This study was carried out on a sedimentary core (PCSC-4) retrieved from Pântano do Sul beach (27∘46′36.49′′ S, 48∘31′45.96′′ W; Fig. 1), located at the southernmost part of Santa Catarina Island. The core was drilled using a Russian peat corer on a peat deposit, reaching a maximum depth of 650 cm. The sampling site is located approximately 1 km from the current coastline and ca. 1 m above the present sea level. The 50 cm long sediment sections were sealed and transported to the Laboratório de Palinologia Marleni Marques Toigo at the Federal University of Rio Grande do Sul.

Four bulk organic-rich sediment samples were selected along the core and analysed with the accelerator mass spectrometry (AMS) at the Center for Applied Isotope Studies (CAIS) Laboratory of the University of Georgia (USA) for radiocarbon dating. Sample selection was made after the palynological analyses that aimed at obtaining ages for significant changes in the palynological record. The radiocarbon dates were calibrated using the Southern Hemisphere calibration curves (SHCal20; Hogg et al., 2020), rounded to the nearest decade and reported as calendar years before present (cal yr BP). The age–depth model was constructed with the software RStudio (RStudio Team, 2021), using the Clam 2.4.0 package (Blaauw, 2010) with linear interpolation (Fig. 3).

Grain size analyses and calculation of organic matter content were made for 64 samples with 10 cm intervals along the core. The samples were equally separated into two sub-samples to determine the grain size analyses and the organic matter content in the sediment. These analyses, as well as the calculations of the statistical parameters, were carried out at the Center for Studies of Oceanic and Coastal Geology (CECO) at the Federal University of Rio Grande do Sul. All the samples were dried in an oven at 40 ∘C and were then weighted. The analyses were performed by a sieving–pipetting method, following the statistical parameters of Folk and Ward (1957) and the textural classification of Shepard (1954). To determine the organic matter content, the samples were calcined in muffle at 550 ∘C over 4 h and were weighed before and after the calcination. The organic matter content of the sediments was determined by the loss of ignition after this process.

A total of 33 samples with 3 cm3 of sediment were collected at 20 cm intervals from the top to the bottom of the core (0–640 cm) added to the basal sample (650 cm) for total organic carbon (TOC), total nitrogen (TN) and δ13C analyses. At first, the samples were dried in an oven at 60 ∘C and weighted. Samples were treated with 10 % HCl to eliminate carbonate and were then washed with Milli-Q water until the pH reached 5. Samples were dried in a freeze-dryer, weighted again and then homogenized to be analysed in the elemental analyser coupled to isotope ratio mass spectrometry (EA-IRMS) in the Hinrichs Laboratory at the University of Bremen. TOC and TN values are expressed as a percentage of dry weight, and δ13C is expressed in delta-per-mil notation, with an accuracy of ±0.17 ‰ with respect to the VPDB standard. The C/N (weight / weight) was calculated using the elemental results ratio.

A total of 66 samples of 3 cm3 each were obtained throughout the core for pollen and spore analyses, with 10 cm spacing between them. After a preliminary taxonomic recognition, samples where dinoflagellate cysts were recorded (650–310 cm) were resampled for more detailed analyses, totalling 35 samples with the same spacing and bulk volume of sample (3 cm3). One Lycopodium clavatum spore tablet (18.584 ± 371 spores) was added to each sample before the chemical processing of both pollen and spore samples and dinoflagellate samples to allow concentration calculations (Stockmarr, 1971).

The pollen and spore samples were processed following standard preparation techniques (Faegri et al., 1989), using HF (40 %), HCL (10 %), KOH (10 %) and acetolysis. To concentrate the material, samples were sieved using a <250 µm sieve; ZnCl2 was used for heavy-liquid separation, checking the residues to be sure that no material was lost in the separation. Slides were prepared from drops of the final residue, mounted with Entellan.

