Late Holocene vegetation dynamics and lake geochemistry
Fig. 1. Location map of the
Taitao-Coyhaique area, XI Region, Chile. Key to numbers representing
sites referred to in the text: 1, Laguna Stibnite (Lumley
& Switsur 1993); 2, Istmo de Ofqui I and II (Heusser
1964); 3, Río Témpanos (Heusser
1964); 4, Interfluctuational section (Heusser
1964); 5, Juncal Alto (Naranjo
& Stern 1998). The Liquiñe-Ofqui fault system (LOFS) is
shown as a dotted line.
The vegetation of the lake catchment is North Patagonian rain forest dominated by evergreen broadleaf and conifer taxa (Gajardo 1995, Veblen et al. 1983). The lowlands of this region support a dense forest of Nothofagus nitida, N. betuloides, Podocarpus nubigena, Weinmannia trichosperma, Drimys winteri, Caldcluvia paniculata, and Pseudopanax laetevirens (botanical nomenclature follows Marticorena & Quezada 1985). The understorey of these forests, particularly where light can easily penetrate, are frequently occupied by bamboo (Chusquea spp.). Poorly drained sites, including the waterlogged margins of Laguna Miranda, are dominated by Pilgerodendron uviferum and Tepualia stipularis. The branches of Nothofagus spp. are occasionally observed to be infested with the semiparasitic plant Misodendron spp. Potamogeton spp. was common around the margins of the lake.
Fieldwork was conducted in the southern hemisphere summer of 1995, with the logistic support of Raleigh International and CONAF. Parallel cores were taken from the centre of Laguna Miranda with a modified Livingstone corer. An undisturbed core of the uppermost sediment was obtained from the centre of the lake using a clear plastic piston corer. Each core was described in detail and scanned by a magnetic susceptibility core scanning loop at 10-20 mm intervals. This identifies changes in the abundance of ferrimagnetic minerals, which is used as an indication of changing erosional input to the lake, and to assist in the location of basaltic tephras. Three major tephra layers were located in this way and the glass shards extracted (H2O2 digestion) and mounted for electron microprobe analysis (EMPA, Department of Earth Sciences, University of Cambridge), following the guidelines set down by Froggatt (1992) and Hunt & Hill (1993). The amount of organic matter was determined by loss-on-ignition at 550°C. Chronological control is provided by using a combination of AMS/bulk radiometric 14C dating (Table 1) and the dates are reported throughout this paper as calibrated years before present (present is 1950 AD, Stuiver & Reimer 1993).
Geochemical analysis of lake sediments follows a fractional method introduced by Engstrom & Wright (1984) and modified by Lumley (1993), providing information on the chemical environment in the lake in relation to erosional inputs and redox conditions within the surrounding landscape (identifying the elements Mn, Fe, Al, Mg, Na, K, Ca, Ti, and Si). The wet-chemical extraction technique separates acid-soluble "authigenic fraction" and biogenic (diatom) silica from the clastic mineral "allogenic fraction". Element analysis of the separate fractions was carried out on an ICPES (Inductively Coupled Plasma Emission Spectrometer, University of London, Royal Holloway, UK).
Pollen analysis follows the standard acetolysis method described by Faegri & Iversen (1989). Pollen identification and nomenclature follows that set out in authored reference material (Heusser 1971, Villagran 1980, Zhou & Heusser 1996) and regional reference collections held at the Department of Plant Sciences, University of Cambridge (Appendix 1). Pollen counts are expressed as percentages of the total pollen sum (excluding pollen of aquatic vascular plants and all spores) which reaches a minimum of 300 in all samples. Charcoal particle concentrations were calculated following the point counting method outlined by Clark (1982). Charcoal analysis provides basic data on the abundance of charcoal in the sediment, from which the extent and intensity of fires around the site can be inferred. Numerical zonation and principal components analysis (PCA) were performed with only major taxa whose pollen or spore values exceeded 5% at least once. Numerical zonation employed optimal splitting by sum-of-squares analysis to partition the Laguna Miranda data into 3 significantly different pollen zones (Bennett 1996). PCA is used to reduce the pollen and spore data to a two-dimensional plot and the resulting data set is displayed as a biplot for samples and taxa (Birks and Gordon 1985). All numerical analyses, including zonation and principal component analysis (PCA), have been implemented within PSIMPOLL, a c program for plotting pollen data, developed by Bennett (1994).
