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Marine plankton is an important component of the global carbon cycle. Whereas the production and seafloor export of organic carbon produced by the plankton, the biological pump, has received much attention, the long-term variability in plankton calcification, controlling the carbonate counter pump, remains less well understood. However, it has been shown that changes in pelagic calcification (biological compensation) could affect the ocean's buffering capacity and thus regulate global carbon budget on geological timescales. Here we use Neogene pelagic sediments deposited on the Ceara Rise in the tropical Atlantic to characterize the variability in pelagic carbonate production with a focus on warm climates. A re-evaluation of published records of carbonate accumulation at the Ceara Rise reveals a systematic increase in sedimentation rates since the late Miocene, but the carbonate accumulation rate does not show a clear trend. Instead, we observe substantial orbital timescale variability in carbonate accumulation, combined with a trend towards less carbonate on average at sites located below 4 km, likely due to the effect of carbonate dissolution. To evaluate long-term changes against possible orbital-scale variability, we generated new high-resolution records of carbonate accumulation rate at Ocean Drilling Program (ODP) Site 927 across two Quaternary interglacials (MIS 5 and MIS 9), the Pliocene warm period (MIS KM5) and the Miocene Climatic Optimum (MCO). We observe that the highest carbonate accumulation rates occurred during the Pliocene but that each of the studied intervals was characterized by large-magnitude orbital variability. Prominent variations in carbonate accumulation prior to the Quaternary preservation cycles appear to follow Earth obliquity and eccentricity. These results imply that pelagic carbonate accumulation in the tropical ocean, buffered from large temperature changes, varied on orbital timescales. The magnitude of the orbital-scale variability was similar or even higher than the long-term mean differences among the studied intervals. Since preservation can be excluded as a driver of these changes prior to the Quaternary, the observed variations must reflect changes in the export flux of pelagic biogenic carbonate. We conclude that the overall carbonate production by pelagic calcifiers responded to local changes in light, temperature, and nutrients delivered by upwelling, which followed long orbital cycles, as well as to long-term shifts in climate and/or ocean chemistry. The inferred changes on both timescales were sufficiently large such that when extrapolated on a global scale, they could have played a role in the regulation of the carbon cycle and global climate evolution during the transition from the Miocene warm climates into the Quaternary icehouse.

We present a global atlas of downcore foraminiferal oxygen and carbon isotope ratios available at https://doi.pangaea.de/10.1594/PANGAEA.936747 (Mulitza et al., 2021). The database contains 2,108 published and previously unpublished stable isotope downcore records with 362,067 stable isotope values of various planktonic and benthic species of foraminifera from 1,265 sediment cores. Age constraints are provided by 6,153 uncalibrated radiocarbon ages from 598 (47 %) of the cores. Each stable isotope and radiocarbon series is provided in a separate netCDF file containing fundamental meta data as attributes. The data set can be managed and explored with the free software tool PaleoDataView. The atlas will provide important data for paleoceanographic analyses and compilations, site surveys, or for teaching marine stratigraphy. The database can be updated with new records as they are generated, providing a live ongoing resource into the future.

The causes of the Mid-Pleistocene Transition, the shift from ∼41-ky to 100-ky interglacial–glacial cycles and more intense ice ages, remain intensely debated, as this fundamental change occurred between ∼1,250 and 650 ka without substantial changes in astronomical climate forcings. Recent studies disagree about the relative importance of events and processes in the Northern and Southern Hemispheres, as well as whether the shift occurred gradually over several interglacial–glacial cycles or abruptly at ∼900 ka. We address these issues using a north-to-south reconstruction of the Atlantic arm of the global meridional overturning ocean circulation, a primary means for distributing heat around the globe, using neodymium (Nd) isotopes. Results reveal a period of intense erosion affecting the cratonic shields surrounding the North Atlantic between Marine Isotope Stages (MIS) 27 and 25 (∼980 and 950 ka), reflected by unusually low Nd isotope ratios in deep North Atlantic seawater. This episode preceded a major ocean circulation weakening between MIS 25 and 21 (950 and 860 ka) that coincided with the first ∼100-ky-long interglacial–glacial onset of Northern Hemisphere glaciation at around 2.4 to 2.8 Ma. The data point to a Northern Hemisphere–sourced initiation for the transition, possibly induced through regolith loss and increased exposure of the crystalline bedrock, which would lead to increased friction, enabling larger ice sheets that are characteristic of the 100-ky interglacial–glacial cycles.