Characteristics and origin of rock varnish from the hyperarid coastal deserts of northern Peru
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<ul><li><p>QUATERNARY RESEARCH 35, 116-129 (1991) </p><p>Characteristics and Origin of Rock Varnish from the Hyperarid Coastal Deserts of Northern Peru </p><p>CHARLES E. JONES' Department of Geology, Stanford University, Stanford, California 94305 </p><p>Received November 22, 1989 </p><p>The characteristics of a new type of rock varnish from the hyperarid coastal deserts of northern Peru, combined with laboratory experiments on associated soil materials, provide new insights into the formation of rock varnish. The Peruvian varnish consists of an Fe-rich, Mn-poor component covering up to 95% of a varnished surface and a Fe-rich, Mn-rich component found only in pits and along cracks and ridges. The alkaline soils plus the catalytic Fe oxyhydroxides that coat much of the varnish surfaces make the Peruvian situation ideal for physicochemical precipitation of Mn. However, the low Mn content of the dominant Fe-rich, Mn-poor component suggests that such precipitation is minor. This, plus the presence of abundant bacteria in the Mn-rich varnish and the recorded presence of Mn-precipitating bacteria in varnish elsewhere, suggests that bacteria are almost solely responsible for Mn-precipitation in rock varnish. A set of experiments involving Peruvian soil samples in contact with water-CO, solutions indicates that natural fogs or dews release Mn but not Fe when they come in contact with eolian materials on rock surfaces. This mechanism may efficiently provide Mn to bacteria on varnishing surfaces. The lack of Fe in solution suggests that a large but unknown proportion of Fe in varnish may be in the form of insoluble Fe oxyhydroxides sorbed onto the clay minerals that form the bulk of rock varnish. The results of this study do not substantively change R. I. Doms paleoenvironmental interpretations of varnish Mn:Fe ratios, but they do suggest areas for further inquiry. 0 1991 University of Washington. </p><p>INTRODUCTION </p><p>A variety of new paleoenvironmental and age-dating techniques are based on the physical and chemical properties of rock varnish (e.g., Harrington and Whitney, 1987; Dorn et al., 1987a; Dorn, 1988). It is therefore essential to understand thor- oughly the formation and subsequent alter- ation of rock varnish. This paper describes a new variety of rock varnish from the hy- perarid coastal deserts of Peru, presents ex- perimental evidence for a physicochemical mechanism that enhances Mn relative to Fe, and takes a closer look at the controls on the Mn:Fe ratios in rock varnish. This work is particularly relevant to paleoenvi- ronmental interpretations of the variations in Mn:Fe ratios in rock varnishes (Dorn, 1984, 1988, 1990). </p><p> Present address: Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, U.K. </p><p>Rock varnish is generally classified as one of two types. Most research has fo- cused on the Fe- and Mn-rich varnishes commonly found in semiarid to arid envi- ronments with generally weakly acidic to weakly alkaline soil conditions (e.g., Engel and Sharp, 1958; Dorn and Oberlander, 1982; Taylor-George et al., 1983). These dusky-brown to black coatings, 500 km thick, are found on stable rock surfaces and consist of approximately 30% Mn and Fe oxides (birnessite and hematite) and 70% mixed-layer illite/montmorillonite clay minerals (Potter and Rossman, 1977, 1979). A variety of microcolonial fungi and bacte- ria appear to be common, if not ubiquitous, inhabitants of these varnish surfaces (Krumbein and Jens, 198 1; Dorn and Ober- lander, 1981a; Taylor-George et al., 1983; Palmer et al., 1985). </p><p>The second type of varnish, Fe-rich but Mn-poor, has received less attention and seems to come in a number of varieties. </p><p>116 0033-5894191 $3.00 Copyright 0 1991 by the University of Washington. </p></li><li><p>PERUVIAN ROCK VARNISH 117 </p><p>Fe-rich bottomcoat varnishes (e.g., Potter and Rossman, 1977) apparently are chemi- cally and mineralogically similar to Mn-rich varnishes except for the lack of Mn oxides. The Fe-Si varnishes of Smith and Whalley (1988), probably Oberlander (1982), possi- bly Glasby et al. (1981), Johnston and Cardile (1984), and Johnston ef al. (1984) appear, based on low A&O, concentrations, to have much lower concentrations of clay minerals. Possibly related to the Fe-Si var- nishes are the nearly pure silica glazes of Fisk (1971), Fat-r and Adams (1984), Curtiss ef al. (1985), and Smith and Whalley (1988). </p><p>The ongoing debate (e.g., Smith and Whalley, 1988; Dorn, 1989) regarding the genesis of Mn-rich rock varnish centers around the mechanisms of Mn concentra- tion and precipitation at the rocks surface. Elvidge and Iverson (1983) present a purely physicochemical model of varnish forma- tion involving pH fluctuations at the var- nish surface. Dorn and Oberlander (1981a), on the other hand, support the view that the concentration and precipitation of Mn is handled by mixotrophic bacteria which ox- idize Mn as part of their life processes. </p><p>GEOGRAPHIC SETTING OF THE