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the Creative Commons Attribution 4.0 License.
| 04 Sep 2024
A 10 m vertical displacement on the Romanian Black Sea coast during modern history
Abstract. Sea level reconstructions in the Black Sea basin and elsewhere rely on the identification of sea level markers and on the understanding of their post-genetic vertical movements. We present here evidence of a fast, bi-directional vertical displacement on the western Black Sea shore at Mangalia, Romania. We argue that an area situated near the shoreline was submerged 4 meters, subsequently filled with marine silts and sands, then uplifted by 10 m, where it currently stands. Radiocarbon dating of several types of materials from the infill, as well as archaeological evidence, indicate that this displacement occurred during the 18th–19th century. While performing radiocarbon dating, we found that near shore clam shells can show a 14C reservoir age offset of ~900 years probably due to the hard water effect, adding more complications to the already problematic dating of Black Sea coastal sediments. Our findings offer strong evidence of short-term, local tectonic movements that should be considered when past sea levels are calculated, while at the same time serve a warning for urban and marine development planners.
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RC1: 'Comment on egusphere-2024-2385', Anonymous Referee #1, 09 Oct 2024
The manuscript by Drăgușin et al. presents stratigraphy and new radiocarbon ages of charcoal, plant remains, bone, and bivalve shells from an archaeological site in Mangalia, on the Black Sea coast of Romania. The authors interpret that the site underwent marine inundation and sedimentation followed by subsequent uplift of ~10 m to its present position sometime within the past 200-400 years. Although the study may be of regional archaeological significance, the tectonic interpretations of the study are extremely poorly supported and unlikely to be accurate. I recommend rejection of the article.
The evidence presented in the article fails to support the extraordinary claim that the study area underwent ~10 m of tectonic uplift in the past 200-400 years. Such an uplift rate (25-50 mm/yr) would place the study area among the most rapidly deforming regions on Earth, such as the Himalaya. The authors’ evidence falls short of supporting this claim in several major ways:
- No supporting evidence for major tectonic uplift is provided, such as major earthquakes or proximity to an active plate boundary (the study area is southeast of the Carpathians). Such extraordinary postulated active deformation would leave many indicators in the geomorphic and geologic records, of which none are presented. In fact, existing geological studies of the region contradict the authors’ assertion and reveal a relatively tectonically stable setting in which Cenozoic deformation has been minor (e.g., Konerding et al., 2010, Geological Society of London special pub.).
- The authors’ conclusion of ~10 m of tectonic uplift is based on a single small site with no correlation or connection across the broader region.
- The conclusion that the site was below sea level is based entirely on the presence of two bivalve shells in a stratigraphic layer at the site that was deposited after the year 1600 AD. The authors say there is no evidence that the shells were transported to the site by humans or a major storm, but there is no evidence for or against their claim. No sedimentological or stratigraphic evidence of any kind is brought to bear on the assertion that the sediments were deposited in a marine setting.
In addition to these major problems, the manuscript is poorly organized, with paragraphs and sentences that jump abruptly between topics. Many key topics, such as an introduction to the regional tectonics, a sedimentological description of the units, and others, are missing.
Citation: https://doi.org/10.5194/egusphere-2024-2385-RC1 -
AC1: 'Reply on RC1', Virgil Dragusin, 07 Nov 2024
Reply to Anonymous Referee #1
We thank the reviewer for their time and effort spent for reviewing our manuscript, and will address their comments below.
Reviewer comment (RC): Although the study may be of regional archaeological significance, the tectonic interpretations of the study are extremely poorly supported and unlikely to be accurate.
Author response (AR): The present study is not a classical tectonic or structural study. It is a paleogeography study. Its aim is to describe and to present a hypothesis for the existence of marine sediments at 10 m above sea level. The study led to the conclusion that, taking into account the human, tsunami, and tectonic hypotheses, the most probable was the tectonic one, but it was not our aim to describe in detail the tectonic mechanism. To add some context, at the end of section 3. Evolution, we now added: “The uplifted might have taken place along a normal fault, part of the N-S trending fault system that determines the active subsidence of the southern Dobrogea shoreline identified by Visarion (1977).”
Visarion, M., Sandulescu, M., Dragoescu, I., Draghici, M., Cornea, I., and Popescu, M.: Map of recent crustal vertical movements, scale 1: 1000000, Institute of Geology and Geophysics, Bucharest, 1977.
RC: Such an uplift rate (25-50 mm/yr) would place the study area among the most rapidly deforming regions on Earth, such as the Himalaya
AR: We have no information that this uplift, as well as the submergence, were continuous, but probably happened in discrete time intervals. Thus, calculating an average uplift rate is not suitable for the characterization of these vertical movements.
RC: No supporting evidence for major tectonic uplift is provided, such as major earthquakes or proximity to an active plate boundary (the study area is southeast of the Carpathians). Such extraordinary postulated active deformation would leave many indicators in the geomorphic and geologic records, of which none are presented.
AR: As our manuscript provides the first evidence of such an event, there cannot be any evidence, other than the data we present, to support our conclusion. Regarding the proximity to an active plate boundary, one should consider this event as being an intraplate tectonic event and we now provide at the end of the introduction examples of such occurrences. The last paragraph of the introduction now states:
“Here we present a marine sediment deposit situated 10 m above sea level at Mangalia and argue that a fast, localized, high amplitude tectonic movement that took place in the 18th-19th centuries could be responsible for its displacement. Mangalia does not sit in a tectonically active region, such as those close to tectonic plate boundaries. Nevertheless, earthquakes and tectonic displacements can take place at plate interiors, which are in general considered as tectonically stable. Two such intraplate events stand out in modern history. In 1811-1812 a sequence of earthquakes with magnitudes of 7.0-7.5 took place in New Madrid, USA (Johnston and Schweig, 1996; Mueller et al., 2004) while in 1819 an earthquake with a magnitude of 7.5 took place at Rann of Kutch, India (Bilham, 1999; Rajendran and Rajendran, 2001). These two spots are situated in regions of failed rifts (Ervin and McGinnis, 1975; Bhattacharya et al., 2019) similar to our study area in the Black Sea basin, which started rifting during the early Tertiary (Maynard and Erratt, 2020). Failed rifts are formerly active rifts that did not develop fully and did not transform into oceanic basins (Burke, 1977), but can concentrate and release intraplate stress. As the recurrence time of intraplate earthquakes is on the order of hundreds and even thousands of years, their geomorphological evidence might be effaced by erosional or depositional processes (Crone et al., 1997).”
