Multi-proxy speleothem-based reconstruction of mid-MIS 3 climate in South Africa
Abstract. The southern coast of South Africa displays a highly dynamical climate as it is at the convergence of both the Atlantic and Indian Ocean, and it is located near the subtropical/temperate zone boundary with seasonal influence of easterlies/westerlies. The region hosts some key archeological sites with records of significant cognitive, technological and social developments. Reconstructions of the state and variability of past climate and environmental conditions around sites of archeological significance can provide crucial context for understanding the evolution of early humans. Here we present a short but high-resolution record of hydroclimate and temperature in South Africa. Our reconstructions are based on trace elements, calcite and fluid inclusion stable isotopes, and fluid inclusion microthermometry from a speleothem collected in Bloukrantz Cave, in the De Hoop Nature Reserve in the Western Cape region of South Africa.
Our record covers the time period from 48.3 to 45.2 ka during Marine Isotope Stage 3. Both 𝛿18Oc and 𝛿 13Cc show strong variability and covary with Sr/Ca. This correlation suggests that the control on these proxies originates from internal cave processes such as Prior Calcite Precipitation, which we infer to be related to precipitation amount. The hydroclimate indicators furthermore suggest a shift towards overall drier conditions after 46 ka, coincident with a cooling in Antarctica and drier conditions in the eastern part of South Africa corresponding to the Summer Rainfall Zone.
Fluid inclusion-based temperature reconstructions show good agreement between the oxygen isotope and microthermometry methods, and results from the latter display little variation throughout the record, with reconstructed temperatures close to the present-day cave temperature of 17.5 °C. Overall, the BL3 record thus suggests stable temperature from 48.3 to 45.2 ka whereas precipitation was variable with marked drier episodes on sub-millennial timescales.
Jenny Maccali et al.
Status: final response (author comments only)
RC1: 'Comment on cp-2023-1', Kerstin Braun, 02 Feb 2023
- AC1: 'Reply on RC1', Jenny Maccali, 02 May 2023
RC2: 'Comment on cp-2023-1', Nick Scroxton, 28 Feb 2023
- AC2: 'Reply on RC2', Jenny Maccali, 02 May 2023
Jenny Maccali et al.
Jenny Maccali et al.
Viewed (geographical distribution)
The manuscript “Multi-Proxy speleothem-based reconstruction of mid-MIS 3 climate in South Africa” presents new records of speleothem stable isotopes, strontium concentrations and temperature reconstructions based on fluid inclusion methods from Bloukrantz Cave, South Africa. In my opinion this manuscript addresses relevant scientific questions within the scope of Climate of the Past. Overall I think this manuscript would be a valuable publication in Climate of the Past after some major additions outlined below.
This is the first record of speleothem trace element compositions and fluid inclusion-based temperature reconstructions from this region. Since this is a very short and highly resolved record comparison to existing proxy records (which are usually much lower resolved) and the conclusions that can be reached are somewhat limited, nevertheless, I think this is a very relevant and important dataset and that the conclusions are substantial. The methods and assumptions used in the manuscript are valid and clearly outlined and their descriptions are sufficiently complete to allow for traceability of results. The results also support the interpretations and conclusions.
There is a clear distinction between previous work (including proper citations) and the addition of the current work. The title and abstract are clear and reflect the content of the article. I only have minor comments about the overall presentation and language (see below); symbols and abbreviations are correctly defined and used and references are appropriate except for a few minor additions/comments below.
I have on major technical correction regarding the dating data tables in the supplementary materials. In the current form there is not enough information about the U-Th ages to judge their quality or to re-calculate them in the future e.g. if half-lives of some of the involved isotopes are updated. Ideally the following parameters (and their uncertainties) should be reported for each dating analysis: depth in sample (this is provided already), weight, uncorrected age result, 238U concentration, 232Th concentrations, 230Th concentration, 230Th/232Th ratio, 230Th/238U ratio, 234U/238U ratio, corrected age, reference (are ages before 1950, 2000, or relative to the year when the age was measured [give the year in this case]).
I also have two main points that I think could be improved in the discussion
First, I think the section ‘Hydroclimate reconstructions’ is well written, but in it’s comparison to other proxies it focusses a lot on very distant records in the summer rainfall region. The authors mention other speleothem records from the South African south coast in the Introduction and I understand that these records are very low resolved at the time when the sample used here was formed preventing a direct comparison. There are also two records from the Little Karoo that are not mentioned in the paper (Talma, A. S., & Vogel, J. C. (1992). Quaternary Research, 37(2), 203–213. https://doi.org/10.1016/0033-5894(92)90082-T; Chase, B. M. et al. (2021). Geology, 1. https://doi.org/10.1130/G49323.1). I think that something might might be learned from comparing the range of d18O and d13C values at the different caves sites – do they overlap or not and what might be the reasons? I also think that the Talma & Vogel (1992) record has a decent resolution for the Holocene section that overlaps with this new record and a comparison of that section might be feasible (the Chase record is a composite that mostly replicates the Talma & Vogel record in the Holocene). I think it is also worth mentioning that the pattern of the presented record with increasing stable isotope values during phases of global cooling is also found in previously published records (e.g. most of the published records from this region have somewhat higher values for d18O and d13C during cooler phases like MIS 4 than the warmer MIS 5), despite the very different resolution. The previously published records have been interpreted in a very different way, but this similarity in the relation to global change might suggest that the processes mentioned here also affected the other records.
