# Enhanced continental weathering (δ7Li,εNd) during the rise of East African complex polities: an early large-scale anthropogenic forcing?

Carlo Mologni, Marie Revel, Luc Bastian, Germain Bayon, Delphine Bosch, Lamya Khalidi, Nathalie Vigier

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## ► To cite this version:

Carlo Mologni, Marie Revel, Luc Bastian, Germain Bayon, Delphine Bosch, et al.. Enhanced continental weathering (δ7Li,εNd) during the rise of East African complex polities: an early large-scale anthropogenic forcing?. Comptes Rendus. Géoscience, 2022, 354 (G2), pp.319-337. 10.5802/cr-geos.169. hal-03864320

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Distributed under a Creative Commons Attribution 4.0 International License
# Comptes Rendus
## Géoscience
### Sciences de la Planète

Carlo Mologni, Marie Revel, Luc Bastian, Germain Bayon, Delphine Bosch, Lamya Khalidi and Nathalie Vigier

**Enhanced continental weathering (δ<sup>7</sup>Li, εNd) during the rise of East African complex polities: an early large-scale anthropogenic forcing?**

Volume 354 (2022), p. 319-337

Published online: 3 November 2022

https://doi.org/10.5802/crgeos.169

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This article is licensed under the
CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL LICENSE.
http://creativecommons.org/licenses/by/4.0/

<figure><img src="image_4.png" /></figure>
~1 ka BP) periods. Therefore, our results provide direct support to the hypothesis of an early and large scale anthropogenic forcing on continental chemical weathering. A comparison with previously published records for Central Africa, Central Europe and China suggests that the impact of the intensification of early agriculture on the ECZ may have operated at a global scale starting around four thousand years ago.

**Keywords.** Chemical weathering, Li isotope, Nd isotope, Nile deep sea fan, Anthropogenic forcing, Aksumite civilization, Earth Critical Zone.

**Note.** Submission by invitation of the editorial board.

Manuscript received 31 May 2022, revised 30 September 2022, accepted 3 October 2022.

# 1. Introduction

The Early Anthropogenic Hypothesis (EAH) considers that human activities related to early agriculture, herding and deforestation may have played a crucial role in driving the onset of global warming between 7 and 5 ka BP, as inferred from the increase of atmospheric CO₂ and CH₄ emissions recorded in high-latitude ice cores [Ruddiman, 2003, Ruddiman et al., 2020]. However, the impact of human activities on landscapes, and how they affected the Earth Critical Zone [ECZ; Lin, 2009] since prehistoric times, remains poorly documented. Recently, a number of geochemical proxies of past continental chemical weathering have emerged. These have great potential to provide key information on the ECZ dynamics. Among them, Li isotopes have been successfully applied to fine-grained sediment records of river-borne particles exported to deltas, alluvial terraces and lakes [Bastian et al., 2017, Rothacker et al., 2018, Yang et al., 2021]. A covariation of Li isotopic compositions with various climate proxies during the Quaternary period has demonstrated the rapid response of soil/rock weathering to major precipitation changes, in particular during arid/humid precessional cycles in the tropics and during glacial/interglacial cycles at higher latitudes [Bastian et al., 2019, 2017, Yang et al., 2021, Rothacker et al., 2018, Dosseto et al., 2015]. In soils, the lightest lithium isotope (⁶Li) is preferentially incorporated into secondary clay mineral phases as a result of mass-dependent isotope fractionation occurring during mineral/water interactions [Hindshaw et al., 2018, Vigier et al., 2008]. The δ⁷Li values of clay minerals can reflect the degree of continental weathering expressed as the ratio between dissolution rate of primary minerals versus clay neoformation rates [Bastian & Mologni et al., 2021, Pogge von Strandmann et al., 2017, 2020, Vigier et al., 2009]. In large continental-scale basins, where chemical and physical erosion operate under steady-

state conditions, this ratio can be related to the intensity of chemical weathering, expressed as the ratio of silicate chemical weathering over total denudation [W/D ratio; Bouchez et al., 2013, Caves Rugenstein et al., 2019, Dellinger et al., 2017]. The provenance of fine-grained sediment in the Nile delta can be identified through its neodymium isotopic signature (εNd) [Bastian & Mologni et al., 2021, Blanchet, 2019, Blanchet et al., 2014, Revel et al., 2015, Weldeab et al., 2014]. As no significant fractionation is observed during chemical weathering, εNd reflects the isotopic composition of the parent bedrock and the origin of the material. Since the Nile basin encompasses the Precambrian African basement as well as the Ethiopian and Somalian Cenozoic basalts, which have contrasting isotopic compositions, the Nd isotope composition of sediment loads is suitable to distinguish their source regions [Bayon et al., 2015, Blanchet, 2019, Grousset et al., 1988]. Our approach combines the use of neodymium and lithium isotope proxy records, together with measured sedimentation rates, in order to provide constraints on past hydrological conditions, sediment transport, and chemical weathering intensity in the Nile River watershed.

Major hydro-climate variability related to changes in insolation and monsoon dynamics were observed over tropical Africa during the Quaternary period, during which time ~21 ka cycles of intense humid periods alternated with arid periods [Blanchet, 2019, Blanchet et al., 2021, Kutzbach et al., 2020, McGee and deMenocal, 2017, Ménot et al., 2020, Pausata et al., 2020]. This variability has been documented in several high-temporal resolution marine sediment records retrieved from the Nile Deep Sea Fan [NDSF; Bastian & Mologni et al., 2021, Bastian et al., 2017, Blanchet et al., 2015, Ménot et al., 2020, Mologni et al., 2020, Revel et al., 2015].

Between ~3 and ~1 ka BP, the Nile River Basin was home to Pre-Aksumite cultures followed by the
Aksumite Kingdom, known to be one of the most important ancient East African civilizations [Anfray, 1967, Fattovich, 2010, Gerlach, 2015, Japp, 2019, Phillipson, 2012]. Ancient Egyptian civilization was established further northward along the Nile mainstream and delta, and its emergence as early as 5000 cal. years BP has been associated with changes in the annual cycle of the Nile summer floods [e.g. Manning et al., 2017, Sheisha et al., 2022] suggesting a relationship between the end of the African Humid Period (AHP) and the Protodynamic period in Egypt.