Bhattacharya, F., Chauhan, G., Prasad, A. D., Patel, R.C., and Thakkar, M.G.: Strike-slip faults in an intraplate setting and their significance for landform evolution in the Kachchh peninsula, Western India, Geomorphology, 328, 118-137, doi: 10.1016/j.geomorph.2018.12.006, 2019.
Bilham, R.: Slip parameters for the Rann of Kachchh, India, 16 June 1819, earthquake, quantified from contemporary accounts, in: Coastal Tectonics, edited by Stewart, I. S., and Vita-Finzl, C., Geological Society, London, Special Publications, 146, 295-319, doi: 10.1144/GSL.SP.1999.146.01.18, 1998.
Burke, K.: Aulacogens and Continental Breakup, Annual Review of Earth and Planetary Sciences, 5, 371-396, doi: 10.1146/annurev.ea.05.050177.002103, 1977.
Crone, A. J., Machette, M. N., and Bowman, J. R.: Episodic nature of earthquake activity in stable continental regions revealed by palaeoseismicity studies of Australian and North American quaternary faults, Australian Journal of Earth Sciences, 44(2), 203–214, doi: 10.1080/08120099708728304, 1997.
Ervin, C. P., and McGinnis L. D.: Reelfoot Rift: reactivated precursor to the Mississippi embayment. GSA Bulletin, 86 (9), 1287–1295, doi: 10.1130/0016-7606(1975)86<1287:RRRPTT>2.0.CO;2, 1975.
Johnston, A. C., and Schweig, E. S.: The enigma of the New Madrid earthquakes of 1811–1812, Annual Review of Earth and Planetary Sciences, 24, 339-384, doi: 10.1146/annurev.earth.24.1.339, 1996.
Maynard, J. R., and Erratt, D.: The Black Sea, a Tertiary basin: Observations and insights, Marine and Petroleum Geology, 118, 104462, doi: 10.1016/j.marpetgeo.2020.104462, 2020.
Mueller, K., Hough, S., and Bilham, R.: Analysing the 1811–1812 New Madrid earthquakes with recent instrumentally recorded aftershocks, Nature, 429, 284–288, doi: 10.1038/nature02557, 2004.
Rajendran, C. P., and Rajendran, K.: Characteristics of deformation and past seismicity associated with the 1819 Kutch earthquake, northwestern India, Bulletin of the Seismological Society of America, 91, 3, 407–426, doi: 10.1785/0119990162, 2001.
RC: In fact, existing geological studies of the region contradict the authors’ assertion and reveal a relatively tectonically stable setting in which Cenozoic deformation has been minor (e.g., Konerding et al., 2010, Geological Society of London special pub.).
AR: We are aware of the existing paradigm of tectonic stability and in the Introduction section we stated: “The region of Southern Dobrogea (Romania) is considered tectonically stable, as the geological history of this region has been interpreted not to show any vertical displacements or faulting since before the Miocene (Dinu et al., 2005). […] The perception of long term tectonic stability influences the understanding of sea level indicators or of the dynamic of coastal settlements.”
We now modified this paragraph, and it now states: “The region of Southern Dobrogea (Romania) is part of the Moesian Platform and is considered by some authors to be tectonically stable, as they interpreted the geological history of this region not to show any vertical displacements or faulting since before the Miocene (Dinu et al., 2005; Konerding et al., 2010). Other authors have shown that the shoreline area is experiencing subsidence at rates exceeding 1.5 mm/year (Visarion, 1977). More recent measurements (Munteanu, 2009) indicate that southern Dobrogea has a southeastwardly displacement, which might be concordant with extensional processes in Vrancea (Ioane and Stanciu, 2018). Additionally, in the offshore region of southern Dobrogea, earthquakes were recorded during modern and historical times and some of them even produced relatively small tsunami waves, whose run-ups did not generally surpass 2.5 m (Papadopoulos et al., 2011).”
Konerding, C., Dinu, C., and Wong, H. K.: Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf, in: Sedimentary Basin Tectonics from the Black Sea and Caucasus to the Arabian Platform, edited by Sosson, M., Kaymakci, N., Stephenson, R. A., Bergerat, F. and Starostenko, V., editors. Geological Society, London, Special Publications, 340, 159–180, doi: 10.1144/SP340.9 0305-8719/10/$15.00, 2010.
Ioane, D., Stanciu, I. M.: Extensional tectonics in Vrancea zone (Romania) interpreted on recent seismicity, geophysical and GPS data. 18th International Multidisciplinary Scientific GeoConference Earth & Geosciences SGEM2018 Albena, Conference Proceedings, 18, 1.1, 787-794, Applied and Environmental Geophysics, doi: 10.5593/sgem2018/1.1, 2018.
Munteanu, L.: The kinematics of the tectonic blocks in the Vrancea area using modern high-precision satellite technology (in Romanian), in: Researches on the Romanian earthquakes disaster management, edited by Marmureanu, G., Tehnopress, Iași, 482 – 546, 2009.
RC: The authors’ conclusion of ~10 m of tectonic uplift is based on a single small site with no correlation or connection across the broader region.
AR: The reviewer does not define what “the broader region” means. In the Introduction, we made a connection with the northern Black Sea shore where tectonic is seen as recurrent: “Recently, several studies revealed that the northern Black Sea coast experienced recurrent tectonic activity (Ovsyuchenko et al. 2017; 2018; 2019; 2020). Ovsyuchenko et al. (2018) offer a good example of tectonic effects on ancient settlements: Akra, on the shore of the Kerch Strait, was rapidly submerged when the wall of a strike-slip fault (with a normal fault component and a steep dip) sunk by a few meters.”
RC: The conclusion that the site was below sea level is based entirely on the presence of two bivalve shells in a stratigraphic layer at the site that was deposited after the year 1600 AD.
