Fire in the Genesis of Laguna Merin Basin Mounds.
The Visualization through the thermoluminescence Dating Techniques
Roberto Bracco Boksar`1*, Christopher Duart`e2, Ofelia Gutiérrez3, Andreina Bazzino3, Natalia Alonso2 and Y Daniel Panario3
1Department of Humanities and Education Sciences, Universidad de la República, Uruguay
2 Department of Sciences, University of the Republic, Uruguay
3Department of Sciences, University of the Republic, Montevideo, Uruguay
Submission: July 09, 2019; Published:November 14, 2019
*Corresponding author:Department of Humanities and Education Sciences, Universidad de la República, Montevideo, Uruguay
How to cite this article:Roberto Bracco Boksar, Christopher Duarte, Ofelia Gutiérrez, Andreina Bazzino, Natalia Alonso, Y Daniel Panario. Fire in the
Genesis of Laguna Merin Basin Mounds.The Visualization through the thermoluminescence Dating Techniques. Glob J Arch & Anthropol . 2019; 11(1):
555804. DOI: 10.19080/GJAA.2019.11.555805
The study of the genesis of the mounds of the Laguna Merín basin focused on the contribution of sediments and discarded elements. Investigations based on the geochemistry of the matrix led to consider the role of fire in its elevation. We try to contrast their presence using the techniques of luminescence dating. If different fractions of the matrix present ages or paleodosis OSL/TL statistically not differentiable, then the bleaching agent would be heat. The hypothesis was confirmed in mounds of three archaeological sites located in the southern area of the basin. We can verify that we are faced with recursive practices, which produce accumulations of thermo-altered sediments. This led us to earth ovens and Australia’s oven mounds. Prehistoric oven earths have been identified in Uruguay. The oven mounds are a powerful ethnographic-archaeological analogue that illustrates the formation processes of thermo-altered sediment accumulations, while allowing addressing socio-economic and symbolic aspects. Finally, from the implications of the hypothesis, we pointed out the pertinence of approaching the mounds in two scales. One would correspond to the behaviors that elevated them and the other to their reality as patches within the landscape.
The investigation of the genesis of Laguna Merin Basin (LMB) earth mounds, has paid special attention to the contribution of sediments and discarded elements. This becomes evident when they are characterized as earth works that integrate waste of human activities as well as human burials [1-5]. Researches focusing on the geochemistry of the matrix have led us to consider the role of fire in the construction processes . In this article we present a series of luminescent data that verify the hypothesis that fire was present in the construction processes. At the same time, we interpret this new evidence in the light of earth ovens technology, which leads us to an ethnographic-archaeological analogue: the oven mounds of Australia.
Luminescence dating techniques are based on the property of some minerals, such as quartz and feldspar, of accumulating ionizing energy and releasing it when stimulated with light or
heat (bleaching). This energy (background radiation) comes in nature, from unstable natural isotopes and from the universe
(cosmic rays). The amount of accumulated energy (paleodose) is proportional to the intensity of background radiation and to the time elapsed since the mineral was last bleached; therefore, if paleodose and radiation intensity are estimated, the time elapsed since bleaching can be calculated [7,8]. The bleaching is total when the mineral is exposed to enough temperature “restarting the thermoluminescent and luminescent clock”, and partial when exposed to light “re-starting the luminescent clock”. In the latter case there remains an energy remnant that is only released if the mineral is heated. If the event to be dated is contemporary with a heat bleaching, both the technique of thermoluminescence dating (TLD) or the technique of dating by optical stimulation (OSLD) can be used. In the case that the event to be dated is coeval with a bleaching by light, only the OSLD dating technique is used, using TLD would estimate a higher paleodose (older age) since it will integrate the energy remnant
that was not released with the sample’s exposure to light only.
This indirectly allows us to know the bleaching agent. When
OSLD and TLD paleodoses are statistically indistinguishable
they are indicating heat bleaching.
