# Interacting drivers of Holocene climate change in southwestern Africa: the influence of insolation, rainbelt dynamics and upwelling

Brian M Chase, Andrew S Carr, Arnoud Boom, Manuel Chevalier, Maarten Blaauw, Michael E Meadows

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► To cite this version:

Brian M Chase, Andrew S Carr, Arnoud Boom, Manuel Chevalier, Maarten Blaauw, et al. Interacting drivers of Holocene climate change in southwestern Africa: the influence of insolation, rainbelt dynamics and upwelling. Quaternary Science Advances, 2026, 21, pp.100312. 10.1016/j.qsa.2025.100312. hal-05443198

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# Interacting drivers of Holocene climate change in southwestern Africa: the influence of insolation, rainbelt dynamics and upwelling

Brian M. Chase<sup>a,b,*</sup>, Andrew S. Carr<sup>c</sup>, Arnoud Boom<sup>c</sup>, Manuel Chevalier<sup>d</sup>, Maarten Blaauw<sup>e</sup>, Michael E. Meadows<sup>b,f</sup>

## ARTICLE INFO

### Keywords:

Holocene

Namibia

Palaeoclimate

Climate dynamics

Rock hyrax middens

Stable isotopes

## ABSTRACT

Understanding regional-scale patterns of long-term climate variability is essential for identifying the drivers of past environmental change. In southern Africa, the continent is often divided into three rainfall zones—summer, winter, and aseasonal—but this framework fails to capture the finer dynamics of transitional areas where tropical, subtropical, and temperate systems converge. This study examines Holocene climate variability along the western margin of the southern African monsoon region using a new 7300-year nitrogen isotope record from rock hyrax middens at Omanyne-4 in northern Namibia. Unlike other Namib Desert records that indicate progressive aridification through the Holocene, the Omanyne-4 sequence shows a long-term trend toward increasing humidity, consistent with insolation-driven enhancement of tropical and Indian Ocean moisture advection. Comparisons with records from northern Namibia, Botswana, and western Zambia reveal a coherent pattern of mid-to late Holocene humidification across the northwestern interior, in contrast to coastal aridification. Periods of both in-phase and antiphase variability with other regional records highlight the role of the Angola-Benguela Front and associated upwelling dynamics in modulating Namibian hydroclimate. These results delineate distinct Holocene climate response regions in northern Namibia and Botswana and demonstrate the non-linear nature of regional responses to insolation forcing and underscore the importance of coastal–inland atmospheric interactions in shaping long-term hydroclimate variability in southwestern Africa.

## 1. Introduction

Identifying coherent palaeoclimate response regions—areas that react similarly to changes in global and regional climate systems—is essential for understanding the drivers of climate change and the relevance of individual proxy records. In studies of long-term climate change in southern Africa, the subcontinent is often broadly divided into three rainfall zones: summer, winter, and aseasonal (Chase and Meadows, 2007; Cockcroft et al., 1987; van Zinderen Bakker, 1976). This classification corresponds to the region's main moisture-bearing systems, with tropical convective systems bringing summer rainfall to much of southern Africa, while the southwestern Cape receives mainly

winter rainfall from the mid-latitude westerly storm track (Tyson, 1986; Tyson and Preston-Whyte, 2000). Although this rainfall seasonality-based framework may be useful at a general level (Chase et al., 2017), it falls short in capturing or predicting climate dynamics at finer spatial scales—especially where tropical, subtropical, and temperate mid-latitude systems interact (Chase et al., 2019a; Chase and Quick, 2018).

In this paper, we examine the Holocene boundary between tropical and subtropical climate systems along the southwestern margin of the southern African monsoon region (as defined by Chase et al., 2022; Wang and Ding, 2008) in Namibia. With the exception of its southwestern coastal zone, Namibia lies within the summer rainfall region

<footnote><sup>a</sup> Institut des Sciences de l'Evolution-Montpellier (ISEM), Université de Montpellier, Centre National de la Recherche Scientifique (CNRS), IRD, Montpellier, France</footnote>

<footnote><sup>b</sup> Department of Environmental and Geographical Science, University of Cape Town, South Lane, Upper Campus, 7701, Rondebosch, South Africa</footnote>

<footnote><sup>c</sup> Institute for Environmental Futures, School of Geography, Geology and the Environment, University of Leicester, Leicester, LE1 7RH, UK</footnote>

<footnote><sup>d</sup> DataSharp Academy, Gert-Marcus-str. 13a, 22529, Hamburg, Germany</footnote>

<footnote><sup>e</sup> School of Natural and Built Environment, Geography, Archaeology and Palaeoecology, Queen's University Belfast, Belfast, BT7 1NN, Northern Ireland, UK</footnote>

<footnote><sup>f</sup> School of Geographic and Ocean Sciences, Nanjing University, Nanjing, China</footnote>

<footnote>* Corresponding author. Institut des Sciences de l'Evolution-Montpellier (ISEM), Université de Montpellier, Centre National de la Recherche Scientifique (CNRS), IRD, Montpellier, France.</footnote>

<footnote>E-mail address: Brian.Chase@umontpellier.fr (B.M. Chase).</footnote>
(sensu Chase and Meadows, 2007). However, proxy records from across this region reveal contrasting interpretations of past environmental change. Marine core leaf wax δD and pollen data — interpreted as reflecting variability in the Namib Desert, the Namibian Plateau and the western Kalahari — suggest generally dry conditions throughout the Holocene, particularly during the early Holocene, with some evidence of increased humidity during the mid-Holocene, followed by renewed aridification over the past 2000 years (Collins et al., 2014; Shi et al., 2000). In direct contrast, terrestrial records from the adjacent Namib Desert indicate a wetter early Holocene, a gradual trend toward aridification through the mid- and late Holocene, and a return to more humid conditions in the last millennium (Chase et al., 2009, 2010, 2019b). These terrestrial patterns align with changes in coastal upwelling intensity, suggesting that the same mechanisms shaping present-day aridity were active in the past.

