Beiner & Rabinovich, 2014: BONES AND SEDIMENTS
DISCUSSION
Yosef, 1990; Weiner et al., 1993). Another
approach relies on measuring bone crystallinity,
which is also considered to be connected with
Because of the potential complex interaction
degradation of bone protein (e.g., Trueman et
between bone and sediment inherited in their
al., 2004) and is even rate-limited by collagen
chemical components, humidity, temperature
decomposition in the early stages of diagenesis
and so on, no simple model can predict how
(Trueman et al., 2008:165). However, it has
exactly bones are preserved in the sediment.
been stated that although loss of collagen
Theories on this subject relate to the action of
weakens the bone structure, destruction is
water, moving organic constituents out from
mainly
a factor
of
physical
weathering
the bone and depositing soluble minerals from
(Trueman et al., 2004:729). Another statement
the surrounding soil matrix within bone pores
is that long-term preservation, again mainly of
(Trueman,
2004:732;
Schweitzer
et
al.,
organic constituents, depends on the rate at
2008:160). As a result, ancient bones end up
which bone becomes a closed system (Trueman
as bio-apatites with abundant authigenic
et al., 2008:165).
mineral phases both in larger pore spaces in
Studies on the relationship between bone
cancellous bone and in smaller vascular pores
diagenesis and depositional environment often
(Chavagnac et al., 2007:178). Chemical change
aim at paleoenvironmental, paleoclimatic and
begins immediately once bones are removed
paleoecological reconstructions. However, in
from their in vivo context (Trueman et al.,
some cases it may be possible to reverse the
2008:160), and organic "leaching" appears to
relationship and gain some information on the
occur very quickly once the bone is deposited,
effect of depositional environments on bone
as shown by bone exposure experiments.
preservation. For example, different minerals
Samples collected five years or more after
identified on and within archeological bone
death already exhibited low organic content,
may be indicative of the type of taphonomic
and bones exposed for 26 years or less already
process,
e.g.,
the
mineral
Trona
undergo considerable physical and chemical
(Na
3(CO3)(HCO3)2H2O) is formed on bone
changes (Trueman et al., 2004:726, 729).
surfaces via evaporation of water containing
Generally speaking, modern bone is composed
dissolved calcium and sodium (Trueman et al.,
of (soluble) carbonated hydroxyl apatite (Berna
2004:732), indicating that the bone was
et al., 2004), whereas ancient bones typically
exposed to air, resting on the soil surface.
contain fluorinated apatite (Chavagnac et al.,
Following this line of thought, it is interesting to
2007:178), also known as francolite (Berna et
note the finds from the chemical analysis of
al., 2004:868). However, it is recognized that
bones
from
Revadim.
Chemical
analysis
regions of a single bone can vary greatly in
indicated the presence of the mineral dahllite,
preservation (Schweitzer et al., 2008:160). For
and that no collagen was left (Rabinovich et al.,
example, loss of organic content in the same
2012:7). Bone crystallinity was assessed
bone can differ considerably, with bone surface
according to parameters set by Weiner and
losing double the organic content as sub-
Bar-Yosef (1990), and the results indicated
surface regions of the same bone (Trueman et
severe bone diagenesis, with varying degrees
al., 2004:726). Preservation in different parts
of manganese oxide accumulation (Rabinovich
of the skeleton may be affected by various
et al., 2012:7). Dahllite tends to accumulate
factors, ranging from anatomical characteristics
mainly on exterior surfaces and its presence
such as tooth, tusk or skull morphology versus
may possibly indicate exposure and weathering
long bone composition (Rabinovich et al.,
processes, as shown by experiments on freshly
2012:2) to microbial action, biogeochemical
exposed bone (Trueman et al., 2004:725).
reactions, cell or tissue breakdown, acid
In the EEA locality, previous excavations
formation, and molecular breakdown processes
revealed information on the geology of the site.
(Schweitzer et al., 2008:161). The latter
Generally speaking, sediment accumulation
taphonomic pathways combine to degrade all
processes by lake margins create good burial
organic remains completely. When bone is well
and preservation potential (Feibel, 2004: 22).
preserved, it may be assumed either that the
Lake margins exhibit complex sedimentary
diagenetic processes were halted at an early
components, deposited either by water or by
stage, or that mineralization proceeded faster,
air, and modified by the fluctuating lakeshore
preserving the shape of the bone (Schweitzer
(Feibel, 2004:22). Although EEA is within a
et al., 2008:161).
formation
characterized
by
a
lacustrine
Current
methods
for
assessing
bone
environment, this particular site exhibits
preservation
tend
to
heavily
emphasize
sediments
with
well-developed
paleosols
collagen preservation (e.g., Weiner and Bar-
(Feibel, 2004:23). Out of 20m of sediments
25
Journal of Paleontological Techniques