Several hypotheses have been put forward to explain the onset of intensive glaciations on Greenland, Scandinavia, and North America during the Pliocene epoch between 3.6 and 2.7 million years ago (Ma). A decrease in atmospheric CO2 may have played a role during the onset of glaciations, but other tectonic and oceanic events occurring at the same time may have played a part as well. Here we present detailed atmospheric CO2 estimates from boron isotopes in planktic foraminifer shells spanning 4.6–2.0 Ma. Maximal Pliocene atmospheric CO2 estimates gradually declined from values around 410 μatm to early Pleistocene values of 300 μatm at 2.0 Ma. After the onset of large‐scale ice sheets in the Northern Hemisphere, maximal pCO2 estimates were still at 2.5 Ma +90 μatm higher than values characteristic of the early Pleistocene interglacials. By contrast, Pliocene minimal atmospheric CO2 gradually decreased from 310 to 245 μatm at 3.2 Ma, coinciding with the start of transient glaciations on Greenland. Values characteristic of early Pleistocene glacial atmospheric CO2 of 200 μatm were abruptly reached after 2.7 Ma during the late Pliocene transition. This trend is consistent with the suggestion that ocean stratification and iron fertilization increased after 2.7 Ma in the North Pacific and Southern Ocean and may have led to increased glacial CO2 storage in the oceanic abyss after 2.7 Ma onward.

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An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752‐1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

Abstract. A major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG) and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001). The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3) and 2.7 Ma (glacial MIS G6/4) (Lisiecki and Raymo, 2005). Various models (sensu Driscoll and Haug, 1998) and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2–3°C) (Bartoli et al., 2005). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007), which probably led to an enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS), and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~2 psu from 3.25/3.16–3.00 Ma, right after the first but still reversible attempt of closing the CAS.