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QUATERNARY RESEARCH 35, 116-129 (1991)

Characteristics and Origin of Rock Varnish from the Hyperarid Coastal Deserts of Northern Peru

CHARLES E. JONES' Department of Geology, Stanford University, Stanford, California 94305

Received November 22, 1989

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. Dom’s paleoenvironmental interpretations of varnish Mn:Fe ratios, but they do suggest areas for further inquiry. 0 1991 University of Washington.

INTRODUCTION

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 thoroughly the formation and subsequent alter- ation of rock varnish. This paper describes a new variety of rock varnish from the hyperarid coastal deserts of Peru, presents experimental 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 paleoenvironmental interpretations of the variations in Mn:Fe ratios in rock varnishes (Dorn, 1984, 1988, 1990).

’ Present address: Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, U.K.

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 environments 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, <2 to >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 bacteria 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).

The second type of varnish, Fe-rich but Mn-poor, has received less attention and seems to come in a number of varieties.

116 0033-5894191 $3.00 Copyright 0 1991 by the University of Washington.

PERUVIAN ROCK VARNISH 117

Fe-rich bottomcoat varnishes (e.g., Potter and Rossman, 1977) apparently are chemically 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), possibly 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 varnishes are the nearly pure silica glazes of Fisk (1971), Fat-r and Adams (1984), Curtiss ef al. (1985), and Smith and Whalley (1988).

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 concentration and precipitation at the rock’s surface. Elvidge and Iverson (1983) present a purely physicochemical model of varnish formation involving pH fluctuations at the varnish 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 oxidize Mn as part of their life processes.

GEOGRAPHIC SETTING OF THE SAMPLING SITE

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.

The rocks of the area are predominantly

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 contain almost no organic matter (<0.2% organic carbon; J. Noller, personal communication, 1988).

METHODOLOGY

The samples were collected from stable desert pavements on the alluvial fans. The cobbles with the most even varnish development were selected for general observations and chemical analyses. These cobbles were oblate, approximately 7 to 15 cm in diameter and 2 to 4 cm thick, and had flat but uneven surfaces. The varnished surfaces were generally 1 to 3 cm above the soil line on the clast. An AMR 1000 scan- ning electron microscope equipped with an energy dispersive X-ray analysis system (SEM/EDS) was used for identification and description of the varnish components. A KEVEX 8000 computer system converted EDS spectra to semiquantitative (standard- less) results under a l-km beam at 20 kV accelerating voltage. Both backscattered electron imaging (BSI) and secondary electron imaging (SEI) were used in obtaining photomicrographs. Polished thin-sections were mounted in epoxy and all samples were carbon-coated.

It was not possible to collect samples of eolian dust. As a substitute, peds from the vesicular A-horizon were collected from the surface at each varnish collection site. Because similar vesicular deposits are found in pits on varnish surfaces, it seems likely that this material is representative of the eolian dust normally in contact with varnish surfaces. In the laboratory these soil samples were sieved together through a 120~pm polypropylene mesh.

In nature, the eolian material on a rock surface is soaked periodically in CO,- buffered rainwater or dew solutions. The

118 CHARLES E. JONES

following experiments were performed to test the ability of these mildly acidic solutions to release Fe and Mn from clay mineral 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.

It was thought that the initial concentration of clay minerals on a varnish surface might affect the efficiency of the buffering solution and the amount of Fe and Mn released 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 1000°C to remove the water from the clays. About 6.5 wt% of water 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%.

DESCRIPTION OF PERUVIAN TWO-COMPONENT ROCK VARNISH

Peruvian rock varnish consists of two components: a thin (co.1 rJ,m) Fe-rich (lO-20%), Mn-poor (~3%) coating that cov-

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 rock’s surface and is found mostly in depressions and along cracks. In addition, a thriving community of microcolonial fungi and bacteria inhabits these varnish surfaces.

Mn-Poor Component

The Mn-poor component varies in color according to the degree of varnish development, ranging from rust-orange to dark reddish-brown to deep reddish-purple (Mun- sell’s 7.5 YR 515 to 5 YR 516 to 2.5 YR 313, respectively). Under a binocular microscope it appears resinous and has a Moh’s hardness of approximately 6. The SEM reveals 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 imaging normally allows varnishes in thin- section to be clearly distinguished from their substrates by highlighting the compo- sitional contrasts between varnish and substrate. In this case, however, the varnish is so thin (estimated at ~0.1 pm) that it is often barely visible.

Chemical characterization of this component is difficult. The hardness and thinness of the varnish make it difficult to remove mechanically without significant contamination from the substrate. SEM/EDS spot analyses perpendicular to the varnish surface 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 chemistry 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,/


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.

