful supplement to geologic evaluation of liquefaction potential. The geostatistical analysis provides an estimate of the continuous volume of liquefiable soil along with an assessment of confidence in an interpolation. The probability of liquefaction volumes compare well with those predicted using geologic interpretations.     

Catastrophic precipitation‐triggered lahar at Casita volcano,Nicaragua: occurrence,bulking and transformation

A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small ?ank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide‐debris avalanche evolved on the ?ank to yield a watery debris ?ood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery ?ow entrained (bulked) enough sediment to transform entirely to a debris ?ow. The debris ?ow, 6 km downstream and 1·2 km wide and 3 to 6 m deep, killed 2500 people, nearly the entire populations of the communities of El Porvenir and Rolando Rodriguez. These ‘new towns’ were developed in a prehistoric lahar pathway: at least three ?ows of similar size since 8330 ¹⁴C years bp are documented by stratigraphy in the same 30‐degree sector. Travel time between perception of the ?ow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the ?ow wave occurred with hydraulic continuity and without pause or any extraordinary addition of water. The precipitation trigger of the Casita lahar emphasizes the need, in volcano hazard assessments, for including the potential for non‐eruption‐related collapse lahars with the more predictable potential of their syneruption analogues. The ?ow behaviour emphasizes that volcano collapses can yield not only volcanic debris avalanches with restricted runouts, but also mobile lahars that enlarge by bulking as they ?ow. Volumes and hence inundation areas of collapse‐runout lahars can increase greatly beyond their sources: the volume of the Casita lahar bulked to at least 2·6 times the contributing volume of the ?ank collapse and 4·2 times that of the debris ?ood. At least 78 per cent of the debris ?ow matrix (sediment < ?1·0Φ; 2 mm) was entrained during ?ow. Copyright © 2004 John Wiley & Sons, Ltd.