scholarly journals Seasonal changes in glacial polynya activity inferred from Weddell Sea varves

2013 ◽  
Vol 9 (5) ◽  
pp. 5123-5156 ◽  
Author(s):  
D. Sprenk ◽  
M. E. Weber ◽  
G. Kuhn ◽  
V. Wennrich ◽  
T. Hartmann ◽  
...  
Keyword(s):  
Grain Size ◽  
Bottom Water ◽  
Chemical Elements ◽  
Winter Season ◽  
Ice Sheet ◽  
Weddell Sea ◽  
Water Production ◽  
Thin Sections ◽  
Fine Grained ◽  
Coastal Polynya

Abstract. The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium to coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, chemical elements, Fe, Ti, Rb, and K are elevated as well. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of glacially enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. If this is correct, silty layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom water formation and increased sediment transport. Vice versa, finer-grained clayey layers were then deposited during summer, when coastal polynya activity was likely reduced.

scholarly journals Seasonal changes in glacial polynya activity inferred from Weddell Sea varves

2014 ◽  
Vol 10 (3) ◽  
pp. 1239-1251 ◽  
Author(s):  
D. Sprenk ◽  
M. E. Weber ◽  
G. Kuhn ◽  
V. Wennrich ◽  
T. Hartmann ◽  
...  
Keyword(s):  
Grain Size ◽  
Bottom Water ◽  
Winter Season ◽  
Ice Sheet ◽  
Weddell Sea ◽  
Water Production ◽  
Thin Sections ◽  
Fine Grained ◽  
Thermohaline Convection ◽  
Coastal Polynya

Abstract. The Weddell Sea and the associated Filchner–Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium-to-coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, Fe, Ti, Rb, and K are elevated. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity during the LGM. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. Under these conditions, lighter coarser-grained layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom-water formation and increased sediment transport. Vice versa, darker finer-grained layers were then deposited during less windier season, mainly during summer, when coastal polynya activity was likely reduced.

scholarly journals Strain-Rate and Grain‒Size Effects in Ice

1987 ◽  
Vol 33 (115) ◽  
pp. 274-280 ◽  
Author(s):  
David M. Cole
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Transition Point ◽  
Boundary Migration ◽  
Peak Stress ◽  
Strain Rates ◽  
Thin Sections ◽  
Fine Grained ◽  
Lower Strain ◽  
Polycrystalline Ice

AbstractThis paper presents and discusses the results of constant deformation-rate tests on laboratory-prepared polycrystalline ice. Strain-rates ranged from 10−7to 10−1s−1, grain–size ranged from 1.5 to 5.8 mm, and the test temperature was −5°C.At strain-rates between 10−7and 10−3s−1, the stress-strain-rate relationship followed a power law with an exponent ofn= 4.3 calculated without regard to grain-size. However, a reversal in the grain-size effect was observed: below a transition point near 4 × 10−6s−1the peak stress increased with increasing grain-size, while above the transition point the peak stress decreased with increasing grain-size. This latter trend persisted to the highest strain-rates observed. At strain-rates above 10−3s−1the peak stress became independent of strain-rate.The unusual trends exhibited at the lower strain-rates are attributed to the influence of the grain-size on the balance of the operative deformation mechanisms. Dynamic recrystallization appears to intervene in the case of the finer-grained material and serves to lower the peak stress. At comparable strain-rates, however, the large-grained material still experiences internal micro-fracturing, and thin sections reveal extensive deformation in the grain-boundary regions that is quite unlike the appearance of the strain-induced boundary migration characteristic of the fine-grained material.

scholarly journals Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves 

2021 ◽  
Author(s):  
Meghana Ranganathan ◽  
Brent Minchew ◽  
Colin Meyer ◽  
Matej Pec
Keyword(s):  
Grain Size ◽  
Ice Sheet ◽  
Shear Zones ◽  
Dislocation Creep ◽  
Model Parameters ◽  
Localized Deformation ◽  
Ice Shelves ◽  
Fine Grained ◽  
Initiation And Propagation ◽  
Rapid Deformation

<p>The initiation and propagation of fractures in floating regions of Antarctica has the potential to destabilize large regions of the ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the creep and toughness of ice. Here we derive a model for dynamic recrystallization in ice and other rocks that includes a novel representation of migration recrystallization, which is absent from existing models but is likely to be dominant in warm areas undergoing rapid deformation within the ice sheet. We show that, in regions of elevated strain rate, grain sizes in ice may be larger than expected (~15 mm) due to migration recrystallization, a significant deviation from solid earth studies which find fine-grained rock in shear zones. This may imply that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law exponent of n=4 in these regions. Further, we find from existing models that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, we expect that this increase in grain size makes the margins of fast-flowing glaciers less viscous and more vulnerable to fracture than we may suppose from standard model parameters.</p>

scholarly journals Ice-Fabric Study of the Mawson Region, East Antarctica

1969 ◽  
Vol 8 (53) ◽  
pp. 253-276 ◽  
Author(s):  
K. Kizaki
Keyword(s):  
Grain Size ◽  
Grain Growth ◽  
Ice Sheet ◽  
Secondary Recrystallization ◽  
East Antarctica ◽  
Strain Rates ◽  
Fine Grained ◽  
Single Maximum ◽  
Multiple Maxima

