Dansgaard-Oeschger cycles of the penultimate and last glacial period recorded in stalagmites from Türkiye

Domain: earth_science
Published: 2024-02-08
Editorial outcome: major_revision

Abstract
The last glacial period is characterized by abrupt climate oscillations, also known as Dansgaard-Oeschger (D-O) cycles. However, D-O cycles remain poorly documented in climate proxy records covering the penultimate glacial period. Here we present highly resolved and precisely dated speleothem time series from Sofular Cave in northern Türkiye to provide clear evidence for D-O cycles during Marine Isotope Stage (MIS) 6 as well as MIS 2-4. D-O cycles are most clearly expressed in the Sofular carbon isotope time series, which correlate inversely with regional sea surface temperature (SST) records from the Black Sea. The pacing of D-O cycles is almost twice as long during MIS 6 compared to MIS 2-4, and could be related to a weaker Atlantic Meridional Overturning Circulation (AMOC) and a different mean climate during MIS 6 compared to MIS 2-4, leading most likely to a higher threshold for the occurrence of D-O cycles.

Introduction
The last glacial period is characterized by centennial- to millennial-scale climate oscillations known as Dansgaard-Oeschger (D-O) cycles. At least 25 D-O cycles were identified in Greenland ice cores and characterized by warm Greenland interstadials (GI) and cold Greenland stadials (GS) 1–3. Though the timing, duration, and spatial extent of D-O cycles are well-documented, uncertainties exist regarding their triggers and pacing. Proposed causes for D-O cycles include changes in the Atlantic Meridional Overturning Circulation (AMOC) and solar and volcanic forcing 4–9. While D-O cycles are well documented for the last glacial period 1, 10–15, there are only very few terrestrial proxy records which provide unambiguous evidence for their existence during the preceding glacial periods 12–14, 16–19. This knowledge gap limits our ability to evaluate the influence of different glacial boundary conditions on D-O variability, which would contribute to a better understanding of their driving mechanisms. The penultimate glacial period corresponding to Marine Isotope Stage (MIS) 6 (191-130 kyrs BP) was most likely one of the strongest Quaternary glaciations, with much greater ice sheet extent throughout Eurasia 20, 21. Though MIS 6 is not directly covered by Greenland ice core records, a synthetic δ18O Greenland record suggests the occurrence of D-O like cycles during the penultimate glacial 22. Further records are needed to better understand D-O cycles and to characterize their pattern, timing, and spatial extent. Such information can be obtained from speleothems, which are known to be an ideal archive for identifying D-O cycles 15. However, speleothem-based climate reconstructions covering the penultimate and preceding glacial periods are still rare and most of them do not display a clear D-O pattern in their isotopic profiles 11, 23–29. At present, D-O cycles have been explicitly identified in tropical and subtropical speleothems from Sanbao Cave 16 and Huagapo Cave 17, respectively, but additional precisely dated and highly resolved paleoclimate records are urgently needed to verify the synthetic Greenland ice core record and to develop a more detailed stratigraphy for D-O cycles during MIS 6.

Here we present an extended Sofular record for MIS 2-4 and parts of MIS 6/7 to show D-O cycles in very close detail. Complementing previous studies 30, 31, we present so far unpublished δ13C isotope profiles to provide further evidence for a very close atmospheric coupling between the eastern Mediterranean and the North Atlantic during the penultimate glacial period. Because of the very strong atmospheric connection between the North Atlantic region and Northern Türkiye 15, 30, 31, Sofular stalagmites are very well suited to investigate millennial-scale variability for glacial periods preceding MIS 2-4.

Results
Studysite
Sofular Cave (41°25’N, 31°56’E) is located in the North Anatolian mountains in Türkiye, ~10 km inland from the southern Black Sea coast (Fig. 1). The cave is within the Lower Cretaceous limestones. Mean annual temperature and total precipitation average ~13.3°C and ~1,200 mm yr−1, respectively. Although ~75% of total annual precipitation occurs between September and April, infiltration of rainwater occurs most likely throughout the year. Moisture originates predominantly from the Black Sea and, to a lesser extent, from the Marmara and Mediterranean Sea 30, 31. The soil above the cave is of heterogeneous thickness and covered by dense vegetation of deciduous trees (e.g., oaks, elms, and beeches), shrubs, and marginal grasslands.

Chronology
The chronology of the Sofular record is based on 150 230Th ages, all of which are in stratigraphic order. Uranium concentrations of ~0.5 ppm and low common thorium result in very precise 230Th ages for all stalagmites with typical age uncertainties of ~0.5%. The Sofular record covers the last 205,000 years before present (BP=1950) almost continuously, except for two gaps between 24.1-21.6 kyrs BP and 160-134 kyrs BP (Figs. 2 and 3).

Sofular isotope profiles
The average temporal resolution of the entire Sofular record is around 40 years. Including previous studies 30, 31, 8540 stable oxygen (δ18O) and carbon (δ13C) isotope measurements were performed. Additional high-resolution data were obtained from stalagmites So-4, So-13 and So-57, and we now present the complete Sofular δ13C record. δ18O and δ13C values range from −7.6‰ to −17.8‰ (VPBD) and −2.7‰ to −10.6‰ (VPBD), respectively (Fig. 2). Oxygen and carbon isotope profiles of all stalagmites are very similar, with correlation coefficients (r) of stalagmites covering MIS 1-4 (So-1, So-2, So-4, So-13) typically ranging from ~0.4 to ~1.0 (determined using iscam 32) (Supplementary Fig. 1a, b). For MIS 6 (So-4 and So-57) correlation coefficients range from ~0.7 to ~1.0 (Supplementary Fig. 1c).

For both MIS 2-4 and MIS 6, isotope profiles show distinct centennial- to millennial-scale variability and D-O like patterns, which are, even on multi-decadal timescales, very similar to those recorded in Greenland ice cores (Fig. 3). The strong resemblance between the Sofular and NGRIP isotope records demonstrates the very close and rapid atmospheric coupling between the North Atlantic and Black Sea region 15, 30, 33.

D-O related variations of 0.5 to 1.5‰ in speleothem δ18O are most likely caused by multiple factors such as air temperature, variations in δ18O of the moisture source, seasonality, and amount of precipitation above Sofular Cave 30, 31. On orbital time-scales fluctuations in Sofular δ18O values track changes in the oxygen isotopic composition of Black Sea surface water through the so-called water vapor source effect 30, 31. δ18O values of around −8.5 ± 1‰ mark periods when the Black Sea was connected with the Mediterranean Sea when the sea level was higher than the Bosporus sill depth of ~35 meters below current sea level 31. More negative δ18O values are therefore indicative of a Black Sea “Lake” 34 without inflow of saline Mediterranean water. Pronounced negative excursions in δ18O of up to −17.8‰, however, indicate the inflow of isotopically depleted meltwater from the Caspian Sea and rivers entering the Black Sea “Lake” 34 (Supplementary Fig. 2). Thus, the water vapor source effect in combination with variable mixing times of the Black Sea 35 affected D-O related fluctuations in δ18O during MIS 2-4 and particularly during MIS 6. This effect is also apparent in Sofular stalagmites So-1 and So-2 δ18O profiles which show a suppressed Bølling-Allerød and Younger Dryas isotopic pattern 31 (Fig. 2).

In contrast, D-O cycles are more clearly visible in the Sofular δ13C isotope profiles indicating a high sensitivity of δ13C to D-O climate variability. Millennial-scale δ13C fluctuations in Sofular speleothems are similar between the last and penultimate glacial ranging from −5‰ to −10‰. Though So-4 and So-57 δ13C profiles show an almost identical isotopic pattern, δ13C values of stalagmite So-57 are on average 1.3‰ more positive. This isotopic offset is most likely related to sample-specific effects caused by lower drip rates and stronger fractionation due to CO2 degassing. In order to reduce sample-specific effects, age uncertainties and to increase the signal-to-noise ratio, we developed a stacked δ13C record of MIS 1-4 and MIS 6 using the program called intra-site correlation age modeling (iscam) 32. Iscam was designed to synchronize overlapping time series from the same site within their age uncertainties. D-O cycles in the Sofular δ13C stacked record are characterized by high amplitude shifts of up to 6‰ (Fig. 3a). We suggest that these shifts are primarily caused by temperature- and moisture-related changes in vegetation and soil microbial activity, with higher vegetation density and soil respiration rates associated with warmer and wetter GIs (Fig. 3a). Our assumption is based on the following lines of evidence. Firstly, a close coupling between climate and soil respiration rates is directly supported by paired δ13C and 14C measurements in stalagmite So-1 36. The paired measurements reveal a rapid response of soil respiration rates and microbial activity across the Bølling-Allerød and Younger Dryas in stalagmite So-1, with enhanced decomposition of soil organic matter (SOM) under warmer and wetter climatic conditions 36.

