Lacasa-Marquina et al., 2014: IN SITU CONSERVATION STRATEGIES
DISCUSSION
Bones are highly vulnerable to external climate
The depositional and burial conditions favored
changes, especially to RH oscillations, due to
preservation
of
sub-fossil
remains.
their property of hygroscopicity, that is, the
Stratigraphic evidence, sedimentary context,
ability to absorb or release moisture with
and taphonomic data show that mostly all the
changing humidity, swelling or shrinking in
alterations documented on the bones did not
response to variations in their moisture
occur during their accumulation and burial
content. Thus, the risk of physicochemical
processes. Thus, the alterations due to
damage occurs when sub-fossil remains are
biostratinomic and diagenetic processes do not
subjected to a fluctuation more rapid than their
entirely justify their poor state of preservation.
ability to respond evenly (Michalski, 2009).
Moreover, the physical properties of the soil
During burial, bones reach a physicochemical
context ­ texture, structure and permeability ­
equilibrium with the soil context, characterized
and also its chemical conditions ­ pH, RRR, and
by almost stable environmental conditions.
matrix composition ­ helped to preserve the
However, at the moment of their unearthing
sub-fossils and, consequently, a wealth of
during fieldwork, sub-fossil specimens are
outstanding information.
The
sedimentary
abruptly exposed to new and fluctuating values
matrix of level B is mainly formed by silt and
of climatic parameters. The adaptation process
clay, which constitute soils characterized by
to these new parameters causes the rapid
their low-moderate permeability and their
drying of the bones, and consequently, it
ability to retain water. The mineral composition
generates physical damage in the form of
of sediments and the low RRR recorded in the
exfoliation, cracks, and splits.
area have favored a tendency towards neutral-
The results of the preliminary climatic study
basic pH of the soil, optimizing the conditions
show that during the fieldwork season the
for preservation of sub-fossil bones. Similarly,
weather was one of the driest and warmest
the calcium carbonate layer on bones plays a
throughout 2012. Thus, in principle, it may be
dual role of preservation, as a structural
thought that the climate conditions would be
reinforcement and protective coat of the
adverse for the preservation of the bones. In
surface, and as a buffering agent of RH.
particular, the diurnal time interval from 1 pm
Nevertheless, some physical properties of the
to 4 pm represents a higher risk of degradation
sedimentary matrix could become a cause of
as it shows the lowest values of RH and the
degradation when the sub-fossils are extracted
highest of T, so the disparity between the
from their soil context. The prismatic structure
values of climate parameters from air and soil
of the silt and clay matrix, combined with their
context is increased. Also, the results on
sudden desiccation process when exposed to
fluctuation dynamics demonstrate that the
the sub-aerial context, constitute a risk factor
climate values of RH are highly and rapidly
for the fragile mechanical stability of bones.
changeable, both at daily and hourly cycles.
Additionally, some of the recognized physical
Additionally, 34% of the values of these data
damage, including fractures, split line cracks,
are outside of the proper range of preservation
exfoliation, delamination, and the presence of
for bones defined by the general criteria
roots, are due to post-depositional processes,
(Michalski, 2009). Nevertheless, the standard
mainly sedimentary compaction.
deviation and the graphical distribution of
Regarding the climatic study, it should be noted
histograms show a high dispersion of the data
that the reference data taken from the Sétif
typical for outdoor environments (Figures 8, 9).
International Airport weather station have been
Therefore,
in
order
to
obtain
more
very useful for carrying out a pre-analysis on
representative
conclusions,
it
would
be
the climate dynamics of the region. However,
advisable
to
define
an
environmental
as stated before, we must bear in mind the
monitoring plan, which would cover a sampling
distance between the two locations. Meanwhile,
interval of one sample of RH and T per hour.
the results of our climatic statistical analysis,
Thus, based on the hygroscopic property of
allow us to draw conclusions about the
bones, their high degree of vulnerability to
potential risk of degradation by assessing the
rapid fluctuations of RH, and the results of the
magnitude of the daily and hourly fluctuations;
environmental analysis, we can infer that the
verifying if the maximum and minimum values
climatic factor represents the main risk of
are within the proper range of preservation;
degradation for bones. The combination of two
evaluating the representativeness of the data;
factors
­
the
contrasting
environmental
and determining the unsuitable time intervals
conditions between air and soil, and the
for the development of fieldwork.
need for bones to reach an equilibrium with the
12
Journal of Paleontological Techniques