Global sea-level rise and the disappearance of tidal notches

Global sea-level rise and the disappearance of tidal notches

Global and Planetary Change 92–93 (2012) 248–256 Contents lists available at SciVerse ScienceDirect Global and Planetary Change journal homepage: ww...

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Global and Planetary Change 92–93 (2012) 248–256

Contents lists available at SciVerse ScienceDirect

Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha

Global sea-level rise and the disappearance of tidal notches N. Evelpidou a,⁎, I. Kampolis a, 1, P.A. Pirazzoli b, 2, A. Vassilopoulos c, 3 a b c

Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Panepistimiopolis 15784 Athens, Greece Laboratoire de Geographie Physique, 1 Place Aristide Briand, 92195 Meudon Cedex, France Geoenvironmental Institute, Naxou 2–4, 15238, Chalandri, Athens, Greece

a r t i c l e

i n f o

Article history: Received 26 September 2011 Accepted 21 May 2012 Available online 27 May 2012 Keywords: sea-level indicators biological erosion rate limestone dissolution tectonic movement global isostatic adjustment Greece

a b s t r a c t The recent rise in global sea level is causing the disappearance of an important geomorphological sea-level indicator, the tidal notch. Tidal notches have often been used in carbonate coasts for Quaternary and late Holocene sea-level reconstructions and estimation of tectonic movements, especially in uplifting areas. In this paper, we review the rates of tidal notch development, and examine the recent gradual depletion of this feature, during at least the last century, and its relation to the increasing rates of sea-level rise. Some examples of tidal notch development are provided with fossil submerged notches from Greece. Although tidal notches are no longer forming in the present-day mid-littoral zone, underwater marks on carbonate cliffs may still provide evidence of submerged tidal notches corresponding to former sea-level positions, or of recent vertical shoreline displacements of seismic origin. © 2012 Elsevier B.V. All rights reserved.

1. Introduction 1.1. What a tidal notch is A marine notch or nip is an undercutting several centimeters to several meters deep, left by sea erosion in coastal zones. Notches are often horizontal and developed with continuity; therefore they are easy to see especially in uplifting coastal areas. Emerged notches have often been used to infer former Quaternary and Holocene sealevel positions. The term ‘tidal notch’, contrary to surf notches, abrasion notches and structural notches, was introduced into the geomorphological literature by Pirazzoli (1986) to refer to midlittoral erosion marks left by sea level especially on limestone rock formations. Tidal notches generally appear as undercuts on rock cliffs. Their height corresponds roughly to that of the local midlittoral zone (i.e.: tidal range plus average wave height). They typically form a recumbent V-shaped or U-shaped profile with a vertex (apex, retreat point) located near Mean Sea Level (MSL) (Fairbridge, 1952; Hodgkin, 1964), a roof near the highest tide level and a base near the lowest tide level.

⁎ Corresponding author. Tel.: + 30 2107274297. E-mail addresses: [email protected] (N. Evelpidou), [email protected] (I. Kampolis), [email protected] (P.A. Pirazzoli), [email protected] (A. Vassilopoulos). 1 Tel.: + 30 2107274297. 2 Fax: + 33 1 4507 5830. 3 Tel.: + 30 210 8056448. 0921-8181/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2012.05.013

Several examples of tidal notches, usually raised and often associated with upper sublittoral marine fossils allowing their age estimation by C14 or U-series dating, can be found in the literature (e.g. Mac Fadyen, 1930; Kuenen, 1933; Carobene, 1972; Thommeret et al., 1981; Mourtzas and Stavropoulos, 1989; Pirazzoli et al., 1989, 1991, 1994; Liew et al., 1993; Stiros et al., 1994a, 1994b, 2000; Morhange et al., 2006). They usually date from periods of relative sea-level stability during Quaternary interglacial or interstadial stages. Late Holocene notches are common, especially in tectonically uplifted areas, or in far field areas far from former ice sheets that emerged due to Global Isostatic Adjustment (GIA). Some examples of Quaternary raised notches are shown in Fig. 1. 1.2. How tidal notches are formed and modified The shape of a tidal notch profile can be modified by gradual sealevel change, while rapid relative sea-level movements greater than the midlittoral zone-height may preserve the full profile of a notch for a long time after emergence or submergence (Pirazzoli, 2005; Evelpidou et al., 2011a,b) (Fig. 2 and Table 1). Though the deepening of a tidal notch may be attributed to various chemical, physical, biological or mechanical processes, the predominant agents are usually assumed to be solution and bioerosion. However, tidal-notch development cannot be ascribed to rainfall, because they can be found also in very arid coastal areas like the Red Sea (Guilcher, 1955). Solution by effluent ground water may occur only in very localized coastal sections near springs. Higgins (1980) ascribes the formation of notches to the proximity of coastal springs. In this case, a visor will be formed just above the water

