Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations

Domain: earth_science
Published: 2023-08-05
Editorial outcome: major_revision

Abstract
Qunf Cave oxygen isotope (δ18O) record from southern Oman is one of the most significant of few Holocene Indian summer monsoon cave records. However, the interpretation of the Qunf δ18O remains in dispute. Here we provide a multi-proxy record from Qunf Cave and climate model simulations to reconstruct the Holocene local and regional hydroclimate changes. The results indicate that besides the Indian summer monsoon, the North African summer monsoon also contributes water vapor to southern Oman during the early to middle Holocene. In principle, Qunf δ18O values reflect integrated oxygen-isotope fractionations over a broad moisture transport swath from moisture sources to the cave site, rather than local precipitation amount alone, and thus the Qunf δ18O record characterizes primary changes in the Afro-Asian monsoon regime across the Holocene. In contrast, local climate proxies appear to suggest an overall slightly increased or unchanged wetness over the Holocene at the cave site.

Introduction
Speleothem oxygen isotope (δ18O) records in the Asian summer monsoon domain are important to understand monsoon variability on a wide range of timescales1–5. The Asian summer monsoon contains two large monsoon subsystems, the East Asian summer monsoon and the Indian summer monsoon. The speleothem δ18O records from both regions show a broadly similar pattern on the orbital scale, following Northern Hemisphere summer insolation (NHSI), nearly in phase at the precession band (~20-ka cycles, ka: thousand years)1,6–10, which supports the conventional monsoon hypothesis11—the insolation hypothesis12. A large number of publications clearly show that the enhanced Asian summer monsoon from the precession minimum (Pmin) or NHSI maximum to the precession maximum (Pmax) or NHSI minimum, corresponds to the negative δ18O excursions in the speleothem records, and vice versa1,3,11,13–19. These data-model comparisons validate the interpretation that speleothem δ18O records from the Asian summer monsoon domain reflect regional Asian summer monsoon intensity or the scale of summer monsoon circulations on orbital scales1,10. However, the relation between the speleothem δ18O records and precipitation amount at the cave site may be complex1–3. A number of studies suggested that δ18O records from the Asian continent may partially reflect changes in moisture source, isotope fractionation at the source area, transport pathways, as well as the isotopic composition of the Bay of Bengal surface water13,20–22. When the Asian summer monsoon is strong, the spatial scale of the summer monsoon circulation expands, and more remote moisture is transported inland, resulting in lower precipitation oxygen isotope (δ18Op) values in continental regions and vice versa1–3,16,23–26. Theoretically, speleothem δ18O, as a proxy of the δ18Op at the cave site, is related to oxygen-isotope fractionations integrated from tropical oceans to the cave site along the moisture transport trajectory25,26. Besides, the influence of other factors on speleothem δ18O, such as the mixing of different moisture sources and rainfall amount, should not be precluded.

The Holocene, from ~11.7 ka BP (before present, where present = 1950 CE) to the present, spans about half of a precession cycle, approximately from Pmin (NHSI maximum) to Pmax (NHSI minimum). While a large set of Holocene speleothem records are available in the East Asian summer monsoon domain3,27,28, similar records in the Indian summer monsoon domain remain sparse and segmented. The well-known Indian summer monsoon record comes from Qunf Cave in Oman, southeastern Arabian Peninsula29. The Qunf δ18O record covers most of the Holocene and has been widely used as a typical Indian summer monsoon record. It has been highly cited during the past two decades and remains a unique Holocene δ18O record in the area up till now. Previous studies on the Qunf δ18O record, and additional speleothem records from Oman and Yemen, suggested that these records reflect changes in Indian summer monsoon precipitation amount during the Holocene29–32. The sharp decrease of the speleothem δ18O from 10.3 to 9.6 ka BP was interpreted to indicate a rapid northward shift of the summer position of the intertropical convergence zone (ITCZ) and the associated Indian summer monsoon rainfall, leading to a rapid increase in summer monsoon precipitation. After ~8 ka BP, the speleothem δ18O record shows an almost linear response to the orbitally-induced variation in NHSI at 30°N, which implies a gradual southward retreat of the ITCZ and gradual weakening of the Indian summer monsoon in response to a decrease in summer insolation29. This interpretation seem to be consistent with the conventional wisdom that increased NHSI enhances the land-sea thermal contrast and, thus, the summer monsoon intensity11. However, this classical notion of monsoon has been challenged by a number of recent modeling studies33–35.

Marine records from the Arabian Sea near Qunf Cave, such as the upwelling-driven biological productivity, foraminifera assemblage, particle size, and oxygen minimum zone records were also interpreted as proxies of the Indian summer monsoon intensity36–38. A few marine upwelling records from the Arabian Sea show a consistent Holocene pattern with the speleothem records39. However, some marine upwelling records in the Arabian Sea suggest intensified Indian summer monsoon winds throughout the entire Holocene and show a significant lag to NHSI and the Indian summer monsoon intensity change inferred by speleothem δ18O records36,40–42. These observations were interpreted to be caused by the latent heat from the southern Indian Ocean and the global ice volume besides insolation36,42,43. Alternatively, the apparent lag to the NHSI is interpreted as the length of late summer and the effect of the Atlantic Meridional Overturning Circulation37,38. Hsu et al.44 were the first to show that their climate model actually reproduced this opposite response between land and ocean to NHSI at the precession band. This study was followed by multiple climate model simulations that confirmed the results from refs. 3,16,17,45–47. Later, ref. 2 coined the term “land-sea precession phase paradox” in the research forefront of the Asian summer monsoon, referring to the contrast between speleothem and marine proxy records. Also, Ruddiman48 argued that the Arabian Sea summer monsoon proxies are not tightly coupled to monsoon intensity but are influenced by other processes. Recently, refs. 45,46 explained the mechanisms for the opposing marine and terrestrial proxy response in the Indian summer monsoon domain. It appears that the upwelling-based proxies in the Arabian Sea are not linked to the Indian monsoon rainfall over India at precession timescales47. Changes in the low-level jet could cause a stronger wind stress curl during weaker summer insolation and lead to strong upwelling. This interpretation appear to largely reconcile the so-called “sea-land precession-phase paradox”, as the monsoon wind strength over the oceans is not necessarily coupled with monsoon precipitation over the Asian continent1,2. However, in light of these recent discussions and data-model comparisons16,21,25, critical issues remain regarding: (1) What is the hydroclimate significance of Qunf δ18O variation? (2) What is the local hydroclimate variation pattern across the Holocene? (3) What is the relation between the Qunf δ18O record and NHSI? (4) Is precipitation at Qunf Cave sourced from single or multiple moisture sources? These questions call for further investigations, especially with speleothem multi-proxy data.

In this study, we provide multiple proxies in addition to δ18O from the stalagmite Q5 from Qunf Cave, southern Oman, including (234U/238U)0, δ13C, trace elements, fluid inclusion δDfi, and Δ'17O records to reconstruct the local and regional hydroclimate history across the Holocene. We then compare the Qunf Cave record with climate model simulations to evaluate alternative interpretations of Qunf δ18O values.

Results
Climate conditions and chronology
Qunf Cave (17°10′N, 54°18′E; 650 m above sea level), from which stalagmite Q5 was collected, is located in southern Oman (Fig. 1), at the northern fringe of the present summer ITCZ and the northern limit of the Indian summer monsoon. At present, summer monsoon rainfall accounts for more than 90% of the total annual rainfall amount (400–500 mm at the cave site)29. The average summer (June, July, and August, JJA) rainfall recorded at the Salalah GNIP station (17°02′N, 54°07′E) is 1.65 mm d−1 (1988–1989), and the JJA rainfall of 1989 is 3.43 mm d−1 at Qairoon Hairiti GNIP station (17°15′N, 54°05′E). The Somali jet brings considerable moisture to the Qunf Cave site during the summer, mainly from the tropical South Indian Ocean across the equator to the western Arabian Sea (Fig. 1). In comparison with the dry years, the wet years in the area have more local moisture contributions, accompanying with less precipitation over South Asia. Meanwhile, the moisture tagging analysis and simulations also indicate that the Red Sea, the Persian Gulf, Mediterranean and Iranian Plateau provide considerable moisture to the Qunf Cave site (Fig. 1 and Supplementary Fig. 1). Q5 grew from 10.8 to 0.4 ka BP with a hiatus from 2374 (±143) to 840 (±56) year BP (Supplementary Fig. 2), i.e., almost the entire Holocene.

Moisture sources revealed by multi-proxy records and climate models simulation
Fluid inclusion δDfi values provide the isotope composition of the cave dripwater from which the speleothem calcite precipitated, and may contain information on the conditions of dripwater. Q5 δDfi data show a distinct trait. During the Holocene Humid Period in Southern Arabia (~10.5 to ~6 ka BP)30,32,49,50, δDfi values are more depleted, with a mean δDfi value of ~2.07‰. After ~6 ka BP, δDfi shows an increasing trend, with an average value of ~9.46‰ (Fig. 2a). Consistently, the δDfi data from Hoti Cave (northern Oman), which sits close to Qunf, also show a significant change in the moisture source, seasonality, and amount of rainfall above the cave around 6 ka BP49. Moreover, the slope of the ln(δ18Oc + 1)-ln(δ17Oc + 1) line shifts at ~6 ka BP from 0.510 to 0.523 (Supplementary Fig. 4a, b). δ18Oc-δ17Oc data thus suggest that either moisture source conditions or the moisture sources have changed at ~6 ka BP in southern Oman. Although previous studies based on δ18Oc records (including Q5) consider that the Indian summer monsoon (or the Indian Ocean) was the sole moisture source in southern Oman29–31,51, our new data seem to imply an additional moisture source, yet unknown, that could be involved during the Holocene Humid Period49.

Δ'17O is defined as the deviation of triple oxygen isotope data from a reference relationship between speleothem δ17Oc and δ18Oc52 (See supplementary text), which is related to distinct hydroclimate dynamics53–56. Speleothem Δ'17O is not sensitive to Rayleigh distillation from the ocean to the continent57 (Supplementary Fig. 5a). Therefore, it reflects mainly the original Δ'17O signal of water vapor, which in turn allows a calculation of the relative humidity at the moisture source52,58–60. In addition, other factors might also contribute to some extent to the Δ'17O variation, such as the mixing of water vapors (between the original air masses and continental recycling moisture along air-mass trajectories), re-evaporation of precipitation (depending on the re-evaporation rate and downdraft intensity), and convections61,62. Collectively, the process of stronger continental recycling and re-evaporation of precipitation may cause higher Δ'17O (Supplementary Fig. 5b). Q5 Δ'17O data show higher values (−225 per meg) during ~8-6 ka BP compared to early (−259 per meg) and late (−245 per meg) Holocene (Fig. 2b). Between 8 and 6 ka BP, the relative humidity of the moisture source area inferred by Δ'17O is lower (~60%) (Fig. 2c). From ~6 ka BP to the present, the calculated relative humidity of Q5 moisture source fluctuated around 80%.

To further understand the hydroclimate conditions at Qunf Cave, we compared the EC-Earth simulated summer (JJA) hydrological conditions between 8K (8 ka BP) and PI (pre-industrial period). Our model simulations indicate that during the 8K period, the North African summer monsoon—characterized by the southwesterly wind extending across the continent and reaching the southern Arabian Peninsula was considerably stronger (Fig. 3a). Although it is possible that the ~900-m high Dhofar mountain could prevent some of the moisture from the African continent to reach the Qunf Cave area, both the summer monsoon wind from the Arabian Sea in the south and the African continent in the west could have contributed water vapor to southern Oman at 8K. This is in line with an expansion of the North African summer monsoon across the Red Sea during the high NHSI time16,63. The model simulations reveal a ~6% increase in relative humidity at the equatorial Atlantic, Arabian Sea and Qunf Cave site from 8K to PI (Fig. 3b). The Δ'17O-based reconstruction of relative humidity at the moisture source shows a ~15% increase from middle to late Holocene (Fig. 2c). The direction of these Holocene trends is consistent. Taking into account the uncertainties of the modeled and reconstructed relative humidity, these datasets show strong similarities.