Further inland, to the east of the Great Escarpment, records in northeastern Namibia and northern Botswana point to a long-term increase in humidity over the course of the Holocene (Cordova et al., 2017; Sletten et al., 2013), consistent with increasing austral summer insolation and distinct from the patterns observed in the marine records or the Namib Desert. Climate model simulations fail to capture these spatial nuances, instead showing the southeast trade winds extending inland to around 22°E, causing a general drying trend across Namibia during the mid-to late Holocene (He and Clark, 2022; Liu et al., 2009; Singarayer and Burrough, 2015).

To resolve these apparent contradictions and better define the boundaries between regional climate response zones, we present a new 7300-year record of aridity variability obtained from rock hyrax midden from northern Namibia—located at the interface between the Southeast Atlantic Anticyclone and the African tropical rainbelt. This record allows us to: 1) delineate distinct Holocene climate response regions in northern Namibia and Botswana, 2) describe the observed variability in terms of shifts in the position and strength of dominant atmospheric circulation systems, and 3) identify the broader global climate drivers behind these regional changes.

## 2. Northern Namibian climate regimes

Although northern Namibia falls within the summer rainfall zone, it sits at the intersection of three major climate systems. To the southwest lies the Benguela upwelling region and the Namib Desert, where cold coastal waters—brought to the surface by southeast trade winds—enhance subtropical high-pressure systems, suppress convection, and produce extremely arid conditions (Nicholson, 2011; Tankard and Rogers, 1978). To the east, the Kalahari Desert receives rainfall driven by tropical easterly flow from the Indian Ocean and the formation of tropical-temperate troughs. These troughs form when mid-latitude weather systems interact with tropical convection, generating cloud bands across the interior (Harrison, 1984; Hart et al., 2013). To the north, rainfall is influenced by low-level westerly winds that transport moisture from the tropical Atlantic across Angola and into the southern Africa interior (Howard and Washington, 2019).

In palaeoclimate research, precipitation in the summer rainfall zone is often associated with the position of the Intertropical Convergence Zone (ITCZ) and, in the west, the Congo Air Boundary (CAB). These features are often considered as zones of atmospheric instability and enhanced convection (e.g. Burrough and Thomas, 2013; Railsback et al., 2016). However, as more detailed datasets have become available, our understanding of these systems has evolved. Notably, Nicholson (2018) challenges the conventional view of the ITCZ as a low-level convergence zone that brings high rainfall and migrates seasonally between hemispheres. She highlights that the zone of maximum low-level convergence is often spatially offset from the zone of peak precipitation. Drawing on Hartman's (2021) work, Nicholson emphasizes the importance of mesoscale convective systems, which readily develop in near-saturated tropical air masses under seasonally warmer conditions. Based on this

more nuanced understanding, Nicholson proposes the term *tropical rainbelt* to describe the seasonally shifting zone of high precipitation (Nicholson, 2009a, 2009b) — a term we adopt in this paper.

Similarly, updated interpretations of the CAB suggest it is not a zone of maximum convection and precipitation. Instead, it represents the sharp southern boundary of the tropical rainbelt—at times less than 100 km wide ((Howard and Washington, 2019). The CAB marks the interface between moist air masses transported inland from the tropical Atlantic by low-level westerlies, and drier easterly air masses that have traversed the continent from the Indian Ocean (Howard and Washington, 2019; Torrance, 1979). This boundary is most distinct during austral winter and autumn. By November and December, however, rising temperatures increase the humidity of the easterlies, reducing the humidity gradient between the two air masses to the point where the CAB becomes indistinct (Howard and Washington, 2019) (Fig. 1).

The climate conditions within, and the boundaries between, northern Namibia's climate domains are determined by the dynamic interactions between the strength and position of the South Atlantic Anticyclone, the low-level westerlies, and the westward extent (and humidity of) of the tropical easterlies (Howard and Washington, 2019; Leroux, 2001; Nicholson, 2018). In eastern Namibia, precipitation mainly originates from the distant Indian Ocean, but as these airmasses cross the continent, much of their moisture is lost over eastern southern Africa (Tyson and Preston-Whyte, 2000). A particularly notable feature of the region is the northeast-southwest precipitation gradient along the western margin of the interior plateau and the mountains of the Great Escarpment—referred to as the *Kalahari Discontinuity* by Howard and Washington (2019). This gradient is influenced by land-sea temperature contrasts (Rife et al., 2010), which are significantly influenced by changes in the strength of the South Atlantic Anticyclone, which drives coastal upwelling. During periods of reduced upwelling and warmer sea-surface temperatures, the humidity gradient weakens, allowing more moisture to penetrate westward into the Namib Desert from the interior. Conversely, periods of increased upwelling strengthen the coastal high-pressure cell and block the incursion of moist air from the interior.

In terms of long-term climate change, numerous terrestrial records from the southwest continental margin confirm that during the Holocene rainfall in the Namib Desert is closely tied to variability in coastal upwelling intensity and the strength of the South Atlantic Anticyclone (Chase et al., 2009, 2019b, 2023b), but the extent of this influence remains to be clearly defined. Located at in NW Namibia, the Omanyne site studied here is strategically located at the boundary of the (aridity enhancing) South Atlantic Anticyclone and the tropical rainbelt, proving an opportunity to consider the relative roles of these systems in driving regional patterns of Holocene hydrological change, across the region and thus enabling a synoptic scale understanding of the connections to changes in global and regional climate systems.

