Sea level since the Last Glacial Maximum from the Atlantic coast of Africa

Domain: earth_science
Published: 2025-02-10
Editorial outcome: major_revision

Abstract
Constraining sea level at the Last Glacial Maximum (LGM) is spatially restricted to a few locations. Here, we reconstruct relative sea-level (RSL) changes along the Atlantic coast of Africa for the last ~30 ka BP using 347 quality-controlled sea-level data points. Data from the continental shelves of Guinea Conakry and Cameroon indicate a progressive lowering of RSL during the LGM from −99.4±5.2 m to −104.0±3.2 m between ~26.7 ka and ~19.1 ka BP. From ~15 ka to ~7.5 ka BP, RSL shows phases of major accelerations up to ~25 mm a−1 and a significant RSL deceleration by ~8 ka BP. In the mid to late Holocene, data indicate the emergence of a sea-level highstand, which varied in magnitude (0.8±0.8 m to 4.0±2.4 m above present mean sea level) and timing (5.0±1.0 to 1.7±1.0 ka BP). We further identified misfits between glacial isostatic adjustment models and the highstand, suggesting the interplay of different ice-sheet meltwater contributions and hydro-isostatic processes along the wide region of Atlantic Africa are not fully resolved.

Introduction
Projections of future sea levels rely on the understanding the relationship between sea levels with past climate and global ice volumes1. In particular, reconstructions of relative sea-level (RSL) change from far-field regions (i.e., located far from extinct ice sheets) since the Last Glacial Maximum (LGM) provide fundamental constraints to global ice volumes1,2 and isostatic response of the solid Earth to the large ice-ocean mass redistribution3,4. Most published sea-level records are temporally restricted to the Holocene5 (last ~11.7 ka BP) with very few extending to the LGM2,6,7 (e.g., last ~30 ka BP). Indeed, there is an absence of quality-controlled LGM data from the Atlantic Ocean except for the Barbados record (Fig. 1a), which has been central to constraining the timing and magnitude of the LGM lowstand2,8,9 and the existence of Meltwater Pulse events including 1b (MWP1b)10,11. Data on the spatial and temporal variability of the mid-Holocene sea-level highstand are also only presently available for the South American and Caribbean coasts of the western Atlantic Ocean12–14.

Here, we compiled a quality-controlled RSL database since the LGM from the Atlantic coast of Africa (Fig. 1a), a passive margin mostly characterized by minor late Quaternary tectonic activity15–17. We produced 234 Sea Level Index Points (SLIPs that constrain RSL in time and space with quantifiable uncertainty18) and 113 terrestrial and marine limiting points (points that constrain the upper and lower boundary of RSL, respectively18). We clustered the database into 15 regions (Fig. 1b) based on their geographic location19–21 (see Methods). We quantified RSL changes using an Error-In-Variables Integrated Gaussian Process (EIV-IGP) model22 and compared the reconstructed RSL histories with GIA predictions from the ICE-6G (VM5a)23 and ANU24,25 models to: (1) constrain the timing and magnitude of RSL changes during the LGM (~29.5 ka to ~19.0 ka BP); (2) quantify magnitudes and rates of postglacial RSL change; and (3) probabilistically define the timing and magnitude of the Holocene sea-level highstand in the last 8.0 ka BP.

Results
Sea-level evolution during the Last Glacial Maximum and the Late-glacial period

The timing, magnitude and rate of RSL changes during the LGM and late-glacial period (i.e., from the onset of the LGM at ~29.5 ka BP to the beginning of the main phase of deglaciation at ~16.5 ka BP24,26,27) represents a key time period to constrain the progressive melting of polar ice-sheets27.

The production of a suite of SLIPs (n=4) and marine limiting points (n=5) collected from the continental shelf of Guinea Conakry (region 8), Ivory Coast (region 9) and Cameroon (region 10) constrain RSL change in the Atlantic Ocean between ~29.5 ka and ~16.5 ka BP (Fig. 2). During the LGM period (~29.5 ka to ~19.0 ka BP), RSL fell from −99.4±5.2 m at 26.7±0.6 ka BP to −103.1±3.2 m at 24.2±0.9 ka BP (Fig. 2). Between ~24 ka and ~20 ka BP, RSL is constrained by a single marine limiting point above −111±2.4 m at 22.1±0.8 ka BP while a SLIP shows RSL was at −104±3.2 m at 19.1±0.8 ka BP. The EIV-IGP model indicates rates of RSL fall of 1.0±4.3 mm a−1 between ~27 ka and ~24 ka BP, and 0.8±4.4 mm a−1 between ~24 ka and ~19 ka BP. In the Late-glacial period (19 ka to 16.5 ka BP), SLIPs and marine limiting data indicate RSL rose from above −99±1.2 m at 17.8±0.7 ka BP to above −89±1.2 m at 16.3±0.7 ka BP at a maximum rate of 9.0±4.7 mm a−1.

The lowering of RSL from ~26.6 ka to ~19.8 ka BP indicated by Atlantic coast of Africa data suggests a sea-level minimum at the end of the LGM (~21 ka to ~20 ka BP), which has also been inferred from the records collected in the Indo-Pacific regions6,7,28. This lowering supports an increasing ice volume driven by the eastward and southward expansion of the Scandinavian ice sheet as well as the southward advance of the Laurentide ice sheet24,25. The lack of data between 22 ka and 20 ka BP did not allow the identification of an abrupt ~40 m drop in RSL at ~21 ka BP that was recorded in the Great Barrier Reef7. These data represent the first Atlantic Ocean evidence of the sea-level lowering trend during the LGM. Previously, the Barbados record, was the single LGM dataset available for the Atlantic Ocean2,8,10 but its accuracy in defining the timing and magnitude of the LGM was debated for possible tectonic influence8 as well as for the potential presence of allochthonous dated material from downslope transportation of the coral sea-level indicators29.

Rates of sea-level rise during the main phase of deglaciation

A major phase of deglaciation and consequent increase in global mean sea level occurred between ~16.5 ka and ~7.0 ka BP24 from a reduction of land-based ice volume of ~45×106 km3.

The presence of numerous SLIPs (n=42) from Senegal (region 7), northern Gulf of Guinea (region 9) and Congo (region 11) enabled the quantitative assessment of magnitudes and rates of RSL change along a large portion of the Atlantic coast of Africa. In Senegal, RSL rose from −53.6±3.5 m at 14.8±0.2 ka BP to −39.2±4.0 m at 11.5±0.2 ka BP and to −1.24±0.7 m at 7.0±0.2 ka BP (Fig. 3a). The 24 SLIPs record a progressive increase in sea-level rate rising at 9 to 11 mm a−1 between 11.5 ka and 8.5 ka BP and decreasing to <7.0±1.0 mm a−1 after 8.0 ka BP (Fig. 3b). In the northern Gulf of Guinea, RSL was stable at −61.6±3.0 m between 14.0 ka and 13.0 ka BP (Fig. 3c). RSL rose to −44.9±4.0 m at 12.0±0.2 ka BP and to −37.2±3.0 m at 11±0.2 ka BP. Younger SLIPs indicate RSL rose to −6.1±0.2 m at 8.0±0.2 ka BP and finally to −1.6±0.7 m at 7.0±0.2 ka BP. The 9 SLIPs indicate two phases of major acceleration with rates of rise up to 25.2±11 mm a−1 between 12.6 ka and 12.1 ka BP and up to 11.6±11 mm a−1 between 10.0 ka and 8.0 ka BP (Fig. 3d). After 8.0 ka BP, RSL rates were <7.0±2 mm a−1 at 7.5 ka BP. In Congo, RSL rose from −70.7±3.9 m to −24.7±1.7 m between 13.3±0.2 ka and 10.0±0.2 ka BP (Fig. 3e). Younger SLIPs indicate RSL rose to −4.3±2.0 m at 8.0±0.2 ka BP and reached present sea-level at 7.0 ka BP. The 9 SLIPs show sea-level rose at rates from 13.4±3.4 mm a−1 at 13.0 ka BP to 14.8±1.8 mm a−1 at 11.4±0.2 ka BP (Fig. 3). This was followed by progressive decrease in rising rates from 10.3±1.4 mm a−1 after 9.0 ka BP to 6.0±1.0 mm a−1 after 8.0 ka BP, respectively (Fig. 3f).

