Holocene paleoenvironmental reconstruction based on the lacustrine evolution of three Cameroonian lakes (SW, Africa)

doi:10.21203/rs.3.rs-4978739/v1

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Holocene paleoenvironmental reconstruction based on the lacustrine evolution of three Cameroonian lakes (SW, Africa)

https://doi.org/10.21203/rs.3.rs-4978739/v1

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Lakes are the most attractive and expressive characteristics of a landscape. Lacustrine sediments provide a historical record of the conditions of the surrounding catchment area. The aim of this study is to reconstruct the Holocene lacustrine evolution of 3 lakes located in Yaoundé, Dizangué and Ngaoundéré regions (Cameroon, SW-Africa) using a multiproxy approach including analysis of sediments, diatoms, pollen, with absolute chronologies based on radiocarbon dates. Sediment cores were recovered from a raft using polyvinyl chloride (PVC) pipe. Sediment grains are generally sub-rounded and rounded indicative of both aeolian and fluvial transportation. Pollen and diatom data revealed major hydrological changes and strong fluctuations between wet conditions during the "Medieval Warm Period" (1100 − 800 yrs BP), and dry conditions during the "Little Ice Age" (500 − 300 yrs BP). Four distinct paleoclimatic phases were identified from the diatom assemblage of lakes from Yaoundé and Dizangué (South) and Ngaoundéré (North). The diatom assemblages in Simbock Lake (southern Cameroon) indicate high water levels initially, followed by a transition marked by increased aerophilic species, potentially indicating episodic drying or reduced water supply. Furthermore, a pH and trophic inversion suggests eutrophication due to nutrient influx. In the Ossa Lake complex (southern Cameroon), four phases reveal changes from oligotrophic to eutrophic conditions, with Aulacoseira species dominating, and the evolution of acidic to clear waters. Lake Ngaoundaba in the Adamawa (northern Cameroon) exhibits three phases, indicating stable oligotrophic conditions, episodic drying, and a rise in lake level with increased tychoplanktonic species, particularly Aulacoseira distans. These hydrological changes have controlled the dynamics of tropical rainforests during the Little Ice Age (LIA) and the Medieval Warming (MW) in this part of Africa, resulting in their expansion during periods of heavy rainfall and contraction during periods of reduced rainfall.

Sedimentology

diatoms

palynology

paleoenvironmental reconstruction

lacustrine sediments

Cameroon

Cameroon stands out as one of the most biodiverse areas in Africa, situated on the eastern edge of the Guinean Forests biodiversity hotspot in West Africa—an area harboring one of the planet's most endangered ecosystems (Myers et al. 2000). Biotic proxy records have unveiled dramatic fluctuations in the climate of South West Africa, though the details of these records vary (Lézine et al. 2019). Around 3000 to 2500 cal. BP, a climatic shift occurred, characterized by cool temperatures and locally wet conditions, fostering the persistence of rainforest refuges (Elenga et al. 1996; Bertaux et al. 2000). As we move into the early to mid-Holocene, evidence suggests wetter climate conditions, leading to the expansion of rainforests. However, after 2500 cal. BP, a trend toward drier conditions or a more seasonal climate emerges in the northern part of the forest domain, as indicated by various studies (Kossoni et al. 2010; Richards et al. 2021). Recognizing the sedimentological characteristics of lacustrine sediments, an archival record of past climate-vegetation relationships, is essential for comprehending their complexity. This understanding is crucial for interpreting anthropogenic-induced changes in lake trophic states revealed by lacustrine archives, providing insights into present-day processes and chemical fluxes concerning pre-industrial conditions and long-term variations.

Recent studies of lakes in Cameroon are mainly focused on water quality (Kwon et al. 2012; Kamtchueng et al. 2016; Awo et al. 2020; Noa Tang et al. 2022), heavy metal contamination, fish species and pollution (Ekengele et al. 2008; Barkai et al. 2014; Ekoa Bessa et al. 2018; Bon et al. 2020), paleoenvironmental reconstruction using palynology, diatoms and radiocarbon dating (Kossoni et al. 2010; Nguetsop et al. 2013; N’nanga et al. 2019; Richards et al. 2021) and rarely multiproxy studies (Ekoa Bessa et al. 2018, 2021a,b;nanga et al. 2021).

Multiproxy studies are a rarity in this context (Ekoa Bessa et al. 2018, 2021a,b;nanga et al. 2021). However, the current research seeks to bridge this gap by employing a multiproxy approach, including sedimentology, diatoms, and palynological analyses. We aim to retrace and compare the evolution and history of environmental change in three lakes: artificial Lake Simbock, subsidence-formed Lake Ossa Complex, and crater-formed Lake Ngaoundaba during modern to sub-modern periods. This selection allows us to investigate how different intensities of anthropogenic processes influence sedimentary dynamics, a topic that remains poorly understood.

This study contributes to a better understanding of the evolution of lakes formed in different contexts and recent climatic and hydrological events in Cameroon and Central Africa. Given the relative '‘lack of work in tropical regions'' noted by Escobar et al. (2020), this research has the potential to significantly advance paleolimnological investigations in tropical areas.

2.1. Lake Simbock

The Simbock lakes, located in the southwestern part of Yaoundé (Fig. 1a, b), constitute a group of six lakes, with only the last lake being explored in the context of this study. The studied lake has a surface area of 0.46 ha and 6835.5 m³ of water with an average depth of 1.5 m (Ekoa Bessa et al., 2018). It shows a considerable reduction in several parameters (surface, volume and depth) compare to others lakes in the region due to rapid urban expansion. The climate is humid tropical with four seasons, an average annual temperature of 24°C, and an average annual rainfall of 1523 mm (Molua 2006). This sector is characterized by semi-caducified dense rainforest, aligning with the drier peripheral semi-evergreen Guineo-Congolian rainforest classification as defined by White (1983). Additionally, it includes semi-deciduous forest featuring Sterculiaceae and Ulmaceae. Increasing human activities have led to the degradation of the area, transforming it from its original state as a natural forest ecosystem to a secondary forest. This secondary forest is now characterized by the presence of grasses, lianas, palms, shrubs, and a sparse population of large trees. The surrounding landscape features food crop plantations interspersed with patches of secondary forest or shrubby savannah. In this zone, various edaphic formations may be found, particularly in swampy areas where Raphia, Mitragyna stipulosa, and Uapaca thrive. Manihot esculenta, Arachis hypogaea, Zea mays, and Theobroma cacao are cultivated on hillsides, while vegetables such as Daucus carota and Solanum lycopersicum are grown in marshy areas. The Simbock lakes were established along the Mefou watercourse in the 1980s by an individual with the dual purpose of promoting tourism and facilitating fish farming. Although the creation of these lakes initially brought about changes to the watercourse, inadequate maintenance over time resulted in alterations to the lakes' morphology, including erosion, flooding, and other relief modeling phenomena. This anthropic history delineates the evolution of the Simbock lakes and their surrounding environment, highlighting the impact of human activities such as urban expansion and agricultural practices.

