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The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean

The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean
The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean

Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600–900 m, 35 pmol kg−1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207Pb = 1.1818–1.1824, 208Pb/207Pb = 2.4472–2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of ∼50–60 years. In contrast, North Atlantic Deep Water (2000–4000 m water depth) featured lower Pb concentrations and isotope compositions (206Pb/207Pb = 1.1762–1.184, 208Pb/207Pb = 2.4482–2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of ∼80–100 years. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distributions is particularly apparent in seawater in the Tropical Atlantic Ocean which is ventilated from the southern hemisphere. In particular, South Atlantic Central Water and Antarctic Intermediate Water were dominated by anthropogenic Pb emitted during the last 50–100 years, despite estimates of much older average ventilation ages in this region.

Anthropogenic Pb, Ocean circulation, Pb isotopes, Transient tracers
0016-7037
36-51
Bridgestock, Luke
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Rehkämper, Mark
08591eef-a2d5-48de-9fc6-b5c0cf54f6e5
van de Flierdt, Tina
aa665040-dc12-4a4a-bc7f-9be69da6b280
Paul, Maxence
80b65f26-9dd5-48cc-bf7d-171920915053
Milne, Angela
39b44fbd-8f5e-40ea-80c9-244c20998020
Lohan, Maeve C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9
Bridgestock, Luke
e42dd641-55c5-4c8a-ae45-314db73a4e3b
Rehkämper, Mark
08591eef-a2d5-48de-9fc6-b5c0cf54f6e5
van de Flierdt, Tina
aa665040-dc12-4a4a-bc7f-9be69da6b280
Paul, Maxence
80b65f26-9dd5-48cc-bf7d-171920915053
Milne, Angela
39b44fbd-8f5e-40ea-80c9-244c20998020
Lohan, Maeve C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9

Bridgestock, Luke, Rehkämper, Mark, van de Flierdt, Tina, Paul, Maxence, Milne, Angela, Lohan, Maeve C. and Achterberg, Eric P. (2018) The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean. Geochimica et Cosmochimica Acta, 225, 36-51. (doi:10.1016/j.gca.2018.01.018).

Record type: Article

Abstract

Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600–900 m, 35 pmol kg−1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207Pb = 1.1818–1.1824, 208Pb/207Pb = 2.4472–2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of ∼50–60 years. In contrast, North Atlantic Deep Water (2000–4000 m water depth) featured lower Pb concentrations and isotope compositions (206Pb/207Pb = 1.1762–1.184, 208Pb/207Pb = 2.4482–2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of ∼80–100 years. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distributions is particularly apparent in seawater in the Tropical Atlantic Ocean which is ventilated from the southern hemisphere. In particular, South Atlantic Central Water and Antarctic Intermediate Water were dominated by anthropogenic Pb emitted during the last 50–100 years, despite estimates of much older average ventilation ages in this region.

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Accepted/In Press date: 14 January 2018
e-pub ahead of print date: 2 February 2018
Published date: 15 March 2018
Keywords: Anthropogenic Pb, Ocean circulation, Pb isotopes, Transient tracers

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Local EPrints ID: 418090
URI: https://eprints.soton.ac.uk/id/eprint/418090
ISSN: 0016-7037
PURE UUID: f54c6f3d-1aa4-4c65-bf5f-3a6a7f3f035b

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Date deposited: 22 Feb 2018 17:30
Last modified: 13 Mar 2019 18:50

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Author: Luke Bridgestock
Author: Mark Rehkämper
Author: Tina van de Flierdt
Author: Maxence Paul
Author: Angela Milne
Author: Maeve C. Lohan

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