Secondly, soil respiration rate is closely linked to temperature and soil moisture conditions on a range of timescales 37, 38, a relationship that is supported by modeling experiments 39 and speleothem records from western Europe 40–42. Thirdly, the Sofular δ13C stacked records show an inverse correlation (r2) of ~0.7 (MIS 2-4) and ~0.5 (MIS 6) with sea surface temperature (SST) records from the Black Sea (gravity core GC25-1 & MSM 33) 33, 43 (Fig. 4, Supplementary Figs. 3 and 4). Such a close coupling between air temperature above Sofular Cave and Black Sea SSTs can be expected as the cave site is only ~10 km inland. Therefore, positive SST shifts of up to 4°C associated with GIs 33 (Fig. 3) would lead to a marked increase in temperature and precipitation above Sofular Cave, thereby promoting a change in the type and density of vegetation 44 and an increase in soil respiration rates due to higher soil microbial activity and faster decomposition of SOM 36. In contrast, cold-dry climatic conditions would affect vegetation density and type and reduce CO2 production due to lower microbial activity, and sparser vegetation is expected to result in lower soil pCO2, lower contribution of biogenic carbon, and, consequently, higher stalagmite δ13C values.

The Sofular stack (Fig. 3a) exhibits the most positive δ13C values of around −3‰ at ~62 kyrs BP, concomitant with a major advance of the Eurasian Ice Sheet (EIS) 45, 46 and Heinrich event 6. In the Black Sea region, this interval was dominated by steppe pollen taxa 44 and higher ice-rafted debris in Black Sea sediments 47. Heinrich events H5–H0 are also well expressed in the Sofular δ13C profile, and characterized by marked SST decreases of the Black Sea (Fig. 3a) and reduced precipitation in the Eastern Mediterranean 33, 47, 48. Climate simulations suggest colder and drier conditions induced by a cyclonic atmospheric circulation anomaly modulating the eastward advection of cold air over Eurasia caused by a weakening of the AMOC during Heinrich events and GSs 49–52.

D-O cycles during MIS 6
During MIS 6, a D-O like climate variability is apparent in the δ13C records of stalagmites So-4 and So-57. Similar to MIS 2-4, D-O cycles during MIS 6 are characterized by abrupt negative shifts of up to 4‰ in δ13C (Fig. 3b) in response to warmer and more humid climatic conditions and higher soil respiration rates, dense vegetation and higher proportions of C3 plant vegetation 36. Phases of increasing temperature and higher effective moisture are broadly consistent with pollen evidence (dec. Quercus, Betula, Pinus) from Lake Van 53 (Supplementary Fig. 2a).

D-O cycles in the Sofular stack mirror those of the synthetic Greenland ice core δ18O record 22 (Fig. 3b), which was tuned to the absolutely-dated Sanbao Cave record 11, 54. Radiometrically dated records showing a clear D-O like pattern are currently very rare and their temporal resolution and/or chronological precision are not sufficient to identify D-O cycles 26, 27, 55, 56. Thus, records that can be used for a detailed comparison to Sofular are rather scarce. Suitable time series are speleothem δ18O records from Sanbao Cave 16 in China and Huagapo Cave 17 in Peru (Fig. 3b). Taking advantage of the precise and absolute chronologies of these stalagmite records, including our Sofular stack, we can gain further information on the pacing and timing of D-O like events during MIS 6. The comparison between the records shows age offsets of several centuries for D-O 6.6, 6.8, 6.9 and 6.12 (Supplementary Fig. 5). Higher age offsets between the records of almost two millennia are observed for D-O 6.7, 6.10, 6.11, 6.13 and 6.14, likely caused by a combination of uncertainties related to the age models and uncertain assignment of D-O events.

For the time interval between 200 and 160 kyrs BP, the average pacing of D-O cycles is ~4.16 kyrs in the Sofular stack (Fig. 5b, Supplementary Fig. 6), very similar to the pacing of ~4.24 kyrs and ~4.03 kyrs in the Huagapo and Sanbao speleothem records, respectively. All speleothem-based estimates for the D-O pacing are in agreement with a pacing of ~4.00 kyrs in the synthetic Greenland ice core. In contrast, the average D-O pacing recorded by the Sofular stack during MIS 2-4 is ~2.07 kyrs (Fig. 5a, Supplementary Fig. 7), and therefore only half as long compared to MIS 6. The Speleothem Interstadial Onset Compilation data set (SIOC19) and Greenland ice cores show an almost identical D-O pacing of ~2.04 kyrs and ~2.09 kyrs respectively (Fig. 5a). Thus, the absolutely dated Sofular stack record presented here provides additional evidence for a longer pacing of D-O cycles during MIS 6. Since D-O events are closely linked to changes in the strength of AMOC, which in turn is dependent on the rate of North Atlantic Deep Water (NADW) formation and sea-ice extent 52, the longer pacing of D-O cycles is most likely directly related to a different AMOC setting during MIS 6. Benthic foraminifera isotope data from the Portuguese margin suggest a shallower and weaker AMOC cell during MIS 6 than during MIS 3 13. This is in agreement with longer bipolar seesaw events 57, greater sea ice cover, and lower amplitude SST variability in the South Atlantic 58. Furthermore, iceberg discharge into the North Atlantic was muted during the first half of MIS 6 and ‘classic’ Heinrich events are missing 13, 57. Overall, there is strong evidence for a weaker AMOC during the penultimate glaciation. AMOC weakening leads to a considerable cooling in the North Atlantic realm and sea-ice advance over the Labrador and Nordic Seas, which in turn increases surface albedo and reduces heat loss from the ocean to the atmosphere 52. As a result of these different mean climatic and glacial boundary conditions during MIS 6, thresholds for triggering D-O cycles were most likely higher and led to less frequent and longer pacing of D-O cycles. However, additional and more detailed reconstructions of AMOC intensity during MIS 6 are urgently needed to prove our hypothesis.

In conclusion, the stacked Sofular δ13C record shows D-O climate variability for the last and penultimate glacial in great detail and thereby provides additional evidence for D-O variability in the synthetic Greenland ice core record. The comparison of absolutely dated speleothem records covering MIS 6, including the Sofular stack, reveals an agreement in both the timing and pacing of D-O cycles. Thus, there is mounting evidence for a significantly longer pacing of D-O cycles during MIS 6 compared to MIS 2-4. This could be related to a generally weaker and shallower AMOC and reduced northward heat transport and significant cooling of the North Atlantic realm, thereby inducing a lower pacing of D-O events during MIS 6.

Methods
Stalagmite samples
A total of six stalagmites from Sofular Cave (So-1, So-2, So-4, So-6, So-13, and So-57) with heights ranging between 0.8 and 1.7 meters were analyzed. So-1, So-2, and parts of So-4 and So-6 have been investigated in previous studies 30, 31, while this study provides complementary δ13C profiles on So-4 and So-6, and previously unpublished data of stalagmites So-13 and So-57.

230Th dating and age model development
U-series dating (230Th) was performed on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo-Finnigan-Neptune) at the Department of Geology and Geophysics, University of Minnesota and Institute of Global Environmental Change, Xi’an Jiaotong University (Supplementary Dataset 1). Further 230Th dating on stalagmites So-1 and So-2 was done on a Nu Instruments® MC-ICP-MS at the Geological Institute, University of Bern (Supplementary Dataset 2). Detailed information on analytical procedures is provided in Supplementary Texts 1 and 2 accompanying this article. Age models of all stalagmites were constructed using the StalAge algorithm 59 and the Sofular stacked record was developed using the iscam algorithm 32.

Stable isotope analysis
Stable isotope analyzes were performed on a Finnigan Delta V and Delta V Plus IRMS equipped with an automated carbonate preparation system (GasBench-II) at the Institute of Geological Sciences, University of Bern, and Department of Environmental Sciences, University of Basel. The precision of δ13C and δ18O measurements is 0.06% and 0.07% (1σ-error) respectively.

