Dimethyl sulfide (DMS), produced by ocean phytoplankton, is a crucial climate-relevant gas. Upon release into the atmosphere, it oxidizes to methane sulfonic acid (MSA) and non-sea-salt sulfate (nssSO₄²⁻), influencing cloud condensation nuclei formation and radiative forcing. DMS accounts for a substantial portion (10-40%) of global sulfur flux. Oceanic DMS emission is determined by the gas transfer coefficient (influenced by wind speed), DMS solubility, and water/air DMS concentrations. Phytoplankton-produced dimethylsulfoniopropionate (DMSP) is converted to DMS by bacterial enzymes, making chlorophyll-a (Chl-a) concentration a useful proxy for DMS levels. Existing remote sensing algorithms for estimating sea surface DMS concentration have limitations, particularly at regional scales and regarding seasonal variations. The Arctic, experiencing rapid warming and sea ice decline, is a region of particular interest. Reduced sea ice cover increases light penetration, stimulating phytoplankton production and potentially DMS emissions. While satellite and model-based estimations indicate increased DMS emissions in recent decades due to sea ice reduction, observational data supporting this are limited. Long-term, continuous aerosol monitoring is crucial for understanding DMS's climatic impact. MSA in aerosols acts as a proxy for oceanic DMS emissions due to its near-exclusive origin from DMS oxidation. While some Arctic stations show increased MSA since 2000, long-term data remain sparse. Ice cores provide valuable records of past aerosol history; however, postdepositional loss in low-accumulation areas necessitates careful site selection. The southeast Greenland Ice Sheet (SE-Dome) offers high accumulation and low temperature, minimizing postdepositional loss, making it an ideal location for ice core analysis to reconstruct seasonal MSA variations and infer DMS emission changes.

Previous research highlights the critical role of DMS in climate regulation (Charlson et al., 1987; Pandis et al., 1994; Simó, 2001). Studies have emphasized the environmental constraints on DMS production and removal (Stefels et al., 2007), the sensitivity of Arctic sulfate aerosol and clouds to DMS changes (Mahmood et al., 2019), and the relationship between DMS flux and wind speed (Liss & Merlivat, 1986; Huebert et al., 2010; Bell et al., 2013, 2017). Gali et al. (2018, 2019) used remote sensing to estimate DMS concentrations, revealing a potential decadal increase in Arctic emissions, though observational evidence was limited. The impact of Arctic sea ice decline on phytoplankton production and subsequent DMS emissions has been explored (Stroeve et al., 2012; AMAP, 2017; Arrigo & van Dijken, 2011; Bélanger et al., 2013; Hayashida et al., 2020). Studies using Arctic aerosol MSA concentrations have suggested an increase since 2000, linking it to changes in sea ice extent (Sharma et al., 2012), though long-term data remain scarce (Pei et al., 2021). Ice core studies provide valuable historical records of aerosol deposition (Oyabu et al., 2015; Iizuka et al., 2018; Kuramoto et al., 2011), with considerations for postdepositional loss (Curran & Jones, 2000; Weller et al., 2004). Previous work on the SE-Dome ice core highlighted its high accumulation rate and low temperature, suitable for preserving volatile species (Iizuka et al., 2017, 2018; Furukawa et al., 2017). Studies on MSA in Greenland ice cores have shown long-term trends (Legrand et al., 1997; Maselli et al., 2017; Osman et al., 2019).

