Hader Sheisha
email: author.sheisha@cerege.fr

David Kaniewski

Nick Marriner

Morteza Djamali

Gamal Younes

Zhongyuan Chen

Gad El-Qady

Amr Saleem

Alain Véron

Christophe Morhange

DK, NM, MD, CM. Methodology: HS, DK, NM, MD, GY, GEQ, AS, CM. Investigation: HS, DK, NM, MD, GY, ZC, GEQ Hs

Nile waterscapes facilitated the construction of the Giza pyramids during the 3rd millennium BCE

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The pyramids of Giza originally overlooked a now defunct arm of the Nile. This fluvial channel, the Khufu branch, enabled navigation to the Pyramid Harbor complex but its precise environmental history is unclear. To fill this knowledge gap, we here use pollenderived vegetation patterns to reconstruct 8000 years of fluvial variations on the Giza floodplain. After a high-stand level concomitant with the African Humid Period, our results show that Giza's waterscapes responded to a gradual insolation-driven aridification of East Africa, with the lowest Nile levels recorded at the end of the Dynastic Period. The Khufu branch remained at a high-water level (~40% of its Holocene maximum) during the reigns of Khufu, Khafre and Menkaure, facilitating the transportation of construction materials to the Giza Pyramid Complex.

Significance

The pyramids of Giza constitute one of the world's most iconic cultural landscapes and have fascinated humanity for thousands of years. Indeed, the Great Pyramid of Giza (Khufu Pyramid) was one of the Seven Wonders of the Ancient World. It is now accepted that ancient Egyptian engineers exploited a former channel of the Nile to transport building materials and provisions to the Giza plateau. However, there is a paucity of environmental evidence regarding when, where and how these ancient landscapes evolved. New palaeoecological analyses have helped to reconstruct an8000-year fluvial history of the Nile in this area, showing that the former waterscapes and higher river levels around 4500 years ago facilitated the construction of the Giza Pyramid Complex.

The Nile River, with its dynamic floods and migratory behavior, played a key role in shaping ancient Egyptian history [START_REF] Williams | The Nile basin: Quaternary Geology, Geomorphology and Prehistoric Environments[END_REF]. The pyramids of Khufu, Khafre and Menkaure, located on the Giza Plateau, overlook the west bank of the Nile (Egypt). Built during the Fourth Dynasty (Old Kingdom 2686-2160 BCE) [START_REF] Shaw | The Oxford History of Ancient Egypt[END_REF], they are one of the world's most iconic cultural landscapes and constitute ancient engineering feats that have fascinated humanity for millennia. To edify the plateau's pyramids, tombs and temples, it now seems that ancient Egyptian engineers took advantage of the Nile and its annual floods [START_REF] Bell | The oldest records of the Nile floods[END_REF][START_REF] Hassan | Historical Nile floods and their implications for climatic change[END_REF], using an ingenious system of canals and basins that formed a port complex at the foot of the Giza plateau [START_REF] Lehner | On the waterfront: canals and harbors in the time of Giza pyramid-building[END_REF][START_REF] Lehner | Profane Landscapes, Sacred Spaces[END_REF]. The harbor complex currently lies >7 km west of the present-day Nile, but it is widely accepted that Fourth Dynasty engineers exploited a now defunct branch of the Nile [START_REF] Butzer | Urban geoarchaeology and environmental history at the Lost City of the Pyramids, Giza: synthesis and review[END_REF], which flowed along the western edge of the Nile floodplain during the Egyptian Old Kingdom and that we here refer to as the Khufu branch. These canals and basins had to be deep enough to accommodate shallow-draft vessels all year round. Larger cargo ships could probably only navigate during the flood season (August-October), when water levels in the Nile channel rose by ~7 m [START_REF] Cooper | The Medieval Nile[END_REF][START_REF] Tallet | Les Papyrus de la Mer Rouge I[END_REF].