The postglacial records from Senegal, the northern Gulf of Guinea, and Congo offer the opportunity to investigate the evidence for MWP events including MWP1b, the existence of which has been debated10,11,29. MWP1b is a rapid acceleration of sea-level identified in the Barbados record between 11.5 ka and 11.0 ka BP10,29. Evidence for this major acceleration, however, is not recorded in Senegal (Fig. 3b) or Congo (Fig. 3f). An acceleration is recorded in the northern Gulf of Guinea (Fig. 3d), but its timing (12.6 ka to 12.1 ka BP) is ~1.0 ka older than the reported occurrence of MWP1b derived from the Barbados record10. This reconstruction, however, is based on a limited number of SLIPs (n=6) between ~15 ka and ~10 ka BP, some with large temporal (i.e., ±0.6 ka) and vertical (i.e., ±1.7 m) uncertainties. Our interpretation of rapid increases in sea level during this period are, therefore, restrained until further evidence is found.

In Senegal and in the northern Gulf of Guinea we observe peak rates of sea-level rise preceding the 8.2 ka cooling event triggered by North Atlantic Freshwater Input following the collapse of North American proglacial Lake Agassiz/Ojibway and of the Hudson Bay Ice Saddle30–32. The low temporal resolution of our sea-level data, however, does not allow for a clear identification of the rapid sea-level acceleration preluding the 8.2 ka event as identified in the Netherlands33,34, Scotland32,35, Chesapeake Bay36, and the Mississippi delta37. Indeed, the quantitative reconstructions from Senegal and northern Gulf of Guinea clearly identify a major deceleration of rising rates after ~8.0 ka (Fig. 3b, d), whose timing is consistent with the global decrease in rates between 8.2 ka and 6.7 ka BP consequent to the final phase of North American deglaciation24.

Timing and magnitude of the sea-level highstand

In far-field regions, the coupled activity of ocean syphoning (i.e., the migration of water from far-field regions into areas vacated by forebulge collapse and subsidence at the periphery of deglaciation centres to maintain dynamic equilibrium3,12) and continental levering (i.e., vertical land motion of continental margins due to the increasing ocean loading caused by rising sea levels, which induces subsidence of offshore regions and an uplift of onshore regions3,12) result in the emergence of a sea-level highstand. Understanding variability in the timing and magnitude of highstand formation is fundamental to constrain Earth and ice model parameters in GIA modelling3,38.

The dense concentration of SLIPs (n=203, Fig. 1b) in all 15 regions of the Atlantic coast of Africa during the mid to late-Holocene enables a quantitative assessment of magnitudes and rates of RSL change as well as comparisons with predictions from the ICE-6G and ANU models (Fig. 4 and Supplementary 1).

In Mauritania and Senegal (regions 5 and 7), the probability of a highstand ≥1 m is ≤50% while along most of northernmost sector of Atlantic coast of Africa (regions 1 to 4), the Sierra Leone and northern Gulf of Guinea (regions 8 and 9) and the southernmost sector (regions 11, 13 and 14) the probability of a highstand ≥1 m is ≥50% (Fig. 4). Both GIA models predict the formation of a highstand above present level in most of the Atlantic coast of Africa (see Supplementary Figs. 2, S2_1 and S2_2). However, both the timing and magnitude of the highstand show differences between the ICE-6G and ANU predictions. ICE-6G predicts a single maximal highstand followed by a continuous drop in RSL for the remaining part of the Holocene. The maximal highstand is generally predicted at ~6.0 ka BP and its magnitude is ≥1 m in most of the regions with the exception of the northernmost portion of Atlantic coast of Africa (northern Morocco, region 1 and northern Atlantic Sahara, region 3) where the highstand is predicted above present level but ≤1 m and in the offshore archipelagos of the Canary Islands and Cabo Verde (regions 2 and 6) where no highstand is predicted. ANU predictions, however, show a more variable pattern of RSL change. At latitudes higher than −25°N (e.g., region 1 and 3) and in the offshore archipelagos of the Canary Islands and Cabo Verde (regions 2 and 6), the maximal highstand is predicted at ~2.0 ka BP and always <1 m above present level. The remaining regions are generally characterized by a highstand at ~6.0 ka BP, which is followed by minor fluctuations in RSL until a second highstand which formed at ~2.0 ka BP. The elevation of the highstand at ~6.0 ka BP is ≥1 m above present level only in Mauritania, Senegal, southern Namibia and South Africa (regions 5, 6, 14 and 15). The highstand at ~2.0 ka BP is always ≤1 m and its maximal elevations (0.5 to 0.7 m above the present datum) are predicted in the coastal sector between Senegal and Cameroon (regions 7, 8, 9, 10).

Our RSL reconstructions indicate the establishment of mid-Holocene highstands ≥ 1m in the northernmost sector of Atlantic coast of Africa at latitudes between 35°N and 26°N (Fig. 4). Both ICE-6G and ANU model predictions show misfits in both the timing and the magnitude of this highstand formation (Fig. 4). This misfit may be due to the neglection of 3D (i.e., lateral heterogeneous) Earth structure, which is important for continental-scale studies of GIA39,40. Notably, the potential lateral viscosity variations inferred from shear velocity anomalies in seismic tomography model across the Atlantic coast of Africa are significant41. Alternatively, it might be an underestimation of the ice-equivalent sea level contribution from North America which is not compensated by an Antarctic contribution in both of the GIA models (Supplementary Figs. 2, S2_1 and S2_2). This underestimation of the ice-equivalent sea level contribution would result in the formation of a mid-Holocene highstand in the northernmost sector of Atlantic coast of Africa whose chronology is comparable with the highstand observed at lower latitudes (−8° to −5°N; Guyana and Suriname) and South America13.

