MIS 5.5 Highstand and Future Sea Level Flooding at 2100 and 2300 in Tectonically Stable Areas of Central Mediterranean Sea: Sardinia and the Pontina Plain (Southern Latium), Italy

Abstract

Areas of the Mediterranean Sea are dynamic habitats in which human activities have been conducted for centuries and which feature micro-tidal environments with about 0.40 m of range. For this reason, human settlements are still concentrated along a narrow coastline strip, where any change in the sea level and coastal dynamics may impact anthropic activities. We analyzed light detection and ranging (LiDAR) and Copernicus Earth observation data. The aim of this research is to provide estimates and detailed maps (in three coastal plain of Sardinia (Italy) and in the Pontina Plain (southern Latium, Italy) of: (i) the past marine transgression occurred during MIS 5.5 highstand 119 kyrss BP; (ii) the coastline regression occurred during the last glacial maximum MIS 2 (21.5 krs cal BP); and (iii) the potential marine submersion for 2100 and 2300. The objective of this multidisciplinary study is to provide maps of sea level rise future scenarios using the IPCC RCP 8.5 2019 projections and glacio-hydro-isostatic movements for the above selected coastal zones (considered tectonically stable), which are the locations of touristic resorts, railways and heritage sites. We estimated a potential loss of land for the above areas of between about 146 km² (IPCC 2019-RCP8.5 scenario) and 637 km² along a coastline length of about 268 km.

1. Introduction

Sea level rise is one of the most important consequences of climate change [1] which strongly affects global communities and human settlements along the coast. The main responsible factors are global warming, which is driving the melting of ice and the thermal expansion of the oceans [1] and finally the geological vertical movements of the land along the coastal zones, which may accelerate (or decrease) the submersion of low-elevated coasts [2,3,4,5]. In this scenario, the coastal lowlands will be more prone to marine flooding during extreme sea events, threatening many highly populated areas of the Mediterranean coasts [3,4].

Future warming on the Mediterranean Sea could provide a higher sea level rise than published in the 2019 IPCC report [1]. Much research was recently published on the effects of sea level rise on the Mediterranean coasts [2,3,4,5]. For much of the Italian coast, adding local factors (tectonic and isostasy) to the global climate signal projections for future sea level rise results in an increasing vulnerability of the littoral zones and coastal plains. In the above studies, in order to provide the relative sea level rise projections for the preparation of flooding scenarios maps at 2100, the authors applied a multidisciplinary approach that includes sea level rise estimates as obtained by the IPCC (www.ipcc.ch—accessed on 20 June 2021), Rahmstorf 2007, detailed geomorphological information and land topography; coastal inundation projection maps were produced through the use of available high-resolution lidar data.

In this research we use the methodologies tested in previous works [2] where the relative sea level is the sum of eustasy, isostasy and tectonic movement. We used the IPCC 2019 AR 8.5 projections for the sea level rise expected at 2100 [6]. The objective of our work is to highlight at what altitude can the fossil signs of maximum marine ingression (forms and aged deposits) now be observed, which occurred during MIS 5.5 compared with the sea level projections for 2100 and 2300 (IPCC 2019) in the four coastal sites under study (Figure 1). We also take in consideration the MIS 5.5 maximum highstand comparing the observed data in field with the ice model [7] projections for Sardinia and the Pontina Plain. In addition, on the basis of the bathymetry and the available data and models, we positioned the 21.5 ka cal BP coastline reached during the LGM (last glacial maximum) on the continental shelf in front of the Plains. Furthermore, on the basis of the IPCC projections we have calculated (adding the geological vertical movements) the altitude at which the sea will be reached (according to the AR 8.5 scenario) at 2100 and 2300. We have chosen these sites because they are located in Sardinia, one of the most stable areas of the Mediterranean and in Southern Lazio, a similar stable coastal area [8] but with high rates of subsidence partly due to tectonics and partly due to the drying of the peat drained by the reclamation of the 1930s and 1940s.

2. Materials and Methods

An innovative methodology made for the first time for drafting the sea flooding maps, was to “look” not only to the future (2100, 2300) but also to the past. We indagated in the studied areas in order to understand what the effects of the maximum transgression of the last highstand were (MIS 5.5 that occurred between 126 and 119 ka BP); this choice is due to very specific reasons. There is a generalized belief that since the maximum insolation on Earth ceased about 6000 years ago (climatic optimum) we are “walking” towards an ice age. The durations of the last warm periods (MIS 5.5, MIS 9, MIS 11, etc.), have always been around 10–11 thousand years [13]. However, if it is true that at 119 ka BP there was a maximum of insolation of about 550 W/m², calculated for 65° in June, it is also true that during the climatic optimum (about 6, ka BP) l insolation was of a lesser entity: 520 W\m². Subsequently: (1) the insolation has been decreasing, and (2) the climate does not seem to have cooled. In a quoted article [14], entitled “An exceptional long Interglacial period?”, the authors show that due to a particular movement of eccentricity the “warm-hot” climatic period, instead of taking us towards an ice age (as happened in the last 4 highstand on earth), should last at least another 50 ka (Figure 1): “But more recent studies point toward a different future: a long interglacial that may last another 50,000 years”.

At 119 ka BP, the insolation was higher than today, the sea was about 8 m higher [7], but the CO₂ content in the atmosphere never exceeded 295 ppm. Meanwhile, today, with a lower insolation, but a much higher content of CO₂, 415 ppm, we can also observe a sea level 8 m lower than 119 ka BP and a rise that exceeds the acceleration of 3 mm/year. The IPCC 2019 projections estimate (using the RCP 8.5 scenario) a rise higher than one meter for the next 79 years (2100). The CO₂ increase would also further delay the triggering of a cold period.

