Relative sea-level fluctuations of the last millennium recorded by intertidal algal rims (NW Sardinia,Mediterranean Sea)

Sampling, structure analysis and dating

The areal distribution of Lithophyllum byssoides rims was surveyed by kayak between March and October 2023 along approximately 24 km of coastline around the Capo Caccia peninsula (Figures 1 and 2(a)). Sampling focused on two main sites, O1 and N1, which are located in areas less exposed to hydrodynamic effects caused by the interaction between wave action and cliff morphology (Figures 1(b), 2(b) and (c)). To minimize vertical uncertainty, sampling was avoided in areas near cave entrances, narrow inlets, or major rock fractures, where local hydrodynamics could affect the vertical growth of the algal rims (Figure 2(d)). Rims were sampled with a hammer and chisel during calm sea conditions. Their elevation measured using a metric rope and GPS, is expressed relative to Biological Mean Sea Level (BMSL) defined as the lowest elevation of the presence of living L. byssoides thalli at each site. The nearest Porto Torres tide-gage record, the global EGM96 geoid and the Italian national vertical datum (Genova, 1942) were used only as auxiliary references for measurement consistency checks and were not adopted as reference datums for RSL index-point calculations.

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Figure 2.

Bio-geomorphological survey. (a) Panoramic view of Liyhophyllum byssoides rim around Capo Caccia cliffs (person on kayak for scale); (b) Detail view acquired during sampling of O1 rim growing on a small wave-cut platform. Note a well-developed micro lagoon on the left. Scale bar = 10 cm; (c) Detail view acquired during sampling of N1 rim growing within a well-developed tidal notch. Scale bar = 10 cm; (d) Detail view of a Liyhophyllum byssoides rim growing around large rock fracture. Note the hydrodynamic effects on rim highlighted by the sea water cascade. (e) Panoramic view of a cliff fully exposed to major storms. Note the rim ruptures and multiple interruptions.

A total of two samples were collected (O1: 24.2 × 20.1 cm; N1: 23.8 × 16.1 cm; dimensions measured along the longest vertical and horizontal axes, respectively). The algal rim samples were longitudinally sliced following their natural growth direction, and a total of 35 standard petrographic thin sections were prepared (14 from N1; 21 from O1; Figures 3–5, SM1). Investigation of algal builders, analysis of rim inner structures and growth pattern along with identification of sediment clasts, matrix and cement compositions were carried out on petrographic thin sections using transmitted-light optical microscope and a Scanning Electron Microscope (SEM; JEOL JCM-700; Figure 3). A morphometric analysis was conducted to quantitatively measure conceptacle chamber dimensions and to qualitatively observe algal thalli cell morphology, following standard taxonomic criteria (Chamberlain, 1994; Woelkerling, 1983). The description of algal macro- and microstructures followed the taxonomy and methodology of Woelkerling et al. (1993) and Di Geronimo et al. (2002). Distinct growth phases were identified based on: (i) erosional or growth discontinuities, (ii) sandy drapes or silica-rich muddy matrix intercalations and (iii) differences in cementation degree.

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Figure 3.

Thin section. (a) General overview with growth directions arrow. (b) Bio-siliciclastic trapped sediment details. (c) Secondary electron image of L. byssoides thallus and sediment. (d) Detail of L. byssoides conceptacle. (e) Reddish fine matrix in the trapped sediment, right below the N1 erosional surface. (f) Intra-thallus cement details with L. byssoides thalli cells.

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Figure 4.

O1 site. Geometry, vertical position and internal structure of the Lithophyllum byssoides algal rim, measured relative to the local Biological Mean Sea Level (BMSL), independently defined at the site from the lower living limit of L. byssoides. The rim position at the time of sampling is shown with respect to the Highest Astronomical Tide (HAT), with the Reference Water Level (RWL) indicated as the midpoint of the biologically defined indicative range (IR = HAT to BMSL). Rim thickness, internal pattern of growth and erosional surface dividing the rim growth phases (first–fourth phase). Note erosional surfaces zooms (E1–E3), together with the position of radiocarbon-dated samples.

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Figure 5.

