Geología

CAVE SITES IN THE SIERRA DE ATAPUERCA

Introduction: Karst formation

Karst is the name of a region of the Alps which forms part of present-day Slovenia and gives its name to a characteristic type of relief. It is a calcareous terrain in which dissolving processes predominate over erosive processes, and where surface runoff of waters has been substituted by a rapid infiltration and subterranean circulation of these same waters, through fissures and cavities in the bedrock. By extension, the term karst has been adopted internationally to denote any region with similar characteristics. These generally occur in carbonated rocks, and principally in limestone or dolomite, although these processes can develop in other lithological settings such as ice, gypsum, salts, and even, in exceptional cases, in quartzite and other siliceous rocks.

The formation of the karst in this Mesozoic mountain at Atapuerca is characterized by the relative unimportance of superficial formations (exokarsts), presenting only modest lapiaces (areas where the rock forms thin vertical laminas) and small dolinas (subterranean chambers which open to the outside) located at the summit of the sierra and by the extensive development of subterranean formations (endokarsts). Large underground chambers, frequently surpassing 20 meters in height, eventually formed as the water level of the Arlanzón River, which marked the regional water table, gradually lowered, depositing its terraces in the river valley.

The karst system of Cueva Mayor-Cueva del Silo is located in the southern sector of the Sierra de Atapuerca. With 3,700 meters of subterranean passages, it constitutes one of the most important cave systems in the Duero River Basin. The lowering of the water table, and the forces generated by water action carved out subhorizontal chambers with no predominant orientation of the galleries. However, judging from the directions of the water circulation, an area of primary water emergence can be identified in the Cueva del Silo, situated at the beginning to the Valhondo valley, at the headwaters of the Pico River.

The galleries in these subterranean cavities are distributed in three clearly differentiated levels, marking the successive drops in the water table of the Arlanzón River, which gradually left the upper conduits inactive. Subsequent collapses and erosive processes caused the collapse of the roofs of these conduits and facilitated access to the interior of the caves by animals and humans since Lower Pleistocene times, at least one million years ago. From this moment, the entrances began filling, little by little, with sediments, branches and other surface materials brought in mainly through the action of surface water runoff and small scale flooding, until the cave mouths were completely sealed toward the end of the Middle Pleistocene (around 128,000 years ago). Some of the galleries are still accessible today, mainly through the Portalón of Cueva Mayor, by a small, narrow passageway which opens between the sediments and infilling, dating to the last 4,000 years, which are preserved in this entrance. The natural beauty of the galleries is due in no small part to the large number of spectacular stalagmites and stalactites present in the interior of the cave system. Unfortunately, the presence and activity of humans in the cave during more than 500 years has caused irreversible damage to this once pristine beauty.

The occupation and subsequent filling of the cave entrances has preserved traces of the past. The cave sites in the Sierra de Atapuerca contain one of the richest archeological and paleontological records documenting the course of human evolution in Europe during the Lower and Middle Pleistocene. The cave sites (Dolina, Galería and Elefante) that appeared during construction of the railway trench (“Trinchera del Ferrocarríl”), along with the Sima de los Huesos, span a time range from more than one million years ago until around 128,000 years ago. The present day entrances of Cueva Mayor, Cueva del Silo and Cueva del Mirador, as well as other caves such as Cueva Peluda and Cueva Ciega contain evidence of Holocene occupations.

At the beginning of the 20th Century, the construction of a trench which cut into the southwest border of the Sierra de Atapuerca, for the passing of a narrow mining railway, brought to light the presence of numerous karst chambers, many of them filled to the roof. Subsequent excavations and research have centered on three sites: Gran Dolina, Galería and Sima del Elefante.

