Abstract
The Upper Palaeolithic in Europe was a time of extensive climatic changes that impacted on the survival and distribution of human populations. During the Late Glacial Maximum (LGM), southern European peninsulas were refugia for flora, fauna, and human groups. One of these refugia, the Cantabrian region (northern Atlantic Spain), was intensively occupied throughout the Upper Palaeolithic. Characterising how climatic events were expressed in local environments is crucial to understand human and animal survival. La Riera Cave (Asturias) has a rich geo-cultural sequence dating between 20.5kyr BP to 6.5kyr BP and represents an ideal location in which to explore this. Stable isotope analysis of red deer and ibex is used alongside other environmental and climatic proxies to reconstruct Late Upper Palaeolithic conditions. Results show that during the LGM, ibex adapted their niche to survive, and became a major prey species for humans. The diverse environmental opportunities offered in the high-relief and coastal environs of La Riera may help to explain the high human population levels in the Cantabrian Region throughout the Late Upper Palaeolithic. Despite fluctuating conditions, herbivores and humans had the flexibility and resilience to adapt, demonstrating the importance of southern European refugia for the survival of different species.
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Introduction
The settlement of Europe during the Last Glacial (MIS2) period (c.30-10kyr BP) was directly affected by the climate which resulted in cycles of expansion and contraction of populations, linked to the ebb and flow of ice sheets1,2,3. Central and northern Europe during the Last Glacial Maximum (LGM) (c. 21-18kyr BP) were mostly covered by ice sheets or polar desert that pushed populations into the south, with Iberia and southwestern France, Italy and the Balkan peninsulas acting as refugial regions. Within these locations less drastic temperatures, associated with milder conditions, facilitated survival of a variety of plant and animal species4,5,6,7,8,9,10,11,12,13. Of these areas, the Cantabrian region, which occupies the northern Atlantic coast of the Iberian Peninsula (Fig. 1) has been demonstrated to have been a genetic refuge for salmon14, red deer15 and humans16,17. At least 55 archaeological sites dating to the Solutrean (24kyr BP) have been recorded in the Vasco-Cantabrian region, concurrent with a large increase in the number archaeological occupation levels recorded within many sites13,18, indicative of a shift of surviving populations in Europe to the region10,11,19,20,21. During the Magdalenian (c. 17-11.5kyr BP), a further population expansion is observed with a dramatic increase in the number of archaeological sites recorded, linked to ameliorated climatic conditions21,22 as identified in proxies such as ice and marine cores. This region has been described as a “dynamic cultural center in the Upper Palaeolithic world of western Europe”23, as reflected by and, in part, due to its extensive and rich cave art and portable art. These phenomena clearly demonstrate the importance of the region. Characterisation of how global climatic events that occurred throughout this period were expressed locally, and the impact that they had on ecological niches, adaptability and resilience of populations, are key to understanding why this region functioned as a refugium throughout the Late Pleistocene, enabling the survival of humanity throughout these climatically challenging conditions.
Location of La Riera cCave within the Cantabrian Region, Northern Spain (Panel A) with other contemporary archaeological sites, together with available environmental and climatic proxy records, referred in the text (Panel B). Panel C shows the outside view of La Riera Cave in the summer of 1976 and Panel D displays the stratigraphy sequence of squares G/H at the site (Photos by L.G. Straus). Panel E is a re-drawing of the stratigraphy at La Riera (based on the original version by G.A. Clarke82).
Ice core evidence from Greenland24,25 provides information on larger climatic events that occurred throughout MIS2/1, including the Last Glacial Maximum, Heinrich 1 event, Greenland Interstadial 1, Younger Dryas, and eventually the onset of the Holocene. The integration of ice and marine cores with terrestrial records has helped to refine the stratigraphy of these events25,26,27. These larger scale climatic records are physically far removed from conditions experienced on regional and local levels in northern Spain. Speleothem records28, lake sediments29,30, ostracod evidence from the continental shelf31 reflect data closer to study areas, enabling regional-scale environmental reconstructions, but these are still distant from the sites occupied by human groups. More localised environmental indicators such as pollen, found within cave sites, can inform on past vegetation32 and have been analysed in Asturian cave sites33,34 but must be used with caution as they can be subject to taphonomic and diagenetic alterations35,36. Both macro- and micro- mammal remains are ubiquitously found on these sites37,38,39,40,41,42,43 and are often less affected by taphonomic and sampling biases than other environmental proxies.
Bone collagen δ13C and δ15N analyses have been abundantly applied in European settings to explore palaeoclimates and palaeoenvironments44,45,46,47 and has yielded successful results for the Middle and Upper Palaeolithic of northern Spain26,48,49,50,51,52. Analysis of δ13C and δ15N in bone collagen is a powerful tool for reconstructing palaeoenvironments, especially when combined with other available environmental proxies53. Carbon and nitrogen in animal bones derives directly from consumed diet54, with a fractionation effect55, and shows long-term dietary behaviour of animals during the period of bone growth, typically representing the lifetime of each individual animal56,57.
A multitude of climatic and environmental factors can affect the δ13C and δ15N values of plants, and thus the animals that consume them. Regarding δ13C values, temperature has been seen to affect values both positively and negatively58, although the δ13C values of plants appear to have a greater correlation with mean annual precipitation than mean annual temperature, and typically higher levels of precipitation result in lower plant δ13C values59,60. Increased tree cover can result in lower δ13C values in relation to more open environments linked to a combination of closed carbon cycling of CO2 under the forest’s canopy limiting atmospheric exchange and recycling of CO2 from decomposing leaf matter61,62,63,64. Plants growing at higher altitude can typically produce elevated δ13C values linked to decreased temperatures and changing CO2 partial pressure65, and has been observed archaeologically to produce elevated δ13C in the fauna that reside at higher altitudes66. Global scale conditions such as a change in atmospheric CO2 levels can also impact on δ13C values of plants67,68, and thus be expressed in fauna. Larger-scale drivers and climatic events such as this would be expected to be observable in multiple species within a site, and typically should be observed across different geographical locations.
A negative correlation between plant δ15N values and annual precipitation has been observed in a variety of ecosystems69,70,71. Higher temperatures and lower water availability are thought to produce higher animal δ15N values in hot/arid potentially due to physiological stress due to heat and lack of water72,73,74, although research has suggested that this changes could be linked to dietary shifts associated with these conditions75,76. Colder temperatures are frequently associated with lower δ15N values69 due to reduced activity of nitrogen fixing mycorrhizae77,78. There is a negative relationship between soil δ15N values and altitude79. Different geographical locations, can have exhibit variation in baseline δ15N due to the nature of soil types within a given region, which affects plant communities, and nitrogen-fixing mycorrhizae present80.
