![Stratigraphic column from the central zone of the Ossuary Gallery (Sector 3), with a brief description of the main sedimentary units and their genetic interpretation. Modified from Cañaveras et al. [16,18].](https://www.researchgate.net/publication/355051624/figure/fig2/AS:1076217360986114@1633601622450/Stratigraphic-column-from-the-central-zone-of-the-Ossuary-Gallery-Sector-3-with-a_Q320.jpg)
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geosciences
Article
Micromorphological Study of Site Formation Processes at El
Sidrón Cave (Asturias, Northern Spain): Encrustations over
Neanderthal Bones
Juan Carlos Cañaveras 1, Sergio Sánchez-Moral 2, Elsa Duarte 3, Gabriel Santos-Delgado 4, Pablo G. Silva 5,
Soledad Cuezva 6,Ángel Fernández-Cortés7, Javier Lario 8, María Concepción Muñoz-Cervera 1and
Marco de la Rasilla 3, *
Citation: Cañaveras, J.C.;
Sánchez-Moral, S.; Duarte, E.;
Santos-Delgado, G.; Silva, P.G.;
Cuezva, S.; Fernández-Cortés, Á.;
Lario, J.; Muñoz-Cervera, M.C.;
Rasilla, M.d.l. Micromorphological
Study of Site Formation Processes at
El Sidrón Cave (Asturias, Northern
Spain): Encrustations over
Neanderthal Bones. Geosciences 2021,
11, 413. https://doi.org/10.3390/
geosciences11100413
Academic Editors: Jesús F.
JordáPardo and Jesus Martinez-Frias
Received: 29 July 2021
Accepted: 22 September 2021
Published: 3 October 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
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iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Department of Environmental and Earth Sciences, University of Alicante, Campus San Vicente del Raspeig,
E-03690 Alicante, Spain; jc.canaveras@ua.es (J.C.C.); mc.munoz@ua.es (M.C.M.-C.)
2Department of Geology, MNCN-CSIC, C/ JoséGutiérrez Abascal, 2, E-28006 Madrid, Spain;
ssmilk@mncn.csic.es
3Department of History, University of Oviedo, C/Amparo Pedregal, s/n, E-33011 Oviedo, Spain;
elduarma@gmail.com
4Department of Cartography and Terrain Engineering, University of Salamanca, Plaza de la Merced, s/n,
E-37008 Salamanca, Spain; gsd@usal.es
5Department of Geology, University of Salamanca, Plaza de la Merced, s/n, E-37008 Salamanca, Spain;
pgsilva@usal.es
6Department of Geology, Geography and Environment, University of Alcaláde Henares,
Campus Científico y Tecnológico, E-28802 Alcaláde Henares, Spain; soledad.cuezva@uah.es
7Department of Biology and Geology, University of Almería, Ctra. Sacramento, s/n, E-04120 Almería, Spain;
acortes@ual.es
8Faculty of Science, Universidad Nacional de Educación a Distancia (UNED), Avda. Esparta s/n,
E-28232 Las Rozas, Spain; javier.lario@ccia.uned.es
*Correspondence: mrasilla@uniovi.es
Abstract:
El Sidrón Cave is an archaeological and anthropological reference site of the Neanderthal
world. It shows singular activity related to cannibalisation, and all existing processes are relevant to
explain the specific behaviour of the concerned individuals. This paper presents geoarchaeological
data, primarily based on mineralogical and petrographic techniques, from an investigation of the
nature of the encrustations or hard coatings that affect a large part of the Neanderthal bone remains
and their relationship with the depositional and post-depositional processes at the archaeological
site. Crusts and patina were found to be numerous and diverse, mainly composed of calcite and
siliciclastic grains, with different proportions and textures. The analysis indicated different origins
and scenarios from their initial post-mortem accumulation to the final deposit recovered during the
archaeological work. The presence of micromorphological features, such as clotted-peloidal micrite,
needle-fibre calcite (NFC) aggregates, clay coatings, iron–manganese impregnation, and/or adhered
aeolian dust may indicate that a significant proportion of the remains were affected by subaerial
conditions in a relatively short period of time in a shelter, cave entrance, or shallower level of the
karstic system, prior to their accumulation in the Ossuary Gallery.
Keywords:
cave sediment; karst; geoarchaeology; palaeoanthropology; Middle Palaeolithic; Mouste-
rian; Iberian Peninsula
1. Introduction
The Neanderthal fossils from the archaeological site of El Sidrón cave comprise, for the
moment, the largest and most complete anthropological collection of these species found on
the Iberian Peninsula. They consist of ~2530 skeletal remains belonging to 13 individuals
with familial relationships and evidence of cannibalism [
1
–
9
]. However, Mousterian lithic
artifacts are quite scarce (~400), and they are made from local chert and quartzite, indicating
Geosciences 2021,11, 413. https://doi.org/10.3390/geosciences11100413 https://www.mdpi.com/journal/geosciences
Geosciences 2021,11, 413 2 of 17
short and expedient behaviour. The high refitting rate of lithics (20%) unequivocally proves
a single archaeological deposit [
10
,
11
]. Animal bones are very scarce, and they are not
related to human activities [12].
A multi-dating approach has been undertaken at the site, and it has given a consistent
date of 48,400
±
3200 years BP for the archaeological and the fossil assemblage, which
places it between Heinrich’s H4 and H5 events of the paleoclimatic stage MIS 3 [
12
–
14
]. It
is precisely this period in which a good percentage of the so-called “classic Neanderthals”
are concentrated, among which we can place the El Sidrón Neanderthal group.
The specificities of this collection (more human bones than lithic and unconsumed
fauna) are opposed to a Neanderthal permanent site, as determined by the deposition
pattern in the small Ossuary Gallery [
1
,
6
,
10
]. Although the preservation of the bones is
generally fairly good, with very limited trampling or erosion and no carnivore or rodent
toothmarks, bones and lithics are not in their original location. Geo–archaeo–stratigraphic
analyses have suggested that the bones went into the cave in a massive water-driven
deposit and fell into the Ossuary Gallery through a vertical shaft, probably resulting from
a flood event after a thunderstorm [15–19].
This rapid event into the cave allowed for the good preservation of both sediments and
archaeological remains. This preservation is a common feature related to caves and rock
shelters because they are little-exposed to open-air alterations, so data regarding human
past activities and the local environment can be obtained [
20
–
24
]. To a large extent, the
general good preservation of fossil remains is due to their rapid incorporation into an en-
dokarstic context, where micro environmental stability conditions favour the preservation
of bone fragments [
21
–
24
]. Alteration processes begin immediately after the sedimentary
input is accumulated in an archaeological deposit, and several environmental factors,
such as groundwater and sediment composition, pH, redox potential, temperature, and
biological activity, can determine the preservation of archaeological bones [
25
–
30
]. Once
the sediments and the archaeological remains are deposited, the taphonomic processes,
related to cultural and environmental factors, are diverse in each site [24].