AR: Our interpretation of the marine origin of the entire deposit of silts and sands is based on the ostracod and foraminifera faunal assemblage, as well as on the mineralogy of these deposits, as presented in the main text and supplementary materials. The two bivalves are discussed in the context of their relevance for radiocarbon dating of the top of the sand layers. In stratigraphy and chronology, sediments containing shells in anatomical connection are considered in situ and not reworked, thus suitable for stratigraphic/chronologic correlation.
We modified the text, in order to outline that the ostracod and foraminifer assemblage was analyzed from the silt layers, thus proving a clearer image for the marine origin not only of the sands but also of the silts. The text now reads: “The silts of Unit 3 contain a rich foraminiferal assemblage of Ammonia beccarii (Linnaeus, 1758) and Elphidium sp., which are common to the Black Sea shallow waters, as also identified south of our site, on the Bulgarian coast (Temelkov et al., 2006 ). Ostracods, although in smaller numbers, are well represented by individuals of the genera Loxoconcha and Leptocythere. Shallow, brackish water species such as Cyprideis cf. littoralis and Amnicythere multituberculata (Livental, 1929) are also present. The ostracod association could represent a “Cyprideis-Loxonconchidae assemblage” specific for low mesohaline and shallow water conditions (Grossi et al., 2015). The molluscs in Unit 2 pertain to the species Abra alba (Wood, 1802) and the shallow water inhabitant Cerastoderma edule (Linnaeus, 1758). The faunal composition of the sediments suggest a marine origin and indicate a shallow, near-shore environment, as also shown by our sedimentological analysis.”
Grossi, F., Gliozzi, E., Anadon, P., Castorina, F. and Voltaggio, M.: Is Cyprideis agrigentina Decima a good paleosalinometer for the Messinian Salinity Crisis? Morphometrical and geochemical analyses from the Eraclea Minoa section (Sicily), Palaeogeography, Palaeoclimatology, Palaeoecology, 419, 75 – 89, doi: 10.1016/j.palaeo.2014.09.024, 2015.
Linnaeus, C.: Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, 10th revised edition, 1, 824, Laurentius Salvius, Holmiae (Stockholm), 1758.
Livental, V. E.: Ostracoda of Akchagilian and Apsheronian beds of the Babazan Section. Izvestiya Azerbajdzahnskogo Politekhnischeskogo Instituta, 1, 1 - 58 (in Russian), 1929.
Temelkov, B. K., Golemansky, V. G., and Todorov, M. T.: Updated check-list of the recent foraminifera from the Bulgarian Black Sea coast, Acta Zoologica Bulgarica, 58, 1, 17-36, 2006.
Wood, W.: Observations on the hinges of British Bivalve shells, Transactions of the Linnean Society of London, 6, 154-176, 1802.
RC: The authors say there is no evidence that the shells were transported to the site by humans or a major storm, but there is no evidence for or against their claim. No sedimentological or stratigraphic evidence of any kind is brought to bear on the assertion that the sediments were deposited in a marine setting.
AR: We are now detailing why we discard the tsunami and human factors. In the Introduction, we modified a phrase that now states:
“Additionally, in the offshore region of southern Dobrogea, earthquakes were recorded during modern and historical times and some of them even produced relatively small tsunami waves, whose run-ups did not generally surpass 2.5 m (Papadopoulos et al., 2011).”
In section 3. Evolution, we added the next paragraph:
“Human activity is not known to deposit such well sorted sediments and well defined lithological units, thus an anthropogenic origin of the deposit in highly unlikely. On the other hand, Black Sea tsunamis were not recorded to have run-ups higher than 2.5 m, thus it is also unlikely that such a recent event could have deposited sediments at 10 m altitude without leaving any trace in the wider region. Around the world, tsunami deposits do not exceed thicknesses of more than several tens of centimeters, although exceptional values of 150 cm were documented (Peters and Jaffe, 2010). The sediment sequence described here, with its identical stratigraphy throughout the site, is much thicker and better sorted. More importantly, tsunami deposits are capped by the fine fraction (Peters and Jaffe, 2010), whereas the deposit described here has the silts at the bottom and the sands at the top. Also, it lacks pebbles and cobbles, which are common offshore this area and would have been transported by such an immense wave.”
Peters, R., and Jaffe, B. E.: Identification of tsunami deposits in the geologic record; developing criteria using recent tsunami deposits, U.S. Geological Survey Open-File Report 2010-1239, 2010.
RC: In addition to these major problems, the manuscript is poorly organized, with paragraphs and sentences that jump abruptly between topics. Many key topics, such as an introduction to the regional tectonics, a sedimentological description of the units, and others, are missing.
AR: The manuscript is formatted in a concise and engaging form and is aimed at a wide and diverse scientific audience, thus had to present a broad range of results that are all important to the conclusions. This resulted in a very dense manuscript containing all the necessary information to support the conclusions. The Supplementary Materials offer the reader further information, especially field images and analytical results.
In the section 2.1. Sediment infill, we added the text: “The silts and clays of Unit 3 indicate deposition in a low energy marine environment such as a lagoon or a semi-enclosed gulf. The barrier of the lagoon/gulf area could have been represented by the loess deposits in TSC13 whose top is at a higher altitude than the top of Unit3. The sands of Unit 2 imply that higher energy water entered the area leading to shore progradation, probably after the barrier was breached.”
With the information we provided above and in reply to the second reviewer, the text now provides more insight into the regional tectonics and sedimentology.
We hope, and we already stated in our manuscript, that our study will bring attention to the issue of earthquakes in this region. For this, we find inspiring the words of Seth Stein and Stéphane Mazzotti who, at the start of the Preface to their edited book titled Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, write: “Earthquakes within continental plates are an embarrassing stepchild of modern earthquake seismology.” The authors end the Preface with the following words: “Continental intraplate earthquakes will remain a challenge to seismologists, earthquake engineers, policy makers, and the public for years to come. Still, our sense is that significant progress toward understanding and addressing this challenge is now being made.”
Stein, S., and Mazzotti, S.: Preface, in: Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, edited by: Seth Stein, Stéphane Mazzotti, v-vi, Geological Society of America, doi: 10.1130/978-0-8137-2425-6.v, 2007.
