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.

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.

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.