Magnetic anomalies in the Eastern Caribbean

Despite the many different studies on the origin and evolution of the Caribbean Plate, no proposal has been widely accepted so far. A key element within this field of research is the characterization of the plate subsurface oceanic crust, as it would clarify the conditions in which it originated, the geological period when it formed and its possible geographical location at this first evolution stage. Based on partial results of this research work, we can say that the conclusions of previous studies are valid to a great extent, namely the NE–SW orientation of the striped magnetic anomalies in the Venezuelan Basin’s western section and the existence of W–E preferentially oriented stripes parallel to the Leeward Antilles. The magnetic response of the triangular section in the southeast of the Venezuelan Basin represents cretaceous magnetic quiet zone (CMQZ) and therefore shows a considerable attenuation of the stripe pattern, indicating that the whole East Caribbean subsurface features oceanic crustal material. As for the period recorded by the Caribbean magnetic stripes, we propose the interval between chrons M23 and M0, and part of CMQZ. The wavelengths of the identified anomalies suggest that the ridge associated with the formation of Caribbean ocean floor was slow-spreading when compared to average currently active ridges.     

Fault-controlled Soil CO2 Degassing and Shallow Magma Bodies: Summit and Lower East Rift of Kilauea Volcano (Hawaii), 1997

Soil CO₂ flux measurements were carried out along traverses across mapped faults and eruptive fissures on the summit and the lower East Rift Zone of Kilauea volcano. Anomalous levels of soil degassing were found for 44 of the tectonic structures and 47 of the eruptive fissures intercepted by the surveyed profiles. This result contrasts with what was recently observed on Mt. Etna, where most of the surveyed faults were associated with anomalous soil degassing. The difference is probably related to the differences in the state of activity at the time when soil gas measurements were made: Kilauea was erupting, whereas Mt. Etna was quiescent although in a pre-eruptive stage. Unlike Mt. Etna, flank degassing on Kilauea is restricted to the tectonic and volcanic structures directly connected to the magma reservoir feeding the ongoing East Rift eruption or in areas of the Lower East Rift where other shallow, likely independent reservoirs are postulated. Anomalous soil degassing was also found in areas without surface evidence of faults, thus suggesting the possibility of previously unknown structures. Received: November 2003, revised: January 2005, accepted: January 2005     

Moisture source in the Hyblean Mountains region (south-eastern Sicily, Italy): Evidence from stable isotopes signature

Here the authors present results of an isotope study on precipitation collected during a 2-a period from a rain-gauge network consisting of 6 stations located at different elevations in the Hyblean Mountains (HM) region, in south-eastern Sicily. The slope of the local meteoric water line (δD = 6.50 δ¹⁸O + 9.87) obtained for the region suggests that precipitation is affected by evaporation during rainfall events. The main variations in rainwater isotope composition are due to seasonal effects and elevation. An average ²H excess value of +21.2‰ was found for precipitation events less affected by evaporation (i.e. when the rainfall was >65 mm/month). The spatial distribution of O isotope composition of precipitation shows a negative gradient from east and south to the inner areas. The depositional rate of Cl, used as a tracer of the origin of air masses, is highest at the coastal rain-gauges (SR and MRG stations) and lowest on the northern flank of the HM region (SC station). Based on these findings, a model is proposed for the origin of precipitation in the HM region, which assumes that a Mediterranean-derived component is the main source of moisture in the studied area. D/H and ¹⁸O/¹⁶O ratios of inferred meteoric recharge waters were also compared with the isotope composition of waters collected from the main local springs and wells. The best linear fit of the δ¹⁸O vs δD relationship for Hyblean groundwater is δD = 4.85 δ¹⁸O–2.01. The enrichment of heavy isotopes in Hyblean groundwater is probably due to evaporation occurring after precipitation events or to a recharging contribution from surface waters (lakes or rivers) enriched in heavy isotopes.     