The Aksumite kingdom was mostly located in the Ethiopian highlands, an area almost exclusively composed of Cenozoic basaltic rocks (Ethiopian Traps), Mesozoic sediments underlain by the Pan-African basement (Figure 1b). The Ethiopian Traps are characterized by a radiogenic neodymium (Nd) isotopic signature with an εNd ranging between ~0~7 [Garzanti et al., 2015], which differs largely from the εNd composition of the detrital sediment derived from other branches of the Nile catchment (Figure 1a). As a consequence, the Nd isotopic composition of the fine-grained sediment deposited at the NDSF can be used to identify the contribution of river-borne particles transported by the Blue Nile and the Atbara/Tekeze headwaters. These drain the Ethiopian Highlands which were occupied by the territories of the Pre-Aksumites and Aksumite Kingdom, as well as the White Nile branch (draining the Central African Craton), and those derived from aeanolian inputs from the nearby Saharan Metacraton [Mologni et al., 2020, Bastian et al., 2017, Blanchet et al., 2013, Revel et al., 2015]. Previous studies on the combined use of lithium and neodymium isotopes in clay-size fractions of sediment deposited in the NDSF indicated a close relationship between hydrological changes, continental chemical weathering and the delivery of material derived from the Ethiopian Traps over the past 100 to 1 ka [Bastian & Mologni et al., 2021]. In this study, we reconstruct the variability of sediment provenance and chemical weathering in the Nile Basin using a marine sediment record covering the last 9000 years. This record cover to the period associated with the rise of the first East African complex polities, namely the Pre-Aksumite cultures (~3 to ~2 ka BP) and the Aksumite Kingdom (~2 to ~1 ka BP) that proceeded it [Fattovich, 2010, Phillipson, 2012], which are contemporaneous with the Ancient Egyptian Dynastic, Ptolemaic and Roman

civilizations [Manning et al., 2017]. In addition to discussing original data on marine sediments, we use a wealth of information provided by a number of Holocene paleo-pedological [Bard et al., 2000, Gebru et al., 2009, Terwilliger et al., 2013, 2011], archeological and paleoclimatic [Costa et al., 2014, Jaeschke et al., 2020] investigations conducted in the Ethiopian Highlands to discuss potential impacts of intensified land-use by Aksumite and Late Egyptian complex polities on sediment transport and chemical weathering intensity.

## 2. Materials and methods

The Nile Delta deep sea cores we sampled display irregular shoreline protuberances directly under the influence of the Nile River's hydro-sedimentary activity [Mologni et al., 2020]. A substantial fraction of the fine-grained particulate load delivered from the Nile River is stored on the Egyptian continental slope in the form of mixed deposits of fine-grained turbidites and hemipelagic sediments. In this study, we investigate two well-dated sediment sequences: core MS27PT [N31° 47.90'; E29° 27.70'; 1389 m water depth; Bastian & Mologni et al., 2021] and core MD04-2726 [N31° 51.00'; E29°47.49'; 1058 m water depth; Mologni et al., 2020], which provide a continuous record of clastic particles exported from the Nile river basin over the last ~100 ka and 11 ka, respectively. The age models of these sediment records are based on 59 previously published radiocarbon dates [Bastian & Mologni et al., 2021, Mologni et al., 2020, Revel et al., 2015, 2010]. In this study, we present new results obtained from the upper 2 m of MD04-2726, focusing on the finest (<2 µm) clay-size detrital fraction. Because the clay-size fractions deposited near the mouth of rivers include secondary clay minerals initially formed in soil weathering profiles, they are thus well suited for the investigation of past silicate chemical weathering in river catchments. Additionally, due to their small size and density, clays are expected to be efficiently delivered from source to sink [Bastian et al., 2017].

### 2.1. Lithium isotope analyses

Chemical purification of Li was performed at the LOV (Laboratoire Océanographique de Villefranche-sur-Mer) in a clean laboratory. A solution containing
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**Figure 1.** (a) Map of the Nile River basin and location of cores MS27PT (N31° 47.90′; E29° 27.70′, 1389 m water depth) and MD04-2726 (31° 51.000′ N, 29° 47.490′ E, 1058 m water depth) in the NDSF. Three main sources of sediment are identified in the Nile basin: the basaltic rocks (red) of the Ethiopian traps (Highlands), which are drained by the Blue Nile, the Atbara/Tekeze and Sobat rivers located in tropical latitudes (around 5 to 15° N); the Precambrian metamorphic rocks (green) of the Central African Craton located in the Ugandan headwaters region of the White Nile and drained by the Bahr el Jebel River; and the Saharan Metacraton sources [Abdelsalam et al., 2002, Grousset and Biscaye, 2005, Scheuvens et al., 2013]. εNd values for the Victoria, White and Blue Nile River mud samples are from Garzanti et al. [2015]. (b) Map of the Blue Nile and Atbara River basins with the location of the main Aksumite cities [black triangles, after Fattovich, 2010, Phillipson, 2012], modern towns (grey squares), the limits of the Nile river catchment (black line), the urbanized Aksumite territories (red dotted line), the expansion of the Aksumite kingdom [brown dotted line; after Fattovich, 2010, Phillipson, 2012], Mai Maikden (MMIII, brown circle) and Adi Kolen (AKIII, green circle) soil sequence studies presented in Figure 3 [Gebru et al., 2009, Terwilliger et al., 2011, 2013].

~60 ng of lithium was introduced on a cationic resin column (AG50X12) and Li was eluted using titrated ultrapure 1.0 N HCl [Vigier et al., 2008]. This separation was performed twice to ensure perfect Li–Na separation. LiCl solution was then

evaporated to dryness and re-dissolved in 0.05 N HNO₃ for isotope analyses. Lithium isotope analyses (δ⁷Li = ((7Li/6Li)/(7Li/6Li)LSVEC – 1) × 1000, LSVEC being the international standard) were performed at the Ecole Normale Supérieure de Lyon (CNRS-INSU
National Facilities) using a Neptune Plus (Thermo-
Fisher) multi-collector inductively coupled plasma
spectrometer (MC-ICP-MS) along with a sample-
standard bracketing technique. A combination of Jet
and X cones were used, as well as an Aridus II desol-
vating system, resulting in a sensitivity of 1 V⁷Li/ppb
[Balter and Vigier, 2014, Thibon et al., 2021]. Before
analyses, Li fractions were diluted to match 4 ppb
Li. Total procedural blanks were negligible (<10 pg
Li), representing ~0.02% maximum of the total Li
fraction for each sample. The accuracy of isotopic
measurements was assessed several times during
each measurement session using reference Li⁷-N
solution [Carignan et al., 2007] and other reference
materials [BE-N and JB-2 rocks and seawater, see
Verney-Carron et al., 2015].