This research is based on the method described in [3], and [15], to ensure homogeneity and enable a comparison with previous results, extending the study toward other central Mediterranean coastal plains, besides Italy. The sea level change along the Mediterranean coast is the sum of eustatic, glacio-hydro-isostatic and tectonic factors. The first is time dependent while the latter two also vary with location, and they consist to sum the different components of sea level rise in the following main steps: (a) the IPCC AR5 projections (RCP-8.5 upper limits scenarios reported in the IPCC 2019); (b) the long-term land vertical movements from geological data [7,8,16]; (c) the glacio-hydro-isostatic movement (GIA) [2]; (d) by combining eustatic, isostatic and tectonic data projected up to 2100, and 2300, we provided the expected sea levels at 2100 and 2300 for the investigated coastal areas and the expected inland extent of related marine flooding.

We underline our choice to use the global IPCC AR5 data for the projections to 2100, even if some works have been published for the Mediterranean that take into account SST, evapotranspiration and salinity [17] and publish projections a few decimeters lower than the global data we used. However, SSTs in the Mediterranean show very different values across the whole basin and the available data [17] relate to the entire Mediterranean. We therefore decided to use IPCC AR5 which contains a literature of thousands of publications. After submitting the manuscript, the IPCC AR6 report was published where regional projections are also taken into consideration. In particular, for the Mediterranean there is the Cagliari data which are 0.78 ± 0.3 m [18]. This number (also considering the error) is quite similar to that expected for Cagliari (

Table 1. a: site; b: (digital terrain model, DTM) resolution and accuracy; c: epoch of DTM; d–g: IPCC projection AR5-RCP8; h: glacial isostatic adjustment (GIA) rate [2]; i: vertical tectonic rate mm/year; l–o: relative sea level rise for 2100, 2300, IPCC AR5-RCP 8.5 scenarios.

N	a Site	b DTM Resolution ± Vertical Accuracy (m)		c Map Year	d Projection 2100 IPCC 2019 AR 8.5, Upper Likely Range mm	E Projection 2100 IPCC 2019 AR 8.5, Median Range mm	F Projection 2300 IPCC 2019 AR 8.5, Upper Likely Range mm	g Projection 2300 IPCC 2019 AR 8.5, Median Range mm	h GIA mm/y	i Vertical Movements mm/y	l Total mm IPCC 2100 Upper Likely Range	m Total mm IPCC 2100 Median Range	n Total mm IPCC 2300 Upper Likely Range	o Total mm IPCC 2300 Median Range
1	Cagliari	LIDAR 5 × 5 (±0.2)	EMODNET 85 × 85 (±1.0)	2008	1100	840	5200	3700	0.57	-	1152	893	5252	3752
2	Oristano	LIDAR 1 × 1 (±0.2)	EMODNET 85 × 85 (±1.0)	2008	1100	840	5200	3700	0.57	-	1152	893	5252	3752
3	Olbia	LIDAR 1 × 1 (±0.2)	EMODNET 85 × 85 (±1.0)	2008	1100	840	5200	3700	0.62	-	1157	897	5256	3756
4	Pontina	25 × 25 ±1	EMODNET 85 × 85 (±1.0)	2010	1100	840	5200	3700	0.44	−0.1	1149	889	5249	3749

: IPCC 2100 median range 0.89).

The choice of the study areas was decided by several different factors: (i) tectonics (stable areas only); (ii) exposure; (iii) max fetch; (iv) sedimentological material; (v) wave energy flux; (vi) bedrock; and (vii) geomorphological features. Our maps show the maximum sea level height expected for 2100 and 2300 for the above referenced climatic projections and the corresponding flooded area. Sea level rose rapidly between 21 and about 4 ka years ago. Over the last 100 years the sea level has risen globally by 18 cm, and by 14 cm (on average 1.4 mm /year) in the Mediterranean Sea. The little differences between the global and a lower rise of the Mediterranean, considered a closed basin, are due to evapotranspiration and to an increasingly limited flow of rivers.

We remark that our analysis does not take into account hydrodynamics models, and the contribution of sediment flow from rivers, coastal erosion and all the possible anthropic defences that may change the estimated extension of the flooded areas proposed in this study.

2.1. Field Work

2.1.1. Pontina Plain

For the Pontina Plain, some papers published in the 1930s and 1950s were carefully considered, and the altitude of the fossil deposit containing Persistrombus latus or Senegalese fauna was also revised in field (with respect to reviews [8] and [19]), lowering it from +10 to +5.3 m. In addition, some new lagoonal fossil layers containing Cerastoderma edulis have been found in the field. These lagoonal levels outcropping in the eastern portion of the Pontina Plain, between +4 and −2 m, were outcrops only if incised by the 1930s’ “Bonifica” channels whose cleaning revealed the fossil levels. We checked what was published in [20], in 3 outcrops on the field. The altitudes were re-measured on Google Earth maps (which presents a very precise altimetry in the flat areas: see also Materials and Methods [15]).

2.1.2. Sardinia

As part of this work, the stratigraphic and chronological point of view of deposits containing Persististrombus latus, Patella ferruginea, etc., have been reviewed, and detected in previous studies [21,22,23] at altitudes between +4.0 m and +8.0 m a.s.l. The levels containing lagoon fossils with Cerastoderma edulis were found in the plains of Cagliari (Elmas-Assemini plane) and of Oristano (north of the Cabras lagoon). The respective elevations were determined through measurements with a DGPS antennas: Trimble R8s model. The planimetric and altimetric surveys were carried out operating in static mode. The raw DGPS data have been processed using the Trimble Business Center software (V. 2.50). The processed data have an accuracy of ±1 ÷ 2 cm.

