Chlorine isotopes constrain a major drawdown of the Mediterranean Sea during the Messinian Salinity Crisis

Domain: earth_science
Published: 2024-11-18
Editorial outcome: minor_revision

Abstract
Hydrological restriction from the Atlantic Ocean transformed the Mediterranean Sea into a giant saline basin during the Messinian Salinity Crisis (5.97–5.33 million years ago). It is still unclear if the deposition of nearly one million km3 of evaporite salts during this event was triggered by a major (≥1 km) evaporative drawdown, or if it took place in a brine-filled Mediterranean connected to the Atlantic. Here we present evidence for a two-phase accumulation of the Mediterranean salt layer based on the chlorine stable isotope composition of halite. During the first phase, lasting approximately 35 kyr, halite deposition occurred only in the eastern Mediterranean, triggered by the restriction of Mediterranean outflow to the Atlantic, in an otherwise brine-filled Mediterranean basin. During the second phase, halite accumulation occurred across the entire Mediterranean, driven by a rapid (<10 kyr) evaporative drawdown event during which sea-level dropped 1.7–2.1 km and ~0.85 km in the eastern and western Mediterranean, respectively. During this extreme drawdown event, the eastern Mediterranean basin lost up to 83% of its water volume, and large parts of its margins were desiccated, while its deep Ionian and Herodotus sub-basins remained filled with >1 km-deep brine.

Introduction
Today, more freshwater is extracted from the Mediterranean Sea by evaporation than is added by continental runoff1. Blocking the net import of Atlantic Ocean waters through the Gibraltar sill would result in a Mediterranean sea level fall at a rate of about 0.5 m yr−1 (ref. 2).

During the Messinian Salinity Crisis (MSC), the deposition of nearly one million km3 of evaporite salts3,4 (mainly gypsum, CaSO4.2H2O, and halite, NaCl) is a testimony of highly restricted hydrological exchange with the Atlantic Ocean. Whether or not evaporite deposition was accompanied by a major (≥1 km) sea level drawdown, however, is still debated. Estimates of the magnitude of the base-level fall during the MSC rely on geological indicators such as deeply incised canyons on land5–9, widespread erosional surfaces on the margins10–13 and in the basins14–16 and evaporite facies from drilled cores17–20. However, these indicators are not interpreted unequivocally21–24 and drawdown estimates range from ~200 m to ~2 km9,11,14,21,25–27. Existing hydro-chemical models cannot solve the controversy because they show that the complete MSC halite body can be deposited in scenarios that include periods of Mediterranean isolation and sea level drawdown28, but also in scenarios involving exclusively a partially restricted, brine-filled Mediterranean9,10,29.

We use the chlorine stable isotope composition of halite to estimate the volumetric halite precipitation rate in the Mediterranean basins during the MSC. Compared to other geochemical tracers such as strontium isotopes30,31, chloride has the advantage of being a major constituent of halite and, for all practical purposes, is derived solely from inflowing seawater. Laboratory contamination issues are also minimal for this reason. At the basis of our approach is the chloride isotope reservoir effect which results from the preferential incorporation of the heavy isotope 37Cl (as opposed to the lighter isotope 35Cl) into precipitating halite32. The extent of the isotope reservoir effect depends on the balance between the rate of halite precipitation, which tends to decrease the 37Cl/35Cl ratio of dissolved chloride, and the rate of addition of dissolved chloride from the Atlantic Ocean, that tends to push Mediterranean 37Cl/35Cl towards heavier global ocean values. The faster the precipitation, the stronger the 37Cl-depletion of the precipitating halite. Evaporative drawdown of a halite-saturated water column33 results in very high rates of halite precipitation, and in the most extreme dissolved Cl, and thus halite, 37Cl-depletion.

Results
Chloride isotope composition of halite
We analyzed 60 drill cuttings from the Aphrodite well (Levantine basin, eastern Mediterranean) starting from the onset of halite precipitation and spanning a thickness of 1393 m up core (Fig. 1). All analytical results are presented in Supplementary Tables 1–3. In the Levantine basin, the Messinian evaporite package is divided into the following seven seismo-subunits34. Four transparent units are composed mainly of halite. Two intercalated, highly reflective units are composed of halite with a higher, albeit laterally variable, clay content. A seventh unit is composed of a mixture of anhydrite, clays and sands, separated from the underlying units by the Intra-Messinian Truncation Surface (IMTS, interpreted as a dissolution surface)34 (Fig. 1d). Bulk XRD analyses of the Aphrodite cuttings (Fig. 2) show that, within seismo-stratigraphic units 1 to 4 and the bottom 360 m of seismo-stratigraphic unit 5 (3943–2700 m depth interval), halite dominates the mineral assemblage (>80 wt-%), with minor (<20 wt-%) amounts of anhydrite (CaSO4) and only trace amounts of CaCO3 and quartz. In the topmost 140 m of seismo-stratigraphic unit 5, the abundance of halite decreases sharply, and that of all the other minerals increases. No samples are available from the well above 2550 m depth, hence no samples are available from unit 6. The drill cuttings also contain sylvite (KCl) and barite (BaSO4) (Supplementary Fig. 1) which were artificially added to the drilling fluids to control their salinity and density, respectively.

Halite grains for Cl-isotope analyses were selected from the drill cuttings under a binocular microscope, excluding grains that contained evident signs of impurities. We cannot exclude, however, that minor amounts of Cl-bearing sylvite (KCl) contributed to the Cl-isotope signature we measured. Mass-balance calculations, however, suggest that the contribution is negligible (see supplementary materials). The δ37Cl values at the base of the halite layer (0.4‰ vs standard mean ocean chloride, hereafter SMOC) are in isotopic equilibrium with oceanic dissolved chloride (0‰ vs SMOC) (Fig. 2g). Subsequent halite, forming Levantine basin seismo-stratigraphic units 1 to 4 and the bottom 360 m of seismo-stratigraphic unit 5 (3943–2700 m depth interval), is slightly depleted in 37Cl (average δ37Cl = 0.31‰ vs SMOC) compared to equilibrium with global ocean dissolved chloride. We interpret this as reflecting a modest isotope reservoir effect, in which the rate of dissolved chloride sequestration into halite becomes important relative to the influx of chloride from the Atlantic. In the topmost 140 m of seismo-stratigraphic unit 5, δ37Cl values decrease abruptly, reaching a minimum of −0.30‰ vs SMOC at 2630 m depth (Fig. 2g). This is interpreted to reflect a sudden increase in the rate of halite precipitation relative to seawater chloride replenishment.

We also analyzed 9 halite samples from drill cores obtained by drilling the topmost few tens of meters of the deep-basin Mediterranean halite layer during Deep Sea Drilling Project (DSDP) Legs 13 and 42A from the Florence Rise (Site 376, eastern Mediterranean), Ionian Abyssal Plain (Site 374, eastern Mediterranean) and west Sardinian margin (Site 134, western Mediterranean). A stratigraphic correlation between these sites and the Aphrodite section is not possible, although the position of the DSDP halite samples at the top of the halite body implies that they represent the final stages of the high-salinity evaporite phase of the MSC (Fig. 2). Halite from DSDP samples shows a range of 37Cl depletions from near-equilibrium with oceanic Cl at Site 376 (0.33 < δ37Cl ‰ vs SMOC < 0.37), to a strong 37Cl-depletion at Site 374 (−0.14 < δ37Cl ‰ vs SMOC < 0.04). Halite from DSDP Site 134 has intermediate δ37Cl (0.08‰ vs SMOC) (Fig. 2). The 37Cl-depleted isotope signature of halite from DSDP sites cannot be influenced by the composition of drilling fluids, because the halite sub-samples were obtained by mechanically breaking solid halite layers in the DSDP cores, rather than extracted from an admixture of drill cuttings and drilling fluids like for the Aphrodite samples.

Two-phase deposition of the Mediterranean halite layer
We used a numerical model to quantify halite precipitation rates and the extent of sea-level drawdown during the MSC. The model is based on mass balances of water and dissolved ions (35Cl−, 37Cl−, Na+, Mg2+, SO4 2−, Ca2+, K+ and HCO3 −) in the western and eastern Mediterranean basins and is forced by the evaporative flux, continental runoff and hydrological restriction at the Gibraltar sill. Evaporation and continental runoff fluxes are based on the Miocene General Circulation Model results of Simon et al.35. Starting from initial chemical and hydrologic conditions corresponding to the modern Mediterranean, the model is integrated forwards in time for 20 kyr, after which the hydrological connectivity at Gibraltar is impeded, salinity rises, and evaporitic minerals precipitate. With a salinity-corrected evaporation rate starting at the average value (1 m yr−1) reported in Simon et al.35 values of continental runoff and hydrological restriction at Gibraltar are found to obtain a good match between simulated and observed4,25 halite volumes and halite δ37Cl values. A full description of the numerical model, model forcing and model constraints is given in the supplementary material.

The best fit of the model to the volume and δ37Cl values of halite (Fig. 3) is obtained if a two-phase hydrological restriction-isolation scenario at Gibraltar is considered. First, the deep-water outflow at Gibraltar is restricted but not blocked, leading to the precipitation of gypsum, followed by the precipitation of halite. This halite is characterized by a modest chloride isotope reservoir effect, and decreasing halite δ37Cl from the value in equilibrium with oceanic dissolved chloride (0.4‰ vs SMOC) to the average value measured from seismo-stratigraphic unit 1 to the lower 360 m of unit 5 (0.31‰ vs SMOC) (Fig. 3h). In this first phase, lasting about 35 kyr, halite precipitates exclusively in the brine-filled eastern Mediterranean; halite does not precipitate in the western Mediterranean (Fig. 3e) because the brine transferred from the eastern to the western Mediterranean basins across the Sicily sill is in thermodynamic equilibrium with halite, but not supersaturated.

After this first phase, the deep outflow at Gibraltar is blocked and the inflow of Atlantic water into the Mediterranean is stopped, leading to the complete isolation of the Mediterranean Sea from the Atlantic Ocean. As a result, the water level in the entire Mediterranean basin drops until the depth of the Sicily sill is reached, and then continues to fall independently in the western and eastern Mediterranean basins, forced by their respective freshwater budgets (Fig. 3b). The rate of water level fall is very rapid, with 50% of the total drawdown taking place in less than 3.5 kyr, 80% in less than 6 kyr, and the total drawdown attained in less than 10 kyr. Evaporative drawdown causes an abrupt increase in the rate of halite precipitation in the eastern Mediterranean, and to the initiation of halite precipitation in the western Mediterranean (Fig. 3e). The increased halite precipitation rate results in a strong Cl-isotope reservoir effect and an abrupt drop in the δ37Cl of precipitating halite (Fig. 3h).

In the eastern Mediterranean basin, ~ 40% of the total halite volume formed in the standard model run accumulates in 35 kyr, while the total halite volume accumulates in ~ 50 kyr, in agreement with a “rapid36” (~50–60 kyr), rather than a “slow37” (~600 kyr), mode of halite accumulation. This time frame is consistent with the widely accepted stratigraphic position of the halite deposit being accumulated during phase 2 of the MSC across the Mediterranean basins (5.60–5.55 Ma; e.g. CIESM38; Roveri et al.3; Ryan et al.39) and, more specifically, in the Levant Basin (Manzi et al.36).

After roughly 10 kyr from the beginning of the second phase, the calculated volume of halite in the western (74,900 km3) and eastern (492,300 km3) Mediterranean basins approaches the estimates of Heida et al.25 (110,000–115,000 km3) and Haq et al.4 (585,000–657,000 km3), respectively, while the δ37Cl of halite in the eastern (−0.30 ‰ vs SMOC) and the western (0.08 ‰ vs SMOC) Mediterranean matches the minimum values measured from the upper 140 m of unit 5 in the Aphrodite well and the top of the salt layer at DSDP Site 374 (both in the eastern Mediterranean) and the value measured in the western Mediterranean at DSDP Site 134, respectively (Fig. 3h). Sensitivity tests (see supplementary material section) show that only a small range of continental runoff values and hydrological restriction at Gibraltar results in a good match of the model to the halite δ37Cl data and halite volume estimates4,25, and that within this range, the calculated time to accumulate the halite body and the extent of the drawdown change little.

Our results solve a controversy regarding the mode of halite precipitation – deep basin, deep water40 vs deep basin shallow water17 – that is outstanding since the discovery of the deep basin evaporites during DSDP Leg 1317 (Fig. 4). We show that in the eastern Mediterranean approximately 40% of the halite deposit was formed in a water-filled Mediterranean basin, connected to the Atlantic Ocean by a two-way hydrological exchange, corresponding to the deep-basin, deep-water model40. The remaining 60% of the eastern halite deposit was formed by evaporative drawdown of a halite-saturated water body, which is reminiscent of the deep-basin, shallow water model17, except that halite precipitation takes place during the drawdown, rather than in a deep, desiccated basin after the drawdown took place.

