Changes in the high latitude Southern Hemisphere through the Eocene-Oligocene Transition:a model-data comparison

International audienceAbstract. The global and regional climate changed dramatically with the expansion of the Antarctic Ice Sheet at the Eocene–Oligocene transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth's oceans and atmosphere at the EOT, we compiled a database of 10 ocean and 4 land-surface temperature reconstructions from a range of proxy records and compared this with a series of fully coupled, low-resolution climate model simulations from two models (HadCM3BL and FOAM). Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. While certain climate model simulations show moderate–good performance at recreating the temperature patterns shown in the data before and after the EOT, in general the model simulations do not capture the absolute latitudinal temperature gradient shown by the data, being too cold, particularly at high latitudes. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support for previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to the opening of the Drake Passage

Warming drove the expansion of marine anoxia in the equatorial Atlantic during the Cenomanian leading up to Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE 2) (∼ 93.5 Ma) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water-column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 Myr of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) content is high but variable (0.41 wt %–17 wt %) across the Cenomanian and increases with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ∼ 35 to 40 ◦C as well as the episodic occurrence of 28,30- dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water-column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water-column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming

An astronomically dated record of Earth's climate and its predictability over the last 66 million years.

Much of our understanding of Earth's past climate comes from the measurement of oxygen and carbon isotope variations in deep-sea benthic foraminifera. Yet, long intervals in existing records lack the temporal resolution and age control needed to thoroughly categorize climate states of the Cenozoic era and to study their dynamics. Here, we present a new, highly resolved, astronomically dated, continuous composite of benthic foraminifer isotope records developed in our laboratories. Four climate states-Hothouse, Warmhouse, Coolhouse, Icehouse-are identified on the basis of their distinctive response to astronomical forcing depending on greenhouse gas concentrations and polar ice sheet volume. Statistical analysis of the nonlinear behavior encoded in our record reveals the key role that polar ice volume plays in the predictability of Cenozoic climate dynamics

Orbitally Paced Carbon and Deep-Sea Temperature Changes at the Peak of the Early Eocene Climatic Optimum

The late Paleocene to early Eocene warming trend was punctuated by a series of orbitally paced transient warming events, associated with the release of isotopically light carbon into the ocean-atmosphere system. These events occurred throughout the early Eocene, critically persisting during onset, peak, and termination of the early Eocene climatic optimum (EECO) and the onset of the middle Eocene cooling. Here we present a ~5.2-Myr-long high-resolution benthic foraminiferal stable-isotope record spanning the peak of the early Eocene “hothouse” from Ocean Drilling Program Leg 208 Site 1263. Our new oxygen isotope record confirms the presence of short-term warming events during the peak and termination of the EECO, previously described in coeval bulk carbonate records. The degree of change between deep-sea temperature and concurrent carbon release during these events is consistent with previous findings for Eocene thermal maximum 2 to 3, suggesting that the orbitally forced processes that triggered these perturbations in the exogenic carbon pool were similar. Additionally, the long-term background carbon isotope signature reveals a rapid enrichment of up to ~1.0‰ across the peak warmth of the EECO, ~51.6 Ma, without a corresponding shift in the oxygen record, suggesting a decoupling from climate. We speculate that this carbon shift reflects a non-recurrent adjustment in the mean (steady) state of the deep-ocean carbon reservoir due to a significant change in carbon source/sink, the biological pump, and/or ocean circulation during the extreme greenhouse conditions of the EECO