Ocean dynamics in the equatorial Pacific drive tropical climate patterns that affect marine and terrestrial ecosystems worldwide. How this region will respond to global warming has profound implications for global climate, economic stability and ecosystem health. As a result, numerous studies have investigated equatorial Pacific dynamics during the Pliocene (5.3–2.6 million years ago) and late Miocene (around 6 million years ago) as an analogue for the future behaviour of the region under global warming1,2,3,4,5,6,7,8,9,10,11,12. Palaeoceanographic records from this time present an apparent paradox with proxy evidence of a reduced east–west sea surface temperature gradient along the equatorial Pacific1,3,7,8—indicative of reduced wind-driven upwelling—conflicting with evidence of enhanced biological productivity in the east Pacific13,14,15 that typically results from stronger upwelling. Here we reconcile these observations by providing new evidence for a radically different-from-modern circulation regime in the early Pliocene/late Miocene16 that results in older, more acidic and more nutrient-rich water reaching the equatorial Pacific. These results provide a mechanism for enhanced productivity in the early Pliocene/late Miocene east Pacific even in the presence of weaker wind-driven upwelling. Our findings shed new light on equatorial Pacific dynamics and help to constrain the potential changes they will undergo in the near future, given that the Earth is expected to reach Pliocene-like levels of warming in the next century.
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The code used in this paper to produce pH from d11B (and to produce all the proxy-related figures) is publicly available as Matlab scripts on GitHub (https://github.com/Maddie-Sh/ShankleEtAl2021_Pliocene-pH). The CESM 18.104.22.168 code is available from https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2_1. The Python code used to create select model figures is available at https://github.com/nburls/ShankleEtAl2021. Source data are provided with this paper.
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The authors acknowledge and appreciate the aid of J. Robbins, W. Strojie, D. Asael and S. Zhang in supervising clean lab chemistry, boron and trace element analysis, and data processing. The authors also appreciate discussion with J. Rae. This work was supported by NSF Award 1602557 and 170251 to P.M.H., NSF Award 1844380 and 2002448 to N.J.B. and Sloan Ocean Fellowships to P.M.H. and N.J.B. Additional funding to A.V.F. was provided by the Guggenheim Fellowship and the ARCHANGE project (ANR-18-MPGA-0001, France). We acknowledge high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the NSF.
The authors declare no competing interests.
Peer review information Nature thanks Kelsey Dyez, Julia Tindall and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 pCO2 from this study’s western equatorial Pacific (WEP) pH values match previous δ11B-derived studies.
a, b, pCO2 estimates (blue markers and shaded areas showing 68% and 95% confidence intervals) from this study’s pH data from the WEP, a region in equilibrium with the atmosphere37, assuming a modern-like alkalinity of 2,275 ± 200 μmol kg−1 (a) versus a modern-like calcite saturation state (Ω = 5 ± 2 (error bars), Ω = 4 (lower black squares) and Ω = 6 (upper black squares); b). Grey data points show δ11B-deried estimates of pCO2 from previously published studies, with shading showing reported confidence intervals51,84,111,112,113,114,115.
Extended Data Fig. 2 δ11B-reconstructed pH matches model output over 25–75 m depth (top left, bottom); reconstructed pH data from ~3 Ma compared to model output also provided for reference (top right).
a, Modelled pH along the equator (at 55 m depth (red solid and blue dashed lines) and across the 25–75 m depth range (shaded areas)) and δ11B-derived pH (circles with 2σ uncertainty from a Monte Carlo simulation, as in Fig. 2b). Markers placed at their approximate longitudes (159.362°E for western ODP Site 806 and 269.182°E for eastern ODP Site 846). The observed (see Methods) modern gradient in surface pH averaged across the 25–50 m depth range is included for reference (yellow crosses), as well as the range of pH from available modern observations (vertical yellow bars on crosses). b–g, Climatological model output of pH and d11B-reconstructed pH with proxy data (circles) from ~3 Ma instead of ~6 Ma (b, c) and model output (contours) at distinct depths of 25 m ad 55 m (bracketing O. universa’s depth habitat) instead of averaged over the 25–55 m range (d–g). In all panels, more acidic waters (or more acidic waters in the past) are shown in yellow-green colours.
