Southern Hemisphere subtropical front impacts on Southern African hydroclimate across the Mid-Pleistocene Transition

Domain: earth_science
Published: 2025-04-12
Editorial outcome: major_revision

Abstract
Southern African (SA) hydroclimate is largely shaped by the interactions of atmospheric circulations, e.g., Hadley Circulation, and oceanic elements, like the Benguela Upwelling System (BUS), Agulhas System, and Antarctic Circumpolar frontal system. Large-scale changes to the Meridional Temperature Gradient (MTG) influence both the atmospheric and oceanic components of the hydroclimate system, and thus impact hydroclimate over SA. We present a leaf wax hydroclimate record from ODP 1084, in the BUS, which reveals that changes in the isotopic signature of precipitation over SA coincide with a strengthening of the MTG across the Mid-Pleistocene Transition (MPT). We use HadCM3 simulations to demonstrate the sensitivity of winter rainfall to shifts in the MTG during the MPT. Given the ongoing impacts of climate change on water resources in SA, awareness of the relationship between rainfall and shifts in Hadley Circulation could provide insight into past water availability and aid regional adaptation efforts.

Introduction
The Mid-Pleistocene Transition (1.2–0.7 Ma, MPT) represents a time of planetary transition, from milder 40 thousand-year (kyr) glacial cycles to more extreme 100 kyr glacial cycles. The cause of this shift is debated, but appears to be linked to changes in the carbon cycle (see Herbert (2023) and the references therein). Several lines of evidence suggest that these climate changes may have perturbed the local ecologies of several regions, including Southern Africa. For example, isotopic measurements of faunal remains (e.g., grazer tooth enamel, ostrich egg shells) suggest dietary changes in grazers and increases in relative humidity around 0.9–0.8 Ma1, which, paired with phytolith morphotype analyses from Wonderwerk Cave2,3 and sedimentological data from Mamatwan Mine4, suggests shifts in moisture availability during the MPT. However, the scarcity and coarseness of these records makes it difficult to ascertain whether this is a signal of local variability, a regional trend, or whether a moisture shift is part of an overarching climate regime change.

There are compelling reasons to think that the MPT would affect climate across Southern Africa (SA, Fig. 1a). One prominent hypothesis about the MPT climate shift posits that expanded sea ice near Antarctic strengthened Southern Ocean (SO) stratification and induced ocean circulation changes5–7. This would impact global and regional climates, especially in SA, where the surrounding ocean is a confluence of the Benguela Upwelling System (BUS), the Agulhas System, and the Antarctic circumpolar frontal system (AFS). The intensity of the Agulhas Leakage (AL), warm and salty masses of water which escape the retroflection of the Agulhas Current, is hypothesized to be modulated by variations of the westerly wind belt and AFS8. Proxy records associate intervals of reduced AL with northward shifts of the AFS, which occurred over the MPT and during post-MPT glacial periods9–11. Northward migrations of the AFS and diminished AL are linked to the intensification of winter rainfall over SA on millennial timescales12,13, however, there is debate over precisely how the westerlies, AFS, and AL influence SA rainfall in the modern14–16.

A more robust understanding of the complex SA hydroclimate system is vital for assessing past and future changes in hydroclimate. Here, we demonstrate that substantial shifts in the leaf wax isotopic signatures of precipitation and vegetation over Southern Africa coincide with significant shifts in the position of the AFS across the MPT. We use HadCM3 simulations to test for a regionally coherent mechanism which reconciles the different perspectives gained from the terrestrial versus marine records (e.g., assess whether a shift in hydroclimate related to large-scale circulation shifts). In particular, we test the hypothesis that northward shifts in the AFS resulted in an expansion of the winter rainfall zone (WRZ) and WRZ-type vegetation (i.e., C3 plants) from 1.5 to 0.9 Ma, and that a sudden withdrawal of the AFS after 0.9 Ma prompted a contraction of WRZ rainfall and vegetation. We conclude by discussing the parallels between the mechanism presented here and that which drove extreme drought in SA in recent years; we raise the question of whether the relatively short-lived, but intense, drought events we see in the modern have the potential to become longer-lived features of the future climatology.

Results
Leaf-Wax analysis of sediment from ODP 1084
ODP 1084 (25° 30.8345’S, 13° 01.6668E) is located in the central upwelling cell of the BUS (Fig. 1a); age control is derived from shipboard magnetostratigraphy and nannofossil datums17. Terrigenous materials are transported to marine sediments via aeolian processes18 and fluvial transport from the Orange River watershed19. Sediments from the watershed enter the water column at the Orange River delta, approximately 250 km south of ODP 1084; a portion of this sediment is carried northward to ODP 1084 via the Benguela Current20 while the rest is transported southward via a coastal countercurrent21. The large source area for ODP 1084 means that contributions from a variety of biomes, including the Succulent Karoo, Nama Karoo, Kalahari Savannah, and grasslands, are integrated into a single signal, which ultimately represents regional trends, rather than localized variability.

Paired carbon and hydrogen isotopic analyses of plant leaf waxes are commonly used to infer the relative proportions of C3 to C4 vegetation and the hydrogen isotopic signature of precipitation, respectively, especially in Africa (e.g., Feakins et al.22; Tierney et al.23; Lupien et al.24). Such methodology is appropriate for regions with little or no presence of CAM vegetation, but may not work well in Southwestern Africa, where CAM plants are more prominent25,26; the broad range of δ13Cwax values that CAM plants display make them impossible to differentiate from C3 and C4 using δ13Cwax alone27. Furthermore, there is high uncertainty regarding the apparent fractionation of hydrogen in CAM plants, which is well constrained for C3 and C4 vegetation. To examine the impact of overlooking the potential influence of CAM plants on δDwax, we performed a carbon-correction using a standard methodology to infer the hydrogen isotopic signature of precipitation (see Methods for details) and found that the correction artificially smooths the δDprecord (Fig. S5). As such, we utilize the raw, uncorrected δDwax data in our statistical analyses.

While we cannot reliably use δ13Cwax on its own to estimate vegetation changes in this region, we may be able to characterize overall biome transitions by leveraging the ratios between individual n-alkanes. Empirical studies have demonstrated that the ratios between individual n-alkanes are sensitive to environmental changes (e.g., ref. 28), and biomes in Southwestern Africa can be characterized by these ratios29. We use ratios of different n-alkanes (e.g,. Norm31 and Norm33) to roughly track the contribution of the WRZ taxa to our record because these metrics reflect the relative wax contribution of different biomes to the sediment (see methods for more details).

Finally, we use simulations from the Hadley Centre Coupled Model 3 (HadCM3) to examine the dynamics driving changes in δDwax. Despite modifications aimed toward more accurately capturing the meridional temperature gradient (MTG)30, HadCM3 remains particularly biased toward warmth over parts of the SO and BUS. Comparing the Pre-industrial control run to observational data (Fig. S2), we find the model simulates a weaker temperature gradient over the AFS and weaker westerlies than those observed in the modern. Considering this bias, along with the limitation of a low spatial resolution, we use the model to examine the sensitivity of rainfall to changes in the AFS, rather than treating the model output as an exact picture of MPT climate (see Methods for further analytical and model details). Given the more southerly placement of the SO boundary in the model, we expect rainfall responses to be similarly displaced.

Coordinated changes in Southern African hydroclimate during the MPT
Lomb-Scargle (LS) periodogram analysis on δDwax (Fig. S6) indicates possible low-frequency orbital influence in the 400-kyr and 100-kyr bands from 0.8 Ma to present, though confidence is low (less than 75%). Given the prominence of orbital signals in other African hydroclimate records (e.g., refs. 31–33, the lack of orbital variability in our record is probably due to low sampling resolution (>21-kyr between many samples, Fig. S1). Rather than orbital-scale variability, we observe broad trends in the δDwax and δ13Cwax records over the past 3.4 Ma (Fig. 2a, b). We also observe relatively abrupt enrichment in both δDwax and δ13Cwax records over the MPT, particularly at ~0.85–0.75 Ma.

The overarching trends in δDwax and δ13Cwax are likely related to long-term changes in both regional and global SSTs. Varimax factor analysis (see Methods for details) of our records and several regional proxy records (Fig. 3) identifies two primary modes of variability which explain ~60% of the variance in the data. The first mode (Factor 1; Fig. 3a, c), which explains 42.62% of the variance, captures the long-term trends in BUS temperatures, hydroclimate, and vegetation, and dust accumulation; Factor 1 thus links the intensification of the BUS to the overall aridification of the African continent34,35. The secondary mode of varibility (Factor 2; Fig. 3b, d) explains 20.56% of the variance in the data and indicates that changes in the MTG are strongly tied to changes in the nutrient availability within the BUS (ODP 1084 Alkenone MAR) and relatively abrupt variations in δDwax. The abrupt enrichment in δDwax around ~0.9–0.75 Ma is thus likely influenced by similarly stark changes in ocean conditions. The changes in δ13Cwax are probably tied to changing hydroclimate, though δ13Cwax loads less strongly on Factor 2.

Due to its stratiform character and relatively depleted vapor source (the South Atlantic), precipitation in the WRZ is depleted in deuterium relative that of the summer rainfall zone (SRZ), which is generally convective and utilizes a relatively enriched vapor source (the Indian Ocean)36–38. An isotopic gradient exist within the SRZ, as precipitation moves inland from the coast and undergoes fractionation by both the amount and continental effects29,36,38,39; due to its smaller spatial extent and lower overall precipitation volume, the isotopic signature of rainfall in the WRZ is less impacted by these effects38,40. The vegetation of each rainfall zone is distinct: the Fynbos, Succulent and Nama Karoo biomes (mainly C3 and CAM plants) dominate in the WRZ, while grassland and savannah (mostly C4 plants) are more prevalent in the SRZ26. Norm31-Norm33 approaches zero at ~0.9 Ma, which may indicate an increased contribution of waxes from grassland and savannah biomes (Fig. S7). We therefore interpret the abrupt enrichment of the ODP 1084 δDwax and δ13Cwax signals as contraction of the winter rainfall zone and associated vegetation types from ~0.9–0.75 Ma. This interpretation is consistent with multiple terrestrial records from the Kalahari which indicate a substantial hydroclimate and vegetation shift occurred around 0.9–0.8 Ma1,41.

Antarctic front migration mechanism
Assessment of regional proxy records presents a compelling mechanism for this shift in seasonality. Around 1.5 Ma, Southern Hemisphere extratropical temperatures cooled rapidly, falling below preindustrial levels (Fig. S742,43). This cooling resulted in a strengthening of the MTG (Figs. 2h, S6) and allowed for an expansion of sea ice (Fig. 2m7), which prompted a reorganization of ocean circulation6,7,44,45. In the surface waters, a steeper temperature gradient over the SO shifted the AFS northward; the presence of foraminifera generally found in Antarctic and sub-Antarctic waters (e.g., refs. 11, 46, 47.), along with records of opal deposition48–50 and primary productivity11,51, indicate a more northerly presence of the AFS prior to ~1 Ma. The northward presence of the AFS may have also created a choke-point at the Agulhas Plateau which limited AL (Fig. 2l10,) and, combined with lower sea levels and weakened Indonesian Throughflow, caused a build-up of warmer, saline waters in the western Indian Ocean (Fig. 2k52,). The strengthened MTG, expansion of sea ice, and limited AL probably impacted deep water circulation as well: foraminiferal trace elements and authigenic Nd isotope proxies indicate increased carbon storage and weaker Atlantic Meridional Overturning Circulation over the MPT, culminating in a meridional overturning minimum at ~0.9 Ma6,44.

Changes of the MTG influence the width (and intensity) of atmospheric Hadley circulation, with models and modern observations demonstrating that a weaker gradient drives an expansion of the Hadley cell53,54 and a poleward propagation of the westerly storm track53,55–58. By this same mechanism, a steepening of the MTG from ~1.5 to 1.1 Ma and from ~1 to 0.8 would cause a contraction of the Hadley cell and an equatorward propagation of the westerly storm track. As winter storms travel further north, the WRZ would expand and the proportion of winter to summer rain would increase; an increase in isotopically depleted winter rains would contribute to the gradual depletion of deuterium in δDwax prior to 0.9 Ma38.

