Here, we compare the ocean overturning circulation of the early Eocene (47–56 Ma) in eight coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP) and investigate the causes of the observed inter‐model spread. The most common global meridional overturning circulation (MOC) feature of these simulations is the anticlockwise bottom cell, fed by sinking in the Southern Ocean. In the North Pacific, one model (GFDL) displays strong deepwater formation and one model (CESM) shows weak deepwater formation, while in the Atlantic two models show signs of weak intermediate water formation (MIROC and NorESM). The location of the Southern Ocean deepwater formation sites varies among models and relates to small differences in model geometry of the Southern Ocean gateways. Globally, convection occurs in the basins with smallest local freshwater gain from the atmosphere. The global MOC is insensitive to atmospheric CO2 concentrations from 1× (i.e., 280 ppm) to 3× (840 ppm) pre‐industrial levels. Only two models have simulations with higher CO2 (i.e., CESM and GFDL) and these show divergent responses, with a collapsed and active MOC, respectively, possibly due to differences in spin‐up conditions. Combining the multiple model results with available proxy data on abyssal ocean circulation highlights that strong Southern Hemisphere‐driven overturning is the most likely feature of the early Eocene. In the North Atlantic, unlike the present day, neither model results nor proxy data suggest deepwater formation in the open ocean during the early Eocene, while the evidence for deepwater formation in the North Pacific remains inconclusive.

Ocean acidification alters the dissolved inorganic carbon chemistry of seawater and can reduce the calcification rates of tropical corals. Here we explore the effect of altering seawater pCO2 on the skeletal morphology of 4 genotypes of massive Porites spp. which display widely different calcification rates. Increasing seawater pCO2 causes significant changes in in the skeletal morphology of all Porites spp. studied regardless of whether or not calcification was significantly affected by seawater pCO2. Both the median calyx size and the proportion of skeletal surface occupied by the calices decreased significantly at 750 µatm compared to 400 µatm indicating that polyp size shrinks in this genus in response to ocean acidification. The coenosteum, connecting calices, expands to occupy a larger proportion of the coral surface to compensate for this decrease in calyx area. At high seawater pCO2 the spines deposited at the skeletal surface became more numerous and the trabeculae (vertical skeletal pillars) became significantly thinner in 2 of the 4 genotypes. The effect of high seawater pCO2 is most pronounced in the fastest growing coral and the regular placement of trabeculae and synapticulae is disturbed in this genotype resulting in a skeleton that is more randomly organised. The study demonstrates that ocean acidification decreases the polyp size and fundamentally alters the architecture of the skeleton in this major reef building species from the Indo-Pacific Ocean.

The early Eocene Climatic Optimum (EECO) (~49-53 Ma) presents an ideal test bed to explore climate interactions in a high CO 2 world (pCO 2 > 1000 ppm [1]). Here we investigate deep ocean circulation during the EECO, employing the Nd isotope fingerprint of water masses as reconstructed using fish debris and foraminifera, at sixteen global DSDP, ODP and IODP sites. Our data reveal a distinct dichotomy between the Atlantic and the Pacific Oceans in both average ε Nd(t) and time-dependent variability. Pacific data disclose relatively constant site-specific signatures, suggesting that the observed latitudinal trend constitutes a long-term feature of the EECO. Through comparison with existing data, we note a latitudinal gradient towards more radiogenic signatures from the south to the north (ε Nd(t) = -4.6 ±0.3, DSDP Site 287 24°4S and ε Nd(t) = -1.9 ± 0.4, ODP Site 883 [2], 38°9N). Although this observation alone is inconclusive in distinguishing between North and South Pacific deep-water formation/export, the data strengthen the notion of a Pacific sector Southern Ocean deep-water source [3].In contrast, Atlantic Ocean signatures display time-dependent variability throughout the EECO. South Atlantic signatures largely reflect Southern Ocean export, characterised by Nd isotope values between -10.4 ± 0.4 and -8.6 ± 0.3 in the Atlantic and Indian sector. The observed amplitude of change in South Atlantic sites is 1.2 epsilon units, which is insufficient to explain large variability in Nd isotope signatures observed within the North Atlantic. DSDP Site 549 reveals a shift from -6.6 ± 0.2 [4] to -10.3 ± 0.2 between ~53.9 and 47.6 Ma, possibly due to a significant change in weathering inputs. Whether transient or long-term, strengthened export of highly unradiogenic Baffin Bay outflow water offers a feasible explanation for these North Atlantic observations. We will furthermore interrogate whether our new data support multiple, separate, Southern Ocean deep water sources during the EECO and potential export from the North Atlantic.