Mid–late Holocene event registered in organo-siliciclastic sediments of Lagoa Salgada carbonate system, southeast Brazil

The formation of the Paraíba do Sul river delta plain on the coast of Rio de Janeiro state, Brazil, gave rise to diverse lagoons formed under different sea level regimes and climate variations. Sedimentary core lithology, organic matter geochemistry, and isotopic composition (δ¹³C and δ¹⁵N) were analyzed to interpret the sedimentation of the paleoenvironment of the Lagoa Salgada carbonate system. Different lithofacies reflect variations in the depositional environment. The abundance of silt and clay between 5.8 and 3.7 kyr enhances the interpretation of a transgressive system, which promoted the stagnation of coarse sediment deposition due to coast drowning. Geochemistry data from this period (5.8–3.7 kyr) suggest the dominance of a wet climate with an increase of C₃ plants and a marked dry event between 4.2 and 3.8 kyr. This dryer event also matches with previously published records from around the world, indicating a global event at 4.2 ka. Between 3.8 and 1.5 kyr, Lagoa Salgada was isolated; sand and silt arrived at the system by erosion with the retreat of the ocean and less fluvial drainage. Geochemistry from this moment marks the changes to favorable conditions for microorganisms active in the precipitation of carbonates, forming microbial mats and stromatolites in the drier phase.

1 Introduction

A severe and prolonged drought around the world characterizes the 4.2 ka climatic event and is reflected in proxy records from North America (Booth et al., 2005; Bradley and Bakke, 2019), Asia (Perşoiu et al., 2019; Scuderi et al., 2019), Africa (Damnati et al., 2012; Gasse, 2000), South America (Tapia et al., 2003), the Arabian Sea (Giesche et al., 2019), and Antarctica (Staubwasser and Weiss, 2006). This significant aridification event in the mid–late Holocene is recognized in lake sequences; ice cores; and in speleothem, dust, and sediment samples. This drought was one of the most pronounced climatic events of the Holocene, after 8.2 ka, which was associated with the collapse of several human civilizations in many sites in the world such as in North Africa, the Middle East, and Asia (Cullen et al., 2000; Gasse, 2000; Weiss et al., 1993).

The 4.2 event has been the focus of several studies. However, the forcing mechanisms behind this event are still unknown. Some authors try to explain the drought and increase in aridity as a result of the weakening of the Asian monsoon (Giesche et al., 2019; Kathayat et al., 2018; Wang, 2005) due to the southward migration of the Intertropical Convergence Zone (ITCZ). Others suggest a prolonged northward shift of the mean position of the ITCZ (Li et al., 2018), being in contrast with the southward shift of the tropical rain belt. The irregular fluctuation of atmospheric pressure over the North Atlantic Ocean, changing the direction of the cyclonic North Atlantic westerlies (Cullen et al., 2002; Kushnir and Stein, 2010), has been argued as another mechanism that results in megadrought around 4.2 ka, as well as the El Niño–Southern Oscillation (ENSO) conditions linked with drought in the monsoon region contributing to aridity in tropical South America during the same period (Davey et al., 2014).

In subtropical South America these events remain uncertain (Deininger et al., 2019), thus climate reconstructions in the region are essential to understand the geography of the changes in hydrological regimes. Southeastern Brazil is directly influenced by the convective rain belt of the South Atlantic Convergence Zone (SACZ), from the western Amazon to southeastern Brazil and the South Atlantic. The SACZ is the main component of the South American monsoon system (SAMS) (Jones and Carvalho, 2002), which is influenced by solar variability, enhancing evaporation and near-surface moisture. This process is also reinforced by the southward movement of the Intertropical Convergence Zone (ITCZ) during periods of increased solar irradiance (Haug et al., 2001), strengthening the SAMS, and bringing moisture to southeastern Brazil via SACZ. Thus, paleoenvironmental studies in lakes and lagoons in coastal areas of southeastern Brazil constitute a powerful tool to understand the changes in the hydrological cycle throughout time.