## 3. Site description, material and methods

### 3.1. Site description

This study describes results from a series of adjacent rock hyrax middens collected from the Kaokoland region of northwestern Namibia (18.843°S, 13.163°E; 450 m.a.s.l.), located 25 km east of Purros and 75 km from Mowebaai on the Atlantic coast. This region is characterised by a "hot desert" climate (BWh Koppen-Geiger classification; Peel et al., 2007), with mean monthly temperatures ranging from 16.9 °C in July to 22.7 °C in February. Mean annual rainfall at the site is only ~100 mm/yr, and is highly seasonal, falling mostly during the austral summer months of January–March (Hijmans et al., 2005). As noted above, this moisture is presently transported via long-distance easterly flow from the SW Indian Ocean and through recurved low-level westerly flow off the tropical Atlantic associated with the Angolan Low (Crétat et al., 2019; Tyson and Preston-Whyte, 2000). Of these airmasses, the
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Map of the northern Namibian study region showing sea-surface temperatures, regional aridity index, summer atmospheric circulation (white arrows) and marine and terrestrial sites referenced in the text (SIN: Lake Sinda (Vincens et al., 1998); KIT: Kitina (Vincens et al., 1999); SON: Lake Songolo (Vincens et al., 1999); GeoB6518-1 (Schefuß et al., 2005); ODP1078C (Dupont and Behling, 2006); MUJ: Mujimbeji dambo (Burrough and Willis, 2015); MUM: Mumbwana dambo (Burrough and Willis, 2015); MUL: Mulele dambo (Burrough and Willis, 2015); OKA: Gauxa Lagoon GL2, Ncamasere Valley NL2-4 (Nash et al., 2006); MAB: Mababe Depression (Burrough et al., 2009a); DBC: Drotsky's & Bone Caves (Brook et al., 1996); NGA: Lake Ngami (Cordova et al., 2017); DAC: Dantes Cave (Sletten et al., 2013); ETO: Etosha Pan (Rust, 1984); OKO: Okomutati (Gil-Romera et al., 2007); OKA: Okanjombo (Gil-Romera et al., 2007); GeoB1023-5 (Kim and Schneider, 2003); OMA: Omanyne (this study); VRE: Vrede (Gil-Romera et al., 2006); AUS: Austerlitz (Chase et al., 2010); ABA: Aba-Huab (Chase et al., 2010); SPZ: Spitzkoppe (Chase et al., 2009); WIN: Windhoek (Scott et al., 1991); STM: Stampriet Aquifer (Stute and Talma, 1998); BLA: Blasskränz (Scott et al., 2022); ZIZ: Zizou (Chase et al., 2023; Chase et al., 2019b); ODP1084B (Farmer et al., 2005)). The Omanyne site is marked with a star, and colour coding for the sites relates to the early to late Holocene anomaly in humidity for terrestrial records (green = more humid late Holocene, orange = more arid late Holocene) and changes in sea-surface temperatures for marine records (red = warmer late Holocene, blue = cooler late Holocene). Grey denotes an ambiguous signal.</figcaption></figure>

proximal westerlies are substantially more humid compared to the easterlies, as the latter dry crossing the continent and Great Escarpment (Howard and Washington, 2019; Tyson and Preston-Whyte, 2000).

The site – referred to as Omanyne-4 – is located on a northwest-facing limestone cliff on the southern flank of a broad east-west trending valley immediately above an ephemeral stream that drains the 21 km² montane basin to the south. The vegetation is characterised by Namibian Savanna Woodlands, which are part of the Southwest African Coastal Drylands bioregion (Dinerstein et al., 2017). Coarse alluvial sediments limit vegetation development on the plains adjacent to the site, which are characterised by sparse, ephemeral grasses (*Stipagrostis* spp.). Along drainages, localised groundwater supports some shrubs and

small trees (particularly *Colophospermum* and *Vachellia*).

## 3.2. Midden collection

The Omanyne-4 middens were collected following the methods developed and described by Chase et al. (2020, 2009, 2019b, 2012). Comprising an extensive midden complex sheltered in a vertical fissure in the cliff (Fig. 2), the individual sections (Table 1) were selected for their thickness and high hyraceum (crystallised urine) content. Proxies obtained from hyraceum are considered to represent environmental conditions more clearly than samples containing high proportions of faecal pellets, which may 1) include a degree of dietary bias, 2) lack the
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** The Omanyne hyrax midden site: (A) viewed from the north and (B) the view from the site looking northward along the adjacent valley.</figcaption></figure>

stratigraphic integrity of hyraceum-rich middens, and 3) are poor temporal integrators, as they reflect the discrete periods of food consumption (Carr et al., 2016; Chase et al., 2012). The midden sections were cut perpendicular to the stratigraphy using an angle grinder and transported back to the laboratory, where each section was cleaned using progressively finer grades of sandpaper prior to sampling.

### 3.3. Chronology

Radiocarbon age determinations for the Omanyne middens (n = 24) were processed at the ¹⁴CHRONO Centre, Queen's University Belfast using accelerator mass spectrometry (AMS) (Table 1). Samples were pre-treated with 2 % HCl for 1 h at room temperature to remove carbonates and dried at 60 °C. They were then weighed into quartz tubes with an excess of CuO, sealed under vacuum and combusted to CO₂. The CO₂ was converted to graphite on an iron catalyst using the zinc reduction method (Slota et al., 1987). The radiocarbon ages were corrected for isotope fractionation using the AMS measured δ¹³C.

The ages were calibrated using the SHCal20 calibration data (Hogg et al., 2020). Post-bomb ages were calibrated using CALIBomb (Reimer and Reimer, 2023) and the SHZ1_2 post-bomb calibration dataset (Hua et al., 2021). The rbacon v.3.0.0 R package (Blaauw and Christen, 2011; Blaauw et al., 2020) was used to generate the age-depth models for the OMA2014-4-back, OMA2014-4-right top and OMA2014-2b-lower middens sections, while the clam 2.6.2 R package (Blaauw, 2010) was used for midden sections OMA2014-4-right lower and OMA2014-2b top, which experienced more significant changes in accumulation rates, rendering them less suitable for Bayesian methods that assume more consistent accumulation (Fig. 3).

### 3.4. Stable nitrogen isotopes

The stable nitrogen isotope composition of 586 hyraceum samples (approx. 2 mg) was measured at the School of Geography, Geology and the Environment, University of Leicester. Samples were obtained from two offset tracks using a 1 mm drill, creating a quasi-continuous record of overlapping samples. Isotope ratios were measured on a Sercon 20-20 continuous flow isotope ratio mass spectrometer. For the stable isotope analyses, the standard deviation derived from replicate analyses of homogeneous reference material (Casein protein CatNo.B2155 Batch no. 114859 Elemental Microanalysis Ltd UK) was better than 0.2 ‰. The results are expressed relative to atmospheric nitrogen (Fig. 4).

