The annual cycle of the biological carbon pump in the subpolar North Atlantic
The biological pump is particularly undersampled in high latitude regions that experience deep winter convection such as the subpolar North Atlantic, despite the outsized importance of such regions in sequestering carbon in the deep ocean on centennial to millennial timescales. My current research uses data from the Ocean Observatories Initiative (OOI) array of sensors on moorings and autonomous robotic vehicles in the subpolar North Atlantic, which provide the first simultaneous year-round observations of biological carbon cycling processes in both the surface ocean and at depth in this critical region. My analysis has shown that much of the carbon that previous studies have observed sinking from the surface during the spring bloom is respired below the surface and ventilated back to the atmosphere during deep winter mixing (extending to >1000 meters), significantly reducing biological carbon sequestration.
Oxygen measurements from autonomous sensors on the OOI array provide a powerful tool to constrain the annual magnitude and seasonal timing of the biological carbon pump (determined as annual net community production; ANCP) and its influence on air-sea carbon dioxide flux. However, oxygen sensors have to be very carefully calibrated to produce high quality data suitable for calculating ANCP by oxygen mass balance. To improve the accuracy and utility of the oxygen data from the OOI array, I received funding from NSF (with David Nicholson, WHOI) to deploy two gliders configured for air calibration of their oxygen sensors when surfacing between profiles. These air-calibrated gliders were deployed on OOI cruises that I joined along with my students in June 2018 and August 2019.
The new data we are currently collecting will provide important observational constraints on the subpolar North Atlantic biological pump and its influence on carbon cycling by addressing: (1) What is the annual magnitude and seasonal progression of biological carbon export, including thermocline remineralization and winter ventilation, in the subpolar North Atlantic? (2) What is the role of biological carbon export in driving air-sea carbon dioxide flux in the subpolar North Atlantic? This analysis will also provide mechanistic insights relevant to the biological pump in other high latitude regions that experience deep winter convection.
Palevsky, H. I. and D. P. Nicholson (2018), The North Atlantic biological pump: Insights from the Ocean Observatories Initiative Irminger Sea Array. Oceanography, 31(1):42-49, https://doi.org/10.5670/oceanog.2018.108.
Sensitivity of the estimated rates and spatial patterns of the biological pump to the choice of export depth horizon
Estimated rates and efficiency of organic carbon export via the biological pump are sensitive to differences in the depth horizons used to define export, which often vary across methodological approaches. Using a global earth system model (CESM-BEC), I have compared estimates of the rate and efficiency of particulate organic carbon export across a range of depth horizons commonly used in both observational- and model-based research. My analysis showed that these differing choices of depth horizon yield widely varying global rates and efficiencies of export. This work also showed that analyses of the biological pump that do not account for winter ventilation of respired carbon can overestimate of the global rate and efficiency of the biological pump and produce strong discrepancies in the spatial patterns of export, due to pronounced influence of winter ventilation in high-latitude regions that experience deep winter mixing.
This has important implications for how we use global earth system models to project changes to export flux under future climate change scenarios, since model output for export flux is often interpreted only at a fixed depth horizon of 100 m, without accounting for winter ventilation. I am currently analyzing model output simulating export rates and efficiency for the 21st century under the IPCC's RCP 8.5 carbon emissions scenario to determine the sensitivity of our future projections of the biological pump to the choice of export depth horizon.
Palevsky, H. I. and S. C. Doney (2018). How choice of depth horizon influences the estimated spatial patterns and global magnitude of ocean carbon export flux. Geophysical Research Letters, 45, https://doi.org/10.1029/2017GL076498.
The North Pacific biological pump: Rates, efficiency and influence on ocean uptake of carbon dioxide
The North Pacific features a band of strong atmospheric carbon dioxide uptake at the transition between the subarctic and subtropical gyres, making it a region of particular interest to quantify the mechanisms of ocean carbon uptake. My PhD dissertation assessed the rates and efficiency of biological carbon export and evaluated the relative roles of biology, physical circulation, and temperature-driven solubility changes in driving ocean carbon uptake throughout the full annual cycle across the North Pacific basin (35°N – 50°N, 142°E – 125°W).