Changes in the environmental conditions (wet or dry) are registered in lake sediment as well as the dynamics and processes that occurred in the water column. Some studies of lake systems have considered the stable isotopes of C and N in sediments as proxies of organic matter (OM) cycles in aquatic systems over time (Salomons and Mook, 1981). Other studies consider the vegetation changes as a result of the climate alteration (wet or dry conditions) (Rossetti et al., 2017) or still the environmental succession stages influenced by sedimentological processes and different communities of primary producers (Duarte et al., 2018). Many studies involving OM have been done to characterize past and recent depositional environments (Megens et al., 2002; Pessenda et al., 2004; Salomons and Mook, 1981). The percentage of total organic carbon (% TOC) and the C:N ratio can also indicate the productivity and OM sources in paleoclimatic interpretations (Hartmann and Wünnemann, 2009). Thus, the objectives of this work were to evaluate the depositional processes related to sea level changes during the marine and lacustrine stage and to interpret the Holocene climatic changes during the last 5.8 kyr. This work will contribute to the growing literature on the mid–late Holocene climate variability and environmental processes in South America.

2 Material and methods

Sediment core S-15 was sampled from Lagoa Salgada in Rio de Janeiro State, Brazil (21^∘54^′46.30^′′ S, 41^∘0^′41.70^′′ W), recovering 212 cm length using a vibracore sampler (Fig. 1). Samples were collected every 2 cm for total organic carbon (C_org) and carbon stable isotopes (δ¹³C_org, δ¹⁵N) on bulk organic matter and every 4 cm for grain-size analysis. Sixteen samples throughout the core were analyzed for Fe∕Ca ratio. Due to the lack of preserved material in the majority of the core, except for a few individuals of Ammonia beccarii and Quinqueloculina seminulum, foraminiferal faunal assemblage was not considered. However, we compare our core with previously published data and respective authors are mentioned in Fig. 1 and Table 1.

Figure 1 Location map of study area. (a) Brazil within South America. Black box indicates the location of (b). Numbers refer to sites mentioned in Table 1. (b) Southeastern Brazil with state capitals and sites mentioned in the text. (c) Location of core S-15 within Lagoa Salgada. Digital Globe image used as background. Note the seasonal low lake level. Image acquired 31 May 2017.

Table 1 Locations of published records cited.

^* Gulf of Oman is not presented on the map.

2.1 Radiocarbon and age model

Radiocarbon analyses were performed at the Arizona Accelerator Mass Spectrometry Facility and BETA Analytic Inc., using ¹⁴C accelerator mass spectrometry (AMS). The age model is based on 11 radiocarbon dates from organic material of bulk dried sediment samples and converted to calendar age (Table 2). Radiocarbon dates were calibrated using the R script BACON version 2.2 with IntCal 13 calibration curve to convert to calendar age. The parameters used were mem.mean = 0.7, acc.shape = 0.8, and t.a = 33 and t.b = 34 (Fig. 2).

Figure 2 Bayesian age–depth model performed with BACON (Blaauw and Christeny, 2011) for the core S-15 (red line) and uncertainty (smooth gray curve) from Lagoa Salgada, Rio de Janeiro state, Brazil, with sedimentation rate (cm kyr⁻¹) (blue line). Black stars indicate the position of the 11 radiocarbon dates measured. The inset shows the iteration history.

Table 2 Ages, ¹⁴C, obtained from the dating of bulk organic matter to core S-15.

2.2 Grain-size analysis

About 2 g of dried sample was decarbonated using HCl for several hours, centrifuged, and washed with distilled water. Hydrogen peroxide (H₂O₂) was also added to remove the organic matter. After these processes, about 30 mL of deflocculant solution (Na₁₆O₄₃P₁₄ – 4 %) was added for 24 h (Barbosa, 1997). The grain-size measurements were performed using a laser particle analyzer (CILAS 1064) which has a detection range of 0.02–2000 µm, using the grain-size statistics method of Folk and Ward (1957) performed in GRADISTAT software version 8.0 (Blott and Pye, 2001).

