# Plant evidence of environmental and hydrological changes in Vichama, Peru (1800–1500 BCE)

Marco Yseki, Claudia Morales, Aldemar Crispin, Tatiana Abad, Alonso Prado, Alexander Zuñiga, Marco Machacuay, Bruno Turcq, Matthieu Carré, Pedro Novoa, et al.

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## ► To cite this version:

Marco Yseki, Claudia Morales, Aldemar Crispin, Tatiana Abad, Alonso Prado, et al.. Plant evidence of environmental and hydrological changes in Vichama, Peru (1800–1500 BCE). Journal of Archaeological Science: Reports, 2025, 68, pp.105473. 10.1016/j.jasrep.2025.105473. hal-05343422

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1 Plant Evidence of Environmental and Hydrological Changes in Vichama, Peru (1800–1500 BCE)

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4 Marco Yseki¹*, Claudia R. Morales², Aldemar Crispin¹, Tatiana Abad¹, Alonso Prado¹, Alexander Zuñiga¹, Marco Machacuay¹, Bruno Turcq³, Matthieu Carré³,⁴, Pedro Novoa¹,⁵ and Ruth Shady¹,⁵

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8 ¹Zona Arqueológica Caral, Unidad Ejecutora 003, Ministerio de Cultura del Perú, Lima, Peru

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10 ²Laboratorio de Palinología y Paleobotánica, Universidad Peruana Cayetano Heredia, Lima, Peru

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12 ³LOCEAN-IPSL, Laboratoire d'Océanographie et du Climat: Expérimentation et Approches Numériques, IRD/Sorbonne Université/CNRS/MNHN, Paris, France

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14 ⁴Centro de Investigación para el Desarrollo Integral y Sostenible (CIDIS), Universidad Peruana Cayetano Heredia, Lima, Perú

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16 ⁵Escuela Profesional de Arqueología, Facultad de Ciencias Sociales, Universidad Nacional Mayor de San Marcos, Lima, Peru

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19 *Email: kyseki@zonacaral.gob.pe

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22 Abstract

23 Reconstructing past climatic fluctuations and their impact on human subsistence is essential for understanding long-term resilience and adaptation strategies. This study reconstructs past vegetation and environmental changes at Vichama (1800–1500 BCE), a pre-Hispanic agro-fishing settlement on Peru’s north-central coast. Our results indicate that the clay used in the preparation of floor mortars was collected from nearby wetlands. 24 25 26 27 Pollen analysis of mortars spanning six architectural periods (P1 to P6) reveals a major shift in coastal wetland vegetation. During P1, P2, and P6, pollen assemblages were dominated by hydrophilic wetland taxa, primarily sedges and cattails (41.4–90.5%), 28 29 30 31 indicating humid conditions. In contrast, P3 to P5 show a sharp decline in these taxa (4.8– 32.8%) alongside a significant increase in wild Poaceae pollen (45.5–90.2%), reflecting 32 33 a transition to drier and more saline conditions. Pollen data also support the use of these 34 wetlands for cultivation, underscoring their strategic role in local subsistence practices.

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The relative chronology of P4 and P5, the periods with the strongest signal of aridity in our pollen record, places them between ~1650 and 1550 BCE. This timing coincides with evidence of droughts in the Andes, suggesting that reduced precipitation in the upper watershed led to lower river discharge, diminished groundwater infiltration, and the contraction of wetlands near Vichama. These drier conditions correspond with agricultural stress indicators, such as smaller guava seeds and reduced crop richness, and may also be reflected in Vichama's friezes depicting famine and social distress.

**Keywords:** Vichama, paleoenvironmental, pollen, archaeology, Peru

**Highlights**

* Pollen in mortars floor documents 300 years of vegetation change at Vichama, Peru

* Decline wetland pollen and rise of Poaceae indicate arid periods at Vichama

* Smaller guava seeds indicate agricultural stress during dry periods at Vichama

## 1. Introduction

Projections of future climate change effects in Peru remain highly uncertain (Salzmann et al., 2013; Heidinger et al., 2018; Vicente-Serrano et al., 2018; Wongchuig et al., 2018; Potter et al., 2023) posing significant challenges, particularly for population reliant on climate-sensitive sectors like agriculture and fisheries. Effective planning for sustainable adaptation requires identifying critical climatic thresholds that may disrupt social or ecological systems and developing culturally informed strategies to mitigate their effects (Rick & Sandweiss, 2020; Burke et al., 2021). In this context, integrating natural climate archives with archaeological records provides valuable insights into how ancient societies responded to past climatic variability. These insights contribute to refining models of human-environment interactions and identifying ecological or social tipping points that may lead to systemic reorganization (Rick & Sandweiss, 2020; Burke et al., 2021).

