A Cool, Nutrient‐Enriched Eastern Equatorial Pacific During the Mid‐Pleistocene Transition

Abstract

The emergence of high‐amplitude, low‐frequency glacial‐interglacial cycles during the mid‐Pleistocene climate transition (MPT; 800–1,200 ka) is associated with global cooling. In the eastern equatorial Pacific, sea surface temperatures cooled, and the upwelling‐induced cold tongue expanded significantly during the MPT. Here we use sedimentary records of iron, biogenic silica, and nutrient‐nitrogen consumption to evaluate biogeochemical changes hypothesized to accompany the cold tongue expansion. Our results suggest that the eastern equatorial Pacific of the MPT hosted surface waters with higher nitrate contents and biogenic silica production relative to the last 600 ka. Increased production occurred despite low iron supply. We attribute this to enhanced upwelling and nutrient enrichment of thermocline waters, both likely related to the northward migration of Southern Ocean fronts. The return of these fronts to their southward positions after the MPT may be associated with stronger drawdown of nutrients and, potentially, atmospheric CO2 in the Southern Ocean.

Pliocene Weakening of Gradients in Temperature but Not in Productivity in the Eastern Equatorial Pacific

“…(2019) for Sites 849 and U1338, and from Robinson et al. (2019) for Site 846. Alkenone SST and productivity data are from Lawrence et al.…”

“…Robinson et al. (2019) provide the Site 846 raw XRF scanning data and report calibrated weight percentage concentrations of element oxides according to a log‐ratio calibration equation. Oxides were converted to elements according to known conversion factors before any element ratio comparisons were undertaken (e.g., the oxide SiO 2 was multiplied by 0.4674 to convert to the element Si).…”

“…Lyle et al (2019) provide the Site 849 raw XRF scanning data and report calibrated weight percentage concentrations of element oxides according to a normalized median-scaled calibration. Robinson et al (2019) provide the Site 846 raw XRF scanning data and report calibrated weight percentage concentrations of element oxides according to a log-ratio calibration equation. Oxides were converted to elements according to known conversion factors before any element ratio comparisons were undertaken (e.g., the oxide SiO 2 was multiplied by 0.4674 to convert to the element Si).…”