A new Proceedings of the National Academy of Sciences paper authored by Ursula Jongebloed, graduate student in UW Atmospheric Sciences, Becky Alexander, PCC Director and UW Professor in Atmospheric Sciences, and colleagues challenges previous work suggesting that North Atlantic primary productivity is declining.

Because certain phytoplankton emit dimethyl sulfide (DMS) which reacts in the air to form methanesulfonic acid (MSA) and sulfate, MSA has been used as a proxy for phytoplankton primary productivity in the past. A study on MSA levels preserved in Greenland ice cores showed a decline over the industrialized era, which led to conclusions that primary production in the North Atlantic was declining. However, Jongebloed et al., argue that to get the full picture of the primary production trends, we need to include both MSA and sulfate.

The previous study had assumed that MSA and sulfate would be produced from the oxidation of DMS in constant levels – but from their own ice cores with data over a longer period of time, Jongebloed et al. instead found that sulfate derived from phytoplankton had actually increased over the industrial era, while MSA had decreased. When combined, the decline in MSA is essentially ‘offset’ by the increase in sulfate, indicating that phytoplankton populations in the North Atlantic may in fact be relatively stable since the mid-1800s. This is not to say that the authors assert that phytoplankton are safe: as a host of other anthropogenic-driven changes threaten phytoplankton, it is vital that research from ice cores, models, and other estimates continue to be analyzed to ensure that we remain alert to how primary production may change in the future.

Abstract: “Marine phytoplankton are primary producers in ocean ecosystems and emit dimethyl sulfide (DMS) into the atmosphere. DMS emissions are the largest biological source of atmospheric sulfur and are one of the largest uncertainties in global climate modeling. DMS is oxidized to methanesulfonic acid (MSA), sulfur dioxide, and hydroperoxymethyl thioformate, all of which can be oxidized to sulfate. Ice core records of MSA are used to investigate past DMS emissions but rely on the implicit assumption that the relative yield of oxidation products from DMS remains constant. However, this assumption is uncertain because there are no long-term records that compare MSA to other DMS oxidation products. Here, we share the first long-term record of both MSA and DMS-derived biogenic sulfate concentration in Greenland ice core samples from 1200 to 2006 CE. While MSA declines on average by 0.2 µg S kg^–1 over the industrial era, biogenic sulfate from DMS increases by 0.8 µg S kg^–1. This increasing biogenic sulfate contradicts previous assertions of declining North Atlantic primary productivity inferred from decreasing MSA concentrations in Greenland ice cores over the industrial era. The changing ratio of MSA to biogenic sulfate suggests that trends in MSA could be caused by time-varying atmospheric chemistry and that MSA concentrations alone should not be used to infer past primary productivity.”

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