At El Sidrón cave, cultural and animal factors are absent because an important part
of the karst conduits of the cave, and more specifically the Ossuary Gallery, were isolated
from the human and animal activity after the accumulation of the massive water-driven
deposit. The subsequent natural factors have consisted of low-energy processes typical of
a vadose environment, evinced by, among others, encrustation. A significant number of
the human fossil remains are coated in authigenic mineral concretions, with abundant fine
detrital material adhered. Different types of isolated or laminar mineral (such as carbonate
and Fe–Mn oxides) concretions can be distinguished. Establishing the depositional and
post-depositional history of such crusts is necessary in order to evaluate a detailed contex-
tualization of the fossils and to improve our understanding of the formation processes of
the site. This paper presents geoarchaeological data related to hard coatings (crusts) and
patina covering Neanderthal bones from a mineralogical and petrographic study in order
to further investigate the circumstances of the original deposition, the post-depositional
processes, and the preservation of the archaeological site.
2. Geological Setting and Sediment Sequence
El Sidrón cave was developed in Oligocene carbonate conglomerates alternating with
fine to medium-grained sandstones. These carbonate successions show approximate E–W
directions, dipping 20–30
◦
to the north. The karst system, with a development of 600 m
(about 3700 m in galleries) and a height difference of 30–32 m between the highest galleries
and the spring, is divided into four levels with a main E–W direction; these were generated
according to the evolution of the regional drainage system. The Main Gallery (Gallery of
the River) and its transverse tributaries (i.e., Ossuary Gallery) are located in the second
level, just above the active (phreatic) level. The Neanderthal bone assemblage is located in
the Ossuary Gallery, an N–S oriented passage that is ~28 m long and 12 m wide (Figure 1).
Geosciences 2021,11, 413 3 of 17
Geosciences 2021, 11, x FOR PEER REVIEW 3 of 18
in the Ossuary Gallery, an N–S oriented passage that is ~28 m long and 12 m wide (Figure
1).
Figure 1. El Sidrón cave: (A) Geographical location of the El Sidrón cave; (B) cave map with the location of the main
galleries; (C) excavation plan of the Ossuary Gallery.
The sedimentary infill in the Ossuary Gallery shows great complexity, thus making
it hard to define a stratigraphic column representing the whole gallery. Five main units
corresponding to events with different hydrodynamic and sedimentary characteristics are
better documented in the central zone of the Ossuary Gallery [16,19,31] (Figure 2). From
bottom to top, these are:
• Unit 0, a unit of massive mud. No clear sedimentary structures can be distinguished.
In a preliminary approach, they seem to be sediments deposited through a low en-
ergy outflow or backswamp conditions.
• Unit I, a unit of laminated fine sands and mud, with cross-stratification. It includes
low-intensity fluvial–karstic material with a relative increase in energy at the top.
Figure 1.
El Sidrón cave: (
A
) Geographical location of the El Sidrón cave; (
B
) cave map with the location of the main
galleries; (C) excavation plan of the Ossuary Gallery.
The sedimentary infill in the Ossuary Gallery shows great complexity, thus making
it hard to define a stratigraphic column representing the whole gallery. Five main units
corresponding to events with different hydrodynamic and sedimentary characteristics are
better documented in the central zone of the Ossuary Gallery [
16
,
19
,
31
] (Figure 2). From
bottom to top, these are:
•Unit 0, a unit of massive mud. No clear sedimentary structures can be distinguished.
In a preliminary approach, they seem to be sediments deposited through a low energy
outflow or backswamp conditions.
•
Unit I, a unit of laminated fine sands and mud, with cross-stratification. It includes
low-intensity fluvial–karstic material with a relative increase in energy at the top.
•
Unit II, a unit of poorly sorted gravels, sands, and mud. It represents the lower limit
of the ‘fossiliferous units’ (units where Neanderthal bones are embedded) so far. The
fluvial–karstic materials originated from a high energy event and are clearly erosive
Geosciences 2021,11, 413 4 of 17
to underlying sediments, especially in the eastern and central parts of the gallery. This
unit corresponds to a diamicton facies.
•
Unit III, a unit of massive clays with dispersed levels of gravels, sands, and silts.
Interbedded silts and fine sands showing fluid escape structures are common. At
the base, this unit is very similar to Unit II and the grain size diminishes towards
the top in general terms. In the western part of the gallery, the grain size of the unit
is also coarser, with a predominance of pebble and gravel deposits. At the top of
the unit, a prominent feature is the existence of calcareous crusts (IIIc) of variable
thickness and texture, with a horizontal arrangement and a high lateral continuity.
These speleothemic crusts (flowstone) reach a greater development and thickness
towards the east wall of the gallery (Figure 3).
•
Unit IV, a unit of massive mud with some interbedded sands. These sediments formed
in a very low energy fluvial–karstic environment and correspond to the final infill
episode in the gallery, which can be regarded as still in progress.
Geosciences 2021, 11, x FOR PEER REVIEW 4 of 18
• Unit II, a unit of poorly sorted gravels, sands, and mud. It represents the lower limit
of the ‘fossiliferous units’ (units where Neanderthal bones are embedded) so far. The
fluvial–karstic materials originated from a high energy event and are clearly erosive
to underlying sediments, especially in the eastern and central parts of the gallery.
This unit corresponds to a diamicton facies.
• Unit III, a unit of massive clays with dispersed levels of gravels, sands, and silts. In-
terbedded silts and fine sands showing fluid escape structures are common. At the
base, this unit is very similar to Unit II and the grain size diminishes towards the top
in general terms. In the western part of the gallery, the grain size of the unit is also
coarser, with a predominance of pebble and gravel deposits. At the top of the unit, a
prominent feature is the existence of calcareous crusts (IIIc) of variable thickness and
texture, with a horizontal arrangement and a high lateral continuity. These speleo-
themic crusts (flowstone) reach a greater development and thickness towards the east
wall of the gallery (Figure 3).
• Unit IV, a unit of massive mud with some interbedded sands. These sediments
formed in a very low energy fluvial–karstic environment and correspond to the final
infill episode in the gallery, which can be regarded as still in progress.
Figure 2. Stratigraphic column from the central zone of the Ossuary Gallery (Sector 3), with a brief description of the main
sedimentary units and their genetic interpretation. Modified from Cañaveras et al. [16,18].