N1 site. Geometry, vertical position and internal structure of the Lithophyllum byssoides algal rim relative to the local Biological Mean Sea Level (BMSL). The rim position at the time of sampling is shown with respect to the Highest Astronomical Tide (HAT), with the Reference Water Level (RWL) defined as the midpoint of the biologically constrained indicative range. Note rim thickness, internal pattern of growth and erosional surface dividing the rim growth phases (third to fourth phase), including portions dominated by subtidal coralline crustose algae (CCA). Erosional surfaces zoom (E), together with the position of radiocarbon-dated samples.

Six algal thalli from each sample were selected for radiocarbon dating (¹⁴C), sampled along the growth direction (Figures 4 and 5). Dating has been performed by the Beta Analytic laboratory (Miami, USA), before the measurement, the micritic matrix and bioclastic-rich sand were removed from the chosen thalli. Radiocarbon ages were calibrated using the MARINE20 curve (Heaton et al., 2020) and corrected for the western Mediterranean marine reservoir effect (∆R = −184 ± 22; Faivre et al., 2019b). Following the conceptual approach proposed by Basso et al. (2022), vertical accumulation was quantified where continuous rim growth could be demonstrated within a single growth phase. This approach was applied only to the lower zone of N1 rim (see chapter 4.5 and Figure 5) where no evidence of erosional truncation or growth hiatuses was observed. In this case, rims accumulation rate was therefore calculated using the vertical distance between dated samples belonging to the same growth phase and progressing toward the top living surface of the bioconstruction.

Analysis of relative sea-level index points

The Relative Sea-level (RSL) was calculated for all the ¹⁴C dated thalli (12 in total) and estimated using the following equation (Shennan and Horton, 2002; subsequently modified by Vacchi et al., 2016):

R S L i = A i − R W L i

where A i is the altitude; R W L i is the Reference Water Level of sample i ;both expressed relative to Biological mean sea level BMSL of O1 and N1 study sites (Table 2). As an example, a dated thallus at site N1 has a measured elevation of 7 cm below the N1 BMSL, thus, A is −7 cm. For Lithophyllum byssoides, the indicative range (IR) yields a Reference Water Level (RWL) of 25 cm relative to BMSL (midpoint of the IR; Figure 4). The relative sea-level position is therefore calculated as: RSL = A − RWL = −7−25 = −32 cm. Following Vacchi et al. (2016), the Reference Water Level (RWL) represents the midpoint of the indicative range (IR) of a given sea-level indicator and reflects its indicative meaning rather than a measured elevation. For Lithophyllum byssoides bioconstructions, the indicative range is biologically defined and spans the intertidal zone between the Highest Astronomical Tide (HAT) and the Biological Mean Sea Level (BMSL; Figures 4 and 5). Along Capo Caccia limestone cliffs, observations of rim position indicate a typical vertical dispersion of approximately 20 cm, consistent with previous studies, and this range is conservatively expressed as an uncertainty of ±10 cm around the RWL. Within this uncertainty, BMSL is assumed to approximate present-day mean sea level, thereby minimizing site-specific hydrodynamic conditions (Faivre et al., 2019a, 2021, 2023, 2024; Rovere et al., 2016; Vacchi et al., 2016). Unlike biological or sedimentary index points, terrestrial limiting points (TLP) do not require the definition of a full indicative range and provide only a unidirectional constraint on past sea level. Therefore, the coastal quarry surfaces south of Capo Caccia (Figure 1; Pascucci et al., 2019) are treated here as terrestrial limiting points providing an upper limit to relative sea level, indicating that sea level could not have exceeded the quarry working surface elevation during their period of use (Pascucci et al., 2019; Vacchi et al., 2016). A vertical uncertainty of ±10 cm was adopted, reflecting the limited functional tolerance of quarry working surfaces and the good preservation state of the structures, in line with previous archeological sea-level studies in the Mediterranean (e.g. Antonioli et al., 2007). In addition, all the markers were adjusted for glacial isostatic adjustment (GIA) using a constant isostatic vertical rate of −0.4 ± 0.15 mm/yr, as suggested by Spada and Melini (2022), based on an ensemble of ANU S4 (E1) and ICE-6G C (VM5a) Earth models (Lambeck and Purcell, 2005; Peltier et al., 2015, respectively).