As mentioned previously, the model’s resolution limitations and temperature bias result in a more poleward-displaced SO position in PI simulations compared to observations, and any rainfall response to changes in the SO is expected to be similarly displaced. The HadCM3 simulations feature northward shifts of the subtropical front and a strengthening of the southern westerlies which result in both a zonal increase and intensification of winter rainfall over the waters south of Africa and parts of SA during the MPT (Fig. 1b, c). The model results thus demonstrate the sensitivity of winter rainfall to dynamical changes in the westerlies and SO. Further work examining this response in models of higher resolution and with the ability to trace water isotopologues is needed.

Frontal system retreat
From ~1.1 to 1 Ma and ~0.8–0.75 Ma, the Southern Hemisphere MTG rapidly weakened (Figs. 2h, S7). The first weakening, from ~1.1 to 1 Ma, occured as tropical SSTs rapidly decreased, is probably due to an increase of equatorial upwelling as the Hadley Circulation intensified59–61. Despite a weaker MTG, the proxy evidence indicates that the AFS remained in a northerly position from ~1.1 to 1 Ma. From ~0.8 to 0.75 Ma, SSTs in both the tropics and Southern Hemisphere Atlantic extratropics increased-the weakening of the MTG appears to have been driven by differences in the rates of warming, as the extratropics appear to have warmed more rapidly than the tropics (Fig. S7). During this interval, sea ice retreated7 and AL resumed10. The coincident decrease and increase in the respective accumulation rates of biogenic silica and alkenones at ODP 1084 suggest the withdrawal of polar nutrients from the BUS17,49,60 as the AFS retreated.

As the AFS and winter storm track migrated southward, the extent of the WRZ would be limited; the abrupt enrichment in δDwax at ODP 1084 and ODP 108162 is consistent with an increase in the proportion of summer rainfall38. The stark fluctuation in δ13Cwax values over this interval indicate a vegetation shift over the leaf wax source area, and our inference that wax contribution from grassland and savannah biomes increased at ~0.8 Ma is consistent with an expansion of the SRZ. Improved sampling resolution during the Late Pleistocene is needed to further examine the impacts of glacial/interglacial variability on δDwax and δ13Cwax after the MPT.

Comparison to terrestrial and tropical records
The interpretation of the ODP 1084 leaf wax records presented here agrees with multiple terrestrial records from the Kalahari which indicate a substantial hydroclimate shift occurred around 0.9–0.8 Ma1,4,41,63. For example, sedimentalogical evidence from Mamatwan Mine, ~1000 km inland from ODP 1084, points to the presence of a shallow water body from ~1.5 to 1 Ma4,63. The remains from lechwe antelope (Kobus leche), which require standing bodies of water, have been found at Wonderwerk Cave1 and regions of the Ghaap Escarpment64, indicating that similar shallow water bodies were present throughout the southern Kalahari during this interval. Just as the extension of the WRZ, and adjacent yearly rainfall zone, would help sustain these shallow waters, the contraction of these zones could help explain their disappearance after 0.9 Ma.

The mechanism presented here also supports tropical vegetation reconstructions for the MPT. Leaf wax records from ODP 1077, near the equator, indicate an increase (decrease) in tropical savannah (C3 rainforest taxa) in the Congo Basin from ~0.95–0.85 Ma, and a reverse of this trend after ~0.85 Ma, which the authors link to tropical SSTs65. East African soil carbonates demonstrate large inter-site variability; most sites in East Africa display a broad trend toward tropical C4 savannah over the last ~4 Ma, however, the records from several locations feature substantial C3 incursions (e.g., Maslin et al.66, Lüdecke67, Quinn et al.68). The shifting MTG and Hadley circulation would have impacted the placement of the Intertropical Convergence Zone, water availability, and subsequent ecosystems variation in the tropics.

Implications for hominin evolution
Elucidation of past hydroclimate change may be useful to contextualize hominin evolution in SA. Hominin individuals living in this region would have experienced abrupt, but extensive landscape change alongside dwindling freshwater resources and a coincident vegetation shift. At the same time, there is a relative reduction in artifact densities in the corresponding strata of Wonderwerk Cave2,69, as well as the last appearance of Paranthropus boisei in the SA archaeological record (e.g., Herries 2022). However, more rigorous work examining the impact of precipitation and landscape change on Paranthropus is necessary.

Implications for future climate
A deeper understanding of the mechanism presented in this study is vital for understanding and predicting modern and future climate events. From 2015 to 2018, the Day-Zero Drought devastated water supplies across southwestern Africa70–72. Intense scrutiny of this drought has pointed to an expansion of the Hadley cell and subsequent poleward displacement of the mid-latitude westerlies as the primary driver57,73–75. At the synoptic scale, the Hadley expansion may be increasing the frequency of South Atlantic Anticyclone ridging high-pressure systems57,75,76, ultimately resulting in a delayed and shortened wet-season75,76. Outside of SA, the response of other Mediterranean-type climates to changes in Hadley circulation is varied and subject to regionally specific drivers77. Compilation of hydrogen isotopic records of precipitation from Chile and South Australia (the other Southern Hemisphere Mediterranean-type climate zones) are thus needed to interrogate the local hydroclimate response to changes of the Hadley circulation and AFS over both modern and paleo-timescales.

The results presented in this paper bolster the view that long-term shifts in the Southern Hemisphere storm tracks may ultimately be responsible for hydroclimate shifts in SA. Given the similarities of these past mechanisms to those observed in the modern, further work is needed to examine whether the relatively short-lived drought events we see in the modern have the potential to become longer-lived features of the future climatology. While we cannot prevent the next extreme drought from occurring, an enhanced understanding of the underlying mechanics of regional hydroclimate may provide additional insight which improves our projections of regional climate, aids mitigation efforts, and strengthens the resilience of SA communities to the effects of anthropogenic climate change.

Summary
This study highlights substantial shifts in rainfall seasonality and vegetation cover which are associated with migrations of the AFS over the MPT. We use HadCM3 simulations to further demonstrate that SA precipitation is sensitive to variations in the westerlies and AFS; this work would benefit from additional analyses using isotope-enabled simulations with higher spatial resolution. Our records supplement the existing terrestrial records of hydroclimate change in SA. We present a mechanism of hydroclimate change during the MPT which is analogous to that of the Day-Zero Drought, suggesting that these impactful drought events we see in the modern record have the potential to become persistent features of the region’s climatology. Continued investigation of this mechanism in the past and present is therefore needed to improve our projections of, and preparations for, future extreme climate events.

Methods
Sample preparation and analytical methods
Samples were freeze-dried and homogenized, and total lipids (TLEs) were extracted and purified via column chromatography. Concentrations of C29 and C31 alkanes were quantified using a Trace 1310 GC-FID, and their hydrogen and carbon isotopic com-position were measured using a Thermo Scientific Trace 1310 GC and a PTV injector connected via an IsoLink II pyrolysis furnace to a Delta V Plus isotope ratio mass spectrometer (GC-IRMS). H2 and CO2 gases calibrated to an n-alkane standard (A7 mix provided by Arndt Schimmelmann at Indiana University) provided references for each analysis and were used to monitor drift. H3 factor measured daily over the course of the hydrogen analyses averaged 4.703 ppm/V over a range of 1–8 V. Samples were run in triplicate for δD to obtain a precision better than 3‰ (1-σ), and duplicate or triplicate for δ13C to obtain a precision better than 0.2‰ (1-σ).

We calculated a mean carbon preference index >1, indicating terrestrial rather than petrogenic inputs78, and an mean average chain length (ACL) of 29.35 ± 0.089 (1-σ). Because concentrations of the C29 and C31 alkanes were similar (within 2.5 ng/g of one another), and because previous studies from SWA have focused on the C31 alkane as a proxy for precipitation (e.g., refs. 35,62), we report isotopic results from this chain length.

Inferring δD of precipitation from leaf waxes
The carbon isotopic measurements of 70 samples were paired with 106 hydrogen isotopic measurements of the C31 alkane (δ13Cwax and δDwax, respectively) to infer the hydrogen isotopic signature of precipitation, δDp. δDwax is offset from the hydrogen isotopic signature of the environmental source waters, generally assumed to be precipitation; this apparent fractionation, εp−w, varies systematically across plant clades79,80, with graminoids generally having a larger εp−w than those of eudicots79,81.

Within a Bayesian framework, we use δ13Cwax and empirically derived constraints on εp−w from modern grasses and eudicots (Table S123,) to infer the proportion of graminoids in our sample and weigh our εp−w accordingly. The resulting εp−w accounts for the variable fractionation between graminoids and eudicots (Eq. 1).

ε = (f C4) * ε C4 + (1 − f C4) * ε C3 (1)

We then use our weighted εp−w to infer δDp (Eq. 2).

δD(precip) = (1000 + δD wax) / (ε / 1000 + 1) − 1000 (2)

This approach has been widely applied to infer δDp from leaf wax δDwax in a variety of paleoclimatic studies (e.g., Tierney et al.23, Bhattacharya et al.82, Windler et al.83).

n-Alkane ratio calculation
Herrmann et al.29 measured n-alkanes in soils, sediments, and river deposits from several biomes across Southern Africa and calculated the ratio of n-alkane homologues C29 and C31 and C29 and C33 as

Norm31 = C31 / (C29 + C31) (3)

and

Norm33 = C33 / (C29 + C33) (4)

where Cx is the concentration of the n-alkane with x carbon atoms. Their results showed that in samples from the Grassland and Savannah biomes (which are mainly SRZ flora), Norm31 ≈ Norm33, but for samples from the Fynbos, Succulent Karoo, and Nama Karoo biomes (primarily in the WRZ), Norm31 > Norm33.

Based on their results, we tentatively infer the relative contributions of SRZ flora vs WRZ flora by taking the difference of Norm31 and Norm33. If Norm31-Norm33 ≈ 0, we infer that wax contributions mostly came from the Grassland or Savannah biomes. If Norm31-Norm33 > 0, we infer that wax contributions include more contributions from the Fynbos, Succulent, or Nama Karoo biomes.

Using this index, we infer more contribution from the Fynbos, Succulent Karoo, and/or Nama Karoo biomes 1.5–1 Ma, and decreasing contribution from these biomes after 1 Ma. The agreement of this inference with evidence of vegetation changes from the Kalahari interior (e.g., phytolith morphotpe analyses2,3, dietary changes in grazers1), lends credence to this index. Further work need to be done to examine whether this index is viable outside of southwestern Africa.

Varimax factor analysis
Varimax Factor Analysis (VFA) is a type of matrix decomposition similar to Principal Component Analysis (PCA); the goal of both techniques is to reduce the number of variables while retaining as much of the original information as possible84,85. However, VFA assumes there is an unobserved variable (or latent factor) which exerts a causal influence upon the observed variables85-this assumption ultimately allows for a more simple interpretation of the underlying structure of the correlations between measured variables.

We performed this analysis using the scikit-learn python toolkit86. The dataset was first standardized and normalized using the scikit-learn pre-processing tools, then the VFA was performed using the appropriate functions from the scikit-learn decomposition module. The results are listed in Table S2.

Model description and results
We utilize a suite of simulations from the Hadley Centre Coupled Model 3 (HadCM3). HadCM3 is a coupled climate model comprised of a 3-dimensional dynamical atmosphere component with a resolution of 2.5° × 3.75° and 19 vertical levels, and an ocean model with a resolution of 1.25° × 1.25° and 20 vertical levels (see Valdes et al.87 for a detailed overview). These simulations cover the last 3.6 Ma and were run every 4 ka using a configuration of HadCM3 which includes a tiling of surface types within MOSES 2.1d88 and additional atmospheric and vegetation moisture stress parameterizations to better simulate a ‘Green Sahara’ during the mid-Holocene89. The model also includes several parameterizations to better resolve the ‘Cold-Pole Paradox’30: major parameterizations include modified cloud condensation nuclei density and cloud droplet effective radius, which better aligned the simulated meridional temperature gradient with proxy observations; adjustments to the ocean model to removed nonlinear instabilities; and the addition of sea ice, which was calculated on a zero-layer model and allowed for partial sea-ice coverage.

