Manganese and iron rock varnishes have been noted to occur in a wide range of environmental settings from well-studied examples in deserts around the globe to less studied examples in rainforests of multiple continents, near active glaciers, and on rocks exposed in rivers [7]. Given the latter, it is perhaps not entirely surprising to learn of rock varnish formation in Washington, DC, which is in the humid subtropical zone according to the Köppen climate classification. Average total precipitation in Washington is ~1000 mm per year, or roughly ten times greater than that for an urban desert location, e.g., Las Vegas, NV.
Rock varnish on the Seneca sandstone described here shares certain similarities with desert varnish: (1) high luster, nearly metallic in places; (2) Mn-rich particle sizes in the nanometer range from <20 to 200 nm; (3) high enrichment in Mn as well as Pb, Cu, and Zn (Table 2); (4) deposition on a lithological substrate containing a low bulk concentration of Mn (Table 1); and (5) mineralogical association with Al-rich silicate dust minerals intimately mixed with the Mn-rich coating (Fig. 8c).
Despite these parallels, rock varnish on the Smithsonian Castle sandstone possesses differences relative to varnish formed in arid settings, namely: (1) Mn is found in two zones, [i] dispersed along grain boundaries and pores in the uppermost 200–250 μm of the rock surface, and [ii] thin and discontinuous surface deposits ≪1 μm in thickness or roughly three orders of magnitude thinner than typical desert varnish. In cross section it is difficult to measure, or even identify, a surface deposit; (2) deposits have no discernable vertically definable substructure as opposed to microstratified desert varnish described in the literature [8, 13]. Differences in both the thickness and lack of microstratigraphy may be attributed to the age disparity between the architectural varnish and those formed over geological periods of time. In this regard Smithsonian varnish may serve as an example of the earliest stage of desert varnish formation.
Varnish minerals are either poorly crystalline, present at levels below the detection limit for x-ray diffraction (~5 volume%), or possibly both. Previous researchers have encountered a similar lack of diffracted x-ray signal, even for more developed desert varnish where the coating is significantly more massive compared to samples in this study [8]. Many Mn oxides formed in nature, e.g., birnessite, are poorly crystalline in general, which may well have compounded our inability to collect structural information [21]. Transmission electron microscopy or synchrotron-based x-ray methods represent useful follow-up methods to characterize the rock varnish in this study [12, 22, 23].
The composition of the varnish likely represents multiple phases, a Mn-rich oxide and an Al-rich silicate mineral. The aluminous phase is either intimately mixed with Mn oxide or results from sampling a clastic mineral grain substrate. Unmixing the convolved chemistry by extrapolating negatively correlated Mn–Si and Fe–Si to a zero Si concentration end-member yields a Mn/Fe ratio of 20. Similar negative correlations between Si and Mn have been noted in desert varnish [8, 24]. The Mn-rich component within the nanophase mineral mixture is most likely either birnessite or todorokite.
$${\text{Avg}}.{\text{ composition:}} \;{\text{Na}}_{0. 2} {\text{Ca}}_{0. 1} {\text{Mg}}_{0. 1} {\text{Al}}_{0. 1} {\text{Si}}_{0. 5} {\text{Mn}}_{ 1. 9} {\text{Fe}}_{0. 5} {\text{O}}_{ 6. 7}$$
$${\text{Birnessite:}}\;\left( {{\text{Na}},{\text{ Ca}}} \right)_{0. 5} \left( {{\text{Mn}}^{ 4+ } ,\;{\text{Mn}}^{ 3+ } } \right)_{ 2} {\text{O}}_{ 4} \cdot 1. 5 {\text{H}}_{ 2} {\text{O}}$$
$${\text{Todorokite:}}\;\left( {{\text{Mn}}^{ 2+ } ,{\text{ Ca}},{\text{ Na}},{\text{ K}}} \right)\left( {{\text{Mn}}^{ 4+ } ,\;{\text{Mn}}^{ 2+ } ,\;{\text{Mg}}} \right)_{ 6} {\text{O}}_{ 1 2} \cdot 3 {\text{H}}_{ 2} {\text{O}}$$
Potter and Rossman [9] found that different terrestrial weathering environments produced distinct mineralogy. If birnessite is the Mn-rich phase in the Smithsonian Castle varnish, the result is consistent with subaerial Mn varnish formed in stream deposits where birnessite is found in association with minor silicate minerals.