Despite Peru's rich archaeological heritage, studies explicitly linking pre-Hispanic societies to past climate change remain limited. Bridging this gap is essential for understanding how ancient populations adapted to environmental pressures and for informing present-day resilience strategies.
The agro-fishing city of Vichama, located on Peru's north-central coast, was a prominent settlement in the lower Huaura River Valley between 1800-1500 BCE (Figs. 1a and 1b) (Shady et al., 2008, 2013, 2014, 2015). Excavations conducted by the *Zona Arqueológica Caral* (ZAC) suggest that Vichama was founded by a society that emerged after the Late Caral period, maintaining core elements of Caral's cultural legacy while developing a distinct regional identity (Shady et al., 2008, 2013, 2014, 2015). Its inhabitants relied on a broad spectrum of surrounding ecosystems, including the sea, rocky and sandy shores, wetlands, and fog oases, for subsistence (Shady et al., 2008, 2013, 2014, 2015). Archaeobotanical evidence indicates that they cultivated a wide range of crops, such as fruit trees, chili peppers, legumes, tubers, and cucurbits (Yseki et al., 2025), and also consumed marine resources, as attested by fish and mollusk remains recovered from public buildings (Shady et al., 2008, 2013, 2014, 2015).

Archaeological evidence further suggests that Vichama's inhabitants likely endured episodes of famine and food scarcity, potentially driven by environmental stressors. In the Main Building (Sector A) of Vichama, archaeologist from the ZAC uncovered a series of 34 mural reliefs known as “The Dance of Life and Death” (Shady et al., 2014, 2015). These reliefs depict skeletal adult figures with pronounced ribs on the lower level, and youthful individuals with circular cavities in their abdomens on the upper level (Fig. 1c), which are interpreted as symbols of hunger and malnutrition. Complementary archaeobotanical data show a marked decline in the ubiquity of edible plant macroremains during architectural periods 3, 4, and 5 (Yseki et al., 2025), supporting the hypothesis of diminished food availability. If prolonged drought or agriculturally unfavorable conditions occurred during this interval, they may have reduced both the frequency and productivity of crops, thereby limited yields and diminished the size and quality of fruits, tubers, and seeds (Daryanto et al., 2016; Fernandes et al., 2018; Zhang et al., 2022; Sun et al., 2024). During a later construction period, the aforementioned murals were deliberately buried, and a new ceremonial hall was built in their place. Its iconography, featuring a toad with human hands emerging from the earth and a lightning bolt above its head (Fig. 1d), reflects Andean cosmological associations with rainfall, fertility, and agricultural abundance (Grebe, 1989; Isasmendi and López Campeny, 2022; Venturoli, 2005). This structure likely functioned as a ritual space for invoking precipitation and ensuring water availability for agricultural production.

Understanding how these environmental dynamics shaped Vichama's subsistence activities offer broader insights into ancient resilience and cultural adaptation to climate
variability. However, reconstructing such local environmental conditions is hindered by
the lack of high-resolution paleoclimate records for Peru's central coast during the site's
occupation. To address this gap, this study analyzes pollen preserved in mortared floor
sediments from stratigraphically secure contexts within three public buildings at
Vichama, the Main Building (Sector A1), the Public-Residential Building (Sector E), and
the Building of the Offerings (Sector K) (Fig. 1b). These samples span all six architectural
periods (P1 to P6), allowing reconstruction of vegetation dynamics and, by proxy,
changes in humidity and water availability over the site's 300-year occupation. To assess
the potential agricultural impacts of these environmental shifts, we also analyze
morphometric variation in guava seeds, the most frequent macrobotanical remains at the
site (Yseki et al., 2025), by measuring seed length and width. Previous studies have shown
that water availability significantly influences size and crop performance (Daryanto et al.,
2016; Fernandes et al., 2018; Zhang et al., 2022; Sun et al., 2024). By integrating
palynological and morphometric data, this study provides a comprehensive reconstruction
of past ecological conditions and explores the interplay between climate, water resources,
and agricultural production at Vichama.

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117 2. Materials and methods

118 2.1. Study area

119 2.1.1. Vichama archaeological site

120 The Vichama archaeological site is located 127 km north of Lima metropolis, in the
121 district of Végueta (Fig. 1). The site covers an area of 136.6 ha, extending along the entire
122 perimeter of Halconcillo Hill, a rocky formation with an elevation of 114m. Excavations
123 conducted by ZAC identified nine areas of activity, including stone architecture bonded
124 with mortar (Vi-01) shown in Fig. 1, ash deposits and shell mounds (Vi-02 and Vi-03),
125 as well as cemeteries containing ceramics (Vi-04, Vi-05, Vi-06, Vi-07, and Vi-08) and
126 Vi-09, tapial architectural (adobe walls) (Shady et al., 2008, 2013, 2015). The present
127 study focuses on the Vi-01 component, Vi-01 covers an area of 25 ha on the northern
128 slope of the Halconcillo Hill and comprises 29 buildings distributed in eight architectural
129 complexes, with four in the upper half and four in the lower half of the settlement (Shady
130 et al., 2008, 2013, 2015) (Fig. 1).

131 The Vichama occupation sequence is supported by both relative and absolute
132 chronological evidence. Radiocarbon dates obtained by the ZAC archaeologists, under
133 the direction of Ruth Shady, place the site's occupation between 1800 and 1500 BCE,
134 providing the basis for its absolute chronology (see Fig. 2 in Yseki et al. (2025)).