Despite these modifications, HadCM3 remains particularly biased toward warmth over parts of the Southern Ocean - comparison to COBE SST data90 demonstrates that HadCM3 simulates a substantially weaker temperature gradient over the AFS. Considering this bias, along with the limitation of a low spatial resolution, we use the model to examine whether a northward shift of the Southern Ocean is simulated during the MPT and whether this northward movement is associated with any change in the WRZ, rather than treating the model output as an exact picture of MPT climate.

We select 39 simulations from six categories: Pre-MPT Interglacials and Pre-MPT Glacials, MPT Interglacials and MPT Glacials, and Post-MPT Interglacials and Post-MPT Glacials. Each category is comprised of select model runs from within the specified interval and a complete list of these simulations can be found in Table S3.

We assess whether the selected model simulations capture shifts in SA precipitation that are consistent with δDp across the MPT. We focus on zonal winds and precipitation, and we use the line of zero WSC (WSC=0)91,92 as indicator of the northern edge of the AFS (i.e., the STF). We considered using the position of the 11 deg C isotherms as a marker of the AFS91,93,94), however Southern Ocean temperature biases in the model87 mean the isotherm may not actually correspond to the location of the front.

The average latitudinal position of WSC=0 across all model runs is 49.48°S - any model run with a WSC=0 placement north of 49.48°S was classified as northward-shifted and those with a WSC=0 placement south of 49.48°S was considered southward-shifted (Fig. 1b). Most Pre- and Post-MPT glacal runs were classified as northward-shifted, as are most MPT glacial and interglacial runs. Pre- and Post-MPT interglacial runs were largely categorized as southward-shifted.

