Physical and Biogeochemical Controls on the Distribution of Dissolved Cadmium and its Isotopes in the Southwest Pacific Ocean

M. Sieber; Tim M. Conway; G. F. de Souza; H. Obata; S. Takano; Y. Sohrin; D. Vance

doi:10.1016/j.chemgeo.2018.07.021

Physical and Biogeochemical Controls on the Distribution of Dissolved Cadmium and its Isotopes in the Southwest Pacific Ocean

M. Sieber, Tim M. Conway, G. F. de Souza, H. Obata, S. Takano, Y. Sohrin, D. Vance

University of South Florida

Research output: Contribution to journal › Article › peer-review

Abstract

<a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/cadmium" title="Learn more about Cadmium from ScienceDirect's AI-generated Topic Pages"> Cadmium </a> <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/stable-isotope" title="Learn more about Stable Isotope from ScienceDirect's AI-generated Topic Pages"> stable isotope </a> ratios (δ 114 Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/trace-metal" title="Learn more about Trace Metal from ScienceDirect's AI-generated Topic Pages"> trace metal </a> cadmium (Cd) throughout the oceans. Here, we present a meridional <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/transect" title="Learn more about Transect from ScienceDirect's AI-generated Topic Pages"> transect </a> of dissolved Cd and δ 114 Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg −1 ) and a homogeneous δ 114 Cd signature (+0.26 ± 0.06&permil;, 2SD, n = 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ 114 Cd of +0.26 ± 0.10&permil; (2SD, n = 436). Higher in the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/water-column" title="Learn more about Water Column from ScienceDirect's AI-generated Topic Pages"> water column </a> , depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg −1 ) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg −1 in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ 114 Cd, with values higher (+0.4 to +0.6&permil;) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ 114 Cd values (up to +0.9&permil;) in the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/ocean-surface" title="Learn more about Ocean Surface from ScienceDirect's AI-generated Topic Pages"> surface ocean </a> . Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/phytoplankton" title="Learn more about Phytoplankton from ScienceDirect's AI-generated Topic Pages"> phytoplankton </a> , and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/antarctic-intermediate-water" title="Learn more about Antarctic Intermediate Water from ScienceDirect's AI-generated Topic Pages"> Antarctic Intermediate Water </a> (AAIW) and Subantarctic <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mode-water" title="Learn more about Mode Water from ScienceDirect's AI-generated Topic Pages"> Mode Water </a> (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ 114 Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/tropics" title="Learn more about Tropics from ScienceDirect's AI-generated Topic Pages"> lower latitude </a> Pacific as these water masses travel northward. Overall, the distribution of Cd and δ 114 Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ 114 Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ 114 Cd signature of equatorial <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/intermediate-water" title="Learn more about Intermediate Water from ScienceDirect's AI-generated Topic Pages"> intermediate water masses </a> shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/remineralization" title="Learn more about Remineralization from ScienceDirect's AI-generated Topic Pages"> remineralization </a> . We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ 114 Cd signal. These observations enable us to constrain the net δ 114 Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotopic-composition" title="Learn more about Isotopic Composition from ScienceDirect's AI-generated Topic Pages"> isotopic compositions </a> to deep waters. Second, at GP19 stations close to the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/equator" title="Learn more about Equator from ScienceDirect's AI-generated Topic Pages"> equator </a> , subtle Cd <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/depletion" title="Learn more about Depletion from ScienceDirect's AI-generated Topic Pages"> depletion </a> (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North Pacific. Discerning the effects of such processes on Cd isotopic distributions is an important step to a more detailed understanding of the biogeochemical cycling of Cd in the modern ocean, and the application of δ 114 Cd as a tracer of past deep <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/water-circulation" title="Learn more about Water Circulation from ScienceDirect's AI-generated Topic Pages"> water circulation </a> . This article is part of a special issue entitled: “Cycles of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/trace-element" title="Learn more about Trace Element from ScienceDirect's AI-generated Topic Pages"> trace elements </a> and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotope" title="Learn more about Isotope from ScienceDirect's AI-generated Topic Pages"> isotopes </a> in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Original language	American English
Journal	Chemical Geology
Volume	511
DOIs	https://doi.org/10.1016/j.chemgeo.2018.07.021
State	Published - Apr 20 2019

Keywords

Trace metals
Biogeochemistry
GEOTRACES
GP19 section
Water mass mixing
Complete utilization

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https://doi.org/10.1016/j.chemgeo.2018.07.021