2.2. Lake Ossa Complex

The Ossa Lake Complex is a set of 7 subduction lakes including Ossa, Mevia, Mwembe, Nsipé, Mboli and two other small lakes. Lake Ossa, 20 km west of Edéa, is at an altitude of 8 m.a.s.l. and an average depth of 8 m. Its surface area covers about 4,500 ha for a catchment area of 190 km² (Fig. 1a, c), the highest point 165 m a.s.l. (Wirrmann 1992). The climate of the region is characterized by an average annual rainfall of about 2,535 mm and a long rainy season from March to November, mitigated by a reduction of precipitation (June) and interrupted by a long dry season (December to February). The annual mean temperature is 27.2°C. Two forest formations predominate in South-Cameroon: The Biafran evergreen forest, is very diversified and has a high abundance of Caesalpiniaceae and the Atlantic coastal forest, with abundant Lophira alata (Ochnaceae), Sacoglottis gabonensis (Humiracaceae), Coula edulis (Olacaceae) and Cynometra hankei (Caesalpiniaceae). Around the lake, the Atlantic coastal forest dominates. Since the beginning of the century, large rubber plantation (Hevea basiliensis) and palm oil (Elaeis guineensis) gradually developed on the western shoreline. Lake Ossa is situated approximately 35 km from the Atlantic shore, where multiple lakes emerged along the valley floor. This area was dammed during the Late Pleistocene by alluvial terraces (details of the formation of Ossa Lake Complex can be seen in Giresse et al. 2005). In the vicinity of this lake, local communities engage in agriculture and operating small village businesses.

2.3. Lake Ngaoundaba

The Ngaoundaba Lake is located less than 100 km to the most northerly patches of humid forest (Fig. 1a, d). It is about 17 ha in surface area at 1,160 m of altitude, with a maximum depth of 62 m and relative depth of 17 m (Kling 1988). The Adamawa Plateau where the lake is located is in the altitudinal tropical climate, which is a transition between the Equatorial climate in the south and the Sahelian climate to the north. There are two seasons: a rainy season (April to October) and a dry season (November to March). Mean temperatures vary between 23 and 26°C and the mean annual precipitation is about 1,500 mm. The precipitation reaches 1,589 mm in Ngaoundéré (Suchel 1998). The Adamawa region is covered by Sudanese type of woody savanna characterized by two main trees: Daniella oliveri (Caesapiniaceae) and Lophira lanceolata (Ochnanceae). Northwards, the height, and density of the vegetation drop, giving way to herbaceous savanna. During the Eocene to Pleistocene transition, phreato-magmatic explosive events led to the formation of numerous crater lakes in the Ngaoundéré volcanic zone (Temdjim 2005). In the geological context of the Ngaoundéré volcanic zone, the abundance of crater lakes, such as Lake Ngaoundaba, implies a sedimentary history reflecting volcanic activity marked by the interaction between magma and water. This phenomenon may be attributed to volcanic occurrences in an environment rich in groundwater or surface water sources. Ngaoundaba Lake, dominated by swimming and tourism, and is one of the attractions of the Ngaoundaba ranch, for renowned tourism. It is one of the crater lakes in the Adamawa regions in Cameroon, known as a maar (Ohba et al. 2013). Specifically, the Adamawa region stands out as an oasis of relatively untouched natural beauty in a landscape marked by sparse human settlements, with expensive savannah stretching.

The geological formations in Cameroon are predominantly Precambrian, comprising the Basement Complex, resulting from the remobilization of the Archean basement under the categories of "Intermediate Series" and the "Dja Formation" (Bessoles 1969). The Ntem Complex, part of the Cameroonian portion of the Congo Craton, is characterized by Archean and Palaeoproterozoic rocks, including tonalites, trundjemites, granites, granulites, and green rocks. Sedimentary basins are distributed in the coastal (Douala and Mamfe basins), southern (Mimtom basin), northern (Benoue basin), and southern Chad regions. The coastal sedimentary basin along the Gulf of Guinea emerged during the Cretaceous, gradually deepening towards the ocean. It comprises black marl, clay, and sandstones. The Benue basin, characterized by detrital sediments of Cretaceous and Quaternary ages, overlays the crystalline basement. The Quaternary deposits form terraces along river edges and consist of argillaceous materials in flood zones. The basin in the northern extreme is part of the Chadian basin, covering Tertiary, Quaternary, and current alluvial deposits with sandy, clayey to sandy-clay, sandstone, and limestone formations.

The Yaoundé series, where Lake Simbock is located, belongs to the ancient metamorphic basement of the Cameroon Basement Complex. It consists of Pan-African chain formations, encompassing paraderived and orthoderived deformations. Paraderived metamorphites include micaschists and quartzites, while orthoderived formations consist of pyroclastics such as pyroxene gneisses and plagioclases, along with associated pyroxene and amphibole gneisses, pyroxenites, and biotites (Owona et al. 2003).

Lake Ossa, situated near the continental edge of the coastal sedimentary basin, is influenced by major fault clusters. The emplacement of Lake Ossa is likely related to the release of ancient fractures, resulting in horsts and grabens. The sedimentary rocks in the catchment area are characteristic of the Dizangue series, primarily comprising cross-stratified ferruginous sandstones of Paleocene age.

The Adamawa plateau, characterized by numerous craters, including the Ngaoundaba Crater, features extensive basaltic flows accompanied by trachytes and trachyphonolites (Temdjim 2005; Ngounouno 2000). The geological formations consist of the Pan-African granite-gneissic basement, represented by granites of Ordovician age, gneisses, Pan-African migmatites, basalts, trachytes, and phonotrachytes (Eno Belinga 1984; Ngounouno et al. 2000), with metadiorites in enclaves of the Paleoproterozoic basement (Toteu et al. 1994).

4.1. Core collection and chronology

For this study, a sediment core was collected in all the three selected lakes during different periods in September 2017 from Lake Simbock, in November 2018 from Lake Ossa Complex, and in September 2019 from Lake Ngaoundaba. The equipment includes a raft that was mounted on sites and polyvinyl chloride (PVC) pipes were used to retrieve core sediments manually. In the Lake Simbock in Yaoundé, the 119 cm long core sediment (NR) was retrieved at 150 cm water depth in the center of the lake. In Lake Ossa Complex, the sediment core (LO) was collected near the center at around 1 km from the outlet, at 450 cm water depth, with 150 cm long. The last core: namely AZ of 96 cm long retrieved at 8 m depth, not far from the center at around 140 m from the border of the Ngaoundaba Lake, but close to an eutrophication area.