Citation: https://doi.org/10.5194/egusphere-2024-2385-AC1
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RC2: 'Comment on egusphere-2024-2385', Anonymous Referee #2, 19 Oct 2024
The manuscript EGUSPHERE-2024-2385 deals with the structural and stratigraphic evolution of Mangalia, located in the western Black Sea shore in Romania. The authors claim a fast, localized, high amplitude tectonic movement that took place in the 18th-19th centuries to explain the existence of marine sediment deposit situated 10 m above sea level. Even though the paper addresses a topic within the scope of ESurf Letters, and new data were collected and several analyses performed (i.e., radiocarbon dating, grain size analysis, mineralogy, stable isotope measurements, ground penetrating radar and electrical resistivity tomography) there are not enough evidence in the results to sustain authors’ claim. Therefore, my suggestion is to reject the paper.
In particular, a proper stratigraphic data description and interpretation is missing. The lithological, planetological and sedimentological description is not properly structure and detailed. Furthermore, the interpretation is almost completely missing and the reader is not able to understand why at some point authors talk about marine and loess deposits without any clear data evidence and interpretation of them.
Furthermore, a structural analysis is essential to prove a 10 m displacement occurred only few hundred years ago, that for sure affected the area both in terms of structures (i.e., evidence for folds and faults) and in stratigraphy (i.e., post-depositional and/or syn-depositional deformation). This is completely missing in this paper.
Last, the paper is unfortunately poorly structures. The amount of collected data is huge, but this is not evident from the main manuscript and these data are not really used to sustain authors’ claim. Only in the materials and methods section there is a list of all the data and methodology performed, which are only available in the supplementary material.
Citation: https://doi.org/10.5194/egusphere-2024-2385-RC2 -
AC2: 'Reply on RC2', Virgil Dragusin, 07 Nov 2024
Reply to Anonymous Referee #2
We thank the reviewer for their time and effort spent for reviewing our manuscript, and will address their comments below.
Reviewer comment (RC): The manuscript EGUSPHERE-2024-2385 deals with the structural and stratigraphic evolution of Mangalia […]. […] there are not enough evidence in the results to sustain authors’ claim.
Author response (AR): The present study is not a classical tectonic or structural study. It is a paleogeography study. Its aim is to describe and to present a hypothesis for the existence of marine sediments at 10 m above sea level. The study led to the conclusion that, taking into account the human, tsunami, and tectonic hypotheses, the most probable was the tectonic one, but it was not our aim to describe in detail the tectonic mechanism.
By submitting our manuscript to ESurf and agreeing to an open review, we were hoping to receive from the scientific community feedback on other hypotheses that should have been taken into account. As of now, there is no other working hypothesis than the three mentioned above.
RC: […] there are not enough evidence in the results to sustain authors’ claim. In particular, a proper stratigraphic data description and interpretation is missing. The lithological, planetological and sedimentological description is not properly structure and detailed. Furthermore, the interpretation is almost completely missing
AR: The reviewer does not offer a model or a solution for “a proper stratigraphic data description”. The way in which the manuscript was written, in a Letter format, was aimed at maintaining the concise and engaging writing style required by the journal for this type of paper.
The lithology of two profiles is depicted in the lithological columns in Fig. 2. The reviewer does not define “properly structure and detailed”. As mentioned above, this might be due to the written style.
In the section 2.1. Sediment infill, we added the interpretation of the depositional environment: “The silts and clays of Unit 3 indicate deposition in a low energy marine environment such as a lagoon or a semi-enclosed gulf (Boyd et al., 1992). The barrier of the lagoon/gulf area could have been represented by the loess deposits in TSC13 whose top is at a higher altitude than the top of Unit3. The sands of Unit 2 imply that higher energy water entered the area leading to shore progradation, probably after the barrier was breached.”
Boyd, R., Dalrymple, R., Zaitlin, B. A.: Classification of clastic coastal depositional environments, Sedimentary Geology, 80, 3–4, 139-150, doi: 10.1016/0037-0738(92)90037-R, 1992.
In order to offer the reader better access to the grain size data, we moved the grain size data table for TSC3 to the main text.
We do not have the necessary space to make an extensive paleontological discussion of the marine faunal assemblage, but we modified the related paragraph, which now states: “The silts of Unit 3 contain a rich foraminiferal assemblage of Ammonia beccarii (Linnaeus, 1758) and Elphidium sp., which are common to the Black Sea shallow waters, as also identified south of our site, on the Bulgarian coast (Temelkov et al., 2006). Ostracods, although in smaller numbers, are well represented by individuals of the genera Loxoconcha and Leptocythere. Shallow, brackish water species such as Cyprideis cf. littoralis and Amnicythere multituberculata (Livental, 1929) are also present. The ostracod association could represent a “Cyprideis-Loxonconchidae assemblage” specific for low mesohaline and shallow water conditions (Grossi et al., 2015). The molluscs in Unit 2 pertain to the species Abra alba (Wood, 1802) and to the shallow water inhabitant Cerastoderma edule (Linnaeus, 1758). The faunal composition of the sediments suggest a marine origin and indicate a shallow, near-shore environment, as also shown by our sedimentological analysis.”
RC: the reader is not able to understand why at some point authors talk about marine and loess deposits without any clear data evidence and interpretation of them.
AR: The reviewer’s comment might be due to the adjacent mentioning of the continental loess deposits that are at the base of the marine sequence, thus in section 2.1. Sediment infill, we modified the text to become clearer: “Unit 4 is most probably a continental deposit. It is defined in TSC6 by the presence of a 0.5 m thick loess-like yellowish silt (Unit 4a) followed by 0.4 m of paleosol-like reddish clay (Unit 4b).” We hope this clarifies the matter that the silt mentioned here is not similar to the silt described from the marine deposit. The further description of its mineralogy, which is similar to loess/paleosol deposit in the region, further supports our interpretation and clarifies it to the reader.
RC: Furthermore, a structural analysis is essential to prove a 10 m displacement occurred only few hundred years ago, that for sure affected the area both in terms of structures (i.e., evidence for folds and faults) and in stratigraphy (i.e., post-depositional and/or syn-depositional deformation). This is completely missing in this paper.
AR: As mentioned above, this study does not aim to disentangle the geological processes behind these events. We provide strong evidence that supports the hypothesis of tectonic movements, whose study can be the subject of future work.
RC: Last, the paper is unfortunately poorly structures. The amount of collected data is huge, but this is not evident from the main manuscript and these data are not really used to sustain authors’ claim. Only in the materials and methods section there is a list of all the data and methodology performed, which are only available in the supplementary material.
