Advances in measurements of particle cycling and fluxes in the ocean

Abstract

The sinking flux of particles is an important removal mechanism of carbon from the surface ocean as part of the biological pump and can play a role in cycling of other chemical species. This work dealt with improving methods of measuring particle export and measuring export on different scales to assess its spatial variability. First, the assumption of 238 U linearity with salinity, used in the 238 U-234 Th method, was reevaluated using a large sample set over a wide salinity range. Next, neutrally buoyant and surfacetethered sediment traps were compared during a three-year time series in the subtropical Atlantic. This study suggested that previously observed imbalances between carbon stocks and fluxes in this region are not due to undersampling by traps. To assess regional variability of particle export, surface and water-column measurements of 234 Th were combined for the first time to measure fluxes on ~20 km scales. Attempts to relate surface properties to particle export were complicated by the temporal decoupling of production and export. Finally, particle export from 234 Th was measured on transects of the Atlantic Ocean to evaluate basin-scale export variability. High-resolution sampling through the water-column allowed for the identification of unique 234 AcknowledgementsI feel so very lucky to have had the opportunity to work with my advisor, Ken Buesseler. Most graduate students are not required to travel extensively and spend weeks at sea with their advisors, but as an oceanography student, I was fortunate to have Ken as both a mentor and a friend. In addition to the many things Ken has taught me about science, I hope he has also instilled in me his skills of diplomacy that I got to witness for five years as his student. I also blame Ken, but thank him, for developing my interest in politics, particularly as it relates to science. Without his encouragement and support, I might not have had the courage to apply for my Knauss Fellowship. Outside of the office, I have to thank both Ken and his wife, Wendi, for being such good friends to Paul and myself. It was so special to have you be such an integral part of our wedding and I look to you as role models for our relationship.My secondary support system was my thesis committee, made up of Phoebe Lam, Dave Glover, and Mick Follows. Your continued support, suggestions, and encouragement over the last three years have helped me get to this point. Thank you also, to the chair of my defense, Carl Lamborg, for his support over my time at WHOI. In addition to those mentioned above, through various projects, I have had the opportunity to work with a large group of scientists whose support and assistance I appreciate including Ken Sims, Matthew Charette, Bill Jenkins, Rachel Stanley, Jim Valdes, Rod Johnson, Mike Lomas, Dave Siegel, Debbie Steinberg, Hugh Ducklow, Hein de Baar, Micha Rijkenberg, and Pere Masque. Those who I may have omitted here have been acknowledged in the relevant thesis chapters.My time at WHOI would not have been the same without th...

Decoupling of net community and export production on submesoscales in the Sargasso Sea

“…Export values derived from single‐bottle surface 234 Th deficits were well correlated to export measured at the PPZ depth ( R 2 = 0.72, n = 20), similar to observations at the West Antarctic Peninsula by Owens [] (Table and Figure ). The regression line between C export at the base of the mixed layer and at the base of the PPZ had a negative intercept (Figure ).…”

The imbalance of new and export production in the western Antarctic Peninsula, a potentially “leaky” ecosystem

“…Application of a temporally varying N: 234 Th ratio would have led to a slight decrease in calculated N flux (using temporally varying N:Th ratios, seasonally averaged gravitational flux would be 16.3 ± 1.2, compared to 20.4 ± 2.4 mg N d −1 with a constant N:Th ratio), acting to further increase the discrepancy between new production and vertical fluxes. Our average C: 234 Th ratio (177 µg C dpm −1 ) also fell on the slightly high end of the values measured by Owens [] on two cruises in the WAP. However, it is important to note that mesozooplankton in other regions have been measured to have very high C: 234 Th ratios (Stukel et al, in review), which means that export mediated by any trophic levels feeding upon them will be essentially invisible to 234 Th‐based methods.…”

High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean

“…Significant deficits of 234 Th with respect to 238 U were present in surface waters down to 100-170 m at all stations, with 234 Th/ 238 U ratios averaging 0.67 70.11 throughout the upper 100 m. In most cases, the base of these deficits matched well with the base of the primary production zone (PPZ, Fig. 2), defined as the depth at which fluorescence reaches 10% of its maximum value (Owens, 2013). The PPZ extended, on average, down to 117 712 m, and the NPP at 100 m (deepest depth for NPP determination) ranged from 0.09 to 0.44 mmol C m À 3 d À 1 , confirming that primary production was occurring at least until 100 m (C. Hoppe, pers.…”

“…238 U was derived from salinity (Owens et al, 2011). Primary production zone (PPZ, dashed grey line) is defined as the depth at which fluorescence reaches 10% of its maximum value (Owens, 2013).…”