Between −15°N and −0° (Regions 6 to 10, Fig. 4 and Supplementary Figs. 1, S1_2) data indicate RSL reached its maximal elevation above the present sea-level in the late Holocene (~2.0 to ~1.7 ka BP). The emergence of this highstand is not predicted by ICE-6G while ANU model predicts a late Holocene highstand in all these regions but at elevations always ≤1 m (Fig. 4). However, the probabilistic analysis (Fig. 4) suggests this highstand exceeded 1m in Cabo Verde Islands (Region 6, highstand ≥1 m = 92%), Sierra Leone (Region 8, highstand ≥1 m = 77%) and Gulf of Guinea (region 9 = highstand ≥1 m = 91%). With the sole exception of the volcanic archipelago of Cabo Verde, the current elevation of this highstand is difficult to explain solely through post-depositional vertical ground movements, given the low Quaternary tectonic activity observed along the Atlantic coast of Africa15,16,42. The emergence of this highstand is most likely driven by hydro-isostatic processes (including ocean syphoning and continental levering), whose timing and magnitude is mainly controlled by the deglaciation history and Earth (i.e., viscosity) structure38. For example, the meltwater input from Antarctic ice sheet diminished over the last ~7.5 ka, although the exact timing and relative contributions are not fully resolved23,43,44. The timing of the Antarctic ice-sheet melting represents a major difference between the models employed in our analysis. In ICE-6G, the Antarctic contribution to meltwater input is minimal in the late Holocene while for ANU the contribution lasted until ~2.0 ka BP (Supplementary 2, Figure S2_3). The isostatic response of the far-field regions of Atlantic coast of Africa supports that ice volumes were still changing in some regions of Antarctica in the late-Holocene44–46. In particular, the Antarctic Thermal Optimum47,48 stimulated melt of the western Antarctic ice sheet until to 2.0 ka BP24, although increased ice accumulation rates in the interior of western Antarctica from warmer temperatures and higher precipitation had implied an ice mass expansion49. Our data imply that the dynamic contributions from Antarctica may be missing (for ICE-6G) or underestimated (for ANU models) from the ice histories used in global GIA models in the mid to late Holocene50,51.

Methods
Compilation of a deglacial sea-level database for the Atlantic coast of Africa

The Atlantic Africa database of SLIPs and marine and terrestrial limiting points was assembled following protocols described by the International Geoscience Programme (IGCP) projects52, and INQUA project HOLSEA5,53. A SLIP estimates the unique position of RSL in space and time with corresponding vertical and temporal uncertainties18,21. Where a suite of SLIPs exists for a locality or region, they indicate changes in RSL through time and allow the magnitude and rate of RSL change to be estimated19. Limiting points provide an upper (terrestrial or upper limiting data points) or lower (marine or lower limiting data points) bound on the past position of RSL at a given point in space and time52.

The sea-level data points were extracted from a wide range of published studies carried out along the Atlantic Africa coasts since the 1960s54–77. A wide range of sea-level indicators characterize the Atlantic coast of Africa (Supplementary Tables 3, S3_2). These can be subdivided in: i) low-energy sea-level indicators derived from cores or exposures found in coastal marshes, sebkhas and lagoons54–57,77; ii) low-energy sea-level indicators derived from off-shore coring performed along the continental shelf58–60; iii) high-energy sea-level indicators derived from beachrocks or loose beach deposits including beach ridges61–64,77 and iv) sea-level indicators derived from fossil bioconstructions65,66. The indicative meaning of SLIPs (i.e., the quantitative relationship between the dated facies and the contemporary tidal frame52,53) were estimated using published data relating the modern elevational distribution of faunal assemblages54,67,68 and/or based on the sediment bedding architecture (for beachrocks and loose beach deposits). Detailed descriptions of the indicative meaning of the different sea-level indicators are provided in Supplementary Tables 3, S3_2. Marine limiting points were mainly from samples found in infralittoral (subtidal) sedimentary facies59,69,70 while terrestrial limiting points were from samples found in facies dominated by freshwater plant macrofossils as well as dune and fluvial facies66,71,72. Samples provided by archaeological middens from prehistoric coastal settlements73,74 were transformed into terrestrial limiting points with the exception of a suite of data from the Banc d’Arguin (Mauritania) which have a quantitative relationship with the former tidal frame and were transformed in SLIPs75. Data from Namibia and South Africa were extracted from the recent database produced using the Holsea guidelines76.

We subdivided the SLIPs derived from cores into basal and intercalated categories to assess the influence of sediment compaction78. Basal samples are those recovered from within the sedimentary unit that overlies the incompressible substrate and are less prone to sediment compaction. SLIPs derived from beach deposits, beachrocks and vermetid reefs are classified as basal because they are compaction-free78. Where stratigraphic information was unavailable for a SLIP, we conservatively interpreted it as intercalated.

We calibrated the age of all samples in the Atlantic Africa database using CALIB 8.2 with a 2σ range and employed the IntCal2079, SHCal2080 and Marine2081 calibration curves for terrestrial samples and marine samples, respectively. Where available, information on the necessary reservoir correction was taken from the Marine Reservoir Database81. All SLIPs are presented as calibrated years before present (BP), where the reference epoch is 1950 AD. A concern with old radiocarbon ages is the correction for isotopic fractionation82. This became a standard procedure in most laboratories by the late 1970s, but some laboratories have only applied this correction since the mid-1980s18. In the database, a significant number of samples were analysed before 1990 and, therefore, we followed the procedure of the HolSEA protocol5 to correct for isotopic fractionation. We finally discarded 87 SLIPs and limiting points because of the following reasons: (i) difficulties in definition of the indicative meaning, (ii) difficulties in defining the elevation of the sample, (iii) major compaction of intercalated samples, (iv) anomalous RSL when compared to coeval data from the same region.

The Atlantic Africa has a N-S orientation except for the northern side of the Gulf of Guinea (e.g., from Ivory Coast to the Niger delta), which runs W-E. We clustered the RSL data into 15 regions based and on their distance from the palaeo-ice sheets19–21. A detailed description of the tectonic, geographic, and climatic setting of these regions is provided in Supplementary 3.

Statistical modelling and GIA predictions

In each of the 15 regions, the quantitative reconstruction of RSL evolution was performed by applying the EIV-IGP model22. The EIV (errors-in-variables)83 accounts for error due to radiocarbon age uncertainties, and the IGP (integrated Gaussian process) is useful for modelling non-linear trends in data. The EIV-IGP model evaluates RSL and its rate of change over time by assuming a prior Gaussian process specified by a mean function and a covariance function that smooths the RSL reconstructions. The model is flexible, capable of accommodating missing data, and enables probabilistic inferences about RSL change over time22. We modelled RSL changes every 150 years and the final accuracy of our reconstruction is dependent on the number of SLIPs in a given timespan and their vertical and horizontal uncertainty.

To better examine the spatial and temporal variability of the sea-level highstand, we used the EIV-IGP model to analyze the probability of RSL being ≥1 m which is the average elevation predicted by the suite of GIA models along the Atlantic coast of Africa (see supplementary 2).

