The use of paleoclimate simulations to refine the environmental and chronological context of archaeological/paleontological sites
Abstract. This study illustrates the strong potential of combining paleoenvironmental reconstructions and paleoclimate modeling to refine the paleoenvironmental and chronological context of archaeological and paleontological sites. We focus on the El Harhoura 2 cave (EH2), an archeological site located on the North-Atlantic coast of Morocco that covers a period from the Late Pleistocene to the mid-Holocene. On several stratigraphic layers, inconsistencies are observed between species- and isotope-based inferences used to reconstruct paleoenvironmental conditions. The stratigraphy of EH2 also shows chronological inconsistencies on older layers between age estimated by Optical Stimulated Luminescence (OSL) and Combination of Uranium Series and Electron Spin Resonance methods (combined US-ESR). We performed paleoclimate simulations to infer the global paleoclimate variations over the EH2 sequence in the area, and we conducted a consistency approach between paleoclimate reconstruction estimated from simulations and available from EH2 paleoenvironmental inferences. Our main conclusion show that the climate sequence based on combined US-ESR ages is more consistent with paleoenvironmental inferences than the climate sequence based on OSL ages. We also evidence that isotope-based inferences are more congruent with the paleoclimate sequence than species-based inferences. These results highlight the difference in scale between the information provided by each of these paleoenvironmental proxies. Our approach is transferable to other sites due to the increase number of available paleoclimate simulations.
Léa Terray et al.
Status: final response (author comments only)
RC1: 'Comment on cp-2022-81', Patrick Bartlein, 17 Jan 2023
- AC2: 'Reply on RC1', Léa Terray, 24 Feb 2023
RC2: 'Comment on cp-2022-81', Chris Brierley, 23 Jan 2023
- AC1: 'Reply on RC2', Léa Terray, 24 Feb 2023
Léa Terray et al.
Léa Terray et al.
Viewed (geographical distribution)
This is an interesting paper that describes the use of climate-model simulations to discriminate among differing chronologies and paleoenvironmental reconstructions at an archaeological site in Morocco. This goal is generalizable to other situations, and the involvement of climate-model simulations in that discrimination allows us to talk about potential mechanistic explanations for different interpretations. The particular approach of running a “current” version of a model (in this case LMDZOR6A) using as boundary conditions output from what might be called “legacy” simulations (somewhat reminiscent of reanalysis simulation) is also generalizable, and provides a way to get added value from older simulations.
My main concern with the paper is that the data-analytical methods used to compare the simulations and reconstructions are inadequately motivated and explained. The text basically says “Look at the figures.” but the figures themselves have some issues, and their legends/captions don’t provide much help with interpreting or understanding the results. In the specific comments below, I’ve asked a number of questions about the figures that if answered or explained would easily address that concern.
line 48: “atmosphere-alone simulations”. This implies to me that SSTs were fixed, and the usual assumption would be that were fixed for modern conditions. I think it would be important early on in the paper to describe that while the LMDZOR6A simulations are atmosphere only, the SSTs used as boundary conditions (along with other forcing) were ultimately derived from coupled simulations.
line 69: “Then, the climate described by the global simulations cannot faithfully represent the magnitude of the differences between the different paleoclimate periods, nor microclimate variations at EH2.” There are two issues here: GCM simulations obviously cannot simulate microclimatic variations, but if they can’t “represent the magnitude of the differences between two different paleoclimatic periods” then what good are they? I think climate models passed the test of being able to simulate climates different from present long ago.
line 132: “actualist approach” A little more explanation would be good. I was unable to locate Stoetzel et al. (2009), Stoetzel et al. (2014) is basically a review, and that leaves Stoetzel et al. 2011 as the main reference to the approach. But the approach is not really described in depth there, nor do we have a good idea of how well it works in practice.
line 295: “There are also important changes in the magnitude of the seasonal temperature variation from June to October and of the seasonal precipitation variation from October to May between these two periods.” If by “these two periods” is meant 115 ka and present, then at some part of the lower temperatures at 115 ka could be related to the calendar effect. At 115 and 116 ka, perihelion occurred in early January and late December respectively, similar to present, but eccentricity was much greater, at its maximum over the last glacial cycle at 115 ka. This led to months in the second half of the year (July – December) beginning and ending farther from the June solstice, and so the calendar effect could have resulted in apparently colder-than-present conditions during that same interval (Bartlein and Shafer, 2019, Fig. 2 and Fig. 11d) that could have reversed the sign of the “real” temperature difference. I don’t think that’s the case here, but it might be good to indicate that the potential role of the calendar effect was considered and discounted.
line 331: As described in its legend or in the text, Figure 8 is incomprehensible. How were the climate variables “centered and reduced”? What was the “common scaling” (z-scores, as implied by line 278)? If the variables are centered, why is a progressive as opposed to diverging color scheme used? What observations are the means and standard deviations calculated over? Why is the top half of the figure brown, and the bottom blue? Why do the variable labels alternate in color? What does “Actual” mean?
line 338: (Top) What do the axes represent, and what are the numbers in parentheses? Why do the points plotted here differ from those on the biplot for DH1? (Bottom) How are “contributions” calculated? Typically, component loadings (correlations between the original variables and components) would be plotted in this context, and ordering the variables by contribution makes it difficult to compare the analyses.
Line 385 (and Fig. 10): The motivation for, design, and interpretation of the 2B-PLS analysis needs to be improved. What are the two blocks of variables? (It looks like the isotopes and climate PC’s in one analysis, and THI categories and climate PC’s in the other, but why use the components and not the original climate variables?). Are there really no significant pair-wise correlations between the principal components and environmental variables, except for a few for DH1 isotopes? What are the little scatter diagrams? What is the basis for the statement that “there is a statistically significant covariation between THI values and all climate variables”? (line 386).