Second, I think the section on “Significance for the archaeological record” should be rephrased. I think this section is not very clear in its current form and it could be improved. E.g. The Howieson’s Poort is a complex microlithic technology dating to late MIS 4 that can be found at many archaeological sites in southern Africa. It’s disappearance at the beginning of MIS 3 and the decrease of site use intensity on the south coast may suggest a decrease of interactions between different hunter-gatherer groups and possibly a population decline and/or shift of activities towards inland locations.
Line 33-35: in sentence “Homo sapiens was anatomically modern as early as …” I think it is convention to italicize species names, also I would delete the words ‘behavioral proxies in’ later in this sentence
Line 38: “Episodes of significant cultural changes, …” there is an ‘e.g.’ before the second citation at the end of this sentence, either delete it or move it in front of the first one.
Lines 68ff: “In the speleothem record, this is illustrated…” add some information and references to Talma & Vogel (1992) and Chase et al., (2021) here.
Line 85: “The theoretical background of this approach dates back to the 1960s…” – all the citations that follow are from the 40s’ and 50s’
Line 114: Chapter 2.2 Sample description: the authors mention the calcite fabrics throughout this chapter, but only one image of the fabric is included in the Supplementary Materials. I think a few examples of the different fabrics mentioned here in the Supplement would be useful.
Starting in Line 215: sections 3.2 Trace elements and 3.3 Stable isotopes as well as Figure 2: the text in the results describes the Sr/Ca record and both stable isotope records in terms of their temporal changes, yet, the figure that presents the results (Figure 2) plots them against depths. And the Sr/Ca and d13C records are not plotted against age in any of the figures. I think this should be harmonized, either by changing figure 2 to be plotted against age (the plot against depth could still be presented in the supplement if needed) or by referring to depths here (ages could be mentioned along with the depths maybe in parentheses). I also think that the order of Sr/Ca, d13C and d18O in Figure 2 from top to bottom should be the order in which they are mentioned in the text.
Line 119: “After the hiatus, the Sr/Ca signal drops markedly and shows little variation with an average of 348,…” This sentence sounds like there is little change for the whole Holocene section and that the values stay below what they were before the hiatus. I don't think this is entirely true. I would rephrase to something like: "After the Hiatus, the Sr/Ca signal drops markedly and shows little variations with an average values of 348 between depths of 200 and 150 mm/ages of __ to__ ka. Between 150 and 70mm (__-__ka) values gradually increase to ~500. The top 70mm (__ to __ ka) show some of the highest variability of the record with averages around 500 (not sure about that value).
Line 336 ff: “The inferred overall drying observed in our record…” I think the publication by Engelbrecht, F. A., et al. (2019. Quaternary Science Reviews, 226, 105879. https://doi.org/10.1016/J.QUASCIREV.2019.105879) could be cited here. They show that a northward shift of the westerlies during the LGM might reduce the amount of winter rainfall along a very narrow stretch of the south coast due to downwind effects along the Cape Fold Mountains.
Line 383: “Here, the water content displays little variation…” refer to Fig 4 along with Fig. S7 in this sentence.
I have two general suggestions:
Line 217: Figure 1: blue shading indicates bathymetry of the surrounding oceans; is the current shown as SAC (South Atlantic Current) not the Antarctic Circumpolar Current? The SAC would be the section in the south Atlantic that also represents the southern branch of the subtropical gyre that then is deflected north into the BC, the ACC is what continues east.
Figure 2: Line 726: “Dashed Lines indicate the isotope transects.” Add ‘along the main growth axes of the speleothem’ since in the text this is what they are said to indicate.
Line 730: “Dashed lines indicate the onset of darker layers…” It looks like the dashed lines were taken from the depth along the isotope profile but were not adjusted for the difference in depth in the Sr profile; are the depths of the dark layers known for the TE profile and could this be adjusted similar to what is shown with the grey boxes?
Line 744: Figure 4: in the text the authors often refer to the outlier data cluster in this plot and that they represent a specific age range. I think these outliers should be marked here, maybe just with a simple circle around the younger samples that are more offset from the meteoric water lines or by using different symbols for them?
Line 749: Figure 5: I think more recent versions of the EDML d18O record do not have the gap between 45 and 44 ka, see here: EPICA Community Members (2010): Stable oxygen isotopes of ice core EDML. doi:10.1594/PANGAEA.754444
I recommend adding a title page to the file that states the title of the paper and names of authors and corresponding author.
Caption to Fig S2: correct ‘Mars’ to ‘March’.