In addition to environmental and atmospheric drivers affecting plants (and their consumers), changing environmental conditions can also affect faunal behaviour, in terms of the dietary and habitat choices of animals. Animal diets may shift to include different plant species consumed, or parts of plants, with differing isotopic signatures resulting from plant physiological variations58. Due to the complex processes influencing how localised and regional conditions are expressed within the isotopic values of plants, and the animals that consume them, temporal trends in δ13C and δ15N values can be expressed differently between regions, and changing conditions do not always affect every species in the same way46,81. Wider environmental indicators can be used to facilitate interpretations of isotopic results to understand the trends that are visible.
When applied to bones with evidence of anthropogenic modifications (e.g., exhibiting cut marks, marrow cracking, hammer blows), bone collagen δ13C and δ15N analysis can provide direct evidence of past environments during the times when these animals were consumed and is one of the few proxies that can inform on how climatic events were experienced by humans and animals on a local scale. This study combines newly generated δ13C and δ15N evidence from La Riera, one of the most important, and well-published Upper Palaeolithic sites in the Cantabrian region with wider climatic and environmental proxies from local, regional and global records to explore environmental changes throughout the archaeological sequence to understand how humans and animals adapted and became resilient to the changing conditions that they faced.
La Riera Cave
La Riera (Posada de Llanes, Asturias) (Fig. 1) was discovered and first excavated by Conde de la Vega del Sella in 1916 and was re-excavated by L.G. Straus and G.A. Clark in 1976–79. All materials discussed in this paper relate to the findings from the 70 s excavations. The site contains a long sequence of archaeologically rich levels dating from the Solutrean to the Asturian (Mesolithic) cultural periods, C14-dated between 20–6.5kyr BP82 (Table 1). La Rieralies c. 30 m above sea level on the south-facing edge of a low ridge on the narrow coastal plain facing the steep, nearby cliffs of the 800 m a.s.l. Sierra de Cuera, a coastal mountain range north of the massive Picos de Europa chain. Currently located 2 km from the shore, during the LGM, when sea level was approximately 120 m lower, the site was around 10 km from the shore, roughly two hours walk from the Late Glacial littoral82. The location of the site means that hunting areas available included the montane habitats of the Sierra, the low, rolling coastal plain, the valleys of the small Calabres and Bedón rivers, and their estuaries offering a selection of wild terrestrial, marine, and riverine resources82
.
To date, La Riera remains one of the most extensively studied sites in the Cantabrian Region, and was a landmark study, as it provided detailed scientific analysis, including extensive analyses of palaeoenvironmental remains including: pollen records, deposited sediments, bird bones, macro- and micro-fauna, and even a pioneering analysis of mollusc shell isotopes34,37,83,84,85,86. Several series of radiocarbon dates82,87,88 were undertaken both by the excavators and subsequent researchers, and two new dates for Levels 23 and 2489 run for this study have resolved some previous dating discrepancies in chronologies at the site (Table 1).
Although not permanently occupied, La Riera was repeatedly visited and inhabited, during different seasons, by human groups throughout the Upper Palaeolithic and Mesolithic and was an important hunting and gathering base82. La Riera was as part of a network of Upper Palaeolithic sites across the Cantabrian region. The Solutrean (Levels 4 to 17) dates to between 20.5-17kyr BP, and the earliest Solutrean occupations are interpreted as representing mainly a hunting camp for the procurement of red deer and mainly ibex90. The site then became a multi-purpose residential camp, still exploiting large numbers of red deer and ibex with fish and shellfish also being collected90. The Lower Magdalenian (Levels 18–20) date to between 17-14kyr BP and the Middle and Upper Magdalenian (Levels 21–26) span between 14.5-13kyr BP. The Azilian, a form of local Mesolithic industry, (Levels 27–28) falls between around 10.5-9kyr BP, and at the end of the sequence, Level 29 (attributed to the Asturian, also a local Mesolithic industry) is a shell midden (‘conchero’) formed between roughly 9-6.5kyr BP. Towards the end of the sequence, faunal evidence suggests stricter bi-seasonality, with occupation focussed on the summer and winter90. The faunal record for La Riera shows evidence of intensive carcass processing, including marrow extraction from the low-yield phalanges and mandibles of red deer, and large numbers of individuals per level were recorded37 despite the relative small portion of the site excavated, reflecting intensive hunting strategies. Most notably from the Upper Magdalenian levels onwards (Level 24–29) there is increasing evidence of slaughtering Cervus hinds and their young, alongside a more diverse faunal suite, including greater frequencies of birds, fish, and marine molluscs37, suggesting increasing levels of dietary diversification and further intensification of diet throughout the Upper Palaeolithic.
La Riera is 26 km west of coastal El Pindal cave, famous for its Magdalenian cave art and yielding a speleothem record now used as a palaeoenvironmental proxy28,91,92 (Fig. 1). Lake Enol, situated at c. 1000 m above present sea level about 20 km WSW in the Picos de Europa of Asturias (Fig. 1) contains valuable lake sediment information regarding temperatures throughout the Late Pleistocene27,30. The relative proximity of La Riera to sites with additional environmental proxies, facilitates a more nuanced interpretation of the past conditions experienced by the human groups that occupied La Riera.
Results and Discussion
Overview of the δ13C and δ15N data
In total 128 specimens (72 red deer and 56 ibex) were analysed for δ13C and δ15N in bone collagen from 10 different archaeological levels at La Riera, from the Solutrean to the Asturian period, covering a temporal span of almost 15kyr BP (Table 1). Summary statistics of these results are shown in Table 2. Preservation at the site was exceptional and collagen was successfully extracted and analysed from 121 specimens (94.5%). All specimens analysed had atomic C:N ratios between 3.2–3.5, falling within the accepted ranges for well-preserved collagen93, with 110 specimens of these having ratios between 3.2–3.4, adhering to more rigid criteria of Van Klinken94. The vast majority of specimens had >35%C and %N values between 10–16%, and 109 of the specimens collagen yields were >1% (based only on the >30Ka fraction, rather than total collagen), indicative of well-preserved in vivo collagen94, as detailed in the Supplementary Information (SI1).