The effect of migratory shorebirds on the benthic species of three southwestern Atlantic Argentinean estuaries

We experimentally evaluate the effect of migratory shorebirds on the benthic fauna of three southwestern Atlantic Argentinean stop-over and wintering sites: Bahía Samborombon (35°30′–36°22′S, 57°23′W), Mar Chiquita coastal lagoon (37°40′S, 57°26′W), and Bahía Blanca (38°48′–39°25′S, 50°–62°25′W). The experiments consisted of exclusion ceilings and controls (both 1 m²), with 10 replicates each and aligned at the same tidal level. During December 1994–May 1995 these experiments were done, twice in Bahía Samborombon, twice at two sites (Sotelo and Celpa) 4 km apart in Mar Chiquita coastal lagoon, and three times in Bahía Blanca. Three polychaete species (Laeonereis acuta, Nephtys fluviatilis, andHeteromastus similis) were found in similar densities in both areas of the Mar Chiquita lagoon.L. acuta andN. fluviatilis were affected in Sotelo, but there was no treatment effect in Celpa. In Sotelo the most abundant shorebirds were Hudsonian godwit (Limosa haemastica), American golden plover (Pluvialis dominica), and White-rumped sandpiper (Calidris fuscicollis), and in Celpa the most abundant shorebirds were White-rumped sandpiper, two-banded plower (Charadrius falklandicus), and lesser yellowlegs (Tringa flavipes). The largest polychaete densities were in the upper sediment layer (0–6 cm), which was also the most affected layer. Three polychaete species were found in Bahía Samborombon (L. acuta, H. similis, andNeanthes succinea), but onlyH. similis showed a treatment effect. The most abundant shorebirds in Samborombon were white-rumped sandpiper and two-banded plover. No effect was detected in Bahía Blanca, where the most abundant shorebirds were white-rumped sandpiper and American golden plover. In all cases, the species affected by shorebirds were the most abundant species. From the two sites of Mar Chiquita, there was a treatment effect only in Sotelo, which was also the area with higher shorebirds counts. However, much lower densities observed in Samborombon (similar to the unaffected area of Mar Chiquita) also produced a significant decrease in infaunal abundance. These evidences also suggest that just selection of study sites may produce different views of the interaction between shorebirds and benthic species.     

Population Abundances, Tidal Movement, Burrowing Ability and Oxygen Uptake of Emerita analoga (Stimpson) (Crustacea, Anomura) on a Sandy Beach of South-Central Chile

Abstract. Field sampling and other experiments were carried out during February 2001 to determine whether different morphodynamic characteristics occurring within an intermediate sandy beach of southern Chile (ca. 39°S) convey differences in population abundance, tidal movement, burrowing ability and oxygen uptake of the anomuran crab Emerita analoga (Stimpson, 1857). Crabs were collected along transects extended between the lowest swash levels and the retention zone above the effluent line of the south and north end of the beach. Burrowing times of nearly 70 crabs collected at each study site were measured in saturated sands collected from the lowest swash level of each site. Oxygen uptake of crabs was measured in incubation glass bottles. The intertidal zone of the north end of the beach was wider (56 m) and flatter (1/14) than that of the south end (45 m and 1/9, respectively). In general, the swash zone of the north end was significantly wider than the south end throughout the sampling period. The frequency of swashes and number of swash crossings above the effluent line, plus up‐wash speed, were usually higher at the steeper south end of the beach. The mean population abundance of E. analoga per linear metre of beach was significantly higher at the north end, whereas density per square metre was significantly higher at the south end. No differences were found in biomass figures. Although the highest abundance of crabs at the north end was usually observed at the lowest swash levels, similar population abundances occurred along all the tidal levels sampled at the south end. Burrowing times of crabs collected from both ends of the beach increased significantly with increasing carapace length and body mass. The mean burrowing time of crabs collected at the south end of the beach was shorter than that of those collected at the north end. Oxygen uptake of E. analoga was positively and exponentially correlated with the size of individuals collected from both ends of the beach. Results of ANCOVA showed no significant difference between the regression lines obtained for the oxygen uptake of crabs collected at both ends of the beach. It is concluded that physical features of each end of the beach seem not to differ enough to produce differences in oxygen uptake of E. analoga, or in the biomass, population structure and body size of crabs, within a single beach of south‐central Chile.     