To evaluate the dynamical and hydroclimate responses to simulated shifts in the STF, we subtract the average precipitation and wind fields of the southward-shifted models from the northward-shifted models (Fig. 1c). As expected, the westerly wind belt is shifted northward and strengthened in northward-shifted runs. These model runs simulate less terrestrial rainfall than the southward-shifted runs, particularly over the SRZ. However, the WRZ does expand zonally and experiences a slight increase in rainfall, and precipitation south of Africa increases substantially.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "• What are the noteworthy results?\nThis paper uses dD and d13C plant wax record from marine core ODP 1084 to trace the expansion and contraction of the\nWinter Rainfall Zone (WRZ) over South Africa during the Mid-Pleistocene Transition (1.2-0.7 Ma). The proxy results are\nfurther supported by model results from MadCM3 simulations which also show an increase in WRZ precipitation tied to\nnorthward migration of the subtropical front. The authors then make a case that the terrestrial environmental changes in\nhydroclimate and thus vegetation contributed to hominin turnover, specifically of Paranthropus.\n• Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work\nis not original, please provide relevant references.\nThis study (as illustrated by Figure 2) compiles a new dD and d13C record with other proxy data.\n• Does the work support the conclusions and claims, or is additional evidence needed?\nThe title suggests an integration of paleo-hydroclimate data with hominin records. I think more evidence was needed to\nstrengthen this relationship as it felt like the authors pointed to correlation in timing in a short paragraph but could have\nexplored the causation in more depth.\n• Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nGiven the role CAM plants play in the WRZ, I am concerned about how the lack of constraint on their relative abundance\ncomplicates the interpretation of the isotope wax records.\n• Is the methodology sound? Does the work meet the expected standards in your field?\nI am not an expect in these analyses but from my perspective I had no concerns or issues as this is a robust team of\ncoauthors and the relevant literature was cited.\n• Is there enough detail provided in the methods for the work to be reproduced?\nYes\nLine comments are included in the uploaded PDF.", "reviewer_2": "The authors present a new leaf wax hydroclimate record from ODP 1084 in the Benguela Upwelling System, integrating it\nwith other published datasets and model simulation to elucidate significant shifts in Southern Hemisphere climate system\nacross the Mid-Pleistocene Transition (MPT). They assert that these shifts have had a profound impact on the early human\nmigration in the South Africa. This study is interesting and contributes valuable insights into the climate evolution of the\nSouthern Hemisphere during the MPT. Some suggestions are provided below.\nThe core data, specifically the leaf wax δDp from ODP 1084, indicate a persistent negative trend before 0.9 Ma, reflecting an\nincrease in regional precipitation. Conversely, the δ13Cwax values were consistently positive, suggesting a deterioration in\nvegetation. These observations imply a northward shift of the Southern Hemisphere climate system prior to 0.9 Ma,\nassociated with the expansion of the Antarctic ice sheet and sea ice, followed by a southward shift after 0.9 Ma. While\nthe identification of this climatic phenomenon is compelling, the linkage to early human evolution appears somewhat\nspeculative.\nThe authors should delve more deeply into the mechanisms driving the transition of the Southern Hemisphere climate\nsystem around 0.9 Ma. Notably, global average SSTs have remained relatively stable in the long-term trend post-0.9 Ma\n(Clark et al., 2024, Science). The glacial stages of Marine Isotope Stages (MIS) 18, 16, and 14 appear less developed\ncompared to MIS 20, MIS 22, and MIS 24, suggesting a general warming trend in the global climate. However, sea level\nreconstructions (Rohling et al., 2014, Nature) indicate that sea levels during MIS 18, 16, and 14 are comparable to\nthose of MIS 22, with MIS 16 even being about 20 meters lower than MIS 22. This discrepancy implies that while the\nAntarctic ice sheet and associated sea ice may have shrunk during the mid-MPT, there was a significant expansion of the\nNorthern Hemisphere ice sheets post-0.9 Ma.\nCould it be that the entire climate zone shifted south after 0.9 Ma? An examination of the SST gradients between the\nhemispheres and related Northern Hemisphere data might provide further insights. Therefore, it is recommended that the\nauthors explore the relationship between global climate evolution and Antarctic climate evolution, and offer a revised\nperspective on the mechanisms underlying the MPT. Early human evolution should be considered as supplementary\nevidence rather than a primary conclusion.\nThere are several suggestions for improvements:\n1. Line 131-133 “SRZ. We therefore interpret the abrupt enrichment of the ODP 1084 δDp and δ13Cwax signals as\ncontractions of the winter rainfall zone and associated vegetation types after 0.9 Ma.”\nContraction and/or Migration?\n2. Fig. 1, Figure caption, “Dashed blue contours over the land denote the extent of the winter, yearly, and summer rainfall\nzones (WRZ, YRZ, and SRZ, respectively).” However, there is only two blue contours, should be three? Need more\ninformation for Fig 1b and c.\n3. Fig. 2. You should add the global SST records in Figure 2, such as the SST records from the Clark et al., 2024, since you\nsuggest your  D and  13C records are likely related to the long-term changes in SSTs.\n4. Fig. 2. The Figure Caption is confusing. For example，(n) is the ACC strength from Lamy et., 2024 ? (o) is the dust MAR\nrecord from ODP 1090?\n5. Line 139 “intensification of the westerlies (Fig. 2n, 1090 dust MAR, Martínez-Garcia et al., 2011)”. The author did not\npropose in the original paper that the dust MAR of ODP 1090 could be used as a proxy for the southern westerly winds.\n6. Line 141, “Changes included a reduction in Antarctic Circumpolar Current strength (Fig. 2n, Lamy et al., 2024)”, should be\nFig. 2o?", "reviewer_3": "Review of Rubbelke et al.,: “Southern Hemisphere Subtropical Front impacts\non Southern African Hydroclimate and early Hominins Across the Mid-Pleistocene Transition”.\nI have read this manuscript draft with interest and I especially appreciate the data-model integration attempt. However, I am\nuncertain if the data presented and the context presented sustains the claim made warranting publication in Nature comm.\nHere my main concerns:\n1. The HadCM3 results do not support that the WRZ expanded/contracted.\n2. The terrestrial sources areas/catchments of the material arriving at ODP site 1084 are not described/determined in detail.\n3. The climate-human interaction scenario hypothesised is lacking details making it a less convincing case.\nRegarding point 1:\nThe model simulates an increase in rainfall in the WRZ during northward shifts of the front but no expansion of this rainfall\nzone (Line 342-346). At the same time rainfall amounts decrease in the SRZ in those scenarios. The authors however use\nthe expansion of the WRZ before 0.9 Ma and a contraction thereafter to argue their case. As I see it does the model not\nsupport this hypothesis. On the contrary, if the WRZ did not expand the catchment areas to ODP 1084 (and cited\narcheological sites) would reside under SRZ influence the entire time being exposed to the dynamics of the SRZ . How is\nthis reconciled with the data and the hypothesized dynamics?\nRegarding point 2:\nIt is no new information that the transport of terrestrial material from land to marine core sites is a complex process,\nespecially in areas such as the BUS. However, the authors only dedicated line 102 to 107 to this topic. What are the actual\nsource area of the n-alkane components?\nWork on shorter time scales in this areas ( Chase, B.M., et al., 2019, Orbital controls on Namib Desert hydroclimate over the\npast 50,000 years: Geology, v. 47, p. 867–871, https:// doi .org /10 .1130 /G46334.1 ) has revealed contrasting results\nbetween marine and terrestrial records from the Namib desert and offshore. Lim et al., 2016; Shi et al., 2001 showed that\npollen from increasingly remote sources are included in the deposition during periods of intensified upwelling. Can changes\nin BUS upwelling dynamics over the MPT be an alternative explanation for the trands in the data presented or can that be\nrules out?\nRegarding point 3:\nThe “implications for Hominin Evolution paragraph” is kept rather short and speculative. We are in an era in which the\ngeoscience community has to collaborate closely with the archaeological one to address these topics/questions. Publishing\non that topic without having any co-author from that discipline as part of the team is not commendable. I suggest ground-\ntruthing the hypothesized climate impact on hominin evolution with experts from that field.\nMinor comments:\n• Fig. 1 captions b and c are reversed/mixed up.\n• Line 190: over what time frame does the shift in the data occur? That would be useful to state because it also related to the\nquestion of: On which timescales people would be affected by climate change?\n• Line 113: is that model based on work in the Sahara applicable to South Africa?"}, "author_response": "Author’s responses are in green.\nWe thank the reviewers for their constructive feedback and incorporate their input throughout the\npaper. Particularly useful were the comments regarding CAM plants, the HadCM3 simulations, and\nthe connection to human evolution. We’ve reframed our discussion of these points in the main text\nand our responses to specific comments are notated below:\nReviewer #1 (Remarks to the Author):\n• What are the noteworthy results?\nThis paper uses dD and d13C plant wax record from marine core ODP 1084 to trace the expansion\nand contraction of the Winter Rainfall Zone (WRZ) over South Africa during the Mid-Pleistocene\nTransition (1.2-0.7 Ma). The proxy results are further supported by model results from MadCM3\nsimulations which also show an increase in WRZ precipitation tied to northward migration of the\nsubtropical front. The authors then make a case that the terrestrial environmental changes in\nhydroclimate and thus vegetation contributed to hominin turnover, specifically of Paranthropus.\n• Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature? If the work is not original, please provide relevant references.\nThis study (as illustrated by Figure 2) compiles a new dD and d13C record with other proxy data.\n• Does the work support the conclusions and claims, or is additional evidence needed?\nThe title suggests an integration of paleo-hydroclimate data with hominin records. I think\nmore evidence was needed to strengthen this relationship as it felt like the authors pointed to\ncorrelation in timing in a short paragraph but could have explored the causation in more\ndepth.\nWe’ve restructured much of the text to emphasize the climate/ocean front story, with less of a focus\non the hominin impacts. As a whole, the discussion has been altered to reflect that this mechanism\nhelps explain terrestrial hydroclimate records and coheres with vegetation records in the tropics\n(Section 2.5), explain how this mechanism may have impacted hominins in the past (Section 2.6),\nand expand on the fact that this mechanism is impacting people in the modern and will likely\ncontinue to impact people in the future (Section 2.7).\n• Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\nGiven the role CAM plants play in the WRZ, I am concerned about how the lack of constraint\non their relative abundance complicates the interpretation of the isotope wax records.\nThis feedback was particularly important for our revisions, so we thank the reviewer for this\ncomment. The methods and main text have been updated to include a discussion of whether\nperforming a carbon-correction of dD is appropriate for this region. Upon revisiting the carbon-\ncorrecting method, we decided to use the raw, uncorrected dD_wax in our plots and statistical\nanalyses. We’ve also updated this section to include the use of n-alkane ratios to tentatively infer\nchanges in vegetation (see lines 133-146 in the updated main text, and 4.1.2 in the methods section\nfor more details).\n• Is the methodology sound? Does the work meet the expected standards in your field?\nI am not an expect in these analyses but from my perspective I had no concerns or issues as this is\na robust team of coauthors and the relevant literature was cited.\n• Is there enough detail provided in the methods for the work to be reproduced?\nYes\nAbstract\nGiven the role the HadCM3 simulation play in the interpretations of this\npaper, I’m surprised this is not mentioned in the abstract.\nThank you for pointing this out – we’ve updated the abstract to reflect our discussion of the\nHadCM3 runs.\n1. Introduction\nThe references given (1-3) are all in China. Given they are supposed to\nsupport a broad picture of hominin evolution, I think the authors should\ndiversify the geographic distribution of the references, including some from\nEastern Africa and the Middle East given that these regions have robust\nfossil and archaeological records, and were key corridors for hominin\nmigration and movement.\nThis is a good point and we’ve updated the citations to expand the geographic distribution of the\nreferences. Please see the first paragraph of the main text.\nLines 70-71: Where are these data published? Citation needed….\nCitations have been updated, please see line 71 in the text.\nLines 82-83: Given the broad scope of this journal, I think description of\nwhat the AL is and why it impacts climate would be helpful context for\nreaders. By contrast, a compelling case is made for both the mechanism\ncontrolling and the impact of the AFS… as written the AL feels like an\nafterthought so if it’s essential to the story, that should be clarified.\nTaking this into account, we’ve revised the second-to-last paragraph of the introduction,\nparticularly lines 85-90. Please let us know if this needs to be addressed or clarified further.\n2. Results and Discussion\nLine 102: Coordinates for ODP 1084 are only reported here to the minute…\nthe location must be more precise than that! Please check… a quick Google\nsearch lead me to this- 25°30.8345´S, 13°1.6668´E.\nThanks for pointing this out, we’ve updated the text accordingly.\nLines 104-105: Tighten up the word choices in this sentence… Instead of\n“results” what about “is sourced”. Consider “integrate” rather than\n“represent”. Both would clarify what you mean and more accurately\nrepresent the relationships.\nTo address this comment, as well as comments from other reviewers, we’ve revised much of this\nparagraph to include a more thorough discussion of the leaf wax source area (Lines 110-118). We’ve\nalso updated Figure 1 to highlight the extent of the Orange River Basin.\nLine 113-114: “ empirically derived constraints on εp−w…” I’d argue that this isn’t\nclear what these are and thus their significance is unclear as written.\nWe’ve updated the methods section to further discuss the carbon-correction. This section (lines\n120-132) have been revised to address the reviewer’s concerns about the influence of CAM plants.\nLine 129-133: I’m concerned about the complicating factor of the CAM\nvegetation in the WRZ on the isotope signature since the authors mention is\nis a main feature of the WRZ vegetation. What percentage of the biomass is\nCAM? How is this accounted for in the analyses since CAM plants can have\nisotopic signatures that can span the C3 and C4 zones and might complicate\nthe interpretation of dD? I just found that this is addressed in section 4.3\nand I definitely appreciate this additional information but am still concern\nabout how CAM plants impact the record given the lack of constrain on the\nrelative abundance.\nAs mentioned above, we’ve revised lines 120-132 to address whether or not the standard method of\ncarbon-correcting dD is appropriate for this region. Upon revisiting the carbon-correcting method,\nwe decided to us the raw, uncorrected dD_wax in our plots and statistical analyses. We’ve also\nupdated this section to include the use of n-alkane ratios to tentatively infer changes in vegetation\n(see lines 133-146 in the updated main text, and 4.1.2 in the methods section).