The dinoflagellate cyst samples were prepared using similar procedures. However, to avoid damage to the cysts, the samples were not prepared using hot acids, KOH and acetolysis. The dinoflagellate cyst samples were decalcified with diluted HCl (10 %) and treated with HF (40 %) to remove silicates. After chemical treatments, the samples were sieved over a 20 µm mesh screen, and residues were transferred to an Eppendorf vial, where the material was concentrated in 1 mL portions. Slides were mounted with glycerin jelly for microscopic analysis.

Pollen and spore samples were counted until reaching a minimum of 300 pollen grains monitored by saturation curves. The other palynomorphs (i.e. spores, algae, acritarchs and microforaminiferal linings) and L. clavatum spores were counted in parallel. Concentrations (palynomorphs cm-3) were calculated using the L. clavatum spores as reference values.

Dinoflagellate cyst samples were counted until reaching their saturation curves. The total dinoflagellate sum adds all counted dinoflagellate cysts, and the relative abundances of each taxa in the dinoflagellate analyses are indicated as a percentage of the total dinoflagellate sum. Concentrations (dinoflagellate cysts cm-3) were calculated using the L. clavatum spores as reference values.

To integrate the dinoflagellate cysts and pollen and spore counts, we used the ratio of L. clavatum counts from the pollen and spore sample and the corresponding dinoflagellate cyst samples as a conversion factor. The dinoflagellate cyst counts were multiplied by this ratio and added to the final integrated diagram.

The total sum represents the sum of all palynomorphs (including dinoflagellate cysts), whereas the pollen sum refers to the total amount of pollen grains. The relative abundances of pollen grains were calculated as a percentage of the pollen sum, whereas the relative abundances of the other palynomorphs were calculated in relation to the total sum.

The environmental zones were established from changes in the palynomorph assemblages and from cluster analysis based on percentage values of the total sum. The depth-constrained cluster analysis (CONISS) was performed using the Edwards & Cavalli-Sforza's chord distance square-root transformation. Cluster analyses, percentage and concentration diagrams were constructed using the Tilia version 1.7.16 (Grimm, 2011). For the principal component analyses (PCA), we used the software Canoco (Šmilauer and Lepš, 2014) and PAST 4.03 (Hammer et al., 2001). Multivariate analyses were performed on palynological relative-abundance data.

The taxonomic determinations of the pollen and spores were based on comparison with modern equivalents in palynological reference collections (MP-Pr slides of the LPMMT/IGeo/UFRGS) and from the literature (e.g. Hooghiemstra, 1984; Neves and Lorscheitter, 1992; Herrera and Urrego, 1996; Lorscheitter et al., 1998; Colinvaux et al., 1999; Macedo et al., 2009; Cancelli et al., 2012). Dinoflagellate cysts were identified following the online key for dinoflagellate cyst determinations (Zonneveld and Pospelova, 2015, and references therein). Dinoflagellate cysts were grouped according to their life strategies, photosynthetic taxa (Operculodinium centrocarpum, O. israelianum, Spiniferites spp., Spiniferites mirabilis and Pentapharsodinium dalei) and heterotrophic taxa (Brigantedinium spp., Leipokatium invisitatum, Polykrikos kofoidii, P. schwartzii, Protoperidinium spp. and Selenopemphix nephroides).

The radiocarbon dating results are presented in Table 1, including uncalibrated and calibrated ages obtained from four selected samples. Calibrated ages indicate that the deposition of the studied core occurred entirely during the middle-to-late-Holocene interval, where the lowermost level (650 cm) has an age of 6520 cal yr BP, whereas the uppermost level (55 cm depth) revealed an age of 380 cal yr BP. The remaining samples presented intermediate ages.

The core consists of unconsolidated sediments composed of medium sand, fine sand, silt and clay added to variable amount of organic matter (Fig. 4). The organic matter is dominant (>80 %) from 220 cm to the top of the core. Regarding the distribution of clastic sediments, in general, there is a mixture of silt, clay and fine to medium sand from the base up to 220 cm (see distribution in Fig. 3). Localized calcareous shells in living position and shell fragments occur, scattered from the base until 400 cm of depth.