The site of Gran Dolina (TD)

This site is a sedimentary infilling of the entrance of a large cave whose section presents a stratigraphic sequence 16 meters deep and is divided into 11 levels. The basal part (levels TD1 and TD2) contains sterile sediments deriving from the inside of the cave, and apparently the cave was not open to the exterior at this time. Beginning in levels TD3-TD4 (some 900,000 years old) the opening of the cave mouth allowed outside sediments and materials to fill the cave, and this process of infilling continued through to level TD 11 (around 100,000 years ago).

Level TD6 stands out as having yielded 84 human remains, in a test pit of some six square meters, representing a new species of human, Homo antecessor, together with faunal remains, pollen and stone tools. The TD6 remains date to more than 780,000 years ago, given that the sediments in level TD7, higher up in the stratigraphic sequence, show a change in the magnetic polarity which coincides with the Matuyama-Brunhes limit marking the beginning of the Middle Pleistocene.

In level TD10 (dated to around 370,000 years ago), the use and occupation of the cave by Middle Pleistocene humans intensified, and there are abundant remains from outside the cave preserved in the sediments. Later, during level TD11 times, the cave became entirely filled to the roof, sealing off the entrance.

The Galería Complex (TG)

This site consists of a horizontal chamber and a complex of sinkholes formed in the limestone bedrock which have been completely filled with sediments. Five phases have been identified in the formation process of the site. The first phase (TG I) concerns the base of the sedimentary sequence and consists of facies typical of the interior of caves. Phases two through five (TG II-V) correspond to times when the cave was open to the outside, containing sediments which derive from the exterior, and was used by both animals and humans.

The archaeological levels (TG II-V) span the time period from greater than 350,000 years ago for the lowest level to 110,000 years ago for the uppermost level, and begin with a layer of guano in the base of TGII. Level TGIII stands out due to the presence of up to 13 human occupation levels. The stone tools, made on flint and quartzite, are associated with abundant faunal remains (horses, deer, bison and rhinoceroses) which were consumed by both humans as well as other carnivores, such as bears, lions, cuons, foxes, lynxes and wildcats.

The Sima del Elefante (TE)

A little over a million years ago, one of the many subterranean galleries in the Sierra de Atapuerca opened up to the outside world, forming the entrance to a cave. From that moment, clay, sand and rocks began accumulating inside the cave giving rise to a long and complex sedimentary sequence known as the Sima del Elefante. The scientific value of this site is enormous since it provides information on a critical period, around one million years ago, in human evolution, when the planet’s climate began to experience pronounced oscillations, giving rise to profound changes in the ecostystems.

The excavations carrried out in the lower levels of the Sima del Elefante have yielded abundant remains of small rodents, as well as deer, primitive bison, hippopotamuses, rhinoceroses, and even macaques and beavers. Carnivore remains representing extinct wolves, foxes, bears and mustelids are also frequent. The important discovery of the bones of an osprey (a type of eagle), together with those of turtles, beavers and hippopotamuses indicate a landscape dominated by vast bodies of water and riverine ecosystems.

But was the Sierra inhabited by hominids a million years ago? The Sima del Elefante has yielded possible evidence of a human presence at Atapuerca during this remote period, but the scarcity of the material urged caution in its interpretation. Nevertheless, during the 2000 field season, an important discovery was brought to light in the lower levels. A small flint flake, indisputably knapped by human hands, confirmed the presence of human groups in the Sierra at least a million years ago. Who were these hominids? What was their role in those ecostytems? The answers to these questions could still be buried in the sediments of this fascinating site.

The Sima de los Huesos site (SH)

This site is located deep inside the Cueva Mayor, some 500 meters from the current entrance, at the bottom of a 13-meter-deep vertical shaft. The fossil-bearing sediments were capped by a speleothem which is in isotopic equilibrium, indicating a minimum age of 350,000 years old and corresponding to the Middle Pleistocene

This site contains abundant human remains representing, at the current stage of excavation, to at least 28 skeletons of the species Homo heidelbergensis, a direct ancestor of the Neandertals. Further, thousands of bear bones corresponding to more than 100 individuals have also been recovered, belonging to a species (Ursus deningeri) which is the precursor of the cave bear, as well as smaller numbers of remains of lions, lynxes and foxes.

