Arctic freshwater outflow suppressed Nordic Seas overturning and oceanic heat transport during the Last Interglacial

Domain: earth_science
Published: 2024-10-27
Editorial outcome: minor_revision

Abstract
The Last Interglacial period (LIG) was characterized by a long-term Arctic atmospheric warming above the preindustrial level. The LIG thus provides a case study of Arctic feedback mechanisms of the cryosphere-ocean-climate system under warm climatic conditions. Previous studies suggested a delay in the LIG peak warming in the North Atlantic compared to the Southern Ocean and evoked the possibility of southward extension of Arctic sea ice to the southern Norwegian Sea during the early LIG. Here we compile new and published proxy data on past changes in sea ice distribution, sea surface temperature and salinity, deep ocean convection, and meltwater sources based on well-dated records from the Norwegian Sea. Our data suggest that southward outflow of Arctic freshwater suppressed Nordic Seas deep-water formation and northward oceanic heat transport during the early LIG. These findings showcase the complex feedback interactions between a warming climate, sea ice, ocean circulation and regional climate.

Introduction
The Arctic cryosphere is transforming rapidly in response to ongoing climate change with profound implications for regional and global climate, future sea level rise and stability of ecosystems1,2. For example, it has been suggested that positive heat and freshwater flux anomalies in the Arctic are the primary cause of the suggested recent slow-down of the Atlantic Meridional Overturning Circulation (AMOC), a crucial regulator of the earth’s climate and fundamental for the mild climate of northwest Europe3. This has also been linked to the so called subpolar North Atlantic “Warming Hole” where the ongoing warming is slower than elsewhere on the globe or has even cooled down over recent years3. A conceptual explanation of these effects is that the associated northward oceanic meridional transport of heat and salt within the AMOC is balanced by a southward flow of cold deep water that is mainly formed in the Nordic Seas (the Greenland, Iceland, and Norwegian Seas) and the subpolar gyre. Thus, a southward advection of Arctic positive buoyancy (heat and freshwater) anomalies may have suppressed the deep-water formation in these key areas and thus altering the AMOC and the associated northward oceanic heat transport. However, there is no agreement regarding the relative role of individual components of Arctic climate change (e.g., melting of the Greenland Ice Sheet, sea ice reduction-induced freshwater and heat flux anomalies) to the suggested slowdown of the AMOC (refs. 3–5) or the associated changes in deep ocean convection6–8. A broader paleoclimatic perspective, permitting a wider range of boundary conditions and different rates of climate change to be investigated, is a key for understanding the interplay and feedback mechanisms between changes in the Arctic cryosphere, global ocean- and atmosphere circulation, and climate.

The Last Interglacial period (LIG; ~128–117 ka, thousands of years before present) was characterized by a warmer-than-present global climate, a smaller ice volume and a higher sea level9. It also provides a case study of long-term polar atmospheric warming above the pre-industrial level10,11 and the response of the Arctic climate system and strength of the deep ocean convection in the Nordic Seas to this warming. However, proxy studies gave conflicting results on changes in Arctic Ocean sea ice cover during the LIG for the central Arctic Ocean (comprising the Eurasian and Canadian Basins)12–17. For example, sea ice biomarker proxies suggest the existence of a permanent sea ice cover all year round in the central Arctic Ocean during the LIG12, while the occurrence of sub-polar planktic foraminifera in sediments assigned to the LIG from the central Arctic argue for sea ice free summers13. Further, recent studies have questioned the LIG age of these records from the central Arctic Ocean18,19. Also, because of the extremely low sedimentation rates in the central Arctic Ocean, the temporal resolution and age uncertainties in these marine proxy records do not allow for detailed reconstructions of the development of oceanographic changes e.g., due to varying insolation forcing across the LIG. Studies from the central and northern Nordic Seas have mostly focused on identifying the LIG climate optimum and comparing with the Holocene climate state or variability, and suggested sea ice free conditions all year round during the LIG peak warming17,20–22. Higher resolution and well-dated records from the southern Norwegian Sea and the subpolar North Atlantic suggested a delay in the LIG peak warming in the North Atlantic compared to the Southern Ocean23–26. Further, planktic foraminiferal assemblages from the southern Norwegian Sea suggest summer temperatures below 5°C (for example, ref. 26 based on sediment core MD95-2009, see Fig. 1a for core location), which raises the possibility that winter sea ice may have expanded much further south in the Nordic Seas during a globally warmer-than-present time interval compared to the historical and preindustrial periods. The early LIG cooling in the southern Norwegian Sea is also supported by dinocyst assemblage analyses in core MD95-2009 (ref. 27). Although the early LIG cooling was attributed to ice sheet melting and suppression of Nordic Seas deep convection25,27,28, neither the sources of meltwater nor deep water formation processes have been constrained by proxy data. Nevertheless, a more recent study based on diatom assemblages in the nearby core JM-FI-19PC suggested that the sea surface during the early LIG was warm29 (see Fig. 1a for core location), but a detailed comparison between these sea surface temperature proxies (i.e., diatom, dinocyst, and planktic foraminifera) is still lacking. Also, a more direct proxy for sea ice such as the biomarker IP25 (a C25 Isoprenoid Lipid; ref. 30) has not yet been applied on the Norwegian Sea sediment records that record the early LIG cooling.

Ultimately, the question of the early LIG cooling in the Norwegian Sea and potential responsible processes remains an open debate. In this study, we utilize a multi-proxy approach (diatom, dinocyst, and planktic foraminiferal assemblages, sea ice biomarkers, planktic foraminiferal Na/Ca and Ba/Ca, and benthic foraminiferal assemblages) to reconstruct the development of sea ice, sea surface temperature, deep ocean convection as well as changes in freshwater input and their sources during the LIG in the Norwegian Sea. We focus on three sediment cores retrieved from the southern Norwegian Sea (Fig. 1a) because they can uniquely be compared with high confidence to their counterparts from the North Atlantic Ocean due to the consistent presence of a chronostratigraphic tephra layer23,24,31 dated to 123.7 ka (ref. 32, Fig. 1; see “Methods”). Figure (1b, c) shows that the three sediment cores share the same characteristic patterns in oxygen isotope ratios (δ18O) measured in planktic (surface ocean dwellers) and benthic (seafloor dwellers) foraminifera across the penultimate glacial maximum, penultimate deglaciation, the LIG and the last glacial inception of marine isotope stages (MIS) 6 to MIS 5d, providing confidence in comparing the different types of proxy data between the three records.