Bio-geomorphological survey

Results of the kayak survey show an almost continuous intertidal algal rim around the main promontories of the study area (Figures 1(b) and 2(a)). Prominent and well-developed bioconstructions occur along the northern and western coasts of Capo Caccia (Figure 1), where they are subject to consistent wave action due to direct exposure to the prevailing Mistral winds (NW) and associated swells. This observation aligns with previous studies, which indicate that the growth, thickness and morphology of L. byssoides rims are strongly influenced by local wave energy exposure (Faivre et al., 2013; Sechi et al., 2020). The thickest and largest rims develop at cave entrances and within large rock fractures, where non-breaking wave run-up causes an increase in water level against the cliff face (Figure 2(d)). Nevertheless, bioconstructions growing on cliffs fully exposed to severe swells tend to be thinner and show signs of breakage and discontinuity (Figure 2(e)). Rim thickness and lateral continuity increase in particular geomorphic settings, such as rocky inlets with partial protection and a deeper seabed in front. These conditions promote stable environments that favor both vertical and seaward lateral accretion of the rim structure.

Rim inner structure and builder

Thin section and SEM analyses reveal that both the O1 and N1 rims consist of several generations of anastomosing algal thalli with characteristic upward-facing, fan-shaped branches that define the predominant upward and seaward growth directions of the rim (Figure 3(a); Mannino, 2003; Woelkerling et al., 1983, 1993; dryades.units.it). These bioconstructions are formed by a continuous superposition of new thalli above the dead ones following a slow-rising sea level (keep-up like model), as generally proposed by Pérez and Picard (1964) and (Adey, 1986). Based on these structural features, the bioconstructions are classified as framestones, as the in-situ fossil Crustose Coralline Algae (CCA) thalli form a rigid framework, with bio-siliciclastic sandy sediments trapped between different algal generations (Figure 3(b)). Conceptacle chambers morphometry (typically ~100–160 µm in average width; Pezzolesi et al., 2017) led to the taxonomic identification of L. byssoides as the dominant framework builder of the studied bioconstructions (Figure 3(c) and (d)). The trapped sediment is composed of fine-grained bioclastic sand, including fragments of coralline algae, bivalves, gastropods, bryozoans, foraminifera and serpulids, along with minor quartz and feldspar grains. In some areas of the O1 and N1 inner rims, a silica-rich, reddish terrigenous muddy matrix is abundant (Figure 3(e)). Typical intertidal calcite spar cement, with inter- and intraparticle equant crystals, welds the sediment to the algal thalli, partially filling the voids and consolidating the rim framework (Figure 3(f)). The high complexity of the rim inner structure is primarily due to algal growth and infilling within cavities produced by bioeroders (e.g. Lithophaga lithophaga). The identification of conceptacle chamber orientation (Woelkerling, 1983; Chamberlain, Y.M. 1994), along with the morphometric analysis of algal thalli, has proven essential for interpreting bioconstruction with stratigraphic coherence. Site-specific inner structures reveal multiple growth phases and shifts in sediment infill and cementation, as outlined below.

Study site O1

Site O1 is located on the west coast of the Capo Caccia peninsula near Isola Piana, in a rocky embayment that is open to the west but partly sheltered from NW-incoming Mistral winds. The seafloor here reaches approximately 7 m depth (Figure 1(a) and d). It features a well-developed rim, 24 cm thick which grows on a small wave-cut platform, a few decimeters wide, forming a micro-lagoon on its upper surface near the cliff (Figures 2(b) and 4). It is attached to the vertical substrate through the land-facing side and the sea-facing side features a protuberance in its upper part. The most abundant top side coverage consisted of living individuals of Lithophyllum byssoides surrounded by few other Crustose Coralline Algae (CCA; i.e. Lithophyllum incrustans, Lithophyllum papillonis). The sea side of the rim, immediately below the lower limit of living Lithophyllum byssoides individuals (BMSL), is characterized by a typically subtidal assemblage dominated by Ceramium and Corallina elongata and exhibits particularly intense bioerosion compared to the upper surface, with cavities mostly occupied by Lithophaga lithophaga and Patella caerulea.

A general fruticulose aspect with a multi-layered structure and the branch shaped thalli has been observed from the polished rim slice (Figure 4). O1 shows three barely developed erosional surfaces, delineating four different inner zones (Figure 4). Furthermore, a millimeters-thick white crust of closely patched laminar sets of other CCA developed inside the micro lagoon on the top of the land side of the rim. The lower unconformity is highlighted by a higher degree of cementation and a diffuse silty-clay reddish matrix in the rim below the surface (Figures 3(e) and 4). In the inner zone above the latter surface, cementation is markedly lower and the reddish muddy matrix is absent. The middle and upper unconformities are dominated by rough surfaces with several cavities filled by biogenic and siliciclastic sediments (Figure 4). The inner zone between these unconformities is rich in sediments and muddy reddish matrix, whereas the outermost part shows a well-developed sediment drape lying on the upper surface.