A series of trends were observable throughout the sequence due to changing δ13C and δ15N values of the species analysed. To facilitate discussion the sequence has been divided into interpretative zones (Z1-Z5) based on key characteristics of the data including; changing statistical relationships of the δ13C or δ15N values of each species, increases/or decreases in the isotopic values of both species, observable by statistically significant differences between archaeological levels, and in one instance notably large ranges in isotopic values of both species observed. A thorough explanation of the key characteristics of each interpretative zone is as follows: Zone (Z1), at the bottom of the stratigraphic sequence, dating to the Solutrean, is characterised by partitioning of the red deer and the ibex, with the latter having 15-N depleted values. Zone 2 (Z2, Middle into Upper Magdalenian) is marked by an increase in the δ15N value of the ibex, followed by stability, in both δ13C and δ15N values. Zone 3 (Z3, Upper Magdalenian) is differentiated by a small increase in the δ13C values of both species observed between Levels 23 and 24. Zone 4 (Z4, Azilian) is marked by a dramatic decrease in the δ13C values in both species. Finally, Zone 5 (Z5, Asturian) exhibits similar δ13C and δ15N values to the preceding levels (27 and 28), but is marked as being different due to a larger range in the δ15N values of both the red deer and ibex in comparison to the rest of the sequence. The isotope data are correlated with environmental proxies on a local, regional and global scale to determine how trends observed relate to wider environmental and climatic trends. Raw data are provided in SI1–10.
The results and interpretation are framed by exploring the patterns in the stable isotope record at La Riera, outlining the conditions experienced continentally within the region and the implications that this had for human groups, before being drawn together in the final discussion.
Zone 1: The Solutrean (Levels 7-13: 20- 17kyr BP)
The Solutrean levels (7, 11 and 13) exhibit a marked difference in the δ15N values of ibex and red deer, with red deer having consistently higher δ15N values by at least 1‰, and no overlap in standard deviation is seen between the two species (Fig. 2). A statistically significant difference in in the δ15N was seen between the two populations for each of these levels (Table 3). From Level 18 onwards, no difference in the δ15N values of the two species was observed (Fig. 2, Table 3). The ibex have the lowest range in δ15N values seen within the sequence at La Riera during this time (Table 2).
Mean δ13C values (left) and δ15N values (right) of red deer (denoted by black circles) and ibex (denoted by grey circles) from the Upper Palaeolithic levels sampled throughout the sequence at La Riera. Error bars show 1σ.
The δ13C values show little difference between the two species. Only in Level 11 were the mean ibex values 0.7‰ lower than the red deer (Table 1; Fig. 2), and the populations were statistically different (p = 0.01) (Table 3). The mean δ13C value of ibex was also slightly higher within Level 13 (Table 2), although there was no statistically significant difference (p = 0.07) (Table 3). There is a decrease in δ13C values of the deer between levels 7 and 11, which was statistically significant (p = 0.02) (Table 4).
The difference in δ15N values indicates niche partitioning of ibex and red deer. Pollen records available from Levels 6–15 indicate a predominance of Ericacea (heather/heath plants), accounting for between 60–90% of the pollen recorded, and little tree cover, with arboreal pollen accounting for <5%34 (Fig. 3). Microtus oeconomus, a species adapted to cold environments, is identified in Level 737. Montane species (ibex) are exploited in greater quantities (74% of NISP in Level 7, declining to 30% and 15% throughout this zone), although red deer are still present in great numbers (25% in Level 7, increasing to 80–90% in Levels 11 and 13 this zone)37 (Figs. 3 and 4).
Multi-scale environmental and climatic comparisons throughout the Upper Palaeolithic on a local, regional and global level. Stable isotope δ13C and δ15N values of red deer and ibex sampled at La Riera are shown alongside additional environmental proxies from the site34,37, and isotopic values from red deer across the Cantabrian Region26,48,49,52.Wider continental environmental indicators28,92, offshore proxies31 and NGRIP ice core evidence25, and climatic events141 are also displayed to further contextualise trends observed through time.
The %NISP of each of key prey species exploited within the sampled levels in this study showing the variation of each species through time, based on data produced by Altuna37.
Within the Solutrean Z1 lower δ15N values in the red deer were noted at El Mirón in comparison to values within Z2 from the site (p = 0.00) (Table 5; Fig. 3), which could suggest colder, drier conditions during this time. Typically El Mirón δ15N values are lower than those seen at La Riera (mean values range between 2.4–2.8‰ in Z1-4), indicative of wider, baseline differences between these two geographically distinct locations (Table 5: Fig. 3). Antoliñako Koba δ13C values were higher in Z1, compared to those seen later on in the sequence (Table 5; Fig. 3) which could indicate colder/drier conditions.
The water temperature estimates for Lake Enol are calculated at 1 °C between 20-17kyr BP, (Fig. 3), with no change registered in the Ca precipitation record, although during cooler times, this record becomes less precise27. Speleothem evidence from El Pindal cave shows some of the lowest δ13C values and a hiatus in growth between 18.2-15.4kyr BP28 (Fig. 3). This interruption is interpreted as belonging to one of the coldest and potentially relatively driest parts of the sequence28 (Fig. 3). Lower δ18O values in the NGRIP record at 20kyr BP indicate that this part of the sequence corresponds to the end of the LGM, which concluded at c. 19kyr BP, when the onset of Heinrich 1 event marks a climatic improvement24, with a slight increase in δ18O values (Fig. 3). The ostracod evidence, from the Basque continental shelf, also attests to generally cooler sea temperatures31 (Fig. 3).
Interpretation
The Solutrean was a cold period, representing the latter stages of the LGM and start of the Heinrich 1 event, as seen continentally and in ice core and offshore marine records. The consistently lower δ15N values of ibex during this time, in comparison to the red deer (and in relation to the ibex analysed from later in the sequence) is distinctive at La Riera. Higher altitudes are associated with depleted δ15N values of soils and plants79, and it has been proposed that animals feeding at altitudes may have produced lower δ15N values of faunal, as seen in Northern Europe, and at the Cataluña site of Balma Guilanya during the 20-10kyr BP95. Typically higher altitudes are also associated with elevated δ13C values in plants65, which has been archaeologically observable within ibex at the site of Valdegoba in central Northern Spain66, and is not evident at La Riera. There is no difference between the ibex and the red deer δ13C values throughout the sequence, suggesting that both species are feeding at isotopically similar altitudes, making this less likely to be the cause for the lower values of the ibex. Furthermore, studies of Mediterranean vegetation indicate that both species density and diversity declines with altitude96, suggesting that scaling higher altitudes would be less profitable for ibex, especially during the hostile conditions of the Solutrean. Thus, alternative drivers are responsible for the difference in δ15N values of the two species during the Solutrean.