Data availability
Source data for Fig. 2 and Fig. 3 are referenced in the Source data provided with this paper and are available online on the NOAA paleoclimate database (https://www.ncei.noaa.gov/access/paleo-search). Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Held et al.\nHeld et al., present d18O and d13C records from Sofular cave speleothems. The speleothems cover 0-\n80 ka, 120-135ka and 160-220ka. The speleothems resolve the DO variability of the last deglaciation,\nlast glacial period as well as MIS6. In addition, using a SST record from the Black Sea, the authors\nderive a transfer function to be able to estimate changes in air temperature above the cave. On top of\nevidencing D-O variability during MIS6, they suggest that the D-O pacing was twice as long during\nMIS6 than MIS2-4. They attribute the main differences in D-O variability between MIS6 and MIS2-4 to\nthe different extent of the Eurasian ice-sheet (EIS).\nThe MIS6 record presented is very interesting and will strongly benefit the community. While I agree\nwith the last sentence of the manuscript that the newly presented Sofular cave record could become\none of the key paleoclimate timeseries for DO study given its resolution and coverage, additional work\nneeds to be done on the manuscript before publication. Please find some comments/suggestions\nbelow.\n1) The authors attribute the different expression of DO variability during MIS6 to the larger extent of\nthe Eurasian ice-sheet. First, MIS6 is taken as one period, whereas during its 60 kyrs conditions most\nlikely varied significantly and probably so did the EIS. The authors should thus precisely present the\nevidence for a larger EIS and more clearly provide information on its possible growth and decay during\nMIS6. I can see that an EIS volume curve is included in Fig. S4 (the reference to the EIS study is\nmissing), but for the period of availability of the Sofular record (ie 160-200ka), I am not sure the EIS\nis larger than during MIS3.\nL. 193-196: to which “atmospheric teleconnections” are the authors referring to? This sentence needs\nto be more precise.\nL. 254-256: While the speleothems provide temperature estimates, the study cannot attribute the\ndifferences between MIS2-4 and MIS6 to the EIS.\nOn a similar topic, the authors mention in the abstract and L. 159-161 that the different D-O pacing at\nMIS2-4 and MIS6 is due to ice-sheet dynamics, but there is no information in the manuscript on how\nthis conclusion is reached.\nL. 231-232: I do not understand how/why it is suggested that it causes longer transition time to\ninterstadial.\nL. 253-254: How can it be concluded that the longer DO in MI6 are due to a feedback between AMOC\nintensity and ice-sheet dynamics?\n2) A lot of climate modelling work has been done to understand D-O variability as well as the climatic\nimpact of changes in the Atlantic Meridional Overturning Circulation. The authors should familiarize\nthemselves with the results of these studies to help with the understanding of their records and put\ntheir record in the context of the dynamical conclusions that have been drown by these studies. A\nrecent review on the topic (Menviel et al., 2020, Nature Reviews) could be a good entry point. The\nstudy also includes extensive references on appropriate climate modelling studies such as Stouffer et\nal. 2006, Kageyama et al., 2013…\nFor example, L. 115-118: previous studies (including Fleitmann et al. 2009, but also Menviel et al.,\nchanges, DO variability and climate change in Turkey).\nL. 168-169 could be rephrased as modelling studies have shown that an AMOC shutdown reduces the\nmeridional oceanic heat transport to the North Atlantic, thus contributing to an increase in sea-ice\ncover in the North Atlantic.\nThe paragraph L. 225 thus needs to be revisited. While the temperature changes mentioned are\ncorrect, they do not simply depend on latitude, but on the processes at play. References to additional\nclimate model simulations would strengthen the study.\n3) Transfer function\nIf I understand correctly and based on Fig. 5, the authors derive different transfer functions for MIS2-\n4 and MIS6. What is the rationale behind this? The use of different transfer functions makes me doubt\nyour result, i.e. the differences in temperature estimates between MIS2-4 and MIS6. I think you\nshould i) re-assess whether the use of 2 different transfer functions is appropriate, ii) present an\nuncertainty band with an estimate based on the same transfer functions.\n4) Paragraph starting L. 170: I find this paragraph on DO rebound events a bit confusing. In addition,\nI do not understand how the authors can conclude that longer DO cycles could favour rebound events\n(L. 172-173).\nMinor points:\nThe text is sometimes hard to follow due to language issues, for example L. 95-96, I don’t think the\nuse of “whereas” is appropriate here.\nL. 161-162: Correlation is a strong word here\nL. 218-222: I find these sentences hard to understand. Please break them down and improve the\nlogical flow.\nL. 232: The transition with “However” lacks a logical flow.", "reviewer_2": "Review of Dansgaard-Oeschger cycles of the penultimate and last glacial period recorded in\nstalagmites from Turkey by Held et al.\nThis manuscript presents new speleothem records that purport to show DO-type variability across the\nsample region during the penultimate glacial period (MIS 6). Compared with results from the same\ncave covering MIS 3, the new results are less continuous and the correspondence between carbon and\noxygen isotopes observed during MIS 3 breaks down completely. The new results are a useful addition\nto the growing collection of records covering MIS 6 and provide a useful source of absolute age\nconstraint (which is rare). However, the implications and conclusions drawn from the dataset are not\nwell developed and thus the potential impact of the study seems to be below that which could be\npossible.\nIn particular, the main conclusions seem to concern the duration of DO cycles (or events) and their\nmagnitude, in terms of temperature change. The first of these could be important for improving our\nunderstanding of abrupt climate variability under different boundary conditions but the discussion is\npoorly developed and does not represent a step forward in understanding. The second conclusion does\nnot seem to hold water, given that other temperature records from the same interval do not appear to\nshow increased temperature variability. This point is not addressed in the study and really needs to\nbe, before any useful conclusions can be drawn about regional differences.\nBelow I outline my comments and questions in order of their appearance.\nComments and questions:\nLine 23 “there are almost no proxy records which provide unambiguous evidence for their existence\nduring preceding glacial periods” – This is not the case. Some of the papers you cite later provide such\nevidence. Here is a short list of examples: Martrat et al., 2004; 2007; Barker et al., 2015, Cortina et\nal., 2015; Cheng et al., 2016\nLine 88 “The new Sofular δ18O and δ13C records show characteristic D-O cycles, which are, even on\nmultidecadal timescales, remarkably similar to those recorded in Greenland ice cores” – Considering\nthe number of proxy records that look like Greenland d18O I would not say that this dataset stands\nout as being particularly remarkable. At the least you should quantify how good the agreement is, and\nmore importantly, you need to describe those parts of the record that differ from the variability\nrecorded in Greenland. For example, the amplitude of individual events is much smaller relative to the\nbaseline difference between (some) stadial events in the Sofular record compared to that seen at\nGreenland. E.g. compare stadial values before and after DO8, or during H events 4, 5, 5a and 6, which\ncontrast with the relative constancy of the Greenland record for the same events.\nSame point on line 130 “Considering the very high coincidence of abrupt climate events between\nGreenland ice cores and Sofular stalagmites…”. Use of the term ‘high confidence’ demands statistical\nquantification.\nLine 104 “Such a climate amelioration would not only promote a change in the type and density of\nvegetation but also increase soil respiration rates due to higher soil microbial activity and faster\ndecomposition of SOM” - How fast would this change occur? Does this limit the rapidity of changes\nthat can be recorded?\nLine 111 “The Sofular record exhibits the most positive δ13C values of around -3‰ at ~62 ka BP,\nconcomitant with the maximum extent of ice sheets in Eurasia and Heinrich event 6 (Fig. 2).” - This is\nconfusing and conflicts with the evidence in Fig. 2 and the cited papers. Fig 2 shows maps for the LGM\nand MIS 6, not H6 or MIS 4 (which was smaller in extent than the LGM, according to the cited paper.\nLine 115 “Since Heinrich events (HE) were most likely restricted to the midlatitude North Atlantic,\nevidence…”. This is not true, the effects of H events were certainly not restricted to the mid northern\nlatitudes but are seen in records around the Earth. For example, Brazilian speleothems show growth\nperiods associated with H events (Wang et al., 2004), surface ocean frontal shifts in the SE Atlantic\nwere more accentuated for H events than non-H events (Barker and Diz, 2014), Chinese speleothems\nshow more pronounced variations associated with H events (Wang et al., 2001). H events are also\nmore pronounced than non-H events in the deep ocean e.g. the deep South Atlantic (Gottschalk et al.,\n2015) and the NE Atlantic (Henry et al., 2016).\nLine 136 “During MIS 6, a D-O like climate variability is most clearly visible in the δ13C records” - But\nwhy does d18O not show these events when it does so clearly during MIS 3? This demands some\nexplanation.