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The high-nutrient, low-chlorophyll region of the northeastern (NE) subarctic Pacific is one of the most biologically productive marine ecosystems in the world, supporting fisheries worth over $5 billion annually. Phytoplankton are the primary producers in this ecosystem and are also a major source of biogenic sulfur emissions, important in Earth's climate system. However, variability in marine primary production through time is not well constrained. Here we establish methanesulfonic acid (MSA) concentrations in the Denali ice core as a proxy for marine primary production in the NE Pacific. Using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT; Stein et al., 2015, ) modeling, we identify moisture source regions for the core site and correlate Sea-Viewing Wide Field-of-View Sensor-derived chlorophyll a concentrations with ice core MSA. From 1998 to 2007 we find that areas of significant positive correlation overlap with the HYSPLIT-inferred moisture source region in the western Gulf of Alaska on an annual basis (r=0.85, p<0.001). We identify an MSA response to a localized bloom related to ash deposition from a 2009 Mt. Redoubt eruption. An anomalous upwelling-driven bloom in spring 2008 did not impact the ice core MSA record due to unfavorable transport conditions. Despite this, we observe that bloom events are rarely missed in the MSA record, which we attribute to the consistent and high snow accumulation rate at the ice core drill site. Our findings suggest that Denali ice core MSA is a reliable recorder of changes in marine primary production through time in the NE subarctic Pacific.

Plain Language Summary The base of the marine food web is composed of single-celled photosynthetic organisms that are collectively termed primary producers. Because these microscopic organisms support all marine life, changes in their biomass can impact the entire food web. Over the past three decades, satellite data have shown that primary producers are declining around the world with some of the greatest declines occurring in the North Pacific Ocean. The reasons for these declines may include changes in ocean temperatures, nutrient availability, and wind-driven ocean mixing, all of which are related to climate. To place these changes within a longer-term context, we seek to validate regionally a proxy tool by measuring a chemical produced by phytoplankton called methanesulfonic acid (MSA). MSA is transported through the atmosphere by storms and deposited on mountain glaciers in the North Pacific region. We measured MSA in a new ice core from Denali National Park, Alaska. We describe strong, statistically significant correlations between ice core MSA concentrations and chlorophyll concentrations in the western Gulf of Alaska. We suggest that the ice core MSA proxy record can help us understand how primary production in this region has changed through time.