## 4. Statistical methods

To explore our results and facilitate meaningful comparisons with

other records, we have used Monte Carlo Single Spectrum Analysis (MC-SSA) (Allen and Smith, 1996) and semblance analysis (Cooper and Cowan, 2008). MC-SSA is used to decompose time series into orthogonal components (trend, oscillations, noise) using SSA and then test the significance of the components vs a red-noise (AR(1)) null hypothesis model using Monte-Carlo surrogates. A bootstrap method (moving-block bootstrap, to preserve temporal correlation) is applied to estimate time-wise uncertainty bands for the reconstructed components. In this study, we use the first eigencomponent and the first two oscillatory pairs (a pair being two eigencomponents that together represent a quasi-periodic oscillation in the data) - (components 2,3 and 4,5) - to create reconstructions that distil the most significant trends from the records analysed.

Semblance analysis is a means of quantifying the local phase relationship between two signals across time and scale, providing a measure of whether they vary in-phase, anti-phase, or incoherently. The method used here (Cooper and Cowan, 2008) uses Morlet Continuous Wavelet Transforms (CWTs) to decompose each signal into time-scale space, allowing the local phase differences to be extracted and converted into semblance values that quantify their coherence.

Prior to these analyses, all records were linearly interpolated to a 10-yr resolution. The analyses were carried out using R (R Core Team, 2025), and the packages Rssa (Golyandina et al., 2018) and biwavelet (Gouhier et al., 2024).

## 5. Results and interpretive basis

### 5.1. Chronology

Radiocarbon analyses indicate that the Omanyne-4 hyrax middens accumulated during the last 7340 years (Table 1; Fig. 3). The age-depth models for the middens suggest continuous deposition, but with some marked changes in accumulation rates in OMA2014-4-right lower (1.98 years/mm to 35.32 years/mm) and OMA2014-2b top (3.08 years/mm to 48.34 years/mm). Considered together, these data indicate a decline in accumulation rates between 3600 and 4300 cal BP, but not a cessation of midden development. Taken as a whole, the composite record has an average resolution of 12.6 years between mean sample ages.

### 5.2. Stable nitrogen and their interpretation

The stable isotope samples from the Omanyne midden sections were combined and sorted according to age as estimated by the age model for each section. Overall, the δ¹⁵N values from the Omanyne middens vary from 11.3 to 23.5 ‰ (Fig. 4). The results are consistent with the region's arid environment, where hyraceum δ¹⁵N values are expected to be
<table><caption>**Table 1**
Radiocarbon ages and calibration information for the Omanyne-4 rock hyrax middens.</caption><thead><tr><th>Sample</th><th>Depth range (mm)</th><th>14C age yr BP</th><th>1 sigma error</th><th>calibration data</th><th>95.4 % (2σ) lower cal range BP</th><th>95.4 % (2σ) upper cal range BP</th><th>relative area under distribution</th><th>median probability (cal BP)</th></tr></thead><tbody><tr><td colspan="9"><strong>OMA2014-4_back</strong></td></tr><tr><td>UBA-26731</td><td>1.491-5.253</td><td>F14C 1.0823</td><td>0.0023</td><td>SHCal20; SHZ1_2</td><td>-54.42</td><td>-52.49</td><td>93.1 %</td><td>-53.45</td></tr><tr><td>UBA-32400</td><td>35.495-39.044</td><td>789</td><td>34</td><td>SHCal20</td><td>-51.84<br>-8.54</td><td>-51.71<br>-8.41</td><td>1.3 %<br>5.6 %</td><td></td></tr><tr><td>UBA-32401</td><td>64.388-69.996</td><td>1113</td><td>39</td><td>SHCal20</td><td>651<br>924</td><td>731<br>1058</td><td>96.1 %<br>100.0 %</td><td>975</td></tr><tr><td>UBA-26732</td><td>80.502-84.123</td><td>1356</td><td>19</td><td>SHCal20</td><td>1178<br>1223<br>1259</td><td>585<br>1251<br>1286</td><td>3.9 %<br>15.6 %<br>30.3 %</td><td>1213</td></tr><tr><td colspan="9"><strong>OMA2014-4_right_top</strong></td></tr><tr><td>UBA-26733</td><td>2.514-6.250</td><td>1031</td><td>18</td><td>SHCal20</td><td>805<br>900<br>948</td><td>867<br>931<br>955</td><td>52.1 %<br>44.7 %<br>3.1 %</td><td>867</td></tr><tr><td>UBA-32402</td><td>27.730-31.394</td><td>1348</td><td>30</td><td>SHCal20</td><td>1177</td><td>1291</td><td>100.0 %</td><td>1227</td></tr><tr><td>UBA-32403</td><td>48.348-51.724</td><td>2056</td><td>37</td><td>SHCal20</td><td>1883<br>2073</td><td>2058<br>2083</td><td>98.7 %<br>1.3 %</td><td>1963</td></tr><tr><td colspan="9"><strong>OMA2014-4_right_lower</strong></td></tr><tr><td>UBA-32403</td><td>0-3.682</td><td>2056</td><td>37</td><td>SHCal20</td><td>1883<br>2073</td><td>2058<br>2083</td><td>98.7 %<br>1.3 %</td><td>1963</td></tr><tr><td>UBA-32404</td><td>33.574-36.895</td><td>2774</td><td>36</td><td>SHCal20</td><td>2757</td><td>2932</td><td>100.0 %</td><td>2829</td></tr><tr><td>UBA-32405</td><td>66.642-72.202</td><td>3097</td><td>29</td><td>SHCal20</td><td>3169</td><td>3363</td><td>100.0 %</td><td>3271</td></tr><tr><td>UBA-32406</td><td>108.664-111.841</td><td>3163</td><td>29</td><td>SHCal20</td><td>3230<br>3429</td><td>3404<br>3440</td><td>98.2 %<br>1.8 %</td><td>3341</td></tr><tr><td>UBA-32407</td><td>109.169-112.780</td><td>3224</td><td>33</td><td>SHCal20</td><td>3271<br>3338<br>3473</td><td>3285<br>3456<br>3481</td><td>2.3 %<br>96.5 %<br>1.2 %</td><td>3404</td></tr><tr><td>UBA-26734</td><td>150.902-156.967</td><td>4394</td><td>24</td><td>SHCal20</td><td>4848<br>5005</td><td>4981<br>5037</td><td>91.6 %<br>8.4 %</td><td>4923</td></tr><tr><td colspan="9"><strong>OMA2014-2b_top</strong></td></tr><tr><td>UBA-26722</td><td>5.142-8.976</td><td>2798</td><td>21</td><td>SHCal20</td><td>2777</td><td>2941</td><td>100.0 %</td><td>2851</td></tr><tr><td>UBA-31525</td><td>69.456-72.594</td><td>3125</td><td>22</td><td>SHCal20</td><td>3212</td><td>3373</td><td>100.0 %</td><td>3292</td></tr><tr><td>UBA-31527</td><td>116.777-72.594</td><td>3445</td><td>33</td><td>SHCal20</td><td>3405<br>3445</td><td>3426<br>3575</td><td>3.6 %<br>96.4 %</td><td>3510</td></tr><tr><td>UBA-31526</td><td>168.107-171.680</td><td>3326</td><td>27</td><td>SHCal20</td><td>3498<br>3564<br>3745<br>3792</td><td>3507<br>3727<br>3771<br>3825</td><td>1.2 %<br>87.1 %<br>3.6 %<br>8.0 %</td><td>3654</td></tr><tr><td>UBA-31528</td><td>181.615-185.188</td><td>3923</td><td>33</td><td>SHCal20</td><td>4156<br>4228</td><td>4202<br>4418</td><td>8.3 %<br>91.7 %</td><td>4320</td></tr><tr><td>UBA-26723</td><td>252.465-256.125</td><td>4495</td><td>23</td><td>SHCal20</td><td>4962</td><td>5289</td><td>100.0 %</td><td>5129</td></tr><tr><td colspan="9"><strong>OMA2014-2b_lower</strong></td></tr><tr><td>UBA-26723</td><td>0.890-4.496</td><td>4495</td><td>23</td><td>SHCal20</td><td>4962</td><td>5289</td><td>100.0 %</td><td>5129</td></tr><tr><td>UBA-31529</td><td>61.329-65.3769</td><td>5139</td><td>43</td><td>SHCal20</td><td>5725<br>5973</td><td>5938<br>5986</td><td>98.6 %<br>1.4 %</td><td>5823</td></tr><tr><td>UBA-31658</td><td>109.259-112.227</td><td>6356</td><td>58</td><td>SHCal20</td><td>6797<br>6841<br>7100</td><td>6819<br>7076<br>7157</td><td>3.5 %<br>83.2 %<br>13.3 %</td><td>6955</td></tr><tr><td>UBA-31530</td><td>151.074-154.311</td><td>6126</td><td>40</td><td>SHCal20</td><td>7024<br>7067</td><td>7047<br>7117</td><td>2.0 %<br>4.7 %</td><td>7241</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued)**</caption><thead><tr><th>Sample</th><th>Depth range (mm)</th><th>14C age yr BP</th><th>1 sigma error</th><th>calibration data</th><th>95.4 % (2σ) lower cal range BP</th><th>95.4 % (2σ) upper cal range BP</th><th>relative area under distribution</th><th>median probability (cal BP)</th></tr></thead><tbody><tr><td rowspan="5">UBA-26724</td><td rowspan="5">159.797-163.124</td><td rowspan="5">6736</td><td rowspan="5">29</td><td rowspan="5">SHCal20</td><td>7156</td><td>7362</td><td>87.0 %</td><td rowspan="5">7571</td></tr><tr><td>7379</td><td>7421</td><td>6.3 %</td></tr><tr><td>7487</td><td>7499</td><td>1.8 %</td></tr><tr><td>7500</td><td>7621</td><td>92.8 %</td></tr><tr><td>7635</td><td>7661</td><td>5.4 %</td></tr></tbody></table>

