the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2023
Mesoscale permeability variations estimated from natural airflows in the decorated Cosquer Cave (SE France)
Abstract. Conservation of decorated caves is highly dependent on airflows in the karst network and through the surrounding host rock. Airflows are driven by pressure gradient and influenced by the shape of the karst conduits and the permeability of the carbonate rock massif. The Cosquer cave is an Upper Paleolithic decorated cave, half drowned in a coastal karst, where conservation is also dependent on the cave’s pools connected to the sea. Hydroclimatic data, such as air pressure and temperature and water level inside and outside the cave have been measured for several years to identify the main processes governing the water level variations, the airflows and the air renewal. Data show an unusual behavior for a karst: the karst air pressure is nearly always higher than the atmospheric pressure. As a result, the water level in the cave is below the sea level. The daily variations of the sea tide provide an assessment of the cave volume above the pools water level. Although the cave air is confined by the rock and the seawater, there are also external air inflows during short pressurization events. Moreover, the carbonate rocks effective permeability to air at the massif scale is inferred from the cave air pressure decrease over the summer season, by applying Darcy’s law in a partially-saturated medium. Six years of data show that permeability varies from year to year, and according to the cumulated rainfalls during the spring and summer. The driest years are correlated with a higher permeability, a faster air pressure decrease in the cave and a faster rise of the pools water level. In the future, in the context of climate change, a perturbation of the rock permeability is then expected in the near surface caves, which will impact airflows in decorated caves and may alter their fragile hydroclimatic stability.
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RC1: 'Comment on egusphere-2023-2380', Anonymous Referee #1, 06 Mar 2024
Review of Mesoscale permeability variations estimated from natural airflows in the decorated Cosquer Cave (SE France) by Pellet et al.
Understanding the processes driving the ventilation dynamics in decorated caves is key to ensure their conservation as even minor shifts in the cave’s climatic equilibrium may lead to microbial outbreaks deteriorating the artwork. Pellet et al. present a six-year monitoring study from Cosquer cave, a major paleolithic site on the Mediterranean coast. The only access to the cave is a flooded passage, thus prohibiting any major exchange with the outside environment. The study encompasses air pressure, water levels and cave air temperature. The authors confirm that the cave air pressure is always higher than in the outside atmosphere keeping the water level below the main paintings. Analyzing the effect of tides on the cave air pressure, the authors calculate a cave volume of c. 5000m3. Although largely isolated from the outside, short pressurization events during the winter season reveal sporadic ingress of external air. These events are used to assess the permeability of the host rock and, thus, may serve as reference for low permeable karst volumes in a broader context.
The paper is generally well-written although a cross-check by a native speaker would be recommended (cf. punctuation and use of articles); the figures are of good quality. However, a few points need clarification before being acceptable for publication:
- Water level: much of the discussion relates to the air pressure in the main chamber. A key point here is that the higher pressure keeps the water level below the mean sea-level. However, no information is given about how the elevation of the water table inside the cave was determined. Is there an altimetric survey available? If so, please indicate uncertainties. Also, what is the source of the temperature fluctuations measured in the cave air? Does it correlate to the seawater temperature and are these temperature fluctuations compensated in the pressure analysis?
- Short-pressurization events: the authors associate short pressurization events with wave activity, however without providing any quantitative data with respect to this wave activity. An alternative interpretation would relate the pressurization events to aquifer recharge after rainfall episodes. Although the author state that addressing more closely the effect of sea waves is out of scope of this paper it would make sense at least to address the effect of hydrology. In absence of surface runoff one would expect that the water infiltrates into the subsurface (as shown on Fig.10), thus pushing interstitial air towards the cave. Accordingly, it would be helpful to plot rainfall distribution together with the effective hydrological recharge along the year and discuss correlations (?) with high-frequency pressure changes in the cave.
- The conceptual model summarized in figure 10 does not match with some of the statements and interpretations made along the paper. In particular, the justification for an upper pathway several meters below sea-level is unclear and inconsistent with Figure2b where the latter is placed at the sea-level. How would waves at the sea-surface propagate into this underwater gallery? Also, Fig 10 suggests there are some water inlets draining from the surface. This makes sense but again raises the question of hydrological recharge on the cave air pressure.