Drawdown of the Mediterranean sea
The water level drawdown we constrain is significant in the western Mediterranean (~850 m), and extreme in the eastern Mediterranean (~2100 m), implying that western and eastern sub-basins lost 43% and 83% of their water volume, respectively. In the eastern Mediterranean, large parts of the continental margins would have been completely desiccated, though the deep Ionian, Herodotus and Antalya basins were still covered by a >1 km-thick brine layer. Our model also predicts that the drawdown in the eastern Mediterranean resulted in the precipitation of the Mg-K-salts epsomite (MgSO4.7H2O; 46,700 km3, Fig. 3f) and kainite (KMgClSO4.3H2O; 23,200 km3, Fig. 3g), although Mg-K salts were not detected by XRD in our samples, possibly due to a lateral heterogeneity of brine composition in the eastern Mediterranean, or their dissolution in the drilling fluids or during the washing of drill cuttings. The sequence of precipitation halite-epsomite-kainite is consistent with that observed in seawater evaporation experiments where the minerals are separated from the evaporating brine as they precipitated (“fractional precipitation)41.

In an attempt to obtain a more conservative estimate of the drawdown extent, we increased the continental freshwater input to the eastern Mediterranean basin in order for the model to reproduce the second-most negative halite δ37Cl data point measured from cuttings of the Aphrodite well (δ37Cl = −0.19‰ vs SMOC; Fig. 2e, star). In these conditions, a more limited drawdown of ~1700 m is calculated (Fig. 3b, green line and star). Taken together these model runs imply that the eastern Mediterranean experienced a large-magnitude drawdown of 1.7–2.1 km during the MSC.

Independent evidence for the occurrence of a sea-level drop and for the subaerial exposure of the basin margins come from the decrease in halite concentration and the peak in Quartz abundance (up to 58 wt-%) in the top 140 m of Unit 5, suggesting significant terrigenous input into the basin during drawdown. The erosion that took place concomitant with the short-lived sea-level drawdown resulted in only a limited accumulation of halite during drawdown at the Aphrodite site (upper 1/3 of Unit 5 above the black horizon traced in the seismic line displayed in Fig. 1d and Fig. 2), compared to the underlying, thick halite layer deposited under a full Mediterranean water column (Units 1–4 and lower 2/3 of Unit 5), leading to a non-linear relation between thickness and volume.

The drawdown of 1.7–2.1 km estimated in this work contrasts with the smaller-magnitude (600 m) drawdown estimated from the restoration of the geomorphological base level of the buried Messinian Nile canyon9, although the canyon development cannot be precisely dated. If the two estimates refer to the same drawdown event, this implies that the large drawdown was too short-lived to be recorded in the Nile valley geomorphological profile. However, our work shows a complete disconnection of the eastern Mediterranean from the western Mediterranean, implying that, following the first, short-lived, large-scaled drawdown, water levels in the eastern Mediterranean would have varied under the influence of an oscillating freshwater budget35, and of the hydrological connection to the Paratethys drainage system42. This hydrological mode, which lasted until the reconnection with the Atlantic via the western Mediterranean, possibly as late as the Zanclean (5.33 Ma)43,44, leaves ample time for intermediate water levels such as those deduced from the study of topographic profile of the Messinian Nile Canyon9 to develop.

Consequences of Mediterranean Sea drawdown
Our results have broader implications for the biological, geologic and climatic evolution of the Mediterranean realm, and beyond. In the western Mediterranean sub-basin, the 0.85 km sea-level drawdown we constrain would have resulted in the formation of a land-bridge connecting Africa and Europe through the Betic region and the Balearic Islands. This land bridge would have allowed continental-terrestrial vertebrate fauna to colonize the Balearic islands, a migration that is known to have taken place during the Messinian based on paleontological evidence45. A drawdown of 1 km in the western and eastern Mediterranean may have resulted in enough lithospheric unloading to trigger a magmatic pulse in the form of volcanism and dykes in the pan-Mediterranean area at the end of the MSC46. Our evidence for drawdowns of ~0.85 km and 1.7–2.1 km in the western and eastern Mediterranean, respectively, make this scenario even more likely. Alongside lithospheric unloading, the huge size of the Mediterranean depression created by MSC water level drawdown - corresponding to a volume loss of 69% of the Mediterranean water body - would have generated planetary-scale climate effects47, inducing changes in precipitation patterns, a scenario suggested by rainfall proxy data47.

Methods
X-Ray diffraction (XRD)
The XRD measurements were carried out on an Empyrean diffractometer (@Malvern-Panalytical), equipped with a copper tube and a PIXcel multi-channel detector. The configuration is Bragg-Brentano θ - θ and the sample holder is a spinner. The mineral phases are identified using Highscore plus (@Malvern-Panalytical) software and the ICSD-Panalytical database. Semi-quantitative evaluation of mineral abundances is obtained by scaling the intensities of the principal mineral diffraction peaks to those of reference materials applying reference intensity ratio (RIR) coefficients.

Cl-isotope measurements
We measured the chloride isotope composition of halite grains hand-picked under a binocular microscope, which overwhelmingly reflects that of halite. First, the halite grains were dissolved in distilled water. Then, dissolved Cl ions were precipitated into AgCl and the AgCl converted into CH3Cl48–50. The δ37Cl of CH3Cl was measured with a Delta plus XP or Delta V ThermoFisher dual-inlet gas source mass spectrometer. The δ37Cl compositions are reported in ‰ vs SMOC. The external reproducibility of the internal IPGP standard seawater dissolved chloride (Atlantique 2) was ±0.025‰ (1σ, n = 48); note that this S.D. value is smaller than the size of the δ37Cl symbols in Fig. 2g.