SAMPLING SITE </p><p>Varnish samples were collected from a set of alluvial fan terraces situated between the Rio Sechin and Rio Casma, roughly 20 km east of the coastal town of Casma, Peru (9.5 S lat., 360 km north of Lima). The field area, located in the hyperarid coastal deserts of South America, has received over the last 20 years an average of less than 5 mm of rain/year; El Nina events of a magnitude like that of the 1982-1983 event bring catastrophic flooding on average at least once every 50 years (Wells, 1987). A significant source of moisture for varnish formation may be early morning fogs and dews. During July and August of 1986, dense morning fogs were common and oc- casionally were observed forming puddles on rock surfaces. </p><p>The rocks of the area are predominantly </p><p>andesite and rhyolite. Accordingly, the soils contain quartz, plagioclase, potassium feldspar, pyroxene, Fe-Ti oxides, kaolin- ite, and mixed-layer illite/montmorillonites (J. Noller, personal communication, 1988). In accordance with the barren conditions of the field area, the upper soil horizons con- tain almost no organic matter (</p></li><li><p>118 CHARLES E. JONES </p><p>following experiments were performed to test the ability of these mildly acidic solu- tions to release Fe and Mn from clay min- eral surfaces. First, all labware was acid- washed for at least 24 hr in a 1:2 HCl + water solution. Then seven sets of 50.00-g soil samples were thoroughly mixed in 100 ml of deionized water saturated with CO, and left to stand for a period ranging from 22 to 213 hr. Every 24 hr the CO, was re- plenished so as to keep the pH below about 6. (Without the CO, buffer, the solutions rapidly shifted to pH 9 or more.) At the end of the experiment all samples were shaken vigorously, allowed to settle for 15 min, and forced through a IO-km polyethylene filter. The solutions were then acidified to pH < 1 to prevent adsorption of Fe or Mn onto the container walls. </p><p>It was thought that the initial concentra- tion of clay minerals on a varnish surface might affect the efficiency of the buffering solution and the amount of Fe and Mn re- leased into solution. To test this, a series of solutions with 5.00 and 50.00 g of clay were prepared as above. The bulk Mn:Fe ratio of the soil was determined by sub-boiling a 50- g sample in concentrated H,SO, to remove all oxides from the mineral surfaces. The concentrations of Fe and Mn in all solutions were determined on a Perkin-Elmer 403 atomic absorption spectrophotometer in flame mode. In addition, some soil was passed through a 38-pm brass sieve to ob- tain a finer size fraction. This sample was baked overnight at 1000C to remove the water from the clays. About 6.5 wt% of wa- ter and other volatiles was lost. A fused disk was prepared from this material and analyzed in a Philips PW 1400 X-ray spec- trometer calibrated using international standards. The major element totals sum- med to 100.5%. </p><p>DESCRIPTION OF PERUVIAN TWO-COMPONENT ROCK VARNISH </p><p>Peruvian rock varnish consists of two components: a thin (co.1 rJ,m) Fe-rich (lO-20%), Mn-poor (~3%) coating that cov- </p><p>ers 75-90% of a varnished surface and a thick (averaging 100 km) Fe-rich (7-15%), Mn-rich (lO-20%) varnish that coats the re- mainder of the rocks surface and is found mostly in depressions and along cracks. In addition, a thriving community of microco- lonial fungi and bacteria inhabits these var- nish surfaces. </p><p>Mn-Poor Component </p><p>The Mn-poor component varies in color according to the degree of varnish develop- ment, ranging from rust-orange to dark red- dish-brown to deep reddish-purple (Mun- sells 7.5 YR 515 to 5 YR 516 to 2.5 YR 313, respectively). Under a binocular micro- scope it appears resinous and has a Mohs hardness of approximately 6. The SEM re- veals surface textures that range from the rough, irregular surfaces of the underlying crystalline substrate to smoother surfaces formed by thicker deposits that largely bury the rough crystalline surfaces (Fig. 1). Un- der the SEM, back-scattered electron imag- ing normally allows varnishes in thin- section to be clearly distinguished from their substrates by highlighting the compo- sitional contrasts between varnish and sub- strate. In this case, however, the varnish is so thin (estimated at ~0.1 pm) that it is often barely visible. </p><p>Chemical characterization of this compo- nent is difficult. The hardness and thinness of the varnish make it difficult to remove mechanically without significant contami- nation from the substrate. SEM/EDS spot analyses perpendicular to the varnish sur- face are dominated by the bulk composition of the substrate due to penetration of the electron beam through the varnish. These analyses show a SiO,/Al,O, ratio of about 8: 1, and the concentrations of K, Ca, and Ti oxides follow the trend K,O > CaO > TiO,. Both results mirror the chemistry of the substrate and contrast with the chemis- try of the Mn-rich varnishes, which show a 2: 1 ratio of SiO,/Al,O, and the trend CaO > K,O > TiO,. Because there is not enough A&O, in the varnish to distort the SiO,/ </p></li><li><p>PERUVIAN ROCK