Ba proxies have been broadly used to reconstruct past oceanic export production. However, the precise mechanisms underlying barite precipitation in undersaturated seawater are not known. The link between bacterial production and particulate Ba in the ocean suggests that bacteria may play a role. Here we show that under experimental conditions marine bacterial biofilms, particularly extracellular polymeric substances (EPS), are capable of bioaccumulating Ba, providing adequate conditions for barite precipitation. An amorphous P-rich phase is formed at the initial stages of Ba bioaccumulation, which evolves into barite crystals. This supports that in high productivity regions where large amounts of organic matter are subjected to bacterial degradation, the abundant EPS would serve to bind the necessary Ba and form nucleation sites leading to barite precipitation. This also provides new insights into barite precipitation and opens an exciting field to explore the role of EPS in mineral precipitation in the ocean.
The pigments, binders and execution techniques used by the Nasrids to polychrome carpentry in the Hall of the Mexuar Palace at the Alhambra (Granada, Spain) were studied using optical microscopy, scanning electron microscopy with EDX analysis, selective staining techniques and gas chromatography -mass spectrometry. This pioneering investigation presents the first results of a research project devoted to filling gaps in the knowledge of Nasrid art, traditionally approached by stylistic studies. Moreover, it is essential for the polychromy conservation of the studied artworks, and will help to clarify historical and painting uncertainties in the Alhambra monument. The palette consists of a limited range of colours: white (lead-base pigment), red (cinnabar and red lead), blue (lapis lazuli), black (carbon-based) and false gold (golden tin). Tempera grassa was the painting technique identified. Two types of grounds were used: (i) gypsum in calligraphy decoration for the false gold technique, and (ii) synthetic minium in geometric drawings in carpentry. Organic insulating layers of linseed oil were used between paint strata. Artists applied synthetic minium to protect the wood ( Juglans regia and conifer) against attack by xylophages. To lighten the surface darkened by this ground layer, powdered tin was added to achieve a metallic lustre.
One of the latest
volcanic features of the Erta Ale range at the Afar Triangle (NE Ethiopia)
has created a polyextreme hydrothermal system located at the Danakil
depression on top of a protovolcano known as the dome of Dallol. The
interaction of the underlying basaltic magma with the evaporitic salts
of the Danakil depression has generated a unique, high-temperature
(108 °C), hypersaline (NaCl supersaturated), hyperacidic (pH
values from 0.1 to −1.7), oxygen-free hydrothermal site containing
up to 150 g/L of iron. We find that the colorful brine pools and mineral
patterns of Dallol derive from the slow oxygen diffusion and progressive
oxidation of the dissolved ferrous iron, the iron-chlorine/-sulfate
complexation, and the evaporation. These inorganic processes induce
the precipitation of nanoscale jarosite-group minerals and iron(III)-oxyhydroxides
over a vast deposition of halite displaying complex architectures.
Our results suggest that life, if present under such conditions, does
not play a dominant role in the geochemical cycling and mineral precipitation
at Dallol as opposed to other hydrothermal sites. Dallol, a hydrothermal
system controlled by iron, is a present-day laboratory for studying
the precipitation and progressive oxidation of iron minerals, relevant
for geochemical processes occurring at early Earth and Martian environments.
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