Holocene environmental and climate evolution of Central West Patagonia as reconstructed from lacustrine sediments from Meseta Chile Chico (46.5o S, Chile)

Franco et al., 2023

General comments

 A 3 m-long composite lake sediment record (LME-CP) was analysed from Laguna Meseta (LME-CP) in Central Western Patagonia at the eastern margin of the North Patagonian Icefield (NPI). Based on a multiproxy lake sediment analysis over the past ~10 ka, the authors discuss the sedimentation dynamic to reconstruct the glacial and environmental history of the area. The sedimentation dynamic was then correlated to environmental changes around the study area, specifically, regional glacial oscillation and paleoclimate proxies, with the objective of deriving insights into the Holocene climate variability in Central Western Patagonia. The authors discuss, infer, and conclude that the major environmental changes during the middle Holocene are mainly controlled by precipitation variability linked with the evolution of Southern Hemisphere Westerly Winds (SHWW).

The manuscript is well presented and structured, with fluent and precise language.  Additionally, the problem, statement, and objectives are clearly explained. The manuscript provides results from a new location in the east of the NPI that contributes to improving the knowledge base concerning the evolution of the SHWW and adds relevant information to the understanding of past climate in the region. The evolution of the SHWW is a key component of the South American climatic systems. This manuscript contributes results that support a better understanding of the behaviour of the evolution of the SHWW through the Holocene and therefore, is within the scope of this journal. The title’s manuscript clearly reflects the scope of the research, focusing on where the study was performed, the type of records found, and the period when is the environmental reconstructions were made. The abstract provides precise and complete information about the content of the manuscript, however, one suggestion for modification is, “3 m-long continuous sediment record” should be changed to “3 m-long composite sediment record” or just “3 m-long sediment record”.

The methods area is clearly outlined and organised, and the description of the experiments is sufficiently complete and precise to allow for their reproduction. However, the construction of the “composite sediment records” is not totally explained in the methods sections, but this issue can be resolved with a brief explanation.

The manuscript’s figures accompany the text well, and these have detailed descriptions, which allow for a better understanding of the problems in the study area context. The manuscript is well presented, and the article is very well structured. The problem statement is clearly explained, as are the objectives, area of study, and the method, the latter, permits the reproduction of the results. The manuscript presents a well pool of multiproxy results, which support the interpretation, and reaches adequate conclusions.

The interpretation of the geochemical data is clear and thoroughly discussed, and this allows the reader to appreciate how the authors constructed a discussion and reached the conclusion. The number of references is adequate; however, it is recommended that the authors check the format of the references. There is a minor change to make, for instance, the “Https://” is sometimes presented before the paper’s doi and other cases not.

Specific comments

Line 132 mentions that three sediment cores were collected, and one of them will be used in the future for pollen analysis. Thus, the lector assume that two cores were utilised to construct the “3 m-long composite sediment record” (Laguna Meseta, LME-CP). Then, it is mentioned that the composite sediment records were constructed with additional help of six visible tephra as marker horizons for correlations between core sections. I understood (or I assumed) how the LME-CP was constructed after reading the results (Fig. 3) and Table 2 (AMS radiocarbon dates). I suggest adding a brief explanation in section 3.1 to clarify how the LME-CP was constructed, for example, mentioning how many overlapping drives of sediment (how long) were cored with a xxx corer from a depth of c xxx producing a composite lake sediment of 300 cm depth (Fig. 3), and that correlations of the overlapping core sections were based on (lithological changes?), and the correlation among sediments cores were made by…etc.

In general terms, tephrochronology is the use of the tephra layers (volcanic ash) as “isochronous” (same time) connections or correlations of deposits from one place to another. It forms a widespread marker horizon or isochron in lake or marine sediments (among other sediments). The authors in this study detected six tephra layers using XRF scanning. This tephra layer detection is not associated with any geochemical quantified data, nor was the data compared with previous work. The authors mention that “the results show a non-uniform composition of these layer, reflecting different volcanic resources”. There is not enough evidence to make that claim, and furthermore, this could perfectly be the result of a different eruption from the same volcanic resource. We don’t know until the geochemistry is quantified. In brief, I suggest rephrasing the idea.