## 2.2. Neodymium isotope analyses

Neodymium isotopic ratios were measured at the Pôle Spectrométrie Océan (Brest, France). For convenience, Nd isotopic ratios results are expressed as: $εNd = [[^{143}Nd/^{144}Nd(meas.)/^{143}Nd/^{144}Nd(CHUR)] - 1] \times 10^4$. The CHUR (Chondritic Uniform Reservoir) value is 0.512638 [Jacobsen and Wasserburg, 1980]. Neodymium was purified using conventional ion chromatography [Bayon et al., 2012]. Nd isotopic compositions were determined using sample-standard bracketing by analysing JNdi-1 standard solutions every two samples. Mass bias corrections were made using the exponential law considering $^{146}Nd/^{144}Nd = 0.7219$. Mass-bias corrected values for $^{143}Nd/^{144}Nd$ were normalized to a JNdi-1 value of $^{143}Nd/^{144}Nd = 0.512115$ [Tanaka et al., 2000]. Repeated analyses of bracketed JNdi-1 standard solutions during the course of this study yielded $^{143}Nd/^{144}Nd$ of $0.512117 \pm 0.000012$ (2 SD, *n* = 38), corresponding to an external reproducibility of $\sim \pm 0.23ε$ (2 SD).

# 3. Results: Li and Nd isotopic compositions of clay fractions

Bastian & Mologni et al. [2021] show how the Nd
isotopic composition of clay-size fractions from core
MS27PT varies from εNd < -3 during humid periods
to -8 during arid periods, following the variability of
insolation between precession maxima and minima
periods between 100 and 3 cal. ka BP (Figure 2).

Our new results show that the εNd values of clays separated from core MD04-2726 vary between -2.3 to -6 at the end of the AHP (Table 1). This range of εNd values is similar to one observed in the core MS27PT over the last 100 ka insolation cycles. Surprisingly, after 2.3 ka, the Nd isotopic composition of clay-size fractions from both cores increases gradually but significantly, to reach a maximum radiogenic εNd value of -1 around 400 years ago.

The δ⁷Li composition of clays from core MS27PT varies between 4‰ and 1‰ during the last 100 ka (Table 2), following the alternation between insolation minima and maxima periods. As for Nd isotopes, after 3 ka, clay δ⁷Li reached values that are unprecedented in the last 100 ka, and it decreased gradually, to reach a value of -2.5‰ 400 years ago.

# 4. Discussion

## 4.1. Evidence for non-climatic forcing on chemical weathering since 3000 years cal. BP

A striking result of our study is that since 3000 cal.
years BP, the Li isotopic composition of clay-size
fractions has decreased to negative values, differ-
ing substantially from the observed δ⁷Li trend ob-
served over the last 100 cal. ka BP [Figure 2; Bas-
tian & Mologni et al., 2021]. This observation implies
(1) that a new forcing gradually appeared around
~3000 cal. BP, resulting in an increase in the inten-
sity of chemical weathering, and (2) that this forc-
ing dominated other controlling factors that were
at play before, namely those related to insolation-
driven hydro-climate variability and ocean dynamics
[see e.g. Bastian & Mologni et al., 2021].

Estuarine and deltaic processes may contribute
to an *in situ* weathering of primary minerals de-
posited in the NDSF [Zhang et al., 2022a,b]. How-
ever, it has been demonstrated by Bastian et al. [2017]
that variations in δ⁷Li recorded in the NDSF clay frac-
tion cannot be explained by diagenetic processes.
Early clay diagenesis and seawater isotopic fraction-
ation is normally characterised by higher δ⁷Li than
continental weathering products [Wei et al., 2020],
which can even be associated with Li absorption
from seawater [δ⁷Li = 31.2; Misra and Froelich, 2012].
More negative δ⁷Li recorded at the top of the core
allow us to exclude this effect for the NDSF In-
deed, small quantity of organics in sediments could
<figure><img src="image_6.png" /></figure>

**Figure 2.** A selection of paleoenvironmental proxy records for East Africa. (a) δDwax of Lake Dendi [Jaeschke et al., 2020]; (b) July insolation in watt/m² at 10° N [Berger and Loutre, 1991]; (c) εNd values of clay-size fractions of cores MS27PT (dark green points) and MD04-2726 (light green points) cores; (d) δ<sup>7</sup>Li values of clay-size fractions of cores MS27PT (red points) and MD04-2726 (orange points) cores. Black crosses and reversed triangles indicate <sup>14</sup>C calibrated ages of MS27PT and MD04-2726 cores respectively. S1, S3 and S4 refer to Sapropel 1, 3 and 4 respectively; AHP refers to “African Humid Period” and LGM to “Last Glacial Maximum” period.

favour the stabilization of deposited primary phases
[Krissansen-Totton and Catling, 2017, Michalopoulos
and Aller, 2004].

The provenance of clays transported by the Nile River can be inferred from Nd isotopes. The clay-size fractions deposited during the last 3000 years at sites MD04-2726 and MS27PT display a trend towards markedly increasing εNd values similar to the highly radiogenic values typical of the Ethiopian Traps that are generally encountered during past African Humid Periods. The same trend towards more radiogenic values (up to εNd ~ 0.4) is also recorded for the last 1880-year interval of core P362/2-33, a nearby marine sediment core retrieved from the NDSF Rosetta system [Blanchet et al., 2015, 2013]. Over the last 100 ka, the time intervals corresponding to the de-

position of clays with radiogenic (high) εNd sig-
natures were systematically associated with an in-
crease in sedimentation rates (up to ~100 cm/ka),
best explained by higher physical erosion rates in the
Ethiopian Traps area [Bastian & Mologni et al., 2021,
Revel et al., 2010]. In contrast, since 3000 years cal.
BP, the observed increase in εNd values has been
associated with relatively low sedimentation rates
[>2 cm/ka; Revel et al., 2015] and low terrigenous
fluxes [<4 g/cm²/ka; Revel et al., 2014], in agreement
with lower insolation and precipitation patterns (Fig-
ure 2). As previously shown in Bastian & Mologni
et al. [2021] and Mologni et al. [2020], this may result
from low fluvial transport of fine-grained sediments
still active during arid climates, since more that 80%
of the sediment deposited at the NDSF originates
<figure><img src="image_7.png" /></figure>