The influence of the Indian summer monsoon and its interaction with the North African summer monsoon may not be ignored as the Arabian Sea is located right in the confluence region between African and Asian summer monsoon systems13,16,63,64. The North African summer monsoon has effective control over the Eastern Mediterranean deep water ventilation on orbital time-scales through changing the discharge of the Nile River. This results in density stratification and breakdown of deep water formation reflected by sapropel layers S1 in Ocean Drilling Program Site 968 from ~10.2 to 6.5 ka BP65–68. This strong North African summer monsoon period correspond precisely with the lowest δ18Oc period in the Q5 record (Fig. 4). Furthermore, the Soreq and Jeita cave δ18Oc records from the Levant are also sensitive to changes in δ18O of the Eastern Mediterranean surface seawater. More negative δ18Oc values are indicative of higher Nile discharge from the early to middle Holocene69,70 (Fig. 4). Additionally, δDwax records from eastern Africa, which reflect North African summer monsoon variations, also exhibit a trend similar to the Q5 δ18Oc record71 (Fig. 4). After ~6 ka BP, the North African summer monsoon fringe retreated from the southern Arabian Peninsula/northeastern Africa associated with the termination of the Mediterranean sapropel S1 period and the termination of the Holocene African Humid Period (Fig. 4). The Qunf multi-proxy datasets, combined with a set of model simulations on the Holocene hydroclimate evolution in the region, allow for a thorough comparison between observed and modeled Holocene hydroclimate conditions. We propose that there was a large-scale reorganization of atmospheric circulation in the region. In addition to the water vapor transported by the Indian summer monsoon from the adjacent Arabian Sea, the North African summer monsoon may have contributed remote moisture from the tropical Atlantic via Northeast Africa to Qunf Cave during the early and middle Holocene. This finding offers robust evidence supporting earlier model results16,17. In this regard, the shift in δDfi and Δ'17O data around 6 ka BP could indicate a decrease/withdrawal in the contribution of the North African summer monsoon63.

Understanding of Qunf δ18Oc record
δ18Oc values of stalagmite Q5 show an abrupt decrease (from −0.2 to −2.5‰) that occurred from ~10.8 to 9 ka BP, followed by a relatively stable period (~−2‰) between 9 and 6 ka BP, and then by a persistent increase of δ18Oc from −2‰ to ~0‰ till a hiatus from ~2.4 to 0.8 ka BP. After the hiatus, δ18Oc decreased from ~0‰ to ~−1‰ over ~400 years (Fig. 2d). δ18Oc shows the same trends as δDfi, supporting that the δ18Oc reflects dripwater δ18O variability. Currently, the effect of the Indian summer monsoon is prevailing over the southern Arabian Peninsula (Fig. 1). The long-term increasing trend of the Qunf δ18Oc record from ~6 ka BP to the present may indicate an overall Indian summer monsoon (or summer monsoon circulation) waning process8,49,72. A significant mid-Holocene positive shift of δ18Oc at ~6 ka BP is also found in Hoti Cave in northern Oman (Fig. 4), marking the southward retreat of the ITCZ and the associated Indian summer monsoon rainfall belt and a change in the seasonality of rainfall32,49. As aforementioned, water vapor from the equatorial Atlantic could be transported across the African continent by strong North African summer monsoon wind to Qunf Cave during the early to middle Holocene. Thus, from ~10 to ~6 ka BP, southern Oman was likely affected by both the Indian summer monsoon and likely, but to a lesser extent, the North African summer monsoon, both resulting in the more negative δ18Op owing to larger-scale summer monsoon circulations and associated remote moisture1–3, as well as the increased local precipitation amount effect (“amount effect”)29–31.

Taking a broader perspective, the monsoons over the African continent and the Indian Ocean exhibit remarkably similar hydroclimate trends during the middle to late Holocene. This suggests that they were sensitive to, or driven by, a common forcing mechanism on suborbital timescales73,74. More broadly, the entire North African summer monsoon and Asian summer monsoon (Indian summer monsoon and East Asian summer monsoon) may be viewed as a large unified summer monsoon system. In this interpretative framework, the Qunf δ18Oc record shows the Holocene hydroclimate variation of this “super-monsoon” or Afro-Asian monsoon regime63,64, since the Qunf δ18Oc variations appear to follow the overall changes of the Afro-Asian monsoon intensity according to both empirical and model results as aforementioned.

The Qunf δ18Oc record mostly reflects large-scale monsoon circulation changes, making local hydroclimate proxies an essential complement. Combining the (234U/238U)0, δ13C, and trace element records from Q5 (Fig. 2) with the simulation results (Fig. 3 and Supplementary Fig. 7) enables us to reconstruct local precipitation minus evaporation (P-E) over the Holocene. Our simulation results show that both the North African summer monsoon and Indian summer monsoon were much stronger at 8K than at PI. Notably, the summer precipitation amount at Qunf Cave was higher at 8K. This may lead to increased water flow rates in Qunf Cave’s epikarst, resulting in lower (234U/238U)0 values, as (234U/238U)0 is influenced by water flow rates and water–rock interaction times75–77.

However, the summer P-E appear to be slightly lower over a large part of the Arabian Peninsula, suggesting a strong evaporation condition (Fig. 3, Supplementary Fig. 7). These results are consistent with Q5 δ13C and trace elements ratio data. Lower summer temperature and slightly increased P-E could favor increased organic materials and reduced prior calcite precipitation (PCP), resulting in more negative δ13C values in the late Holocene78–80. Q5 δ13C values range from -6.4‰ to 1.1‰, with relatively stable values (~−1‰) from ~10.8 to 6 ka BP and a decreasing trend from ~−1‰ to ~−5‰ since ~6 ka BP (Fig. 2), suggesting slightly lower P-E (or effective wetness) in the early and middle Holocene compared to the late Holocene. Trace element ratios (Mg/Ca, Sr/Ca, and Ba/Ca) show a significant decreasing trend from 10.8 to ~6 ka BP, followed by low and stable ratios after ~6 ka BP (Fig. 2). PCP often occurs in the vadose zone of many cave systems, and increases with reduced effective infiltration, affecting trace element ratios in cave drip waters81–83. The slightly lower P-E condition at 8K (or the early to middle Holocene, Supplementary Fig. 7) suggests increased evaporation under higher summer insolation, promoting PCP and resulting in observed higher trace element ratios. The low and stable trace element ratios after ~6 ka BP could be due to the expected trend of reduced summer evaporation caused by decreased NHSI from the early/middle to late Holocene.

Both model simulation and proxy records show a similar increasing trend in local hydroclimate conditions at the cave site from 8K to PI, the P-E was lower during the early to middle Holocene compared to the late Holocene (See supplementary text). However, given the variety of factors and multiplicity of proxy interpretations, further studies remain a prerequisite to understanding the Holocene hydroclimate history in the region.

The Indian summer monsoon variations across the Holocene
The understanding of monsoons as an integral component of the global atmospheric circulation and hydroclimate is becoming prevalent33,35. Of note are three primary theoretical concepts of monsoons: one based on convective quasi-equilibrium (CQE)84,85, another founded on the moist static energy budget86,87, and a third that frames the monsoon as an extension of the zonal-mean ITCZ34,35. While these theoretical frameworks help explain certain aspects of modern monsoon variability, especially regarding monsoon rainfalls and thermodynamics, the interpretations of paleoclimate variations across various timescales are still in development45,46,88,89.

Physically, the summer barometric differentials and other boundary conditions on a large spatial-scale, regardless of their cause, drive the Asian summer monsoon circulation (e.g., at 850 hPa) (Supplementary Fig. 8) and the associated large-scale vapor flux (Supplementary Fig. 9). In terms of precipitation δ18Op at the cave site, prior to the onset of the Asian summer monsoon, the moisture source is relatively proximate, resulting in heavier δ18Op. After the summer monsoon onset, the spatial-scale of monsoon circulation and moisture from remote sources increased dramatically (Supplementary Figs. 8 and 9), both leading to lighter precipitation δ18Op at the cave site. Consequently, cave δ18Oc mainly indicates the dynamic aspect of the monsoon circulation90, with a small portion of the variability explained by local rainfall amount or thermodynamics. Local rainfall amount or P-E is, however, better reflected by other proxies [e.g., trace elements, (234U/238U)0, and δ13C].

As the Indian summer monsoon is an interhemispheric monsoon system, the interhemispheric differential in tropical insolation (or Summer Inter-Tropical Insolation Gradient, SITIG91) is critical, reflecting both “pull” and “push” forcings from Northern and Southern Hemisphere that propel monsoon changes1. Thus, we use the interhemispheric differential in tropical insolation (30°N-30°S) as an integrated insolation forcing of the monsoon, consistent with previous evidence, such as the Asian summer monsoon variation pattern over the MIS31. The Qunf δ18Oc record suggests an Indian summer monsoon waning trend from the middle Holocene to the present, broadly following SITIG. This observation is consistent with many other Indian summer monsoon records, such as the Sahiya and Mawmluh records from northern India72,92, the Tianmen record from the southern Tibetan Plateau93, and some of the upwelling records from the Arabian Sea and its adjacent area39 (Fig. 4).

In the early 1990s, ref. 36 studied a set of upwelling proxies from marine sediment cores from the Arabian Sea. They raised a hypothesis that changes in the Indian summer monsoon were driven not only by NHSI, but also by the latent heat release of moisture originating from the southern Indian Ocean and the global ice volume affecting the Indian summer monsoon precession phases. Sea surface temperature variation over the south subtropical Indian Ocean controls the latent heat effect on the Indian summer monsoon, which has a significant phase lag to the NHSI maxima at the precession band36,40,94–96. Multiple upwelling records indeed show a significant phase lag (up to 8–11 ka) or nearly anti-phase to NHSI at the precession band40–42. Model simulations reveal contrasting hydroclimatic responses between land and ocean to NHSI changes at the precession band16,17,44. This phenomenon has been referred to as the “land-sea precession phase paradox”2. In the Indian summer monsoon regions, Jalihal et al.46,47,97 investigated the opposing marine and terrestrial responses, both empirically and theoretically. They discovered that, in addition to NHSI, precipitation changes over the ocean are influenced by changes in surface energy fluxes and vertical stability. This view largely reconciles the apparent discrepancy between the Qunf δ18Oc record and some previous marine upwelling records from the Arabian Sea in terms of the precession phase paradox.

Overall, our speleothem multi-proxy records and modeling results provide valuable insights into the interpretation of the Qunf δ18Oc record and the Holocene hydroclimate history in the southern Arabian Peninsula. During the early to middle Holocene, corresponding to the Holocene Humid Period in southern Arabia, the North African summer monsoon, in addition to the Indian summer monsoon, may likely contribute water vapor to the Qunf Cave area as well. After ~6 ka BP, Qunf Δ'17O data suggest a possible reduction in moisture delivered by the North African summer monsoon, higher δ18Oc values suggest that only the Indian summer monsoon prevails with water vapor mainly derived from the proximate Arabian Sea like today. Our results are consistent with the interpretative framework that in addition to the amount effect, speleothem δ18Oc values are essentially related to oxygen-isotope fractionations integrated during the moisture transport from the source to the cave site. More broadly, the Qunf δ18Oc record may be viewed theoretically as an archive indicating the change of the vast monsoon regime, the unified North African summer monsoon-Asian summer monsoon system. Although the Qunf δ18Oc have lighter values and the precipitation amount might increase from ~10 to ~6 ka BP, Qunf (234U/238U)0, δ13C, and trace element ratios (Mg/Ca, Sr/Ca, Ba/Ca) seem to suggest a lower P-E condition. This requires further investigation.

Methods
Modern source–receptor relationship
To establish a modern source–receptor relationship for moisture transport in the study region, the Lagrangian trajectory model FLEXPART, driven by ERA-Interim reanalysis, was used. Based on ERA-interim precipitation (1979–2019), we created wet and dry composites from the three driest (−1.5σ) and wettest (+1.5σ) June-July-August (JJA) years in our study area (17°N−17.75°N, 54°E-54.75°E). FLEXPART simulations were performed during JJA in these years.

