Rock varnish microlaminations from northern Tianshan, Xinjiang and their paleoclimatic implications

Spatial, temporal and geographic considerations of the problem of rock varnish diagenesis

“…The contrast between the light and scanning electron microscope observations of microsedimentary basins Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010), and the opposing conclusion reached by Garvie et al (2008) that paleoenvironmental interpretations of varnish are specious, exemplify the relevance of an epistemological perspective.…”

“…Scenario 3: if Garvie et al (2008) collected varnish that originated in a rock joint, exposed by spalling (Coudé-Gaussen et al, 1984;Villa et al, 1995;Dorn, 1998;Cerveny et al, 2006), then disequilibrium through an adjustment to the subaerial environment would be expected. Liu and colleagues are explicit in that they do not interpret paleoenvironmental changes from varnishes in these contexts Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010).…”

“…In warm deserts, however, a century of anecdotal observations of Quaternary and anthropogenic rock surfaces suggests it takes perhaps 10 4 years to coat completely many rock surfaces (Schweinfurth, 1903;Basedow, 1914;Blackwelder, 1948;Iskander, 1952;Denny, 1965;Hayden, 1976). It was not, however, until the compilation of the microlamination data set by Liu and colleagues Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010) that it was possible to obtain rates of accretion of varnish formed on subaerial surfaces in warm desert environments. Knowing the age of the underlying geomorphic surface and thicknesses of varnishes accumulated in over 10,000 microsedimentary basins led to the first comprehensive understanding of accumulation rates; subaerial Mn-rich varnish in the Basin and Range of the western USA, for example, accretes at the order of a few micrometers per millennia (Dorn, 1998;.…”

“…see Dorn, 1998: 223). For example, Garvie et al's (2008) sample 1 is from a site context that would not be used by Liu and colleagues for paleoenvironmental analysis Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010), because it derives from too small a desert pavement clast (Garvie et al, 2008). The photograph of varnish shown in Garvie et al (2009: Figure A) shows ground-line band varnish, a unique type of varnish recognized by Engel and Sharp (1958); such a sample is completely unsuitable for paleoenvironmental analysis because it is heavily influenced by soil effects.…”

“…Using a mix of light microscopy, X-ray mapping, backscattered electron microscopy, T. Liu and colleagues only analyzed varnishes carefully sampled from subaerial rock varnishes at the rock-atmosphere interface, well above soil surfaces, and from geomorphic positions that could not have originated in a rock fractureinformation provided in their methodological discussions. Liu and colleagues have been careful to point out that they avoided meterscale geomorphic conditions that would lead to varnish instability, as well as centimeter-scale microenvironmental conditions that would lead to biogeochemical mobilization of varnish constituents Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010). In other words, Liu and colleagues clearly established the spatial and temporal scales of samples of a specific type of rock varnish collected from very specific geomorphic settings.…”