Geology Explained

  Summary New research provides evidence that a lack of metals in aquatic environments results in higher emissions of nitrous oxide (N 2 O), a greenhouse gas associated with global warming. General aspects of the denitrification process Did you know that nitrous oxide (N 2 O) has a higher global warming potential (at least 265 times more) than the well-known greenhouse gas (carbon dioxide, CO 2 ) 1 mainly produced by the burning of fossil fuels? Since 57% of global N 2 O emissions are from natural soils and oceans, the global N 2 O emissions into the atmosphere are mainly regulated by biogeochemical denitrification of nitrate (NO - 3 ) to nitrogen (N 2 ) mainly by microbes in aquatic systems 2 . Other non-biological denitrification processes (e.g., dissimilatory nitrate reduction to ammonium, DNRA) also occur in nature. Biological denitrification is normally a four-stage process involving the reduction of nitrate  (NO - 3 ) to nitrite  (NO - 2 ), nitric oxid...

 The Himyarite kingdom (once situated in what is now Yemen; Fig. 1) was one of the most powerful polities in Arabia until around 525 CE 1 . Figure 1. Map showing the Himyar and the Aksum kingdom (Source: University of Basal). Besides trading aromatics and metals, agriculture was central to the economic prosperity of this Jewish kingdom 1,2 . The ancient terraced agricultural fields, dams (Fig. 2) and irrigation systems affirm that water was a limited resource, and the importance of agriculture to this kingdom 3 . The reasons for the demise of this state spanning about 140 years are still debated and various socio-economic and political factors have been proposed among them the annexation of Himyarite by Aksum (a state situated in what is now Ethiopia) in 525 CE, internal political conflict, and the collapse of the irrigation system 4 . Figure 2. Ancient remains Great Dam of Marib which formed part of the irrigation structures of the Himyarite kingdom (Source: Wikipedia) Dr...

Current understanding of the formation of porphyry deposits Porphyry deposits are a major source of copper (Cu), gold (Au), and molybdenum (Mo) (Fig. 1). These deposits contribute 75%, 50%, and 20% of the Cu-, Mo-, Au produced in the world, respectively 1 . Figure 1. Molybdenite (dark grey molybdenum-sulphide mineral) hosted in quartz (grey) veins in a porphyry deposit (Source 2 ) Typically, porphyry deposits form in magmatic arcs (oceanic and continental) above subduction zones in a series of processes beginning with (i) the expulsion of metal-enriched fluids from the subducting slab into the mantle wedge resulting in partial melting of the mantle wedge, followed by, (ii) the migration and stalling of such magmas in the magma chambers in the mid to lower crust, and subsequently, (iii) the migration of metal-rich fluids produced during the crystallisation of magmas in magma chambers into the upper crust where they form pipe- and dyke-like porphyry deposits  3 , 4 (Fig. 2). F...

What are TTGs? A suite of granitoids comprising tonalites , trondhjemite and granodiorites collectively referred to as tonalite-trondhjemite-granodiorite (TTG) granitoids are the main constituents of Archean terranes 1 . TTGs were mostly emplaced in the Archean and are not generated in modern-day subduction zones 1 . Thus, these rocks have invariably undergone multiple episodes of metamorphism and usually occur as strongly deformed gneisses (Fig. 1). Figure 1. TTG specimen (Source: Wikipidea) Geochemically, TTGs are sodic (K 2 O/Na 2 O<0.5), contain high contents of light rare earth elements (LREE) and low contents of high rare earth elements (HREE) resulting in strongly fractionated REE patterns (average La/Yb N of 32.4), exhibit negative Nb-Ti anomalies, and lack pronounced Eu and Sr anomalies 2 . The fractionated REE patterns and the Nb-Ti anomalies are interpreted to be indicative of the presence of garnet, amphibole and/or rutile in the melt residue 3 , whereas the Sr and...