135 Within this temporal range, ZAC archaeologists, identified a series of minor and major
136 architectural changes based on stratigraphic relationships, construction overlaps, wall
137 alignments, and physical modifications to structures. Minor changes include the addition
138 of interior partition walls, the remodeling of floors, and other modifications or structural
139 additions that addressed the evolving needs of specific social groups or individuals
140 managing the buildings. While these changes altered internal layouts, they largely
141 preserved the foundational structure and architectural components of the original
142 constructions. In contrast, major changes involved the construction of entirely new
143 buildings over previous ones. These transformations often included the remodeling of
144 facades, sometimes involving a shift in orientation by up to 90 degrees, as well as the
145 partial demolition of earlier structures and the addition of new features such as circular
146 plazas. Due to the extent of these modifications, ZAC archaeologists classify them as
147 distinct "architectural periods". Based on these observations, six major changes or
148 architectural periods (P1-P6) have been identified at Vichama. These were primarily
149 defined through the stratigraphic superposition of buildings identified in Sector A, the
150 Main Building of the lower half of the settlement. Notably, the periods form three
151 interconnected architectural pairs: P1-P2, P3-P4, and P5-P6. In each pair, the first
152 structure served as the foundation for the subsequent one. P1 and P2 are characterized by
153 laterally extended plans, whereas P3 and P4 exhibit a longitudinal elongation. In P5 and
154 P6, the orientation of main façade shifted from northeast to southeast (a 90-degree turn),
155 while the overall design returned to a lateral extension, including a central staircase and
156 sequence of overlapping platforms. The remaining buildings investigated were correlated
157 with the six-periods sequence based on architectural characteristics and stratigraphic
158 evidence, enabling the establishment of a general chronological framework for Vichama.
159 Excavations were conducted under the authorization of the Peruvian Ministry of Culture
160 (Directorial Resolution N° 202-2020), following the guidelines and regulations set by
161 both the Ministry of Culture and the ZAC.

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163 2.1.2. Regional settings

164 Vichama is located on Peru's north-central coast, 6.5 km northwest of the Huaura River
165 and 1.5 km from the Pacific coast, in the inter-basin area between the Huaura and Supe
166 rivers (Shady et al., 2008, 2013, 2014, 2015). The central Peruvian coast experiences
minimal annual precipitation (14 mm), with intermittent coastal river flows (Lagos et al., 2008). However, during the austral summer, monsoonal precipitation in the Andes increases river discharge (Garreaud et al., 2009; Vuille et al., 2012). Intense El Niño events occasionally cause extreme flooding and elevated sediment discharges (Bourrel et al., 2015; Guzman et al., 2020; Rau et al., 2017; Yseki et al., 2022). Despite being situated in the arid Peruvian desert, Vichama is located in an area with a relatively shallow water table (0.87 m), which enables groundwater to surface in lower-lying zones and promotes the development of coastal wetlands near the archaeological site (INRENA et al., 2005; Santos & Peña, 2016; Shady et al., 2008, 2014). Previous studies have documented distinct plant communities in coastal wetlands of Peru, shaped by variations in hydrological regime, salinity, and substrate composition (Ramirez et al., 2010; Aponte Ubillús & Ramírez Huaroto, 2011; Aponte & Cano, 2013; Aponte & Ramírez, 2014). Vegetation in these coastal wetlands exhibits a clear spatial organization along moisture and salinity gradients, giving rise to distinct plant communities across the landscape. Permanently or seasonally inundated zones are dominated by tall, rhizomatous emergent macrophytes such as sedges and cattails, which form dense, monodominant stands with high ground cover (Ramirez et al., 2010; Aponte Ubillús & Ramírez Huaroto, 2011; Aponte & Cano, 2013; Aponte & Ramírez, 2014). These taxa are indicative of freshwater to slightly brackish conditions and fulfill essential ecological functions, including sediment stabilization and the maintenance of humid microhabitats. Toward the outer margins of the wetlands, where soils exhibit higher salinity and improved drainage, vegetation shifts to halophytic species, including salt-tolerant grasses (Ramirez et al., 2010; Aponte Ubillús & Ramírez Huaroto, 2011; Aponte & Cano, 2013; Aponte & Ramírez, 2014). Additional herbaceous taxa associated with ruderal or anthropogenically disturbed environments are also present. These include members of the Chenopodiaceae-Amaranthaceae group and various Asteraceae species, which are generally of low to medium stature and inhabit zones with fluctuating moisture regimes. Their occurrence suggests dynamic vegetation responses to both natural hydrological variability and possible human impacts, including cultivation, pasturing, or construction activities (Ramirez et al., 2010; Aponte Ubillús & Ramírez Huaroto, 2011; Aponte & Cano, 2013; Aponte & Ramírez, 2014). The list of plant species recorded in coastal wetlands near the Vichama site is provided in Supplementary Table S1.
## 2.2. Pollen Analysis

The archaeological sediment samples (10 grams) analyzed in this study correspond to clay-rich sediments used in the preparation of floor mortars. These samples were collected from well-defined construction contexts associated with the mortar floors of the Main Building (Sector A1), the Public-Residential Building (Sector E), and the Building of the Offerings (Sector K). All six architectural periods identified at Vichama (P1-P6), spanning approximately 300 years of occupation between 1800 and 1500 BCE, were represented in the sampling strategy (Supplementary Fig S1-S5).