Our work hypothesis is: if the OSLD and TLD paleodoses of
the same fractions of the matrix coming from the same level,
are statistically not differentiable, then the matrix was exposed
to heat, at enough temperature so that the entire luminescent
record had been bleached. This hypothesis has already been
contrasted in the García Ricci and Los Ajos sites (Bracco et al.
2019) (Figure 1).
Data presented here comes from mounds 1, 2 and 3 of
the Pelotas sites (33o27’26.26 “S-53o50’28.05” W). This
archaeological site is composed of 9 mounds of 1.5 to 3.5 meters
in height. It is located on the right bank of the homonymous
stream, on a plain that develops at an elevation of 11 meters
above sea level in the department of Rocha, Uruguay. With the
assistance of a soil sampler (AMS™), successive vertical samples
of the matrix were taken in the center of the mounds, jacketed
in PVC tubes of 15cm in length, from peak to base. Samples
were processed in the Laboratory of Luminescence of Faculty of
Sciences, University of the Republic. Under appropriate artificial
red light (645-700nm) the fine sand and silt fractions were
separated and treated to measure their luminescent signals with
a Daybreak 1100 automatic reader. To estimate the equivalent
dose [7,9] a Daybreak ™ Model 801E irradiator equipped with a
90Sr beta source was used (0.0597 Gy/s September 2000).
The TLD measurements were performed under a nitrogen
atmosphere, following the MAAD method . The region of the
spectrum used to determine the equivalent dose was selected
by the plateau method . The OSLD measurements were made
following the MAAD and MARD method . Series of 6 aliquots
were measured to estimate the intensity of natural signal and
of the 4 irradiation steps. To reduce dispersion caused by
differences in sensitivity or load, the OSLD measurements of
the sand fraction included a previous measurement of 0.15
seconds of all aliquots. From these data, a correction factor was
calculated. In all cases, only the series of measurements that had
a CV ≤ 5% were accepted as valid, ruling out a maximum of two
measurements, otherwise, the series was repeated.
The χ ^ 2 values allow us to observe that paleodoses
determined by duplicate for the same fractions of the matrix
coming from the same level, are statistically not differentiable
for TLD or OSLD (Table 1). Accordingly, weighted average TLD
and weighted average OSLD from samples of the same fractions
and the same provenance, are also not statistically differentiable.
Paleodoses determined from the silt fraction are higher than
those determined from the sand fraction because in the sand
fraction the record of the alpha particles was eliminated .
Paleodoses estimated by OSLD and TLD techniques for
the different fractions of the mound’s matrix and for the same
levels are statistically not differentiable, so we conclude that the bleaching agent was heat. The peaks of the spectra indicate that
the minimum heating temperature reached for the matrix was in
the order of 380oC (Figure 2). Many human practices produce
thermo-alteration of sediments, however, those that recur in
the same place, during very long periods of time, originating
mounds, are not so common. The oven mounds, Australian earth
mounds, are concordant with both aspects, thermo-altered
and accumulated sediments. Earth mounds are a characteristic
archaeological feature of the lower basins of the West,
South Alligator and Murray rivers of southern and northern
Australia, their research has been based on ethnographic and
archaeological information [11,12]. They have circular or oval
plants, reaching 200 meters long, and 2 meters height. They are
found in floodplains or in their limits. They are recognized as
multifunctional sites that were occupied seasonally . The
spatial distribution of the mounds suggests an economy focused
on aquatic ecosystems. This is supported by ethnohistorical
references . In the north, the earliest chronologies of earth
mounds reach 4600 years 14C AP . In the south they reach
4330 years 14C AP. Their elevation was progressive, some took
short periods of 300 years and others longer periods, above
2000 years  (Figure 2). The presence of heat accumulator
fragments made of clay, noted by Jones et al. , indicates that
the main mechanism of mound growth is the sequential use
of earth oven technology during long periods, supplemented
with occupational waste. Brockwell  reports, for the
northern region, different researchers who, based on the same
evidence, propose the same principal mechanism of elevation.