2.3 Total organic carbon and stable carbon and nitrogen isotopes in bulk organic matter

Sediment samples for C_org, δ¹³C_org, and δ¹⁵N were dried at 40 ^∘C, powdered, and homogenized with an agate mortar. Samples were decalcified with a 1 N HCl solution for several hours, centrifuged, washed with distilled water, and subsequently dried at 40 ^∘C. About 30 mg of the dried material was weighed in tin capsules and analyzed at the University of California Stable Isotope Facility (Davis, USA), using a MICRO cube elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany.) interfaced to a PDZ Europa 20–20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). The long-term standard deviation was 0.2 ‰ for δ¹³C_org. The δ¹³C_org were given as per mill in relation to Vienna Pee Dee Belemnite (VPDB) and the δ¹⁵N were given as per mill in relation to the air.

The carbon accumulation (C_org accumulation) was determined using the following equation by Thunell et al. (1992), Eq. (1):

$$\begin{array}{}\text{(1)}& C_{\mathrm{org}}\mathrm{accumulation}\left(\mathrm{g}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{kyr}}^{-\mathrm{1}}\right)=\mathit{\rho }\mathrm{SR}\left(C_{\mathrm{org}}\right),\end{array}$$

where ρ is density (in g cm⁻³), SR is the sedimentation rate (in cm kyr⁻¹), and C_org represents the total organic carbon content.

About 20 mg of the sample was dried, crushed, and placed in a specific container to analyze the carbonate content. The analysis was performed every 2 cm using an inorganic carbon analyzer (TOC-V with ASI-V SSM 5000, Shimadzu).

2.4 Fe∕Ca analysis

The Fe∕Ca analysis was performed on 16 dried and powdered samples using an X-ray fluorescence (XRF) spectrometer, Epsilon 3 (PANalytical), at the Universidade Federal Fluminense, Brazil.

3 Results

The sediment core S-15 recovered the last 5.8 kyr. The sedimentation rate ranged from 10 to 250 cm kyr⁻¹. Sedimentation rate increased between 5 to 4 kyr (from 10 to 250 cm kyr⁻¹) with a posterior decrease during 4 to 3.7 kyr (from 250 to 140 cm kyr⁻¹) and an increase between 3.7 and 3.5 kyr (140 to 160 cm kyr⁻¹) (Fig. 2).

Grain-size analysis shows an increase in fine sediments, clay (10 %), and silt (70 %), between 5.8 and 3.7 kyr, with sand decreasing from 100 % to 20 %. Between 3.7 and 0 kyr the opposite trend occurred with an increase in sand grains (∼84 %) and a decrease in clay (∼1 %) and silt (∼15 %).

An increase in the Fe∕Ca ratio was observed (9 to 15) between 5.8 and 3.7 kyr with a posterior decrease (15 to 4) toward the top. The iron and calcium alone showed an opposite trend with a decrease between 6 and 3.7 kyr, an increase between 3.7 and 3 kyr, and posterior decrease toward the top (Fig. 3).

Figure 3 Comparison between sea level changes with sedimentologic records over the past 6 kyr. (a) Sea level (m) (Castro et al., 2014); (b) Ca (%), (c) Fe∕Ca, (d) Fe (%), (e) sand (%), (f) silt (%), and (g) clay (%). The gray and yellow bars indicate two different stages in the last 6 kyr, marine (I) and lacustrine (II) stages, respectively.

Carbonate content showed an increase from 10 % to 50 % between 5.8 and 3.7 kyr. In the interval from 3.7 to 3 kyr, a decrease in the carbonate occurred (from 50 % to 20 %) with a posterior increase toward the top (∼80 %) (Fig. 4b).

Figure 4 Comparison between sea level changes with geochemical records over the past 6 kyr. (a) Sea level (m) (Castro et al., 2014); (b) CaCO₃ (%), (c) C∕N, (d) TOC (%), (e) δ¹⁵N (‰), and (f) δ¹³C (‰). The gray and yellow bars indicate two different stages in the last 6 kyr, marine (I) and lacustrine (II) stages, respectively.

