New View of Biological Carbon Pump

By ooistaff | April 5, 2023 |

Hilary I. Palevsky, Dept. of Earth & Environmental Sciences, Boston College, Boston, MA, USA. Extracted from OOI Science Plan, 2021.

The ocean’s biological carbon pump plays an important role in the global carbon cycle by transferring photosynthetically ﬁxed organic carbon from the surface into the deep ocean, sequestering it from contact with the atmosphere (Le Moigne, 2019; Volk and Hoffert, 1985). Historically, shipboard measurements of the biological pump’s rates and mechanisms have been concentrated in the spring and summer during the period of peak photosynthetic production (e.g., the North Atlantic Bloom Experiments), with observations of the full seasonal cycle limited to time-series sites in regions more conducive to year-round shipboard sampling (e.g., the Hawaii Ocean Time Series and Bermuda Atlantic Time Series). However, a growing body of work has shown that year-round observations are needed to fully constrain the biological pump, especially in regions such as the OOI array sites that experience strong seasonality in both biological and physical processes (e.g., Boyd et al., 2019; Palevsky and Doney, 2018).

Autonomous biogeochemical sensors deployed at the OOI arrays capture high temporal- resolution year-round data throughout the water column that can be used to improve our constraints on rates and mechanisms of the biological carbon pump in regions that have historically been under- sampled. Dissolved oxygen data from the ﬁrst two years of observations at the Global Irminger Sea Array in the subpolar North Atlantic provide an example of the new insights into the biological pump enabled by the OOI (Fig. to the right; Palevsky and Nicholson, 2018). Surface measurements show the seasonal cycle expected based on numerous prior studies of the strong spring bloom in this region (e.g., Briggs et al., 2011), with the bloom driving oxygen super-saturations that indicate net photosynthetic production and export of organic carbon from the stratiﬁed seasonal mixed layer. However, subsurface proﬁler observations show that much of the organic carbon exported from the surface is remineralized within the seasonal thermocline and ventilated back to the atmosphere during deep mixing the subsequent winter, rather than being sequestered long-term. This interplay between the biological processes driving seasonal export and the physical processes driving winter ventilation is being further explored at the Irminger Sea Array by considering interannual variability in subsurface respiration and winter convection (Wanzer, 2019) and by employing a new approach to oxygen calibration using gliders with modiﬁed sensor mounts (Nicholson and Feen, 2017) that will provide the high-accuracy data needed to constrain the rate of air-sea oxygen exchange and the total amount of carbon sequestered below the winter ventilation depth.

Beyond the work to date focused on dissolved oxygen data at the Irminger Sea Array, a strength of the OOI program is that every array combines sensors for multiple biogeochemical tracers – including nitrate, carbon (pH and pCO2), and bio-optical measurements of chlorophyll and backscatter from particles, as well as oxygen – providing unprecedented temporal resolution and depth-resolved coverage for multi-tracer year-round observations. This combination of multiple tracers offers the potential for greater mechanistic understanding of the biological pump by quantifying the separate contributions of particulate and dissolved organic matter to the total organic carbon ﬂux, and distinguishing among ﬂuxes driven by gravitational settling, eddy-driven subduction, and cycles of mixed layer deepening and detrainment (e.g., Lacour et al., 2019; Llort et al., 2018). The full depth coverage achieved by including biogeochemical sensors across all platforms – including surface and subsurface moorings, proﬁling moorings, and autonomous vehicles – provides opportunities to consider not only biological carbon ﬂux from the surface ocean, but also transfer effi ciency through the mesopelagic and effectiveness of long-term sequestration below the winter ventilation depth. The high temporal resolution of measurements (~minutes to hours across platforms) also opens opportunities to consider processes such as rapid bloom onset in spring and mixing/re-stratiﬁcation events in winter that are more diffi cult to capture using methods such as Biogeochemical-Argo ﬂoats, which provide broader spatial coverage than possible with moored platforms but must sample less frequently in order to last multiple years (Claustre et al., 2020).

Finally, the OOI Program offers the opportunity to compare detailed time-series observations of the biological pump across multiple sites, complementing both ship-based process studies (e.g., EXPORTS; Siegel et al., 2016) and more globally wide-spread observations from Biogeochemical-Argo ﬂoats and satellites. The OOI arrays represent a diverse set of complementary physical and biogeochemical settings that together could be used to better constrain how interactions between biological and physical processes inﬂuence the biological pump. The two Southern Hemisphere sites, though now decommissioned, provided data in two highly undersampled regions: a site of high biological productivity and strong currents and eddies in the Argentine Basin, and a region of strong heat and carbon ﬂuxes and deep winter convection in the Southern Ocean. At the Northern Hemisphere Global Arrays, the Irminger Sea site features both the classic North Atlantic seasonal spring bloom and exceptionally deep winter mixing, while Station Papa at a similar latitude in the subarctic Northeast Paciﬁc provides a contrasting physical setting with a strong halocline that restricts winter mixing and a more tightly coupled ecosystem during the productive season. The Pioneer and Endurance Coastal Arrays, as well as the Oregon slope proﬁling moorings on the Regional Cabled Array, capture the spatial and temporal variability of two very different, but both highly dynamic and productive coastal margins, providing new constraints on coastal biological carbon ﬂuxes. Continued observations and new syntheses of OOI data across sensors and sites promise many new and important insights into our regional and global understanding of the biological pump and its role in the ocean carbon cycle.