Extended Data Fig. 3 Extended physical oceanographic model output links pH changes to a Pacific meridional overturning circulation.
a–d, Timeseries of PMOC strength (blue lines), zonal Pacific pH gradient (ΔpH, black lines), and zonal Pacific SST gradient (ΔSST, red lines) for the model control run (a, b) and early Pliocene/late Miocene run (c, d). PMOC strength (Max PMOC Streamfunction) is defined as the maximum streamfunction north of 25°N and below 500 m depth in sverdrups (1 Sv = 106 m3 s−1). The pH gradient is defined as the pH difference between a western (5°S–5°N, 150–170°E) and eastern Pacific box (5°S–5°N, 260–280°E), taken at 55 m depth. The SST gradient is defined as the SST difference between the same boxes taken at the surface. Note that while the zonal SST gradient equilibrates within ~500 years in the early Pliocene/late Miocene experiment (d) and is hardly influenced by the appearance of the PMOC between ~800–1,600 years, the zonal pH gradient increases in phase with the PMOC (c). e–h, Zonally averaged streamfunction over the Pacific (e, g) and Atlantic (f, h) basins for the control run (e, f) and early Pliocene/late Miocene (g, h) runs of the model Positive (green values) denote clockwise circulation, and negative (pink) values denote counterclockwise rotation. Panel g is the same as Fig. 4b in the main text.
Extended Data Fig. 4 This study’s ~1 Ma samples fall in different stages of the glacial–interglacial cycles, and δ11B-pH core top calibration used in this study recreates observed ambient pH.
a, b, High-resolution sea surface temperature records from western equatorial Pacific site ODP 806 G. ruber-derived Mg/Ca data116 (a) and eastern equatorial Pacific ODP Site 846 alkenone (UK′37) data117 (b). In both panels the ages of the ~1 Ma samples in this study are overlain as green lines, falling at different points in the glacial–interglacial cycles. c, Core top, net tow, and sediment trap d11B data36,109,110 converted to pH using the calibration of Henehan et al.36 shows good agreement with reported in situ pH. For the odd point to the far right (dark orange, x), Raitzsch et al109. calculated in situ pH using an anomalous low value of alkalinity (~200 μmol kg−1 lower than alkalinity for the location reported in datasets such as GLODAPv266. Recalculating pH using the same in situ temperature and salinity as Raitzsch et al. but an alkalinity value derived from GLODAPv2 (2,325 μmol kg−1) brings the point in agreement with the other data (empty dark orange circle). For the odd point in the lower part of the figure (dark green, x), Guillermic and colleagues110. report an unrealistic value for temperature at this site: ~18 °C whereas the same site (in the central Indian Ocean) appears to be closer to ~26 °C on average according to GLODAPv2. When we re-calculate δ11B-pH derived pH using GLODAPv2’s 26 °C, this brings the point into better agreement with the other data (empty dark green circle). The line of best fit and 2σ shading was calculated with the original odd data points, however, and note that the core top data is still in good agreement with reported in situ pH following this calibration.
a–d, Observed pH (or pH calculated from alkalinity and total dissolved inorganic carbon (DIC)) from all cruises within 5° latitude and longitude of our study sites (pale profiles) during neutral-ENSO (a), El Niño (b) and La Niña conditions (c). Thick lines in panels a–c show average profiles from all stations on a given cruise. d, Average pH-depth profiles from all neutral-ENSO observations. Modern reference values of pH (that is, diamonds on the y axis in Fig. 2) were derived from averaging panel d’s profiles over 25–50 m depth (approximate depth habitat of O. universa). e–g, Modern pH derived from cruise-based observations of alkalinity and total DIC. Data compiled from the GLODAPv2 dataset66 and plotted using the Ocean Data View software (R. Schlitzer, Ocean Data View, https://www.osv.awi.de, 2018). Overlain are this study’s sites (black diamonds) as well as the site of the additional 3 Ma pH values51 included in Fig. 2 (blue square).
Extended Data Fig. 6 An enhanced early Pliocene/late Miocene zonal pH gradient is observed under different treatments of the data.
δ11B-derived pH from O. universa from the east (blue) and west (red) with 2σ uncertainty from a Monte Carlo simulation (empty circles; averages at ~1 Ma, ~3 Ma and ~6 Ma in filled circles) showing: all observations of modern pH used in producing average modern pH reference values (diamonds) given as individual dashes (a), the range of pH from using varied SST records in the pH calculation (grey bars) (b), pH as calculated using the SST record derived from calibrating this study’s Mg/Ca data with the BAYMAG calibration88 (c), and pH as calculated using dissolution-corrected118 Mg/Ca SSTs from this study (yellow squares) (d). In panel b, pH ranges cover pH calculated using: TEX86-derived3 SSTs, Mg/Ca-derived SSTs (this study) using a linear correction for Mg/CaSW, and Mg/Ca-derived SSTs (this study) using a power-law correction87 for Mg/CaSW. Both Mg/Ca-SST calibrations were done using the Mg/CaSW record of Fantle and DePaolo81.
