Thermohaline circulation crisis and impacts during the mid-Pleistocene transition

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Science  18 Jul 2014:
Vol. 345, Issue 6194, pp. 318-322
DOI: 10.1126/science.1249770

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  1. Fig. 1 Meridional hydrographic profile in the present-day South Atlantic showing the different water masses based on the salinity distribution (World Ocean Atlas).

    ODP sites 1088 and 1090 are located at the upper and lower boundaries of the southward-flowing NADW wedge, which is partly mixed with northward-flowing CDW and Antarctic Intermediate Water (AAIW). White isolines correspond to the modern South Atlantic seawater εNd (29) and show that NADW has more-negative εNd (–10 to –10.5), whereas present-day CDW and AAIW have higher εNd (–8.5 to –9), which reflects the mixing of Atlantic and Pacific source waters (23). psu, practical salinity unit.

  2. Fig. 2 Glacial-interglacial εNd in the South Atlantic (Cape Basin) at mid-depth (site 1088) and deep (site 1090) sites through the MPT (~1100 to 600 ka).

    Red and blue lines connect data for samples from interglacial and glacial maximum, respectively. Error bars indicate the propagated error (2σ) of the εNd measurements. (A and B) ODP site 1088 (purple) and site 1090 (pink) benthic foraminiferal δ18O records (21, 28). MIS numbers of interglacials are listed. (C) Sites 1088 (yellow) and 1090 (green) benthic δ13C isotope records plotted in the same scale, showing the small δ13C amplitude at site 1088 compared with 1090. The vertical gray bar indicates the time interval between MISs 24 and 22, when a drastic change in the εNd values took place at both sites, linked to a major weakening of NASW advection into the South Atlantic.

  3. Fig. 3 Estimation of the mixing proportions of NASW and NPSW water masses during glacial (blue) and interglacial (red) time periods using εNd and Nd concentrations and assuming binary mixing for (A) sites 1088 and (C) 1090.

    The Atlantic-Pacific mixing curve is based on the following parameters: for NADW, εNd = –13 and [Nd] = 22 pmol/kg; for NPW, εNd = –4.5 and [Nd] = 42 pmol/kg (23). The horizontal dashed lines and gray bar correspond to modern mixing ratios for the Cape Basin (εNd = –10.3), Southern Ocean (εNd = –9), and the very high εNd values in the South Atlantic during Heinrich events (εNd = –6.9 to –7.3) (24). (B) Yellow circles correspond to atmospheric Pco2 estimates based on B-isotope reconstructions (16) and compared with ice-core atmospheric Pco2 (35). Dashed lines indicate pre- and post-MPT Pco2 averages (2σ deviations), indicating a net drop of 40 parts per million by volume (ppmv), mostly driven by glacial Pco2 drop (16). Also shown are Pco2 reconstructions based on benthic foraminiferal δ13C (gray) (36), and two model outputs based on the LR04 stack [light blue and magenta (19, 32)]. Error bars indicate the propagated εNd error (2σ) into the mixing calculation (supplementary materials).

  4. Fig. 4 Schematic illustrations of the changes in South Atlantic THC across the MPT.

    The pre-MPT time period of MISs 31 to 25 (~1100 to 950 ka) is characterized by substantial export of NASW to the South Atlantic during both (A) glacials and (B) interglacials. εNd values indicate little glacial-interglacial variability at mid-depth (site 1088) and important NASW contributions during glacials. In the deep ocean (site 1090), the difference between glacials and interglacials is larger, but pre-MPT glacials show substantially more NASW than post-MPT glacials. The post-MPT time period since MISs 21 to 15 (~860 to 600 ka) is characterized by (C) strong THC during interglacials, like the pre-MPT period, but (D) weak THC during glacials. (E) The MIS 24 to 22 interval, centered at ~900 ka, shows an unprecedented weakening of NASW, similar to post-MPT glacials. MIS 23 is unique as an interglacial with THC like post-MPT glacial periods.

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