Rate and Apparent Quantum Yield of Photodissolution

We quantified rates of photochemical dissolution (photodissolution) of organic carbon in coastal Louisiana suspended sediments, conducting experiments under well-defined conditions of irradiance and temperature. Optical properties of the suspended sediments were characterized and used in a radiative transfer model to compute irradiances within turbid suspensions. Photodissolution rate increased with temperature (T), with activation energy of 32 ± 7 kJ mol−1, which implicates indirect (non-photochemical) steps in the net reaction. In most samples, dissolved organic carbon (DOC) concentration increased approximately linearly with time over the first 4 h of irradiation under broadband simulated sunlight, after higher rates in the initial hour of irradiation. Four-hour rates ranged from 2.3 µmol DOC m−3 s−1 to 3.2 µmol DOC m−3 s−1, but showed no relation to sample origin within the study area, organic carbon or reducible iron content, or mass-specific absorption coefficient. First-hour rates were higher—from 3.5 µmol DOC m−3 s−1 to 7.8 µmol DOC m−3 s−1—and correlated well with sediment reducible iron (itself often associated with organic matter). The spectral apparent quantum yield (AQY) for photodissolution was computed by fitting DOC photoproduction rates under different spectral irradiance distributions to corresponding rates of light absorption by particles. The photodissolution AQY magnitude is similar to most published dissolved-phase AQY spectra for dissolved inorganic carbon photoproduction, which suggests that in turbid coastal waters where particles dominate light absorption, DOC photoproduction from particles exceeds photooxidation of DOC

Rate and apparent quantum yield of photodissolution of sedimentary organic matter

Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 57 (2012): 1743-1756, doi:10.4319/lo.2012.57.6.1743.We quantified rates of photochemical dissolution (photodissolution) of organic carbon in coastal Louisiana suspended sediments, conducting experiments under well-defined conditions of irradiance and temperature. Optical properties of the suspended sediments were characterized and used in a radiative transfer model to compute irradiances within turbid suspensions. Photodissolution rate increased with temperature (T), with activation energy of 32 ± 7 kJ mol−1, which implicates indirect (non-photochemical) steps in the net reaction. In most samples, dissolved organic carbon (DOC) concentration increased approximately linearly with time over the first 4 h of irradiation under broadband simulated sunlight, after higher rates in the initial hour of irradiation. Four-hour rates ranged from 2.3 µmol DOC m−3 s−1 to 3.2 µmol DOC m−3 s−1, but showed no relation to sample origin within the study area, organic carbon or reducible iron content, or mass-specific absorption coefficient. First-hour rates were higher—from 3.5 µmol DOC m−3 s−1 to 7.8 µmol DOC m−3 s−1—and correlated well with sediment reducible iron (itself often associated with organic matter). The spectral apparent quantum yield (AQY) for photodissolution was computed by fitting DOC photoproduction rates under different spectral irradiance distributions to corresponding rates of light absorption by particles. The photodissolution AQY magnitude is similar to most published dissolved-phase AQY spectra for dissolved inorganic carbon photoproduction, which suggests that in turbid coastal waters where particles dominate light absorption, DOC photoproduction from particles exceeds photooxidation of DOC.We would like to acknowledge funding support from the National Science Foundation Chemical Oceanography program (L.M. and M.L.E.), a National Aeronautics and Space Administration Earth Systems Science Graduate Fellowship (M.L.E.), and the Office of Naval Research Environmental Optics program (E.B.)

Role of iron and organic carbon in mass-specific light absorption by particulate matter from Louisiana coastal waters

Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 57 (2012): 97-112, doi:10.4319/lo.2012.57.1.0097.We investigated the influences of organic content and mineralogical composition on light absorption by mostly mineral suspended particles in aquatic and coastal marine systems. Mass-specific absorption spectra of suspended particles and surface sediments from coastal Louisiana and the lower Mississippi and Atchafalaya rivers were measured with a centered sample-mount integrating sphere and analyzed in conjunction with organic carbon (OC), hydrochloric acid– (HCl-) extractable iron, and dithionite-extractable iron contents. Compositions and absorption properties were comparable to published values for similar particles. Dithionite-extractable iron was strongly correlated with absorption at ultraviolet (UV) and blue wavelengths, while OC and HCl-extractable iron were weakly but positively correlated. Oxidative removal of OC from sediments caused small and variable changes in absorption, while dithionite extraction of iron oxides strongly reduced absorption. Shoulders in the absorption spectra corresponded to absorption bands of iron oxide minerals, and their intensities were well correlated to dithionite-extractable iron contents of the samples. These findings support a primary role for iron oxide and hydroxide minerals in the mass-specific absorption of mostly inorganic particles from the terrestrially influenced coast of Louisiana. Riverine particles had higher dithionite-extractable iron contents and iron oxide–specific absorption features than did marine particles, consistent with current knowledge regarding differential transport of particulate iron oxides and hydroxides through estuarine salinity gradients and reductive alteration of these oxide phases on the Louisiana shelf. The quantifiable dependence of UV absorption features on iron oxide content suggests that, under certain conditions, in situ hyperspectral absorption measurements could be designed to monitor water-column iron mineral transport and transformation.We would like to acknowledge funding support from the National Science Foundation Chemical Oceanography program (L.M. and M.L.E.), a National Aeronautics and Space Administration Earth Systems Science Graduate Fellowship (M.L.E.), and the Office of Naval Research Environmental Optics program (E.B. and C.R.)

Role of Iron and Organic Carbon in Mass-specific Light Absorption by Particulate Matter from Louisiana Coastal Waters

We investigated the influences of organic content and mineralogical composition on light absorption by mostly mineral suspended particles in aquatic and coastal marine systems. Mass-specific absorption spectra of suspended particles and surface sediments from coastal Louisiana and the lower Mississippi and Atchafalaya rivers were measured with a centered sample-mount integrating sphere and analyzed in conjunction with organic carbon (OC), hydrochloric acid– (HCl-) extractable iron, and dithionite-extractable iron contents. Compositions and absorption properties were comparable to published values for similar particles. Dithionite-extractable iron was strongly correlated with absorption at ultraviolet (UV) and blue wavelengths, while OC and HCl-extractable iron were weakly but positively correlated. Oxidative removal of OC from sediments caused small and variable changes in absorption, while dithionite extraction of iron oxides strongly reduced absorption. Shoulders in the absorption spectra corresponded to absorption bands of iron oxide minerals, and their intensities were well correlated to dithionite-extractable iron contents of the samples. These findings support a primary role for iron oxide and hydroxide minerals in the mass-specific absorption of mostly inorganic particles from the terrestrially influenced coast of Louisiana. Riverine particles had higher dithionite-extractable iron contents and iron oxide–specific absorption features than did marine particles, consistent with current knowledge regarding differential transport of particulate iron oxides and hydroxides through estuarine salinity gradients and reductive alteration of these oxide phases on the Louisiana shelf. The quantifiable dependence of UV absorption features on iron oxide content suggests that, under certain conditions, in situ hyperspectral absorption measurements could be designed to monitor water-column iron mineral transport and transformation

The Neutrally Buoyant Sediment Trap: Two Decades of Progress

The biological carbon flux from the ocean’s surface into its interior has traditionally been sampled by sediment traps, which physically intercept sinking particulate matter. However, the manner in which a sediment trap interacts with the flow field around it can introduce hydrodynamic biases, motivating the development of neutral, self-ballasting trap designs. Here, the performance of one of these designs, the neutrally buoyant sediment trap (NBST), is described and evaluated. The NBST has been successfully used in a number of scientific studies since a prototype was last described in the literature two decades ago, with extensive modifications in subsequent years. Originated at Woods Hole Oceanographic Institution, the NBST is built around a profiling float and carries cylindrical collection tubes, a feature that distinguishes it from other neutral traps described in the literature. This paper documents changes to the device that have been implemented over the last two decades, including wider trap tubes; Iridium Communications, Inc., satellite communications; and the addition of polyacrylamide gel collectors and optical sedimentation sensors. Information is also provided with the intent of aiding the development of similar devices by other researchers, including the present adaptation of the concept to utilize commercially available profiling float hardware. The performance of NBSTs built around commercial profiling floats is comparable to NBSTs built around customized floats, albeit with some additional operational considerations. Data from recent field studies comparing NBSTs and traditional, surface-tethered sediment traps are used to illustrate the performance of the instrument design. Potential improvements to the design that remain to be incorporated through future work are also outlined