The FLEXPART simulation was driven by ERA-interim through 6-hourly analyzes (at 00.00, 06.00, 12.00, and 18.00 UTC) and 3-hourly forecasts at intermediate times (at 0300, 0900, 1500, and 2100 UTC), with 1°×1° resolution on 60 model levels98. Surface moisture flux is calculated over an area (A), where E-P for the total particles residing over A is given by:

E-P ≈ 1/A * K * (e-p) (1)

where e-p is the rate of moisture change along the trajectory99. The simulation releases 50,000 particles, and K is the number of N particles that resides over A. This approach divides the whole atmosphere’s mass into small elements distributed homogeneously in the atmosphere according to the atmospheric mass distribution. These are then moved with the mass-consistent winds and are based on mass-consistent turbulence and convection parameterizations100,101. We calculated the mean net moisture flux based on E-P and backtracked elements that had a loss (E-P<0) over the Qunf Cave region (17°N−17.75°N, 54°E-54.75°E) for 12 days.

230Th dating
A total of 50 new 230Th dates were obtained from stalagmite Q5. Powder sub-samples were hand-drilled parallel to the growth band near the stalagmite axis. The 230Th dating work was performed at Xi’an Jiaotong University Isotope Laboratory using multi-collector inductively coupled plasma mass spectrometers (Thermo-Finnigan Neptune-plus). We used standard chemistry procedures to separate U and Th for dating102. A triple-spike (229Th–233U–236U) isotope dilution method was employed to correct instrumental fractionation and determine U-Th isotopic ratios and concentrations. Details about the instrumental setup are provided by refs. 103,104. All ages are in stratigraphic order within dating uncertainties. The age model was constructed using the Constructing Proxy Records from Age (COPRA) program105. Q5 δ18Oc and δ13C records were adjusted to the new age model (Supplementary Fig. 2).

Trace element analysis
The trace elements were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Thermo Scientific iCAP DUO 6300) in Instituto Pirenaico de Ecologia, Spanish Scientific Research Council (IPE-CSIC). The drilled powder (100–150 mg) was placed in tubes cleaned with 10% HCl, rinsed with MilliQ-filtered water, and dissolved in 1.5 mL of 2% HNO3 (Tracepur) immediately before analysis. Samples were run at average Ca concentrations of 200 ppm. Calibration was conducted offline using the intensity ratio method described by ref. 106. The results are shown by molar ratios of Mg, Ba, and Sr relative to Ca.

Triple oxygen isotope analysis
The δ17Oc of Q5 was measured at Xi’an Jiaotong University Isotope Laboratory. After drilling the carbonate powder of the sample, we add in phosphoric acid (H3PO4, 1.92 g/ml, ~104%) at 25°C to extract CO2. Then the CO2 was equilibrated with an equal amount of the O2 gas for 30 min under 750°C to reach a Pt-catalyzed equilibrium by an O2–CO2 Pt-catalyzed oxygen-isotope equilibration reaction system. In this system, the two post-equilibration gases were separated from each other cryogenically. Finally, δ17Oc was obtained through measurements of the resultant O2 and CO2 by a Thermo Scientific MAT 253 mass spectrometer60. The calculation of Δ'17O and the relative humidity at the moisture source follows the method from ref. 60 (See supplementary text).

Fluid inclusion measurements
Q5 δDfi was analyzed at Xi’an Jiaotong University Isotope Laboratory. The Q5 calcite blocks were crushed at a temperature of ~120°C. The liberated water was then transported to a wavelength-scanned cavity ring-down spectroscopy system (Picarro L2140-i analyzer). The analytical method is described in ref. 107, the maximum error on the reproducibility of crushed speleothem samples for this system is 2‰ for δDfi (1 SD). δDfi values are reported on the Vienna Standard Mean Ocean Water to SLAP2 (VSMOW2–SLAP2) scale.

Model simulations
The simulations based on the fully coupled Earth system model EC-Earth are used in this study. A consortium of European research institutions develops EC-Earth to build a fully coupled Atmosphere-Ocean-Land-Biosphere Earth system model for seasonal to decadal climate prediction and future climate projections108. The atmospheric model of EC-Earth is based on the IFS, including a land model H-TESSEL, developed at the European Centre for Medium-Range Weather Forecasts (ECMWF)98. In addition, the dynamic vegetation model LPJ-GUESS109 is coupled to the land component. The ocean component is the Nucleus for European Modelling of the Ocean (NEMO)110 and includes a sea-ice model LIM3111. We use the CMIP6 configuration of EC-Earth3-veg-LR, in which the atmosphere and land model has a T159 horizontal spectral resolution (roughly 1.125°–125 km) with 62 vertical levels. The NEMO and LIM have a nominal horizontal resolution of 1°, and NEMO has 75 vertical levels. The coupling between the atmosphere and ocean/sea ice is through the Ocean Atmosphere Sea Ice Soil coupler (OASIS, version 3.0)112.

We simulate the 8 ka BP (8K) and a pre-industrial control simulation (PI) using EC-Earth3-veg-LR. The simulations follow the CMIP6-PMIP4 protocol for each experiment setup113. The PI and 8K simulations have the same boundary conditions except for the orbital forcing and greenhouse gas concentration differences (Supplementary Table 1). The orbital forcing is computed in the IFS component according to ref. 114, as described in ref. 115. The greenhouse gas concentration for PI follows CMIP6 and for 8K using the reconstruction from ref. 116.

The initial condition for PI simulation with EC-Earth-veg-LR is taken from a previous PI run with EC-Earth3-LR115. The simulation reaches the quasi-equilibrium after 300 years (meets the criteria global mean surface temperature trend <±0.05K per century113, and the simulation continues running for another 700 years. The initial condition for the 8K simulation is taken from the equilibrium state of PI simulation, with changed orbital forcing and Green House Gas concentration. For the 8K condition it takes around 300 years to reach equilibrium, and we run for another 700 years. The last 200 years’ outputs are used to analyze both simulations.