Attempts are made to test the relation predicted by Brace (1960) between strain-rates and the ice-fabric patterns obtained at Mawson station, east Antarctica. These orientation fabrics not only are hardly related to the prediction by Brace (1960) or Kamb (1959) but also change easily within a strain grid with 100m diagonals.Stable patterns of two- and three-maximum fabrics are confirmed. The latter is common and stable in the coarse ice at the surface of the ice sheet. It is apparent that the fabric patterns are generally related to the grain-size. The single-maximum fabric always occurs in fine-grained ice, then more maxima are formed in the course of grain growth.It appears that syntectonic-secondary recrystallization is effective in producing the orientation fabrics with two, three and multiple maxima. Also, the maxima always shift away from the pole of foliation as grain-size increases and there are several stable positions of maximum such as 0°, 17°, 23° and 30°. It is expected that further stable angles would occur with coarser crystals as found in temperate glaciers.

scholarly journals Strain-Rate and Grain‒Size Effects in Ice

1987 ◽  
Vol 33 (115) ◽  
pp. 274-280 ◽  
Author(s):  
David M. Cole
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Transition Point ◽  
Boundary Migration ◽  
Peak Stress ◽  
Strain Rates ◽  
Thin Sections ◽  
Fine Grained ◽  
Lower Strain ◽  
Polycrystalline Ice

AbstractThis paper presents and discusses the results of constant deformation-rate tests on laboratory-prepared polycrystalline ice. Strain-rates ranged from 10−7 to 10−1s−1, grain–size ranged from 1.5 to 5.8 mm, and the test temperature was −5°C.At strain-rates between 10−7 and 10−3 s−1, the stress-strain-rate relationship followed a power law with an exponent of n = 4.3 calculated without regard to grain-size. However, a reversal in the grain-size effect was observed: below a transition point near 4 × 10−6 s−1 the peak stress increased with increasing grain-size, while above the transition point the peak stress decreased with increasing grain-size. This latter trend persisted to the highest strain-rates observed. At strain-rates above 10−3 s−1 the peak stress became independent of strain-rate.The unusual trends exhibited at the lower strain-rates are attributed to the influence of the grain-size on the balance of the operative deformation mechanisms. Dynamic recrystallization appears to intervene in the case of the finer-grained material and serves to lower the peak stress. At comparable strain-rates, however, the large-grained material still experiences internal micro-fracturing, and thin sections reveal extensive deformation in the grain-boundary regions that is quite unlike the appearance of the strain-induced boundary migration characteristic of the fine-grained material.

scholarly journals Ice-Fabric Study of the Mawson Region, East Antarctica

1969 ◽  
Vol 8 (53) ◽  
pp. 253-276 ◽  
Author(s):  
K. Kizaki
Keyword(s):  
Grain Size ◽  
Grain Growth ◽  
Ice Sheet ◽  
Secondary Recrystallization ◽  
East Antarctica ◽  
Strain Rates ◽  
Fine Grained ◽  
Single Maximum ◽  
Multiple Maxima

Attempts are made to test the relation predicted by Brace (1960) between strain-rates and the ice-fabric patterns obtained at Mawson station, east Antarctica. These orientation fabrics not only are hardly related to the prediction by Brace (1960) or Kamb (1959) but also change easily within a strain grid with 100m diagonals.Stable patterns of two- and three-maximum fabrics are confirmed. The latter is common and stable in the coarse ice at the surface of the ice sheet. It is apparent that the fabric patterns are generally related to the grain-size. The single-maximum fabric always occurs in fine-grained ice, then more maxima are formed in the course of grain growth.It appears that syntectonic-secondary recrystallization is effective in producing the orientation fabrics with two, three and multiple maxima. Also, the maxima always shift away from the pole of foliation as grain-size increases and there are several stable positions of maximum such as 0°, 17°, 23° and 30°. It is expected that further stable angles would occur with coarser crystals as found in temperate glaciers.

WEAK PLIOCENE BOTTOM CURRENTS IN THE JANE BASIN, NW WEDDELL SEA BASED ON MULTIVARIATE ANALYSIS OF GRAIN SIZE SPECTRA

2017 ◽  
Author(s):  
Melissa R. Luna ◽  
◽  
Suzanne O'Connell ◽  
Joseph D. Ortiz ◽  
Michael C. Wizevich
Keyword(s):  
Grain Size ◽  
Multivariate Analysis ◽  
Weddell Sea ◽  
Size Spectra ◽  
Bottom Currents

scholarly journals Ventilation of the abyss in the Atlantic sector of the Southern Ocean

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Camille Hayatte Akhoudas ◽  
Jean-Baptiste Sallée ◽  
F. Alexander Haumann ◽  
Michael P. Meredith ◽  
Alberto Naveira Garabato ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Ocean ◽  
Analytical Framework ◽  
Weddell Sea ◽  
Global Ocean ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Atlantic Sector

AbstractThe Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m$$^{-3}$$ - 3 $$\gamma _{n}$$ γ n is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.

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