British Columbian continental shelf as a source of dissolved iron to the subarctic northeast Pacific Ocean

Abstract

[1] The distribution of dissolved (<0.4 mm) iron (Fe) across the continental shelf and slope of Queen Charlotte Sound on the west coast of Canada was examined to estimate the potential of these waters as a source of Fe to the Fe-limited waters of the subarctic northeast Pacific. Iron profiles obtained in shelf, slope, and offshore waters demonstrate decreasing concentrations of Fe with distance from the continent. Within 50 m of the shelf sediments dissolved Fe concentrations were 5.3 ± 0.3 nM. This signal was detected, although attenuated by 80%, along the isopycnal surface at offshore stations 40-50 km seaward of the shelf break, strongly suggesting cross-shelf transport of an Fe-rich plume originating in low dissolved oxygen (<3 ml L À1 , <130 mmol kg À1 ) waters in subsurface water over the continental shelf. Several physical mechanisms that may cause these Fe-enriched waters to advect offshore in this region (i.e., tidal currents and Ekman transport in the bottom boundary layer, coastal downwelling/relaxation from upwelling, and the formation of anticyclonic, westward-propagating, coastal eddies) are discussed. We suggest that strong tidal currents over broad continental shelves may play a key role in Fe supply to ocean basins.

Drivers of Spatiotemporal Distributions of Macro‐ and Micronutrients (Fe, Ni, Cu, Zn) in the Northern Gulf of Alaska

“…Nearshore bottom samples (≤5 m from the seafloor) exhibited the largest [dFe] range in summer with less variability in spring and fall (Figure 5, Figure S5 in Supporting Information ). This near‐bottom dFe signal was carried offshore along an isopycnal layer σ θ ∼26.5–27.5 (e.g., Figure 9d), similar to observations on the British Columbia continental shelf/slope (Cullen et al., 2009), with NGA subsurface [dFe] gradually diminishing in the offshore direction (Figures 5d–5f). While this pattern was clear over the Seward and Middleton Island Lines, near‐seafloor samples from the Kodiak Island Line shelf were not elevated in [dFe] (∼0.7–∼1.2 nM) relative to background offshore observations in spring or summer (Figure S5d and S5e in Supporting Information ).…”

“…This pool of dFe represents a potential source to basin waters, particularly when downwelling conditions facilitate its offshore transport (e.g., summer 2022 and 2023). Similar continental subsurface input of dFe from the Gulf of Alaska shelf was found southeast in British Columbia (Cullen et al., 2009; σ θ 26 to 27) and Southeast Alaska (GEOTRACES Intermediate Data Product Group, 2021) and further west off Kodiak Island (Sieber et al., 2024). Together, these observations indicate a spatially extensive shelf source of dFe to the central Gulf of Alaska.…”

Drivers of Spatiotemporal Distributions of Macro‐ and Micronutrients (Fe, Ni, Cu, Zn) in the Northern Gulf of Alaska

“…Nearshore bottom samples (≤5 m from the seafloor) exhibited the largest [dFe] range in summer with less variability in spring and fall (Figure 5, Figure S5 in Supporting Information ). This near‐bottom dFe signal was carried offshore along an isopycnal layer σ θ ∼26.5–27.5 (e.g., Figure 9d), similar to observations on the British Columbia continental shelf/slope (Cullen et al., 2009), with NGA subsurface [dFe] gradually diminishing in the offshore direction (Figures 5d–5f). While this pattern was clear over the Seward and Middleton Island Lines, near‐seafloor samples from the Kodiak Island Line shelf were not elevated in [dFe] (∼0.7–∼1.2 nM) relative to background offshore observations in spring or summer (Figure S5d and S5e in Supporting Information ).…”

“…This pool of dFe represents a potential source to basin waters, particularly when downwelling conditions facilitate its offshore transport (e.g., summer 2022 and 2023). Similar continental subsurface input of dFe from the Gulf of Alaska shelf was found southeast in British Columbia (Cullen et al., 2009; σ θ 26 to 27) and Southeast Alaska (GEOTRACES Intermediate Data Product Group, 2021) and further west off Kodiak Island (Sieber et al., 2024). Together, these observations indicate a spatially extensive shelf source of dFe to the central Gulf of Alaska.…”

Fluxes of particulate iron from the upper ocean around the Crozet Islands: A naturally iron-fertilized environment in the Southern Ocean

“…The release of Fe to biota via conversion of litho to BioFe has been previously suggested by Lam et al [2006], Lam and Bishop [2008], Frew et al [2006], and more recently Cullen et al [2009]. The present work strongly supports this hypothesis and the data also suggests that the BioFe derived from lithogenic iron has a longer residence time in the upper ocean than does the lithogenic iron which is not accessed by biota.…”

Trace Element Distributions in the Water Column near the Deepwater Horizon Well Blowout