2.2. Digital Terrain Models

To map the sea level rise scenarios, a data set of high-resolution topography based on light detection and ranging (LiDAR) observations produced by different agencies from 2008 to 2019 was used (Table 1). The extracted digital terrain models (DTM) were obtained at variable spatial resolutions depending on the data set and in the range at about 20 cm of mean vertical resolution [24]. The details of the characteristics of the DTM are described in Table 1 and in the maps available in the online supporting material.

To link the land surface to the seafloor along the coasts and represent MIS 2 sea level, the bathymetric data were obtained from the European Marine Observation and Data Network (EMODnet, http://portal.emodnet-bathymetry.eu/ [25]—(accessed on 16 December 2020). The DTM has been generated from selected bathymetric survey data sets, composed by DTMs and satellite derive bathymetry (SDB) data products, while gaps of no data coverage were completed by integrating the GEBCO digital bathymetry. The spatial resolution of this DTM was 85 × 85 m while the vertical resolution was 1 m. However, the use of different resolution DTMs for the emerged and submerged part did not lead to differences in the elaboration of flood models; this is mainly due to the fact that the high-resolution DTM was available for the entire emerged sector, up to the current coastline.

Marine and terrestrial topographic data were co-registered and georeferenced into the same UTM-WGS84 (Zones 32 and 33) reference frame, and the shoreline position was determined relative to the epoch of the surveys for each area. The details regarding the link to the website from which we downloaded or requested the digital data are also described in the online supporting material.

The DTMs were mapped and analyzed by Global Mapper Software^® (www.globalmapper.com [26]—accessed on 10 June 2020) (Version 21, Hallowell, ME, USA) to create 3D high-resolution maps of the investigated areas, on which the position of the present-day coastline and its potential position in 2100 as a result of relative sea level rise are shown by contour lines. The DTMs with contour lines and submerged surfaces were represented using the color-shaded option and exported as georeferenced images through the GIS composer (Table 1 and

Table 2. Modelled and observed altitude during LGM (21.5 kyrs cal BP) and MIS 5.5 (119 Kyrs BP).

Sites	Altitude LGM Model, [2]. m	Altitude LGM Model, [27]. m	Observed LGM Altitude. m	Altitude MIS 5.5 Model, [7]. m		Observed MIS 5.5 Altitude. m This Paper
Cagliari	−129.2	−122	−125	+8.4		+4.8
Oristano	−126.8	−124	−130	+8.6		+6.5
Olbia	−127.5	−122	−126	+8.5		+4.9
Pontina	−117,8	−117	-----	+8.2		+7.96, −12

).

3. Results

3.1. Geomorphology and Altitude of MIS 5.5 in Sardinia Sites

3.1.1. Cagliari Coastal Plains

The coastal sector of Campidano graben of Cagliari (Figure 2), is articulated in two coastal plains with different geodynamic evolutions: the eastern plain is set on a graben structure active until the upper Miocene, while the western plain is set on a graben active until the lower-middle Pleistocene.

The stratigraphy section of Sant’Elia Cape represents a closure towards the sea of the Cagliari hills horst structure, where A. Lamarmora, in 1856, surveyed for the first time the last interglacial stratigraphic succession. Later, this stratigraphic section was indicated as the locus typicus of the “Tyrrhenian plane” [21], an attribution confirmed by U/Th isotopic dating corals [8,22,23]. The maximum sea ingression altitude of the sea during MIS 5.5 formed two deep bays separated by the Cagliari hills promontory and the paleo island of Sant Elia and Sella del Diavolo reliefs. Afterwards, sedimentation from two tributary rivers (Rio Mannu-Cixerri to the west and Riu Saliu and Cungiaus to the east) led to the emersion of the Sa Illetta barrier islands forming the actual Santa Gilla Lagoon (western sector) and to the formation of the Is Arenas littoral spit closing the Molentargius paleo-lagoon (eastern sector). Evidence of the shoreline of MIS 5.5 maximum ingression was reported by Segre [28], in the Elmas coastal plain, behind the innermost part of the Santa Gilla lagoon, represented by a coastal gravel deposit in a fossiliferous sandy matrix of Cerastoderma glaucum.

During observations for the study of the Holocene sea rise, the site described by Segre [28] and Orrù [29] was detected; in the present work, the inner margin of the MIS 5 transgression was recognized in both Santa Gilla and Molentargius coastal plains, represented by a bank erosion incised in Middle Pleistocene alluvial deposits (MIS 6) on which they rest in onlap.

The MIS 5.5 marine littoral depositional terraces are composed by predominantly quartzose gravels at the elevation of +3.8 m in the western sector (Elmas-Assemini plain), (Figure 3 and Figure 4, Section 5) and of fossiliferous sandstones with inclined lamination at the maximum elevation of 4.80 for the eastern sector (Pirri-Monserrato plain) (Figure 4, Section 4). The lower elevation of the Elmas marine terrace could be related to the greater incidence of the western plain subsidence, as hosted in a structure with recent tectonic activity.

The paleo barrier island of Sa Illetta has a maximum elevation of MIS 5 littoral deposits at +4.30 m in correspondence with a bioconstructed reef with corals (Cladocora coespitosa) and red algal associations of Litophyllum [30] (Figure 4, Section 2 and Figure 5). Comparative U/Th and aminostratigraphic analyses on corals have attributed an age of −149 10 kyrs BP [23]); a similar chronostratigraphic placement based on ESR analyses on Arca noae [30] presented littoral deposits of the Is Arenas paleo-beach that reached a maximum altitude of +4.5 m (Figure 4, Section 3).