The GIA predictions were obtained by solving the gravitationally and topographically self-consistent Sea-Level Equation (SLE)84. The SLE, which describes the spatiotemporal variations of sea-level associated with the melting of late Pleistocene ice sheets, has been solved numerically using the SELEN4 open-source code85,86. SELEN4 assumes a laterally homogeneous, spherical, incompressible, and self-gravitating Earth characterized by a Maxwell rheology. It includes the effects of rotational feedback on sea level87 and accounts for horizontal migration of shorelines88. We have implemented in SELEN4 realizations of three different GIA models: i) the ICE-6G23 spatio-temporal evolution of ice sheets, coupled with a five-layer approximation of the VM5 viscosity profile88, in which the thickness of the elastic lithosphere has been kept constant to 90 km, the upper mantle and transition zone viscosities have been set to 5×1020 Pa·s, while the lower mantle has been approximated with two homogeneous layers of viscosities 1.5×1021 Pa·s and 3.2×1021 Pa·s89; (ii) two versions of the ANU global GIA model, corresponding to the “high-viscosty” and “low-viscosity” solutions obtained by Kurt Lambeck and collaborators24, in which the thickness of the elastic lithosphere and upper mantle viscosities have been set to 60 km and 1.5×1020 Pa·s, respectively, while for the lower mantle we considered both the low-viscosity (2.0×1021 Pa·s, ANUlv) and the high-viscosity (7.0×1022 Pa·s, ANUhv) values. In all the figures of the manuscript, we only show the ANUhv predictions which showed a better agreement with the suites of sea-level data. For all the considered GIA models, the present-day topography has been prescribed according to the bedrock version of the ETOPO1 global topographic model90, integrated with the Bedmap2 relief91 in the Antarctica region (i.e., below 60°S latitude). The numerical solutions of the SLE have been obtained on a global geodesic grid with spatial resolution of ~40 km and include harmonic terms up to harmonic degree LMAX=512, corresponding, by Jeans rule, to the minimum wavelength of ~80 km over the Earth’s surface.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The submitted manuscript covers a topic, deglacial sea level rise, that is within the scope of the journal and has been awaiting solution for a long time.\nOur understanding of the topic has long been hindered by the sparsity of far-field observations and the difficulty in putting these observations into the context of other observations. Deglacial sea level rise happened with such a blistering pace that our lack of understanding puts us in a compromised situation when considering the future.\nThe new observations produced by this manuscript provide new information about deglacial sea level rise, they are taken in a region without such observations before, and they have a good amount of redundancy by use of many field sites up and down the western African coast. I found many questions and discrepancies when reading the manuscript that need to be addressed if the reader is to truly understand the implications of the work. The new observations tend to open up as many new questions as they answer, as can be expected, but some readers may begin to question the robustness of the observational techniques overall. The manuscript would be well served by offering more information in the main text about why these inferences are reliable.\nIssues:\nI had many issues in checking the consistency of the main text and the figures. Some of the discrepancies may be due to my specific printer or computer. Other issues suggest a problem in the versioning of the manuscript. Yet other issues may require more attention to detail from the authors.\nL47, L51: Figure 1b instead of 1a?\nL65: how is the duration of the LGM defined?\nL68-71: These sentences appear to restate the information in Figure 1c. A callout to this figure would be helpful. The text mentions five data points but I could only find four in the figure.\nL75: Note that these corrections can be larger than 10 meters. The reader would benefit from a discussion of the processes that are being corrected/adjusted. It also raises the question of circularity when the new African observations are being adjusted by ICE-6G and then they are later compared to that same product.\nL78: What specifically is meant by \"good agreement\" when comparing African and Barbados records? I see that the limiting points produced from Africa at about 22 ka and 24 ka are on the wrong side of the Barbados record. The conflict is as large as 10 meters.\nL89: What is meant by the \"expanded Atlantic Ocean sea-level record\"? How were the Barbados and African records combined when they have different characteristics including the sampling frequency/time resolution.\nL101: The Senegal record has a progressive increase in sea-level rise, but this occurs during the previously-mentioned deceleration phase of the Atlantic African records overall. How should we interpret the spatial variability in the new observations? It is worth explaining why the sea-level data is reliable.\nL131: Other reconstructions also have the 8.2 ka cooling event, but are they similar or consistent with the new data?\nFigure 2: I only see one panel for this figure. I do not see a Figure 2b. Apologies if this is my mistake.\nWhere do these conflicting records leave us in our understanding of deglacial sea-level rise? What should the non-expert take away from this study?\nThe reader would benefit by more information regarding the EIV-IGP model in the main text. How does the method deal with spatial inhomogeneities in the Atlantic African records? Do the error bars depend upon the time resolution of the statistical output? Are any assumptions about the temporal correlation of the underlying process made? If we are to interpret the statistical estimate as the sea level at one instant in time, that could be very different from interpreting it as the running average of sea level of a 1,000 year time interval. Some of this information in the supplementary material should be highlighted in the main text.\nMinor issues:\nL29: reconcile\nL92: 21 ka BP?\nL145- L148: I did not understand what this sentence was driving at.\nL163: Include a reference for ocean siphoning.\nFigure 3: It may be misleading to use both 1-sigma and 2-sigma error bars in the same figure.\nFigure 3: The red solid line appears blue. The gray solid line appears red. The blue square is gray. The arrow is not found.\nFigure 4: The black line appears red. Can the result of the EIV-IGP model also be plotted in the same form as the ICE-6G model for easier comparison? The current comparison between a statistical quantity and the ICE-6G model is not straightforward.", "reviewer_2": "In this study, the authors have compiled existing data from the Late Pleistocene and Holocene that indicate the position of sea level in space and time across the Atlantic coast of Africa (a geographic region that is under-represented in understanding past environmental change). The authors use standard methods and produce a new sea-level curve for the region. While the methods and results are sound and presented well, the manuscript reads very specific to the paleo sea-level discipline and does not necessarily provide a significant change in our understanding of meltwater contributions despite the title. In particular, the significance of this compilation study could be improved by comparing their results to more than one GIA model that might incorporate broader variability in ice volume change over the deglaciation (currently the authors use only ICE6G without much justification as to why).\nBelow I have made some other specific comments roughly ordered by section:\nAbstract\nI would expect a broader abstract that tells the reader a bit more about the implications of this compilation study – currently it reads well for a discipline-specific audience, but the significance may be unclear to others.\nMain\nMid- and Late Holocene are formal subdivisions of the Holocene epoch and should be capitalized (i.e., Late Holocene instead of late Holocene)\n‘Eustatic’ should be defined explicitly in a RSL context given ‘new’ definitions/terminology (e.g., barystatic; Gregory et al. 2019)\nLine 41: perhaps rephrase to explain the Barbados record without the adjective ‘seminal’? I’m not sure it’s necessary\nLine 43: what’s meltwater pulse 1b? Paleo and RSL people know but not a broader audience. A half sentence to explain would make this clearer.\nBroadly, I would include a little more about the big picture. For example, why do we need to know the timing and magnitude of the LGM lowstand? Why is the Mid-Holocene highstand important? What are some big questions that still remain in this space that this new curve helps to answer (or at least partially answer)? What’s the significance of this information?