**Figure 3.** A diagram illustrating paleoenvironmental proxies in association with an archaeological timeline for East Africa during the Holocene. (a) δD<sub>wax</sub> of Lake Dendi [Jaeschke et al., 2020]; (b) sedimentation rate of core MD04-2726; (c) εNd (light green) and δ<sup>7</sup>Li (orange) in clay-size fractions of core MD04-2726; (d) charred organic matter (%) of Mai Maikden (MMIII, green line) and Adi Kolen (AKIII, brown line) soil profiles [Gebru et al., 2009, Terwilliger et al., 2011, 2013]; (e) δ<sup>13</sup>C of Mai Maikden (MMIII, green line) and Adi Kolen (AKIII, brown line) soil profiles [Gebru et al., 2009, Terwilliger et al., 2011, 2013]. Changes in: (f) Human culture [after Fattovich, 2010, Hensel et al., 2019, Khalidi et al., 2020, Ménard et al., 2014, Phillipson, 2012], (g) Subsistence economy [after Bard et al., 2000, Boardman, 1999, Khalidi et al., 2020, Lesur et al., 2014], (h) Vegetation [after Darbyshire et al., 2003, Terwilliger et al., 2013, Jaeschke et al., 2020] across the Ethiopian highlands.
<table><caption>**Table 1.** $\delta^7\text{Li}_{\text{clay}}$ and $\varepsilon\text{Nd}_{\text{clay}}$ data for clays from the MD04-2726 core</caption><thead><tr><th>ID Clay sample</th><th>Depth (cm)</th><th>Age (years cal. BP)</th><th>δ<sup>7</sup>Li (‰)</th><th>εNd</th><th>2σ</th></tr></thead><tbody><tr><td>C5</td><td>1</td><td>501</td><td>-2.64</td><td>-1.14</td><td>0.29</td></tr><tr><td>C7</td><td>5</td><td>751</td><td>-2</td><td>-1.53</td><td>0.25</td></tr><tr><td>C14</td><td>9</td><td>1000</td><td>-2.04</td><td>-3.03</td><td>0.25</td></tr><tr><td>C3</td><td>13</td><td>1248</td><td>0.93</td><td>-4.11</td><td>0.19</td></tr><tr><td>C6</td><td>17</td><td>1497</td><td>0.076</td><td>-5.53</td><td>0.26</td></tr><tr><td>C9</td><td>21</td><td>1801</td><td>0.78</td><td>-5.7</td><td>0.3</td></tr><tr><td>C13</td><td>25</td><td>2269</td><td>0.21</td><td>-6.48</td><td>0.22</td></tr><tr><td>C15</td><td>29</td><td>2737</td><td>1.39</td><td>-6.89</td><td>0.24</td></tr><tr><td>C11</td><td>33</td><td>3115</td><td>2.525</td><td>-6.25</td><td>0.26</td></tr><tr><td>C2</td><td>37</td><td>3290</td><td>1.74</td><td>-6.69</td><td>0.23</td></tr><tr><td>C4</td><td>41</td><td>3409</td><td>1.69</td><td>-6.33</td><td>0.25</td></tr><tr><td>C1</td><td>45</td><td>3810</td><td>2.67</td><td>-5.63</td><td>0.37</td></tr><tr><td>C18</td><td>49</td><td>4212</td><td>1.59</td><td>-4.93</td><td>0.23</td></tr><tr><td>C16</td><td>53</td><td>4613</td><td>1.79</td><td>-5.09</td><td>0.24</td></tr><tr><td>C8</td><td>57</td><td>4948</td><td>0.92</td><td>-4.8</td><td>0.21</td></tr><tr><td>C10</td><td>61</td><td>5215</td><td>2.67</td><td>-4.61</td><td>0.26</td></tr><tr><td>C17</td><td>65</td><td>5482</td><td>0.74</td><td>-4.35</td><td>0.23</td></tr><tr><td>C12</td><td>69</td><td>5750</td><td>2.3</td><td>-4.48</td><td>0.26</td></tr><tr><td>C20</td><td>128</td><td>7564</td><td>2.03</td><td>-4.09</td><td>0.19</td></tr><tr><td>C19</td><td>186</td><td>8543</td><td>2.29</td><td>-3.06</td><td>0.26</td></tr></tbody></table>

in the Ethiopian Highlands [Garzanti et al., 2015]. This can be explained by Blue Nile seasonal discharge, supported by summer monsoon precipitations even during arid periods [Harrower et al., 2020, Sulas, 2014].

Overall, the radiogenic $\varepsilon$Nd signature of clay-size fractions in cores MD04-2726 and MS27PT indicate a Blue Nile/Ethiopian Traps provenance over the last ~3000 years. A reworking of Ethiopian clay can be excluded because it would result in a shift toward lower, rather than higher $\varepsilon$Nd values. Previously deposited sediments with high radiogenic $\varepsilon$Nd values could have been reworked by the intensive irrigation management conducted along the Nile mainstream during the Ptolemaic or Roman periods, concomitant with the development of the Aksumite civilization [Flaux et al., 2012, Pennington et al., 2017, Stanley and Bernhardt, 2010]. However, in that case, one would have expected a shift towards lower $\varepsilon$Nd values. Indeed, basaltic Nd values ($\varepsilon$Nd = -2 to ~0) of Ethiopian Traps origin would have been diluted by

the input of sediments with the lowest $\varepsilon$Nd values originating from the White Nile River ($\varepsilon$Nd = -13.3) and from aeolian sources, which are not preserved [$\varepsilon$Nd = -15; from the Saharan Metacraton; Abdelsalam et al., 2002, Garzanti et al., 2015].

Altogether, the low $\delta^7\text{Li}$ and high $\varepsilon$Nd signatures of the Late Holocene support a strong non-climatic forcing of chemical weathering conditions in the Ethiopian Traps. These results are at odds with what would normally be expected based on the arid hydroclimatic conditions in East Africa during this period and based on chemical weathering trends observed over the last 100 ka. In consequence, we suggest that anthropogenic control was a probable driving force of chemical weathering in the Ethiopian Highlands.