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@article{23d31da0ecdb48c99b36ced51e836b46,

title = "Physical and Biogeochemical Controls on the Distribution of Dissolved Cadmium and its Isotopes in the Southwest Pacific Ocean",

abstract = " Cadmium stable isotope ratios (δ 114 Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive trace metal cadmium (Cd) throughout the oceans. Here, we present a meridional transect of dissolved Cd and δ 114 Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg −1 ) and a homogeneous δ 114 Cd signature (+0.26 ± 0.06&permil;, 2SD, n = 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ 114 Cd of +0.26 ± 0.10&permil; (2SD, n = 436). Higher in the water column , depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg −1 ) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg −1 in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ 114 Cd, with values higher (+0.4 to +0.6&permil;) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ 114 Cd values (up to +0.9&permil;) in the surface ocean . Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by phytoplankton , and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ 114 Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the lower latitude Pacific as these water masses travel northward. Overall, the distribution of Cd and δ 114 Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ 114 Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ 114 Cd signature of equatorial intermediate water masses shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to remineralization . We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ 114 Cd signal. These observations enable us to constrain the net δ 114 Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different isotopic compositions to deep waters. Second, at GP19 stations close to the equator , subtle Cd depletion (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North Pacific. Discerning the effects of such processes on Cd isotopic distributions is an important step to a more detailed understanding of the biogeochemical cycling of Cd in the modern ocean, and the application of δ 114 Cd as a tracer of past deep water circulation . This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González. ",

keywords = "Trace metals, Biogeochemistry, GEOTRACES, GP19 section, Water mass mixing, Complete utilization",

author = "M. Sieber and Conway, {Tim M.} and {de Souza}, {G. F.} and H. Obata and S. Takano and Y. Sohrin and D. Vance",

year = "2019",

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doi = "10.1016/j.chemgeo.2018.07.021",

language = "American English",

volume = "511",

journal = "Chemical Geology",

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TY - JOUR

T1 - Physical and Biogeochemical Controls on the Distribution of Dissolved Cadmium and its Isotopes in the Southwest Pacific Ocean

AU - Sieber, M.

AU - Conway, Tim M.

AU - de Souza, G. F.

AU - Obata, H.

AU - Takano, S.

AU - Sohrin, Y.

AU - Vance, D.

PY - 2019/4/20

Y1 - 2019/4/20

N2 - Cadmium stable isotope ratios (δ 114 Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive trace metal cadmium (Cd) throughout the oceans. Here, we present a meridional transect of dissolved Cd and δ 114 Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg −1 ) and a homogeneous δ 114 Cd signature (+0.26 ± 0.06&permil;, 2SD, n = 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ 114 Cd of +0.26 ± 0.10&permil; (2SD, n = 436). Higher in the water column , depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg −1 ) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg −1 in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ 114 Cd, with values higher (+0.4 to +0.6&permil;) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ 114 Cd values (up to +0.9&permil;) in the surface ocean . Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by phytoplankton , and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ 114 Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the lower latitude Pacific as these water masses travel northward. Overall, the distribution of Cd and δ 114 Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ 114 Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ 114 Cd signature of equatorial intermediate water masses shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to remineralization . We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ 114 Cd signal. These observations enable us to constrain the net δ 114 Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different isotopic compositions to deep waters. Second, at GP19 stations close to the equator , subtle Cd depletion (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North Pacific. Discerning the effects of such processes on Cd isotopic distributions is an important step to a more detailed understanding of the biogeochemical cycling of Cd in the modern ocean, and the application of δ 114 Cd as a tracer of past deep water circulation . This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

AB - Cadmium stable isotope ratios (δ 114 Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive trace metal cadmium (Cd) throughout the oceans. Here, we present a meridional transect of dissolved Cd and δ 114 Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg −1 ) and a homogeneous δ 114 Cd signature (+0.26 ± 0.06&permil;, 2SD, n = 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ 114 Cd of +0.26 ± 0.10&permil; (2SD, n = 436). Higher in the water column , depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg −1 ) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg −1 in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ 114 Cd, with values higher (+0.4 to +0.6&permil;) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ 114 Cd values (up to +0.9&permil;) in the surface ocean . Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by phytoplankton , and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ 114 Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the lower latitude Pacific as these water masses travel northward. Overall, the distribution of Cd and δ 114 Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ 114 Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ 114 Cd signature of equatorial intermediate water masses shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to remineralization . We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ 114 Cd signal. These observations enable us to constrain the net δ 114 Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different isotopic compositions to deep waters. Second, at GP19 stations close to the equator , subtle Cd depletion (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North Pacific. Discerning the effects of such processes on Cd isotopic distributions is an important step to a more detailed understanding of the biogeochemical cycling of Cd in the modern ocean, and the application of δ 114 Cd as a tracer of past deep water circulation . This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

KW - Trace metals

KW - Biogeochemistry

KW - GEOTRACES

KW - GP19 section

KW - Water mass mixing

KW - Complete utilization

UR - https://digitalcommons.usf.edu/geo_facpub/1491

UR - https://doi.org/10.1016/j.chemgeo.2018.07.021

U2 - 10.1016/j.chemgeo.2018.07.021

DO - 10.1016/j.chemgeo.2018.07.021

M3 - Article

VL - 511

JO - Chemical Geology

JF - Chemical Geology

ER -

Physical and Biogeochemical Controls on the Distribution of Dissolved Cadmium and its Isotopes in the Southwest Pacific Ocean

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