Scholten Jan C.

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Jan C.

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  • Article
    GEOTRACES radium isotopes interlaboratory comparison experiment
    (Association for the Sciences of Limnology and Oceanography, 2012-06) Charette, Matthew A. ; Dulaiova, Henrieta ; Gonneea, Meagan E. ; Henderson, Paul B. ; Moore, Willard S. ; Scholten, Jan C. ; Pham, Mai Khanh
    In anticipation of the international GEOTRACES program, which will study the global marine biogeochemistry of trace elements and isotopes, we conducted a multi-lab intercomparison for radium isotopes. The intercomparison was in two parts involving the distribution of: (1) samples collected from four marine environments (open ocean, continental slope, shelf, and estuary) and (2) a suite of four reference materials prepared with isotopic standards (circulated to participants as 'unknowns'). Most labs performed well with 228Ra and 224Ra determination, however, there were a number of participants that reported 226Ra, 223Ra, and 228Th (supported 224Ra) well outside the 95% confidence interval. Many outliers were suspected to be a result of poorly calibrated detectors, though other method specific factors likely played a role (e.g., detector leakage, insufficient equilibration). Most methods for radium analysis in seawater involve a MnO2 fiber column preconcentration step; as such, we evaluated the extraction efficiency of this procedure and found that it ranged from an average of 87% to 94% for the four stations. Hence, nonquantitative radium recovery from seawater samples may also have played a role in lab-to-lab variability.
  • Preprint
    Contribution of Th-230 measurements to the estimation of the abyssal circulation
    ( 2007-01-02) Marchal, Olivier ; Francois, Roger ; Scholten, Jan C.
    An inverse finite-difference model of the abyssal circulation in the North Atlantic Ocean is developed in order to evaluate the dynamical information contained in measurements of thorium-230 (230Th). The model has a very coarse resolution and is based on lowest order balances for planetary flows. The naturally occurring 230Th differs from more conventional oceanic tracers in several respects, e.g., its production (by 234U radioactive decay) is globally uniform to a good approximation and its removal can be understood in terms of a simple reversible exchange with particles sinking slowly to the seafloor. The time required for 230Th to reach steady state with respect to particle exchange is estimated to increase with depth, reaching O(10) yr below 1000 m. In the North Atlantic 230Th activities at distant locations share a similar increase with depth in the upper 1000m—a pattern consistent with a reversible exchange—but show drastic differences in the abyssal interior. Two inversions are conducted in order to determine whether the 230Th differences reflect the effects of the circulation—by preventing the slow attainment to steady state w.r.t. particle exchange in deep water—and provide complementary information about the abyssal flow. In a first inversion, observations of density from a hydrographic compilation and of volume transports at specific locations are combined with the dynamical balances in order to infer the basin-scale flow. The inferred flow displays the western boundary current and coherent structures in the abyssal interior with low statistical significance. In a second inversion, the flow is further constrained by the 230Th measurements and the condition that 230Th divergence by the flow field and particle sinking must be locally balanced by 230Th production from 234U decay. The addition of 230Th leads to the estimation of a larger amplitude of the integrated meridional transports below 1000 m (by 2–9 Sv), where the range reflects the uncertainties in the large scale 230Th distribution and in the radiochemical balance. This result is interpreted as a correction by 230Th for the tendency of inverse geostrophic models to lead to the inference of a vanishing circulation when horizontal density gradients are insignificant.
  • Preprint
    Preparation of Mn-fiber standards for the efficiency calibration of the delayed coincidence counting system (RaDeCC)
    ( 2010-04) Scholten, Jan C. ; Pham, Mai Khanh ; Blinova, Oxana ; Charette, Matthew A. ; Dulaiova, Henrieta ; Eriksson, Mats
    Precise measurements of the short lived radium isotopes 223Ra and 224Ra by means of the delayed coincidence counting system (RaDeCC) rely on an efficiency calibration of this system using Mn-fiber standards for which radium activities are exactly known. We prepared seventeen different standards by placing Mn-fibers in seawater spiked with various amounts of 227Ac (with 223Ra in radioactive equilibrium), 228Th (in radioactive equilibrium with 232Th and 224Ra) and 226Ra. We tested for quantitative adsorption of 227Ac and 228Th on the Mn-fibers by: (1) measuring 227Ac and 232Th in the residual solutions after preparing the Mn-fiber standards and (2) monitoring their 223Ra and 224Ra activities over a period of ~100 days. In the residual solutions, the activities of 227Ac and 232Th were < 1.0 % and < 5.3 %, respectively, of the activities initially added to the Mn-fibers. Our results indicate that Milli-Q water washing of the Mn-fibers is the major source of our observed losses of thorium. Measurements of 227Ac standards over 1½ years indicate a significant decrease of measurable 223Ra with time prohibiting the long-term use of 227Ac Mn-fiber standards. We found the 224Ra efficiency to be independent of the range of 227Ac, 228Th and 226Ra activities on the Mn-fibers standards used. The efficiency determination for 223Ra, however, may be biased in the case of relatively high 224Ra activities due to insufficient correction of chance of coincidence. Thus we suggest using a single 227Ac Mn-fiber standard for the efficiency determination for 223Ra.
  • Article
    Radium isotopes as submarine groundwater discharge (SGD) tracers: review and recommendations
    (Elsevier, 2021-05-14) Garcia-Orellana, Jordi ; Rodellas, Valenti ; Tamborski, Joseph ; Diego-Feliu, Marc ; van Beek, Pieter ; Weinstein, Yishai ; Charette, Matthew A. ; Alorda-Kleinglass, Aaron ; Michael, Holly A. ; Stieglitz, Thomas ; Scholten, Jan C.
    Submarine groundwater discharge (SGD) is now recognized as an important process of the hydrological cycle worldwide and plays a major role as a conveyor of dissolved compounds to the ocean. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) are widely employed geochemical tracers in marine environments. Whilst Ra isotopes were initially predominantly applied to study open ocean processes and fluxes across the continental margins, their most common application in the marine environment has undoubtedly become the identification and quantification of SGD. This review focuses on the application of Ra isotopes as tracers of SGD and associated inputs of water and solutes to the coastal ocean. In addition, we review i) the processes controlling Ra enrichment and depletion in coastal groundwater and seawater; ii) the systematics applied to estimate SGD using Ra isotopes and iii) we summarize additional applications of Ra isotopes in groundwater and marine studies. We also provide some considerations that will help refine SGD estimates and identify the critical knowledge gaps and research needs related to the current use of Ra isotopes as SGD tracers.
  • Article
    Erratum : GEOTRACES radium isotopes interlaboratory comparison experiment
    (Association for the Sciences of Limnology and Oceanography, 2012-06) Charette, Matthew A. ; Dulaiova, Henrieta ; Gonneea, Meagan E. ; Henderson, Paul B. ; Moore, Willard S. ; Scholten, Jan C. ; Pham, Mai Khanh
    In our original paper, Charette, M. A., H. Dulaiova, M. E. Gonneea, P. B. Henderson, W. S. Moore, J. C. Scholten, and M. K. Pham. 2012. GEOTRACES radium isotopes interlaboratory comparison experiment. Limonol. Oceanogr.: Methods 10:451, the incorrect headers were used for Table 9.
  • Preprint