The vast majority of the anthropological and archaeological material is concentrated
between squares E-H/10–E-H/4 in Unit III, which corresponds to Sector 3 (Figure 3). Con-
sidering the whole sediment, this unit is made up of poorly sorted gravelly muddy sands.
The mineralogy of the fine fraction is markedly siliceous, with quartz (70–85%) and clays
(5–25%) as dominant mineral phases [3,4,16]. Feldspars and calcite (mainly bioclast and
rock fragments) usually do not exceed 10% and 5%, respectively. The clay fraction is
mainly composed of kaolinite (25–75%), illite (20–50%) and smectite (5–25%). Sand-size
grains are usually angular-to-subangular in shape, with a typology concordant with host
rock (Oligocene arenites from Pudinga de Posadas Fm.) [16–18]. Subrounded gravel-sized
fragments of Santonian limestones (biopelmicrites and biopelsparites), also from the em-
bedding rock (Pudinga de Posadas Fm.), are common at the base of the unit. Micromor-
phological characters that reflect post-depositional processes, whether edaphic or not, are
Figure 2.
Stratigraphic column from the central zone of the Ossuary Gallery (Sector 3), with a brief description of the main
sedimentary units and their genetic interpretation. Modified from Cañaveras et al. [16,18].
The vast majority of the anthropological and archaeological material is concentrated
between squares E-H/10–E-H/4 in Unit III, which corresponds to Sector 3 (Figure 3).
Considering the whole sediment, this unit is made up of poorly sorted gravelly muddy
sands. The mineralogy of the fine fraction is markedly siliceous, with quartz (70–85%)
and clays (5–25%) as dominant mineral phases [
3
,
4
,
16
]. Feldspars and calcite (mainly
bioclast and rock fragments) usually do not exceed 10% and 5%, respectively. The clay
fraction is mainly composed of kaolinite (25–75%), illite (20–50%) and smectite (5–25%).
Sand-size grains are usually angular-to-subangular in shape, with a typology concordant
with host rock (Oligocene arenites from Pudinga de Posadas Fm.) [
16
–
18
]. Subrounded
gravel-sized fragments of Santonian limestones (biopelmicrites and biopelsparites), also
from the embedding rock (Pudinga de Posadas Fm.), are common at the base of the unit.
Micromorphological characters that reflect post-depositional processes, whether edaphic
or not, are very scarce in the sedimentary fill of the gallery in general and, particularly, in
Unit III [
1
,
10
]. These are restricted to clay/silt translocation processes that can be observed
Geosciences 2021,11, 413 5 of 17
as coatings around voids and the iron–manganese staining of some levels that delineate
fluid escape structures [3,4,10,16].
Geosciences 2021, 11, x FOR PEER REVIEW 5 of 18
very scarce in the sedimentary fill of the gallery in general and, particularly, in Unit III
[1,10]. These are restricted to clay/silt translocation processes that can be observed as coat-
ings around voids and the iron–manganese staining of some levels that delineate fluid
escape structures [3,4,10,16].
The geological analysis of the sediments suggests that all the archaeological record
(the Neanderthal and the lithic remains) dropped into the cave from a higher level in the
karstic system via a vertical shaft in a massive flow deposit as a result of a collapse after a
high-energy event, probably a thunderstorm [16,19,31]. Several pieces of evidence suggest
that the archaeological and anthropological remains were deposited near-simultaneously
shortly before the high energy event: marks left by the mentioned gnawing of carnivores
and rodents are absent, articulated Neanderthal bones are present, and a high refitting
rate of the lithic industry has been observed (studies are ongoing) [11]. Additionally, the
relatively good condition of the bones indicates that they came from the outside, but they
must have been deposited in a protected environment (e.g., a surficial gallery near the
entrance or a rock shelter) and, given the scant traces of alteration documented on the
bones, their exposure time in surface conditions must have been very short [2,12,32].
Figure 3. Excavation plan of Sector 3 with the location of the upper calcareous crust subunit in yellow (Unit III) and sam-
pling zone detailed in the lithostratigraphic sections (square F8).
The morphology of the Ossuary Gallery (i.e., width, length, and sinuosity) has influ-
enced the hydrodynamic behaviour of the cavity, resulting in steep energy from south to
north, which is reflected in the complex distribution of different sediment facies. The spe-
cial configuration of the bottom of the gallery (sponge-work) has determined the complex
Figure 3.
Excavation plan of Sector 3 with the location of the upper calcareous crust subunit in yellow (Unit III) and
sampling zone detailed in the lithostratigraphic sections (square F8).
The geological analysis of the sediments suggests that all the archaeological record
(the Neanderthal and the lithic remains) dropped into the cave from a higher level in the
karstic system via a vertical shaft in a massive flow deposit as a result of a collapse after a
high-energy event, probably a thunderstorm [
16
,
19
,
31
]. Several pieces of evidence suggest
that the archaeological and anthropological remains were deposited near-simultaneously
shortly before the high energy event: marks left by the mentioned gnawing of carnivores
and rodents are absent, articulated Neanderthal bones are present, and a high refitting
rate of the lithic industry has been observed (studies are ongoing) [
11
]. Additionally, the
relatively good condition of the bones indicates that they came from the outside, but they
must have been deposited in a protected environment (e.g., a surficial gallery near the
entrance or a rock shelter) and, given the scant traces of alteration documented on the
bones, their exposure time in surface conditions must have been very short [2,12,32].
The morphology of the Ossuary Gallery (i.e., width, length, and sinuosity) has in-
fluenced the hydrodynamic behaviour of the cavity, resulting in steep energy from south
to north, which is reflected in the complex distribution of different sediment facies. The
special configuration of the bottom of the gallery (sponge-work) has determined the
complex geometry of its sediment infill, but, in turn, has favoured the preservation of
Geosciences 2021,11, 413 6 of 17
archaeo–anthropological material. In this sense, many of these fossiliferous deposits have
been trapped in rock nooks and then rested (well-protected) from episodes of sediment
reworking and destruction, which are common in karst dynamics.
3. Materials and Methods
In order to document the site formation processes operating at the El Sidrón archaeo-
logical site, the sediments that fill the Ossuary Gallery, including those that constitute the
host sediment of the samples under study in the present work, have been characterized
both in situ and in the laboratory (granulometric and petrographic analysis, mineralogical
and geochemical characterization, etc.) [1,3,4,16,19].
A total of 8 samples corresponding to bone fossil remains (Figure 4) and 5 samples
corresponding to black coatings and or impregnations were mineralogical and texturally
studied (Sid. 01, 02, 04, 05, and 06). The samples were selected as the most represen-
tative of the El Sidrón archaeological record in order to study their depositional and
post-depositional evolution; they were also selected because doing so would not negatively
interfere with the anthropological and palaeogenetic studies. All of these samples came
from Unit III and square F8, located in Sector 3 at the central part of Ossuary Gallery
(coinciding with the upper part of Unit III), where a great number of human remains have
been found [11,19].
