Alkaline phosphatase activity and its relationship to inorganic phosphorus in the transition zone of the North-western African upwelling system

The enzymatic activity of alkaline phosphatase (APA) was studied in the transition zone between the African upwelling system and the open ocean waters of the Canary Islands region. This region is recurrently dominated by the presence of upwelling filaments that may transport nutrient-enriched waters out into the open ocean before nutrients become exhausted by plankton. Turnover rates by APA were generally low in the whole region, but detectable in all the measurements carried out. On average, turnover rates were higher in the upwelling stations, and APA in those waters seemed to be mainly generated by heterotrophic bacteria to supply easily assimilable organic C. APA outside the upwelling area showed an inverse hyperbolic relationship with increasing phosphate, suggesting the presence of both constitutive and Pi-inducible APA. In these offshore waters, a threshold of 0.1 μM of phosphate could be defined for the regulatory function of Pi on APA. Thus, APA in nutrient-poor waters seemed to be induced to compensate for Pi-deficiency. Turnover rates in the filaments showed basal (probably constitutive) levels, whereas they increased in the surrounding waters, where phosphate concentration presumably did not satisfy plankton P-demands. The fertilising effect of the filaments and associated cyclonic eddies extended to at least 175 km offshore, where basal alkaline phosphatase activities were still found. The magnitude of this effect depends probably on the intensity of upwelling events and the degree of recirculation of filament water back to the coastal jet.     

Distribution and transport of organic matter along a filament-eddy system in the Canaries – NW Africa coastal transition zone region

The distribution and transport of chlorophyll a (Chla), particulate (POC) and dissolved (DOC) organic carbon, and the respiratory ETS activity of the microplankton community were studied along a filament-eddy system located in the transition zone between the NW Africa upwelling and Canary Islands waters. Two independent filaments (F1 and F2) stemming from the coastal jet, between Cape Juby and Cape Bojador, merged about 100 km offshore, turning southward and onshore forced by the circulation of a recurrent oceanic cyclonic eddy. In general, the coastal upwelling waters presented higher Chla, but lower POC, DOC and ETS activity than filament waters. However, differences in organic carbon distribution and respiratory activity were observed among stations from the two filaments. The bio-chemical fields were strongly influenced by a complex sub-mesoscale hydrography resulting from the interaction of cyclonic and anticyclonic island eddies with the filaments. The combined F1 + F2 filament system transported 97.1 kg s⁻¹ of excess (non-refractory) total organic carbon (e-TOC), a value comparable to other published estimates from upwelling filaments in the NE Atlantic. About 90% of e-TOC was exported as DOC, since eddy re-circulation precluded the offshore transport of POC. Assuming that the calculated transport of e-TOC is representative of the annual average, the yearly offshore transport (3.1 x 10⁹ kg C) would represent about 25% of the upwelling primary production of the region of study.     

Open registers: past, present and future

The debate about open registers (ORs) is not a new matter, and throughout centuries States have taken advantage of other flags for their own profit in order to overcome any kind of restrictions. To understand this system in depth it is necessary to know about its origins, development and consolidation. Eradicating substandard registers calls for the application of specific directions like the ones presented, which are a result of many years of researching and working for international organizations aiming at the modernization and consolidation of some ORs. This has led to an increase in the levels of safety and credibility of the maritime administrations where they were applied.     

Metamorphic and deformational imprint of Cambrian–Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, Spain)

The high-temperature metamorphism recorded in the Valuengo and Monesterio areas constitutes a rare occurrence in the Ossa-Morena Zone of Southwest Iberia, where low-grade metamorphism dominates. The metamorphism of the Valuengo area has been previously considered either Cadomian or Variscan in age, whereas that of Monesterio has been interpreted as a Cadomian imprint. However, these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution. The metamorphism of the Valuengo and Monesterio areas affects Late Proterozoic and Early Cambrian rocks, and is syn-kinematic with a top-to-the-north mylonitic foliation, which was subsequently deformed by early Variscan folds and thrusts. The U–Pb zircon age (480±7 Ma) we have obtained for an undeformed granite of the Valuengo area is consistent with our geological observations constraining the age of the metamorphism. We propose that this high-temperature metamorphic imprint along a NW–SE ductile extensional shear zone is related to the crustal extension that occurred in the Ossa-Morena Zone during the Cambro-Ordovician rifting. In the same way, the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one.     

