The speed and timing of climate change: Detailed ice core stable isotope records from NorthGRIP, Greenland and Mt. Moulton, West Antarctica

Objective projections of future climate change require a detailed understanding of the natural variability and behavior of the climate system observed in the past. This dissertation endeavors to characterize the speed and timing of past climate changes on both orbital and extremely abrupt time scales from new high-resolution stable isotope time series from ice cores collected at NorthGRIP in Greenland and Mt. Moulton in West Antarctica.;Greenland ice cores are uniquely suited for study of abrupt climate events because relatively high snow accumulation rates allow single years to be identified well into the last glacial period. Deuterium (deltaD) and deuterium excess time series at near annual resolution or better across the three major abrupt transitions of the last glacial termination are examined from the precisely dated NGRIP ice core. These records have been synchronized to within a few years or less to similar isotopic series developed from the GISP2 ice core which was collected 320 km SSE of the NGRIP site. 300-500 year time intervals were examined in detail at the end of the Younger Dryas (YD, 11.7 ka), the beginning of the YD (12.9 ka), and at the onset of Bolling (14.7 ka). Transition timing, duration and amplitudes were determined for each transition at both sites via a statistical ramp fitting approach. At each transition a complete mode shift between two climate states recorded by deltaD or excess, or both, was detected in as little as a single year with one exception at the cooling at the onset of the YD at GISP2. NGRIP and GISP2 share much common variance, sometimes in remarkable detail, in the timing and evolution of the climate transitions. Complete mode shifts in excess were synchronous to within a year or better between NGRIP and GISP2 for the two cold-to-warm transitions and represent a significant regional reorganization of atmospheric circulation delivering moisture from lower latitude source regions to the high latitudes of Greenland. At the end of the YD in Greenland, warming commenced at or just after the excess shift and was identical in detail over NGRIP and GISP2 lasting 41-46 years. At the onset of Bolling, warming over Greenland was recorded differently at NGRIP and GISP2, where at NGRIP it was extremely abrupt (1-3 years) compared to decades long warming at GISP2. The deltaD transitions marking the cooling at the onset of the YD are more difficult to precisely detect with high resolution isotopic data, but share a common character between NGRIP and GISP2 and occur on the order of 100 years or less. The results are discussed in the context of abrupt climate changes having both a temporal and geographic anatomy that can change from one event to the next. The emerging picture is one in which not all abrupt climate transitions are regionally uniform across Greenland and where a single sequence of events as recorded by a variety of proxy data cannot describe all abrupt climate changes.;Ice cores collected from the summit of Mount Moulton in West Antarctica offer an opportunity to develop independently dated paleoclimate and atmosphere records spanning at least the last two glacial-interglacial transitions. A continuous 250-meter horizontal ice sequence from exposed blue ice contains stratigraphically ordered volcanic ash layers (tephra) that have been precisely dated using 40Ar/39Ar. An oxygen isotope (delta 18O) profile of Moulton ice was developed spanning 10.5 to 135.6 ka with six dated tephra layers providing a time scale. The oxygen isotope profile yields a detailed record of climate variations that includes the penultimate glacial-interglacial transition (Termination II) and last interglacial climate for the first time from ice in West Antarctica. While the oxygen isotope profile shares a common evolution and many shared features with records from around Antarctica including the EPICA cores, it also shows some unique features. For example, the coldest parts of the last glacial period are punctuated by rapid negative shifts of isotope values. A similar abrupt event described at Siple Dome around 22 ka suggests that abrupt climate change may be a regional phenomenon for West Antarctica, one which has not been recorded elsewhere on the continent. The results also point to complex elevation histories related to insolation variations at West Antarctic ice core sites. During the last interglacial period coherent series of methane concentration and oxygen-18 content of the atmosphere from a vertical ice core directly into the blue ice support the integrity of the Moulton isotope record over this interval and provide a means to cross date with other cores. A tephra layer dated 135.6 +/- 0.9 ka coincides with the onset of Termination II. Thus, Termination II at Mount Moulton starts no earlier than 135.6 +/- 0.9 ka which is consistent with the timing predicted by Milankovitch forcing of ice ages.