Research ArticleOCEANOGRAPHY

Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

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Science Advances  09 Dec 2020:
Vol. 6, no. 50, eabd1654
DOI: 10.1126/sciadv.abd1654
  • Fig. 1 Modern hydrography and nutrient content of the northern Pacific and Atlantic Oceans.

    (A) Sea surface salinity, with sea surface temperature contours for 5° and 10 °C. (B) Surface phosphate concentration, with a sea surface height contour to denote the subpolar gyre boundary. (C) Subsurface phosphate concentration along a zonal section indicated by the dashed line in (B), with pre-bomb radiocarbon age contours. Note the relative isolation of the subpolar gyre in the North Pacific, which allows pooling of cold, fresh water in the surface, and old, nutrient-rich waters to upwell from below. In contrast, the active overturning circulation of the North Atlantic flushes warm, salty, nutrient-poor water from the subtropics through the upper reaches of this well-ventilated basin. Salinity, temperature, and phosphate data are gridded annual averages from WOA09; radiocarbon data from Global Ocean Data Analysis Project version 2 [GLODAPv2 (101)]; and sea surface height from (159).

  • Fig. 2 Reconstructions of changes in ventilation, export productivity, temperature, and salinity at the LGM relative to the Holocene.

    Red indicates an increase at the LGM relative to the Holocene, blue a decrease, and white no clear change outside of 1σ uncertainty (see the Supplementary Materials for details of data compilation). (A) Ventilation proxies, including δ13C (circles), radiocarbon (diamonds), and a variety of redox tracers (squares), taken from sites above 20°N. (B) Export productivity proxies, including sediment core opal (circles) and biogenic barium (squares) contents. (C) Sea surface temperature proxies, including planktic foraminiferal Mg/Ca (circles) and alkenone saturation index UK’37 (diamonds). (D) Change in sea surface salinity, derived from paired δ18O and Mg/Ca data on planktic foraminifera, corrected for whole-ocean δ18O and salinity changes due to ice volume. At the LGM, North Pacific intermediate waters are better ventilated, and productivity in the subpolar North Pacific is lower. An increase in salinity is seen throughout the North Pacific, and although the subtropical gyre cools, the subpolar North Pacific shows an anomalous warming, despite peak glacial conditions. See fig. S1 for an alternative presentation of these data.

  • Fig. 3 Profiles and meridional sections of proxies for deep ocean circulation.

    The δ18O profile (A) uses Cibicidoides spp. (circles) and Uvigerina spp. (squares), with Uvigerina data corrected for vital effects by –0.47‰ (160); the δ13C profile (B) uses Cibicidoides spp. only; profile data are from the NW Pacific (6). LGM δ13C data (in B and D) and δ18O are corrected for the –0.34 and 1‰ whole ocean changes in δ13C and δ18Osw (17). Generalized additive model fits are shown for Holocene δ13C and δ18O (dashed lines) and LGM δ13C (solid line) (161). The thin dotted-dashed line through the LGM δ18O data in (A) is the Holocene δ18O fit with a +0.6 ‰ offset; while the Holocene δ18O profile increases smoothly with depth, the LGM data exhibit a marked transition at ~2000 m, indicative of a water mass boundary. 14C data in (C) are shown as 14C age relative to the contemporaneous atmosphere, with modern data from GLODAPv2 and LGM data from benthic foraminfera (see Materials and Methods for details). δ13C section data in (D) are from the western Pacific (17)—see inset map. Locations of sediment core data are shown in black dots and areas of poor data coverage in white, and further details are given in figs. S2 and S3.

  • Fig. 4 cGENIE Earth system model experiments illustrating the impact of changes in North Pacific overturning on nutrient concentrations.

    Each symbol (A to C) represents a 5000-year-long model experiment under preindustrial (orange triangles) or glacial (blue diamonds) boundary conditions. Experiments are forced by decreasing the atmospheric freshwater flux into the North Pacific, which increases surface salinity and enhances PMOC (fig. S6). Output is plotted against the maximum meridional stream function in the North Pacific below 280 m. Phosphate concentrations are shown for (A) the surface layer (top 80 m) and (B) intermediate depths (928 to 1158 m) in the subpolar gyre (grid cells spanning 43°N to 66°N and 115°W to 225°W). Convection frequency (C), a count of convection events throughout the water column per model time step, is shown for the same region. Meridional sections of phosphate concentration (shading) at 165°W and Pacific meridional overturning stream function (contours) are shown in (D), for an experiment with a salinity forcing of −0.19 Sv and a PMOC of 8 Sv [indicated by a dashed line in (A) to (C)] that produces the best fit to our LGM data (Fig. 5 and fig. S10); anomalies of phosphate and overturning from the glacial base state are shown in (E); and the spread of North Pacific water at 1000-m depth, predominantly on the basin’s western boundary, is shown in (F).

  • Fig. 5 Simulated δ13C profiles in cGENIE compared to benthic δ13C data in the NW Pacific.

    Data (6) are shown as symbols with a general additive model fit shown by the gray dashed line. cGENIE simulations are given by the colored lines. To aid comparison, all profiles are shown as δ13C anomalies from the mean δ13C for each profile. Left: Late Holocene data and the experiments run under preindustrial boundary conditions. Right: LGM data and experiments run under glacial boundary conditions. The legend lists the maximum North Pacific overturning for each experiment. The upper portion of the LGM δ13C profile shows the closest match to the simulation run with an Atlantic to Pacific freshwater forcing of −0.19 Sv, which has a PMOC of 8 Sv. As the lower portions of the profiles likely remain ventilated by Southern Ocean waters, and as we have not made any changes to the Southern Ocean in our simulations, some offset at depth is to be expected.

  • Fig. 6 Schematic of ocean circulation and productivity in the modern and LGM North Pacific.

    The modern North Pacific (left) lacks vigorous local ventilation (thin black arrows), because of low salinity in surface waters of the subpolar gyre (SPG). This is a result of high net precipitation (P-E) and minimal exchange with the saltier waters of the subtropical gyre (STG). The modern subpolar North Pacific is thus dominated by upwelling of nutrient- and CO2-rich subsurface waters (gray vertical arrow), driving relatively high export productivity (green wavy arrows) and CO2 outgassing. During the LGM (right), our data compilation suggests that ventilation at intermediate depths was enhanced, export productivity was reduced, and subpolar surface waters were saltier and warmer (red wavy arrows). This is consistent with an invigorated meridional overturning circulation, with enhanced formation of intermediate waters and advection of warm, salty, and nutrient-depleted subtropical waters to high latitudes, analogous to a shallower version of the overturning circulation seen in the modern North Atlantic.

Supplementary Materials

  • Supplementary Materials

    Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

    J. W. B. Rae, W. R. Gray, R. C. J. Wills, I. Eisenman, B. Fitzhugh, M. Fotheringham, E. F. M. Littley, P. A. Rafter, R. Rees-Owen, A. Ridgwell, B. Taylor, A. Burke

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    • Figs. S1 to S14
    • Tables S1 and S2
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