The lower δ15N may be linked to changing patterns of vegetation consumption, resulting in the niche of the ibex being altered to adapt to the cold conditions of the Solutrean. The lower ibex δ15N values occur alongside increased quantities of Ericaceaous (heath) plants34 (Fig. 3). Ericaceaous plants typically occupy regions with lower quality, acidic soils and have mycorrhiza which enhance their ability to absorb nitrogen from soils97. The larger proportions of ericaceous plants suggest more hostile environmental conditions were experienced during these levels. Studies of recent red deer and ovicaprids in Scotland have shown that red deer can digest heather better than ovicaprids98. Ecological research into the feeding behaviour of ovicaprids and red deer in mosaic heath landscapes showed that the dietary selection of ovicaprids was heavily affected by the presence of heather, resulting in the two species consuming different diets99, and could explain the observed niche partitioning in these archaeological specimens during the Solutrean at the site. Also, ecological studies of ibex have demonstrated that they are pushed into alternative niches when competing with livestock100, and a similar effect might be observed when faced with red deer as a competitor. Red deer are an ecologically flexible species, thriving in open grasslands, in addition to forested environments101,102,103. Their combination of grazing and browsing behaviour means that they have great flexibility in their dietary behaviour, already observed archaeologically104, and thus may have been able to continue existing the ericaceous landscape that proved challenging to the ibex. Ibex are known to have a high plasticity in the feeding strategies depending on ecological constraints105, and it is possible that the ibex populations were being pushed away from the ericaceous vegetation communities of the 15N depleted heathlands and were consuming a different diet. The mobility of ibex reduces when food sources are scarcer and encounter rates are low106 and they have a reduced dietary breadth in the winter months107. Their high representation within the zooarchaeological record suggests their niche habitat was well-known to Solutrean hunters as a reliable and consistent source of animal protein and may explain the draw of people to La Riera during these colder stages.
Zone 2: The Lower Magdalenian and Upper Magdalenian (Levels 18-23: 16-15kyr BP)
Mean δ15N values of ibex in Lower Magdalenian Level 18 are 1.3‰, higher than in Level 13 (Fig. 2, Table 2), and the populations were statistically different (p = 0.02; Table 3). This difference disappears after the formation of Level 13 and the values of red deer and ibex track each other relatively closely in mean values, ranges and standard deviations (Table 2), and the diet of both species becomes isotopically indistinguishable. An increase in mean δ13C values of red deer is seen between Levels 13 and 18, although this was not statistically significant (p = 0.07) (Table 4). Throughout the Lower Magdalenian and into the Upper Magdalenian (Levels 18, 19, 23) (16-15kyr BP), the δ15N and δ13C values of both the red deer and the ibex remain stable (Table 2, Fig. 2). Red deer and ibex populations within Levels 18–19 and Levels 19–23 were statistically similar (Table 4).
With the conclusion of the Solutrean, from Level 16 onwards, there was a large decrease in Ericaceae and heath was replaced by steppic vegetation (including Liguliflorae, Tubuliflrae, Chenopodiaceae, and Artemisia)34 (Fig. 3). Arboreal pollen increased slightly from before, but remains low, accounting for less than 10% of the total pollen record between Levels 16–2334. Lowland species (notably red deer) are the most commonly exploited game (70%), but with montane (steep, rocky habitat-preferring) species (predominantly ibex) also representing a valuable resource within these levels (Fig. 4)37,38.
The wider isotopic data from the Cantabrian Region red deer also show little temporal change in δ13C values in the Lower Magdalenian to the Upper Magdalenian (17-12kyr BP) (Table 5; Fig. 3). Within Z2, the El Mirón δ13C values indistinguishable from those seen in Z1 (p = 0.798) (Table 5). The mean δ15N value at El Mirón is 2.8‰ which is 0.4‰ higher than observed in Z1 (Table 5). Lake Enol temperatures of 2 °C are estimated during this period, warmer than previously observed27. This period in the NGRIP ice core record belongs to Heinrich 1b event, followed by Greenland Interstadial 1, which represents a warmer climate (Fig. 3). A decrease in the “Northern Guest” benthic foraminifera between 16-15kyr BP, indicate warmer sea bottom temperatures off the Basque shelf, and warmer seawater surface temperatures are suggested by the lower δ18O values and decline in N. pachyderma sin. %32.
Interpretation
Climatic amelioration is attested within the La Riera sequence by a shift in vegetation, that occurs with the change to GI1 signalled by a decline in heath plants and an increase in steppic plants, then suggesting a warmer and slightly drier environment. The δ15N values of the ibex increase in this zone, and they become isotopically indistinguishable from the red deer. Warmer conditions which may explain the increased δ15N values of the ibex, as a product of increased soil microbial activity77,78, although this would be expected to affect the red deer values also. The lack of covariance in the δ15N values of the La Riera red deer suggests that drivers behind this change are specific to the niche of the ibex. The shift in the ibex δ15N values may be linked to dietary change of the ibex related to the reduction in Ericacea, allowing the ibex could expand into regions that were previously less desirable, adapting their niches to suit these new environmental conditions. The consistency in the stable isotope values of both species throughout Levels 18–23 indicates that the environment was relatively stable within these levels, as is mirrored in continental stable isotope results across the region. The increase in red deer bones within the zooarchaeological assemblage suggests that there is a shift in hunting focus, associated within these milder conditions.
Zone 3: Upper Magdalenian (Level 24: 13.5kyr BP)
During the Upper Magdalenian occupation of the site, between Levels 23 and 24, an increase in δ13C is observed, and mean values in both red deer and ibex are higher within Level 24 (Table 2, Fig. 2), which was statistically significant for both species (Table 4). A slight increase in δ15N values is visible for both species (Table 2, Fig. 2), although this was not statistically significant for red deer (p = 0.61) or ibex (p = 0.11) (Table 4). A stratigraphic hiatus was recorded between these two levels and new dating of ungulate bones from these levels (Table 1), revealed a 2,000-year gap between them, with Level 24 marking a restart in the archaeological sequence82,89.