Sediment, tephra analysis and chronology
The sediment stratigraphies for two cores from Laguna Miranda are shown in Fig. 2. The sediment is brown gyttja with thin green to grey inorganic inclusions. The upper 0.75 m core overlaps with the 4,35 m long Livingstone piston core to give a 4.60 m long sediment record at the site. Penetration of the core to greater depth was not possible due to increased sand (tephra) content at the base.
Fig. 2. a) Sediment characteristics,
stratigraphic correlations between cores, and b) sediment chronology
from Laguna Miranda.
A total of 22 distinct inorganic layers, all containing volcanic glass shards, were identified as tephra deposited directly from atmospheric fallout, or from remobilised deposits within the lake catchment. Three tephra deposits, Mir-3 (olive silty sand, 15 mm), Mir-2 (olive silty sand, 8 mm) and Mir-1 (Olive grey silty sand, 80 mm) that gave strong magnetic susceptibility signals and were clearly visible in the core, were selected for tephra analysis. Geochemical compositions of these different tephra layers are shown in Appendix 2. The analysis show that the three tephras contain a suite of glass shards derived from both dacitic-andesite type eruptions (SiO2 content of 60-65%, and a high K2O content), and basaltic-andesite type eruptions (SiO2 of around 50-55 % and a high TiO2 content). Comparison with geochemical data from the nearest volcanic sources (Austral Volcanic Zone, 49°-55°S, and the Southern Southern Volcanic Zone, 41.5°-46°S, see Futa & Stern 1988, López-Escobar et al. 1993, Haberle & Lumley 1998, Stern 1990, 1991, Naranjo et al. 1993, Naranjo & Stern 1998) show that the distinct geochemical signature from Volcán Hudson eruptions (Fig 3a) is consistent with those obtained from tephras deposited in Laguna Miranda (Fig. 3b).
Fig. 3. a) SiO2 versus
TiO2 and K2O for selected volcanic sources in
Southern South America. Data for the Hudson, Maca, Cay volcanoes in
the South Southern Volcanic Zone (SSVZ) and the Lautaro, Burney and
Aguilera volcanoes in the Austral Volcanic Zone (AVZ) is from Futa
& Stern (1988), Haberle
& Lumley (1998), López-Escobar
et al. (1993), Stern
et al. (1993). b) SiO2 versus TiO2 and
K2O for analysis on the three Laguna Miranda tephras
compared to the tephra analysis envelope for Hudson Volcano (data
given in Appendix
Radiocarbon dating indicates that average rates of sedimentation at Laguna Miranda were close to linear throughout the last 4800 years (Fig. 2). The temporal resolution of the cores varies from 9 to 14 year 10mm-1 depending on the depth. Table 2 lists the inferred age of inorganic (tephra) layers, showing that major phases with high frequency deposition events occur between 2800 - 3100 cal yr BP and 4400 - 4830 cal yr BP. The tephras identified at this site have not been dated directly, but there is a strong correspondence between the ages determined for two Hudson Volcano tephras recorded 70 km to the southwest in Laguna Stibnite (1690-1530 cal yr BP and 2790-2490 cal yr BP, Haberle and Lumley 1998), one recorded 80 km to the east of Hudson Volcano at Junco Alto (3885 cal yr BP, Naranjo & Stern 1998), and those recorded at Laguna Miranda (1695 cal yr BP, 2800 cal yr BP and 3820 cal yr BP). These results may provide additional chronological control when compared to regional tephrochronology being developed from lake sediment records (e.g., Haberle & Lumley 1998, Naranjo & Stern 1998).
Sediment chemistry (Fig. 4)
Authigenic fraction: Fe, Al, Ca and Si dominate the authigenic chemistry of the Laguna Miranda sediments (Fig. 4a). Ca and Mn concentrations are high between ca. 4800 and 4000 cal yr BP, though subsequently decreasing towards the top of the core. The Fe:Mn ratio shows a steady increase upwards through the core. i, Fe, Al and Si reach maximum concentrations between 4000 cal yr BP and 1200 cal yr BP, when there is a striking decrease in the concentrations of all these elements.This coincides with a decrease in the total authigenic content and an increase in sediment organic content, Mg and the Fe:Mn ratio.
