The Chemical, Physical, and Microbial Origins of Pleistocene Cherts at Lake Magadi, Kenya Rift Valley

The cherts in the Magadi basin have long been considered to be a product of the diagenesis of the rare, inorganically-precipitated, soft sodium-silicate mineral, magadiite (forming “Magadi-type chert”). While this process is known to have occurred in the basin, not all chert necessarily formed from diagenetic conversion of this mineral, as there are several features to suggest other mechanisms of silica deposition alternative to that of magadiite. Lake Magadi, in the southern Kenya rift, shows a highly geochemically evolved closed lake system rich in dissolved Na-CO3-Cl, that has produced many unique sedimentary deposits throughout its geological history. Sediments deposited in the Magadi Basin from the mid- to late Pleistocene host many distinct varieties of lacustrine chert, which formed as a result of tectonic, climatic, microbial, and geochemical processes. The form, distribution, morphology, and abundance of cherts in the Magadi basin has drawn many researchers to consider their origin and diagenesis. The aim of this study is to reassess ideas about the origins of the siliceous sediments and rocks exposed in the Magadi basin. Of the four recognized sedimentary units in the Magadi Basin, chert occurs in three of them. The oldest cherts, found in the Oloronga Beds (800 to 300 ka), are diatomaceous in composition, and were excluded from this study. A staggering amount of the chert in the Magadi Basin (95%) belongs to the Green Beds (98 to 40 ka), a recently defined sedimentary unit. Features in many of the Green Beds cherts imply a potential microbial influence during precipitation of the precursor material, and also a possible relationship with hot springs. The High Magadi Beds (24 to 9 ka) are host to definitive Magadi-type cherts, with distinct macro- and microscopic features allowing for their identification (reticulate patterning, and “grid-like” precipitation of chalcedony, respectively). The Green Beds cherts have not been studied in detail since the introduction of this previously undefined sedimentary unit (Behr & Röhricht, 2000; Behr 2002). Microscopic evidence of preserved microbial remains in Magadi chert is indistinct, but macroscopic evidence of microbial influence is extensive (e.g., petee and tepee structures in chert;couplets and triplets of thin, banded chert layers). Extensive bedded chert layers crop out in a few locations around the basin, displaying evidence of a shallow, evaporative environment (e.g., preserved salt crystal casts on chert surfaces). Geothermal influence at the time of silica precipitation is inferred by macroscopic, geochemical, and analogous observations (e.g., a possible hot spring vent mound composed of chert; geochemical signatures; modern Na-Al-Si gels precipitating near hot spring inflow in Nasikie Engida). Perennial lagoons, sustained by warm springs issuing along the faulted margins of the basin, are observable to this day. Spring activity is locally observable, as perennial peripheral lagoons, and can also be inferred as a past process. Geothermal activity occurs along long-lived fault lines, and the surface expression of these tectonic features is distinctly discernible across the modern landscape (horsts and graben tectonism that can be playfully referred to as a “piano key landscape”). Whereas “Magadi-type chert” was once used to describe all chert in the basin, and was also thought to form in a deep, stratified lake, many lines of evidence exist to suggest siliceous-gel-like precursors, and a shallow, evaporative, and geothermally-influenced environment

Geomorphology from space: A global overview of regional landforms

This book, Geomorphology from Space: A Global Overview of Regional Landforms, was published by NASA STIF as a successor to the two earlier works on the same subject: Mission to Earth: LANDSAT views the Earth, and ERTS-1: A New Window on Our Planet. The purpose of the book is threefold: first, to serve as a stimulant in rekindling interest in descriptive geomorphology and landforms analysis at the regional scale; second, to introduce the community of geologists, geographers, and others who analyze the Earth's surficial forms to the practical value of space-acquired remotely sensed data in carrying out their research and applications; and third, to foster more scientific collaboration between geomorphologists who are studying the Earth's landforms and astrogeologists who analyze landforms on other planets and moons in the solar system, thereby strengthening the growing field of comparative planetology

EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE DURING MESOZOIC TETHYAN RIFTING: CONSTRAINTS FROM THE EXTERNAL LIGURIAN MANTLE SECTION (NORTHERN APENNINE, ITALY)

Our study is focussed on mantle bodies from the External Ligurian ophiolites, within the Monte Gavi and Monte Sant'Agostino areas. Here, two distinct pyroxenite-bearing mantle sections were recognized, mainly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records reactive melt infiltration under plagioclase-facies conditions. This process involved both peridotites (clinopyroxene-poor lherzolites) and enclosed spinel pyroxenite layers, and occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was replaced by Ca-rich plagioclase and new orthopyroxene, typically associated with secondary clinopyroxene. The reactive melt migration caused increase of TiO2 contents in relict clinopyroxene and spinel, with the latter also recording a Cr2O3 increase. In the Monte Gavi peridotites and pyroxenites, geothermometers based on slowly diffusing elements (REE and Y) record high temperature conditions (1200-1250 °C) related to the melt infiltration event, followed by subsolidus cooling until ca. 900°C. The Monte Sant'Agostino mantle section is characterized by widespread ductile shearing with no evidence of melt infiltration. The deformation recorded by the Monte Sant'Agostino peridotites (clinopyroxene-rich lherzolites) occurred at 750–800 °C and 0.3–0.6 GPa, leading to protomylonitic to ultramylonitic textures with extreme grain size reduction (10–50 μm). Compared to the peridotites, the enclosed pyroxenite layers gave higher temperature-pressure estimates for the plagioclase-facies re-equilibration (870–930 °C and 0.8–0.9 GPa). We propose that the earlier plagioclase crystallization in the pyroxenites enhanced strain localization and formation of mylonite shear zones in the entire mantle section. We subdivide the subcontinental mantle section from the External Ligurian ophiolites into three distinct domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain, characterized by subsolidus static formation of plagioclase, i.e. the Suvero mantle section (Hidas et al., 2020), (2) a plagioclase mylonite domain experiencing melt-absent deformation and (3) a nearly undeformed domain that underwent reactive melt infiltration under plagioclase-facies conditions, exemplified by the the Monte Sant'Agostino and the Monte Gavi mantle sections, respectively. We relate mantle domains (1) and (2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of anhydrous plagioclase mylonites. Hidas K., Borghini G., Tommasi A., Zanetti A. & Rampone E. 2021. Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy). Lithos 380-381, 105855

