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| Titel |
Composition and microstructure of marine ice in the Southern McMurdo ice Shelf, Antarctica |
| VerfasserIn |
I. Koch, S. Fitzsimons, N. Cullen, M. Hambrey, D. Samyn, J.-L. Tison |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250058816
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| Zusammenfassung |
| Marine ice forms from a mixture of sea water and glacial melt water at the base of ice shelves
and is thought to enhance ice shelf stability by adding dense ice mass or filling bottom
crevasses, rifts or inverted depressions. Although widespread in Antarctic ice shelves, the
physical and chemical properties of marine ice remain poorly understood. The aim of the
study is to understand processes of marine ice formation and its influence on the behavior of
the Southern McMurdo Ice Shelf (SMIS). The SMIS is a small ice shelf (~5000 km2)
adjacent to the western margin of the Ross Ice Shelf that flows southwest at velocities
between 4 and 7 m a-1. As the ice becomes grounded in the south it slows to 1 to 2 m a-1
and a broad zone of dark ice crops out on the surface of the ice shelf. This ice contains
marine debris and macrofossils and has a negative surface mass balance of -0.16 m
a-1.
Eight ice cores between 2.5 and 10 m in length were extracted from the ice
shelf surface at a distance of 0.5 to 6 km from shore from within and beyond the
dark ice zone. Ice from within the dark ice zone is more depleted in heavy isotopes
than sea water but more enriched than glacier ice (-4 to +3 0/00δ18O and -35to +25
0/00δD), with a total salinity between 0.03 and 0.9 ppt. In vertical thin section,
granular or elongated ice crystals 2 – 15 mm in size commonly show single-maximum
anisotropic fabrics, characteristic of frazil ice crystals derived from a supercooled
mix of sea- and glacier water at the bottom of the ice shelf. Occasionally fold-like
textures of frazil ice crystals with multiple maxima ice fabrics indicate exposure to
post-accumulation stress. Ice extracted ~2 to 6 km away from shore marks the
boundary with meteoric ice. The meteoric ice shows equigranular ice crystals with
air bubbles, weaker or isotropic ice crystal fabrics and low salinities of < 0.01
ppt on average. The ice is enriched in heavy isotopes with ratios plotting along
the meteoric water-line of δD = 8.18 δ18O + 7.53 (r = 0.99). Blocks of ice were
also cut from an ice-cored moraine at the southern edge of the ice shelf. In the
moraine ice, water isotope ratios also indicate a mixed sea- and glacier water origin,
with -8 to -12 0/00δ18O, -80 to -100 0/00δD and salinities of 0.03 to 0.4 ppt. The
moraine ice crystals commonly have a size of 20 – 60 mm, with multiple maxima
crystal-orientation fabrics indicating recrystallization and grain growth within this old marine
ice.
Data from this study suggests that marine ice forms in thick layers of frazil ice crystals
below SMIS and becomes deformed at the ice margin or after grounding as evident
from complex multiple maxima orientation fabrics. Marine ice is characterized
by higher salinity, more enriched δ18O and δD values than meteoric ice, and ice
fabrics that have a strong preferred orientation with frequently elongated crystals. |
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