Their relative chronology (Supplementary Table S2) was based on archaeological stratigraphy and contextual associations, supported by multiple radiocarbon dates obtained by the ZAC. It is important to note that the analysis of pollen from floor mortars does not reflect human activities directly associated with the use of the floors or interior spaces. This is because the mortar layer is overlain by a thin finishing plaster of different color and only the underlying sediment used in the preparation of the mortar was analyzed. As a result, the pollen assemblages recovered from the mortars primarily reflect the vegetation of the surrounding environments from which the clay was originally sourced for construction purposes, rather than the activities that took place within the architectural spaces.

Sample processing was carried out at the Palynology and Paleobotany Laboratory of Cayetano Heredia Peruvian University (UPCH) following standard procedures (Traverse, 1998). This process primarily involved the removal of organic matter, carbonates, and silicates through a series of chemical reactions using potassium hydroxide, hydrochloric acid, and hydrofluoric acid, respectively. Grains of pollen and spores were counted until a total of 200 of these elements was reached, or until 1,000 Lycopodium marker spores had been counted, whichever occurred first. The percentage of each taxon was calculated by dividing the number of pollen grains or spores of that taxon by the total number of palynomorphs (grains of pollen and spores) in the sample and multiplying the result by 100. Palynomorph identification was conducted using a Nikon Eclipse E200 microscope. Taxonomic classification of pollen and spores was based on reference database (Bush & Weng, 2007), including the collection of the Palynology and Paleobotany Laboratory of UPCH.

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235 2.3. Guava seeds measurements

236 The length and width of 960 guava (Psidium guajava) seeds from architectural periods 1
237 to 6 of the Vichama settlement were measured using a digital caliper. Guava seeds were
238 selected for analysis as they represented the only whole and most ubiquitous botanical
239 macro-remains recovered across all six periods studied (Yseki et al., 2025). The
240 measurements were used to evaluate potential changes in seed size related to
241 environmental variability and water stress. Prior to statistical testing, data normality was
242 evaluated using the Shapiro-Wilk test, which indicated that both length and width values
243 were non-normally distributed (p < 0.05). Therefore, non-parametric analyses were
244 conducted. Overall differences in seed length and width among the six archaeological
245 phases (P1-P6) were examined using the Kruskal-Wallis test, which revealed a highly
246 significant difference (p < 0.001). When the Kruskal-Wallis test indicated significance,
247 Dunn's post-hoc test with Bonferroni correction was applied to perform pairwise
248 comparisons between phases. To summarize and visualize these results, a Compact Letter
249 Display (CLD) was generated based on the adjusted p-values from Dunn's test. In this
250 display, phases sharing the same letter do not differ significantly (p > 0.001), whereas
251 those assigned different letters indicate statistically significant differences (p< 0.001). A
252 conservative significance threshold of p < 0.001 was adopted within the CLD framework
253 to ensure a robust criterion and reduce the risk of Type I errors associated with multiple
254 comparisons. All statistical analyses and plots were performed in R version 4.3.0, using
255 the packages ggplot2, ggpubr, dunn.test, and rcompanion.
256

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257 3. Results

258 3.1.Botanical Taxa Identified

259 All analyzed samples yielded at least 200 pollen and spore grains, except for two samples
260 (Supplementary Table S3). A total of 1813 pollen grains were recovered from the eleven
261 analyzed samples, representing 25 botanical taxa characterized by diagnostic pollen
262 grains. Among these, pollen grains from edible plants were identified, including Psidium
263 sp. "guava", Ipomoea sp. "sweet potato", Pachyrhizus sp. "jíquima" and Canavalia sp.
264 "jack bean". Pollen grains from cultigens of industrial importance, such as Gossypium sp.
265 "cotton", were also present. Additionally, pollen grains from trees such as Alnus sp. and
266 Podocarpus sp., coastal wetland plants including members of the family Cyperaceae
267 "sedge", Typha sp. "cattail", Phyla sp., and Alternanthera sp., were identified. Wild
268 herbaceous plants were represented by taxa such as Asteraceae, Cactaceae,
Nyctaginaceae, Caryophyllaceae, Solanaceae, *Ambrosia* sp., and *Chenopodium-Amaranthus* sp., along with pollen grain of wild Poaceae "grasses" (Supplementary, Table S3 and Fig. S6). In addition to the identified taxa, some pollen grains lacked distinctive morphological features necessary for confident taxonomic assignment and were thus classified as morphotypes. These undetermined grains represent between 0 and 12.5% of the total pollen count across the eleven analyzed samples (Supplementary, Table S3 and Fig. S6).