The lack of thermophilic species in Level 24 in the palynological record suggests a cooler environment. There is a small increase in arboreal plants, steppic taxa continue to be represented, and generally tree cover remains low (Fig. 3) with ferns present only in the upper parts of level 2434. Previous interpretations of the level suggested that it was formed during a cold spell within the Allerød34,82. Reindeer was present within this level (Fig. 4)37 and sedimentary evidence indicated a colder environment than in the previous level85. There is evidence of a greater exploitation of montane species (mostly represented by ibex, accounting for 40% of the NISP) (Fig. 4).
On a wider regional level, continental records at Lake Enol show a slight temperature increase of 2–4 °C at around 14-13kyr BP91 (Fig. 3). There are few comparative isotopic results from this period, with only 3 specimens from El Mirón (in the montane interior of Cantabria, c. 160 km to the east) dating to c.13kyr BP, slightly later than the specimens from La Riera. The δ13C values are very similar to those observed previously in the sequence, although the mean δ15N value is slightly lower at this time (Table 5; Fig. 3). At 14kyr BP, the El Pindal Cave δ13C and δ18O records show lower values, but increase again at 13kyr BP28,91, indicative of changing conditions. The NGRIP record shows the end of GI1, where a slight decrease in δ18O values, indicative of a cooler environment24 (Fig. 3). There are no “Northern Guest” benthic foraminifera between 15-12.5kyr BP (Fig. 3), suggesting warmer sea bottom temperatures, and this phase is part of a longer trend of declining N. pachyderma sin percentages31.
Interpretation
The shift in the δ13C values of the red deer between Levels 23 and 24 appears to be a product of colder, and possibly drier environmental conditions. Positive and negative correlations are seen between temperature and δ13C values81, and elevated δ13C can be indicative of drier conditions59,60 (also suggested by the small increase in δ15N values, which typically can become elevated when exposed to more arid conditions). Given that this shift coincides with a cooler period, as marked by the NGRIP δ18O at the end of GI1, and wider environmental evidence from the site suggests colder environments, a decreasing in temperature, and possible drier environments are potentially responsible for the increase in δ13C values. The increased exploitation of ibex in this level (42% of NISP in Level 24, compared to 13% of NISP in Level 23, Fig. 4), a characteristic also seen in the Solutrean levels, indicate a tendency to hunt greater quantities of montane animals during these more rigorous conditions. This zone reflects a short duration of time, with few contemporary isotopic specimens from other sites available to date (Table 5, Fig. 3). It is possible that La Riera was occupied in favour of other sites due to the prevalence of ibex that had found a niche in order to succeed in the area, even during periods of environmental stress.
Zone 4: Azilian (Levels 27 and 28: 12-9kyr BP)
With the transition to the Azilian (Level 27) a dramatic decrease in the δ13C values is evident for both main game species, with mean values becoming more negative at this point, by 1‰ for red deer and 1.5‰ for ibex, and stay at these lower values throughout the Azilian and Asturian (Levels 27–29) (Table 2, Fig. 2).The δ15N values of both species remain at a consistent level in comparison to the previous levels sampled (Table 2, Fig. 2).
Between Levels 27 and 28 (c. 12-9kyr BP) a shift in δ15N was observed in red deer. The mean δ15N value in Level 28 is 0.8‰ lower than Level 27 (Table 2, Fig. 2), and the populations show a statistically significantly difference (p = 0.02, Table 3). Values remain at this lower level within Level 29. A similar effect is also observed within the ibex, with the mean δ15N value being 0.9‰ lower, although this is not statistically significant(p = 0.15), potentially due to the smaller sample size available for Level 28.
The palynological record shows a small increase in arboreal woodland species such as oak, elm and hazel, in addition to ferns (Polypodium vulgare), suggesting a warmer, more humid environment during Levels 27 and 2834 (Fig. 3). Dormouse (Glis glis), which inhabits more temperate and moist environments, was present37. Macromammal assemblage continue to show dominance in the exploitation of ungulates that inhabit mixed and wooded environments, notably both red deer and the appearance of wild, for the first time, in the sequence within Levels 27 and 28 boar (Fig. 4). An increase in animals exploited from open landscapes is evident (notably bovines accounting for 10% of the NISP in Level 28)37 (Figs. 3 and 4).
Other isotopic records from the Cantabrian Region are more limited for this temporal period. The mean value of specimens from Antoliñako Koba (c. 190 km east on a mountaintop near the coast) is 0.7‰ lower in Z4 than the mean values seen in Z1, the only other zone for which data are available from this site (Table 5). At El Mirón within Z4, the mean δ13C values and δ15N values are indistinguishable from to those seen within Z3 and no statistically significant differences were observed between populations between these zones, and it is only within Z5 when decreased δ13C values and increased δ15N values are evident at El Mirón (Table 5), indicative of changing Holocene conditions. Between 12-9kyr BP. Lake Enol temperature estimates average temperature of 10 °C, which is warmer than observed earlier in the sequence27. At El Pindal Cave from 12kyr BP onwards, there is a continual increase in the δ13C record.
The NGRIP data for this period is marked by the brief lowering of δ18O values marking the Younger Dryas, followed by the large increase in δ18O values that indicate the onset of the Holocene (Fig. 3). No “Northern guests” are seen in the Basque continental shelf cores, indicating warmer sea bottom temperatures31. The slow, but steady decline in the N. pachyderma sin ‰ and the steady increase in the δ18Olob record is also indicative of warmer sea temperatures.
Interpretation
This zone corresponds with the start of the Holocene and is marked by more negative δ13C values of both species at La Riera. This pattern appears to extend further and is seen in the herbivore bone collagen record within wider Europe datasets, where after 14kyr BP there is a trend towards decreased δ13C values49. This change may in part be related to increased concentration in atmospheric CO2 alongside a possible increased water availability108,109, as further evidenced by the increased presence of arboreal pollen, and forest dwelling species within the wider environmental records at the site. The lower δ15N values within Leve 28 support this interpretation, as there is a correlation between lower δ15N values and wetter environments69, which are also reflected in El Mirón later, in Z5. This indicates a possible delayed response in nitrogen to the changing climate52. The increased suite of vegetation present in the pollen record, indicates greater habitat diversity, which in turn is reflected in the change in the suite of animals exploited in Level 28. In Level 28, a shift towards exploiting larger areas of the landscape was observed, with the inclusion of plain species (bison) which are infrequently present in assemblages before this point (Fig. 4). Diet in general becomes broader, with larger quantities of marine shells appear in the malacological record86. The Holocene in Asturias, and in the whole Cantabrian region, is marked by a change in the suite of macrofauna fauna present, with reindeer, bison and cave bear disappearing and increasing numbers of forest- dwelling species such as wild boar and roe deer38,110, which are more suited to these temperate environments.