Numerical model
A full description of the numerical model is provided in the supplementary material.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present new chlorine isotope data and some supporting seismic information to provide definitive evidence that the Messinian Salinity Crisis occurred with isolation from the Atlantic Ocean and significant drawdown. The strongest data for these results were the chlorine isotope data and the supporting geochemical modeling. The use of the seismic data was not as effective as desired and may not be needed to support the author’s goals. There are better, easier-to-interpret illustrations of erosion surfaces in past literature.\n\nIn addition to grammatical and stylistic corrections, this text's primary areas for improvement are further attention to detail in the figures and addressing some unknowns in the methodology section. The figures should be redrafted to minimize ambiguity. Some additional effort could be made to emplace the stratigraphic units within the timescale of the presented model. In addition to Manzi et al. 2021, is there more supporting information for the timescale of halite deposition your work determines? The results of your work could be further emphasized by highlighting its alignment with other chronological constraints and adding more information on the data underpinning those results.\n\nMethodological questions\nHow was the 20 kyr time step for the model decided? Why not less?\nIn the FE section, I do not see any explanation for variation in surface area as the Mediterranean was drawn down with evaporation. How big of a difference would this have been, why is it not considered here?\nLines 586-587 is not a complete sentence. In the FRTOT and FRWMed section, Lines 587-589 state that evaporation would have been higher over the eastern Mediterranean than over the western Mediterranean, yet, in the FE section, the evaporation rate is set equal for the eastern and western Mediterranean, please elaborate a little more.\nLines 607 to 617: In this section only the seismic mobile unit is used for the western basin, leading the a halving of estimated evaporite volume from a prior study. However, the values from the same study (Haq et al., 2019) that were not used for the western basin were used for the eastern basin. The logic here needs more explanation.\nLines 637-638 discuss the potential reworking of halite for unit 6. Unit 5, with its erosional surface would also likely have some reworking of halite, why is this not mentioned in the model discussion?\nThe methods section is inconsistent in its use of 1000’s separator. Please correct. Furthermore, please consistently label the units (no units on lines 596-597).\nThis manuscript has several data presentation elements that could be greatly improved.\nThere is inconsistent punctuation with units, in some areas there is no space between a number and its unit, in others, there is. Please be consistent.\nThe figures are not consistently lettered off. Figures 1 and 2 were missing letters for their different subsections. Figures 3 and 4 used different styles to identify subsections (A) vs (a.)\nItems within the figures were not clearly labeled. Figure 1a shows some red lines in the northwestern area, but I could not deduce from context clues if those were important information or if they were left on by accident. The boxes in the inset in Fig 1a were seismic data areas, but that was not made clear until Figure 4. The y-axis labels Figure 1b and c were 2-4 sizes smaller than the other axes, making them almost impossible to read without great magnification. Similarly, Unit 7 in Figure 1c has a different text size. Furthermore, the unit of (km) in Figure 1b needs more clarification, km below msl? I did not see this made clear in the text (this comment also applies to Figure 2). Figure 1b would be easier to read if it had a key, similar to Figure 1a, but the description in the text is acceptable. Figure 1c shows a white box that I assume is the seismic shown in Figure 2, but that was never specified.\nFigure 1a could be improved by splitting it into two segments, one with the modern topography with the sills and subbasins labeled. This article should be more accessible to a broader audience unfamiliar with the specific location of the Sicily sill or the subbasins (e.g., Ionian, Herodotus, and Antalya). Similarly, the Eastern and Western basins should be highlighted on the map. The second part could show the data highlighted in the key of Figure 1a, currently it is challenging to interpret the colours in the figure because of differences in transparency (e.g. the marginal basin deposit is shown as solid in the key and transparent in the map, making matching the colours between the two difficult).\nFigure 2 shows FBI, which is not defined anywhere in the text. Stratigraphic is also misspelled in Figure 2. The data presented in Figure 2 do not support Unit 3 as terrigenous material bearing, neither does the presented stratigraphic column, an additional line about lateral heterogeneity and a reference would clarify this.\nFigure 3 might be improved by annotating (H or another part of the figure) with the Units corresponding with the model over time. The green line in Figures 3B and H is only explained in the text; it should also be explained in the caption.\nFigure 4 should be redrafted. As presented, it is hard to identify the data it is intended to highlight. I do not like how the coordinate system between it and Figure 1a differs (a matter of opinion). I also do not like how it attempts to hide the fact that some of the seismic data are in depth and some are in time in Figure 4A; the two can only be differentiated by inspecting Figures 4B to D. This could be avoided through clearer labeling or by using a different colour scheme for the two types of data. I suggest that Figure 4a be greatly zoomed in to highlight the data of interest, as it is shown now, half of the space shown does not relay helpful information (the erosion surface) similarly, it is unclear if the erosion surface features shown in 4a are from erosion or later deformation (which is highlighted in Fig 1c). It is not clear that the three seismic x-lines are needed. One excellent example might suffice here, ideally the example would be clearer than the examples shown here.\nThe pale white boxes were difficult to find (again requiring magnification of the screen), and the truncations might be better highlighted by arrows showing where the reflector has gone absent. As this figure currently stands in the text, I am not sure it is needed, if it is kept, it should be clarified at what water depth this subaqueous erosion occurred within and the importance of this surface be highlighted further in the text.\nLines 154-156 discuss the mode of salt precipitation models this work addresses: “deep basin, deep water vs. deep-basin, shallow water.” (Note inconsistent hyphenation.) If space allows, adding an illustrative figure of this concept may be helpful for readers.\nSpecific line items\nLine 55: “strongest to “stronger”\nLine 211: “and” to “an”\nLine 483: “form” to “from”\nSupplementary Table 5: for reference/notes for 37Cl/35Cl ratio in SMOC standard (SMOC) it states “from somewhere”. Please add the appropriate reference.\nExtended data Figure 1: This table is unnecessary because the same data is present largely in Figure 2, and the remaining data is shown in Supplementary Table 1.", "reviewer_2": "Aloisi and others present some very interesting new d37Cl and other data from a commercial borehole in the eastern Mediterranean. Along with already-known volumes of the evaporites deposited, the authors attempt to reproduce by numerical modelling the conditions in the basin that led to these d37Cl and evaporite volumes. The work suggests an early period during the MSC in which sea-level changed little (units 1-early 5), followed by a late Messinian larger drawdown. The magnitude of that drawdown is compared with morphologic evidence from seismic reflection data collected in the east basin. The MSC is very widely known and the debate over deep versus shallow drawdown has persisted, so I expect many researchers will be interested in these results.\n\nThe modelling of the -0.2 to -0.3 ppt d37Cl measurements constrains the sea-level drawdown. To assess this important outcome, I deliberately read the article first without referring to the electronic supplement. However, I found it extremely hard to assess this claim from the article contents alone. There are critical assumptions made that are only provided in the electronic supplement. I strongly urge the authors to consider withdrawing their article and submitting it instead to a journal that would allow a more traditional structure, so that the model can be explained more fully (including assumptions) before the modelling results are presented. The key is in the name “supplement” – these should be for information that embellishes the story, not information that is so critical for understanding it.\n\nIt would help reassure readers if a simple \"back of envelope\" type calculation could be provided to show that the results are in the right general ball-park. In other modelling involving finite-difference calculations, we often use a steady-state solution to show where incremental steps are heading towards. Here, I wondered if a rough calculation could be done with the -0.2 to -0.3 ppt d37Cl in terms of salinity and proportion of seawater volume evaporated. Without that, there's a danger of the result being viewed as a \"black box\" calculation that few will understand and hence the results not fully accepted.\n\nIt would be helpful to have a figure showing all or as many parameters as possible with mathematical symbols. Extended Data Figure 2, for example, has terms shown as the same for both basins but they are different and is missing functionality. E varies between the basins, similarly \"drawdown\", P...\n\nI missed whether the smaller deviations of d37Cl from equilibrium global ocean values are addressed. I may have simply overlooked this, though the model outputs do not reflect the variation from equilibrium in early unit 1 to 0.3 ppt at just before the blue horizontal dashed line in Figure 2.\n\nThe d37Cl measurements seem to me the most important, though I didn't feel I knew how these were made exactly. For example, at one point they are described as \"bulk\" measurements - does that mean measured samples included impurities and if so how many?\n\nThe English is mostly very good, although the text and figure captions include some very long sentences, which make the text hard to read. Please consider breaking them up. Commonly, where semicolons are used, sentence breaks could make the reader’s work easier.\n\nMany researchers prefer a writing style in which the observations are described first, then the authors provide their interpretations. Within this draft, the two are inter-mingled (e.g., around line 82). You might consider separating them, which would be easier in the different journal format I suggest.\n\nDetailed comments:\nLine #\n“Modelling of chlorine isotopes suggests Mediterranean Sea drawdown history during the Messinian Salinity Crisis”\n28 no need for “completely”?\n51 37Cl relative to the lighter isotope 35Cl into …\n52, 54 that -> which\nFour transparent … are composed…\nTwo intercalated ….are composed of …\nA seventh unit is composed ..\n67-68. Please add the measurement method (XRD) to the caption to Figure 2 (e.g., carbonate can be derived from the bomb method instead).\n74 …depth, hence no samples are available from unit 6.\n75 that -> which\n80 please insert a comma before “is”\n82. Please break sentence after chloride.\nPlease separate your observations from interpretations.\nWe interpret this as reflecting a modest isotope effect, in which the rate of ….\n84 decrease\n97 please use consistent language:\n… has intermediate d37Cl (0.08 …)\n121 …while halite is not precipitated in the …\n189 same depth at which bulk\n196 to->with\n203 incision -> development\n210 that -> which\n211 leaves ample time for intermediate …\n216-7 .. the 0.85-km … suggested drawdown would have formed a land bridge …\nI have looked over the literature on Africa-Arabia migrations over the Miocene-Pliocene. Surprisingly, there is very little evidence of the Red Sea varying in obstruction. That may be because of the Sinai route, though it could also suggest that many species are able to cross sea barriers over long geological times.\n222 would -> may\n- Without references, this is speculative. Indeed, if it occurred slowly so that diffusive cooling also affected magma reservoirs, it need not necessarily have triggered eruptions.\nFigures:\n434. Please cite the source of the map (it looks like GeoMapApp).\nPlease insert \"evaporite\" above \"thickness\" in key.\n- Similarly for ROP\n443 reflector -> reflection\n444. Please break sentence instead of using semicolon.\n450 in Figure 2 on right, it is unclear what “equilibrium with seawater” means.\n455 …(halite) compositions are in …. Sediment, corrected for barite (…)… originating from drilling fluids.\nPlease replace the semicolons with sentence breaks – they make the caption hard to follow.\nYellow layers…. – these might be better written on the figure on the right and instead put the text “DSDP samples….” Moved to the figure caption. Otherwise, reader has to flip back and forth to caption.\n493 survey datasets\n493 (b) to (d) Three seismic profiles flattened at the intra-unit 5 reflection from t he Sara-Mira….. , respectively.\nWhite dashed rectangles in (b) to (d) highlight areas of truncation …\nElectronic supplement:\nFigure 1:\n... have been added to the samples from the drilling fluids.\nNumerical model\n504 that simulates -> to simulate\n.... basins. Those basins variedly exchange water ....\n512 delete \"can further be\"\n531 that-> which\n578 .. model was run (...) with a constant freshwater flux of ****\nThis lies within the range....\n- i.e., state the flux please. Text is confusing here.\n583 \"is varied to fit the data\" Please state if you are varying it over time or it has a constant value in each run but is adjusted until the model outputs (d37Cl) match the data.\n586-589. This is stated \"the fact that\" but there is no citation / evidence given. I suggest instead:\nIn the modern M...., evaporation is stronger over the eastern basin than its western basin (Mariotti ...). Due to the greater isolation of the eastern basin from the Atlantic Ocean, we suspect that this was also the case in the Messinian, so we maintained ....\n(or other reason - at the moment there is no justification)\n594 was chosen equal to the modern .... hydrological flux with the Atlantic\n597 that -> which\n602 delete \"down\"\n602-603 There is no justification for these two stages of isolation. In practice, two-way flow is unlikely to stop unless the water depth over the sill were extremely small. In the work on the Red Sea, we know that two-way flow persisted through the Last-Glacial Maximum (some species survived - Fenton et al.), even though water depth over Hanish Sill became very small (Lambeck et al.). What would be the implications of a different onset of the crisis?\n612 estimated\n(study was carried out in the past - similarly for other citations)\n617 and consider that -> , hence it would have been six times smaller than the 600,000 km3 of the eastern Med...\n638-641. Doesn't this imply that the integrated inflow flux of the model is too small? I find this section confusing.\n657 keep track of -> track\n663 ion concentration\n680 ...Cl of 0.31.... (Brand et al....).\n700-704 Please break sentence.\nThis is good, though I had earlier not realised that varied surface area was considered, hence please update text earlier so the starting assumptions are clearer.\n713-714 Why have you assumed the water flux would be linear with density contrast? Also, turbulence varies with velocity contrast, hence turbulent \"friction\" is non-linear with velocity.