Results
Sea surface temperature development during the LIG
Faunal and floral assemblages including planktic foraminifera, dinoflagellate cysts, and diatoms have been widely used to reconstruct past changes in surface ocean conditions29,32–34. Planktic foraminiferal assemblage studies from the southern Norwegian Sea found that during the penultimate deglaciation (~138–128 ka) and the early LIG (~128–123.5 ka), the polar species Neogloboquadrina pachyderma constituted ~90–100% of the planktic foraminiferal assemblages (ref. 23, Fig. 1d) suggesting that summer sea surface temperatures were below 5°C (ref. 26). Diatom assemblages29 when placed on similar age scales as the planktic foraminiferal data (see “Methods”) also suggest a delay in the LIG warming peak and that Holocene-like SST only established at ~123.5 ka (Figs. 2e, f; 3a). This is also supported by the dinocyst assemblages as the warm Atlantic water indicator species such as Operculodinium centrocarpum significantly increased in abundance only at ~124 ka, whereas the early LIG was dominated by the colder water species Brigantedinium spp. (ref. 27, see Fig. 2g). Further, a close inspection of the planktic foraminiferal and diatom assemblages (Fig. 3a, b) reveals three distinct phases of sea surface temperature development during the LIG namely at ~128–126.5 ka, ~126.5–123.5 ka, and ~123.5–117 ka. During the earliest LIG (~128–126.5) both diatom and planktic foraminiferal assemblages were dominated by cold species, similar to their compositions during terminations II (Fig. 2e, f, g). The assemblage compositions of the earliest LIG are also similar to the assemblage compositions during the cold glacial stadial periods23,29 when the Norwegian Sea was covered by perennial or seasonal sea ice35. This suggests that summer sea surface temperatures were below ~5°C during both the latest part of penultimate deglaciation and earliest LIG and raises the possibility that winter sea ice may have persisted during the earliest LIG—a possibility that we will investigate latter.

During the second phase of the LIG (~126.5–123.5 ka), the cold-water diatom species were gradually being replaced by warm diatom species (Fig. 3a), but the planktic foraminiferal assemblages remained dominated by the polar species N. pachyderma similar to the earliest LIG (Fig. 3b). Yet, both assemblages indicate colder surface ocean compared to the pre-industrial and the entire interval of the Holocene (Figs. 3a, b). Below ~5°C, the planktic foraminiferal assemblages are dominated by N. pachyderma (~90–100%) (e.g., ref. 32) and thus changes in temperature from subfreezing temperature to ~5°C may not be recorded. Similar discrepancies between planktic foraminiferal and diatom assemblages were also previously related to changes in the thermal structure in the upper water column as diatoms reflect surface water conditions (0–50 m) and planktic foraminifera record subsurface waters (e.g., ref. 36). Additionally, the planktic foraminiferal assemblages could be partly affected by dissolution during this interval in which less abundant and more dissolution prone subpolar planktic species are more affected as outlined by Zamelczyk et al. (ref. 37) for the last glacial period. Further, we observe a remarkable short-term decrease in the % N. pachyderma at 124.5 ka in only one sediment core (ref. 31, Fig. 2f). Although this may suggest a rapid and large amplitude change in sea surface temperature, a careful comparison with sedimentological indicators suggests that the increase in the subpolar species is due to bioturbation and thus not reflecting temperature change (presence of an ash pod indicates this is the case; c.f., ref. 38, Supplementary Fig. 1). Overall, summer sea surface temperature during this time interval (~126.5–123.5 ka) was increasing but was lower than the pre-industrial and Holocene range.

The third LIG phase is the late LIG ~123.5–117 ka when the percentage of warmer-water indicating planktic foraminiferal and diatom species reached similar or higher values compared to the Holocene (Fig. 3a, b). Interestingly, a recent study from the Labrador Sea also suggested three LIG phases particularly the earliest LIG (128–126.5 ka) when the Labrador Sea was seasonally covered by sea ice and that persistent inflow of warm water only occurred at 124–116 ka (ref. 39). In conclusion, planktic foraminiferal and diatom assemblages collectively suggest: 1) Norwegian Sea summer sea surface temperature did not change significantly between Termination II and the earliest last interglacial (~128–126.5 ka) with temperature <5°C; 2) a transitional period (~126.5–123.5 ka) with colder-than-present, but increasing, summer sea surface temperature; and 3) persistent warm Atlantic inflow and similar water column thermal structure to present-day during 123.5–117 ka.

Sea ice distribution and seasonality during the LIG
Did winter sea ice expand to the southern Norwegian Sea during the globally warmer-, but regionally colder-than-present early LIG (~128–123.5 ka), in particular during the coldest interval 128–126.5 ka? To answer this question, we measured the biomarker IP25 and sterol concentrations in two of the three studied sediment cores. The biomarker IP25 is biosynthesized by a few diatom species that live in sea ice40, and thus its presence is an indicator of seasonal sea ice30. Complete absence of IP25 indicates either permanent sea ice (because of the too limited light and nutrient availability) or year-round open ocean conditions (due to the absence of ice algae). In such case, the additional view on phytoplankton biomarkers such as brassicasterol and dinosterol can distinguish between the permanent sea ice versus open ocean conditions. Taken in consideration both IP25 and sterol concentrations, sea ice indices PIIB25 and PIID25 were suggested as semi-quantitative indicators of sea ice changes on a scale that ranges from zero (open ocean conditions) to 1 (perennial sea ice) (ref. 41). High IP25 concentrations, low brassicasterol concentrations and sea ice index values of ~0.4–0.8 during the penultimate deglaciation (~130–128 ka) suggest seasonal sea ice conditions (Fig. 2b, c). This is further supported by the high abundance (~60–90%) of the dincoyst species Islandinium minutum (ref. 27, see Fig. 2g), which may indicate prolonged periods of sea ice coverage42. During the entire LIG, IP25 is absent, the concentrations of brassicasterol are at maximum and the sea ice index values are zero (Fig. 2; Supplementary Fig. 2) indicating open water conditions all year round. Overall, the sea ice development from the penultimate deglaciation and across the LIG is also in agreement with the sea ice related diatom species and dinocyst assemblages (refs. 27, 29, Fig. 2; Supplementary Fig. 2). After the end of the LIG, the biomarker (IP25, PIIB25) data and sea ice-indicating diatom species suggest differences in the timing of the appearance of sea ice. The abundance of sea ice indicating diatom species starts to increase (close to the penultimate deglaciation levels) immediately at the end of LIG at ~116.5 ka (Fig. 2b), whereas IP25 was absent until ~114 ka (Fig. 2c, d). It could be that winter sea ice started to extend southward at ~116.5 ka, but it only reached the Faroe Islands margin at 114 ka or later – such a prolonged open ocean corridor during the last glacial inception may have provided moisture for ice sheet growth24,43,44.