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Fig. 1. Some examples of raised Quaternary notches. a) Remnants of at least five uplifted tidal notches appear on the limestone cliffs at Cape Ladiko (Rhodes Island, Greece) between + 1.6 and + 3.75 m. They have all been displaced by coseismic uplift movements during the last 6000 years (Pirazzoli et al., 1989); b) Holocene (slightly emerged) and of marine isotope stage5 (at + 8 to + 10 m, dated about 122 ka) notches from Rurutu (a raised atoll of the Austral Islands, French Polynesia) (Pirazzoli and Veeh, 1987) (photo P.A.P No. 5795, 1980); c) Raised notch of marine isotope stage 5 from the northeast coast of Barbados (Schellmann and Radtke, 2002) (photo P.A.P. No. G851, 2002); d) marine notch at about 350 m in altitude ascribed to marine isotope stage 17 (estimated about 730 ka) from Sumba Island, Indonesia (C. Jouannic gives scale) (after Pirazzoli et al., 1993).

surface (Fig. 2f′), which should not be confused with a tidal notch (Evelpidou et al., 2011b). The ability of sea water to dissolve calcium carbonate is uncertain. Alkaline water of tropical seas, supersaturated with CaCO3, is evidently unable to do this, though periodic under-saturation of sea water may occur in tidal pools at night (Emery, 1946), but not in open waters in the vicinity of notches (Emery, 1962). According to some authors, undercutting would result from occasional under-saturation of open coastal waters, either because of water turbulence (Kaye, 1957), currents, humic and soil acids washed in from the land, or because of surface film effects due to gaseous exchange with the air (Emery, 1962), but these phenomena cannot be reproduced in the laboratory (Revelle and Emery, 1957). From a marine biological standpoint, the tidal notch development may be considered a consequence of midlittoral bioerosion. In the midlittoral zone various parallel vegetational belts are well developed. The process of bioerosion caused by Cyanobacteria, patellaceous

gastropods (limpets) and chitons (Laborel and Laborel-Deguen, 2005) contributes to notch formation by eating the vegetational belts, eroding the underlying rock, abrading the surface with their hard teeth and radulas and enabling the development of an undercut in limestone cliffs with a maximum erosion rate near MSL. In non-sandy sites, according to Trudgill (1976), the effect of grazing would contribute 64% to the process of notch formation. Some estimates of erosion rates associated to biologic activity are available. They range from 1.4 mm/yr for dense field populations (100 m²) of Echinometra Lucuntur (Focke, 1978), to 0.51 to 0.75 mm/yr for the grazers Patella coerulea and 1.1 mm/yr for sea urchins in the North Adriatic (Torunski, 1979). Unfortunately, tidal notches cannot be dated directly and information about the duration of various sea-level positions can only be deduced from assumptions about the rates of intertidal undercutting. The rate of maximum undercutting (near MSL) varies with rock type and local climate. It has been roughly estimated of the order of 1 mm/yr

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Fig. 2. Theoretical graphic schemes of tidal notch profiles, on a vertical carbonate cliff from different combinations of relative sea-level changes in sheltered areas. The maximum bioerosion rate is assumed to be 0.5 mm/yr and the tidal range 40 cm. A period of 2000 years has been considered with relative sea level stable or rising at a rate smaller than the bioerosion rate, followed by a recent short period of sea-level rise at a rate faster than the bioerosion rate. The most recent continuous sea level rise has resulted to the absence of a present-day notch. For explanation regarding each notch type see Table 1.