A&O, ratio of the substrate, it appears that there are only minor amounts of clay minerals (SiO,/Al,O, ratios of 2: 1 to 1: 1) in the varnish.

The inadequacy of the chemical analyses makes classification of this Fe-rich varnish tentative. Some Fe-rich varnishes, apparently 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.

Mn-Rich Component

The Mn-rich component occurs as an

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 surrounding 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 botryoidal, relatively continuous intrapit layering that locally gives way to a series of oxide mounds surrounded by detrital materials (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).


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.

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.

Biological Component The varnish surfaces are inhabited by a

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

found on both varnished and unvarnished rock surfaces and in appearance are somewhat 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 adsorb 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, immediately surrounding individual MCF specimens.

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


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 individual bacteria.

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 varnishes commonly found in semiarid to arid environments. It therefore seems reasonable to conclude that observations and gen- eralizations based on Peruvian Mn-rich varnish should help explain the formation of Mn-rich varnishes in general. The Mn-poor component appears to have analogs described 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 varnishes of Oberlander (1982), Smith and Whalley (1988), and this study; and the nearly pure silica glazes of Fisk (1971), Fat-r

EXPERIMENTAL RESULTS

The results from the H,SO, treatment of the clays suggest a bulk Mn:Fe ratio of 153. The X-ray fluorescence results give 6.11 wt% Fe and 0.13 wt% Mn. This gives a bulk Mn:Fe ratio of 1:47. For this paper it will be assumed that a Mn:Fe ratio of 150 is appropriate for the surrounding Peruvian soils as well as the eolian material supplied to the varnish surfaces.

To simulate the effects of dew or fog moistening eolian materials on a varnishing surface, soil material from the alluvial fan surfaces was placed in contact with CO*- charged water for 22 to 213 hr. Figure 3 shows the amount of Fe and Mn released into solution as a function of time in solution. The concentration of Fe generally re- mains constant at 0 ppm. The two excep- tions, at 96.5 and 213 hr of soaking time, are probably due to contamination from a faulty autopipet. The concentration of Mn,

1.0

0.8

0.6

0 40 80 120 160 200

Time (hours)

FIG. 3. Amounts of Fe and Mn released into solution as a function of time. The squares represent Fe and the circles represent Mn concentrations (ppm). The analytical errors are approximately ?5%.


on the other hand, shows an exponential increase with time. This curve may be described by

[Mn] = 0.135e0.1’9’ (r = 0.97),

where [Mn] is the concentration of Mn in ppm and t is time in hours. Thus, even though the bulk Mn:Fe ratio of the oxides sorbed onto the Peruvian soil mineral surfaces is Mn:Fe = 150, no Fe is released into solution over the time interval studied, while the amount of Mn released increases exponentially with time.

Figure 4 shows the effect of an increase in the amount of soil per unit volume of solution on the amount of Mn released into solution. The IO-fold increase in soil concentration, from 50 to 500 g/liter, does not hamper the buffering action of the dissolved CO2 and results in a ca. 50% increase in Mn concentration (e.g., from 0.096 to 0.14 ppm). Again, no Fe was released into solution.

FORMATION OF ROCK VARNISH

Most recent researchers agree that the components of rock varnish are externally derived and are transported to varnish surfaces via eolian activity (e.g., Potter and

Rossman, 1977; Perry and Adams, 1978; Elvidge and Iverson, 1983; Dorn and Ober- lander, 1982). Also, it is agreed that detrital clay minerals are “fixed” to the rock surface through the precipitation of Fe and Mn oxyhydroxides. Because there is no evidence for crystallinity on most varnish surfaces down to the 100-A resolution level of the SEM (Potter and Rossman, 1977), it appears that only the smallest clay particles are incorporated into the varnish. While isolated pockets of visible detritus do occur, the bulk of the detrital material is generally blown away and recirculated by the wind. The debate concerning the formation of rock varnish historically concerns two problems: (1) how to enhance Mn relative to Fe from an average crustal Mn:Fe ratio of 1:59 (Krauskopf, 1967) to varnish ratios commonly in excess of Mn:Fe = 1: 1, and (2) how to precipitate Mn under the weakly acidic to weakly alkaline surface conditions found in most regions in which Mn-rich varnish is common. In addition, the results of this study suggest a third problem: if the CO,-buffered solutions are not acidic enough to release Fe into solution, where does the Fe in rock varnish come from?

