Particulate fluxes and circulation in a changing Arctic Ocean: tracer data and modeling

27 March 2020

A complete review of published and new water column profiles of thorium-230 (²³⁰Th) and protactinium-231 (²³¹Pa) concentrations and neodymium (Nd) isotopic compositions collected in the Amerasian Basin of the Arctic Ocean between 1983 and 2015 was performed by Grenier and co-workers (2019, see reference below). This review allowed them to identify regional and temporal variability of geochemical and physical oceanic properties, among which was a notable regional decrease in ²³⁰Th and ²³¹Pa concentrations with time. They associated this decrease to an increase in the last decades of:

lateral and vertical particulate fluxes, resulting from enhanced sediment resuspension and productivity, respectively;
ocean mixing;

in response to increasing summer retreat of sea ice and enhanced coastal erosion.

These hypotheses were corroborated and complemented by a companion modeling paper, in which Yu and co-workers (2020) performed the first modeling of ²³⁰Th in the Arctic Ocean by coupling a physical model (NEMO configuration ANHA4) to a chemical scavenging one (²³⁰Th model). Their approach enabled them to characterize the relative contribution of the parameters – enhancement of particle fluxes vs. enhancement of ocean mesoscale mixing and advection– driving the modeled ²³⁰Th concentration decrease between 2002 and 2015.

2020, Grenier — **Figure:** **(a-b) From Grenier et al. (2019): regional and temporal variability of particulate fluxes and water mixing between the boundary and central part of the Canada Basin.** (a) Comparison of ²³⁰Th_dconcentration (in fg kg^-1) vertical profiles and potential temperature (in °C)-salinity profiles (with superimposed isopycnal σ₀) at stations of very close location but of different sampling year. The ²³⁰Th minimum observed between 500 and 1000 m at CB4 (2015), not observed at station 3 (2000), suggests increased lateral advection of particles from the margins toward the central basin (corroborated by an associated maximum of particulate Fe at CB4, not shown here). The decrease in concentrations below 500 m between station 2700 in 2007 and L1.1 in 2009 suggests an increased contribution of boundary, 230Th-depleted water, mixing with overlying ²³⁰Th-enriched central water. These latter can also be identified in potential temperature-practical salinity profiles by thermohaline intrusions (McLaughlin et al., 2009), which are smoothed below 0.4°C (i.e. below 600 m depth) due to mixing with boundary water. (b) Schematic representation of the Canada Basin areas as defined from the ²³⁰Th profile features. Each station location is characterized following its degree of impact by coastal processes: weak, strong, or intermediate (white, black, and black/white filled circles, respectively), closely related to the extension of the boundary, northern, and mixed water areas. Color contours refer to the year of station sampling. Larger circles, cut in two halves, represent temporal variability of the spatial extension of boundary processes from the stations visited twice (a).
**(c) From Yu et al. (2020): modeled contribution of sea ice retreat and ocean mixing increase to ²³⁰Th decrease.** Time series of yearly and regionally averaged ²³⁰Th concentrations at 500 m in the Canada Basin. (1) Base Run (variable ocean flow and sea ice) versus Exp. 1, in which the ocean flow repeats 2002 and thus does not change with time. (2) Base Run versus Exp.2, where the sea ice is held at the 2002 level. These two comparisons show that the particle flux increase –defined as fully dependant on sea ice retreat– accounts for ~70% of the modeled ²³⁰Th concentration decrease since 2002, while the remaining ~30% decrease results from increases in ocean mesoscale mixing and advection.

References:

Grenier, M., François, R., Soon, M., Rutgers van der Loeff, M., Yu, X., Valk, O., Not, C., S. Bradley Moran, S. B., Edwards, R. L., Lu, Y., Lepore, K., & Allen, S. E. (2019). Changes in Circulation and Particle Scavenging in the Amerasian Basin of the Arctic Ocean over the Last Three Decades Inferred from the Water Column Distribution of Geochemical Tracers. Journal of Geophysical Research: Oceans, 124(12), 9338–9363. DOI: https://doi.org/10.1029/2019JC015265

Yu, X., Allen, S. E., François, R., Grenier, M., Myers, P. G., & Hu, X. (2020). Modeling Dissolved and Particulate Th in the Canada Basin: Implications for Recent Changes in Particle Flux and Intermediate Circulation. Journal of Geophysical Research: Oceans, 125(2). DOI: https://doi.org/10.1029/2019JC015640

McLaughlin, F. A., Carmack, E. C., Williams, W. J., Zimmermann, S., Shimada, K., & Itoh, M. (2009). Joint effects of boundary currents and thermohaline intrusions on the warming of Atlantic water in the Canada Basin, 1993–2007. Journal of Geophysical Research, 114, C00A12. DOI: https://doi.org/10.1029/2008JC005001

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