Geosciences 2021, 11, x FOR PEER REVIEW 6 of 18
geometry of its sediment infill, but, in turn, has favoured the preservation of archaeo–
anthropological material. In this sense, many of these fossiliferous deposits have been
trapped in rock nooks and then rested (well-protected) from episodes of sediment rework-
ing and destruction, which are common in karst dynamics.
3. Materials and Methods
In order to document the site formation processes operating at the El Sidrón archae-
ological site, the sediments that fill the Ossuary Gallery, including those that constitute
the host sediment of the samples under study in the present work, have been character-
ized both in situ and in the laboratory (granulometric and petrographic analysis, miner-
alogical and geochemical characterization, etc.) [1,3,4,16,19].
A total of 8 samples corresponding to bone fossil remains (Figure 4) and 5 samples
corresponding to black coatings and or impregnations were mineralogical and texturally
studied (Sid. 01, 02, 04, 05, and 06). The samples were selected as the most representative
of the El Sidrón archaeological record in order to study their depositional and post-depo-
sitional evolution; they were also selected because doing so would not negatively interfere
with the anthropological and palaeogenetic studies. All of these samples came from Unit
III and square F8, located in Sector 3 at the central part of Ossuary Gallery (coinciding
with the upper part of Unit III), where a great number of human remains have been found
[11,19].
Figure 4. Bone remains with sampled calcareous concretions. The locations of both XRD analyses (dots) and thick sections
(yellow rectangles) are given. See table 2 for A, B, C, D.
The detailed locations and characteristics of bone samples are given in Table 1. The
collected samples were micromorphologically and compositionally characterized using
different analytical techniques.
Figure 4.
Bone remains with sampled calcareous concretions. The locations of both XRD analyses (dots) and thick sections
(yellow rectangles) are given. See Table 2 for A, B, C, D.
The detailed locations and characteristics of bone samples are given in Table 1. The
collected samples were micromorphologically and compositionally characterized using
different analytical techniques.
Geosciences 2021,11, 413 7 of 17
Table 1. Analysed samples indicating their locations in the Ossuary Gallery.
Sample Square Sub-
Square X (cm) Y (cm) Z (cm) Neanderthal Bone
(Anatomical Part)
280 F8 5 38 48 144.0 Indeterminate
282 F8 6 51 82 133.0 Indeterminate
299 F8 2 21 60 149.0 Incisor
304 F8 4 57 30 150.5 Vertebra
317 F8 2 17 59 156.0 Scapula
324 F8 2/3 20 70 139.5 Indeterminate
696 F8 7 80 18 135.0 Vertebra
709 F8 9 81 80 151.0 Rib
X-ray diffraction, with quartz as an internal standard, was used to determine the
mineral composition of the powdered samples. The analyses were performed by using a
PHILIPS PW-1710 XR-diffractometer from Museo Nacional de Ciencias Naturales (MNCN-
CSIC, Madrid) operating at 40 kV and 30 mA under monochromatic CuK
α
radiation. The
diffraction patterns were obtained with a continuous scan from 3
◦
2
θ
to 60
◦
2
θ
, with a 0.01
◦
2
θ
resolution. The XPOWDER
®
program [
33
] was used to evaluate the semi-quantitative
mineral compositions of the samples.
Petrographic and micromorphological conclusions were made based on the exami-
nation of standard and double-polished thin sections with conventional transmitted light
microscopy (Zeiss Assioskop with a digital camera). The samples were preliminarily
observed under a stereoscopic microscope at low magnification.
To complete the textural and compositional characterization of the samples, etched and
unetched specimens of rock fragments and polished thin sections were studied using an
FEI QUANTA 200 scanning electron microscope, with an analytical X-ray energy dispersive
analysis system (EDS) of the MNCN-CSIC laboratory working at 30 kV.
4. Results
All the bone remains from El Sidrón cave are embedded in a dense, poorly sorted,
sandy-silt matrix with a porphyritic, coarse-/fine-related distribution (coarser fragments
floating in a finer matrix). They are highly dehydrated, crumbly, and have multiple mi-
crocracks that, in some cases, have become cracks and fractures (Figure 5A,B). The degree
of physical deterioration is variable, from fragments that present a marked fragmentation
that makes their morphological study impossible to those that fully retain their morphol-
ogy [
2
,
34
]. Additionally, a large amount of the bones appear to be coated with authigenic
mineral coatings of different types and developments that are sometimes interbedded with
different type of crusts.
Calcite is the most common authigenic mineral associated with bone remains at the
site. Additional clay-rich and/or Fe–Mn-rich coatings and associated structures have been
recognized (Table 2). Calcite is found as a micro–mesocrystalline precipitates both at bone
surfaces and in the bone mass itself, as sparry calcite completely filling osteonal cavities,
and along structural weakness in the bone (Figure 5C–F).
Geosciences 2021,11, 413 8 of 17
Geosciences 2021, 11, x FOR PEER REVIEW 8 of 18
Figure 5. Microphotographs of indeterminate bone fragments: (A) microcracks affecting bone surface (Sample 280); (B)
detail of splintered bone fragment (Sample 324); (C,D) calcite aggregates and Fe–Mn precipitates, respectively, filling os-
teonal cavities (Sample 282); (E,F) thin silty micritic crust on bone surface, with detail of calcite crystals filling osteonal
channels and other micropores (Sample 282). Microphotographs (A–C,E) were taken under plane-polarized light; (D,F)
were taken under crossed nicols.
Table 2. Mineral composition of the analysed samples. See Figure 4 for the location of each sam-
ple.
Sample Crust Subtypes Calcite (%) Quartz (%) Feldspars (%) Hydroxyl-Apatite (%)
280-A T 53 39 8
282-A T 62 33 <5
299-A T 64 36
299-B Cm 87 13
304-A Cp 68 32
304-B T 80 20
304-C T 71 12 17
317-A Cm 79 21
324-A T 45 34 21
696-A Cm 96 4
696-B T 63 37
696-C Cp 78 22
696-D T 67 33
709-A T 52 48
709-B T 50 50
(T) Silty–sandy calcite crust; (Cp) sparitic calcite crust; (Cm) micritic calcite crust.
At bone surface, several types of calcareous crusts have been discriminated, mainly
attending to the type of cementing phase and the amount and nature of the grains. Some
of the studied bone fragments show several associated crust types. A schematic represen-
tation of each type, as well as the distribution and spatial relationship of each of these
types in the studied samples, is shown in Figure 6.
