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

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. 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 trasgression occurred during MIS 5.5 higstand 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 [1]projections and glacio-hydro-isostatic movements for the above selected coastal zones, 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 [1]) and 637 km2 along a coastline length of about 268 km.


Introduction
Future warming on Mediterranean Sea could provide higher sea level rise than published on 2019 IPCC report [2]. Many research was recently published on the effects of sea level rise on the Mediterranean coasts [3,4,5,6]. In this research we use the methodologies tested in previous works [3] 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 [1].
The objectives of our work is to highlight at what altitude can now be observed the fossil signs of maximum marine ingression (forms and aged deposits) occurred during MIS 5.5 comparing with the sea level projections for 2100 and 2300 (IPCC 2019) in the 4 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 Pontina Plain. Also, 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.
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 were the effects of the maximum transgression of the last highstand, (MIS 5.5 that occurred 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 duration of last warm periods (MIS 5.5, MIS 9, MIS 11, etc.), has always been around 10-11 thousand years [9]. But, if it is true that 119 ka BP there was a maximum of insolation of about 550 W\m 2 , calculated for 65 ° in June, it is also true that during the climatic optimu (about 6, ka BP) l insolation was of lesser entity: 520 W\m 2 . Subsequently: 1) the insolation has been decreasing, 2): the climate does not seem to have cooled). In a quoted article [10], 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 higstand 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".  [1,2]; eccentricity and insolation from [12]. 119 ka BP the insolation was a higher than oday, the sea was about 8 meters higher [7], but the CO 2 content in the atmosphere has never exceeded 295 ppm, while today, with a lower insolation, but a content of CO 2 much higher, 415 ppm, we can also observe a sea level 8 meters lower than 119 ka BP and a rise that exceeds the acceleration of 3 mm / year. IPCC 209 projections estimates (using the RCP 8.5 scenario) a rise higher than one meter for the next 79 years (2100).
The CO 2 increase would also further delay the triggering of a cold period.

Figure 2
Overview map of the Central Mediterranean region showing the locations (red dots) of the coastal plains studied.

Materials and Methods
This research is based on the method described in [4], and [14], to ensure homogeneity and enable a comparison with previous results, extending the study toward other central Mediterranean coastal plains, besides Italy. Sea-level change along the Mediterranean coast is the sum of eustatic, glaciohydro-isostatic, and tectonic factors. The first is time dependent while the latter two also vary with location, consists 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 report IPCC 2019), the long term land vertical movements from geological data [7,8,15];c) the glacio-hydro-isostatic movement (GIA) [3]; (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 investigate coastal areas and the expected inland extent of related marine flooding. 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 reference climatic projections and the corresponding flooded area. Sea level rose rapidly between 21 until 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 Mediterranean sea, the 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.

Pontina Plain
For this Plain, some papers published in the 1930s and 1950s were carefully considered, the altitude of fossil deposit containing Persistrombus latus or Senegalese fauna, was revised also in field (respect the review [8] and [16]), lowering it from +10 to +5.3 m. In addition, some new lagoonal fossil layers containing Cerastoderma edulis have been founded in the field. These lagoonal levels outcropping in the eastern portion of Pontina plain, between +4 and -2 m, outcrops only if incised by the the 1930s "Bonfica" channels whose cleaning reveals the fossil levels. We checked what published in [17], in 3 outcrops on field. The altitudes were re-measured on google Earth maps (which presents a very precise altimetry in the flat areas see also Material and Method [14]).

Sardinia
As part of this work, the stratigraphic and chronological point of view of deposits containing Persististrombus latus, Patella ferruginea, etc., have been reviewed, detected in previous studies [18,19,20] 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 antenna -Trimble R8s model. The planimetric and altimetric survey were carried out operating in static mode. The raw DGPS data have been processed using the Trimble Busines software (V. 2.50). The processed data have an accuracy of ± 1 ÷ 2 cm.

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 dierent 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 [21]. The details of the characteristics of the DTM are described in Table 1 and in the maps available in the online supporting material.

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 August 2021
To link the land surface to the seafloor along the coasts and represent MIS 2 sea level, the bathymetric data were obtained from European Marine Observation and Data Network (EMODnet, http://portal.emodnet-bathymetry.eu/ [22]). 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.
DTMs were mapped and analyzed by Global Mapper Software® (www.globalmapper.com [23]) (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 GIS composer (Tables 1,2).

Cagliari coastal plains
The coastal sector of Campidano graben of Cagliari (Figure 2), is articulated in 2 coastal plains with different geodynamic evolution: the eastern plain is set on a graben structure active until the upper Miocene, while western plain is set on a graben active until the lower-middle Pleistocene.
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 Intergacial stratigraphic succession. Later, this stratigraphic section was indicated as locus typicus of the "Tyrsrhenian plane" [18], an attribution confirmed by U/Th isotopic dating corals [8,19,20]. During observations for the study of the Holocene sea rise, the site described by Segre [24] and Orrù [25]     Geophysical surveys (seismic profile, [25]) allowed to follow the paleo-valleys (mostly buried) carved during the MIS 2, these cross the whole platform until they reach the shelf break where contribute to the construction of low stationing terraces (LGM palaeo-sealevels); sometimes are involved in mass gravitational movements [29].
A Punic paleo-sea level (2500 BP) buried at -2 m is preserved in the innermost area of the Santa Gilla lagoon [30,31,32].