substantially higher than from middens in more humid regions (cf. Chase et al., 2015; Chase et al., 2022). The δ¹⁵N value of the most recent hyraceum sample (13.38 ‰; age range CE, 2016; CE, 1984) is consistent with δ¹⁵N measurements of modern pellets from the site (13.5 ± 1.5 ‰ (n = 81)) and δ¹⁵N measurements of modern plants from the foraging range (12.5 ± 2.7 ‰ (n = 36)), with a diet-faeces offset of 1.0 ± 3.1 ‰ associated with fractionation (Carr et al., 2016).

Variations in midden δ¹⁵N are understood, as supported by previous studies on nitrogen isotopes in hyraceum (e.g. Chase et al., 2019a; Chase et al., 2009; Chase et al., 2019b; Chase et al., 2012), to indicate fluctuations in water availability and aridity. This pattern is typically linked to a more open nitrogen cycle in arid environments. In humid regions, nitrogen is continuously cycled between living and decomposing organic matter, whereas in drier areas, a larger proportion of nitrogen moves into mineral pools, where it is more prone ammonia volatilisation and gaseous loss of the lighter ¹⁴N isotope (Austin and Vitousek, 1998). As a result, soil δ¹⁵N values tend to increase with aridity (Carr et al., 2022; Handley et al., 1999; Murphy and Bowman, 2009). This correlation between aridity and soil δ¹⁵N extends to plant and animal tissues, including herbivore excrement such as hyraceum (e.g. Aranibar et al., 2008; Carr et al., 2016; Craine et al., 2009; Hartman, 2011; Hartman and Danin, 2010; Murphy and Bowman, 2006; Newsome et al., 2011; Swap et al., 2004).

Overall, the mid-Holocene is characterised by relatively high δ¹⁵N values, while δ¹⁵N values decrease across the late Holocene. The transition from the mid-to late Holocene registers the highest variance in the record, from ~4300 to 3200 cal yr BP, with some of the lowest δ¹⁵N values of the record (~4200-4100 cal yr BP) being followed by some of the highest (4000-3800 cal yr BP).