In the present study, six ¹⁴C AMS dates (Table 1) were carried out in the Poznań Radiocarbon Laboratory (Poz-) from two cores LO (Lake Ossa) and AZ (Lake Ngaoundaba). Profile collected from Lake Simbock (NR core) was not subjected for dating as this lake was constructed less than 30 years ago. The absolute chronology and sedimentation rates of LO and AZ cores is based on the Bayesian age-depth models, carried out using the OxCal 4.4 software (Bronk Ramsey 1995; 2006). The P_Sequence function (parameters: k0 = 1, log10(k/k0) = 1 and interpolation = 1 cm) was applied (Bronk Ramsey 2008; Bronk Ramsey and Lee 2013) with the IntCal20 calibration curve (Reimer et al., 2020). The µ value from the models rounded to tens was presented as calendar age in this paper.

The obtained radiocarbon dates are used to construct an age-depth model, illustrating the relationship between the age of sediment layers and their depth in the core. The OxCal software performs Bayesian statistical analysis, incorporating prior knowledge to refine estimations. The P_Sequence function models event sequences with variable intervals, allowing for precise modeling of sedimentation rates over time. Calibration with the IntCal20 curve adjusts models by considering variations in atmospheric C-14 levels, providing accurate estimations in calendar years. Bayesian modeling results are presented as µ values, representing sedimentation rates and indicating the speed of sediment accumulation over time.

4.2. Diatoms analysis

A total of 43 samples were analyzed for diatoms. The sampling was made every 10 cm from each core. For the preparation of the diatoms, the sediments were gently treated with dilute hydrogen peroxide (H₂O₂) following the method described in Nguetsop et al. (2011). Diatom identification, and counting of diatoms followed the procedure described in Nguetsop et al. (2011, 2013). For quantitative analyses, the number of valves counted per sample ranged from 685 to 2948, higher than what is recommended in Battarbee (1986), because a few species dominated the samples.

4.3. Pollen analysis

Nine samples for pollen analysis (3 per profile) were collected at depths of 98 − 96 cm, 64 − 62 cm, and 17–19 cm for the NR core; 149 − 147 cm, 72 − 70 cm, and 12 − 10 cm for the LO core; and 87 − 85 cm, 43 − 41 cm, and 7 − 5 cm for the AZ core. They were prepared using standard laboratory procedures: adding 10% HCl to dissolve carbonates, heated in 10% KOH to remove the humic fraction and at least 24-hour treatment with HF to remove the mineral fraction followed by acetolysis (Berglund and Ralska-Jasiewiczowa 1986). Pollen and spores were counted with a biological microscope under 400 × and 1000 × magnification from the entire area of cover slide until at least 200–250 pollen grains and spores were obtained (cf. Maley and Brenac 1998). Pollen grains were identified using atlases (Maley 1970; Sowunmi 1973, 1995; Gosling et al. 2013; Schülerand and Hemp 2016; Willis 2020; https://globalpollenproject.org/). Percentages of pollen grains, spores and alga were calculated on the basis of the ratio of an individual taxon to the TPS (total pollen sum); the TPS consists of the sum of AP (arboreal pollen) and NAP (non-arboreal pollen but excluding any taxa originating from aquatic and wetland plants as well as spores and algae).

5.1. Chronology, lithology and sedimentation rates

The results of ¹⁴C measurements and age-depth modelling are summarized in Table 1 and Fig. 3 and their captions. The model from AZ and LO profiles revealed high agreements (A_model) exceeding 60% recommended for age-depth models (Bronk Ramsey 2008). These were 102.2% for the model from AZ profile and 100.6% LO profile.

The NR core from Lake Simbock exhibits three distinct units: (i) 119 − 67 cm, characterized by organic lamination, light brown color (10YR 7/3), sandy texture, and organic laminas containing wood fragments, roots, and leaves with thin ochre and muscovite flakes; (ii) 67 − 17 cm, featuring a low content of organic matter, clayey sand, and a brown color (10YR 5/3); and (iii) 17 − 0 cm, consisting of silty clay with a greenish-brown color (10YR 3/4) (Fig. 3). Grain sizes are heterogeneous from base to the top, with a predominance of sub-rounded grains over sub-angular grains.

The LO profile from Lake Ossa Complex displays three distinct layers: (i) 150 − 102 cm, characterized by a dark grey color (2.5Y 4/1) and silty clay texture; (ii) 102 − 20 cm, exhibiting a grayish-brown color (2.5Y 5/2) with silty mud content; and (iii) 20 − 0 cm, composed of silt and displaying a dark grayish-brown color (2.5Y 4/2) (Fig. 3). This sediment core has a high percentage of organic matter, and the grains are sub-rounded to sub-angular with few amounts of rounded and angular shapes. The lacustrine sediments (150 − 20 cm; 527 − 400 cal. BP) revealed fast accumulation rate ranging between 1 and 1.4 cm/yr, whereas deposition of sediments (400 − 69 cal. BP) was distinctly slower reaching ca. 0.04–0.05 cm/yr.

Three distinct lithological layers are identified in the AZ core from Lake Ngaoundaba: (i) 96 − 57 cm, characterized by dark gray (2.5Y 4/1) silty clay; (ii) 57 − 20 cm, featuring black (5Y 2.5/1) material, with 32 cm of organo-silty mud; and (iii) 20 − 0 cm, the third layer, 20 cm thick and exhibiting a dark grayish-brown color (2.5Y 4/2) (Fig. 3). All the grains in the AZ core are sub-rounded. The sedimentation rate in the section between 96 and 20 cm (918 − 60 cal. BP) is approximately 0.1 cm/yr, while the sediment section on the top (60–70 cal. BP) shows a slower rate ranging between 0.05 and 0.06 cm/yr.

5.2. Hydrological changes

5.2.1. Diatoms and hydrological variations

The analysis of diatom communities in sedimentary cores from three different lakes are shown in Fig. 4: Lake Ngaoundaba (Fig. 4a), Lake Simbock (Fig. 4b), and Ossa Lake complex (Fig. 4c). A temporary list of actually determined diatom species with their autoecology and habitat for varied samples of the selected lakes from Cameroon (current analysis) is proposed in Table 2.