The existence of a paleo-channel, canals and harbors is now supported by recent archaeological findings [START_REF] Lehner | On the waterfront: canals and harbors in the time of Giza pyramid-building[END_REF][START_REF] Lehner | Profane Landscapes, Sacred Spaces[END_REF], as well as research related to the 'Lost City of the Pyramids' (the Heit el-Ghurab site) [START_REF] Butzer | Urban geoarchaeology and environmental history at the Lost City of the Pyramids, Giza: synthesis and review[END_REF][START_REF] Lehner | Giza Reports, I. Project history, survey, ceramics, and main street and gallery iii.4 operations[END_REF][START_REF] Lehner | Cities and Urbanism in Ancient Egypt[END_REF]. Core drillings and subterranean engineering works for Giza's modern urban projects have yielded stratigraphic evidence consistent with a paleo-branch of the Nile. In particular, Old Kingdom structures unearthed during these interventions offer insights into the local cultural waterscape at the time of the pyramid builders. Furthermore, the Wadi el-Jarf Papyri, discovered at a Khufu-age port on the Red Sea coast [START_REF] Tallet | Les Papyrus de la Mer Rouge I[END_REF][START_REF] Tallet | Note d'information : Les Papyrus de la Mer Rouge (Ouadi el-Jarf, Golfe de Suez)[END_REF][START_REF] Tallet | Toponymie et Perception de l'Espace en Egypte de l'Antiquité au Moyen-Âge[END_REF], attest to the existence of such a harbor complex, called Ro-She Khufu ('Entrance to the Lake…' or '…Basin of Khufu'). The Journal of Merer, a large corpus of these papyri, describes the transport of limestone from Toura, ~17 km from the Giza Plateau, to the construction site of the Great Pyramid of Khufu. A striking parallel exists between the names of the basins and waterways in the papyri and the spatial organization of Giza's Fourth Dynasty waterscape, as reconstructed by archaeologists [START_REF] Lehner | On the waterfront: canals and harbors in the time of Giza pyramid-building[END_REF].

The fluvial-port-complex hypothesis postulates that pyramid builders cut through the western levee of the Khufu branch of the Nile and dredged basins down to river depth in order to harness the annual 7-m rise of the flood like a hydraulic lift, bringing the higher water levels to the base of the Giza plateau [START_REF] Lehner | On the waterfront: canals and harbors in the time of Giza pyramid-building[END_REF][START_REF] Lehner | Profane Landscapes, Sacred Spaces[END_REF]. In this way, it was possible to transport supplies and building materials directly to the pyramid complex. These canals and ports would also have been key for the longerdistance transport of materials via the Nile, facilitating transport logistics and for the continuous supply of the plateau's diverse building projects and linking Giza with nearby cities in the Memphite region and on the delta. Nonetheless, the interactions between ancient Egyptian societies and environmental changes along the Khufu branch are complex and poorly understood.

We here evaluate its water levels during the last 8000 years, with a particular focus on the Dynastic Period and the Old Kingdom, using sediment cores and long-term ecosystem dynamics. Our data shed new light on previous reconstructions because environmental and historical data are more closely coupled; such resolution has never been achieved in this region. Furthermore, our sequence can be extrapolated to upstream and downstream areas of the Egyptian Nile, allowing comparison with other archaeological sites.

To estimate variations in the water level of the Khufu branch, five cores were drilled on the present Giza floodplain, east of the pyramid complex (Fig. 1 and SI Appendix Fig. S1 and Fig. S2).

Bioindicators (pollen grains) were extracted from cores G1 and G4 (14) situated in what has been geographically defined as the Khufu branch of the Nile [START_REF] Lehner | On the waterfront: canals and harbors in the time of Giza pyramid-building[END_REF][START_REF] Lehner | Profane Landscapes, Sacred Spaces[END_REF]. We identified 61 taxa (SI Appendix Fig. S3) grouped into seven pollen-derived vegetation patterns (PdVs) based on their ecological affinities (Table S1). Among the defined PdVs, the Cyperaceae (papyrus, sedge; located on the banks of the Nile), tropical Nilotic taxa (pollen carried downstream by the Nile from tropical areas of the watershed), and helophytes reveal the presence of a permanent waterbody on the Giza floodplain (Table S1). The other PdVs (upland terrestrial herbs, date palm-willow, cereals and ferns) correspond to a more terrestrial influence. We used these PdV time series (the associated dating error is averaged to ± 80 years for the whole sequence) to perform a principal component analysis (PCA; SI Appendix Fig. S4), using PCA-Axis 1 as a proxy for Holocene Khufu-branch levels (Fig. 2). The negative values of PCA-Axis 1 are loaded by plants located on the Nile floodplain while the positive scores are weighted by taxa deriving from beyond the floodplain (SI Appendix Fig. S4). The highest water levels are attested by high abundances of Cyperaceae and helophytes, with a higher input of tropical pollen from the Nile river, while the low levels are framed by an increase in terrestrial taxa (SI Appendix Fig. S5). Nile flow variations, as deduced from the PCA-Axis 1, are correlated with changes in the lithofacies (SI Appendix Fig. S6).