# 6. Discussion

## 6.1. Climate variability at orbital timescales

Considered in its broader spatial context, Omanyne is situated on the southwestern margin of the southern African monsoon region (sensu Wang and Ding, 2008; see Chase et al., 2022), near the southern limit of the African tropical rainbelt (Fig. 1). At the onset of the Holocene, orbital precession resulted in peak summer insolation in the Northern Hemisphere. This induced a northward shift of the rainbelt, resulting in more humid conditions across northern and eastern Africa during the African/Holocene Humid Period (Braconnot et al., 2012; deMenocal et al., 2000; Garcin et al., 2018; Kutzbach, 1981; Schefuß et al., 2005; Shanahan et al., 2015). By contrast, much of the southern African monsoon region experienced relatively dry conditions, reflecting the hemispheric antiphase response to orbital forcing, with monsoon intensity tracking local summer insolation maxima. (Chase, 2021; Chase et al., 2022; Chevalier and Chase, 2015; Holmgren et al., 2003; Konecky et al., 2011; Partridge et al., 1997; Schefuß et al., 2011).

Records from the southwestern margin of Africa — including hyrax middens from Aba Huab, Austerlitz (20.5°S), Spitzkoppe (21.8°S), Zizou (24.1°S), Pella (29°S), De Rif (32.4°S), and Groenfontein (32.8°S)— show progressive aridification from the early to late Holocene (Chase, 2010; Chase et al., 2009, 2010, 2019b, 2023a). These trends are strongly linked to changes in the Benguela upwelling system, where orbital forcing intensified latitudinal intra-hemispheric temperature gradients,

strengthening the SE Atlantic Anticyclone and coastal high-pressure systems. These changes promoted the inland advection of cold, dry air and blocked the incursion of moisture from the north and east. Simultaneously, in the interior, warming in the SW Indian Ocean and continental interior enhanced easterly moisture flow and convection (Chase, 2021; Chase et al., 2019b, 2022).

However, the δ¹⁵N record from Omanyne-4, despite its proximity to the Namib Desert, diverges sharply from the southern hyrax midden records. Instead, it shows a long-term increase in humidity across the mid- and late Holocene, likely reflecting enhanced tropical moisture advection driven by rising summer insolation (Fig. 5A and B). This stands in strong antiphase to, for example, the Spitzkoppe record (Fig. 5C; Chase et al., 2019b), which aligns with sea-surface temperatures and upwelling indicators from adjacent marine cores (Fig. 5E; Farmer et al., 2005). In contrast to this difference in terrestrial records, sea-surface temperature records from the northern Benguela (e.g., GeoB 1023-5) show progressive cooling during the mid-to late Holocene (Fig. 5F; Kim and Schneider, 2003), but this signal does not appear to influence conditions at Omanyne.

Further north, equatorial records from Barombi-mbo (Garcin et al., 2018; Maley and Brenac, 1998), Ngaoundaba (Schaaff et al., 2025), CEN-17.4 (Garcin et al., 2022), Lake Sinnda (Vincens et al., 1998), Kitina (Vincens et al., 1999) and marine core GeoB6518-1 (Schefuß et al., 2005) show evidence of mid-to late Holocene aridification driven by declining summer insolation and a southerly shift of the tropical rainbelt at the end of the African Humid Period (Figs. 1 and 5). At orbital timescales, these records express trends opposite to those observed in the Omanyne-4 record, supporting the model of a southerly displacement of the rainbelt, consistent with insolation forcing.

While evidence from the eastern southern African monsoon zone supports a southward-shifting rainbelt (Chase et al., 2022; Chevalier and Chase, 2015; Holmgren et al., 2003; Schefuß et al., 2011), the western and interior regions remain underrepresented in palaeoclimate records. In the Kalahari, studies of dunes and palaeolake shorelines (e.g. Burrough et al., 2009b; O'Connor and Thomas, 1999; Stokes et al., 1997; Thomas and Burrough, 2012) provide insights into landscape change, but these discontinuous 'geoproxy' archives often lack clear climatic interpretations (Burrough and Thomas, 2013; Chase, 2009; Thomas, 2013). At some sites, however, such as Lake Ngami in Botswana (Fig. 1), geoproxy records (Burrough et al., 2007) have been paired with continuous sedimentary and palaeoecological records to provide enhanced insight into environmental change and landscape response (Cordova et al., 2017). Here, pollen-based climate reconstructions suggest limited precipitation from ~12,500-10,000 cal BP followed by progressively wetter conditions across the Holocene. As the lake is fed in part by rivers flowing in from the Angolan Highlands, it is possible that this may reflect, at least in part, conditions upstream. However, pollen from plants characteristic of the Angolan Highlands (e.g. Brachystegia) and Okavango Delta (e.g. Hyphaene) is almost entirely absent from the Ngami record, suggesting the dominance of local and regional influences (Cordova et al., 2017). This pattern of variability may also be reflected in the distribution of palaeolake shoreline ages from the site for this period (Burrough et al., 2007). While of much lower resolution than the Omanyne-4 record, similar patterns of variability can be observed in the Omanyne-4 δ¹⁵N and Lake Ngami precipitation reconstruction records, both in the long-term trend as well as perhaps at millennial timescales,
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Distribution in time and depth of 14C ages for the Omanyne-4 rock hyrax midden sections with age-depth model (grey), overlaying the calibrated distributions of the individual dates calculated using the R packages Bacon v2.5.8 (Blaauw and Christen, 2011) and clam v2.3.2 (Blaauw, 2010).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** The δ¹⁵N data from the Omanyne-4 rock hyrax middens. The data points from each midden section included in the composite δ¹⁵N record are shown overlaying the composite result (in grey) derived using Gaussian kernel smoothing of the age-ranked data (Rehfeld et al., 2011). The variance of the record was calculated over time using the R package Zoo (Zeileis and Genthendieck, 2005) with a 10-point moving window, and the 95 % credible interval associated with the age-depth model for each sample from each midden are indicated.</figcaption></figure>

particularly regarding peaks in humidity between ~3000 and 5000 cal BP and after ~1500 cal BP (Fig. 5I). These humid periods are also recorded in speleothem records from Dante's Cave (Figs. 1 and 5H; Sletten et al., 2013). Further support can be found in the sedimentological and palynological data from the Okavango Panhandle (Nash et al., 2006) – the conduit through which Lake Ngami and other lakes of the Makgadikgadi Basin are fed – and from peatlands in western Zambia (Burrough and Willis, 2015), which also indicate a general comparable pattern of progressively wetter conditions across the mid-to late Holocene. Brook et al. (1998) also infer a general trend of increasingly wetter conditions across the Holocene based on numbers of speleothem ages from nearby Drotsky and Bone caves, with lower numbers of ages apparently corresponding with periods of decreased humidity at Omanyne-4, centred at ~1400, 2000, 3900, 5400, 6000 and 7200 cal BP).

