Post by 1dave on Jul 4, 2017 16:50:41 GMT -5
Desert Varnishes grow fastest in damp environments, slower in arid areas, staying on physically stable rock surfaces that are less subject to frequent wind abrasion, heavy precipitation, or rock fracturing.
Different rocks have varying abilities to accept and retain varnish.
Limestones typically do not retain varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form on.
Basalt, fine grained quartzites, sandstones, and metamorphosed shales often develop shiny, dense and black varnishes and retain them because of their high resistance to weathering.
During the Little Ice Age varnishes in many areas resulted in the accretion of three very narrow secondary black bands - generally accepted as the "Little Ice Age Signal" (Liu and Broecker 2007).
Composition:
A major part of desert varnish is wind driven clay wetted by dew, which then catches additional substances Such as Manganese, Iron, other heavy metals, and bits of vegetation that chemically react together when the rock reaches high temperatures in the desert sun.
In iron rich but poor manganese conditions, orange hematite varnishes develop. In extremely dry areas orange hematite may change to black goethite.
Black desert varnish often has an unusually high concentration of manganese. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese can be 50 to 60 times more abundant.
Alternating humid and arid climates probably accounts for Mn/Fe fluctuations.
Multiple researchers have since confirmed that lead and other pollutants contaminate the active surface of desert varnish and other iron-rich rock coatings.
Electron microprobe profiles reveal that lead is a contaminant in the uppermost surfaces of rock varnishes, but these concentrations drop to background levels below the very surface of natural rock coatings that have formed since lead additives were introduced into gasoline in 1922. (Dorn 1998, 139)
Radioactive cesium from nuclear bomb tests, lead released from automobiles, and zinc from smelters were found in the surface-most layer of varnish (Fleisher et al. 1999), including in eastern California distant from cities (Broecker and Liu 2001).
Another study (Wayne, Diaz, and Orndorff 2004) noted that “the surface layers of all varnish samples studied display an extreme enrichment in Pb that is not always reflected in the abundances of most other trace elements. Varnish Pb isotope signatures contain a distinct atmospheric Pb component, relative to those of the substrate rock.”
Still others reported similar surficial contamination by lead and other heavy metal contaminants (Thiagarajan and Lee 2004; V. F. Hodge et al. 2005; Wayne et al. 2006; Spilde, Boston, and Northup 2007; Nowinski et al. 2010).
alliance.la.asu.edu/dorn/Authenticating_PG_2012.pdf
Methods of Authenticity Testing.
Two different methods are used in combination to place arid-region inscriptions in three possible time groupings:
Each testing starts by measuring lead profiles of rock coatings formed on top of the engravings.
1. If the profile only shows contamination from twentieth-century automotive lead pollution, then the authentication effort need go no further.
2. The varnish microlamination (VML) technique is used to assess whether the engraving formed during the Little Ice Age only if lead concentrations drop to natural low levels underneath a contaminated surface layer.
(1) carved during the period of twentieth-century automotive lead and other heavy metal pollution;
(2) engraved in the late nineteenth or earliest twentieth century before lead and other heavy metal pollution but after the Little Ice Age ended by ca. 1850; or
(3) inscribed during the Little Ice Age that started in the mid-fourteenth century and ended by the
middle of the nineteenth century.
The background amount of lead vapor in the air constantly fluctuates from volcanoes to modern industrial uses.
Only the most modern layers show a rise in the lead content from three separate samples from the 1776 inscription from the Escalante expedition.
Different rocks have varying abilities to accept and retain varnish.
Limestones typically do not retain varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form on.
Basalt, fine grained quartzites, sandstones, and metamorphosed shales often develop shiny, dense and black varnishes and retain them because of their high resistance to weathering.
During the Little Ice Age varnishes in many areas resulted in the accretion of three very narrow secondary black bands - generally accepted as the "Little Ice Age Signal" (Liu and Broecker 2007).
Composition:
A major part of desert varnish is wind driven clay wetted by dew, which then catches additional substances Such as Manganese, Iron, other heavy metals, and bits of vegetation that chemically react together when the rock reaches high temperatures in the desert sun.
In iron rich but poor manganese conditions, orange hematite varnishes develop. In extremely dry areas orange hematite may change to black goethite.
Black desert varnish often has an unusually high concentration of manganese. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese can be 50 to 60 times more abundant.
Alternating humid and arid climates probably accounts for Mn/Fe fluctuations.
Multiple researchers have since confirmed that lead and other pollutants contaminate the active surface of desert varnish and other iron-rich rock coatings.
Electron microprobe profiles reveal that lead is a contaminant in the uppermost surfaces of rock varnishes, but these concentrations drop to background levels below the very surface of natural rock coatings that have formed since lead additives were introduced into gasoline in 1922. (Dorn 1998, 139)
Radioactive cesium from nuclear bomb tests, lead released from automobiles, and zinc from smelters were found in the surface-most layer of varnish (Fleisher et al. 1999), including in eastern California distant from cities (Broecker and Liu 2001).
Another study (Wayne, Diaz, and Orndorff 2004) noted that “the surface layers of all varnish samples studied display an extreme enrichment in Pb that is not always reflected in the abundances of most other trace elements. Varnish Pb isotope signatures contain a distinct atmospheric Pb component, relative to those of the substrate rock.”
Still others reported similar surficial contamination by lead and other heavy metal contaminants (Thiagarajan and Lee 2004; V. F. Hodge et al. 2005; Wayne et al. 2006; Spilde, Boston, and Northup 2007; Nowinski et al. 2010).
alliance.la.asu.edu/dorn/Authenticating_PG_2012.pdf
Methods of Authenticity Testing.
Two different methods are used in combination to place arid-region inscriptions in three possible time groupings:
Each testing starts by measuring lead profiles of rock coatings formed on top of the engravings.
1. If the profile only shows contamination from twentieth-century automotive lead pollution, then the authentication effort need go no further.
2. The varnish microlamination (VML) technique is used to assess whether the engraving formed during the Little Ice Age only if lead concentrations drop to natural low levels underneath a contaminated surface layer.
(1) carved during the period of twentieth-century automotive lead and other heavy metal pollution;
(2) engraved in the late nineteenth or earliest twentieth century before lead and other heavy metal pollution but after the Little Ice Age ended by ca. 1850; or
(3) inscribed during the Little Ice Age that started in the mid-fourteenth century and ended by the
middle of the nineteenth century.
The background amount of lead vapor in the air constantly fluctuates from volcanoes to modern industrial uses.
Only the most modern layers show a rise in the lead content from three separate samples from the 1776 inscription from the Escalante expedition.
