Stefanie Ann Brachfeld

Holocene history of the Larsen-A Ice Shelf constrained by geomagnetic paleointensity dating

A sedimentary record collected from beneath the former Lar- sen-A Ice Shelf reveals the Holocene history of the Larsen-A re- gion. The record begins with the transition from grounded ice to a floating ice shelf, completed by 10.7 6 0.5 ka, and ends with the modern recession. The record contains several late Holocene dia- tomaceous ooze layers that suggest proximity to productive open- water events. Radiocarbon ages obtained from these sediments were complicated by the presence of detrital and reworked carbon. We have eliminated these complications and constructed a chro- nology for the Larsen-A Ice Shelf history via tuning of the geo- magnetic field paleointensity record with a reference curve. This approach provides chronological control to sediment sequences that lack appropriate material for radiocarbon dating. Geomag- netic paleointensity features with wavelengths of 2-3 k.y. can be recognized and interhemispherically correlated, illustrating the po- tential to use geomagnetic paleointensity variations as a global cor- relation and dating tool at sub-Milankovitch time scales.

A 13 200 year history of century to millennial-scale paleoenvironmental change magnetically recorded in the Palmer Deep, western Antarctic Peninsula

Abstract A 13 200-yr record of magnetic parameters from the Palmer Deep, western Antarctic Peninsula, records a sequence of five distinct shifts in glacimarine sedimentation coupled with century-scale variations in paleoproductivity. The five major shifts are manifested as abrupt, order of magnitude changes in low-field magnetic susceptibility, accompanied by changes in magnetic particle size and mineralogy. The Late Holocene (3.4–0 ka), the Early Holocene (11.5–9 ka), and the Last Glacial Maximum (prior to 13.2 ka) are intervals of strong low-field magnetic susceptibility and are characterized by multi-domain (MD) magnetite. MD magnetite is associated with zones of abundant gravel grains and is interpreted here as an indicator of material transported as ice-rafted debris. Deglaciation (13.2–11.5 ka) and the Middle Holocene (9–3.4 ka) were times of enhanced productivity. The Middle Holocene marks the onset of century-scale productivity cycles seen in high-field magnetic susceptibility, which is responding to variations in biogenic silica. Deglaciation and the Middle Holocene interval contain pseudo-single domain magnetite and titanomagnetite, respectively. These observations are surprising given the abundance of coarse MD magnetite available in local source rocks. The magnetic mineral assemblage in the Deglaciation interval, however, can be explained by density sorting in meltwater plumes. During the Middle Holocene, the magnetic mineral assemblage suggests the reduction or cessation of locally derived terrigenous sediment, and by inference, the reduction or cessation of iceberg generation. The Early Holocene–Middle Holocene shift in terrigenous sedimentation may be responding in part to sea level, which controls the position of the grounding line. The Palmer Deep Late Holocene interval coincides with the Neoglacial period, a time of glacier re-advances worldwide.

Glacial morphology and post-glacial contourites in northern Prince Gustav Channel (NW Weddell Sea, Antarctica)

We present the results of a marine geophysical investigation of the northern Prince Gustav Channel. By comparative analysis of multibeam bathymetric data, single channel seismic reflection profiles, underway chirp sonar data, ADCP current data and sediment coring, we define the main morphological elements of the area. In particular we define the glacial morphogenesis in relation to the excavation of inner shelf basins and troughs along structural discontinuities and lithologic boundaries. We identify streamlined surfaces that testify to the grounding of ice and past ice flow directions. These glacial forms are found only on glacial tills preserved in the deepest part of the basins, while net erosion to bedrock has occurred elsewhere. Since the Last Glacial Maximum (LGM), the relict glacial morphology has been draped by hemipelagic and diatomaceous mud, and bottom currents have played a major role in focusing sedimentation within small depocentres, that we define as contouritic drifts. Based on shallow sediment architecture and supported by direct measurements, we propose that the direction of bottom water flow is from the outer shelf into the Prince Gustav channel as a result of a combination of tidal currents and ice shelf-related thermohaline circulation.

Corrigendum to ''A 13 200 year history of century- to millennial-scale paleoenvironmental change magnetically recorded in the Palmer Deep, western Antarctic Peninsula'' (Earth Planet. Sci. Lett. 194 (2002) 311^326) C

This article contained an error in Fig. 2. Thecorrect depth^age equation for the interval be-tween turbidite 2 (T2) and turbidite 3 (T3)(33.22 to 40 mcd) is:y¼ 3613:4 þ187:27x; R¼ 0:90974Fig. 2 is reprinted below with the correctedequation. The authors would like to apologizefor any inconvenience caused by this error.