## 3.2. Paleobotanical Environmental Reconstruction

During periods P1 and P2, pollen from hydrophytic coastal wetlands, primarily sedges and cattails, accounted for 41.4-90.5% of the total assemblage. In contrast, Poaceae pollen, mainly wild Poaceae species with grain sizes smaller than 30 µm, was notably less abundant (0.9-24.1%), while tree pollen (0.5-2.8%) and herbaceous pollen (0.5-15.5%) were scarce. Cultivated pollen accounted for 0.5-4.6%, with evidence of guava, cotton, sweet potato, and jíquima cultivation (Fig. 2 and Supplementary Table S3). In contrast, during P3, there is a decline in hydrophytic wetland taxa (32.8%) and herbaceous pollen (6.9%), accompanied by an increase in wild Poaceae (44.62%). This pattern intensifies in P4 and P5, which show a substantial reduction in wetland taxa (4.8-11.1%) and herbs (2.4-4.3%), along with a marked increase in wild Poaceae (81.1-90.2%). Cultivated pollen also declined during P4 and P5, ranging from 0% to 0.81%, with cotton being the only identified cultivated taxon (Fig. 2 and Supplementary Table S3). Finally, during period P6, the final period of settlement occupation, the pollen record indicates an increase in coastal wetland taxa (43.8-64.1%) and herbaceous pollen (7.5-20.3%), accompanied by a reduction in Poaceae pollen (18.1-29.1%). The abundance of cultivated plant pollen during this period ranged from 0.5% to 5.8%, with evidence of guava, cotton, sweet potato, jíquima, and jack bean cultivation (Fig. 2 and Supplementary Table S3).

Principal component analysis (PCA) of pollen data corroborates the observed vegetation dynamics across periods P1-P6. Principal Component 1 accounts for 98.2% of the dataset variance and correlates positively with grassland pollen and negatively with wetland pollen. The PCA results (Supplementary Fig. S7) reveal two distinct clusters along PC1. The first cluster includes samples from P3, P4, and P5, characterized by a higher prevalence of Poaceae pollen. The second cluster includes samples from P1, P2, and P6, distinguished by a higher abundance of coastal wetland pollen.
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303 3.3. Water Stress and Size Effects in Guava

304 A total of 960 guava seeds from the six architectural periods (P1–P6) of the Vichama
305 settlement were measured to assess changes in seed dimensions through time
306 (Supplementary Table S4). The Kruskal–Wallis test revealed highly significant
307 differences among periods for both seed length (χ² = 148.20, df = 5, p < 0.001) and seed
308 width (χ² = 55.42, df = 5, p < 0.001). Pairwise Dunn's post-hoc tests with Bonferroni
309 correction confirmed that several periods differed significantly from one another.
310 According to the Compact Letter Display (CLD), seed length showed clear separation
311 into three main groups: P1 and P2 (group a) had the largest median lengths; P3 (group b)
312 displayed intermediate values; and P4 and P5 (group c) contained the smallest seeds. P6
313 (group a) exhibited median lengths similar to P1 and P2 (Fig. 3a). For seed width, the
314 CLD indicated fewer differences among periods. Most phases shared the same statistical
315 group (a), except Period 4, which formed a distinct group (b) characterized by smaller
316 median widths (Fig. 3b). Overall, variation in guava seed size across the sequence reflects
317 noticeable fluctuations, with the most reduced dimensions recorded in Periods 4 and 5.
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318

319 4. Discussion

320 The floor mortars samples analyzed in this study were recovered three buildings in
321 Vichama (Sectors A, E, and K) spanning all six architectural periods of the site (P1–P6),
322 with occupation beginning around 1800 BCE and abandonment occurring around 1500
323 BCE, see Fig. 2 in Yseki et al. (2025). Although Vichama is located in the arid Peruvian
324 desert, it lies in an area with a relatively high-water table (0.87 m), allowing groundwater
325 to emerge in low-lying areas and contributing to the formation of coastal wetlands near
326 the site (INRENA et al., 2005; Santos & Peña, 2016; Shady et al., 2008, 2014). It is likely
327 that between 1800 and 1500 BCE, these wetlands covered a larger area than today, as
328 modern agricultural expansion has significantly altered the landscape. Given this
329 environmental context, it is plausible that the ancient inhabitants of Vichama sourced
330 clay-rich sediments from the surface layers of nearby wetlands for the preparation of floor
331 mortars. These mortars are approximately 2 cm thick. Considering that coastal wetlands
332 in Peru exhibit an average sedimentation rate of 0.5 cm per year (Escobedo, 2021), the
333 collection of the uppermost 5 cm of sediment from wetland margins would encompass
334 roughly a decade of environmental deposition. Given the thinness of the mortars and the
fine stratification of wetland deposits, the use of these surface layers for construction
would preserve a highly localized and temporally constrained record. As a result, the
pollen assemblages embedded in the mortars can be interpreted as a proxy for short-term
vegetation dynamics, reflecting the composition of local plant communities and the
prevailing hydrological conditions at the time of clay collection and mortar preparation
during each architectural period. The presence of sedges and cattails, which are
characteristic taxa of coastal wetlands along the Peruvian coast (Ramirez et al., 2010;
Aponte Ubillús & Ramírez Huaroto, 2011; Aponte & Cano, 2013; Aponte & Ramírez,
2014), in floor mortars across all analyzed periods and their high frequencies during P1,
P2 and P6 (41.4–90.5%) (Fig. 2), support the interpretation that the ancient inhabitants of
Vichama used clay-rich sediments from wetlands in the preparation of floor mortars.
However, during the remaining periods (P3, P4, and P5), especially during P4 and P5, we
observed a marked decline in hydrophytic wetland plant pollen (4.8–11.8%) alongside a
corresponding increase in Poaceae pollen (81.1–90.2%), suggesting a shift in the
composition of coastal wetland during these periods (Fig. 2).

