the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Improving the age constraints on the archeological record in Scladina Cave (Belgium): new speleothem U-Th ages and paleoclimatological data
Abstract. The sedimentary sequence in Scladina Cave (Belgium) is well-known for its rich Middle Paleolithic assemblages and its numerous faunal remains. Of particular interest is the presence of a nearly complete mandible of a Neandertal child. To place all these finds in the correct chronostratigraphic context, various dating techniques have been applied over the past decades. This resulted in a reasonably well-constrained age model, roughly spanning the last glacial cycle. Age constraints of the lower part of the Scladina sequence as well as from the underlying Sous-Saint-Paul Cave were however absent until now. Previous attempts to date several speleothem layers in Scladina Cave, using U-Th dating were only partly successful, presumably because diagenetic alteration of speleothem material compromised the ages. In the present study we re-assessed U-Th dating of various speleothem levels in Scladina Cave, applying state-of-the-art U-Th dating, and carefully selecting material that experienced little to no diagenetic alteration. The new results constitute a robust age framework for the Scladina sequence, which provides precisely dated stratigraphic anchor points that improve the previous age model. Furthermore, new U-Th analyses, for speleothems from the lower part of the Scladina sequence and from the Sous-Saint-Paul sequence, document Middle Pleistocene ages, making this one of the longer fossil-rich cave sedimentary sequences in NW Europe. The new data confirm that speleothem deposition predominantly took place in periods of warmer climate, while siliciclastic sediments characterize the colder intervals. New speleothem ages further suggest that the Neandertal mandible found in the sequence, and previously placed in Marine Isotope Stage 5a or 5b, could potentially be as old as Marine Isotope Stage 5d.
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RC1: 'Comment on cp-2024-27', Anonymous Referee #1, 04 Jun 2024
This manuscript is focused on a new dating exercise of an existing archaeological sequence in Scladina Cave, Belgium. The authors provide new U-Th dates that result more accurate than previous ones, particularly due to an exhaustive examination of speleothem samples using trace elements and fluid inclusions to select the best intervals. Although my review is positive and consider this manuscript an interesting piece of work and an advance in the knowledge of an important and classic sequence, I have some comments and requests that should be considered before publication.
- I miss more discussion on how the U-Th technique has improved in the last decades and how these improvements have a consequence in a better chronology of the considered sequence. This is not explained and would be useful. Besides, the quality of the U-Th dates has to be evaluated according to Table 2 numbers and not once checking if they coincide or not with other studies (ej. line 484 where a date is discarded because there are not other examples of speleothem grow in Europe).
- I also miss more information about which diagenetic processes have been detected or approached by trace elements and isotopes. It is not clear to me what type of alterations (recrystallization? Fluid movements?) are to be avoided for reaching a better dating and why. In fact, some of the explanations are about discoloration or black layers outside the growh axis where sampling is carried out. Also it is said that samples were covered by siliciclastic sediments (line 244), how does it affect the speleothems? Please, provide references of similiar studies.
- I wonder how important are trace element results in this manuscript. It seems to me that those results are quite obvious and probably not too important (eg. avoid black layers when sampling is a quite usual procedure). Please, be more concrete about the utility of these proxies here.
- I think that some interpretations that are based on only one date should be avoided or at least moderated. I am talking about climate inferences from growth cessation of only one sample, for example (lines 457, line 464, etc).
- Detailed discussion about the final chronology and how it may change previous hypothesis would be needed as a kind of final conclusion of this manuscript.
Some other minor comments:
- line 31: be more concrete about the "state-of-the-art" U-Th dating
- line 90: you need to add the mthods to prepare thin slides
- figure 3: numbers are too small. It may be better to split the stratigraphic column in two and make a squared-figure.
- section 2.2.: please, indicate which sample was dated in every lab.
- section 2.3. can be added at the end of section 2.2.
- section 2.6: please indicate in which lab where the fluid inclusion analyses carried out
- section 2.8: this does not belong to materian and methods. Cave site and regional climate should be section 3.
- caption fig. 4. "shows clear discoloration towards the right side of the stalagmite". Is this important if you don't sample it?
- section 3.2. too long and unclear. Please, consider to include all that information about every speleothem in a table.
- table 2: be sure that units are the same in Melbourne and Xian labs. (eg. activity versus atomic ratios).