Zone 5: Asturian (Level 29: 8.6-6.5 kyr BP)
Within Level 29 (a shell midden deposit), the δ15N ranges for both species are larger than seen in any other period (red deer 2.5‰, ibex 2.7‰, Table 2). This indicates a greater inter-individual variability within both red deer and ibex populations. The δ13C values remain at a similar level to those observed within Z4.
The pollen record for Level 29 is similar to that of Level 28, with the presence of abundant tree pollen, dominated by hazel, with oak, elm, alder and birch also represented. In addition, higher instances of ferns and slightly higher proportions of arboreal pollen are observed, indicative of a moist and temperate environment34 (Fig. 3). Sediment composition also suggests that the level was formed under temperate and humid conditions85.
Red deer of El Mirón (9kyr BP) have higher δ15N with mean value of 4‰, which is 1.2‰ than seen within Z3, and δ13C are 1‰ lower (Table 5). Lake Enol has records of water temperature of 12–13 °C at 8.5 to 8kyr BP, before dropping slightly to 10–11 °C (Moreno et al. 2014). In the El Pindal Cave speleothem record, there is a decrease in δ13C and δ18O during the last part of the record at c. 8kyr BP indicative of a cooler environment than seen previously in the sequence and may be reflecting the 8.2kyr BP event27.
The higher δ18O values in the NGRIP record are part of the continued trend for warmer Holocene climates24 (Fig. 3). The marine core influx of benthic “Northern Guest” species c. 9.3kyr BP and 7.8kyr BP indicate cooler sea bottom waters and the sharp decrease in δ18Olob is thought to relate to the early Holocene cooling events26,31.
Interpretation
This level corresponds to the shell midden (‘conchero’) that capped the infilling of the site. The larger ranges in the δ15N values might indicate that animals were consuming plants from a broader environmental area, encompassing a range of isotopically different locations. These larger ranges could be related to the accumulation of midden deposits over a long timescale, resulting in a wider variety in values observed. The longer spans of time represented in the midden layers, in comparison to the lower levels, could have resulted in a higher variation in values, representing a series of environmental fluctuations within levels. An alternative explanation for the diversity in stable isotope values is that the animals hunted from this level came from isotopically diverse areas of the landscape and could be indicative of procurement of resources from larger catchment areas. The concept of base-camps being used, with hunting parties likely travelling within a 15–40 km walking distance of the site has been proposed as a model of site use111, a with coastal-inland movements aimed at maximising exploitation of local resources41. Animals gradually accumulated, from different parts of the landscape at La Riera, could reflect exploitation from several isotopically distinct locations.
Discussion
Throughout the sequence at La Riera a series of changes in environment were detected in the bone collagen δ13C and δ15N isotopic records, and other environmental proxies showed shifts in both animal niches and human hunting habits, which has global significance for enhancing our understanding of the importance of European refugia during the Late Pleistocene.
At the beginning of the sequence, during the Solutrean (20.5-17kyr BP, Zone 1), conditions appeared to be cold and arid, with ericaceous plants dominating the landscape. Throughout Z1, during relatively hostile Late Glacial conditions, ibex were able to adapt their niche and survive. The high-relief environments surrounding La Riera enabled them to successfully alter their feeding behaviour in response to those adverse environmental conditions. Niche partitioning of herbivores is a characteristic observed throughout the Pleistocene, both isotopically45,66,112 and using dental microwear113. Archaeological studies have indicated that niche partitioning may not necessarily be predictable through time45. Adapting feeding habits in relation to changing environmental conditions can be a valuable survival strategy, reducing competition for resources, potentially reflecting mosaic environments. Despite the pronounced effects of the climatic severity, the suite of environments surrounding La Riera provided alternative habitats for ibex, outside of the niche of the red deer, enabling them to thrive and thus to be available to be hunted extensively by humans near the site37,90. The predictable presence of ibex surrounding La Riera environs may have been one of the major attractions of hunter-gatherer populations during times of hardship.
Studies of Late Glacial macromammals imply that local hunting strategies were adapted to the local conditions114, and at La Riera the results indicate that the availability of ibex during this time of climatic turmoil may have allowed hunters to adapt their strategies. This may, in part explain, the success of this region for human and animal populations during the LGM as attested by genetic studies14,15,17 in validation of the classic archaeological refugium theory. Both red deer and ibex were crucial to the survival of Solutrean populations of Cantabrian Spain, although with fluctuations as to which species was dominant21. The environments surrounding La Riera cave would have offered a reliable source of game, a highly favourable characteristic for Late Glacial populations21 and may have acted as a hub or base camp for Solutrean bands inhabiting the area.
The climatic improvement that occurred at the end of the LGM throughout the Lower-Upper Magdalenian (16-15kyr BP, Zone 2) is represented by a shift in the niche of the ibex, causing them to have more similar δ13C and δ15N values to those of the red deer. This change in values also coincides with a change in vegetation away from ericaceous plants, towards more steppic conditions, and suggests that plants in the landscape could successfully support both species. The consistency in both the δ13C and δ15N isotopic values throughout this period suggests that, the Cantabrian region offered environmental predictability which may explain why this area was so densely occupied with a remarkable increase in the number of archaeological sites, estimated at 10–15 per millennium10,11,13,19,20. Stability in the resource availability, during this newly improved climate and environment, would have been a major draw of the region to human populations.
Colder conditions observed in Level 24 (13.5kyr BP, Zone 3) coincide with a shift towards greater consumption of ibex, a trait observed during the Solutrean at La Riera when the environment was also characterised as being cold. This suggests that La Riera inhabitants were able to adapt their hunting strategies towards targeting different prey when conditions were harsher, indicating human resilience. This cool spell was not reflected in three other sites in the Cantabrian region, where stable isotope records were available and may reflect series of microenvironments or resource ‘patches’ facilitating survival of different species in around the region effectively, buffering the varying climatic conditions. This environmental buffering, enabled by differential expressions of climate on a local level across the region, may have allowed animals and humans to shift locations depending on resource availability. The movement of populations and the expansion of wider territories into both, coastal and inland areas, has long been discussed as a model of occupation in the region115,116,117. By having the flexibility to change location, even during times of environmental hardship, the occupants of the Cantabrian Region refugium were able to survive, and even thrive. La Riera was a location that offered abundant game, even during colder more challenging conditions and demonstrates its importance in the network of sites in the local and regional framework.