Constraints on the sources of potential freshwater anomalies and implications
What caused the regional cooling during the early LIG? The early LIG interval (~128–124 ka) was characterized by high summer solar insolation at high northern latitudes – significantly higher than pre-industrial levels (ref. 45, Fig. 1f). Previous studies suggested the potential cooling of the early LIG may have been caused by persistent melting of ice sheets25,27,28 that could have disturbed open ocean convection in the Nordic Seas and the subpolar North Atlantic and thus suppressed the northward oceanic heat transport. Yet, proxy evidence for an early LIG meltwater event (i.e., a decrease in salinity) and its source is not well-constrained. To date, it has proven difficult to develop a reliable independent proxy for sea surface salinity. The Na/Ca in planktic foraminifera is proposed as a proxy for surface ocean salinity, although non-salinity parameters may also play a role46,47. Using Na/Ca measured in shells of N. pachyderma as a qualitative indicator for sea surface salinity, it suggests lower salinity during the early LIG compared to the late LIG (Fig. 4a, b). This is also supported with the dinocyst assemblages that are dominated by Brigantedinium spp. (mostly Brigantedinium simplex) during the early LIG (ref. 27, see Figs. 2g, 4a), which likely indicate lower salinities and colder-than-present, but increasing, temperatures48,49—compared with the late LIG, which is dominated by warmer water species such as O. centrocarpum (ref. 27 Fig. 2g).

What was the source of the freshwater anomaly in the southern Norwegian Sea during the early LIG? Constraining the freshwater sources during the early LIG could provide invaluable insights on cryosphere-ocean circulation-climate interactions under interglacial conditions. Potential freshwater sources include glacial runoff (and melting of icebergs), increased river input to the region, local precipitation, meltwater from sea ice, and/or a decrease in the inflow of northward Atlantic surface water. The early LIG in the southern Norwegian Sea is characterized by relatively high planktic foraminiferal δ18O and seawater δ18O (Fig. 4f; ref. 24) as well as a complete absence of Ice Rafted Debris (IRD) (Fig. 4d), which do not support a freshwater source of ice sheet/glacier runoff as ice-sheet-sourced meltwater would decrease surface ocean δ18O and release ice rafted debris. A proxy that has been used to identify meltwater of continental origin is Ba/Ca measured in planktic foraminiferal shells. Glacial runoff and iceberg melt or increased riverine input would increase the Ba concentrations in the surface ocean50, which is thought to be recorded in the shells of planktic foraminifera51. For example, relative increases in Ba/Ca values in N. pachyderma from the central Arctic Ocean have been used to indicate meltwater of a continental origin during the last deglaciation52. However, recent studies have shown that Ba/Ca in non-spinose planktic foraminiferal species are much higher than expected, which was related to the foraminiferal calcification microenvironment taking place in marine organic aggregates meaning that shell Ba/Ca ratios are also dependent on primary productivity53. This makes a quantitative assessment of a potential freshwater anomaly based on foraminiferal Ba/Ca difficult, and we will thus use the non-spinose N. pachyderma Ba/Ca as a qualitative indicator of freshwater of continental origin. Our Ba/Ca data measured in N. pachyderma show extremely elevated values (30–60 umol/mol) during the penultimate deglaciation (Fig. 3c), likely suggesting influence of glacial runoff from ice sheets. This is also supported by low planktic foraminiferal δ18O and sea surface δ18O (Fig. 4f; ref. 24) as well as high IRD content (Fig. 4d). Across the LIG, planktic Ba/Ca are relatively low and stable (6 µmol/mol, standard deviation = 0.3 µmol/mol, n = 15; Fig. 4c) which does not support significant influence of meltwater of continental origin (e.g., from ice sheets) during the early LIG compared to the late LIG. A freshwater source that may not result in elevated surface ocean Ba concentration or lower seawater δ18O (i.e., in agreement with our planktic foraminiferal δ18O and Ba/Ca) is sea ice sourced meltwater54,55. We therefore suggest that enhanced melting of sea ice in the central Arctic Ocean during the early LIG, likely due to the high solar insolation, was probably the main source of meltwater carried southward with the EGC and transported to the southern Norwegian Sea via the East Icelandic Current (Fig. 1a). This also suggests that this enhanced melting of central Arctic Ocean sea ice during the early LIG resulted in southward advection of buoyancy anomalies potentially reducing open ocean convection and the associated northward ocean heat transport.

One remaining question is if we can provide evidence for a reduction in open ocean convection during the early LIG compared to late LIG and pre-industrial intervals? Benthic foraminiferal assemblages have been suggested to be sensitive to changes in deep water formation rates and/or processes. For example, previous studies from the Norwegian Sea found a close correlation between the composition of benthic foraminiferal assemblages and climate for the past 150 kyr—and identified three main benthic foraminiferal assemblages as “interglacial”, “glacial stadial” and “glacial interstadial” fauna23,56. The Interglacial assemblage consists of Cassidulina neoteretis, Cassidulina reniforme, Melonis barleeanus, and Islandiella norcrossi. However, these studies did not examine in detail the potential changes in the benthic fauna within the interglacial periods. Our results show that the Holocene benthic fauna is dominated by C. neoteretis and C. reniforme constituting together ~60–80% with up to ~20% M. barleeanus (Fig. 3c). This faunal composition is similar to the late LIG, but the relative abundances of these species for the early LIG are distinctly different. During the early LIG, both C. neoteretis and C. reniforme constitute <10% of the benthic foraminiferal assemblages with up to ~50% M. barleeanus (Fig. 3c). Although, these differences in the foraminiferal fauna are not well understood, they point to different bottom water conditions (likely linked to deep water formation processes and/or rates) compared to the Holocene and late LIG. Changes in productivity and food availability are unlikely explanations for these changes as infaunal benthic δ13C values are the same for both the late and early LIG (see Supplementary Fig. 3). The benthic foraminiferal fauna in the early LIG resemble those of the Younger Dryas stadial (12.8–11.5 ka; ref. 57), when deep water formation in the Nordic Seas is thought to be suppressed58. Nevertheless, distinct differences exist between the two time intervals e.g., during the early LIG there was open ocean conditions all-year round, no IRD, and no reported relatively low planktic and benthic δ18O in the southern Norwegian Sea, which is in contrast to the Younger Dryas interval24,35,57. Future studies may utilize more proxies such as the water mass mixing proxy, Neodymium isotopes, to further elucidate the differences between reconstructed (de)glacial and interglacial variability in the Nordic Seas deep ocean circulation.