Only one tephra layer was geochemistry identified and compared with previous data from H1 (Hudson 1 eruption) and M1 (Mentolat 1). The results from this study show similar geochemical characteristics as T5 (H1, Hudson 1) (Table 1: Fig. 4). Then, the authors mention a mean age of 8,415 cal yr BP (Stern et al., 2016), which was drawn in figure 6 but was not included in the age-depth model display. My point here is to ask why the authors do not include H1 to display the age-depth model and completely apply the tephrochronology concept? I suggest writing a brief phrase explaining why H1 age was not included in the age-depth model. For example, an answer could be associated with any of the following: the limits of the tephra layer are not clear, the previous H1 age is too old/young compared with this study, or another motive. This explanation is very useful because tephrochronology is frequently not used in  new data and/or  in the other data we want to compare, and in my opinion, tephrochronology is underused.

Format comments

Line 65, should say (Fig. 2a)

Line 66, should say (Fig. 1c)

Line 66, should say (….: De Cruz and Suárez, 2008)

Line 91, at the beginning of the manuscript the references are organised by alphabetical order, then by year? should be consistent throughout the ms.

Line 132, include the point after (Fig. 2a)

Line 133, should say (Fig 2b)

Line 263, should say (Fig. 1b)

Line 277, should say (Fig. 4a)

Line 284, should say (Fig. 4b)

Line 308, should say (Fig. 5a)

Lines 310, should say (Fig. 5b)

Line 332, should say (Fig. 3)

Line 352, should say (Fig.7a)

Line 358 and 375, should say (Fig. 7b), should be consistent throughout the ms. Please check.

Line 405, should say 2,500 cal yr BP.

Line 448, Nothofagus, italic

Line 500, Nothofagus, italic

Line 663, in some references “the https://” missing, should be consistent throughout the ms.

Review on the manuscript "Holocene environmental and climate evolution of Central West Patagonia as reconstructed from lacustrine sediments of Meseta Chile Chico (46.5°S, Chile) by C. Franco et al.

The manuscript "Holocene environmental and climate evolution of Central West Patagonia as reconstructed from lacustrine sediments of Meseta Chile Chico (46.5°S, Chile)" by C. Franco et al. focuses on the reconstruction of Holocene environmental changes in West Patagonia using a 3-m-long lacustrine sediment succession. Overall, although the manuscript is well written, I ask for major revisions to better constrain the aim of this study, to improve the structure of the manuscript, and to make data and results reproducible. Particularly the (tephro-)chronology needs a thorough revision and clearer discrimination of raw data and modelled data. The tephrostratigraphy is weak, but an improvement might be beyond the scope of this manuscript and is time consuming. However, based on the weak tephrostratigraphy a correlation of tephra layers in the lacustrine sediments with those known from other records should be more carefully addressed and discussed.

The abstract provides a short introduction and summarizes the main findings of the study, i.e. the environmental changes over the last c. 10,000 years. Although most of the abstract reads well and is coherent, it is not clear to me how the authors come to the conclusion in the last sentence that environmental conditions during the Holocene were mainly controlled by precipitation variability and oscillations of the Southern Hemisphere Westerly Winds. While this might be correct, in the text previously, changes in sedimentation characteristics were explained with a broader variety of environmental conditions, including shifts in lacustrine productivity and variations in allochthonous sediment input, likely caused by colder and/or wetter conditions. The reduction to one of these factors in the last sentence is surprising and difficult to follow. I suggest some minor rewording here and there to emphasize the role of precipitation changes and the Westerly Winds.

The introduction needs major revision. The first of a total of three paragraphs deals with a period that is not covered by the sediment succession from Meseta Chile Chico. The two sentences of the second section describe the state of research on Holocene climate variability and on glacial advance and retreat in the region. The third paragraph defines the aims of the study. This introduction is completely ignoring the existing knowledge on Holocene environmental changes, as it is derived from numerous marine and terrestrial records in the region and its realm (including e.g. Zolitschka et al. 2013, and references therein). Not a single note is found to precipitation changes and oscillations of the Westerly Winds, which according to the abstract are the factors controlling environmental, climate (and glacier?) changes in the region. There is a distinct need to span here the broader frame for the purpose of the new study, the new data and their interpretation. Some of the missing information is provided later, in chapter 2.2 ("Previous paleoenvironmental reconstructions"). However, also this chapter contains a lot of information from the pre-Holocene, a period, which is not covered with the sediment succession from the Laguna Meseta. I suggest to streamline the content a bit (also with respect to the knowledge on oscillations of the wind systems) and move large parts into the introduction.