### 4.1.1. Concomitant increase of land use and chemical weathering since 3000 BP in the Ethiopian highlands

East Africa is widely known as one of the cradles of humankind because the volcano-tectonic formation
<table><caption>**Table 2.** $\delta^7\text{Li}_{\text{clay}}$ and $\varepsilon\text{Nd}_{\text{clay}}$ data measured in the clay fractions from the MS27PT core</caption><thead><tr><th>ID</th><th>Clay sample</th><th>Depth (cm)</th><th>Age (years cal. BP)</th><th>δ<sup>7</sup>Li (‰)</th><th>2σ</th><th>εNd</th><th>2σ</th></tr></thead><tbody><tr><td>A28</td><td></td><td>2</td><td>1258</td><td>-1.15</td><td>0.01</td><td>-5.69</td><td>0.1</td></tr><tr><td>A24</td><td></td><td>7</td><td>2640</td><td>0.47</td><td>0.06</td><td>-6.97</td><td>0.14</td></tr><tr><td>A33</td><td></td><td>11</td><td>3449</td><td>1.45</td><td>0.01</td><td>-7</td><td>0.2</td></tr><tr><td>A30</td><td></td><td>17</td><td>4923</td><td>1.72</td><td>0.01</td><td>-5.01</td><td>0.12</td></tr><tr><td>A2</td><td></td><td>22</td><td>5925</td><td>2.02</td><td>0.06</td><td>-4.51</td><td>0.16</td></tr><tr><td>A22</td><td></td><td>35</td><td>7430</td><td>1.47</td><td>0.02</td><td>-3.64</td><td>0.13</td></tr><tr><td>A46</td><td></td><td>39</td><td>7582</td><td>1.85</td><td>0.02</td><td>-2.45</td><td>0.14</td></tr><tr><td>A34</td><td></td><td>52</td><td>7946</td><td>1.82</td><td>0.02</td><td>-3.62</td><td>0.17</td></tr><tr><td>A29</td><td></td><td>54</td><td>8007</td><td>1.8</td><td>0.01</td><td>-3.81</td><td>0.12</td></tr><tr><td>L21</td><td></td><td>70</td><td>8403</td><td>2.11</td><td>0.06</td><td></td><td></td></tr><tr><td>A48</td><td></td><td>82</td><td>8551</td><td>2.14</td><td>0.01</td><td>-2.1</td><td>0.13</td></tr><tr><td>L22</td><td></td><td>95</td><td>8712</td><td>2.47</td><td>0.04</td><td></td><td></td></tr><tr><td>A3</td><td></td><td>102</td><td>8891</td><td>2.83</td><td>0.02</td><td>-4.2</td><td>0.19</td></tr><tr><td>A39</td><td></td><td>111</td><td>8980</td><td>1.95</td><td></td><td></td><td></td></tr><tr><td>A35</td><td></td><td>114</td><td>8999</td><td>2.49</td><td>0.01</td><td>-2.27</td><td>0.12</td></tr><tr><td>L23</td><td></td><td>120</td><td>9068</td><td>1.88</td><td>0.08</td><td></td><td></td></tr><tr><td>A57</td><td></td><td>137</td><td>9203</td><td>1.79</td><td>0.01</td><td>-2.98</td><td>0.17</td></tr><tr><td>A5</td><td></td><td>152</td><td>9338</td><td>2.41</td><td>0.02</td><td>-3.01</td><td>0.23</td></tr><tr><td>A47</td><td></td><td>172</td><td>9498</td><td>1.72</td><td>0.01</td><td>-1.73</td><td>0.17</td></tr><tr><td>A58</td><td></td><td>194</td><td>9683</td><td>2.04</td><td>0.01</td><td>-2.34</td><td>0.27</td></tr><tr><td>L24</td><td></td><td>206</td><td>9820</td><td>2.43</td><td>0.06</td><td></td><td></td></tr><tr><td>L26</td><td></td><td>243</td><td>11,165</td><td>1.81</td><td>0.05</td><td></td><td></td></tr><tr><td>L27</td><td></td><td>255</td><td>11,601</td><td>1.89</td><td>0.06</td><td></td><td></td></tr><tr><td>A6</td><td></td><td>263</td><td>11,892</td><td>2.27</td><td>0.02</td><td>-0.98</td><td>0.17</td></tr><tr><td>A31</td><td></td><td>269</td><td>12,074</td><td>2.15</td><td>0.01</td><td>-2.82</td><td>0.2</td></tr><tr><td>A44</td><td></td><td>274</td><td>12,256</td><td>1.73</td><td>0.01</td><td>-2.65</td><td>0.23</td></tr><tr><td>A27</td><td></td><td>278</td><td>12,522</td><td>1.66</td><td>0.02</td><td>-1.99</td><td>0.12</td></tr><tr><td>A25</td><td></td><td>281</td><td>12,695</td><td>1.53</td><td>0.02</td><td>-2.76</td><td>0.11</td></tr><tr><td>A7</td><td></td><td>283</td><td>12,810</td><td>1.79</td><td>0.02</td><td>-2.11</td><td>0.15</td></tr><tr><td>A54</td><td></td><td>289</td><td>13,309</td><td>1.99</td><td>0.02</td><td>-1.97</td><td>0.2</td></tr><tr><td>A56</td><td></td><td>296</td><td>14,588</td><td>1.62</td><td>0.02</td><td>-3.65</td><td>0.14</td></tr><tr><td></td><td></td><td>296</td><td>14,934</td><td>1.98</td><td>0.04</td><td>-4.24</td><td>0.12</td></tr><tr><td>A8</td><td></td><td>298</td><td>15,280</td><td>2.58</td><td>0.02</td><td>-5.44</td><td>0.17</td></tr><tr><td>A45</td><td></td><td>300</td><td>16,635</td><td>2.03</td><td>0.02</td><td>-5.25</td><td>0.16</td></tr><tr><td>A9</td><td></td><td>300</td><td>17,989</td><td>3.08</td><td>0.02</td><td>-6.88</td><td>0.14</td></tr><tr><td>A55</td><td></td><td>301</td><td>18,855</td><td>2.59</td><td>0.01</td><td>-5.63</td><td>0.22</td></tr><tr><td>A32</td><td></td><td>303</td><td>20,588</td><td>2.61</td><td>0.02</td><td>-5.97</td><td>0.15</td></tr><tr><td>B6</td><td></td><td>304</td><td>21,024</td><td>2.22</td><td>0.03</td><td>-6.63</td><td>0.31</td></tr><tr><td>B4</td><td></td><td>305</td><td>21,461</td><td>2.52</td><td>0.04</td><td>-6.01</td><td>0.14</td></tr></tbody></table>