AR: As we made several additions to the text, we aimed at clarifying most of the reviewer’s concerns. The data and images in the supplementary materials are not crucial for supporting our conclusion, but are readily available for readers. Moreover, where we used descriptions in the text, we also cited specific values, as in the case of mineralogy.
Citation: https://doi.org/10.5194/egusphere-2024-2385-AC2
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AC2: 'Reply on RC2', Virgil Dragusin, 07 Nov 2024
Status: closed
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RC1: 'Comment on egusphere-2024-2385', Anonymous Referee #1, 09 Oct 2024
The manuscript by Drăgușin et al. presents stratigraphy and new radiocarbon ages of charcoal, plant remains, bone, and bivalve shells from an archaeological site in Mangalia, on the Black Sea coast of Romania. The authors interpret that the site underwent marine inundation and sedimentation followed by subsequent uplift of ~10 m to its present position sometime within the past 200-400 years. Although the study may be of regional archaeological significance, the tectonic interpretations of the study are extremely poorly supported and unlikely to be accurate. I recommend rejection of the article.
The evidence presented in the article fails to support the extraordinary claim that the study area underwent ~10 m of tectonic uplift in the past 200-400 years. Such an uplift rate (25-50 mm/yr) would place the study area among the most rapidly deforming regions on Earth, such as the Himalaya. The authors’ evidence falls short of supporting this claim in several major ways:
- No supporting evidence for major tectonic uplift is provided, such as major earthquakes or proximity to an active plate boundary (the study area is southeast of the Carpathians). Such extraordinary postulated active deformation would leave many indicators in the geomorphic and geologic records, of which none are presented. In fact, existing geological studies of the region contradict the authors’ assertion and reveal a relatively tectonically stable setting in which Cenozoic deformation has been minor (e.g., Konerding et al., 2010, Geological Society of London special pub.).
- The authors’ conclusion of ~10 m of tectonic uplift is based on a single small site with no correlation or connection across the broader region.
- The conclusion that the site was below sea level is based entirely on the presence of two bivalve shells in a stratigraphic layer at the site that was deposited after the year 1600 AD. The authors say there is no evidence that the shells were transported to the site by humans or a major storm, but there is no evidence for or against their claim. No sedimentological or stratigraphic evidence of any kind is brought to bear on the assertion that the sediments were deposited in a marine setting.
In addition to these major problems, the manuscript is poorly organized, with paragraphs and sentences that jump abruptly between topics. Many key topics, such as an introduction to the regional tectonics, a sedimentological description of the units, and others, are missing.
Citation: https://doi.org/10.5194/egusphere-2024-2385-RC1 -
AC1: 'Reply on RC1', Virgil Dragusin, 07 Nov 2024
Reply to Anonymous Referee #1
We thank the reviewer for their time and effort spent for reviewing our manuscript, and will address their comments below.
Reviewer comment (RC): Although the study may be of regional archaeological significance, the tectonic interpretations of the study are extremely poorly supported and unlikely to be accurate.
Author response (AR): The present study is not a classical tectonic or structural study. It is a paleogeography study. Its aim is to describe and to present a hypothesis for the existence of marine sediments at 10 m above sea level. The study led to the conclusion that, taking into account the human, tsunami, and tectonic hypotheses, the most probable was the tectonic one, but it was not our aim to describe in detail the tectonic mechanism. To add some context, at the end of section 3. Evolution, we now added: “The uplifted might have taken place along a normal fault, part of the N-S trending fault system that determines the active subsidence of the southern Dobrogea shoreline identified by Visarion (1977).”
Visarion, M., Sandulescu, M., Dragoescu, I., Draghici, M., Cornea, I., and Popescu, M.: Map of recent crustal vertical movements, scale 1: 1000000, Institute of Geology and Geophysics, Bucharest, 1977.
RC: Such an uplift rate (25-50 mm/yr) would place the study area among the most rapidly deforming regions on Earth, such as the Himalaya
AR: We have no information that this uplift, as well as the submergence, were continuous, but probably happened in discrete time intervals. Thus, calculating an average uplift rate is not suitable for the characterization of these vertical movements.
RC: No supporting evidence for major tectonic uplift is provided, such as major earthquakes or proximity to an active plate boundary (the study area is southeast of the Carpathians). Such extraordinary postulated active deformation would leave many indicators in the geomorphic and geologic records, of which none are presented.
AR: As our manuscript provides the first evidence of such an event, there cannot be any evidence, other than the data we present, to support our conclusion. Regarding the proximity to an active plate boundary, one should consider this event as being an intraplate tectonic event and we now provide at the end of the introduction examples of such occurrences. The last paragraph of the introduction now states:
“Here we present a marine sediment deposit situated 10 m above sea level at Mangalia and argue that a fast, localized, high amplitude tectonic movement that took place in the 18th-19th centuries could be responsible for its displacement. Mangalia does not sit in a tectonically active region, such as those close to tectonic plate boundaries. Nevertheless, earthquakes and tectonic displacements can take place at plate interiors, which are in general considered as tectonically stable. Two such intraplate events stand out in modern history. In 1811-1812 a sequence of earthquakes with magnitudes of 7.0-7.5 took place in New Madrid, USA (Johnston and Schweig, 1996; Mueller et al., 2004) while in 1819 an earthquake with a magnitude of 7.5 took place at Rann of Kutch, India (Bilham, 1999; Rajendran and Rajendran, 2001). These two spots are situated in regions of failed rifts (Ervin and McGinnis, 1975; Bhattacharya et al., 2019) similar to our study area in the Black Sea basin, which started rifting during the early Tertiary (Maynard and Erratt, 2020). Failed rifts are formerly active rifts that did not develop fully and did not transform into oceanic basins (Burke, 1977), but can concentrate and release intraplate stress. As the recurrence time of intraplate earthquakes is on the order of hundreds and even thousands of years, their geomorphological evidence might be effaced by erosional or depositional processes (Crone et al., 1997).”
Bhattacharya, F., Chauhan, G., Prasad, A. D., Patel, R.C., and Thakkar, M.G.: Strike-slip faults in an intraplate setting and their significance for landform evolution in the Kachchh peninsula, Western India, Geomorphology, 328, 118-137, doi: 10.1016/j.geomorph.2018.12.006, 2019.