Data availability
Source data for Fig. 2 and Fig. 4 are referenced in the Source data provided with this paper. The absolute 230Th dates and multi-proxy data time series are available online on the NOAA paleoclimate database (https://www.ncei.noaa.gov/access/paleo-search/study/38279). FLEXPART model and documentation can be found at https://www.flexpart.eu/ (last accessed: 2023-03-09). Climate model simulation data is published on Zenodo (https://zenodo.org/record/8121547). Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Reviewer’s comments on “New insights into speleothem oxygen isotope deciphered by multi-proxy and climate model simulations in southern Oman” Ye Tian et al.\nGeneral comments\nTian et al. in this manuscript, have attempted to interpret multiple proxies taken from a cave in Oman, in the context of Indian monsoon over the Holocene. They have tried to substantiate and elucidate their results with climate model simulations. They find that the source of moisture changed across the Holocene, due to changes in large scale monsoon circulation. While interesting, I do not find this result to be novel enough to justify publication in a high impact journal and warrants a rejection. I find a bit of ambiguity/lack of clarity in some of the sentences which allows for a benefit of doubt. Hence, I want to give the authors a chance to respond to the following major concerns, and I recommend a major revision instead.\nMajor concerns\n1. Novelty of the work: Battisti et al. 2014, have already established that Qunf cave proxy does not represent precipitation intensity. It merely captures the isotope fractionation of water vapor - implying the source of moisture as the driver of δ18O. A number of recent literature (eg. Dayem et al. 2010, Krklec et al. 2014, Kathayat et al. 2020; to name a few) have demonstrated that isotope fractionation of oxygen from speleothems are related to changes in the source of moisture. I find the current study one of the many of this growing body of literature on this aspect.\n2. Reconstruction of Humidity: The authors have calculated humidity based on ∆’17O and temperature from a transient simulation TraCE-21k. One should be careful while combining parameters from proxy and a climate model. Moreover, it is not clear, how the temperature was used in their equation. The authors should also show RH calculated entirely from the output of TraCE-21k. Does RH calculated from TraCE-21k match the reconstructed RH?\n3. Interpretation based on relative humidity: The authors have shown that RH in the proxy was different in early Holocene compared to the late Holocene. This can be due to either a change in local RH or a change in source of moisture or a combination of these two factors. Since, there is no definite way to isolate these two effects on the proxy, the authors have used EC-Earth simulations. It is a well known fact that most CMIP models simulate a constant RH (Eg. Gormon & Muller 2010, Allan et al. 2020, Douville et al. 2022). Consistent with this, EC-Earth shows no changes in RH, except over North African summer monsoon region and the Arabian peninsula.\nThis is primarily due to the fact that these regions are deserts in the control. The large change in the hydrology during the early Holocene leads to large change in RH, in these regions. The model shows an increase in RH as expected. However, figure 2c suggests that the proxy has a recorded a decrease in RH. The direction of change suggested by the proxy and model are not consistent. This inconsistency has not been discussed. A change in moisture source surely cannot have such a drastic impact. Even if I assume the authors interpretation is correct, Qunf is too close to the Arabian Sea (roughly 15 km) - I’d still say that moisture from the Arabian Sea dominates. In fact, a visual examination of figure 3b suggests that almost entire moisture and winds into the cave site are from the Arabian Sea.\nSuggestion: use FLEXPART to identify the source. Or the tagging techinque used in Kathayat et al. 2020.\n4. Use of FLEXPART The authors have used FLEXPART in the modern context only. They have identified moisture sources for precipitation at the site of the cave based on modern data.\nThey have used two yrs - one with lowest and another highest annual precipitation in modern climate. For more robustness I recommend that the authors instead use a composite of high and low rainfall years (defined by precip greater than or lower than 1 sigma for example). As acknowledged by the authors, the sources of moisture change across the Holocene. Still, the authors use results based on modern day FLEXPART analysis to interpret their proxy. I suggest that the authors do the analysis with the 8ka and PI simulations to make a strong case.\n5. Insufficient literature Line 85-88 and Lines 221 - 225: I have been following the recent developments in the debate between marine and terrestrial proxies of monsoons quite closely. This debate has been largely resolved recently. Therefore, it is important for the benefit of the readers to have a couple of additional sentences. This will help the readers get a perspective of why it is important to isolate processes in the Arabian Sea and those related to Qunf - which is quite essential for this paper. The following papers must be included: Hsu et al 2010 was the first to suggest the opposing response of land and ocean monsoons, followed by Battisti et al. 2014, Bosmans et al. 2018.\nJalihal et al. 2019 & Jalihal et al. 2020, were the first papers to connect it to the opposing marine and terrestrial proxy response, and provide a mechanism for the opposite response.\nLater, Cheng et al, 2021 coined the term ”land-sea precession phase paradox”.\nMinor points\n1. Obsolete theory of monsoon: Line 69-70: The authors refer to the enchancing land-sea thermal contrast as major cause of stronger monsoon. Over the last couple of decades, studies have discredited the land-sea thermal contrast theory of monsoons and moist static energy based theories have gained momentum (see review papers eg. Gadgil 2018, Biasutti et al. 2018, Hill 2020). This has been used to understand orbital-scale monsoon variability (D’Agostino et al. 2019, 2020, Jalihal et al 2019, 2020). Accordingly text needs to be modified at appropriate places eg. lines 210-213.\n2. Line 35: Include citations to Mohtadi et al. 2014, Seth et al. 2019\n3. Line 53: As noted later in the literature, it is not just the fractionations integrated from tropical oceans to cave site, but mixing from other moisture sources etc. It will be good to mention these factors in the introduction itself.\n4. Line 101: ISMt -> ISM\n5. Line 166-168: How is this statement different from the main conclusion of Battisti et al. 2014?\nPerhaps, a more correct statement would be that the authors have an observational evidence for a result propsed by Battisti et al. based on models.\n6. Line 174-175: ”Currently, the ISM is prevailing along the southern Arabian Peninsula”. This statement is wrong. How can Indian summer monsoon be prevalent over the Arabian peninsula?\nI think the authors mean, the effect of ISM is prevalent over the Arabian peninsula.", "reviewer_2": "In “New insights into speleothem oxygen isotope deciphered by multi-proxy records and climate model simulations in southern Oman,” Tian et al. present new fluid inclusion dD and carbonate D17O measurements from south Oman stalagmite Q5, paired with air parcel back trajectory analysis and climate model simulations, to comment on interpretation of the published Q5 d18Oc record, particularly with respect to Indian Summer Monsoon strength (and North African summer monsoon strength) captured in southern Oman over the Holocene. The paper concludes that the Q5 d18Oc record indicates the change of a super-monsoon regime across the Holocene.\nThe fluid inclusion dD and carbonate D17O measurements are an excellent addition to this significant speleothem d18O record (published almost 2 decades ago now) using the latest advances in the field, and the authors’ moisture trajectory hypothesis discussion, which is well supported by the new proxy records and model analysis, will be of interest to many in the community interested in African and Asian summer monsoon mechanisms. However, there were several instances in the text in which I found the authors taking too much liberty in brushing over topics without much explanation, assuming too much in my opinion that the reader would take the authors at their word, which I don’t really appreciate, particularly in a journal such as Nature Communications. I detail these below. I thus propose a major revision before publication in this journal.\nMajor comments:\n1. The methods of the FLEXPART analysis are poorly described, and thus it is unclear to me how ‘E-P diagnosed from 12-days back trajectories using FLEXPART’ shown in Fig.1 was calculated, or what the figure is even displaying. The figure caption does not state explicitly from which point the back trajectories were back tracked from, or on what dates, nor otherwise refer the reader to the methods section. In the methods FLEXPART section, there is limited information on how the FLEXPART simulation was performed: “We calculated the mean net moisture flux based on E-P and backtracked elements that had a loss (E-P < 0) over the Qunf Cave region (16-18°N, 52-56°E) for 12 days.” I understand picking out Qunf cave area air parcel elements with an E-P<0 during JJA, and then performing a back trajectory analysis on each of them, but I don’t understand the jump from having 1000s(?) of air parcel back trajectories to the map display shown in Fig. 1. This is missing from the methods.\n2. D17O was not introduced at all– I imagine this proxy is not familiar to most broad audience readers. Providing a list of references for the reader to investigate and learn about on their own is not good enough for a Nat Comms article in my opinion.\n3. There is no information included in the article (or the supplement) on how relative humidity was calculated from the D17O proxy. In the “Triple oxygen isotope analysis” methods section (methods was not referenced in the main article text), there is an equation listed, but no citation with this equation, and no explanation on where it comes from. The sentence prior to the equation says that temperature was calculated from TraCE-21k… is the equation dependent on T? If so, what is its sensitivity to T? TraCE-21k is relatively low resolution, and this site is near the coast. Also, why TraCE-21k when EC-Earth simulations were run specifically for this paper? Further, not all the variables in the equation are explicitly stated. Overall, I found this section of the methods incredibly confusing.\n4. Line 187 introduces the terms “East African monsoon” and “West African monsoon” without any explanation… until this point the African moisture has been described as the “North Africa summer monsoon”. I’m confused. This is another example of the text using terms without any description.\n5. Line 195 states “the proxies of the local hydroclimate conditions are then a critical complement” to the regional d18Oc, dD, D17O proxies. The authors however do not include any explanation in the main text on how their local hydroclimate proxies (234U/238U, d13C, Mg/Ca, Sr/Ca, or Ba/Ca) are interpreted. There is text in the supplement, but in my opinion if the information is ‘critical’, the proxy interpretation should not be 100% exported to the supplement section of the article. Indeed, the interpretation of these proxies is quite complex and interesting in this situation— there is an early-late Holocene pattern in d13C, U234/U238, and Mg/Ca, Sr/Ca, and Ba/Ca, but the different proxies do not complement each other in a simple manner. In my opinion the section on the local hydroclimate proxies needs to either be given an appropriate amount of discussion text in the manuscript (including perhaps the summary paragraph from the supplement text), or removed and published as a separate article.\n6. Paragraph beginning Line 214 discusses Clemens et al. work, who have a hypothesis on the mechanisms responsible for ISM strength. The citations listed are very old (Clemens et al., 1991; Clemens and Prell, 2003), which I found a bit disingenuous, particularly as this paper’s senior author’s recent work on monsoon strength mechanisms is well cited in the paper (e.g. Cheng et al., 2022; Cheng et al., 2021). Perhaps in this paragraph the authors could comment on Clemens et al., Science Advances, 2021 to acknowledge/address more up-to-date research by this group?\n7. In the 230Th Dating methods section, the authors state “All ages are in stratigraphic order within dating uncertainties.” This is a false statement, as demonstrated in Fig. S1. There are several ages from Fleitmann et al. 2003 which appear to be ~500 yr offset from the new U-Th ages—though I cannot comment on exact differences because none of the U-Th data is provided in a table. I find it disappointing that the group of authors, which includes Fleitmann as second author, have completely ignored this mismatch, and acted in the manuscript as if it doesn’t exist. A section is needed in the supplement describing why some previously published Q5 U-Th ages have been ignored in the final age model for this study, and if the new Q5 age model modifies any previously published age-sensitive proxy interpretations from Q5 d18Oc and d13C.\nMinor comments:\n-NHSI, first appearing on Line 38, is not defined. From what latitude, and what date(s) or longitude of perihelion(s), is the insolation calculated?\n-Line 141- what is a significant change? Please quantify\n-Given that Africa and Asia both start with ‘A’, I would avoid using the monsoon acronyms: NASM, ASM, ISM etc. It makes the text unnecessarily confusing.\n-Line 174 – include the period of the -2‰ to 0‰ increase, as it does not continue all the way to the present. Likewise Line 175/176 is false: there is a hiatus at 2 ka, and the d18Oc values beginning at 1ka decrease through their ~500 yrs of growth.