Figure 5.
Microphotographs of indeterminate bone fragments: (
A
) microcracks affecting bone surface (Sample 280); (
B
)
detail of splintered bone fragment (Sample 324); (
C
,
D
) calcite aggregates and Fe–Mn precipitates, respectively, filling
osteonal cavities (Sample 282); (
E
,
F
) thin silty micritic crust on bone surface, with detail of calcite crystals filling osteonal
channels and other micropores (Sample 282). Microphotographs (
A
–
C
,
E
) were taken under plane-polarized light; (
D
,
F
)
were taken under crossed nicols.
Table 2. Mineral composition of the analysed samples. See Figure 4for the location of each sample.
Sample Crust
Subtypes Calcite (%) Quartz (%) Feldspars
(%)
Hydroxyl-Apatite
(%)
280-A T 53 39 8
282-A T 62 33 <5
299-A T 64 36
299-B Cm 87 13
304-A Cp 68 32
304-B T 80 20
304-C T 71 12 17
317-A Cm 79 21
324-A T 45 34 21
696-A Cm 96 4
696-B T 63 37
696-C Cp 78 22
696-D T 67 33
709-A T 52 48
709-B T 50 50
(T) Silty–sandy calcite crust; (Cp) sparitic calcite crust; (Cm) micritic calcite crust.
At bone surface, several types of calcareous crusts have been discriminated, mainly
attending to the type of cementing phase and the amount and nature of the grains. Some of
the studied bone fragments show several associated crust types. A schematic representation
of each type, as well as the distribution and spatial relationship of each of these types in
the studied samples, is shown in Figure 6.
Geosciences 2021,11, 413 9 of 17
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Figure 6. Distribution of the different types of carbonate crusts (hard coatings) recognized over the Neanderthal bones
(square F8): (a) horizontal distribution and crust associations; (b) vertical distribution and thickness of the crusts. See
Figures 1 and 3 for locations in the Ossuary Gallery map.
4.1. Calcite Crusts with Abundant Siliciclastic (Terrigenous) Grains
Calcite crusts with abundant siliciclastic (terrigenous) grains are the most abundant
and occur in direct contact with the bone in most samples. Their content of clay and/or
Fe–Mn oxides–hydroxides is variable. Two subtypes can be distinguished:
• Silty (orange) crusts discontinuously adhere to some of the bones and locally infil-
trate through cracks and fractures. They consist of micritic crusts of a yellowish-or-
ange hue about 50–500 µm thick, directly adhering to bone surface (Figure 7A). These
crusts are quite dense and compact, and they are mainly composed of micritic ce-
ment, with clays (predominantly illites), some iron oxide, and a few small (25–50 µm)
terrigenous grains (such as quartz and feldspars).
Figure 6.
Distribution of the different types of carbonate crusts (hard coatings) recognized over the Neanderthal bones
(square F8): (
a
) horizontal distribution and crust associations; (
b
) vertical distribution and thickness of the crusts. See
Figures 1and 3for locations in the Ossuary Gallery map.
4.1. Calcite Crusts with Abundant Siliciclastic (Terrigenous) Grains
Calcite crusts with abundant siliciclastic (terrigenous) grains are the most abundant
and occur in direct contact with the bone in most samples. Their content of clay and/or
Fe–Mn oxides–hydroxides is variable. Two subtypes can be distinguished:
•
Silty (orange) crusts discontinuously adhere to some of the bones and locally infiltrate
through cracks and fractures. They consist of micritic crusts of a yellowish-orange hue
about 50–500
µ
m thick, directly adhering to bone surface (Figure 7A). These crusts
are quite dense and compact, and they are mainly composed of micritic cement, with
clays (predominantly illites), some iron oxide, and a few small (25–50
µ
m) terrigenous
grains (such as quartz and feldspars).
•
Sandy (yellowish) crusts up to 2–3 cm thick, directly developed on the surface of the
bones or on the previously described silty crusts. Their colour is lighter, and their
clays and iron oxide content are lower and more dispersed. On the contrary, the
content of terrigenous grains is higher (Figure 7B). The nature of the grains is mostly
quartz, with a very variable size (40–800
µ
m) and the majority being 50–250
µ
m thick.
Quartz grains seem to display a bimodal sorting with fine, subangular (dominant),
and coarser rounded grains. Feldspars, metamorphic rock fragments, and carbonate
Geosciences 2021,11, 413 10 of 17
bioclasts are also present, although to a lesser extent, as are bone fragments (chips) of
varying size and morphology (Figure 7C,D). The size of the calcitic cement crystals is
microsparitic to mesosparitic (40–100
µ
m), and an increase in the size of the detrital
grains and in the porosity can be observed as one moves away from the bone surface.
There are darker (orange) areas that are irregularly dispersed and about 50 µm thick;
these correspond to a higher content of clays and smaller size of the quartz grains and
crystals of calcite cement. Voids are scarce and mostly correspond to regular vugs or
planes. Associated with large voids, discontinuous clayey cutans (clay coatings) can
be observed, as can some calcitic cement fillings consisting of palisades (sometimes
radially arranged) composed of calcitic tabular crystals with a maximum length of
0.5–0.6 mm. This type of crust is the most abundant at the studied sector of the site.
Geosciences 2021, 11, x FOR PEER REVIEW 10 of 18
Figure 7. Microphotographs of feature characteristics of calcite crusts with abundant siliciclastic grains: (A) thin silty mi-
critic crust (Si) on bone surface; a net contact with the overlying sandy and more porous crust can be observed (Sample
280). (B) Detail of sandy crust with a high proportion of subangular quartz grains in a micrite matrix (Sample 709). (C)
Bioclast (echinoderm) (eq) and bone fragments (bi) in sandy micritic crust (Sample 324). (D) Bone fragments (bi) in sandy
micritic crust (Sample 324). All microphotographs were taken under plane-polarized light.
• Sandy (yellowish) crusts up to 2–3 cm thick, directly developed on the surface of the
bones or on the previously described silty crusts. Their colour is lighter, and their
clays and iron oxide content are lower and more dispersed. On the contrary, the con-
tent of terrigenous grains is higher (Figure 7B). The nature of the grains is mostly
quartz, with a very variable size (40–800 µm) and the majority being 50–250 µm thick.
Quartz grains seem to display a bimodal sorting with fine, subangular (dominant),
and coarser rounded grains. Feldspars, metamorphic rock fragments, and carbonate
bioclasts are also present, although to a lesser extent, as are bone fragments (chips)
of varying size and morphology (Figure 7C,D). The size of the calcitic cement crystals
is microsparitic to mesosparitic (40–100 µm), and an increase in the size of the detrital
grains and in the porosity can be observed as one moves away from the bone surface.