Although the fundamental drivers of Holocene climate variability in the southern African monsoon region can be broadly associated with changes in summer insolation, it is evident from the Omanyne data that there is not a simple linear relationship between insolation and climate change (Fig. 5A and B). Kim et al. (2003) argued that one of the principal features of regional atmospheric and oceanic circulation dynamics is the Angola-Benguela Front (Fig. 1), where the cold waters of the Benguela upwelling system converge with warm tropical waters transported southward by the Angola Current. This system is strongly influenced by the SE Atlantic trade winds, which drive upwelling along the Namibian coast and modulate the intensity of the Angola Current and transport of
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** The Omanyne-4 δ¹⁵N record (B) compared with (A) summer insolation at 25°S (Laskar et al., 2004), (C) the Spitzkoppe 2012-1 δ¹⁵N record (Chase et al., 2019b), (D) a sea-surface temperature reconstruction from marine core ODP 1084b (Farmer et al., 2005), (E) a sea-surface temperature reconstruction from marine core GeoB 1023-5 (Kim et al., 2003), (F) the marine core GeoB 6518-1 δD record (Schefuß et al., 2005), (G) a sea-surface temperature reconstruction from marine core ODP 1078C (Kim et al., 2003), (H) the Dantes Cave δ¹⁸O record (Sletten et al., 2013), and (I) the Lake Ngami pollen-based wet season precipitation reconstruction (Cordova et al., 2017). For ease of comparison, the Omanyne-4 δ¹⁵N data are shown in grey with each record.</figcaption></figure>

heat into the eastern equatorial Atlantic (Kim et al., 2003). During periods of increased SE trade wind strength, the temperature gradient across the Angola-Benguela Front steepens and concurrently the Namib Desert is predicted to experience more arid conditions while the low-level westerlies transport warmer, moister air across Angola into northern Namibia. During the mid-to late Holocene, coastal upwelling intensity and the temperature gradient across the Angola-Benguela Front is weakest between ~5.5 and 4 ka, but strengthens progressively across the late Holocene (Kim et al., 2003). These changes correspond to variations in humidity at Omanyne, indicating that climate change at the site during this period may be strongly affected by moisture transport via the low-level westerlies (Fig. 6).

As northern Namibia and Botswana have been the focus of studies using climate models to explore palaeohydrological dynamics, we have compared our results from Omanyne with simulations of precipitation obtained from the TraCE-21k-II (He and Clark, 2022) and HadCM3

models (Beyer et al., 2020; Singarayer and Valdes, 2010). In marked contrast with the Omanyne record, both models simulate a decline in precipitation across the mid-to late Holocene (Fig. 7). The source of this discrepancy is difficult to assess at present because the simulations are relatively coarse (0.5° HadCM3 (Beyer et al., 2020) and 3.75° for TraCE-21k-II (He and Clark, 2022)), limiting evaluation of model performance in complex, transitional climate regions. Singarayer and Burrough (2015) noted that PMIP3 mid-Holocene simulations perform particularly poorly in this region, with low inter-model agreement arising in part from differences in parameterisations and in the strength of land-atmosphere and ocean-atmosphere coupling. Problems have also been documented in the southeast African monsoon domain (Chevalier et al., 2017), reinforcing the view that the complexity of the southern African climate system, as noted by Singarayer and Burrough (2015), can undermine the reliability and utility of regional simulations. Accordingly, interpretations of past environmental change in southern
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** The Omanyne-4 δ¹⁵N record compared with the sea-surface temperature gradient across the Angola-Benguela Front (Kim et al., 2003). The records are shown as MC-SSA reconstructions including the first eigencomponent and oscillatory pairs 2–3 and 4–5.</figcaption></figure>

Africa based on these simulations should be treated cautiously and, where possible, corroborated with independent proxy evidence.

## 6.2. Climate variability at millennial to centennial timescales

Exploring patterns of climate variability at millennial to centennial timescales is inherently restricted by the region's limited number of higher resolution records spanning the mid-through late Holocene. Previous studies of such records – from hyrax midden sites at Marakabi and Spitzkoppe – have indicated that during the mid-Holocene the eastern southern African monsoon region and the Benguela region experienced similar multi-centennial to millennial scale patterns of climate variability until ~5 ka, roughly coincident with the end of the African Humid Period, after which time a distinct antiphase relationship was established (Chase et al., 2022). Here, we expand this analysis (Fig. 8) to include the Cold Air Cave δ¹³C record and the Omanyne δ¹⁵N record. Using Monte Carlo Single Spectrum Analysis (MC-SSA) (Allen and Smith, 1996), we have identified the first eigencomponent of each record (i.e. multi-millennial tendency) (Fig. 8A), highlighting the general similarity in trends between records from the eastern southern African Monsoon region (Marakabi and Cold Air Cave) and Omanyne, and the broadly antiphase trend exhibited at Spitzkoppe. At finer timescales, shown as MC-SSA reconstructions including the first eigencomponent and oscillatory pairs 2–3 and 4–5 and Morlet Continuous Wavelet Transform (CWT) periods of 500–2000 years (Fig. 8B), it is apparent that similarities at centennial to millennial scales exist, but they are not consistent across the mid-to late Holocene. To more clearly highlight the relationships between these records, we applied semblance analysis (Cooper and Cowan, 2008) to identify correlations between individual CWT (Fig. 8C).