Two main models are proposed to over-

024 -

020 -

016 -

z

8 5 012 -

0.08 -

004 -

000 1 1 1 I I I I

0 10 20 30 40 50 60

Time (hours)

FIG. 4. Amounts of Mn released from solutions containing 50 and 500 g clay/liter (lower and upper lines, respectively). No Fe was detected in this experiment. Analytical errors are approximately 25%.


come the problems of enhancement and precipitation of Mn in rock varnish. The physicochemical model of Elvidge and Iverson (1983) is based on the following reasoning: Mn is considerably more soluble than Fe at the Eh and pH conditions found at the Earth’s surface (e.g., Krauskopf, 1967). Thus, when rain or dew wets a rock surface dusted with clay minerals, the slightly acidic waters (pH -5.7 in equilib- rium with atmospheric CO,) will release Mn and Fe into solution with a Mn:Fe ratio appropriate for desert varnish. As the water evaporates and the clays buffer the solution to make alkaline values, Mn and Fe will precipitate and thus fix a proportion of the clays onto the rock surface.

Dorn and Oberlander (1981a) note an im- portant objection to the physicochemical model: Mn-rich varnishes are also found in humid environments having only weakly acidic to neutral conditions that would be unable to produce the alkalinity required to precipitate Mn. Instead, Dorn and Ober- lander propose a biological model in which bacteria act as the sole agents of Mn concentration and precipitation. In support of this model they cultured bacteria taken from natural varnish surfaces and produced an artificial Mn-rich varnish that was similar chemically, mineralogically, and mor- phologically to natural varnishes. Taylor- George et al. (1983) and Palmer et al. (1985) have also isolated Mn-oxidizing bacteria from a variety of varnish surfaces but were unable to produce artificial varnishes.

In this paper I propose a model that uses physicochemical processes to create a varnishing solution with an enhanced Mn:Fe ratio and biological processes to precipitate and possibly influence the Mn:Fe ratio of the material incorporated into the varnish.

Enhancement of Mn over Fe The first problem is to explain the con-

siderable enhancement of the Mn:Fe ratio from an average crustal value near I:59 (Krauskopf, 1967) to varnish Mn:Fe ratios

that regularly exceed 1: 1 (Dorn, 1984, 1988, 1990). I begin with the physicochemical process responsible for the high Mn:Fe ratios of the varnishing solution.

The two experiments described above were carried out to test the ability of clays to provide Fe and Mn under the mildly acidic conditions of water buffered with atmospheric CO,. The bulk Mn:Fe of the Pe- ruvian soil samples is approximately 1:50, which is close to the average crustal value of Mn:Fe = I:59 (Krauskopf, 1967) and to the 1:60 ratio found by Elvidge and Iverson (1983) for dust samples in Arizona. In a CO,-buffered solution the abundant Fe oxyhydroxides are apparently impervious to carbonic acid attack, while the much less abundant Mn oxyhydroxides steadily release Mn as a function of the length of time in solution. This implies not only that natural physicochemical processes can efficiently fractionate Mn relative to Fe, but also that there may be some difficulty in getting dissolved Fe into the varnishing solutions. The problem of Fe in varnish will be discussed below.

The existence of Mn-rich varnishing solutions is consistent with observed varnishes. In the Peruvian case, the varnish clasts are flat-lying cobbles with somewhat irregular horizontal surfaces. When a heavy dew or fog allows moisture to accumulate on the rock surface, the clays release their Mn into solution and the solution accumu- lates in the pits and depressions. When the water evaporates, the Mn may precipitate either biologically or physicochemically. Because the pits on the Peruvian cobbles are commonly 1 mm deep and l-2 mm in diameter, it is likely that once dissolved material precipitates in a pit, it will remain in that pit. Thus, one would expect pits to contain Mn-rich varnishes, and as we have seen, the Peruvian pit varnishes are Mn- rich (lO-20%) with high Mn:Fe ratios (generally between 1:2 and 3:l and higher).

By contrast, it appears that biological fractionation has little control over the


Mn:Fe ratio of the varnishing solutions. If we assume that bacteria do not dissolve the Mn or Fe oxyhydroxides sorbed on eolian mineral surfaces, that bacteria are able to fractionate Mn only by selective precipitation from the varnishing solution, and that no physicochemical process fractionates Mn relative to Fe, then the Mn:Fe ratio of the varnishing solutions should reflect the ratios of the ambient material, i.e., Mn:Fe = 150 to 1:60. Thus, when bacteria selec- tively pull Mn out of a varnishing solution to produce an Mn-rich varnish, an Mn- depleted solution is left behind to flow into the pits and depressions. Evaporation of the water in the pits should then produce an Mn-poor varnish, with an Mn:Fe ratio at least as low as 150, thus contradicting the observed Mn-rich pit varnishes.

