## Introduction

Atmospheric iodine, primarily emitted from oceans, forms new aerosol particles in the atmosphere1,2,3,4,5,6,7 and efficiently destroys ozone in the troposphere and lower stratosphere8,9,10,11, which reduces ozone radiative forcing12,13,14,15. In the Arctic environment, iodine has recently been identified as a significant source of cloud condensation nucleii (CCN), with the potential to influence clouds formation3. Iodine in the atmosphere is increasing globally (3-fold since the 1950s), as independently evidenced by polar and alpine ice core and tree ring measurements, following anthropogenic ozone pollution and global warming16,17,18. In the Arctic, a coastal ice core from Greenland revealed that ocean primary productivity controlled atmospheric iodine variability during the Holocene (i.e., last 11,700 years)19. Before the climatically stable Holocene, the Arctic underwent abrupt environmental changes featuring vast ice sheet growth and marked climate events during the Last Glacial Period (LGP) (115–11.7 kyr before 2000 CE (b2k)) and experienced temperatures warmer than today during the Last Interglacial Period (Eemian, 130–115 kyr b2k). However, to date, the natural evolution of Arctic iodine before the Holocene, in the absence of anthropogenic forcings, and its coupling with climate variability remain unknown.

In this work, we report the iodine levels in the Arctic during the Last Glacial Cycle (LGC) (last 127 kyr) using records from two Greenland ice cores (NEEM and RECAP) drilled in northwestern and eastern coastal Greenland, respectively (Fig. 1). Both ice cores provide continuous paleoenvironmental records back to the Eemian interglacial period20,21,22. We provide evidence that ocean-ice-atmosphere exchange of biogenic iodine in the Arctic Ocean dominated the atmospheric iodine budget during periods of rapid climatic changes. The results show that abrupt warming and sea ice retreat in the Arctic preceded maximum iodine levels, reaching peak concentrations during interglacial periods.

## Results and discussion

### Present-day iodine emission sources and depositional fluxes over Greenland

Atmospheric iodine in Greenland is controlled by a complex interplay involving iodine emissions from the oceans, as well as particle-bound iodine compounds related to dust and/or sea spray aerosol (ssa) variability23. The oceans, which are the largest reservoirs of iodine on Earth24, are known to be the dominant source of atmospheric iodine25,26,27. The major oceanic sources of gas-phase iodine are (i) inorganic, ozone-induced, hypoiodous acid and molecular iodine emissions and (ii) organic iodine from the metabolic activity of primary producers (phytoplankton and macro- and micro-algae). Furthermore, atmospheric iodine can also be related to mineral dust by promoting iodate stability in the ice matrix and/or favouring the adsorption of iodine gas-phase molecules to dust particles during atmospheric transport10. On the other hand, ssa expelled from the ocean surface during wave breaking incorporate iodine in their composition and are a substrate where gaseous iodine species undergo heterogeneous recycling reactions28,29,30.

We used a global chemistry climate model17 to understand present-day iodine levels in Greenland (see Methods). Present iodine depositional fluxes at the RECAP drilling site (71°18′ N; 26°43′ W; 2315 m a.s.l.) are ~51% higher than at NEEM (77°45′ N, 51°06′ W; 2484 m a.s.l.) (Supplementary Fig. 1). This is explained by the different ocean iodine emission strengths at the source regions of air masses that influence both sites (the Canadian Arctic, Baffin Bay and Hudson Bay for NEEM31 and the North Atlantic and Arctic Ocean for RECAP32). Note also that RECAP is closer to the coast (30 km from the North Atlantic Ocean) than NEEM (350 kms from the Baffin Bay). Our model results show higher iodine emissions in the RECAP source regions (50N–80N, 45W–12E; mean iodine emissions of 9.1 × 10−14 kg m−2 s−1) than in the NEEM source areas (50N–80N, 120W–45E; mean iodine emissions of 3.5 × 10−14 kg m−2 s−1) (Fig. 1).