\nLine 137 “D-O cycles during MIS 6 are characterized by abrupt negative shifts of up to 4‰ (Fig. 3) in\nresponse to warmer and more humid climatic conditions and higher soil respiration rates, denser\nvegetation and higher proportions of C3 plant vegetation.” – what is the evidence for this?\nLine 157 “The average pacing of D-O cycles is almost twice as long (3.83 ka) during MIS 6 compared\nto MIS 2-4 (2.07 ka) (Fig. 4).” – I have several concerns about this statement and therefore the\narguments that build on it. Firstly, from Fig. 4 it looks like you are describing the duration of individual\nDO events rather than the time between events (which is implied by the term ‘pacing’) – the\ndistinction is very important for any discussion of the underlying causes. You repeatedly use the term\n‘DO cycles’ (which would include both stadial and interstadial event) but if you are actually referring to\nthe duration of DO events, this needs to be made clear. Secondly, I could not find any description of\nhow the duration was measured for each dataset. This needs to be shown in detail to allow the reader\nto assess how robust the finding is. Thirdly (and this is related the second point), looking at Figs 2 and\n3, it is not at all obvious that your calculated durations are correct. For example in Fig 4, DO6.6 lasts\nfor something like 3-5kyr but most of the records in Fig 3 show it to be 2-3kyr.\nLine 161 - The following discussion requires a lot of development before it might represent a step\nforward in understanding. At the moment it is just a collection of vague possibilities, with no thread,\nor coherent argument.\nLine 180 “…the Huagapo δ18O record reveals a pronounced asymmetric D-O pattern during MIS 6\nsuggesting a significant influence of different regional boundary conditions on D-O events.” - Or it\ncould just reflect differences in the recording of DO events by different archives and proxies. This is a\ncrucial question.\nLine 187 Please stipulate which data presented here were published previously in the Badertscher\n2011 paper.\nLine 207 and below - This exercise makes very little sense to me. You are calibrating to a very low\nresolution record and you get a completely different calibration for MIS 6 vs MIS 2 (which should tell\nyou something is wrong). Also, you omit to mention the numerous temperature (and equivalent)\nreconstructions from MIS 6 which are nicely summarized in the Wegwerth paper you cite for the Black\nSea temperature record. None of these other reconstructions suggest a larger amplitude of\ntemperature change associated with DOs of MIS 6. So are you challenging all of these records or are\nyou suggesting that the larger magnitude of DO warming during MIS 6 was limited to a region around\nyour sample site?\nMinor comments\nLine 38 “a very tied atmospheric teleconnection” – poor wording\nLine 81 “The Sofular record covers the last 80,000 years and the period between 130,000 and\n200,000 years before present almost continuously. Hiatuses exist between 21.6 - 24.1 ka BP and 134\n- 160 ka BP (Figs. 2 and 3).” – A gap of 26kyr within a 70kyr interval is hardly ‘almost continuous’.\nReferences\nBarker, S., Chen, J., Gong, X., Jonkers, L., Knorr, G., & Thornalley, D. (2015). Icebergs not the trigger\nfor North Atlantic cold events. Nature, 520(7547), 333-336.\nBarker, S., & Diz, P. (2014). Timing of the descent into the last ice age determined by the bipolar\nseesaw. Paleoceanography, 29(6), 489-507.\nCheng, H., Edwards, R. L., Sinha, A., Spötl, C., Yi, L., Chen, S., Kelly, M., Kathayat, G., Wang, X., & Li,\nX. (2016). The Asian monsoon over the past 640,000 years and ice age terminations. Nature,\n534(7609), 640-646.\nCortina, A., Sierro, F. J., Flores, J. A., Martrat, B., & Grimalt, J. O. (2015). The response of SST to\ninsolation and ice sheet variability from MIS 3 to MIS 11 in the northwestern Mediterranean Sea (Gulf\nof Lions). Geophysical Research Letters, 42(23), 10,366-310,374.\nMartrat, B., Grimalt, J. O., Lopez-Martinez, C., Cacho, I., Sierro, F. J., Flores, J. A., Zahn, R., Canals,\nM., Curtis, J. H., & Hodell, D. A. (2004). Abrupt temperature changes in the Western Mediterranean\nover the past 250,000 years. Science, 306(5702), 1762-1765.\nMartrat, B., Grimalt, J. O., Shackleton, N. J., de Abreu, L., Hutterli, M. A., & Stocker, T. F. (2007). Four\nclimate cycles of recurring deep and surface water destabilizations on the Iberian margin. Science,\n317(5837), 502-507.\nWang, X. F., Auler, A. S., Edwards, R. L., Cheng, H., Cristalli, P. S., Smart, P. L., Richards, D. A., &\nShen, C. C. (2004). Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate\nanomalies. Nature, 432(7018), 740-743.\nWang, Y. J., Cheng, H., Edwards, R. L., An, Z. S., Wu, J. Y., Shen, C. C., & Dorale, J. A. (2001). A high-\nresolution absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science,\n294(5550), 2345-2348.", "reviewer_3": "Held and co-authors present high-resolution and, as for these archives possible, extremely well dated\nspeleothem isotope data from NW-Anatolia reaching back into the penultimate glacial period, to\nprovide evidence and discuss Dansgaard-Oeschger-type climate changes during MIS 6 in comparison\nto the well-known D-O variability of the last glacial. Focus is mainly on the speleothem carbon isotope\nsignal fluctuating in function of soil pCO2, which in turn depends on temperature and moisture\navailability, both affecting vegetation and soil microbial activity. The authors observe an immediate\nresponse in the d13C signal to the North Atlantic climate cycles, even recording in detail the\nprecursor-type events. MIS 6 variations are compared to other stalagmite data and the synthetic\nGreenland ice core record derived from Antarctic ice cores. For some of the identified D-O events,\nage\noffsets are small, for others the assignments are ambiguous and age offsets are relatively large. Latter\nis particularly the case, where the offset in the isotope signals between two different stalagmites of\nthe same cave (So-4 and So-57) is very large. The authors suggest that D-O climate variability and\nduration as recorded in Anatolia is strongly linked to insolation changes and the size of the European\nIce Sheet, by affecting the teleconnection strengths to the North Atlantic system. In a last part of the\nmanuscript, the d13C signal is calibrated against Black Sea surface temperatures to discuss\ntemperature variability at the cave site.\nSince such detailed D-O observations for MIS 6 are rather sparse, this contribution is highly relevant\nfor the comprehensive understanding of the dynamics of rapid climate change and I would very much\nlike to see this work published in Nature communications after moderate revisions.\nBesides some following general remarks, please find some detailed comments below.\nFirst, being aware of the previous studies on the Sofular speleothems, I would ask the authors to more\nprecisely state which data are original to this study and which data were used from previously\npublished work. In particular, MIS 2-4 data and corresponding discussion is reiterated in several places\nand could perhaps be shortened.\nSecond, the authors convincingly use the d13C as the better and more immediate recorder of\nenvironmental changes above the cave. But this strength is also a weakness at the same time, when\nit comes to reproducibility, absolute values and amplitudes. The authors indeed discuss the complexity\nof the d13C signal formation, but at the end they calculate an average signal from two very different\nrecords, and, even more important, they use this record to linearly calibrate and extract temperature\ninformation, knowing that distinct hydroclimatic changes and shifts in the vegetation contributed\nsignificantly to the signal formation. Detailed reconstructions of the MIS 2-4 Anatolian vegetation\ndynamics, e.g., share the overall D-O pattern, but with significant differences as well (Shumilovskikh\net al. 2014, Climate of the Past). Therefore, absolute temperatures as well as the amplitude (I would\nexpect higher amplitudes on the continent and at this elevation) of the changes may differ significantly\nfrom the proposed temperature record, which is linked straight forward to Black Sea surface\ntemperatures, without considering differences in elevation (ca. 500 m vs sea level) and a certain\ncontinentality and seasonality (since Black Sea temperatures are assumed to represent annual\naverages and soil activity is definitively biased towards the warm season). Although I find the idea to\nextract temperatures from the stalagmite d13 a very interesting exercise, there are too many\nuncertainties and unknowns involved and I recommend to rather stay with the original d13C. Unless\none were to take the certainly promising approach of extracting organic biomarker (GDGCs) from\nstalagmites to reconstruct soil/atmospheric temperatures above the cave (e.g., Zang et al. 2023, DOI\n10.3389/fevo.2023.1117599).\nThe authors discuss the MIS 6 d18O data noting that there is no asymmetry (as perhaps expected) in\nthe D-O cycles and that large changes in the Black Sea hydrology (e.g., marine and melt water\ninflows) matter as well. Reference to two Black Sea studies could strengthen the discussion here, since\nthey showed (1) that the glacial Black Sea d18O source signal has undergone a strong modification\ncompared to the Greenland ice cores, due to the variable mixing time of the large Black Sea reservoir\nsmoothing and delaying the signal - a one to one pattern cannot be expected (Wegwerth et al. 2019,\ncommunications earth and environment), and (2) that three large meltwater pulses arrived in the\nBlack Sea during the penultimate glacial.\nComments:\nL 22: change to “(AMOC) and solar and volcanic forcing.\nL 63-67, 79-85: Please make clear, which part of the data are original to this paper.