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numerous discipline-specific results and deglaciation and flotation of the ice sheet collapse during the mid-Holocene (Balco et
publications, we focus here on the cross- progressed (Fig. 2). Flow reorientation al., 2013).
disciplinary results of the project. These during retreat, generally from flow that In contrast, the Larsen B embayment
studies focus on past climate variability included a component of alongshore flow was continuously occupied by an ice shelf
from ice core records, current climate toward flow more directly offshore, for at least 12,000 years prior to its 2002
changes, seabed landforms (a window on reflected the changing ice sheet geometry disintegration (Domack et al., 2005a;
past ice-flow patterns), and marine geo- as the grounding line of the ice sheet Rebesco et al., 2014). Absolute diatom
logic core analysis (combining climatic, neared the modern coastline (Fig. 2). abundance in Larsen B sediment cores
glacial, biological, and oceanographic Strong elongation of the seabed features increased sharply upon ice-shelf breakout;
histories preserved in sedimentary strata). indicates rapid ice flow during the glacial however, pre-breakup Holocene sediments
Extensive sea ice and landfast-ice cover in maximum period. Several possible LGM- are almost completely depauperate
the area of the Larsen Ice Shelf forced parts era ice-shelf collapses are noted near the (Domack et al., 2005a; Rebesco et al.,
of each major cruise to include work on the continental shelf break in the form of ice- 2014). This suggests either limited contri-
western side of the Antarctic Peninsula. berg furrows oriented sub-parallel to the bution from Weddell Sea waters to the sub-
These western Antarctic Peninsula data seabed lineations. The arrangement of sea- ice cavity or that these waters were diatom
have supported a comparison across the bed features indicates a sudden discharge poor, which could be attributable to heavy
drainage divide between the warmer, of many icebergs whose drift is still par- sea-ice cover in the Weddell Sea limiting
wetter western Antarctic Peninsula and tially controlled by surrounding grounded primary productivity.
the colder, dryer Larsen side of the ice. Ongoing ice retreat is governed in part Even during times of extended Holocene
Antarctic Peninsula. by reorganization of flow patterns accom- ice-shelf cover, styles of sediment accumu-
panying grounding line movement. lation differ between the two Larsen
SEAFLOOR RECORDS OF Marine sediment core data document a embayments. Though ice-shelf–free condi-
CHANGES SINCE THE LAST major difference in the long-term histories tions are recorded only in mid-Holocene
GLACIAL MAXIMUM (LGM) of the Larsen A and Larsen B embayments sediments from the Larsen A embayment,
Multibeam sonar mapping was con- (Fig. 3). The former Larsen A experienced the consistent presence of diatom valves in
ducted on both sides of the Antarctic periods of shelf removal during the mid- sediment cores indicates their advection
Peninsula and merged with existing multi- to late Holocene (Brachfeld et al., 2003). into the sub-ice cavity throughout the
beam mapping data of the area (Lavoie et Cosmogenic-nuclide exposure ages from Holocene (Brachfeld et al., 2003). The
al., 2015). This work shows that an exten- coastal sites support a Larsen A ice-shelf Larsen A area is connected to the Bransfield
sive system of outlet glaciers and lateral ice
domes extended from the present coastline
during the LGM, reaching the shelf break
in at least some areas on each side of the
Antarctic Peninsula. Evidence for flowing
grounded ice in the Larsen B embayment
was found as deep as 1100 m below mod-
ern sea level. However, some areas that are
inland of the maximum grounding line, and
thus were overridden by glacial ice, show
no evidence of having had grounded ice on
the seafloor. Rather, these areas show flat-
lying sedimentary layering interpreted as
subglacial lake deposits formed when ice
was grounded farther offshore but not in
the deepest parts of the inland basin
(Rebesco et al., 2014). A sudden drop in
elevation in one area of Crane Glacier just
inland of a set of exposed lake deposits
within the fjord seabed was interpreted as a
subglacial lake drainage event induced by
recent (post-ice-shelf disintegration) sur-
face slope changes (Scambos et al., 2011).
When expanded during the LGM, ice
was grounded on the eastern continental
shelf for several hundred kilometers
beyond the current glacier grounding lines Figure 2. Geomorphic features mapped on the seafloor of the Larsen A and Larsen B embayments,
which were used for reconstructing paleo-ice flow patterns on the shelf. Features are mapped across
(Lavoie et al., 2015; Campo et al., 2017). the area where multibeam data were collected. Gaps in the feature mapping largely represent areas
Glacial geomorphic features on the sea- where no geophysical data could be collected due to extensive ice cover. Thin solid lines are 500 m
bathymetry contour. Blue lines represent modern ice divides. Dashed lines represent paleo-ice divides
floor record shifting ice-flow patterns as (Lavoie et al., 2015). Based on mapping from Campo et al. (2017). MSGL—mega-scale glacial lineations.

6 GSA Today | August 2019