\n-Fluid inclusions methods section says the reproducibility of crushed speleothem samples is 2‰, but the results shown in Fig 2 have variable error bars, many much less than 2‰. Should the methods section say “maximum 2‰”?\n-Lines 326/327 are confusing. Was the CMIP6 configuration of EC-Earth3-veg-LR used for both the PI and 8k simulation? If so this could be stated more clearly. If not, why not? Is there a difference between the CMIP6-PMIP4 and the CMIP6 protocol?"}, "author_response": "Point-by-point responses\nReviewer #1 (Remarks to the Author):\nGeneral comments\nTian et al. in this manuscript, have attempted to interpret multiple proxies taken from a\ncave in Oman, in the context of Indian monsoon over the Holocene. They have tried to\nsubstantiate and elucidate their results with climate model simulations. They find that\nthe source of moisture changed across the Holocene, due to changes in large scale\nmonsoon circulation. While interesting, I do not find this result to be novel enough to\njustify publication in a high impact journal and warrants a rejection. I find a bit of\nambiguity/lack of clarity in some of the sentences which allows for a benefit of doubt.\nHence, I want to give the authors a chance to respond to the following major concerns,\nand I recommend a major revision instead.\nMajor concerns\n1. Novelty of the work: Battisti et al. 2014, have already established that Qunf cave\nproxy does not represent precipitation intensity. It merely captures the isotope\nfractionation of water vapor - implying the source of moisture as the driver of δ18O. A\nnumber of recent literature (eg. Dayem et al. 2010, Krklec et al. 2014, Kathayat et al.\n2020; to name a few) have demonstrated that isotope fractionation of oxygen from\nspeleothems are related to changes in the source of moisture. I find the current study\none of the many of this growing body of literature on this aspect.\nThis is a very good question.\nThe Holocene δ18O record of stalagmite Q5 is one of the most important records\nc\nin the Asian summer monsoon/Indian summer monsoon regions, which is rather unique\nin the Arabian Peninsula till today. The broad scientific significance of this record is\nattested by more than 1700 citations (Google Scholar), since Fleitmann et al. (2003)\npublished it in Science. In the past two decades, the fast developments in both proxy\nand modelling research forefronts have made it possible to further study this unique site\nwith multi-proxy and model-data comparison approaches, which is critical to\nunderstand a number of scientific issues in debate.\nAt present, the interpretation of the climate significance of speleothem δ18O alone\nc\nremains tentative. While a set of modelling studies had been carried out to resolve this\nissue (e.g., Battisti et al., 2014; Bosmans et al., 2018; Hsu et al., 2010; Jalihal et al.,\n2019; Jalihal et al., 2020; Jalihal et al., 2022), the Holocene evolution of the Indian\nsummer monsoon across the Arabian Peninsula is still ambiguous, manifesting for\ninstance, a so-called “sea-land precession-phase paradox”, particularly for the\nHolocene (e.g., Cheng et al., 2021, 2022). This is largely because of the lack of proxy-\nbased evidence besides the δ18O record published two decades ago, thus calling for\nc\nmulti-proxy data from the region in order to make a new in-depth and comprehensive\n1\ndata-model comparison.\nMost of the previous studies up till now focused mainly on analysis of modern\nmoisture sources and hydroclimate modelling (e.g., Battisti et al., 2014; Dayem et al.,\n2010; Kathayat et al., 2021; Krklec and Dominguez-Villar, 2014). In this study,\nadditional to modern hydroclimate analyses, we provided substantial new proxy-based\nevidence, including (234U/238U) , 13C, trace element, 17O and fluid inclusion dataset,\n0\nand new modelling results, such as precipitation, 850 hPa wind, and near-surface\nrelative humidity. This allows us not only to reconstruct the Holocene hydroclimate\nevolution more robustly, but also to accomplish a critical comparison between new\nmulti-proxies and model results for the critical site.\nAccordingly, we have made the following changes:\nLines 29-30 in the abstract: “This study thus provides critical proxy data in support\nof previous and new model results.”\nLines 64-66 in the introduction: “It has been highly cited during the past two\ndecades and remains a unique Holocene δ18O record in the area up till now.”\nc\nLines 107-108 in the introduction: “These questions call for further investigations,\nespecially with multi-proxy data.”\nLines 309-311 in the conclusion section: “Our multi-proxy records and new\nmodelling results provide new insights into the interpretation of the Qunf δ18O record\nc\nand the Holocene hydroclimate history in the southern Arabian Peninsula.”\n2. Reconstruction of Humidity: The authors have calculated humidity based on ∆’17O\nand temperature from a transient simulation TraCE-21k. One should be careful while\ncombining parameters from proxy and a climate model. Moreover, it is not clear, how\nthe temperature was used in their equation. The authors should also show RH calculated\nentirely from the output of TraCE-21k. Does RH calculated from TraCE-21k match the\nreconstructed RH?\nWe agree with the reviewer.\nTemperature is used to calculate the δ18O fractionation (18α ) between\ncarbonate/water\npaired cave dripwater and the carbonate precipitation from the water by the following\nequation (Kim and O'Neil, 1997):\n1000ln18 =18.03 ( 103T−1) −32.42 (1)\ncarbonate/water\nWe agree with the reviewer’s comment that combining parameters from proxy and\na climate model might bring in some uncertainties. In the revised manuscript, we used\nthe summer SST from ODP site 723A (Naidu and Malmgren, 2005), instead of the\nTraCE-21k temperature, to calculate RH. The calculated RH results from the SST and\nTraCE-21k are similar within the uncertainty (Fig. R1).\ntemperature from the TraCE-21, EC-Earth and summer SST in ODP site 723A. Individual\nerrors are standard errors of the corresponding measurements propagated from δ17O and\nc\nδ18O of speleothem carbonates (Sha et al., 2020; Sha et al., 2021). The triple oxygen\nc\nisotope compositions of parent water are calculated by the Monte Carlo model (MC=1000)\nin combination with statistical estimates of uncertainty.\nWe have added more information and related equations in the revised manuscript\nand supplementary file to provide the details about the calculation of ∆’17O and RH.\n3. Interpretation based on relative humidity: The authors have shown that RH in the\nproxy was different in early Holocene compared to the late Holocene. This can be due\nto either a change in local RH or a change in source of moisture or a combination of\nthese two factors. Since, there is no definite way to isolate these two effects on the proxy,\nthe authors have used EC-Earth simulations. It is a well-known fact that most CMIP\nmodels simulate a constant RH (e.g., Gormon & Muller 2010, Allan et al. 2020,\nDouville et al. 2022). Consistent with this, EC-Earth shows no changes in RH, except\nover North African summer monsoon region and the Arabian Peninsula.\nThis is primarily due to the fact that these regions are deserts in the control. The large\nchange in the hydrology during the early Holocene leads to large change in RH, in these\nregions. The model shows an increase in RH as expected. However, figure 2c suggests\nthat the proxy has a recorded a decrease in RH. The direction of change suggested by\nthe proxy and model are not consistent. This inconsistency has not been discussed. A\nchange in moisture source surely cannot have such a drastic impact. Even if I assume\nthe authors interpretation is correct, Qunf is too close to the Arabian Sea (roughly 15\nkm) - I’d still say that moisture from the Arabian Sea dominates. In fact, a visual\nexamination of figure 3b suggests that almost entire moisture and winds into the cave\nsite are from the Arabian Sea.\nSuggestion: use FLEXPART to identify the source. Or the tagging technique used in\nKathayat et al. 2020.\n(1) In our study, the ∆’17O-based reconstruction of the RH at the moisture source\nshows a ~15% increase from the middle to late Holocene (Fig. 2c), which is apparently\nconsistent with our model results for the ultimate ocean sources (Fig. 3a). On the other\nhand, the simulated RH at the Arabian Sea and Qunf Cave site increases by ~6% from\n8K to PI (Figs. 3a and R2). This is consistent with our speleothem proxy records\n[(234U/238U) , δ13C, and trace element ratios (Mg/Ca, Sr/Ca, Ba/Ca)] that also indicate\n0\na drier condition (low P-E) at the Qunf Cave site during the early to middle Holocene\ncompared to the late Holocene. Thus, both model simulation and proxy record show a\nsimilar increasing trend from 8K to PI in terms of local hydroclimate conditions at the\ncave site. Nonetheless, we admit that the model simulation of RH remains a challenge,\nespecially regarding its uncertainties.\nIn the revised manuscript, we have added the above discussions.\nLines 169-174: “The model simulations reveal a ~6% increase in relative\nhumidity at the equatorial Atlantic, Arabian Sea and Qunf Cave site from 8K to PI (Fig.\n3a). The ∆’17O-based reconstruction of relative humidity at the moisture source shows\na ~15% increase from middle to late Holocene (Fig. 2c). The direction of these\nHolocene trends is consistent. Taking into account the uncertainties of the modelled and\nreconstructed relative humidity, these datasets show strong similarities.”\nLines 249-252: “Both model simulation and proxy records show a similar\nincreasing trend in local hydroclimate conditions at the cave site from 8K to PI, the\nearly to middle Holocene was drier (lower P-E) during early to middle Holocene\ncompared to the late Holocene (See supplementary text).”\nFigure R2. Model result of 8K-PI near-surface relative humidity (% in colour-scale).\nAlso shown is the EC-Earth grid cell (green box) containing the Qunf Cave site (red star).\n(2) Currently, the FLEXPART cannot provide the analysis on moisture back\ntrajectory. Since the EC-Earth has not yet implemented the water tagging diagnostics\n4\nin its simulations and output, we instead use the iCESM from Hu et al. (2019) for the\nanalysis, which is similar to that used in Kathayat et al. (2021). The iCESM tagging\nresults show that besides the Arabian Sea as the dominant moisture source for the Qunf\nCave site, North Africa and the Arabian Peninsula contribute a large percentage of\nmoisture to the Qunf Cave site (Fig. R3). Therefore, we suggest that when NHSI was\nhigh during the early and middle Holocene, the North African monsoon may likely\ncontribute more water vapor to the Qunf Cave site.\nFigure R3 has been added to the supplementary material as Supplementary Fig. 1.\nWe have added the above information in the revised main text (lines 123-126):\n“Meanwhile, the moisture tagging analysis and simulations show that the African\ncontinent, Mediterranean and Iranian Plateau also provide considerable moisture to\nthe Qunf Cave site (Fig. 1), particularly from North Africa and the Arabian Peninsula\nduring summer (JJA) (Supplementary Fig. 1).”\nFigure R3. Seasonal moisture contribution from each source to the Qunf Cave site in the\niCESM simulation (1954-2012).\n(3) The wind arrows in Fig. 3b show a strong westerly-southwesterly wind\nanomaly from the African rain belt towards the Arabian Peninsula and Oman. A number\nof previous model studies (e.g., Patricola and Cook, 2007; Battisti et al., 2014; Hu et\nal., 2019) also suggest that during the high NHSI periods, the moisture transformed\nfrom the tropical Atlantic across Northeast Africa as far as East Africa and the Arabian\nPeninsula appears to increase. The study from Hoti Cave speleothem, Northeast Oman,\nalso suggests that “greatly enhanced rainfall was caused by an intensification and\ngreater northward extension of the African and Indian monsoons into Arabia”\n(Fleitmann et al., 2022). Overall, the Arabian Sea is likely the dominant moisture source\nduring the Holocene. In the meantime, moisture originating from the North African\nsummer monsoon may also contribute to the precipitation at the cave site, although it\nis difficult to precisely assess the percentage of the contribution of each moisture source.\nHowever, we suspect that the contribution of the moisture from the North African\n5\nmonsoon in the middle Holocene would be higher than or at least comparable to today’s\npercentage inferred by the moisture tagging simulations (Figure R3).\n4. Use of FLEXPART The authors have used FLEXPART in the modern context only.\nThey have identified moisture sources for precipitation at the site of the cave based on\nmodern data.\nThey have used two yrs - one with lowest and another highest annual precipitation in\nmodern climate. For more robustness I recommend that the authors instead use a\ncomposite of high and low rainfall years (defined by precip greater than or lower than\n1 sigma for example). As acknowledged by the authors, the sources of moisture change\nacross the Holocene. Still, the authors use results based on modern day FLEXPART\nanalysis to interpret their proxy. I suggest that the authors do the analysis with the 8ka\nand PI simulations to make a strong case.\nThis is a good suggestion.\nWe have rerun the FLEXPART simulations based on ERA-interim precipitation\n(1979-2019) and identified the wet and dry composites from the three driest and three\nwettest (>1σ) June-August (JJA) years in our study area (17°N-17.75°N, 54°E-54.75°E),\nrespectively. The results show that the African Continent, Mediterranean and Iranian\nPlateau also contribute considerable moisture to the Qunf Cave site, consistent with our\nprevious result (Fig. 1).