The early LIG represents a case study of a prolonged warmer-than-preindustrial global climate mainly due to stronger northern hemisphere solar insolation. Our findings suggest that southward flux of Arctic buoyancy anomalies (likely related to enhanced melting of sea ice in the central Arctic Ocean) during the Early LIG (~128–124 ka) have altered deep water formation rates and/or processes in the Nordic Seas and thus suppressed northward ocean heat transport (i.e., reduced Nordic Seas heat pump). This highlights the sensitivity of Nordic Seas overturning circulation and regional climate to buoyancy anomalies under interglacial conditions. Satellite observations indicate that the sea ice cover in the Arctic Ocean has declined substantially during the past four decades and it is predicted that consistently ice-free summer conditions will occur by mid-century ~2050 (e.g., refs. 59, 60) with serious implications for the climate and ecosystems, though in unpredictable ways (e.g., refs. 3–8). Our study showcases the complex feedback interactions between a warming climate and Arctic sea ice, and identify the early LIG as a key time interval for data-model intercomparison efforts to better understand and constrain the impacts of a changing cryosphere on regional and global climate.

Methods
Chronology
We used the age models of Capron et al. (ref. 26) for sediment cores MD95-2009 and ENAM33. In brief, the North Atlantic ENAM33 core was transferred onto the AICC2012 ice core chronology61 by aligning its sea surface temperature to both Greenland ice core δ18O (as a proxy for Greenland air temperatures) and global abrupt methane increases. This is based on the observation that during the last glacial abrupt climate change, North Atlantic Sea surface temperature increased (semi) simultaneously with both air temperatures over Greenland and atmospheric methane25,26,62,63. For the Norwegian Sea MD95-2009 core, Capron et al. (ref. 26) used several lines of evidence to place MD95-2009 on the age model of ENAM33, which we also adopted here to update the age models for the other Norwegian Sea sediment cores LINK16 and JM11-FI-19PC (Supplementary Fig. 4; Supplementary Table 1). Capron et al. (ref. 26) identified the following tie points to reconstruct the age model of MD95-2009: (1) the onset of deglacial decrease in benthic δ18O records, which is dated at 138.2 ± 4 ka; (2) the biostratigraphic link of disappearance of Atlantic benthic foraminiferal species group in both MD95-2009 and ENAM33 marking the onset of the LIG (ref. 23), which is dated at 128 ± 1.5 ka in ENAM33 and also matches a remarkable increase in benthic δ18O in the Norwegian Sea cores (ref. 23, Supplementary Fig. 4b); (3) the ash layer 5e-Low/BasIV identified in both ENAM33 and MD95-2009 (ref. 23) and dated to 123.7 ± 2 ka in ENAM33 (ref. 26). This tephra layer is also present in cores LINK16 and JM11-FI-19PC (refs. 24, 31); and (4) a pronounced cooling in MD95-2009 SST record is tied to the corresponding enhanced cooling in the NGRIP ice core at 116.7 ± 2 ka marking the end of the LIG, which can be transferred to other Norwegian Sea sediment cores by aligning % N. pachyderma and planktic δ18O records (Supplementary Fig. 4a, c). For details see Govin et al. (ref. 25) and Capron et al. (ref. 26). Sediment core JM-FI-19PC does not cover the entire penultimate deglaciation and its bottom part is dated to 130 ka based on the correlation of its planktic and benthic δ18O records with the MD95-2009 records; this tie point is also transferred to sediment core LINK16 (Supplementary Fig. 4). Age models were then constructed by linear interpolation (i.e., assuming constant sedimentation rate) between tie-points.

Biomarker analyses
In sediment cores JM-FI-19PC and LINK16 analyses of brassicasterol (24-methylcholesta-5, 22E-dien-3β-ol), dinosterol (4a-23,24-trimethyl-5a-cholest-22E-en-3β-ol), IP25, and C37 methyl alkenones were carried out on freeze-dried and homogenized sediments. The samples were extracted with dichlormethane/methanol (2:1, v/v) by ultrasonication (3 × 15 min). For quantification of the lipid compounds, the internal standards 7-HND (7-hexylnonadecane), C36 n-alkane, and androstanol (5α-androstan-3β-ol) were added prior to any analytical step. The extracts were separated into hydrocarbon and sterol (alkenone) fractions by open silica gel column chromatography using 5 ml n-hexane and 9 ml ethyl acetate/n‐hexane, respectively. Furthermore, an aliquot portion of the sterol fraction was derivatized with 200 µl bis-trimethylsilyl-trifluoracet-amid (BSTFA) (60°C, 2 h).

After extraction with hexane, analyses of sterols and IP25 were carried out by gas chromatography-mass spectrometry (GC–MS) using an Agilent 6850 GC (30 m DB-1MS column, 0.25 mm inner diameter, 0.25 µm film thickness) coupled to an Agilent 5975C VL mass selective detector. Alkenones were analysed by GC Agilent 6890A FID equipped with a cold injection system (60 m DB-1MS column, 0.32 mm i.d., 0.25 µm film thickness; cold). In both cases helium was used as carrier gas. For sterols and IP25, individual compound identification was based on comparisons of their retention times with those of reference compounds and on comparisons of their mass spectra with published data30,64,65. IP25 was quantified using its molecular ion m/z 350 in relation to the abundant fragment ion m/z 266 of 7-HND. Brassicasterol and dinosterol were quantified as trimethylsilyl ethers using the molecular ions m/z 470 and m/z 500, respectively, in relation to the molecular ion m/z 348 of androstanol (for further details see ref. 66). The PIP25 indices were calculated after Müller et al. (ref. 41) using the following equation: PIP25 = conc. IP25 / [conc. IP25 + (conc. phytoplankton biomarker × c)], where c is a balance factor, calculated by the ratio of mean IP25 concentration to mean phytoplankton marker concentration, to counterbalance the higher concentrations of sterols compared to IP25. Open water phytoplankton markers brassicasterol and dinosterol were used to calculate PIIB25 and PIID25 indices, respectively. As both sea ice indices run almost parallel, only PIIB25 is shown.

Faunal assemblages
In this study we use published and new benthic foraminiferal assemblage records; data from core LINK16 (Fig. 3c, right plot) are new and data from core JM-FI-19PC (Fig. 3c, left plot) are shown in the supplements in Ezat et al. (ref. 67), but not discussed there. When available, at least 300 benthic foraminiferal specimens from the size fraction >100 µm were counted and identified to species level.

Planktic foraminiferal assemblages discussed in this study are published in Abbott et al. (ref. 31) (sediment cores LINK16), Ezat et al. (ref. 24) (sediment core JM11-FI-19PC), Rasmussen et al. (ref. 23) (sediment cores MD95-2009 and ENAM33). In these studies, at least 300 planktic foraminiferal specimens from the size fraction >100 µm were counted and identified to species level. The relative abundance of planktic foraminiferal species in core JM11-FI-19PC was not quantified for the penultimate deglaciation and LIG parts, but our visual inspection confirms the same patterns as shown from the nearby core MD95-2009. In particular, we did not observe presence of subpolar planktic foraminiferal species in JM11-FI-19PC during the early LIG similar to MD95-2009.

Diatom data discussed in this study are published and described in Hoff et al. (ref. 29).