(Laborel et al., 1999); however, this is only a first order value, while lower rates are generally observed in hard limestones, especially in non-tropical areas. Other estimates range from 0.2 to 5 mm/yr, depending on lithology, location, and duration of bioerosion (for references, see Pirazzoli, 1986, Table 1; Laborel et al., 1999, Table 1; Evelpidou et al., 2011a). Bioerosion rates reported from the Mediterranean area vary between 0.2 and about 1.28 mm/yr. In Table 2 the first columns summarize the development of the tidal notch profile that can be theoretically expected for certain periods (e.g. one century, one millennium) of stable sea level in areas where the midlittoral zone range is 30 cm (common in many Mediterranean sites) or 100 cm (common in many oceanic sites). In these situations,

bioerosion/dissolution rates are 0.3, 0.5, or 1.0 mm/yr, respectively. It can be seen that the height of the notch does not change with time, but the inward depth of its profile increases in proportion to the duration of the stable sea level. The last three columns of Table 1 repeat the same calculations for a gradual relative sea-level rise of 0.6 mm/yr. Notch profiles disappear with bioerosion/dissolution rates smaller than the rate of sealevel rise, while, for greater rates of bioerosion/dissolution, only the difference between this rate and that of sea-level rise will contribute to the development of the notch profile. The final result will be an increase in the height of the notch, proportional to the sea-level rise, whereas the shape of the notch profile will not become deeper.

Table 1 Different tidal notch types referred in this paper. For the graphic schemes of their profiles see Fig. 2. Notch type Characteristics a′ b′ c′

d′ e′ e″ f

Interpretation

Reclined U-shaped notch profile with the height of the notch roof (Hr) This fossil notch has been preserved underwater after a rapid subsidence movement, greater very similar to the height of the notch floor (Hf). than the tidal range, which followed a former relative sea-level stability. Two submerged fossil notches. The two fossil notches are preserved underwater after two rapid subsidence movements, greater than the tidal range. Fossil notch higher than the tidal range with two vertices, separated The notch sunk because of a rapid subsidence, smaller than the tidal range, preceded and by an undulation in the notch profile. followed by a relative sea-level stability. Thus, the two vertices indicate the former and the following MSL positions. A double vertex profile may also be produced, without any rapid vertical displacement, if, for example, the sea level fluctuates around two modes due to seasonal phenomena. Fossil notch higher than the tidal range and of limited depth. It has been derived due to gradual relative sea-level rise, at a rate smaller than the bioerosion rate. Fossil notch with a greater height than the tidal range and Hr b Hf. This type derives from a gradual relative sea-level rise, followed by relative sea-level stability. Fossil notch with a greater height than the tidal range and Hr > Hf. This type derives from a relative sea-level stability followed by a gradual relative sea-level rise. Submerged fossil notch with a kind of visor just above the water The notch sunk due to a rapid subsidence movement, greater than the tidal range. The visor surface. is produced by dissolution in areas where fresh water flows.

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Table 2 Theoretical profile of tidal notch development in relation to bioerosion/dissolution rates, height of the midlittoral zone (tide plus regular waves) and stable or slightly rising relative sea level. Height of the mid-littoral zone (cm)

Bioerosion/dissolution rate (mm/yr)

Time of relative stable sea level 100 yr Notch height (cm)