Fig. 4. Sediment chemistry of Laguna
Miranda samples against lake depth and inferred age (note
independent scaling for each element). a) Authigenic fraction, b)
Biogenic and Allogenic fraction.
Biogenic fraction: Biogenic Si shows a general trend of increasing concentrations towards the top of the core (Fig. 4b). Two periods in which biogenic Si increases markedly are ca. 2000 cal yr BP and after ca. 1200 cal yr BP.
Allogenic fraction:The allogenic fraction is dominated by Si, Al, Fe and Ca (Fig. 4b), which is similar to the elemental composition of the three tephra deposits. Total allogenic content is highest in the basal sediments, between ca. 4800 - 4000 cal yr BP, and higher in the core 1800 - 1200 cal yr BP. All elements follow a similar pattern of change as shown in the total allogenic curve.
Pollen, spores and microscopic charcoal (Fig. 5)
Zone 1 (ca 4800 - 2100 cal yr BP): Pollen spectra are dominated by Nothofagus dombeyi type (50-70%), Podocarpus nubigena (15-25%), and Filicales (30-40%). Podocarpus nubigena and Weinmannia trichosperma have relatively high percentages between ca. 4800 - 4000 cal yr BP, followed by a gradual decline towards the upper part of the core. Other tree taxa remain in consistently low abundances. The first appearance of Gramineae pollen occurs after ca. 2400 cal yr BP. The ratio between Misodendron and Nothofagus pollen shows only minor fluctuations. Microscopic charcoal is also low in abundance with only a slight rise between ca. 4000 - 3300 cal yr BP.
Fig. 5. Pollen percentage diagram of
combined cores from Laguna Miranda against depth and inferred age.
Pollen counts are expressed as percentages of the total pollen sum,
excluding pollen of aquatic vascular plants and all spores. Selected
taxa percentages are drawn with an exaggeration of x10 (single
line). Nothofagus to Misodendron ratio as an indicator
of relative Nothofagus infestation. Charcoal is expressed as
a percentage of the total pollen sum. For explanation of sedimentary
column see Fig. 2a.
Zone 2 (ca 2100 - 200 cal yr BP): This zone is characterised by a rise in the percentage abundance of Gramineae. Cyperaceae and the aquatic, Potamogeton, increase after 1200 cal yr BP. Other taxa are relatively unchanged. The are slight increases in Pilgerodendron uviferum and Tepualia stipularis, and a decrease in Nothofagus dombeyi type towards the top of the zone. The ratio between Misodendron and Nothofagus continues to show only minor fluctuations. Microscopic charcoal remains very low.
Zone 3 (ca 200 cal yr BP to present): Pilgerodendron uviferum pollen becomes important in this zone. Tepualia stipularis, Maytenus and Lepidothamnus fonkii show slight increases in representation. Microscopic charcoal is very low.
Principal Components Analysis (PCA): The major patterns of variation in the pollen data were summarised by means of PCA, with pollen taxa accounting for significant sample variance displayed as a biplot (Fig. 6). The sample distribution shows a U-shaped shift through time showing the three phases of vegetation change: (i) early dominance of Nothofagus forest with Podocarpus, and to a lesser extent Weinmannia and Pilgerodendron important taxa, (ii) reduced influence from all forest taxa except Nothofagus under the influence of increased Gramineae, and (iii) the shift to Nothofagus forest including Pilgerodendron and Weinmannia as important components of the forest. High negative loadings for Nothofagus and Misodendron in axis 2 reflect changes in the forest dominant Nothofagus, and the close correspondence between abundance of the semiparasitic plant Misodendron and its host Nothofagus.
Fig. 6. Principal Components Analysis
of pollen data from Laguna Miranda (taxa with relative abundance of
<5% were excluded from the analysis). Eigenvalues are expressed
as proportions of total variation with axis scale linear and plot
centred on origin. The two dimensional representation accounts for
around 82% of the original variability (Axis 1 = 62%, Axis 2 = 20%).