To do this, I used data collected on sixteen container ship transects between Hong Kong and Long Beach, CA from 2008-2012, where my research group measured the the surface concentration of dissolved inorganic carbon (DIC), the partial pressure of carbon dioxide to determine air-sea carbon dioxide flux, and triple oxygen isotopes and oxygen/argon dissolved gas ratios as non-incubation based geochemical tracers of gross primary production and net community production. My results demonstrated that the physical dynamics of winter ventilation are a key mechanism controlling the rate and efficiency of the biological pump in the North Pacific, and that biological carbon export is the dominant process driving ocean carbon uptake in the eastern north Pacific while physical processes drive the strong ocean carbon sink in the western North Pacific.
Palevsky, H. I., P. D. Quay, D. E. Lockwood, and D. P. Nicholson (2016), The annual cycle of gross primary production, net community production and export efficiency across the North Pacific Ocean, Global Biogeochemical Cycles, 29, doi: 10.1002/2015GB005318.
Palevsky, H. I. and P. D. Quay (2017), Influence of the biological pump on ocean carbon uptake over the annual cycle across the North Pacific Ocean, Global Biogeochemical Cycles, 31, 81-95, doi: 10.1002/2016GB005527.
Satellite algorithms and global biogeochemical models as tools to estimate primary and export production: A comparison with geochemical methods
Due to the scarcity of geochemical measurements with broad spatial and temporal coverage such as those in my dissertation work, current global-scale estimates of the strength and efficiency of the biological pump rely on dynamic numerical ocean models with biogeochemical and marine ecosystem components, or empirically-derived models based on satellite observations of ocean temperature and sea surface color.
While these are promising tools, few independent direct observational estimates have been available to validate model predictions. I compared chemical tracer-based estimates of photosynthetic production and carbon export rates in the North Pacific with multiple commonly-used model estimates, and found that no single satellite algorithm or model can reproduce seasonal and annual geochemically-determined primary and net community production rates throughout the North Pacific basin. The high latitude regions show large primary production discrepancies in winter and spring and strong effects of deep winter mixed layers on annual net community production that cannot be accounted for in current satellite-based approaches. These results underscore the need to evaluate satellite- and model-based estimates using multiple productivity parameters measured over broad ocean regions and throughout the full annual cycle, including during winter ventilation, in order to accurately estimate the rate and efficiency of carbon sequestration via the ocean’s biological pump.
Palevsky, H. I., P. D. Quay, and D. P. Nicholson (2016), Discrepant estimates of primary and export production from satellite algorithms, a biogeochemical model, and geochemical tracer measurements in the North Pacific Ocean, Geophysical Research Letters, 43, doi:10.1002/2016GL070226.
Fine-scale structure of phytoplankton communities and their role in biological carbon export and ocean carbon uptake
By combining continuous underway dissolved gas measurements to quantify the rate and efficiency of carbon uptake and biological carbon export with novel fine spatial-scale applications of flow cytometry to identify phytoplankton populations, I uncovered a hot spot of intense biological carbon export and oceanic carbon dioxide uptake in the Gulf of Alaska characterized by a phytoplankton community distinct from phytoplankton in surrounding regions. These results indicate the potential significance of both fine-spatial scale processes and of unique phytoplankton assemblages in driving the biological carbon pump.
Palevsky, H. I., F. Ribalet, J. E. Swalwell, C. E. Cosca, E. D. Cokelet, R. A. Feely, E. V. Armbrust, P. D. Quay (2013) The influence of net community production and phytoplankton community structure on CO2 uptake in the Gulf of Alaska. Global Biogeochemical Cycles, 27, 664-676, doi: 10.1002/gbc.20058. [pdf]
Inside the Atlantic Cod Fishery: In Search of a Sustainable Future
I traveled to Iceland, Denmark, Scotland, Norway and Newfoundland to study fisheries science, the fishing industry, and management policies as a Thomas J. Watson Fellow. This project included two fisheries research cruises, one with the Marine Research Institute of Iceland's Northern Shrimp Survey and one with the Scottish Fisheries Research Services as part of the International Bottom Trawl Survey, and first-hand research in fishing communities while living in the homes of fishing families in Peterhead, Scotland, the Lofoten Islands of Norway and Fogo Island, Newfoundland. I also contributed to a published book chapter on European fisheries policy in collaboration with researchers in Denmark and Norway. You can read full details of my experience from my fellowship blog.