The geochemical data are presented as individual profiles along the studied core (Fig. 4) and as the binary plot of δ13C × C/N (Fig. 5). The total organic carbon (TOC) concentration varies from 0.5 % to 49.6 % and shows two main intervals separated by a gradual transition between them. The interval from the base of the core up to 240 cm shows an average value of 5.8 %, whereas between 220–200 cm, the average is 24.5 %. Samples from 180 cm to the top present an average TOC value of 44.6 %. The total nitrogen (TN) ranges from a minimum of 0.04 % at 650 cm depth to 2.1 % at 60 cm depth. C/N ratios (weight / weight) show nearly constant values of ca. 15 from the base up to 240 cm, followed by an abrupt increase from 240 to 180 cm (15.2–47.8) and a subsequent subtle decrease from 180 cm to the top (47.8–26.2; Fig. 4). The δ13C results are in the range of -12.2 ‰ to -29.6 ‰. The δ13C values are higher at the base of the sediment core (650–240 cm depth), with a range of -12.2 ‰ to -20.9 ‰, followed by a downward trend towards the top of the core.

A total of 114 distinct palynomorphs were identified along the 650 cm of the studied core, including pollen grains (59 taxa), spores (16), freshwater algae (4), marine algae (1), acritarchs (3), dinoflagellate cysts (10), indeterminate spores (8) and indeterminate pollen grains (13), as well as, fungi, microforaminiferal linings, scolecodonts and copepod eggs.

The palynological diagrams show the distribution of palynomorphs in the samples, grouped according to their ecological affinities (habit or habitat; Figs. 6–9). Both visual examination and the cluster and PCA analyses (Fig. 10) show that the samples can be grouped into four zones with characteristic species associations. For simplification purposes, the results will be presented according to the four zones discussed in Sect. 5 in ascending stratigraphic order.

High percentages of marine palynomorphs, including dinoflagellate cysts (8 %–78 %), microforaminiferal linings (<8 %), acritarchs (<17 %) and Cymatiosphaera (<5 %), are evidence of sea waters reaching the sampling site. Microforaminiferal linings are abundant in estuarine marshes, with variable salinity water influence (Batten, 1996). Species of the acritarch genus Micrhystridium are characteristic of shallow coastal water associations (Montenari and Leppig, 2003; Félix and Souza, 2012). The prasinophyte Cymatiosphaera, on the other hand, is known to be typically associated with marine water (Mudie et al., 2021).

The dinoflagellate cysts include photosynthetic (Operculodinium centrocarpum, O. israelianum, Spiniferites spp., S. mirabilis and P. dalei) and heterotrophic taxa (Brigantedinium spp., Leipokatium invisitatum, Polykrikos kofoidii, P. schwarzii, Protoperidinium spp. and Selenopemphix nephroides; Fig. 9). All the above-mentioned dinoflagellate cysts are commonly registered in the South Atlantic Ocean, as well as in coastal sites (Zonneveld et al., 2013). The presence of heterotrophic taxa in the association suggests high nutrient inputs in the waters. The photosynthetic association is dominated by Operculodinium centrocarpum and Spiniferites spp. On a global scale, the species Operculodinium centrocarpum has a cosmopolitan distribution (e.g. Zonneveld et al., 2013). However, in the western South Atlantic, this species is typically present in the relatively warm waters of the BC (Gu et al., 2019). Throughout this zone, specimens of Operculodinium israelianum and Spiniferites mirabilis are registered, which are typical for warm, temperate waters (Zonneveld et al., 2013). The dinoflagellate cyst association therefore indicates that marine waters entering into the study site had their origin in the relatively warm, saline BC waters.

Even though, nowadays, waters of the BCC and MC can seasonally reach the coast at the same latitude as that of the core position, we did not observe any evidence that this has been the case in Zone I (Fig. 2). Species that are characteristically abundant in MC and BCC waters, such as Selenopemphix antarctica and/or Impagidinium variaseptum (Zonneveld et al., 2013; Gu et al., 2019), were not observed in the studied material. The presence of a warm, temperate dinoflagellate cyst association, characteristic for high nutrient concentrations, implies that the lagoon waters were relatively warm and nutrient rich in this zone.