### The Greenland iodine records

Total iodine and sodium ([I], [Na]) concentrations from NEEM and RECAP ice core samples were determined at 1.10 m and 0.55 m mean intervals of melted ice, respectively (Supplementary Figs. 2-4). Calcium ([Ca]) concentrations were also used as proxies of mineral dust in Greenland ice cores33,34. These geochemical measurements along with other ice cores and marine paleoceanographic archives allow the reconstruction of Arctic iodine variability and environmental sources during the LGC.

Maximum iodine levels were found during the Eemian in the NEEM ice core, with a total mean iodine depositional flux (Iflux) of 9.6 µg m−2 yr−1 coinciding with the highest Arctic Ocean primary productivity inferred from the phytoplankton biomarker brassicasterol35 (Fig. 2). Our analysis suggests that these high Eemian iodine levels resulted from the interplay of (i) enhanced biogenic iodine emissions driven by primary productivity; (ii) higher sea surface temperatures (SSTs) in the Arctic and the North Atlantic36,37, which facilitates sea-air transfer of volatiles; and (iii) dominant open water conditions in the Arctic, as reflected by the low PBIP25 values. PBIP25 has been used as a proxy of the intensity of sea ice cover in the Arctic38, as well as in Baffin Bay31,39 since it accounts for the algal and phytoplankton (brassicasterol) biomarkers activity, which allows to assess the spatial and temporal extent of the sea ice cover40 (Fig. 2). The Arctic sea ice extent was significantly reduced during the Eemian, and minimum sea ice concentrations towards almost ice-free summers occurred in wide areas as far north as the northern Barents Sea as a consequence of the strong inflow of warm Atlantic waters38, thus facilitating springtime blooms and oceanic iodine production. Indeed, summer North Atlantic SSTs during the Eemian were 2 °C greater than at present36,37. Furthermore, a mean global sea level 6–8 m above the present level41,42,43 (Fig. 2) triggered the flooding of most parts of the shallow Siberian marginal seas, eventually resulting in more productive coastal areas in the Arctic. At NEEM, the highest Iflux was recorded at ~122 kyr b2k concomitant with maximum levels of biological productivity, sea level rise and SST. The coupling of all these drivers during the Eemian would have eventually released large amounts of iodine into the Arctic atmosphere, resulting in the largest iodine emissions on record. The significant decrease in NEEM and RECAP Iflux recorded during the last glacial inception at ~120 kyr b2k is correlated with an increase in sea ice31,38 and a decrease in both SST and brassicasterol (Fig. 2), as also indicated by independent modelling experiments that resemble pre-industrial scenarios41.

Ice core iodine showed reduced values throughout the LGP, with mean NEEM and RECAP Iflux of 3.8 and 5.1 µg m−2 yr−1, respectively (Fig. 2). The LGP is characterized by abrupt climatic variability marked by Dansgaard/Oeschger (D/O) events. These climatic phases consist of warm Greenland interstadials and cold Greenland stadials that are well recorded in Greenland ice cores and marine sediments from the North Atlantic region and the Nordic seas44,45,46,47,48,49. To investigate in detail the iodine variability across the rapid D/O transitions, additional high-resolution iodine measurements were carried out in the NEEM ice core from 34 to 42 kyr b2k (D/O 7 to 10), with a 3- to 10-year resolution (Fig. 3 and Supplementary Fig. 4). In order to differentiate the main sources of iodine that determine Iflux variability, we have decoupled oceanic iodine emissions (Iocean) from dust-related iodine (Idust) (‘Methods’). The sub-decadal resolution measurements of iodine during this period show the concomitant evolution of Iocean, Arctic Ocean primary productivity and sea ice variability46,48,49 (Fig. 3). Each D/O event showed a similar sequence: (i) iodine emissions increased concurrently with a reduction in sea ice at the onset of the interstadials; (ii) iodine emissions decreased during the late interstadial periods as the climate cooled and sea ice gradually increased; and iii) negligible iodine was released from the oceans with the development of thicker sea ice conditions during the cold stadial periods.