The samples are identified according to their respective zone and
the midpoint of samples from a single zone (mean of principal
components) are joined up in stratigraphic order (thick stippled
line). The directions of influence for each pollen taxa used in this
analysis are overlain on the stratigraphic sample data in this
The lake sediment record begins around 4800 cal yr BP at which time a North Patagonian rainforest, including a mixture of shade-tolerant and shade-intolerant species, was already established around the lake. Burning appears to have been minimal throughout the record. Between 4800 - 2100 cal yr BP very little change is evident in the forest composition, though there is a gradual decline in Podocarpus over this time and a reduction in the importance of Weinmannia trichosperma after 4000 cal yr BP. High allogenic inputs into the lake, particularly prior to 4000 cal yr BP, are probably derived from direct airfall of tephra and subsequent inwashing and reworking via overground water flow. Similarly, authigenic minerals formed in the waterlogged organic catchment soils, and mobilised by eluvial processes, are likely to have been derived from weathered products of older tephra deposits. The concentration of biogenic Si is at its lowest during this period, despite the high mineral inputs from tephra airfall events. Biogenic Si tends to be negatively correlated with total allogenic inputs suggesting the addition of tephra derived minerals resulted in an aquatic environment that was relatively poorly suited for the growth of diatoms and other aquatic organisms.
Between 2100 - 200 cal yr BP there are important changes in both the vegetation and geochemical record. Gramineae appears around 2400 cal yr BP and becomes increasingly important after 2100 yr BP. These changes may relate to the earlier rise in authigenic minerals, particularly Fe and the Fe:Mn ratio, indicative of reduced soil redox conditions during the development or expansion of waterlogged soils (Mackereth 1966, Pennington et al. 1972, Engstrom & Wright 1984). With continued addition of mineral nutrients to the environment from tephra fall and high levels of disturbance, there would have been an increase in the availability of sites for invasion. Nothofagus and Gramineae (likely the Chusquea bamboo) may have been best suited to take advantage of the disturbance regime and changing soil conditions.
The striking reduction in total authigenic elements in the sediment after 1200 cal yr BP coincides with a sharp reduction in the addition of tephra to the sediment. The increases in Cyperaceae and Potamogeton, around the edge of the lake, are indicative of progressively shallow lake margin and possibly marginal peat development. This is also supported by the rise in Fe:Mn ratio in response to a further reduction in soil redox conditions. The development of a lake margin peat, today occupied by Chusquea bamboo and the tree taxa Pilgerodendron and Tepualia, may also have contributed to the reduction in both total authigenic and allogenic elements by restricting mobility of inwashing allogenic and authigenic minerals. Dense growth of Chusquea bamboo may have acted as a filter to inwashing minerals and also utilised authigenic minerals in plant growth. Lake productivity increases during this period as biogenic Si concentrations reach a maximum. The last 200 cal years sees a rise in Pilgerodendron, presumably expanding around the lake margin as open sites become available for tree growth. There are no marked changes in sediment chemistry or microscopic charcoal to suggest a possible cause for this change. Szeicz (unpublished results) has examined this period in high resolution, comparing Pilgerodendron tree-rings analysis to determine stand age and pollen results over the last 400 years, and suggests that the most recent expansion of Pilgerodendron at the northern and eastern margins of this site may be a response to periodic tectonic induced watertable changes or is part of a long-term trend in gymnosperm growth around a shallowing lake margin. Fluctuations in pollen taxa percentages over this time may suggest that periodic disturbance still plays an important part in local vegetation dynamics, despite the lack of erosion or tephra evident in the lake sediment record.
DISCUSSION AND CONCLUSIONS
This study has focussed on late Holocene environmental change recorded in a lake catchment with minimal human disturbance. Despite the long history of human occupation in southern Chile that reaches into at least the late glacial transition (Dillehay 1989) the influence of fire, which in this high rainfall environment is most likely associated with human activity, is negligible in the Laguna Miranda record (low charcoal levels throughout). The occupation of the channel islands by amerindians (Chono or Wayteca), whose subsistence was based primarily on fishing and coastal resource exploitation, appears to be relatively late, with sparse archaeological remains dating to no earlier than 5800 cal yr BP (Cardenas et al. 1993). Amerindian impact on this environment may have been focussed on the coastal margins and occasional excursions to the interior of the islands of the Taitao-Chonos region. Lake sediments under investigation from the northern Chonos islands show high levels of charcoal over at least the last 4000 years, suggesting much greater impact from human activity and possibly higher populations living in this environment relative to the Taitao Peninsula region (Haberle, unpublished results). Currently, human settlement is very sparse and clearance for grazing or agriculture is non-existent, though some areas have been burnt over the last 150 years for the purpose of timber extraction. So, what was the role of natural disturbance, in the absence of human activity, on the vegetation and sediment history of Laguna Miranda over time scales of several millennia?
