On the age of the hominid fossils at the Sima de los Huesos, Sierra de Atapuerca, Spain:...

On the Age of the Hominid Fossils at the Sima de los Huesos,Sierra de Atapuerca, Spain: Paleomagnetic Evidence

JOSEP M. PARES,1* ALFREDO PEREZ-GONZALEZ,2 ARLO B. WEIL,1AND JUAN LUIS ARSUAGA3

1Department of Geological Sciences, University of Michigan, Ann Arbor,Michigan 48109-10632Departamento de Geodinamica, Facultad de CC. Geologicas, UniversidadComplutense, 28040 Madrid, Spain. E-mail: [email protected] de Paleontologıa, Facultad de CC. Geologicas,Universidad Complutense, 28040 Madrid, Spain.E-mail: [email protected]

KEY WORDS Middle Pleistocene; Neanderthal; Spain; remanentmagnetization; Matuyama-Brunhes

ABSTRACT We report new paleomagnetic data for the Middle Pleisto-cene hominid-bearing strata in the Sima de los Huesos, North Spain. Sedi-ments (brown muds with human and bear fossils and the underlying sterileclayey and sandy unit) preserve both normal and reversed magnetic compo-nents. The sterile unit has exclusively reversed magnetization, dating back tothe Matuyama Chron, and thus is Lower Pleistocene in age. The overlyingfossiliferous muds have a dominant normal magnetization that overprints apartially resolved reversed magnetization. These data are compatible withone of the reversal events that occurred during the Brunhes Chron. Com-bined with the existing U-series dates and evidence from the macro- andmicrofauna, these paleomagnetic results suggest an age of the hominid fossilsbetween 325 to 205 ka, whereas the underlying sand and silts are older than780 ka. Am J Phys Anthropol 111:451–461, 2000. © 2000 Wiley-Liss, Inc.

The Sierra de Atapuerca in north centralSpain contains a rich variety of karst fis-sures and cavities of a phreatic-vadose re-gime. Several caves of this karst systemcontain archaeological and paleontologicalremains that have been the focus of numer-ous studies since the 1980s (see Aguirre etal., 1990; Arsuaga et al., 1997 and refer-ences therein). In particular, the Sima delos Huesos has been systematically exca-vated since 1984 (see a review of the re-search in Journal of Human Evolution,1997, v. 33). The Sima de los Huesos (“Pit ofthe Bones”) hominids are the largest andbest preserved sample of Middle Pleistocenehumans from one site, and they are thoughtto document an early stage in Neanderthalevolution (Arsuaga et al., 1993). A detaileddescription of the geological and karst de-velopment of the paleontological site can be

found in Perez-Gonzalez et al. (1995), Zazoet al. (1983), and Martin et al. (1981). Thehominids have recently been dated byBischoff et al. (1997), using U-series andESR analyses, and these provide a mini-mum age of 284 ka. In addition, rodents(Cuenca-Bescos et al., 1997) give an age ofSaalian (oxygen isotope stage; OIS 8-7), andthe carnivore (Garcia et al., 1997) assem-blage correlates with OIS 7 or older (.200ka), in agreement with the U-dating. Wecarried out a paleomagnetic study on the

Grant sponsor: Consejerıa de Cultura y Turismo (Junta deCastilla y Leon); Grant sponsor: DGES; Grant numbers: PB96-0815, PB96-1026-C03; Grant sponsor: Unidad Asociada CSIC-UCM.

*Correspondence to: Josep M. Pares, Department of GeologicalSciences, University of Michigan, 2534 C.C. Little Building, AnnArbor, MI 48109-1063. [email protected].

Received 14 May 1999; Accepted 7 December 1999.

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 111:451–461 (2000)

© 2000 WILEY-LISS, INC.

fossiliferous clay (“arcilla roja,” matrix ofthe breccia of the human bones) and on theunderlying deposits in order to establish themagnetic record and further constrain theage of the hominid fossils.