Lake Simbock (NR core)

The core from Simbock Lake, analyzed at various depths, reveals a diversity of 84 diatom species belonging to 5 families and 20 genera (Fig. 4b). Throughout the NR core, there is a significant and noticeable variation in the relative abundance of ecological groups that indicate habitat preferences (tychoplanktonic and planktonic, benthic, epiphytic, aerophilic), trophic status (eutrophic and oligotrophic), and pH preferences (alkaliphilic, acidophilic, indifferent) with respect to depth. This variation is particularly prominent, with numerous changes observed. However, it's worth noting that the variation is less pronounced for benthic organisms (Fig. 4b). Table 2 further details the distribution and significance of various diatom species, contributing valuable insights into the ecological dynamics of lakes at different sedimentary layers. Between 90 and 70 cm: Aulacoseira distans dominates, suggesting a relatively deep and stable lacustrine environment. From 70 and 30 cm: A transition is observed with a significant increase in Eunotia incisa around 70 cm, indicating episodic drying periods. High concentrations of epiphytes and aerophiles correspond to lower lake levels. Between 30 and 10 cm: Aulacoseira distans persists as the dominant species, indicating an increase in the lake level. The relatively high concentrations of aerophiles suggest relatively more acidic water conditions implies a correlation between the abundance of aerophilic species and the acidity of the water in a given environment. The persistence of Aulacoseira distans at different depths suggests its adaptability to variable conditions. Aerophilic and epiphytic diatoms seem to be sensitive indicators of changes in lake level and environmental conditions (Fig. 5a).

From the diatom ecological groups, the water level in Lake Simbock was high which is indicated by domination of tycho-planktonic taxa. However, the high percentages of aerophilic species in 72 cm, 54 cm, and between 27–34 cm levels suggest a decrease in water level probably due to a decrease in water supply to the lake by the tributaries, and/or drastic dry seasons as observed in the last decade in West Central Africa (Lézine et al. 2019). It may also arise from increased erosion strength, potentially leading to the displacement of these organisms from the catchment areas into the lake. An inversion of the pH and water trophy (generally oligotrophic and acidic) is observed between 62 and 89 cm and between 8 and 27 cm. The lake waters became eutrophic and alkalphilic due to the supply of the lake water by nutrients drained from the watershed (Fig. 5a).

Ossa Lake Complex (LO core)

From the 14 samples analyzed in the sedimentary core LO, 98 species belonging to 9 family and 23 genera were identified (Fig. 4c).

Phase 1

150 − 115 cm (ca. 530 ± 20–490 ± 20 cal. BP)

The diatom assemblages were dominated by the oligotrophic acidophilic tychoplanktonic species A. distans var. humilis characteristic for oligotrophic waters (Genkal and Chekryzheva 2016). The base of this core is also dominated by A. muzzanensis which is considered as an eutrophic, planktonic taxa, encountered in the plankton of lakes (Shoeman, 1973). The maximum abundance (54%) reached ca. 530 cal. BP (150 cm). It co-occurred with the benthic species Pinnularia stomatophora and the epiphytic species Gomphonema gracile, with their highest abundances at (34 and 18%, respectively). The low pH percentage was thus indicated by both benthic and epiphytic species (Fig. 5b). The average values in planktonic and tychoplanktonic diatoms indicate a decrease in the lake level, with a transparent water column that can allow the development of benthics. At the bottom of the lake and probably in the littoral zones, benthic diatoms (Pinnularia stmatophora and Pinnularia viridiformis), epiphytes (Gomphonema gracile) are developed (Fig. 5b). Epiphytes become relatively important ca. 520 cal. BP, indicating a drop in the lake level associated with well-marked dry seasons.

Phase 2

from 115 to 65 cm (ca. 500 ± 20–450 ± 40 cal. BP)

Among planktonic taxa mainly A. muzzanensis, A. ambigua, and A. italica dominates (Fig. 5b). The species Aulacoseira granulata was particularly abundant ca. 470 cal. BP (90 cm) and reaches 77%. During this period, the increase in abundance of planktonic, eutrophic, alkaliphilic taxa simultaneous to the decrease in oligotrophic, acidophilic, tychoplanktonic species indicates a relatively high-water level, a relatively high pH and waters significantly more concentrated with organic matter compared to the previous lake episode. In addition, the low abundance of A. distans var. humilis acidophilic to the benefit of A. italica would indicate an evolution toward waters that were first neutral and then alkaline. The water column was probably less stable compared to the previous period. This trend shows a change from an oligotrophic environment to more eutrophic conditions.

Phase 3

from 65 to 45 cm (ca. 450 ± 40–430 ± 50 cal. BP)

A. granulata var. valida (12–33%) dominates with some fluctuations. The degraded frustules suggest that they were possibly brought from the Sahara by wind (Nguetsop et al. 2010, 2011). Benthic (P. viridiformis and P. stomatophora), epiphytic (Cymbella silesiaca) and aerophilic (Eunotia incisa, E. glacialis) diatoms became important. This feature indicates a drop-in lake level associated with well-marked dry seasons in which the harmattan penetrated as far as the Gulf of Guinea. The waters remained oligotrophic and acidic (Fig. 5b).

Phase 4

from 35 to 10 cm (ca. 410 ± 60–170 ± 60 cal. BP)

This phase similarly as phase 2 is characterized by the dominance of diatoms of the genus Aulacoseira. However, the high percentages of A. muzzanensis, A. ambigua at the beginning are followed by ever-increasing number of benthic and acidophilic aerophilic species such as P. viridiformis and E. glacialis ca. 170 ± 60 cal. BP (10 cm). These data reflect a deeper lake environment than during the previous period with stable and nutrient-rich waters, which gradually evolved toward acidic and clear waters (Fig. 5b).

Lake Ngaoundaba (AZ core)

The diatom microflora analyzed in 10 samples yielded 84 species belonging to 5 families and 20 genera.

There are three major phases in the diatom communities were identified in this lake (Fig. 4a).

Phase 1

Between 96 and 85 cm (1030 ± 60–920 ± 40 cal. BP)

A. distans is dominates. This tychoplanktonic species prefers oligotrophic acidic waters preferably in tropical lakes and swamps (Gasse 1987). Its relatively high abundance (28%) in plankton in Adamawa crater lakes (Kom et al. 2018) is interpreted as a characteristic of deep and stable waters, thus a relatively deep lake environment with a stable and oligotrophic water column (Fig. 5c).

Phase 2

Between 85 and 45 cm (ca. 920 ± 40–520 ± 30 cal. BP)

Aerophilics Eunotia incisa reach their highest abundance ca. 770 cal. BP (70 cm) indicating at least episodic drying. The high abundances of epiphytes (8%) and aerophiles (20%) correspond to the lowest lake levels, probably due to climate variability on a multiannual scale for some years. A. distans and A. distans var. africana were abundant (Fig. 5c), which suggests stable and oligotrophic waters (cf. Hustedt 1927).

