Geochemical-Microscopical Characterization of the Deterioration of Stone Surfaces in the Cloister of Santa Maria in Vado (Ferrara, Italy)

Santa Maria in Vado is a monument in the rich artistic heritage of the city of Ferrara (north of Italy). In this paper we want to investigate the state of conservation of tombstones, cloister and the entrance to the basilica, in order to keep them in the best possible state for the future generations. From the chemical characterization, the state of conservation was determined focusing on the biodeteriogenic and non-biodeteriogenic factors, which determine a series of unwanted changes in the physical, mechanical and above all aesthetic properties of the material, often closely connected with the environment and conservation conditions. On the macroscopic observation, the state of conservation of the tombstones appeared to be very deteriorated through aesthetic and structural damage. In detail, the stereo microscope observation of samples collected from the tombstones show the presence of efflorescence probably caused by the abundant of water that bring the salts present inside the rock into solution. Relating the columns, μ-XRF analysis confirm the carbonate composition of samples and presence of iron and sulfur. Finally, SEM observation highlighted the presence of black crust on arch samples and the presence of pollen on the black crust and spheroidal particles probably related to atmospheric pollution.

Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ³³S reservoir of sulfate aerosols?

To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric SO2 cycle, we measured the oxygen (δ17O, δ18O, and Δ17O) and sulfur (δ33S, δ34S, δ36S, Δ33S, and Δ36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The δ18O and δ34S values, ranging between 7.5 ‰ and 16.7±0.5 ‰ (n=27, 2σ) and between −2.66 ‰ and 13.99±0.20 ‰, respectively, show anthropogenic SO2 as the main sulfur source (from ∼2 % to 81 %, average ∼30 %) with host-rock sulfates making the complement. This is supported by Δ17O values (up to 2.6 ‰, on average ∼0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Negative Δ33S and Δ36S values between −0.34 ‰ and 0.00±0.01 ‰ and between −0.76 ‰ and -0.22±0.20 ‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the SO2 oxidation on the building stone, leading to 33S depletion in black crust sulfates and subsequent 33S enrichment in residual SO2. Except for a few samples, sulfate aerosols mostly have Δ33S values > 0 ‰, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative Δ33S reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.

The multi oxygen isotope analyses on black crust from Sicily highlight the volcanic emission influence from Mount Etna on urban areas

<p>This study reports on measurements of Δ<sup>17</sup>O (derived from the triple oxygen isotopes) in sulphate from black crust sampled in Sicily. Atmospheric oxidants, such as O<sub>3</sub>, H<sub>2</sub>O<sub>2</sub>, OH and O<sub>2</sub> carry specific <sup>17</sup>O-anomalies, which are partly transferred to the sulphate during sulphur gas (e.g. SO<sub>2</sub>) oxidation. Hence, the Δ<sup>17</sup>O in sulphate can be used as a tracer of sulphur oxidation pathways. So far, this method has been mostly applied on sulphate from aerosols, rainwaters, volcanic deposits and ice cores. Here we propose a new approach, that aims to investigate the dominant oxidants of gaseous sulphur precursors into sulphate extracted from black crust material. Black crusts are mostly found on building/monument/sculpture and are the result of the reaction between sulphur compounds (SO<sub>2</sub>, H<sub>2</sub>SO<sub>4</sub>) and carbonate (CaCO<sub>3</sub>) from the substrate, which leads to the formation of gypsum (CaSO<sub>4</sub>, 2H<sub>2</sub>O). Sicilian black crust from sites under different emission influences (anthropogenic, marine and volcanic) were collected. Multi oxygen and sulphur isotope analyses were performed to better assess the origins of black crust sulphate in these different environments. This is crucial for both a better understanding of the sulphur cycle and the preservation of historical monument.</p><p>Multi sulphur isotopes show mostly negative values ranging from -0.4 ‰ to 0.02 ‰ ± 0.01 and from -0.59 ‰ to 0.41‰ ± 0.3 for Δ<sup>33</sup>S and Δ<sup>36</sup>S respectively. This is unique for natural samples and different from sulphate aerosols measured around the world (Δ<sup>33</sup>S > 0‰). This tends to indicate that sulphate from black crust is not generated by the same processes as sulphate aerosols in the atmosphere. Instead of SO<sub>2</sub> oxidation in the atmosphere, dry deposition of SO<sub>2</sub> and its oxidation on the substratum is preferred. The multi oxygen isotopes show a clear dependence with the geographical repartition of the samples. Indeed, black crusts from Palermo (the biggest Sicilian city) show small <sup>17</sup>O-anomalies ranging between -0.16 ‰ to 1.02 ‰ with an average value of 0.45 ‰ ± 0.26 (n=12; 2σ). This is consistent with Δ<sup>17</sup>O values measured in black crust from the Parisian Basin (Genot et al., 2020), which are also formed in an environment influenced by anthropogenic and marine emissions. On the other hand, samples from the eastern part of the Mount Etna region, which are downwind of the volcanic emissions, show the highest <sup>17</sup>O-anomalies ranging from 0.48 ‰ to 3.87 ‰ with an average value of 2.7 ‰ ± 0.6 (n=11; 2σ).</p><p>These results indicate that volcanic emissions influence the oxygen isotopic signature of black crust sulphate. In standard urban areas, SO<sub>2</sub> deposited on the substratum is mostly oxidised by O<sub>2</sub>-TMI and H<sub>2</sub>O<sub>2 </sub>to generate the black crust. Yet, under the influence of volcanic emissions, O<sub>3</sub> may play the main role in the SO<sub>2</sub> oxidation.</p>

Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative ∆³³S-reservoir of sulfate aerosols?

To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric SO2 cycle, we measured the oxygen (δ17O, δ18O and ∆17O) and sulfur (δ33S, δ34S, δ36S, ∆33S and ∆36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW-SE cross-section in the Parisian basin. The δ18O and δ34S, ranging between 7.5 and 16.7 ± 0.5 ‰ (n = 27, 2σ) and between −2.6 and 13.9 ± 0.2 ‰ respectively, show anthropogenic SO2 as the main sulfur source (from 2 to 81 %, in average ~30 %) with host-rock sulfates making the complement. This is supported by ∆17O-values (up to 2.6 ‰, in average ~0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Both negative ∆33S-∆36S-values between −0.34 and 0.00 ± 0.01 ‰ and between −0.7 and −0.2 ± 0.2 ‰ respectively were measured in black crusts sulfates, that is typical of a magnetic isotope effect that would occur during the SO2 oxidation on the building stone, leading to 33S-depletion in black crust sulfates and subsequent 33S-enrichment in residual SO2. Given that sulfate aerosols have mostly ∆33S > 0 ‰ and no processes can yet explain this enrichment, resulting in a non-consistent S-budget, black crust sulfates could well represent the complementary negative ∆33S-reservoir of the sulfate aerosols solving the atmospheric SO2 budget.

The Interpretation of Biogeochemical Growths on Gold Coins from the SS Central America Shipwreck: Applications for Biogeochemistry and Geoarchaeology

Black crusts that formed on gold coins recovered from the 1857 shipwreck of the SS Central America played a key role in their preservation in a near original state. Within a few years of the sinking, the significant quantities of iron and steel on the shipwreck produced laminar geochemical precipitates of fine-grained iron minerals on the coins. This coating served to armor the coins from future chemical or biological attacks. Once coated, the coins were colonized by at least two distinct populations of gold-tolerant bacteria that precipitated abundant nanoparticulate gold in the black crust material and produced biomineralized bacteria in a web-like mat. Above this middle layer of black crust, the outer layer consisted of a geochemical reaction front of euhedral crystals of iron sulfate and iron oxy-hydroxide species, formed by the interaction of seawater with the chemical wastes of the bacterial mat. Understanding this process has application for assessing the diverse and extreme conditions under which nano-particulate gold may form through biological processes, as well as understanding the conditions that contribute to the preservation or degradation of marine archaeological materials.