Preservation of Human Tracks in Arid Salina Environments

Observations detail modern human track preservation in arid salina settings where sulfate crust formation modifies track morphology.

Introduction

Until recently, the record of fossil to subfossil human footprints has been largely neglected. Despite anecdotal reports of human tracks from Mexico and elsewhere in Central and North America, documentation of sites has been sparse at best. Some historical reports even suggest that collected samples, mainly from the central Mexico Basin, are now missing (Avaleyra-Arroyo de Anda, 1950). Other reports include the possible existence of human tracks from the state of Jalisco (Rodríguez-de la Rosa et al., 2004) and Quevedo-Lara (1998) mentioned human tracks in association with equid tracks from sedimentary rocks in the state of Oaxaca. Lockley et al. (2008) also mention sites in Guerrero, Chihuahua and Sonora.

Due to this poor record, the authors’ attention was drawn to two lithified hominid-track-bearing slabs of tufa curated by the Museo Regional de La Laguna and on display at the Museo del Desierto in Saltillo, Coahuila (Gonzalez et al., 2009). Although known to have been found in the early 1960s in the Municipio de Cuatro Cienegas (Rodríguez-de la Rosa et al., 2004), the precise locality of origin was unknown at the time our search began. Similarly, the purported age of the tracks as (∼10,000 B. C. E) was unconfirmed.

As a result, in the spring of 2005, the authors attempted to relocate the site. Although this effort was unsuccessful, partly because local residents claimed to recollect the discovery site being in a salina area, we were nevertheless able to make observations on the preservation of recently made tracks in salt pan deposits near Cuatro Cienegas (Fig. 1). Subsequently, in the spring of 2006, the senior author, working with another team from the Museo del Desierto in Saltillo, Coahuila, was successful in locating hominid tracks in lithified tufa deposits at another locality nearby (Gonzalez et al., 2006a, 2006b, 2009). By this time, a first draft of this paper on the salina tracks had been submitted for review. Our brief study provides us with the opportunity to outline a distinct mode of preservation, in a depositional setting quite different from that of the track-bearing tufa deposits, but within the same localized geographic area.

To the best of our knowledge, there have been no previous descriptions of modern human tracks preserved in association with salt precipitates in saline substrates.

Materials and Methods

Recently made human tracks were located in an arid area known as Los Mesquites, approximately 5 km southwest of the city of Cuatro Cienegas, in Central Coahuila, Mexico (Fig. 1). The site is located at latitude 26◦55.373′ North and longitude 102◦06.344′ West. A single human trackway composed of several individual tracks was observed and at least five footprints were cast with plaster; a single latex mould was also taken on site (Fig. 2).

We had not previously attempted to make a latex mold of tracks in gypsum, but the method worked well, and the latex dried quickly as a result of a combination of wind, warm air temperatures, and the astringent qualities of the gypsum, which evidently helped draw moisture from the latex. Likewise, we had not previously tried to cast tracks in gypsum with plaster of Paris; this also worked well. The plaster, while fluid, was not so saturated as to dissolve the salt crust, and after it had set up, we were able to remove the gypsum layer (only a few mm in thickness) by washing it off in the lab.

We collected one track by trenching around it and removing it as a block of sediment, with the latex mold still in place on the surface. When the mold was removed, as was done with the plaster casts, the outline of the underprints below the salt crust was clear and the good quality of preservation easily seen.

Results

A single human trackway made by a barefoot individual was located at Los Mesquites (Fig. 2A); the steps are 70 cm in length from heel to heel and 72 cm from the tip of toe I to the same point in the next footprint. The tracks average 25 cm in length and 10.5 cm in width (Figs. 2B, C).

The substrate in the study area is covered by a thin white, crystalline crust of gypsum. Most of the substrate immediately beneath this crust is of a homogeneous texture and of a light brown-yellowish color; it is composed of a mixture of sand and a high percentage of sulfate crystals with a preponderance of gypsum. Saturation levels of the sand are also very high as the evaporite minerals evidently act as an impermeable cap that prevents further evaporation. This role of salt as an impermeable barrier to fluid migration is well known in petroleum geology and has been cited in many standard texts.

The uppermost layers (top few centimeters) are a much darker brownish color. Within this layer and close to the surface there is a darker gray-black layer one centimeter thick and rich in organic material. Close to the substrate surface there is a very thin green lamina suggesting the proliferation of algae. These most superficial centimeters suggest the accumulation of water that permitted the concentration of organic material as well as the growth of the algae at or near the substrate/salina surface.

In regard to preservation, the tracks were made at the site when the substrate was very saturated and cohesive. Thus, the preservation of the tracks is very clear, showing all diagnostic features (heel, arch, ball, big and little toes; Fig. 2). However, due to the arid conditions and evapotranspiration of the water, a white surface crust evidently formed soon after the time when the individual walked in the area leaving the tracks.

Within the track, the surface growth of sulfate crystals (or surface crust) was modified by the pressure caused by the foot’s registration on the substrate. Thus, the more compacted areas beneath the big toe, ball, and heel of the foot are smoother than the surrounding area (Figs. 2B, C). This suggests that evaporite crystals grew more slowly in these areas where the sediment had been compacted, thus reducing local porosity and permeability.