Phase 3

Between 45 and 10 cm (ca. 520 ± 30–110 ± 40 cal. BP)

As in phase 1, the oligotrophic acidophilic tychoplanktonic species Aulacoseira distans are dominated and reached its maximum (72%) ca. 270 cal. BP (20 cm). It is associated here with relatively high contents of aerophilic diatoms (up to 14% ca. 370 cal. BP; 30 cm) represented by E. incisa. Overall, the increase in tychoplanktonic species indicates a rise in the lake level compared to the previous episode, in relatively more acidic waters. However, the planktonic species remain low, indicating a moderate rise (Fig. 5c).

5.2.2. Vegetation changes – palynological analysis

Figure 6 provides insights into the vegetation and environmental changes in three different lakes: Lake Ngaoundaba (AZ core; Fig. 6a), Lake Simbock (NR core; Fig. 6b), and Lake Ossa (LO core; Fig. 6c), based on pollen and other sedimentary records.

Lake Simbock (NR core)

The arboreal taxa were dominated by Alchornea type (15.5–27.9%), Macaranga (5.8–10%), Pycnanthus (4.4–6.2%), Combretaceae (0-5.6%), Elaeis guineensis (3-6.9%), Musanga type 1 (0–7%), Nauclea (1.5–3.2%) and Celtis types (1.5–3.9%) Bosqueia (0.9–2.1%), Trema guineensis (0.7–1.5%), and Mallotus types. This set of taxa indicate prevalence of pioneer forests with admixture semi-deciduous forests (Fig. 6b). Herbaceous, swamp and aquatic taxa are rare. Grasses (Poaceae) were more frequent and probably widespread at the beginning of the lake functioning (pollen: max. 4.5%). Algae are rarely present in pollen spectra as single specimens of Scenedesmus on the top of sediments. In the topmost deposits also, testate amoebae Arcella and Assulina cf. muscorum are identified (Fig. 6b).

Lake Ossa Complex (LO core)

During the period of ca. 530 ± 20 cal. BP local forests were well-developed (AP sum ca. 87%). The dominant taxa are: Macaranga type (25%), Musanga type 1 (6.8%), Alchornea type (7.7%) and Elaeis guineensis (7.2%). This set of pollen taxa indicates rather domination of pioneer type of forest (Maley and Brenac 1998). Additional arboreal taxa are: Hymenocardia type, Mallotus type, Urticaceae, Nauclea type, Uapaca, Celtis, and Syzygium. These taxa set shows semi-deciduous, Biafran and evergreen forests (Maley and Brenac 1998). Among herbs the most frequent are Poaceae (< 40 µm; 7.7%). Elevated levels of algae characterize an eutrophic state in the lake: Botryococcus (56%, max), Tetraedron (24%), Scenedesmus (14.5%) and Pediastrum (18.6%). The relative abundance of algae indicates the presence of open waters in the section from which the sample was collected (Fig. 6c).

The next pollen spectrum, representing age of ca. 440 ± 40 cal. BP was characterized by the immense increase in pollen percentages of Poaceae (< 40 µm, 65.6%) and spores of Polypodiaceae monolete (62.8%). This is indicated by a strong increase in forest openness (deforestation) and/or a decrease in the water table and expansion of marshlands composed of grasses. As there is no distinct increase in the number of taxa related to open land, perhaps the later explanation seems to be more probable. Perhaps expansion of grasses influenced negatively on Cyperaceae which percentages tends to two-fold decline (to 5.9%). It reflects the regional forest canopy disturbance in response to drier conditions identified by Ngomanda et al. (2005) ca. 550 − 220 cal. BP (Fig. 6c).

About 360 ± 70 cal. BP the amount of arboreal pollen (71.3%) increased, but was still lower in comparison to the spectrum reflecting period ca. 530 ± 20 cal. BP. In comparison to period of ca. 530 − 520 ± 20 cal. BP percentages of pioneer arboreal taxa Macaranga type (5.6%) and Elaeis guinensis are decreased substantially. This shows that deforestation in Lake Ossa vicinity perhaps affected the most species producing these pollen taxa. More frequent bigger pollen grains of Poaceae (> 40 µm) reaching 9% may suggest introduction of some cultivated grasses in the lake vicinity (Fig. 6c).

Lake Ngaoundaba (AZ core)

About 1030 ± 60 cal. BP, Syzygium (13.5%), Ficus (6.6%), Chassalia type (3.5%), and Nauclea type (3%) were the most frequent arboreal, which indicates semi-deciduous, Biafrean and evergreen forest (cf. Maley and Brenac, 1998). In comparison to spectra from Lake Ossa pioneer forest taxa were rarer. Among them, Alchornea type (0.4%), Musanga type 1 (2.2%), and Mallotus type (0.4%) dominates. Arboreal pollen taxa representing the mosaic forest-savanna such as Lophira cf. alata were rare. This taxon nowadays actively involved in savanna colonization in the absence of fire (White et al. 2000). Montane forests were recorded by single pollen grains of Phoenix cf. reclinata. High openness of the landscape was marked by prominent shares of Poaceae (< 40 µm, 45.9%). The frequent occurrence of grasses might have been connected with the presence of belt of rushes or marshland adjacent to the lake. This was also marked by the presence of Cyperaceae (28.8%). Shallow waters were colonized by Nymphaeaceae (pollen: 9.2%; hair basal cells: 73.4%). Their presence reveals probably limited development of algae, which are represented by sparse Pediastrum and Spirogyra (Fig. 6a).

In the spectrum representing age of ca. 750 ± 40 cal. BP, pioneer forest taxa were more frequent in comparison with ca. 1030 ± 60 cal. BP. Musanga type 1 (6.3%) and Alchornea type (1.2%) became more frequent among pioneer forest taxa. Increase in percentages of Nauclea (4.4%), Bridelia (1.3%) and Lannea types (1.3%) point to expansion of semi deciduous, Biafrean and evergreen forests. Low percentages of Podocarpus (0.6%) and Phoenix cf. reclianta (0.6%) indicated presence small patches of montane forests far from the site (cf. Maley and Brenac, 1998). In open land vegetation, species of grasses of bigger size of pollen grains (> 40 µm; 17%) are identified. Potential decrease in water table was manifested by the increase in percentages of Cyperaceae (36.7%) and Nymphaeaceae basal cells (206%) (Fig. 6a).