Our score-based reconstruction shows that the water level of the Khufu branch (termed K-1) was higher during the African Humid Period [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF][START_REF] Tierney | Rainfall regimes of the Green Sahara[END_REF], with a local termination estimated at 3550±80 BCE (5500±80 BP) and a later peak recorded at 2950±80 BCE (Fig. 2) after a significant drop at 3450-3250±80 BCE. During this period, it has been shown that conglomerations of predynastic and early dynastic settlements (3550-3250 BCE) existed along the east bank of the Khufu branch and on the floodplain between the paleo-channel and the pyramid plateau [START_REF] Jones | A new Old Kingdom settlement near Ausim: Report of the archaeological discoveries made in the Barakat Drain Improvements Project[END_REF][START_REF] El-Sanussi | A site of the Maadi culture near the Giza pyramids[END_REF]. We suggest that the environmental attractivity of Giza during the fifth millennium resulted from a decrease in fluvial levels following the end of the African Humid Period [START_REF]End of the African Humid Period[END_REF]. This higher stand level at Giza, also supported by Nile Delta sedimentation rates (correlation R 2 = 0.75, SI Appendix Fig. S7) [START_REF] Marriner | Nile delta subsidence: quantifiable links with Holocene compaction and changes in sediment supply?[END_REF][START_REF] Marriner | ITCZ and ENSO-like modulation of Nile delta hydro-geomorphology during the Holocene[END_REF], is attributed to increased rainfall over Lake Tana (correlation R 2 = 0.86, SI Appendix Fig. S7), the source of the Blue Nile [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF] and Lake Victoria (correlation R 2 = 0.84, SI Appendix Fig. S7), the source of the White Nile [START_REF] Stager | A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa[END_REF]. Nile flow is mediated by monsoon activity in tropical Africa.

The Holocene intensity of the East African Monsoon, as reconstructed at Lake Abiyata [START_REF] Chalié | Late Glacial-Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)[END_REF] and Lake Rutundu [START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF] in Ethiopia (Fig. 3), is modulated by the north-south movement of the Intertropical convergence Zone (ITZC), with higher rainfall levels during the early Holocene consistent with a more northward migration of the monsoon rain belt driven by higher insolation [START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF]. We found that the Khufu branch has recorded the same trend (Fig. 2), with a high-stand level that correlates with greater summer insolation (correlation R 2 = 0.88, SI Appendix Fig. S7) and a northward migration of the ITCZ (correlation R 2 = 0.83, SI Appendix Fig. S7), which reinforced the intensity of the East African Monsoon (correlation R 2 = 0.68, SI Appendix Fig. S7) over the Nile Valley and created higher flow regimes in the Khufu branch (Fig. 2). The peak in the Khufubranch level at 2950±80 BCE (Fig. 2), while not documented in Lake Abiyata (Fig. 3), is recorded at Lake Rutundu, Lake Tana and Lake Victoria, and on the Nile Delta (Figs. 2 and3). Higher Khufu-branch levels at Giza are also in phase with higher levels in North African lakes (high and intermediate lakes; Fig. 3) [START_REF] Tierney | Model, proxy and isotopic perspectives on the East African Humid Period[END_REF]. The most significant negative fluctuations in Khufu-branch levels during this period (~8000-5500 years ago) are correlated with volcanic forcing (correlation R 2 = -0.30; SI Appendix Fig. S7) leading to diminished Nile summer flooding [START_REF] Manning | Volcanic suppression of Nile summer flooding triggers revolt and constrains interstate conflict in ancient Egypt[END_REF]. Moreover, a drought episode recorded in Lake Tana at 5550 BCE, due to a southward shift in the monsoon front, is also documented by a drop in the Khufu-branch level at Giza 5550±80 BCE (Fig. 2).