The presence of freshwater algae indicates freshwater influence despite the significant marine contribution. In addition, Botryococcus is a euryhaline freshwater algae that may have its photosynthetic activity inhibited, directly or indirectly, by the water salinity (Tyson, 1995). Also, Batten and Grenfell (1996) cite Botryococcus occurrence in calm or stagnant waters, such as lakes, swamps or marshes, although it can withstand relatively higher salinity from other environments such as mangroves and estuaries. Therefore, the low concentrations of this freshwater algae in this zone might be caused by the presence of marine influence. The observed palynomorph association suggests that calm conditions and mixing of marine and freshwater prevailed, which is typical for a lagoon environment.

Pollen grains (herbs, trees and shrubs) were transported by streams and/or wind and were deposited in the lagoon in low concentrations (Fig. 7). The herb taxa are mostly represented by Poaceae, Amaranthus and Chenopodiaceae, Apiaceae, Asteraceae subf. Asteroideae, and Cyperaceae (Fig. 8). These herbs are better adapted to sandy soils and include many halophytes (Lorscheitter, 2003). This suggests that saline soil conditions prevailed in the margins of the lagoon.

In this zone, the pollen of the tree and shrub taxa are mainly composed of Alchornea, Arecaceae, Myrtaceae and Celtis. The presence of Celtis and Trema, pioneer taxa, suggests an unstable environment subject to ecological succession. These pioneer taxa, together with the tree and shrub taxa (Alchornea, Myrtaceae and Arecaceae) and the halophytes herbs taxa (e.g. Amaranthus and Chenopodiaceae), indicate seasonally flooded open restinga in the surrounding area, with sandy soil subject to salinity (Freitas and Carvalho, 2012). Indeterminate pollen grains occur at higher concentrations in this zone and the subsequent Zone II in comparison to the two upper zones. This can be explained by the input of pollen grains transported by streams and wind into the lagoon partly damaging some of the grains. Alternatively, the presence of indeterminate pollen in this phase may be related to the transport of organic particles and the granulometry of sediments (fine sands and silts) in the very redeposition of lacustrine–lagoonal sediments configuring a high-energy and oxygenated environment that is more greatly influenced by marine currents than by the continental (terrestrial biomass) contribution. However, the predominance of fine sand, silt and clay sediment, as well as the presence of preserved calcareous shells in living position, could indicate deposition in a predominantly calm water body. Therefore, we interpret that the water body was likely calm with sporadic higher-energy events (e.g. storms). The pollen grains that originated from the Andes (Nothofagus and Alnus) are expected to be transported over long distances by air dispersion to the deposition site. These palynomorphs have been previously found in many Quaternary sedimentary profiles from the southern Brazil coastal plain (e.g. Cancelli et al., 2012; Diniz and Medeanic, 2012; Masetto and Lorscheitter, 2016; Kuhn et al., 2017; Silva et al., 2021).

The combination of high δ13C values (-19 to -12) and low C/N (14–16) ratios indicates mixtures of terrestrial and marine palynomorphs and shows little variation throughout this zone (Figs. 4, 5). Low C/N ratios are attributed to the presence of nitrogen-enriched freshwater algae organic matter (Meyers, 1994, 1997; Wilson et al., 2005). Among terrestrial plants, higher δ13C values indicate the predominance of C4 plants over C3 plants. This can be explained by the deposition of herb remnants from the borders of the lagoonal body. TOC values do not exceed 10 %. Such low TOC values are typical of lagoonal and estuarine environment (Tyson, 1995; Lorente et al., 2014).