The neutrally buoyant sediment trap: two decades of progress

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Estapa, M., Valdes, J., Tradd, K., Sugar, J., Omand, M., & Buesseler, K. The neutrally buoyant sediment trap: two decades of progress. Journal of Atmospheric and Oceanic Technology, 37(6), (2020): 957-973, https://doi.org/10.1175/JTECH-D-19-0118.1.The biological carbon flux from the ocean’s surface into its interior has traditionally been sampled by sediment traps, which physically intercept sinking particulate matter. However, the manner in which a sediment trap interacts with the flow field around it can introduce hydrodynamic biases, motivating the development of neutral, self-ballasting trap designs. Here, the performance of one of these designs, the neutrally buoyant sediment trap (NBST), is described and evaluated. The NBST has been successfully used in a number of scientific studies since a prototype was last described in the literature two decades ago, with extensive modifications in subsequent years. Originated at Woods Hole Oceanographic Institution, the NBST is built around a profiling float and carries cylindrical collection tubes, a feature that distinguishes it from other neutral traps described in the literature. This paper documents changes to the device that have been implemented over the last two decades, including wider trap tubes; Iridium Communications, Inc., satellite communications; and the addition of polyacrylamide gel collectors and optical sedimentation sensors. Information is also provided with the intent of aiding the development of similar devices by other researchers, including the present adaptation of the concept to utilize commercially available profiling float hardware. The performance of NBSTs built around commercial profiling floats is comparable to NBSTs built around customized floats, albeit with some additional operational considerations. Data from recent field studies comparing NBSTs and traditional, surface-tethered sediment traps are used to illustrate the performance of the instrument design. Potential improvements to the design that remain to be incorporated through future work are also outlined.Funding supporting this work has come from multiple sources over the years: the NSF Chemical Oceanography and Carbon and Water programs (most recently OCE-1660012 and OCE-1659995), the NASA Ocean Biology and Biogeochemistry and New Investigator programs (80NSSC17K0662 and NNX14AM01G), and the Woods Hole Oceanographic Institution Technology Award

Decoupling of Net Community Production and Export Production at Submesoscale Fronts in the Sargasso Sea

Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux-regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed-layer biogeochemical parameters and proxies for NCP and EP during 10, high-spatial resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common but rarely did the same parameters covary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in \u3e5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However, when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect-averaged EP determinations were slightly smaller than similar NCP values (Type-II regression slope of 0.93, standard deviation = 0.32) but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations

Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Estapa, M., Buesseler, K., Durkin, C. A., Omand, M., Benitez-Nelson, C. R., Roca-Marti, M., Breves, E., Kelly, R. P., & Pike, S. Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa. Elementa: Science of the Anthropocene, 9(1), (2021): 00122, https://doi.org/10.1525/elementa.2020.00122.Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.The authors would like to acknowledge funding support from the NASA EXPORTS program (Award 80NSSC17K0662) for all sediment trap data presented here. Net primary production data collection was supported by EXPORTS (Award 80NSSC17K568) to Oregon State University. Thorium data collection was supported by EXPORTS (Award 80NSSC17K0555) to KB, CRBN, and L. Resplandy

Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Roca-Marti, M., Benitez-Nelson, C. R., Umhau, B. P., Wyatt, A. M., Clevenger, S. J., Pike, S., Horner, T. J., Estapa, M. L., Resplandy, L., & Buesseler, K. O. Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa. Elementa: Science of the Anthropocene, 9(1), (2021): 00166, https://doi.org/10.1525/elementa.2020.00166.Fluxes of major bioelements associated with sinking particles were quantified in late summer 2018 as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign near Ocean Station Papa in the subarctic northeast Pacific. The thorium-234 method was used in conjunction with size-fractionated (1–5, 5–51, and >51 μm) concentrations of particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) collected using large volume filtration via in situ pumps. We build upon recent work quantifying POC fluxes during EXPORTS. Similar remineralization length scales were observed for both POC and PN across all particle size classes from depths of 50–500 m. Unlike bSi and PIC, the soft tissue–associated POC, PN, and TPP fluxes strongly attenuated from 50 m to the base of the euphotic zone (approximately 120 m). Cruise-average thorium-234-derived fluxes (mmol m–2 d–1) at 120 m were 1.7 ± 0.6 for POC, 0.22 ± 0.07 for PN, 0.019 ± 0.007 for TPP, 0.69 ± 0.26 for bSi, and 0.055 ± 0.022 for PIC. These bioelement fluxes were similar to previous observations at this site, with the exception of PIC, which was 1 to 2 orders of magnitude lower. Transfer efficiencies within the upper twilight zone (flux 220 m/flux 120 m) were highest for PIC (84%) and bSi (79%), followed by POC (61%), PN (58%), and TPP (49%). These differences indicate preferential remineralization of TPP relative to POC or PN and larger losses of soft tissue relative to biominerals in sinking particles below the euphotic zone. Comprehensive characterization of the particulate bioelement fluxes obtained here will support future efforts linking phytoplankton community composition and food-web dynamics to the composition, magnitude, and attenuation of material that sinks to deeper waters.The authors would like to acknowledge support from the National Aeronautics and Space Administration as part of the EXport Processes in the Ocean from RemoTe Sensing program awards 80NSSC17K0555 and 80NSSC17K0662. They also acknowledge the funding from the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study for MRM and KOB, the National Science Foundation Graduate Research Fellowship Program for AMW, and the Ocean Frontier Institute for MRM

High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport processes in the ocean from RemoTe sensing field campaign

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Buesseler, K. O., Benitez-Nelson, C. R., Roca-Marti, M., Wyatt, A. M., Resplandy, L., Clevenger, S. J., Drysdale, J. A., Estapa, M. L., Pike, S., & Umhau, B. P. High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport processes in the ocean from RemoTe sensing field campaign. Elementa: Science of the Anthropocene, 8(1), (2020): 030, https://doi.org/10.1525/elementa.030.The EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program of National Aeronautics and Space Administration focuses on linking remotely sensed properties from satellites to the mechanisms that control the transfer of carbon from surface waters to depth. Here, the naturally occurring radionuclide thorium-234 was used as a tracer of sinking particle flux. More than 950 234Th measurements were made during August–September 2018 at Ocean Station Papa in the northeast Pacific Ocean. High-resolution vertical sampling enabled observations of the spatial and temporal evolution of particle flux in Lagrangian fashion. Thorium-234 profiles were remarkably consistent, with steady-state (SS) 234Th fluxes reaching 1,450 ± 300 dpm m−2 d−1 at 100 m. Nonetheless, 234Th increased by 6%–10% in the upper 60 m during the cruise, leading to consideration of a non-steady-state (NSS) model and/or horizontal transport, with NSS having the largest impact by decreasing SS 234Th fluxes by 30%. Below 100 m, NSS and SS models overlapped. Particulate organic carbon (POC)/234Th ratios decreased with depth in small (1–5 μm) and mid-sized (5–51 μm) particles, while large particle (>51 μm) ratios remained relatively constant, likely influenced by swimmer contamination. Using an average SS and NSS 234Th flux and the POC/234Th ratio of mid-sized particles, we determined a best estimate of POC flux. Maximum POC flux was 5.5 ± 1.7 mmol C m−2 d−1 at 50 m, decreasing by 70% at the base of the primary production zone (117 m). These results support earlier studies that this site is characterized by a modest biological carbon pump, with an export efficiency of 13% ± 5% (POC flux/net primary production at 120 m) and 39% flux attenuation in the subsequent 100 m (POC flux 220 m/POC flux 120m). This work sets the foundation for understanding controls on the biological carbon pump during this EXPORTS campaign.The authors would like to acknowledge support from the National Aeronautics and Space Administration (NASA) as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program awards 80NSSC17K0555 and 80NSSC17K0662; the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study for KOB and MRM, and the National Science Foundation Graduate Research Fellowship Program (NSF-GRFP) for funding and support of AW