\nI think some justification of correcting the ICE-6G model is required I think – why this model? Why might it be good to use/not good to use for this specific region? And why not ICE-7G? I know some of the detail is in the methods instead but the main text feels a bit empty without it. Same goes for comparison to the Barbados record – what are some limitations to using this record as the ‘benchmark’? Perhaps phrasing more to test the hypothesis of occurrence of MWP1b, rather than just referring to the ‘Barbados’ record is more impactful.\nLine 81: ‘these data’ – not super clear as written especially because the next sentence also uses a ‘this’… I’d suggest prefacing ‘the Atlantic Africa data’ or similar.\nIf the main results of this paper are to compare existing, quality-controlled data to a GIA model, then hypothesis testing of meltwater pulses and highstands would be better achieved by comparing to a suite of GIA models (or rather, a suite of GIA models that incorporate difference ice sheet histories, particularly the behaviour of Antarctica in the Holocene). There are many assumptions about ice volume histories in a single GIA model and therefore conclusions about meltwater and rates should address these assumptions.\nFigures\n1a) should Papua Nuova Guinea be Papua New Guinea?\n1b) needs more text in the caption to explain what the blue RSL bar represents? It’s not clear.\n1c) there is no caption explaining figure 1c (or it seems merged with 1b?)\nSupplementary Files\nFile 3, figure 2 – caption needs a lot more explaining.\nShallow and open lagoon environments – lagoons can be deeper than a few metres, particularly during early stages of post-glacial flooding before sedimentation catches up (indeed this is highly site-specific). I’m not sure the reference water level and indicative range of the shallow lagoon facies represents this adequately.", "reviewer_3": "Vacchi et al., “New constraints on LGM sea levels and meltwater contributions derived from Atlantic Africa”\nSummary and key results:\nThe authors have compiled, recalibrated and quality checked previous sea-level data from the western (Atlantic) margin of Africa, and following comparison with other sea-level data and GIA predictions (using the ICE-6G spatial and temporal ice history, and laterally homogenous rheology) for the time interval (~0 to 30 ka), the authors highlight three main findings; (1) that the new data compilation shows no evidence for progressive descent, nor step into the LGM (cf. Lambeck et al., 2014 and Yokoyama et al., 2018 respectively); (2) evidence for intervals of high rates of sea-level rise (SLR) during the deglacial, and; (3) the timing and magnitude of the mid-Holocene sea-level highstand along the coast of western Africa and potential Antarctic contributions.\nThe new data compilation is most welcome addition; this region is a notable “data gap”, and the compilation is a valuable addition and makes accessible some good but old publications and data. However, whilst the new compilation and results are nicely presented (barring concerns relating to Figure 2, see comments below), there is relatively light interrogation of the data/results, and only perfunctory discussion of the wider implications. The findings are not particularly novel (see detailed comments following), but this is a solid contribution but perhaps submission to Nature Communications is a little premature.\nIn more detail:\n1) The total ice volume magnitude of the LGM sea-level lowstand (the “missing ice” problem) and the evolution of ice growth is longstanding in the literature (Andrews, 1992), with considerable body of work attempting to address the issue discrepancies between potential ice histories, which continues to be highly debated (e.g., Gowan et al., 2022; Yokoyama et al., 2022). The authors note that their compilation of sea-level indicators from the western coast of Africa the presence of a small number (n=4) of marine limiting points that are more consistent with the sea-level history of Barbados, rather than the more recent records from both Tahiti (Bard et al., 1990; Deschamps et al., 2012) and the Great Barrier Reef (GBR) (Webster et al., 2018; Yokoyama et al., 2018), with a rather bland statement at lines 90 to 92 that these data points do not show a “step” into the LGM (cf. Yokoyama et al., 2018) nor progressive lowering (cf. Lambeck et al., 2014). However, the few data points that exist are marine limiting points (i.e., sea level is somewhere – undetermined, and unknowable – above the data point), and so they lack the vertical precision to be able differentiate between possible sea-level/ice histories. Eyeballing the data (& trusting the quality of the data, apologies but my French is insufficient to gauge this, and the tectonic setting of the area is outside my expertise), could they not equally support the alternative ice histories (e.g., 2-step drop Yokoyama et al., 2018)? The authors might benefit from adding Yokoyama et al. (2018) global mean sea level reconstruction to their plot.\nThe discussion of the dataset within the context of other available datasets is rather cursory – leaving aside the lack of precision from marine limiting data (I do agree that they are valuable, just that they are incapable to testing the question you are trying to answer – i.e., deciphering between LGM ice volumes and evolution of sea-level lowering into the LGM). Other datasets also do not fit with the to the prima facie Barbados record (see comment below) – the Bonaparte Gulf, Red Sea and other data also suggest a the drop into the LGM. I think you would benefit from placing the Atlantic Africa data within the wider context of substantial existing data, and exploring the potential reasons for the discrepancies between the datasets.\nAs an example, there are well documented concerns regarding the Barbados coral record e.g., potential for ex-situ corals and downslope transportation (Blanchon et al., 2021; Blanchon and Perry, 2004), tectonic setting (high-viscosity slab, Austermann et al., 2013; core location on either side of an extensional tectonic feature, Bard et al., 2010), or mantle viscosity structure (Nakada et al., 2016). Further, the older Barbados data (~19 to 26 ka) are different coral derived from deeper settings rather than from the reef crest and these are associated with significantly greater palaeo water depths (~20 m Diploria sp., and 50 m for O. annularis, compared to 5 m for A. palmata from which most of the younger portion of the record is derived). Interestingly, if we look at extended figure 7 from Yokoyama et al. (2018), once these deeper-dwelling species (with their much greater palaeo water depths) are GIA corrected, these older Barbados data just overlap with their RSL predictions based on an ice history derived from the GBR coral data (i.e., drop into the LGM at 22 to 20.5 ka).\nThis brings me onto Figure 2. The discussion in this section of the manuscript is really hampered by the quality of this figure, particularly the lack of error bars, and palaeowater depths for corals (Abbey et al., 2013; Abdul et al., 2016; Bard et al., 2010; Deschamps et al., 2012; Fairbanks et al., 2005; Thomas et al., 2012; Yokoyama et al., 2018 etc.), and indicative meaning for sedimentary archives such as the Sunda Shelf (Hanebuth et al., 2009, 2000) and the Bonaparte Gulf (De Deckker and Yokoyama, 2009; Ishiwa et al., 2019, 2016; Yokoyama et al., 2000) etc. Also, is the data presented from the Bonaparte Gulf complete? There seems to be missing data between the 22 to 28 ka age range (e.g., figure 4 in Ishiwa et al., 2019). Please add these carefully derived palaeowater depths/indicative meanings to the plot. Further, this is relative sea level – GIA complicates the interpretation of sea-level data at specific locations. It is rather disingenuous to just plot all of these datasets on top of one another, and claim that the newly compiled Atlantic coast data compares well to the Barbados data.\nAdditionally, Papua New Guinea is misspelt, and I think you actually mean the data from the Huon Peninsula data of Cutler et al. (2003) – correct?\nThis last point brings me to the sloppy referencing of the original studies in the manuscript (i.e., the compilation of the older data is repeatedly referred to as new data, which it is not), but especially here in Figure 2. Please ensure that the original authors contributions (i.e., sources for the data you have compiled) are properly cited, ensuring that this work properly recognised. I know that Nat Comms limits references to 100, and you currently have 86 in the main body of the manuscript (and you have compiled data from 56 works) but there must be some way to address this. This issue is most apparent in the Figure 2 where you also have not properly referenced the authors for the key deglacial records (Barbados, Tahiti, Sunda Shelf etc.) and instead erroneously refer to Shaw’23. This is not acceptable and must be rectified. I’m certain that there is a solution that properly acknowledges the contribution of the original authors – perhaps the journal can suggest an acceptable solution.