Sea Ice control on CO2 release from the deep ocean in the subarctic Pacific Ocean / / by Savannah Worne

Savannah Worne is a PhD student from the University of Nottingham, working with the British Geological Survey as part of the Envision Doctoral Training Partnership. Her project focuses on reconstructing sea ice in the Bering Sea over the last million years. Through analysing the chemical composition of sediments from the bottom of the ocean, Savannah has been working towards trying to understand how increased/decreased amounts of sea ice control primary productivity, nutrient cycling, the upwelling of deep ocean water, and the impacts of these processes on the exchange of CO2 between the ocean and the atmosphere through time. Here, Savannah tells us more... 

Over the last million years, the earth’s climate has alternated between cold (glacial) and warm (interglacial) states, primarily driven by changes in the amount of solar energy the earth receives. The pace and extent of these glacial / interglacial cycles is considered to be controlled by how much CO2 there is in the atmosphere, as this determines how global temperature and the polar ice sheets respond to solar energy. Ice core records have previously been used to assess how the amount of atmospheric CO2 in the atmosphere has changed over the last 800,000 years (black line in figure 2) but the cause of the variation is disputed.

Fig 1: Bering Strait sediment cores from the bottom of the ocean (2009), used to investigate past oceanography and climate change

Some believe that that the amount of CO2 released from the deep ocean is a dominant control on glacial-interglacial atmospheric CO2 variability, particularly in the subantarctic region of the Southern Ocean 1. However, there is growing evidence to suggest that the subarctic Pacific Ocean is also a key region for deep ocean CO2 ventilation 2. Therefore we set out to investigate how deep water upwelling and CO2 release from the Bering Sea has varied over the last 850,000 years. The Bering Sea is the most northerly region in the subarctic Pacific Ocean, with North Pacific Deep Water (NPDW), which is rich in nutrients and CO2 upwells at the Bering shelf slope.

In our new paper, we analyse sediments collected by the International Ocean Discovery Program (IODP) in 2009 during Expedition 323. Using geochemical proxies for nutrient utilisation and primary productivity, we created a novel nutrient 'upwelling index' which provides a semi-quantitative assessment of deep water upwelling strength (see the red line in Figure 2). High upwelling index results indicate periods of increased deep water upwelling, bringing nutrients and CO2 to the surface. Alternatively, low upwelling index values represent periods where deep water upwelling is suppressed, causing nutrients and CO2 to be trapped in the deep Bering basin.

Fig 2: Strong correlation between atmospheric pCO2 record (L├╝thi et al, 2008) and the nutrient upwelling index for the Bering Sea (Worne et al., accepted)

We found that during glacial periods, when the sea level dropped more than 50m, deep water upwelling was reduced (represented as the blue bars in Figure 2). We hypothesised that this was due to increases in sea ice during cold glacial periods. As sea ice forms on the northern Bering shelf, the salts in the surface sea-water are excluded causing the waters which underlie sea ice to become more saline. This results in the formation of a current called North Pacific Intermediate Water which prevents upwelling of the deep waters which lie underneath. We suggest that this would have reduced the release of CO2 from the ocean to the atmosphere causing a significant decrease in atmospheric CO2 during glacials.

These findings lend support to the suggestion that polar/sub-polar sea ice played a major and the resultant intermediate water formation, modulated deep water upwelling and ocean-atmosphere CO2 exchange on glacial-interglacial timescales.

The JOIDES Resolution Ship, which sailed to the Bering Sea in 2009 during IODP Expedition 323

1. (Sigman et al., 2010)
2. (Gray et al., 2018; Jaccard et al., 2010; Kender et al., 2018; Rae et al., 2014) 

Further reading:

Coupled Climate and Subarctic Pacific Nutrient Upwelling Over the Last 850,000 Years