Thus, it appears likely that the physicochemical process described above is the dominant mechanism enhancing the Mn:Fe ratio of the varnishing solutions. Bacterial enhancement of this ratio is possible only if bacteria on varnish surfaces are somehow able to attack the Mn oxyhydroxides sorbed on mineral surfaces.

Precipitation of Mn

While the ability of bacteria from natural varnish surfaces to precipitate Mn is well demonstrated (Dorn and Oberlander, 1981a; Taylor-George et al., 1983; Palmer et al., 1985), it is not known what proportion of the Mn, if any, is precipitated physicochemically. It is therefore appropriate to discuss some indirect evidence provided by the Peruvian varnish that bears on this point.

The Peruvian environment is ideally suited for the physicochemical precipitation of Mn. First, the Peruvian soil material is highly alkaline; a pH of 9 or more is readily obtained when 50 g of soil is placed in 100 ml of deionized water. Mn is known to precipitate rapidly at pH >8.5 and at slower rates at lower pH values (Morgan et al., 1985; Bartlett, 1986). Thus, in Peru the

model of Elvidge and Iverson (1983) could work: a slightly acidic dew in contact with clay minerals will release some Mn into solution. As the dew evaporates, the alkaline Peruvian material shifts the pH of the solution up to 9 or more, forcing the Mn to precipitate.

This process is facilitated by the highly catalytic Fe and Mn oxyhydroxides that cover Peruvian rock surfaces as components of rock varnish. These catalysts dra- matically accelerate the rate of Mn precipitation by incorporating Mn2+ into the surface matrix of disordered oxyhydroxides (Hem, 1978; Wilson, 1980; Crerar, 1980; Morgan et al., 1985; Rott, 1985). This facil- itates the precipitation of Mn at lower pH values by giving the captured Mn2+ time to oxidize without being carried away by excess moisture.

Despite these favorable conditions, the evidence suggests that physicochemical precipitation of Mn is insignificant. Some 75-95% of the Peruvian varnish surface is covered with the Mn-poor component, which is rich in catalytic Fe oxyhydroxides (10-20 wt% Fe). The eolian materials in Peru will make this varnishing surface quite alkaline. If physicochemical oxidation of Mn occurs, then one would expect these alkaline, catalytic surfaces, over which the Mn-rich varnishing solutions flow, to be rich in Mn oxides. Instead, they contain <3 wt% Mn, implying that physicochemical precipitation of Mn is at best a minor process, especially considering that even this Mn may be due to bacterial oxidation.

In contrast to the Peruvian case, most Mn-rich varnishes are found in semiarid to arid environments and a few develop under humid conditions (Dorn and Oberlander, 1981a). The soils in these environments generally produce weakly acidic to weakly alkaline conditions on the rock surfaces (Dorn and Oberlander, 1982). Under these pH conditions the physicochemical precipitation of Mn, if it occurs at all, will occur very slowly (Morgan et al., 1985). Thus, if


inorganic precipitation of Mn is insignili- cant under the alkaline conditions in Peru, it is almost certainly insignificant under the more acidic conditions in which most Mn- rich varnishes form.

Biological Enhancement of the Mn:Fe Ratio

Because Mn-precipitating organisms form the link between a varnishing solution and the final varnish product, it seems clear that they should have a role in determining the final Mn:Fe ratio of a varnish layer. Dorn and Oberlander (1981a, b, 1982) used SEM/EDS spot analyses to compare average varnish surfaces with varnish deposited immediately around visible bacteria. While all EDS spectra show considerable concentration of Mn and Fe immediately surrounding the bacteria, the relative enhancement of Mn and Fe appears to vary. Some spectra show a clear enhancement of Mn over Fe (Dorn and Oberlander, 1981b, Fig. 2 (top); Dorn and Oberlander, 1982, Figs. 2c, 3d, 4c) while others imply little or no change in the Mn:Fe ratio (Dorn and Ober- lander, 1981a, Fig. Ic; Dorn and Ober- lander, 1982, Fig. 3b) or even an enhancement of Fe relative to Mn (Dorn and Ober- lander, 1981b, Fig. 2, bottom). Thus, at present it appears that biological controls on the Mn:Fe ratio in varnish may vary, perhaps according to the composition of bacterial communities and the environmental conditions under which they live. More work clearly needs to be done to elucidate the possible ranges of biological control on the final Mn:Fe ratios of rock varnish.