GEOLOGICAL SETTING AND METHODS

The Sierra de Atapuerca is made of Me-sozoic limestones and is located at thenortheastern border of the Iberian Meseta,in the Duero Basin, one of its Tertiary con-

tinental basins (Fig. 1). The karst where theAtapuerca archaeological sites are located(Elefante, Galeria, Dolina, and CuevaMayor) is developed in the Upper Creta-ceous (Turonian-Santonian) limestones,and these represent a marine transgressionof an inner platform of 45–70 m thickness.Unconformably overlying the Cretaceouslimestones are calcareous conglomeratesand red Oligocene sandstones (Olive et al.,1990; Pineda and Arce, 1997). The Atapu-

Fig. 1. a: Location of Atapuerca paleoanthropological site. b: Interpretive east-west cross section ofthe Sima de los Huesos according to Bischoff et al. (1997). The sections that are object of this study areareas A, B, and SRB. See text for details.

452 J.M. PARES ET AL.

erca endokarst has a phreatic origin (e.g.,Cueva Mayor) and is a covered karst(Torres, 1976), whose main developmentphase was synchronous to the sedimenta-tion of the Aragonian or Middle Vallesianlimestones (Zazo et al., 1983, 1987). Accord-ing to these authors, Cueva Mayor and thecaves of Galerıa, Dolina, and Elefante of theabandoned railway trench would be senileforms, along with terraces T1 and T2, pre-sumably of Lower Pleistocene age.

The paleomagnetic sampling was carriedout in two areas within the Sima de losHuesos (Fig.2). The site consists of a 2–4-m-diameter vertical shaft that extends down-ward 13 m to terminate at the top of amoderate ramp or “rampa” (for a more de-tailed description of the site, see Arsuaga etal., 1997; Bischoff et al., 1997). The ramp isa 9-m-long irregular linear chamber 1–2 mhigh by 2–3 m wide, inclined approximately32° to the northwest, with two excavatedareas (areas A and B, Fig. 1) at its base.Area A is at the west edge of the ramp, andthe stratigraphic section comprises two dif-ferent levels: a lower level of semiconsoli-dated sand and silt, and an upper level ofmatrix-supported breccia of human bonesand bear bones, and fragments of spe-leothem, known as “arcilla roja” (red clay)(Arsuaga et al., 1997; Bischoff et al., 1997).Area B, located at the north and at the footof the ramp, has only the upper level ofarcilla roja, which reaches up to 75 cm inthickness. In addition to areas A and B, themiddle part of section SRB was also sam-pled for paleomagnetism.

Clay and silt deposits at Sima de los Hue-sos are sufficiently soft that the samplescould be obtained by simply hammering abrass tube with a reinforced stainless steeltip into the cleaned outcrop surface. A stan-dard orientation device (compass-inclinom-eter) was used to measure the azimuth anddip of the in situ cylinders. Sediment sam-ples were stored in cylindrical plastic boxesand were impregnated with a water solutionof sodium silicate (1:1) for solidification.

Natural Remanent Magnetization (NRM)of the 30 samples was measured on twodifferent SQUID magnetometers: 1) aGM400 three-axes magnetometer housed ina three-pair Helmholtz coil at the CSIC Pa-

leomagnetic Laboratory in Barcelona, and2) a 2G three-axes cryogenic magnetometerin the field-free room at the University ofMichigan’s paleomagnetic laboratory. Thenoise level of these magnetometers is 7mA/m, well below the magnetization inten-sity of the measured samples. Standard cy-lindrical 2.54-cm paleomagnetic cores wereprogressively demagnetized in a TSD-1 fur-nace (Schonstedt in Barcelona and in anASC Thermal Demagnetizer at the Univer-sity of Michigan). Least-squares fits to thelinear demagnetization trajectories for boththe low- and the high-temperature compo-nents in each sample were made by princi-pal components analysis (Kirschvink, 1980).

RESULTS

The initial intensity of the Natural Rema-nent magnetization ranged from 2.5–16mA/m for the silty and clayey sand under-lying the fossiliferous mud, and from 2–17mA/m for the clays of the hominid-bearingarcilla roja. Representative orthogonal pro-jection plots (Zijderveld, 1967), includingnormalized magnetization (M[T]/M[20°C])plotted vs. demagnetization temperature,are presented in Figures 3–5, and equalarea projections of the characteristic rema-nent magnetization (ChRM) directions forthe three separate sedimentary units aregiven in Figure 6. Sand and silt samplesfrom the lower part of the section at area Aand section SRB show two magnetic compo-nents upon thermal demagnetization (Fig.3). A low-temperature (LT) component isunblocked at temperatures up to 250°C andoften, but not always, has a northerly anddownward direction, i.e., conforming to thepresent-day field. A high-temperature (HT)component is defined above 300°C with asoutherly and upward direction, representa-tive of a reversed geomagnetic field. Thisbicomponent behavior has been previouslyobserved in other stratigraphic sectionswithin the Atapuerca site and reveals thatthere has been a widespread secondaryoverprint by the Brunhes normal polaritychron on an HT component that is reversed(Pares and Perez-Gonzalez, 1995). The re-sults for the arcilla roja at areas A and B aresummarized in Figures 4 and 5. Stepwiseprogressive thermal demagnetization al-