\nL 102-109: Evidence for vegetation chages come from, e.g. Shumilovkikh et al.,. add ref\nL 115-117: This link has been discussed widely by Wegwerth et al. 2015 and 2016, add refs\nL 120-121: Please indicate more precisely precursor-type events in Fig. 2\nSuppl. Fig. 1: no element ratio but only the concentrations of Mg and Sr is shown. As it is now, the\nstatement is not supported.\nL 124-126 and Supp. Fig. 1b: this figure should contain a direct and detailed comparison between the\nSofular and NGRIP isotope data to support the statement\nL 141-145: Offset between So-4 and So-57 is discussed, however the stacking approach is rather\nquestionable, since the patterns and amplitudes are significantly different. For this discussion, a\ncomposit is not needed.\nL 139-140: Include pollen data von Lake Van into the graph\nL 148: Statement is misleading. Of course one can compare various records to the Sofular data even if\nthey are of lower resolution and weaker in stratigraphy. In that case, one has to consider the\nrespective limitations.\nFig. 4 caption: change to ...synthetic ice core.\nL 161-162: There are many D-O events not fitting into this picture. Please discuss.\nL 176-178: See general comment on the d18O record\nL 183-186: In fact, three periods of melt water input into the Black Sea are hypothesized for MIS 6\nL 225-236: Regional patterns in D-O associated temperature changes have been summarized and\nthoroughly discussed in Wegwerth et al. 2016.", "reviewer_1_2nd": "2nd review of Held et al.\nIn their revised version, Held et al addressed my comments related to the link with modelling and\nrelationship between d13C and temperature. Please find below a few minor comments that should be\naddressed before publication.\nAbstract, L. 12-15: A weaker AMOC as the reason behind the longer DO pacing is an hypothesis that\ncannot be tested here. I thus suggest to carefully phrase it as such (and also L. 227-230). In addition\nhere “background conditions” are also mentioned whereas they are not mentioned L. 227-230 (unless\nI misunderstood what you meant by background conditions).\nL. 22: There is a lot of evidence for DO cycles being associated with AMOC changes. What caused\nthese AMOC changes is unclear, and volcanic forcing is one of the hypothesis being put recently\nforward but has only limited evidence, but not solar cycles.\nL. 23-24: I understand there are few terrestrial records of the penultimate glacial period which present\nDO variability but since they look at pollen, Margari et al. 2010 and Tzedakis et al. 2009 should be\nadded there (and not only for the last glacial period). Martrat et al., 2007 also present data for the\npenultimate glacial period and Antarctic ice core records also provide information on DO variability,\nsince they form the basis of the synthetic Greenland record.\nL. 31: I don’t think further records are needed to “confirm” DO cycles, but additional high-resolution\nrecords like the one you are providing are needed to better understand these cycles.\nL. 197: What do you mean by age offsets?\nFigure 5: Please provide information on how you calculated DO pacing.\nL. 215: I am not sure that's the case for the periods of interest and note that warmer conditions in SO\nwould be expected with a weaker AMOC\nL. 224: Please replace “confirms” by “provides additional evidence”\nL. 227-228: “This could be related” iof “most likely”", "reviewer_2_2nd": "Review of Revised version: Dansgaard-Oeschger cycles of the penultimate and last glacial period\nrecorded in stalagmites from Turkey by Held et al.\nIn this revised version of their manuscript, the authors have removed many of the difficult sections of\nthe original version, which I commend. In particular, the authors now suggest that the longer\ntimescale of DO variability during MIS 6, was due to a weaker and shallower mode of the AMOC.\nHowever, while this may have been the case, we have no direct evidence for the state of the AMOC\nduring MIS 6 (in fact the state of AMOC during the LGM is still a matter of strong debate). The\nevidence cited here is all circumstantial, based on benthic oxygen isotopes, surface temperature and\nCO2 records, and not proxies of AMOC strength or geometry. I therefore think that the authors have\nto tone down this argument. Perhaps they could argue that their results (longer timescale of DO\nvariability) might reflect a difference in the underlying structure and vigor of the AMOC during MIS 6,\nbut that this conclusion awaits future direct and quantitative reconstructions of AMOC during that\ninterval.\nOther than this point, I don’t have major concerns about the ms as presented.", "reviewer_3_2nd": "The revised manuscript and rebuttal letter adresses most of the comments and suggestions of the\nthree reviewers and I very much acknowledge the effort of the authors to improve the manuscript .\nParticularly the temperature calibration approach has been removed from the manuscript together\nwith the associated discussion.\nTwo issues raised in the first instance have not been properly adressed for my opinion.\nFirst, more a technical aspect but mentioned by all reviewers, was to stipulate which data were\nalready published in previous publications. In the rebutal letter this has been made more or less clear,\nbut it is not yet satisfactorily addressed in the manuscript itself.\nSecond, I cannot follow the authors response on the stacking procedure of their TWO partly very\ndifferent MIS6 d13C records (n=2!). Since the records are partly very different, the stacking clearly\naffects the D-O assignment and interpretation. The mentioning of SPECMAP (n=5) and LR04 stack\n(n=57) is not very helpfull here because they come along with a much more robust statistics! An\naverage of 2 is probelmatic here.\nThese and some additional minor comments are included in the attached annotated pdf of the\nresubmitted paper."}, "author_response": "Responses to Reviewers\nWe would like to thank all reviewers for their thoughtful and constructive comments, most of which\nhelped to significantly improve the quality of our manuscript. Please find our responses (in black) to\nthe reviewer comments (in blue).\nBased on their comments and suggestions, we did a major revision of our manuscript, and also added\nand changed most figures. We also adjusted the general structure to address the comments of the\nreviewers. In particular, we agree with the concerns of the reviewers that the temperature\nreconstruction is subject to major uncertainties and limitations. We therefore removed this part of\nthe manuscript as it was too controversial. To improve and sharpen the content of our manuscript,\nwe acquired new additional data by adding another stalagmite So-13 and by improving the chronology\nof So-57 with more 230Th ages.\nReviewer: 1\n1) The authors attribute the different expression of DO variability during MIS6 to the larger extent of\nthe Eurasian ice-sheet. First, MIS6 is taken as one period, whereas during its 60 kyrs conditions most\nlikely varied significantly and probably so did the EIS. The authors should thus precisely present the\nevidence for a larger EIS and more clearly provide information on its possible growth and decay during\nMIS6. I can see that an EIS volume curve is included in Fig. S4 (the reference to the EIS study is\nmissing), but for the period of availability of the Sofular record (ie 160-200ka), I am not sure the EIS\nis larger than during MIS3.\nWe apologize that we didn’t make it clearer and we agree with reviewer 1 that the greater extent of\nthe EIS is not responsible for the longer pacing of D-O events during MIS 6. In order to clarify this, we\nexpanded the discussion on the potential causes for longer D-O pacings in terms of changes in the\nAMOC strength and bipolar sea saw mechanism related to the interhemispheric coupling.\nL. 193-196: to which “atmospheric teleconnections” are the authors referring to? This sentence needs\nto be more precise.\nWe are referring to the atmospheric teleconnection between the North Atlantic and Eastern\nMediterranean. We changed the sentence accordingly.\nL. 254-256: While the speleothems provide temperature estimates, the study cannot attribute the\ndifferences between MIS2-4 and MIS6 to the EIS.\nThis is now obsolete as we discarded our controversial temperature reconstruction.\nOn a similar topic, the authors mention in the abstract and L. 159-161 that the different D-O pacing at\nMIS2-4 and MIS6 is due to ice-sheet dynamics, but there is no information in the manuscript on how\nthis conclusion is reached.\nWe would like to thank reviewer 1 for this thoughtful comment. We have refined the discussion on\nthe relationship between longer D-O pacings and changes in the Atlantic Meridional Ocean Circulation\n(AMOC). We included additional references (Menviel et al., 2020; Shin et al., 2020; Margari et al.,\n2010; Gottschalk et al., 2020) to highlight and discuss important components contributing to\ndifferences in the AMOC state during MIS 2-4 and MIS 6.\nL. 231-232: I do not understand how/why it is suggested that it causes longer transition time to\ninterstadial.\nWe do not discuss this matter in the revised version of the manuscript, also because we removed the\nquantitative temperature reconstruction.\nL. 253-254: How can it be concluded that the longer DO in MI6 are due to a feedback between AMOC\nintensity and ice-sheet dynamics?\nSee comment above. In the revised version we do not discuss the duration of D-O events but rather\nfocus on their pacing.\n2) A lot of climate modelling work has been done to understand D-O variability as well as the climatic\nimpact of changes in the Atlantic Meridional Overturning Circulation. The authors should familiarize\nthemselves with the results of these studies to help with the understanding of their records and put\ntheir record in the context of the dynamical conclusions that have been drown by these studies. A\nrecent review on the topic (Menviel et al., 2020, Nature Reviews) could be a good entry point. The\nstudy also includes extensive references on appropriate climate modelling studies such as Stouffer et\nal. 2006, Kageyama et al., 2013…\nWe thank reviewer 1 for this interesting reference and we have expanded our discussion to address\nthe potential causes of AMOC changes on D-O climate variability.