\nPerhaps a linear flux with salinity contrast is the most practical, though could be discussed.\n755 - presumably only equal until water level reaches the Sicilian sill.\n784 delete \"according to\"\n791 perhaps indicate where mminx is taken from (seems arbitrary otherwise).\n(This appears later at line 840.)\n849 suing?\nPerhaps delete \"suing ... Mathematica\"\n983 composition of halite-dominated particles of the...\n(if correct?)\n1040 \"from somewhere\" needs to be replaced by a citation.\nbottom width of Sicily sill - there should be a reference for this relatively well-studied feature.\n1077 \"Atmopheric Pressure\" in right column\nThis is a major effort to carry out, which is reflected in the length of the electronic supplement. Certainly these d37Cl data and the illustrative modelling would be important contributions to the literature. I look forward to seeing them published.\nNeil C. Mitchell\nUniversity of Manchester, March 2024\nFenton, M., Geiselhart, S., Rohling, E.J. and Hemleben, C., 2000. Aplanktonic zones in the Red Sea. Mar. Micropal., 40: 277-294.\nLambeck, K., Purcell, A., Flemming, N.C., Vita-Finzi, C., Alsharekh, A.M. and Bailey, G.N., 2011. Sea level and shoreline reconstructions for the Red Sea: isostatic and tectonic considerations and implications for hominin migration out of Africa. Quat. Sci. Rev., 30: 3542-3574.", "reviewer_3": "Aloisi et al. in “Chlorine isotopes constrain a major drawdown of the Mediterranean Sea in the late Miocene” present a fascinating new chlorine isotope dataset from halite. The data appear to be of high quality and will help advance our understanding of the depositional history of the thick evaporite sequences in the Mediterranean basin — and therefore our ability to quantitatively estimate the sea level changes during the Messinian salinity crisis. I am supportive of using chlorine isotope data to model the hydrogeochemical changes in the Mediterranean basin, although I think additional sedimentological and petrographic data could be used to rule out the contribution of other processes and non-halite mineral phases to the large and rapid decline in chlorine isotope data shown by coupled sudden changes in the bulk mineralogy and chlorine isotope values. Although I have a few comments, questions and suggestions, as detailed below, I am very supportive of publication and I think this could be a novel and important contribution.\n\nThe manuscript presents stable chlorine isotope data from the analysis of bulk samples from core cuttings in the Levantine basin in the eastern Mediterranean and three DSDP sites in the eastern and western Mediterranean basins. Chlorine isotope values of bulk samples are suggested to reflect those of halite — the dominant chlorine-bearing mineral. Chlorine isotope data from the bottom ~1240m interval, dominated by halite (>80 wt.%), show a slight upward decreasing trend from 0.4‰ to 0.3 ‰ (Δ= –0.1‰). The top 140m, however, shows an abrupt decrease in chlorine isotope data to a minimum of –0.3‰ (Δ= –0.6‰), coinciding with a sharp decrease in halite fraction (<30 wt.%), increase in detrital quartz (up to 48 wt.%), anhydrite (up to 35 wt.%), and artificially added drilling fluid (BaSO4 - 33 wt.% and KCl- 6 wt.%). The authors associate the abrupt decrease of chlorine isotope data with a rapid increase in halite precipitation rate and water level drawdown throughout the Mediterranean basin.\n\nA method section on sample preparation and processing procedures for the chlorine isotope analysis is missing. This section would help the reader understand how the authors overcame the contamination from drilling fluids in the top halite section, specifically sylvite (KCl), which could impact the results. Depending on the sylvite-to-halite ratios, the bulk analysis could give a falsely low chlorine isotope value because the chlorine isotope composition of sylvite is low (–0.123‰) compared to halite (0.351‰).\n\nSimilarly, kainite (KMgClSO4·3H2O) has a significantly low chlorine isotope composition (–0.42‰), and it tends to lower the bulk isotope values (Eggenkamp et al., 2016). In this study, simulated evaporation of Mediterranean seawater using PHREEQC predicted the precipitation of kainite, and the numerical model involves kainite co-precipitation with halite during restricted Atlantic flow to the Mediterranean. Early studies also show kainite as the dominant potash mineral in many eastern Mediterranean basins (Lugli et al., 1999). The authors mentioned that kainite and other Mg-K salts were undetected in the XRD results, but it is possible that the peak intensity of halite masked the peaks of other minor minerals such as kainite. Petrographic observations could be used to examine the presence/absence of kainite and other potash salts, commonly preserved as solid inclusions or daughter crystals in fluid inclusions. Given the significant difference in the chlorine isotope fractionation factor between kainite and halite, I believe this needs to be addressed in the main model or model sensitivity tests. How could the precipitation and recycling of kainite during the rapid drawdown and erosion of the marginal deposits influence the model? This discussion could be extended to the recycling of marginal halite deposits. Petrographic observations would be useful to examine the primary versus detrital nature of the top 140 m of the Aphrodite core (recent work by Lowenstein et al. (2021) on criteria for recognizing in situ vs. clastic halites might be a relevant reference on this topic). Increased detrital quartz, gypsum, and decreased halite fractions suggest recycling, which could overestimate the mass of evaporites formed during the late stage of the Messinian salinity crisis.\n\nLine 146-148: Line 91 indicates that the DSDP sites represent the final stage of the evaporite phase of the MSC with depleted δ37Cl; however, higher chlorine isotope compositions of halites from DSDP site 376 suggest the opposite.\n\nFigure 3. Simulated evaporation of modern seawater, including Mediterranean seawater, reaches halite saturation at a degree of evaporation of ~10.7 or a salinity of ~373 g/kg (Babel and Schreiber, 2014). However, the model here shows halite precipitation occurring between 260 and 300 g/kg, which is lower than the halite saturation.\n\nFigure 3 Caption B: Change meters to kilometers.\n\nLine 511: The assumption of ion-free freshwater riverine runoff could be reasonable during the first brine-filled Mediterranean phase, but this assumption might not be accurate during the second stage, where the recycling of marginal evaporite deposits was significant during the rapid drawdown. This scenario could be tested in model sensitivity."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript is much improved and presents a clear presentation of the measurements, model, and results. I have made some minor comments and suggestions in the attached document.", "reviewer_2": "I don't have time to re-review the manuscript in detail unfortunately. However, reading the manuscript and browsing the response to reviews, it is clear that the authors have thought about the feedback carefully and taken some time in carrying out the revisions.\n\nThe authors misunderstood one of my comments. I was not suggesting they find a steady state solution to the equations (I was rather mentioning steady states used in other systems using differential equations as a way to check model outputs). However, I can see from the equations and box model that a check like this is not easy to find. The model outputs essentially are untested, but future researchers will be able to attempt modelling of their own to reproduce the d37Cl trends. It looks like there is enough information in the supplement to do that.\n\nGiven that the many input parameters, which each have uncertainties, I am still not entirely sure how to interpret the estimated drawdown values. However, the reader can see the assumptions and parameters used. Testing may take some further effort.\n\nMinor comment:\nFigure 4: In the figure, the sequence is separated into stages, but in the caption they are phases. It would be best to stick with one of either of these two terms - changing terms confuses readers.\n\nI find the results very interesting and look forward to seeing them in print.", "reviewer_3": "The revised version of the Aloisi et al. manuscript, \"Chlorine Isotopes Constrain a Major Drawdown of the Mediterranean Sea During the Messinian Salinity Crisis,\" is almost ready. I have a follow-up comment and a minor quibble.\n\nA new section has been added to the supplementary material, which estimates the contribution of sylvite from drilling fluid to the measured chloride-isotope values using a simple isotope mass balance calculation. Could you include these results in Table 1 for each sylvite-halite proportion based on the XRD data?\n\nAdditionally, the authors requested more references regarding the salinity or degree of evaporation (DE) which halite precipitation begins. The degree of evaporation is more commonly reported than salinity. Evaporation studies of modern seawater in solar evaporation ponds and simulations using geochemical models (PHREEQC, EQL-EVP, HMW, Geochemist's Workbench) consistently show that halite first begins to precipitate at a DE >10, with an average of ~10.6 (McCaffrey et al., 1987; Amdouni, 2000; Timoffeeef et al., 2001).\n\nFor reference, when I ran the EQL-EVP model at 25 °C to simulate the evaporation of modern seawater (using Risacher and Clement, 2001), it yielded a corresponding salinity (TDS) of 373.3 g/l or 302.2 g/kg and DE of 10.7 at the onset of halite precipitation. Both EQL-EVP and PHREEQC rely on the Pitzer dataset (Pitzer, 1973) to calculate the molal concentration (mol/kg H₂O) of dissolved species (Mg²⁺, Ca²⁺, Na⁺, K⁺, SO₄²⁻, Cl⁻, alkalinity) and the moles of salts precipitated at each evaporation step. Both EQL-EVP and PHREEQC must provide similar salinity and DE at each evaporation stage.\n\nImportantly, the simulated seawater evaporation at 25 °C using these computer programs closely matches observed brine evolution trends and mineral sequences in modern systems (Timofeeff et al., 2001; Weldeghebriel et al., 2022).\n\nTherefore, the salinity of 260 g/kg in your model corresponds to a DE of 8.9, which is lower than the DE of 10.6 typically associated with the first halite saturation stage (McCaffrey et al., 1987; Babel and Schreiber, 2014). I have attached the EQL-EVP simulated run for a reference.\n\nAdditional notes:\n• Line 65: the Messinian evaporite package\n• Line 80-81: … the drill cuttings… binocular microscope, excluding grains…\n\nReferences:\nAmdouni, R., 2000. Chemical study of free brines in the solar salt works of Sfax saline (S.E. Tunisia). In: 8th World Salt Symp. pp. 501–506.\nBabel M. and Schreiber B. C., 2014. Geochemistry of evaporites and evolution of seawater. In Treatise on Geochemistry. Elsevier, Oxford, pp. 483–560.\nMcCaffrey, M.A., Lazar, B., Holland, H.D., 1987. The evaporation path of seawater and the coprecipitation of Br- and K+ with halite. J. Sediment. Petrol. 57, 928–937.\nPitzer, K.S., 1973. Thermodynamics of electrolytes. I. Theoretical basis and general equations. J. Phys. Chem. 77, 268–277Risacher, F., Clement, A., 2001. A computer program for the simulation of evaporation of natural waters to high concentration. Comput. Geosci. 27, 191–201.\nTimofeeff, M.N., Lowenstein, T.K., Brennan, S.T., Demicco, R.V., Zimmermann, H., Horita, J., Von Borstel, L.E., 2001. Evaluating seawater chemistry from fluid inclusions in halite: examples from modern marine and nonmarine environments. Geochim. Cosmochim. Acta 65, 2293–2300.\nWeldeghebriel, M.F., Lowenstein, T.K., García-Veigas, J., Cendón, D.I., 2022. [Ca2+] and [SO42-] in Phanerozoic and terminal Proterozoic seawater from fluid inclusions in halite: The significance of Ca-SO4 crossover points. Earth Planet. Sci. Lett. 594, 117712."}, "author_response": "Response to the reviewers’ comments to manuscript NCOMMS-24-09842-T\nsubmitted by Aloisi et al. to Nature Communications\nKey to reading this document:\n Reviewers’ comments are in blue italics\n Authors’ answers are in black\n OM = original manuscript\n RM = revised manuscript\n Note that Microsoft Word skips line numbers from one page to another when\nusing track changes. Reference to line numbers in the text below refers to the\nWord document with track changes.\nReviewer #1 (Remarks to the Author):\nThe authors present new chlorine isotope data and some supporting seismic\ninformation to provide definitive evidence that the Messinian Salinity Crisis occurred\nwith isolation from the Atlantic Ocean and significant drawdown. The strongest data\nfor these results were the chlorine isotope data and the supporting geochemical\nmodeling. The use of the seismic data was not as effective as desired and may not\nbe needed to support the author’s goals. There are better, easier-to-interpret\nillustrations of erosion surfaces in past literature.\nWe agree that the strongest point of the paper is made with the chlorine isotopes,\nand that the seismic data somewhat distracts the reader from the main point we\nmake using the chloride isotopes and the modelling. After in-depth discussion among\nco-authors, we have decided to omit most of the seismic data and discussion from\nour manuscript. This resulted in omitting figure 4 and the text in lines 188 to 200 in\nthe OM. Reference to a seismic reflector occurring at the depth of the erosional\nevent (erosion is deduced from XRD data) is kept in figures 1 and 2 and in one\nsentence in the text (lines 262 to 263 in the RM), because this reflector marks the\nerosional event. The result of this change is a manuscript that is easier to follow and\nmore focussed on the new chlorine isotope approach.\nIn addition to grammatical and stylistic corrections, this text's primary areas for\nimprovement are further attention to detail in the figures and addressing some\nunknowns in the methodology section. The figures should be redrafted to minimize\nambiguity.\nWe agree with this recommendation and redrafted most figures, addressing all\ninconsistencies (see below for details of how this was done).\nSome additional effort could be made to emplace the stratigraphic units within the\ntimescale of the presented model. In addition to Manzi et al. 2021, is there more\nsupporting information for the timescale of halite deposition your work determines?\nThe results of your work could be further emphasized by highlighting its alignment\nwith other chronological constraints and adding more information on the data\nunderpinning those results.\nThe widely accepted stratigraphic position of the halite deposit in the Mediterranean\nis during Stage 2 of the MSC (5.60-5.55 Ma). Indeed, this stratigraphic position dates\nback to well before the study by Manzi et al., 2021. It was agreed in the CIESM\nConsensus Report of 2008 (CIESM 2008. The Messinian Salinity Crisis from mega-\ndeposits to microbiology - A consensus report. N° 33 in CIESM Workshop\nMonographs [F. Briand, Ed.], 168 p., CIESM Publisher, Monaco.) and published in\nthe milestone review paper by Roveri et al., 2014. More recently, Bill Ryan has re-\naffirmed the stratigraphic position of the halite deposit in his extensive review: Ryan,\nW.B.F., 2023. 50th anniversary review of the Mediterranean desiccation hypothesis.\nRiv. Nuovo Cim., 46(4-5), 164-291. More specifically for the Levant Basin, Manzi et\nal., 2021 have validated the stratigraphic position of the halite using industrial wells\nin the same study area as our study. We have made wider reference to the\nstratigraphic location of the halite deposit in the revised text (lines 188 to 206 in the\nRM):\n“This time frame is consistent with the widely accepted stratigraphic position of the\nhalite deposit being accumulated during phase 2 of the MSC across the\nMediterranean basins (5.60-5.55 Ma; e.g. CIESM38; Roveri et al.3; Ryan et al.39) and,\nmore specifically, in the Levant Basin (Manzi et al.36).”