PALEOMAGNETIC AGE OF HOMINIDS AT SIMA DE LOS HUESOS 453

Fig. 2. View of the sedimentary record in the Simade los Huesos. a: View to the west of area A, showinglaminated sandy silts overlaid by the red clay unit (“ar-cilla roja”). The contact between these two units is ero-sional, and is marked by a clast of a plate of speleothem(at right). The red clays contain both human and bear

bones, as well as fragments of speleothem. b: View tothe northwest of area B, showing mud breccia of bearand human bones and contact (white dots) with under-lying laminated sandy-silts. Photographs by JavierTrueba.


Fig. 3. Results of progressive thermal demagnetiza-tion displayed by vector end point diagrams (Zijderveld,1967) of representative samples for the sand and silt ofArea A. Each data point represents the natural rema-nent magnetization (NRM) end vector for individualdemagnetization steps projected onto the horizontal

(solid symbols) and vertical (open symbols) plane. Num-bers adjacent to magnetization directions indicate thedemagnetization temperatures in degrees Celsius. Ini-tial value of magnetization intensity (NRM) is alsoshown.


lows us to distinguish two different types ofbehavior in the mud samples from the hom-inid breccia. The first set of samples is char-acterized by a sharp decrease of intensity upto 300°C that corresponds to a northerly anddownward component on the orthogonalplots (Fig. 4, sample HUE1-8A; Fig. 5, sam-ple HUE1-11A). The second set of samplesalso displays a relatively sharp decrease inintensity up to 300°C; however, at this tem-perature there is still about 10–15% of theinitial intensity left (Fig. 4, sample HUE1-7A; Fig. 5, sample HUE1-9B). Further in-creases in temperature failed to decrease

the intensity and resulted in a spuriousmagnetization above ca. 500°C. The demag-netization paths on the orthogonal dia-grams for this set of samples pass close tothe origin, revealing that there is an unre-solved high-temperature component whichis masked by the LT component. The orthog-onal projections indicate that this magneti-zation has reverse polarity: the path of thehorizontal component of magnetization isdirected towards the southern quadrant,whereas the vertical component tends to goto the upper hemisphere (i.e., negative).Only very few samples show a high-temper-

Fig. 4. A: Results of progressive thermal demagne-tization of representative samples from clays of area A.Same symbols as in Fig. 3. B: Plot of the intensitydecay curves during thermal treatment. Intensity isexpressed as a normalized value (M[T]/M[20°C]). C:Equal-area, lower-hemisphere projection of demagneti-

zation end vectors for two samples. Note the conver-gence of the two great circles towards a south andupwards magnetization direction, revealing an unre-solved high-temperature reversed magnetization (refer-ence direction by Pares and Perez-Gonzalez, 1995). Seetext for details.


ature component that is unambiguously re-solved (Fig. 4, sample HUE1-6A). In thissample, the remanence remains stable athigh temperature and has a well-definedsoutherly and upwards direction, i.e., a re-versed magnetization. In this particularsample there is also a low-temperaturenortherly and downward direction compo-nent that parallels a normal geomagneticfield, not included in Figure 4 for clarity.

The limited number of samples displayingthe unresolved HT component prevents theapplication of great-circle methods to deter-mine its direction. Nonetheless, to judgefrom the demagnetization circles on a ste-reographic projection (Figs. 4, 5), there is ahigh-temperature component of reversedpolarity.

We cannot determine the origin of rema-nent magnetization in the clays (i.e., chem-

Fig. 5. A: Results of progressive thermal demagne-tization of representative samples from clays of area B.Same symbols as in Fig. 3. B: Plot of the intensitydecay curves during thermal treatment. Intensity isexpressed as a normalized value (M[T]/M[20°C]). C:Equal-area, lower-hemisphere projection of demagneti-

zation end vectors for three samples. Note the conver-gence of the great circles towards a south and upwardsmagnetization direction, revealing an unresolved high-temperature reversed magnetization (reference direc-tion by Pares and Perez-Gonzalez, 1995).


ical vs. detrital). There are, however, nu-merous studies on clays from caves (Turnerand Lyons, 1986; Lovlie and Sandnes, 1987;Papamarinopoulus et al., 1991; Valen et al.,1995) that show that reliable geomagneticsecular variation curves can be recovered.The existence of these smoothing curves in-dicates that the magnetization in cave clays

is stable, and that subsequent alteration ofthe magnetic signal is unlikely.