Focussing on longer periods, from 1 to 2 kyr (at shorter periods the reliability of the results diminishes as the demands on individual age models increase), it can be seen that a strong positive correlation exists between the Cold Air Cave and Marakabi records, indicating that despite a degree of regional heterogeneity (cf. Loftus et al., 2025) climate variability in the eastern southern African monsoon region is likely responding to a common driver. Both of these records are generally in-phase with the Spitzkoppe record prior to ~5 ka, but a strong antiphase relationship is established after this time (Fig. 8C), consistent with the previously published results of Chase et al. (2022). These authors suggest that this dynamic is associated with coeval inverse changes in atmospheric pressure due to changes in interhemispheric temperature gradients, with higher pressure over the SE Atlantic Ocean resulting in

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Left: The TraCE-21k-II simulated precipitation anomaly between 1000 cal BP and 8000 cal BP. The position of the Omanyne site is indicated by the star. Right: First eigencomponents derived from Monte-Carlo Single Spectrum Analysis (Allen and Smith, 1996) of the Omanyne-4 δ¹⁵N record (this paper) and precipitation changes at the Omanyne-4 site simulated by the TraCE-21k-II (He and Clark, 2022) and HadCM3 models (Singarayer and Valdes, 2010).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** (A) First eigencomponents derived from Monte-Carlo Single Spectrum Analysis (Allen and Smith, 1996) of the Omanyne-4 δ¹⁵N record (this paper), Spitzkoppe δ¹⁵N (Chase et al., 2019b), Cold Air Cave δ¹³C (Holmgren et al., 2003) and Marakabi δ¹⁵N (Chase et al., 2022) records. (B) Reconstructions comprising the first eigencomponent and oscillatory pairs 2–3 and 4–5 and Morlet Continuous Wavelet Transform (CWT) of the same records. (C) Semblance analysis (Cooper and Cowan, 2008) of pairs of these records, identifying correlations between them (red = 1, blue = -1) at 500 to 2000-year periods.</figcaption></figure>

increased upwelling intensity creating drier conditions along the western continental margin while at the same time lower atmospheric pressure over the continent favouring increased convection and the incursion of moist tropical air masses in the east.

The relationship between the Omanyne and Spitzkoppe records is more variable, likely due to the site's position adjacent to the Angola-Benguela Front and the proximity of the Benguela upwelling system and the low-level westerlies. Until ~5.5 ka, a generally in-phase
relationship is observed at 1–2 kyr periods, transitioning to primarily antiphase from 5.5 ka to ~3 ka, after which an in-phase relationship begins to progressively reestablish across an increasingly wide range of frequencies (Fig. 8C). Despite the distinct differences in trends over longer time periods, the in-phase response between Omanyne and Spitzkoppe at shorter frequencies indicates that changes in upwelling intensity play a role in modulating climate variability at Omanyne through changes in the associated coastal high-pressure cell that blocks incursions of moisture into the Namib Desert. In contrast, the period of antiphase conditions between Omanyne and Spitzkoppe from 5.5 ka to ~3 ka suggests that during this time the low-level westerlies penetrated further into the region. This correlates with a period of relatively elevated sea-surface temperatures in the central Benguela upwelling region (Farmer et al., 2005) (Fig. 5D), indicating a likely weakening of the coastal high-pressure cells and the potential for increased moisture transport to Omanyne. Similarities with the Cold Air Cave and Marakabi records (Fig. 8C) at these frequencies during this period may also indicate that moisture transport to Omanyne was not limited to low-level westerly influence, and that easterly moisture transport was enhanced, weakening the humidity gradient along the Congo Air Boundary and resulting in increased convection across northern Namibia.

## 7. Conclusions

The 7300-year record from Omanyne provides critical insight into the spatial complexity of Holocene hydroclimate variability in northern Namibia, a region situated at the intersection of tropical and subtropical atmospheric circulation systems. In contrast to hyrax midden records from sites along the southwestern continental margin, which show progressive aridification linked to Benguela upwelling and strengthening of the South Atlantic Anticyclone, the Omanyne δ¹⁵N record indicates a long-term trend toward increased humidity throughout the mid-to late Holocene. This pattern reflects enhanced tropical moisture advection via the low-level westerlies and demonstrates that the southwestern margin of the southern African monsoon region responded in strong antiphase to the adjacent Namib Desert.

Comparison with palaeoclimate archives from the Kalahari, Okavango, and eastern southern African monsoon zone further highlights the existence of distinct climate response regions within the summer rainfall zone. While insolation forcing set the boundary conditions for Holocene hydroclimate variability, our results show that the balance between coastal upwelling, the position of the tropical rainbelt, and the penetration of tropical versus subtropical air masses determined regional expressions of climate change. At centennial–millennial time-scales, these interactions generated shifting in-phase and antiphase relationships among sites, underscoring the sensitivity of boundary regions like Omanyne to subtle changes in atmospheric circulation and ocean–atmosphere coupling across the Angola–Benguela Front.

By documenting the heterogeneous response of northern Namibia to Holocene climate forcing, this study demonstrates the limitations of broad, seasonality-based rainfall classifications and emphasizes the need for finer-scale differentiation of palaeoclimate response regions in southern Africa. Our findings not only reconcile previously contradictory interpretations of proxy records but also provide a more nuanced framework for evaluating the regional impacts of external and internal climate drivers. This work underscores the importance of strategically located high-resolution records for disentangling the mechanisms governing climate variability at the margins of the African monsoon system—an understanding that is essential for placing current and future hydroclimatic changes into a long-term perspective.

### CRediT authorship contribution statement

**Brian M. Chase:** Writing – original draft, Visualization, Validation, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Andrew**

**S. Carr:** Writing – original draft, Validation, Methodology, Investigation. **Arnoud Boom:** Writing – original draft, Validation, Methodology, Data curation. **Manuel Chevalier:** Writing – original draft, Validation, Methodology, Investigation. **Maarten Blaauw:** Validation, Methodology. **Michael E. Meadows:** Writing – original draft, Validation, Investigation.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This research was funded, in whole or in part, by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013), ERC Starting Grant “HYRAX”, grant agreement no. 258657. A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.

### Data availability

Data will be made available on request.

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