Nutrient dynamics in Mediterranean temporary streams: A case study in Pardiela catchment (Degebe River, Portugal)

Most of the streams in the Mediterranean region are temporary, following predictable seasonal of flooding and drying, with a transition from lotic conditions to shallow lentic conditions. The goal of our study was to assess the nitrogen and phosphorus dynamics in channel-bed processes of temporary streams between floods. Results show that, during winter, temperatures ranged between 9.5 and 11.2 °C and oxygen concentration ranged from 8.0 to 9.5 mg L⁻¹, whereas, during summer, temperatures varied between 21.2 and 26.8 °C and oxygen between 1.2 and 5.3 mg L⁻¹, with oxygen depletion in the pools during the night. The nitrate concentrations were far more abundant during winter (February), while ammonium concentration increased after stream fragmentation into pools (especially in July when oxygen depletion conditions favoured ammonification). Results on sediment profiles showed that the most active sediment layers for NH₄-N are the top 2–3 cm, corresponding to the sediment depositional sites of the stream. Phosphate concentrations had larger variability, yet concentrations decreased from winter to spring and increased again in summer, when the shallow water pools were formed. Sediment profiles at the sediment depositional sites showed that PO₄-P was more dynamic in the first 6 cm.

In Mediterranean temporary streams, nutrient dynamics vary seasonally, as the system transits from lotic conditions to shallow lentic conditions, evidencing the regeneration of nutrients from organic and inorganic matter during the flow cessation period.     

Late Pleistocene–Holocene environments in Valle Carbajal, Tierra del Fuego, Argentina

The authors discuss Late Pleistocene–Holocene depositional environments in one of the Fuegian Andes valleys on the basis of palynological, geomorphological, and sedimentological analyses from two sites located near the Beagle Channel. The results obtained at these localities reinforce and refine the Late Pleistocene–Holocene climatic pattern previously recorded there. A colder period, associated with the Younger Dryas stadial event, is suggested by low Nothofagus pollen frequency, and communities of grass, low scrub, and shrub heath expanded into the low/middle slopes (10,310 ¹⁴C yr BP). By ca. 9500 ¹⁴C yr BP, warmer and drier conditions occurred, as evidenced by the development of open-grown vegetation in the valley floors (pollen zone O-3), followed by the expansion of open Nothofagus woodland (pollen zone O-2) in the middle Holocene. The milder climate subsequently changed, as indicated by the spreading of the closed forest and mire (pollen zone O-1), to more humid and cooler conditions during the last ca. 5000 yr BP.     

The Alleret maar (Massif Central, France): A new lacustrine sequence of the early Middle Pleistocene in western Europe

The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) ⁴⁰Ar/³⁹Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages.     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.     

The isotopic composition of oxygen and carbon in the hyaloclastites from the Mt. Iblei volcanic area,Eastern Sicily: A preliminary study

Oxygen and carbon isotopic analyses were carried out for some typical submarine volcanic products (a lava flow, a pillow fragment and four hyaloclastite breccias) from the northwestern zone of the Mt. Iblei volcanic complex, eastern Sicily. The δ¹⁸O value of the perental basaltic magma (6.0 ± 0.2‰), estimated from the analyses of some fresh unaltered glassy samples of various type, lies in the values range of primary basalts. Appreciably higher δ¹⁸O values, probably due to low-temperature exchanges with sea water, have been found for lava samples and the interior of the pillow fragment. The δ¹⁸O and δ¹³C of the calcites of the groundmass of the hyaloclastite samples, ranging from 30.59 to 33.65 and from ?2.99 to 0.46‰ respectively, are typical of low-temperature marine carbonates. Because calcite is one of the last minerals to form. these results suggest that the hyaloclastites studied formed entirely in a submarine environment. The¹⁸O/¹⁶O ratios recorded in the silicate portions of the matrices of the hyaloclasites (δ¹⁸O=13.99 to 16.61) are interpreted as the result of halmyrolytic processes occurring at temperatures somewhat higher than that of the sea floor.     