During the age of ca. 240 ± 60 cal. BP, arboreal vegetation probably expanded (65.4%). There was a distinct increase in Podocarpus percentages (3.8%, montane forests). Nauclea (5.9%), Bridelia (1.8%), Syzygium (18.8%), Lophira cf. alata (5.3%), Celtis (1.8%) and Uapaca types (1.2%) became more frequent or appeared in comparison with the period of 750 ± 40 ca. BP. This set of taxa is rather characteristic of semi-deciduous, Biafran and evergreen forests (cf. Maley and Brenac 1998). This forest expansion probably was the cost of pioneer forest taxa among which Musanga type 1, Alchornea type, were less frequent. The lake revealed shallowing and probable increase in trophy manifested by spread of Cyperaceae, Nymphaeaceae (pollen: 68.6%), Haloragaceae (6.4%) and more frequent presence of algae Scenedesmus (197%) and Botryococcus (11.1%) (Fig. 6a).

Based on the ecology of the diatom associations in the sites, reconstruction of certain factors such as paleobathymetry, water column stability, and water chemistry can be inferred. Variations in paleobathymetry can be evaluated by studying the specific content of each sample, especially the distribution of diatoms according to their ecological affinity (Nguetsop et al. 2010). The water level of the lakes depends on their water balance: their water supply is regulated by direct precipitation in the lakes and by the amount of water brought from their basins or catchments through drainage. Water loss in lakes is due to evaporation and infiltration. Precipitation depends on rainfall, while evaporation is related to the energy received from the sun. Thus, the hydrological phases defined by the diatom species assemblages from this study revealed four main periods of limnological change in the south and three in the north.

In the south, four major fluctuations in Simbock and Ossa Lake levels were identified: i) a drop in the water level is observed at the beginning of the phase between ca. 530 − 500 cal. BP (150 and 120 cm) ; ii) an increase in the water level between ca. 500 − 450 cal. BP (120 and 70 cm); iii) a drop in the water level is recorded between ca. 450 − 410 cal. BP (70 and 30 cm) and iv) a rise in water level is recorded between ca. 410 cal. BP to present day ( 30 and 10 cm) and marked by the dominance of planktonic diatoms (Fig. 7). Similar evolution of the hydrological levels of these two ecosystems from ca. 500 cal. BP is probably due to the synchronous response of these sites to the displacement of ITCZs (Ngeutsop et al. 2004).

Three major fluctuations have been individualized in the Lake Ngaoundaba (North): i) a high lake level recorded between ca. 1030 − 870 cal. BP (96 and 80 cm); ii) a low lake level recorded between ca. 870 − 570 cal. BP (80 and 50 cm) and iii) a high lake level is observed from ca. 570 − 110 cal. BP (50 to 10 cm) (Fig. 8). Major trends in climate evolution and comparisons with other paleodata from some regional reference sites were done in two main areas, the south (Lake Simbock and Lake Ossa) and the Adamawa (Ngaoundaba Lake) Cameroon.

South -Cameroon (Lakes Simbock and Ossa)

The palynological assemblages retrieved from Lake Simbock and Lake Ossa cores exhibit significant variations, offering a valuable tool for reconstructing the hydrological and climatic history of the region over the past 500 years. Figure 7 illustrates the variations in water level depicted by planktonic and tychoplanktonic diatoms, harmattan dust intensity, and rainfall at Ossa. Additionally, it presents the evolution of water level and vegetation in Paurosa, along with the vegetation evolution in Barombi Mbo (Maley and Brenac 1998) and Bambili (Lézine et al. 2012).

At the base of the Lake Ossa core (ca. 500 − 450 cal. BP), oligotrophic acidophilic tychoplankton prevails, coupled with elevated proportions of benthic and epiphytic diatoms, indicating a decline in lake level. This scenario is marked by a stable, clear, and nutrient-deficient water column. Sediment and diatom analyses during this period highlight a low lake level characterized by high percentages of tychoplanktonic and planktonic diatoms, accompanied by a diatom peak brought by the wind (Nguetsop et al. 1998). Low water levels around the Gulf of Guinea are evident in most lake sites since ca. 400 cal. BP. Palynological data from Cameroon reveal herbaceous savannas at Barombi Mbo Nyabessan (Maley and Brenac 1998; Ngomanda et al. 2009) and a forest opening in Gabon (Ngomanda et al. 2009). Bambili's pollen spectrum from this period indicates a significantly impoverished forest environment, with declines observed in Podocarpus and Schefflera (Lezine et al. 2012). In Sinnda, around 500 cal. BP, the replacement of forest taxa by Poaceae, Cyperaceae, and Pteridophyte spores suggests a significant open environment or water shallowing, potentially indicating the expansion of a belt of rushes. Considering the low temporal and sampling resolution observed at the Lake Ossa site, this phase of low-level is contemporaneous with a lacustrine episode known in some sites as the "Petit Age de Glace" (Little Ice Age), such as Nguéné and Kamalété in Gabon. The temporal shift may be attributed to chronological uncertainties in age patterns between sites (Anchukaitis and Tierney 2013) or other site-specific parameters influencing the recording of hydrological fluctuations and responses of palynological assemblages.

From ca. 450 cal. BP, diatom assemblages are dominated by planktonic, eutrophic, and alkaliphilic species, indicating a significant change in the lake environment. These assemblages suggest a deep environment characterized by a turbid water column, potentially becoming even more turbid. These conditions globally reflect a relatively high balance; eutrophic conditions may be associated with more erosive rains (abundance of alkaliphilic diatoms), leading to soil leaching and the transport of organic matter to the lake basin. In South-Cameroon, between ca. 500 and 450 cal. BP, a hydromorphous forest occupied a large area in both Dizangué and Yaoundé. The minimal presence of herbaceous elements, particularly Poaceae, implies the absence of a grassland environment. Such a palaeobotanical environment would indicate a humid climatic episode with precipitation likely higher than today. Similar data from Lake Barombi-Mbo (Maley and Brenac 1987; Giresse et al. 1994) around 550 cal. BP indicates a significant lake-level decline associated with deforestation. In the sediment sequences of Lakes Simbock and Ossa, and other neighboring paleoecological sites in Gabon, Cameroon, and Congo, a humid trend marked by forest recolonization is observed. For example, Bambili in West Cameroon shows an increase of up to 40% in taxa characteristic of montane and sub-montane forests, including Scheffera sp, Ilex sp, and Macaranga sp (Lezine et al. 2012). Similarly, pollen diagrams from low-latitude sites in Gabon (Nguéné, Kamalété, and Maridor Lakes) and Congo (Kitina Lake, Mopo Mbai depression) indicate higher percentages of dense forest taxa after ca. 500 cal. BP (Elenga et al., 1996; Ngomanda et al. 2009; Sutra 2013).