- line 333: which would be the isotopic values you would expect if there was a "clear diagenetic imprint"? why? Please, explain.
- line 434: the title is not correct to me. This is not a paleoclimate reconstruction but just few data on particular time windows. This has to be clear. These records do not overlap, they do not provide a continuous sequence.
- line 458 and beyond: talking about growth cessation and its association with climate deterioration with only one sample is no a robust argument. Please, moderate your interpretations about past climates.
- line 475: Sancho et al., (2015) can be also cited as evidences of a quite wet MIS6 in the Mediterranean region:
Sancho, C., Arenas, C., Vázquez-Urbez, M., Pardo, G., Lozano, M. V., Peña-Monné, J. L., Hellstrom, J., Ortiz, J. E., Osácar, M. C., Auqué, L., and Torres, T.: Climatic implications of the Quaternary fluvial tufa record in the NE Iberian Peninsula over the last 500 ka, Quaternary Research, 84, 398–414, https://doi.org/10.1016/j.yqres.2015.08.003, 2015.- line 490: please, indicate the uncertainty of the date. I doubt that its precission was so high to associate it to the transition from GI-21 to GS-20.
- lines 490-495: overinterpretation.
- section 4.5 may be earlier to make clear what is new in this manuscript and how it has changed previous conclusions about the evolution of Neanderthas in Scladina cave.
- line 557: "A minority" is too optimistic, I am afraid.
Citation: https://doi.org/10.5194/cp-2024-27-RC1 -
RC2: 'Comment on cp-2024-27', Anonymous Referee #2, 17 Jun 2024
This paper presents some new U-Th dating results obtained on speleothems intercalated in a significant archaeological stratigraphy. The authors focus on the methods they used to scrutinize the speleothems and target the best sections in order to avoid the influence of diagenetic processes on the dating results. They obtain a new chronology, more complete and robust that the one previously available.
General comments: The description of the sampling, which is core to the rest of the discussion, is insufficient; some extra figures and tables should be provided to make all the information available.
The palaeoclimatological interpretations are beyond the scope of the paper, and rest on scarce and discontinuous data (dating mostly). They should be kept to a minimum by comparison with the benthic stack or the SST as in Fig. 6. Further, the section 4.4 discussing the box plots of stable isotope data is arguably the weakest part of the paper and should probably be removed. As a consequence, “palaeoclimatological data” should be withdrawn from the title.
In order to support the significance of this work, it would be useful to provide a comparison of previous dates/age models with the one produced here, and to explain the most obvious consequences for the hypotheses of the archaeologists, especially regarding the age of the Neanderthal mandible.
Specific comments and technical corrections:
Fig. 1, Left: Font is too small
Fig 2 A: Not sure I understand this cross section. What is shown in brown? Sediment fill? How then is it possible to have bedrock less than a meter thick separating it from Sous Saint Paul cave? Has this lower cave been entirely filled? More textual description of the site would be useful here.
Line 62: Typo Sous Saint Paul cave
Line 105: “2012#1&2 are two pieces of the same stalagmite in stratigraphic continuity with 2012#1”. There is no “2012#1” listed in table 1.
Section 2.2 U-series analyses: The dating was performed in 4 different labs and it seems, while reading this section, that the information provided on the methods are either incomplete, inconsistent or redundant. Hence this section would be greatly improved and be of greater interest to the reader if re-written, showing what is common between the different labs (overall, the chemistry is quite similar) and enhancing the differences (type of spike, standard, instrument?). The information about sample size and extraction is provided, briefly, only for those ran at MPIC; it is important, however, to document this information for all the samples (e.g. how? powder or prisms? dental drill or saw or corer or else? position on speleothems?).
Line 159: “corrected for detrital contamination assuming a 232Th/238U weight ratio of 3.8 for the detritus”. Please cite a reference?
Line 171: Correct “The in-house CaCO3 standards VICS (…) was analyzed”
Line 178: “A single dripwater sample”: please elaborate, e.g. if it’s coming from a vial collecting monthly drip water or just the last hour or minutes of dripping.
Line 186: ”0.5-1.0 g chips of speleothem calcite” please elaborate on how these “chips” were detached from the speleothem, and in what specific locations (a figure would be useful).
Line 205: “Drip rates in the cave have been observed to be variable”: is there any reference or data to support this? “This likely relates to the comparatively small hydrological catchment area”: in comparison with what?