30

100

0.3 0.5 1.0 0.3 0.5 1.0

30

100

Time of relative sea-level rise of 0.6 mm/yr 1000 yr

Inward notch depth (cm) 3 5 10 3 5 10

100 yr

Notch height (cm) 30

100

1.3. The recent eustatic sea-level rise Two recent studies (Jevrejeva et al., 2008 and Kemp et al., 2011) allow for a quantification of the global sea level rise and its rates during the last two millennia and especially during the last two centuries. The first study, by Kemp et al. (2011), is a reconstruction of sea-level change over the past 2100 years using salt-marsh proxy records from North Carolina. They suggest that sea level was stable from at least BC 100 to AD 950, then increased at a rate of 0.6 mm/yr during about 400 years (i.e. about 24 cm), followed by a further period of stability, or slightly falling sea level that persisted until the late 19th century. Since then, sea level is estimated by Kemp et al. (2011) to have risen at an average rate of 2.1 mm/yr. There is good agreement between the marsh-reconstructed sea level curve and recent compilations of global tide-gauge data and modeling studies. In particular, according to Jevrejeva et al. (2008), sea level acceleration appears to have started at the end of the 18th century, with a rise of 6 cm during the 19th century (i.e. at an average rate of 0.6 mm/yr), then 19 cm in the 20th century (i.e. at a rate of 1.9 mm/yr). According to other views, the acceleration in the rate of rise starts during the second half of the 19th century, around 1865 (Kemp et al., 2011), or around 1890 (Woppelmann et al., 2006; Pouvreau, 2008; Gouriou, 2012). Since 1993, according to altimetric satellites, the rate of the global sea-level rise has been estimated to be approximately 3.3 ± 0.4 mm/yr (Cazenave and Llovel, 2010). Although it is known that the global sea-level rise did not occur uniformly in all coastal regions, it can be assumed that in the Mediterranean it followed, more or less and eventually with a slight delay, the global trend (e.g. Antonioli et al., 2009; Lambeck et al., 2011; Evelpidou et al., in press). If only the time period with tide-gauge records is considered, it appears that the global sea-level rise since 1870 (estimated to +1.7 ± 0.3 m by Church and White, 2006), has been a little slower along the Atlantic coasts of Europe: 1.44 ± 0.13 mm/yr since 1860 at Liverpool (Woodworth, 1999), and 1.32 ± 0.07 since 1861 at Brest (Pouvreau, 2008). Among the longest tide-gauge records available in the Mediterranean at the PSMSL data bank, Marseille (France) shows between 1885 and 2009 (6 annual values missing) a rate of relative sea-level rise 1.239 mm/yr (R²=0.6814), while in Italy, Trieste (between 1905 and 2011) (6 annual values missing) shows a similar rate of about 1.229 mm/yr (R²=0.522). All the above rates of sea-level rise are clearly greater than the expected rates of bioerosion in carbonate rocks; in the Mediterranean rates above 1 mm/yr are therefore a critical threshold for notch formation. Smaller rates of sea-level rise can also be critical if they exceed the local bioerosion rate. However if they remain less than the bioerosion rate, they contribute to the increase of the height of the notch profile, without submerging it. 1.4. Other possible causes of local relative sea-level rise Ongoing glacial isostatic adjustment is known to have been active, at variable rates, all over the world since the last glacial maximum. Apart

1000 yr

Inward notch depth (cm)

Notch height (cm)

Inward notch depth (cm)

Notch height (cm)

Inward notch depth (cm)

30 50 100 30 50 100

– – 36 – – 106

– – 4 – – 4

– – 90 – – 160

– – 4 – – 4

from areas of former ice sheets, this adjustment, according to GIA models, has been causing, after the mid-Holocene stabilization of the eustatic sea level, relative sea-level rise in many coastal regions, at least in Western Europe, Mediterranean, U.S.A. and Caribbean area. For the last millennia, however, the GIA rate of gradual relative sealevel rise did not seem to have exceeded a fraction, or even a small fraction, of mm/yr (Stocchi and Spada, 2009; Evelpidou et al., in press), though short term changes in regional sea level cannot be excluded. Tectonic motions may be locally important, especially in areas located near plate boundaries. They may result in sudden co-seismic vertical movements of uplift or subsidence, or even in subsidence over wide areas affected by the spreading of the crust or by an extensional tectonic regime related to a nearby subduction zone; such subsidence, in areas like the Cycladic plateau in Greece, may appear gradual over the long term, e.g. with average values of 0.34 to 0.60 mm/yr during the last 400 ka (Lykousis, 2009). However, increasing evidence provided by submerged beachrocks (e.g.: Desruelles et al., 2004) and by submerged tidal notches (e.g.: Evelpidou et al., 2011b) suggest that periods of relative sea-level stability may have alternated with events of rapid subsidence, while evidence of emergence seems missing from the Cycladic islands. Estimating the rates of relative sea-level change is difficult for most of the Quaternary period, even if fossil notches representing the peak of certain interglacial stages exist (e.g. Fig. 1b–d). Nonetheless, notches do suggest periods of relative sea-level stability that may have lasted at least some millennia, based on the inward depth of the fossil notch profiles. Similar raised notches dating from past interglacial peaks may be observed in many uplifting areas. Recently some authors have noted the absence of a present-day tidal notch in several Mediterranean coastal areas: in Croatia (Ambert, 1978; Dalongeville, 1978; Pirazzoli, 1980), in Sicily at Taormina (Kershaw and Antonioli, 2004), in the Gulf of Trieste (Antonioli et al., 2004) and in Greece (Evelpidou et al., 2011a,b). This absence has been generally attributed to tectonic subsidence trends. The aim of this paper is to describe the recent depletion of tidal notches, that may be a consequence of increasing rates of sea-level rise. For this purpose, we compare the above estimations of sealevel rise with recent coastal and submarine observations from Greece where we have verified in several coastal areas that no tidal notch appears today near present sea level, while at most sites fossil submerged tidal notches exist. 2. Material and methods Field work took place in October 2009, July 2010, August 2010 and August 2011, in order to collect information on existing or submerged tidal notches along the northern coasts of the Corinth Gulf, the Euboean Gulf and several Cyclades islands (Antiparos, Paros, Iraklia, Naxos, Keros, Sifnos, Anafi, Ios, Sikinos, Folegandros, and Amorgos) (Fig. 3). The systematic underwater survey along the carbonate cliffs that seemed favorable to bioerosion in the midlittoral zone, was carried