Fe in Rock Varnish The results of the experiments in this pa-

per suggest that while carbonic acid solutions can release Mn into solution, they do not release Fe into solution. This presents the problem of the source of Fe in rock varnish. One possibility is that clay minerals in rock varnish have enough Fe oxyhydroxides sorbed onto their surfaces to account for the -8-20% Fe content of rock var-

nishes. A second possibility is that carbonic acid is not the only solvent in most varnishing solutions. Humic and other organic acids may be quite effective in releasing Fe into solution. There is some evidence for both possibilities.

Mn-rich rock varnishes generally contain roughly 70% clay minerals and between -8 and 20% Fe (Potter and Rossman, 1977; Engel and Sharp, 1958; Hooke et al., 1969; Potter and Rossman, 1979; Smith and Whalley, 1988). If all of the Fe is associated with clay minerals, then the clay minerals alone would contain about IO-22% Fe. Is this more Fe than is normally associated with clay minerals? An XRF analysis of the Peruvian soil used in the experiments above shows that the <38-Fm size fraction contains 6.11 wt% Fe. For smaller size fractions, the number of broken bonds at clay mineral edges increases with decreasing particle size. Thus, we might expect the relative proportion of absorbed elements, such as Fe, to increase with decreasing particle size (Corstea, 1968). In a study on the amount of Fe that different size fractions of montmorillonite clays can adsorb, Corstea (1968) found that the 0.2- to 2-km fraction can contain - 18 wt% adsorbed Fe while the <0.2-km fraction can contain -20 wt% Fe. While these results are not necessarily representative of naturally occurring Fe on clays, they do suggest that it is reasonable for most if not all Fe in rock varnish to be adsorbed onto the surfaces of clay minerals.

Some evidence also exists for dissolved Fe in varnishing solutions. The Fe-rich, Mn-poor bottomcoat varnishes analyzed by Potter and Rossman (1979) appear to be similar to Mn-rich topcoat varnishes except that they do not contain much Mn. Without the Mn oxides fixing the clay minerals to the rock surface, it appears necessary to have Fe oxides acting as the “glue.” This implies that, like the Mn oxides of the topcoat, the Fe oxides precipitated out of aqueous solution and thus fixed the clay minerals onto the rock surface. As a second


line of evidence, the SEM/EDS spot analyses of Dorn and Oberlander (1981a, b, 1982; see discussion above) imply bacterial concentration of Fe, which suggests either that bacteria actively precipitate (or coprecipi- tate with Mn) dissolved Fe from solution or that the freshly precipitated Mn oxyhydroxides associated with bacteria are able to scavenge Fe oxyhydroxide particles sus- pended in the varnishing solution. As a third line of evidence, the Peruvian Mn- poor varnish, which covers 7595% of a varnished surface, shows relatively high concentrations of Fe compared to the substrate. This suggests that some Fe has precipitated from the varnishing solutions that flow across the rock surface. The extreme thinness of this component (co.1 pm) implies that the amount of dissolved Fe is quite small, especially compared with the amounts of Mn required to produce the -lOO-pm-thick Mn-rich varnish component.

In conclusion, a large proportion of the Fe found in Mn-rich rock varnish may be in the form of Fe oxides associated with the clay minerals in the varnish. The amount of Fe in varnish that precipitated from a varnishing solution is difficult to constrain. The estimates range from the very small amounts required to form the Peruvian Mn- poor varnishes to the larger amounts apparently required to form the orange bottomcoat varnishes. Future work should verify the amount of Fe found in the <0.2+m size fractions of natural eolian materials and look into possible sources of dissolved Fe.

INTERPRETATION OF THE MN:FE RATIO IN ROCK VARNISH

Variations in the Mn:Fe ratio through successive layers of rock varnish are interpreted by Dorn (1984, 1990) as recording fluctuations in paleoalkalinity levels. This interpretation is thought to be independent of the two competing models of varnish formation. Under the biological model, if bacteria are inhibited by alkaline conditions,

they will precipitate less Mn per unit time and form a relative Mn-poor varnish layer. Under the physicochemical model, highly alkaline conditions are thought to inhibit the pH-Eh fluctuations necessary to con- centrate Mn, and again a Mn-poor varnish will result (Dorn, 1990). The alkalinity fluctuations are thought to be tied to the abun- dance of alkaline aerosols, which in turn is related to the degree of plant cover over dusty soils and to the flooding or desicca- tion of playa lakes (Dorn, 1990). Thus, the variations of the Mn:Fe ratio in rock varnish may ultimately be tied to fluctuations between warmer-drier climates, with re- duced plant cover and more exposed alkaline playa sediments, and cooler-wetter climates, with increased plant cover and plu- vial lakes (Dorn, 1984, 1990).