Measuring Geological Time

A fundamental problem facing all paleontologists is how to date the sites they work at and the fossils they recover. The classic, or paleontological, dating method attempts to situate them within their ancient environment, with the animals and plants that surrounded them. The dating of a site by its fossils is known as biochronology, and the presence of a species named Mimomys savini in the sediments of the Gran Dolina has been used to chronologically place the fossils which are contemporaneous with this rodent.

However, other dating methods are based mainly on radioactive decay of certain natural elements which occurs at known rates in the environment. This form of measuring geological time is known as Geochronometry. Biochronology and Geochronometry together form the science of Geochronology. The natural elements which spontaneously change from one form to another are known as radioactive elements because, upon doing so, they emit radiation. However, they aren’t dangerous to humans, unlike the nuclear bombs which, based on the same principles, have been developed for destructive purposes.

We have seen that the radioactive elements change from one form to another at a constant rate known to physicists. At the moment when a rock was formed or an animal died, only the initial element existed, which we will call the parent element. When the object to date is analyzed, whether a rock or an animal, and it is discovered that part of the parent element has changed into an offspring element we have a way to measure the amount of time that has transpired. The half-life of a radioactive element is the amount of time necessary for half of the original parent element to change into an offspring element.

Dating methods based on the radioactive decay of elements can be used on different types of rock as well as directly on the fossils excavated from paleontological sites. The methods known as Potssium-Argon dating (K/Ar) Argon-Argon dating (Ar39/Ar40) and fission-track dating are used on volcanic rock. Although this type of rock is frequent in Africa and on Java, it is generally not found in archaeological sites in Europe. Other radiometric methods rely on different radioactive elements, such as C14 or Uranium Series dating. The C14 technique (the first method to be developed) can only be applied to organic materials, whether animal or vegetal, but is very accurate and provides a direct date of the object in question. Unfortunately, it yields reliable results only up to about 50,000 years ago. As water courses through the limestone bedrock in karstic cave systems (see the discussion of karst formation), it forms speleothems, or stalactites and stalagmites, through the precipitation of dissolved calcium carbonate. In certain favorable situations when the carbonate crystals are sufficiently pure, speleothems can be dated by the Uranium series method. This technique has a maximum reliable limit of up to 350,000 years ago. In the Sima de los Huesos, the paleontological deposits containing human fossils are covered by a speleothem which is at least this old.

However, in many cases, archaeological sites don’t contain any datable speleothems. In these circumstances, paleontologists must resort to other techniques, such as electron spin resonance (ESR) or thermoluminescence (TL), and its variants known as optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL). All of these techniques are related, relying on similar assumptions, but can be applied to different materials. ESR is generally used on tooth enamel from faunal remains, while the luminescence methods are applied to burnt flint instruments or a variety of sediments which have been exposed to sunlight. In more modern contexts, they can also be applied to ceramics and pottery. All these techniques rely on the assumption that a mineral, such as flint, just like a tooth or bone, acts as a natural trap which accumulates the radiation to which it is exposed over a long period of time. Radiation, as discussed previously, is a direct consequence of the transformation of one element into another, and the amount of radiation received by an object is a measure of the amount of time which has transpired, so that more radiation means a longer period of time. Nevertheless, we will see that dating a piece of flint is not the same as dating a tooth.