Stable Oxygen and Carbon Isotope Analyses
For Core LINK16, ~25 pristine specimens of the planktic foraminiferal species Neogloboquadrina pachyderma and ~15 pristine specimens of the benthic foraminiferal species Melonis barleeanus were picked from the size fraction 150–250 um for stable oxygen and carbon isotope analyses. Stable isotope measurements were performed at the Mass Spectrometer Laboratory at UiT the Arctic University of Norway on a Thermo Scientific MAT 253 IRMS with a Gasbench II. The precision of the instrument is 0.07‰ for δ13C and 0.08‰ for δ18O and results are reported on the VPDB standard. Planktic and benthic δ18O data from JM11-FI-19PC core are published in Ezat et al. (ref. 24) and MD95-2009 data are published in Rasmussen et al. (ref. 23).

Planktic foraminiferal Na/Ca and Ba/Ca analyses
Only pristine 60–160 specimens of the planktic foraminiferal species N. pachyderma with no visible signs of dissolution were selected from the size fraction 150–250 µm for minor/trace element analyses from sediment core JM11-FI-19PC. The specimens were gently crushed, weighed, and cleaned following the “full cleaning” procedure of Martin and Lea (ref. 68). This included the following steps: 1) Milli-Q water and methanol rinses to remove clay contaminates; 2) oxidation of organic matter by buffered H2O2; 3) reduction step with buffered solution of anhydrous hydrazine to remove Mn-Fe oxide coatings; 4) treatment with alkaline diethylene-triamine-pentaacetic acid (DTPA) to remove sedimentary barium contaminants e.g., barite; and 5) brief weak acid leach. The cleaned foraminiferal samples were then dissolved and analyzed on iCAPQ Inductively Coupled Plasma Mass Spectrometry (ICP-MS) at Lamont Doherty Earth Observatory (LDEO) at Columbia University, USA. Based on repeated measurements of in-house standard solutions, the average relative precision is 0.8% and 2.1% for Ba/Ca and Na/Ca, respectively. Blank samples were analyzed within every batch of samples in order to monitor potential contamination from reagents and vials. The Mg/Ca and B/Ca data were previously published in Ezat et al. (ref. 24) and the Ba/Ca and Na/Ca data are new to this study. As discussed in Ezat et al. (ref. 24), the Mg/Ca based temperatures also indicate early LIG cooling in line with the faunal planktic foraminiferal and diatom assemblages.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review: Last Interglacial Arctic Sea ice, ocean circulation and regional climate\nAuthors: Mohamed M. Ezat1*, Kirsten Fahl2, Tine L. Rasmussen\nGeneral. This paper addresses the complexity of the last interglacial period in the Arctic Ocean and adjacent Nordic Sea with particular emphasis on AMOC circulation. It is an important topic for understanding modern Arctic warming and AMOC slowdown and the paper is well written and the study carefully planned and carried out.\nPoints for authors to consider:\nMost important, the title says Arctic sea ice and refers to the central Arctic early in the paper. But the studied cores are from Norwegian Sea/Iceland area. While we recognize the close link for inflowing Atlantic Layer water, outflowing cold E Greenland water, etc, between Arctic Ocean proper and Nordic Seas [Greenland, Ic eland, Norwegian Seas], Perhaps clarity for those readers unfamiliar with this complex system would benefit from definitions early in the paper. This is subtle, Arctic Ocean sea ice flowing out into the Nordic Seas, should be clarified.\nI also noticed that many studies of the LIG in the central and western Arctic are not cited. Recognizing, central arctic sedimentation is slower and intermittent, there are nonetheless records. Check deVernal, Vermassen, Polyak Cronin and others on interglacial conditions in the central Arctic [including a summer ice free LIG]. This also raises the question: if the authors make direct comparisons between LIG and Holocene interglacial, perhaps the large Holocene literature for Greenland and the Arctic should be covered in more detail.\nThe discussion of benthic foram assemblage variability within the interglacial is important as McManus pointed out many years ago for the N Atlantic. Do the authors view MIS5 variability – prominent in the three LIG phases they define - to be related to sea or land ice and Heinrich type millennial bahaviour? The similarity of LIG to the YD seems important to me.\nIn summary, I think this statement in discussion section justifies publication of this study:\n“This highlights the sensitivity of Nordic Seas open ocean convection and regional climate to buoyancy anomalies under interglacial conditions.”\nMinor points\nLIG is defined as 129-117 ka early then 128–117 ka in figure caption 3.", "reviewer_2": "Last Interglacial Arctic Sea ice, ocean circulation and regional climate\nThis manuscript argues that freshwater outflow from the Arctic, derived from melting sea ice, delayed warming during the last interglacial (LIG) by slowing deep-water formation in the Nordic seas.\nIt presents foram assemblage and isotope data, trace metal chemistry and biomarker-based sea ice reconstructions from three sites in the southern Nordic Seas, adjacent to Iceland. The late onset of warming during the LIG is a well-documented occurrence in Nordic Sea records – but I believe that the authors suggestion that it is caused by outflow of sea-ice derived meltwater is a unique argument.\nOverall, I think it is an intriguing hypothesis and easy-to-read manuscript, but there are a number of shortcomings that I think the authors could easily correct in a revised submission. I do think the topic, and manuscript would have broad appeal and be a significant contribution to Arctic and sub-Arctic paleoceanography.\n1. The map embedded in figure 1 lacks a bit of detail. I think the authors need to better elevate the past work that has been done on this topic, and show some of the key records from the Nordic seas where this division between early and late LIG proxy data exists. This is especially true for some of the records mentioned in the manuscript but have no geographic information attached to them. For example, Interpreting a SST record from the Nordic seas requires one to know which water mass it was likely sitting in (Atlantic, Arctic?)\n2. I think it is necessary to show (at least in the supplementary information) the sterols concentrations in the cores. Currently we are only shown the IP25 ug/g and the PBIB25 index. It is not possible to see if there is any biomarker-based evidence for open water productivity during the LIG – which is kind of important for deciding whether the IP25 records for sea ice are reliable or not. In a way this is buried in the PBIP25 index, but it would also make sense to show the raw sterol concentrations somehwere.\n3. On lines 198-200 the authors discuss interpreting the %C37:4 and Na/Ca concentrations of foram shells as proxies for sea surface salinity. This seems to make sense, but can they also comment on how this fits with existing dinocyst-based SST reconstructions from the Nordic Seas?\n4. Another ‘proxy’ that needs a better introduction/explanation is Ba. On Line 70 the authors write: “Glacial runoff and iceberg melt would increase the Ba concentrations in the surface ocean38.” This is a key piece of the argument, and as such I would like to see a more substantial statement here, that perhaps includes some typical concentrations compared to open ocean sites, but also some examples of where this has been applied. Ultimately, it is the Ba data that seems to be used to rule out other glacial sources for meltwater or river outflow etc. Can the authors develop these arguments in a more convincing and rigorous way? Ultimately, at the end of the manuscript, I am not clear on why it must be Arctic sea ice melt that added freshwater to the Nordic seas to suppress deep-water convection. The reason is the lack of detail in how some of the results and proxies are presented. They lack of bit of rigor in their explanations that could easily be fixed.