Study site N1

Site N1, located on the north coast near Punta del Gallo, lies within a northwest-facing rocky cove that is slightly exposed to Mistral winds and characterized by a shallow seafloor, approximately 3 m deep (Figure 1(a) and (c)). It exhibits a 24 cm thick rim that stands at the base of a well-developed tidal notch (Figures 2(c) and 5). The land-facing side is attached to the vertical substrate, while the sea-facing side features an upper protuberance and a highly porous surface, with millimeter- to centimeter-sized cavities, many of which are occupied by Lithophaga lithophaga. Externally, the rim is predominantly covered by subtidal Crustose Coralline Algae (e.g. Lithophyllum incrustans, Figure 2(b)), and is generally sparsely colonized by living Lithophyllum byssoides, which occurs only with a few individuals above the top surface.

From the polished rim slice, the multi-layered structure and the branch shaped thalli give a general fruticulose aspect to the inner pattern (Figure 5). The presence of a well-developed erosional surface draped by a centimeter thick sandy accumulation, allows us to subdivide the N1 slice into two inner zones (Figure 5). Below the erosional surface, sample N1 shows well-formed cement crystals, with algal skeleton cells mostly filled, reducing the porosity, along with a higher concentration of silty-clay reddish matrix (Figure 3(e)). Above the erosional surface, there are more interspaces between the thalli, and the trapped sediments are generally less cemented with conceptacle chambers often empty. Moreover, a distinctive white crust, ranging from a few millimeters to 1 cm in thickness, has developed on the seaward side of the rim (Figure 5). This encrustation is composed of tightly packed thalli of subtidal Crustose Coralline Algae (CCA), forming a continuous and visually striking white layer. Its presence indicates a shift from intertidal to subtidal assemblages, as the crust is composed of CCA species typically found in deeper, permanently submerged environments.

Chronology of rim growth

To reconstruct the chronological framework of rim accretion, a total of 12 Lithophyllum byssoides thalli fragments were collected from the O1 and N1 sites for radiocarbon dating (Figures 4 and 5 and Table 2). For O1, six samples were extracted from distinct inner zones of the rim (Figure 4). Two fragments, from the innermost land-side portion, yielding calibrated radiocarbon ages of 1050 ± 78 and 1060 ± 78 cal AD, respectively. Two additional samples, taken from the mid-zone between the lower and middle erosional surfaces, returned ages of 1630 ± 82 and 1631 ± 82 cal AD. Two thalli collected from just below and above the uppermost erosional surface were dated to 1720 ± 83 and 1881 ± 63 cal AD, respectively.

Six thalli fragments were dated to constrain the accretional history of the N1 rim (Figure 5). Three samples collected from the lower zone, beneath the erosional surface, yielded ages of 1701 ± 88, 1713 ± 87 and 1740 ± 80 cal AD. The lower zone of N1 rim segment shows continuous accretion between 1701 and 1740 AD with no evidence of erosional truncation or growth hiatuses. Over this interval, the bioconstruction records a vertical accretion of 16 cm, corresponding to an average accumulation rate of ~4 mm/yr (16 cm over ~40 years). This value represents a phase-specific bioconstruction accumulation rate, applicable only to this sample and to this time span due to the consistent succession of the dated thalli. This value is reported as an indicative mean accumulation rate, given that radiocarbon age uncertainties. In contrast, three samples retrieved from above the erosional surface yielded younger ages of 1909 ± 41, 1920 ± 30 and 1924 ± 26 cal AD.

The stratigraphic distribution shows O1 ages gathering into four coherent growth phases (Figure 4), with each age group falling within the 68.2% probability intervals and showing no overlap between age clusters (Table 1). The N1 rim also displays two distinct age clusters (Figure 5), with no overlap in their calibrated age probability distributions (68.2% probability intervals; Table 1). This chronological separation mirrors the observed sedimentological discontinuity, suggesting a pause or shift in rim growth, followed by renewed accretion in subsequent, progressively younger phases. The vertical and lateral arrangement of the dated material—from older layers at the base to younger ones toward the seaward margin—confirms a preferential accretional growth trajectory oriented seaward and upward, from the substrate toward the outer surface of the rims.

