A Holocene history of upwelling along the northern California coast

Spring upwelling along the California margin brings cold, nutrient-rich waters from the deep Pacific Ocean into the coastal zone. This has the paired effects of (1) driving a strong, seasonal peak in marine ecosystem productivity & biodiversity, and (2) cooling the adjacent US West Coast (Fig. 1). Upwelling is responsible for generating the “classic” San Francisco fog, and helps to support a vibrant commercial fishing industry that was valued at over $250 million in California alone in 2013. Because of the myriad impacts of upwelling on the US West Coast, understanding potential changes in upwelling is important, but climate modeling efforts show strongly conflicting projections for the next century.

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To better understand future upwelling dynamics under predicted warmer North Pacific Ocean conditions, a team of researchers from the USGS, Idaho State University, the Lamont-Doherty Earth Observatory, and the Skidaway Institute of Oceanography analyzed a marine sediment record from the northern California continental slope (569 m water depth) that spans the last 7,500 years as a potential analogue for future conditions.

Several key measurements were made, including opal concentrations (a proxy for diatom productivity, which is one of the key phytoplankton groups that make up the base of the California marine food web) and stable ð¹³C and ð¹⁵N isotope measurements (proxy indicators of nutrient cycling in the water column). Statistical analyses of these data discovered that upwelling intensity along the northern California margin has undergone at least four shifts since the early Holocene, with the most intense upwelling period starting at ~2900 calibrated years before present (cal yr BP) and extending into modern times (Fig. 2). This same interval is also identified as the onset of “modern” upwelling along the California margin.

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These results were further verified by both marine and terrestrial microfossil indicator species within the same sediment samples: a diatom associated with coastal upwelling conditions (Rhizosolenia sp.), and pollen from the coastal redwood (Sequoia sp). Coastal redwood trees collect much of their moisture from fog, which is associated with spring and summer upwelling, thus making the presence of Sequoia sp. pollen an indirect indicator of upwelling as well.

Comparing these new upwelling data against a nearby pre-existing record of Holocene sea surface temperatures (SST), the study found that the onset of modern California upwelling occurred at the same time as an approximate +1.5°C warming. This result supports projections of future upwelling intensification along the US West Coast. Because it is difficult to extrapolate these local data to the entire 3,000-km-long region, this work highlights the continued need for paleoceanographic exploration of the North Pacific Ocean. 

The paper “A Holocene record of ocean productivity and upwelling from the northern California continental slope” was published in Quaternary International.

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