Optically Stimulated Luminescence (OSL) Dating of Alluvial Deposits from the Cahuachi Archaeological Site (South Peru)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

   Cahuachi is a significant archaeological site associated with the Nasca culture in Peru. The decline of this culture has several hypotheses, one of which suggests a connection to ENSO events that brought sandy gravel deposits. In this context, determining the burial ages of these deposits is crucial for validating the hypothesis. In this study, the authors collected six sediment samples for luminescence dating from five different locations within the study area. While some details about the OSL measurements were provided and the obtained OSL ages are considered reliable, additional information is needed.

 

Specifically, it is important to know whether large or small aliquots were used, as this affects the De distribution. The luminescence properties of the quartz samples for the SAR protocol should be detailed, including the OSL decay and dose-response curves, as well as the results of preheating and dose recovery tests. The authors mentioned that samples NZ7 and NZ8 were saturated; it would be helpful to see their dose-response curves to confirm this. Given that all samples should come from similar sources, it is puzzling why samples NZ7 and NZ8 are saturated while others are not. Typically, alluvial samples are poorly bleached before burial, leading to significant scatter in De values. Therefore, well-bleached grains should be selected for age calculation using various models. In this study, the FMM model was used only for sample NZ11. What are the overdispersion values for these samples?

 

Regarding dose rates, the cosmic ray dose rate and other parameters should be explained and provided. Since all samples were taken from sandy gravel or gravelly sand layers, which complicate gamma dose rate calculations, the authors should clarify their method for calculating gamma dose rates. Additionally, sedimentary sections for all sampling points and a location map should be included. Some specific points are listed below.

 

Line 101: change “cm-wide” to “cm-diameter”.

Line 107: why did you analyzed grain size and mineral of the sediments?

Line 248: 90Sr/90Y, superscript 90.

Line 252: space between “to 3” and “40”, superscript 40.

Line 255: give the reason about 30% of the saturation water content.

Lines 288 and 294: delete BP, BP is only used for radiocarbon dates.  

Figures 2–6, the top right insets should be explained in the caption.  

 

 

                                                                                                                                                                                                                                                    

Author Response

Cahuachi is a significant archaeological site associated with the Nasca culture in Peru. The decline of this culture has several hypotheses, one of which suggests a connection to ENSO events that brought sandy gravel deposits. In this context, determining the burial ages of these deposits is crucial for validating the hypothesis. In this study, the authors collected six sediment samples for luminescence dating from five different locations within the study area. While some details about the OSL measurements were provided and the obtained OSL ages are considered reliable, additional information is needed.

Reply: Thank you for the time and effort in reviewing our manuscript. The information you requested has been added (see below).

Specifically, it is important to know whether large or small aliquots were used, as this affects the De distribution. The luminescence properties of the quartz samples for the SAR protocol should be detailed, including the OSL decay and dose-response curves, as well as the results of preheating and dose recovery tests. The authors mentioned that samples NZ7 and NZ8 were saturated; it would be helpful to see their dose-response curves to confirm this. Given that all samples should come from similar sources, it is puzzling why samples NZ7 and NZ8 are saturated while others are not. Typically, alluvial samples are poorly bleached before burial, leading to significant scatter in De values. Therefore, well-bleached grains should be selected for age calculation using various models. In this study, the FMM model was used only for sample NZ11. What are the overdispersion values for these samples?

Reply: As written in line 232 (first version; line 229 revised version) and following, small aliquots were used in order to minimize any heterogeneity in the distribution of equivalent dose among the different quartz grains. Smaller quantities of grains could not be used to avoid having OSL signals that were too weak. 

Some examples of OSL decay and dose response curves were added in the new Figure 10. Also an example of dose recovery test at different temperatures was added in Appendix (new Figure A6).

The only sample that was in saturation was NZ1a: its OSL shine-down curves were reported in Figure 10a. For this sample, 70% of the aliquots were at saturation. This can be due to a poorly bleaching before burial or to an intrinsic characteristics of the quartz grains. The overdispersion values for all samples were added in Table 2. Sample NZ1a has the lowest OD value of all samples. This could be an indication that the sample has been well bleached and the fact that it is close to saturation is therefore linked to the intrinsic luminescence characteristics of the quartz contained in it.

In general, as shown in Figure 11, the E.D. distributions are non skewed. The arithmetic mean was used for E.D. calculations with the exception of sample NZ11 for which the FMM model was used due to the dual distribution of E.D. obtained.

Regarding dose rates, the cosmic ray dose rate and other parameters should be explained and provided. Since all samples were taken from sandy gravel or gravelly sand layers, which complicate gamma dose rate calculations, the authors should clarify their method for calculating gamma dose rates. Additionally, sedimentary sections for all sampling points and a location map should be included.

Reply: A sentence about the contribution of the cosmic rays to the dose rates was added at the end of paragraph 2.4.1.

The alpha counting measurements for the determination of U and Th were carried out on a representative sample of the total grain size of each sample. In this case the heterogeneity of the sample is not a problem for the effect of gamma radiation, which has a penetration depth of about 30 cm.

Sedimentary sections of the investigated alluvial deposits are now reported in the new Figure A3, while the location of the sampling points is in Figure 1.

Some specific points are listed below.

Line 101: change “cm-wide” to “cm-diameter”.

The change was made

Line 107: why did you analyzed grain size and mineral of the sediments?