\n2) Rates of sea-level rise (SLR) during the deglacial: The use of the EIV-Gaussian process model (Cahill et al., 2015) means that you have sea-level reconstructions that account for vertical and temporal uncertainties, and allow for a more robust estimates of the rates of change. This section is fine, again nicely presented but with rather perfunctory discussion of your result. I wasn’t really sure what was new here – can the location of these sites shed more light on the potential sources of the SLR (i.e., the highly debated meltwater pulse 1b, and the rather more well documented SLR rises around the 8.2 ka event) (I appreciate that this may require further analysis), or are they additional lines of evidence for intervals of high rates of sea-level rise? Perhaps the comment (Bard et al., 2016) and reply (Mortlock et al., 2016) on Abdul et al. 2016 might be useful to consider in your discussion of the Barbados rates given in Figure 3 but not really discussed in the main manuscript?\n3) Contribution of Antarctica to mid-Holocene sea-level highstand along the Atlantic coast of Africa: Again this section is nice, but lacks a link to the implications of the new compilation as much of what is covered in this section is not new. For example, previous work has highlighted that non-unique solutions (due to the combined dependence of ice history and rheology) in global GIA modelling makes partitioning between the difference ice sheets difficult (e.g., Caron et al., 2017) resulting in significant variation magnitude and spatial pattern of deformation (e.g., Whitehouse et al., 2019). Coupled with this is the considerable lateral variations globally (and in Antarctica e.g., Martín‐Español et al., 2016) which could introduce biases (e.g., Yousefi et al., 2021), mean that Antarctic mass contributions (Jones et al., 2022; Sproson et al., 2022) may not be accounted for in the ICE-6G ice history used by the authors. This is to say that a body of literature already exists to suggest that the potentially dynamic contributions from Antarctica may be missing from the ice histories used in our global GIA models (and indeed there is marked differences deglaciation histories between used in GIA modelling e.g., Argus et al., 2014; Ivins et al., 2013; Lambeck et al., 2014; Whitehouse et al., 2012).\n4) Figures: What is the uncertainty level of the data shown in your figures, ±1σ, 2σ? Please add this information to each figure caption (it’s there for some, but not for others).\nFigure 1: (i) are there no vertical uncertainties for the terrestrial and marine limiting points, or are they within the symbol? If the former, add them to the plot. If the latter please state in the caption. (ii) The caption is also missing information for panel c – please add this. (iii) Figure 1B: misleading subdivision, esp. lower end i.e., 1 to 15 SLIPs, many of these have 5 or fewer datapoints – can you add 1 to 5 or 1 to 10 datapoint subdivision.\nFigure 2: (i) why are there no vertical uncertainties shown for the data? Corals (Tahiti, GBR, Barbados) have palaeowater depths, and careful facies analysis for the Bonaparte Gulf (De Deckker and Yokoyama, 2009; Ishiwa et al., 2019, 2016; Yokoyama et al., 2000) etc. This becomes important for your discussion of the drop (or not) into the LGM. (ii) Cite the original authors and their publications for the deglacial data not Shaw! (iii) There is no figure 2b, remove “a” from caption and from the text of the main manuscript. (iv) PNG spelling.\n5) Other minor comments:\nLine 48: you did not produce this data, rather it is a compilation (with recalibration of 14C dates) of data from other studies and these authors should be properly acknowledged. Change “produced” to “compiled”.\nLine 76: (i) remove “input” from “ice equivalent sea level input” (as it doesn’t make sense with the minus sign that follows); (ii) There is no figure 2b, so remove the “A” in “(Fig. 2A)”\nLine 96: I think you are refereeing to Fig 1C rather than 1B here?\nLine 116: this is not new data, rather a compilation. Please amend wording to make this clear.\nSOM3: spelling in figure caption (Figure 1; line 4 “Norther Emisphere”"}, "author_response": "Response to the reviewers\nGeneral comments\nWe thank the three reviewers for their very constructive comments. We have significantly\nmodified the resubmitted manuscript.\nWe accepted all minor comments and corrections suggested by the reviewers with all changes\nhighlighted in yellow in the resubmitted manuscript.\nA detailed point-by-point responses to the reviewer’s major comments are appended below.\nReviewer 1\n1. Consistency of the main text and the figures.\nI had many issues in checking the consistency of the main text and the figures. Some of the\ndiscrepancies may be due to my specific printer or computer. Other issues suggest a problem in the\nversioning of the manuscript. Yet other issues may require more attention to detail from the\nauthors.\nWe have made a major effort to crosscheck the correspondence among text and figures.\n2. GIA correction of the LGM data and agreement between model and data\nL75: Note that these corrections can be larger than 10 meters. The reader would benefit from a\ndiscussion of the processes that are being corrected/adjusted. It also raises the question of\ncircularity when the new African observations are being adjusted by ICE-6G and then they are\nlater compared to that same product.\nL78: What specifically is meant by \"good agreement\" when comparing African and Barbados\nrecords? I see that the limiting points produced from Africa at about 22 ka and 24 ka are on the\nwrong side of the Barbados record. The conflict is as large as 10 meters.\nL89: What is meant by the \"expanded Atlantic Ocean sea-level record\"? How were the Barbados and African records\ncombined when they have different characteristics including the sampling\nfrequency/time resolution.\nThe reviewer is correct, and we made major changes in the section related to the Last Glacial\nmaximum (LGM) (lines 61-86). The correction for glacial isostatic adjustment (GIA) can be\nvery important and this may change according to the model used for this correction. For this\nreason, we only discussed the anatomy of the LGM as derived from our dataset and how this\ncompares with the other LGM records. This was possible because we produced four new sea-\nlevel index points (SLIPs).\n3. Progressive increase in the sea-level rising rate before the 8.2 event and spatial variability in\nthe new observations\nL101: The Senegal record has a progressive increase in sea-level rise, but this occurs during the\npreviously-mentioned deceleration phase of the Atlantic African records overall. How should we\ninterpret the spatial variability in the new observations? It is worth explaining why the sea-level\ndata is reliable.\nWe revised the discussion in the text (lines 116-128). We underlined the presence of a sea-level\nacceleration before the 8.2 event both in Senegal and in northern Gulf of Guinea. However,\nwe also stated that the resolution of our quantitative reconstruction in this period did not\nprovide clear evidence of a sea-level jump preluding the 8.2 event as recorded in other sea-\nlevel records.\n4. Other reconstructions with the 8.2 ka cooling event.\nL131: Other reconstructions also have the 8.2 ka cooling event, but are they similar or consistent\nwith the new data?\nThis point is important and we have modified the text. Many papers identified the sea-level\njump preluding the 8.2 ka and we discussed this in lines 129-132 with appropriate references.\n5. More information regarding the EIV-IGP model in the main text.\nThe reader would benefit by more information regarding the EIV-IGP model in the main text. How\ndoes the method deal with spatial inhomogeneities in the Atlantic African records? Do the error\nbars depend upon the time resolution of the statistical output? Are any assumptions about the\ntemporal correlation of the underlying process made? If we are to interpret the statistical estimate\nas the sea level at one instant in time, that could be very different from interpreting it as the running\naverage of sea level of a 1,000 year time interval. Some of this information in the supplementary\nmaterial should be highlighted in the main text.\nThe EIV-IGP model has been used quantify late Quaternary relative sea-level changes in\nmany coastal regions. We followed the reviewer suggestions to provide some additional detail\nin the methods section (lines 244-247) with appropriate references.\nReviewer 2.\n1. Broader significance of our paper.\nI would expect a broader abstract that tells the reader a bit more about the implications of this\ncompilation study – currently it reads well for a discipline-specific audience, but the significance\nmay be unclear to others.\nI would include a little more about the big picture. For example, why do we need to know the timing\nand magnitude of the LGM lowstand? Why is the Mid-Holocene highstand important? What are\nsome big questions that still remain in this space that this new curve helps to answer (or at least\npartially answer)? What’s the significance of this information?