a–c, Mg/Ca-derived SSTs for eastern (blue) and western (red) equatorial Pacific at ~1 Ma, ~3 Ma and ~6 Ma calculated using the O. universa calibration of Anand et al.89, using a linear (dark squares) and power-law (light squares) relationship between Mg/CaSW and Mg/Catest, and using the Bayesian calibration BAYMAG88, also corrected for Mg/CaSW. Error bars denote calibration errors on O. universa calibration (squares) and 95% confidence intervals on the BAYMAG-calibrated data (circles). Dark squares are plotted at corrects ages; other markers are offset for ease of viewing. d–f, δ11B-derived pH at the time points calculated using SST records from UK′377,119 (crosses, using the SST calibration of Conte et al.120), TEX863 (asterisks, using the SST calibration of Kim et al.120), and Mg/Ca data (this study, using the O. universa SST calibration of Anand et al.89) (assuming a linear (circles) and power-law87 (triangles) relationship between Mg/CaSW and Mg/Catest). Note that because the western-site UK′37 record119 (red crosses, bottom panels) only extends back to ~5.3 Ma, we assume the maximum temperature calculable by this calibration (~28.5 °C121) for our western points at ~6 Ma (red crosses, panel f), towards which the record of Pagani et al.119 was trending and had nearly approached even by ~5.3 Ma (~28.3–28.4 °C)). All error bars on pH depict uncertainty (2σ) returned from a Monte Carlo simulation.
Extended Data Fig. 8 Different Mg/Caseawater records influence sea surface temperatures but have only a modest impact on pH.
a, Various published Mg/CaSW reconstructions11,81,88,122 (lines) plotted alongside proxy-based estimates (orange points) from carbonate veins123,124 (orange triangle and diamond) and fluid inclusions82,83 (orange circle and square) with error reported in those studies (where there are error bars). b–d, Using different Mg/CaSW records only slightly affect SSTs and even less so pH results. b, SSTs calculated from T. sacculifer Mg/Ca data1 using the species-specific calibration of Dekens et al.125 and various Mg/CaSW reconstructions11,81,88,122. c, SSTs calculated from O. universa Mg/Ca data (this study) using the species-specific calibration of Anand et al.89 a and various Mg/CaSW reconstructions11,81,88,122. d, δ11B-derived pH (this study) using the various SST reconstructions in panel c. Note that the Mg/CaSW records of O’Brien et al.11 and Evans et al.122 (blue and purple lines in panel a) do note extend back far enough to apply to the ~6 Ma data, ending at ~4.8 Ma and ~5 Ma, respectively.
Extended Data Fig. 9 Different sea surface temperature proxies record varying reductions in the zonal sea surface temperature gradient, and a dissolution correction and different Mg/CaSW record is applied to this study’s data.
a, b, SST records from the eastern (blue) and western (red) equatorial Pacific. a, SST records which evince a collapse of the modern zonal SST gradient include: T. sacculifer Mg/Ca data1 calibrated and corrected for Mg/CaSW with the Bayesian BAYMAG calibration88 with 95% confidence intervals (pale blue and red bands); UK′37 data7,119 using a global ocean annual-mean calibration126 with its associated 1 s.e. (±1.1 °C) (dark blue and red bands, dashed line shows upper saturation limit of the proxy); T. sacculifer Mg/Ca data1 using a species-specific calibration125 and corrected for Mg/CaSW by O’Brien et al.11 (bright blue and red lines, no uncertainty reported); and O. universa Mg/Ca data (this study) using a species-specific calibration89 and the Mg/CaSW record of Fantle and DePaolo81, assuming a linear relationship between Mg/CaSW and Mg/Catest (square markers with calibration error in error bars). Note that the three Mg/Ca records in this panel all use different Mg/CaSW records11,81,88. As the goal of this figure is to depict the range of temperature (and temperature gradient) reconstructions from the literature, we have not standardized them all to the same Mg/CaSW record. b, SST records which evince the modern zonal SST gradient being roughly maintained back into the early Pliocene/late Miocene, derived from TEX86 data3 using the calibration of Kim et al.121, with its associated calibration error (±2.5 °C) (blue and red bands). c, SSTs calculated from O. universa Mg/Ca data in this study both with (solid squares) and without (empty squares) applying the dissolution correction of Regenberg and colleagues118. d, Western equatorial Pacific (WEP) SSTs according to T. sacculifer Mg/Ca data1 (lines) versus O. universa (this study, points). Depending on choice of Mg/CaSW record (light versus darker colours), even T. sacculifer may record cooler-than-modern temperatures in the WEP. Average annual modern WEP SST (from GLODAPv266) given in the dashed line.
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Shankle, M.G., Burls, N.J., Fedorov, A.V. et al. Pliocene decoupling of equatorial Pacific temperature and pH gradients. Nature 598, 457–461 (2021). https://doi.org/10.1038/s41586-021-03884-7
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