350

351 4.1. Climate and Hydrology as Drivers of Coastal Wetland Vegetation Change
352 Our pollen record reveals a shift in coastal wetland vegetation near Vichama that begins
353 in P3 and becomes more pronounced in P4 and P5. This transformation is characterized
354 by an inverse relationship between the abundance of Poaceae and hydrophytic wetland
taxa, particularly sedges and cattails (Fig. 2), which are typically associated with high-
355 moisture environments (Caramanica et al., 2020). In P3, a decline in sedge and cattail
356 pollen, alongside an increase in Poaceae, suggests the initial transition from hydrophilic
357 wetland vegetation to grass species adapted to drier, sandier, or more saline conditions.
358 By P4 and P5, Poaceae dominate the pollen assemblage (81.1–90.2%), while hydrophytic
359 wetland taxa become notably scarce (4.8–11.1%). These patterns indicate a significant
360 change in vegetation structure, likely linked to reduced water availability in the
361 surrounding wetlands. Due to the limitations of light microscopy, Poaceae pollen could
362 not be identified beyond the family level. While some hydrophilic grasses, such reed, can
363 occur in coastal wetlands, they were not recorded in the modern vegetation surveys of the
364 study area (Ramirez et al., 2010; Aponte Ubillús & Ramírez Huaroto, 2011; Aponte &
365 Cano, 2013; Aponte & Ramírez, 2014). However, their presence in the past cannot be
366 entirely ruled out. Nevertheless, the strong inverse relationship observed between the
367 abundance of Poaceae and hydrophytic taxa suggests that hydrophytic grasses were not
368
the dominant contributors to the Poaceae assemblage during P4 and P5. Moreover, most of the measured Poaceae grains were smaller than 30 microns, a size typically associated with small wild grasses (Schüler & Behling, 2011; Jan et al., 2015; Radaeski & Bauermann, 2017; Radaeski et al., 2020). Therefore, although the presence of hydrophytic Poaceae cannot be fully excluded, the decline of humid-indicator taxa (sedges and cattails) supports the interpretation that the observed increase in Poaceae reflects the expansion of non-hydrophytic grasses under more arid environmental conditions during P4 and P5. Taken together, these patterns suggest that the methodological limitation does not compromise the validity of our paleoenvironmental interpretation. In addition to limitations related to taxonomic resolution, the number of pollen grains counted per sample represents another constraint that may influence the robustness of paleoenvironmental interpretations. In this study, eleven sediment samples corresponding to the six architectural periods at Vichama were analyzed, covering approximately 300 years of occupation. Nine of these samples exceeded the commonly recommended threshold of 200 counted palynomorphs. Two samples, corresponding to periods P1 and P5, yielded lower counts of 102 and 123 grains, respectively. However, for P1, two additional samples provided consistent results. In the case of P5, although only one sample was available, the relative proportions between Poaceae and hydrophytic taxa were similar to those observed in P4. These patterns suggest that the overall pollen signal reflects coherent shifts in vegetation dynamics within the coastal wetlands surrounding Vichama. To refine these findings, future research should include a larger number of samples and integrate high-resolution paleoclimatic data from nearby lakes or wetlands.

The vegetation shift recorded in our pollen data indicates a sustained decline in moisture availability and a disruption of the hydrological balance of coastal wetlands in the study area. This ecological transition involved the replacement of obligate hydrophytic taxa, such as sedges and cattails, by wild grasses tolerant to drought and salinity. In this hyperarid setting, where local precipitation is minimal (Lagos et al., 2008), coastal wetlands of the study area rely primarily on groundwater recharge derived from the infiltration of discharge from the Huaura River, which originates in the Andes (Santos & Peña, 2016; INRENA et al., 2005). A decrease in rainfall in the upper watershed likely reduced river discharge, which in turn limited aquifer recharge and lowered the regional groundwater table. This reduction in subsurface flow may have destabilized the wetland
402 hydrology by decreasing the persistence of shallow water bodies that are essential for
403 hydrophilic plant communities. Simultaneously, increasing aridity may have intensified
404 surface evaporation, accelerating soil desiccation. High solar radiation and persistent
405 coastal winds can raise evapotranspiration rates, particularly in areas with limited
406 vegetative cover or shallow water bodies. In addition, during arid periods and in the
407 hyperarid conditions of the study area, enhanced aeolian activity may have contributed to
408 sand deposition along wetland margins. The accumulation of sediment over saturated
409 soils can hinder upward capillary water movement and modify soil texture and drainage
410 properties. These physical alterations may have further intensified environmental stress
411 on wetland vegetation, disrupting hydrological connectivity and promoting the expansion
412 of drought-tolerant species over obligate hydrophytes.
413