\nLines 142-144: I think it would be helpful to add in discussion of what the\nexpansion of the WRZ results in depleted dDp. This could be added into the\nIntroduction in lines 93-97 or developed in this section but that linkage has\nnot been thoroughly made in my opinion.\nThe depletion of winter rainfall is largely due to the stratiform nature of the rainfall and a more\ndeuterium depleted vapor source. This has been updated in lines 185-187.\nLines 144-147: These feel redundant with the previous sentence. Can this be\ncombined into one that points to the hypothesized relationship and the\nultimate confirmation through the application of the HadCM3 model?\nTo address this comment, and feedback from the other reviewers, much of this section has been\nrestructured. Specifically, lines 227-235 discuss our use of the model to confirm our hypothesis.\n2.3 Antarctic Front Migration Mechanism\nIssue in lines 147 about significance of model result given small spatial\nextent… I appreciate the authors transparency about the potential impact of\nthe resolution of the model given the limited spatial extent. Is there some\nway to test the sensitivity to a change of only a few grid cells? Are there\nother applications of this or similar models that are significant? How might\nyou test/validate whether or not this truly is a statistically significant impact\non climate?\nWe’ve expanded our discussion of how/why we use the model (Lines 147-158). We mention that\ndue to the poor spatial resolution and substantial temperature bias over the Southern Ocean, the\nentire system in the model is displaced poleward. We emphasize that we are testing the sensitivity\nof the model’s precipitation to perturbations in the model’s ocean and atmosphere. We’ve updated\nlines 227-235 to reflect this, and we also call for further work examining this mechanism in models\nwith higher resolution and the ability to trace water isotopologues.\nLine 151: I don’t see where SO is previously defined but I’m guessing it\nstands for Southern Ocean??\nThanks for catching this – yes it is the abbreviation for Southern Ocean. We’ve updated the text to\ndefine it in the introduction.\nLines 169-169: Is the change is dD just a function of change in seasonality\n(ie when the rain is falling) or is there also an effect of how much rain is\nfalling? Given the generally slow sedimentation rates in marine cores, I\nwould be curious for more justification for this being a seasonal signal vs an\namount effect. Can you explain more about how you ruled out the latter?\nWhile a slight isotopic gradient exist within the SRZ, as precipitation moves inland from the coast\nand undergoes fractionation by both the amount and continental effects, integration of leaf waxes\nfrom the entire Orange River drainage basin means any gradient is consolidated into a single\nregional signal which is enriched in deuterium, relative to the WRZ. Work with other\npaleohydroclimate archives from the yearly rainfall zone (at the boundary between the SRZ and\nWRZ) have shown that long-term changes in the proportion of summer to winter rainfall are\nrecorded (e.g. Braun et al. 2017, Harris et al. 2010).\nPlease see lines 180-187 in the updated main text.\n2.5 Implications for Hominin Evolution\nBased on the title of this paper, I anticipated this link would be robustly\ndeveloped. There is really just one paragraph (lines 190-202) that talks\nabout the links with hominins. I think the proposed connection with P. boisei\nand P. robustus is an interesting connection but given the lateral shift of the\nWRZ and SRZ, the connection to why this would have cause extinction needs\nto be further developed.\nThis was a common concern among the reviewers and one we appreciate. We agree that our\ndiscussion of hominin evolution is rather speculative. We’ve reframed our discussion a bit to reflect\nthat this mechanism helps explain terrestrial hydroclimate records and coheres with vegetation\nrecords in the tropics (Section 2.5), explain how this mechanism may have impacted hominins in\nthe past (Section 2.6), and expand on the fact that this mechanism is impacting people in the\nmodern and will likely continue to impact people in the future (Section 2.7).\nLines 190-191: Citations are definitely needed here because others would\nargue that variability (rather than rapid linear change) drove hominin\nevolution and that even catastrophic events like the Toba eruption had\nessentially no impact on hominin population size (see Yost et al. 2018).\nWe’ve updated some of the citations and updated the section to reflect our position that this\nmechanism was occurring during the MPT, and given the impact of this mechanism on modern\npopulations, it’s not unreasonable to speculate that changes in water availability and vegetation\ncover may have impacted past hominin populations. We emphasize that future work is needed to\nmore deeply examine the impact this mechanism may have on different hominin groups.\nLines 198 and 201: P. robustus has an extra “s” in the text.\nThanks for catching this -It’s been corrected in the text.\n3. Conclusion\nIt’s unclear to me what is meant by the “observed gap in the hominin fossil\nrecord”. Is this something specific to this region? This is also mentioned in\nlines 66-67 and again needs more support as this is highly contested.\nThere is an interval of time during which no anthropological remains have been recovered from\nSouthern Africa and this has been referred to as a “gap in the fossil record” by a few studies (\nProfico et al. 2016, Hu et al. 2023, Vincenzo et al, 2023).\nHowever, we’ve revised this as “Our records supplement the existing terrestrial records… and may\nbe useful for contextualizing the evolutionary and dispersal patterns of early hominins in Southern\nAfrica” (see lines 330-332).\n4. Methods\n4.2 Spectral Analyses\nI’m really curious for the authors’ thoughts on why the ODP 1084 dD record\nis so smooth and doesn’t show orbital cyclicity. This seems like it could be an\nreally interesting story that should be highlighted rather than buried into this\nsection. How does this compare with other southern hemisphere records\n(only an example from eastern Africa is cited)? Some important ideas are\nrolled into section 4.3… together these two sections I believe deserve to be\nhighlighted in this study.\nWe’ve included much of section 4.3 in the main text (see our above comments about CAM plants)\nand we’ve updated lines 163-169 to discuss orbital cyclicity.\nEssentially, we discuss that our periodogram does indicate orbital influence of the 400-kyr and 100-\nkyr bands, but confidence is low. Given the prominence of orbital signals in other African records,\nwe expect the lack of signal in our record is due to low sampling resolution.\n4.4 Varimax Factor Analysis\nThis is an interesting choice and not something I’m familiar with. I would be\ncurious to hear more about why the assumption of an unobserved variable is\nthe appropriate choice here.\nWe chose to use VFA because there are several latent variables that are not constrained or are\npoorly constrained by the data. These latent variables could include biotic interactions, pCO2\nlevels (for which there are a number of proxies, each with different degrees of uncertainty), etc.\nFigures\nFigure 1: Fig 1b and 1c are flipped relative to the figure caption…. For the\nbar and wicker plot, can you explain the n = x further in the caption? Are\nthese module runs? Something else?\nN is the number of model runs used for each time interval. This has been updated in the figure 1\ncaption.\nFigure 2- Multiple records are not labeled correctly when comparing the\ncaptions to the figure (2o, r)… this should be closely double checked for\nfurther issues.\nFixed -see updated caption.\nFigure 2a and 2b: Since most of this figure is compiling previous results, I\nthink it would be helpful to emphasize that these are the new data from this\nstudy.\nFixed -see updated caption.\nFigure 2c: I understand that this is using the latitude of “diatom mats”…\ncould you clarify what a laminated mat is? How are these intervals of\nlaminated (diatomite?) to track the position of the Arctic Polar Front?\nDiatom mats are essentially dense/thick layers of biogenic opal from diatom (plankton) shells.\nDeposition of these mats tracks the location of maximum nutrient availability. As the APF moves\nnorth or south, the nutrients and plankton within those waters also move. We’ve revised the figure\ncaption to include this clarification, as well as correcting mistakes in the caption.\n2d: missing from the figure caption… is it ODP 1081?\nFixed -see updated caption.\n2p: P. bosei isn’t on this figure and should be given the discussion in section\nFixed -see updated caption.\nFigure 3:Key is needed for the colored lines in a and b…\nFixed -see updated caption.\nReferences\nThere are multiple proper nouns that should be capitalized in the titles of\nreferences.\nLine 496 check capitalization\nLine 502- China should be capitalized as should the time period\nLine 506- looks like capitalization is needed in the title of all the proper\nnouns in the papers… I’m not going to mention others from here on but all\nshould be checked for issues.\nThank you for catching all these issues -I think this was an issue with the pdf compilation in Latex. It\nshould be fixed now!\nReviewer #2 (Remarks to the Author):\nThe authors present a new leaf wax hydroclimate record from ODP 1084 in the Benguela Upwelling\nSystem, integrating it with other published datasets and model simulation to elucidate significant\nshifts in Southern Hemisphere climate system across the Mid-Pleistocene Transition (MPT). They\nassert that these shifts have had a profound impact on the early human migration in the South\nAfrica. This study is interesting and contributes valuable insights into the climate evolution of the\nSouthern Hemisphere during the MPT. Some suggestions are provided below.\nThe core data, specifically the leaf wax δDp from ODP 1084, indicate a persistent negative trend\nbefore 0.9 Ma, reflecting an increase in regional precipitation. Conversely, the δ13Cwax values were\nconsistently positive, suggesting a deterioration in vegetation. These observations imply a\nnorthward shift of the Southern Hemisphere climate system prior to 0.9 Ma, associated with the\nexpansion of the Antarctic ice sheet and sea ice, followed by a southward shift after 0.9 Ma. While\nthe identification of this climatic phenomenon is compelling, the linkage to early human evolution\nappears somewhat speculative.\nThe authors should delve more deeply into the mechanisms driving the transition of the Southern\nHemisphere climate system around 0.9 Ma. Notably, global average SSTs have remained relatively\nstable in the long-term trend post-0.9 Ma (Clark et al., 2024, Science). The glacial stages of Marine\nIsotope Stages (MIS) 18, 16, and 14 appear less developed compared to MIS 20, MIS 22, and MIS\n24, suggesting a general warming trend in the global climate. However, sea level reconstructions\n(Rohling et al., 2014, Nature) indicate that sea levels during MIS 18, 16, and 14 are comparable to\nthose of MIS 22, with MIS 16 even being about 20 meters lower than MIS 22. This discrepancy\nimplies that while the Antarctic ice sheet and associated sea ice may have shrunk during the mid-\nMPT, there was a significant expansion of the Northern Hemisphere ice sheets post-0.9 Ma.\nCould it be that the entire climate zone shifted south after 0.9 Ma? An examination of the SST\ngradients between the hemispheres and related Northern Hemisphere data might provide further\ninsights. Therefore, it is recommended that the authors explore the relationship between global\nclimate evolution and Antarctic climate evolution, and offer a revised perspective on the\nmechanisms underlying the MPT. Early human evolution should be considered as supplementary\nevidence rather than a primary conclusion.\nWe agree that our discussion of hominin evolution is rather speculative. We’ve reframed our\ndiscussion a bit to reflect that this mechanism helps explain terrestrial hydroclimate records and\ncoheres with vegetation records in the tropics (Section 2.5), explain how this mechanism may have\nimpacted hominins in the past (Section 2.6), and expand on the fact that this mechanism is\nimpacting people in the modern and will likely continue to impact people in the future (Section 2.7).\nWe’ve also expanded our discussion of the large-scale dynamical mechanisms behind the\nhydroclimate shift. Namely, we explain that cooling in the Southern Hemisphere extratropics\nallowed for expansion of sea ice, reorganization of ocean circulation, and overall shift of the\nmeridional temperature gradient. Changes to the gradient would’ve driven changes in atmospheric\ncirculation that ultimately resulted in hydroclimate shifts. Please see our updated sections 2.3 and\n2.4.\nThere are several suggestions for improvements:\n1. Line 131-133 “SRZ. We therefore interpret the abrupt enrichment of the ODP 1084 δDp and\nδ13Cwax signals as contractions of the winter rainfall zone and associated vegetation types after\n0.9 Ma.”\nContraction and/or Migration?\nWe only have evidence thus far to suggest a contraction since wehaven’tconstrained the southern\nedge of this zone’s boundary, which would be far over the Southern Ocean.\n2. Fig. 1, Figure caption, “Dashed blue contours over the land denote the extent of the winter, yearly,\nand summer rainfall zones (WRZ, YRZ, and SRZ, respectively).” However, there is only two blue\ncontours, should be three? Need more information for Fig 1b and c.\nWe’ve updated this caption to reflect that the blue contours denote the extent of the winter and\nsummer rainfall zones, and the yearly rainfall zone is a transitional zone which falls between them.\nWe’ve also updated this caption to provide further information on panels b and c.\n3. Fig. 2. You should add the global SST records in Figure 2, such as the SST records from the Clark\net al., 2024, since you suggest your D and 13C records are likely related to the long-term changes\nin SSTs.\nWe’ve updated this figure and our Varimax factor analysis to include changes in the Southern\nHemisphere temperature gradient.\n4. Fig. 2. The Figure Caption is confusing. For example，(n) is the ACC strength from Lamy et., 2024\n? (o) is the dust MAR record from ODP 1090?\nThis figure has been updated to include the SH temperature gradient and no longer includes the\ntemperature record from ODP 1087 or the record from Lamy et al. We’ve updated the caption\naccordingly.\n5. Line 139 “intensification of the westerlies (Fig. 2n, 1090 dust MAR, Martínez-Garcia et al., 2011)”.\nThe author did not propose in the original paper that the dust MAR of ODP 1090 could be used as a\nproxy for the southern westerly winds.\nAs we mention above, we have revised sections 2.3 and 2.4, so this line is no longer in the text.\n6. Line 141, “Changes included a reduction in Antarctic Circumpolar Current strength (Fig. 2n,\nLamy et al., 2024)”, should be Fig. 2o?\nThis record (as well as the SST record from ODP 1087) has been removed from the figure to make it\neasier to follow.\nReviewer #3 (Remarks to the Author):\nReview of Rubbelke et al.,: “Southern Hemisphere Subtropical Front impacts\non Southern African Hydroclimate and early Hominins Across the Mid-Pleistocene Transition”.\nI have read this manuscript draft with interest and I especially appreciate the data-model\nintegration attempt. However, I am uncertain if the data presented and the context presented\nsustains the claim made warranting publication in Nature comm.\nHere my main concerns:\n1. The HadCM3 results do not support that the WRZ expanded/contracted.\n2. The terrestrial sources areas/catchments of the material arriving at ODP site 1084 are not\ndescribed/determined in detail.\n3. The climate-human interaction scenario hypothesised is lacking details making it a less\nconvincing case.\nRegarding point 1:\nThe model simulates an increase in rainfall in the WRZ during northward shifts of the front but no\nexpansion of this rainfall zone (Line 342-346). At the same time rainfall amounts decrease in the\nSRZ in those scenarios. The authors however use the expansion of the WRZ before 0.9 Ma and a\ncontraction thereafter to argue their case. As I see it does the model not support this hypothesis.