Stable isotope record, carbonate and organic carbon content of the Miocene section of IODP Site 321-U1337

The Miocene Climatic Optimum (~17-14.7 Ma) represents one of several major interruptions in the long-term cooling trend of the past 50 million years. To date, the processes driving high-amplitude climate variability and sustaining global warmth during this remarkable interval remain highly enigmatic. We present high-resolution benthic foraminiferal and bulk carbonate stable isotope records in an exceptional, continuous, carbonate-rich sedimentary archive (Integrated Ocean Drilling Program Site U1337, eastern equatorial Pacific Ocean), which offer a new view of climate evolution over the onset of the Climatic Optimum. A sharp decline in d18O and d13C at ~16.9 Ma, contemporaneous with a massive increase in carbonate dissolution, demonstrates that abrupt warming was coupled to an intense perturbation of the carbon cycle. The rapid recovery in d13C at ~16.7 Ma, ~200 k.y. after the beginning of the MCO, marks the onset of the first carbon isotope maximum within the long-lasting "Monterey Excursion". These results lend support to the notion that atmospheric pCO2 variations drove profound changes in the global carbon reservoir through the Climatic Optimum, implying a delicate balance between changing CO2 fluxes, rates of silicate weathering and global carbon sequestration. Comparison with a high-resolution d13C record spanning the onset of the Cretaceous Oceanic Anoxic Event 1a (~120 Ma ago) reveals common forcing factors and climatic responses, providing a long-term perspective to understand climate-carbon cycle feedbacks during warmer periods of Earth's climate with markedly different atmospheric CO2 concentrations.

Sedimentology and stable carbon and oxygen isotope record of the Central Arctic

Stable oxygen and carbon isotope and sedimentological-paleontological investigations supported by accelerator mass spectrometry 14C datings were carried out on cores from north of 85°N in the eastern central Arctic Ocean. Significant changes in accumulation rates, provenance of ice-rafted debris (IRD), and planktic productivity over the past 80,000 years are documented. During peak glacials, i.e., oxygen isotope stages 4 and 2, the Arctic Ocean was covered by sea ice with decreased seasonal variation, limiting planktic productivity and bulk sedimentation rates. In early stage 3 and during Termination I, major deglaciations of the circum-Arctic regions caused lowered salinities and poor oxygenation of central Arctic surface waters. A meltwater spike and an associated IRD peak dated to ~14-12 14C ka can be traced over the southern Eurasian Basin of the Arctic Ocean. This event was associated with the early and rapid deglaciation of the marine-based Barents Sea Ice Sheet. A separate Termination Ib meltwater event is most conspicuous in the central Arctic and is associated with characteristic dolomitic carbonate IRD. This lithology suggests an origin of glacial ice from northern Canada and northern Greenland where lower Paleozoic platform carbonates crop extensively out.

Stable carbon and oxygen isotope record of foraminifera from ODP holes 104-643A and 104-644A

Continuous sediment sections spanning the last 2.8 Ma have been studied using stable isotope stratigraphy and sedimentological methods. By using paleomagnetic reversals as a chronostratigraphic tool, climatic and paleoceanographic changes have been placed in a time framework. The results show that the major expansion of the Scandinavian Ice Sheet to the coastal areas occurred in the late Neogene period at about 2.8 Ma. Relatively high-amplitude glacials appeared until about 2 Ma. The period between 2.8 and 1.2 Ma was marked by cold surface water conditions with only weak influx of temperate Atlantic water as compared with late Quaternary interglacials. During this period, climatic variations were smaller in amplitude than in the late Quaternary. The Norwegian Sea was a sink of deep water throughout the studied period but deep water ventilation was reduced and calcite dissolution was high compared with the Holocene. Deep water formed by other processes than today. Between 2 and 1.2 Ma, glaciations in Scandinavia were relatively small. A transition toward larger glacials took place during the period 1.2 to 0.6 Ma, corresponding with warmer interglacials and increasing influx of temperate surface water during interglacials. A strong thermal gradient was present between the Norwegian Sea and the northeastern Atlantic during the Matuyama (2.5-0.7 Ma). This is interpreted as a sign of a more zonal and less meridional climatic system over the region as compared with the present situation. The transition towards more meridionality took place over several hundred thousand yr. Only during the last 0.6 Ma has the oceanographic and climatic system of the Norwegian Sea varied in the manner described from previous studies of the late Quaternary.