\nFigure 1. Qunf Cave (yellow asterisk) JJA evaporation minus precipitation (E-P)\ndiagnosed from 12-days back trajectories in June-August using the FLEXPART for\n(a) highest precipitation years (1983, 1986 and 1996) and (b) lowest precipitation years\n(1999, 2000 and 2012). Positive values (red) indicate a net moisture supply, while\nnegative values (blue) indicate water vapor condensation and precipitation. Sites of other\ncaves (yellow circles) and marine sediment cores (green boxes) discussed in the text that\ncontain important Holocene paleoclimate records (discussed in the text) are also shown:\nHT, Hoti Cave; JT, Jeita Cave; SQ, Soreq Cave; TM, Tianmen Cave; SH, Sahiya Cave;\nML, Mawmluh.\nIn general, the FLEXPART simulations do not provide direct explanations for our\nproxies, neither for PI scenarios, but rather for setting a baseline for the current\n6\natmospheric circulation and moisture sources to the Qunf Cave site. That is, the\nFLEXPART cannot perform back trajectory analysis for 8K and PI. Instead, however,\nour EC-Earth simulations show precipitation and wind differences between 8K and PI,\nsuggesting a stronger southwesterly wind strength and more precipitation over North\nAfrica during 8K compared with PI. Besides, speleothem fluid inclusion δD and triple\noxygen isotope data provide a line of evidence that the North African summer monsoon\nmay possibly contribute more precipitation at the Quaf Cave site during the early to\nmiddle Holocene, although the Indian Ocean remains an important source.\n5. Insufficient literature Line 85-88 and Lines 221 - 225: I have been following the\nrecent developments in the debate between marine and terrestrial proxies of monsoons\nquite closely. This debate has been largely resolved recently. Therefore, it is important\nfor the benefit of the readers to have a couple of additional sentences. This will help the\nreaders get a perspective of why it is important to isolate processes in the Arabian Sea\nand those related to Qunf - which is quite essential for this paper. The following papers\nmust be included: Hsu et al 2010 was the first to suggest the opposing response of land\nand ocean monsoons, followed by Battisti et al. 2014, Bosmans et al. 2018. Jalihal et\nal. 2019 & Jalihal et al. 2020, were the first papers to connect it to the opposing marine\nand terrestrial proxy response, and provide a mechanism for the opposite response.\nLater, Cheng et al, 2021 coined the term “land-sea precession phase paradox”.\nWe thank the reviewer for his/her good suggestion. Accordingly, we have added\nthe above content suggested by the reviewer in the revised introduction and discussion\nsections.\nLines 90-94: “Hsu et al. (2010) were the first to show that their climate model\nactually reproduced this opposite response between land and ocean to NHSI at the\nprecession band. This study was followed by multiple climate model simulations that\nconfirmed the results from Hsu et al. (Zhang et al., 2021; Battisti et al., 2014; Bosmans\net al., 2018; Jalihal et al., 2019, 2020 and 2022). Later, Cheng et al. (2021) coined the\nterm ‘land-sea precession phase paradox’ in the research forefront of the Asian summer\nmonsoon, referring to the contrast between speleothem and marine proxy records.”\nLines 96-98: “Recently, Jalihal et al.(2019,2020) explained the mechanisms for\nthe opposing marine and terrestrial proxy response in the Indian summer monsoon\ndomain.”\nLines 288-294: “Model simulations reveal contrasting hydroclimatic responses\nbetween land and ocean to NHSI changes at the precession band (Hsu et al., 2010;\nBattisti et al., 2014; Bosmans et al., 2018). This phenomenon has been referred to as\nthe “land-sea precession phase paradox” (Cheng et al., 2022). In the ISM regions,\nJalihal et al. (2019, 2020, 2022) investigated the opposing marine and terrestrial\nresponses, both empirically and theoretically. They discovered that, in addition to NHSI,\nprecipitation changes over the ocean are influenced by changes in surface energy fluxes\nand vertical stability.”\n1. Obsolete theory of monsoon: Line 69-70: The authors refer to the enhancing land-\nsea thermal contrast as major cause of stronger monsoon. Over the last couple of\ndecades, studies have discredited the land-sea thermal contrast theory of monsoons and\nmoist static energy-based theories have gained momentum (see review papers eg.\nGadgil 2018, Biasutti et al. 2018, Hill 2020). This has been used to understand orbital-\nscale monsoon variability (D’Agostino et al. 2019, 2020, Jalihal et al 2019, 2020).\nAccordingly text needs to be modified at appropriate places eg. lines 210-213.\nThis is indeed a good suggestion.\nWe agree with the reviewer. For over 300 years (Halley, 1686), the monsoon has\nbeen considered to be a gigantic land–sea breeze. The traditional conception of\nmonsoons as land-sea breezes have been increasingly challenged by the modern\nperspective of monsoons as an integral component of the global atmospheric circulation\nand hydroclimate (e.g., Gadgil et al., 2018; Hill et al., 2019). Of note are three primary\ntheoretical conceptions of monsoons: one based on convective quasi-equilibrium (CQE)\n(e.g., Emanuel et al., 1994; Nie et al., 2010), another founded on the moist static energy\n(MSE) budget (e.g., Neelin et al., 1987; Chou et al., 2001), and one that frames the\nmonsoon as an extension of the zonal-mean ITCZ (e.g., Biasutti et al., 2018; Hill 2019).\nWhile those theoretical frameworks can better explain some important aspects of the\nmodern monsoon variability, especially regarding monsoon rainfalls and\nthermodynamics, the interpretations of paleoclimate variations on a wide range of\ntimescales are still in development (e.g., D’Agostino et al. 2019, 2020; Jalihal et al.,\n2019, 2020). Physically, the summer barometric differentials and other boundary\nconditions in a vast spatial-scale, regardless of the cause of it, would drive the Asian\nsummer monsoon circulation (e.g., at 850 hPa) (Fig. R4) and associated large-scale\nvapor flux (Fig. R5). In terms of precipitation δ18O at the cave site, prior to the onset of\nthe Asian summer monsoon, the moisture source is rather close and thus precipitation\nis characterized by heavier δ18O. After the summer monsoon onset, the spatial-scale of\nmonsoon circulation and moisture from remote sources increase dramatically (Figs. R4\nand R5), both resulting in the lighter precipitation δ18O at cave site. As such, cave δ18O\nin principle, indicates mainly the dynamic aspect or the monsoon circulation, rather\nthan a direct proxy of local rainfall amount or thermodynamics that are shown via other\nproxies [e.g., trace elements, (234U/238U) , and δ13C].\nSupplementary Fig. 8 and Supplementary Fig. 9.\nAccordingly, we have expanded our main text to reflect the development in\nmonsoon theory:\nLines 76-78: “However, some model simulations reveal distinct regional\ndivergences of the spatial patterns of rainfall and wind across the precession cycle in\nthe Asian summer monsoon domain.”\nLines 256-273: “The understanding of monsoons as an integral component of the\nglobal atmospheric circulation and hydroclimate is becoming prevalent (e.g., Gadgil et\n8\nal., 2018; Hill et al., 2019). Of note are three primary theoretical concepts of monsoons:\none based on convective quasi-equilibrium (CQE) (e.g., Emanuel et al., 1994; Nie et\nal., 2010), another focused on the moist static energy (MSE) budget (e.g., Neelin et al.,\n1987; Chou etal., 2001), and a third that frames the monsoon as an extension of the\nzonal-mean ITCZ (e.g., Biasutti et al., 2018; Hill 2019). While these theoretical\nframeworks help explain certain aspects of modern monsoon variability, especially\nregarding monsoon rainfalls and thermodynamics, the interpretations of paleoclimate\nvariations across various timescales are still in development (e.g., Jalihal et al., 2019,\n2020; D’Agostino et al. 2019, 2020).\nPhysically, the summer barometric differentials and other boundary conditions on\na large spatial-scale, regardless of their cause, drive the Asian summer monsoon\ncirculation (e.g., at 850 hPa) (Supplementary Fig. 8) and the associated large-scale\nvapor flux (Supplementary Fig. 9). In terms of precipitation δ18O at the cave site, prior\np\nto the onset of the Asian summer monsoon, the moisture source is relatively, resulting\nin heavier δ18O . After the summer monsoon onset, the spatial-scale of monsoon\np\ncirculation and moisture from remote sources increase dramatically (Supplementary\nFigs. 8 and 9), both leading to lighter precipitation δ18O at the cave site. Consequently,\np\ncave δ18O mainly indicates the dynamic aspect of the monsoon circulation, with a small\nc\nportion of the variability explained by local rainfall amount or thermodynamics. Local\nrainfall amount or P-E is, however, better reflected by other proxies [e.g., trace\nelements, (234U/238U) , and δ13C].”\nseason (June to July) during 2018 to 2022 from ERA5.\n(b) monsoon season (June 1 to July 15) during 1981 to 2010 from the Nation Climate Centre of\nChina.\n2. Line 35: Include citations to Mohtadi et al. 2014, Seth et al. 2019\nThe two references have been cited in line 36.\n3. Line 53: As noted later in the literature, it is not just the fractionations integrated\nfrom tropical oceans to cave site, but mixing from other moisture sources etc. It will be\ngood to mention these factors in the introduction itself.\nWe have revised the text to mention these factors in the introduction (lines 54-57):\n“Theoretically, speleothem δ18O , as a proxy of the δ18O at the cave site, is related\nc p\nto oxygen-isotope fractionations integrated from tropical oceans to the cave site along\nthe moisture transport trajectory. Besides, the influence of other factors on speleothem\nδ18O , such as mixing of different moisture sources and rainfall amount, should not be\nc\ndismissed.”\n4. Line 101: ISMt -> ISM\nAll acronyms for monsoons have been changed to full spelling in the revised\nmanuscript.\n5. Line 166-168: How is this statement different from the main conclusion of Battisti et\nal. 2014? Perhaps, a more correct statement would be that the authors have an\nobservational evidence for a result proposed by Battisti et al. based on models.\nGood suggestion.\nIndeed, in this study we present multi-proxy evidence from Qunf Cave, a set of\nmodel simulations on the Holocene hydroclimate evolution focused in the region, and\nthus a close comparison between the observations and Holocene hydroclimate\ncondition model results. Our study provides an ideal complement to Battisti et al. (2014),\nwhich focus on the δ18O interpretation in the pan-Asian climatic system based on the\nc\n10\n“high-insolation” (218 ka BP) and “low-insolation” (207 ka BP) experiments, the high\nand low extrema in JJA insolation over the past 950,000 years, especially compared to\nthe Holocene. Regarding the δ18O interpretation and the moisture contribution from\nc\nthe tropical Atlantic via northeast Africa to the Arabian Peninsula, our results are\nconsistent with the conclusion of Battisti et al. (2014) based on the experiments of “high\nand low extrema”.\nWe have revised the statement (lines 189-196) as:\n“The Qunf multi-proxy dataset, combined with a set of model simulations on the\nHolocene hydroclimate evolution in the region, allow for a thorough comparison\nbetween observed and modelled Holocene hydroclimate conditions. We propose that,\nin addition to the water vapor transported by the Indian summer monsoon from the\nadjacent Arabian Sea, the North African summer monsoon may have contributed\nremote moisture from the tropical Atlantic via northeast Africa to Qunf Cave during the\nearly and middle Holocene. This finding offers robust evidence supporting earlier\nmodel results (Battisti et al., 2014; Bosmans et al., 2018)”\n6. Line 174-175: “Currently, the ISM is prevailing along the southern Arabian\nPeninsula”. This statement is wrong. How can Indian summer monsoon be prevalent\nover the Arabian peninsula?\nI think the authors mean, the effect of ISM is prevalent over the Arabian peninsula.\nWe agree with the reviewer. This statement (lines 202-203) has been revised to:\n“Currently, the effect of the Indian summer monsoon predominates over the\nsouthern Arabian Peninsula.”\nReferences\nBar-Matthews, M., Ayalon, A., Matthews, A., Sass, E., Halicz, L., 1996. Carbon and oxygen isotope\nstudy of the active water-carbonate system in a karstic Mediterranean cave: Implications for\npaleoclimate research in semiarid regions. Geochimica Et Cosmochimica Acta 60, 235-241.\nBattisti, D.S., Ding, Q., Roe, G.H., 2014. Coherent pan-Asian climatic and isotopic response to orbital\nforcing of tropical insolation. Journal of Geophysical Research: Atmospheres 119, 11,997-912,020.\nBiasutti, M., Voigt, A., Boos, W.R., Braconnot, P., Hargreaves, J.C., Harrison, S.P., Kang, S.M., Mapes,\nB.E., Scheff, J., Schumacher, C., Sobel, A.H., Xie, S.P., 2018. Global energetics and local physics\nas drivers of past, present and future monsoons. Nature Geoscience 11, 392-+.\nBosmans, J.H.C., Erb, M.P., Dolan, A.M., Drijfhout, S.S., Tuenter, E., Hilgen, F.J., Edge, D., Pope, J.O.,\nLourens, L.J., 2018. Response of the Asian summer monsoons to idealized precession and obliquity\nforcing in a set of GCMs. Quaternary Science Reviews 188, 121-135.\nCheng, H. et al., 2022. Milankovitch theory and monsoon. The Innovation 3, 100338.\nCheng, H., Zhang, H., Cai, Y., Shi, Z., Yi, L., Deng, C., Hao, Q., Peng, Y., Sinha, A., Li, H., Zhao, J.,\nTian, Y., Baker, J., Perez-Mejías, C., 2021. Orbital-scale Asian summer monsoon variations:\nChou, C., Neelin, J. D., & Su, H., 2001. Ocean‐atmosphere‐land feedbacks in an idealized monsoon.\nQuarterly Journal of the Royal Meteorological Society, 127(576), 1869-1891.\nD'Agostino, R. et al., 2020. Contrasting Southern Hemisphere Monsoon Response: MidHolocene\nOrbital Forcing versus Future Greenhouse Gas-Induced Global Warming. J. Clim. 33, 9595-9613,\ndoi:10.1175/Jcli-D-19-0672.1.\nD'Agostino, R., Bader, J., Sordoni, S., Ferreira, D. & Jungclaus, J., 2019. Northern Hemisphere\nMonsoon Response to Mid-Holocene Orbital Forcing and Greenhouse Gas-Induced Global\nWarming. Geophys. Res. Lett. 46, 1591-1601, doi:10.1029/2018gl081589.