\nMethodological questions\nHow was the 20 kyr time step for the model decided? Why not less?\nThe time step in model calculations is 5 years, not 20,000 years. This is stated in line\n849 of the OM.\nIn the FE section, I do not see any explanation for variation in surface area as the\nMediterranean was drawn down with evaporation. How big of a difference would this\nhave been, why is it not considered here?\nIn the FE section (page 24 of the OM), no mention of the effect on evaporation of the\nsurface area decrease during Mediterranean sealevel drawdown is made because\nFE is the linear evaporation rate (in m yr-1), which does not depend on the surface\narea of the evaporating water body.\nIn the RM, we specified “linear evaporation rate” in line 999 to make this clear.\nThe effect of surface area on the volumetric (in m3 yr-1) evaporation rate, instead, is\nexplained in lines 523-526 of the OM:\n“The modern hypsometry of both sub-basins (Meijer and Krijgsman, 2005) is\nconsidered, such that drawdown does not continue indefinitely but an equilibrium\nwater height is reached when evaporation from the reduced basin surface area\nequals fresh-water input by continental runoff.”\nLines 586-587 is not a complete sentence.\nThe sentence was corrected grammatically.\nIn the FRTOT and FRWMed section, Lines 587-589 state that evaporation would\nhave been higher over the eastern Mediterranean than over the western\nMediterranean, yet, in the FE section, the evaporation rate is set equal for the\neastern and western Mediterranean, please elaborate a little more.\nSince we do not have an estimate of the linear evaporation rate in the western vs.\neastern Mediterranean basins during the Messinian, we made the assumption of an\nequal linear evaporation rate over the western and eatern Mediterranean basins\nduring the Messinian. The chosen value (1 m yr-1) corresponds to the average late\nMiocene linear evaporative flux over the entire Mediterranean basin (Simon et al.,\n2017). We have added this explanation in lines 1001 to 1004 of the RM:\n“The value of 1 m yr-1 corresponds to the average late Miocene linear evaporative\nflux over the entire Mediterranean basin (Simon et al., 2017). We use this average\nvalue of 1 m yr-1 for both the eastern and western Mediterranean sub-basins.”\nWith this assumption, to fit the halite volume and 37Cl constraints, we have to\nimpose a west-east distribution of continental runoff that overestimates the runoff\nfeeding the western basin (%F = 0.52 in our model), compared to the GCM\nRWMed\nmodel runs of Marzocchi et al. (2019) (0.13 < %F < 0.36).\nRWMed\nAs stated in the text (lines 1043 to 1045 of the RM), the origin of this discrepancy\nlikely lies in the fact that, similar to the modern situation (Mariotti et al., 2002), also\nduring the Messinian the linear evaporation rate in the eastern basin was higher than\nin the western basin.\nNote that since the freshwater budget is the result of the combination of runoff and\nevaporation, this modelling strategy has no effect on the computed rise in salinity,\nsaturation states and volumes of evaporitic minerals and 37Cl signatures calculated\nby our model.\nLines 607 to 617: In this section only the seismic mobile unit is used for the western\nbasin, leading the a halving of estimated evaporite volume from a prior study.\nHowever, the values from the same study (Haq et al., 2019) that were not used for\nthe western basin were used for the eastern basin. The logic here needs more\nexplanation.\nFor the western basin, Haq et al. (2019) include the seismic unit underlying the halite\n“mobile” unit (MU) in their evaporite volume calculations. However, it is very likely\nthat this lower unit is not composed of evaporites, but of clastic deposits (Lofi et al.,\n2005, already cited in the OM). We therefore preferred to use the more recent\nvolumetric estimate of Heida et al. (2021) for the western Mediterranean basin. This\nwas specified by adding the following sentence in lines 1087 to 1095 of the RM:\n“The assumption for picking base and top of evaporites by Haq et al. (2019) was that\nthe entire evaporitic sequence of the western Mediterranean is made of halite\nsandwiched by two gypsum layers (the so-called seismic reflection ‘trilogy’), while in\nthe Levant Basin evaporitic sequence is composed almost exclusively by halite\n(Mobile Unit MU). However, in line with Lofi et al. (2005), later research indicates that\nthe lower gypsum seismic unit (LU) detected exclusively in the western\nMediterranean probably consists of resedimented terrigenous material thus\nsubstantially reducing the volume of halite compared to the estimation of Haq et al.\n(2019) in the Western Mediterranean (Heida et al., 2021; Ryan 2023).”\nLines 637-638 discuss the potential reworking of halite for unit 6. Unit 5, with its\nerosional surface would also likely have some reworking of halite, why is this not\nmentioned in the model discussion?\nIn the RM, we mention the terrigenous origin of part of the material composing unit 5\nin lines 256 to 259:\n“Independent evidence for the occurrence of a sea-level drop and for the subaerial\nexposure of the basin’s margins come from the decrease in halite concentration and\nthe peak in Quartz abundance (up to 58 wt-%) in the top 140 m of Unit 5,\nsuggesting significant terrigenous input into the basin during drawdown.”\nThe methods section is inconsistent in its use of 1000’s separator. Please correct.\nA comma is used as a 1000’s separator throughout the RM.\nFurthermore, please consistently label the units (no units on lines 596-597).\nChange made\nThis manuscript has several data presentation elements that could be greatly\nimproved. There is inconsistent punctuation with units, in some areas there is no\nspace between a number and its unit, in others, there is. Please be consistent.\nA space now always separates numbers and their units in the RM.\nThe figures are not consistently lettered off. Figures 1 and 2 were missing letters for\ntheir different subsections. Figures 3 and 4 used different styles to identify\nsubsections (A) vs (a.)\nInconsistencies were addressed in the figures of the revised manuscript. Thank you\nfor bringing this to our attention.\nItems within the figures were not clearly labeled. Figure 1a shows some red lines in\nthe northwestern area, but I could not deduce from context clues if those were\nimportant information or if they were left on by accident.\nThe red lines illustrated Messinian canyons. We removed them from Fig. 1 for clarity\nand to avoid any confusion. Given that in the RM we have excluded the in-depth\ndiscussion of seismic data, we have also simplified panel b of Fig. 1, omitting the\npositioning of seismic cubes, while we kept the position of the seismic profile\npresented in panel d.\nThe boxes in the inset in Fig 1a were seismic data areas, but that was not made\nclear until Figure 4.\nPlease see our answer to the previous comment.\nThe y-axis labels Figure 1b and c were 2-4 sizes smaller than the other axes, making\nthem almost impossible to read without great magnification. Similarly, Unit 7 in\nFigure 1c has a different text size.\nThank you for this suggestion, we increased the font size (to 8).\nFurthermore, the unit of (km) in Figure 1b needs more clarification, km below msl? I\ndid not see this made clear in the text (this comment also applies to Figure 2). Figure\n1b would be easier to read if it had a key, similar to Figure 1a, but the description in\nthe text is acceptable.\nWe added a note in the caption stating that vertical distances are in km below sea\nlevel. A “lithology” key has been added to Fig. 1c.\nFigure 1c shows a white box that I assume is the seismic shown in Figure 2, but that\nwas never specified.\nYes, it represents the seismic shown in Figure 2. We specified this in Fig. 1’s cross\nsection.\nFigure 1a could be improved by splitting it into two segments, one with the modern\ntopography with the sills and subbasins labeled. This article should be more\naccessible to a broader audience unfamiliar with the specific location of the Sicily sill\nor the subbasins (e.g., Ionian, Herodotus, and Antalya).\nWe labeled the Sicily sill on Fig. 1a. The subbasins were already labelled but not\nmentioned in the caption. We have corrected this in the RM.\nSimilarly, the Eastern and Western basins should be highlighted on the map. The\nsecond part could show the data highlighted in the key of Figure 1a, currently it is\nchallenging to interpret the colours in the figure because of differences in\ntransparency (e.g. the marginal basin deposit is shown as solid in the key and\ntransparent in the map, making matching the colours between the two difficult).\nWe adjusted the key to make the colours match. More generally, we reshaped Fig. 1,\nand we believe it is more accessible to a broad audience and is now easier to\nunderstand.\nFigure 2 shows FBI, which is not defined anywhere in the text.\nWe added the definition of the FBI (Foraminifer Barren Interval) in the figure caption,\nwith the reference to Manzi et al. (2021) where the environmental and stratigraphic\ninterpretation of the FBI is given in full.\nStratigraphic is also misspelled in Figure 2.\nThis has been corrected.\nThe data presented in Figure 2 do not support Unit 3 as terrigenous material bearing,\nneither does the presented stratigraphic column, an additional line about lateral\nheterogeneity and a reference would clarify this.\nIn the description of the lithological origin of Levantine Basin seismic units (lines 70\nto 74 of the RM), we specified:\n“In the Levantine basin, the Messinian evaporites package is divided into the\nfollowing seven seismic subunits34. Four transparent units are composed mainly of\nhalite. Two intercalated, highly reflective units are composed of halite with a higher,\nalbeit laterally variable, clay content. A seventh unit…”\nFigure 3 might be improved by annotating (H or another part of the figure) with the\nUnits corresponding with the model over time. The green line in Figures 3B and H is\nonly explained in the text; it should also be explained in the caption.\nIn panel h, we have added horizontal bars that correspond to the timing of\naccumulation of units 1 to the bottom 2/3 of unit 5, and the top 1/3 of unit 5. The\ngreen lines in panels b, e and h have been explained in the figure caption.\nFigure 4 should be redrafted. As presented, it is hard to identify the data it is\nintended to highlight. I do not like how the coordinate system between it and Figure\n1a differs (a matter of opinion).\nFigure 4 has been excluded from the RM following the suggestion of reviewer #1 to\nomit most of the seismic data and discussion from our manuscript.\nI also do not like how it attempts to hide the fact that some of the seismic data are in\ndepth and some are in time in Figure 4A; the two can only be differentiated by\ninspecting Figures 4B to D. This could be avoided through clearer labeling or by\nusing a different colour scheme for the two types of data.\nFigure 4 of the OM has been omitted in the RM; please see our answer to the\nprevious comment.\nI suggest that Figure 4a be greatly zoomed in to highlight the data of interest, as it is\nshown now, half of the space shown does not relay helpful information (the erosion\nsurface)\nFigure 4 of the OM has been omitted in the RM; please see our answer to the\nprevious comment.\nsimilarly, it is unclear if the erosion surface features shown in 4a are from erosion or\nlater deformation (which is highlighted in Fig 1c). It is not clear that the three seismic\nx-lines are needed. One excellent example might suffice here, ideally the example\nwould be clearer than the examples shown here.\nFigure 4 of the OM has been omitted in the RM; please see our answer to the\nprevious comment.\nThe pale white boxes were difficult to find (again requiring magnification of the\nscreen), and the truncations might be better highlighted by arrows showing where\nthe reflector has gone absent.\nFigure 4 of the OM has been omitted in the RM; see our answer to the previous\ncomment.\nAs this figure currently stands in the text, I am not sure it is needed, if it is kept, it\nshould be clarified at what water depth this subaqueous erosion occurred within and\nthe importance of this surface be highlighted further in the text.\nFigure 4 has been excluded from the RM following the suggestion of reviewer #1 to\nomit most of the seismic data and discussion from our manuscript.\nLines 154-156 discuss the mode of salt precipitation models this work addresses:\n“deep basin, deep water vs. deep-basin, shallow water.” (Note inconsistent\nhyphenation.) If space allows, adding an illustrative figure of this concept may be\nhelpful for readers.\nThe RM includes a new figure 4 that presents a sketch summarising the results of\nour work. Specifically, if presents the two phases of halite accumulation, first in a\nfilled, but hydrologically restricted, Mediterranean basin, then in an isolated basin\nundergoing evaporative drawdown. For clarity, this sketch also presents the classical\nMSC chrono-stratigraphy after CIESM (2008), Ryan et al. (2014) and Ryan (2023),\nwhich is also mentioned in the text of the manuscript.\nSpecific line items\nLine 55: “strongest to “stronger”\nLine 211: “and” to “an”\nLine 483: “form” to “from”\nThese changes have been made\nSupplementary Table 5: for reference/notes for 37Cl/35Cl ratio in SMOC standard\n(SMOC) it states “from somewhere”. Please add the appropriate reference.\nThe reference Brand et al. (2014) has been added.\nExtended data Figure 1: This table is unnecessary because the same data is present\nlargely in Figure 2, and the remaining data is shown in Supplementary Table 1.\nWe prefer leaving Extended data Figure 1 in the manuscript because it presents\nuncorrected XRD mineral abundance data, whereas in Figure 2 the mineral\nabundance data is corrected to account for the fact that sylvite and barite have been\nadded to the drilling fluids.\nReviewer #2 (Remarks to the Author):\nAloisi and others present some very interesting new d37Cl and other data from a\ncommercial borehole in the eastern Mediterranean. Along with already-known\nvolumes of the evaporites deposited, the authors attempt to reproduce by\nnumerical modelling the conditions in the basin that led to these d37Cl and evaporite volumes.\nThe work suggests an early period during the MSC in which sea-level changed little\n(units 1-early 5), followed by a late Messinian larger drawdown. The magnitude of\nthat drawdown is compared with morphologic evidence from seismic reflection data\ncollected in the east basin. The MSC is very widely known and the debate over deep\nversus shallow drawdown has persisted, so I expect many researchers will be\ninterested in these results.\nThe modelling of the -0.2 to -0.3 ppt d37Cl measurements constrains the sea-level\ndrawdown. To assess this important outcome, I deliberately read the article first\nwithout referring to the electronic supplement. However, I found it extremely hard to\nassess this claim from the article contents alone. There are critical assumptions\nmade that are only provided in the electronic supplement. I strongly urge the\nauthors to consider withdrawing their article and submitting it instead to a journal that\nwould allow a more traditional structure, so that the model can be explained more\nfully (including assumptions) before the modelling results are presented. The key\nis in the name “supplement” – these should be for information that embellishes the\nstory, not information that is so critical for understanding it.