There are darker (orange) areas that are irregularly dispersed and about 50 µm thick;
these correspond to a higher content of clays and smaller size of the quartz grains
and crystals of calcite cement. Voids are scarce and mostly correspond to regular
vugs or planes. Associated with large voids, discontinuous clayey cutans (clay coat-
ings) can be observed, as can some calcitic cement fillings consisting of palisades
(sometimes radially arranged) composed of calcitic tabular crystals with a maximum
Figure 7.
Microphotographs of feature characteristics of calcite crusts with abundant siliciclastic grains: (
A
) thin silty micritic
crust (Si) on bone surface; a net contact with the overlying sandy and more porous crust can be observed (Sample 280).
(
B
) Detail of sandy crust with a high proportion of subangular quartz grains in a micrite matrix (Sample 709). (
C
) Bioclast
(echinoderm) (eq) and bone fragments (bi) in sandy micritic crust (Sample 324). (
D
) Bone fragments (bi) in sandy micritic
crust (Sample 324). All microphotographs were taken under plane-polarized light.
4.2. Calcite Crusts without (Or with Few) Siliciclastic (Terrigenous) Grains
Two subtypes of calcite crusts without (or with few) siliciclastic (terrigenous) grains
can be distinguished:
•
Sparitic crusts alternating (sometimes erosively) with terrigenous-rich crusts. They
consist of layers of palisades composed of millimetre-thick calcite crystals that alter-
nate with bands rich in terrigenous grains (Figure 8A,B). Together, they constitute
a 2–3 cm thick banded precipitate. There are areas of compact palisades showing
banding growth and very porous areas showing the growth of large clustered or
arborescent crystals that are somewhat zoned and sometimes present displacing tex-
tures
(Figure 8C)
. Remobilized areas can also be observed with crystals or aggregates
of broken and moved crystals, as well as local patinas (cutans) of clays and oxides
(Figure 8D). The crystals that make up the palisades—both the compact and arbores-
Geosciences 2021,11, 413 11 of 17
cent ones—usually show scalenohedral terminations, and morphologies similar to
regrown skeletal crystals and/or calcitic rafts can be observed (Figure 9).
•
Micritic crusts, normally with a compact and massive microstructure and locally
characterized by the presence of an irregular lamination involving the alternation
of (1) dark laminae (0.05–0.2 mm thick) of dense micrite and (2) laminae of variable
thickness (0.1–1 mm) consisting of less dense, clotted-to-peloidal micrite–microsparite
that locally present with a wavy–cloudy structure (Figure 10A,B). Areas with the
presence of dispersed terrigenous grains (mainly quartz) of variable size (25–100
µ
m)
are present. Peloidal or spherical structures have diameters ranging between 5 and
80
µ
m (Figure 10B). In some cases, acicular crystals (1–2
µ
m thick and approximately
10
µ
m long) are present in a random deposition (Figure 9C). These are whisker or
needle-fibre calcite (NFC) morphologies (Figure 10C). They are arranged to fill small
pores or partially cover the large ones in association with the clayey patina (clay
coatings and cutans) (Figure 9D). In some cases, their recrystallization to microsparite
crystals is intuited. In the sometimes-transitional contact zones with the yellow–orange
terrigenous-rich crusts, the abundance of fibrous textures, NFC, is significantly higher.
In some cases, an undulating banded arrangement can be observed (Figure 9A).
Geosciences 2021, 11, x FOR PEER REVIEW 11 of 18
length of 0.5–0.6 mm. This type of crust is the most abundant at the studied sector of
the site.
4.2. Calcite Crusts without (Or with Few) Siliciclastic (Terrigenous) Grains
Two subtypes of calcite crusts without (or with few) siliciclastic (terrigenous) grains
can be distinguished:
• Sparitic crusts alternating (sometimes erosively) with terrigenous-rich crusts. They
consist of layers of palisades composed of millimetre-thick calcite crystals that alter-
nate with bands rich in terrigenous grains (Figure 8A,B). Together, they constitute a
2–3 cm thick banded precipitate. There are areas of compact palisades showing band-
ing growth and very porous areas showing the growth of large clustered or arbores-
cent crystals that are somewhat zoned and sometimes present displacing textures
(Figure 8C). Remobilized areas can also be observed with crystals or aggregates of
broken and moved crystals, as well as local patinas (cutans) of clays and oxides (Fig-
ure 8D). The crystals that make up the palisades—both the compact and arborescent
ones—usually show scalenohedral terminations, and morphologies similar to re-
grown skeletal crystals and/or calcitic rafts can be observed (Figure 9).
Figure 8. Microphotographs of feature characteristics of sparitic crusts: (A,B) columnar fabric composed of elongate calcite
crystals with irregular boundaries, some of them dissolved and filled by detrital grains (Sample 304); (C) detail of arbo-
rescent-zoned calcite crystals (Sample 304); (D) detail of erosive contact (arrows) between sparitic laminar layers and over-
laying sandy micritic crust (Sample 696). Microphotographs (A,C,D) were taken under plane-polarized light; (B) was taken
under crossed nicols.
Figure 8.
Microphotographs of feature characteristics of sparitic crusts: (
A
,
B
) columnar fabric composed of elongate
calcite crystals with irregular boundaries, some of them dissolved and filled by detrital grains (Sample 304); (
C
) detail of
arborescent-zoned calcite crystals (Sample 304); (
D
) detail of erosive contact (arrows) between sparitic laminar layers and
overlaying sandy micritic crust (Sample 696). Microphotographs (
A
,
C
,
D)
were taken under plane-polarized light; (
B
) was
taken under crossed nicols.
Geosciences 2021,11, 413 12 of 17
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Figure 9. SEM micrographs of sparitic calcite crust: (A,B) partially dissolved scalenohedral calcite crystals (Sample 304);
(C) detail of calcite scalenohedral terminations (Sample 696); (D) microcrystalline aggregate with needle-fibre calcite
(NFC) crystals (Sample 696).
• Micritic crusts, normally with a compact and massive microstructure and locally
characterized by the presence of an irregular lamination involving the alternation of
(1) dark laminae (0.05–0.2 mm thick) of dense micrite and (2) laminae of variable
thickness (0.1–1 mm) consisting of less dense, clotted-to-peloidal micrite–micro-
sparite that locally present with a wavy–cloudy structure (Figure 10A,B). Areas with
the presence of dispersed terrigenous grains (mainly quartz) of variable size (25–100
µm) are present. Peloidal or spherical structures have diameters ranging between 5
and 80 µm (Figure 10B). In some cases, acicular crystals (1–2 µm thick and approxi-
mately 10 µm long) are present in a random deposition (Figure 9C). These are
whisker or needle-fibre calcite (NFC) morphologies (Figure 10C). They are arranged
to fill small pores or partially cover the large ones in association with the clayey pat-
ina (clay coatings and cutans) (Figure 9D). In some cases, their recrystallization to
microsparite crystals is intuited. In the sometimes-transitional contact zones with the
yellow–orange terrigenous-rich crusts, the abundance of fibrous textures, NFC, is sig-
nificantly higher. In some cases, an undulating banded arrangement can be observed
(Figure 9A).