\nOn the contrary, if the WRZ did not expand the catchment areas to ODP 1084 (and cited\narcheological sites) would reside under SRZ influence the entire time being exposed to the\ndynamics of the SRZ . How is this reconciled with the data and the hypothesized dynamics?\nThank you for this feedback. We’ve reframed our discussion of the model to emphasize that we are\ntesting the sensitivity of rainfall to changes in the ocean fronts, rather than treating the model\noutput as an exact snapshot of MPT climate. Comparing the PI to modern SSTs, HadCM3 is biased\ntoward warmth over the Southern Ocean (particularly south of Africa), resulting in a shallower AFS.\nWe’ve added Figure S2 to illustrate this bias in the model. As the Southern Ocean is essentially\nmore poleward in the model, the response of the hydroclimate system to changes in the SO are\nsimilarly displaced poleward. We do however show a similar mechanism in the model with sea-ice\nexpansion in the Southern Ocean and the associated change in the MTG. If this warm bias in the\nmodel were removed we would likely see the expected response.\nWe’ve expanded our discussion of how/why we use the model (Lines 147-158). We mention that\ndue to the poor spatial resolution and substantial temperature bias over the Southern Ocean, the\nentire system in the model is displaced poleward. We emphasize that we are testing the sensitivity\nof the model’s precipitation to perturbations in the model’s ocean and atmosphere. We’ve updated\nlines 227-235 to reflect this, and we also call for further work examining this mechanism in models\nwith higher resolution and the ability to trace water isotopologues.\nRegarding point 2:\nIt is no new information that the transport of terrestrial material from land to marine core sites is a\ncomplex process, especially in areas such as the BUS. However, the authors only dedicated line\n102 to 107 to this topic. What are the actual source area of the n-alkane components?\nWe’ve updated the text to extend our discussion of the source area and added the Orange River\nBasin to Figure 1a.\nWork on shorter time scales in this areas ( Chase, B.M., et al., 2019, Orbital controls on Namib\nDesert hydroclimate over the past 50,000 years: Geology, v. 47, p. 867–871, https:// doi .org /10\n.1130 /G46334.1 ) has revealed contrasting results between marine and terrestrial records from the\nNamib desert and offshore. Lim et al., 2016; Shi et al., 2001 showed that pollen from increasingly\nremote sources are included in the deposition during periods of intensified upwelling. Can changes\nin BUS upwelling dynamics over the MPT be an alternative explanation for the trends in the data\npresented or can that be rules out?\nThe results from Chase et al. are very interesting and raise important questions regarding transport\nvs signal. We agree that if the signal indicating change in rainfall seasonality was restricted to only\nthe marine core, the potential change in transport pathways would need additional consideration.\nHowever, given the broad source area and the agreement of our marine signal with terrestrial\nrecords which reflect factors such as local herbivory and vegetation (see Section 2.5), we are\nconfident that our signal most likely reflects a regional reorganization of hydroclimate rather than\njust a change in transport to marine depositional settings.\nRegarding point 3:\nThe “implications for Hominin Evolution paragraph” is kept rather short and speculative. We are in\nan era in which the geoscience community has to collaborate closely with the archaeological one\nto address these topics/questions. Publishing on that topic without having any co-author from that\ndiscipline as part of the team is not commendable. I suggest ground-truthing the hypothesized\nclimate impact on hominin evolution with experts from that field.\nThis was a common concern among the reviewers and one we appreciate. We agree that our\ndiscussion of hominin evolution is rather speculative. We’ve reframed our discussion a bit to reflect\nthat this mechanism helps explain terrestrial hydroclimate records and coheres with vegetation\nrecords in the tropics (Section 2.5), explain how this mechanism may have impacted hominins in\nthe past (Section 2.6), and expand on the fact that this mechanism is impacting people in the\nmodern and will likely continue to impact people in the future (Section 2.7).\nGiven the pace of anthropogenic climate change and the degree to which communities are being\ninfluenced by extreme weather/climate events, an improved understanding of how large-scale\nhemispheric temperature gradients impact regional-scale hydroclimate is desperately needed. This\npaper exemplifies the variety of timescales across which this mechanism may operate. The\ndiscussion of hydroclimate dynamics, as well as the discussion of how these dynamics impact\nboth landscape and people (past, present and future) may be of interest to several disciplines,\nincluding paleo- and modern climatologists, archaeologists, ecologists, policy researchers, etc.,\nmaking Nature Communications a great avenue for sharing this work.\nMinor comments:\n• Fig. 1 captions b and c are reversed/mixed up.\nWe’ve revised this figure and updated the caption accordingly.\n• Line 190: over what time frame does the shift in the data occur? That would be useful to state\nbecause it also related to the question of: On which timescales people would be affected by\nclimate change?\nAs mentioned above, we now include a discussion of how this mechanism is impacting modern\npopulations in Southern Africa. In the modern, the hydroclimate response to perturbations of the\nHadley cell was rapid (2015-2017).\n• Line 113: is that model based on work in the Sahara applicable to South Africa?\nThe version of HadCM3 utilised here does include the recent updates that simulate a ‘Green\nSahara’. However, we emphasize that we are testing the sensitivity of the model’s precipitation to\nperturbations in the model’s ocean and atmosphere. Additionally, this model has been used for\nglobal studies (e.g https://www.nature.com/articles/s41586-023-05712-6) and should be\napplicable to Southern Africa."}, {"round": 2, "reviewer_comments": {"reviewer_1": "In the initial draft, I raised concerns about the following two areas:\n1) Is potential complication from CAM plants in the interpretation of the isotope records\n2) The lack of meaningful connections with the hominin record\nI appreciate the authors’ work trying to address both areas. For the first point, I think this manuscript is definitely improved. In\nthe second area, I still have concerns.\n1) The revision to the methods and text addressed by concerns about CAM plants. I sounds lines 120-133 really helpful and\nthe Methods sited on line 131 the pointed to the use of the ratios between n-alkanes like a good solution to get that this issue\nin a quantitative way. One thing that did come up for me related to the core itself is there is not information/citation provided\npointing towards the original age model for ODP 1084. I’m wondering if this is standard in the ocean drilling community (it’s\nnot in the terrestrial coring community!)? In particular, in section 2.2 where the low sampling resolution is evoked to explain\nthat lack of orbital-scale variability (lines 160-162), not having any information about what the sampling resolution for this\ncore actually is really hinters the ability to asses the chronological constraints on the types of timescales that might resolve\nthings like increase climate variability comparable to records such as those cited in lines 279-282. Also related to\nunderstanding better where on land this core is sampling, I think it would be helpful to add the drainage basin on the Orange\nRiver to figure 1a and tighten up some of the text in section 2.1(lines 112-114…. I’m assuming by “sediments from the\nwatershed enter the water column…” you are describing the delta and “material is carried…” is referring to suspended\nsediment from the Orange River. In short, I think some minor revisions to this area are still needed but the main issue\nhas been addressed.\n2) In terms of the connection with hominins/humans, the connection still feels forced. I like the new title and the revised\nabstract! I think this is much more in line with what this paper does really well and where it’s main story lies. However, the\ntransition from those two elements to the first paragraph where hominin evolution is introduced (lines 063-064) feels really\nabrupt! That’s the first time it’s mentioned and then it goes on to be a substantial thread through line 077. I fear that in trying\nto appease the reviewers, the authors lost their voice and the narrative of the story. I think that the point about how this might\nbe impactful to people in the future is an interesting one. The “Day Zero Drought” was a really dramatic example of modern\nvulnerability to climate change in Cape Town. However, given the low-resolution of the sampling, this type of event would be\ncompletely missed by the ODP 1084 record! I suggest keeping the reference to the “Day Zero Drought” and then enhancing\nthat by pointing out this event, so impactful to us today, is far less extreme and long-lasting that what is recorded in the\ncore…. that is truly noteworthy (and scary!). To summarize, this paper is robust in it’s treatment of the climate/ocean story\n(that’s it’s strength!) but if hominins are going to remain in here (appreciate the expanded references 1-15) and humans\nbrought in, the text needs reworking to make this come through coherently.", "reviewer_2": "I appreciate the authors' efforts to address the reviewers' comments. I would like to recommend acceptance of this\nmanuscript.", "reviewer_3": "While the details presented in the manuscript have increased and improved the paper I am not convinced about the novelty\nof the work that justifies publication in this journal.\nIn the revised version the interpretations are less based/ or supported on/by the model. As I pointed out in my first review\n(see below point 1) the model did not support a shift in the rainfall zones. Now the authors admit/state that there are model\nbiases: “…due poor spatial resolution and substantial temperature bias over the Southern Ocean, the entire system in the\nmodel is displaced poleward.”\nRegarding point 1:\nThe model simulates an increase in rainfall in the WRZ during northward shifts of the front but no expansion of this rainfall\nzone (Line 342-346). At the same time rainfall amounts decrease in the SRZ in those scenarios. The authors however use\nthe expansion of the WRZ before 0.9 Ma and a contraction thereafter to argue their case. As I see it does the model not\nsupport this hypothesis. On the contrary, if the WRZ did not expand the catchment areas to ODP 1084 (and cited\narcheological sites) would reside under SRZ influence the entire time being exposed to the dynamics of the SRZ . How is\nthis reconciled with the data and the hypothesized dynamics?\nIf the model does not support the hypothesis, neither does it have the spatial resolution that is needed for an area like SA\n(where large spatial heterogeneity exists on a small scale) nor is it isotope-enabled to help explain the isotope interpretation.\nHence why even use the model result at all?\nSecond, the human-climate link over the MPT instead of exploring and expanding on that it was shortened and put on the\nside. Here, the novelty for publication in Nat com could have arisen from if that link was explored in more depth and in\ncollaboration with experts from the other field.\nOther comments on the content:\nLine 206 onwards: The authors fail to explain what the isotopic signal of the rainfall in the area of the rivers they are\napparently tracing is driven by? In areas where different moisture sources overlap, like in South Africa, isotope distributions\nare affected by many processes and not easily to interpret, certainly not by fixed 'end-members'. One of the best isotope\nmaps for South Africa is found in West et al. (Journal of Geochemical Exploration, 2014,\ndoi.org/10.1016/j.gexplo.2014.06.009) which is very similar to the gridded data by Bowen et al. (2003). For instance, what\nhas been found for the Orange river catchment is that an increase of Atlantic moisture (not westerly related) leads to isotopic\nenrichment in the Late Holocene (Arnault et al., 2022, DOI: 10.1175/JCLI-D-22-0041.1).\nHow can the dDwax record be reconciled with those findings and modern isotope data from the area? Can the “observed\nrelatively abrupt enrichment in δDwax record over the MPT at ∼0.85-0.75 Ma derived from increase in Atlantic summer\nmoisture rather than an extension of the SRZ?\nLine 211 and S7: When the calculated Norm31−Norm33 approaches zero, it indicates that the normalized concentrations of\nC31 and C33C n-alkanes are nearly equal. This could reflect homogeneous plant inputs, minimal environmental change, or\na mixed vegetation signal. The result should be contextualized with other proxies to fully understand its implications. In\nsome cases, post-depositional processes like microbial degradation or diagenesis could also alter the n-alkane distribution,\npotentially leading to equalized concentrations and a Norm31−Norm33 value close to zero. How are the carbon preference\nindex (CPI) in that interval?\nLine 302: “Some sort of vegetation shift”- that does not sound like a confident statement being convinced by their own\ninterpretation."}, "author_response": "We appreciate the reviewers’ careful and thoughtful feedback and address their concerns\nbelow. The changes have greatly improved the manuscript, and allowed us to clarify some key\nfeatures of the study:\nWe have updated the introduction and section 2.7 to further emphasize the relevance of this\nstudy to modern events, such as the Day-Zero drought. Additional modifications to the main\ntext include references to the ship-board age model and explicitly discussing the sampling\nresolution we use. We also wrote an additional section of the SI in which we explain that\ndespite the biases inherent in all models, they are useful for examining causality and\ndynamical mechanisms of the climate system.\nReviewer #1 (Remarks to the Author):\nIn the initial draft, I raised concerns about the following two areas:\n1) Is potential complication from CAM plants in the interpretation of the isotope records\n2) The lack of meaningful connections with the hominin record\nI appreciate the authors’ work trying to address both areas. For the first point, I think this\nmanuscript is definitely improved. In the second area, I still have concerns.\n1) The revision to the methods and text addressed by concerns about CAM plants. I sounds lines\n120-133 really helpful and the Methods sited on line 131 the pointed to the use of the ratios\nbetween n-alkanes like a good solution to get that this issue in a quantitative way. One thing that did\ncome up for me related to the core itself is there is not information/citation provided pointing\ntowards the original age model for ODP 1084. I’m wondering if this is standard in the ocean drilling\ncommunity (it’s not in the terrestrial coring community!)?\nWhile in the terrestrial community age models tend to be redone often, for ODP cores we tend\nto refer to the original sources for the age models. But, the reviewer is right that these should\nhave been cited. We’ve updated the text to include references to the age model derived from\nshipboard magnetostratigraphy as well as in some cases benthic foraminifiera oxygen isotope\nstratigraphy, and nannofossil datums (see lines 110-111 in the updated text). Additionally, we\ncite the studies which further refine the age models for these sites (e.g. Etourneau et al.).\nIn particular, in section 2.2 where the low sampling resolution is evoked to explain that lack of\norbital-scale variability (lines 160-162), not having any information about what the sampling\nresolution for this core actually is really hinters the ability to asses the chronological constraints on\nthe types of timescales that might resolve things like increase climate variability comparable to\nrecords such as those cited in lines 279-282.