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Table 1.

Calibrated radiocarbon ages of Lithophyllum byssoides samples. Calibrated calendar ages obtained from AMS radiocarbon dating of Lithophyllum byssoides samples used in this study. Radiocarbon analyses were performed by Beta Analytic. Calibration was carried out using the MARINE20 calibration curve with the appropriate regional marine reservoir correction (ΔR), applying the High Probability Density (HPD) method. Ages are reported as median values of the calibrated age distributions together with the corresponding 68.2% probability intervals (see Supplemental Material for more details).

Sample ID	Lab. No.	Selected material	cal 14C age AD (median)	Cal age range (68.2%, cal AD)
N1	Beta-669808	L. byssoides thallus	1701	1613–1790
N3	Beta-669809	L. byssoides thallus	1713	1626–1800
N9	Beta- 669812	L. byssoides thallus	1740	1660–1820
N10	Beta-705827	L. byssoides thallus	1909	1896–1939
N13	Beta-705829	L. byssoides thallus	1920	1890–post 1950
N14	Beta-669813	L. byssoides thallus	1924	1898–post 1950
O1	Beta-705029	L. byssoides thallus	1050	972–1128
O2	Beta-705034	L. byssoides thallus	1062	984–1141
O7	Beta-705030	L. byssoides thallus	1630	1549–1714
O11	Beta-705031	L. byssoides thallus	1631	1548–1715
O16	Beta-705032	L. byssoides thallus	1720	1637–1804
O20	Beta-705033	L. byssoides thallus	1881	1818–post 1950

Climate control on algal rim evolution over the last 1000 years

The intertidal algal rims studied at Capo Caccia (Sardinia, Mediterranean Sea) clearly display four distinct phases of Lithophyllum byssoides growth, each separated by marked erosional surfaces. These growth phases correspond to periods of relative sea-level rise, whereas the erosional surfaces—draped in bio-siliciclastic sand-sized sediments and reddish muddy matrix—point to prolonged (multi-decadal) episodes of subaerial exposure and relative sea-level fall. Together with AMS radiocarbon ages, these features indicate four sub-centennial phases of sea-level rise and three sea-level falls over the last millennium, which can be tied to European summer temperature anomaly reconstructions (Luterbacher et al., 2016; Wanner et al., 2022; Figure 6).

