I. Nicholas McCave (Cambridge, UK)
With 2 Figures
In a depth transect from 1200 m to 4900 m in the SW Pacific east of New Zealand, benthic carbon and oxygen isotopic profiles reflect the structure of water masses at present and in-ferred for the past. Between Holocene and Last Glacial Maximum (LGM) these have retained a constant structure of Lower Circumpolar Deep Water-Upper Circumpolar Deep Water/
North Pacific Deep Water-Antarctic Intermediate Water (LCDW, UCDW, NPDW, AAIW) with little apparent change in the depths of water mass boundaries between glacial and in-terglacial states (Fig. 1). Among the lowest values of LGM benthic δ13C in the world ocean (–1.03 ‰ based on Cibicidoides wüllerstorfi) occur here at ~2200 m. Comparable values occur in the Atlantic sector of the Southern Ocean (Hodell et al. 2006), while those from the North Pacific are distinctly higher, suggesting that the Southern Ocean, not Glacial NPDW was the source for the unventilated/nutrient-enriched water seen here. Oxygen and carbon isotopic data are compatible with a glacial cold deep water mass of high salinity, but lower nutrient content (or better ventilated), below ~3400 m depth. A stratum ~1500 –2000 m thick of old (ventilation age ~2.7 ka vs. 1.6 ka at present [Skinner et al. 2015]), nutrient-rich water (mean δ13C of – 0.84 ‰) entering the Pacific during the LGM is indicated by these data. This contrasts with the South Atlantic where unventilated/nutrient-enriched water with δ13C ~ 0.9
‰ extends down to the sea bed. The Ross Sea is the most likely source of the ventilated gla-cial deep water with δ13C of – 0.31 ‰ entering the Pacific below ~3500 m.
McCave et al. (2008) regarded the S Atlantic as the major source for the low δ13C water flowing in the ACC over the major sills above 3500 m depth around Kerguelen and the Mac-quarie-Balleny Ridges to arrive east of New Zealand in the deep western boundary current (DWBC). The top of this water mass at Chatham Rise was ~ 2000m (Fig. 1, 2). Neutral density surfaces slope downwards to the north, so that, presuming a similarly shaped glacial density structure (c.f. Ferrari et al. 2014), this low δ13C water, mixing with higher δ13C Ross Sea Bottom Water (RSBW) would become the bottom and deep water north of the equator, occupying much of the North Pacific, albeit with higher δ13C (Fig. 2). For a 50/50 RSBW/
UCDW mixture that would be 0.58 ‰, but data from the mid-latitude N Pacific gives values of ~0.2 ‰ (Herguera et al. 1992, Matsumoto et al. 2002), less even than the RSBW inflow value at Chatham rise (Fig. 1), so there remains an unresolved problem in relating the inflow to the interior of the deep N Pacific. The 2000 m boundary to the top of this low δ13C water appears to be universal (Atlantic and Pacific) and justified on theoretical grounds (Curry and Oppo 2005, Herguera et al. 1992, Ferrari et al. 2014). Flow speed results from both the Indian Ocean and SW Pacific DWBCs indicate little change in the flow, which could be in-terpreted as flux, between glacial and the present (McCave et al. 2008, Thomas et al. 2007).
The present inflow in the Pacific DWBC is ~ 16 Sv (Whitworth et al. 1999).
I. Nicholas McCave
156 Nova Acta Leopoldina NF 121, Nr. 408, 155 –157 (2015)
Fig. 1 Averaged depth profiles of δ18O and δ13C for the SW Pacific down the north side of Chatham Rise (40°– 42°S;
178°E–168°W). Left, Holocene; right, LGM. Oxygen, blue; carbon, green. No corrections for relation to modern water values or glacial terrigenous carbon influence have been applied. Note that the scale for δ18O runs continuously across the whole diagram with each panel 1.5 ‰ wide, and that the scale for δ13C is such that each panel is 3 ‰ wide.
N and S for AAIW sites refer to positions north and south of Chatham Rise.
Fig. 2 Glacial water masses deduced for the Chatham rise transect superimposed on the neutral density field (γn) of the N–S western Pacific WOCE line P15 along 170°W (Talley 2007). The top of the low δ13C watermass at 2000 m (gUCDW) is continued to the north at the same level, but the base at ~3400 m is shown descending to the north along present γn of 28.14 kg/m3. Below that is glacial Ross Sea Bottom Water (gRSBW) with mixing symbols along the upper interface. It is presumed that a similar transformation to North Pacific Deep Water occurred in the glacial as at present (gNPDW), the Pacific being a ‘dead-end’ ocean. The possibility of a North Pacific convection cell leading to intermediate water production is not shown.
A Carbon Isotope Perspective on the Glacial Circulation of the Deep Southwest Pacific
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References
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McCave, I. N., Carter, L., and Hall, I. R.: Glacial-interglacial changes in water mass structure and flow in the SW Pacific Ocean. Quat. Sci. Rev. 27, 1886 –1908 (2008)
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(Series eds. by M. Sparrow, P. Chapman and J. Gould). 326 pp. Southampton, U. K.: International WOCE Project Office 2007
Thomas, A. L., Henderson, G. M., and McCave, I. N.: Constant bottom-water flow into the Indian Ocean for the past 140 ka indicated by sediment 231Pa/230Th ratios. Paleoceanography 22, PA4210 (2007)
Whitworth, T., Warren, B. A., Nowlin, W. D., Rutz, S. B., Pillsbury, R. D., and Moore, M. I.: On the deep western-boundary current in the Southwest Pacific Basin. Progr. Oceanogr. 43, 1–54 (1999)
Prof. I. Nicholas McCave, Sc.D.
University of Cambridge
Godwin Laboratory for Palaeoclimate Research Department of Earth Sciences
Downing Street Cambridge, CB2 3EQ UK
Phone: +44 1223 333422 Fax: +44 1223 333450 E-Mail: mccave@esc.cam.ac.uk
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