Interestingly, NEEM and RECAP Iflux and Iocean do not follow the same trend during D/O events (Fig. 3). In the context of strongly reduced iodine emissions during the LGP due to the extensive development of perennial sea ice in the Canadian Arctic31, a significant fraction of iodine registered in the NEEM ice core was strongly controlled by dust and, to a lesser extent, by ssa. This is corroborated by the robust statistical correlations between [I] and [Ca] and between [I] and [Na] from D/O 7–10 (r > 0.75 and 0.78, respectively). These correlations are also seen in RECAP during the LGP (r > 0.88 and 0.87, respectively). Sea spray aerosol emissions (indicated by [Na] concentrations in both ice cores) show a 50% increase during the LGP33 (Supplementary Figs. 2 and 3), thereby contributing to higher deposition of ssa-related iodine during this period. On the other hand, [Ca] concentrations in Greenland ice cores show a major increase during stadials33 (Supplementary Figs. 2 and 3) due to enhanced mineral dust associated with increased aridity in East Asia and enhanced atmospheric transport mechanisms33,50,51. The higher Idust values found in NEEM and RECAP during the coldest phases of the LGP are in agreement with the highest iodine fluxes recorded in the Talos Dome ice core (eastern Antarctica) between 16.8 and 33.8 kyr b2k10. This Antarctic ice core reported a correlation between iodate, the most stable iodine species in the atmosphere, and atmospheric dust, indicating uptake of gas-phase iodine molecules into fine dust particles during atmospheric transport10.

The largest iodine variability occurred during the LGP-Holocene transition when iodine emissions strongly fluctuated following the main climatic phases of the Last Glacial Termination. NEEM and RECAP Iocean showed very similar evolution patterns, with minimum Iocean values during Heinrich event 1 (H1, 16.8 kyr b2k) (Fig. 4), when surface waters cooled and a thick perennial sea ice cover reached both the Norwegian Sea and Baffin Bay31,46,52. The Iocean increased in both ice cores at the onset of the Bølling (14.7–14.1 kyr b2k) warming in the Arctic, slightly decreased during the Intra-Allerød Cold Period (14.1–13.9 kyr b2k) and increased again during the Allerød (13.9–12.9 kyr b2k) warm period (Fig. 4). The increase in Iocean values during the Bølling-Allerød period was synchronous with an abrupt increase in ocean productivity as far north as the Fram Strait53. NEEM and RECAP Iocean moderately decreased during the last cold spell of the Last Glacial Termination, the Younger Dryas (12.9–11.7 kyr b2k).

Oceanic iodine emissions increased in both ice cores at the onset of the Holocene (~11 kyr b2k; Fig. 4) resulting from the interplay of different factors: (i) enhanced marine primary productivity in the Arctic46,54,55,56; (ii) dominant open water conditions in the subpolar North Atlantic46; (iii) abrupt sea level rise (~120 m) during the Last Glacial Termination57,58 resulting in more shallow oceanic domains increasing iodine emission from coastal areas; and iv) maximum levels of solar irradiance in the Arctic that would have also increased oxidative stress in algae, leading to an enhancement of biological iodine production in the ocean and subsequent release into the Arctic atmosphere59. Iflux strongly increased in RECAP from 11 to 9 kyrs b2k, while the NEEM Iflux increase was significantly more gradual at that time (Fig. 2), most likely due to the lower productivity and delayed sea ice retreat in Baffin Bay during the early Holocene60 (Fig. 4). Indeed, this area remained densely sea ice covered until ~7.4 kyr b2k, when warmer North Atlantic waters penetrated Baffin Bay, increasing regional bioproductivity61 (Fig. 4). The mean Holocene iodine flux in NEEM is 57.9% of that in RECAP (mean iodine fluxes of 5.1 and 8.8 µg m−2 yr-1, respectively), in agreement with present-day reactive iodine deposition at both sites obtained from atmospheric chemistry transport model results (Supplementary Fig. 1).