\nFor example, L. 115-118: previous studies (including Fleitmann et al. 2009, but also Menviel et al.,\nchanges, DO variability and climate change in Turkey).\nAddressed in the revised version, see also comment above.\nL. 168-169 could be rephrased as modelling studies have shown that an AMOC shutdown reduces the\nmeridional oceanic heat transport to the North Atlantic, thus contributing to an increase in sea-ice\ncover in the North Atlantic.\nRevised.\nThe paragraph L. 225 thus needs to be revisited. While the temperature changes mentioned are\ncorrect, they do not simply depend on latitude, but on the processes at play. References to additional\nclimate model simulations would strengthen the study.\nDue to the reviewers' concerns about the uncertainty of the temperature dataset, we decided to\nexclude it from the revision and refrain from further discussion.\n3) Transfer function\nIf I understand correctly and based on Fig. 5, the authors derive different transfer functions for MIS2-\n4 and MIS6. What is the rationale behind this? The use of different transfer functions makes me doubt\nyour result, i.e. the differences in temperature estimates between MIS2-4 and MIS6. I think you should\ni) re-assess whether the use of 2 different transfer functions is appropriate, ii) present an uncertainty\nband with an estimate based on the same transfer functions.\nThis comment is now obsolete because we decided to remove the controversial quantitative\ntemperature reconstruction, a transfer function is therefore not used in the revised manuscript. We\nonly show a crossplot between Black Sea surface temperature and stalagmite δ13C to highlight the\nstrong influence of temperature on carbon isotope values.\n4) Paragraph starting L. 170: I find this paragraph on DO rebound events a bit confusing. In addition, I\ndo not understand how the authors can conclude that longer DO cycles could favour rebound events\n(L. 172-173).\nWe recognized that this discussion is not highly relevant for the core of our manuscript, as the main\nfocus is on D-O variability during MIS 6. For this reason, we have decided to remove the discussion of\nprecursor and rebound from the revised version.\nMinor points:\nThe text is sometimes hard to follow due to language issues, for example L. 95-96, I don’t think the\nuse of “whereas” is appropriate here.\nCorrected.\nL. 161-162: Correlation is a strong word here\nDue to a high uncertainty in the previously presented relationship between D-O pacings and insolation\nand the fact that we cannot provide a clear conclusion and further discussion on the current data\nbasis, we decided to discuss the potential causes for longer D-O cycles from a different point of view\nfocusing on interlinked changes in the AMOC-sea-ice-atmosphere system.\nL. 218-222: I find these sentences hard to understand. Please break them down and improve the\nlogical flow.\nObsolete, the sentence belonging to the quantitative temperature estimates is removed from the\nrevised manuscript.\nL. 232: The transition with “However” lacks a logical flow.\nRemoved.\nReviewer: 2\nLine 23 “there are almost no proxy records which provide unambiguous evidence for their existence\nduring preceding glacial periods” – This is not the case. Some of the papers you cite later provide such\nevidence. Here is a short list of examples: Martrat et al., 2004; 2007; Barker et al., 2015, Cortina et al.,\n2015; Cheng et al., 2016\nChanged, we are now stating “…only very few terrestrial records…” to avoid any misunderstanding.\nLine 88 “The new Sofular δ18O and δ13C records show characteristic D-O cycles, which are, even on\nmultidecadal timescales, remarkably similar to those recorded in Greenland ice cores” – Considering\nthe number of proxy records that look like Greenland d18O I would not say that this dataset stands\nout as being particularly remarkable. At the least you should quantify how good the agreement is, and\nmore importantly, you need to describe those parts of the record that differ from the variability\nrecorded in Greenland. For example, the amplitude of individual events is much smaller relative to the\nbaseline difference between (some) stadial events in the Sofular record compared to that seen at\nGreenland. E.g. compare stadial values before and after DO8, or during H events 4, 5, 5a and 6, which\ncontrast with the relative constancy of the Greenland record for the same events.\nWe have changed the wording from “remarkably” to “very” to avoid any misunderstandings.\nSame point on line 130 “Considering the very high coincidence of abrupt climate events between\nGreenland ice cores and Sofular stalagmites…”. Use of the term ‘high confidence’ demands statistical\nquantification.\nWe are not using the term “High confidence” in our manuscript.\nLine 104 “Such a climate amelioration would not only promote a change in the type and density of\nvegetation but also increase soil respiration rates due to higher soil microbial activity and faster\ndecomposition of SOM” - How fast would this change occur? Does this limit the rapidity of changes\nthat can be recorded?\nThe time lag between the climate amelioration and the vegetation change is relatively short for most\nof the observed DO events, as suggested by the rapid δ13C changes observed and also by the good\nsynchronicity between δ13C and δ18O records. Furthermore, pollen records from the Black Sea area\n(Shumilovskikh et al., 2014) and Lake Van show an almost synchronous and immediate response of\nvegetation within decades to D-O events. Other records from the Mediterranean (Allen et al., 1999)\nand marine pollen data from the Iberian Margin (Sanchez Goni et al., 2002) show very similar short\ntime lags between climate and vegetation changes.\nLine 111 “The Sofular record exhibits the most positive δ13C values of around -3‰ at ~62 ka BP,\nconcomitant with the maximum extent of ice sheets in Eurasia and Heinrich event 6 (Fig. 2).” - This is\nconfusing and conflicts with the evidence in Fig. 2 and the cited papers. Fig 2 shows maps for the LGM\nand MIS 6, not H6 or MIS 4 (which was smaller in extent than the LGM, according to the cited paper.\nWe apologize for this confusing statement, we changed the wording to “…a major advance of the\nEurasian Ice Sheet (EIS)…”.\nLine 115 “Since Heinrich events (HE) were most likely restricted to the midlatitude North Atlantic,\nevidence…”. This is not true, the effects of H events were certainly not restricted to the mid northern\nlatitudes but are seen in records around the Earth. For example, Brazilian speleothems show growth\nperiods associated with H events (Wang et al., 2004), surface ocean frontal shifts in the SE Atlantic\nwere more accentuated for H events than non-H events (Barker and Diz, 2014), Chinese speleothems\nshow more pronounced variations associated with H events (Wang et al., 2001). H events are also\nmore pronounced than non-H events in the deep ocean e.g. the deep South Atlantic (Gottschalk et al.,\n2015) and the NE Atlantic (Henry et al., 2016).\nWe agree with reviewer 2 that this statement is misleading. We removed the sentence.\nLine 136 “During MIS 6, a D-O like climate variability is most clearly visible in the δ13C records” - But\nwhy does d18O not show these events when it does so clearly during MIS 3? This demands some\nexplanation.\nIn the revised version we have expanded the discussion to include further explanations for the lack of\na clear D-O pattern in the Sofular oxygen isotope records. The new paragraph states that: “…Thus, the\n“water vapour source effect” in combination with variable mixing times of the Black Sea34 dampened\nD-O related fluctuations in δ18O during MIS 6. This effect is also apparent in Sofular stalagmites So-1\nand So-2 δ18O profiles which show a suppressed Bølling-Allerød and Younger Dryas isotopic pattern29\n(Fig. 2).”\nLine 137 “D-O cycles during MIS 6 are characterized by abrupt negative shifts of up to 4‰ (Fig. 3) in\nresponse to warmer and more humid climatic conditions and higher soil respiration rates, denser\nvegetation and higher proportions of C3 plant vegetation.” – what is the evidence for this?\nAs the same climatic and environmental factors like an increase in temperature, moisture and soil\nrespiration were responsible for δ13C fluctuations during MIS 2-4, it is therefore most likely that the\nsame factors influenced δ13C values during MIS 6. Furthermore, the close correlation between δ13C\nand Black Sea sea surface temperatures supports our assumption. Finally, the statement is supported\nby the following sentence regarding the pollen record of Lake Van. We added “Phases of increasing\ntemperature and higher effective moisture are consistent with pollen evidence (dec. Quercus, Betula,\nPinus) from Lake Van53.”\nLine 157 “The average pacing of D-O cycles is almost twice as long (3.83 ka) during MIS 6 compared to\nMIS 2-4 (2.07 ka) (Fig. 4).” – I have several concerns about this statement and therefore the\narguments that build on it. Firstly, from Fig. 4 it looks like you are describing the duration of individual\nDO events rather than the time between events (which is implied by the term ‘pacing’) – the\ndistinction is very important for any discussion of the underlying causes. You repeatedly use the term\n‘DO cycles’ (which would include both stadial and interstadial event) but if you are actually referring to\nthe duration of DO events, this needs to be made clear. Secondly, I could not find any description of\nhow the duration was measured for each dataset. This needs to be shown in detail to allow the reader\nto assess how robust the finding is. Thirdly (and this is related the second point), looking at Figs 2 and\n3, it is not at all obvious that your calculated durations are correct. For example in Fig 4, DO6.6 lasts\nfor something like 3-5kyr but most of the records in Fig 3 show it to be 2-3kyr.