Research ArticlePALEOCEANOGRAPHY

Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene

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Science Advances  13 Sep 2017:
Vol. 3, no. 9, e1700156
DOI: 10.1126/sciadv.1700156
  • Fig. 1 Changes in Northern Pacific climate over the last 6 million years.

    (A) Benthic δ18O record (62) showing glacial cycles in global ice volume and deep-ocean temperature with smoothed trend line superimposed. PDB, Pee Dee Belemnite. (B) Magnetic susceptibility of ocean sediments at ODP Site 882 (8) in the subarctic Pacific (see Fig. 3 for site location). This nondimensional variable increases in the presence of ice-rafted debris from Northern Hemisphere glaciers. (C) Biogenic opal (8) and (D) CaCO3 mass accumulation rates (MAR) at ODP Site 882 (11). The light gray line in (D) indicates the range in high-latitude North Atlantic, Holocene CaCO3 accumulation from International ODP Site U1313 (63). (C and D) These data indicate that, during parts of the Pliocene, the North Pacific was both opal- and carbonate-rich, requiring that the previously noted vertical exchange supplying silicate to surface waters also included vertical mixing that reached to the seafloor at the core site (3244 m). (E) Focusing in on the 2.73-Ma transition, CaCO3 starts to decline before the opal flux, with discrete peaks that decline in amplitude until the sediments were essentially CaCO3-free. This signals a decline in the depth of winter deep convection events over a period when wintertime surface/subsurface exchange was still adequately great to import high concentrations of silicate to the surface.

  • Fig. 2 Redox-sensitive trace metal records from ODP Site 882 provide complementary evidence that deep-ocean oxygen concentrations were higher before the 2.73-Ma transition.

    (A) U and V as a function of Al concentrations with values before the 2.73-Ma transition indicated by green shading. wt %, weight %. (B) Al, (C) authigenic U (aU) and V, and (D) opal and organic carbon (Corg) changes across the 2.73-Ma transition, before which U and V concentrations were low despite the export and diagenetic remineralization of organic matter being high, suggesting oxygenated conditions.

  • Fig. 3 NPDW formation in Pliocene simulation.

    (A and B) Pliocene changes in large-scale SST gradients and meridional atmospheric circulation (red arrows) support deep-water formation and ocean overturning cells in both the Atlantic and Pacific basins. (C) Changes in sea surface salinity (SSS) between the preindustrial and Pliocene-like simulations. (D and E) January-February-March climatological mixed-layer depths (MLD) for (D) the preindustrial control and (E) the Pliocene-like simulation. We see the appearance of substantially deeper mixed-layer depths due to winter deep-water formation in the North Pacific. ODP Site 882 (50°22′N, 167°36′E), from which the records in Figs. 1 and 2 originate, is in the region of simulated NPDW formation.

  • Fig. 4 Weak SST gradients and subsurface warming in Pliocene simulation.

    Annual mean SSTs for (A) the preindustrial control and (B) the Pliocene-like experiment. Note the reduced meridional and zonal large-scale SST gradients within the Pliocene-like simulation [see the study by Burls and Fedorov (34) for a detailed comparison with available SST reconstructions from locations around the globe]. (C and D) Zonal-mean ocean temperature change between the Pliocene-like and control simulations for (C) the Pacific and (D) the Atlantic. Superimposed circles show estimated Pliocene temperature changes based on available reconstructions. Consistent with North Atlantic temperature reconstructions from Site 607 (36) and Site 926 (37), the deep ocean is 2° to 3°C warmer in Pliocene simulation. In line with the estimates by Lear et al. (37) from Site 806, waters between 2000 and 3000 m in the Pacific are 2° to 3°C warmer. Below 3000 m, the deep-ocean warming is less in the Pacific Ocean than in the Atlantic, with bottom-water temperatures only between 1° and 2°C warmer than the control, which is arguably consistent with recent Pliocene bottom-water temperature estimates (40).

  • Fig. 5 PMOC and NPDW formation in Pliocene simulation.

    Indo-Pacific zonal-mean stream functions for (A) the preindustrial control and (B) the Pliocene-like simulation. The adjusted oceanic state within our simulation with a Pliocene-like SST pattern supports meridional overturning in the Pacific basin. Zonal-mean Pacific salinity for (C) the preindustrial control and (D) the Pliocene-like simulation shows NPDW (fresher than other deep waters) forming in the SNP and filling the North Pacific between ~1000 and 3000 m. AAIW, Antarctic Intermediate Water; NPIW, North Pacific Intermediate Water; PDW, Pacific Deep Water; CDW, Circumpolar Deep Water.