Bilham, R.: Slip parameters for the Rann of Kachchh, India, 16 June 1819, earthquake, quantified from contemporary accounts, in: Coastal Tectonics, edited by Stewart, I. S., and Vita-Finzl, C., Geological Society, London, Special Publications, 146, 295-319, doi: 10.1144/GSL.SP.1999.146.01.18, 1998.
Burke, K.: Aulacogens and Continental Breakup, Annual Review of Earth and Planetary Sciences, 5, 371-396, doi: 10.1146/annurev.ea.05.050177.002103, 1977.
Crone, A. J., Machette, M. N., and Bowman, J. R.: Episodic nature of earthquake activity in stable continental regions revealed by palaeoseismicity studies of Australian and North American quaternary faults, Australian Journal of Earth Sciences, 44(2), 203–214, doi: 10.1080/08120099708728304, 1997.
Ervin, C. P., and McGinnis L. D.: Reelfoot Rift: reactivated precursor to the Mississippi embayment. GSA Bulletin, 86 (9), 1287–1295, doi: 10.1130/0016-7606(1975)86<1287:RRRPTT>2.0.CO;2, 1975.
Johnston, A. C., and Schweig, E. S.: The enigma of the New Madrid earthquakes of 1811–1812, Annual Review of Earth and Planetary Sciences, 24, 339-384, doi: 10.1146/annurev.earth.24.1.339, 1996.
Maynard, J. R., and Erratt, D.: The Black Sea, a Tertiary basin: Observations and insights, Marine and Petroleum Geology, 118, 104462, doi: 10.1016/j.marpetgeo.2020.104462, 2020.
Mueller, K., Hough, S., and Bilham, R.: Analysing the 1811–1812 New Madrid earthquakes with recent instrumentally recorded aftershocks, Nature, 429, 284–288, doi: 10.1038/nature02557, 2004.
Rajendran, C. P., and Rajendran, K.: Characteristics of deformation and past seismicity associated with the 1819 Kutch earthquake, northwestern India, Bulletin of the Seismological Society of America, 91, 3, 407–426, doi: 10.1785/0119990162, 2001.
RC: In fact, existing geological studies of the region contradict the authors’ assertion and reveal a relatively tectonically stable setting in which Cenozoic deformation has been minor (e.g., Konerding et al., 2010, Geological Society of London special pub.).
AR: We are aware of the existing paradigm of tectonic stability and in the Introduction section we stated: “The region of Southern Dobrogea (Romania) is considered tectonically stable, as the geological history of this region has been interpreted not to show any vertical displacements or faulting since before the Miocene (Dinu et al., 2005). […] The perception of long term tectonic stability influences the understanding of sea level indicators or of the dynamic of coastal settlements.”
We now modified this paragraph, and it now states: “The region of Southern Dobrogea (Romania) is part of the Moesian Platform and is considered by some authors to be tectonically stable, as they interpreted the geological history of this region not to show any vertical displacements or faulting since before the Miocene (Dinu et al., 2005; Konerding et al., 2010). Other authors have shown that the shoreline area is experiencing subsidence at rates exceeding 1.5 mm/year (Visarion, 1977). More recent measurements (Munteanu, 2009) indicate that southern Dobrogea has a southeastwardly displacement, which might be concordant with extensional processes in Vrancea (Ioane and Stanciu, 2018). Additionally, in the offshore region of southern Dobrogea, earthquakes were recorded during modern and historical times and some of them even produced relatively small tsunami waves, whose run-ups did not generally surpass 2.5 m (Papadopoulos et al., 2011).”
Konerding, C., Dinu, C., and Wong, H. K.: Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf, in: Sedimentary Basin Tectonics from the Black Sea and Caucasus to the Arabian Platform, edited by Sosson, M., Kaymakci, N., Stephenson, R. A., Bergerat, F. and Starostenko, V., editors. Geological Society, London, Special Publications, 340, 159–180, doi: 10.1144/SP340.9 0305-8719/10/$15.00, 2010.
Ioane, D., Stanciu, I. M.: Extensional tectonics in Vrancea zone (Romania) interpreted on recent seismicity, geophysical and GPS data. 18th International Multidisciplinary Scientific GeoConference Earth & Geosciences SGEM2018 Albena, Conference Proceedings, 18, 1.1, 787-794, Applied and Environmental Geophysics, doi: 10.5593/sgem2018/1.1, 2018.
Munteanu, L.: The kinematics of the tectonic blocks in the Vrancea area using modern high-precision satellite technology (in Romanian), in: Researches on the Romanian earthquakes disaster management, edited by Marmureanu, G., Tehnopress, Iași, 482 – 546, 2009.
RC: The authors’ conclusion of ~10 m of tectonic uplift is based on a single small site with no correlation or connection across the broader region.
AR: The reviewer does not define what “the broader region” means. In the Introduction, we made a connection with the northern Black Sea shore where tectonic is seen as recurrent: “Recently, several studies revealed that the northern Black Sea coast experienced recurrent tectonic activity (Ovsyuchenko et al. 2017; 2018; 2019; 2020). Ovsyuchenko et al. (2018) offer a good example of tectonic effects on ancient settlements: Akra, on the shore of the Kerch Strait, was rapidly submerged when the wall of a strike-slip fault (with a normal fault component and a steep dip) sunk by a few meters.”
RC: The conclusion that the site was below sea level is based entirely on the presence of two bivalve shells in a stratigraphic layer at the site that was deposited after the year 1600 AD.
AR: Our interpretation of the marine origin of the entire deposit of silts and sands is based on the ostracod and foraminifera faunal assemblage, as well as on the mineralogy of these deposits, as presented in the main text and supplementary materials. The two bivalves are discussed in the context of their relevance for radiocarbon dating of the top of the sand layers. In stratigraphy and chronology, sediments containing shells in anatomical connection are considered in situ and not reworked, thus suitable for stratigraphic/chronologic correlation.