\nIn the revised manuscript, we have addressed all the concerns regarding model\nassumptions raised by Reviewer #2. Our detailed responses to these concerns are\nprovided below. We believe that, by addressing these issues, the claims made in the\nmanuscript can be fully evaluated through the main text. In addition, to make it clear\nthat the numerical model plays an important role in our work, we added reference to\nit in the abstract (lines 20 to 21 of the RM):\n“Here we present evidence based on the chlorine stable isotope composition of\nhalite interpreted using a geochemical model for a two-phase accumulation of the\nMediterranean salt layer”\nIt would help reassure readers if a simple \"back of envelope\" type calculation could\nbe provided to show that the results are in the right general ball-park. In other\nmodelling involving finite-difference calculations, we often use a steady-state solution\nto show where incremental steps are heading towards. Here, I wondered if a rough\ncalculation could be done with the -0.2 to -0.3 ppt d37Cl in terms of salinity and\nproportion of seawater volume evaporated. Without that, there's a danger of the\nresult being viewed as a \"black box\" calculation that few will understand and hence\nthe results not fully accepted.\nConcerning the suggestion of carrying out a “back of envelope” calculation,\nunfortunately the chloride isotope system is in a strongly non-steady-state behavior,\nwhich precludes simple analysis. If a near steady-state situation was present, we\nwould have avoided the more complicated non-steady state model we developed.\nIt would be helpful to have a figure showing all or as many parameters as possible\nwith mathematical symbols. Extended Data Figure 2, for example, has terms shown\nas the same for both basins but they are different and is missing functionality. E\nvaries between the basins, similarly \"drawdown\", P...\nExtended Data Figure 2 was modified and symbols that represent water fluxes in\nequations 1 and 2 of the supplementary material were added to the figure.\nReference to these two equations is made in the figure caption.\nI missed whether the smaller deviations of d37Cl from equilibrium global ocean\nvalues are addressed. I may have simply overlooked this, though the model outputs\ndo not reflect the variation from equilibrium in early unit 1 to 0.3 ppt at just before the\nblue horizontal dashed line in Figure 2.\nThe numerical model indeed reproduces the small negative Cl-isotope depletion\nfrom unit 1 to the bottom 2/3 of unit 5. This is shown in panel h of Fig. 3, where the\nlabel AP-phase 1 indicates this small negative Cl-isotope deviation. In lines 113 to\n119 of the OM, we state that halite precipitating with this small negative Cl-isotope\ndepletion is deposited during a first phase of restriction:\n“The best fit of the model to the volume and 37Cl values of halite (Fig. 3) is obtained\nif a two-step hydrological restriction-isolation scenario at Gibraltar is considered.\nFirst, the deep-water outflow at Gibraltar is restricted but not blocked, leading to the\nprecipitation of gypsum, followed by the precipitation of halite. This halite is\ncharacterized by a modest chloride isotope reservoir effect and decreasing\nhalite 37Cl from the value in equilibrium with oceanic dissolved chloride (0.4\n‰ vs SMOC) to the average value measured from seismo-stratigraphic unit 1\nto the lower 360 m of unit 5 (0.31 ‰ vs SMOC) (Fig. 3h).”\nThe d37Cl measurements seem to me the most important, though I didn't feel I knew\nhow these were made exactly. For example, at one point they are described as\n\"bulk\" measurements - does that mean measured samples included impurities and if\nso how many?\nFor the halite chloride isotope analyses, we hand-picked halite grains under the\nbinocular microscope. We cannot a-priori exclude the presence of minor Cl-bearing\nimpurities – most probably sylvite (KCl), that was detected in the XRD analyses - that\nmight have contributed to the “bulk” Cl-isotope signature.\nNote that while the halite grains selected for the Cl-isotope analyses were hand-\npicked under a binocular microscope, the sub-samples for the XRD analyses were\ncollected to be the most representative as possible of the macroscopic lithological\nheterogeneity of the drill cutting. Consequently, in the RM, we:\n(i) refer to “bulk” XRD analyses (RM line 75), and chloride-isotope analyses carried\nout on “halite-grains handpicked under the binocular microscope” (RM line 467),\n(ii) explain that we cannot exclude minor Cl-bearing impurities (e.g. KCl) in the halite\ngrains that will contribute to the chloride-isotope signature (RM, lines 94 to 97),\n(iii) in the supplementary material section (lines 884 to 932 of the RM),\npresent an isotope mass balance to estimate the effect of KCl admixture with NaCl on the\nchloride-isotope data from the Aphrodite 2 hydrocarbon well where we know that\nKCl was added to the drilling fluids. The result is that the effect is very small. We\nadded the following text to support these first three points (lines 93 to 97 in the\nRM):\n“Halite grains for Cl-isotope analyses were selected from drill cuttings under the\nbinocular microscope taking care to exclude grains that contained evident signs\nof impurities. We cannot exclude, however, that minor amounts of Cl-bearing\nsylvite (KCl) contributed to the Cl-isotope signature we measured. Mass-balance\ncalculations, however, suggest that the contribution is negligible (see\nsupplementary materials).”\n(iv) add that in the Levantine basin, we measured an extremely 37Cl-depleted halite\n(37Cl = - 0.14 ‰ vs SMOC), which cannot be ascribed to the presence of\nsylvite in drilling fluids because the halite samples were obtained by\nmechanically breaking solid halite layers in the DSDP cores, rather than\nextracted from an admixture of drill cuttings and drilling fluids. We added the\nfollowing text in lines 137 to 141 of the RM:\n“The 37Cl-depleted isotope signature of halite from DSDP site 374 cannot be\ninfluenced by the composition of drilling fluids, because the halite sub-samples\nwere obtained by mechanically breaking solid halite layers in the DSDP cores,\nrather than extracted from an admixture of drill cuttings and drilling fluids like for\nthe Aphrodite samples.”\n(v) Finally, in the OM, “bulk” was used also to indicate the “bulk halite precipitation\nrate”. We corrected this to “volumetric precipitation rate” (line 48 in the RM),\nto\navoid ambiguity with the use of “bulk” when referring to the XRD analyses.\nThe English is mostly very good, although the text and figure captions include some\nvery long sentences, which make the text hard to read. Please consider breaking\nthem up. Commonly, where semicolons are used, sentence breaks could make the\nreader’s work easier.\nThe figure captions were revised.\nMany researchers prefer a writing style in which the observations are described first,\nthen the authors provide their interpretations. Within this draft, the two are inter-\nmingled (e.g., around line 82). You might consider separating them, which would be\neasier in the different journal format I suggest.\nWe have decided to resubmit a revised manuscript to Nature Communications,\nmaking a concerted effort to clarify our data description and model analysis\napproach. Although the data and interpretation are still somewhat interwoven in the\nmain text, we believe that presenting them in a successive, block format improves\nthe narrative flow.\nDetailed comments:\nLine #\ntitle instead of Late Miocene (which may sound like you have found another crisis).\nAlso, you have also suggested there was a period with no drawdown. I therefore\nsuggest instead:\n“Modelling of chlorine isotopes suggests Mediterranean Sea drawdown history\nduring the Messinian Salinity Crisis”\nWe feel that, next to the chloride isotope method, the focus on the large drawdown in\nthe title showcases what we think is the most original result we present. We would\ntherefore prefer to keep the large drawdown result in the title. We agree that the term\nMessinian Salinity Crisis should appear, and propose the following title:\n“Chlorine isotopes constrain a major drawdown of the Mediterranean Sea during the\nMessinian Salinity Crisis”\n28 no need for “completely”?\nWe agree that “large parts of its margins were nearly completely desiccated” is\nredundant, and have removed “nearly completely”.\ncontamination (not that I expect it would change the results much). This is another\nreason why it would be better to have methods /samples explained first in the main\ntext.\nThis issue was addressed in detail, please see our answer to the same question in\nthe “Remarks to the author”, above.\n51 37Cl relative to the lighter isotope 35Cl into …\n52, 54 that -> which\nfollowing” before “seven”.\nFour transparent … are composed…\nTwo intercalated ….are composed of …\nA seventh unit is composed ..\nto exactly which panel you are referring to.\n67-68. Please add the measurement method (XRD) to the caption to Figure 2 (e.g.,\ncarbonate can be derived from the bomb method instead).\n74 …depth, hence no samples are available from unit 6.\n75 that -> which\n80 please insert a comma before “is”\n82. Please break sentence after chloride.\nPlease separate your observations from interpretations.\nWe interpret this as reflecting a modest isotope effect, in which the rate of ….\n84 decrease\n97 please use consistent language:\n… has intermediate d37Cl (0.08 …)\nAll of the above suggestions were implemented.\nensure a period of near-global values, followed by negative values, the model\nincludes two stages of differing flow restriction at Gibraltar. First, …\nWe preferred to stick to our original wording of this sentence in the RM.\n121 …while halite is not precipitated in the …\n189 same depth at which bulk\ndeformation. The results in Figs. 4b-4d reveal truncations of reflections that ….\n196 to->with\n203 incision -> development\n210 that -> which\n211 leaves ample time for intermediate …\nAll of the above suggestions were implemented.\nthey are not consequences of your modelling results?\nThe published scenarios introduced in the last paragraph – migration of mammals\nfrom Africa to Europe, pulse of Mediterranean volcanism and large-scale climate\neffects - rely on the occurrence of a large sea-level drawdown in the Mediterranean\nSea during the Messinian Salinity Crisis. Since our chloride-isotope work provides\nevidence for such and extreme drawdown event, it makes those scenarios more\nplausible. We thought that this consequence out our work was worthwhile to be\nhighlighted as a closing of our work.\n222 would -> may\n- Without references, this is speculative. Indeed, if it occurred slowly so that diffusive\ncooling also affected magma reservoirs, it need not necessarily have triggered\neruptions.\nReference to Sternai et al. 2017 was added.\nFigures:\n434. Please cite the source of the map (it looks like GeoMapApp). Please insert\n\"evaporite\" above \"thickness\" in key.\nThank you for this comment. The bathymetry and onshore background map have\nindeed been taken from GeoMapApp. The following text has been added to the\ncaption of Figure 1 “Background relief and bathymetry maps have been taken from\nhttps://www.geomapapp.org/.”\nWe also added “Evaporites” above thickness.\n- Similarly for ROP\n443 reflector -> reflection\n444. Please break sentence instead of using semicolon.\nThe above suggestions were implemented.\n450 in Figure 2 on right, it is unclear what “equilibrium with seawater” means.\nThis is explained in the figure caption of the RM: “the red dashed line in panel g\nrepresents the equilibrium chlorine stable isotope composition of halite precipitating\nfrom seawater”\n455 …(halite) compositions are in …. Sediment, corrected for barite (…)…\noriginating from drilling fluids.\nPlease replace the semicolons with sentence breaks – they make the caption hard to\nfollow.\nThis change was implemented.\nprecipitation from an infinite reservoir, i.e., global seawater?\nThere must be a misunderstanding here because we do not talk about a “reference”\ncomposition in line 457 of the OM. In the RM, lines 730-731, we state: “the red\ndashed line in panel g represents the equilibrium chlorine stable isotope composition\nof halite precipitating from seawater”. This should be easily understandable for a\ngeochemist.\nYellow layers…. – these might be better written on the figure on the right and instead\nput the text “DSDP samples….” Moved to the figure caption. Otherwise, reader has\nto flip back and forth to caption.\nWe prefer to keep our version of the figure.\n493 survey datasets\n493 (b) to (d) Three seismic profiles flattened at the intra-unit 5 reflection from t he\nSara-Mira….. , respectively.\nWhite dashed rectangles in (b) to (d) highlight areas of truncation …\nThe above suggestions were implemented.\nElectronic supplement:\nFigure 1:\n... have been added to the samples from the drilling fluids.\nNumerical model\n504 that simulates -> to simulate\n.... basins. Those basins variedly exchange water ....\n512 delete \"can further be\"\nThe above suggestions were implemented.\nplate-tectonic movements and the deposition of Plio-Pleistocene sediments has only\nmade a minor change in overall volume, which may indeed be the case though could\nbe justified. Similarly, isostatic effects of removing so much water are ignored. This\nought to be justified.\nTo our knowledge, in addition to the modern hypsometric curve of the Mediterranean\nSea that we used in our modeling exercise, there exists a hypsometric curve for the\nTortonian Mediterranean (late Miocene, from 11.6 to 7.2 Myr; Meijer and Krijgsman,\n2005, EPSL, 240 (2), 510-520). The hypsometry during the Messinian Salinity Crisis\nwas likely intermediate between the modern and Tortonian hypsometries, with the\nlargest difference being that during the Tortonian the Tyrrhenian Sea had not opened\nyet (Meijer and Krijgsman, 2005). We chose the approach of Meijer and Krijgsman\n(2005) who used the modern hypsometry with a fully formed Tyrrhenian basin. Note\nthat overall, the hypsometric curves for present day and for the Tortonian have\nsimilar shapes (Meijer and Krijgsman, 2005). We added the following text in lines\n958 to 962 of the RM:\n“The model considers the modern hypsometry of both Mediterranean sub-basins,\nwhich is likely very similar to the Messinian hypsometry (Meijer and Krijgsman,\n2005). Thus, drawdown does not continue indefinitely but an equilibrium water height\nis reached when evaporation from the reduced basin surface area equals fresh-\nwater input by continental runoff.”\n531 that-> which\nWas it assumed to be constant and if so what are the consequences for the model\noutputs?\nIn line 1006 of the RM, we specified:\n“the total evaporation rate in a water-filled Mediterranean is”\n578 .. model was run (...) with a constant freshwater flux of ****\nThis lies within the range....\n- i.e., state the flux please. Text is confusing here.\nThe above suggestion was implemented.\n583 \"is varied to fit the data\" Please state if you are varying it over time or it has a\nconstant value in each run but is adjusted until the model outputs (d37Cl) match the\ndata.\nThe following addition was made in lines 1029 to 1033 of the RM:\n“The partition of the total continental runoff flux between western and eastern\nMediterranean basins (%F ) is constant in time. Its value is determined by\nRWMed\nfitting the calculated halite volume and halite 37Cl to the volume estimated\nbased on seismics (Haq et al., 2019; Heida et al., 2021) and the measured halite\n37Cl values.”\n586-589. This is stated \"the fact that\" but there is no citation / evidence given. I\nsuggest instead:\nIn the modern M...., evaporation is stronger over the eastern basin than its western\nbasin (Mariotti ...). Due to the greater isolation of the eastern basin from the Atlantic\nOcean, we suspect that this was also the case in the Messinian, so we maintained\n....