In this zone, the predominance of fine sediments and the occurrence of freshwater algae and marine palynomorphs still indicates the presence of a calm brackish water body (Fig. 11; Zone II). However, the significant reduction of the marine indicators (e.g. microforaminiferal linings, acritarchs and Cymatiosphaera) and the disappearance of dinoflagellate cysts point to a progressive reduction of sea water input into the water body. Towards the end of this zone (ca. 1520 cal yr BP), the lagoon was probably disconnected from the sea (Fig. 11; Zone II). This is supported by the observation of the increase in the abundance of Botryococcus towards the top of the zone, suggesting that freshwater inputs into the lagoon progressively decreased its salinity, transferring the lagoon into a freshwater lake. Previous studies suggest that an increase of Botryococcus concentration might be related to a decrease of the water level in a lake (Tyson, 1995). It is therefore likely that, during the process of the closing of the lagoon and the freshening of the waters, the water table dropped consistently.

No significant changes were observed in the spore–pollen assemblage. However, the decrease of Amaranthus and Chenopodiaceae throughout this zone suggests a progressive desalination of the soil of the adjacent areas of the lagoonal body. The C/N ratios remain at low values of ca. 15 % throughout this zone, indicating that freshwater algae are still major contributors to the total organic matter. The binary plot of δ13C × C/N shows a trend towards lower δ13C values, likely related to the increase of freshwater phytoplankton input.

The transition from Zone II to III is marked by a drastic decline of freshwater algae concentrations and the disappearance of Botryococcus, Pseudoschizaea rubina and Zygnema. This reduction suggests a reduced freshwater input into the area. Moreover, marine palynomorphs are no longer recorded in this zone because of the disconnection with the sea that occurred in the previous zone.

The development of soils rich in organic matter at the site is evidenced by the increase of fungi. The abundance of fungal fragments is indicative of aerobic biodegradation of plant remains (Sebag et al., 2006). Additionally, the increase of the organic matter in the sediment and the high values of TOC (average of ca. 44 %) indicate the development of soils rich in organic matter in this zone.

High percentages of Poaceae and Cyperaceae taxa and δ13C enrichment from the beginning of this zone up to 140 cm of depth can be observed in this zone (Figs. 4, 8). In particular, Cyperaceae is an emergent aquatic plant common in restinga vegetation in lagoon environments, marshes and swampy lowlands (Falkenberg, 1999). This suggests that the herbs that previously occupied the margins of the lagoon advanced and colonized the palaeo-lagoon area and the environment of ongoing humid soil conditions. The concentration values for trees and shrubs remain constant throughout this zone, suggesting that the input of arboreal pollens that were transported from adjacent areas covered by Atlantic rainforest did not change (Fig. 11; Zone III). Consequently, the observed decrease of the relative abundances of trees and shrubs associated with increasing herbs supports our previous suggestion of a significant increase in the development of herbs in the palaeo-lagoon. The strong increase in the C/N ratios (Fig. 4) in this zone indicates an input of carbon-enriched material and suggests the increase of dense vegetation in the adjacent areas. The C/N ratios greater than 20 originated from vascular land plants (Meyers, 1994), and δ13C values around -25 ‰ indicate an influence of C3 plants (Meyers, 1997). The binary plot of δ13C × C/N also shows that the organic matter was influenced by C3 plants (Fig. 5). In the upper part of the section related to this zone, Poaceae and Cyperaceae (herbs) decreased their relative abundances, whereas Myrtaceae and Arecaceae (trees and shrubs) increase, showing a succession from herbal to arboreal taxa of the restinga forest.

This zone is characterized by a decrease in the relative abundances of herbs in favour of trees and shrubs. This suggests that forests developed in the palaeo-lagoon. The high relative abundances of arboreal taxa typical of the Atlantic rainforest (Arecaceae, Ilex and Myrtaceae) marks the consolidation of this arboreal forest in the area during this zone. In addition, the increase of epiphytes, lianas and climbers (mainly Cucurbitaceae taxa) suggest the advance of arboreal components in the vegetational succession of the forest as well (IBGE, 2012; Sevegnani and Schroeder, 2013).