Figure 9.
SEM micrographs of sparitic calcite crust: (
A
,
B
) partially dissolved scalenohedral calcite crystals (Sample 304); (
C
)
detail of calcite scalenohedral terminations (Sample 696); (
D
) microcrystalline aggregate with needle-fibre calcite (NFC)
crystals (Sample 696).
Geosciences 2021, 11, x FOR PEER REVIEW 13 of 18
Figure 10. Microphotographs of feature characteristics of micritic crusts: (A,B) compact micritic crust composed of the
alternation of dense and clotted laminae, with details of peloidal structures (Sample 304); (C) NFC crystals in a random
aggregate (Sample 696); (D) laminar and discontinuous clay (hypo-) coatings (arrows) (Sample 696). Microphotographs
(A,B,D) were taken under plane-polarized light; (C) was taken under crossed nicols.
4.3. Black Crusts and Patina
Black crusts and patinas are common in the studied samples, covering clasts of the
conglomeratic host rock or the filling sediments, as well as upholstering walls and delim-
iting fluid escape structures. These layers are usually very thin, smaller than 1 mm, and
when their composition is identifiable in XRD, it can be seen that they are composed of
manganese oxides, manganese hydroxides (mainly birnessite), and iron minerals (such
goethite and ferrihydrite). From a textural point of view, massive crypto-microcrystalline
aggregates are predominant (Figure 11A,B), but laminar and botryoidal textures covering
grains (quartz, bone fragments, etc.) or pores are also present (Figure 11C,D). Semi-quan-
titative chemical analyses (EDSs) seem to indicate that iron-rich mineral phases predomi-
nate in grain and clast coatings, while manganese precipitates predominate in crusts and
impregnations in fine sediments (Table 3).
Figure 10.
Microphotographs of feature characteristics of micritic crusts: (
A
,
B
) compact micritic crust composed of the
alternation of dense and clotted laminae, with details of peloidal structures (Sample 304); (
C
) NFC crystals in a random
aggregate (Sample 696); (
D
) laminar and discontinuous clay (hypo-) coatings (arrows) (Sample 696). Microphotographs
(A,B,D) were taken under plane-polarized light; (C) was taken under crossed nicols.
Geosciences 2021,11, 413 13 of 17
4.3. Black Crusts and Patina
Black crusts and patinas are common in the studied samples, covering clasts of
the conglomeratic host rock or the filling sediments, as well as upholstering walls and
delimiting fluid escape structures. These layers are usually very thin, smaller than 1 mm,
and when their composition is identifiable in XRD, it can be seen that they are composed
of manganese oxides, manganese hydroxides (mainly birnessite), and iron minerals (such
goethite and ferrihydrite). From a textural point of view, massive crypto-microcrystalline
aggregates are predominant (Figure 11A,B), but laminar and botryoidal textures covering
grains (quartz, bone fragments, etc.) or pores are also present (
Figure 11C,D
). Semi-
quantitative chemical analyses (EDSs) seem to indicate that iron-rich mineral phases
predominate in grain and clast coatings, while manganese precipitates predominate in
crusts and impregnations in fine sediments (Table 3).
Geosciences 2021, 11, x FOR PEER REVIEW 14 of 18
Figure 11. Fe–Mn deposits: (A) iron-rich patina covering a siliciclastic grain (Sample Sid-04); (B) anhedral masses of Fe
and Mn oxides (Sample Sid-06); (C,D) ferruginous cements with a botryoidal texture (Sample Sid-05).
Table 3. Chemical composition (EDS) of black impregnations and coatings. (Sid 01, 02, and 06): black
impregnations (mottling); (Sid 04 and 05): grain coatings.
Sid 01 Sid 02 Sid 04 Sid 05 Sid 06
O 49.45 50.64 48.63 47.15 53.50
C 15.41 21.65 11.01 15.58 8.79
Si 2.41 5.72 4.46 3.30 1.24
Al 4.11 4.55 3.80 2.89 6.47
Mg 0.46 0.38 0.19 0.29 -
Fe 3.26 3.91 28.98 18.02 10.10
Mn 18.56 8.51 1.05 10.28 17.10
Ca 3.00 3.94 1.36 2.00 2.55
K - 0.47 0.38 0.34 0.04
P 0.38 0.25 0.14 0.16 0.21
F 3.03 - - - -
5. Discussion and Conclusions
Different types of crusts (and coatings) have been observed to be attached to bone
fragments in the Ossuary Gallery sedimentary infill at the El Sidrón Cave archaeological
Figure 11.
Fe–Mn deposits: (
A
) iron-rich patina covering a siliciclastic grain (Sample Sid-04); (
B
) anhedral masses of Fe and
Mn oxides (Sample Sid-06); (C,D) ferruginous cements with a botryoidal texture (Sample Sid-05).
Table 3.
Chemical composition (EDS) of black impregnations and coatings. (Sid 01, 02, and 06): black
impregnations (mottling); (Sid 04 and 05): grain coatings.
Sid 01 Sid 02 Sid 04 Sid 05 Sid 06
O 49.45 50.64 48.63 47.15 53.50
C 15.41 21.65 11.01 15.58 8.79
Si 2.41 5.72 4.46 3.30 1.24
Al 4.11 4.55 3.80 2.89 6.47
Mg 0.46 0.38 0.19 0.29 -
Fe 3.26 3.91 28.98 18.02 10.10
Mn 18.56 8.51 1.05 10.28 17.10
Ca 3.00 3.94 1.36 2.00 2.55
K - 0.47 0.38 0.34 0.04
P 0.38 0.25 0.14 0.16 0.21
F 3.03 - - - -
Geosciences 2021,11, 413 14 of 17
5. Discussion and Conclusions
Different types of crusts (and coatings) have been observed to be attached to bone
fragments in the Ossuary Gallery sedimentary infill at the El Sidrón Cave archaeological
site. From a mineralogical point of view, these crusts are mainly composed of calcite
(cements) and quartz (detrital), with lower proportions of feldspars (detrital) and iron and
manganese oxides (such as patina and concretions), the latter not quantifiable by XRD.
