Microbial Carbonates

Beachrocks are coastal sedimentary formations resulting from a relative rapid cementation of beach sediments
by the precipitation of carbonate cements. These lithified structures are not usually observed at
temperate settings. The present work is focused on the occurrence of a significant intertidal cementation
in sand-gravel beaches formed among 43 ◦N latitude coastline, close to the Nerbioi-Ibaizabal
estuary (Bilbao, Bay of Biscay, North of Spain). Raman micro-spectroscopy combined with SEM-EDX
analyses and petrographic descriptions have been applied for the determination of the cement generations
and the cemented materials compositions of the beachrock outcrops. In general terms, the cements
described were: Cement Generation 1 (CG 1, aragonite, high-magnesium calcite and silicate mixtures),
Cement Generation 2 (CG 2, aragonite) and Cement Generation 3 (CG 3, mixtures of CaCO3 polymorphs
and iron oxides). The rest of the interstitial porosity of the rocks appeared either empty or filled with
heterogeneous cemented mixtures of previously reworked compounds. The mineralogy, the regular distribution
and the isopachous character of the carbonate cements together with the accurate cementation
at advanced seaward bands propose a possible marine-phreatic context for the beachrock formation.
However, the impure cements and the materials covering the interstitial porosity seem to be the result
of both, the weathering actions consequences and the surface alterations of specific grains. Moreover,
the presence of modern cemented materials (e.g. slag, bricks and pebbles) suggest a recent formation of
the phenomenon.

Microbialites are rock-like underwater structures that look like reefs
but are made entirely of millions of microbes. These structures
are very ancient and can be found in di􀀀erent environments on
every continent on Earth. Mexico has many microbialite reefs,
in desert valleys, crater-lakes, and coastal lagoons. Science helps
us to understand the microbes that build microbialites, to know
whether the same kinds of microbes make up microbialites in
di􀀀erent regions of the world, and to figure out how the microbes
organize into microbialites. Many things are damaging microbialites
in Mexico, including poorly planned development, pollution from
lack of sewage treatment, too much water usage, and the fertilizers
used for agriculture. Policies that regulate development are urgently
needed to help save these diverse and ancient microbial reefs.

Geochemical signatures and carbonate microfacies highlight contrasts between two distinctive mid–late Triassic reef communities in the Dolomite Alps, Italy. In the first community, sponges, bryozoans, calcified cyanobacteria and problematic organisms (Archaeolithoporella, Shamovella), together with a variety of micritic fabrics, formed compact reefs in high energy shallow-water at the margins of high-rise Ladinian–Carnian carbonate platforms. Debris from these margins created steep foreslopes, and some large blocks of the allochthonous material (Wengen–Cassian formations, Cipit Boulders) were buried in basinal shales that protected them from subsequent alteration and regional dolomitization. In the second and slightly younger community, small Carnian patch reefs (Heiligkreuz Formation, Alpe di Specie) developed in quieter shallow water, where they too were protected against alteration by enclosing shales. They were constructed mainly by scleractinian corals, sponges and red algae, and contain relatively large framework cavities with clotted-peloidal micrite. These early examples of coralgal reefs have broad similarities to present-day examples, whereas the community represented by the Cipit Boulders has more in common with Late Permian reefs. Both bioconstructions preserve primary microfabrics and biomarkers. The Cipit Boulder samples contain bacterial , mainly cyanobacterial biomarkers, lack specific molecules typical of sulfate-reducing bacteria (SRB), and have Rare Earth Element (REE) values indicative of oxic conditions. These signatures are consistent with their original high-energy platform margin location, compact structure, and presence of calcified cyanobacteria such as Cladogirvanella and Girvanella. In contrast, the coralgal patch reefs contain SRB biomarkers, lack specific molecules typical of cyanobacteria, and have REE values indicative of sub-oxic conditions. These signatures are consistent with their lower energy depositional conditions and well-developed skeletal framework that created protected low-oxygen micro-habitats. The SRB biomarkers can be linked to the associated clotted-peloidal fabrics which resemble those commonly present in younger coral-reef frameworks. These details of redox conditions and bacterial processes underscore the important biotic, structural and environmental changes that affected shallow marine reefs during the Triassic.