Flint was one of the most frequently used raw materials for making stone tools during the Paleolithic. If at some moment in the remote past, a prehistoric hominid dropped or threw a piece of flint into the campfire, the high temperature would free all the energy (radiation) which the flint had previously accumulated since its geological formation millions of years before. This is referred to as zeroing the radiometric clock. From this point forward, the flint will begin to accumulate radiation again at a rate which varies according to the particular conditions of the sediments in which it is buried. After excavation, geochronologists heat the burnt flint to a temperature greater than 450º C in the laboratory. This causes the flint to release all the energy which it had accumulated since being buried at the site, emitting a light whose intensity is proportional to the amount of energy accumulated. The age of the site is obtained by dividing the total accumulated energy of the flint, known as the paleodose, by the annual radiation dosage which the flint received. If it had received a low annual dosage at the site and had accumulated a large amount of energy, the age of the fire which originally heated the flint, and hence the archaeological level in which it was found, is greater. If the annual dosage was large, the age of that same fire made by Prehistoric humans was younger, even if the paleodose of the flint is the same. This is why it is so important to accurately measure the annual dosage. The annual dosage, in turn, depends on several internal and external factors. The internal radioactivity depends on the concentration of radioactive elements in the material itself, while the external radioactivity is a function of both the concentration of radioactive elements in the surrounding soil and cosmic rays from the environment. The radioactive elements present in the soil are variants, known as isotopes, of uranium (U), thorium (Th) and potassium (K). All these intervening variables can be measured, and after including them in a mathematical formula, an age for the flint is determined.

If no burnt flint is available, electron spin resonance (ESR) can be applied to fossilized bones and teeth recovered from the archaeological site. Tooth enamel, which is formed of more than 96% hydroxyapatite, is generally used, and the basic equation is the same as in the case of TL just described. That is, the age of the fossil = paleodose / annual dosage. Nevertheless, the way of measuring the paleodose of the fossil is different, and doesn’t rely on heating the material.

Speleothems can also be dated by this technique. However, there is a fundamental difference between dating a stalactite and a fossil tooth. The main problem with applying the ESR method to teeth stems from the external radiation which the fossil has received, produced by the decay of a radioactive isotope of uranium. During the formation of a stalactite by precipitation of the calcium carbonate dissolved in water, the radioactive uranium which was also dissolved in the water, is also absorbed by the rock. However, living mammals don’t have radioactive uranium in their tissues, and their fossilized remains must have incorporated it after they had died and were buried at the site. The ESR method considers several simple models of uranium acquisition (uptake) after the death of the animal. The most frequently employed models assume either an early uptake (EU) of uranium after death or a continuous (linear) uptake (LU). In reality, however, the situation is often much more complex, with different episodes of uranium uptake occurring at irregular intervals.

For this reason, we can’t speak of absolute dates when we work with fossils, but rather only of radiometric dates, whose reliability depends on whether the assumptions of the model approach reality. Recently, Uranium Series dating is being combined with the ESR method to obtain more reliable chronologies of faunal remains. In the time periods where they overlap (i.e. less than 350,000 years ago) the Uranium Series date can help identify the correct uptake model to use when calculating the ESR date. Theoretically, the ESR technique can also provide dates beyond the limit of Uranium Series (350,000 years ago), and older dates are being obtained whose precision is still untested. At some of the sites in the Sierra de Atapuerca, it is being applied to fossils which are known to be 800,000 years old and seems to yield reliable results. Ultimately, the application of several dating techniques to a variety of materials at the same site will provide the best chronology possible.

Finally, we should mention a method which provides neither an absolute nor a relative date, but can still help us ascertain the age of a site. Paleomagnetism consists of establishing the orientation of the Earth’s magnetic poles when the site was formed. Although it seems hard to believe, the north and south magnetic poles have reversed their positions numerous times during the history of the Earth, and these changes have been accurately dated. Thus, geological time can be divided into periods of normal polarity, the present situation, and reversed polarity. If we can identify the “fossil” polarity of a site, or of a particular level at a site, we can rule out those time periods when the polarity was the opposite. At a site with many levels, more than one polarity change can often be detected, which makes it easier to solve the question of its geological age. This change in the polarity of the magnetic field has been identified in the Gran Dolina, confirming the previous biochronological dates and establishing that the fossils representing Homo antecessor are at least 780,000 years old.