\n6. Finally, I wonder about the concluding sentiment here – that persistent melting of Arctic sea ice during the the early LIG was a significant source of meltwater to the Nordic Seas. Would it be possible to quantify (back of the envelope) how much sea ice melt would be needed? The reason I ask is that today, and probably during most Quaternary interglacials, there has been persistent melting of winter sea ice in the Arctic. So conceptually, how does this differ from what the authors envisage during the LIG? This is a critical question that needs to be resolved before a final decision can really be made on the impact and viability of this manuscript. Currently, it is a rather qualitative description – and I think it needs to be a bit more.\nOverall, I am very positive about this contribution and feel the authors should be given a chance to re-submit after a moderate revision. I have attached an annotated PDF with some additional comments and edits."}, "author_response": "The authors wish to thank the two reviewers for their very constructive comments. Below we explain how we implemented each comment in the manuscript; our responses are in bold and italic font.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReview:\nLast Interglacial Arctic Sea ice, ocean circulation and regional climate\nAuthors: Mohamed M. Ezat1*, Kirsten Fahl2, Tine L. Rasmussen\nGeneral. This paper addresses the complexity of the last interglacial period in the Arctic Ocean and\nadjacent Nordic Sea with particular emphasis on AMOC circulation. It is an important topic for\nunderstanding modern Arctic warming and AMOC slowdown and the paper is well written and the\nstudy carefully planned and carried out.\nPoints for authors to consider:\nMost important, the title says Arctic sea ice and refers to the central Arctic early in the paper. But the\nstudied cores are from Norwegian Sea/Iceland area. While we recognize the close link for inflowing\nAtlantic Layer water, outflowing cold E Greenland water, etc, between Arctic Ocean proper and\nNordic Seas [Greenland, Ic eland, Norwegian Seas], Perhaps clarity for those readers unfamiliar with\nthis complex system would benefit from definitions early in the paper. This is subtle, Arctic Ocean sea\nice flowing out into the Nordic Seas, should be clarified.\nWe changed the title accordingly to “Arctic freshwater outflow suppressed Nordic Seas overturning\nand oceanic heat transport during the Last Interglacial”. Also, we now defined what we mean by\ncentral Arctic Ocean, Nordic Seas earlier in the ‘introduction section’, and also the reference to\ndifferent areas clearer throughout the manuscript (see e.g., lines 44, 60, 20).\nI also noticed that many studies of the LIG in the central and western Arctic are not cited.\nRecognizing, central arctic sedimentation is slower and intermittent, there are nonetheless records.\nCheck deVernal, Vermassen, Polyak Cronin and others on interglacial conditions in the central Arctic\n[including a summer ice free LIG]. This also raises the question: if the authors make direct\ncomparisons between LIG and Holocene interglacial, perhaps the large Holocene literature for\nGreenland and the Arctic should be covered in more detail.\nWe are now referring and discussing more studies from the central Arctic Ocean, including Polyak\net al., 2013; Nørgaard-Pedersen et al., 2007; Adler et al., 2009, Kageyama et al., 2021, Stein et al.,\n2017; Vermassen et al., 2023; Hillaire-Marcel et al. 2017; Razmjooei et al., 2023; de Vernal et al.,\n1994 (see e.g., line 58-67). Regarding the comparison to the Holocene, our study is about the LIG\ndevelopment in the region but the comparison to the Holocene is only to show that that the early\nLIG was colder (and has different benthic foraminiferal assemblages) compared to the late LIG and\nthe Holocene at the studied area. We have now changed the title of the caption of figure 3 to make\nthis point clearer.\nThe discussion of benthic foram assemblage variability within the interglacial is important as\nMcManus pointed out many years ago for the N Atlantic. Do the authors view MIS5 variability –\nprominent in the three LIG phases they define - to be related to sea or land ice and Heinrich type\nmillennial bahaviour? The similarity of LIG to the YD seems important to me.\nWe thanks the reviewer for this comment which enabled us to provide more insightful information.\nThe glacial stadials (including Heinrich stadials) are characterized by distinctly different benthic\nforaminiferal assemblages compared to the early and late LIG – which is described in details in the\nreferences we refereed in this part (Rasmussen et al., 1996, 1999, 2003). Despite the similarity in the\nbenthic assemblages between the deglacial YD event and the early LIG, clear differences exist\nbetween the two time intervals which we have now highlighted in the manuscript: “Nevertheless,\ndistinct differences exist between the two time intervals e.g., during the early LIG there was open\nocean conditions all-year round, no IRD, and no reported relatively low planktic and benthic δ18O\nin the southern Norwegian Sea, which is in contrary to the Younger Dryas interval57,24,35. Future\nstudies may utilize more proxies such as the water mass mixing proxy, Neodymium isotopes, to\nfurther elucidate the differences between reconstructed (de)glacial and interglacial variability in the\nNordic Seas deep ocean circulation (see lines 265-270)”.\nIn summary, I think this statement in discussion section justifies publication of this study:\n“This highlights the sensitivity of Nordic Seas open ocean convection and regional climate to\nbuoyancy anomalies under interglacial conditions.”\nMinor points\nLIG is defined as 129-117 ka early then 128–117 ka in figure caption 3.\nWe have now made it consistent throughout referring to the LIG as ~128-117 ka – as the onset of\nLIG in the Nordic Seas is defined at 128 ±1.5 ka in Capron et al 2014 – which we also utilized in\nour age model (see Methods).\nReviewer #2 (Remarks to the Author):\nLast Interglacial Arctic Sea ice, ocean circulation and regional climate\nThis manuscript argues that freshwater outflow from the Arctic, derived from melting sea ice, delayed\nwarming during the last interglacial (LIG) by slowing deep-water formation in the Nordic seas.\nIt presents foram assemblage and isotope data, trace metal chemistry and biomarker-based sea ice\nreconstructions from three sites in the southern Nordic Seas, adjacent to Iceland. The late onset of\nwarming during the LIG is a well-documented occurrence in Nordic Sea records – but I believe that\nthe authors suggestion that it is caused by outflow of sea-ice derived meltwater is a unique argument.\nOverall, I think it is an intriguing hypothesis and easy-to-read manuscript, but there are a number of\nshortcomings that I think the authors could easily correct in a revised submission. I do think the topic,\nand manuscript would have broad appeal and be a significant contribution to Arctic and sub-Arctic\npaleoceanography.\n1. The map embedded in figure 1 lacks a bit of detail. I think the authors need to better elevate the past\nwork that has been done on this topic, and show some of the key records from the Nordic seas where\nthis division between early and late LIG proxy data exists. This is especially true for some of the\nrecords mentioned in the manuscript but have no geographic information attached to them. For\nexample, Interpreting a SST record from the Nordic seas requires one to know which water mass it\nwas likely sitting in (Atlantic, Arctic?)