These results indicate the existence of different phases and/or mechanisms of carbonate
(calcite) crusting, characterized by variations in the detrital aggregate/cement ratio from the
internal to external zones of the bony substrate, as well as by granulometric and textural
differences. These differences indicate diverse scenarios, from the initial post-mortem
accumulation to the final deposit in the Ossuary Gallery, as well as eventual alterations
linked to the changes in the hydrodynamic regime of the gallery.
There is no correlation with the depth at which each sample was found (i.e., the
deepest one was not the one with a thickest and largest crusty development). Likewise,
there is no clear pattern in the spatial distribution of the crusts according to their typology.
Only the crusts that are richer in carbonate (sparitic subtype) were found to be located
closer to the cemented flowstone layer at the top of Unit III. The detrital-rich crusts are
directly attached to the surface of the bones, and the calcitic crusts (both the micritic and
sparitic subtypes) are located on top of the bones.
Micritic crusts show diagenetic microfabrics as clotted-to-peloidal micrite–microsparite,
NFC, and clay coatings or cutans that point to microbial biological activity in a subaerial
environment [
24
,
35
]. Clotted peloidal fabrics are common in microbial formations such as
travertines, stromatolites, and thrombolites [
36
]. The observed NFC aggregates rarely com-
pletely fill the pores in which they occur, creating a fine interlacing partial infilling. NFC
usually forms in the early phase of pedogenesis and precipitates as cement in vadose condi-
tions [
37
]. NFC can also be found on cave walls in association with speleothems [
38
]. NFC’s
origin has been discussed for many years, but several recent studies have supported argu-
ments for its directly or indirectly biogenic origin [
38
–
40
], although the micro-organisms
responsible for its formation have still not been identified [
40
]. However, some studies
have suggested that NFCs are largely a product of abiogenic vadose precipitation that
involved little or no biological influences [41].
On the other hand, the sparitic crusts formed by palisades of calcitic tabular crystals
correspond to episodes of net speleothemic precipitation. The relative proximity of these
sparitic coatings to the flowstone deposits that culminate in Unit III (Figures 3and 6)
could be related to the percolation of carbonate-rich water through the sediment and the
precipitation of calcite coatings at lower levels. However, the orientation and geometry of
these coatings in the studied samples (they do not show pendant geometry or parallel to
the surface) indicate that their formation was mostly prior to their arrival to the Ossuary
Gallery and to the formation of the speleothems that are associated with its sedimentary
infill. Detritus within or associated with these precipitates could have originated from a
variety of sources, including air-born silts and clays near cave entrances, transportation by
cave ventilation, or fine-grained sediments carried through fractures by infiltrating waters
or suspended by floodwaters [42].
Calcite crusts with abundant siliciclastic (terrigenous) grains are the most abundant
and most in contact with the bones, which are commonly fragmented and disarticulated.
In several samples, these crusts are covered by calcite crusts subtypes (massive peloidal mi-
critic and porous micritic with NFC) (Figure 6) whose formation occurred under subaerial
conditions close to the surface in a phase prior to the arrival of the bones to the Ossuary
Gallery. The silty (orange) crusts are adhered more or less continuously to the surface of
some bones, with their grain size (clay and silt) and the texture (angular and well-sorted)
being possibly related to an aeolian origin [
29
,
43
,
44
]. Likewise, the existence of accumula-
tions of iron oxides–hydroxides associated with these crusts fits well with an environment
of subaerial exposure [
29
,
45
]. Sandy (beige) crusts, which are the most abundant and
often intercalated with the other types of crusts, contain depositional and post-depositional
Geosciences 2021,11, 413 15 of 17
hydromorphic features (i.e., layered clay coatings, extensive iron–manganese impregnation,
and desiccation cracks) [34].
Finally, the development of Fe–Mn oxides precipitates as grain (including bone frag-
ments) coatings and disperse impregnations on groundmass may have resulted from a
hydromorphic process, indicating the movement of water through the profiles under the
influence of a shallow groundwater table in an oxic cave environment [
46
,
47
]. Fe and Mn
oxide deposits formed in this way are common in caves and thought to be primarily medi-
ated by microbial activity [
48
,
49
]. Likewise, the formation of these oxides, together with
the reprecipitation of calcite as void coats and infillings and the presence of clay-cutans,
could indicate soil-formation processes [50,51].
Regarding sediments, no micromorphological features (e.g., hydromorphism and
bioturbation) that point to the development of in situ pedogenetic processes in any of the
units that make up the sedimentary fill of the site have been recognized. Features such as
clay coatings and/or silt cappings may have originated from drip-water that percolates
through the sediment and redistributes fine-grained detritus around grains or filling pores.
In summary, the analysis of the crusts adhered to the Neanderthal bones at the Ossuary
Gallery has indicated that some of the skeletal remains remained in a surface environment
(aeolian patina, illuviation–eluviation features, superficial biogenic crusts, etc.) earlier
than their deposition inside the cave. These subtle soil-forming processes must have
occurred in areas close to the outside of the karstic system such as cave entrances or
rockshelters. Given the few traces of alteration that the bones present, their permanence
in superficial conditions must have been short [
19
,
32
]. This intermediate storage, like the
most superficial location (shelter or entrance to a gallery), is situated in a vadose context,
and they are both distant and disconnected from the hydrodynamically active zone of
the El Sidrón karst system. At present, that entrance is covered by colluvial deposits and
soils on which the current forest is developing. During their post-depositional history,
the paleontological bone assemblage suffered from surface bleaching, the loss of organic
components, progressive cracking and splintering, and carbonate concretion.
Author Contributions:
Conceptualisation, J.C.C., S.S.-M., and M.d.l.R.; investigation, J.C.C., S.S.-M.,
E.D., G.S.-D., M.C.M.-C., P.G.S., S.C., Á.F.-C., J.L., and M.d.l.R.; writing—original draft, J.C.C. and
M.d.l.R. All authors have read and agreed to the published version of the manuscript.
Funding:
This work has been supported since 1999 through different research contracts between the
Government of the Principality of Asturias, the University of Oviedo, the University of Alicante, the
National Museum of Natural Sciences (CSIC, Madrid), and the University of Salamanca.
Data Availability Statement: Not applicable.
Acknowledgments:
We want to thank the magnificent team of archaeologists for the invaluable
help that they have given us over all these years; they patiently and conscientiously made our work
infinitely easier. We also thank Jesús Jordá, Guest Editor of this Special Issue, and the reviewers for
their constructive suggestions. In memoriam to our dear Javier Fortea (1946–2009), without whom
this work would have been far from perfect, and Manolo Hoyos (1944–1999), a master geologist and
pioneering geoarchaeologist.
Conflicts of Interest: The authors declare no conflict of interest.
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