414

4.2. Regional Climate Variability and Its Impact on Vegetation Change

415 Our results indicate that the hydrological imbalance observed in the coastal wetlands
during P3 to P5 was most likely caused by reduced groundwater recharge, resulting from
416 lower Huaura River discharge due to diminished precipitation in the Andean headwaters.
417 This decline in rainfall may be linked to a weakening of the South American Summer
418 Monsoon (SASM), a climatic system known to fluctuate at multidecadal to millennial
419 timescales (Garreaud et al., 2009; Vuille et al., 2012). It is plausible that such a weakening
420 occurred during P3 to P5, contributing to the hydrological stress reflected in our record.
421 Given that Vichama was occupied for approximately 300 years (1800–1500 BCE), and
422 the observed vegetation changes likely unfolded over multidecadal timescales,
423 meaningful comparison requires high-resolution paleoclimate records. Unfortunately,
424 most available datasets do not offer sufficient temporal resolution to capture variability
425 at this scale. Furthermore, while several datasets exist for eastern South America,
426 paleoclimate information from the western slopes of the Andes remains limited. A
427 valuable exception is the high-resolution peat record from Cerro Llamoca (14°S, 4450
428 m), which documents arid conditions and a weakened SASM between ~1650 and 1550
429 BCE (Schitte et al., 2015). While the Cerro Llamoca record offers high chronological
430 resolution, uncertainties in its age-depth model must be considered when drawing
431 comparisons with our pollen sequence, which lacks internal absolute radiocarbon dates.
432 Nonetheless, the Vichama sequence is anchored by a consistent relative chronology based
433 on well-defined archaeological stratigraphy, including construction overlaps, wall
434
alignments, and floor remodeling, as well as absolute radiocarbon dates obtained by ZAC.
435

Despite these limitations, the temporal overlap between the arid phase documented at
436

Cerro Llamoca and periods P3 to P5 at Vichama suggests a potential regional climatic
437

connection (Fig. 4). Taken together, these observations suggest that a broader reduction
438

in precipitation likely affected the upper reaches of watersheds along the north-central
439

and southern Peruvian coast between 11 and 14°S. This regional-scale decline in rainfall
440

would have reduced river discharge and groundwater infiltration in the lower Huaura
441

basin. In turn, this would have caused wetland contraction and a shift in plant
442

communities, as documented in the pollen record during P3 to P5 (Fig. 2), highlighting
443

the sensitivity of coastal ecosystems to Andean hydroclimatic variability. This case
444

highlights the potential of integrating stratigraphic archaeological records with
445

paleoenvironmental proxies to reconstruct fine-scale climate-ecological interactions in
446

ancient coastal societies.
447

448

448

4.3. Evidence of Agricultural Practices and Environmental Stress

Vichama was strategically located in an area that facilitated agriculture due to the high-water table and the emergence of groundwater, which contributed to the formation of coastal wetlands near the archaeological site, possibly used as cultivation fields. Excavation at Vichama have uncovered a diverse array of plant remains, including seeds, fruits, roots, and leaves from food crops spanning all six architectural periods (Yseki et al., 2025). Identified remains include fruit trees (e.g., guava and lucuma), root crops (e.g., sweet potato and achira), squashes, capsicum peppers, and legumes (e.g., pacae, common bean, lima bean and jack bean). Pollen analysis further confirms the presence of cultivated plants such as guava, cotton, sweet potato, jíquima, and jack bean. Most of these taxa had already been identified in the macrobotanical assemblage (Yseki et al., 2025), with the exception of jíquima, which appears only in the pollen record. This correspondence between datasets reinforces the evidence for local agricultural production at Vichama. The occurrence of pollen from these cultivated species in the analyzed clay-rich sediments suggests that farming activities were carried out in close proximity to coastal wetlands. This interpretation is supported by the physical characteristics of crop pollen, which tend to be relatively large in size, as in the case of cotton and sweet potato, and are not easily dispersed over long distances (Fish, 1994), indicating localized cultivation near wetland environments.

450

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452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467
Periods of reduced humidity and decreased groundwater levels, as inferred from our pollen record during P3–P5, likely posed significant challenges to agriculture. This is reflected in the decline in the ubiquity of macrobotanical remains of food plants (Yseki et al., 2025) and reduced crop richness in our pollen record (Fig. 2). Additionally, the reduced guava seed length observed in P4-P5 provides further evidence of environmental stress, as seeds from these periods were significantly smaller (Fig. 3). This trend aligns with previous studies demonstrating that reduced water availability and hydric stress can negatively impact fruit size, crop productivity and overall plant health (Daryanto et al., 2016; Fernandes et al., 2018; Zhang et al., 2022; Sun et al., 2024). Despite the presence of agricultural resources during P3–P5, as indicated by our pollen record (Fig. 2) and previous macro-botanical evidence from Vichama (Yseki et al., 2025), our results suggest that agricultural production may have declined during these drier periods, likely impacting the living conditions of its ancient inhabitants. These environmental challenges may be reflected in the murals of Sector A, which depict themes possibly related to food scarcity (Fig. 1c). However, the resilience of the population allowed for continued occupation despite adverse conditions.

