Clevenger Samantha J.

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Samantha J.

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Now showing 1 - 4 of 4
  • Article
    Review of the analysis of Th-234 in small volume (2-4 L) seawater samples: improvements and recommendations
    (Springer, 2021-06-24) Clevenger, Samantha J. ; Benitez-Nelson, Claudia R. ; Drysdale, Jessica A. ; Pike, Steven M. ; Puigcorbé, Viena ; Buesseler, Ken O.
    The short-lived radionuclide 234Th is widely used to study particle scavenging and transport from the upper ocean to deeper waters. This manuscript optimizes, reviews and validates the collection, processing and analyses of total 234Th in seawater and suggests areas of further improvements. The standard 234Th protocol method consists of scavenging 234Th from seawater via a MnO2 precipitate, beta counting, and using chemical recoveries determined by adding 230Th. The revised protocol decreases sample volumes to 2 L, shortens wait times between steps, and simplifies the chemical recovery process, expanding the ability to more rapidly and safely apply the 234Th method.
  • Article
    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
    (University of California Press, 2020-12-10) Buesseler, Ken O. ; Benitez-Nelson, Claudia R. ; Roca-Martí, Montserrat ; Wyatt, Abigale M. ; Resplandy, Laure ; Clevenger, Samantha J. ; Drysdale, Jessica A. ; Estapa, Margaret L. ; Pike, Steven M. ; Umhau, Blaire P.
    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.
  • Article
    Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa
    (University of California Press, 2021-06-28) Roca-Martí, Montserrat ; Benitez-Nelson, Claudia R. ; Umhau, Blaire P. ; Wyatt, Abigale M. ; Clevenger, Samantha J. ; Pike, Steven M. ; Horner, Tristan J. ; Estapa, Margaret L. ; Resplandy, Laure ; Buesseler, Ken O.
    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.
  • Thesis
    Investigating the ocean’s biological pump using thorium-235 and polonium-210
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2024-02) Clevenger, Samantha J. ; Buesseler, Kenneth O.
    Oceans play a critical role in the Earth’s ability to regulate atmospheric carbon dioxide, absorbing approximately one-third of all anthropogenically-emitted CO2. The biological pump is a mechanism which controls oceanic uptake of CO2 and sequestration of carbon into deeper waters. It is a complex web of processes starting with biota in the upper ocean which transform CO2 into particulate organic carbon (POC) via photosynthesis, a portion of which is ultimately sequestered from the atmosphere. Due to the massive scale of these processes, small changes in the strength of the biological pump can greatly impact atmospheric CO2 levels. Tools used to quantify the flux of biologically relevant elements such as POC and biogenic silica via the biological pump are radioisotopes – naturally-occurring clocks which allow us to look back in time to study oceanic processes. In this thesis, the two radioisotope pairs 234Th/238U and 210Po/210Pb are utilized. Chapter 2 is an update of the 234Th/238U measurement method which simplifies the protocol, and allows for expansion of use. Chapter 3 presents an unprecedentedly extensive, non-steady state study of 234Th/238U to assess POC and biogenic silica flux through NASA’s EXPORTS project. The study involved tracking an eddy for one month in the North Atlantic Ocean to quantify export over the development of a dual-phase plankton spring bloom. In Chapter 4, this study is expanded to add a non-steady state application of the 210Po/210Pb tracer to look farther back in the history of POC and biogenic silica export. Extensive comparison of the two tracer systems results in new-to-the-field conclusions about the concurrent use of tracers. This thesis culminates in Chapter 5, which compares three 234Th/238U studies with community structure data to make inferences about POC export as it relates to who is present in the ecosystem. The EXPORTS North Atlantic dataset is a “strong” export endmember, WHOI’s Ocean Twilight Zone (OTZ) project offers an “intermediate” export case in the Northwest Atlantic Slope, and the EXPORTS North Pacific study is a “weak” export endmember. Taken together, this thesis greatly expands the use of 234Th/238U and 210Po/210Pb tracers to investigate the biological pump in diverse environments.