Description: Climate proxies indicate coupling between changes in atmospheric pCO2, global temperatures and ice volume over much of the Cenozoic. Evidence has been presented for decoupling of these factors in the Miocene, though the cause of the apparent decoupling was uncertain. Here, we revisit Deep Sea Drilling Program (DSDP) Site 608 (24-9 Ma) in the North Atlantic, to provide improved constraints on sea surface temperatures (SSTs) using the TEX86 and Uk'37 proxies, and use these to recalculate atmospheric pCO2. From the Oligocene/Miocene boundary to the middle Miocene Climatic Optimum (MCO, ~23.03 to ~14.5 Ma), SSTs at Site 608 were upwards of 30°C, nearly 15 °C warmer than modern. During the Middle Miocene Climatic Transition (MMCT), starting at ~14.5 Ma, SSTs cooled by ~ 6 °C These lower SSTs persisted until the end of our record (9 Ma). Our organic proxy derived SST estimates are considerably higher than those previously calculated from planktonic foraminiferal oxygen isotope data, leading to reassessed pCO2 estimates from alkenones ~65 to ~175 ppm higher than previously calculated, with other assumptions held constant. A pCO2 decrease from an average of ~430 ppm in MCO to ~305 ppm after the MMCT, in step with the ~6 °C SST cooling, demonstrates coupling of climate and the carbon cycle, as well as a highly sensitive climate system.

Description: The Palaeocene/Eocene thermal maximum, 55 million years ago, was a brief period of widespread, extreme climatic warming (Zachos et al., 2003; Kennett and Stott, 1991, doi:10.1038/353225a0; Tripati and Elderfield, 2005, doi:10.1126/science.1109202), that was associated with massive atmospheric greenhouse gas input (Dickens et al., 1995, doi:10.1029/95PA02087). Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition (Backman et al., 2006, doi:10.2204/iodp.proc.302.2006). We show that sea surface temperatures near the North Pole increased from 18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations (Shellito et al., 2003, doi:10.1016/S0031-0182(02)00718-6), but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms -perhaps polar stratospheric clouds (Sloan and Pollard, 1998, doi:10.1029/98GL02492) or hurricane-induced ocean mixing (Emanuel et al., 2004, doi:10.1175/1520-0469(2004)061〈0843:ECOTCI〉2.0.CO;2)- to amplify early Palaeogene polar temperatures.

Description: Falling atmospheric CO2 levels led to cooling through the Eocene and the expansion of Antarctic ice sheets close to their modern size near the beginning of the Oligocene, a period of poorly documented climate. Here we present the first record of climate evolution across the entire Oligocene (33.9-23.0 Ma) based on new TEX86 Sea Surface Temperature (SST) estimates from southwestern Atlantic Deep Sea Drilling Project Site 516 (paleolatitude ~ 36°S) and western equatorial Atlantic Ocean Drilling Project Site 929 (paleolatitude ~ 0°), combined with a compilation of existing SST records and climate modeling. In this relatively low CO2 Oligocene world (~300-700 ppm), warm climates similar to those of the late Eocene continued with only brief interruptions, while the Antarctic ice sheet waxed and waned. SSTs are spatially heterogenous, but generally support late Oligocene warming coincident with declining atmospheric CO2. This Oligocene warmth, especially at high latitudes, belies a simple relationship between climate and atmospheric CO2 and/or ocean gateways, and is only partially explained by current climate models. Though the dominant climate drivers of this enigmatic Oligocene world remain unclear, our results help fill a gap in understanding past Cenozoic climates and the way long-term climate sensitivity responded to varying background climate states.