The C∕N ratio ranged from 7 to 23 showing a variation between allochthonous and autochthonous organic material. Between 5.8 and 3.7 kyr the mean value was around 13. Between 3.7 and 3.2 kyr an increase in the values (∼18) occurred with a posterior decrease toward the present (Fig. 4c).

The total organic carbon ranged from 0.1 % to 2 % with an increase in the values between 4.7 and 3.7 kyr (Fig. 4d). The δ¹⁵N and δ¹³C showed the same trend increasing toward the present. The δ¹⁵N ranged from 5 ‰ to 15 ‰ and δ¹³C ranged from −40 ‰ to −12 ‰ (Fig. 4e, f).

4 Discussion

4.1 Sedimentary processes

Lagoa Salgada paleohydrodynamics shows two distinct stages during the mid–late Holocene. The first stage comprised the period between 5.8 and 3.7 kyr (marine stage) and the second stage from 3.7 to 1.5 kyr (lagoonal stage).

The marine stage (5.8 to 3.7 kyr) was characterized by a predominance of fine sediments (Fig. 3) and a gradual increase in the sediment deposition toward 3.7 ka (Fig. 2). According to Castro et al. (2014, 2018) and Suguio et al. (1985), the maximum Holocene transgression occurred at ∼5 ka when the sea level reached ∼3 m above the modern (Fig. 5a), causing the submergence of the coastal area. However, the evolution of the Paraíba do Sul river delta on the coastal plain formed the Lagoa Salgada initially as an intralagoonal system in a drowned coast around 3900 BP (Lemos, 1995).

Figure 5 Relative sea-level variation curve for (a) the coast of the Rio de Janeiro state, Brazil (Castro et al., 2014), and (b) Salvador, Bahia state, Brazil (Martin and Suguio, 1992).

In the first stage, the wet condition is dominant in almost all of the period, with a punctual change from 4.2 to 3.7 kyr. The gradual increase in the wet condition fed the river and increasing fine river discharge and organic material deposition (Figs. 3 and 4), indicating changes in the climate condition. The climate changes are inferred by the modification of vegetation type entering the system and in the source of material deposited (Fig. 4c, e, f). The S-15 core shows that between 5.8 and 3.7 kyr the Lagoa Salgada was submerged within a drowning estuary and river flow stagnation occurred with fine sediment decantation in the environment during this period of high sea level (Fig. 3).

Low values of δ¹³C ($\sim -\mathrm{25}$ ‰) and high C∕N ratio from organic material (greater than 10) (Meyers, 1997) register an elemental contribution from cellulosic land plants (C₃) to the total organic matter input preserved in the sediments, which were less susceptible to degradation. The δ¹³C and C∕N ratio of the core S-15 show the dominance of C₃ plants between 5.8 and 4.8 kyr with mixed sources between 4.8 and 3.7 kyr, when a small increase in δ¹⁵N occurred, indicating a contribution of another source, such as phytoplankton ($\sim -\mathrm{19}$ ‰) and C₃ plants ($\sim -\mathrm{25}$ ‰) (Fig. 4c, f), within a period of humid conditions. The δ¹⁵N shows the source and quality of the organic matter and the influence of terrestrial organic material during 5.8 to 4.8 kyr (Fig. 4e). This influence is observed by low δ¹⁵N values near to 0 ‰ (Schulz and Zabel, 1999), in which part of the nitrogen demand could come from atmospheric fixation. Unfortunately, nitrogen isotopes cannot discriminate between primary producers that overlap in carbon isotope values. While more detailed pollen analysis could be used to differentiate vegetation types and discriminate the sources of organic matter in sediment, isotopic composition values of Lagoa Salgada were in general consistent with pollen analyses made in cores collected from Lagoa Santa and Lagoa dos Olhos (Table 1), also in southeastern Brazil, showing the development of semideciduous forest. The increasing humidity favored the vegetation changes with the predominance of C₃ plants during 7 to 4 kyr (De Oliveira, 1992; Ledru et al., 1998).