\nWe have better stated both in the abstract (lines 19-20) and in the main text (lines 35-38) why\nour results are important for a broader audience. The abstract is much stronger because we\ncan constrain the LGM data from the presence of a suite of new SLIPs.\nWe have better explained the importance of the timing and magnitude of the LGM lowstand\n(lines 61-64) and the Mid-Holocene highstand (lines 135-142) important.\n2. Selection of the GIA model\nIf the main results of this paper are to compare existing, quality-controlled data to a GIA model,\nthen hypothesis testing of meltwater pulses and highstands would be better achieved by comparing\nto a suite of GIA models (or rather, a suite of GIA models that incorporate difference ice sheet\nhistories, particularly the behaviour of Antarctica in the Holocene). There are many assumptions\nabout ice volume histories in a single GIA model and therefore conclusions about meltwater and\nrates should address these assumptions.\nThe main results of the paper were not the tuning of a GIA model. But the comparison with\nICE-6G allowed a better understanding of RSL at the LGM.\nThe ICE-6G ice model is largely constrained by the coral reconstructions from Barbados in\nthe Atlantic Ocean. Since our records are also from the Atlantic Ocean (off the Africa coast),\nwe compared the SLIPs with ICE-6G_C model predictions.\nAdditionally, the ICE-6G_C global ice model is widely used and freely accessible (from the\nwebsite: https://www.atmosp.physics.utoronto.ca/~peltier/data.php), allowing others to easily\nreplicate our results.\n3. Figures.\nWe have changed the caption of Figure 1. We have added further details in the caption of\nSupplementary Figure 3. More generally, we have improved the clarity of the figures.\n4. Indicative range of the lagoons\nShallow and open lagoon environments – lagoons can be deeper than a few metres, particularly\nduring early stages of post-glacial flooding before sedimentation catches up (indeed this is highly\nsite-specific). I’m not sure the reference water level and indicative range of the shallow lagoon\nfacies represents this adequately.\nThe reviewer is correct that indicative meaning can be subject to change through time,\nespecially prior to the Holocene.\nBut we used an indicative meaning (o to -2 m), which is supported by modern lagoons\ndescribed in Morocco, Mauritania, Senegal, Benin and South Africa. The indicative meaning\nis similar to those used along the coast of Atlantic Europe (see Artola et al., 2019 in QSR) and\ndefined in the Atlas of deglacial sea levels (Khan et a., 2019).\nReviewer 3\n1. Quality of the LGM dataset\nThe total ice volume magnitude of the LGM sea-level lowstand (the “missing ice” problem) and the\nevolution of ice growth is longstanding in the literature (Andrews, 1992), with considerable body of\nwork attempting to address the issue discrepancies between potential ice histories, which continues\nto be highly debated (e.g., Gowan et al., 2022; Yokoyama et al., 2022). The authors note that their\ncompilation of sea-level indicators from the western coast of Africa the presence of a small number\n(n=4) of marine limiting points that are more consistent with the sea-level history of Barbados,\nrather than the more recent records from both Tahiti (Bard et al., 1990; Deschamps et al., 2012)\nand the Great Barrier Reef (GBR) (Webster et al., 2018; Yokoyama et al., 2018), with a rather\nbland statement at lines 90 to 92 that these data points do not show a “step” into the LGM (cf.\nYokoyama et al., 2018) nor progressive lowering (cf. Lambeck et al., 2014). However, the few data\npoints that exist are marine limiting points (i.e., sea level is somewhere – undetermined, and\nunknowable – above the data point), and so they lack the vertical precision to be able differentiate\nbetween possible sea-level/ice histories. Eyeballing the data (& trusting the quality of the data,\napologies but my French is insufficient to gauge this, and the tectonic setting of the area is outside\nmy expertise), could they not equally support the alternative ice histories (e.g., 2-step drop\nYokoyama et al., 2018)? The authors might benefit from adding Yokoyama et al. (2018) global\nmean sea level reconstruction to their plot.\nFollowing the reviewer’s very important comments we revised the text and figures as follows:\n1. We produced a new SLIPS. Our new data are in line with a progressive lowering of\nthe relative seas level during the LGM, providing the first evidence of a late sea-level\nminimum (e.g., at ca 20 ka BP) in the Atlantic Ocean. We modified the text ins both in\nthe main text (lines 64-81) and in the abstract (lines 22-24). Our data cannot confirm\nthe 2-step drop found Yokoyama et al., 2018 because we have a lack of data between 22\nand 20 ka BP. We modified the text at lines 80-82 to clearly state this limitation.\n2. We did not correct the relative sea-level data for GIA as in our original submission,\nbecause this correction may be different according to the different models available in\nliterature.\n3. We have discussed the relationship between our record and the Barbados record (lines\n82-86) and updated the references.\n4. We have replaced Figure 2 which is now focused solely on our datasets from Atlantic\nAfrica.\n2. Rates of sea-level rise (SLR) during the deglacial\nRates of sea-level rise (SLR) during the deglacial: The use of the EIV-Gaussian process model\n(Cahill et al., 2015) means that you have sea-level reconstructions that account for vertical and\ntemporal uncertainties, and allow for a more robust estimates of the rates of change. This section is\nfine, again nicely presented but with rather perfunctory discussion of your result. I wasn’t really\nsure what was new here – can the location of these sites shed more light on the potential sources of\nthe SLR (i.e., the highly debated meltwater pulse 1b, and the rather more well documented SLR\nrises around the 8.2 ka event) (I appreciate that this may require further analysis), or are they\nadditional lines of evidence for intervals of high rates of sea-level rise? Perhaps the comment (Bard\net al., 2016) and reply (Mortlock et al., 2016) on Abdul et al. 2016 might be useful to consider in\nyour discussion of the Barbados rates given in Figure 3 but not really discussed in the main\nmanuscript?\nBy applying the EIV-Gaussian process model to the SLIPs we are able to quantify the rates\nand magnitude of sea level which includes the uncertainties in the data.\nWith new SLIPs at the LGM we are able to quantify lowering of the relative seas level\nWe highlighted the discrepancies between our data and the MWP1b in the Barbados record.\nHowever, there are some limits on our dataset (lines 118-120) and, in our opinion, this\nprecludes any deeper analysis. We state this in the manuscript (lines 117 to 121)\nWe have modified the text to better discuss the observed increase in the sea-level rising rates\nprior to the 8.2 ka BP (lines 124-129) and the major deceleration observed at 8.0 ka (lines 129-\n132).\n3. Contribution of Antarctica to mid-Holocene sea-level highstand along the Atlantic coast of\nAfrica\nContribution of Antarctica to mid-Holocene sea-level highstand along the Atlantic coast of Africa:\nAgain this section is nice, but lacks a link to the implications of the new compilation as much of\nwhat is covered in this section is not new. For example, previous work has highlighted that non-\nunique solutions (due to the combined dependence of ice history and rheology) in global GIA\nmodelling makes partitioning between the difference ice sheets difficult (e.g., Caron et al., 2017)\nresulting in significant variation magnitude and spatial pattern of deformation (e.g., Whitehouse et\nal., 2019). Coupled with this is the considerable lateral variations globally (and in Antarctica e.g.,\nMartín Español et al., 2016) which could introduce biases (e.g., Yousefi et al., 2021), mean that\nAntarctic mass contributions (Jones et al., 2022; Sproson et al., 2022) may not be accounted for in\nthe IC‐E-6G ice history used by the authors. This is to say that a body of literature already exists to\nsuggest that the potentially dynamic contributions from Antarctica may be missing from the ice\nhistories used in our global GIA models (and indeed there is marked differences deglaciation\nhistories between used in GIA modelling e.g., Argus et al., 2014; Ivins et al., 2013; Lambeck et al.,\n2014; Whitehouse et al., 2012).\nAccording to the reviewer’s suggestion, we made changes to this section. We better explore\nthe causes of the misfit between data and model both at lines 155-164 and at lines 174-185.\nWe focused our discussion on the potential Antarctic contribution and on the neglection of 3D\n(i.e., lateral heterogeneous) Earth structure, which needs to be included in GIA modelling for\na continental scale study.\n4) Figures\nFigures: What is the uncertainty level of the data shown in your figures, ±1σ, 2σ? Please add this\ninformation to each figure caption (it’s there for some, but not for others).