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Fig. 3. Location map.

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out by boat. This enabled the verification that no tidal notch was developed at the present sea level, while at all investigated sites, along the coasts, submerged notches at various depths were observed. Notch geometries (e.g. retreat point elevation, depth and height) were also measured by using the methods of Pirazzoli (1986). Several measurements were performed on each location improving their accuracy by multiple measurements. Geographic locations of measured notches were recorded as Long/Lat coordinates with average accuracy of ±5 cm using GPS. Depths were referred to the sea level at the time of the measurement and subsequently corrected by comparison with the tide-gauge records or with the astronomical tidal predictions, also taking into account the real meteorological conditions. Various tidal notch profiles for submerged notches, from different combinations of RSL in sheltered areas, have been summarized graphically by Evelpidou et al. (2011a,b). They allow to qualitatively distinguish, e.g. type-a′, type-b′ or type-c′ profiles (resulting from a co-seismic subsidence event), from type-d′ profiles (resulting from an apparently gradual relative sea-level rise) and from type-e′ or type e″ profiles (stable sea level preceded or followed by gradual relative sea-level rise). Herein, the graphically presented notch profiles are adapted to the absence of a present-day tidal notch in Fig. 2, in order to present most of the cases discussed. 3. Results There are several examples from Greek carbonate coasts of the Gulf of Euboea, the Gulf of Corinth and Cyclades Islands (Fig. 3) which demonstrate areas where tidal notches could not develop recently in the midlittoral zone, but where submerged fossil notches could be found at some depth. Our assertion that the absence of present-day tidal notches can be ascribed to the recent global sealevel rise is based mainly on a comparison with the available chronology of the global sea-level rise, and on two main examples. At two sites in particular (Atalandi Mines and Keros), we found chronological benchmarks permitting to ascribe the submergence of the tidal notch to the period of the recent global sea-level rise or even to seismic effects of a recent earthquake. In the first one, at Atalandi Mines (Gulf of Euboea), a conspicuous pre-existing tidal notch has been submerged by a coseismic subsidence in 1894 (Evelpidou et al., 2011a), while no tidal notch could develop in the present mid-littoral zone since that time. In the second one, at Keros Island (Cyclades), the now submerged notch could be dated as still active less than three centuries ago and as specified below a strong seismic event occurred in 1956, providing a second chronological benchmark (Evelpidou et al., 2012a) (see below). At other sites, we observed fossil notches at various depths, which were unnoticed until now. Their presence testifies the great potential value and usefulness of submerged tidal notches as accurate sea-level indicators for the study of relative sea-level changes and past tectonic events. For each case study the discussion includes the interpretations that can be deduced from submerged tidal notches after accurate measurement of their size, when recent amounts of relative sealevel rise are also taken into account. 3.1. Atalandi Mines (Euboea Gulf) The site of Atalandi Mines (Gulf of Euboea, central Greece) is significant because there is no indication of a modern notch (Fig. 4a). In the same place, a fossil submerged tidal notch exists (Evelpidou et al., 2011a, Fig. 5), with a vertex at −75 ± 10 cm, after a co-seismic subsidence estimated of about 53 cm occurred in 1894. Local spring tidal range is 70 cm; water level is approximately at MSL. Gradual sea-level rise since 1894 can be estimated to be of 2.1 mm/yr (ca. 22 cm), i.e. much faster than the potential bioerosion rate.