\nWe have now described the geographic location of these records in the manuscript (see lines 76-77,\n83-88) and referred to the map. Also, the reviewer kindly made relevant comments in an annotated\nfile, please see our responses to these respective comments below.\n2. I think it is necessary to show (at least in the supplementary information) the sterols concentrations\nin the cores. Currently we are only shown the IP25 ug/g and the PBIB25 index. It is not possible to see\nif there is any biomarker-based evidence for open water productivity during the LIG – which is kind of\nimportant for deciding whether the IP25 records for sea ice are reliable or not. In a way this is buried\nin the PBIP25 index, but it would also make sense to show the raw sterol concentrations somehwere.\nWe are now showing the sterol concentrations and other sea ice indicators in the Supplemental\nFigure 2. We also included the sterol concentrations in the discussion (see lines 158-173).\n3. On lines 198-200 the authors discuss interpreting the %C37:4 and Na/Ca concentrations of foram\nshells as proxies for sea surface salinity. This seems to make sense, but can they also comment on how\nthis fits with existing dinocyst-based SST reconstructions from the Nordic Seas?\nWe followed the reviewer’s suggestion and now added the available dinocyst data from our studied\narea (e.g., see Figure 2g), which indeed agree with other data/proxies discussed in our manuscript.\nNow we are discussing the dinocysts within the 3 different sections of ‘temperature’, ‘sea ice’ and\n‘salinity’ discussions (e.g., see lines 113-116, 170-171, 204-208).\n4. Another ‘proxy’ that needs a better introduction/explanation is Ba. On Line 70 the authors write:\n“Glacial runoff and iceberg melt would increase the Ba concentrations in the surface ocean38.” This is\na key piece of the argument, and as such I would like to see a more substantial statement here, that\nperhaps includes some typical concentrations compared to open ocean sites, but also some examples\nof where this has been applied. Ultimately, it is the Ba data that seems to be used to rule out other\nglacial sources for meltwater or river outflow etc. Can the authors develop these arguments in a more\nconvincing and rigorous way? Ultimately, at the end of the manuscript, I am not clear on why it must\nbe Arctic sea ice melt that added freshwater to the Nordic seas to suppress deep-water convection. The\nreason is the lack of detail in how some of the results and proxies are presented. They lack of bit of\nrigor in their explanations that could easily be fixed.\nWe have now added more information about foraminiferal Ba/Ca as a proxy for freshwater of\ncontinental origin (see lines 218-228). Also, in response to this comment, we have now significantly\nrestructured the discussion part of early LIG freshening and its sources to make our arguments\nclearer. We are now first discussing salinity changes across LIG using %C and Na/Ca as well as\n37:4\npublished dinocyst data from the area. Then we discuss potential sources for early LIG freshening\nby listing different sources and how different proxies are in apparent disagreement with all\npotential sources except for sea ice melt (i.e., enhanced melting of central Arctic sea ice and\nsouthward export via EGC and EIC). Finally, we discuss the evidence for a changing deep water\nformation/condition across the LIG.\n6. Finally, I wonder about the concluding sentiment here – that persistent melting of Arctic sea ice\nduring the the early LIG was a significant source of meltwater to the Nordic Seas. Would it be possible\nto quantify (back of the envelope) how much sea ice melt would be needed? The reason I ask is that\ntoday, and probably during most Quaternary interglacials, there has been persistent melting of winter\nsea ice in the Arctic. So conceptually, how does this differ from what the authors envisage during the\nLIG? This is a critical question that needs to be resolved before a final decision can really be made on\nthe impact and viability of this manuscript. Currently, it is a rather qualitative description – and I think\nit needs to be a bit more.\nWe have now replaced the word ‘persistent’ with ‘enhanced’, please see also our response to the\nrespective comment in the annotated PDF below. We have now added more text to this part further\nhighlighting the fact that stronger northern hemisphere solar insolation during the early LIG than\nduring the Holocene. We also agree that our hypothesis should be more tested by e.g., numerical\nmodelling, and we are currently in a contact with earth system modelling group to take it further;\nwe refer to this in the manuscript via “Our study showcases the complex feedback interactions\nbetween a warming climate and Arctic sea ice and identify the early LIG as a key time interval for\ndata-model intercomparison efforts to better understand and constrain the impacts of a changing\ncryosphere on regional and global climate”.\nOverall, I am very positive about this contribution and feel the authors should be given a chance to re-\nsubmit after a moderate revision. I have attached an annotated PDF with some additional comments\nand edits.\nWe transferred the additional comments from the annotated pdf with their respective line numbers\nthere to this file – please, see below. As above, in our answer, the line numbers refer to lines in the\nrevised manuscript.\nLine 20. I am not aware of issues with correalting records. Dating is certainly an controversial issue -\nbut perhaps more acutely, the kind of resolution one needs to look at a few thousand year offsets in\nclimate phenomena simply have not be recovered from the central Arctic Ocean, and in the absence of\na continuous isotope stratigraphy would be difficult to date at such a resolution. The use of 'correlation'\nhere does not seem to capture these aspects of the problem.\nAt the same time (and perhaps this is discussed later) - even in low sedimentation rate settings, where\npresumably most of the terrigenous material on the seafloor is deposited from sea ice transport, open\nwater conditions in the central Arctic even for a small portion of the LIG would/could leave a\ndiscernible signal in either biomarker concentrations or microfossil populations. For example, in the\nappearance in subpolar specialists in different taxa that would indicate a substantially different\nenvrionment than exists in any modern or Holocene analogues.\nTherefore, it is not sufficient to dismiss Arctic marine sediment archives because of these perceived\nlimitations. Any inference of sea-ice free conditions in the Arctic (based on sub-Arctic records) needs\nto be anchored by a discussion of the available proxy data and interpretations that exist from the\nArctic. I have not read beyond the abstract yet, so I mention this now and will see how this aspect is\ndealt with by the authors.\nWe totally agree that the central Arctic records provide invaluable insights. We didn’t edit the\nsentence as it is now deleted in order to meet the maximum word limit (150 words) for the abstract\nLine 27. Does this mean that northward heat transport was also suppressed.\nYes – we slightly edited the sentence to make it clearer (lines 28-30).\nLine 42: Here it would be good to define what is moving south. Positive bouyancy anomalies is not so\ndescriptive -is it less saline and warmer surface water (causing positive bouyancy anomalies???\nDone (line 45).