Quantifying the seasonal variations in fluvial and eolian sources of terrigenous material to Cariaco Basin,Venezuela

The varved sediments that accumulate in the Cariaco Basin provide a detailed archive of the region’s climatic history, including a record of the quantity of fluvial and wind-transported material. In this study, we examine the sedimentological characteristics (clay mineralogy and grain size) of both surface sediments and sinking lithogenic material collected from sediment trap samples over a three-year period from 1997 to 2000. Data from biweekly sediment trap samples show a tri-modal particle size distribution, with prominent peaks at 2, 22 and 80 μm, indicating sediment contributions from both eolian and fluvial sources. The clay mineralogy of the water column samples collected from 1997 to 1999 also shows distinctive characteristics of eolian and fluvial material. An examination of surface sediment samples from the Cariaco Basin indicates that the Unare River is the main source of riverine sediments to the eastern sub-basin. By combining these sedimentological proxies, we estimate that ～10% of the terrigenous material delivered to the Cariaco Basin is eolian, while ～90% is fluvial. This represents an annual dust accumulation rate of ～0.59 mg/cm²/yr. Since aerosols are closely linked to climate variability, the ability to quantify paleo-dust fluxes using sedimentological characteristics will be a useful tool for future paleoclimate studies looking at sub-Saharan aridity and latitudinal migration of the Intertropical Convergence Zone.     

Habitat provision of barnacle tests for overcrowded periwinkles

In habitats where competition for space is a shaping force of animal distribution such as in the intertidal rocky ecosystem, new habitats are readily taken by colonising species. We examined the importance of empty Chthamalus spp. tests as a habitat for the intertidal common periwinkle Melaraphe neritoides on Portuguese rocky shores. The role played by the space between neighbouring barnacles as a habitat for other species has been largely studied with regard to how an ecosystem functions, whereas the equivalent role of empty barnacle tests remains largely unknown. The small periwinkle is one of the most abundant snails in European rocky shores and is an important prey for key mobile predators. Biological facilitation is common in the rocky intertidal zone, where biological structures often potentiate the abundance of other species. The role played by barnacles as biological facilitators through habitat provision is not fully understood. In this study, the abundance and morphometric features of empty barnacle tests and their occupants were examined across shore levels and shores with differing exposures, as these are important gradients explaining barnacle distribution. The availability of empty barnacle tests was also experimentally manipulated to examine the percentage, time and length of occupation. Empty barnacles were more abundant on the midshore of sheltered shores and barnacle tests were wider on the upper shore but taller on the midshore. The minimum barnacle test occupation rate by the periwinkle was 70%. Barnacle shell height was an important factor determining snail occupation, where taller barnacles harboured significantly more, but not necessarily larger, snails. Snails outside the barnacle tests were significantly larger than those found within, suggesting that this crustacean group has an increased importance as a habitat for juvenile snails, thus potentially influencing the population dynamics of M. neritoides. We found that occupation of experimentally created empty barnacles was extremely high (70%) on the day after, and remained at 100% after 3 months of monitoring. Our study is the first of its kind to focus on the features of barnacle tests that snails occupy and their occupying snail traits. According to our results, it is clear that barnacles have an important role in providing additional habitat for young gastropods. The small periwinkle is an important grazer and prey for intertidal and subtidal foraging predators; hence, the potential refuge role of barnacle tests for juvenile M. neritoides may be important in the dynamics of intertidal communities.     

The 2013 Ibiza Calibration Campaign of Jason-2 and SARAL Altimeters

This study presents the results of the 2013 Ibiza (Western Mediterranean) calibration campaign of Jason-2 and SARAL altimeters. It took place from 14 to 16 September 2013 and comprised two phases: the calibration of the GNSS (Global Navigation Satellite System) buoys to estimate the antenna height of each of them and the absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was performed at ～ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and SARAL nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and SARAL, respectively. The GNSS data were processed using precise point positioning technique. The biases found are of (?0.1 ± 0.9) and (?3.1 ± 1.5) cm for Jason-2 and SARAL, respectively.