In summary (Fig. 6), the demagnetizationdata reveal that the sand and silt unit in thelower part of the stratigraphic section inarea A (the base of the entire cave sequenceso far exposed), and the clay breccia at sec-tion SRB, have a reversed magnetization.

Fig. 6. Equal-area, lower-hemisphere stereographicprojections of characteristic remanent magnetization(ChRM) directions for the studied units and correspond-ing magnetic polarity. Each symbol on the stereo-graphic projections corresponds to an individual sam-ple. Solid (open) symbols are projections onto the lower(upper) hemisphere and thus correspond to samples

with normal (reverse) polarity. Squares are the meanChRM direction and they have the associated 95% con-fidence circles. The reversed directions (open symbols)show more scatter around the reference direction due tothe contamination of the normal magnetic field. Thetriangles correspond to the reference direction (Paresand Perez-Gonzalez, 1995).


The overlying arcilla roja, the matrix of thebreccia of hominid and bear bones, in bothareas A and B, has a dominant normal po-larity magnetization. However, there is alsoevidence for a reverse polarity magnetiza-tion, and we interpret this as the primarymagnetization.

DISCUSSION

The sand and silts exposed on the lowerpart of area A are considered to be the baseof the entire cave-sequence so far exposed(Bischoff et al., 1997). Its extension acrossthe Sima de los Huesos is unclear, and thelack of continuous exposure precludes cor-relation between the various excavated pitswithin the shaft. The top of the sand and siltunit is an irregular erosional surface,marked by the presence of a clast of a plateof fragile “cornflake” speleothem. Frag-ments of this speleothem are found withinthe next overlying 20-cm-thick hominid-bearing clay (Fig. 1). U-series dating ofthese speleothem clasts gives an age in ex-cess of 350 ka (Bischoff et al., 1997); there-fore, the underlying sand and silt should beolder than that age. Given that they displayreversed magnetization, the most conserva-tive hypothesis is that the unit of sand andsilts in the lower section of area A was de-posited during the Matuyama Chron (.780ka). In addition, the reversed magnetizationdisplayed by the clay breccia at section SRBwould also suggest a Matuyama age for thedeposit. The arcilla roja, which is the matrixfor the breccia of human and bear bones,overlies the deposit in both areas A and Band is thus younger. As shown before, allfossil evidence shows that the arcilla rojahas a Middle Pleistocene age. Thereafter,any normal polarity observed in the arcillaroja has to correspond to the BrunhesChron. Thus, the traces of a reversed mag-netization in the arcilla roja at areas A andB are an indication that some magnetiza-tion acquisition took place during a shortreversed geomagnetic event within theBrunhes Chron. In the studied arcilla rojasamples (areas A and B), it turns out thatthe LT component has normal polarity,whereas the HT is reversed. On the conser-vative assumption that the LT component isyounger than the HT one, this implies that

after the acquisition of the reverse HT com-ponent, the wet and unconsolidated mudhaving the LT component recorded a normalgeomagnetic field. We think that at thatstage, some magnetic grains were still ableto rotate after the polarity reversal. Thelock-in of the magnetization in sediments isa process that can take place over a period ofthousands of years, longer than the dura-tion of the magnetic polarity change. As aconsequence, some of the particles depositedwhen the geomagnetic field is reversed maystill be growing or reorienting when the geo-magnetic field reverses its polarity to nor-mal. In addition, the sediments with a pri-mary reversed magnetization could alsohave been partially overprinted as a conse-quence of the ensuing higher-intensity nor-mal geomagnetic field acting on the still un-consolidated sediment, by realigning somefraction of magnetic grains. Both the shortduration and presumably low intensity ofthe geomagnetic field during the reversedevent and diagenesis in the sediment ex-plain how the reverse magnetization signalis obscured by the normal Brunhes magne-tization. The existence of local areas of highporosity with indications of water drainagewould probably restrict the zones with well-developed HT and LT components.