\nWe apologize for the incorrect representation of Figure 4 and the resulting confusion. In fact, Figure\n4 shows the pacing, and we corrected the Y-axis title. A detailed illustration showing how the pacing\nis determined is now available in Supplementary Figure 6 and 7.\nLine 161 - The following discussion requires a lot of development before it might represent a step\nforward in understanding. At the moment it is just a collection of vague possibilities, with no thread,\nor coherent argument.\nWe agree with this comment to a certain extent, but think that the discussion does contribute to\nfurther understanding. Of course, we cannot draw any final conclusions at this stage, as the data base\nand evidence for MIS 6 is still sparse. As reviewer 2 mentioned at the introduction, absolutely dated\nrecords covering MIS 6 are currently rare.\nLine 180 “…the Huagapo δ18O record reveals a pronounced asymmetric D-O pattern during MIS 6\nsuggesting a significant influence of different regional boundary conditions on D-O events.” - Or it\ncould just reflect differences in the recording of DO events by different archives and proxies. This is a\ncrucial question.\nObsolete, the sentence belonging to the different D-O characteristics is removed from the revised\nmanuscript.\nLine 187 Please stipulate which data presented here were published previously in the Badertscher\n2011 paper.\nWe have modified the introduction and methods to stipulate which data were already published in\nprevious publications. The δ13C record for time interval between 50 and 80 kyr was never presented\nin a publication, the same is true for the MIS 6 records. We also added two new stalagmite records\n(So-13 and So-57). Overall, the majority of data is original and unpublished. We adjusted the text to\nhighlight the originality of our dataset.\nLine 207 and below - This exercise makes very little sense to me. You are calibrating to a very low\nresolution record and you get a completely different calibration for MIS 6 vs MIS 2 (which should tell\nyou something is wrong). Also, you omit to mention the numerous temperature (and equivalent)\nreconstructions from MIS 6 which are nicely summarized in the Wegwerth paper you cite for the Black\nSea temperature record. None of these other reconstructions suggest a larger amplitude of\ntemperature change associated with DOs of MIS 6. So are you challenging all of these records or are\nyou suggesting that the larger magnitude of DO warming during MIS 6 was limited to a region around\nyour sample site?\nWe agree that the current data are of limited suitability for a calibration and we removed this part\nfrom the revised version. We only show a crossplot between Black Sea SST’s and Sofular δ13C to\ndemonstrate that temperature (and precipitation) have an effect on soil respiration rates and\nvegetation above the cave.\nMinor comments\nLine 38 “a very tied atmospheric teleconnection” – poor wording\nModified.\nLine 81 “The Sofular record covers the last 80,000 years and the period between 130,000 and 200,000\nyears before present almost continuously. Hiatuses exist between 21.6 - 24.1 ka BP and 134 - 160 ka\nBP (Figs. 2 and 3).” – A gap of 26kyr within a 70kyr interval is hardly ‘almost continuous’.\nModified.\nReviewer: 3\nFirst, being aware of the previous studies on the Sofular speleothems, I would ask the authors to more\nprecisely state which data are original to this study and which data were used from previously\npublished work. In particular, MIS 2-4 data and corresponding discussion is reiterated in several places\nand could perhaps be shortened.\nWe have modified the introduction and methods accordingly. Sofular δ13C isotope profiles for MIS 4\nand MIS 6 were never presented in a publication. In the revised version we also added a new\nstalagmite records (So-13) and we improved the chronology of So-57 with new absolutely dated 230Th\nages. We adjusted the text to highlight the originality of our dataset.\nSecond, the authors convincingly use the d13C as the better and more immediate recorder of\nenvironmental changes above the cave. But this strength is also a weakness at the same time, when\nit comes to reproducibility, absolute values and amplitudes. The authors indeed discuss the complexity\nof the d13C signal formation, but at the end they calculate an average signal from two very different\nrecords, and, even more important, they use this record to linearly calibrate and extract temperature\ninformation, knowing that distinct hydroclimatic changes and shifts in the vegetation contributed\nsignificantly to the signal formation. Detailed reconstructions of the MIS 2-4 Anatolian vegetation\ndynamics, e.g., share the overall D-O pattern, but with significant differences as well (Shumilovskikh\net al. 2014, Climate of the Past). Therefore, absolute temperatures as well as the amplitude (I would\nexpect higher amplitudes on the continent and at this elevation) of the changes may differ significantly\nfrom the proposed temperature record, which is linked straight forward to Black Sea surface\ntemperatures, without considering differences in elevation (ca. 500 m vs sea level) and a certain\ncontinentality and seasonality (since Black Sea temperatures are assumed to represent annual\naverages and soil activity is definitively biased towards the warm season). Although I find the idea to\nextract temperatures from the stalagmite d13 a very interesting exercise, there are too many\nuncertainties and unknowns involved and I recommend to rather stay with the original d13C. Unless\none were to take the certainly promising approach of extracting organic biomarker (GDGCs) from\nstalagmites to reconstruct soil/atmospheric temperatures above the cave (e.g., Zang et al. 2023, DOI\n10.3389/fevo.2023.1117599).\nLike reviewer 2, reviewer 3 questions the development of a quantitative temperature reconstruction\nfrom Sofular stalagmites. We removed this part of the manuscript in order to focus more on the nature\nof D-O events during MIS 6. It is, however, common practices in paleoclimatology to use multiple\nspeleothem records to develop a stacked record. This approach helps to minimize sample-specific\nsignals and therefore to increase the signal to noise ratio. As mentioned above, this is, for instance,\nfrequently done for tree ring records and marine sediment records (e.g., SPECMAP and LR 04 stack).\nThe authors discuss the MIS 6 d18O data noting that there is no asymmetry (as perhaps expected) in\nthe D-O cycles and that large changes in the Black Sea hydrology (e.g., marine and melt water\ninflows) matter as well. Reference to two Black Sea studies could strengthen the discussion here, since\nthey showed (1) that the glacial Black Sea d18O source signal has undergone a strong modification\ncompared to the Greenland ice cores, due to the variable mixing time of the large Black Sea reservoir\nsmoothing and delaying the signal - a one to one pattern cannot be expected (Wegwerth et al. 2019,\ncommunications earth and environment), and (2) that three large meltwater pulses arrived in the\nBlack Sea during the penultimate glacial.\nWe thank reviewer 3 for the comment and have expanded the discussion.\nComments:\nL 22: change to “(AMOC) and solar and volcanic forcing.\nCorrected.\nL 63-67, 79-85: Please make clear, which part of the data are original to this paper.\nSee first comment.\nL 102-109: Evidence for vegetation changes come from, e.g. Shumilovkikh et al.,. add ref\nCorrected.\nL 115-117: This link has been discussed widely by Wegwerth et al. 2015 and 2016, add refs\nChanged.\nL 120-121: Please indicate more precisely precursor-type events in Fig. 2\nWe recognized that the paragraph on precursor and rebound events disturbs the general structure\nand that this discussion is not highly relevant for the core of our manuscript. For this reason, we\ndecided to remove the discussion of precursor and rebound from the revised version.\nSuppl. Fig. 1: no element ratio but only the concentrations of Mg and Sr is shown. As it is now, the\nstatement is not supported.\nSee comment above.\nL 124-126 and Supp. Fig. 1b: this figure should contain a direct and detailed comparison between the\nSofular and NGRIP isotope data to support the statement\nSee comment above.\nL 141-145: Offset between So-4 and So-57 is discussed, however the stacking approach is rather\nquestionable, since the patterns and amplitudes are significantly different. For this discussion, a\ncomposit is not needed.\nThe use of a stacked record is particularly advantageous for records with different amplitudes, as\nsample-specific effects are reduced. This improves the reliability of both records in terms of age\nuncertainty and increases the signal to noise ratio. As mentioned above, this is a common and\ndesirable practice in paleoclimatology.\nL 139-140: Include pollen data von Lake Van into the graph\nLake Van is mentioned in one sentence and we therefore decided to include the pollen data from Lake\nVan in Supplementary Figure 2.\nL 148: Statement is misleading. Of course one can compare various records to the Sofular data even if\nthey are of lower resolution and weaker in stratigraphy. In that case, one has to consider the\nrespective limitations.\nModified.\nFig. 4 caption: change to ...synthetic ice core.\nCorrected.\nL 161-162: There are many D-O events not fitting into this picture. Please discuss.\nWe agree with reviewer 3 that the high uncertainty in the previously presented relationship between\nD-O pacing and solar insolation is insufficient for an informed conclusion. Since we cannot provide a\nclear conclusion and further discussion on the currently available data, we decided to consider the\nrelationship of longer D-O cycles and ice dynamics from a different point of view.