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

Abstract

Since the 1980-1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230-thorium ( 230 Th) and 231-protactinium ( 231 Pa) concentrations and neodymium isotopes (expressed as ε Nd ) measured in the Amerasian Basin prior to 2000 are compared to a new, post-2000s data set. The comparison shows a large scale decrease in dissolved 230 Th and 231 Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230 Th and 231 Pa, with ε Nd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing. Plain Language SummaryThe Arctic Ocean is one of the oceanic regions most affected by climate change. The increasing summer retreat of sea ice allows greater atmosphere-ocean exchanges and light penetration, while land-ocean exchanges are expected to increase, due to enhanced continental erosion (notably through permafrost thawing and increased river flow). These changes are driven by climate and in turn impact the climate. This study aims at assessing possible shifts in oceanic circulation and particle fluxes resulting from these climate-driven changes in the Amerasian Basin of the Arctic Ocean, during the last few decades. To achieve this goal, we measured geochemical tracers of circulation and particle fluxes in seawater samples collected in this area between 2005 and 2015, which we compared to published data from samples collected between 1983 and 2000. The primary geochemical tracers studied here are 230 Th and 231 Pa. These radioisotopes are uniformly produced in seawater, from uranium decay, and are strongly reactive with particles. Therefore, their oceanic distribution depends on particle fluxes and circulation. The evolution of their distribution in the Amerasian Basin over space and time documents enhancement of particle flux and lateral mixing in the Amerasian basin of the Arctic Ocean over the last three decades. Key Points: • Concentration decreases of 230 Th d and 231 Pa d suggest an enhancement of particulate scavenging since the 2000s in the Amerasian Basin • Particulate scavenging results from higher productivity and sediment resuspension from the shelves • Post-2...

Modeling Dissolved and Particulate Th in the Canada Basin: Implications for Recent Changes in Particle Flux and Intermediate Circulation

“…Over the period 2002-2015, there is a substantial decrease in the basin-wide 230 Th concentration at 500 m depth (Figure 4). This decrease in 230 Th is also observed in profiles measured in 2015 ( Figure 6 of Grenier et al, 2019). According to the model, by the end of 2015, the area with 230 Th values higher than 0.3 dpm m −3 is less than one third of that in 2002.…”

“…We use observations from several sources: older published data (Bacon et al, 1989;Edmonds et al, 1998;Scholten et al, 1995;Trimble et al, 2004) and more recent data compiled by Valk et al (2018) and Grenier et al (2019). The more recent data were collected during six cruises: R/V Polarstern 2007 (GEOTRACES-IPY Figure 1).…”

“…230 Th, a geochemical tracer with a residence time well matched to decadal variation in the Arctic Ocean, is applied in this study. Observations have shown dramatic changes in the Arctic 230 Th profiles since the 1980s (Grenier et al, 2019). Here we focus on the Atlantic layer of the Canada Basin and use a 230 Th scavenging model coupled to an Arctic Ocean circulation model, to elucidate the ocean dynamics leading to the observed changes.…”

“…The Atlantic layer is warm compared to layers above and below and, if mixed to the surface, could melt much of the remaining ice in the Arctic Ocean (Rudels et al, 2004). Large changes in this layer have been measured both in its warmth and its flux (McLaughlin et al, 2009), and observations also suggest changes in circulation patterns (Grenier et al, 2019;Karcher et al, 2012;McLaughlin et al, 2009). The main flow in the layer is through boundary currents (Woodgate et al, 2007), which are very narrow due to the small Rossby radius in the Arctic.…”

“…Here, we will focus on changes in the Canada Basin where there is a strong contrast in productivity and particle concentration between the continental shelf and the oligiotrophic Beaufort Gyre (Jackson et al, 2010). Thus, we observe strong contrasts in scavenged tracer 230 Th (Grenier et al, 2019).…”