In the chapter on the regional settings, I miss important information about the lake itself, e.g. lake size, what is the bathymetry based on (Fig. 2), what is the hydrology of the lake with respect to temperature, mixis, etc.? What is the trophic state, what is the dimension of the catchment area? What is the predominant vegetation in the catchment area and how dense is the vegetation cover?

In the Methods and Materials chapter, please add some more information to the length of the individual core sections, and how many sections have been recovered at the individual sites. This is important to follow the correlation of individual core sections to a composite record. Adding a table with individual core lengths, field depths, sites and/or tools used would help to better understand the coring procedure and splicing of core sections to the composite record. I also suggest to restructure this chapter, starting in line 138 with a subchapter on scanning and logging (XRF and mag sus), then geochemical analyses on tephra, followed by analyses on discrete samples (grain size and bulk geochemistry), before dating, age-depth modelling and handling with published data is described. With respect to tephra layers, why was only one tephra selected for major and trace element analyses? Can you provide some information how accurate WD XRF is for trace element analyses in lake sediments (particularly with respect to Sr, I assume that calcite precipitation in the lake is limited, but it would be good to discuss this a bit more, probably later in the text)? With respect to chronology and radiocarbon dating, why was bulk sediment used for dating, if plant remains have been detected (chapter 4.1; Fig. 3)?

The subchapter "Tephra records and tephrochronology" (pls reword; see below) needs further elaboration. Tephrochronology is not the right wording here, as tephra was not used to constrain the age model (at least from what I understood when reading the text, however, see also comments below). In contrast, the age model was used to assign one of the tephra layers to a known eruption. Overall, I miss further information on (1) why are Cay and Maca volcanoes not included in the discussion? (2) What is H0-H3 shown in Fig. 4? (3) Do all dots in Fig 4b represent bulk samples? (4) Why are there different markers (dots, diamonds, triangles) used in Fig 4a and 4b? (5) Provide a more nuanced discussion of mismatch between bulk concentrations displayed in Fig 4a. Also, restructuring is needed here, as the age model is not presented yet and information on the activity of the two volcanoes is too late in this chapter. Start directly with activity of volcanoes in the region to confine the origin of T5.

In chapter 4.3 trends of the magnetic susceptibility are described. However, it seems that these results have no further implications. So why are the data shown and described? Either include the results in the discussion or delete mag sus from the entire text, if it is useless.

Chapter 4.5 describes the radiocarbon ages and the establishment of the age-depth model. This is the second major weakness of the manuscript. Although the authors write that "all ages were considered for calculation of the age/depth model" and "T5 ... is used as a temporal control point", it seems that all 14 C ages were regarded as being reliable, except of the age at 240.5 cm composite depth. Moreover, although the authors declare that the age of the potential H1 tephra was used only as a control point, it seems that the age range of the tephra was included in the modelling approach, as the error bar (greyish boundaries in Fig. 6) is wide at the top of the unit F and is not restricted to the age and error bars of sample D-AMS 043189, as this is the case in all other dated horizons. Table 2 does not contain calibrated mean ages, it shows rbacon derived ages that apparently including age information of the T5 tephra. This would explain the discrepancy between the calibrated mean age of sample D-AMS 043190 in Table 2 compared to the location of the respective sample in Fig. 6. If the tephra was included as a single event, ages of the tephra should be completely removed from the rbacon calculation. Although I can partly follow the discussion of the impact of the tephra and potential fall-out deposits on top of T5, I miss a thorough discussion, why the age at 240.5 cm depth is somewhat erroneous in contrast to all other radiocarbon ages. Including sample D-AMS 043190 as a reliable sample would lead to a basal age of T5 of ca. 9400 cal yr BP, which is distinctly older than the published ages of the H1 eruption. This provokes a more nuanced discussion of the tephrostratigraphical correlation. On the other hand, assuming that the 14.5 cm on top of T5 may represent a mixture of erosion of sediments and tephra from the catchment is ok, but also this might contain some time. Extrapolating the sedimentation rate calculated from the two ages on top of the tephra downward and including these 14.5 cm of unit F would provide an age slightly older than that of sample D-AMS 043189, i.e. > 8100 cal yr BP (also the 8257 cal yr BP in Table 2 seem to be biased by including a H1 age into the rbacon modelling). Such an age would at least marginally match with published ages of the H1 tephra. Overall, a substantial revision is needed in this chapter to provide reliable ages and discuss potential errors in the age-depth modelling. Simply stating that one age has a potential offset and this age is not important and therefore was not excluded is not sufficient for a thorough scientific study and a discussion of potential tephra correlation.