The study of the grain size and the mineralogical composition of the samples were important in order to determine whether the samples could be subjected to OSL analysis. In particular, they were carried out to verify the presence of sufficient quartz with a grain size ranging from 180 to 250 microns (see added sentences in lines 41-45 of the revised version).

Line 248: 90Sr/90Y, superscript 90.

The change was made

Line 252: space between “to 3” and “40”, superscript 40.

The change was made

Line 255: give the reason about 30% of the saturation water content.

As stated in the text, the value chosen for F is due to the relatively dry environmental conditions of the site. This value was found to be compatible with the average water content measured in the samples at the time of collection.

Lines 288 and 294: delete BP, BP is only used for radiocarbon dates.

The change was made

Figures 2–6, the top right insets should be explained in the caption.

The caption was modified adding the description of the top right insets.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript reports a well-planned, executed and described work, with high scientific and historical interest. This work is innovative in nature and has a very positive impact on studies already carried out at the archaeological site. I strongly recommend its publication after the minimal corrections suggested in the pdf document. The biggest change indicated is the verification of the way references are cited throughout the text, which were not properly placed (highlighted in orange).

Comments for author File: Comments.pdf

Author Response

The manuscript reports a well-planned, executed and described work, with high scientific and historical interest. This work is innovative in nature and has a very positive impact on studies already carried out at the archaeological site. I strongly recommend its publication after the minimal corrections suggested in the pdf document. The biggest change indicated is the verification of the way references are cited throughout the text, which were not properly placed (highlighted in orange).

Reply: Thank you for your time and effort in reviewing our manuscript. We have checked and modified the references based on your suggestions.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript of Marco Delle Rose et al. is a very interesting study focused on the OSL dating of alluvial deposits from the Cahuachi archaeological site (Nazca River Valley, Peru). I found the article very well-written and engaging. The subject of the study is both intriguing and relevant, making it likely to capture the interest of potential readers of the journal. The background provided within the introduction is comprehensive and establishes a clear context for the research, effectively highlighting the aim of the study. The article engages well with previous studies in the literature, the results are clearly presented, and the conclusions are supported by the findings. In short, the authors did a really great work, the overall quality of the manuscript is excellent.

One limitation of the study is the relatively small number of samples investigated. Nonetheless, the overall approach employed is known to be expensive and time-consuming, which justifies the sample size. Expanding this research in future studies would be beneficial for broader validation and deeper insights.

 

The only point of improvement involves minor stylistic inconsistencies with the citation format in certain parts of the manuscript. Specifically, some references appear to be doubled, in different styles. 

Author Response

The manuscript of Marco Delle Rose et al. is a very interesting study focused on the OSL dating of alluvial deposits from the Cahuachi archaeological site (Nazca River Valley, Peru). I found the article very well-written and engaging. The subject of the study is both intriguing and relevant, making it likely to capture the interest of potential readers of the journal. The background provided within the introduction is comprehensive and establishes a clear context for the research, effectively highlighting the aim of the study. The article engages well with previous studies in the literature, the results are clearly presented, and the conclusions are supported by the findings. In short, the authors did a really great work, the overall quality of the manuscript is excellent.

Reply: Thank you for the time and effort in reviewing our manuscript.

One limitation of the study is the relatively small number of samples investigated. Nonetheless, the overall approach employed is known to be expensive and time-consuming, which justifies the sample size. Expanding this research in future studies would be beneficial for broader validation and deeper insights.

Reply: We agree that expanding the research in future studies would be beneficial for broader validation.

The only point of improvement involves minor stylistic inconsistencies with the citation format in certain parts of the manuscript. Specifically, some references appear to be doubled, in different styles.

Reply: We have checked and modified the references based on your comment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

“The alpha counting measurements for the determination of U and Th were carried out on a representative sample of the total grain size of each sample. In this case the heterogeneity of the sample is not a problem for the effect of gamma radiation, which has a penetration depth of about 30 cm”, Gravel, sand, and clay typically consist of different minerals or chemical components. If the "total grain size of each sample" is ground and counted, the beta dose rate for the sample may be inaccurately estimated. The beta dose of the measured sand-sized grains comes only from the contribution of fine grains due to the short range of beta rays. Although gamma rays have a range of about 30 cm, their contribution to the dose rate should differ between gravels and sands because of their different chemical components.

Author Response

“The alpha counting measurements for the determination of U and Th were carried out on a representative sample of the total grain size of each sample. In this case the heterogeneity of the sample is not a problem for the effect of gamma radiation, which has a penetration depth of about 30 cm”, Gravel, sand, and clay typically consist of different minerals or chemical components. If the "total grain size of each sample" is ground and counted, the beta dose rate for the sample may be inaccurately estimated. The beta dose of the measured sand-sized grains comes only from the contribution of fine grains due to the short range of beta rays. Although gamma rays have a range of about 30 cm, their contribution to the dose rate should differ between gravels and sands because of their different chemical components.

REPLY: Thank you for the time and effort in reviewing our manuscript. The samples were not ground for the alpha counting measurements. The average granulometry of all the samples is between 100 and 400 microns (see figures 7 and 8). Moreover, for sample NZ7, NZ8 and NZ11, grains larger than 500 microns are negligible. In this range, the variation in the beta dose, as shown in the figure (Wintle and Aitken 1977), does not change significantly. For the gamma rays, the minerals Z-effective is the same and consequently their absorption coefficient.

Author Response File: Author Response.pdf