There is general agreement that productivity in high latitudes increased in the late Eocene and remained high in the early Oligocene. Evidence for both increased and decreased productivity across the Eocene-Oligocene transition (EOT) in the tropics has been presented, usually based on only one paleoproductivity proxy and often in sites with incomplete recovery of the EOT itself. A complete record of the Eocene-Oligocene transition was obtained at three drill sites in the eastern equatorial Pacific Ocean (ODP Site 1218 and IODP Sites U1333 and U1334). Four paleoproductivity proxies that have been examined at these sites, together with carbon and oxygen isotope measurements on early Oligocene planktonic foraminifera, give evidence of ecologic and oceanographic change across this climatically important boundary. Export productivity dropped sharply in the basal Oligocene (~33.7 Ma) and only recovered several hundred thousand years later; however, overall paleoproductivity in the early Oligocene never reached the average levels found in the late Eocene and in more modern times. Changes in the isotopic gradients between deep-and shallow-living planktonic foraminifera suggest a gradual shoaling of the thermocline through the early Oligocene that, on average, affected accumulation rates of barite, benthic foraminifera, and opal, as well as diatom abundance near 33.5 Ma. An interval with abundant large diatoms beginning at 33.3 Ma suggests an intermediate thermocline depth, which was followed by further shoaling, a dominance of smaller diatoms, and an increase in average primary productivity as estimated from accumulation rates of benthic foraminifera.

Understanding changes in export production through time provides insight into the response of the biological pump to global climate change, particularly during periods of rapid climate change. In this study we consider what role changes in export production may have had on carbon sequestration and how this may have contributed to the onset of the Eocene‐Oligocene transition (EOT). In addition, we consider if these export production variations are dominantly controlled by orbitally driven climate variability. To accomplish these objectives, we report changes in export production in the Eastern Equatorial Pacific (EEP) from Site U1333 across the EOT reconstructed from a high‐resolution record of marine barite accumulation rates (BAR). BAR fluctuations suggest synchronous declines in export production associated with the two‐step increases in oxygen isotopes that define the transition. The reduction in productivity across the EOT suggests that the biological pump did not contribute to carbon sequestration and the cooling over this transition. We also report a previously undocumented peak in EEP export productivity before the EOT onset. This peak is consistent with export production proxies from the Southern Ocean, potentially implying a global driver for this precursor event. We propose that this enhanced export production and the associated carbon sequestration in the late Eocene may have contributed to the pCO2 drawdown at the onset of Antarctic glaciation.

[1] The late Eocene through earliest Oligocene (40-32 Ma) spans a major transition from greenhouse to icehouse climate, with net cooling and expansion of Antarctic glaciation shortly after the Eocene/Oligocene (E/O) boundary. We investigated the response of the oceanic biosphere to these changes by reconstructing barite and CaCO 3 accumulation rates in sediments from the equatorial and North Pacific Ocean. These data allow us to evaluate temporal and geographical variability in export production and CaCO 3 preservation. Barite accumulation rates were on average higher in the warmer late Eocene than in the colder early Oligocene, but cool periods within the Eocene were characterized by peaks in both barite and CaCO 3 accumulation in the equatorial region. We infer that climatic changes not only affected deep ocean ventilation and chemistry, but also had profound effects on surface water characteristics influencing export productivity. The ratio of CaCO 3 to barite accumulation rates, representing the ratio of particulate inorganic C accumulation to C org export, increased dramatically at the E/O boundary. This suggests that long-term drawdown of atmospheric CO 2 due to organic carbon deposition to the seafloor decreased, potentially offsetting decreasing pCO 2 levels and associated cooling. The relatively larger increase in CaCO 3 accumulation compared to export production at the E/O suggests that the permanent deepening of the calcite compensation depth (CCD) at that time stems primarily from changes in deep water chemistry and not from increased carbonate production.

[1] Atmospheric deposition is an important source of nutrients to the coastal and open ocean; however, its role in highly productive upwelling regions like coastal California has not been determined. Approximately 0.1%-0.2% of new production is attributable to atmospheric deposition of nitrogen (N) annually, but if the estimate is expanded to encompass the effects of iron (Fe), aerosols may support 1%-2% of new production on average, and up to 5% on days with high deposition fluxes. Laboratory culture and in situ incubation experiments confirm the bioavailability of N from dry deposition in this region. A significant positive relationship between aerosol optical thickness and chlorophyll a derived from the Moderate Resolution Imaging Spectroradiometer is observed for the summer months and is stronger offshore than near the coast. Moreover, the portion of productivity supported by atmospheric deposition is higher on days without upwelling and during El Niño periods when nutrient input from upwelling is suppressed, a phenomenon that could become more prevalent due to climate warming. Expanding the results from California, we estimate that dry deposition could increase productivity in other major coastal upwelling regions by up to 8% and suggest that aerosols could stimulate productivity by providing N, Fe, and other nutrients that are essential for cell growth but relatively deplete in upwelled water.