\nThe reviewer makes a great point here – if we want to get at orbital scale resolution, we need a\nhigher resolution of sampling. This would be a great target for future analyses, and is\nsomething the PI plans to undertake in future work. In terms of sampling resolution, in section\n2.2 we point to Figure S1, which graphically demonstrates our sampling resolution – and that it\ncannot clarify orbital-scale variability.\nWe feel that the primary argument in this paper is about lower frequency changes in the\nocean-atmosphere system in the Southern Ocean.\nThis focus is supported by multiple other published sources – for instance, the Clark et al 2024\nsouthern hemisphere tropics – extratropics gradient shows strong low-frequency changes\n(lower than Milankovich frequencies) during the interval between 2 and 1 Ma. These are also\nshown in a new record of ACC changes (Starr et al., 2025, we have now cited this in the text as\nthis is a brand new paper! https://www.science.org/doi/10.1126/sciadv.adp1692).\nOur central argument in the paper is that the mechanistic changes responsible for these past\nlow-frequency changes are similar to hypothesized mechanisms behind future changes,\nsuggesting that we can get ‘state shifts’ to persistent drier conditions in Southern Africa as a\nresult of changes in global boundary conditions (thanks to the reviewer for pointing out how to\nhelp with this framing!)\nAlso related to understanding better where on land this core is sampling, I think it would be helpful\nto add the drainage basin on the Orange River to figure 1a and tighten up some of the text in section\n2.1(lines 112-114…. I’m assuming by “sediments from the watershed enter the water column…” you\nare describing the delta and “material is carried…” is referring to suspended sediment from the\nOrange River. In short, I think some minor revisions to this area are still needed but the main issue\nhas been addressed\nThank you for this feedback – the Orange River watershed was added to Figure 1a during the\nprevious round of revisions, but we’ve made it more bold in the revised figure. We’ve updated\nthe language to further clarify that sediment is entering the water column via the Orange River\ndelta.\n2) In terms of the connection with hominins/humans, the connection still feels forced. I like the new\ntitle and the revised abstract! I think this is much more in line with what this paper does really well\nand where it’s main story lies. However, the transition from those two elements to the first\nparagraph where hominin evolution is introduced (lines 063-064) feels really abrupt! That’s the first\ntime it’s mentioned and then it goes on to be a substantial thread through line 077. I fear that in\ntrying to appease the reviewers, the authors lost their voice and the narrative of the story. I think that the point about how this might\nbe impactful to people in the future is an interesting one. The “Day Zero Drought” was a really dramatic example of modern\nvulnerability to climate change in Cape Town. However, given the low-resolution of the sampling, this type of event would be\ncompletely missed by the ODP 1084 record! I suggest keeping the reference to the “Day Zero\nDrought” and then enhancing that by pointing out this event, so impactful to us today, is far less\nextreme and long-lasting that what is recorded in the core…. that is truly noteworthy (and scary!). To\nsummarize, this paper is robust in it’s treatment of the climate/ocean story (that’s it’s strength!) but\nif hominins are going to remain in here (appreciate the expanded references 1-15) and humans\nbrought in, the text needs reworking to make this come through coherently.\nWe agree that this section felt a bit awkward -we’ve updated the first paragraph to more\nsmoothly transition from introducing the MPT, explaining some of the evidence that suggests\nlarge environmental changes, and then moving into paragraph 2 (formerly paragraph 3 in the\nlast draft) introducing the AFS mechanism.\nWe kept section 2.6 (Implications for Hominin Evolution) because we do think that this record\nis important for contextualizing early hominin evolution. While none of the authors are\narchaeologists or anthropologists, we hope that by keeping this section in the paper, we\ninspire another team to tackle this question and further our understanding of hominin-\nenvironment interactions during the MPT.\nWe’ve updated section 2.7 (Implications for Future Climate) to further emphasize the\nanalogous nature of the mechanisms for Day Zero and the MPT hydroclimate shift, and we call\nfor future work which examines whether the relatively short-lived, though intense, drought in\nSouthern Africa has the potential to become the norm in future climate regimes.\nReviewer #2 (Remarks to the Author):\nI appreciate the authors' efforts to address the reviewers' comments. I would like to recommend\nacceptance of this manuscript.\nReviewer #3 (Remarks to the Author):\nWhile the details presented in the manuscript have increased and improved the paper I am not\nconvinced about the novelty of the work that justifies publication in this journal.\nIn the revised version the interpretations are less based/ or supported on/by the model. As I pointed\nout in my first review (see below point 1) the model did not support a shift in the rainfall zones. Now\nthe authors admit/state that there are model biases: “…due poor spatial resolution and substantial\ntemperature bias over the Southern Ocean, the entire system in the model is displaced poleward.”\nWe appreciate the reviewer’s concerns about models. However, we think that there are some\nkey aspects of climate models that should be clarified, since they may be resulting in some\nconfusion on the part of the reviewer. Almost all state of the art climate models are actually\nbiased, especially when it comes to regional-scale. This is because of the use of\nparameterizations in the atmosphere and ocean, and the computational cost of running global\nmodels. We have a new section in our SI that summarizes the argument below, but we include\nit here as well to make it easier to read and consider when reviewing.\nKim, H., Kang, S.M., Takahashi, K., Donohoe, A. and Pendergrass, A.G., 2021.\nMechanisms of tropical precipitation biases in climate models. Climate Dynamics,\n56(1), pp.17-27.\nPendergrass, A.G., Gleckler, P.J., Leung, L.R. and Jakob, C., 2020. Benchmarking\nsimulated precipitation in earth system models. Bulletin of the American\nMeteorological Society, 101(6), pp.E814-E816.\nLuo, F., Ying, J., Liu, T. and Chen, D., 2023. Origins of Southern Ocean warm sea surface\ntemperature bias in CMIP6 models. npj Climate and Atmospheric Science, 6(1), p.127.\nSchuddeboom, A.J. and McDonald, A.J., 2021. The Southern Ocean radiative bias,\ncloud compensating errors, and equilibrium climate sensitivity in CMIP6 models.\nJournal of Geophysical Research: Atmospheres, 126(22), p.e2021JD035310.\nAs shown in Luo et al. and Schuddeboom et al. (the last two papers cited above), the Southern\nOcean bias is present across almost all CMIP6 models, resulting in biases in the spatial\ndistribution of sea surface temperature. SSTs and winds over the Southern Ocean are tightly\ncoupled - the presence of an anomalously warm Southern Ocean results in an anomalously\nweaker and southerly displaced westerly wind belt. Likewise, the strong coupling between\nwinds and precipitation means that the westerly storm track would also be shifted\nsouthward/poleward.\nIn the text we make note of the coarseness of the grid used in HadCM3; this lower spatial\nresolution has both benefits and costs. High resolution models are extremely expensive to\nrun (e.g. 10 years of model time might take 6 months, and paleo runs require thousands of\nyears per run). Even if computational expense were not a barrier, we’d like to point out that\neven the highest resolution models contain biases. Figure 14 in Chang et al. demonstrates this\npoint exactly.\nChang, P., Zhang, S., Danabasoglu, G., Yeager, S.G., Fu, H., Wang, H., Castruccio, F.S.,\nChen, Y., Edwards, J., Fu, D. and Jia, Y., 2020. An unprecedented set of high resolution\nearth system simulations for understanding multiscale interactions in climate\n-variability and change. Journal of Advances in Modeling Earth Systems, 12(12),\np.e2020MS002298.\nThe lower spatial resolution is beneficial in its speed and computational cost. As paleoclimate\nsimulations generally require hundreds of years of spinup to achieve near-surface\nequilibrium, and longer to equilibrate the oceans. This process is more efficient in models\nwith a lower resolution, as there are fewer calculations to be made.\nIt is thus not an ‘admission’ per se that models have biases – this is fact which is well\nestablished and well known. This is true for coarse and finer resolution models alike.\nGiven the known and well-documented biases above, it’s unrealistic to perform or expect a\npoint-for-point comparison between our proxy data, which incorporates signals from an entire\nwatershed, and a single model grid cell. Instead, we choose to leverage the model to\ndemonstrate the direction of causality and broad spatial patterns of the underlying climate\nmechanisms, as well as reconcile differences in regional and local signals. In this model, only\nglobal boundary conditions are prescribed, and the changes in circulation evolve organically\nin a way that is consistent with the mechanism we hypothesize is at work based on proxy\nrecords. The model thus serves as a test of a qualitative hypothesis – the direction of the\ncausality is exactly what we’d expect even though there isn’t a perfect match between our\nsampling site and model output (which we would not expect anyways given the sources\nabove).\nRegarding point 1:\nThe model simulates an increase in rainfall in the WRZ during northward shifts of the front but no\nexpansion of this rainfall zone (Line 342-346). At the same time rainfall amounts decrease in the\nSRZ in those scenarios. The authors however use the expansion of the WRZ before 0.9 Ma and a\ncontraction thereafter to argue their case. As I see it does the model not support this hypothesis.\nOn the contrary, if the WRZ did not expand the catchment areas to ODP 1084 (and cited\narcheological sites) would reside under SRZ influence the entire time being exposed to the\ndynamics of the SRZ . How is this reconciled with the data and the hypothesized dynamics?\nIf the model does not support the hypothesis, neither does it have the spatial resolution that is\nneeded for an area like SA (where large spatial heterogeneity exists on a small scale) nor is it\nisotope-enabled to help explain the isotope interpretation. Hence why even use the model result at\nall?\nWe thank this reviewer for their feedback and hope that we sufficiently address their concerns\nin our response. As above, we acknowledge that there are model biases, because all models\nhave biases. This is because, at their foundation, models are merely representations and\nparameterizations of how we think the climate system works. In other words, all models are\nwrong, but hopefully some can be useful. To that end, HadCM3 is far from the only model with\ninherent biases; several studies, including those in the studies cited by the reviewer below\n(see section 5a in Arnault et al), demonstrate that both the WRF and iCESM models have\nsubstantial temperature and precipitation biases over Southern Africa, which translate to\nunrealistic precipitation dD fields.\nDespite its biases, the HadCM3 family of models has been used extensively in both the CMIP\nand PMIP experiments, where the overall performance of these models has been vetted and\ntheir skill at reproducing modern-day climate has been evaluated. We also note that there are\nbenefits to a lower-resolution model for paleoclimate applications. Paleoclimate simulations\ngenerally require hundreds of years of spinup to achieve near-surface equilibrium, and longer\nto equilibrate the oceans (even regional models like WRF requires boundary conditions from\nglobal models to run). A coarser resolution means this process is less computationally\nexpensive and allows for repeated simulations aimed at testing the sensitivity of the model to\nvarious perturbations. As we mention in Section 4.3, these model runs are from an ongoing\nproject in which snapshot simulations are run every 4ka for the past 3.6 Ma, with each\nsimulation having time-sensitive boundary conditions. This is a truly unique set of\nsimulations.\nWe again emphasize that we merely using the model to show that simulated rainfall is indeed\nsensitive to changes in the position of the Subtropical Front – that is, in the simulations during\nwhich the STF is shifted northward, the model does in fact produce more rainfall in the WRZ\nand YRZ; both zones are indeed expanded horizontally and there is a considerable increase in\nprecipitation south of the continent, which, in the absence of the Southern Ocean bias, would\nbe shifted northward and fall over the continent.\nSecond, the human-climate link over the MPT instead of exploring and expanding on that it was\nshortened and put on the side. Here, the novelty for publication in Nat com could have arisen from\nif that link was explored in more depth and in collaboration with experts from the other field.\nThe model is simulating a mechanism which is consistent with that proposed in several past\nstudies (e.g. Jiang et al., 2015; Weldeab, et al., 2013; Stuut et al., 2004; etc.). Furthermore, and\nperhaps most importantly, this model demonstrates the precise inversion of the mechanism\nthought to be responsible for the Day-Zero Drought, demonstrating the relevance of this study\nto both paleo and modern climatic studies. We feel that this dynamical implication is one of\nthe major takeaways from this paper, since it suggests that the mechanism for the Day-Zero\nDrought was similar to the mechanism for a long-term state shift during the MPT. We\nencourage further work examining whether the relatively short-lived, extreme events in the\nmodern, such as Day Zero, have the potential to become longer-lived features of the future\nclimate regime.\nOther comments on the content:\nLine 206 onwards: The authors fail to explain what the isotopic signal of the rainfall in the area of the\nrivers they are apparently tracing is driven by? In areas where different moisture sources overlap,\nlike in South Africa, isotope distributions are affected by many processes and not easily to\ninterpret, certainly not by fixed 'end-members'. One of the best isotope maps for South Africa is\nfound in West et al. (Journal of Geochemical Exploration, 2014,\ndoi.org/10.1016/j.gexplo.2014.06.009) which is very similar to the gridded data by Bowen et al.\n(2003). For instance, what has been found for the Orange river catchment is that an increase of\nAtlantic moisture (not westerly related) leads to isotopic enrichment in the Late Holocene (Arnault\net al., 2022, DOI: 10.1175/JCLI-D-22-0041.1).\nHow can the dDwax record be reconciled with those findings and modern isotope data from the area?\nCan the “observed relatively abrupt enrichment in δDwax record over the MPT at 0.85-0.75 Ma derived from increase in Atlantic summer\nmoisture rather than an extension of the SRZ?\nOur conceptual interpretation of the isotope signal is stated in the paragraph beginning in Line\n188 onwards. It is consistent with previous interpretations like those found in Braun et al.,\n2017. Remember that furthermore, it is corroborated by hypothesized vegetation shifts\ninferred from chain length distributions (e.g. Karoo and Fynbos biomes). We apologize for\nreferring to simple ‘end members’ in the previous text – reference to these have been\neliminated. We agree that the West et al. paper is a great interpretation and we have cited it\naccordingly. We also cite Kaseke et al., whose isoscape of Namibia broadly coheres with that\nof West et al.\n(Kaseke KF, Wang L, Wanke H, Turewicz V, Koeniger P (2016) An Analysis of Precipitation\nIsotope Distributions across Namibia Using Historical Data. PLoS ONE 11(5): e0154598.