The results of this study suggest the following model for Mn-rich rock varnish formation. The enhancement of Mn relative to Fe may be effectively carried out by natural fogs or dews coming into contact with the eolian clay minerals present on rock surfaces. These waters are slightly acidic, with pH -5.7 due to dissolved atmospheric C02, and are able to release Mn into solution without releasing Fe. The amount of Mn in solution increases exponentially according to the amount of time the clay minerals spend in contact with the CO,-water solution. While carbonic acid does not appear to release Fe into solution, in nature vari- ous organic acids may act as more effective solvents.

The precipitation of Mn and any dissolved Fe fixes clay minerals and other very small detrital particles to the rock’s surface. The amount of Mn precipitated from solution is bacterially controlled only where varnishing solutions can flow freely over a varnish surface without forming puddles that evaporate to dryness. Where varnishing solutions do accumulate and evaporate to dryness, the Mn:Fe ratio of the re- sultant varnish is independent of the precipitation mechanism and is instead a function of the aforementioned fraction-


ation processes, the amount of clay in the depression, and the size of the “drainage basin” supplying Mn to the depression. Be- cause the physicochemical precipitation of Mn appears to be minimal even on the highly alkaline and catalytic Peruvian varnish surfaces, it is assumed to be an unim- portant process for all varnishes forming under less alkaline conditions. The clay minerals and detrital material not incorporated into the varnish are blown or washed away.

A large proportion of the Fe in rock varnish appears to be associated with the clay minerals in the varnish. Thus, one would expect Fe concentrations to parallel a geo- chemical indicator of clay mineral concentrations such as Al,O,. An unknown proportion of Fe in varnish, ranging from small amounts in the Peruvian topcoat varnish to apparently large amounts in the orange bot- tomcoats of the Western United States, may have precipitated from solution. The controls on dissolved Fe in varnishing solutions are at present unknown.

This model offers an interpretation of Mn:Fe ratios similar to that of Dorn (1984, 1990). During wetter times eolian clay minerals spend more time in contact with dew and fog, thus increasing the amount of Mn put into solution. At the same time, wetter soils, increased plant cover, and flooded playa lakes decrease the total amount of alkaline eolian material settling on varnish surfaces. Less alkaline surface conditions favor Mn-precipitating bacteria (Dorn and Oberlander, 1981a), thus allowing them to take full advantage of the increased levels of dissolved Mn in the varnishing solutions. A reduction in circulation of eolian clays may also reduce the amount of clay minerals incorporated into the varnish, thus re- ducing the amount of Fe in the varnish as well. High Mn production and precipitation plus less Fe incorporation yields a higher varnish Mn:Fe ratio. Conversely, drier conditions reduce the contact time between clay minerals and varnishing solutions, inhibit Mn-precipitating bacteria through in-

creased deposition of alkaline eolian materials, and increase the amount of Fe- bearing eolian clay minerals incorporated into varnish. A low Mn:Fe ratio results.

An interesting problem exists regarding the relationship between the supply of dissolved Mn and the population of varnish bacteria. If varnishing solutions, assumed to be physicochemically derived, always supply an excess of Mn to the bacteria, then the amount of Mn incorporated into varnish is controlled by the total amount of Mn a given population of bacteria can precipitate. The Mn:Fe ratio is thus largely biologically controlled. Conversely, if the population is limited by the supply of Mn, then the Mn:Fe ratio will be controlled largely by physicochemical processes. Climatic conditions that result in higher Mn production will support a larger bacteria population, which in turn will precipitate more Mn and produce a Mn-rich varnish. Environ- mental conditions that restrict the supply will result in a smaller population precipitating less Mn and thus produce an Mn- poor varnish. In this situation the bacteria merely respond to changes in the supply of Mn brought by environmental changes forcing a physicochemical mechanism of Mn production.

CONCLUSION

Mn:Fe ratios in rock varnish potentially offer a useful means of obtaining Quater- nary paleoenvironmental information. However, before these ratios can be widely interpreted with confidence, it is essential to understand in detail what controls Mn and Fe deposition in varnish, particularly as regards changing climatic and environmental conditions. Dorn (1984, 1990) has presented a general model relating climate, environmental change, and Fe and Mn deposition in varnish. Moreover, in support of the model, he has provided empirical cor- relations between Holocene varnish Mn:Fe ratios and such proxy environmental indi- cators as soil pH, aridity indicies, local wa-


ter balances, and the 6i3C of ambient plant matter (Dorn, 1990). This paper, in making a first step toward a more detailed under- standing of Fe and Mn deposition in rock varnish, has raised more questions that it has answered. It is hoped that this paper may stimulate renewed interest in the geochemistry of rock varnish.

