A Glacial-Interglacial Record of the North Pacific Biological Pump for the Past 600,000 Years

Because North Pacific deep water solely originates from other sources, namely the Southern and Atlantic Oceans, it is a prime location for global carbon sequestration and storage. Through photosynthesis and gas exchange, the surface ocean sequesters CO₂ from the atmosphere, and its biological pump subsequently buries the carbon in the deep ocean. Organisms preferentially utilize the lighter ¹²C for photosynthesis, leaving behind relatively ¹³C-enriched surface waters. In contrast, respiration reintroduces ¹²C and produces ¹³C-depleted water, especially in the deep ocean, where photosynthesis is absent. This difference in δ¹³C between surface and deep water is an excellent proxy for characterizing the strength of the biological pump, which our record shows, is relatively strong during glacials and weak during interglacials. We measured δ¹³C in N. pachyderma, sinistral (planktonic) and Uvigerina spp. (benthic) to reflect surface and deep water conditions, respectively. However, several factors complicate our interpretation of δ¹³C and biological productivity. During glacials, the entire ocean negatively shifts ~0.3 ‰ in δ¹³C from terrestrial carbon input. Taking the difference in δ¹³C of benthic and planktonic foraminifera eliminates the struggle of having to characterize the absolute productivity of the surface and deep ocean. But further, the strength of global deep water circulation may affect North Pacific benthic δ¹³C signature, which may become more depleted with increased aging and respiration. On the other hand, the surface ocean is susceptible to δ¹³C variations due to air-sea interaction, in which relatively depleted atmospheric carbon mixes into the surface ocean carbon pool. When untangled from global and local complications in δ¹³C, our record may be able to show glacial-interglacial changes in the strength of the North Pacific biological pump.