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The oxygen isotope composition of water masses in the northern North Atlantic

The oxygen isotope composition of water masses in the northern North Atlantic
The oxygen isotope composition of water masses in the northern North Atlantic
The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, 18O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their 18O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of 18O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in 18O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of 18O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in 18O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ?250 km3 is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, 18O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, 18O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in 18O–salinity space, whereas NWABW lies between NEADW and LSW on 18O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has 18O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.
WATER MASSES, WOCE, OXYGEN ISOTOPES, METEORIC WATER, CRUISE 62 1991, "CHARLES DARWIN"
0967-0637
2265-2286
Frew, R.D.
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Dennis, P.F.
a3284c51-dd05-4e9a-8b3e-126ba20748fa
Heywood, K.J.
4eeebe72-3857-4729-8d6a-7ece46d37cd5
Meredith, M.P.
e750017c-3619-4103-8a9a-dd299173e42b
Boswell, S.M.
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Frew, R.D.
e6f76497-4288-4caa-8ddb-52cb3f246e50
Dennis, P.F.
a3284c51-dd05-4e9a-8b3e-126ba20748fa
Heywood, K.J.
4eeebe72-3857-4729-8d6a-7ece46d37cd5
Meredith, M.P.
e750017c-3619-4103-8a9a-dd299173e42b
Boswell, S.M.
7bdb812b-6553-48ed-b15f-0a09fe3243f6

Frew, R.D., Dennis, P.F., Heywood, K.J., Meredith, M.P. and Boswell, S.M. (2000) The oxygen isotope composition of water masses in the northern North Atlantic. Deep Sea Research Part I: Oceanographic Research Papers, 47 (12), 2265-2286. (doi:10.1016/S0967-0637(00)00023-6).

Record type: Article

Abstract

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, 18O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their 18O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of 18O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in 18O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of 18O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in 18O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ?250 km3 is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, 18O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, 18O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in 18O–salinity space, whereas NWABW lies between NEADW and LSW on 18O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has 18O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.

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Published date: 2000
Keywords: WATER MASSES, WOCE, OXYGEN ISOTOPES, METEORIC WATER, CRUISE 62 1991, "CHARLES DARWIN"

Identifiers

Local EPrints ID: 8903
URI: http://eprints.soton.ac.uk/id/eprint/8903
ISSN: 0967-0637
PURE UUID: 832b54b1-3bf4-4126-b97a-e88ddf04f1ef

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Date deposited: 10 Sep 2004
Last modified: 15 Jul 2019 19:38

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Author: R.D. Frew
Author: P.F. Dennis
Author: K.J. Heywood
Author: M.P. Meredith
Author: S.M. Boswell

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