Coastal submersions in the north-eastern Adriatic during the last 5200 years
Introduction
Relative Sea Level (RSL) rise features prominently among the consequences of climate change (Nicholls et al., 2007; Kopp et al., 2014, Kopp et al., 2017, Rovere et al., 2016). In 2017, more than 600 million people lived in coastal areas (less than 10 m above sea level) and nearly 2.4 billion people (~40% of the world's population) resided within 100 km of the coast (UN Ocean Conference, 2017). The coastal zone accounts for only 20% of all land area (CIESEN, 2000) but population densities in coastal regions are about three times higher than the global average. The risk of coastal flooding and increased saltwater intrusions on coastal areas therefore concerns at least ~10% of the world's population. In the Mediterranean, coastal flooding events are among the most feared hazards as the region is particularly vulnerable to the effects of ongoing climate change (Satta et al., 2017) with densely populated seaboards (Wolff et al., 2020) and mass tourism (Cortés-Jiménez, 2008), infrastructure and heritage sites (Reimann et al., 2018; Anzidei et al., 2020) situated close to the shore and with often limited adaptive capacity (Cramer et al., 2020). By 2025, ~174 million people will live in coastal areas, estuaries and deltas (~33% of the Mediterranean's population; Fabres, 2012). RSL rise and storm-related floods will make low-lying zones and coastal activities increasingly exposed to submersion and beaches vulnerable to erosion (Hzami et al., 2011), and aquifers more sensitive to saltwater intrusion (Nicholls et al., 2021).
Among the threatened areas in the northern Mediterranean, the Gulf of Venice has been identified as a hot spot according to the Coastal Vulnerability Index, the Coastal Exposure Index, and the Coastal Risk Index (Satta et al., 2017; Furlan et al., 2018, Furlan et al., 2021). This situation is worrying as populations and coastal infrastructure in low-elevation areas will potentially face recurrent submersion events (Antonioli et al., 2017; Marsico et al., 2017; Bonaldo et al., 2020). UNESCO World Heritage sites such as the Episcopal Complex of Poreč, the Stato da Terra - Stato del Mar occidental, Aquileia, or Venice (Camuffo et al., 2014) will all be exposed to coastal flooding and erosion with RSL scenarios based on the Representative Concentration Pathways (RCP 2.6, RCP 4.5 and RCP 8.5; Reimann et al., 2018; Zanchettin et al., 2020). While the whole northern Adriatic coastal system seems vulnerable, analysis of future RSL rise and storm surges at a narrow geographic scale (north-western coast - northern coast - north-eastern coast) shows a more contrasting pattern and underlines the uncertainty behind the extent of RSL rise (regional forcings versus remote effects; Scarascia and Lionello, 2013), its impacts, and the severity of storm surges on either side of the Gulf of Venice (Lionello et al., 2012; Conte and Lionello, 2013; Kaniewski et al., 2016; Bonaldo et al., 2020). Bonaldo et al. (2020) also showed a shift in directional wave-energy contributions in the northern Adriatic resulting in a possible local increase in wave climate severity along the northern and north-eastern coasts. This process could be aggravated in northern Croatia by coastal subsidence amounting to 0.4 mm per year (Faivre et al., 2019a, Faivre et al., 2021), which is coherent with the estimated 0.45 ± 0.6 mm per year subsidence in southern Croatia (Faivre et al., 2013; Shaw et al., 2018). Moreover, RSL changes for the last 1500 years along north-eastern coasts seem consistent with shifts in temperature and consequently with periods of rapid climate change (Faivre et al., 2019a).
Here, our aim is to investigate the environmental patterns that may have favoured coastal submersions and increased saltwater inputs inland during the last 5200 years in the north-eastern Adriatic (Istrian peninsula - Croatia, Fig. 1), one of the most vulnerable areas to present-day changes (Bonaldo et al., 2020). Our study explores the role of RSL rise, variations in Mediterranean SST, climate shifts and winter/summer insolation as potential forcing agents that may have promoted coastal submersions during the Mid-to-Late Holocene, and to infer whether present-day trends could play out in a similar manner. By trying to understand how all these environmental drivers may have acted conjointly during the past, this study seeks to identify the processes/patterns behind coastal flooding events.
Section snippets
Marine influence
The term “marine influence” in this study encompasses all the processes leading to increases in seawater in the coastal zones of Croatia (including coastal submersions with temporary flooding or permanent intrusion, and an increase in storm activity at the marine-terrestrial boundary). The difference between saltwater inundations and saltwater intrusions was made based on the pattern of coastal freshwater inputs. When coastal freshwater discharges are reduced, the saltwater penetrates inland
RSL in north-eastern Adriatic
We reconstructed the sea-level evolution of the north-eastern Adriatic Sea by coupling the sea-level data available in Vacchi et al. (2016) with those produced by Covelli et al. (2006), Faivre et al., 2011, Faivre et al., 2019a, Faivre et al., 2019b, Furlani et al. (2011), Trincardi et al. (2011), and Trobec et al. (2018). We thus assembled a set of 43 RSL index points (points that constrain the palaeo mean sea-level (MSL) in space and time; Shennan et al., 2015; Fig. 2a-b) derived from
RSL rise for the last 10,000 years
A total of 43 RSL index points was used to frame the Holocene sea-level evolution of the north-eastern Adriatic (Fig. 2a). At 10,270 ± 16.5 cal. BP, the index point places the RSL at 25.17 ± 1.94 m bcMSL (Fig. 2b) with a rising rate of 3.3 ± 3 mm per year (Fig. 2c). The index points indicate that RSL rose rapidly until 5050 ± 96.5 cal. BP (3.9 ± 1.74 m bcMSL and a rising rate of 2.8 ± 2.2 mm per year), followed by a slowdown in the rising rates. Since this period, index points delineate a
Discussion
As emphasized by the International Panel on Climate Change (e.g. IPCC, 2007, IPCC, 2014), present-day climate change effects are expected to substantially increase global mean sea level (Chang et al., 2011), significantly affecting erosion and flooding of coastal areas in the near future (Aucelli et al., 2017). This rise will also have a severe adverse effect on saltwater intrusion processes in coastal aquifers whereas the need for freshwater is increasing as burgeoning population levels
Conclusions
This study shows that coastal flooding and saltwater intrusions have had a major impact on the northeast Adriatic coastal areas over the past thousand years. Enhanced and recurrent submersions were triggered by several cumulative factors and processes such as RSL variability, warm SSTs, warmer atmospheric temperatures and drier conditions. The drivers behind past coastal submersion are similar to the modern and predicted changes, suggesting that present-day patterns may lead to severe
Data availability
The data supporting our results are provided as Supplementary data to this article.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We wish to thank the Centre Camille Jullian (Aix Marseille Université, CNRS), the Zavičajni musej Poreštine / Museo del territorio parentino (Poreč, Croatie), and the Croatian Science Foundation. A.R. and M.V. acknowledge PALSEA, a working group of the International Union for Quaternary Research (INQUA) and Past Global Changes (PAGES), which in turn received support from the Swiss Academy of Sciences and the Chinese Academy of Sciences. This work is a contribution to the French MITI CNRS
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