\n(or other reason - at the moment there is no justification)\nWe changed the relevant sentence in lines 1043 to 1045 of the RM, to:\n“In the modern Mediterranean, evaporation over the eastern basin is stronger than\nover the western basin (Mariotti et al., 2002). We assume that this was also the case\nin the Messinian, which would explain the discrepancy in %F values.\nRWMed\npredict the geometry of the connection, hence the need to treat this as an adjustable\nparameter.\nWe agree with this statement. Consistently, in our modelling exercise the parameters\nthat control the hydrological exchange across the Gibraltar sill are forcing\nparameters; i.e. they are not imposed a-priori, but they are varied until a good fit of\nthe model results to the available halite volume and 37Cl data is obtained.\n594 was chosen equal to the modern .... hydrological flux with the Atlantic\n597 that -> which\n602 delete \"down\"\nThe above corrections were made.\n602-603 There is no justification for these two stages of isolation. In practice, two-\nway flow is unlikely to stop unless the water depth over the sill were extremely small.\nIn the work on the Red Sea, we know that two-way flow persisted through the Last-\nGlacial Maximum (some species survived - Fenton et al.), even though water depth\nover Hanish Sill became very small (Lambeck et al.). What would be the implications\nof a different onset of the crisis?\nThe existence of two stages of isolation is an outcome of the modelling exercise, not\na model forcing that needs to be justified a-priori (like, for example the magnitude of\nthe freshwater fluxes). In other words, the two stages of isolation are not imposed\nbased on previous knowledge of how hydrological fluxes at the Gibraltar sill evolved\nin time during the MSC. Rather, different histories of hydrological exchange at the\nGibraltar sill are tested, until the model calculated halite volumes and halite 37Cl\nvalues match the volume estimates from the seismics and the experimental 37Cl\ndata. This is explained in lines 592 to 593 of the OM:\n37\n“ is thus varied in order to fit model results to the  Cl halite and to the halite\nvolume estimated in seismic surveys (see below). »\n𝑫𝑬𝑬𝑷\n𝒈𝑾→𝑨\nof the earlier section (reader is expecting text on the constraints here).\n612 estimated\n(study was carried out in the past - similarly for other citations)\n617 and consider that -> , hence it would have been six times smaller than the\n600,000 km3 of the eastern Med...\nThe above suggestion was implemented.\nit back to only new information. The text is missing detailed information on sampling\nand analytical methods.\nWe agree that some of the material presented in this paragraph is a repetition of\nwhat stated in the main text. But we also think that it is necessary in this\nsupplementary material section on data constraints to the model calculations in order\nto follow the logical steps of our modelling exercise. We hope that these few lines will\nnot pose a problem of overall length of the electronic supplement and can be kept.\n638-641. Doesn't this imply that the integrated inflow flux of the model is too small? I\nfind this section confusing.\nWe purposely chose model parameters that underestimate the halite volume\nobserved in seismic surveys because we do not have 37Cl data from unit 6, and part\nof this unit may represent a primary halite precipitate.\nED Figure 2 was modified for consistency of symbols with those used in the text\ndescribing the model (equations 1 and 2 of the supplementary information).\n657 keep track of -> track\n663 ion concentration\n680 ...Cl of 0.31.... (Brand et al....).\n700-704 Please break sentence.\nThis is good, though I had earlier not realised that varied surface area was\nconsidered, hence please update text earlier so the starting assumptions are clearer.\nThe above suggestion was implemented.\n713-714 Why have you assumed the water flux would be linear with density\ncontrast? Also, turbulence varies with velocity contrast, hence turbulent \"friction\" is\nnon-linear with velocity.\nPerhaps a linear flux with salinity contrast is the most practical, though could be\ndiscussed.\nWe assume a proportionality between the water flux and the salinity contrast (e.g.\nequation 16). This approach is the same as that used in the models of Topper et al.\n(2011) and Topper and Meijer (2013) that simulate the water exchange fluxes at the\nGibraltar sill during the Messinian Salinity Crisis. This approximation is sufficient for\nour chemical and isotopic mass balance model where hydrological fluxes at Gibraltar\nare a model forcing, rather than a model outcome.\n755 - presumably only equal until water level reaches the Sicilian sill.\nYes, the sentence in question refers to the case where the water level in the\nMediterranean is higher than the Sicily Sill. This condition is clear from the sentence\nin lines 748 to 750 of the OM:\n“In these conditions, a surface water flux from the western to the eastern\nMediterranean is present only if the water height above the Sicily, SillSIC is greater\nthan 0”\n784 delete \"according to\"\n791 perhaps indicate where mminx is taken from (seems arbitrary otherwise).\n(This appears later at line 840.)\n849 suing?\nPerhaps delete \"suing ... Mathematica\"\nThe above suggestions were implemented except for line 882 where we prefer to\nkeep our text\nThe sensitivity test is actually on water pressure, we have specified this\n983 composition of halite-dominated particles of the...\n(if correct?)\nwe added that the d37Cl analyses are from hand-picked halite grains\n1040 \"from somewhere\" needs to be replaced by a citation.\nReference added\nbottom width of Sicily sill - there should be a reference for this relatively well-studied\nfeature.\nThis is estimated based on the bathymetric chart of the Mediterranean Sea\n1077 \"Atmopheric Pressure\" in right column\nWe specified that we are talking about water pressure\nThis is a major effort to carry out, which is reflected in the length of the electronic\nsupplement. Certainly these d37Cl data and the illustrative modelling would be\nimportant contributions to the literature. I look forward to seeing them published.\nWe thank the reviewer for his detailed revision of our manuscript and for having\nspotted numerous points that needed improvements, as well as plenty smaller\nchanges that needed to be implemented to improve the readability of the manuscript.\nThe effort he put is really appreciated.\nReviewer #3 (Remarks to the Author):\nAloisi et al. in “Chlorine isotopes constrain a major drawdown of the Mediterranean\nSea in the late Miocene” present a fascinating new chlorine isotope dataset from\nhalite. The data appear to be of high quality and will help advance our understanding\nof the depositional history of the thick evaporite sequences in the Mediterranean\nbasin — and therefore our ability to quantitatively estimate the sea level changes\nduring the Messinian salinity crisis. I am supportive of using chlorine isotope data to\nmodel the hydrogeochemical changes in the Mediterranean basin, although I think\nadditional sedimentological and petrographic data could be used to rule out the\ncontribution of other processes and non-halite mineral phases to the large and rapid\ndecline in chlorine isotope data shown by coupled sudden changes in the bulk\nmineralogy and chlorine isotope values. Although I have a few comments, questions\nand suggestions, as detailed below, I am very supportive of publication and I think\nthis could be a novel and important contribution.\nThe manuscript presents stable chlorine isotope data from the analysis of bulk\nsamples from core cuttings in the Levantine basin in the eastern Mediterranean and\nthree DSDP sites in the eastern and western Mediterranean basins. Chlorine isotope\nvalues of bulk samples are suggested to reflect those of halite — the dominant\nchlorine-bearing mineral. Chlorine isotope data from the bottom ~1240m interval,\ndominated by halite (>80 wt.%), show a slight upward decreasing trend from 0.4‰ to\n0.3 ‰ (Δ= –0.1‰). The top 140m, however, shows an abrupt decrease in chlorine\nisotope data to a minimum of –0.3‰ (Δ= –0.6‰), coinciding with a sharp decrease in\nhalite fraction (<30 wt.%), increase in detrital quartz (up to 48 wt.%), anhydrite (up to\n35 wt.%), and artificially added drilling fluid (BaSO4 - 33 wt.% and KCl- 6 wt.%). The\nauthors associate the abrupt decrease of chlorine isotope data with a rapid increase\nin halite precipitation rate and water level drawdown throughout the Mediterranean\nbasin.\nA method section on sample preparation and processing procedures for the chlorine\nisotope analysis is missing. This section would help the reader understand how the\nauthors overcame the contamination from drilling fluids in the top halite section,\nspecifically sylvite (KCl), which could impact the results. Depending on the sylvite-to-\nhalite ratios, the bulk analysis could give a falsely low chlorine isotope value because\nthe chlorine isotope composition of sylvite is low (–0.123‰) compared to halite\n(0.351‰).\nWe have addressed the issue of the possible contribution of sylvite to the chloride-\nisotope signature of halite by:\n(i) specifying in the methods section that halite grains for chloride-isotope analyses\nhave been hand-picked under the binocular microscope to avoid evident\ncontaminations from other minerals;\n(ii) carrying out a worst-case scenario of sylvite (KCl) contamination of halite (NaCl),\nwhere we assume that the hand-picking procedure failed entirely to select halite\ngrains, but incorporated sylvite grains in the proportion determined by the bulk\nXRD measurements of the drill cuttings. The result of this contamination scenario,\ndescribed in detail in a new section of the supplementary material, is that even in\nthis worst-case scenario 37Cl halite values would not have been affected\nconsiderably.\nFor detailed reference to changes in the text, see our detailed response to the same\nquestion by reviewer #2, above.\nSimilarly, kainite (KMgClSO4·3H2O) has a significantly low chlorine isotope\ncomposition (–0.42‰), and it tends to lower the bulk isotope values (Eggenkamp et\nal., 2016). In this study, simulated evaporation of Mediterranean seawater using\nPHREEQC predicted the precipitation of kainite, and the numerical model involves\nkainite co-precipitation with halite during restricted Atlantic flow to the Mediterranean.\nEarly studies also show kainite as the dominant potash mineral in many eastern\nMediterranean basins (Lugli et al., 1999). The authors mentioned that kainite and\nother Mg-K salts were undetected in the XRD results, but it is possible that the peak\nintensity of halite masked the peaks of other minor minerals such as kainite.\nTo understand if kainite went undetected in XRD analyses due to masking by the\npeaks of halite, we systematically looked for major peak position of the minerals we\ndetected in our samples, as well as common evaporitic minerals that were not\ndetected in our samples (see table copied below). We found that main diffraction\npeak of kainite (d = 7.372) has no equivalent in halite, or in other minerals that were\ndetected in our samples with XRD. We therefore conclude that kainite is absent, or\nbelow the detection limit, in the Aphrodite well drill cuttings.\nMineral Formula 1st peak d 2nd peak d 3rd Peak d\nvalue (I/I0) value (I/I0) value (I/I0)\nDetected by XRD\nHalite NaCl 2.821(1) 1.994(0.55) 1.628(0.15)\nQuartz SiO2 3.342(1) 4.257(0.22) 1.8179(0.14)\nCalcite CaCO3 3.035(1) 2.095(0.18) 2.285(0.18)\nBarite BaSO4 3.442(1) 3.101(0.97) 2.12(0.8)\nSylvite KCl 3.15(1) 2.22(0.59) 1.82(0.23)\nAnhydrite CaSO4 3.49(1) 2.849(0.35) 2.328(0.2)\nAkermanite Ca2MgSi2O7 2.87(1) 1.76(0.3) 3.09(0.3)\nVaterite CaCO3 2.73(1) 3.3(1) 3.58(1)\nNot detected in XRD\nGypsum CaSO4.2H2O 7.63(1) 4.28(1) 3.07(0.8)\nEpsomite MgSO4.7H2O 4.21(1) 2.677(0.25) 5.35(0.25)\nHexahydrite MgSO4.6H2O 4.39(1) 5.45(0.5) 5.1(0.45)\nKainite MgSO4.KCl.3H2O 7.372(1) 3.08(0.86) 7.771(0.83)\nCarnallite KMgCl3.6H2O 3.3(1) 2.92(0.7) 4.65(0.5)\nKieserite MgSO4.H20 3.409(1) 3.331(0.9) 4.84(0.9)\nSylvite KCl 3.15(1) 2.22(0.59) 1.82(0.23)\nBischofite MgCl2.6H2O 4.1(1) 2.65(0.75) 2.88(0.5)\nNote: d values are in Angstroms; I/I0 indicates the intensity of a given peak relative to the most\nintense diffraction peak.\nThe major peak position of epsomite (d = 4.21), on the other hand, overlaps with the\nsecond peak of Quartz (d = 4.257), so that, if present in only trace amounts in our\nsamples, we can expect epsomite to be masked by Quartz. Note, however, that the\nsecond and third peaks of epsomite have no equivalent position in the XRD spectra\nof the minerals detected in our samples. Furthermore, epsomite does not contain Cl,\nso that its presence in our samples would not affect the chloride isotope values.\nPetrographic observations could be used to examine the presence/absence of\nkainite and other potash salts, commonly preserved as solid inclusions or daughter\ncrystals in fluid inclusions. Given the significant difference in the chlorine isotope\nfractionation factor between kainite and halite, I believe this needs to be addressed\nin the main model or model sensitivity tests. How could the precipitation and\nrecycling of kainite during the rapid drawdown and erosion of the marginal deposits\ninfluence the model? This discussion could be extended to the recycling of marginal\nhalite deposits.\nOur numerical model runs, that include 17 sensitivity tests, never calculate a\nsupersaturation of the brines with respect to K-Mg salts other than epsomite and\nkainite. Chloride isotope fractionation during the precipitation of kainite is considered\nin our model, where we impose a chloride isotope fractionation factor 37\nKAI-solufion\nequal to 1.00013 (Eggenkamp et al., 1995). We carried out an additional model run\n(sensitivity test 18) to test the effect of kainite precipitation on calculated 37Cl halite\nvalues. In this sensitivity test we considered the absence of chloride isotope\nfractionation between dissolved chloride and kainite (37 = 1). In this sensitivity\nKAI-solufion\ntest, halite precipitated in the moment of maximum drawdown in the eastern basin\nhas a calculated 37Cl value of -0.301165 ‰ vs SMOC, instead of -0.308891 ‰ vs\nSMOC in the standard run. The difference is thus negligible.\nPetrographic observations would be useful to examine the primary versus detrital\nnature of the top 140 m of the Aphrodite core (recent work by Lowenstein et al.\n(2021) on criteria for recognizing in situ vs. clastic halites might be a relevant\nreference on this topic). Increased detrital quartz, gypsum, and decreased halite\nfractions suggest recycling, which could overestimate the mass of evaporites formed\nduring the late stage of the Messinian salinity crisis.\nWe do not agree with this statement. It is indeed possible that part of the halite in the\nupper 1/3 of unit 5 is recycled, either chemically or mechanically. However, recycled\nhalite must have precipitated somewhere in the Levantine basin during the\nMessinian Salinity Crisis. Including it in the basin-scale chloride mass balance is\nlegitimate.\nLine 146-148: Line 91 indicates that the DSDP sites represent the final stage of the\nevaporite phase of the MSC with depleted δ37Cl; however, higher chlorine isotope\ncompositions of halites from DSDP site 376 suggest the opposite.\nHalite in DSDP cores represents the final stage of the evaporite phase, simply\nbecause it lies stratigraphically above all other evaporite deposits. However, it is not\npossible to correlate stratigraphically the various DSDP sites nor to date the samples\nwe measured. In addition, site 376 (2000m depth) is and was much shallower than\nsite 374 (4000m depth) and could have been exposed during the second phase of\nthe evaporative drawdown (Carmerlenghi 2019). The halite measured from DSDP\n376 might thus reflect an earlier stage of the evaporative drawdown, either (i)\nbecause halite formed earlier during the MSC resedimented or reprecipitated at this\nsite or (ii) halite formation at site 376 stopped earlier than at deeper sites because it\nwas exposed subaerially.