The first growth phase, observed in the O1 rim and dated to around 1050 AD, developed during the Medieval Climate Anomaly (900–1225 AD), a prolonged warm period preceding the cold Little Ice Age (Wanner et al., 2022). The available data are insufficient to precisely constrain the entire duration of this growth phase. Furthermore, regional to global reconstructions highlight pronounced spatial and temporal variability in global mean sea level (GMSL) and regional relative sea level during the MCA. These records indicate a relative lowstand around ~1000–1050 AD (Oort Minimum), followed by a relative sea-level rise between ~1100 and 1200 AD (e.g. Faivre et al., 2023; Kopp et al., 2016; Vacchi et al., 2021), occurring during a period when mean air temperatures were comparable to or slightly lower than present-day values (Figure 6; Wanner et al., 2022). Subsequently, a period of both arrested L. byssoides growth and partial erosion of the rim occurred, likely driven by a relative sea-level fall associated with the onset of the Little Ice Age (LIA). Notably, phases of reduced solar activity during the Common Era have been associated with global mean sea level lowering (e.g. Faivre et al., 2023; Kopp et al., 2016), providing a broader climatic context for the observed local RSL behavior. The definitive transition to the European LIA took place with the Wolf and Spörer Grand Solar Minima (GSM; periods of markedly reduced solar activity), each accompanied by clusters of volcanic eruptions (Wanner et al., 2022). The longer LIA cooling phases were typically triggered by such eruptions, which coincided with Grand Solar Minima (Bradley et al., 2016) and were further intensified by internal feedbacks involving the atmosphere, oceans and sea ice. Around 1575 AD, temperatures dropped to −2°C compared to present (Luterbacher et al., 2016), marking one of the coldest intervals of the LIA. The second growth phase took place in the first half of the 17th century during a relatively warm period (−0.3°C below present) between the Spörer and Maunder solar minima (Wanner et al., 2022). Subsequently, a relative sea-level drop frames the Maunder Minimum (air temperature −0.5°C below present), and during the first half of the 18th century, rim accretion resumed with a pronounced relative sea-level rise. This period was characterized by a positive European summer temperature anomaly, with temperatures reaching 1990s levels (Figure 6). This warming was driven by increased solar irradiance (marking the end of the Maunder Minimum) and a reduced incidence of volcanic eruptions (Perry and Hsu, 2000; Warren et al., 2022), creating favorable conditions for the algal rim development. The sampled rims also show evidence of a third severe erosion (Figures 3 and 5), linked to a relative sea-level drop driven by the last cold pulse (Dalton Minimum) of the LIA, occurring between 1800 and 1860 AD (Wanner et al., 2022). In the early 19th century, European summer temperatures fell to −3°C compared to present (Luterbacher et al., 2016; Figure 6), influenced by the Dalton GSM and at least four major volcanic eruptions between 1808 and 1835 AD. The 1815 Tambora eruption, in particular, played a decisive role in this climate cooling (Sigl et al., 2015). This cooling is also reflected in the third and most pronounced advance of the Great Aletsch and Gorner Alpine glaciers in 1860 AD (Holzhauser et al., 2005). With the onset of the Industrial Era after 1860 AD, gradual warming began, likely driven by reduced volcanic activity and increased solar irradiance (Calvin et al., 2023; Wanner et al., 2022; Figure 6). This natural warming set the stage for modern sea-level rise and triggered the fourth phase of rim accretion at Capo Caccia. In the 20th century, greenhouse gas concentrations increased exponentially, and after 1988 AD, the transition to rapid warming accelerated global sea-level rise consistently (Calvin et al., 2023; Luterbacher et al., 2016; Vacchi et al., 2021; Wanner et al., 2022). This phenomenon has led to the ongoing submergence of the N1 rim and the formation of a white crust made by other subtidal Crustose Coralline Algae, clearly visible on its seaward side (Figures 2(c) and 5). In contrast, the O1 rim appears to have maintained active vertical accretion, likely supported by site-specific exposure conditions that promote frequent water renewal within the intertidal zone (Figures 2(b) and 4). Currently, the surface of the N1 rim shows a marked reduction in Lithophyllum byssoides and a profuse presence of other CCA species, typically thriving in subtidal environments (e.g. L. incrustans). This shift in CCA assemblage is consistent with a partial drowning of the N1 rim and thus reflects a biological response to the recent acceleration in relative sea-level rise.

Relative sea-level changes and comparison with proxy-based reconstructions

Sub-centennial sea-level fluctuations during the Late-Holocene remain poorly constrained in the proxy record. To evaluate the broader significance of the four rim growth phases and associated sea-level rises and drops observed at Capo Caccia, these findings are cautiously compared with established Mediterranean and global sea-level reconstructions (Garcia-Artola et al., 2009; Gehrels et al., 2006; Goodwin and Harvey, 2008; Gregory et al., 2006; Leorri et al., 2008; Faivre et al., 2019a, 2023; Kemp et al., 2011; Kopp et al., 2016; Vacchi et al., 2021), as well as with millennial-scale climate variability and solar insolation trends (Figure 6; Luterbacher et al., 2016; Perry and Hsu, 2000; Wanner et al., 2022).