The Brunhes Chron (0–780 ka) appearsto contain several short reversed-polarityevents (e.g., Champion et al., 1988, 1996;Langereis et al., 1997; Lund et al., 1998).However only some of them have sufficientsupport to indicate that they may representactual reversals of the geomagnetic fieldand not fluctuations of the geomagnetic in-tensity. For brevity, not all of these excur-sions can be discussed, and readers are re-ferred to Champion et al. (1988, 1996) andJacobs (1994). Within the interval of inter-est (780–200 ka) there are two recorded andwell-documented excursions that could po-tentially explain our observations (see dis-cussion on events within Brunhes in Lange-reis et al., 1997): namely, Calabrian Ridge 1(CR1) (315–325 ka) and Jamaica/PringleFalls (J/PF) (205–215 ka).

The older age of the arcilla roja given bythe U-series dating, 320 ka (Bischoff et al.,1997), is consistent with the age range ofUrsus deningeri (Blackwell et al., 1990).


The age of the arcilla roja is thus con-strained between 780 ka (the beginning ofBrunhes Chron) and 200 ka (the youngestage given by the U-series dating for thedeposit) (Fig. 7).

A related question is the origin of thearcilla roja deposit. This deposit is charac-terized by a lack of bedding and a chaoticmixing and breaking of bones and spe-leothems. According to Bischoff et al. (1997),a karstic undermining caused collapse ofthe sediment package into the void, rotatingand mixing in the process. Such a processwould have serious implications for the pa-leomagnetic directions recorded in the ar-cilla roja. The postdepositional rotation andbrecciation of the mud unit would have arandomizing effect on the magnetization di-rections. Our data show, however, that themagnetization directions are not scatteredbut are grouped (Fig. 6) in the stereographic

projection. This grouping of the magnetiza-tion directions reveals that the clay deposithas not distorted subsequently to its depo-sition, ruling out the existence of a collapseof the unit that would produce a mixingprocess in the clays.

CONCLUSIONS

The paleomagnetic determinations indi-cate that the sand and silt (basement of thehuman-bearing clays) (area A) and the claybreccia (section SRB) have reversed magne-tization, and thus the most conservative ex-planation is that they record the Matuyamaepoch (Lower Pleistocene). The arcilla rojamatrix of the hominid-bearing clay has anage of one of the Brunhes excursions (CR1or J/PF), as indicated by reverse magnetiza-tion. The rodent fauna (A. bursae andPliomys lenki relictus) seems to indicate anage of Saalian (OIS 8-7) (Cuenca-Bescos etal., 1997), whereas the carnivore fossils cor-relate with OIS 7 or older (.200 ka) (Garcıaet al., 1997). On the other hand, the pres-ence of Ursus deningeri constrains the ageof the arcilla roja as no older than 315 ka.Therefore, either the CR1 or J/PF geomag-netic events may be responsible for the re-versed magnetization in the arcilla roja, asboth of them accommodate the U-series dat-ing. Thus the age of the arcilla roja is either205–325 or 315–325 ka. Further magneticstudies addressing the origin and delay ofmagnetization in endokarst clays will im-prove our understanding of the deposit andrefine correlations between the endo- andexokarst deposits.

ACKNOWLEDGMENTS

Fieldwork in Atapuerca is supported bythe Consejerıa de Cultura y Turismo (Juntade Castilla y Leon). This research was sup-ported by DGES grants PB96-0815 andPB96-1026-C03, and by the Unidad Aso-ciada CSIC-UCM. We thank our three anon-ymous reviewers for comments that led tothe improvement of the manuscript. We arevery grateful to the Edelweiss SpeleologicalGroup, and in particular to A.I. Ortega andM.A. Martın, for their constant help.

Fig. 7. Summary of chronological data for Sima delos Huesos. Black (white) boxes at left represent periodsof normal (reversed) magnetization. Two geomagneticevents within Brunhes are indicated: Calabrian Ridge 1(315–325 ka) and Jamaica/Pringle Falls (205–215 ka).Striped horizontal bars indicate the age range for boththe arcilla roja and the sand and silt unit. The absoluteages for the human fossils are from U-series dating(Bischoff et al., 1997), as discussed in the text. Thelower units at Sima de los Huesos (sand and silt andclay breccia) have reversed magnetization and thushave a Matuyama age, i.e., Lower Pleistocene.


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