ACKNOWLEDGMENTS

I thank T. H. van Andel, G. E. Brown, J. Stebbins, L. E. Wells, J. Noller, and N. W. Jones for their ad- vice and criticism of this work and R. I. Dorn and J. B. Adams for greatly improving the final drafts of this manuscript. I thank S. Fultz for her assistance in identifying the microcolonial fungi and D. GrbiC-GaliC for her help in identifying the varnish bacteria. Fund- ing came from a Firestone Major Grant, administered through Stanford University, and from the Stanford School of Earth Sciences.

Note added in proof. A recent paper by Behra and Sigg (1990) indicates that fog can contain significant quantities of dissolved Fe ‘+ Fog therefore may represent a source of dissolved Fe for varnish surfaces.

REFERENCES Bartlett, R. (1986). Soil redox behavior. In “Soil Phys-

ical Chemistry” (D. L. Sparks, Ed.). CRC Press, Boca Raton, FL.

Behra, P., and Sigg, L. (1990). Evidence for redox cycling of iron in atmospheric water droplets. Na- ture (London) 344, 41w21.

Corstea, D. D. (1968). Formation of hydroxy -Al and -Fe interlayers in montmorillonite and vermiculite: Influence of particle size and temperature. Clays and Clay Minerals 16, 231-238.

Crerar, D. A., Cormick, R. K., and Barnes, H. L. (1980). Geochemistry of Mn: An overview, In “Ge- ology and Geochemistry of Manganese” (I. M. Var- entsov and G. Y. Grasselly, Eds.), Vol. I. E. Schweizerbart’sche Verlagsbuchhandlung, Stutt- gart.

Curtiss, B., Adams, J. B., and Ghiorso, M. S. (1985). Origin, development, and chemistry of silica- alumina rock coatings from the semi-arid regions of the island of Hawaii. Geochimica et Cosmochimica Acta 49, 49-56.

Dorn, R. I. (1984). Cause and implications of rock varnish microenvironmental laminations. Nature (Lon- don) 310, 767-770.

Dom, R. I. (1986). Rock varnish as an indicator of aeolian environmental change. In “Aeolian Geo- morphology” (W. G. Nickling, Ed.), pp. 291-307. Allen and Unwin, London.

Dorn, R. I. (1988). A rock varnish interpretation of

alluvial fan development in Death Valley, Califor- nia. National Geographic Research 4, 56-73.

Dom, R. I. (1989). A comment on “A note on the characteristics and possible origins of desert varnishes from southeast Morocco” by Drs. Smith and Whalley. Earth Surface Processes and Landforms 14, 167-170.

Dorn, R. I. (1990). Quaternary alkalinity fluctuations recorded in rock varnish microlaminations on western U.S.A. volcanics. Palaeogeography, Palaeocli- matology, Palaeoecology, 76, 291-310.

Dorn, R. I., DeNiro, M. J., and Ajie, H. 0. (1987a). Isotopic evidence for climatic influence on alluvial- fan development in Death Valley, California. Geol- ogy 15, 108-I 10.

Dorn, R. I., and Oberlander, T. M. (1981a). Microbial origin of desert varnish. Science 213, 1245-1247.

Dorn, R. I., and Oberlander, T. M. (1981b). Rock varnish origin, characteristics, and usage. Zeitschrift

fur Geomorphologie 25, 42M36. Dorn, R. I., and Oberlander, T. M. (1982). Rock var-

nish. Progress in Physical Geography 14, 308-322. Elvidge, C. D., and Iverson, R. M. (1983). Regenera-

tion of desert pavement and varnish. In “Environ- mental Effects of Off-Road Vehicles” (R. H. Webb and H. G. Wilshire, Eds.), pp. 225-243. Springer- Verlag, New York.

Engel, C. G.. and Sharp, R. P. (1958). Chemical data on desert varnish. Geological Society of Americu Bulletin 69, 487-518.

Farr,T. G., and Adams, J. B. (1984). Rockcoatings in Hawaii. Geological Society of America Bulletin 95, 1077-1083.

Fisk, E. P. (1971). Desert glaze. Journal of Sedimen- tary Petrology 41, 1136-l 137.

Ghiorse, W. C., and Balkwill, D. L. (1983). Enumer- ation and morphological characterization of bacteria indigenous to subsurface environments. In “Devel- opments in Industrial Microbiology,” Vol. 24, Pro- ceedings of the 39th General Meeting of the Society for Industrial Microbiology, St. Paul, MN, Aug. 1982.