The final parts of the sequence at La Riera show two key trends. During the Azilian, in Zone 4 (12-9kyr BP), warmer and wetter Holocene conditions are recorded. These conditions continue during the Asturian (Mesolithic) (8.6-6.5kyr BP, Zone 5) with another trend, that is, an increase in the ranges of δ15N values in both animal species. Hunting animals from a wider catchment area, causing a larger spread in the data, and may indicate increased hunting ranges. Expanded hunting ranges could be due to a need to feed larger numbers of people, to maximise food returns. An increase in human demography is seen at this time, supporting this hypothesis. Other changes in diet are also seen, with a wider dietary breadth in the region throughout the Late Upper Magdalenian, Azilian and Asturian, with a greater number of marine species being exploited, along with lower ranked ungulate prey (such as roe deer), species that are more dangerous and difficult to hunt (e.g. wild boar) and with high percentages of juvenile animals40,118. Additionally, more intensive processing of carcasses, such as marrow cracking of small bones, such as phalanges is seen in La Riera from the Upper Magdalenian onwards37, and at the site of Las Caldas in central Asturias at this time119,120. In the wider region, stone boiling to maximise nutritional output from carcasses121 is observed. There have been questions as to whether this dietary change is result of environmental change or population pressures and over-hunting90,118,122,123. The evidence from earlier in the La Riera sequence suggests that the major ungulate species (particularly ibex) being exploited by humans at the site were quite resilient and able to adapt to changing conditions, even during times of harsh climatic conditions. This suggests that human population pressures on ungulate populations in Upper Magdalenian-Azilian times may have been responsible for this dietary change. The increase in number of sites per millennia recorded in the archaeological record, throughout the Late Pleistocene19,20,22, attests to the opportunities offered by the region, even in the face of increased population size. Whilst the environment played a role in the suite of species on offer during the Pleniglacial, the move towards hunting lower ranked, less sustainable prey is likely a product of an increased human population, causing pressure on resource availability.
During the Mesolithic (Asturian) in the Cantabrian region, populations start to abandon much of the interior, moving away from the densely forested inland areas with limited ungulate prey, likely due to over- exploitation, and towards the coastal ecotone strip with newly formed estuaries and inlets124,125, something which is seen at La Riera, where coastal resources (i.e., fish, molluscs, crustaceans) became major and easily collected food resources86,90,126. This phenomenon has also been observed across the wider Cantabrian region, as attested by the zooarcheological record and by technological specialisation118,126,127,128,129,130, and a marked increase in the number of coastal sites emerging in the Mesolithic131. A decrease in the exploitation of larger mammals is evident in the region118. It is thought that demographic pressures, more than (or at least in combination with) environmental conditions themselves, were likely ultimately responsible for the shift in human diet41,90,132,133,134. Intensive carcass processing, and increased exploitation of smaller species and aquatic resources has been identified across Europe during the latter stages of the Upper Palaeolithic and is often attested to population growth135,136, suggesting that the observations at La Riera are consistent with trends occurring happening on a pan-European scale at this time.
Conclusions
The large-scale climatic changes that dominated the Late Pleistocene have been detected within the bone collagen record δ13C and δ15N correlating with the wider proxies within the archaeological sequence at La Riera, thus providing insights into the conditions experienced by humans occupying this refuge region. The isotopic study has shown that the climatic conditions were responsible for some of the affecting changes in vegetation, and thus plant and faunal δ13C and δ15N values. Behavioural changes associated with those fluctuating conditions were also integral in producing many of the trends observed in the isotopic record at La Riera, influencing both animal feeding behaviour and human hunting strategies. Humans occupying the site and animals living in the locality of La Riera showed the capacity to adapt to these new environments, and to change their feeding behaviour, demonstrating their resilience. Animals might have altered their ecological niches by feeding in different environmental zones, that were readily available for human exploitation, buffering the effects of harsher climatic conditions. The predictability of resources in the environs of the site, even during more environmentally unstable times, may explain why this region was routinely occupied throughout the Upper Palaeolithic. La Riera was just one site in a dense network of Upper Palaeolithic sites in the Cantabrian Region, and by moving between sites and exploiting locally available sources during different seasons and environmental conditions, humans succeeded to survive, and even thrive during the harsh Late Glacial conditions. During the end of the Upper Palaeolithic, in the Azilian and in the Asturian, environments become wetter and increased hunting ranges are seen, in addition to more intensive carcass processing and dietary diversification. Human and animal populations were able to cope with adapting to more hostile conditions earlier in the sequence, suggesting that these changes during early Holocene may be linked instead to pressure on resources, due to an increased human population size. The multi-proxy approach used here shows the value of integrating lines of evidence to characterise how global climatic changes were expressed on continental and local levels and how these affected the Late Glacial human populations, and has wider applications for understanding the importance of European refugia during these times of climatic turbulence.
Materials and Methods
Bone sampling strategy
Specimens were sampled from the archive collections held at the Museo Arqueológico de Asturias in Oviedo. The archaeological material was organised relating to the original numerical level attributions assigned during excavation which are inverse to the levels discussed and explained in the site monograph82. In this study, archaeology excavation levels have been correlated to the levels as discussed throughout the original publication82 to facilitate comparison with the wider palaeoenvironmental datasets available from the site. Levels with secure stratigraphy, pertaining to discrete temporal units, spanning the Solutrean to the Asturian period, were selected to provide insights into changes in environment through time.
Red deer (Cervus elaphus) and Spanish ibex (Capra pyrenaica), the most consistently consumed species throughout the sequence at the site37, and within the wider Cantabrian region during Late Pleistocene118 were selected for analysis. Mature individuals, as identified from state of bone fusion were selected for analysis to prevent any animals with elevated δ15N nursing signatures137,138, which can mask environmental signatures. Bones with evidence of anthropogenic alteration were targeted, including specimens exhibiting cut marks and green fractures indicative of marrow extraction, to ensure that the palaeoenvironmental evidence was directly linked to periods of human activity.