Sources of instability in the catchment include volcanic and tectonic disturbance, changes due to disease, fire, and climate induced changes from drought and wind. The site is prone to earthquakes as it lies close to the main Liquiñe-Ofqui fault system, where evidence for uplift rates of around 10 m per 1000 years have been recorded around Chiloé Island for the late Holocene (Hervé & Ota 1993). Mass movement scars and drowned forests around Laguna San Rafael (Reed et al. 1988), and throughout south-central Chile (Veblen & Ashton 1978), attest to the capacity of tectonic movement to impact forests within the area. The absence of significant eroded soils in the Laguna Miranda sediment demonstrate that no widespread slope failures have taken place during the recorded history of the catchment.
The record of tephra layers found at Laguna Miranda represent only a partial record of the sequence of tephra eruptions at Hudson Volcano during the last 5 millennia. It is likely that there were additional eruptions of Hudson Volcano during this period that were not recorded in the Laguna Miranda sequence, either because the eruptions were small and deposition was not sufficient to be visible in this analysis, or because tephra dispersal was in a different direction during the eruption. The predominant winds in the region favour dispersal in an easterly direction, away from Laguna Miranda, so the tephras record at Laguna Miranda can only be considered to represent a minimum estimate of eruption frequency. Indeed, it is important to note that the lack of tephra layers after 1200 cal yr BP in Laguna Miranda may be a function of dominant dispersal direction rather than lower eruption frequency or magnitude.
What is the impact of tephra deposition on the vegetation around Laguna Miranda? Observations from the forested slopes of Hudson Volcano after the 1991 eruption showed that most of the plants in the Nothofagus forest community survived up to 100 mm thick tephra deposits, though with thicker deposits plant mortality increased (Vogel 1996). Complete destruction of the forest only appeared to occur when buried by more than 1,80 m of tephra. Most of the Laguna Miranda tephra layers are very thin (<80 mm), suggesting thickness of deposits in the catchment and impact from burial may have been minimal. The long-term shift to Nothofagus dominance from 4800 to 1200 cal yr BP is here considered to be a response to periodic volcanic disturbance, in which shade-intolerant Nothofagus species are best adapted. By contrast, if the absence of tephra layers after 1200 cal yr BP in the Laguna Miranda record is taken to indicate a period of relative volcanic, if not tectonic stability, then it is possible that the shift to a more mixed forest with a reduced Nothofagus component was the product of a long period of stability. This appears to be consistent with the response observed in other Chilean Nothofagus communities subjected to variable disturbance regimes (Veblen et al. 1996).
The addition of thin layers of tephra to the Laguna Miranda catchment may have caused damage to vegetation through mechanical and chemical processes, such as defoliation and subsequent prolonged absence of leaves (Rees 1979) or noxious gases and acid-bearing rains (Thorarinsson 1979). Fires associated with volcanic activity may have been a contributory factor to vegetation change, though there is no evidence to support this in the Laguna Miranda record. Disease and dieback are also known to affect Nothofagus forest structure in southern Chile, and in many cases large areas of canopy tree mortality have been observed (Veblen et al. 1996). The infestation of individual Nothofagus trees by the semi-parasitic plants Misodendron appears to have been a persistent feature in the Laguna Miranda catchment. Minor increases in Misodendron infestation may have been due to periodic disturbance events, such as tephra-tectonic activity, climatic variability, natural successional changes and human activity, that may have increased the susceptibility of individual Nothofagus trees to infestation for short periods of time.
Edaphic conditions may change with the addition of mineral nutrients and sand/silt sized particles after tephra deposition, altering the nutrient balance within the forest and influencing regeneration. Rapid rates of sedimentation of around 1 m per 1000 years and the development of waterlogged soils and peats around the margins of Laguna Miranda are likely to be a response to frequent renewal of nutrients from tephra deposition. Some support for this is evident in a comparison of sedimentation rates from a similar lake basin in a more remote region some 120 km from Hudson Volcano. The sedimentation rate at Laguna Stibnite on the Taitao Peninsula is around 28 cm per 1000 years, one quarter the rate recorded for Laguna Miranda (Lumley & Switsur 1993), which may reflect a relatively poor supply of nutrients to the catchments there.