Between 70 and 30 cm in sediment depth, exogenously introduced diatom species dominate assemblages, along with elevated percentages of benthic and aerophilic diatoms, indicating a decrease in water level. These data suggest drier climatic conditions, implying a significant change in seasons, possibly marked by pronounced dry seasons due to a southward shift in the mean position of the Intertropical Convergence Zone (ITCZ). This shift likely led to alterations in precipitation rate and/or its distribution throughout the year. This phase might coincide with a dry period around 250 cal. BP in the same region (Nguetsop et al. 2004). Despite these drier conditions, most Central African sites experienced forest recolonization during this period, as evidenced by dominant tree species like Alchornia sp. over Poaceae in pollen data from Mopo-Bai (North-Congo Brazzaville) and Bambili (West Cameroon). This highlights the complex and heterogeneous responses of terrestrial and aquatic ecosystems in intertropical zones during climatic variations.

From ca. 450 cal. BP to the present, pollen data reveal the retreat of arboreal strata and the regeneration of mature forest at the Dizangué and Yaoundé sites, giving way to hygrophytic and the expansion of the lowland Biafran evergreen rainforest that persists today. This period was characterized by a mosaic of swampy grasslands interspersed with local patches of forests, indicating drier climatic conditions than before. This reconstruction aligns with the contemporary distribution of identified aquatic plants in the fossil spectra. Between ca. 230 and 450 cal. BP, a decline in rainfall coincided with a more contrasting seasonal distribution of precipitation. During this extended dry season, the water level decreased, allowing the development of Xyris. Concurrently, the forest, although relatively open with numerous climbers, remained in the lowest zones of the depression with water. The presence of water in Ossa Lake during this interpreted dry period could be explained by the utilization of underground water flow in these low zones. Similar dry and humid phases were documented in a pollen sequence from Kamlaté and Nguené lakes in Gabon (Ngomanda et al. 2007).

In the southern part of Central Africa, human influence is inferred from various environmental changes documented in the sediment sequences of Lakes Simbock and Ossa. Around ca. 500 − 450 cal. BP, evidence of deforestation is apparent, marked by a decline in lake level. This period is associated with a decrease in arboreal vegetation, such as Podocarpus, and an increase in herbaceous elements, particularly Poaceae. These changes in the pollen record suggest human-induced land use changes, with the clearing of trees and conversion of forested areas into open environments or savannas (Quamar and Kar 2022). The simultaneous increase in herbaceous vegetation and the decline in arboreal elements, along with the presence of Poaceae, implies intentional land clearance, possibly through burning (Garcin et al. 2018; Bayon et al. 2019). This practice is often associated with human activities related to agriculture, indicating that the local population likely used fire to clear land for cultivation. The presence of fire-adapted taxa and the decline in arboreal elements point to anthropogenic modifications in the landscape (Eastwood et al. 2018). The fluctuations in water levels and the dominance of planktonic and tychoplanktonic diatoms in certain periods may be linked to human activities affecting the hydrological balance. Changes in land cover and land use practices can influence water runoff, sedimentation, and overall lake dynamics. The shift from low water levels to more eutrophic conditions may reflect human-induced alterations in the local hydrological regime, potentially related to land use changes and agricultural activities (Bayon et al. 2019). These human-induced modifications, as reflected in the pollen and diatom records, also interact with broader climatic and environmental patterns. For example, the expansion of hydromorphous forests in later phases could be a response to changing human activities, including potential land abandonment or altered land use practices (Eastwood et al. 2018).

Adamawa (Lake Ngaoundaba)

The palynological assemblages along the studied sequence (AZ) exhibit substantial variations, providing insights into the climatic history of Adamawa over the past 1000 years. Figure 8 illustrates variations in water level from planktonic and tychoplanktonic diatoms, water stability at Ngaoundaba, changes in vegetation at Gbali (Sutra 2013) and Mbalang (Nguetsop et al. 2011), and the evolution of the water level in Lake Chad (Maley 2001). Throughout the Holocene, variations in monsoon activity, as detailed by studies such as Shanahan et al. (2006) and Itambi et al. (2010), have influenced alternating wet and dry periods in tropical Africa, impacting water budgets (Nguetsop et al. 2004). In Adamawa, a savannah-forest developed in response to climate from ca. 1030 to 870 cal. BP. The lacustrine domain is presently contracting due to sediment accumulation, possibly exacerbated by high precipitation. Notably, the presence of intraclasts as mud at 90 cm depth ca. 970 cal. BP in Ngaoundaba Lake indicates plant cover during this period. Dominant acidophilic and oligotrophic tychoplanktonic diatoms suggest relatively high, stable, and low turbidity water levels, indicative of a more intense monsoon flow linked to the northward shift of the mean position of the ITCZ (Nguetsop et al. 2004).

Between ca. 700 − 500 cal. BP, planktonic and tychoplanktonic diatoms decrease, accompanied by benthic and aerophilic diatoms, indicating a drop in lake levels and water transparency. The abundance of aerophilic diatoms suggests a strengthening dry season, enabling the development of these species in edge areas during reduced lake areas. This period corresponds to the "Medieval Warming" recorded at various African sites, marked by sudden drops in water levels in Lake Chad (Maley 2004), Lake Mbalang (Nguetsop et al. 2011), and the Paurosa Marsh (Kom et al. 2018). Palynological data from other Central African sites also indicate changes, such as herbaceous savannas at Mbalang and Gbali (Nguetsop et al. 2011; Sutra 2013) and forest openings in Gabon (Ngomanda et al. 2009). The decrease in Marine Surface Temperatures (MST), as explained by Fontaine et al. (1998), is identified as a pivotal factor influencing the reduction in monsoon flow near the Gulf of Guinea during this period, highlighting the intricate dynamics between oceanic conditions and atmospheric processes.