\nL 176-178: See general comment on the d18O record\nModified.\nL 183-186: In fact, three periods of melt water input into the Black Sea are hypothesized for MIS 6\nModified, information provided in Supplementary Figure 2.\nL 225-236: Regional patterns in D-O associated temperature changes have been summarized and\nthoroughly discussed in Wegwerth et al. 2016.\nObsolete, the sentence belonging to the quantitative temperature estimates is removed from the\nrevised manuscript.\nResponses to Reviewers\nWe would like to thank all reviewers for their thoughtful and constructive remarks, which helped to\nfurther improve the quality of our manuscript. Please find our responses (in black) to the reviewer\ncomments (in blue).\nReviewer: 1\nAbstract, L. 12-15: A weaker AMOC as the reason behind the longer DO pacing is a hypothesis that\ncannot be tested here. I thus suggest to carefully phrase it as such (and also L. 227-230). In addition,\nhere “background conditions” are also mentioned whereas they are not mentioned L. 227-230 (unless\nI misunderstood what you meant by background conditions).\nWe toned down the statement. We changed the term “background conditions” to “mean climate” and\nrefer to it again in the discussion L. 225.\nL. 22: There is a lot of evidence for DO cycles being associated with AMOC changes. What caused these\nAMOC changes is unclear, and volcanic forcing is one of the hypothesis being put recently forward but\nhas only limited evidence, but not solar cycles.\nUnfortunately, it is not clear to us how we can address this comment.\nL. 23-24: I understand there are few terrestrial records of the penultimate glacial period which present\nDO variability but since they look at pollen, Margari et al. 2010 and Tzedakis et al. 2009 should be\nadded there (and not only for the last glacial period). Martrat et al., 2007 also present data for the\npenultimate glacial period and Antarctic ice core records also provide information on DO variability,\nsince they form the basis of the synthetic Greenland record.\nWe thank reviewer 1 for this comment, we added the references.\nL. 31: I don’t think further records are needed to “confirm” DO cycles, but additional high-resolution\nrecords like the one you are providing are needed to better understand these cycles.\nChanged.\nL. 197: What do you mean by age offsets?\nWe refer to \"age offset\" as the temporal differences in the chronology of D-O events between the\nSofular, Sanbao, Huagapo and synth. ice core time series.\nFigure 5: Please provide information on how you calculated DO pacing.\nA detailed illustration of how the D-O pacing was determined is given in Supplementary Figures 6\nand 7. This should be sufficient to document our approach to calculate the D-O pacing.\nL. 215: I am not sure that's the case for the periods of interest and note that warmer conditions in SO\nwould be expected with a weaker AMOC.\nWe modified the sentences to “…greater sea ice cover and lower amplitude SST variability …” to avoid\nmisunderstanding.\nL. 224: Please replace “confirms” by “provides additional evidence”\nChanged.\nL. 227-228: “This could be related” iof “most likely”\nChanged.\nReviewer: 2\nIn this revised version of their manuscript, the authors have removed many of the difficult sections of\nthe original version, which I commend. In particular, the authors now suggest that the longer timescale\nof DO variability during MIS 6, was due to a weaker and shallower mode of the AMOC. However, while\nthis may have been the case, we have no direct evidence for the state of the AMOC during MIS 6 (in\nfact the state of AMOC during the LGM is still a matter of strong debate). The evidence cited here is\nall circumstantial, based on benthic oxygen isotopes, surface temperature and CO2 records, and not\nproxies of AMOC strength or geometry. I therefore think that the authors have to tone down this\nargument. Perhaps they could argue that their results (longer timescale of DO variability) might reflect\na difference in the underlying structure and vigor of the AMOC during MIS 6, but that this conclusion\nawaits future direct and quantitative reconstructions of AMOC during that interval.\nWe thank reviewer 2 for this comment. We have toned down the statement.\nReviewer: 3\nTwo issues raised in the first instance have not been properly addressed for my opinion.\nFirst, more a technical aspect but mentioned by all reviewers, was to stipulate which data were\nalready published in previous publications. In the rebuttal letter this has been made more or less clear,\nbut it is not yet satisfactorily addressed in the manuscript itself.\nIn the revised version, we provide detailed information on the use of previously published data and\nrefer in more detail to the novelty of the data in this study.\nSecond, I cannot follow the authors response on the stacking procedure of their TWO partly very\ndifferent MIS6 d13C records (n=2!). Since the records are partly very different, the stacking clearly\naffects the D-O assignment and interpretation. The mentioning of SPECMAP (n=5) and LR04 stack\n(n=57) is not very helpful here because they come along with a more robust significance. In your case,\nmillennial-scale patterns in this interval are partly even anticorrelated.\nWe would like to emphasize that we are following common practice with this approach, and present\nfurther examples that also use this stacking method with a small number of records: Vollweiler et al.\n2006 published in GRL (n=3), Bond et al. 2001 published in Science (n=4). Overlapping periods of\nstalagmite records of the combined Sanbao Cave timeseries (Cheng et al. 2016, published in Nature)\nare also represented by only 2 to a maximum of 3 records. Iscam is a frequently used program\nwhich, on a statistical basis, calculated a correlation coefficient for our record of 0.7 (δ13C) and 0.98\n(δ18O), indicating that there is not that much difference between the two records. Last but not least,\nthe records date back to more than 140,000 years before present and age offsets between the two\nrecords can be expected as dating uncertainties amount to several hundred years. Thus, it is not\nsurprising that two records show small differences in their pattern as both are based on\nindependent age models.\nL. 2: Why not “Turkey”? Did the international english name changed?\nYes, the international name has changed to “Türkiye”.\nFigure 2: Please use references to indicate data that are already published.\nSee first comment.\nL. 109-120: Same as above. While you state in the rebuttal letter that you “have modified the\nintroduction and methods to stipulate which data were already published in previous publications”, I\nfeel this part can still be improved!\nSee first comment.\nL. 125-127: Reference?\nA reference is added in the revised version.\nL. 130-132: This is not only the case for MIS 6 but particularly also for MIS 2-4!! The signal is not only\ndampened but the timing and sawtooth-like D-O pattern is affected.\nWe have changed this sentence. According to Fleitmann et al. (2009) and Corrick et al. (2020), the\ntiming of D-O events during MIS 2-4 is not affected and is similar within the age uncertainties to\nother paleoclimate records discussed in these studies.\nL. 138-141: I still find the stacking of only two records problematic for the MIS 6 interval (175-190)\nand mentioning of SPECMAP and LR04 stacks are not helpful here, since these examples deal with\nn=5 and n=57, respectively, coming along with a more robust significance. In your case,\nmillennial-scale patterns in this interval are partly even anticorrelated.\nSee comment above.\nFigure 4: Selection of correlation points is partly questionable (i.e. So 6.6 low temp. point, So 6.8\nhigh temp. point, So 6.9 high temp. point…)\nWe have revised the correlation. Low temperature point of So 6.6 and high temperature point of So\n6.8 have been changed. We do not recognize any alternative point for the high temperature point of\nSo 6.8.\nL. 183: See comment above.\nBased on the context of our statement, it is unfortunately not clear to us which previous comment\nreviewer 3 is referring to. If it also refers to the stacking approach, please refer to the answer from\nthe main comment at the beginning.\nL. 186-188: Thank you for adding this data to suppl. Figure 2. In fact, So-57 shows an anticorrelated\nsignal on the D-O scale. Negative d13C values correspond to cold/dry conditions in Lake Van. I think\nthat the comparison is not as consistent as stated here.\nWe thank reviewer 3 for this comment. We modified this statement to “…broadly consistent…” to\navoid misunderstandings. Considering the low resolution of the pollen record and the age\nuncertainties of both records (Pollen, Sofular), the overall trend and patterns of the pollen record\nand the Sofular δ13C record are similar. We have modified Supplementary Figure 2 to emphasize the\ntrending in both records.\nL. 196: Please explain the difference between pacing and timing and how exactly you calculated the\npacing.\nThe term \"pacing\" describes the time between D-O events, while \"timing\" refers to a certain point in\ntime, such as the onset or peak of a D-O event. A detailed illustration of how the D-O pacing was\ncalculated is given in Supplementary Figures 6 and 7.\nL. 217: During peak (early) MIS 3, pacing is also closer to 4 ka, but associated rather with the\nwarmest part of MIS 2-4. How does this observation fit to the proposed mechanism?\nD-O variability is assumed to differ significantly between glacials and interglacials, thereby also\nhaving an effect on the threshold triggering D-O events. Our record shows an increase in δ13C values\nto almost Holocene conditions for the early MIS 3. These climate conditions could have caused a\nchange in the AMOC setting leading to a longer pacing."}]

Decision letter