Pteridophytes that are typically associated with the arboreal restinga forest (Blechnum and Polypodiaceae; Falkenberg, 1999) increase their concentration and support the consolidation of the arboreal restinga forest in the region. The isotope data indicate a depletion in the δ13C values during this zone, reaching the lowest value in the uppermost sample (-30). These δ13C values reveal the dominance of C3 plants, which is also indicated in the pollen record. In addition, the high TOC values and abundant organic matter contents in the sediment (∼ 40 and ∼ 90 %, respectively) indicate the occurrence of a dense forest similar to the one that covers the area nowadays.

The observed transition from a lagoonal environment at ca. 6520 cal yr BP to the actual restinga forest in the Pântano do Sul area is interpreted to be directly related to sea level changes throughout the Holocene. In general, there is an observed sequence of decreasing marine water contribution that indicates that the relative sea level was higher than the current sea level in the first zone, successively decreasing during the second zone. Because of the disconnection of the sea and the lagoonal body evidenced by the absence of marine palynomorphs in Zone III and Zone IV, we interpret that the sea level reached the current level at some point during one of these zones, most likely in Zone III. Previous studies on the Brazilian coastal plain suggested the existence of high-frequency oscillations in the relative sea level with two regressive zones during the late Holocene (Suguio et al., 1985; Martin et al., 2003). The authors suggested that the sea level was slightly below its present elevation at ca. 4200–3700 and 2700–2100 yr BP. However, recent studies suggest a regular decline in the relative sea level without significant oscillations during the late Holocene (Angulo et al., 1999, 2006, 2022; Ybert et al., 2003). In particular, Angulo et al. (2006, 2022) suggested that the highstand in the Holocene occurred between 5000 and 5800 yr BP without a distinct peak. The phase succession observed in our study can be better explained by a regular decline without significant oscillations, as proposed by Angulo et al. (2006, 2022; Fig. 12). Moreover, other palynological studies in the southern Brazil coastal plain in the Rio Grande do Sul (e.g. Cordeiro and Lorscheitter, 1994; Lorscheitter and Dillenburg, 1998; Meyer et al., 2005; Masetto and Lorscheitter, 2019) and Santa Catarina states (Behling and Negrelle, 2001; Amaral et al., 2012; Cancelli, 2012; Kuhn et al., 2017; Val-Peón et al., 2019; Cohen et al., 2020; Silva et al., 2021) also identified the marine influence ca. 6000–5000 yr BP at their sites and showed a similar sea level dynamic. In addition, palynological studies of palaeoenvironmental reconstitutions performed in the coastal areas of Uruguay and Argentina also indicated a highstand sea level between ca. 6000–5000 yr BP, followed by a regressive event (e.g. Borel and Gómez, 2006; García-Rodríguez et al., 2010; Mourelle et al., 2015; Vilanova and Prieto, 2012).

In general, the environmental zone succession identified in this study is similar to those presented in previous palaeoenvironmental studies developed on the southern Brazil coastal plain, particularly in the Santa Catarina sector (e.g. Amaral et al., 2012; Cancelli, 2012; Kuhn et al., 2017; Val-Peón et al., 2019; Silva et al., 2021). Most of these studies indicate a succession of three zones from a lagoonal and/or estuarine environment (I) to a transitional and/or swampy regime (II) and an arboreal forest environment (III). However, in this contribution, we were able to define the transition from a herbaceous restinga (Zone III) to the arboreal restinga forest (Zone IV). Some of the above-mentioned studies indicate a maximum relative sea level (RSL) ranging from ca. 5200 to 4500 yr BP (Fig. 13).

The distinct ages for the end of marine influence and the development of the Atlantic rainforest described in the previous studies is probably related to the different distances and/or altitudes of the depositional sites in relation to the current coastline and sea level. Localities nearer to the sea (e.g. Kuhn et al., 2017, and this study) were more affected by the sea level rise and show late development of the Atlantic rainforest in comparison to those located further from the shoreline (e.g. Cancelli, 2012). This supports that the restinga forest development was mainly controlled by edaphic factors and was less sensitive to climate factors during the Holocene (Scheel-Ybert, 2000; Amaral et al., 2012; Melo and Boeger, 2015).