\nLine 54. This sentence could be far more descriptive and informative by saying like . . . \"organic\ngeochemical proxies suggest the existence of a perennial sea ice cover during the LIG (Stein et al\nXXX), while the occurrence of sub-polar plantkic foraminifera in sediments assigned to the LIG from\nacross the central Arctic have supported arguments for sea-ice free conditions (Vermassen et al.,\n2023)\" Otherwise it paints are rather pessimistic picture by not including relevant information.\nDone (see lines 61-64)\nLine 56. This should be re-written - it comes off very awkward. The central point is true, that nobody\nhas tried to dissect LIG records from the central Arctic into 1-2 kyr timeslices because either a) the\nsedimentation rates are too low (less than 1-2 cm/kyr) or b) the recognition of a more pressing\nproblem in that the identification of stratigraphic position for the LIG remains debated as different\ndating tools continue to provide different age assignments to central Arctic sediments (as discussed in\nthe following sentence).\nDone (lines 67-70).\nLine 63. These sentences are really important for framing the past work but they lack detail - for\nexample the location of the studied cores, the conditions that they are exposed to today etc.\nfurthermore - although the difficulty of reconstructing sea ice in the central Arctic has been mentioned,\nthere is no information on what the current status of LIG sea ice reconstructions are in the Nordic\nSeas. To me this seems very surprising given the scope of the paper and could presumably be easily\nfixed.\nWe have now added this information later in this paragraph (see the second last paragraph in the\n“Introduction” section)\nLine 64. it seems really important to provide some additional information here. As the location of this\nSST record is rather critical for interpreting it as reflecting a change in the sea-ice conditions, and why.\nWas it a western (influenced by EGC) or eastern site (influence by Atlantic water inflow) in the Nordic\nseas? I think this information can be worked into this sentence easy enough, and perhaps this site can\nbe named and shown on figure 1?\nDone (see lines 76-77).\nLine 66. Is this the first time that this argument has been put forward to explain the SST data at this\nsite? Or is there a reference for this?\nWe are not aware of any study that explicitly mentioned the possibility of presence of winter sea ice\nin the southern Norwegian during the early LIG\nLine 70. Yes, but where?\nWe now added the core name and referred to its location on the map (see lines 83-84).\nLine 143. It would be very nice to see the concentration data Ip25 and the open-water sterols\ncommented on in the text - or shown in the main paper (not the supp. information) - as a lot of the sea\nice arguments hinges on 'indices' that combine these datasets - and when the indices suggest values of\n'0' it is nice to see why\nWe have now described sterol concentrations in the text and showed it in a Supplemental Figure\nwith more sea ice indicators. Please see also our response comment #2 from Reviewer #2 above.\nLine 152. What is the evidence for this? This sentence loses me a little . . .\nThe sentence in question is now deleted (175-179).\nLine 157. This sounds really important - but it is hard to gauge the novelty here as the authors have not\ntired to summarize the state-of-the-art for sea ice conditions in the Nordic seas during the LIG. This\nreally seems like a large omission that makes it hard to evaluate the manuscript.\nIn also a response to a previous comment, we have now described in the introduction the status of\nNordic Seas sea ice reconstructions during MIS 5 (see our responses above to Reviewer#2\ncomments on lines 63, 66).\nLine 175. Usually this paper is discussing the early or late LIG. It is confusing when the entire 'LIG'\nis referenced. Perhaps, in such a case, instead of 'During the LIG' it should be 'Across the LIG' - but I am\nconfused none-the-less whether this sentence refers to the early LIG or the entire LIG.\nWe mean the entire LIG and we followed the suggestion and replaced “During” with “Across”. We\nhave also edited this part of the argument to make it clearer (lines 232-234). Also, see our response\nto comment #4 of Reviewer #2 above.\nLine 184. This sentence seems to contradict the next. Can the authors better explain why they think the\n18O and Ba data from surface waters around iceland likely indicate the melting of sea ice in the\ncentral Arctic ocean? It's just not clearly there in the two sentences leading up to this pivotal sentence.\nI also think it is important to more explicitly define these proxies - like Ba - with a sentence that\ndescribes what it is a proxy for and why.\nPlease see our response to comment #4 of reviewer #2. We have now added more information about\nforaminiferal Ba/Ca as a proxy for freshwater of continental origin (see lines 218-228). Also, we\nhave now significantly restructured the discussion part of early LIG freshening and its sources\n(Lines 189-270) to make our arguments clearer. We are now first discussing salinity changes across\nLIG using %C and Na/Ca as well as published dinocyst data from the area. Then we discuss\n37:4\npotential sources for early LIG freshening by listing different sources and how different proxies are\nin apparent disagreement with all potential sources except for sea ice melt (i.e., enhanced melting of\ncentral Arctic sea ice and southward export via EGC and EIC). As there is almost no oxygen isotope\nfractionation during sea ice formation, melting of sea ice will result in salinity decrease but almost\nno change in sweater d18O – and the meltwater source is not of a continental origin i.e. not\nexpected to result in elevated seawater Ba (lines 238-240). Finally, we discuss the evidence for a\nchanging deep water formation/condition across the LIG.\nLine 188. When you say persistent - what do you mean? Sea ice melts in the summer rather\npersistently. So what is envisoned during the early LIG? Anything beyond what happens today - or\ndoes 'persistent' imply something extraordinary? Is the 'extraordinary' aspect that sea ice was present in\nthe Arctic during the early LIG? If so, I am not sure anyone has ever argued it was not there through\nthe winter, and it would have always melted back to some extreme in the summer.\nWe thank the reviewer very much for this comment. We meant “enhanced” and not “persistent”. We\nreplaced the word “persistent” by “enhanced” (e.g., lines 240, 248)\nLine 197. What do the existing dinocsyt records from the Nordic sea say about spatial and temporal\npatterns in sea surface salinity during the LIG?\nWe followed the reviewer’s suggestion and now added the available dinocyst data from our studied\narea (e.g., see Figure 2g), which indeed agree with other data/proxies discussed in our manuscript.\nNow we are discussing the dinocysts within the 3 different sections of ‘temperature’, ‘sea ice’ and\n‘salinity’ discussions (e.g., see lines 113-116, 170-171, 204-208).\nLine 217. These are well articulated arguments for a change in deep-water circulation, and the\ncomparison with the YD is intriguing - it would be fantastic to see this mapped out in a more\nsystematic way in another work.\nThank you - we agree.\nResponse letter\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI read the response to both reviewers and the revised manuscript - I find the new draft acceptable and\nrecommend NatComm publish this.\nResponse: thank you for taking the time to read our responses and revised manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I read the response to both reviewers and the revised manuscript - I find the new draft acceptable and recommend NatComm publish this."}, "author_response": "Response: thank you for taking the time to read our responses and revised manuscript."}]

Decision letter