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<table><caption>**Table 2.** (continued)</caption><thead><tr><th>ID Clay sample</th><th>Depth (cm)</th><th>Age (years cal. BP)</th><th>δ<sup>7</sup>Li (‰)</th><th>2σ</th><th>εNd</th><th>2σ</th></tr></thead><tbody><tr><td>A10</td><td>306</td><td>21,897</td><td>3.63</td><td>0.01</td><td>-5.82</td><td>0.16</td></tr><tr><td>B5</td><td>307</td><td>22,334</td><td>2.88</td><td>0.04</td><td>-5.99</td><td>0.14</td></tr><tr><td>B8</td><td>308</td><td>22,770</td><td>4</td><td>0.03</td><td>-6.4</td><td>0.23</td></tr><tr><td>A37</td><td>310</td><td>23,207</td><td>2.65</td><td>0.01</td><td>-5.66</td><td>0.18</td></tr><tr><td>B9</td><td>310</td><td>23,643</td><td>2.51</td><td>0.03</td><td>-5.23</td><td>0.12</td></tr><tr><td>B11</td><td>311</td><td>24,079</td><td>2.87</td><td>0.03</td><td>-5.25</td><td>0.12</td></tr><tr><td>B10</td><td>312</td><td>24,516</td><td>3.16</td><td>0.04</td><td>-4.95</td><td>0.08</td></tr><tr><td>B12</td><td>313</td><td>24,952</td><td>3.12</td><td>0.04</td><td>-5.7</td><td>0.14</td></tr><tr><td>B14</td><td>314</td><td>25,389</td><td>1.77</td><td>0.08</td><td>-6.18</td><td>0.16</td></tr><tr><td>A36</td><td>316</td><td>25,825</td><td>0.93</td><td>0.01</td><td>-4.83</td><td>0.18</td></tr><tr><td>A60</td><td>317</td><td>26,411</td><td>1.92</td><td>0.01</td><td>-5.05</td><td>0.16</td></tr><tr><td>B3</td><td>321</td><td>27,630</td><td>1.65</td><td>0.04</td><td>-3.9</td><td>0.16</td></tr><tr><td>B7</td><td>323</td><td>28,312</td><td>1.86</td><td>0.04</td><td>-4.74</td><td>0.27</td></tr><tr><td>A61</td><td>325</td><td>28,994</td><td>1.74</td><td>0.01</td><td>-4.08</td><td>0.33</td></tr><tr><td>B2</td><td>327</td><td>29,677</td><td>2.09</td><td>0.03</td><td>-4.68</td><td>0.45</td></tr><tr><td>B1</td><td>329</td><td>30,359</td><td>2.02</td><td>0.02</td><td>-3.26</td><td>0.12</td></tr><tr><td>A59</td><td>331</td><td>31,041</td><td>1.3</td><td>0.01</td><td>-4.29</td><td>0.18</td></tr><tr><td>D3</td><td>335</td><td>31,881</td><td>1.53</td><td>0.04</td><td>-3.64</td><td>0.25</td></tr><tr><td>D10</td><td>343</td><td>32,107</td><td>2.12</td><td>0.04</td><td>-3.75</td><td>0.22</td></tr><tr><td>D12</td><td>357</td><td>34,078</td><td>3.09</td><td>0.02</td><td>-4.67</td><td>0.25</td></tr><tr><td>E9</td><td>370</td><td>37,689</td><td>2.48</td><td>0.04</td><td>-3.55</td><td>0.22</td></tr><tr><td>E3</td><td>374</td><td>38,841</td><td>2.83</td><td>0.03</td><td>-4.09</td><td>0.21</td></tr><tr><td>E19</td><td>375</td><td>39,024</td><td>2.56</td><td>0.04</td><td>-4.56</td><td>0.23</td></tr><tr><td>E15</td><td>376</td><td>39,101</td><td>2.82</td><td>0.03</td><td>-4.15</td><td>0.25</td></tr><tr><td>E4</td><td>377</td><td>39,183</td><td>3.31</td><td>0.02</td><td>-4.65</td><td>0.23</td></tr><tr><td>D8</td><td>385</td><td>41,627</td><td>2.34</td><td>0.03</td><td>-4.01</td><td>0.25</td></tr><tr><td>E12</td><td>395</td><td>46,755</td><td>1.52</td><td>0.03</td><td>-3.6</td><td>0.32</td></tr><tr><td>E14</td><td>397</td><td>47,811</td><td>1.72</td><td>0.02</td><td>-4.98</td><td>0.22</td></tr><tr><td>E2</td><td>398</td><td>48,339</td><td>2</td><td>0.03</td><td>-4.66</td><td>0.22</td></tr><tr><td>F6</td><td>399</td><td>48,630</td><td>1.98</td><td></td><td></td><td></td></tr><tr><td>E8</td><td>400</td><td>49,081</td><td>1.81</td><td>0.01</td><td>-3.62</td><td>0.22</td></tr><tr><td>F4</td><td>403</td><td>50,015</td><td>2.27</td><td></td><td></td><td></td></tr><tr><td>F3</td><td>408</td><td>51,572</td><td>2.44</td><td></td><td></td><td></td></tr><tr><td>F2</td><td>414</td><td>53,440</td><td>1.95</td><td></td><td></td><td></td></tr><tr><td>E18</td><td>419</td><td>54,997</td><td>4.27</td><td>0.06</td><td>-2.66</td><td>0.24</td></tr><tr><td>F5</td><td>421</td><td>55,619</td><td>3.34</td><td></td><td></td><td></td></tr><tr><td>F1</td><td>429</td><td>58,110</td><td>1.98</td><td></td><td></td><td></td></tr><tr><td>D7</td><td>440</td><td>61,535</td><td>0.03</td><td>2.7</td><td>-3.29</td><td>0.24</td></tr><tr><td>D9</td><td>443</td><td>62,469</td><td>2.87</td><td>0.02</td><td>-3.03</td><td>0.25</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 2.** (continued)</caption><thead><tr><th>ID Clay sample</th><th>Depth (cm)</th><th>Age (years cal. BP)</th><th>δ<sup>7</sup>Li (‰)</th><th>2σ</th><th>εNd</th><th>2σ</th></tr></thead><tbody><tr><td>D2</td><td>447</td><td>63,715</td><td>2.52</td><td>0.04</td><td>-5.18</td><td>0.26</td></tr><tr><td>E1</td><td>448</td><td>64,026</td><td>3.09</td><td>0.02</td><td>-5.15</td><td>0.24</td></tr><tr><td>E11</td><td>451</td><td>64,960</td><td>2.08</td><td>0.02</td><td>-5.09</td><td>0.23</td></tr><tr><td>D4</td><td>455</td><td>66,206</td><td>2.21</td><td>0.03</td><td>-6.66</td><td>0.23</td></tr><tr><td>D11</td><td>459</td><td>67,275</td><td>1.92</td><td>0.04</td><td>-7.55</td><td>0.25</td></tr><tr><td>D1</td><td>469</td><td>72,290</td><td>2.17</td><td>0.03</td><td>-4.99</td><td>0.25</td></tr><tr><td>D5</td><td>476</td><td>74,500</td><td>2.9</td><td>0.03</td><td>-4.4</td><td>0.27</td></tr><tr><td>E7</td><td>495</td><td>78,500</td><td>3.01</td><td>0.02</td><td>-4.17</td><td>0.24</td></tr><tr><td>E5</td><td>506</td><td>79,600</td><td>3.38</td><td>0.03</td><td>-4.41</td><td>0.25</td></tr><tr><td>D6</td><td>520</td><td>81,000</td><td>3.45</td><td>0.03</td><td>-4.09</td><td>0.24</td></tr><tr><td>D20</td><td>540</td><td>83,000</td><td>3.67</td><td>0.05</td><td>-3.9</td><td>0.23</td></tr><tr><td>D15</td><td>553</td><td>84,300</td><td>3.52</td><td>0.03</td><td>-4.1</td><td>0.25</td></tr><tr><td>E13</td><td>567</td><td>85,700</td><td>3.41</td><td>0.02</td><td>-3.71</td><td>0.23</td></tr><tr><td>E17</td><td>578</td><td>87,320</td><td>2.96</td><td>0.02</td><td>-3.58</td><td>0.23</td></tr><tr><td>D19</td><td>590</td><td>89,374</td><td>2.71</td><td>0.03</td><td>-3.59</td><td>0.24</td></tr><tr><td>D16</td><td>597</td><td>90,455</td><td>3.09</td><td>0.03</td><td>-5.66</td><td>0.23</td></tr><tr><td>D13</td><td>602</td><td>91,201</td><td>2.67</td><td>0.04</td><td>-3.85</td><td>0.26</td></tr><tr><td>D18</td><td>609</td><td>92,080</td><td>3.99</td><td>0.03</td><td>-7.69</td><td>0.23</td></tr><tr><td>D22</td><td>617</td><td>93,085</td><td>2.14</td><td>0.04</td><td>-6.03</td><td>0.26</td></tr><tr><td>E16</td><td>627</td><td>94,341</td><td>2.95</td><td>0.02</td><td>-4.44</td><td>0.26</td></tr><tr><td>D17</td><td>641</td><td>96,100</td><td>3.03</td><td>0.03</td><td>-4.65</td><td>0.26</td></tr><tr><td>E22</td><td>645</td><td>96,603</td><td>3.07</td><td>0.02</td><td>-4.27</td><td>0.2</td></tr><tr><td>E6</td><td>666</td><td>99,304</td><td>3.28</td><td>0.02</td><td>-3.71</td><td>0.22</td></tr><tr><td>D14</td><td>679</td><td>100,874</td><td>2.99</td><td>0.01</td><td>-3.8</td><td>0.24</td></tr><tr><td>E10</td><td>694</td><td>102,758</td><td>3.45</td><td>0.02</td><td>-3.47</td><td>0.25</td></tr><tr><td>D23</td><td>705</td><td>104,140</td><td>2.99</td><td>0.02</td><td>-3.97</td><td>0.2</td></tr><tr><td>D25</td><td>712</td><td>105,020</td><td>2.76</td><td>0.02</td><td>-4.42</td><td>0.25</td></tr><tr><td>D24</td><td>719</td><td>105,899</td><td>3.15</td><td>0.03</td><td>-3.74</td><td>0.23</td></tr></tbody></table>