I could not find code- or data-availability statements. Several R packages are mentioned in the acknowledgments, but I was unable to find a function for 2B-PLS among them. (There is a function in the geomorph package, but that’s not listed.)
Abstract (and throughout): “species- and isotope-based” or “species presence-absence and isotope-based”? (I didn’t get a good sense of whether the paleoenvironmental reconstructions were based on species presence alone or species presence-absence data.)
Abstract: consistent/congruent There is a very subtle distinction between these words, but “congruent” does have a secondary meaning (in geometry) of exact agreement. So, I would just say “consistent.”
Abstract: “We performed paleoclimate simulations…” It would be good to expand on this a little
line 8: replace “on the localities” with “in the vicinity”
line 8: Not parallel. Replace with “inconsistencies between isotope (isotopic?) compositions, differences in type of remains, and variations in stratigraphies…”
line 10: “and prevent proper assessment of”
line 17: “teeth of small mammals”
line 19: “seasonal variations of climate”
line 20: Two of how many layers? (Is two a lot?)
line 26: Delete “global”?
line 34: “quartzitic sediment” or just plain “quartz”?
line 38: “Climate-model simulations”
line 39: “Climate models simulate…”
line 41: “model-data”
line 44: “for testing”
line 45: “depending for the different stratigraphy options” I don’t know what this means.
line 47: “the last ten years”
line 47: “the IPSL model”. But it’s just the atmosphere and land-surface package of the IPSL coupled model (section 2.21), right?
line 53: “hypothetic climate state” I would rephrase this to say “the climate state consistent with the boundary conditions specified for a particular time.”
line 60: I lost track of what “these differences” were.
line 61: “data have different spatial resolutions” Also “used here”
line 66: Replace “hypothesis” with “assumption”
line 68: Replace “would” by “could”
line 73: “we examine the consistency of paleoclimatic simulaitions and paleoenvirontal inferences…”. (The climate-model simulations are just that; they’re not “reconstructions.”)
line 76: “methods”
line 77: “we examine the consistency”
line 82: “follows”
line 83: “run the set of paleoclimatic simulations”
line 91: Fig. 1: The heads of the wind arrows are way too small. I realize most readers would be able to figure things out from the pressure contours, but…
line 92: “precipitation is” (Globally change precipitation from plural to singular.)
line 96: The “humid/arid” tradeoff is climate, but is “open/closed” vegetation cover? Is it the case that closed vegetation occurs during humid times and open during arid. If so, I would reorder the terms. Maybe “… which resulted in alternation between humid/closed and arid/open environments and vegetation-cover. (Or something.)
line 145: I would still call the simulations “simulations”, and not “reconstructions” (and this would be consistent with line 173).
line 181: It might be good to note that these were run with coupled versions of the model.
line 196: “They translate…” Does this mean that the biases are propagated into the paleo simulations of the seasonal cycle of surface air temperature and precipitation?
line 215: This is an odd location for the sub-heading, between the discussion of the bias, and the practical solution (bias-correcting the paleo simulations). I would move it up.
line 222: Replace “hypothesis” with “assumption”.
line 227: Were the SSTs specified as 50-year time series, or long-term means?
line 262: “described below”?
line 265: Why would 2B-PLS be preferred to canonical correlation? (e.g. Rohlf and Corti, 2000, Syst. Biol. 49:740-753).
lines 273-280: But previously you said that the environmental variables were uncorrelated (lines 251-253). The existence of principal components that explain over half of the variance of individual data sets suggests that the variables are indeed correlated.
line 287: How were these dissimilarities calculated? (What metric/which variables?). I’m surprised that the grid cell that EH2 lies in wasn’t lit up for the midH simulation because the climate is so similar to that at present (Fig. 6). The key for the different shades of blue isn’t specified.
line 289: Replace “dynamic” by “atmospheric circulation,” “vertical pressure gradient” by “meriodional pressure gradient,” and “zonal circulation” by “zonal flow.” Same for summer (“zonal” as opposed to “horizontal”.)
line 291: Replace “installation” with “establishment”. (The area represented by the maps is a little small to be able to really see the low, but we know it’s there.)
line 301: Replace “concomitant” with “consistent”.
line 325: I don’t think that what is plotted here are values from a “distance matrix” in the usual sense of an n by n matrix of dissimilarities, but instead a 1 by n vector of dissimilarities (where n is the number of land grid points).
lines 343-355: I guess I see as much difference in the shading on Fig. 8 between L2 and L1 as between some of the other major transitions. I see 15 color changes between L3 and L2 and 17 between L2 and L1. Is this a significant difference?
Line 363: The second component also divides the hot, dry group into two subgroups.
Line 413: “the mixed influence of global and regional”
line 415: “seems to depend more on” (more than on regional processes?)
line 419-420: Not parallel. “increasing greenhouse gas concentrations and the retreating ice sheet”?
line 421: “the amplitude of the variation in obliquity and (climatic) precession”? (Maybe reverse the order: orbital variations first, then insolation and temperature.)
line 428: “the proximity of the high pressure”. It’s not the proximity (the center, presumably) of the high pressure cell that’s important, it’s the magnitude that influences the gradient.
line 435: replace “varies” with “differs”?
line 451: replace “since” with “because”
line 457: replace “as” with “because”
line 483: Another explanation for an increase in steppe over forest is low CO2.
line 501: replace “evidence” with “show”
line 507: replace “thought” with “so”
line 508: “Although our approach…”. Not a real sentence.
line 510: establish what?