During the excavation of the access channel to the container port of Cagliari, a tanatocenosis of Persististrombus latus −10 m was sampled (Figure 4, Section 1). We describe in this paper a new section of the same level at −16.50 m, analysed during a dive survey in the deepening area of the outer channel.

Geophysical surveys (seismic profile, [29]) allowed the following of the paleo-valleys (mostly buried) carved during the MIS 2, these cross the whole platform until they reach the shelf break where they contribute to the construction of low stationing terraces (LGM paleo-sea levels); they are sometimes involved in mass gravitational movements [31].

A Punic paleo-sea level (2500 cal BP) buried at −2 m is preserved in the innermost area of the Santa Gilla lagoon [32,33,34].

Figure 5. (a) A view of the section of Sa Illetta marine interglacial fossiliferous sediments [8]; tidal plain sandy littoral deposit irregularly cemented with burrows bioturbation structures and plane-parallel lamination (Sections 1 and 2); (b) Section 2 bottom 2a, particular of the icnofacias due to the activity of Polychaetes; (c) Section 2 top 2d, bioclastic sandstone well cemented containing Lithtaminium and bioconstructions in Cladocora coespitosa −149 ± 10 kyrs BP [23]; (d) A view of the section of the last interglacial interdidal fossiliferous sediments of Is Arenas (Section 3, 2a) bottom level with burrows bioturbation structures containing Persististrombus latus [23]; (e) At the top conglomeratic heterometric and polygenic fossiliferous level (Section 3, 2b), at the bottom weakly cemented sandstones and sands with channeled flow structures in a tidal plane environment (Section 3, 2a); (f) sub-horizontal laminated sandstone enclosing a forest level containing Arca Noae Limne, Arca tetragona tetragona Poli, Glycimeris sp. 111 kyrs BP [35].

Figure 5. (a) A view of the section of Sa Illetta marine interglacial fossiliferous sediments [8]; tidal plain sandy littoral deposit irregularly cemented with burrows bioturbation structures and plane-parallel lamination (Sections 1 and 2); (b) Section 2 bottom 2a, particular of the icnofacias due to the activity of Polychaetes; (c) Section 2 top 2d, bioclastic sandstone well cemented containing Lithtaminium and bioconstructions in Cladocora coespitosa −149 ± 10 kyrs BP [23]; (d) A view of the section of the last interglacial interdidal fossiliferous sediments of Is Arenas (Section 3, 2a) bottom level with burrows bioturbation structures containing Persististrombus latus [23]; (e) At the top conglomeratic heterometric and polygenic fossiliferous level (Section 3, 2b), at the bottom weakly cemented sandstones and sands with channeled flow structures in a tidal plane environment (Section 3, 2a); (f) sub-horizontal laminated sandstone enclosing a forest level containing Arca Noae Limne, Arca tetragona tetragona Poli, Glycimeris sp. 111 kyrs BP [35].

3.1.2. Oristano North Coastal Plains and Sinis Peninsula

An extensive wave-cut platform, incised in middle Miocene marlstone and upper Pliocene basalts, was detected in the eastern coast of Capo San Marco (southern part of the Sinis peninsula – Figure 6). The littoral deposit of MIS 5.5 is set on the platform, represented by a heterometric conlomerate with large basaltic blocks at the base, followed by micronglomerates and plane-parallel lamination fossiliferous sandstone with Persististrombus latus, Patella ferruginea, Mytilus galloprovincialis up to the altitude of +4.5 m [36]. The inner margin, generally hidden by MIS 5.5 littoral sediments and the overlying eolianites (MIS 4-2), is observable in the southernmost part of Capo San Marco at +6.5 m.

The MIS 5.5 marine-littoral deposits sequence is most preserved along the western coast of San Giovanni di Sinis [37].

The sequence (Figure 7, Section 1) is interrupted by erosion surfaces incised into the continental aeolian and colluvial deposits containing vertebrate and gastropod pulmonate remains from MIS 6. The transgressive succession is characterized from the base by polygenic erometric conglomerates, passing to fossiliferous quartz microconglomerates and fossiliferous sandstones (Arca noae, Mytilus galloprovincialis, Spondylus gaederopus, Patella ferruginea, Thai haemastoma, Dentalium sp.) with plane-parallel and inclined lamination (Figure 8).

The continuity of the sequence is interrupted by numerous erosion surfaces that sometimes retain levels or pockets of sandy paleosols with a strongly oxidized matrix. At the top, the MIS 5.5 sequence is sealed by continental deposits with sandy colluvium and aeolian sandstone cross-lamination (MIS 4-2).

The paleo-littoral bar of the MIS 5.5 that closes the Cabras lagoon is made up of flat-parallel and inclined sandstones that include foreset structures. The summit depositional surface of the MIS 5 deposits is located at altitudes between +4.30 and +4.50 m (Figure 8, Section 2). The significant sections are revealed on the walls of irrigation canals and of ancient Punic-Roman quarries between Cabras and San Salvatore. The inner margin of the maximum marine ingression of MIS 5.5 is masked by slope deposits and river sediments from the Holocene.

3.1.3. Olbia Gulf and Coastal Plains

MIS 5.5 deposits in the Olbia Gulf were first described by Segre [38], which indicated extensive outcrops of conglomerates and fossiliferous sandstones along the northern coast of Golfo Aranci and along the southern coast of Tavolara Island in front of Spalmatore Bay (Figure 9).