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3.2. Ampelos Island (Corinth Gulf) A visor formed by a freshwater spring undercuts at sea level the continental cliffs (Fig. 4b) facing the small Ampelos Island (northern coast of Corinth Gulf). In the lower part of the visor profile, the base of a tidal notch is missing and no other submerged notch can be observed below the visor, suggesting that no recent co-seismic events affected this area (Evelpidou et al., 2011b), or that the effects of freshwater outflow prevented the formation of reliable sea-level indicators. Local spring tidal range is 20 cm. Gradual sea-level rise here can be estimated to about 25 cm since the beginning of the 19th century (i.e. at a rate of 1.2 mm/yr), faster than the potential bioerosion rate, while dissolution by freshwater has shaped continuously the visor and increased its elevation during this period. 3.3. Keros Island (Cyclades) There is no evidence of a present-day tidal notch as of our field surveys in 2010. However, a submerged tidal notch, d′ type, is visible between 20 and 65 (±10 cm) cm below sea level, with a vertex at about −30 to − 40 cm (Fig. 4c). The submerged tidal notch is ascribed not only to the recent global sea-level rise but also, at least in part, to a rapid seismic effect of about 1 dm, produced by the 1956 Amorgos earthquake (Ms = 7.4) (Evelpidou et al., 2012a). Local spring tidal range is about 20 cm. Gradual sea-level rise since 1956, can be estimated to ca. 12 cm (at an average rate of 2.1 mm/yr), i.e. much faster than the potential bioerosion/dissolution rate, preventing the development of a new tidal notch at sea level. According to the inward depth of 15 cm of this notch, its period of development with a possible variability of bioerosion between 0.2 and 1 mm/yr, leads to the assumption that the notch was formed in a uniform lithological, biological and climatic conditions, lasted some centuries and was probably still active around 232 ± 35 years BP (i.e. at AD 1718 ± 35). This radiocarbon age has been deduced from a Cerastoderma shell collected at a depth of 63 cm by core NA2 collected at Naxos near St. Georgios (Evelpidou et al., 2012b). At that time, the submergence of the notch had probably not yet started. In general, the height of the Keros notch was initially increased until the end of the 19th century, partly by the global sealevel rise (when it remained smaller than the bioerosion rate). The submersion started at least around 1865, when the rate of the rise increased (Kemp et al., 2011) and reached up to 14 cm between 1865 and 1956. An additional seismic subsidence of about 10 cm occurred in 1956. Finally, the present situation was attained by taking into account additional 11 cm of global sea-level rise occurred since 1956. 3.4. Ios Island (Cyclades) At Ios Island (southern Cyclades) tidal notches are absent in the present-day mid-littoral zone, as well as at a small depth, with the exception of some marks at certain sites at about − 60 cm. However, a well developed fossil shoreline, e″ type, exists at several sites at a depth of about 3 m (Fig. 4d). The corresponding notch has an inward depth of several decimeters, testifying of a relatively stable sea level, under assumed conditions, during many centuries. The present position of the fossil shoreline can be interpreted as resulting from, still undated, strong co-seismic subsidences, the last of which could only have been occurred a few centuries ago. Local tidal range is about 20 cm. 3.5. Folegandros Island (Cyclades) At Folegandros Island (southern Cyclades) the present-day tidal notch is absent, but a submerged tidal notch of d′ type, can be identified (Fig. 4e) with a roof at a depth of 20± 10 cm, a vertex at 31± 10 cm below sea level and an inward depth of about 19 cm.

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Fig. 4. a. No marks of present-day tidal notch appear in 2009 in the Atalandi Mines coastal area (Gulf of Euboea, central Greece). b. A visor caused by a freshwater spring undercuts at sea level the continental cliffs facing the small Ampelos Island (northern coast of Corinth Gulf). c. Submerged fossil tidal notch at Keros Island (SE Cyclades, Greece), no marks of present-day tidal notch appear here in 2010. d. At Ios Island (southern Cyclades) tidal notches are absent in the present-day mid-littoral zone, and almost absent at a small depth, until a significant fossil tidal notch at a depth of about 3 m. e. At Folegandros Island (southern Cyclades) the present-day tidal notch is also absent. f. Submerged shoreline at Amorgos Island (S–E Cyclades) near Mouros (south coast), while tidal notches are absent in the present mid-littoral zone or at small depth.