\nFigure 3. Simulated evaporation of modern seawater, including Mediterranean\nseawater, reaches halite saturation at a degree of evaporation of ~10.7 or a salinity\nof ~373 g/kg (Babel and Schreiber, 2014). However, the model here shows halite\nprecipitation occurring between 260 and 300 g/kg, which is lower than the halite\nsaturation.\nIn Babel and Schreiber (2004) we have not found the claim that halite precipitates\nfrom evaporating seawater when salinity attains 373 g/kg. Instead, we found the\nfollowing two indications on this threshold: (i) in Table 5 (page 500), halite is reported\nto start precipitating when TDS (total dissolved solids = salinity) is equal to 307.9\ngrams/litre; in seawater evaporation experiments of Fontes and Matray (1993); (ii) in\npage 516, Babel and Schreiber (2004) report a salinity value of ~320‰ at the start of\nhalite precipitation (units of g/kg are equivalent to ‰). No reference is provided for\nthis threshold value.\nNote that the estimate of 307.9 is in grams/litre. These units have to be converted to\ng/kg considering the density of a halite saturated brine (~ 1.2 g/litre), leading to a\nthreshold salinity value of ~ 260 g/kg.\nFigure 3 Caption B: Change meters to kilometers.\nThis was corrected\nLine 511: The assumption of ion-free freshwater riverine runoff could be reasonable\nduring the first brine-filled Mediterranean phase, but this assumption might not be\naccurate during the second stage, where the recycling of marginal evaporite deposits\nwas significant during the rapid drawdown. This scenario could be tested in model\nsensitivity.\nWe agree that, during drawdown, continental runoff would have been enriched in\nions acquired by dissolution of outcropping evaporites. In our modeling exercise, this\nscenario might be relevant during the accumulation of the upper 1/3 of unit 5.\nIndeed, adding Na+ and Cl- ions to freshwater input during this time interval would\nincrease the volume of halite deposited in the basin. But we lack a reliable criterion\nto choose a concentration of dissolved ions for river runoff. Their concentration will\ndepend on the time of contact of runoff with halite outcrops, and the flow rate of\nwater along the outcrops. We feel that simulating this scenario would introduce\ncomplexity that will not provide further insight on the main point we want to make\nwhich is one of halite accumulation rate and extent of sea-level drawdown. In a\nseparate model study of the latest stages of salt giant development, where the\ndeposition of units 6 and 7 I investigated, considering ions dissolved in river water\nwill be necessary. For our modeling scenarios, we prefer to keep the chemistry\nsimpler, and account for the possibility that part of the post-unit 5 halite is a primary\ndeposit by choosing a target halite volume slightly smaller than the estimate from the\nseismic surveys of Haq et al. (2019) (see explanation of this strategy in section 1.3 of\nthe supplementary material).\nReviewer #3 (Remarks on code availability):\nI reviewed the model equations but did not review the code because I do not have\naccess to Mathematica.\nResponse to the reviewers’ comments made in their re-review of manuscript\nNCOMMS-24-09842-T submitted by Aloisi et al. to Nature Communications\nKey to reading this document:\n Reviewers’ comments are in blue italics\n Authors’ answers are in black\nIn submitting the final version of our manuscript we got rid of all the track changes\nsince the additional modifications have all been implemented and are very minor.\nReviewer #1 (Remarks to the Author):\nThe manuscript is much improved and presents a clear presentation of the\nmeasurements, model, and results. I have made some minor comments and\nsuggestions in the attached document.\nThe suggestions have been integrated in the revised manuscript\nReviewer #2 (Remarks to the Author):\nI don't have time to re-review the manuscript in detail unfortunately. However,\nreading the manuscript and browsing the response to reviews, it is clear that the\nauthors have thought about the feedback carefully and taken some time in carrying\nout the revisions.\nThe authors misunderstood one of my comments. I was not suggesting they find a\nsteady state solution to the equations (I was rather mentioning steady states used in\nother systems using differential equations as a way to check model outputs).\nHowever, I can see from the equations and box model that a check like this is not\neasy to find. The model outputs essentially are untested, but future researchers will\nbe able to attempt modelling of their own to reproduce the d37Cl trends. It looks like\nthere is enough information in the supplement to do that.\nGiven that the many input parameters, which each have uncertainties, I am still not\nentirely sure how to interpret the estimated drawdown values. However, the reader\ncan see the assumptions and parameters used. Testing may take some further\neffort.\nMinor comment:\nFigure 4: In the figure, the sequence is separated into stages, but in the caption they\nare phases. It would be best to stick with one of either of these two terms - changing\nterms confuses readers.\nBoth in Figure 4 and in its caption “phases” refers to the phases of halite precipitation\nwe deduce from Cl-isotopes, while “stages” refers to the chrono-stratigraphic stages\nof the Messinian Salinity Crisis that are defined in the literature: CIESM (2006),\nRoveri et al. (2014) and Ryan (2023). This is explained in the caption.\nI find the results very interesting and look forward to seeing them in print.\nReviewer #3 (Remarks to the Author):\nThe revised version of the Aloisi et al. manuscript, \"Chlorine Isotopes Constrain a\nMajor Drawdown of the Mediterranean Sea During the Messinian Salinity Crisis,\"\nis\nalmost ready. I have a follow-up comment and a minor quibble.\nA new section has been added to the supplementary material, which estimates the\ncontribution of sylvite from drilling fluid to the measured chloride-isotope values using\na simple isotope mass balance calculation. Could you include these results in Table\n1 for each sylvite-halite proportion based on the XRD data?\nWe included the numerical results of the contribution of sylvite the 37Cl\nmeasurements in Supplementary Table 2, so that they are next to the measured\n37Cl values.\nNote that the “SD” (= standard deviation columns in Supplementary Tables 2 and 3,\nwhere we present the results of the Cl-isotope analyses, have been erased. This is\nbecause they contained the standard deviation of 10 successive mass spectrometric\nmeasurements, which is an information of limited interest. The external\nreproducibility of the internal IPGP standard (± 0.025 ‰), which encompasses all the\nmeasurement steps and is the parameter of interest, is presented in the section (“Cl\nisotope measurements” in the Online Methods section),\nAdditionally, the authors requested more references regarding the salinity or degree\nof evaporation (DE) which halite precipitation begins. The degree of evaporation is\nmore commonly reported than salinity. Evaporation studies of modern seawater in\nsolar evaporation ponds and simulations using geochemical models (PHREEQC,\nEQL-EVP, HMW, Geochemist's Workbench) consistently show that halite first begins\nto precipitate at a DE >10, with an average of ~10.6 (McCaffrey et al., 1987;\nAmdouni, 2000; Timoffeeef et al., 2001).\nFor reference, when I ran the EQL-EVP model at 25 °C to simulate the evaporation\nof modern seawater (using Risacher and Clement, 2001), it yielded a corresponding\nsalinity (TDS) of 373.3 g/l or 302.2 g/kg and DE of 10.7 at the onset of halite\nprecipitation. Both EQL-EVP and PHREEQC rely on the Pitzer dataset (Pitzer, 1973)\nto calculate the molal concentration (mol/kg H O) of dissolved species (Mg² , Ca² ,\nNa , K , SO ² , Cl , alkalinity) and the moles of salts precipitated at each\nevaporation step. Both EQL-EVP and PHRE₂EQC must provide similar sali⁺nity an⁺d\nD⁺E at⁺ each₄ e⁻vap⁻oration stage. Importantly, the simulated seawater evaporation at 25\n°C using these computer programs closely matches observed brine evolution trends\nand mineral sequences in modern systems (Timofeeff et al., 2001; Weldeghebriel et\nal., 2022).\nTherefore, the salinity of 260 g/kg in your model corresponds to a DE of 8.9, which is\nlower than the DE of 10.6 typically associated with the first halite saturation stage\n(McCaffrey et al., 1987; Babel and Schreiber, 2014). I have attached the EQL-EVP\nsimulated run for a reference.\nWe have checked our salinity calculations and concluded that they are fine. We\nexplain why in the following.\nWe checked our model runs simulating the Mediterranean Salinity Crisis, and halite\nstarts to precipitate at a salinity of 272,2 g_salts/kg_solution. (Note that previous\nreferences in exchanges with reviewer #3 of a halite saturation threshold equal to\n260 g_salts/kg_solution are wrong).\nReviewer #3 provided the results of a model run carried out with the geochemical\nsoftware EQL-EVP where seawater is allowed to evaporate and the chemistry of the\nevolving brine is calculated up to halite saturation and beyond. (We suppose that\nconcentrations provided are in mol/kg_H2O.) We have looked into this calculation,\nand performed an analogous calculation with a simple PHREEQC script to compare\nresults. The outcome of this analysis is presented in the excel file named\n“Evaporation_calc.xls”, that we submit with our answer to the reviewers.\nIn the EQL-EVP calculation provided by reviewer #3, halite starts precipitating at a\ndegree of evaporation (DE) equal to 10,71. (We take DE to be the ratio of the\nremaining mass of water molecules to the initial mass of water molecules.) To this\ndegree of evaporation, corresponds a concentration of Total Dissolved Solids (TDS)\nequal to 373,3 g/litre, or 302,2 g/kg (we suppose these units are\ng_salts/kg_solution).\nHowever, in the simulation provided by reviewer #3, we do not understand how the\nTDS value in units of g/litre is obtained from the composition of the brine in\nmol/kg_H2O.\nOur take on how to obtain TDS in g_salts/kg_solution from the composition of the\nbrine in mol/kg_H2O is as follows:\n(i) calculate the sum of the products of the concentrations of ions in mol/kg_H2O\nand their molar masses (in g/mol), obtaining the concentration of total ions in\ng_salts/kg_H2O,\n(ii) consider a brine containing 1 kg of H2O and calculate TDS in\ng_salts/kg_solution as\n𝑔𝑠𝑎𝑙𝑡𝑠\n𝑔𝑠𝑎𝑙𝑡𝑠 𝑘𝑔𝐻2𝑂\n𝑇𝐷𝑆( 𝑘𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 ) =\n1 𝑘𝑔𝐻2𝑂 +\n𝑔𝑠𝑎𝑙𝑡𝑠\n∙10\n−3\n𝑘𝑔𝐻2𝑂\nWe have performed this calculation based on the brine composition calculated in the\nEQL-EVP model run (see Evaporation_calc.xls, sheet “EQL-EVP”, light brown\nshaded columns). The result is a salinity (TDS) equal to 270,9 g_salts/kg_solution at\nhalite saturation, which is coherent with the salinity at halite saturation calculated by\nour Mediterranean Salinity Crisis simulations (272,2 g_salts/kg_solution).\nBecause our Mediterranean Salinity Crisis model is coupled to PHREEQC\ncalculations, rather than EQL-EVP calculations, we ran a simple PHREEQC script to\nsimulate evaporation of seawater at 25°C and 1 bar. As reviewer #3 points out, both\nthese thermodynamic models use the Pitzer thermodynamic database, and should\ntherefore provide comparable results.\nThe results of the PHREEQC model run are presented in the Evaporation_calc.xls\nexcel file (sheet named “PHREEQC”). The DE value at which halite saturation is\nattained in the PHREEQC model run (10,86) is comparable to that obtained in the\nEQL-EVP model run (10,71). The PHREEQC-computed salinity at halite saturation is\nequal to 271,8 g_salts/kg_solution.\nTo further check that the relation between DE and the concentration of dissolved\nions in the PHREEQC and the EQL-EVP model runs is comparable, we plotted ion\nconcentrations vs DE for the two models. These graphs are presented in the “DE vs\nion concentration” sheet of the Evaporation_calc.xls excel file.\nThis analysis shows that the evolution of ion concentrations with DE is very similar\nfor the two model runs, with the small exception of dissolved Ca, probably due to\ndifferent choices in the equilibrium conditions of the DIC system.\nWe therefore conclude that:\n(i) the evolution of brine chemistry in our PHREEQC calculations is consistent with\nthat observed in the EQL-EVP calculations provided by reviewer #3. In\nparticular, the DE at halite saturation in our PHREEQC evaporation run (10,86)\nis comparable to that obtained by reviewer #3 in the EQL-EVP evaporation run\n(10,71), and with DE values at halite saturation available in the literature from\ncomputer simulations and natural systems (~10.6 ; McCaffrey et al., 1987;\nAmdouni, 2000; Timoffeeef et al., 2001; Weldeghebriel et al., 2022; Babel and\nSchreiber, 2014).\n(ii) the differences in the salinity at halite saturation calculated by reviewer #3\n(302,2 g_salts/kg_solution) and by our calculations (PHREEQC evaporation\nrun: 271,8 g_salts/kg_solution; Mediterranean model: 272,2\ng_salts/kg_solution) are due to differences in the computation carried out to\nobtain TDS (in g_salts/kg_solution) starting from ion concentrations (in\nmol/kg_H2O).\nFinally, in our PHREEQC evaporation model run, we calculated TDS in\ng_salts/liter_solution multiplying the TDS in g_salts/kg_solution by the density of the\nbrine in g_solution/liter_solution. The figure below shows how the TDS calculated in\nthe three units we are considering (g_salts/kg_H2O, g_salts/kg_solution and\ng_salts/litre_solution) evolves with DE during the evaporation of seawater.\nHalite saturation is attained at TDS values of:\n373,3 g_salts/kg_H2O,\n330,3 g_salts/litre_solution,\n271,8 g_salts/kg_solution.\nWe maintain that our salinity calculations are sound as: (i) they are consistent with\nwhat is known of the threshold DE at halite saturation from other computer\nsimulations and from measurements of natural environments; (ii) we think we\ncalculate salinity correctly in units of g_salts/kg_solution starting from brine\ncomposition in mol/kg_H2O.\nAdditional notes:\n• Line 65: the Messinian evaporite package\n• Line 80-81: … the drill cuttings… binocular microscope, excluding grains…\nCorrections made.\nReferences:\nAmdouni, R., 2000. Chemical study of free brines in the solar salt works of Sfax\nsaline (S.E. Tunisia). In: 8th World Salt Symp. pp. 501–506.\nBabel M. and Schreiber B. C., 2014. Geochemistry of evaporites and evolution of\nseawater. In Treatise on Geochemistry. Elsevier, Oxford, pp. 483–560.\nMcCaffrey, M.A., Lazar, B., Holland, H.D., 1987. The evaporation path of seawater\nand the coprecipitation of Br- and K+ with halite. J. Sediment. Petrol. 57, 928–937.\nPitzer, K.S., 1973. Thermodynamics of electrolytes. I. Theoretical basis and general\nequations. J. Phys. Chem. 77, 268–277Risacher, F., Clement, A., 2001. A computer\nprogram for the simulation of evaporation of natural waters to high concentration.\nComput. Geosci. 27, 191–201.\nTimofeeff, M.N., Lowenstein, T.K., Brennan, S.T., Demicco, R.V., Zimmermann, H.,\nHorita, J., Von Borstel, L.E., 2001. Evaluating seawater chemistry from fluid\ninclusions in halite: examples from modern marine and nonmarine environments.\nGeochim. Cosmochim. Acta 65, 2293–2300.\nWeldeghebriel, M.F., Lowenstein, T.K., García-Veigas, J., Cendón, D.I., 2022.\n[Ca2+] and [SO42-] in Phanerozoic and terminal Proterozoic seawater from fluid\ninclusions in halite: The significance of Ca-SO4 crossover points. Earth Planet. Sci.\nLett. 594, 117712.\nReviewer #3 (Remarks on code availability):\nI reviewed the model equations but did not review the code because I do not have\naccess to Mathematica."}]

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