Pressure-Dependence of Rare Earth Element Distribution in Amphibolite- and Granulite- Grade Garnets. A LA-ICP-MS Study

Using an excimer (KrF) laser ablation ICP-MS system, we studied the distribution of REE in garnets from metapelites and metabasites from Ivrea-Verbano (Western Alps, Italy) and from the Peña Negra Anatectic Complex (Central Iberia), finding systematic variations that correlate well with the metamorphic grade. Chondrite-normalized REE patterns of garnets from amphibolite-grade metapelites have lower-than-chondrite levels from La to Sm, a very small or no Eu anomaly, and a steep rise in the abundance of heavy REE as the atomic number increases. Metapelitic garnets from the amphibolite-granulite transition have a marked Eu negative anomaly and are enriched in MREE such that Sm is 10-15 times chondrite and the pattern is almost flat from Dy to Yb-Lu. In garnets from granulite-grade metapelites, the intensity of the Eu anomaly and the relative concentration of Nd, Sm, Gd and Tb increase, with almost flat chondrite-normalized patterns from Sm to Lu. Garnets from mafic granulites are remarkably similar to those of metapelitic garnets equilibrated at the same pressure, except for the Eu anomaly. The apparent paradox of enhanced uptake of larger REE ions with increasing pressure is attributed to the 3M²⁺ 2REE³⁺+ vacancy substitution, which produces a net decrease in the dimensions of the unit-cell of garnet. Variations in REE patterns depend essentially on the pressure and have little dependence on either temperature, bulk-composition of garnet, or REE whole-rock composition, so they could represent a new approach for geobarometric studies. The best numerical parameter to express pressure-related variations of REE distribution in garnets is the Gd/Dy ratio which does not seem perceptibly affected by disequilibrium partitioning. The regression equation between GASP pressure and the average Gd/Dy^garnet is P = 3.6 + 5.6 Gd/Dy. This equation seems to be reliable for garnets: (1)equilibrated within a pressure range of 4-9 kbar, (2) coexisting with modal monazite; and (3) with unit-cell dimensions under 11.46 Å.     

Anomalous alkaline rocks of Soustov, Kola: evidence of mantle-derived metasomatic fluids affecting crustal materials

The intrusive complexes of Gremiakha-Vyrmes and Soustov represent the two extremes of the Early Proterozoic alkaline plutons of Kola, predominantly composed of feldespathoidal syenites. Gremiakha-Vyrmes rocks (zircon age: 1,884Lj Ma) have trace-element and isotope signatures (⁸⁷Sr/⁸⁶Sr_tƸ.704, )Nd_t,-3-1.3) compatible with an ultimate mantle origin. Soustov syenites (zircon age: 1,872NJ Ma) are totally different and show an acute crustal imprint. They have sodaline and analcite instead of nepheline, contain a plethora of REE-HFSE-rich accessories, and are characterised by elevated contents of F, Cl, REE, Y, Th, U, Zr, Hf, Nb, Ta, Sn, Be, Li, Rb, Tl, Pb and Cs, negative Eu anomalies, K/Rb겞-160, Nd/Thƻ, and Nb/Taᄼ, with extremely high ⁸⁷Sr/⁸⁶Sr_t (>0.720) and, at the same time, relatively high )Nd_t (,-1.6-1.7). In this paper, we explore the idea that the anomalous features of Soustov syenites can be explained if we assume they are derived from a metasomatic agent, initially an H₂O-CO₂ supercritical fluid released by alkaline mafic magmas, that was profoundly contaminated during percolation through crustal materials. As percolation advanced, the bulk composition of the fluid solute changed from alkali halides and carbonates to a silica-undersaturated alkaline melt. When the fluid cooled to a temperature of ~550-600 °C, it reached the point at which vapor and melt were no longer miscible and split into two components, a vapour phase and a Cl- and F-rich silica-undersaturated silicate melt that crystallised to produce Soustov syenites. To study this process, we have developed a numerical method for modelling the solute composition of the fluid during the infiltration metasomatism. Our results, using the LREE abundances and the Sr and Nd isotope composition of a Gremiakha-Vyrmes pegmatite as the starting solute composition of the fluid, and the mode and mineral trace-element and isotope composition of a common Kola gneiss as representative of percolated materials, indicate that the fluid would have acquired a signature closely matching Soustov's, even in the case of Nd isotopes, if the gneiss age is 2.9 Ga, near its real age. This model is still a mere working hypothesis that needs further refinements, but may represent a reasonable explanation of the genesis of anomalous alkaline rocks with high ⁸⁷Sr/⁸⁶Sr_t and )Nd_t̾, either saturated or undersaturated, which are difficult to understand in terms of magmatic fractionation/contamination.     