Future research should explore how Vichama's inhabitants adapted their subsistence strategies to mitigate the impacts of prolonged drought during P3-P5. Given that Vichama's economy relied on both agriculture and marine resources (Shady et al., 2008, 2013, 2014, 2015; Yseki et al., 2025), it is plausible that during drier periods, marine resources became an important component of the diet. Additionally, agricultural practices may have adapted through the cultivation of drought-resistant crops, adjustments to planting calendars, or increased trade with populations from different ecological zones. The presence of potatoes in ritual offerings during arid periods, as indicated by the macrobotanical record (Yseki et al., 2025), suggests exchange with highland Andean communities, as potato cultivation would have been unfeasible on the arid Peruvian coast 3,800 years ago. This underscores the role of interregional trade networks in sustaining agricultural production under fluctuating climatic conditions. Furthermore, future isotopic analyses of food remains could provide insights into whether organic fertilizers were used more intensively during dry periods to counteract water stress and maintain soil productivity. Understanding these adaptive strategies will contribute to broader discussions on human resilience and long-term responses to climatic variability in ancient coastal societies.

501
502 CRediT authorship contribution statement

503 **Marco Yseki**: Conceptualization, formal analysis, funding acquisition, investigation, methodology, validation, writing-original draft and writing-review and editing. **Claudia**

504 **R. Morales**: Formal analysis, investigation, methodology, writing-review and editing.

505 **Aldemar Crispin**: Investigation, methodology and writing-review and editing. **Tatiana**

507 **Abad**: Investigation, methodology and writing-review and editing. **Alonso Prado**:

508 Investigation, methodology and writing-review and editing. **Alexander Zuñiga**:

509 Investigation, methodology and writing-review and editing. **Marco Machacuay**:

510 Investigation, methodology, supervision, validation and writing-review and editing.

511 **Bruno Turcq**: Funding acquisition, investigation, methodology, supervision, validation,

512 writing-review and editing. **Matthieu Carré**: Funding acquisition, investigation,

513 methodology, supervision, validation, writing-review and editing. **Pedro Novoa**:

514 Conceptualization, funding acquisition, investigation, methodology, supervision,

515 validation, and writing-review and editing. **Ruth Shady**: Conceptualization, funding

516 acquisition, investigation, methodology, project administration, supervision, validation,

517 and writing-review and editing.

518

519 **Funding**

520 This research was supported by the Programa Nacional de Investigación Científica y

521 Estudios Avanzados - Consejo Nacional de Ciencia, Tecnología e Innovación

522 Tecnológica (PROCIENCIA-CONCYTEC, Grant No. PE501082072-2023).

523

524 **Declaration of competing interest**

525 The authors declare no conflict of interest.

526

527 **Acknowledgements**

528 We extend our special thanks to Rocío Tabra and Josselin Chavez for their assistance in

529 measuring guava seeds. The authors are grateful to Karin Ramirez for help in the

530 preparation of the map of the location of the Vichama site.
531

532 **Data availability**

533 Data is provided within the manuscript or supplementary information files

534

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685
686 **Figures**

687

688 Figure 1. A. Location of Vichama in the Huaura Valley, north-central coast of Peru. B.

689 Map of Vichama showing sectors A1, E1, and K, from which the analyzed sediment
690 samples were collected. C. Examples of mural relief images. D. Image of the mural relief
691 of the frog with human-like hands.

692

693 **Figure 2.** Relative abundances of major pollen groups from Vichama, classified by
694 environmental affinity and growth habit.

695

696 **Figure 3.** Violin plots showing the distribution of guava seed (A) length and (B) width
697 across architectural periods at Vichama (P1–P6). Boxplots within violins indicate the
698 interquartile range and median, while individual points represent measured seeds; red
squares denote group medians. Different letters above violins correspond to the Compact
Letter Display (CLD) derived from the results of Dunn's post-hoc test (with Holm
correction), using a significance threshold of *p* < 0.001 following a Kruskal-Wallis
analysis. Periods sharing the same letter do not differ significantly, whereas those with
different letters indicate statistically significant differences. Letters shown in panels A and
B are independent, as length and width were analyzed separately. The shaded area marks
the driest periods (P4 and P5) inferred from the pollen record, corresponding to reduced
water availability in the region.

707

708

709 **Figure 4.** A. Record of arsenic (As, cps) in the Cerro Llamacoca peatland (14°S, 4450 m a.s.l.). Peaks in As (cps), represented on an inverted scale, indicate dry events (Schittekk et

710
al., 2015). B. Abundance of hydrophytic wetland pollen recorded in archaeological sediments from the Vichama settlement (11°S, 50 m a.s.l., 1800-1500 BCE). P1-P6 represent the architectural periods of the Vichama site. Periods P4, and P5, marked by reduced wetland pollen abundance (indicative of dry conditions at the Vichama site), would correspond to dry events identified by peaks in As in the Andes.

716