ALBERT

Description: Observations of rapid ongoing grounding line retreat, ice shelf thinning and accelerated ice flow from the West Antarctic Ice Sheet (WAIS) may forebode a possible collapse if global temperatures continue to increase. Understanding and reconstructing West Antarctic Ice Sheet dynamics in past warmer-than-present times will inform about its behavior as an analogue for future climate scenarios. International Ocean Discovery Program (IODP) Expedition 379 visited the Amundsen Sea sector of Antarctica to obtain geological records suitable for this purpose. During the expedition, cores from two drill sites at the Resolution Drift on the continental rise returned sediments whose deposition was possibly influenced by West Antarctic Ice Sheet dynamics from late Miocene to Holocene times. To examine the West Antarctic Ice Sheet dynamics, shipboard physical properties and sedimentological data are correlated with seismic data and extrapolated across the Resolution Drift via core-log-seismic integration. An interval with strongly variable physical properties, high diatom abundance and ice-rafted debris occurrence, correlating with partially high amplitude seismic reflection characteristics was identified between 4.2 and 3.2 Ma. Sedimentation during this interval is interpreted as having occurred during an extended warm period with a dynamic West Antarctic Ice Sheet in the Amundsen Sea sector. These records compare to those of other drill sites in the Ross Sea and the Bellingshausen Sea, and thus suggest an almost simultaneous occurrence of extended warm periods in all three locations.

Description: 〈p〉A robust collection of seismic and geomorphic data is used to examine the evolution of the Antarctic Ice Sheet within the Ross Sea Embayment. We use geomorphic data to reconstruct Last Glacial Maximum and post-Last Glacial Maximum ice sheet drainage and demonstrate retreat behaviours for the East Antarctic and West Antarctic sectors of the ice sheet. Using this framework, we then use seismic data and chronostratigraphic information from drill cores to reconstruct the long-term evolution of the ice sheet. Early ice sheet evolution during the Late Oligocene was characterized by isolated ice caps on bathymetric highs, followed by an interval of sediment infilling of rift basins and the development of more subdued relief in the eastern Ross Sea than in the western Ross Sea. Both ice sheets have experienced multiple episodes of expansion across the continental shelf since the Middle Miocene, with the frequency increasing during the Plio-Pleistocene. We conclude that seafloor bathymetry has been the principal control on ice sheet palaeodrainage and retreat behaviour since at least the middle Miocene, demonstrated by broad West Antarctic ice streams loosely guided by south to north cross-shelf troughs, whereas East Antarctic ice streams were funnelled through troughs that merge and converge around banks.〈/p〉

Description: A robust collection of seismic and geomorphic data is used to examine the evolution of the Antarctic Ice Sheet within the Ross Sea Embayment. We use geomorphic data to reconstruct Last Glacial Maximum and post-Last Glacial Maximum ice sheet drainage and demonstrate retreat behaviours for the East Antarctic and West Antarctic sectors of the ice sheet. Using this framework, we then use seismic data and chronostratigraphic information from drill cores to reconstruct the long-term evolution of the ice sheet. Early ice sheet evolution during the Late Oligocene was characterized by isolated ice caps on bathymetric highs, followed by an interval of sediment infilling of rift basins and the development of more subdued relief in the eastern Ross Sea than in the western Ross Sea. Both ice sheets have experienced multiple episodes of expansion across the continental shelf since the Middle Miocene, with the frequency increasing during the Plio-Pleistocene. We conclude that seafloor bathymetry has been the principal control on ice sheet palaeodrainage and retreat behaviour since at least the middle Miocene, demonstrated by broad West Antarctic ice streams loosely guided by south to north cross-shelf troughs, whereas East Antarctic ice streams were funnelled through troughs that merge and converge around banks.

Description: Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.