Here, the transgressive level with large heterometric blocks. both crystalline and carbonate rocks, fossilizes the wave-cut platform set on inequigranular Leucogranites. This basal level is covered by a well-cemented biogenic calcarenite with high fossil content (Conus mediterraneus, Conus vayssieri, Hexaplex trunculus, Cerithium vulgatum, Cerithium rupestre, Bittium reticulatum, Thais haemastoma, Columbella rustica, Arca noae, Barbatia barbata, Spondylus gaederopontaus, Patella ferruginea, Chama gryphina, Venus verrucosa, Chamelea gallina and Tapes sp.) [39].

The marine-littoral sediment sequence is organized at levels and lenses of coarse gravels, alternating with fine-grained sediments up to a maximum height of +5.5 m (Figure 10, Section 1 and Figure 11). MIS 5.5 deposits are sealed upward by strongly cemented continental landslide deposits (MIS 4?).

The sedimentary sequence of MIS 5.5 at Punta La Mandria has a lower thickness and rests discordantly on both Paleozoic granitoid substrate and Middle Jurassic arenaceous dolomitic limestone. The succession is fossilized by both rockfalls and cryoclast deposits represented by alternating Eboulis ordonnés and Grezes litees (LGM).

Data about the elevation of tidal notches incised during the MIS 5.5 highstand are presented. The elevations (Figure 10, Table 2) show a relative tectonic stability of the area, but a slight negative tilting; differing the elevations of the southern cliff to the northern cliff of Tavolara Island. This tendency is more marked towards Capo Figari.

Similar movements, although of greater magnitude, have been found in the Gulf of Orosei and Capo Caccia [7].

3.2. Geomorphology and Altitude of MIS 5.5 Transgression in Pontina Plain

Located in Italy, south of Rome, the Pontina Plain is bordered to the north by volcanoes and pyrso-clastic deposits of Albani hills, to the east by the Lepini and Ausoni mountains (Mesozoic carbonates complexes) and to the west by the Tyrrhenian Sea (Figure 12). This area is characterized by a distensive tectonic regime, that was active during the Pliocene and the Quaternary, linked with the tectonic opening of the Tyrrhenian Sea. In this context, the Pontina Plain represents a sector of particular geological complexity, as the interaction of the sea level changed with neotectonic phenomena.

Figure 12. Main geological outcrops of the Pontina Plain, this map is a compilation of: (i) Italian Geological Survey sheet numbers 170, 158 and 159; (ii) map of the soil [40]; (iii) sinkhole map of Regione Lazio http://www.regione.lazio.it/binary/rl_main/tbl_documenti/AMB_PBL_Carta_Sinkholes_Lazio_2011.pdf accessed on 20 June 2021. The red dots refer to the sites described in

Table 3. On this table all the outcrops dated to MIS 5.5 found studied in the Pontina Plain from Borgo Sabotino (NW) to the Piana di Fondi (SE) and the carbonate promontories of Sperlonga and Gaeta are listed (Figure 13 and Figure 14). In particular, for the lagoon facies sites found in Section D of Figure 12 and published in [20] (Pontinia and Borgo Vodige), an infield check was carried out with respect to the presence of fossil deposits, their share and the fossiliferous association. Section E shows images of channels in the silty portion of the Pontina and Fondi Plain. The flow of the springs in conjunction with moments of high tide can lead to the deposition silts that are flooded and sometimes partially cover the MIS 5.5 fossil lagoon that extends inland for tens of kilometers.

	A	B	C	D	E	F
n.	Site	Coordinates	MIS 5.5 Altitude (m)	Kind of Marker	Age	References
1	Fosso Moscarello Gnif Gnaf, Santa Maria	41.4615 12.8112	10 ± 1	Fossil beach containing Persistrombus latus	Senegalese Fauna Aminoacid	[41,42]
2	Canale Mussolini	41.4483 12.8107	5.1 ± 0.1	Fossil beach containing Persistrombus latus	Senegalese Fauna Aminoacid	[39,43,44]
3	Nuclear power plant Borgo Sabotino	41.4231 12.8053	−4.3 + 5 ± 0.5	Fossil beach containing Persistrombus latus	Senegalese Fauna	[44]
4	Pontinia 1	41.4129 13.0449	+5.3 ± 0.5	Lagoonal facies with Cerastoderma sp.	Geomorphological correlation	[20,45]
5	Pontinia 2	41.4172 13.0600	+4.4 ± 0.5	Lagoonal facies with Cerastoderma	Geomorphological correlation	[20,45]
6	Pontinia 3	41.4323 13.0721	+2.3 ± 0.5	Lagoonal facies with Cerastoderma sp.	Geomorphological correlation	[20,45]
7	Pontinia 4	41.4355 13.0864	+0.8 ± 0.5	Lagoonal facies with Cerastoderma sp.	Geomorphological correlation	[20,45]
7.1	Check in field	41.434771 13.062667	−1 ± 0.5	Cerastoderma e Tapes, travertino con incrostazioni	Geomorphological correlation	This paper
7.2	Check in field	41.3757510 13.1281060	−2 ± 0.5	Lagoonal facies with Cerastoderma sp.	Geomorphological correlation	This paper
7.3	Check in field	41.363875 13.140631	−3 ± 0.5	Lagoonal facies with Cerastoderma edulis, Tapes decussatus, Nassa mutabilis	Geomorphological correlation	This paper
8	Pontinia 5	41.4424 13.0751	−0.5 ± 0.5	Lagoonal facies with Cerastoderma	Geomorphological correlation	[20,45]
9	Mezzaluna core	41 27 47 13 06 01	−14.30 − 11.41 ± 0.5	Venus and Cerastoderma	Pollen Analysis, U\Th and aminoacid	[46]
10	Borgo Vodige 1	41.3571 13.1317	1 ± 0.5	Lagoonal facies with Cerastoderma	Aminoacid	[20,45]
11	Borgo Vodige 2	41.3497 13.1293	−0.6 ± 0.5	Lagoonal facies with Cerastoderma	Aminoacid	[20,45]
12	Borgo Vodige 3	41.350 13.117	−1.80 ± 0.5	Lagoonal facies with Cerastoderma	Aminoacid	[20,45]
13	Terracina	41.288 13.260	7.96 ± 0.1	Tidal notch	Geomorphological correlation at 5 km from aged MIS 5.5 deposit	[7]
14	Fondi APT4	41.0065 13.331	−6\−24	Marsh with Cerastoderma Aminozone E	Aminoacid	[47]
15	Sperlonga	41.229691 13.502001	7.30 ± 0.5		Geomorphological correlation	[8]
16	Sperlonga	41.2187 13.5321	6.53 ± 0.1	Tidal notch	Geomorphological correlation	[47]
17	Gaeta	41.2046 13.5774	5.92 ± 0.1	Tidal notch	Geomorphological correlation	[47]