Although there was an increase in iron in the sediments, indicating high terrestrial input, the Fe∕Ca ratio (Fig. 3c, d) shows an opposite trend. This difference occurred due to the highest amount of calcium deposited in the sediment floor compared to the iron input, which regulates the changes in the Fe∕Ca ratio in this environment. The iron input also promoted an increase in the primary productivity and, consequently, the increase in calcium carbonate during the first stage (Figs. 3c and 4b).

The second stage comprised the period between 3.7 and 1.5 kyr. The formation of the sandy barrier caused by sea transgression favored the creation of lagoonal systems in the delta. During this stage coarse sediments predominate (sand) (∼84 %). The Fe∕Ca ratio was low with a considerable increase in calcium percentage (∼80 %) (Fig. 3). According to Castro et al. (2014), a rapid marine regression occurred between 5.5 and 4.5 kyr. In the S-15 core, marine regression is identified after 3.7 ka when the lake was formed, allowing the input of coarse sediments by erosion with the retreat of the ocean. Lemos (1995) indicated the ages of lake formation at about 2000 and between 3090 and 3900 BP, respectively. The approximate ages were estimated from different strata of the stromatolites at the edge of the lake.

Geomorphological characteristics and seasonal variability modified the geochemistry of the lake, influencing the sedimentation and precipitation of salts and carbonates that formed biosedimentary structures of stromatolites, thrombolites, and oncoids (Silva e Silva et al., 2005, 2008).

Carbonate content shows an increase during the second stage as a result of the increasing biological productivity in the lake, while the C∕N ratio shows mixing between terrestrial plants and phytoplankton as the organic source with decreasing values (∼10); δ¹⁵N and δ¹³C also indicate different sources of organic matter. In this stage, δ¹⁵N and δ¹³C increase towards the top of the core, characterizing changes in vegetation with dominance of C₄ plants. A C₄ plant signature at the top of the core (around 2 to 1.5 kyr), evidenced by increasing δ¹³C ($\sim -\mathrm{10}$ ‰), was also observed by França et al. (2016) and Ledru et al. (1998) in cores collected in lakes from southeastern Brazil, showing a replacement of tropical semideciduous forests by herbaceous vegetation, indicating a relatively dry climate during this period. The climate condition at this time could be influenced by the upwelling system (Laslandes et al., 2006; Nagai et al., 2009, 2016), which favors increasing ocean–land temperature gradient typical of semiarid climates, corroborating the dominance of C₄ plants. The input of saltwater into the coastal wetland and the formation of coastal dunes also can trigger the development of plant communities dominated by C₄ species as a result of the competitive advantages of salt-tolerant species, promoting a regressive succession of vegetation not necessarily influenced by climate changes (Zhou et al., 2018). However, pollen data from cores collected from freshwater lakes in southeastern Brazil (Behling, 1995, 1998; Ledru et al., 1998), with no influence of coastal dynamics, also show changes in vegetation resulting from climate alterations throughout the Holocene. Thus, we consider climate to be the main cause of the changes in vegetation dominance, resulting in the isotopic alteration of the organic matter.

The abrupt change in proxy values in the second stage of the lake show that local climate and the proliferation of microbial communities have modified the geochemistry of the lake and its sedimentation. High δ¹⁵N values also suggest metabolism related to the development of the microorganisms, which gave rise to the stromatolites present in the Lagoa Salgada. The presence of gastropods caused bioturbation in the sediments, affecting the microbial processes and altering the physicochemical properties of the sediment, by favoring the entry of O₂ at the water–sediment interface and N fixation stimulating denitrification (Laverock et al., 2011).

Some species of cyanobacteria have the ability to live in the mud of hypersaline environments and they are halophilic, alkaline (Dupraz et al., 2009), and precipitate carbonates (Xu et al., 2006). Silva et al. (2013) identified 21 species of cyanobacteria in stromatolites of the Lagoa Salgada, with the most representative being Microcoleus chthonoplastes and Lyngbya aestuarii, which are diazotrophic cyanobacteria present in coastal microbial mats. In hypersaline lakes, such as Lagoa Salgada, microbial mats precipitate CaCO₃ as a by-product of CO₂ capture through photosynthesis by cyanobacteria (Jonkers et al., 2003; Ludwig et al., 2005). The precipitation of CaCO₃ that generated the lithification of the microbial mats in the lake are caused by cyanobacteria that increase the pH through photosynthesis in a CaCO₃ supersaturated system (Decho and Kawaguchi, 2003).