Beachrocks are sedimentary structures derived from the precipitation of carbonates in the intertidal zone.
Because of this cementation process, they can involve a significant variety of grain types within their
structures. This is the case for beachrock outcrops located in the vicinity of the Nerbioi-Ibaizabal estuary
(Getxo, Basque Country), an area highly influenced by industrialization. Concretely, a vertical beachrock
outcrop located in Arrigunaga Beach, in the inner part of the mouth of the estuary, was studied using a
novel analytical methodology to simplify and understand the heterogeneity of the deposits. First, a
granulometric separation was carried out followed by the optical microscopic inspection of the fractions
and their Raman spectroscopic analysis, revealing the probable heterogeneity of the main beachrock
constituents (carbonates and metallic compounds) in each fraction. The total carbonates (acid–base
back titration) and the acid-extractable elements (ICP-MS) were quantified, followed by chemometric
analyses (correlation analysis and principal component analysis). The combined use of these
conventional techniques was particularly relevant for concluding that although the main elements were
Fe, Ca, Mg, Mn, Al, Na and K, the key point lies in their distribution in different particle sizes. Indeed,
heavy metals, particularly Fe, were accumulated in the 75–250 mm fraction, in contrast to a higher
content of carbonates and Ca in the <75 mm fraction. The fact that heavy metals were more
concentrated in the coarser fraction led us to believe that an external income of these elements existed,
which is likely to come from industrial wastes. Furthermore, the complementarity of the techniques used
corroborated the isolation of cements in the finest fraction. Thus, the analytical methodology proposed
here helped in distinguishing anthropogenic elements from those comprising the cements, suggesting
that both the trapped materials and the components responsible for the cementation occurred in this
temperate latitude.

The 1200m thick carbonate sequence of the Chudleigh Limestone Subgroup (Gordon Group) near Mole Creek conformably overlies the siliciclastics of the Magog Subgroup (Denison Group) and is conformably overlain by over 45m of siliciclastic Eldon Group correlate. The Chudleigh Limestone Subgroup is divisible into six conformable formations: the Standard Hill Fm. (145m thick), the Ugbrook Fm. (120m), the Sassafrass Creek Fm. (145m) the Dogs Head Fm. (470m) the Mole Creek Fm. (30-75m), the Overflow Creek Fm. (240m) and the Den Fm. (133m). The Den Fm. is conformably and sharply overlain by 15m of siltstone containing Isorthis and then 20m of white quartzite. Both of these units are placed in the Eldon Group by Burrett whilst Banks places the lower unit in the Gordon Group. The Chudleigh Limestone Subgroup contains a record of tropical, shallow water, dominantly peritidal, carbonate environments and faunas lasting about 25 my from the Late Whiterockian (Chazyan) through the Edenian. Deposition and basinal subsidence averaged 0.05mm per year.

La fotosíntesis de las algas calcificantes es un relevante conductor de la formación de carbonatos en las costas. El carbono inmovilizado en los talos calcificados de estas algas es un componente estructural de
diferentes tipos de carbonatos. Entre ellos, la calcita magnesiana es el mineral más comúnmente reportado. El contenido de otros cationes (además del calcio) en estos biominerales les brinda características particulares. Por ello, los biominerales son objeto de investigación actual, en particular frente a los panoramas de cambio ambiental. En este trabajo utilizamos a las algas coralinas de las pozas intermareales de Calerita (Bahía de la Paz, BCS, México) como modelos para estudiar la diversidad mineral (en microescala) del talo de Neogoniolithon thrichotomum, el alga coralina más abundante en las pozas estudiadas. Los resultados preliminares indican la presencia de aragonita, calcita magnesiana con alto contenido de Mg (~ 18%), también ankerita y siderita (Fe-carbonatos) espacialmente segregada
a microescala.
Palabras clave: bio-carbonatos; calcita magnesiana; bio-minerales; carbonatos costeros.

What are Microbialites? WHAT ARE MICROBIALITES? Do you know what a microbialite is? Not many people do. This is MICROBIALITE A rock-like structure made by millions of microbes that precipitate carbonate. because microbialites look like slimy underwater rocks, but they are actually reefs made up of microbes (simple, one-celled organisms). MICROBE An organism that is seen under the microscope. Microbialites are fascinating, because these rock-like structures are made by the interaction of millions of microbes that live in certain aquatic environments. The microbes facilitate the precipitation of minerals from the water, to form the microbialite structure [ ]. Since microbialites are like rocks, they have remained on earth since extremely ancient times. Fossilized microbialites, known as stromatolites (from the Greek strõma, meaning bed or layer, and STROMATOLITE Fossilized microbialites, and are the oldest fossils dated so far. lithos, meaning rock), are the oldest evidence of life on Earth, dating back to. billion years ago [ ]. Cyanobacteria are one type of microbe found in microbialites, and CYANOBACTERIA A phylum of bacteria that can make photosynthesis.