.

Figure 12. Main geological outcrops of the Pontina Plain, this map is a compilation of: (i) Italian Geological Survey sheet numbers 170, 158 and 159; (ii) map of the soil [40]; (iii) sinkhole map of Regione Lazio http://www.regione.lazio.it/binary/rl_main/tbl_documenti/AMB_PBL_Carta_Sinkholes_Lazio_2011.pdf accessed on 20 June 2021. The red dots refer to the sites described in Table 3.

Forms and deposits aged MIS 5.5, in particular fossil tidal notches and Cerastoderma living in lagoons, are considered the best markers to detect even small vertical tectonic movements occurred during the last interglacial [7,8]. As regards the research and studies published for the Quaternary (Upper Pleistocene) of the Pontina Plain, they began in the 1930s and continued until 2018 with the study, analysis and dating of new fossil deposits, cores or fossil tidal notches [7,20,39,43,44,45,46,47,48,49,50,51].

We can divide the Pontina Plain into two completely different geomorphological areas: a sandy aeolian portion called Duna Rossa [52] with developed paleosols that preserved erosion and morphology which extends from the Tyrrrhenian Sea coast up to about half of the Pontina Plain (with altitudes between 15 and 30 m); and a silty peaty portion (subject to strong compaction starting from the 1930s) with altitudes reaching 5 m to the north and up to −3 m to the south near the carbonatic mountains. The negative altitudes are due to: (i) extensive subsidence phenomena caused by the drying of peat up to over 6 m [53]; (ii) sinkhole phenomena in the Pontina Plain eastern portion near the contact with Mesozoic carbonates; (iii) possible direct fault actions that lower the Mesozoic carbonates under the Pontina Plain (Figure 12). The lower elevation portion of the plain is towards the eastern portion which is bordered by the Lepini mountains (Mesozoic Carbonates) which, with direct faults, “descend” under the plain; towards the west it borders the formation of the Duna Rossa, aeolian sands aged from the Middle Pleistocene, MIS 5.5, MIS 4, 3 and 2. These deposits are engraved to the north by some reclamation drainage channels including (at Borgo Sabotino) the “Mussolini channel”, whose excavation allowed the Persistrombus latus findings [8,45] to place them at +10 m; however, after a careful reading of the original paper by the authors of [54] we can conclude that the highest deposits of the central/southern Pontina Plain containing Persistrombus latus are located at +5.3 m, the northern portion, near Rome. Blanc, in 1957, performed at Canale Mussolini, one of the first radiocarbon ages of Europe, dating the deposits as older than MIS 5.5: wood and peat, attributing them to MIS 4. In the higher levels, the authors of [55] published an interesting mammalian fauna with Megaloceras, Equus idruntinus, Ursusu speleus. On the depressed peaty area of the Pontina Plain (Figure 13, Figure 14), the MIS 5.5 deposits dated with aminostratigraphy [20] were found between +3 m to −2 m. These negative altitudes, often present in the Pontina (up to −4 m) derive in part from the reclamation that has left up to 16 m [53] of peat which causes drying to produce a considerable subsidence, especially in correspondence with some areas (former sinkholes) close to the limestones of the Lepini-Ausoni mountains.

3.3. Maps of the Pontina Plain

On the basis of the maximum altitude of the highstand reached during the MIS 5.5 of 8.2 m, [47] and the present altitude of the Pontina Plain maps (Table 1), it was possible to reconstruct the transgressive event that occurred 119 ka BP: a gulf that forwards in a NW direction for about 32 km, with a total flooded area of 401.42 km² (

Table 4. A: coastal sites; B: exposure; C: maximum fetch; D: max fetch; E: coastal material; F: wave energy flux kW\m. This is an average between two types of data in [56]. Maximum waves and energy are found between the Balearic Islands and Capo Caccia with values between 9 and 10.0.; G: geomorphology; H: flooded area during MIS 5.5 km²; I: projection flooded area IPCC 2300, km²; L: projection flooded area IPCC 2100 km². As regards the flooded area of the Pontina Plain on the map (Figure 13), * we divided it into three different areas (Pontina 190, Fondi 64.5 and Laghi Costieri 49.2; M: exposed coastline Length km; N: human-made structures.