Radiocarbon dating by Coimbra et al. (2000) in the stromatolite head of the Lagoa Salgada shows the growth of these structures to have begun around 2200±80 BP and finished around 290±80 BP. They noticed differences in growth rates of stromatolite relating to the organization of the structure, being better structured at the middle of the head than at the top of the structure, with an average growth rate of 0.05 mm yr⁻¹. In the case of the Lagoa Salgada, changes in the environmental dynamics and the development of microbial communities after isolation of the marine influence, shown by changes in vegetation type (C₄ plants) and an increase in CaCO₃ values (80 %), influence the appearance of the stromatolites at around 2800±8 BP.

4.2 The 4.2 event

During the transgressive stage (5.8 to 3.7 kyr) differences in climate conditions are observed in southeastern Brazil (Fig. 6).

Figure 6 Geochemical records from Lagoa Salgada in comparison with other climate records. (a) Gray scale (nm) from Cariaco Basin, Venezuela (Hughen et al., 1996); (b) Carbonate content (CaCO₃) (%) and (c) dolomite (%) from the Gulf of Oman (Cullen et al., 2000); (d) Carbonate content (CaCO₃) (%), (e) clay (%), and (f) Fe (%) from this study; (g) Mg∕Ca, Globigerinoides ruber, from the southwest Brazilian coast (Lessa et al., 2016) and (h) Mg∕Ca speleothem from Botuvera Cave, southwestern Brazil (Bernal et al., 2016). The nine black stars indicate the position of the radiocarbon dates measured in this study. The gray bar emphasizes the period between 4.3 and 3.6 kyr (wet condition) and the yellow bar shows the 4.2 ka event (dry condition).

Geochemistry data show an increase in productivity between 5 and 4.2 kyr with increasing carbonate and organic carbon percentages (Fig. 6d). The enrichment of organic carbon in the sediment floor is also related to increasing deposition of fine sediments (Fig. 6e), which have the ability to adsorb electrolytes and organic material (Busch and Keller, 1981; Cruz et al., 2013, 2018), thus changing the composition of the sediments.

The wet condition of the environment during this period (5–4.2 kyr) was characterized by high carbon accumulation and predominance of C₃ plants (Fig. 4d, f). High humidity during this period is also characterized by decreasing Mg∕Ca ratios in speleothems collected in the Botuvera Cave, southeastern Brazil (Bernal et al., 2016) (Fig. 6g). In that study, Bernal et al. (2016) suggest that most of the changes in rainfall patterns during the Holocene were driven by the intensity of the South Atlantic Monsoon Summer (SAMS). SAMS intensification, influenced by the South Atlantic Convergence Zone (SACZ), protrudes as a lower troposphere convective rain belt from the western Amazon to southeastern Brazil and the South Atlantic (Gandu and Silva Dias, 1998). The precipitation response also results from an adjustment of the Intertropical Convergence Zone (ITCZ), which displaces itself according to cooling in the Northern Hemisphere and changes in the interhemispheric sea surface temperature (SST) (Cvijanovic et al., 2013). The anomalous southward displacement of the ITCZ shown by dry conditions in the Cariaco Basin (Hughen et al., 1996) (Fig. 6a) indicates increased wet conditions during the transition from the middle to the early Holocene in the Southern Hemisphere.

