Variable water column structure of the South Atlantic on glacial-interglacial time scales

Gema Martinez-Mendez, Elizabeth Molyneux, Ian R. Hall, Rainer Zahn

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more delta C-13 enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or 'chemocline', at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal delta C-13. Cd/Ca and derived carbon isotope air-sea exchange signature (delta C-13(as)) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic delta C-13 data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43 degrees S) linearly decrease with water depth, indicative of mixing between upper 'well' and lower 'poorly' ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic delta C-13 data south of the LGM-SAF below 2500 m water depth are uniformly around -0.8 parts per thousand. The apparent delta C-13 gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic delta C-13 and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive delta C-13 and delta C-13(as) signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on delta C-13 must be taken into consideration when assessing the implications of the vertical delta C-13 gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO2 and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Original languageEnglish
Pages (from-to)3379-3387
Number of pages9
JournalQuaternary Science Reviews
Volume28
Issue number27-28
Early online date22 Oct 2009
DOIs
Publication statusPublished - Dec 2009

Keywords

  • Benthic foraminiferal CD/CA
  • sea-surface temperatures
  • deep-water
  • ocean circulation
  • carbon-isotope
  • atmospheric CO2
  • delta-C-13
  • ice
  • variability
  • records

Cite this

Variable water column structure of the South Atlantic on glacial-interglacial time scales. / Martinez-Mendez, Gema; Molyneux, Elizabeth; Hall, Ian R.; Zahn, Rainer.

In: Quaternary Science Reviews, Vol. 28, No. 27-28, 12.2009, p. 3379-3387.

Research output: Contribution to journalArticle

Martinez-Mendez, Gema ; Molyneux, Elizabeth ; Hall, Ian R. ; Zahn, Rainer. / Variable water column structure of the South Atlantic on glacial-interglacial time scales. In: Quaternary Science Reviews. 2009 ; Vol. 28, No. 27-28. pp. 3379-3387.
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T1 - Variable water column structure of the South Atlantic on glacial-interglacial time scales

AU - Martinez-Mendez, Gema

AU - Molyneux, Elizabeth

AU - Hall, Ian R.

AU - Zahn, Rainer

PY - 2009/12

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N2 - The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more delta C-13 enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or 'chemocline', at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal delta C-13. Cd/Ca and derived carbon isotope air-sea exchange signature (delta C-13(as)) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic delta C-13 data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43 degrees S) linearly decrease with water depth, indicative of mixing between upper 'well' and lower 'poorly' ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic delta C-13 data south of the LGM-SAF below 2500 m water depth are uniformly around -0.8 parts per thousand. The apparent delta C-13 gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic delta C-13 and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive delta C-13 and delta C-13(as) signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on delta C-13 must be taken into consideration when assessing the implications of the vertical delta C-13 gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO2 and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

AB - The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more delta C-13 enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or 'chemocline', at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal delta C-13. Cd/Ca and derived carbon isotope air-sea exchange signature (delta C-13(as)) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic delta C-13 data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43 degrees S) linearly decrease with water depth, indicative of mixing between upper 'well' and lower 'poorly' ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic delta C-13 data south of the LGM-SAF below 2500 m water depth are uniformly around -0.8 parts per thousand. The apparent delta C-13 gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic delta C-13 and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive delta C-13 and delta C-13(as) signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on delta C-13 must be taken into consideration when assessing the implications of the vertical delta C-13 gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO2 and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

KW - Benthic foraminiferal CD/CA

KW - sea-surface temperatures

KW - deep-water

KW - ocean circulation

KW - carbon-isotope

KW - atmospheric CO2

KW - delta-C-13

KW - ice

KW - variability

KW - records

U2 - 10.1016/j.quascirev.2009.09.022

DO - 10.1016/j.quascirev.2009.09.022

M3 - Article

VL - 28

SP - 3379

EP - 3387

JO - Quaternary Science Reviews

JF - Quaternary Science Reviews

SN - 0277-3791

IS - 27-28

ER -