Following the African Humid Period, the Khufu branch manifests an important and permanent drop during the Early Dynastic Period (3000-2686 BCE) as reconstructed in the core K-1 (Fig. 4). This drop is correlated with a decline in human occupation in the Eastern Sahara [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: motor of Africa's evolution[END_REF] and a slight regression of sites in Upper and Lower Egypt [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: motor of Africa's evolution[END_REF]. The first dynasties of Egypt played out during a major fall in the Nile level, as attested by the Khufu branch from 2970±80 to 2690±80 BCE, consistent with a hydrological shift at Lake Tana (Fig. 2) and the onset of Nile Delta erosion due to decreasing sedimentation (20) (Fig. 3). This drop of the Khufu-branch level is part of long-term hydrological changes in Eastern Africa mainly triggered by orbitally-driven insolation pacing as well as the variability of humid/arid periods [START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Bastian | Co-variations of climate and silicate weathering in the Nile Basin during the Late Pleistocene[END_REF][START_REF] Garelick | Atmospheric controls on precipitation isotopes and hydroclimate in high-elevation regions in Eastern Africa since the Last Glacial Maximum[END_REF]. At Lake Abhe (Central Afar region), incursion of air moisture masses in the East African Rift System, caused by a high-pressure gradient between the Congo Air Boundary and the Indian Summer Monsoon air masses, resulted in heightened precipitation over Northeastern Africa [START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF]. These conditions supported large waterbodies in the northern East African Rift System until ~2650 BCE [START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF]. At Lake Tana, the persistence of a relatively high waterbody until at least 3000 BCE probably resulted from the same conditions, even if hydrological changes in the lake have occurred since 5850 BCE. This was caused by the southward migration of the ITCZ [START_REF]Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing[END_REF], inducing a reduction in rainfall over the Ethiopian Highlands, with the lowest level of the lake recorded at 2250 BCE [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]. Nile flow (Fig. 2) followed the same trend.

During the first half of the Old Kingdom of Egypt (2686-2440 BCE; Fig. 4), the level of the Khufu branch of the Nile remained relatively constant, characterized by a level at ~40% of that reached during the African Humid Period (Fig. 2). This relatively stable phase is also recorded in North African lake levels (Fig. 3). From the 3 rd to the 5 th dynasties, the Khufu branch clearly offered an environment conducive to the emergence and development of the pyramid construction site, helping builders to plan the transport of stone and materials by boat. As a consequence, the number of archaeological artefacts increased on the Giza plateau [START_REF] Lucarini | The MedAfriCarbon radiocarbon database and web application. Archaeological dynamics in Mediterranean Africa, ca. 9600-700 BC[END_REF], particularly during the Fourth Dynasty. Between the reign of King Unas (accession date 2438-2397 BCE) [START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF] and the end of the Sixth Dynasty, the level (and probably the baseflow) of the Khufu branch attained its earliest low levels (Fig. 4). It has been suggested that a failure of the annual Nile flood caused severe famine episodes [START_REF] Stanley | Nile flow failure at the end of the Old Kingdom, Egypt: strontium isotopic and petrologic evidence[END_REF], leading to the First Intermediate Period (2160-2055 BCE). Regarding the Khufu branch at Giza, the first low point occurred at 2225±80 BCE (Fig. 4), with a level that locally dropped by ~10% compared with the Great Pyramid stage. This low level resulted from a wider-scale climatic event [START_REF] Weiss | Megadrought and Collapse: From Early Agriculture to Angkor[END_REF][START_REF] Kaniewski | The 4.2 ka BP event in the Levant[END_REF] that also affected North African lake levels [START_REF] Tierney | Model, proxy and isotopic perspectives on the East African Humid Period[END_REF].