Is there evidence that climatic fluctuations have influenced vegetation dynamics during the late Holocene? Previously investigated pollen profiles from the region have been interpreted to indicate vegetation responding to changes from warmer-drier to cool-moist conditions during the late Holocene (Heusser 1964, Markgraf 1989). Neoglacial advances have also been proposed for the southern Chilean region, culminating 5750-4450 cal yr BP, 3200-1950 cal yr BP, 1250-950 cal yr BP and 700-50 cal yr BP (Clapperton & Sugden 1988), and suggesting that climates where variable during the mid to late Holocene. The complicated dynamics of tide-water glaciers, such as the San Rafael Glacier some 50 km to the south of Laguna Miranda, make the interpretation of glacial fluctuations in the Laguna Miranda region highly problematic. In the Laguna San Rafael region, high abundance of Weinmannia in the lower levels of Ismo de Ofqui I site and the Interfluctuational section is considered to represent a warmer climate phase following neoglacial retreat from the sites (Heusser 1964). The sites are fragmentary and the dating is highly problematic (Clapperton & Sugden 1988); but, stratigraphic indications from each site point to disturbance, from both volcanic eruptions and nearby physical influences from glacial meltwater flooding, as possible significant factors in local vegetation change during the high Weinmannia phase. Weinmannia trichosperma is a relatively shade-intolerant species that is able to readily invade disturbed sites in much the same way as Nothofagus. Clearly caution must be exercised when inferring climate change from a single species such as Weinmannia, as it may be responding to changing disturbance regimes as much as climate variables.
Over shorter time scales of years to decades, climate variability may have been high during the late Holocene due to an increase in ENSO (El Niño-Southern Oscillation) climate events (Markgraf et al. 1992), though the southwestern region of Chile may not have been as strongly influenced by ENSO events as the subtropical-tropical regions (Ortlieb & Macharé 1993). The impact of these short-term fluctuations on vegetation are poorly understood, though over longer time scales the cumulative impact may be to increase the success of species adapted to frequent disturbance. There is no conclusive evidence of climatically induced forest change at Laguna Miranda. This raises the possibility that short-term climatic disturbances may produce similar responses in the pollen record as does volcanic or tectonic activity.
We would like to thank Raleigh International and CONAF for logistic support and permission to carry out this work in the XI Region, Chile. Many international venturers, including some 25 Chilean venturers, contributed outstandingly to the success of this work in difficult and remote terrain. he funding for this project was provided by the Leverhulme Foundation, UK, and the Queen's University Advisory Research Council, Canada.
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Associate Editor J. Armesto
Received November 28,1997; accepted September 8, 1998
Abundances of major elements in glass shards extracted from Laguna Miranda tephra samples. Tephra glass geochemistry is expressed as percentage weight, normalised to 100 wt%. Analysis was performed at the Department of Earth Sciences, University of Cambridge, using a Cameca SX50 spectrometer microprobe, an electron microscope with three wave dispersive spectrometers and a LINK AN10000 energy dispersive spectrometer running PAP matrix correction software. The probe operated at 20kV, using a 10nA beam current and a 10µm defocused beam to minimise loss of Na and K (50s count time). A mixture of minerals, natural oxides and pure metals were employed as standards that were periodically checked in order to verify internal consistency of the results
Abundancia de elementos principales en fragmentos de cristales extraídos de las muestras de tefra de Laguna Miranda. La geoquímica del cristal de tefra se expresa como peso porcentual, con normalidad de 100 wt%. El análisis se realizó en el Departamento de Ciencias de la Tierra (Department of Earth Sciences), Universidad de Cambridge, usando un espectrómetro microsonda Cameca SX50, un microscopio electrónico con tres espectrómetros dispersores de ondas y un espectrómetro LINK AN1000 dispersor de energía utilizando software PAP de matriz correctora. La sonda operaba a 20kV, usando una corriente de haz de 10nA y un haz desviado de 10µm para minimizar pérdida de Na y K (50s tiempo controlado). Se empleó una mezcla de minerales, óxidos naturales y metales puros como patrón, verificándolos periódicamente a fin de comprobar la consistencia interna de los resultados
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