“…For Fe, the situation is less straightforward. Fe did show a vertical trend of increasing concentrations for near-bottom samples (Figure a), as might be expected in a continental margin environment (e.g., and ). However, there was also a lateral trend in these submerged plume samples with lower dissolved Fe closer to the wellhead during late May 2010 (Figure b).…”

Drivers of Spatiotemporal Distributions of Macro‐ and Micronutrients (Fe, Ni, Cu, Zn) in the Northern Gulf of Alaska

“…Nearshore bottom samples (≤5 m from the seafloor) exhibited the largest [dFe] range in summer with less variability in spring and fall (Figure 5, Figure S5 in Supporting Information ). This near‐bottom dFe signal was carried offshore along an isopycnal layer σ θ ∼26.5–27.5 (e.g., Figure 9d), similar to observations on the British Columbia continental shelf/slope (Cullen et al., 2009), with NGA subsurface [dFe] gradually diminishing in the offshore direction (Figures 5d–5f). While this pattern was clear over the Seward and Middleton Island Lines, near‐seafloor samples from the Kodiak Island Line shelf were not elevated in [dFe] (∼0.7–∼1.2 nM) relative to background offshore observations in spring or summer (Figure S5d and S5e in Supporting Information ).…”

“…This pool of dFe represents a potential source to basin waters, particularly when downwelling conditions facilitate its offshore transport (e.g., summer 2022 and 2023). Similar continental subsurface input of dFe from the Gulf of Alaska shelf was found southeast in British Columbia (Cullen et al., 2009; σ θ 26 to 27) and Southeast Alaska (GEOTRACES Intermediate Data Product Group, 2021) and further west off Kodiak Island (Sieber et al., 2024). Together, these observations indicate a spatially extensive shelf source of dFe to the central Gulf of Alaska.…”

Fluxes of particulate iron from the upper ocean around the Crozet Islands: A naturally iron-fertilized environment in the Southern Ocean

“…The release of Fe to biota via conversion of litho to BioFe has been previously suggested by Lam et al [2006], Lam and Bishop [2008], Frew et al [2006], and more recently Cullen et al [2009]. The present work strongly supports this hypothesis and the data also suggests that the BioFe derived from lithogenic iron has a longer residence time in the upper ocean than does the lithogenic iron which is not accessed by biota.…”

Trace Element Distributions in the Water Column near the Deepwater Horizon Well Blowout

“…For Fe, the situation is less straightforward. Fe did show a vertical trend of increasing concentrations for near-bottom samples (Figure a), as might be expected in a continental margin environment (e.g., and ). However, there was also a lateral trend in these submerged plume samples with lower dissolved Fe closer to the wellhead during late May 2010 (Figure b).…”

Drivers of Spatiotemporal Distributions of Macro‐ and Micronutrients (Fe, Ni, Cu, Zn) in the Northern Gulf of Alaska

“…Nearshore bottom samples (≤5 m from the seafloor) exhibited the largest [dFe] range in summer with less variability in spring and fall (Figure 5, Figure S5 in Supporting Information ). This near‐bottom dFe signal was carried offshore along an isopycnal layer σ θ ∼26.5–27.5 (e.g., Figure 9d), similar to observations on the British Columbia continental shelf/slope (Cullen et al., 2009), with NGA subsurface [dFe] gradually diminishing in the offshore direction (Figures 5d–5f). While this pattern was clear over the Seward and Middleton Island Lines, near‐seafloor samples from the Kodiak Island Line shelf were not elevated in [dFe] (∼0.7–∼1.2 nM) relative to background offshore observations in spring or summer (Figure S5d and S5e in Supporting Information ).…”

“…This pool of dFe represents a potential source to basin waters, particularly when downwelling conditions facilitate its offshore transport (e.g., summer 2022 and 2023). Similar continental subsurface input of dFe from the Gulf of Alaska shelf was found southeast in British Columbia (Cullen et al., 2009; σ θ 26 to 27) and Southeast Alaska (GEOTRACES Intermediate Data Product Group, 2021) and further west off Kodiak Island (Sieber et al., 2024). Together, these observations indicate a spatially extensive shelf source of dFe to the central Gulf of Alaska.…”

Fluxes of particulate iron from the upper ocean around the Crozet Islands: A naturally iron-fertilized environment in the Southern Ocean

“…The release of Fe to biota via conversion of litho to BioFe has been previously suggested by Lam et al [2006], Lam and Bishop [2008], Frew et al [2006], and more recently Cullen et al [2009]. The present work strongly supports this hypothesis and the data also suggests that the BioFe derived from lithogenic iron has a longer residence time in the upper ocean than does the lithogenic iron which is not accessed by biota.…”

Trace Element Distributions in the Water Column near the Deepwater Horizon Well Blowout

“…For Fe, the situation is less straightforward. Fe did show a vertical trend of increasing concentrations for near-bottom samples (Figure a), as might be expected in a continental margin environment (e.g., and ). However, there was also a lateral trend in these submerged plume samples with lower dissolved Fe closer to the wellhead during late May 2010 (Figure b).…”