The Khufu branch of the Nile rose again during the Middle Kingdom (2055-1650 BCE), stabilizing throughout the Second Intermediate Period (1650-1550 BCE), before declining again (after 1350±80 BCE) during the New Kingdom (1550-1069 BCE; Fig. 2). After the reign of king Tutankhamun (accession date 1349-1338 BCE) [START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF], the level gradually declined and reached a low value at 1075-1025±80 BCE, close to the level recorded at 2225±80 BCE (Fig. 2). This shift, also widely recorded in Eastern Mediterranean paleoclimatology [START_REF] Kaniewski | Drought and societal collapse 3200 years ago in the Eastern Mediterranean: a review[END_REF], is associated in Egypt with a weaker East African Monsoon, leading to a fall in level at Lake Tana (Fig. 2) and to a strong decline of North African lake levels (Fig. 3). In 1069 BCE, Egypt transitioned into the Third Intermediate Period, which lasted for 400 years (from the 21 st to 24 th the dynasties) [START_REF] Shaw | The Oxford History of Ancient Egypt[END_REF]. While the level of the Khufu branch again increased during the 22 nd dynasty, from the reign of king Sheshonq I to the reign of king Osorkon IV, the end of the Third Intermediate Period and the Late Period recorded the most important drop in Nile levels at Giza. The level is the lowest documented during the last 8000 years (Fig. 2). This pronounced fall is correlated with an increase in δ 18 O values in teeth and bones from Egyptian mummies [START_REF] Touzeau | Egyptian mummies record increasing aridity in the Nile valley from 5500 to 1500 yr before present[END_REF], suggesting a more arid environmental period (Fig.

4). A less intense East African Monsoon, low sedimentation and increased sediment losses on the

Nile Delta all point to a dry phase that lasted ~600 years. According to Bunbury and Jeffreys (40), the Khufu branch was diminished to a small channel named 'Maaty' by the Middle Kingdom. The level of the Khufu branch was extremely low during the New Kingdom and when Alexander the Great conquered Egypt, at the start of the Ptolemaic Period.

The chronology from the Early Dynastic to the Late Period has been widely debated. It derives from different sources, including textual evidence, astronomical observations and 14 C dating [START_REF] Shaw | The Oxford History of Ancient Egypt[END_REF][START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF][START_REF] Kitchen | The chronology of Ancient Egypt[END_REF][START_REF] Beckerath | Chronologie des pharonischen Ägypten[END_REF][START_REF] Spence | Ancient Egyptian chronology and the astronomical orientation of pyramids[END_REF][START_REF] Gautschy | A new astronomically based chronological model for the Egyptian Old Kingdom[END_REF]. Focusing on the Giza pyramids, the accession date of Khufu (Fig. 4) ranges from 2613 to 2577 BCE (1σ) according to the 14 C-based chronology (34) while other sources place the onset of his reign at 2589 BCE (2), 2554 BCE [START_REF] Beckerath | Chronologie des pharonischen Ägypten[END_REF] or 2480±5 BCE based on astronomical observations [START_REF] Spence | Ancient Egyptian chronology and the astronomical orientation of pyramids[END_REF]. The same inaccuracy is noted for the reign of Khafre or Menkaure. The accession date of Khafre ranges from 2586 to 2548 BCE (1σ) [START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF] or is chronologically centered on 2558 BCE (2), 2522 BCE [START_REF] Beckerath | Chronologie des pharonischen Ägypten[END_REF], or 2448±5 BCE [START_REF] Spence | Ancient Egyptian chronology and the astronomical orientation of pyramids[END_REF]. The accession date of Menkaure ranges from 2564 to 2524 BCE (1σ) [START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF] with centers on 2532 BCE (2), 2489 BCE [START_REF] Beckerath | Chronologie des pharonischen Ägypten[END_REF], or 2415±10 BCE [START_REF] Spence | Ancient Egyptian chronology and the astronomical orientation of pyramids[END_REF]. The wide chronological ranges regarding the accession date of each king suggests that the use of the 14 C chronology [START_REF] Bronk-Ramsey | Radiocarbon-based chronology for Dynastic Egypt[END_REF] is preferable as less subject to interpretation. This is what was used in the present study.

Our 8000-year reconstruction of Khufu-branch levels improves understanding of fluvial landscapes at the time of the construction of the Giza Pyramid Complex, and demonstrates that Old Kingdom engineers harnessed the fluvial environment -the Nile and its annual floods -to exploit the plateau area overlooking the floodplain for monumental construction. We suggest that these paleoclimate data can also be used as a template to understand the waterscapes bordering the earlier Saqqara and Dahshur pyramid complexes, also on the West Bank of the Nile, and respectively 16 km and 22 km south of Giza.