Glasby, G. P., McPherson. J. G., Kohn, B. P., Johnston, J. H., Keys, J. R., Freeman, A. G., and Tricker, M. J. (1981). Desert varnish in southern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics 24, 389-397.

Harrington, C. D., and Whitney, J. W. (1987). Scan- ning electron microscope method for rock varnish dating. Geology 15, 967-970.

Hem, J. D. (1978). Redox processes at surfaces of MnO, and their effects on aqueous metal ions. Chemical Geology 21, 199-218.

Hooke, R. LeB., Yang, H., and Weiblen, P. W. (1969). Desert varnish: An electron microprobe study. Journal of Geology 77, 275-288.

Johnston, J. H.. Cardile, C. M., Coote, G. E., Sparks, R. J., Wallace, G., McMillan, J. W., Pum-


meny, F. C., and Longworth, G. (1984). Desert varnish in Antarctica: A nuclear microprobe and backscattered iron-57 Mossbauer spectroscopic study. Chemical Geology 42, 189-201.

Johnston, J. H., and Cardile, C. M. (1984). The characterization of the iron oxide phase in desert varnish from Antarctica using backscattered conversion electron and X-ray Mossbauer spectroscopy. Chem- ical Geology 45, 73-90.

Krauskopf, K. B. (1967). “Introduction to Geo- chemistry.” McGraw-Hill, New York.

Krumbein, W. E., and Jens, K. (1981). Biogenic rock varnishes of the Negev Desert (Israel): An ecologi- cal study of iron and manganese transformation by cyanobacteria and fungi. Oecologia 50, 25-38.

Morgan, J. J., Sung, W., and Stone, A. (1985). Chem- istry of metal oxides in natural waters: Catalysis of the oxidation of Mn(II) by gamma-FeOOH and re- ductive dissolution of Mn(II1) and Mn(IV) oxides. In “Environmental Inorganic Chemistry” (K. J. Ir- golic and A. E. Martell, Eds.). VCH, Deerfield Beach, FL.

Oberlander, T. M. (1982). Interpretation of rock varnish from the Atacama Desert. Association ofilmer- ican Geographers Abstracts, p. 311.

Palmer, F. E., Staley, J. T., Murray, R. G. E., Coun- sell, T., and Adams, J. B. (1985). Identification of manganese-oxidizing bacteria from desert varnish. Geomicrobiology Journal 4, 343-360.

Perry, R. S., and Adams, J. (1978). Desert varnish: Evidence of cyclic deposition of manganese. Nature (London) 278, 489-191.

Potter, R. M., and Rossman, G. R. (1977). Desert varnish: The importance of clay minerals. Science 196, 14461448.

Potter, R. M., and Rossman, G. R. (1979). The manganese- and iron-oxide mineralogy of desert varnish. Chemical Geology 25, 79-94.

Rott, U. (1985). Physical, chemical, and biological as- pects of the removal of Fe and Mn underground. Water Supply 3, 143-150.

Smith, B. J., and Whalley, W. B. (1988). A noteon the characteristics and possible origins of desert varnishes from southeast Morocco. Eurth Surface Pro- cesses and Landforms 13, 251-258.

Staley, J. T., Jackson, M. J., Palmer, F. E., Adams, J. B., Borns, D. J., Curtiss, B., and Taylor-George, S. (1983). Desert varnish coatings and microcolonial fungi on rocks of the Gibson and Great Victoria Deserts, Australia. BMR Journal of Geology und Geophysics 8, 83-87.

Staley, J. T., Palmer, F., and Adams, J. B. (1982). Microcolonial fungi: Common inhabitants on desert rocks? Science 215, 1093-1095.

Sung, W., and Morgan, J. J. (1980). Kinetics and prod- ucts of ferrous iron oxygenation in aqueous sys- tems. Environmental Science and Technology 14, 561-568.

Taylor-George, S., Palmer, F. E., Staley, J. T., Borns, D. J., Curtiss, B., and Adams, J. B. (1983). Fungi and bacteria involved in desert varnish formation. Microbial Ecology 9, 227-245.

Wells, L. E. (1987). An alluvial record of El Niiio events from northern coastal Peru. Journal of Geo- physical Research 92 (Cl3), 14,463-14,470.

Wilson, D. E. (1980). Surface and complexation effects on the rate of Mn*+ oxidation in natural waters. Geochimica et Cosmochimica Acta 44, 131 l- 1317.

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