Analytical methods
Collagen extraction was undertaken following a modified Longin139 method with an ultra-filtration step140. Samples ranging between 0.7–0.9 g were drilled and cleaned using abrasion. Samples were demineralised in 0.5 M HCL at 6–8 °C for between 3–10 days, and then washed three times using de-ionised water before being gelatinised in a weak acidic solution (pH3 HCL) at 70 °C for 48 hours. Samples were filtered using 5–8 μm Ezee® mesh filters (Elkay Laboratory Products), before being ultra-filtered to separate out the larger > 30ka collagen chains. The > 30ka fraction was then frozen and lyophilized for 48 hours. Between 0.2–0.45 mg of collagen was weighed into tin capsules for analysis of stable carbon (δ13C) and nitrogen (δ15N) isotope ratios. Specimens were analysed in duplicate, using a Delta XP mass spectrometer coupled to a Flash EA 2112 elemental analyser. The δ13C values and δ15N values are reported relative to the V-PDB standard and AIR standards. A series of international and internal standards were used to calculate analytical error which was ± 0.1‰ (1σ) or better. The mean difference observed between duplicate measurements was 0.03 for δ13C, and 0.01 for δ15N. Data were not normally distributed, meaning that the nonparametric Mann-Whitney U test, with a post-hoc Holm-Bonferroni correction141 was used. A p-value of < 0.05 or less was deemed to be indicative of a statistically significant result.
Comparative environmental and climatic proxies
Macromammalian faunal information from La Riera38 was compiled and plotted as %NISP of the major prey species exploited to create Fig. 4, excluding minor species such as carnivores represented by occasional fragments. The full faunal spectrum from La Riera is provided in Supplementary SI2. For the creation of Fig. 3 species were sorted by habitat type for the creation of Fig. 3. Red deer and wild boar were included in the mixed/forest category. Montane species were, predominantly, represented by ibex, which preferentially occupy rocky cliffs and slopes such as in the nearby Sierra de Cuera. Species attributed to open landscapes included horse and bovids (aurochs and bison which are often not possible to speciate further). Microfaunal remains at the site were scarce, likely linked to sampling strategies used to recover materials (i.e., hand-picking from fine-mesh water-screen residues, rather than from flotation or dedicated stratigraphic column sampling), meaning that the assemblage may not be entirely representative of the full suite of species inhabiting the landscape. Environmental indicators species were identified including Glis glis (dormouse), which is characteristic of more temperate environments and Microtus oeconomus (root or tundra vole), indicative of cold (and humid) environments, and only their presence is indicated in Fig. 3. The microfaunal data from La Riera are also provided in Supplementary SI3. Pollen data available from la Riera34 were collated into categories that have been combined into Mediterranean forest, Atlantic forest, heath and steppic environments (see Supplementary SI4). A summary of evidence and interpretations of the sedimentary record is included in SI5 85.
Regional continental environmental indicators were used to explore how the local environmental conditions observed at La Riera relate to wider landscape trends. Lake temperature estimation, based on Ca precipitation in Lake Enol (Picos de Europa, eastern Asturias), provides an additional indicator of temperature changes throughout the study period27 (see Supplementary SI7). Speleothem δ18O and δ13C records from El Pindal cave (also located in Asturias) were collated28 (see Supplementary SI8). Additional bone collagen δ13C and δ15N studies from across the region have been included for comparative purposes. Data was available from La Paloma, roughly 100 km west of La Riera49, El Mirón Cave52 (on the northern edge of the Cantabrian Cordillera in eastern Cantabria, about 160 km east of La Riera and 20 km from the present shoreline at 260 m a.s.l.). Two more sites from the Basque region were also included: Antoliñako Koba26 and the palaeontological site of Kiputz IX48 (all shown in Fig. 1). Data from red deer was included to enable cross-comparisons, but ibex data were not available (see Supplementary SI6). Isotopic data were plotted according to the radiocarbon ages rather than archaeological levels, to enable direct comparisons with results generated from La Riera.
Records available from the continental shelf, including the percentages of Neogloboquadrina pachyderma sin. Ostracods, δ18O values from Globigerina bulloides and interpretations were available31 using data from cores KS05-10, KS05-05, and KS04-16 (see Supplementary SI9). This enables a regional, offshore perspective of environment for comparison, and are the closet available records to this study region. These provide an indication of regional temperature changes.
Finally, NGRIP data δ18O values available25 were plotted at 500-year intervals in Fig. 3 to show general climatic trends throughout the study period (see Supplementary SI10) facilitating correlations of site based, and local data. Global climatic events142 are also marked within Fig. 3.
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Acknowledgements
This research was funded by a Marie Skłodowska-Curie Individual Fellowship grant awarded to JRJ (H2020-MSCA-IF-2014-656122). ABMA was supported by a Marie Curie Career Integration Grant (FP7-PEOPLE-2012-CIG-322112), the Spanish Ministry of Economy and Competitiveness (HAR2012-33956 and Ramon y Cajal (RYC-2011-07690) and Santander Talent Attraction for Research (STAR-1). The new radiocarbon dates obtained by LGS for Levels 23 and 24 were kindly financed by the Stone Age Research Fund (University of New Mexico), J. and R. Auel principal donors. The authors would like to thank L. Agudo Pérez for helping to draw Figs. 1 and 3 and for her assistance sampling the remains from La Riera. Permission to recreate the stratigraphic section from the La Riera monograph was very kindly given by Geoffrey Clark (Arizona State University), editor of the Anthropological Research Papers series. Many thanks to C. Garcia de Castro at the Museum of Asturias for facilitating access to their collections. Special thanks to R. Macdonald and M. Wong (University of British Columbia) for their laboratory and technical assistance.
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Data generation and analysis: J.J. Funding acquisition: J.J. Investigation: J.J., A.B.M.A., M.R. and L.G.S., Methodology: M.R. and J.J. Resources: A.B.M.A. and M.R., Writing – original draft: J.J. Writing – review & editing: J.J., A.B.M.A, L.G.S. and M.R.
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Jones, J.R., Marín-Arroyo, A.B., Straus, L.G. et al. Adaptability, resilience and environmental buffering in European Refugia during the Late Pleistocene: Insights from La Riera Cave (Asturias, Cantabria, Spain). Sci Rep 10, 1217 (2020). https://doi.org/10.1038/s41598-020-57715-2
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DOI: https://doi.org/10.1038/s41598-020-57715-2
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