Materials and Methods

Cores and chronology

A total of five cores was drilled on the Giza floodplain (Fig. 1) in May 2019 under the auspices of the Egyptian National Research Institute of Astronomy and Geophysics. The cores G1 (890 cm depth; 29°59'04"N, 31°09'07"E; 18 m a.s.l.) and G4 (700 cm depth; 29°58'40"N, 31°09'39"E; 19 m a.s.l.) were selected to reconstruct the Khufu-branch levels because these two cores are located inside the supposed Khufu basin. The lithology of cores G1 and G4 is detailed in SI Appendix Fig. S1. The chronology of the two cores is based on radiocarbon ( 14 C) dates of the total organic carbon fraction extracted from the sedimentary deposits. The cores were completely void of plant macrofossils. For dating purposes, we isolated only the most reliable organic-rich sediments, without any enrichment in old carbonates. Clam age-depth modelling [START_REF] Blaauw | Methods and code for 'classical' age-modelling of radiocarbon sequences[END_REF][START_REF] Blaauw | Testing the timing of radiocarbon-dated events between proxy archives[END_REF] was then used to build the age-models for G1 and G4 (SI Appendix Fig. S2). Although based on a few reliable samples, the age models (G1 and G4) were supported by pottery sherds found in the sedimentary deposits. The samples are dated to the Predynastic and Early Dynastic-Old Kingdom periods by comparison with earlier studies at Giza [START_REF] Sowada | The petrography of imported Levantine combed vessels from early Old Kingdom Giza[END_REF] and Saqqara (48). The age-models are also corroborated by comparison with high-resolution sequences such as Lake Tana ( 22) (Fig. 2) or Lake Abiyata (24) (Fig. 3). All dates are expressed as Before Common Era (BCE) and Before Present (BP).

Palaeoecological data

We prepared a total of 109 samples for pollen analysis using standard procedures for clay deposits [START_REF] Faegri | Textbook of pollen analysis[END_REF]. Tablets of Lycopodium spores (about 20,848 spores/tablet) were added to each sample in order to estimate the pollen concentration. Pollen grains were counted under x400 and x1000 magnification using an Olympus microscope. Atlases [START_REF] Bonnefille | Pollens des savanes d'Afrique orientale[END_REF][START_REF] Reille | Pollen et Spores d'Europe et d'Afrique du Nord[END_REF][START_REF] Reille | Pollen et Spores d'Europe et d'Afrique du Nord[END_REF][START_REF] Reille | Pollen et Spores d'Europe et d'Afrique du Nord[END_REF], books [START_REF] Faegri | Textbook of pollen analysis[END_REF][START_REF] Moore | Pollen Analysis[END_REF][START_REF] Punt | The Northwest European Pollen Flora[END_REF], as well as local reference slides were used for pollen identification. Pollen percentages are based on the terrestrial pollen sum, excluding the local helophytes (Alismataceae, Cyperaceae, Sparganium and Typha) and spores of non-vascular cryptogams (ferns). Percentages of helophyte taxa were calculated using a sum that includes local freshwater plants and terrestrial vegetation. Percentages of ferns were calculated using a sum that includes local freshwater plants, terrestrial vegetation and ferns. Palynological slides were counted using a minimum sum of 100 grains. It has been shown that a pollen count of 70-150 grains is statistically significant for a 0.90 reliability [START_REF] Djamali | Statistically significant minimum pollen count in Quaternary pollen analysis; the case of pollen-rich lake sediments[END_REF]. In arid and semiarid areas, pollen contributions from the vegetation cover are low, reducing the sample pollen content [START_REF] Djamali | Statistically significant minimum pollen count in Quaternary pollen analysis; the case of pollen-rich lake sediments[END_REF]. The pollen data are here presented as concentration diagrams (number of pollen grains per cm -3 of sediment; SI Appendix Fig. S3). All the pollen slides are housed at the IMBE laboratory (France). The taxa were then clustered into pollen-derived vegetation groups (PdVs) based on their ecological affinities. Seven PdVs were defined: upland terrestrial herbs, Cyperaceae, helophytes (excluding the Cyperaceae), tropical Nilotic taxa, date palm-willow, cereals and ferns (SI Appendix Table S1).

Composite sequence

The two cores were merged, based on the 14 C chronology, to create a single composite sequence, termed K-1 (for Khufu-1). The paleoecological dataset for this sequence is based on the average (G1 and G4) of each PdV for each sample (SI Appendix Fig. S6).