  • Fig. 6 Simulated North Pacific ventilation changes versus observed Pliocene δ13C differences from the Late Pleistocene interglacial values.

    Pliocene-control changes in ventilation age along (A) 158°E (the longitude of ODP Sites 586 and 1208) and (B) the average between 111° and 123°W (the longitude range of ODP Sites 849, 1014, and 1018). Superimposed circles show Pliocene δ13C differences from Late Pleistocene interglacial values for Sites 586, 849, 1014, and 1018 [see Table 2 of Ravelo and Andreasen (47)] and for Site 2018 [(40); using the same approach as by Ravelo and Andreasen (47) for the 1208 record, with average Holocene values serving as the Late Pleistocene estimate, and 3.1- to 3.37-Ma values as the Pliocene estimate, and with Site 849 used to correct for global ocean δ13C changes]. Note that whereas the PMOC results in varying degrees of enhanced North Pacific ventilation for all depths above ~3500 m, the presence of a deep western boundary current (see fig. S15) leads to increased ventilation in the western Pacific, thus accounting for the higher Pliocene δ13C observed at Site 586.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1700156/DC1

    Supplementary Text

    fig. S1. Deep-ocean ventilation as the driver of CaCO3 cycles.

    fig. S2. Pliocene variations in porphyrin and TOC concentrations at Site 882.

    fig. S3. Oceanic adjustment and the development of a PMOC.

    fig. S4. Atlantic zonal-mean stream functions.

    fig. S5. Simulated changes in ocean salinity.

    fig. S6. Simulated and observed temperature, salinity, and density profiles at Site 882.

    fig. S7. Simulated precipitation minus evaporation changes.

    fig. S8. Atmospheric moisture transport and oceanic salinity budget.

    fig. S9. Simulated surface density change between the preindustrial and Pliocene-like experiments.

    fig. S10. Simulated January-February-March potential density profiles at the approximate location of Site 882.

    fig. S11. Vertical velocities across ~800 m.

    fig. S12. Simulated changes in ocean static stability.

    fig. S13. Zonal mean Pacific vertical diffusivity (cm2/s).

    fig. S14. Vertical diffusivity at ~800 m.

    fig. S15. Change in North Pacific deep-ocean (1000 to 2000 m) currents between the Pliocene and preindustrial control experiments.

    fig. S16. Variability in the PMOC.

    fig. S17. Simulated zonal mean meridional heat transport.

    fig. S18. Simulated surface currents and theoretical wind-driven Sverdrup flow.

    table S1. List of the salt budget variables.

    table S2. The contribution of the change in each variable, δζi, to the change in salinity, ∑iΔζi, of the Subarctic and midlatitude regions between the control and Pliocene simulations, as shown in fig. S8B.

    Site 882 Dataset

    Reference (64)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Deep-ocean ventilation as the driver of CaCO3 cycles.
    • fig. S2. Pliocene variations in porphyrin and TOC concentrations at Site 882.
    • fig. S3. Oceanic adjustment and the development of a PMOC.
    • fig. S4. Atlantic zonal-mean stream functions.
    • fig. S5. Simulated changes in ocean salinity.
    • fig. S6. Simulated and observed temperature, salinity, and density profiles at Site 882.
    • fig. S7. Simulated precipitation minus evaporation changes.
    • fig. S8. Atmospheric moisture transport and oceanic salinity budget.
    • fig. S9. Simulated surface density change between the preindustrial and Pliocene-like experiments.
    • fig. S10. Simulated January-February-March potential density profiles at the approximate location of Site 882.
    • fig. S11. Vertical velocities across ~800 m.
    • fig. S12. Simulated changes in ocean static stability.
    • fig. S13. Zonal mean Pacific vertical diffusivity (cm2/s).
    • fig. S14. Vertical diffusivity at ~800 m.
    • fig. S15. Change in North Pacific deep-ocean (1000 to 2000 m) currents between the Pliocene and preindustrial control experiments.
    • fig. S16. Variability in the PMOC.
    • fig. S17. Simulated zonal mean meridional heat transport.
    • fig. S18. Simulated surface currents and theoretical wind-driven Sverdrup flow.
    • table S1. List of the salt budget variables.
    • table S2. The contribution of the change in each variable, δζi, to the change in salinity, ΣiΔζi, of the Subarctic and midlatitude regions between the control and Pliocene simulations, as shown in fig. S8B.
    • Reference (64)

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