Minor comments:
l.41 underline that the stronger airflow in winter is valid for a descending conduit. In ascending conduits, this would likely happen in summer.
l.63 please state the supposed mechanism driving this airflow. Obviously, this wouldn’t happen without an external force if the cave is over-pressured.
l.91 Cretaceous should be capitalized
l.92 “thin sections observations” delete observations
l.134 “monitoring” instead of “survey”
l.186 delete “in cave”
l.189 tide-related temperature variations “in the cave air” (?)
l.195 phrasing: According the cave air temperature measurements it is rather the wall which stays close to equilibrium with the cave air rather than the opposite.
l.201 Fig 6 suggests that, during low tide, the water level is higher in the cave than at Port Miou. Can you expand on this?
l.205 In Fig 6, you show that the cave water level is c. 5 cm lower than the sea level whereas the fluctuations shown on Fig 7 range in the order of 1 m. What do I get wrong?
Fig.7 I think I get the point but you may want to explain better why the sea-level was c. 0.25 cm higher in 2018 than 2017. What do the horizontal lines (red and black) represent on the figure?
l.241 This variation is
l.290 where does the factor 2 on the right hand-side come from?
l.307 what drives the temperature variations in the cave air? Using a constant Trock is an approximation as we know there are seasonal fluctuations in both, the cave air and the outside atmosphere? what is their effect?
l.325 please edit: ‘equation will, in most cases, yields …’
l.345 unclear, please edit
l.350-351 unclear, please edit. How does this compare with figures in the previous sentence?
l.380 missing article: when the air
l.405 unclear, please rephrase: are your referring to the rapid pressure decay or the slow depressurization.
l.435 have been
l.450 you may want to specifiy that this is during summer
l.452 the pools' reference surface (?)
l.455 should be “and during years”
l.476 waves where not addressed yet, did they? Wouldn't it rather be associated with groundwater recharge?
l.490 Wave heights and direction were not discussed so far
l.502 This upper conduit was not introduced yet
l.550 not connected to the cave. But what about the epikarst, flushing air into the frature during recharge events?
Table1 Please provide also numerical estimates and reference to it if these are used in further calculations
Fig.2a the 3D projection is difficult to read and a classical speleological survey would probably be more helpful here. In particular, it is unclear how P2 is connected to the rest of the chamber. Is this a small "island" in the middle of a pool? What is the grey-scale of the 3rd dimension? Adding some elevation quotes would be useful, or even better, draw some isolines.
Fig. 2b please add a vertical scale to this (nice) illustration
Fig.4C Please plot also the seawater temperature
Fig. 11 interesting figure, but one would also like to see how hydrological recharge correlates with cave Pair rises during the winter months, resp. get an idea of rainfall distribution along the year.
Citation: https://doi.org/10.5194/egusphere-2023-2380-RC1 - AC1: 'Reply on RC1', Hugo Pellet, 16 Apr 2024
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RC2: 'Comment on egusphere-2023-2380', Anonymous Referee #2, 02 Apr 2024
The manuscript as such is interesting in many ways. It deals with the culturally extremely important cave and presents an interesting, unusual and valuable set of data. As such I think authors should be motivated to revise the paper to a publishable version.
I am attaching a commented pdf with more general and detailed comments and suggestions. I did not go into a more detailed review, since the paper first needs a major revision. What follows in this text are just some general observations:
My criticism mainly goes to the clarity of the presentation, which makes the manuscript hard to read. Some important concepts and objectives should be clearly stated earlier in the paper to keep the reader interested. For example, the pressurization events are shown early on, but their essential role -- they are the reason for the long-term overpressurization of the cave (i guess do?) -- is not told.
The derivation of equations (albeit simple) is at some points not clear, therefore it is not possible to judge their correctness (see comments in the PDF) .
The conceptual model (Fig. 10) needs some reformulation and a clearer explanation. As such it is not very convincing, although it is hard to judge if the reason is only poor text or not well elaborated concept.
- AC2: 'Reply on RC2', Hugo Pellet, 16 Apr 2024
Data sets
Port-Miou Observatory Dataset SNO Karst https://data.oreme.org/snokarst/snokarst_map
Meteorological data Meteo-France https://donneespubliques.meteofrance.fr/
Model code and software
Codes for calculations and figures Pellet Hugo https://gitlab.osupytheas.fr/hpellet/chapitre-3-mesoscale-permeability-variations
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