Reconstruction of Holocene variations in the level of the Nile's Khufu branch

All the PdV time series from K-1(expressed as percentages) were regularly interpolated to a 50-yr interval, consistent with the average chronological resolution of cores G1 and G4. The time series were then converted into z-scores and the results used as a data matrix for cluster analyses (with paired group as the algorithm and correlation as the distance measure) to individualize the PdV located inside and outside the Nile floodplain (SI Appendix Fig. S4). The link between these two groups was tested, ranking the Nile floodplain vegetation scores in ascending order and retaining the associated vegetation scores from beyond the Nile floodplain (SI Appendix Fig. S5). The two datasets are depicted with their confidence interval (95%). The two groups were also crosscorrelated to ascertain the best temporal match. The correlation coefficient (R 2 ) is then given, focusing on the Lag0 value (with +0.50 and -0.50 as significant thresholds). A principal component analysis (PCA) was subsequently performed to test the ordination of samples by assessing major changes in the PdV-scores. The first axis (PCA-Axis 1), which carries the maximum variance, has been extracted. The PdVs influenced by the Nile floodplain are loaded by the negative scores of PCA-Axis 1 while the PdVs located outside the Nile influence are loaded by the positive scores (SI Appendix Fig. S4). We then transformed the PCA-Axis1 scores using a Loess smoothing (with a LOWESS algorithm) and performed a bootstrap to estimate a 95% confidence band based on 1000 random replicates. The Loess curve and the 95% confidence band were used as a proxy for the Khufu-branch level (Fig. 2).

Long-term trends

The long-term trends were calculated using polynomial models (order 3, Pvalue < 0.001) and a moving average function (100-yr and 200-yr).

Other proxies

We compared Holocene variations in the level of the Khufu branch using various high-resolution datasets including: the Nile Delta [START_REF] Marriner | Nile delta subsidence: quantifiable links with Holocene compaction and changes in sediment supply?[END_REF][START_REF] Marriner | ITCZ and ENSO-like modulation of Nile delta hydro-geomorphology during the Holocene[END_REF], Lake Tana [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF], Lake Victoria [START_REF] Stager | A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa[END_REF], Lake Rutundu [START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF], the Cariaco basin [START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF], summer insolation [START_REF] Laskar | La2010: A new orbital solution for the longterm motion of the Earth[END_REF], volcanic forcing [START_REF] Manning | Volcanic suppression of Nile summer flooding triggers revolt and constrains interstate conflict in ancient Egypt[END_REF][START_REF] Kobashi | Volcanic influence on centennial to millennial Holocene Greenland temperature change[END_REF], Lake Abiyata [START_REF] Chalié | Late Glacial-Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)[END_REF], African lake levels [START_REF] Tierney | Model, proxy and isotopic perspectives on the East African Humid Period[END_REF], archaeological sites in Upper and Lower Egypt [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: motor of Africa's evolution[END_REF] and Egyptian mummies [START_REF] Touzeau | Egyptian mummies record increasing aridity in the Nile valley from 5500 to 1500 yr before present[END_REF]. To probe the link between these time series and variations in the Khufu branch, cross-correlations (P = 0.05) were calculated (SI Appendix Fig. S7). Positive and negative correlation coefficients are considered, focusing on the Lag0 value.

Data from archaeological sites in Sudan and Egypt

The number of sites in Sudan and Egypt [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: motor of Africa's evolution[END_REF] was converted into a moving 1000-yr sum and transformed into z-scores (Fig. 4). The Khufu branch was converted into a moving 1000-yr average for comparative purposes. The two datasets are depicted with their confidence interval (95%).

Software

All the statistical analyses were performed using R 4.

Fig. 1 .

 1 Fig. 1. Location of the cores on the Giza floodplain. The two cores used to reconstruct Holocene variations in Khufu-branch levels (cores G1 and G4) are located where the Khufu basin was connected to the Nile. The Giza Pyramid Complex currently lies >7 km from the present-day Nile branch. Harbor remains were previously found at the site of G3; the fluvial history derives from the work of Lehner (5, 6, 10, 11).

Fig. 2 .