\ndoi:10.1371/ journal.pone.0154598)\nArnault et al. invokes a southerly displacement of both the Southern Hemisphere westerlies\nand the tropical easterlies to explain a shift in moisture source that resulted in an enrichment\nof dD from mid-Holocene to present. Our HadCM3 simulations show a northward\ndisplacement of the westerlies coincident with a minor southward shift in the tropical\neasterlies -consistent with the Hadley Cell constriction we suggest. If this slight shift in the\ntropical easterlies were substantially contributing deuterium enriched moisture to the Orange\nRiver watershed during the intervals when the STF was in its northerly position, we’d expect\nour dD record to become more enriched during these intervals. Rather, the ~40 permille\ndepletion we observe from ~1.4 to 0.9 Ma indicates that there is a substantial amount of\ndeuterium depleted moisture entering the watershed.\nIt is possible therefore that different processes operate on different timescales, and that the\nshifts we see during the MPT come from a different dynamical mechanism than Arnault’s late\nHolocene changes. We reiterate, however, that both WRF and iCESM have substantial\ntemperature and precipitation biases, as pointed out by Arnault et al., which likely also\ninfluenced dD simulation. Additionally, as Arnault et al. acknowledge in their paper, their WRF\nsimulations were not run to equilibrium, as they were too computationally expensive to run for\nlonger than 10 years, resulting in radiative imbalances. Nevertheless, we hope this work\nmotivates future studies which use either WRF, iCESM, or another isotope-enabled model to\neither substantiate or refute the mechanism we present here.\nLine 211 and S7: When the calculated Norm31−Norm33 approaches zero, it indicates that the\nnormalized concentrations of C31 and C33C n-alkanes are nearly equal. This could reflect\nhomogeneous plant inputs, minimal environmental change, or a mixed vegetation signal. The result\nshould be contextualized with other proxies to fully understand its implications. In some cases,\npost-depositional processes like microbial degradation or diagenesis could also alter the n-alkane\ndistribution, potentially leading to equalized concentrations and a Norm31−Norm33 value close to\nzero. How are the carbon preference index (CPI) in that interval?\nAs stated in the text, the carbon preference index throughout the record was >1, indicating\nterrestrial input, rather than petrogenic input and limited diagenesis (Jeng et al., 2006; Carr et\nal., 2014). The average chain length was 29.35 ± 0.089 and the concentrations of the C29 and\nC31 alkanes were within 2.5 ng/g of one another.\nLine 302: “Some sort of vegetation shift”- that does not sound like a confident statement being\nconvinced by their own interpretation.\nThank you for this feedback, we have updated the text to reflect our confidence in this\nstatement.\nThank you for your feedback, we appreciate the input. Please see our responses below.\nSmall items:\nline 67- phytolyth rather than phytolith\n-Fixed! Thank you for catching this.\nline 69- \"coarseness\"? Isn't it rather that these records are too short to examine trends through\ntime?\nSome records are too short, but some do extend further back in time, but have data which is very\nsporadic, with hundreds of thousands of years or more between datapoints, hence we refer to it as\ncoarse. Also, the limitations of dating some archaeological finds means that the dates we do have\nare imprecise, with rather large error bounds.\nlines 100 to 101- this is a great addition as a segue into the significance of the findings!\nSection 2.6\nlines 285 to 288- I suggest that you make it clearer that the goal of this section is to put forward\nhypotheses based on the correlations you see between the timing shifts in biozones and last\nappearance datums (LADs) for Paranthropus that could be tested by future work. A rigorous\ntreatment of this topic is truly deserving of its own paper in my opinion. For example, it's not clear\nform this short paragraph if the timing of the LADs cited here are regional or across Africa. What\ngaps exist in the paleoanthropological record and do those bias the timing of LADs regionally?\nlines 287 to 289- I think most people would argue that generalist like H. sapiens seems to do\nrelatively well in highly variable conditions.... I think what you really mean here is that specialists\n(such a P. boisei which you mention in a following sentence) disappear as the climate and thus the\nenvironment becomes more variable. See Maslin et al., 2015... Maslin, M. A., Shultz, S., and Trauth,\nM. H., 2015, A synthesis of the theories and concepts of early human evolution: Philos Trans R\nSoc Lond B Biol Sci, v. 370, no. 1663, p. 20140064. This nuance is really important!\nThank you for this important feedback. We’ve incorporated these comments and believe that\nSection 2.6 reads much better now.\nSection 2.7\nlines 323 to 325- This final sentence feels incomplete to me... \"aid in mitigation efforts...\" how? I\nwould really appreciate the authors thoughts here.\nWe agree and we’ve expanded this last sentence to;\nWhile we cannot prevent the next extreme drought from occurring, an enhanced understanding of\nthe underlying mechanics of regional hydroclimate may provide additional insight which improves\nour projections of regional climate, aids mitigation efforts, and strengthen the resilience of SA\ncommunities to the effects of anthropogenic climate change.\n3. Conclusion\nI think backing away from a hominin evolutionary story was the right move given the strengths of the\nauthors and lack of a paleoanthropologist on this team but it still seems like a mention of how a\nrecord of the MPT provides interesting context for hominins, seeding hypotheses that could be\ntested by future work. I think it would be worth incorporating a sentence like that.\nAgreed! The updated conclusion now says:\nThis study highlights substantial shifts in rainfall seasonality and vegetation cover which are\nassociated with migrations of the AFS over the MPT. We use HadCM3 simulations to further\ndemonstrate that SA precipitation is sensitive to variations in the westerlies and AFS; this work\nwould benefit from additional analyses using isotope-enabled simulations with higher spatial\nresolution. Our records supplement the existing terrestrial records of hydroclimate change in\nSouthern Africa and may useful for contextualizing the evolutionary and dispersal patterns of early\nhominins in Southern Africa over the Pleistocene, though more rigorous work examining the impact\nof precipitation and landscape change on Paranthropus is necessary.\nWe present a mechanism of hydroclimate change during the MPT which is analogous to that of the\n“Day-Zero Drought,” suggesting that these impactful drought events we see in the modern record\nhave the potential to become persistent features of the region’s climatology. Continued\ninvestigation of this mechanism in the past and present is therefore needed to improve our\nprojections of, and preparations for, future extreme climate events.\nComments from reviewer-\nAbstract\nGiven the role the HadCM3 simulation play in the interpretations of this\npaper, I’m surprised this is not mentioned in the abstract.\n1. Introduction\nThe references given (1-3) are all in China. Given they are supposed to\nsupport a broad picture of hominin evolution, I think the authors should\ndiversify the geographic distribution of the references, including some from\nEastern Africa and the Middle East given that these regions have robust\nfossil and archaeological records, and were key corridors for hominin\nmigration and movement.\nLines 70-71: Where are these data published? Citation needed….\nLines 82-83: Given the broad scope of this journal, I think description of\nwhat the AL is and why it impacts climate would be helpful context for\nreaders. By contrast, a compelling case is made for both the mechanism\ncontrolling and the impact of the AFS… as written the AL feels like an\nafterthought so if it’s essential to the story, that should be clarified.\n2. Results and Discussion\nLine 102: Coordinates for ODP 1084 are only reported here to the minute…\nthe location must be more precise than that! Please check… a quick Google\nsearch lead me to this- 25°30.8345´S, 13°1.6668´E.\nLines 104-105: Tighten up the word choices in this sentence… Instead of\n“results” what about “is sourced”. Consider “integrate” rather than\n“represent”. Both would clarify what you mean and more accurately\nrepresent the relationships.\nLine 113-114: “ empirically derived constraints on εp−w…” I’d argue that this isn’t\nclear what these are and thus their significance is unclear as written.\nLine 129-133: I’m concerned about the complicating factor of the CAM\nvegetation in the WRZ on the isotope signature since the authors mention is\nis a main feature of the WRZ vegetation. What percentage of the biomass is\nCAM? How is this accounted for in the analyses since CAM plants can have\nisotopic signatures that can span the C3 and C4 zones and might complicate\nthe interpretation of dD? I just found that this is addressed in section 4.3\nand I definitely appreciate this additional information but am still concern\nabout how CAM plants impact the record given the lack of constrain on the\nrelative abundance.\nLines 142-144: I think it would be helpful to add in discussion of what the\nexpansion of the WRZ results in depleted dDp. This could be added into the\nIntroduction in lines 93-97 or developed in this section but that linkage has\nnot been thoroughly made in my opinion.\nLines 144-147: These feel redundant with the previous sentence. Can this be\ncombined into one that points to the hypothesized relationship and the\nultimate confirmation through the application of the HadCM3 model?\n2.3 Antarctic Front Migration Mechanism\nIssue in lines 147 about significance of model result given small spatial\nextent… I appreciate the authors transparency about the potential impact of\nthe resolution of the model given the limited spatial extent. Is there some\nway to test the sensitivity to a change of only a few grid cells? Are there\nother applications of this or similar models that are significant? How might\nyou test/validate whether or not this truly is a statistically significant impact\non climate?\nLine 151: I don’t see where SO is previously defined but I’m guessing it\nstands for Southern Ocean??\nLines 169-169: Is the change is dD just a function of change in seasonality\n(ie when the rain is falling) or is there also an effect of how much rain is\nfalling? Given the generally slow sedimentation rates in marine cores, I\nwould be curious for more justification for this being a seasonal signal vs an\namount effect. Can you explain more about how you ruled out the latter?\n2.5 Implications for Hominin Evolution\nBased on the title of this paper, I anticipated this link would be robustly\ndeveloped. There is really just one paragraph (lines 190-202) that talks\nabout the links with hominins. I think the proposed connection with P. boisei\nand P. robustus is an interesting connection but given the lateral shift of the\nWRZ and SRZ, the connection to why this would have cause extinction needs\nto be further developed.\nLines 190-191: Citations are definitely needed here because others would\nargue that variability (rather than rapid linear change) drove hominin\nevolution and that even catastrophic events like the Toba eruption had\nessentially no impact on hominin population size (see Yost et al. 2018).\nLines 198 and 201: P. robustus has an extra “s” in the text.\n3. Conclusion\nIt’s unclear to me what is meant by the “observed gap in the hominin fossil\nrecord”. Is this something specific to this region? This is also mentioned in\nlines 66-67 and again needs more support as this is highly contested.\n4. Methods\n4.2 Spectral Analyses\nI’m really curious for the authors’ thoughts on why the ODP 1084 dD record\nis so smooth and doesn’t show orbital cyclicity. This seems like it could be an\nreally interesting story that should be highlighted rather than buried into this\nsection. How does this compare with other southern hemisphere records\n(only an example from eastern Africa is cited)? Some important ideas are\nrolled into section 4.3… together these two sections I believe deserve to be\nhighlighted in this study.\n4.4 Varimax Factor Analysis\nThis is an interesting choice and not something I’m familiar with. I would be\ncurious to hear more about why the assumption of an unobserved variable is\nthe appropriate choice here.\nFigures\nFigure 1: Fig 1b and 1c are flipped relative to the figure caption…. For the\nbar and wicker plot, can you explain the n = x further in the caption? Are\nthese module runs? Something else?\nFigure 2- Multiple records are not labeled correctly when comparing the\ncaptions to the figure (2o, r)… this should be closely double checked for\nfurther issues.\nFigure 2a and 2b: Since most of this figure is compiling previous results, I\nthink it would be helpful to emphasize that these are the new data from this\nstudy.\nFigure 2c: I understand that this is using the latitude of “diatom mats”…\ncould you clarify what a laminated mat is? How are these intervals of\nlaminated (diatomite?) to track the position of the Arctic Polar Front?\n2d: missing from the figure caption… is it ODP 1081?\n2p: P. bosei isn’t on this figure and should be given the discussion in section\n2.5.\nFigure 3:Key is needed for the colored lines in a and b…\nReferences\nThere are multiple proper nouns that should be capitalized in the titles of\nreferences.\nLine 496 check capitalization\nLine 502- China should be capitalized as should the time period\nLine 506- looks like capitalization is needed in the title of all the proper\nnouns in the papers… I’m not going to mention others from here on but all\nshould be checked for issues.\nReviewer expertise\nPlease indicate any particular part of the manuscript, data or analyses that\nyou feel is outside the scope of your expertise, or that you were unable to\nassess fully.\nI am not a paleoceanographer and thus I flagged some part of the\nmanuscript where, given my lack of expertise, I was unclear on the context\nand thus the significance of what was being described. I am not qualified to\ncomment directly on the details of these mechanisms. My expertise is in\nterrestrial paleoenvironmental reconstruction in the context of hominin\nevolution."}, {"round": 3, "reviewer_comments": {"reviewer_1": "Small items:\nline 67- phytolith rather than phytolyth\nline 69- \"coarseness\"? Isn't it rather that these records are too short to examine trends through time?\nlines 100 to 101- this is a great addition as a segue into the significance of the findings!\nSection 2.6\nlines 285 to 288- I suggest that you make it clearer that the goal of this section is to put forward hypotheses based on the\ncorrelations you see between the timing shifts in biozones and last appearance datums (LADs) for Paranthropus that could\nbe tested by future work. A rigorous treatment of this topic is truly deserving of its own paper in my opinion. For example, it's\nnot clear form this short paragraph if the timing of the LADs cited here are regional or across Africa. What gaps exist in the\npaleoanthropological record and do those bias the timing of LADs regionally?\nlines 287 to 289- I think most people would argue that generalist like H. sapiens seems to do relatively well in highly variable\nconditions.... I think what you really mean here is that specialists (such a P. boisei which you mention in a following\nsentence) disappear as the climate and thus the environment becomes more variable. See Maslin et al., 2015... Maslin, M.\nA., Shultz, S., and Trauth, M. H., 2015, A synthesis of the theories and concepts of early human evolution: Philos Trans R\nSoc Lond B Biol Sci, v. 370, no. 1663, p. 20140064. This nuance is really important!\nSection 2.7\nlines 323 to 325- This final sentence feels incomplete to me... \"aid in mitigation efforts...\" how? I would really appreciate the\nauthors thoughts here.\n3. Conclusion\nI think backing away from a hominin evolutionary story was the right move given the strengths of the authors and lack of a\npaleoanthropologist on this team but it still seems like a mention of how a record of the MPT provides interesting context for\nhominins, seeding hypotheses that could be tested by future work. I think it would be worth incorporating a sentence like\nthat.", "reviewer_3": "I appreciate the authors' efforts to address the reviewers' comments. I would like to recommend\nacceptance of this manuscript."}, "author_response": ""}]

Decision letter