Impact of geogenic degassing on C-isotopic composition of dissolved carbon in karst systems of Greece

The Earth C-cycle is complex, where endogenic and exogenic sources are interconnected, operating in a multiple spatial and temporal scale (Lee et al., 2019). Non-volcanic CO2 degassing from active tectonic structures is one of the less defined components of this cycle (Frondini et al., 2019). Carbon mass-balance (Chiodini et al., 2000) is a useful tool to quantify the geogenic carbon output from regional karst hydrosystems. This approach has been demonstrated for central Italy and may be valid also for Greece, due to the similar geodynamic settings. Deep degassing in Greece has been ascertained mainly at hydrothermal and volcanic areas, but the impact of geogenic CO2 released by active tectonic areas has not yet been quantified. The main aim of this research is to investigate the possible deep degassing through the big karst aquifers of Greece. Since 2016, 156 karst springs were sampled along most of the Greek territory. To discriminate the sources of carbon, the analysis of the isotopic composition of carbon was carried out. δ13CTDIC values vary from -16.61 to -0.91‰ and can be subdivided into two groups characterized by (a) low δ13CTDIC, and (b) intermediate to high δ13CTDIC with a threshold value of -6.55‰. The composition of the first group can be related to the mixing of organic-derived CO2 and the dissolution of marine carbonates. Springs of the second group, mostly located close to Quaternary volcanic areas, are linked to possible carbon input from deep sources

Impact of Etna’s volcanic emission on major ions and trace elements composition of the atmospheric deposition

Mt. Etna, on the eastern coast of Sicily (Italy), is one of the most active volcanoes on the planet and it is widely recognized as a big source of volcanic gases (e.g., CO2 and SO2), halogens, and a lot of trace elements, to the atmosphere in the Mediterranean region. Especially during eruptive periods, Etna’s emissions can be dispersed over long distances and cover wide areas. A group of trace elements has been recently brought to attention for their possible environmental and human health impacts, the Technology-critical elements. The current knowledge about their geochemical cycles is still scarce, nevertheless, recent studies (Brugnone et al., 2020) evidenced a contribution from the volcanic activity for some of them (Te, Tl, and REE). In 2021, in the framework of the research project “Pianeta Dinamico”, by INGV, a network of 10 bulk collectors was implemented to collect, monthly, atmospheric deposition samples. Four of these collectors are located on the flanks of Mt. Etna, other two are in the urban area of Catania and three are in the industrial area of Priolo, all most of the time downwind of the main craters. The last one, close to Cesarò (Nebrodi Regional Park), represents the regional background. The research aims to produce a database on major ions and trace element compositions of the bulk deposition and here we report the values of the main physical-chemical parameters and the deposition fluxes of major ions and trace elements from the first year of research. The pH ranged from 3.1 to 7.7, with a mean value of 5.6, in samples from the Etna area, while it ranged between 5.2 and 7.6, with a mean value of 6.4, in samples from the other study areas. The EC showed values ranging from 5 to 1032 μS cm-1, with a mean value of 65 μS cm-1. The most abundant ions were Cl- and SO42- for anions, Na+ and Ca+ for cations, whose mean deposition fluxes, considering all sampling sites, were 16.6, 6.8, 8.4, and 6.0 mg m-2 d, respectively. The highest deposition fluxes of volcanic refractory elements, such as Al, Fe, and Ti, were measured in the Etna’s sites, with mean values of 948, 464, and 34.3 μg m-2 d-1, respectively, higher than those detected in the other sampling sites, further away from the volcanic source (26.2, 12.4, 0.5 μg m-2 d-1, respectively). The same trend was also observed for volatile elements of prevailing volcanic origin, such as Tl (0.49 μg m-2 d-1), Te (0.07 μg m-2 d-1), As (0.95 μg m-2 d-1), Se (1.92 μg m-2 d-1), and Cd (0.39 μg m-2 d-1). Our preliminary results show that, close to a volcanic area, volcanic emissions must be considered among the major contributors of ions and trace elements to the atmosphere. Their deposition may significantly impact the pedosphere, hydrosphere, and biosphere and directly or indirectly human health

ENGLISH Английский язык для изучающих географию

Цель пособия – обучить студентов-географов различным видам чтения, переводу, аннотированию иноязычной литературы по специальности, а также сформировать у них навыки устной монологической речи. Тексты являются аутентичными, заимствованными из британских и американских источников. Учебное пособие может быть использовано как для аудиторной, так и для самостоятельной работы. Адресуется студентам географических факультетов университетов, а также всем, кто изучает английский язык и интересуется географией