We modified the text, in order to outline that the ostracod and foraminifer assemblage was analyzed from the silt layers, thus proving a clearer image for the marine origin not only of the sands but also of the silts. The text now reads: “The silts of Unit 3 contain a rich foraminiferal assemblage of Ammonia beccarii (Linnaeus, 1758) and Elphidium sp., which are common to the Black Sea shallow waters, as also identified south of our site, on the Bulgarian coast (Temelkov et al., 2006 ). Ostracods, although in smaller numbers, are well represented by individuals of the genera Loxoconcha and Leptocythere. Shallow, brackish water species such as Cyprideis cf. littoralis and Amnicythere multituberculata (Livental, 1929) are also present. The ostracod association could represent a “Cyprideis-Loxonconchidae assemblage” specific for low mesohaline and shallow water conditions (Grossi et al., 2015). The molluscs in Unit 2 pertain to the species Abra alba (Wood, 1802) and the shallow water inhabitant Cerastoderma edule (Linnaeus, 1758). The faunal composition of the sediments suggest a marine origin and indicate a shallow, near-shore environment, as also shown by our sedimentological analysis.”
Grossi, F., Gliozzi, E., Anadon, P., Castorina, F. and Voltaggio, M.: Is Cyprideis agrigentina Decima a good paleosalinometer for the Messinian Salinity Crisis? Morphometrical and geochemical analyses from the Eraclea Minoa section (Sicily), Palaeogeography, Palaeoclimatology, Palaeoecology, 419, 75 – 89, doi: 10.1016/j.palaeo.2014.09.024, 2015.
Linnaeus, C.: Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, 10th revised edition, 1, 824, Laurentius Salvius, Holmiae (Stockholm), 1758.
Livental, V. E.: Ostracoda of Akchagilian and Apsheronian beds of the Babazan Section. Izvestiya Azerbajdzahnskogo Politekhnischeskogo Instituta, 1, 1 - 58 (in Russian), 1929.
Temelkov, B. K., Golemansky, V. G., and Todorov, M. T.: Updated check-list of the recent foraminifera from the Bulgarian Black Sea coast, Acta Zoologica Bulgarica, 58, 1, 17-36, 2006.
Wood, W.: Observations on the hinges of British Bivalve shells, Transactions of the Linnean Society of London, 6, 154-176, 1802.
RC: The authors say there is no evidence that the shells were transported to the site by humans or a major storm, but there is no evidence for or against their claim. No sedimentological or stratigraphic evidence of any kind is brought to bear on the assertion that the sediments were deposited in a marine setting.
AR: We are now detailing why we discard the tsunami and human factors. In the Introduction, we modified a phrase that now states:
“Additionally, in the offshore region of southern Dobrogea, earthquakes were recorded during modern and historical times and some of them even produced relatively small tsunami waves, whose run-ups did not generally surpass 2.5 m (Papadopoulos et al., 2011).”
In section 3. Evolution, we added the next paragraph:
“Human activity is not known to deposit such well sorted sediments and well defined lithological units, thus an anthropogenic origin of the deposit in highly unlikely. On the other hand, Black Sea tsunamis were not recorded to have run-ups higher than 2.5 m, thus it is also unlikely that such a recent event could have deposited sediments at 10 m altitude without leaving any trace in the wider region. Around the world, tsunami deposits do not exceed thicknesses of more than several tens of centimeters, although exceptional values of 150 cm were documented (Peters and Jaffe, 2010). The sediment sequence described here, with its identical stratigraphy throughout the site, is much thicker and better sorted. More importantly, tsunami deposits are capped by the fine fraction (Peters and Jaffe, 2010), whereas the deposit described here has the silts at the bottom and the sands at the top. Also, it lacks pebbles and cobbles, which are common offshore this area and would have been transported by such an immense wave.”
Peters, R., and Jaffe, B. E.: Identification of tsunami deposits in the geologic record; developing criteria using recent tsunami deposits, U.S. Geological Survey Open-File Report 2010-1239, 2010.
RC: In addition to these major problems, the manuscript is poorly organized, with paragraphs and sentences that jump abruptly between topics. Many key topics, such as an introduction to the regional tectonics, a sedimentological description of the units, and others, are missing.
AR: The manuscript is formatted in a concise and engaging form and is aimed at a wide and diverse scientific audience, thus had to present a broad range of results that are all important to the conclusions. This resulted in a very dense manuscript containing all the necessary information to support the conclusions. The Supplementary Materials offer the reader further information, especially field images and analytical results.
In the section 2.1. Sediment infill, we added the text: “The silts and clays of Unit 3 indicate deposition in a low energy marine environment such as a lagoon or a semi-enclosed gulf. The barrier of the lagoon/gulf area could have been represented by the loess deposits in TSC13 whose top is at a higher altitude than the top of Unit3. The sands of Unit 2 imply that higher energy water entered the area leading to shore progradation, probably after the barrier was breached.”
With the information we provided above and in reply to the second reviewer, the text now provides more insight into the regional tectonics and sedimentology.
We hope, and we already stated in our manuscript, that our study will bring attention to the issue of earthquakes in this region. For this, we find inspiring the words of Seth Stein and Stéphane Mazzotti who, at the start of the Preface to their edited book titled Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, write: “Earthquakes within continental plates are an embarrassing stepchild of modern earthquake seismology.” The authors end the Preface with the following words: “Continental intraplate earthquakes will remain a challenge to seismologists, earthquake engineers, policy makers, and the public for years to come. Still, our sense is that significant progress toward understanding and addressing this challenge is now being made.”
Stein, S., and Mazzotti, S.: Preface, in: Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, edited by: Seth Stein, Stéphane Mazzotti, v-vi, Geological Society of America, doi: 10.1130/978-0-8137-2425-6.v, 2007.
Citation: https://doi.org/10.5194/egusphere-2024-2385-AC1
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RC2: 'Comment on egusphere-2024-2385', Anonymous Referee #2, 19 Oct 2024
The manuscript EGUSPHERE-2024-2385 deals with the structural and stratigraphic evolution of Mangalia, located in the western Black Sea shore in Romania. The authors claim a fast, localized, high amplitude tectonic movement that took place in the 18th-19th centuries to explain the existence of marine sediment deposit situated 10 m above sea level. Even though the paper addresses a topic within the scope of ESurf Letters, and new data were collected and several analyses performed (i.e., radiocarbon dating, grain size analysis, mineralogy, stable isotope measurements, ground penetrating radar and electrical resistivity tomography) there are not enough evidence in the results to sustain authors’ claim. Therefore, my suggestion is to reject the paper.