Higher sand fraction and lower carbonate and iron contents reveal a significant change in the environmental conditions during the interval of 4.2–3.8 kyr, which could be a regional manifestation of the 4.2 ka event in southeastern Brazil. Dry conditions could affect the local vegetation (the mixture of sources shown in Fig. 4f) which leads to the reduction of dense vegetation (C₃ plants), increasing erosion and consequently the accumulation of coarser materials. The drought would be caused by a reduction of the intensity of the SAMS and the possible northward displacement of the ITCZ, shown by increasing wet conditions in Cariaco Basin (Fig. 6a) and drier conditions for the Botuvera Cave in southeastern Brazil (Fig. 6g). In addition, the northward migration of the ITCZ could also have caused the weakening of the upwelling system in southeastern Brazil. Upwelling during this period became limited to the subsurface with warm conditions on the surface waters, shown by increasing Mg∕Ca ratio in planktonic foraminifera (Globigerinoides ruber) (Fig. 6f) (Lessa et al., 2016).

Several other paleoarchives recovered around Asia (Giesche et al., 2019; Kaniewski et al., 2018; Kathayat et al., 2018; Scuderi et al., 2019), Europe (Isola et al., 2019; Zanchetta et al., 2016), and Africa (Gasse, 2000) show this drier event between 4.2 and 3.8 kyr. Arz et al. (2006) suggested that the environmental changes around 4.2 ka is an expression of a major drought event, which strongly affected the Middle Eastern agricultural civilizations. A sediment core recovered in the Gulf of Oman showed a rather abrupt signature with climate changes around 4.2 ka (Cullen et al., 2000) and a prominent spike of CaCO₃ and dolomite indicating the aridity (Fig. 6b, c) during the same dry period shown by the S-15 core indicating it may correspond to a global event.

The increase in the sand fraction in core S-15 also can be explained by an erosional phase that changed local hydrodynamics, leading to an increase in the coarse deposition as the consequence of a regression of the sea level (Fig. 5b) (Martin and Suguio, 1992). This regression also allowed the deposition of terrestrial organic material, shown by an increase in C∕N ratio (Fig. 4c) and a decrease in δ¹⁵N and δ¹³C (Fig. 4e, f), causing a decrease in the carbonate accumulation (Fig. 4b). According to Martin and Suguio (1992), the abrupt fall in sea level, between about 4200 and 3900 BP, provoked an oceanward exit of active distributaries of the intralagoonal delta. However, the lack of data during this period, in both sea level curves (Castro et al., 2014; Martin and Suguio, 1992), make this hypothesis of sea level regression merely speculative and the influence of climate change a more plausible alternative to the environmental changes that occurred during this period.

Between 3.9 and 3.7 kyr, a return to the same environmental conditions before the event is observed with increasing humidity. This period may also have been marked by a new marine transgression, which prevented terrestrial deposition in the study area (Martin and Suguio, 1992).

5 Conclusions

The paleohydrodynamics of Lagoa Salgada shows a clear adjustment with variation in the sea level. During the period of the sea level transgression (5.8 to 3.7 kyr), Lagoa Salgada was submerged, promoting the drowning of a river and the stagnation of coarse sediment contribution, thus increasing decantation of fine sediment and organic material deposition. This period was also characterized by the dominance of C₃ plants and an increase in the sedimentation rate, indicating wetter conditions.

During the transgressive stage (5.8 to 3.7 kyr), a significant change in climate conditions occurred resulting in a period of aridification, from 4.2 to 3.7 kyr. The period between 4.2 and 3.7 kyr was also characterized by changes in the local vegetation, with a reduction of C₃ plants and the accumulation of coarse sediments due to increasing erosion. The drought would be caused by a reduction in the intensity of the SAMS due to the northward displacement of the ITCZ.

The regression of the sea level (3.7 ka to present) promoted the evolution of the Paraíba do Sul river delta on the coastal plain and the formation of the lake system. The lake was formed allowing the input of coarse sediments by erosion with the retreat of the ocean. Abrupt modification in the vegetation type and in the sedimentary deposits was observed in this period with a dominance of C₄ plants and a decrease in the sedimentation rate, indicating a predominance of dry conditions on the environment. With the closure of the Lagoa Salgada by the sandy ridges of the delta, geochemical modifications generated internally in the lake and allowed the appearance of microbial carpets and stromatolites after 2.8 ka.