Thus, crystal growth within the track occurred at a slower rate, whereas outside the track it proceeded at a higher rate. The result of this differential crystal growth is that the tracks, as seen in the crust, mirror the surface texture of the tracks seen in the underlying substrate; that is, within the track, the small crystals appear compacted, whereas in the intervening substrate the crystals are large and ‘coarse’ in appearance, reflecting the more irregular, uncompacted salina surface. Thus, despite the coating of gypsum crystals within the track, it is easy to see the preservation of distinctive track features such as well-defined toes, ball, and heel.

Discussion

All footprints cause compaction of the substrate in the area where they register, and in many cases, the increased compaction leads to enhanced preservation. This even occurs in snow when it becomes compacted into ice, allowing tracks to remain as ice pedestals long after the snow has melted or blown away (Lockley, 1991). Thus, compaction is a universal phenomenon associated with the formation of tracks. Even the Museo del Desierto specimens, which formed as a result of a quite different process—tufa mineralization—(Gonzalez et al., 2009), show enhanced preservation of the compacted areas. Preservation within the tufa tracks contrasts with the preservation of surrounding sediment which is much more deeply weathered.

Despite these universal similarities, each substrate is potentially different in texture, saturation, environmental location, and so forth. Generally speaking, salina or alkali flat deposits provide excellent environments for track formation. However, they do not necessarily represent good environments for long-term preservation. This is because constant remobilization of saline ground water by alternations of flooding and capillary action keep pore water in solution and do not allow the sediment to cement or lithify. This was demonstrated to the senior author during subsequent study (Gonzalez et al., 2009) when a 3 m trench was excavated quite near Los Mesquites (Fig. 3). All the sediment in this trench was unconsolidated sand with poor stratification and no sign of consolidated layers.

Forti and coworkers (2004) described the process of salt crust formation in Cuatro Cienegas. In this process, many of the minerals present in the soil of the Cuatro Cienegas area are slowly diffused by capillarity action due to evapotranspiration of the water. There is a progressive ionic concentration at the air-soil interface and sulfate crystals are precipitated with a clear preponderance of gypsum (CaSO4.2H2O). This crystal deposition progressively forms a superficial white crust composed mainly of minerals such as Bloedite [Na2Mg(SO4)2.4H2O], Epsomite (MgSO4.7H2O) and gypsum (CaSO4.2H2O). This crust growth soon limits the water evaporation, thus stopping the process (Forti et al., 2004). Just as ice prevents evaporation in cold areas, salt can play a similar role in arid areas. It is interesting to note that the sulfate crystal growth process becomes altered by the pressure caused by making footprints. This is a good example of a biogenic process (track formation) subsequently affecting a physical sedimentological process.

The white salt crust, however, remains soluble in the presence of abundant water (rain, condensation, or runoff). It could thus be partially or totally destroyed and the crust formation process could restart. In the long term, the alternation of crust formation and dissolution will deteriorate tracks at or near the surface, with adverse effects on long-term preservation potential. Surface gypsum, like other fine sediment, is also easily removed by wind deflation and may accumulate locally as dunes. In fact, the Cuatro Cienegas area is famous for its gypsum dunes, which are comparable to the equally famous White Sands dunes of New Mexico.

It is not impossible to preserve tracks in such environments. However, they may be poorly preserved in such circumstances. Lucas et al. (2002) reported several dozen mammoth and camel tracks preserved in a 20 cm thick “gypsite” layer in the Upper Pleistocene Otero Formation at the White Sands Missile Range in the Tularosa Basin of southern New Mexico. The Otero Formation represents deposition in Pleistocene Lake Otero during high stand conditions during the Late Wisconsinan. Lucas et al. (2002, p. 288) also noted that many of the tracks are poorly preserved and pedestalled, as a result of compaction of the gypsum, and that “extant oryx are now leaving similar pedestalled tracks on the alkali flats just north of the Pleistocene tracksite.” Due to prolonged and favorable precipitation conditions in Lake Otero, the gypsite layer is thicker than any noted in our observations of deposits in the Cuatro Cienegas area.

Conclusions

1. Salinas, also referred to as salt pans or alkali flats, provide favorable environments for the formation of vertebrate footprints.
2. However, the long-term preservation and lithification potential of tracks in such evaporite deposits is compromised by the alternation of flooding, dissolution, capillary evapotranspiration, and wind deflation.
3. Nevertheless, at least one fossil example of vertebrate tracks in consolidated Pleistocene deposits has been reported.
4. As in most other track-making situations, compaction caused by foot impact is also important in evaporite deposits, and leads to differential preservation of tracked and non-tracked surfaces.

Scientific Citation: Lockley, Martin G. and Rodríguez-de la Rosa, Rubén A. (2009) ‘Preservation of Human Tracks in Arid Environments’, Ichnos, 16:1, 98 — 102.

[GALLERY_CAPTIONS]
1. Locality map showing the Los Mesquites site, approximately 5 km southwest of Cuatro Cienegas in Central Coahuila, Mexico.
2. Modern human trackway preservation: (A) General view of the trackway in the salina; (B, C) Detailed views of individual footprints showing clear anatomical features and differential crystal growth between the compacted track and surrounding substrate.
3. Profile of a 3-meter excavation trench near Los Mesquites, illustrating the unconsolidated nature of the saline sand deposits.