Apart from an extreme enrichment in Mn relative to the Seneca sandstone, the varnish is additionally concentrated in a number of heavy metals, including Pb, Zn, Cu, and Ni. These results are consistent with experiments that have determined divalent metal preferential adsorption for Pb, Zn, and Cu on interlayer crystallographic sites in birnessite [25]. Analytical studies also show a significant enrichment of Mn, Co, Pb, Ni, and Cu in desert varnishes [24, 26].
According to previous investigators, e.g., [7, 10], the mechanism of formation of Mn rock varnishes involves multiple steps: (1) accumulation of externally derived clay-bearing dust on a rock surface [10], (2) transport of Mn to the rock surface in solution within rain/fog droplets, and (3) oxidation and precipitation of Mn oxide cementing previously unconsolidated clay into a heterogeneous nanophase mixture. Step 1 was recently bolstered by the observation that anticorrelated Mn and Si electron microprobe analyses for desert varnish have a common Mn-free end-member regardless of the composition of the varnished rock [24]. Regarding step 2, one may question whether Mn is derived locally from the host rock or externally via atmospheric transportation [8]. In this study we report the presence of hematite, titanite, and rutile in the Seneca sandstone, which may all serve as internal reservoirs of Mn. However, to within the sensitivity of our compositional imaging we find no evidence for solution transportation of Mn in the sandstone. Based upon rare earth element fractionation observed in desert varnish it has been argued that the degree of element fractionation observed is inconsistent with isochemical leaching and reprecipitation within the same rock [26]. Therefore, the source of Mn must be delivered to the rock surface externally via atmospheric transport and surface leaching/dissolution of airborne dust grains. The final step 3 requires oxidation and precipitation of Mn either by a physiochemical process under acidic oxidized conditions in rainwater [26] or via microbially assisted oxidation as favored by other investigators [7, 11, 27]. The microstructural imagery collected on the Smithsonian Castle varnish offers no evidence for microbial entombment in Mn oxide, and the organisms observed on the surface (Fig. 14) grew upon the varnish and therefore may not be related to Mn oxidizing microbes. Apart from local compositional heterogeneity discussed above, another factor to consider regarding the likelihood of varnish development is stone roughness. Rougher areas provide increased surface area for condensation and therefore deposition of atmospheric dust. However, no systematic associations between surface roughness and rock varnish have been noted on the building stone and gateposts.
The patchy distribution of varnish on the Castle is less consistent with a chemical process, which would lead to the development of a more uniform coating, and more suggestive of biological colonization. It seems reasonable that biological mediation must play some significant role in Mn oxidation, if in tandem with abiological processes, in the development of rock varnishes in this study. Furthermore, we cannot state definitively why varnish formation is occurring on the Smithsonian Castle Seneca sandstone as opposed to other building stones on the National Mall. However, it has been noted that
“the irregular onset of bacterial colonization accounts for the puzzling inconsistency in varnish development from stone to stone…” [11].
The rate of subaerially exposed Mn varnish formation in the western USA has been estimated to range between <1 and 40 nm/a [28]. Given that the age of the Castle gatepost exposure to the atmosphere is known to be just under 30 years, a rock varnish thickness range of ~28 nm–1.08 μm would be expected using the rates determined for the western USA. The observed surface coating is closer to the lowest end of that thickness range, although Mn-rich particles sequestered in pores within the sandstone do not contribute to the surface thickness estimate. A rare 20–40 year old rock varnish has been reported forming on steel slag piles in southern California [29], where the substrate material is rich in Mn, possibly influencing the rapid rate of growth. Impressively, in a few short decades stone disfigurement of the Smithsonian’s Castle is readily noticeable and a testament to the strong pigmentation of Mn oxides even when present in small concentrations.
Removal of varnish from the stone may prove problematic. While laser cleaning can be used to remove the exposed coating it will be more difficult to remove Mn oxide within pores without damaging the sandstone.