Accumulation of waste produced using earth ovens (mainly heat
accumulators) in the same place over long periods, is indicated
as the main cause of mounds’ growth.  emphasize that
beyond the choice of location for economic factors and the main
mechanism of growth, the mounds may have been imbued with
a cultural, social and / or spiritual meaning. In the region of the
Reynolds and Alligator rivers they have been used as burial sites
Black & Thoms  (Figure 1) describe a ground furnace as
a cooking device arranged in layers within a pit. It starts with
the fire, arranging the heat accumulators inside and around it.
Once the fire diminishes, the accumulators are placed below and
above the food packaged with vegetables. Layers of vegetables
are often arranged between accumulators and food. Finally, the
oven is sealed with sediment, bark or leather. Heat accumulators
reported for North America and Europe are rock fragments
[13-15]. In Australia, clay balls are used . Ethnographic
information indicates that fragments of termite nest were used
as heat accumulators for the Reynolds River and the Blyth River
regions. According to Black & Thoms  most of the ovens
earth layers are rarely discernible in the archaeological record,
but not the heat accumulators.
Black & Thoms  point out that earth ovens are specialized
plant processing facilities, dating back to the beginning of
the Holocene in America. Thoms  indicates that its use
intensified among hunter-gatherers in western North America
between 4,000- and 2,000-years BP, when abundant and
accessible plant resources, which require prolonged cooking to
increase their nutritional value, were integrated into the diet
 Wandschnider 1997. According to Black & Thoms  sites
of earth ovens are often reused hundreds of times over centuries
or millennia. Some of the elements incorporated during their
use are removed or perishable (food, fuel and packaging).
Others, such as fragmented heat accumulators, remain. For
Uruguay we have two references of earth ovens. Guidón  for
the site Y-58 of Salto Grande, refers to the presence of “culinary
pits”, accumulations of snails accompanied by stones. They
are assigned an age of 3000 years. Consens  in the mound
of Yacaré-Cururú recognizes several ovens formed by stone
accumulations in shallow pit. He interprets them as ovens
used to improve the properties of the nodes destined to make
stone tools. However, these structures are very similar to those
diagnosed as earth furnaces in North America (Figure 2) [15,18]
Other similarities between Laguna Merin basin mounds
and the Australians mounds deserve attention. Bracco et al. 
point out that, for the region of India Muerta-Paso Barranca, the
mounds are aggregated in the areas where the flood is more
extensive, where the supply of economic resources would be
higher. Burned earth was characteristic of the mounds of that
area, which could be explained as remnants of heat containers.
It has also been pointed out that, for their preparation, anthills,
biostructures like termite mounds, were used . On the other
hand, anthracological records [19,20] are concordant with
the vegetal fuel of low caloric potential used in earth furnaces
. The record of silicophytoliths shows a wide use of wild
plants, many of which are cooked for consumption by native
groups [21,22]. Finally, it is pointed out that the mounds of the
Laguna Merín, like the Australians, were elevated over centuries
or millennia, manifesting themselves as the consequence of
a recursive and spatially circumscribed behavior that was
repeated over centuries or millennia [23-25].
Paleodoses estimated by TLD / OSLD of different fractions
of the matrix, for the same levels of the mounds 1, 2 and 3 of
the Pelotas sites, are statistically not differentiable, which allows
us to deduce that the mounds’ matrix was exposed to heat. The
peaks of the TL spectra indicate that the heating temperature
reached at least 380°C. This allows us to infer that the mounds’
growth integrated practices that thermoaltered the matrix. The
recursive use of a space for the make and use of earth ovens
is consistent with this empirical evidence. The earth oven
technology and the Australian mounds ovens (ethnographicarchaeological
analogue) provide a framework to understand
which behaviors could generate accumulations of thermoaltered
sediments, and a scenario to generate hypotheses that
associate technological, economic and social, as well as symbolic
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