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Radium isotopes as tracers of iron sources fueling a Southern Ocean phytoplankton bloom

Radium isotopes as tracers of iron sources fueling a Southern Ocean phytoplankton bloom
Radium isotopes as tracers of iron sources fueling a Southern Ocean phytoplankton bloom
Elevated levels of productivity in the wake of Southern Ocean island systems are common despite the fact that they are encircled by high-nutrient low-chlorophyll (HNLC) waters. In the Crozet Plateau region, it has been hypothesized that iron from island runoff or sediments of the plateau could be fueling the austral summer phytoplankton bloom. Here, we use radium isotopes to quantify the rates of surface-ocean iron supply fueling the bloom in the Crozet Plateau region. A 1-D eddy-diffusion-mixing model applied to a 228Ra profile (t1/2=5.75 years) at a station north of the islands suggests fast vertical mixing in the upper 300 m (Kz=11–100 cm2 s?1) with slower mixing between 300 and 1000 m (Kz=1.5 cm2 s?1). This estimate is discussed in the context of Kz derived from the CTD/LADCP data. In combination with the dissolved Fe profile at this location, we estimated a vertical flux of between 5.6 and 31 nmol Fe m?2 d?1. The cross-plateau gradients in the short-lived radium isotopes, 224Ra (t1/2=3.66 d) and 223Ra (t1/2=11.4 d), yielded horizontal eddy diffusivities (Kh) of 39 and 6.6 m2 s?1, respectively. If we assume that the islands (surface runoff) alone were supplying dissolved Fe to the bloom region, then the flux estimates range from 2.3 to 14 nmol Fe m?2 d?1. If the plateau sediments are considered a source of Fe, and conveyed to the bloom region through deep winter mixing combined with horizontal transport, then this flux may be as high as 64–390 nmol Fe m?2 d?1. Combined, these Fe sources are sufficient to initiate and maintain the annual phytoplankton bloom.
Radium isotopes, Iron, Productivity, Ocean mixing, Southern Ocean
0967-0645
1989-1998
Charette, M.A.
c0850a57-65a0-4429-b2f0-23a18f301f21
Gonneea, M.E.
4892de3e-ffcb-46cf-835d-7030de670c58
Morris, P.J.
efc0a494-3d80-4116-8010-c2dd5d1bbdee
Statham, P.
51458f15-d6e2-4231-8bba-d0567f9e440c
Fones, G.
587ca638-e1ce-4832-b91b-f48f126c1a5e
Planquette, H.
cf8631d8-c16a-4ee4-8713-736766936d24
Salter, I.
805cba99-e754-417b-aac1-cee3a259aced
Naveira Garabato, A.C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Charette, M.A.
c0850a57-65a0-4429-b2f0-23a18f301f21
Gonneea, M.E.
4892de3e-ffcb-46cf-835d-7030de670c58
Morris, P.J.
efc0a494-3d80-4116-8010-c2dd5d1bbdee
Statham, P.
51458f15-d6e2-4231-8bba-d0567f9e440c
Fones, G.
587ca638-e1ce-4832-b91b-f48f126c1a5e
Planquette, H.
cf8631d8-c16a-4ee4-8713-736766936d24
Salter, I.
805cba99-e754-417b-aac1-cee3a259aced
Naveira Garabato, A.C.
97c0e923-f076-4b38-b89b-938e11cea7a6

Charette, M.A., Gonneea, M.E., Morris, P.J., Statham, P., Fones, G., Planquette, H., Salter, I. and Naveira Garabato, A.C. (2007) Radium isotopes as tracers of iron sources fueling a Southern Ocean phytoplankton bloom. Deep Sea Research Part II: Topical Studies in Oceanography, 54 (18-20), 1989-1998. (doi:10.1016/j.dsr2.2007.06.003).

Record type: Article

Abstract

Elevated levels of productivity in the wake of Southern Ocean island systems are common despite the fact that they are encircled by high-nutrient low-chlorophyll (HNLC) waters. In the Crozet Plateau region, it has been hypothesized that iron from island runoff or sediments of the plateau could be fueling the austral summer phytoplankton bloom. Here, we use radium isotopes to quantify the rates of surface-ocean iron supply fueling the bloom in the Crozet Plateau region. A 1-D eddy-diffusion-mixing model applied to a 228Ra profile (t1/2=5.75 years) at a station north of the islands suggests fast vertical mixing in the upper 300 m (Kz=11–100 cm2 s?1) with slower mixing between 300 and 1000 m (Kz=1.5 cm2 s?1). This estimate is discussed in the context of Kz derived from the CTD/LADCP data. In combination with the dissolved Fe profile at this location, we estimated a vertical flux of between 5.6 and 31 nmol Fe m?2 d?1. The cross-plateau gradients in the short-lived radium isotopes, 224Ra (t1/2=3.66 d) and 223Ra (t1/2=11.4 d), yielded horizontal eddy diffusivities (Kh) of 39 and 6.6 m2 s?1, respectively. If we assume that the islands (surface runoff) alone were supplying dissolved Fe to the bloom region, then the flux estimates range from 2.3 to 14 nmol Fe m?2 d?1. If the plateau sediments are considered a source of Fe, and conveyed to the bloom region through deep winter mixing combined with horizontal transport, then this flux may be as high as 64–390 nmol Fe m?2 d?1. Combined, these Fe sources are sufficient to initiate and maintain the annual phytoplankton bloom.

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More information

Published date: 2007
Keywords: Radium isotopes, Iron, Productivity, Ocean mixing, Southern Ocean

Identifiers

Local EPrints ID: 42479
URI: https://eprints.soton.ac.uk/id/eprint/42479
ISSN: 0967-0645
PURE UUID: 61f7fb8e-4dd4-4ebb-a244-397131895191

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Date deposited: 13 Dec 2006
Last modified: 13 Mar 2019 21:11

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