 2 Fig. 2. Reconstructed Khufu-branch variations of the Nile during the last 8000 years. (A)Pollen-derived Khufu-branch levels (K-1, blue line) expressed as Loess smoothing scores (with 2.5 and 97.5 percentiles) compared with Nile Delta sedimentation rates (red line)[START_REF] Marriner | Nile delta subsidence: quantifiable links with Holocene compaction and changes in sediment supply?[END_REF]. Long-term trends are depicted by polynomial models (order 3, Pvalue < 0.001). (B) Variations in Lake Victoria (White Nile)[START_REF] Stager | A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa[END_REF] and Lake Tana (Blue Nile)[START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF] reconstructed as a diatom record (red line) and Ti (mg/g; orange line). Long-term trends are depicted by polynomial models (order 3, Pvalue < 0.001). The Khufu branch is represented by a shaded curve. (C) Summer insolation[START_REF] Laskar | La2010: A new orbital solution for the longterm motion of the Earth[END_REF], migration of the ITCZ[START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF] shown as Wm -2 (red line) and Fe (cps; purple line), and the Eastern African Monsoon shown as δDprecip (‰ VSMOW)[START_REF] Mologni | Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)[END_REF]. Long-term trends are denoted by polynomial models (order 3, Pvalue < 0.001). The Khufu branch is illustrated by a shaded curve. The background colors show the transition from the African Humid Period to the drying trend in Egypt.

Fig. 3 .

 3 Fig. 3. Khufu-branch levels during the Holocene compared with volcanic forcing, East African Monsoon and North African lakes. (A) Volcanic forcing (58) expressed as Wm -2 and shown as events (red line), 100-yr (shaded light purple) and 200-yr (shaded dark purple) moving averages. (B) Pollen-derived Khufu-branch levels (K-1, blue line) expressed as 2.5 and 97.5 percentiles, and as a 200-yr moving average. (C) East African Monsoon (green line) reconstructed from planktonic diatoms recorded at Lake Abiyata -Ethiopia (24). The long-term trend is depicted by a 200-yr moving average (shaded blue line). (D) North African lake levels (blue: intermediate and purple: high lakes) (27) shown as relative abundances. Long-term trends are illustrated by polynomial models (order 3, Pvalue < 0.001). (E) Nile Delta accretionary status (20) shown in mm per century (orange) with the standard deviation (yellow). The background colors show the transition from the African Humid Period to the drying trend in Egypt.

Fig. 4 .

 4 Fig. 4. Khufu-branch variations and North African archaeological sites. (A) Level of the Khufu branch (K-1, blue line) compared to the number of archaeological sites in Sudan and Egypt (light purple line) (29). Archaeological site numbers were transformed into z-scores. The two datasets are shown as 1000-yr averages (with a 95% confidence interval). Long-term trends are represented by polynomial models (order 3, Pvalue < 0.001). (B) Level of the Khufu branch (blue line; Loess smoothing scores with 2.5 and 97.5 percentiles) compared to archaeological sites in Lower and Upper Egypt (orange line) (29). The number of archaeological sites is shown as a 1000yr sum, transformed into z-scores. Long-term trends are illustrated by polynomial models (order 3, Pvalue < 0.001). (C) Khufu-branch levels (2.5 and 97.5 percentiles) during the construction of the pyramids of kings Khufu, Khafre and Menkaure (blue line). The long-term trend is represented by a polynomial model (order 3, Pvalue < 0.001). The background colors show the transition from the African Humid Period to the aridification of Egypt (A and B).

Fig. 5 .

 5 Fig. 5. Khufu-branch variations and Egyptian mummies geochemistry. (A) Pollen-derived Khufu-branch levels (K-1, blue line) expressed as Loess smoothing scores (with 2.5 and 97.5 percentiles). Long-term trends are depicted by a polynomial model (order 3, Pvalue < 0.001). (B) δ 18 O ‰ values (with standard deviation) from Egyptian mummies (teeth and bones; light red) (39). The data are shown as averages and with a polynomial model (order 3, Pvalue < 0.001) to highlight the long-term trend. The background colors show the transition from the African Humid Period to the aridification of Egypt.

Acknowledgments: Drilling permission was obtained with the help of the National Research

Institute of Astronomy and Geophysics (NRIAG, Helwan, Egypt).

Competing interests: Authors declare that they have no competing interests.

Data and materials availability:

All data are available in the main text or the supplementary materials and dataset.

SI Appendix

Figs. S1-7 Table S1 Tabulated data