VARNISH 119 </p><p>FIG. 1. SEM photomicrograph (SEI) of a characteristically botryoidal Mn-rich pit (left-center) surrounded by smoother Mn-poor varnish (right). Round objects are microcolonial fungi. Scale bar is 100 pm. </p><p>A&O, ratio of the substrate, it appears that there are only minor amounts of clay min- erals (SiO,/Al,O, ratios of 2: 1 to 1: 1) in the varnish. </p><p>The inadequacy of the chemical analyses makes classification of this Fe-rich varnish tentative. Some Fe-rich varnishes, appar- ently found predominantly as coatings on the underside of varnished cobbles, contain about 90% clay minerals (Potter and Ross- man, 1977). Other Fe-rich varnishes, found as topcoats, are very rich in SiO,, show a considerable enhancement of Fe with re- spect to the substrate, and contain small amounts of Al,O, as the third principal component. The Fe-rich varnishes of Smith and Whalley (1988), Oberlander (1982), possibly Glasby et al. (1981), and the Peru- vian Mn-poor varnish seem to fit this cate- gory of clay-poor Fe-Si varnishes. </p><p>Mn-Rich Component </p><p>The Mn-rich component occurs as an </p><p>opaque black coating in pits and along cracks and ridges. It may be removed easily with a steel probe. Under the SEM the Mn- rich component is characterized by a dis- tinctive botryoidal surface morphology that frequently appears to overlap the surround- ing Mn-poor varnish (Fig. 1). Thin-sections confirm that the Mn-rich varnish is indeed laterally discontinuous, forming sharp con- tacts with both the barely visible Mn-poor varnish and the substrate (Fig. 2). A well- developed varnish pit averages 70 to 100~pm thick and may be up to 250-km thick. These deposits display wavy to bot- ryoidal, relatively continuous intrapit layer- ing that locally gives way to a series of ox- ide mounds surrounded by detrital materi- als (Fig. 2). An electron microprobe study of the Mn:Fe ratio variations through the thickness of these deposits reveals that the bulk of the Mn:Fe ratios ranges between OS:1 and 3.2:1 (C. E. Jones, unpublished data). </p></li><li><p>120 CHARLES E. JONES </p><p>FIG. 2. SEM photomicrograph (BSI) of a Mn-rich pit (light gray, top) showing layering and stro- matolite-like features in the top left. Weathered substrate immediately below varnish is enriched in characteristic varnish elements: Mn, Fe, and some P. Scale bar (lower right) is 100 km. </p><p>SEM/EDS analyses reveal that the SiO,/ Al,O, ratio found in Mn-rich varnishes is compatible with the 2:l SiO,/Al,O, sand- wich structure of illite/montmorillonite clay minerals. These minerals are the dominant components of Mn-rich varnishes from the Mojave Desert (Potter and Rossman, 1977). Peruvian Mn-rich varnishes contain only trace amounts (at most 0.3%) of Cu, Ni, and Co. </p><p>Biological Component The varnish surfaces are inhabited by a </p><p>profusion of microcolonial fungi (MCF) and baciliococci bacteria. The MCF generally range in size from 50 to 100 urn and are black under a 10x handlens. They are </p><p>found on both varnished and unvarnished rock surfaces and in appearance are some- what similar to those reported from deserts in the western United States and Australia (Staley et al., 1982, 1983; Taylor-George et al., 1983; Dorn, 1986; Dorn, 1990). SEM/ EDS work shows that while MCF may ad- sorb eolian materials onto their surfaces to some extent, they do not generally show MnO, concentrations above l-2%. There are no unusual concentrations of MnO, im- mediately surrounding individual MCF specimens. </p><p>The SEM revealed no bacteria on the varnish surfaces. However, when a small portion of mechanically removed Mn-rich varnish was stained following the method </p></li><li><p>PERUVIAN ROCK VARNISH 121 </p><p>of Ghiorse and Balkwill (1983) and placed and Adams (1984), Curtiss et al. (1985), and under 1000x on a UV-equipped optical mi- Smith and Whalley (1988). These Si glazes croscope, a flourishing population of may be end-member of the Fe-Si varnishes -0.2-Frn baciliococci bacteria was readily or may be a product of special chemical observed. These bacteria are abundant on composition of the substrate and/or sur- both mineral and fungal surfaces. Because rounding eolian materials. they could not be observed under the SEM, it was not possible to find variations in MnO, concentrations surrounding individ- ual bacteria. </p><p>In summary, the chemistry, morphology, biology, and apparent mineralogy of the Mn-rich component in the Peruvian varnish is strikingly similar to the Mn-rich var- nishes commonly found in semiarid to arid environments. It therefore seems reason- able to conclude that observations and gen- eralizations based on Peruvian Mn-rich var- nish should help explain the formation of Mn-rich varnishes in general. The Mn-poor component appears to have analogs de- scribed in other regions, but before a thor- ough model can be constructed more work is needed to establish the relationships among Fe-rich bottomcoat varnishes (e.g., Potter and Rossman, 1977); the Fe-Si var- nishes of Oberlander (1982), Smith and Whalley (1988), and th...</p></li></ul>
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