The Discussion (chapter 6) is very sound and covers a broad suite of environmental factors that control sedimentation in the lake basin. With respect to the above-mentioned broadened discussion of the age model and the reliability of sample D-AMS 043190, the discussion of the Sedimentary environments (chapter 6.2) could be complemented by a discussion of a potential glacial advance in the early Holocene and coarser sedimentation in unit G, probably corresponding with other observations in the broader vicinity (e.g. Douglass et al 2005a and other references therein; glacial advances around 9.4 cal ka BP).

Specific comments:

I cannot follow the order of referencing in the text, is it according to author name or to date? Please check author guidelines.

Figure 1: Check coordinates in Figure 1b. West and South axes are mixed up.

Figure 3: What is the black triangle in T5 indicating? Include in legend. Litho-boundaries seem not match exactly with the boundaries mentioned in the text.

Table 2. An overview of how the individual core segments were spliced to a core composite would help to better evaluate the reliability of the age-depth model.

Line 58/59: ".... and form part of its drainage basin that currently drains into Río Jeinemeni." check sentence

Line 60: What do you mean with "topographic connections" of the glacial cirques?

Lines 83: Include coordinates in Fig. 2a, provide a scale bar in 2b indicating the dimensions of the Laguna Meseta.

Lines 84/85 and also throughout the entire text: Check Fig 1.c or Fig. 1.c etc for correct spelling

Line 140: ...were performed, a 3-m-long ....

Line 146/147: "Due to their compositional variability, no element ratio correlates with all six tephra layers. Therefore, data points corresponding to these layers were discarded for

correlation coefficient calculations." – I assume that the element ratios are based on the XRF scanner data? I do not understand these sentences or the relation to what is written in the XRF scanning chapter. Can you rephrase?

Line 236: delete "sediment"

Line 238: Plant remains are not shown in Figure 3

Lines 244-245: What do you mean with grainsize is very well sorted? I assume that this is true for individual horizons in Unit B, but not for the entire Unit. Pls specify.

Line 247: with plant remains of up to a few millimeters in size

Line 248: Chapter header should be "Tephra layers"

Line 250: XRF scanning results show a heterogenous composition of these layers, reflecting different volcanic sources

Line 261: I have not seen the age model yet. Moreover, I have no information on the activity of Mentolat and Hudson yet. This comes later in the text. Restructuring needed.

Line 289: The highest values of Ca are in unit B, not in T5, please correct, check also for other elements. Rewording needed.

Line 296: " ... is obscured by T3 and T4, values for chemical elements..."

Line 305: I would not say that TN concentrations of <1.7 % are negligible. A high number of lakes has TN concentrations of <1%, so concentrations of up to 1.7 % are substantial. The high concentrations confirm that OM is an important sediment component.

Lines 393–400: Why is Unit G in stage 1 ignored? It stands out for its high clastic sediment input. Include G in the description and discuss, what may have caused this unit. Please check also for the age of unit G with respect to a revision of the age-depth model. Considering sample D-AMS 043190 as a reliable sample would place unit G into a period around 9400 cal yr BP or so.

Line 630: ... fall-out deposits tentatively originating from the H1 eruption of Hudson volcano....