Oristano North coastal plains and Sinis peninsula
An

Geomorphology and altitude of MIS 5.5 in Pontina Plain
Located in Italy, south of Rome, the Pontina Plain 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 Tyrsrhenian sea ( Figure 12).  drainage channels including (at Borgo Sabotino) the "Mussolini channel) whose excavation allowed the Persistrombus latus findings [8,46] place them at +10 meters), but after a careful reading of the original paper by [49] we can conclude that the highest deposits of the  Figure 13) the MIS 5.5 deposits dated with Aminostratigraphy [17] are found between +3 meters to -2 meters. These   Table 3.  Table 3 (Figures 13 and 14). In particular, for the lagoon facies sites found in sector D of the of Figure 12 and published in by [17] (Pontinia and Borgo Vodige), an in-field check was carried out with respect to the presence of fossil deposits, their share, and the fossiliferous association. E 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 some-times partially cover the Mis 5.5 fossil lagoon that extends inland for tens of kilometers.

Maps of the Pontina Plain
On the basis of the maximum altitude of the higstand reached during the MIS 5.5 of 8.2 m, [44] 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 NW direction for about 32 kilometers, with a total flooded area of 401.42 km 2 ( Table 4) and about 100 km of coastline involved in the transgression of 119 ka BP ( Table 2). This high sea stationing event is demon-     Figure 15 The Pontina and Fondi Plains maps showing the potential submersion area using IPCC AR5 RCP 8.5 for 2100 and 2300: The MIS 5.5 extension occurred 119 ka BP.

Maps of Cagliari Plain
The northern sector of the Gulf of Cagliari is characterized by a flood plain with a low slope towards South. In this sector, during MIS 5.5, the maximum altitude of +8.

Figure 17
Map of Cagliari plane, (see also Figure 1 for location). The potential submersion area, using IPCC AR5 RCP 8.5 projections at 2100 and 2300.

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..    Figure 1 for location). The potential submersion area, using IPCC AR5 RCP 8.5 projections at 2100 and 2300.

Maps of Olbia Plain
The

Discussion
The palaeoclimatic events that 125.000 yrs BP have led to a notable ice melting with a global consequence of a sea level rise more than 5-7 meters around the world, are still the subject of scientific debate. As regard 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 [55,56]. Gilford et al 2020 [57] explores the extent to which MIS 5.5 constraints could inform future Antarctic contributions to sea-level rise, forcing Last Interglacial with IPCC RCP8.5, obtaining a sea level over year 2150 of about 5 meters even beyond the 2019 IPCC forecasts.
As regards data and interpretations on insolation due to astronomical variables dynamics of are not univocal, and are difficult to compare with each other due to the different measurement methods and sensors [58,59]. Datasets 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. [60].
While taking into account the IPCC literature which mainly indicates a phase of decrease in insolation in the last 6 ka while the warming recorded in the last 2 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 astronomical origin (Milankovic cycles) characteristic of an Interglacial phase [61,62], a condition that finds support in the data from geo-archaeological indicators of the last 4 Ka [32].
In this case, the increase in CO2 would contribute to global warming by accelerating the upward trend of the sea level. [2].

Figure 21
Map of Olbia plane, (see also Figure 1 for location). The potential submersion area, using IPCC projections at 2100 and 2300.
For all these reasons we think that the approach of using the maximum transgression occurred 119 ka BP has serious scientific motivations. None of the Authors are hoping for a sea level rise above 1 meter in 2100, but at the 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 there future sea level rise.

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

Cagliari coastal plain
The map of the Cagliari Coastal Plain highlights 2 deep inlets separated by the high structural system of 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 (Fig. 3; Fig.   5 -Section 3). In the sector of maximum expansion of the Last Interglacial transgression, strips of the inner margins engraved in MIS 6 alluvial deposits are preserved (Fig. 5 -Section4).
The western bay deepens into the plain for about 15 km. The MIS 5.5 transgression internal margin present a marine terrace at Cerastoderma glaucum [24] which a strip was found at +3.40 m (Fig. 5 -Section 5). The coastal palaeo-spit marine deposits of Sa Illetta, which closed the palaeo-lagoon of Santa Gilla, were found up to an altitude of + 4.30 m, while in correspondence of the 2 lagoon palaeo-mouths of the the MIS 5 fossiliferous levels are found at -2 m ( east lagoon-mouth) and -5 m (west lagoon-mouth -La Plaja coastal spit).

Oristano-Sinis coastal plain
In the sector of Oristano-Sinis the Last Interglacial transgression enters in the coastal plain of Cabras for about 10 km, no inner margin was clearly detected as it is covered by river and slope deposits. A paleo-spit closes the paleo-lagoon of Cabras ( fig. 6) with littoral sediments detected up to an altitude of 4.30 m (Fig. 8 -Section 2).
The maximum altitude of the littoral deposits of MIS 5 was found in San Giovanni di Sinis where sandstone with pianopalallel lamination reaches + 5.50 m (Fig. 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.

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 National Marine Protected Area). (Fig. 9) The upper limit of the Last Interglacial is well marked in the area with by tidal noches, at + 7.5 m at Grotta del Papa (Fig. 11 -e) The MIS 5 fossiliferous deposits of Spalmatore di Terra (Tavolara Island), in high energy facies (conglomerates and microconglomerates) reach + 5.5 m (Fig. 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.

Conclusions
In this work, we have shown a methodology to create precise maps with a potentially expected submersion scenarios for 2100 in 4 selected coastal zones of the Mediterranean basin which are prone to marine submersion under the effects of relative sea-level rise. For these areas, were produced thematic maps that were based on climatic scenarios, tectonics, local geological behavior and the best available digital topography.
Some areas guest 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. 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 maximum altitude that sea level reached during MIS 5.5 can therefore be considered as a reference also for the future sea level rise.