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Abstract. Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols. Ice cores in regions affected by anthropogenic pollution show an industrial-era decline in MSA, which has previously been interpreted as indicating a decline in phytoplankton abundance. However, a simultaneous increase in DMS-derived sulfate (bioSO4) in a Greenland ice core suggests that pollution-driven oxidant changes caused the decline in MSA by influencing the relative production of MSA versus bioSO4. Here we use GEOS-Chem, a global chemical transport model, and a zero-dimensional box model over three time periods (preindustrial era, peak North Atlantic NOx pollution, and 21st century) to investigate the chemical drivers of industrial-era changes in MSA and bioSO4, and we examine whether four DMS oxidation mechanisms reproduce trends and seasonality in observations. We find that box model and GEOS-Chem simulations can only partially reproduce ice core trends in MSA and bioSO4 and that wide variation in model results reflects sensitivity to DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized increase in DMS oxidation by the nitrate radical over the industrial era, which increases bioSO4 production, but competing factors such as oxidation by BrO result in increased MSA production in some simulations, which is inconsistent with observations. To improve understanding of DMS oxidation, future work should investigate aqueous-phase chemistry, which produces 82 %–99 % of MSA and bioSO4 in our simulations, and constrain atmospheric oxidant concentrations, including the nitrate radical, hydroxyl radical, and reactive halogens. 

An industrial-era drop in Greenland ice core methanesulfonic acid (MSA) is thought to herald a collapse in North Atlantic marine phytoplankton stocks related to a weakening of the Atlantic Meridional Overturning Circulation. In contrast, stable levels of marine biogenic sulfur production contradict this interpretation and point to changes in atmospheric oxidation as a potential cause of the MSA decline. However, the impact of oxidation on MSA production has not been quantified, nor has this hypothesis been rigorously tested. Here we present a multi-century MSA record from the Denali, Alaska, ice core, which shows an MSA decline similar in magnitude but delayed by 93 years relative to the Greenland record. Box model results using updated chemical pathways indicate that oxidation by industrial nitrate radicals has suppressed atmospheric MSA production, explaining most of Denali’s and Greenland’s MSA declines without requiring a change in phytoplankton production. The delayed timing of the North Pacific MSA decline, relative to the North Atlantic, reflects the distinct history of industrialization in upwind regions and is consistent with the Denali and Greenland ice core nitrate records. These results demonstrate that multi-decadal trends in industrial-era Arctic ice core MSA reflect rising anthropogenic pollution rather than declining marine primary production. 

Abstract. Carbonyl sulfide (COS) is the most abundant sulfur gas in the atmosphere with links to terrestrial and oceanic productivity. We measured COS in ice core air from an intermediate-depth ice core from the South Pole using both dry and wet extraction methods, recovering a 52 500-year record. We find evidence for COS production in the firn, altering the atmospheric signal preserved in the ice core. Mean sea salt aerosol concentrations from the same depth are a good proxy for the COS production, which disproportionately impacts the measurements from glacial period ice with high sea salt aerosol concentrations. The COS measurements are corrected using sea salt sodium (ssNa) as a proxy for the excess COS resulting from the production. The ssNa-corrected COS record displays substantially less COS in the glacial period atmosphere than the Holocene and a 2 to 4-fold COS rise during the deglaciation synchronous with the associated climate signal. The deglacial COS rise was primarily source driven. Oceanic emissions in the form of COS, carbon disulfide (CS2), and dimethylsulfide (DMS) are collectively the largest natural source of atmospheric COS. A large increase in ocean COS emissions during the deglaciation suggests enhancements in emissions of ocean sulfur gases via processes that involve ocean productivity, although we cannot quantify individual contributions from each gas. 

Abstract A large volcanic sulfate increase observed in ice core records around 1450 C.E. has been attributed in previous studies to a volcanic eruption from the submarine Kuwae caldera in Vanuatu. Both EPMA–WDS (electron microprobe analysis using a wavelength dispersive spectrometer) and SEM–EDS (scanning electron microscopy analysis using an energy dispersive spectrometer) analyses of five microscopic volcanic ash (cryptotephra) particles extracted from the ice interval associated with a rise in sulfate ca. 1458 C.E. in the South Pole ice core (SPICEcore) indicate that the tephra deposits are chemically distinct from those erupted from the Kuwae caldera. Recognizing that the sulfate peak is not associated with the Kuwae volcano, and likely not a large stratospheric tropical eruption, requires revision of the stratospheric sulfate injection mass that is used for parameterization of paleoclimate models. Future work is needed to confirm that a volcanic eruption from Mt. Reclus is one of the possible sources of the 1458 C.E. sulfate anomaly in Antarctic ice cores. 

Abstract. The South Pole Ice Core (SPICEcore) was drilled in 2014–2016 to provide adetailed multi-proxy archive of paleoclimate conditions in East Antarcticaduring the Holocene and late Pleistocene. Interpretation of these recordsrequires an accurate depth–age relationship. Here, we present the SPICEcore (SP19) timescale for the age of the ice of SPICEcore. SP19 is synchronized to theWD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) icecore using stratigraphic matching of 251 volcanic events. These eventsindicate an age of 54 302±519 BP (years before 1950) at thebottom of SPICEcore. Annual layers identified in sodium and magnesium ionsto 11 341 BP were used to interpolate between stratigraphic volcanic tiepoints, yielding an annually resolved chronology through the Holocene.Estimated timescale uncertainty during the Holocene is less than 18 yearsrelative to WD2014, with the exception of the interval between 1800 to 3100BP when uncertainty estimates reach ±25 years due to widely spacedvolcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rateof 7.4 cm yr−1 (water equivalent). The time variability of accumulation rateis consistent with expectations for steady-state ice flow through the modernspatial pattern of accumulation rate. Time variations in nitrateconcentration, nitrate seasonal amplitude and δ15N of N2 in turn are as expected for the accumulation rate variations. The highlyvariable yet well-constrained Holocene accumulation history at the site canhelp improve scientific understanding of deposition-sensitive climateproxies such as δ15N of N2 and photolyzed chemicalcompounds. 

Abstract Variability in sea ice is a critical climate feedback, yet the seasonal behavior of Southern Hemisphere sea ice and climate across multiple timescales remains unclear. Here, we develop a seasonally resolved Holocene sea salt record using major ion measurements of the South Pole Ice Core (SPC14). We combine the SPC14 data with the GEOS‐Chem chemical transport model to demonstrate that the primary sea salt source switches seasonally from open water (summer) to sea ice (winter), with wintertime variations disproportionately responsible for the centennial to millennial scale structure in the record. We interpret increasing SPC14 and circum‐Antarctic Holocene sea salt concentrations, particularly between 8 and 10 ka, as reflecting a period of winter sea ice expansion. Between 5 and 6 ka, an anomalous drop in South Atlantic sector sea salt indicates a temporary sea ice reduction that may be coupled with Northern Hemisphere cooling and associated ocean circulation changes.