At Capo Caccia, the Medieval Climate Anomaly is recorded in the early phase of O1 rim development (ca. 1050–1060 AD). The associated relative sea level index points are positioned above present sea level and, following the Oort Minimum, likely reflect environmental conditions favorable to algal rim accretion (Figures 4 and 6). This interpretation is consistent with previously reported RSL rise rates, ranging between 0.5 and 0.7 mm/yr from algal rim records in the Adriatic Sea (Faivre et al., 2019a, 2023, 2024), as well as comparable rising rates inferred from North Atlantic salt-marsh sedimentary records between ~850 and 1250 AD (Kemp et al., 2011). A more prolonged RSL fall followed between 1200 and 1600 AD, which corresponds to the development of a well-defined lower erosional surface in the O1 rim. During the ca. 1500–1600 AD interval, terrestrial limiting points from the Alghero coastal quarries constrain the local RSL to have remained at least 35 cm below present mean sea level (Pascucci et al., 2019). After correction for glacial isostatic adjustment and considering 10 cm of uncertainty, this constraint corresponds to a terrestrial limiting elevation ranging between 19 and 29 cm, below present mean sea level (Figure 6). This terrestrial limiting point indicates that relative sea level could not have exceeded the elevation of the quarry working surfaces during their period of use (Pascucci et al., 2019). In addition, the occurrence of a reddish matrix within the studied O1 algal rim further suggests extended periods of subaerial exposure, implying that sea level may have dropped by over half a meter during the onset of the Little Ice Age as firstly assumed by Pascucci et al. (2018) working on the sedimentary sequence of Mistral beach-ridge system (W Sardinia). This lowstand aligns with near-stable or slightly negative trends recorded in various Mediterranean and global sea-level reconstructions (Faivre et al., 2019a, 2023; Kemp et al., 2011; Vacchi et al., 2021), reinforcing the climatic significance of this period. A renewed phase of relative sea-level rise is identified between ~1600 and 1630 AD. The onset of this phase is constrained by a terrestrial limiting point (TLP) from the Alghero coastal quarries, while its upper termination is defined by two RSL index points from the O1 rim. This phase records a RSL rise that occurred within a relatively warm interval between the Spörer and Maunder solar minima (Figure 6). This phase was short-lived, as growth ceased again in the latter half of the 17th century—coincident with the Maunder Minimum—when a relative sea-level drop of at least 20 ± 10 cm, producing erosion and subsequent increments of sediment accumulation, likely interrupted and slowed the O1 rim accretion (Figures 4 and 6). Growth resumed during the following warm period (ca. 1700 and 1740 AD), between the Maunder and Dalton minima, indicating a phase of sustained relative sea level rise. While these growth-related relative sea-level rises exceed the frequency and amplitude typically reported for the Late-Holocene, they may reflect brief warm oscillations that are not clearly resolved in the proxy record. In contrast, during this time interval, several studies document a lack of rim formation (Faivre et al., 2019a) or stable to slightly negative sea-level behavior (Kemp et al., 2011). Following 1740 AD, both rims display evidence of a marked depositional hiatus and erosional truncation, suggesting a relative sea-level fall of at least 20 ± 10 cm. Coastal stratigraphy from western Sardinia further supports this interpretation, with records of beach progradation linked to falling sea level and increased sediment supply (Pascucci et al., 2018). This lowstand coincides with the final cold stage of the Little Ice Age, associated with the Dalton Grand Solar Minimum (ca. 1800–1860 AD) and aligns with global reconstructions indicating a significant sea-level drop during this time (Gregory et al., 2006; Kopp et al., 2016). These parallels suggest that the sea-level trend observed at Capo Caccia likely fits within a broader regional, northern hemispheric, pattern. At Capo Caccia the algal rims record a renewed relative sea-level rise between 1881 and 1909 AD, increasing progressively by 1924 AD. These interpretations are in agreement with regional observations from the northeastern Adriatic (Faivre et al., 2019a) and with records from the North Atlantic, where rates of up to 2.1 mm/yr were documented between 1865 and 1892 (Kemp et al., 2011). A broader global acceleration in sea-level rise around 1863 AD is also supported by proxy database analyses (Walker et al., 2022). This early phase of modern sea-level rise likely reflects a combination of reduced volcanic activity and modest increases in solar irradiance following the end of the Little Ice Age. However, these natural forcings remained relatively stable or declined in the following decades, indicating that they cannot account for the pronounced acceleration observed after the mid-20th century (Calvin et al., 2023; Haigh, 2019). According to a growing number of global and relative sea-level reconstructions (Faivre et al., 2019a, 2023; Gregory et al., 2006; Kemp et al., 2011; Kopp et al., 2016; Vacchi et al., 2021), the increase in greenhouse gas concentrations during the industrial era—particularly from the second half of the 20th century—has led to a marked warming of global temperatures. This, in turn, triggered a progressive and widespread acceleration of global sea-level rise through thermal expansion of seawater and increased melting of land-based ice. In this context, Mediterranean sea-level curves show rates ranging between 1 and 3 mm/yr (Faivre et al., 2019a; Vacchi et al., 2021), which are consistent with the presence of well-developed subtidal external white crusts in the N1 rim (Figures 5 and 6). These findings confirm that the growth phases of the algal rims capture the transition from “natural” Holocene sea-level variability to modern, anthropogenically driven sub-centennial sea-level trends.