### Environmental implications

Ocean-emitted iodine leads to new particle formation (NPF) in the atmosphere5,6. Recent field observations have demonstrated that iodine is a significant driver of NPF in the central Arctic3 and is currently considered a potentially relevant source of CCN3—from which marine clouds originate—that scatter incoming radiation and can contribute a cooling effect to the Earth´s radiation budget62. The significant present-day contribution of iodine to NPF leads us to speculate that, in the absence of other human-induced NPF nucleation drivers during pre-industrial times63, oceanic iodine may have played an even larger relative role in total NPF nucleation in a pristine Arctic during the LGC.

We find that the time lag between temperature changes and iodine maximum emission values recorded in Greenland ice cores throughout the LGC strongly depends on the atmosphere-ice-ocean interactions driving warming SST, sea ice thinning rate and algae metabolic activity that eventually control biogenic iodine emissions. Indeed, the highest iodine emission periods during the LGC were preceded by the highest air temperatures that would have first enhanced SST and ocean biological activity and later favoured the transfer of volatile iodine compounds from the ocean to the atmosphere. Together with the efficiency of iodine to destroy ozone and activate CCN in the polar atmosphere, this points to a possible contribution of iodine to ozone loss and new particle formation, and to their associated radiative impacts, during past abrupt climate change periods, such as D/O events and the onset of interglacials. However, this potential contribution of iodine to past ozone loss and atmospheric NPF is currently unknown and warrants further research.

We now turn to the use of past warm intervals recorded in Greenland ice cores as analogues to evaluate potential feedback and thresholds in the Arctic climate system and their implications for future warming impacts. The rate of ongoing atmospheric warming resembles only the abrupt climate changes and sea ice retreat that occurred during the D/O events64, with climate warming of 5–16.5 °C occurring within a century65. The high resolution NEEM iodine dataset during D/O 7–10 show empirical evidence of a fast response of iodine emissions synchronously with the rapid North Atlantic sea ice retreat preceding abrupt Greenland warming during D/O events48. Therefore, one question arising is if Arctic iodine levels are facing this scenario in the near future?

Iodine emissions and depositional fluxes during the Eemian and/or Holocene Thermal Maximum shed light on the possible environmental conditions in an expected ice-free Arctic Ocean and thus remain useful as an observational constraint on projections of future impacts. Thus, while atmospheric iodine depositional fluxes recorded in RECAP during the Holocene Thermal Maximum19 mirror present-day conditions17, iodine levels recorded in NEEM during the mid-Eemian tripled present-day values at this location. Similar trends might be expected in near future environmental scenarios including (i) predicted ice-free summertime conditions in the Arctic Ocean by 2050 CE66 and (ii) 3–4 °C warmer global temperatures with a three- to four-fold amplification in the Arctic by 2100 CE67. However, one important point to consider is that the predicted iodine emissions in the Arctic in the near future based on past analogues represent only the biogenic fraction of the total foreseen atmospheric iodine concentrations in the Arctic. Ozone-induced inorganic iodine emissions during these past warm periods could be considered negligible in the context of the very low ozone concentrations before industrialization19,68. In contrast, at present, ocean release of inorganic iodine is estimated to account for 75% of the total source of atmospheric iodine, with the remaining 25% coming from biogenic emissions69. Furthermore, ozone-related inorganic iodine emissions are predicted to increase during the 21st century15. Therefore, we argue that the future combined (inorganic + biogenic) ice-free Arctic Ocean iodine emissions might not have analogues in the past, which could potentially lead to a near future scenario with the highest iodine levels of the last 127,000 years.

In summary, this study uncovers how past climate changes modulated the variability of the ocean-atmosphere exchange of iodine in the Arctic during the LGC. We show that ocean productivity was the dominant source of Arctic iodine during interglacial periods, leading to highest levels during the Holocene and the Eemian, while dust-related iodine constituted a significant source of iodine in the Arctic atmosphere during the coldest phases of the Last Glacial Period. The mirrored trends of iodine emissions and ocean primary productivity proxies during different past environmental scenarios highlight the key role of ocean biology and sea ice cover in driving iodine variability in the Arctic region. Finally, we conclude that understanding the past variability of iodine as a key environmental element is important to further comprehending the evolution of Arctic atmospheric chemistry on centennial to millennial timescales and its coupling to climate changes.