In particular, a proper stratigraphic data description and interpretation is missing. The lithological, planetological and sedimentological description is not properly structure and detailed. Furthermore, the interpretation is almost completely missing and the reader is not able to understand why at some point authors talk about marine and loess deposits without any clear data evidence and interpretation of them.
Furthermore, a structural analysis is essential to prove a 10 m displacement occurred only few hundred years ago, that for sure affected the area both in terms of structures (i.e., evidence for folds and faults) and in stratigraphy (i.e., post-depositional and/or syn-depositional deformation). This is completely missing in this paper.
Last, the paper is unfortunately poorly structures. The amount of collected data is huge, but this is not evident from the main manuscript and these data are not really used to sustain authors’ claim. Only in the materials and methods section there is a list of all the data and methodology performed, which are only available in the supplementary material.
Citation: https://doi.org/10.5194/egusphere-2024-2385-RC2 -
AC2: 'Reply on RC2', Virgil Dragusin, 07 Nov 2024
Reply to Anonymous Referee #2
We thank the reviewer for their time and effort spent for reviewing our manuscript, and will address their comments below.
Reviewer comment (RC): The manuscript EGUSPHERE-2024-2385 deals with the structural and stratigraphic evolution of Mangalia […]. […] there are not enough evidence in the results to sustain authors’ claim.
Author response (AR): The present study is not a classical tectonic or structural study. It is a paleogeography study. Its aim is to describe and to present a hypothesis for the existence of marine sediments at 10 m above sea level. The study led to the conclusion that, taking into account the human, tsunami, and tectonic hypotheses, the most probable was the tectonic one, but it was not our aim to describe in detail the tectonic mechanism.
By submitting our manuscript to ESurf and agreeing to an open review, we were hoping to receive from the scientific community feedback on other hypotheses that should have been taken into account. As of now, there is no other working hypothesis than the three mentioned above.
RC: […] there are not enough evidence in the results to sustain authors’ claim. In particular, a proper stratigraphic data description and interpretation is missing. The lithological, planetological and sedimentological description is not properly structure and detailed. Furthermore, the interpretation is almost completely missing
AR: The reviewer does not offer a model or a solution for “a proper stratigraphic data description”. The way in which the manuscript was written, in a Letter format, was aimed at maintaining the concise and engaging writing style required by the journal for this type of paper.
The lithology of two profiles is depicted in the lithological columns in Fig. 2. The reviewer does not define “properly structure and detailed”. As mentioned above, this might be due to the written style.
In the section 2.1. Sediment infill, we added the interpretation of the depositional environment: “The silts and clays of Unit 3 indicate deposition in a low energy marine environment such as a lagoon or a semi-enclosed gulf (Boyd et al., 1992). The barrier of the lagoon/gulf area could have been represented by the loess deposits in TSC13 whose top is at a higher altitude than the top of Unit3. The sands of Unit 2 imply that higher energy water entered the area leading to shore progradation, probably after the barrier was breached.”
Boyd, R., Dalrymple, R., Zaitlin, B. A.: Classification of clastic coastal depositional environments, Sedimentary Geology, 80, 3–4, 139-150, doi: 10.1016/0037-0738(92)90037-R, 1992.
In order to offer the reader better access to the grain size data, we moved the grain size data table for TSC3 to the main text.
We do not have the necessary space to make an extensive paleontological discussion of the marine faunal assemblage, but we modified the related paragraph, which now states: “The silts of Unit 3 contain a rich foraminiferal assemblage of Ammonia beccarii (Linnaeus, 1758) and Elphidium sp., which are common to the Black Sea shallow waters, as also identified south of our site, on the Bulgarian coast (Temelkov et al., 2006). Ostracods, although in smaller numbers, are well represented by individuals of the genera Loxoconcha and Leptocythere. Shallow, brackish water species such as Cyprideis cf. littoralis and Amnicythere multituberculata (Livental, 1929) are also present. The ostracod association could represent a “Cyprideis-Loxonconchidae assemblage” specific for low mesohaline and shallow water conditions (Grossi et al., 2015). The molluscs in Unit 2 pertain to the species Abra alba (Wood, 1802) and to the shallow water inhabitant Cerastoderma edule (Linnaeus, 1758). The faunal composition of the sediments suggest a marine origin and indicate a shallow, near-shore environment, as also shown by our sedimentological analysis.”
RC: the reader is not able to understand why at some point authors talk about marine and loess deposits without any clear data evidence and interpretation of them.
AR: The reviewer’s comment might be due to the adjacent mentioning of the continental loess deposits that are at the base of the marine sequence, thus in section 2.1. Sediment infill, we modified the text to become clearer: “Unit 4 is most probably a continental deposit. It is defined in TSC6 by the presence of a 0.5 m thick loess-like yellowish silt (Unit 4a) followed by 0.4 m of paleosol-like reddish clay (Unit 4b).” We hope this clarifies the matter that the silt mentioned here is not similar to the silt described from the marine deposit. The further description of its mineralogy, which is similar to loess/paleosol deposit in the region, further supports our interpretation and clarifies it to the reader.
RC: Furthermore, a structural analysis is essential to prove a 10 m displacement occurred only few hundred years ago, that for sure affected the area both in terms of structures (i.e., evidence for folds and faults) and in stratigraphy (i.e., post-depositional and/or syn-depositional deformation). This is completely missing in this paper.
AR: As mentioned above, this study does not aim to disentangle the geological processes behind these events. We provide strong evidence that supports the hypothesis of tectonic movements, whose study can be the subject of future work.
RC: Last, the paper is unfortunately poorly structures. The amount of collected data is huge, but this is not evident from the main manuscript and these data are not really used to sustain authors’ claim. Only in the materials and methods section there is a list of all the data and methodology performed, which are only available in the supplementary material.
AR: As we made several additions to the text, we aimed at clarifying most of the reviewer’s concerns. The data and images in the supplementary materials are not crucial for supporting our conclusion, but are readily available for readers. Moreover, where we used descriptions in the text, we also cited specific values, as in the case of mineralogy.
Citation: https://doi.org/10.5194/egusphere-2024-2385-AC2
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AC2: 'Reply on RC2', Virgil Dragusin, 07 Nov 2024
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