It would have required a period of some centuries to form under assumed conditions, followed by a period of relative sea-level rise of about 31± 10 cm at a rate faster than the potential bioerosion/dissolution rate, probably during the last two centuries, that might include a small co-seismic subsidence of a few centimeters. The local tidal range is about 20 cm. 3.6. Amorgos Island (Cyclades) At Amorgos Island (S–E Cyclades) near Mouros (south coast), in the absence of tidal notches in the present mid-littoral zone or at

small depth, a well developed tidal notch of e″ type, continuous over at least 100 m, has been cut at 136 ± 10 cm below sea level in a massive crystalline limestone formation (Fig. 4f). Its inward depth of about 28 cm suggests that it required a period of 3 to 14 centuries to form, under assumed conditions. It also suggests that sea level remained almost stable at this level during this period, before a co-seismic subsidence of about 1.1 ± 0.1 m that remains to be dated. This subsidence is probably older than the 1956 earthquake as Stiros et al. (1994a) reported that a slight uplift of 20–30 cm occurred at that time along the south coast of the island.

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4. Discussion and conclusions Our investigation of submerged tidal notches builds on prior studies in the Mediterranean area, especially from the Adriatic and Aegean coasts (e.g. Pirazzoli, 1980; Fouache et al., 2000; Faivre and Fouache, 2003; Antonioli et al., 2004; Benac et al., 2004, 2008; Nixon et al., 2009; Evelpidou et al., 2011a, b; Furlani et al., 2011). In cases where the absence of the present-day tidal notch has been noted by observers in certain places, it has generally been ascribed to gradual tectonic subsidence. In the present study, a correlation between the period of development of a fossil submerged notch and the period of the recent global sea level rise has been demonstrated at Keros, showing that the fossil submerged notch was still active in the 18th century, just before the beginning, of the global sea-level rise. At another site (Atalandi Mines), a fossil tidal notch was submerged by a co-seismic subsidence in 1894 and no new tidal notch could form since that time. At Folegandros, where a submerged fossil notch has also been observed, its depth underwater is almost consistent with the amount of the recent global sea-level rise. Finally, at other sites, observed submerged fossil tidal notches at greater depths, provide evidence of still undated tectonic movements of subsidence. An important point concerning the interpretation of slightly submerged tidal notches is how to distinguish the origin (eustatic or tectonic) of the submergence. Although geographical variations in sea-level trends may exist, in the absence of documented local tectonic trends we propose that recent submergence of the order of at maximum 20 to 30 cm has affected sites like at Folegandros. In contrast, greater submergence would imply a tectonic effect, such as that observed at Ios and Amorgos. The occurrence of recent co-seismic subsidence appears to have caused local submergences of tidal notches up to half a meter (including the effects of the recent global sea-level rise) at several sites of the northern coast of the gulf of Corinth (Evelpidou et al., 2011b), although such effects could not be detected at other sites of the same area, because fresh water from coastal springs tends to influence erosion and coastal geomorphology. 5. Summary and conclusions The fact that during the last two centuries the rate of global sealevel rise has become greater than the natural possibilities of marine bioerosion, causing the disappearance of tidal notches, is producing a lacuna in geologic marks, which should be taken into account in the interpretation of geologic and oceanographic events during the last few centuries. According to climatic models that predict acceleration in global sea-level rise, such lacuna can be expected to widen and complicate the geologic and geomorphologic interpretation of certain tectonic rates and especially of tectonic and seismic events having occurred after the 19th century. According to predictions of near-future acceleration for the next century provided by climatic models, one can expect even that new tidal notches will not develop for an undetermined period of time (e.g. Jevrejeva et al., 2012). Nevertheless, past submerged tidal notches can still be used to understand past changes in relative sea-level. Acknowledgments Part of this work was supported by COST Action ES0701 “Improved constraints on models of Glacial Isostatic Adjustment”. Authors wish to thank the president of the community and Mrs Gavala for the provided facilities at Koufonissia islands, the president of the municipality for facilities provided at Ios Island and the president of the municipality's council Mr. Kyriakos Venios for boat facilities at Ios Island. We wish also to thank the president of the municipality Mr. Giannis Lidis for his provided boat facilities at Folegandros island. Finally, we thank Mr. Theodosis Kritikos for revision of the English text. Constructive comments

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of the editor Thomas M. Cronin and the two anonymous reviewers have been appreciated and contributed to improve this study that is a contribution to the IGCP Project 588 (Preparing for coastal change).

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