of the East African Rift System created the ideal living conditions for early hominins and for the preservation of their remains [WoldeGabriel et al., 2001]. Comprehensive archaeological studies have demonstrated that during the long period of transition from solely hunter-gathering, to herding around 12 to 3 ka cal. BP, populations were relatively well adapted to the effects of hydrological changes in the region as a result of flexible mobility and subsistence strategies [Chritz et al., 2019; Khalidi et al., 2020, Kuper and Kröpelin, 2006, Mologni et al., 2021, Wright et al., 2015]. The Late Holocene witnessed a gradual aridification during which the first East African complex

polities in Highland Ethiopia and Eritrea developed and integrated agriculture into their socio-economic fabric [Boardman, 1999]. The Ethiopian plateau, located in the headwaters region of the Blue Nile, saw the emergence of the first Pre-Aksumite polities around 3000 BP, followed by the foundation of the first Aksumite urban settlements around ~2000 years cal. BP [Fattovich, 2010, Phillipson, 2012]. Extending over the source region of the Blue Nile and Atbara/Tezeke rivers, the pre-Axumite to Axumite period was accompanied by the intensification of agricultural practices [Blond et al., 2018; Figure 1b; Boardman, 1999]. Even if the use of irrigation systems
was not necessary because of abundant seasonal monsoonal rainfall [Harrower et al., 2020], anthropic small-scale management of primary landscapes became widespread across the highland region [Blond et al., 2018, Sulas, 2014]. From ~2600 BP onwards, increasing demographic pressure over the Ethiopian highlands was accompanied by intensified land use and deforestation [Darbyshire et al., 2003], soil degradation [Bard et al., 2000] and water management [Blond et al., 2018, Federica and Innocent, 2018, Sulas, 2014, Sulas et al., 2009]. Organic proxy investigations of soil profiles from the headwaters region of the Atbara/Tekeze rivers (Adi Kolan=AKIII, Mai Maikden=MMIII; Figure 1b) revealed a strong increase in land clearing and anthropogenic fire activity between ~3000 and ~2000 years cal. BP [Gebru et al., 2009, Terwilliger et al., 2013, 2011; Figure 3c, d]. Other proxies such as hydrogen (HI) and oxygen (OI) indices from Rock-Eval pyrolysis also indicated intense chemical alteration of organic matter due to agricultural humidification processes and/or human-induced fires [Terwilliger et al., 2013, 2011]. Such extensive anthropogenic landscape management had a major impact on the pedogenic system, leading to an increase of soil leaching, and potentially explaining the decrease of δ⁷Li values during this period.

In this context, the δ⁷Li proxy evidence for enhanced chemical weathering in the Ethiopian Traps starting ~3000 BP is concomitant with anthropogenic landscape degradation recorded during the Pre-Aksumite and Aksumite periods (Figure 3). During the same period, large erosion rates in the Ethiopian highlands were recorded in Lake Dendi sediment records, in agreement with intensive land-use activity [Jaeschke et al., 2020].

Overall, the concomitance between the intensification of land use beginning around 3000 BP and enhanced soil alteration (inferred from negative δ⁷Li values) suggests that agricultural management systems played a key role in this dramatic shift. The impact of human activities on Li isotopes appears clearly distinct from the effect induced by an increase in precipitation during high insolation periods. It should be specified that the area of the Pre-Aksumite and Aksumite polities to which our results refer are those drained by tributaries of the Nile River, which cover only one part of the overall territory of these polities (Figure 1b). Consequently, our re-

sults reflect the impact of anthropic activities in regions that were part of the territory of the Aksumite kingdom and its precursors, and not necessarily that of the main centres [Fattovich, 2010, Phillipson, 2012].

It is interesting to note that paleolimnological and historical data obtained for the Late Holocene record of Lake Fayum along the northern Nile indicate an increase of land-use since ~4 ka BP, hence 1000 years earlier than the observed εNd shift in our NDSF cores [Butzer, 1997, Hamdan et al., 2020, Hasan et al., 2006]. Similarly, various indices of land-use intensification in the Nile delta have been recorded since ~4 ka BP [Zhang et al., 2022a,b]. The fact that there was a recorded increase of human activities in the Nile delta and no change in εNd signature around 4 Ka BP indicates that the reworking of fine-grained sediments with radiogenic Nd isotopic signatures and erosion processes were negligible during this period. In contrast, the co-evolution of chemical weathering (inferred from δ⁷Li) and intensive land-use patterns associated with pre-Aksumite and Aksumite complex polities (inferred from εNd and sedimentological evidence) over the last 3000 years suggest that the increase of chemical weathering intensity was mostly generated in the Blue Nile basin, and therefore in the basaltic highlands. Further high resolution studies of local pedo-sedimentary archives will be needed in order to more precisely quantify the effects of land use on weathering in this region, and which may be more significant than what has been illustrated by studies of deltaic sediments.