Line 225: Phosphorus.
Fig. 4: What are the holes on the slab? FI samples? If so, how come they were not extracted with a dentist drill in order to follow the laminae and optimize resolution?
Section 3.2: This section is hard to follow without the support of a figure showing the position of the samples or a summarizing table.
Fig. 6: Please indicate the color code for the U-Th dates.
Line 304: Grey bars (not green).
Line 308: Green bars (not grey).
Line 333: “range of values does not directly indicate a clear diagenetic overprinting of the original signal.” What do you mean? What values would you expect if there was diagenetic alteration?
Line 335: Instead of “equilibrium conditions”, this may reflect changes in precipitation conditions, influencing isotopic fractionation.
Fig. 7: Not sure of the pertinence of this figure. What are the actual data encompassed in the box plot representing? We lack information about the sampling (missing in Methods).
Apparent contradiction in the reasoning with line 410: “supports limited or absent diagenetic alteration” and further, line 416: “diagenesis of speleothem calcite weakened fluid inclusion stability”.
Also, this part needs more details: Line 414 “rather resembles diffusive water loss from speleothem calcite as an analytical artefact, reported in a recent study by Fernandez et al. (2023).” And line 416: “facilitating diffusion of water out of the sample before analysis”. What do you mean by “before”? Is it between the sampling and the crushing or is it in the cave setting? What kind of analytical artefact are you referring to?
Section 4.3: “Palaeoclimatological backdrop”: this part suffers from overinterpretation, or simplification, given the available data and their precision. I am not sure that the conclusion (i.e. that speleothems grow during periods of warmer climate) requires such a long presentation; the results of Fig. 6 are, in my opinion, sufficient to draw this conclusion.
Lines 453-455: unclear.
Line 483: “Since some signs of diagenetic overprint have been observed in stalagmite 2010#6, we therefore suspect this particular age may be inaccurate.” Then perhaps mention should be made of this in table 3 (FI results) and table 2 (e.g. add a column to warn about suspicious results or reversed ages).
Section 4.4: This paragraph seems to me the most arguable and the least necessary in this paper. As pointed out before (see comment about Fig. 7), we don’t know how the data were obtained and what they actually represent, which makes the results difficult to discuss. Moreover, it’s hard to comprehend why the isotopic values between a glacial and an interglacial would be less different than between the MIS 5e and the Holocene for example. It’s also hard to believe that the MIS5e was one the driest interval, given the existing literature about LIG palaeoclimate in Europe. The seasonality might indeed be a factor but as you state it, your dataset does not allow you to resolve it. Overall, this part raises more questions than it solves and it does not add anything crucial to the core of your paper, hence I’d suggest to remove it.
Line 516: “Our data allow for several changes and additions to the existing age model”. It would be useful and convincing to add a figure to show this, comparing the previous age model with the one you produced.
Line 536: “to assign ages to the stratigraphy of the underlying Sous-Saint-Paul Cave”. One could argue that you provided a maximum age for the base of the SSP Cave stratigraphy but what do we know about the sedimentary processes and how this gallery was filled, in comparison with the one of Scladinia? Could they fill at the same time or were they necessary filled successively, starting with SSPC?
Conclusion: “Fluid inclusion isotope data of the speleothem samples predominantly plot close to the GMWL” and “fluid inclusion isotope data also provide a clear indication of the good preservation of most of the speleothem material selected, as fluid inclusions are considered to be very sensitive to diagenetic alteration. A minority of the fluid inclusion data trends away from the GMWL”: This seems quite contradictory with what I understand from lines 412-416. Perhaps this latter paragraph should be clarified, or the conclusion should be more nuanced.
Line 549: “One notable consequence of the new U-Th ages is that a juvenile Neandertal mandible, previously placed in Marine Isotope Stage 5a or 5b, could now potentially date back to Marine Isotope Stage 5d”. Please explain why this is important, how it affects existing hypotheses.
Fig A1. Please define the holes that are distinguishable on each slab; it would actually be useful to label each of them to make the information fully available: anyone should be able to see the exact position of any dating sample or FI sample on the slabs.
Line 586: who is “A.J.v.N”?
Data availability: they are not yet available in the Edmond database.
References: Needs a careful check of formatting and homogenizing.
Citation: https://doi.org/10.5194/cp-2024-27-RC2
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