\nDayem, K.E., Molnar, P., Battisti, D.S., Roe, G.H., 2010. Lessons learned from oxygen isotopes in\nmodern precipitation applied to interpretation of speleothem records of paleoclimate from eastern\nAsia. Earth and Planetary Science Letters 295, 219-230.\nEmanuel, K. A., David Neelin, J., & Bretherton, C. S., 1994. On large‐scale circulations in convecting\natmospheres. Quarterly Journal of the Royal Meteorological Society, 120(519), 1111-1143.\nGadgil, S., 2018. The monsoon system: Land–sea breeze or the ITCZ? Journal of Earth System Science,\n127, 1-29.\nHalley, R., 1686. An historical account of the trade winds and monsoons, etc. Phil. Trans. Roy. Soc.\nLondon, 16,153-158.\nHill, S.A., 2019. Theories for Past and Future Monsoon Rainfall Changes. Curr Clim Change Rep 5, 160-\n171.\nHsu, Y.H., Chou, C., Wei, K.Y., 2010. Land-Ocean Asymmetry of Tropical Precipitation Changes in the\nMid-Holocene. Journal of Climate 23, 4133-4151.\nHu, J., Emile‐Geay, J., Tabor, C., Nusbaumer, J., Partin, J., 2019. Deciphering Oxygen Isotope Records\nFrom Chinese Speleothems With an Isotope‐Enabled Climate Model. Paleoceanography and\nPaleoclimatology 34, 2098-2112.\nJalihal, C., Bosmans, J.H.C., Srinivasan, J., Chakraborty, A., 2019. The response of tropical precipitation\nto Earth's precession: the role of energy fluxes and vertical stability. Climate of the Past 15, 449-\n462.\nJalihal, C., Srinivasan, J., Chakraborty, A., 2020. Different precipitation response over land and ocean to\norbital and greenhouse gas forcing. Scientific Reports 10.\nJalihal, C., Srinivasan, J., Chakraborty, A., 2022. Response of the Low‐Level Jet to Precession and Its\nImplications for Proxies of the Indian Monsoon. Geophysical Research Letters 49.\nKathayat, G., Sinha, A., Tanoue, M., Yoshimura, K., Li, H., Zhang, H., Cheng, H., 2021. Interannual\noxygen isotope variability in Indian summer monsoon precipitation reflects changes in moisture\nsources. Communications Earth & Environment 2, 1-10.\nKim, S.-T., O'Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope effects in synthetic\ncarbonates. Geochimica et Cosmochimica Acta 61, 3461-3475.\nKrklec, K., Dominguez-Villar, D., 2014. Quantification of the impact of moisture source regions on the\noxygen isotope composition of precipitation over Eagle Cave, central Spain. Geochimica et\ncosmochimica acta 134, 39-54.\nNaidu, P.D., Malmgren, B.A., 2005. Seasonal sea surface temperature contrast between the Holocene\nand last glacial period in the western Arabian Sea (Ocean Drilling Project Site 723A): Modulated\nby monsoon upwelling. Paleoceanography 20.\nNeelin, J. D., & Held, I. M., 1987. Modeling tropical convergence based on the moist static energy budget.\nNie, J., Boos, W. R., & Kuang, Z., 2010. Observational evaluation of a convective quasi-equilibrium\nview of monsoons. Journal of Climate, 23(16), 4416-4428.\nPatricola, C.M., Cook, K.H., 2007. Dynamics of the West African monsoon under mid-Holocene\nprecessional forcing: Regional climate model simulations. Journal of Climate 20, 694-716.\nSha, L.J., Maha, S., Duan, P.Z., Luz, B., Zhang, P., Baker, J., Zong, B.Y., Ning, Y.F., Brahim, Y.A., Zhang,\nH.W., Edwards, R.L., Cheng, H., 2020. A novel application of triple oxygen isotope ratios of\nspeleothems. Geochimica Et Cosmochimica Acta 270, 360-378.\nSha, L.J., Mahata, S., Duan, P.Z., Zong, B.Y., Ning, Y.F., Zhang, P., Wang, J., Cai, Y.J., Cheng, H., 2021.\nPreparation of high-precision CO2 with known triple oxygen isotope for oxygen isotope analysis.\nIsot Environ Healt S 57, 443-456.\nReviewer #1 (Remarks to the Author):\nGeneral comments:\nOverall, I’m satisfied with the response by the authors, although some do not entirely convince me.\nThe manuscript has also been modified. However, the text needs some minor corrections.\nFurthermore, I would have liked to see iCESM tagging analysis for the mid-Holocene or early Holocene\nto substantiate their results. I do not, however, consider this necessary. Hence, I recommend\npublication only after the authors have addressed the following.\nB) Concerns from the author’s response:\n1. Reply 1: The authors have said that they provide observational evidence for previous model-based\nresults. This implies the insights from speleothem are not ”New,” and a change in title might be\nnecessary.\n2. Reply 2: This is a remarkable result. The match between core & TraCE is good. But which\nsimulations are the EC-Earth? So far, the authors have only described 8ka experiment from EC-Earth.\nWhy is it not shown in the plot?\n3. Reply 3: The authors have claimed that, in the modern climate, other than the Arabian Sea, North\nAfrica, and the Arabian peninsula contribute substantially to the moisture over the Qunf cave.\nBoth these regions are dry in the modern climate. How is then the moisture sourced from\nthese regions? Besides, the authors have not mentioned anywhere in the text the contribution from\nmore local sources such as the Red Sea and the Persian Gulf. Contribution from these regions is\nrelatively high, as can be seen from their figures (Fig. 1, and supplementary Fig. 1). In\nfact, it can be argued that at 8K, the stronger winds from African monsoon pick up moisture from the\nRed Sea and transport it over to the Qunf cave.\n4. Reply 4: The authors have now only used the three highest and lowest precipitation years during\nthe time period (1979-2019). The authors should make a composite of all the years that have precip\ngreater than and less than 1σ.\nC) Concerns from the manuscript and supplementary:\n1. Line 76-78: The authors should instead re-write the sentence saying that the land-sea contrast\ntheory is not the correct explanation. The current sentence is confusing.\n2. Line 138-140: Based on data from northern and southern Oman, the authors suggest that a large-\nscale re-organization of atmospheric circulation must have taken place. The two cites are too close\ntogether to make such a claim. In the later part of the manuscript, the authors have used multiple\nproxies (Fig. 4). These proxies are spread over sufficiently large region to speculate changes in\nlarge-scale circulation. And so this point can be made later in the text.\n3. Line 244-246: The authors suggest that increased evaporation at 8ka promotes PCP and a higher\ntrace elements ratio. However, they have substantiated this with a plot of P-E. Wouldn’t it be better to\nshow a spatial plot of an anomaly in evaporation instead of P-E?\n4. Line 250 contradicts Line 229: In Line 229, authors conclude that precipitation was higher at 8ka,\nwhereas, in Line 250, they say that the early Holocene was drier. In Line 250, I guess they are\nreferring to higher evaporation. However, higher evaporation does not imply a drier climate.\n5. Line 275-277: I do not agree with this statement. Precipitation on orbital scales is driven by local\ninsolation (Q. Wen et al., 2022). It is the insolation over the monsoon domains that’s important and\nnot the gradient in insolation or higher latitude insolation. However, there is still some ambiguity in\nthe literature, and the authors need not change this statement if they do not wish to. It would be good\nto clarify the definition of NHSI in this context.\n6. Line 294-305: I find these lines repetitive. For brevity and for the benefit of the reader, the authors\nmust remove these lines as they have already been discussed in the introduction.\n7. Line 678: Are the changes in Precip over the Qunf cave significant?\n8. Supplementary material Line 101-103: The authors mention that precipitation is notably higher at\n8ka over the region of the cave. However, in Fig. 3b, it appears to have only marginally higher\nprecipitation.\n9. Supplementary fig 8: The authors have used four years to make a climatology plot. They should\nuse at least 30 yrs, as done in supplementary figure 9.\nD) Miscellaneous:\n1. Line 23: Replace ”climate meaning” with a more appropriate word.\n2. Line 25-26: It is not Indian summer monsoon moisture that prevails over southern Oman. In\nthis context, it is better to say moisture from the Arabian Sea prevails in southern Oman.\n3. Line 164: Fig 3b is referred to first. Perhaps, it is better if the author's interchange figures\n3a and 3b for maintaining flow.\n4. Line 322: ”between from”\n5. Line 323: ”a lower P-E. but”\nReviewer #2 (Remarks to the Author):\nThe authors have comprehensively responded to two detailed reviews, and I am satsfied with the\nauthors' modifications in response to my comments (reviewer 2). I can now recommend this\nmanuscript for publication following a few minor comments regarding the manuscript figures:\n1. The top blue/yellow banner in Figure 2 needs to be described in the figure caption. It would also\nbe helpful for the Holocence Humid Period grey bar to be identified in the figure caption.\n2. Please add the name of the model in the Figure 3 caption title, instead of just \"Simulation results\"\nPoint-by-point responses\nReviewer #1\nA) General comments:\nOverall, I’m satisfied with the response by the authors, although some do not entirely convince me.\nThe manuscript has also been modified. However, the text needs some minor corrections.\nFurthermore, I would have liked to see iCESM tagging analysis for the mid-Holocene or early\nHolocene to substantiate their results. I do not, however, consider this necessary. Hence, I\nrecommend publication only after the authors have addressed the following.\nWe are glad that you are overall satisfied with our response. Here are our responses to your\nremaining concerns and suggestions.\nB) Concerns/suggestions from the author’s response:\n1) Reply 1: The authors have said that they provide observational evidence for previous model-\nbased results. This implies the insights from speleothem are not “New,” and a change in title might\nbe necessary.\nThank you for the suggestion. We have changed the title to “Holocene climate change in\nsouthern Oman deciphered by speleothem multi-proxy records and climate model simulations”.\n2) Reply 2: This is a remarkable result. The match between core & TraCE is good. But which\nsimulations are the EC-Earth? So far, the authors have only described 8ka experiment from EC-\nEarth. Why is it not shown in the plot?\nThe RH result calculated by the EC-Earth (blue line in Fig. R1) is very similar to the result\ncalculated by the core and thus covered by the latter (the red line in Fig. R1) in the previous version\nof the figure. We have moved the blue line to the top so that it is visible. Additionally, our EC-Earth\nsimulation does not have the result before 8 ka BP (Fig. R1).\nTraCE-21, EC-Earth and summer SST from ODP site 723A. Individual errors are standard errors of the\ncorresponding measurements propagated from δ17O and δ18O of speleothem carbonates (Sha et al., 2020,\nc c\n2021). The triple oxygen isotope compositions of parent water are calculated by the Monte Carlo model\n(MC=1000) in combination with statistical estimates of uncertainty.\n3) Reply 3: The authors have claimed that, in the modern climate, other than the Arabian Sea, North\nAfrica, and the Arabian Peninsula contribute substantially to the moisture over the Qunf cave.\nBoth these regions are dry in the modern climate. How is then the moisture sourced from\nthese regions? Besides, the authors have not mentioned anywhere in the text the contribution from\nmore local sources such as the Red Sea and the Persian Gulf. Contribution from these regions is\nrelatively high, as can be seen from their figures (Fig. 1, and supplementary Fig. 1). In fact, it can\nbe argued that at 8K, the stronger winds from African monsoon pick up moisture from the Red Sea\nand transport it over to the Qunf cave.\nThis is indeed a good suggestion/comment.\nThe statement about the moisture contribution from North Africa and the Arabian Peninsula is\nbased on the iCESM simulation results, in which the two regions are set as a whole large tagging\nregion. Meanwhile, the FLEXPART results show more detailed information: Besides the Somali jet,\nthe Red Sea, the Persian Gulf, the Mediterranean and Iranian Plateau also provide considerable\nmoisture to the Qunf Cave site (Fig. 1 and Supplementary Fig. 1). Compared to dry years, wet years\nhave more moisture contributions from the local/nearby regions, including the Red Sea and the\nPersian Gulf, which is associated with less precipitation over South Asia (Figs. R2 and 1).\nWe revised the statement (lines 122-126) to: “In comparison with the drier years, the wetter\nyears in the area have more local moisture contributions, accompanied with less precipitation over\nSouth Asia. Meanwhile, the moisture tagging analysis and simulations also indicate that the Red\nSea, the Persian Gulf, the Mediterranean and Iranian Plateau provide considerable moisture to the\nIt is sound that the Red Sea contributed moisture to Qunf Cave through the wind of the North\nAfrican summer monsoon. However, it is difficult to calculate the individual moisture contribution\nfrom the North Atlantic, the North African Continent and the Red Sea, respectively. As such, we\nmerely state that the North African summer monsoon brought moisture from the west to the cave\nsite (including the sources of the North Atlantic, the North African Continent and the Red Sea).\n4) Reply 4: The authors have now only used the three highest and lowest precipitation years during\nthe time period (1979-2019). The authors should make a composite of all the years that have precip\ngreater than and less than 1σ.\nFigure 1 shows the results with the three highest and lowest precipitation years (beyond ±1.5σ),\nrespectively and their difference. In comparison, we provide the results for ±1σ precipitation\nanomaly scenarios: -1σ (1999, 2000, 2001, 2012, and 2017), and +1σ in (1983, 1986, 1988, 1996,\nand 2007) in Figure R2, which is broadly similar to the ±1.5σ scenarios (Fig. 1).