Peraluminous granites frequently with mantle-like isotope compositions: the continental-type Murzinka and Dzhabyk batholiths of the eastern Urals

Murzinka and Dzhabyk are continental-type batholiths of the middle and southern East Uralian domain. They comprise mainly undeformed peraluminous K-rich granites whose elemental composition is similar to some late-Variscan granites of western Europe, but with much more primitive Sr and Nd isotope ratios. Murzinka (254±5 Ma) is composed of silica-rich granites forming two different rock series with a ⁸⁷Sr/⁸⁶Sr_init of 0.709 and 0.704, respectively. Both series have enormous variations in εNd₂₅₅ (–11.9 to –0.1 and –8.9 to +4.1) that reveal derivation from heterogeneous sources. Dzhabyk (291±4 Ma) also comprises two coeval magmas which yielded voluminous granites and quartz-monzonites, respectively, with smaller differences in ⁸⁷Sr/⁸⁶Sr_init and εNd₂₉₀ (~0.7043, +0.8 to +1.6 and ~0.7049, 0.0 to +0.8). Despite their isotope compositions both batholiths lack evidence of genetic involvement of a mantle-derived parental magma. Moreover, we suggest that Dzhabyk granitoids were generated by polybaric partial melting of Paleozoic island-arc material, whereas Murzinka granitoids derived from an extremely heterogeneous source consisting mainly of Paleozoic and Proterozoic metagreywackes. This implies a relative fast reworking of juvenile arc crust and burial of the protoliths during the orogenic evolution of the Urals. Since there is neither evidence of significant extension, nor a direct link with subduction, we suggest that the main cause for late-orogenic anatexis was elevated heat production and fertility in the protolith, perhaps combined with some additional heat from unexposed mafic intrusions.     

Thermochronology and exhumation rates of granitic intrusions at Mesa Central,Mexico

ABSTRACT

The Mesa Central of Mexico (MC) is an elevated plateau located 2000 m above sea level in central Mexico, where intrusions outcrop that register the history of exhumation-erosion occurring during the Late Cretaceous-Paleogene. The tectonic history of the region records formation of the Late Cretaceous-Paleogene ‘Mexican orogen’; this was followed by extension of the entire region and several plutons were then exhumed. The age and magnitude of the crustal uplift and erosion occurring during exhumation has not been addressed to date. Therefore, this study reports the crystallization and cooling ages of two plutons, the Tesorera Granodiorite and the Comanja Granite, and estimates their emplacement depths. Based on these data, the exhumation age of the Tesorera Granodiorite is estimated to be between ~73 Ma and ~63 Ma at an exhumation rate of ~528 m/m. y. and that of the Comanja Granite is 52 Ma and 48 Ma at an exhumation rate of ~2500 m/m. y. Exhumation-erosion event of the Tesorera Granodiorite was located on the trace of the San Luis-Tepehuanes Fault System and that of the Comanja Granite on the a trace of the El Bajío Fault System. Furthermore, the high exhumation rate in the Comanja Granite suggests that gravitational collapse played an important role during exhumation.