A note on the SG(<Emphasis Type="Italic">m</Emphasis>) test

López et al. (Reg Sci Urban Econ 40(2–3):106–115, 2010) introduce a nonparametric test of spatial dependence, called SG(m). The test is claimed to be consistent and asymptotically Chi-square distributed. Elsinger (Reg Sci Urban Econ 43(5):838–840, 2013) raises doubts about the two properties. Using a particular counterexample, he shows that the asymptotic distribution of the SG(m) test may be far from the Chi-square family; the property of consistency is also questioned. In this note, the authors want to clarify the properties of the SG(m) test. We argue that the cause of the conflict is in the specification of the symbolization map. The discrepancies can be solved by adjusting some of the definitions made in the original paper. Moreover, we introduce a permutational bootstrapped version of the SG(m) test, which is powerful and robust to the underlying statistical assumptions. This bootstrapped version may be very useful in an applied context.     

Heat-producing Elements-enriched Continental Mantle Lithosphere and Proterozoic Intracontinental Orogens: Insights from Brasiliano/Pan-African Belts

Collisional and accretionary processes have been recognized in several Precambrian orogens, but the formation of intracontinental (ensialic) orogens has remained elusive. The recognition that modern orogenic belts (as, for example, the mountain belts of central Asia) may form in response to far-field stresses transmitted to the interior of continental plates removed much of the objection against Proterozoic intracontinental deformation. However, the causes responsible for the localization of contractional deformation are still uncertain. Due to the strong dependence of lithosphere rheology on temperature, lithospheric thermal weakening prior to contraction must be a required condition. Studies of dioritic rocks from Borborema Province, northeastern Brazil, and Damara belt, southern Africa, indicate that they are derived from ancient, incompatible elements-enriched subcontinental lithosphere. The concentrations of the heat-producing elements K (0.6 wt %), U (0.02–0.2 ppm) and Th (0.15–1.5 ppm) estimated for this source allow calculations of heat production between 0.09 and 0.25 mW m⁻³. These high values of heat generation may cause increases in temperature of several tens to three hundred degrees Celsius with respect to cratonic mantle lithosphere, leading to great reductions in lithosphere strength. Consequently, continental domains underlain by hot and weak continental mantle may thicken under the action of relatively feeble tectonic forces. High rheological contrasts between these domains and adjacent stiff and cold blocks provide an explanation for formation of intracontinental orogens.     

Flood Events of Selected European Rivers in the Sixteenth Century

The severity and frequency of sixteenth-century floods of the Rhine, the Main, the middle and upper Elbe with its tributaries, rivers of northern and central Italy, the Garonne and rivers in Catalonia and Andalusia are analyzed using documentary evidence. The basic topographical and hydrological characteristics of the rivers investigated as well as the synoptic causes of their flooding during the instrumental period are presented. Different examples of modifications of the run-off process due to anthropogenic activity are discussed. Prevalence in flood occurrence during the second half of the sixteenth century in comparison to the first half is typical for central European and Andalusian rivers (mainly in the 1560s and 1590s) and agrees with the evolution of precipitation patterns. On the other hand, Italian and Catalonian rivers, in part, had a higher occurrence of floods during the first half of the century. Changes in the flooding seasons in both halves of the century are not unambiguous. Results of an analysis on a broader European scale show floods to be a random natural phenomena with limited areal extent defined by the spatial influence of forcing meteorological factors (continuous heavy rains, sudden melting of thick snow cover, etc.). Despite some limitations of documentary evidence, series of reconstructed historical floods are valuable sources of proxy data which can be utilized for the study of the flooding fluctuations in the pre-instrumental period.