A	B	C	D	E	F	G	H	I	L	M	N
Site n°	Coastal Site	Exposure Direction	Max Fetch Km	Coastal Material	Wave Energy Flux kW/m	Geomorphology	Flooded Area MIS 5.5 km2	Flooded Area IPCC 2300 km2	Flooded Area IPCC 2100 km2	Exposed Coastline Length km	Human-Made Structures
1	Pontina Plain *	N-W	308	sand	3–4	Embayed Beach	396.3	303.7	61	104.9	Towns and Agricultural crops
2	Cagliari	S	208	sand	4–5	Barrier and lagoon systems	131.5	87.2	27.5	29.6	Towns and Agricultural crops
3	Oristano	W	357	sand	6–7	Barrier and lagoon systems	376.8	230.0	53.5	102.7	Towns and Agricultural crops
4	Olbia	E	231	sand	3–4	Rias + Barrier and lagoon systems	24.7	16.3	4.53	30.7	Towns, harbor and industrial hub

) and about 100 km of coastline involved in the transgression of 119 ka BP (Table 2). This high sea stationing event is demonstrated by the data (forms and deposits) emerging in the plain and described in the previous chapter. Based on the IPCC 2019 data, in 2300 the maximum elevation of the sea will be 5.249 m higher than in 2019. Based on the elevation of the Pontina Plain maps it was possible to reconstruct the possible future transgressive event (Figure 15), provided that “man” does not carry out any anthropic building (dams, genic interventions (dams, water pumps, etc.). Figure 15 shows a gulf that will extend into the Pontina Plain for about 26 km and that could have a total flooded area of 306.67 km².

3.4. Maps of Cagliari Plain

The northern sector of the Gulf of Cagliari is characterized by a flood plain with a low slope towards the south. In this sector, during MIS 5.5, the maximum altitude of +8.4 m above current sea level originated two bays: one to the west (current Laguna di Santa Gilla) with a retreat of the shoreline of about 16 km and one to the east (current Molentargius) with 6 km of retreat. The Sant’Elia promontory was isolated from the mainland and represented an island or was probably connected to the coast through a tombolo. The map shows that in MIS 5.5, the marine transgression affected the coastal sector wide 27 km with a flooded area of 131.54 km². The same map shows the maximum level of + 5.25 m compared to the sea level for the year 2300 based on IPCC 2019 data; in particular, the map shows a scenario which 87.24 km² of coastal areas submerged; the sea level rise would lead to the demolition of the Holocene littoral spits with a shoreline retreat of 16 km and 14 km, respectively, for the Santa Gilla and Molentargius lagoons and the subsequent formation of bay beaches (Figure 16 and Figure 17).

3.5. Maps of Oristano Plain

In the Gulf of Oristano, the MIS 5.5 shoreline reaches a maximum level of +8.5 m a.s.l. In addition, for this sector the slope is very low; there is a maximum shoreline retreat of about 13 km and a submersion area of about 376.77 km². The map shows the maximum level of +5.25 m above sea level for the year 2300 based on IPCC 2019 data. This scenario would lead the demolition of the Cabras and Santa Giusta littoral spits, the submersion of 230 km² in a sector of coastline that extends for 103 linear km and the retreat of the Tirso River mouth towards the northeast (Figure 18 and Figure 19).

3.6. Maps of Olbia Plain

The Gulf of Olbia is represented by a rias coast engraved on granite lithologies. The slopes are greater than the alluvial plains of Cagliari and Oristano. For this reason, the MIS 5.5 sea level with an altitude of +8.5 m a.s.l. was measured up to a maximum of 2 km from the current shoreline, while the submerged surface during MIS 5.5 was equal to 24.69 km². For this area, the sea level on the basis of the IPCC 2019 data will reach the maximum altitude of +5.25 with the submersion of 16.31 Km² of coastal area. The map shows the demolition of the lagoon behind Le Saline beach, the retreat of the Rio Padrongianus mouth and the retreat of approximately 1 km in correspondence of the city of Olbia (Figure 20 and Figure 21).

4. Discussion

The paleo-climatic events that 125 kyrs BP have led to a notable ice melting with a global consequence of a sea level rise more than 5–7 m around the world, are still the subject of scientific debate. With regard to the CO₂ content in the atmosphere, 119 ka BP, was considerably lower than today, but the insolation was higher. In fact, such a high sea level (and carbon dioxide in the atmosphere) was never reached except in the Pliocene [57]. Gilford et al. 2020 [58] explore the extent to which MIS 5.5 constraints could inform future Antarctic contributions to sea level rise, forcing the last interglacial with IPCC RCP8.5, and obtaining a sea level over year 2150 of about 5 m even beyond the 2019 IPCC forecasts.

As regards data and interpretations on insolation, due to astronomical variables dynamics they are not univocal and are difficult to compare with each other due to the different measurement methods and sensors [59,60]. Data sets of daily SSR records (1953–2013) were interpolated on a regular grid and grouped into two regions, northern and southern Italy, both in all-sky and clear-sky SSR mode [61].

While taking into account the IPCC literature which mainly indicates a phase of decrease in insolation in the last 6 Ka, the warming recorded in the last two centuries is due to the increase in CO₂ (IPCC 2019).

The possibility is also considered that, on the contrary, there is an increase in the insolation of an astronomical origin (Milankovic cycles) characteristic of an interglacial phase [62,63], a condition that finds support in the data from geo-archaeological indicators of the last 4 Ka [33]. In this case, the increase in CO₂ would contribute to global warming by accelerating the upward trend of the sea level [1].