\nThe ±1σ and ±1.5σ scenarios denote the outliers outside 68 % and 86.64 % distribution,\nrespectively. The latter (±1.5σ) in essence shows more extreme cases of stronger-weaker monsoon\nor wetter-drier variability over southern Oman and surrounding areas. In that regard, we would like\nto keep the ±1.5σ results in the manuscript (Fig. 1) and add a plot of their difference (Fig. 1c) as\nwell.\nLines 120-126 are also revised to \" The Somali jet brings considerable moisture to the Qunf\nCave site during the summer, mainly from the tropical South Indian Ocean across the equator to the\nwestern Arabian Sea (Fig. 1). In comparison with the drier years, the wetter years in the area have\nmore local moisture contributions, accompanied with less precipitation over South Asia. Meanwhile,\nthe moisture tagging analysis and simulations also indicate that the Red Sea, the Persian Gulf, the\nMediterranean and Iranian Plateau provide considerable moisture to the Qunf Cave site (Fig. 1\nand Supplementary Fig. 1).\"\n12-day back trajectories in June-August using the FLEXPART. (a) Results for the highest\nprecipitation (> +1.5σ) years (1983, 1986 and 1996). (b) Results for the lowest precipitation (< -1.5σ)\nyears (1999, 2000 and 2012). (c) The difference between wet and dry years. Positive values (red) indicate\na larger net moisture supply, while negative values (blue) indicate higher water vapor\ncondensation/precipitation. The yellow star depicts Qunf Cave (QF). Sites of other caves (yellow circles)\nand marine sediment cores (green boxes) discussed in the text that contain important Holocene\npaleoclimate records (discussed in the text) are also shown: HT, Hoti Cave; JT, Jeita Cave; SQ, Soreq\nCave; TM, Tianmen Cave; SH, Sahiya Cave; ML, Mawmluh.\n12-day back trajectories in June-August using the FLEXPART. (a) Results for the highest\nprecipitation (> +1σ) years (1983, 1986, 1988, 1996 and 2007). (b) Results for the lowest precipitation\n(< -1σ) years (1999, 2000, 2001, 2012 and 2017). (c) The difference between wet and dry years. Positive\nvalues (red) indicate a larger net moisture supply, while negative values (blue) indicate higher water\nvapor condensation/precipitation. The yellow star depicts Qunf Cave.\nC) Concerns/suggestions from the manuscript and supplementary:\n1) Line 76-78: The authors should instead re-write the sentence saying that the land-sea contrast\ntheory is not the correct explanation. The current sentence is confusing.\nWe agree with the reviewer. We have rewritten the sentence (lines 74-77) as: “This\ninterpretation seems to be consistent with the conventional wisdom that increased NHSI enhances\nthe land-sea thermal contrast and, thus, the summer monsoon intensity. However, this classical\nnotion of monsoon has been challenged by a number of recent climate modelling and reanalysis\nstudies.”.\n2) Line 138-140: Based on data from northern and southern Oman, the authors suggest that a large-\nscale re-organization of atmospheric circulation must have taken place. The two sites are too close\ntogether to make such a claim. In the later part of the manuscript, the authors have used multiple\nproxies (Fig. 4). These proxies are spread over sufficiently large region to speculate changes in\nlarge-scale circulation. And so this point can be made later in the text.\nExcellent suggestion. We have revised the sentence (lines 138-139) as: “Consistently, the D\nfi\ndata from Hoti Cave (northern Oman) also show a significant change in the moisture source,\nseasonality, and amount of rainfall above the cave around 6 ka BP.”, and moved the statement about\nthe large-scale circulation to the later part of the section (lines 190-193): “We propose that there was\na large-scale reorganization of atmospheric circulation in the region. In addition to the water vapor\ntransported by the Indian summer monsoon from the adjacent Arabian Sea, the North African\nsummer monsoon may have contributed remote moisture from the tropical Atlantic via Northeast\nAfrica to Qunf Cave during the early and middle Holocene.”\n3) Line 244-246: The authors suggest that increased evaporation at 8ka promotes PCP and a higher\ntrace elements ratio. However, they have substantiated this with a plot of P-E. Wouldn’t it be better\nto show a spatial plot of an anomaly in evaporation instead of P-E?\nThe spatial pattern of anomaly in JJA evaporation between 8K and PI is shown in Figure R3.\nBoth evaporation and precipitation were slightly higher at 8K compared with PI. It appears to be\nbetter to display the difference in their P-E, since it in theory indicates the effective wetness (P-E),\nmore relevant to the hydroclimate changes discussed in the section.\nFigure R3. EC-Earth simulation of evaporation difference between 8K and PI.\n4) Line 250 contradicts Line 229: In Line 229, authors conclude that precipitation was higher at 8ka,\nwhereas, in Line 250, they say that the early Holocene was drier. In Line 250, I guess they are\nreferring to higher evaporation. However, higher evaporation does not imply a drier climate.\nWe suggest that the precipitation (P) – evaporation (E) (P-E) was lower in the early and middle\nimplying that the effective wetness was likely lower, thus causing a relatively dry condition.\nWe have revised the sentence in lines 249-250 to: “the precipitation-evaporation (P-E) was\nslightly lower during the early to middle Holocene compared to the late Holocene.”\n5) Line 275-277: I do not agree with this statement. Precipitation on orbital scales is driven by local\ninsolation (Q. Wen et al., 2022). It is the insolation over the monsoon domains that’s important and\nnot the gradient in insolation or higher latitude insolation. However, there is still some ambiguity in\nthe literature, and the authors need not change this statement if they do not wish to. It would be good\nto clarify the definition of NHSI in this context.\nAs Beck et al. (2018) mentioned: “65°N insolation exhibits nearly identical phase, net-range,\nand pattern of variations as those of the low-latitude (30°N to 30°S) June solar insolation gradient.”\nParticularly, the Indian summer monsoon originates from the Southern Hemisphere near the\nMascarene High and transports moisture across the equator, as far as North India. Thus, the\ninterhemispheric differential in tropical insolation (or Summer Inter-Tropical Insolation Gradient,\nSITIG) may be essential to describe the summer monsoon system. This is because the insolation\ndifference involves both “pull” and “push” forcings from two hemispheres that both propel monsoon\nchanges (e.g., Cai et al., 2006; Cheng et al., 2022; Rohling et al., 2009, and references therein).\nWe have revised the sentence (lines 273-278) to: “As the Indian summer monsoon is an\ninterhemispheric monsoon system, the interhemispheric differential in tropical insolation (or\nSummer Inter-Tropical Insolation Gradient, SITIG, Reichart et al.,1997) is critical, reflecting both\n“pull” and “push” forcings from the Northern and Southern Hemisphere that both propel monsoon\nchanges (Cheng et al., 2022). Thus, we use the interhemispheric differential in tropical insolation\n(30°N-30°S) as an integrated insolation forcing of the monsoon, consistent with previous evidence,\nsuch as the Asian summer monsoon variation pattern over the MIS 3 (Cheng et al., 2022).” In the\nmanuscript, when referring previous studies, we keep the original description of insolation forcing\n(i.e., NHSI in most cases). However, for this study, we use the insolation differential (SITIG) as the\ninsolation forcing in both text and figures.\n6) Line 294-305: I find these lines repetitive. For brevity and for the benefit of the reader, the authors\nmust remove these lines as they have already been discussed in the introduction.\nDone.\n7) Line 678: Are the changes in Precip over the Qunf cave significant?\nThe EC-Earth precipitation difference between 8K and PI at Qunf Cave is ~0.1mm/day. This\nmay not be very large or significant, but regarding the P-E changes, model results show the same\ndirection as those inferred from the multi-proxy records.\n8) Supplementary material Line 101-103: The authors mention that precipitation is notably higher\n7\nat 8ka over the region of the cave. However, in Fig. 3b, it appears to have only marginally higher\nprecipitation.\nThe precipitation at Qunf Cave at 8K was slightly higher than PI, the modelling result shows\na ~0.1mm/day decrease from 8K to PI.\nFollowing the comment, we have changed the sentence in supplementary text (lines 101-102)\nto: “The summer precipitation amount at Qunf Cave was marginally higher at 8K”.\n9) Supplementary fig 8: The authors have used four years to make a climatology plot. They should\nuse at least 30 yrs, as done in supplementary figure 9.\nVery good suggestion. We have replotted this figure using the ERA5 climatology data for 30\nyears (1981 - 2010), as same as supplementary Figure 9.\nSupplementary Figure 8. 850 hPa wind of (a) pre-monsoon season (March to May, MAM), and (b)\nmonsoon season (June to July) during 1981-2010 from ERA5.\nD) Miscellaneous:\n1) Line 23: Replace “climate meaning” with a more appropriate word.\nWe have replaced it with “variability”.\n2) Line 25-26: It is not Indian summer monsoon moisture that prevails over southern Oman. In\nthis context, it is better to say moisture from the Arabian Sea prevails in southern Oman.\nWe have adjusted this sentence to “the moisture from the Arabian Sea prevails in southern\nOman”.\n3. Line 164: Fig 3b is referred to first. Perhaps, it is better if the author's interchange figures\n3a and 3b for maintaining flow.\nDone.\n4. Line 322: “between from”\n5. Line 323: “a lower P-E. but”\nWe have corrected the sentence in line 315 to: “……a lower P-E. This requires further\ninvestigation.”\nReference\nBeck, J.W., Zhou, W.J., Li, C., Wu, Z.K., White, L., Xian, F., Kong, X.H., An, Z., 2018. A 550,000-year\nrecord of East Asian monsoon rainfall from Be-10 in loess. Science 360, 877-+.\nCai, Y., An, Z., Cheng, H., Edwards, R.L., Kelly, M.J., Liu, W., Wang, X., Shen, C.-C., 2006. High-\nresolution absolute-dated Indian Monsoon record between 53 and 36 ka from Xiaobailong Cave,\nsouthwestern China. Geology 34, 621-624.\nCheng, H., Li, H., Sha, L., Sinha, A., Shi, Z., Yin, Q., Lu, Z., Zhao, D., Cai, Y., Hu, Y., Hao, Q., Tian, J.,\nKathayat, G., Dong, X., Zhao, J., Zhang, H., 2022. Milankovitch theory and monsoon. The\nInnovation, 100338.\nRohling, E.J., Abu-Zied, R., Casford, J., Hayes, A., Hoogakker, B., 2009. The marine environment:\npresent and past. The physical geography of the Mediterranean, 33-67.\nThe authors have comprehensively responded to two detailed reviews, and I am satisfied with the\nauthors' modifications in response to my comments (reviewer 2). I can now recommend this\nmanuscript for publication following a few minor comments regarding the manuscript figures.\nWe are glad that you are satisfied with our response. Here are our responses to your following\nsuggestions.\n1. The top blue/yellow banner in Figure 2 needs to be described in the figure caption. It would also\nbe helpful for the Holocene Humid Period grey bar to be identified in the figure caption.\nWe revised the color of Figure 2 and the caption as well:\nfi\nreproducibility of crushed speleothem samples; (b) Q5 Δ’17O; (c) Relative humidity calculated by Q5\nΔ’17O data; (d) δ18O; (e) δ13C; (f) (234U/238U) ; (g) Mg/Ca; (h) Sr/Ca; and (i) Ba/Ca. The top yellow and\nc 0\nblue banners show the dominant monsoon system at the cave site, with ISM stands for Indian summer\nmonsoon, and NASM stands for North African summer monsoon. The Holocene Humid Period is from\n10.5 to 6 ka BP, coinciding with the intensified Northern African and Indian summer monsoons.\n2) Please add the name of the model in the Figure 3 caption title, instead of just \"Simulation results\"\nGood suggestion. We have adjusted the figure caption title to “EC-Earth simulation results\nof summer (JJA) hydroclimate difference between 8K and PI.”.\nReviewer #1 (Remarks to the Author):\nThe authors have responded to my queries satisfactorily and I recommend publication of the\nmanuscript. The authors have not answered my question regarding the EC-Earth simulations. There\nis no description of the simulations from EC-Earth that are used to calculate the RH values for time\nperiod between 0ka and 8ka. The authors have described only PI and 8ka simulations in the main\ntext. Are these similar simulations with different boundary conditions? or are these RH values\nestimated from EC-Earth in some other way. Since, these simulations are not used in the main text,\nthis does not affect my decision on the publication of the manuscript.\nPoint-by-point response\nReviewer 1#\nThe authors have responded to my queries satisfactorily and I recommend publication of the\nmanuscript. The authors have not answered my question regarding the EC-Earth simulations. There\nis no description of the simulations from EC-Earth that are used to calculate the RH values for time\nperiod between 0ka and 8ka. The authors have described only PI and 8ka simulations in the main\ntext. Are these similar simulations with different boundary conditions? or are these RH values\nestimated from EC-Earth in some other way. Since, these simulations are not used in the main text,\nthis does not affect my decision on the publication of the manuscript.\nWe thank the reviewer for overall satisfaction with our responses.\nThe EC-Earth simulations we used to calculate the RH in the last response materials are the same\nas what we used in the main text, e.g. 8K and PI simulations. The calculated RH from 0 to 8ka, as\nshown in our last response, is from a Holocene transient simulation of EC-Earth3 that has not been\npublished yet (only temperature data are used in Askjær et al., 2022), hence it is not used for the\nmanuscript. The boundary conditions of EC-Earth 8K and PI simulations are shown in\nSupplementary Table 1.\nReference:\nAskjær, T. G., Zhang, Q., Schenk, F., Ljungqvist, F. C., Lu, Z., Brierley, C. M., ... & Yang, H. (2022).\nMulti-centennial Holocene climate variability in proxy records and transient model simulations.\nQuaternary Science Reviews, 296, 107801."}]

Decision letter