\nFigure 1: (i) are there no vertical uncertainties for the terrestrial and marine limiting points, or are\nthey within the symbol? If the former, add them to the plot. If the latter please state in the caption.\n(ii) The caption is also missing information for panel c – please add this. (iii) Figure 1B:\nmisleading subdivision, esp. lower end i.e., 1 to 15 SLIPs, many of these have 5 or fewer datapoints\n– can you add 1 to 5 or 1 to 10 datapoint subdivision.\nFigure 2: (i) why are there no vertical uncertainties shown for the data? Corals (Tahiti, GBR,\nBarbados) have palaeowater depths, and careful facies analysis for the Bonaparte Gulf (De\nDeckker and Yokoyama, 2009; Ishiwa et al., 2019, 2016; Yokoyama et al., 2000) etc. This becomes\nimportant for your discussion of the drop (or not) into the LGM. (ii) Cite the original authors and\ntheir publications for the deglacial data not Shaw! (iii) There is no figure 2b, remove “a” from\ncaption and from the text of the main manuscript. (iv) PNG spelling.\nWe provided the uncertainty requested by the reviewer in the caption of Figure 1 and 3.\nIn the Figure 1, the vertical uncertainty is within the symbol. We have corrected the caption\nto have the right correspondence with the different panel. The subdivision of the number of\nSLIPs of panel b was done to have data grouped every 15 SLIPs.\nWe followed the suggestion of the reviewer regarding the significance of Figure 2. The mix\namong index points and limiting points was unclear and was not helpful. Furthermore, the\ncomparison between data corrected for GIA and raw data was misleading. For this reason, we\ndecided to focus the figure on our data and discuss the anatomy of the LGM derived from the\nAtlantic Africa record."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The manuscript presented by Vacchi et al. is very important, detailed work that has been done using standard – but certainly sound – methods. The framing of the study and the clarity of the significance – in particular that related to the LGM – is much improved. I look forward to seeing this published. I have three key points regarding this latest iteration:\n1) I had previously made a suggestion that the study reads well for a discipline-specific audience, but may be less clear to a broader audience. I still believe this is the case and the authors have not changed much to address this (although I’m not sure what was suggested by the editor!)\n2) In addition, in my previous review I queried the use of only ICE-6G as a comparison. I appreciate that the main results are not to tune GIA models but I don’t find the authors response particularly compelling. There’s a discussion on 3D Earth structure and on Antarctic melting history to explain some of the results – why not include some other models that show some different scenarios? I would have expected a bigger suite of models to compare to as the current comparison really raises more questions than provides answers.\n3) The other framing I think that should really be addressed is the statements of ‘new constraints’ (e.g., line 63). These are not new constraints. The interpretation as a SLIP/limiting point is the new – and still very important! - part. I suggest this language be changed to reflect this throughout the manuscript. Especially given the authors were not involved in the collection, analysis, and publication of any of the data that has been re-interpreted and analysed in this manuscript.\nI have noted some more minor things below:\nLines 53-56: perhaps tell us what these time ranges are in numbers as well?\nLine 89 – can we get some details of where this land-based ice estimate number comes from.\nFigure 1A: explanation of the colour bar (e.g., RSL at what specific point -or window- of time? Also refer to the ice extent that appear to be here too?). Papua New Guinea or Papua Nuova Guinea?\nFigure 2: SLD not referred to in caption (guess it’s sea level drop but tell the reader what the acronym is or better yet just write it out)", "reviewer_3": "The authors have addressed my comments on the initial draft of the manuscript, thank you. The revised manuscript has placed the current study within the context of previous work, and gives a much more rounded picture for the general reader.\nA couple of minor issues should be addressed prior to publication: (i) acknowledgement the original authors of the data in both the text of the manuscript and the figure captions (i.e., refer the reader to your method section and the SOM), and; (ii) the text of the captions (see below).\nFiona Hibbert (Reviewer 3)\nMinor comments:\nFigure 1(a): what is the underlying plot (i.e., associated with the colour bar)? Predicted sea level at what time, and from what model? Just needs clarification in the caption.\nFigure 2:\nplease add “region” to the numbers in parentheses for three locations to avoid confusion with references ( as per your other captions).\nLine 512: Consider changing “extension” to “extent” for clarity."}, "author_response": "Response to the reviewers\nGeneral comments\nWe thank the reviewers for their second round of very constructive comments.\nWe accepted all minor comments and corrections suggested by the reviewers with all changes\nhighlighted in yellow in the resubmitted manuscript. We have also edited the figure captions\naccording to the comments of the reviewers.\nReviewer 2 found that our manuscript is very important and that the framing of the study and\nthe clarity of the significance is much improved after our revision.\nWe have made some major changes in order to reply to his/her key points\n1) I had previously suggested the study reads well for a discipline-specific audience, but may be\nless clear to a broader audience.\nWe further modified the introduction of the paper in order to make clearer the global\nsignificance of our paper for a broader audience (lines 33-37). Furthermore, at the beginning\nof section, we underlined the importance of LGM sea-levels (60-63) and mid-holocene\nhighstand (135-142).\n2) In my previous review I queried the use of only ICE-6G as a comparison. I appreciate that\nthe main results are not to tune GIA models but I don’t find the authors response particularly\ncompelling. There’s a discussion on 3D Earth structure and on Antarctic melting history to\nexplain some of the results – why not include some other models that show some different\nscenarios? I would have expected a bigger suite of models to compare to as the current\ncomparison really raises more questions than provides answers.\nWe followed the request of Reviewer 2 and we generated a new suite of GIA predictions using\nthe ANU models which were largely employed (together with ICE6G) in other far field regions.\nThe results of the comparison between data and model are very interesting while ANU\npredictions show more fit with the sea level data in mid Atlantic Africa (predicting a second\npeak of the highstand at about 2.0 ka) while they show major misfits in the northern part of\nAtlantic Africa (lines 150-171)\nThis allowed to better discuss the fit/misfit of the two different models and provide more\ninsights into the meltwater contributions in the different part of Atlantic Africa (lines 176-\n179, lines 193-202 and 205-207).\nThe comparison between the probabilistic analysis of the highstand and the ICE6G and ANU\npredictions is now also showed in the new figure 4 and in its figure caption (lines 566-568).\nWe further updated the methods section with the details of the ANU predictions (lines 285-\n291) and we created new figures in Supplementary 2 and Supplementary 3.\n3) The other framing I think that should really be addressed is the statements of ‘new\nconstraints’ (e.g., line 63). These are not new constraints. The interpretation as a\nSLIP/limiting point is the new – and still very important! - part. I suggest this language be\nchanged to reflect this throughout the manuscript. Especially given the authors were not\ninvolved in the collection, analysis, and publication of any of the data that has been re-\ninterpreted and analysed in this manuscript.\nAccording to the reviewer’s suggestion, we changed the languages along the manuscript. We\nWe clearly stated in the abstract (lines 20-21) that the data we showed are the result of a\ndatabase of quality controlled sea-level data. This is also now specified in the methods section\n(lines 219-221) where we gave credit to the original collectors of the data. We remind that a\nlot of data where not used as sea-level indicator in the original papers. So that, the\ncompilation of the database and the production of SLIP, TLP and MLP represent an original\nwork."}, {"round": 3, "reviewer_comments": {"reviewer_2": "I have only looked at the new figures and text as part of this review rather than the other bits of the manuscript that I assume are the same. It is nice to see the inclusion of one different GIA model that allows the authors to speak more to different ice histories.\nSome (very) minor things:\n1) should be late Holocene rather than late-Holocene (e.g., line 199)\n2) line 220 - should probably be written as \"1960's\""}, "author_response": ""}]

Decision letter