For all these reasons, we think that the approach of using the maximum transgression occurred for 119 ka BP has serious scientific motivations. None of the authors are hoping for a sea level rise above 1 m in 2100, but in light of what happened during MIS 5.5 it seemed useful to have a realistic view of the effects. The maximum altitude that sea level reached during MIS 5.5 can therefore be considered as a reference also for future sea level rise.

4.1. Pontina Plain

The Pontina Plain map (Figure 15) highlights a long paleo-gulf that falls for 33 km. It should also be emphasized that the eastern portion of the Pontina Plain is subject to a subsidence partly induced by the reclamation, by partial drying of the peat and partly to negative tectonic movements, especially in correspondence with the eastern edge of the Pontina Plain in contact with the Mesozoic Carbonate. In some areas of the Pontina Plain, we found lagoonal deposits containing Cerastoderma edulis outcropping in field down to −2 m; these circumstances confirm small but continuous negative vertical movements especially in the southern portion of the Pontina Plain (Figure 15).

4.2. Sardinia

4.2.1. Cagliari Coastal Plain

The map of the Cagliari Coastal Plain highlights two deep inlets separated by the high structural system of the Cagliari hills—Cala Mosca-Sella del Diavolo promontory. The eastern inlet gave rise to the Molentarigius paleo-lagoon, closed by the littoral spit of Is Arenas (Figure 3 and Figure 4, Section 3). In the sector of maximum expansion of the last interglacial transgression, strips of the inner margins engraved in MIS 6 alluvial deposits have been preserved (Figure 4, Section 4).

The western bay deepens into the plain for about 15 km. The MIS 5.5 transgression internal margin presents a marine terrace at Cerastoderma glaucum [28], from which a strip was found at +3.40 m (Figure 5, Section 5). The coastal paleo-spit marine deposits of Sa Illetta, which closed the paleo-lagoon of Santa Gilla, were found up to an altitude of +4.30 m, while in correspondence of the two lagoon paleo-mouths of the MIS 5 fossiliferous levels are found at −2 m (east lagoon-mouth) and −5 m (west lagoon-mouth—La Plaja coastal spit).

4.2.2. Oristano–Sinis Coastal Plain

In the sector of Oristano, the last interglacial transgression entered in the coastal plain of Cabras for about 10 km, and no inner margin was clearly detected as it is covered by river and slope deposits. A paleo-spit closes the paleo-lagoon of Cabras (Figure 6) with littoral sediments detected up to an altitude of 4.30 m (Figure 8, Section 2).

The maximum altitude of the littoral deposits of MIS 5 was found in San Giovanni di Sinis where sandstone with plane-parallel lamination reaches +5.50 m (Figure 8, Section 1), while on the eastern coast of the di Capo San Marco promontory an abrasion platform engraved in the Pliocene basalts with an inner margin at an altitude of +7 m is present.

4.2.3. Olbia Coastal Plain and Tavolara Island

The littoral deposits of the Olbia coastal plain were extensively studied by Aldo Segre in 1954. The significant increase in tourist construction on the coast during about 70 years has obliterated most of the outcrops that have instead been preserved on the Tavolara Island (for decades, a national marine protected area) (Figure 9).

The upper limit of the last interglacial is well marked in the area by tidal notches, at +7.5 m at Grotta del Papa (Figure 11e).

The MIS 5 fossiliferous deposits of Spalmatore di Terra (Tavolara Island), in high energy facies (conglomerates and micro-conglomerates) reach +5.5 m (Figure 11, Section 1).

Fossil deposits similar to those of MIS 3 were crossed by geotechnical cores at an altitude of −4 m in the area of the inner port of Olbia, a port area located in the bottom of the bay set on a deep Ria with a tectonic setting.

After the submission of the manuscript, the IPCC AR6 report was published. This report confirms and studies in depth the previous report (AR5) and also enters into the merits of particular areas such as the Mediterranean. The AR6 report provides, for example, the relative sea level rise of the Mediterranean expected in 2100 in numerous coastal locations, including Cagliari, one of our four sites. As can be seen in Figure 22 from the report released 10 days ago, the forecasts for the RCP8.5 scenario used by us are very close to what we reported in Table 1 for Cagliari. The IPCC value is 0.78 ± 0.3 m; the value used by us for Cagliari for 2100 in Table 1 is 0.89 m.

5. Conclusions

In this work, we have shown a methodology to create precise maps with potentially expected submersion scenarios for 2100 in four selected coastal zones of the Mediterranean basin which are prone to marine submersion under the effects of relative sea level rise. For these areas, we produced thematic maps that were based on climatic scenarios, tectonics, local geological behavior and the best available digital topography.

Some areas are natural high value sites belonging to protected areas or national parks; some are deeply urbanized by residential or touristic settlements; others are characterized by the presence of cultural heritage, and infrastructure such as communication routes and harbors.

From our analysis, we estimated that during MIS 5.5, the whole studied sites were flooded by the sea at an area of 929.3 Km²: 396.3 Km² in the Pontina Plain, 131.5 Km² in the Cagliari Plain, 376.8 for Oristano Plain and 24.7 for the Olbia Plain. The potential loss of land for the above areas in 2300 are 146.5 Km², and in 2100 are 146.5 Km² for the IPCC-RCP8.5 whole sites studied, impacting a coastline length of about 267.9 km. In these coastal areas are often located densely inhabited settlements and infrastructure. The expected scenario and the exposition of the investigated areas to coastal hazards should be considered for cognizant management of the coastal zone.

The comparison between the maximum ingression line of the last interglacial and the present evolutionary trend of submersion of the coastal plains is justified by the fact that, at the current state of knowledge, it cannot be excluded that the maximum altitude the sea level reached during MIS 5.5 can therefore be considered as a reference also for the future sea level rise.