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The Amazon Fan contains sedimentary/acoustic sequences characteristic of many large and small modem mud-rich fans. Analyses of high-resolution single-channel seismic-reflection profiles and 3.5-kHz profiles suggest that fan growth is in part related to sea-level fluctuations and in part related to events such as channel bifurcations and large debris flows that appear unrelated to sea-level position. Sinuous fan channels are perched on top of lens-shaped overbank deposits to form channel-levee systems in the upper and middle fan. Individual channel-levee systems overlap and coalesce to build levee complexes that also stack and overlap, but that are bounded by large debris-flow deposits. Because both channel-levee systems and debris flows can be active at the same time, this depositional pattern does not necessarily develop as a result of sea-level change. The sinuous fan channels appear to be nearly at grade because channel sinuosity varies downfan to keep the along-channel gradient uniformly decreasing downfan. Flat-lying, high-amplitude reflection packets that underlie a channel-levee system and extend downfan to form part of the lower fan may develop when new, oversteepened channels are created as a result of avulsion on the middle fan. This suggests that portions of the lower fan are formed concurrently with channellevee systems. Piston cores from near the most recently active channel suggest that the locus of sedimentation shifted landward as sea level rose at the end of the last glaciation.

The highest resolution Holocene sediment core from the Antarctic Peninsula to date was collected during the fi rst SHALDRIL cruise (NBP0502). Drilling yielded a 108.2-m-long core (87% recovery; site NBP0502-1B) from Maxwell Bay, South Shetland Islands. This high-resolution sediment record comes from a region that is currently experiencing dramatic climate change and associated glacial retreat. Such records can help to constrain the nature of past climate change and causal mechanisms, and to provide a context for evaluating current climate change and its impacts.The base of the drill site sampled till and/or proximal glacimarine sediments resting directly on bedrock. Glacimarine suspension deposits composed of dark greenish gray silty mud with variable diatom abundance and scattered very fi ne sand laminations make up the majority of the sedimentary section. Detailed sedimentological and geochemical analyses, including magnetic susceptibility, total organic carbon (TOC) content, carbon and nitrogen isotopic composition, pebble content, and biogenic silica content, allow subdivision of the glacimarine section into nine units, and seismic facies analyses resulted in the identifi cation of six distinct seismic units. We used 29 radiocarbon ages to construct an age model and calculate sedimentation rates that vary by two orders of magnitude, from 0.7 mm/a to ~30 mm/a. Radiocarbon ages from glacimarine sediments just above the till date back to between 14.1 and 14.8 ka. Thus, ice was grounded in the fjord during the Last Glacial Maximum and eroded older sediments from the fjord. Following initial retreat of grounded ice from Maxwell Bay, the fjord was covered by a permanent fl oating ice canopy, probably an ice tongue. The highest sedimentation rate corresponds to an interval that contains abundant sand laminations and gravelly mud inter vals and likely represents a melt-out phase or period of rapid glacial retreat from 10.1 ka to 8.2 ka. There is no evidence for an early Holocene climatic reversal, as recorded farther south at the Palmer Deep drill site. Minimum sea-ice cover and warm water conditions occurred between 8.2 and 5.9 ka. From 5.9 to 2.6 ka, there was a gradual cooling and more extensive sea-ice cover in the bay. After 2.6 ka, the climate varied slightly, causing only subtle variation in glacier grounding lines. There is no compelling evidence for a Little Ice Age readvance in Maxwell Bay. The current warming and associated glacial response in the northern Antarctic Peninsula appears to be unprecedented in its synchroneity and widespread impact. Modern Climate, Glacial, and Oceanographic ConditionsAtmospheric temperatures for Maxwell Bay average -3 °C, but during the summer months, temperatures commonly rise above freezing For permission to copy, contact editing@geosociety.org

Seismic reflection profiles show at least four major mass-transport deposits (MTDs) on the Amazon Fan that drilling has shown date from the late Pleistocene. Each deposit extends over an area on the order of 10 4 km 2 and is 50-100 m thick. The entire thickness of individual MTDs was penetrated at Sites 931, 933, 935, 936, 941, and 944, and wireline logs were collected at most of these sites. Most deposits consist of large deformed blocks (meters to decameters) of clayey sediment. A little matrix is recognized between blocks, and some weaker smaller blocks are highly deformed. Thin matrix-rich deposits with small clasts near the top of some units are true debris flows. Properties of clasts in the MTDs show a broadly repetitive character vertically within the deposit, on a scale of meters to tens of meters. There is no evidence that a long time span is represented by discontinuities in sediment properties; rather, this repetitive pattern probably represents retrogressive failure from a headwall scarp. Major units 20-50 m thick within the MTDs can be correlated between sites. Sediment properties and microfossils suggest that most sediment was derived from muddy channel-levee deposits on the continental slope, but some sediment (particularly near the base of flows) resembles local deep-water levee sediments. Mass-transport events are inferred to have initiated in slope and upper-fan levee sediments. This sediment was underconsolidated because of rapid prodeltaic deposition during marine lowstands as well as a result of the presence of shallow gas and gas hydrates. Local steepening and weakening by diapiric intrusion may also have facilitated failure. The ages of the mass-transport events may correlate with times of falling sea level, when gas hydrate sublimation could destabilize sediments. MTDs were partly confined by pre-existing channel-levee topography on the fan. In places, high-relief levee deposits were eroded by the mass-transport flow and incorporated in the basal part of the deposit.

The Antarctic shelf is traversed by large-scale troughs developed by glacial erosion. Swath bathymetric, lithologic, and chronologic data from jumbo piston cores from four sites along the East Antarctic margin (Iceberg Alley, the Nielsen Basin, the Svenner Channel, and the Mertz-Ninnis Trough) are used to demonstrate that these cross-shelf features controlled development of calving bay reentrants in the Antarctic ice sheet during deglaciation. At all sites except the Mertz-Ninnis Trough, the transition between the Last Glacial Maximum and the Holocene is characterized by varved couplets deposited during a short interval of extremely high primary productivity in a fjordlike setting. Nearly monospecific layers of the diatom Chaetoceros alternate with slightly more terrigenous layers containing a mixed diatom assemblage. We propose that springtime diatom blooms dominated by Chaetoceros were generated within well-stratified and restricted surface waters of calving bays that were influenced by the input of iron-rich meltwater. Intervening post-bloom summer-fall laminae were formed through the downward flux of terrigenous material sourced from melting glacial ice combined with mixed diatom assemblages. Radiocarbon-based chronologies that constrain the timing of deposition of the varved sediments within calving bay reentrants along the East Antarctic margin place deglaciation between ca. 10,500-11,500 cal yr B.P., postdating Meltwater Pulse 1A (14,200 cal yr B.P.