From 740 cal. BP, high abundances of acidophilic and oligotrophic tychoplanktonics and planktonics were observed, sometimes associated with high epiphytic species (60–70 cm). Waters mixed, likely due to winds, as indicated by high percentages of planktonic species. Around 380 cal. BP, diatoms recorded a low lake level associated with a decrease in water stability. Similar trends were recorded at nearby sites (Gbali, Paurosa, Mbalang) (Nguetsop et al. 2011; Kom et al. 2018). Pollen data indicate an increase in aquatic herbaceous taxa at the expense of forest taxa in Nguéné and Kamalété (Ngomanda et al. 2009). From 170 yrs BP, a eutrophic water body was overgrown at the top, with a decrease in water trophy associated with the appearance of peat patches in potentially more acidic conditions. In woodlands, more frequent Syzygium suggests drier climatic conditions, consistent with pollen sequences from Lake Assom (Ngos et al., 2003) and Lake Fonjak (N’nanga et al. 2019) in the Adamawa Plateau, North-Cameroon. These data reflect a relatively low water balance associated with climate degradation, potentially driven by a decrease in length of rainy seasons, harsh dry seasons (high evaporation), and/or reduced rainfall rates. The magnitude of the decrease in water level varied across sites. This low lake level would be contemporary with a lacustrine episode highlighted in some sites, such as Nguéné and Kamalété in Gabon, referred to as the "Petit Age de Glace" (Little Ice Age). In the Adamawa and the Northern part of Central Africa, from 1030 to ca. 870 cal. BP, the climate was humid in a pioneer forest. From ca. 870 to 570 cal. BP, the conditions were dry, affected by deforestation leading to savannah. From 570 cal. BP to the present day, the climate has been humid and dry, dominated by savannah and some Biafran evergreen rain forest (Fig. 9b).

The sediment sequence from Ngaoundaba Lake in the northern part of Central Africa provides valuable insights into the region's environmental history. Over the past 1000 years, variations in pollen, spore, and algae assemblages reflect significant climatic and ecological changes. Notably, ca. 800 cal. BP, at a depth of 70 cm, the pollen record indicates a sharp decline in Podocarpus and other arboreal vegetation, signaling deforestation. This decline coincides with an increase in herbaceous vegetation, as seen in elevated representation of Asteraceae and notable percentage increases in Poaceae. Algae and spore data further support this shift in local vegetation dynamics (Quamar and Kar 2022). The observed changes in the pollen, spore, and algae assemblages strongly suggest anthropogenic impacts on the environment (Garcin et al. 2018; Bayon et al. 2019). Human activities, such as deforestation likely accompanied by burning practices, emerge as key drivers behind these shifts. This alteration in the vegetation composition aligns with the patterns observed in the sediment records from Lake Ngaoundaba. Together, these findings underscore the intricate relationship between human influence and environmental changes in the northern part of Central Africa over the past millennium.

This study focuses on sedimentological, pollen and diatom data in three lake sediments. Pollen data are of low resolution, they are background. It’s over the last 1000 yrs BP in three regions of Cameroon (Yaoundé, Dizangué and Ngaoundéré) using a multiproxy approach. The sediments from studied lakes vary from one lake to another.

Over the past 500 years in the lakes of southern Cameroon, from the Southern part of West Central Africa, four distinct hydrological phases have been identified. These phases, marked by fluctuations in water levels, indicate alternating wet and dry conditions. The research highlights the influence of climatic factors, including the Medieval Warm Period and the Little Ice Age, on the dynamics of aquatic ecosystems. The paleoecological evidence reveals forest recolonization during humid periods and savannah development during drier intervals.

In the lakes of Adamawa, located in the Northern part of West Central Africa, the study uncovers distinct climatic phases over the past millennium. Dominant tychoplanktonic and planktonic diatoms indicate consistently significant lake levels, with exceptions between approximately 570 and 870 cal. BP. The reconstructed history unveils periods of humid climate fostering pioneer forests, followed by drier conditions leading to deforestation and savannah formation. Subsequently, there is evidence of a return to humid and dry climates dominated by savannah and some Biafran evergreen rainforest.

Noteworthy is the initial limited impact of anthropogenic activities on deforestation during this temporal epoch, suggesting a nuanced reciprocity between human activities and the environment. However, later anthropogenic influences witnessed a notable escalation, characterized by extensive deforestation and associated fires, indicative of shifts in subsistence farming methodologies. This investigation underscores the intricate interplay among climatological, ecological, and anthropogenic factors influencing landscape and vegetation dynamics across temporal dimensions. While the study enhances our understanding, further inquiry and empirical substantiation are necessary to comprehensively grasp the magnitude and character of human-induced environmental perturbations across distinct epochs.

Drawing insights from historical occurrences allows for a more informed approach to addressing contemporary and future challenges associated with environmental preservation and sustainable land utilization within the regional context.

Conflict of interest
I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Funding

This research not received any funding.

Author Contribution

A.Z.E.B and P.K wrote the main manuscript text; B.B prepared figures 8 and 9; N.P-D have made the sampling and core description; T.A and J.S.A-A supervised the work. All authors reviewed the manuscript.

Acknowledgments

The first author acknowledges the financial assistance received in the form of Swiss Government Excellence Scholarship to conduct research at the University of Lausanne under grant N° 2022.0663 of the Federal Commission for Scholarships for Foreign Students (FCS). The authors thank the Associate Editor (Maria I. Velez) and two anonymous reviewers for their constructive comments that substantially improved this manuscript.

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Table 1 and 2 are available in the Supplementary Files section.

No competing interests reported.

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Holocene paleoenvironmental reconstruction based on the lacustrine evolution of three Cameroonian lakes (SW, Africa)

Status:

Version 1

Abstract

Figures

1. Introduction

2. Present-day environment

2.1. Lake Simbock

2.2. Lake Ossa Complex

2.3. Lake Ngaoundaba

3. Geological setting

4. Methods

4.1. Core collection and chronology

4.2. Diatoms analysis

4.3. Pollen analysis

5. Results and interpretations

5.1. Chronology, lithology and sedimentation rates

5.2. Hydrological changes

5.2.1. Diatoms and hydrological variations

5.2.2. Vegetation changes – palynological analysis

6. Discussion

7. Conclusion

Declarations

Conflict of interest
I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Funding

Author Contribution

Acknowledgments

References

Tables

Additional Declarations

Supplementary Files

Status:

Version 1

Holocene paleoenvironmental reconstruction based on the lacustrine evolution of three Cameroonian lakes (SW, Africa)

Status:

Version 1

Abstract

Figures

1. Introduction

2. Present-day environment

2.1. Lake Simbock

2.2. Lake Ossa Complex

2.3. Lake Ngaoundaba

3. Geological setting

4. Methods

4.1. Core collection and chronology

4.2. Diatoms analysis

4.3. Pollen analysis

5. Results and interpretations

5.1. Chronology, lithology and sedimentation rates

5.2. Hydrological changes

5.2.1. Diatoms and hydrological variations

5.2.2. Vegetation changes – palynological analysis

6. Discussion

7. Conclusion

Declarations

Conflict of interest I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Funding

Author Contribution

Acknowledgments

References

Tables

Additional Declarations

Supplementary Files

Status:

Version 1

Conflict of interest
I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.