## Methods

### Ice core chronologies and sampling resolution

NEEM age-depth model was achieved by transferring the annual layer counting from the NGRIP to the NEEM ice cores using 787 tie points (mainly volcanic layers)21. NEEM samples were collected at 1.10 m mean intervals of melted ice where 10 mL of meltwater was collected in an acid-cleaned polyethylene bottle and immediately frozen. Chronological resolution between samples (n = 84) ranges from 4 to 267 years during the Holocene. There is a sampling gap between 3-9 ky b2k where the ice was not analysed due to the poor quality of the core in the “brittle ice” section31. The mean sampling frequency during the Glacial Period (n = 247) is ~400 years, where the highest resolution achieved was between 25 and 50 kyrs b2k with sampling resolution ranging from 60 to 150 years during warmer interstadial periods and colder glacial stadial periods, respectively. Additional sampling was carried out during the intervals 34.5–35.8, 37.6–38.8 and 39.4–41.6 (n = 1304) providing a time resolution ranging from 3 to 10 years during those intervals to better resolve the rapid stadial-interstadial transition.

RECAP age-depth model is based on (i) annual layer counting from the last four millennia using the StratiCounter algorithm70 constrained by volcanic eruption markers and synchronized to the GICC05 Greenland Ice Core Chronology framework71; (ii) a modified Dansgaard-Johnsen ice flow model72 constrained to well-dated age markers from 4048 to 11,703 ka b2k and (ii) linear interpolation between 73 GICC05-modelext age markers between 11,703 and 120,215 years b2k32. Each RECAP sample integrated a 55 cm depth interval, where the resolution in the Holocene (n = 1035) ranged from sub-annual in the upper metres to decadal or centennial resolution during the mid to early Holocene (average resolution of 12 years per sample). Due to the thinning of annual layers according to depth, each sample integrates approximately 470 years during the glacial period (n = 218)32. For details on the age model we refer the readers to Simonsen et al.22.

### Geochemical analyses

Samples from NEEM and RECAP ice cores were collected using continuous flow analysis systems. NEEM samples were sent to the Environmental Analytical Chemistry laboratory of the CNR-ISP and Ca’ Foscari University of Venice for iodine and sodium analyses. Total iodine and sodium concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS). Sodium and iodine detection limits were 1 ppb and 0.005 ppb, respectively. RECAP samples were sent to ISP-CNR and to Curtin University of Technology (Perth, Australia). Total iodine and sodium concentrations in Perth were determined by inductively coupled plasma-sector field mass spectrometry (ICP-SFMS) with sodium and iodine detection limits of 1.1 ppb and 0.002 ppb, respectively. The analytical procedure for both records was performed following Vallelonga et al.73 and Maffezzoli et al.32. Instrumental errors for iodine and sodium concentrations were 5%.

### Iflux, Idust and Iocean calculation

Total iodine depositional fluxes (Iflux) in the ice cores were calculated following Corella et al.19. To estimate Iflux in the RECAP ice core, we re-scaled NGRIP accumulation rates65 by a factor of 436/174 since it is the modern accumulation ratio at the NGRIP and RECAP drilling sites74,75,76. Dust-related iodine (Idust) and oceanic gas-phase iodine emissions (Iocean) were calculated according to Eqs. (1) and (2)