This study utilized a high-resolution ice core from the SE-Dome in southeast Greenland, known for its high accumulation rate and low temperature, minimizing post-depositional loss. The ice core, spanning 1960-2014, was analyzed for MSA concentration using ion chromatography. Annual and seasonal MSA fluxes were calculated by multiplying MSA concentration by the water equivalent of each sample section. The accuracy of the ice core dating was evaluated by comparing δ¹⁸O records from the ice core with those simulated by isotope-enabled climate models, achieving an accuracy of approximately one month. To assess the impact of dating uncertainty on the results, simulations with randomly assigned age errors based on the 95% confidence limits of the age determinations were conducted. The relationship between MSA flux and various factors was investigated. Chlorophyll-a (Chl-a) concentrations, indicating phytoplankton biomass and thus DMS production, were obtained from merged satellite data provided by the European Space Agency GlobColour project, offering improved temporal and spatial resolution. Wind speed data, crucial for determining the gas transfer coefficient, and sea ice concentration data were obtained from the ERA5 reanalysis dataset. The frequency of intermediate wind speeds (8-11 m/s, optimal for DMS emission) was calculated for each period. The sea ice retreat day was defined as the last day when daily sea ice concentration was below 10% during April-September. Backward trajectory analysis, using the NOAA HYSPLIT model and NCEP reanalysis data, was employed to identify the source regions of air masses reaching the SE-Dome. The cumulative distribution function (CDF) of air mass probabilities, weighted by precipitation amounts, was used to define source regions with varying probability levels (CDF20, CDF40, CDF60, CDF80). Statistical analyses, including correlation analysis and Student's t-tests, were performed to assess the relationships between MSA flux and environmental variables.

The mean annual MSA flux from 2002–2014 was significantly higher than the overall mean (1960–2014) and the mean from 1972–2001. A decreasing trend in annual MSA flux was observed from 1960–2001, consistent with previous ice core studies. However, the post-2002 increase is novel. Seasonal analysis revealed a significant increase in summertime (July-September) MSA flux from 2002–2014, which was 3-6 times higher than that from 1972–2001. Springtime (April-June) MSA flux showed a positive correlation with Chl-a concentration in the Irminger Sea, indicating that phytoplankton production is a significant driver of DMS emissions during spring. However, no significant correlation was found between summertime MSA flux and Chl-a concentration, wind speed, or sea ice concentration. Analysis of sea ice retreat dates revealed a marked difference among the three periods studied (1960-1971, 1972-2001, 2002-2014). Sea ice retreated significantly earlier in 2002-2014, especially in the regions identified by backward trajectory analysis as major contributors to MSA deposition at the SE-Dome. This earlier retreat, alongside higher Chl-a concentrations in the same region during 2002-2014, strongly suggests that the enhanced phytoplankton production due to increased light penetration from earlier sea ice retreat is the primary driver of the increased summertime DMS emissions.

The findings demonstrate a significant increase in summertime DMS emissions in the regions surrounding southeast Greenland since 2002. This increase correlates strongly with the earlier retreat of sea ice, allowing for prolonged periods of enhanced phytoplankton production and DMS release. While springtime DMS emissions are primarily driven by phytoplankton productivity in the Irminger Sea, the summertime emissions seem to be more intricately linked to the interplay between sea ice retreat and subsequent phytoplankton blooms in the coastal waters adjacent to the southeast Greenland ice sheet. This observation directly supports model predictions suggesting that sea ice decline will lead to increased DMS emissions in the Arctic. The observed increase in MSA, a reliable proxy for DMS, provides crucial observational evidence to support existing model predictions and enhances our understanding of the Arctic climate system's response to ongoing warming trends. Future research should explore the regional variability in the effects of sea ice reduction on DMS emissions, focusing on the interaction between biological productivity, meteorological conditions, and the transport of DMS and its oxidation products.

This study provides compelling evidence from a Greenland ice core showing a significant increase in summertime DMS emissions since 2002, linked to earlier sea ice retreat and enhanced phytoplankton production. This finding directly supports model projections linking Arctic sea ice decline with increased DMS emissions, contributing significantly to our understanding of climate-ocean interactions in the Arctic. Future research should investigate the spatial and temporal variability of this phenomenon across different Arctic regions and refine our understanding of the complex interplay between sea ice, phytoplankton, and atmospheric processes.

The study relies on MSA in the ice core as a proxy for DMS emissions. While MSA is largely derived from DMS oxidation, other sources, although minor, may exist. The analysis focuses on a specific location in southeast Greenland. While backward trajectory analysis helps to determine source regions, it does not fully capture the complexities of atmospheric transport. The satellite-derived Chl-a data used have some inherent uncertainties, especially in areas with sparse data or cloud cover. The study's focus is on the influence of sea-ice retreat, and other factors such as changes in nutrient availability and ocean currents could also contribute to observed changes in DMS emissions.