## 4.2. Towards a global scale impact of anthropogenic activities on Earth systems since the late Holocene?

As discussed above, the combined Nd and Li isotope records are consistent with an increase of chemical weathering intensity in the Ethiopian highlands (Figure 4a). This is during a period characterized by deforestation, the development of agriculture, water management, and the exploitation of soil resources attributed to pre-Aksumite and Aksumite polities between 3000 and 1000 years cal. BP. This finding is consistent with the hypothesis of an early anthropogenic forcing in the Nile River Basin, adding further evidence to the large scale impact of pre-historic human activities on continental weathering.
<figure><img src="image_8.png" /></figure>

**Figure 4.** Diagram illustrating the intensity of continental chemical weathering worldwide during the Late Holocene. (a) clay $\delta^7$Li of MD04-2726 core (this study); (b) $\epsilon$Hf variations in KZAl-01 core [Congo basin; Bayon et al., 2012]; (c) Bulk sediment Al/K ratio in KZR23 core [Congo basin; Bayon et al., 2019]; (d) silt $\delta^7$Li of Co1260 [Djoran Lake; Rothacker et al., 2018]; (e) Clay $\delta^7$Li evolution determined for the CM97 core [Yangtze basin; Yang et al., 2021].

Several compelling comparisons can be made with other large continental regions. In highland Yemen, a direct correlation has been made between combined settlement increase, development of ter-

raced agriculture, environmental degradation and soil erosion during the Yemen Bronze Age [between 5000 and 4000 BP; Wilkinson, 2005, 1997]. Similarly to Ethiopia, Yemen highland erosion is attributed to
the combination of climatic drying and major soci-
etal changes, which left a heavy human footprint on
the landscape [Bard et al., 2000, Wilkinson, 1997]. In
the Congo Basin, Bayon et al. [2012, 2019] reported
an abrupt event of intense chemical weathering dur-
ing the third millennium BP. During this time in-
terval, the highest aluminum/potassium (Al/K) and
hafnium isotopic (εHf; Figure 4b) ratios of the last 40
thousand years were encountered in deltaic records
recovered near the mouth of the Congo River. In Cen-
tral Africa, this weathering event coincided with a
collapse of the rainforest and accelerated soil ero-
sion, but also with the southward migration of Bantu-
speaking people from a region located between Nige-
ria and Cameroon [Bayon et al., 2019, 2012, Garcin
et al., 2018, Saulieu et al., 2021, Seidensticker et al.,
2021]. While debate exists about the cause of this ma-
jor environmental crisis, several authors have sug-
gested that both the vegetation shift and the chem-
ical weathering event were partly caused by an in-
tensification of iron smelting and land use related to
the Bantu expansion [Bayon et al., 2012, Garcin et al.,
2018, Bayon et al., 2019]. In south-eastern Europe,
Rothacker et al. [2018] reported an anomalous (i.e.
unrelated to the climate trend) increase of δ²³U and
δ⁷Li values in a sedimentary record from Lake Do-
jran (Greece) over the last 3300 years cal. BP. These
isotopic variations have been interpreted as a marker
of both intensive soil erosion and chemical weather-
ing (Figure 4c). Combined with additional biomarker
data and inferences from archaeological studies, this
feature coincided with the first signs of agriculture
in the Dojran basin, as well as the development of
cultivated and ruderal plants. Eventually, in China,
Yang et al. [2021] suggested that small but significant
Li isotope fluctuations recorded in a sediment core
from the Yangtze delta since ~2 ka, were associated
with increase in sedimentation rates, and might be
related to the rapid development of agricultural prac-
tices and other human activities in the lowland re-
gion of the Yangzte River basin (Figure 4d).

Although the number of case studies using chem-
ical weathering proxies remains limited to date, most
of them highlight an early anthropogenic forcing on
continental chemical weathering over the last few
thousand years. Additional high temporal resolution
studies are now needed in order to determine the late
Holocene origin and evolution of chemical weather-
ing at a global scale, and to precisely quantify the

environmental impact caused by the rise of early
polities worldwide.

**5. Conclusions**

Over recent years, arguments in favour of human-
induced climate alteration during prehistoric times
have provided support for the Early Anthropogenic
Hypothesis. However, the impact of human activi-
ties on chemical weathering and on Earth surface
processes remains poorly documented. In this arti-
cle, we have explored the relationships between the
evolution of soil weathering and the rise of early
complex polities and urban societies in East Africa,
namely during the Pre-Aksumite and Aksumite pe-
riods. Using Nd (εNd) and Li (δ⁷Li) isotopes in the
clay-size fraction of marine sediments retrieved from
the Nile Deep Sea Fan, we reconstructed the evo-
lution of continental weathering in the Nile Basin
over the last 9 ka. Our results show a trend towards
higher εNd and very low δ⁷Li values over the last
3000 years cal. BP, indicating both enhanced weath-
ering processes and sediment transport from the area
of the Ethiopian highlands drained by the Blue Nile.
The negative δ⁷Li values, which are unprecedented
over the last 100 ka, suggest that this event cannot
be explained by natural hydroclimatic forcing. In-
stead, using archaeological, paleo-pedological and
paleo-botanical data, we observed that this weather-
ing event was concomitant with the emergence and
rise of the Pre-Aksumite and Aksumite polities, which
developed after 3000 ka BP in the headwaters region
of the Blue Nile. The association between the onset of
intensive agricultural and hydrological management
by Pre-Axumite and Aksumite populations, and en-
hanced chemical weathering in the Blue Nile catch-
ment points towards an early anthropogenic degra-
dation of soils in East Africa. Taken together with
other case-studies and proxy records worldwide, we
posit that human activities have acted as a major
driver of continental chemical weathering since 4.3-
3 ka BP, driving the hypothesis of an Earlier Anthro-
pogenic disruption of the Earth Critical Zone over a
larger scale.

**Conflicts of interest**

Authors have no conflict of interest to declare.
## Acknowledgements

This work was financially supported by BQR Geoazur- OCA, an INSU-SYSTER grant and the ANR INTOCC (ANR-15-CE31-0013). The MS27PT and MD04-2726 cores were recovered during the MIMES and VANIL cruises respectively. We greatly thank Philipe Télouk for his help with the MC-ICP-MS at the CNRS-INSU national service at Lyon.

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