$${{{{{{\rm{I}}}}}}_{{{{{\rm{dust}}}}}}}={{{{{\rm{nssCa}}}}}}\,{{{{{\rm{X}}}}}}([{{{{{\rm{I}}}}}}]{/}[{{{{{\rm{Ca}}}}}}])_{\left.{{{{{\rm{crust}}}}}}\right)}{/}[{{{{{\rm{I}}}}}}]$$
(1)
$${{{{{{\rm{I}}}}}}_{{{{{\rm{ocean}}}}}}}=({{{{{\rm{nssI}}}}}}-({{{{{\rm{nssCa}}}}}}\,{{{{{\rm{X}}}}}}{([{{{{{\rm{I}}}}}}]{/}[{{{{{\rm{Ca}}}}}}])}_{{{{{{\rm{crust}}}}}}})){/}[{{{{{\rm{I}}}}}}]$$
(2)

with non-sea salt iodine (nssI) representing the excess iodine production beyond the iodine production related to ssa. It is calculated as nssI = [I] − [Na] × ([I]/[Na])seawater with a [I]/[Na] seawater concentration ratio of 5.6 × 10−6 77. Non-sea salt calcium (nssCa) represents the non-sea spray aerosol fraction of calcium and is representative of mineral dust input. It is calculated as nssCa = [Ca] − ssCa, where ssCa = [Na] × ([Ca]/[Na])seawater with a [Ca]/[Na] seawater concentration ratio of 0.0391. ([I]/[Ca])crust represents the ratio of both elements in the continental crust (4.75 × 10−5 78). Both, Iocean and Idust correspond to unitless fractional proxies.

### Atmospheric chemistry modelling

We also used the global 3-D chemistry-climate Community Atmospheric Model with chemistry (CAM-Chem) version 4 to estimate (i) oceanic iodine emissions from different source areas in the Arctic and North Atlantic Oceans (Fig. 1) and (ii) the deposition fluxes of reactive iodine (HI + HOI + I2O2 + I2O3 + I2O4 + INO2 + IONO2) in Greenland (Supplementary Fig. 1). CAM-Chem includes a comprehensive benchmark chemistry scheme to simulate the evolution of trace gases and aerosols in the troposphere and stratosphere79,80. The model implements a halogen chemistry scheme for chlorine, bromine and iodine12,81,82. This includes the photochemical breakdown of five very short-lived bromocarbons (VSLBr = CHBr3, CH2Br2, CH2BrCl, CHBrCl2, CHBr2Cl) and four iodocarbons (VSLI = CH3I, CH2ICl, CH2IBr, CH2I2), which are naturally emitted from the ocean into the atmosphere83. Additionally, abiotic oceanic sources of HOI and I2 were included in the lowest layer of the model26 based on laboratory studies of the oxidation of aqueous iodide by atmospheric ozone deposited on the ocean surface25,84. In this simulation, the model was configured with a horizontal resolution of 1.9° latitude by 2.5° longitude and 26 levels, from the surface to 40 km (with eight levels above 100 hPa)17,29,82. At the lower boundary, the time-varying zonally averaged distributions of CO2, CH4, H2, N2O and long-lived halocarbons (CFC-11, CFC-12, CFC-113, HCFC-22, H-1211, H-1301, CCl4, CH3CCl3, CH3Cl and CH3Br) are specified following their observed distribution for 2000 CE85. Monthly mean time variations observed for sea surface temperature and sea ice distribution are also prescribed. To obtain a reasonable representation of the overall stratospheric circulation, the integrated momentum that would have been deposited above the model top is specified by an upper boundary condition. We used the output of a previous work17 for CE 2010. This simulation was run in free-running mode79 with prescribed sea surface temperatures, sea ice and meteorological fields from 1950 to 201086. The simulated dynamics and transport therefore represent the daily synoptic conditions of the observations in CE 2010, allowing the online coupling between the ocean, ice and atmospheric modules of the CESM model79. Abiotic oceanic sources of iodine (HOI and I2) are modelled according to the meteorology (temperature, winds, sea surface temperature and surface pressure) in CE 2010 based on MacDonald et al.84 parameterization. The authors would like to remark that a quantitative assessment of the role of iodine in aerosol radiative forcing (through its influence in NPF in the past) is yet not feasible mainly because there are still remaining chemical mechanistic gaps in our knowledge of the iodine particle formation and growth4,87.