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The chemical, physical and structural properties of estuarine ice in Great Bay, New Hampshire
Estuarine,
Coastal
and Shelf
Science
(1987) 24,833-840
The Chemical, Physical Properties of Estuarine New Hampshire
Debra Walter
and Structural Ice in Great Bay,
A. Meese”sb, Anthony J. Gowb, Paul Ficklin’ and Theodore C. Loder”
A. Mayewski”,
“Glacier Research Group, University of New Hampshire, Durham, New Hampshire 03824, U.S.A.; biJ.S. Army Cold Regions Laboratory, Hanover, New Hampshire 03755, U.S.A.; Survey, Branch of Exploration Geochemistry, Denver, Received
2 July
1985 and in revised
Keywords: chemical properties; New Hampshire
form
Research and’lJ.S. Colorado
and Engineering Geological 80225, U.S.A.
17 FebruarJll986
physical properties;
structure;
estuaries; ice;
The purpose of this study was to provide general information on the chemical, physicat and structural properties of estuarine ice and show how it compares with sea ice found at higher latitudes in order to determine whether the ice in Great Bay can be used as an analog in the study of arctic sea ice. Ice cores and water samples were collected during the 1983-84 winter season at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulfate and silicate were determined for samples chosen on the basis of identifiable stratigraphic layers (i.e. bubble size and shape, sediment layers, etc.). Similarities between ice formation in Great Bay and those in the arctic regions include the nature of the freezing process and the ice types produced. In addition, the distribution and concentration of chemical constituents were found to be similar to those observed in arctic sea ice. Factors affecting the chemistry of the ice in Great Bay include rainfall during the freezing season, the presence of sediment layers in the ice cores, the nature of incorporation of brine into the crystal structure of the ice and the drainage of brine. Introduction
To date, research on sea ice has concentrated on the polar regions (Weeks & Ackley, 1982) and only very limited information is available on this type of ice at the lower latitudes. The purpose
of this study was to provide
general
information
on the chemical,
physical
and
structural properties of estuarine ice, mainly to seehow it compareswith seaice found at higher latitudes and to determine whether the ice in Great Bay can be usedasan analog for studying arctic seaice. During
the 198384
freezing
season (late
December-late
February)
ice cores and
surface-water sampleswere taken 3-4 times per week at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulphate and silicate of both water and ice sampleswere measured. In the case 833
0272-7714!87/0b0833+08
$03.00/O
0 1987 Academic Press Inc. (London) Limited
834
D. A. Meese et al.
MNN ‘\i4-
Greenland 0
I 4
Miles
Figure 1. Location map of Great Bay.
of the seaice, sampleswere chosen using identifiable stratigraphic layers. Results of this study together with data on the crystalline structure of the seaice and entrained debris are presented in this paper. Sampling area The sampling site chosenfor this study wasAdams Point in Great Bay, south-eastern New Hampshire (Figure 1). This site was chosenbecauseof its accessibility and assurancesof a seasonalice cover. In Great Bay, ice usually begins to form in late December and may persist into March. The extent of ice cover varies throughout the freezing seasonand from seasonto season depending on weather conditions. During a mild seasonice may be limited to small inlets and remain relatively thin (l-2 cm), whereas during a harsh winter ice may cover the entire Bay and reach thicknessesof up to 30 cm. Although there appears to be a large inflw of freshwater into the Bay, which would create a brackish water situation, Loder et al. (1983) found that the combined freshwater discharge of the major tributaries entering Great Bay contributes lessthan 2% of the total water volume exchanged during each tidal cycle. Accordingly, mixing in the Estuary is predominately tidal. The tidal current and the turbulence associatedwith it produce a well-mixed water column throughout most of the Bay (Loder et al., 1983). Average January salinity at Furber Strait is 23%, which is lessthan that of the Gulf of Maine (32%0) (Loder et al., 1983) but normal for estuarine conditions.
Properties ojestuarine
835
ice
Methods Sampleswere taken 2-4 times per week during the 9-week sampling period. During each visit two ice cores and two water sampleswere collected. In addition, weather data, ice thickness, ice temperature and water temperature were measured. A total of 45 ice cores and 74 water sampleswere collected. Cores were taken using a t&on-coated SIPRE ice auger. Cores were double bagged in polyethylene bags, transported in a cooler and stored in a freezer at - 20 “C in the Glacier Research Group (GRG) Laboratory, University of New Hampshire. Water sampleswere collected in 30-ml precleaned Nalgene containers and kept frozen until analysis. Containers were precleaned by rinsing 3-4 times with double distilled-deionized water. Each ice core was described structurally (including observations of bubble size and density, brine pocket characteristics and sediment layers). Cores were cut in half lengthwise and were then sectioned for chemical analysis basedon stratigraphy. The outer 1 cm of half of the core wasmelted off using double distilled-deionized water in order to remove any contaminants introduced during coring and preliminary handling. Each section was then placed in a precleaned polystyrene jar. The jars were precleaned by rinsing them 3-4 times with double distilled-deionized water, filled and soakedovernight. The following day, the jars were rinsed 2-3 times and sealedwith plastic tape until use. At the end of the seasonthe sampleswere sent frozen to the Water Laboratory, Branch of Exploration Geochemistry, United States Geological Survey, Denver, where they were analysed for chloride, bromide, phosphate and sulphate using a Dionex Model 212Oi* Ion Chromatograph. A standard eluent of sodiumbicarbonate (0.003 M) and sodium carbonate (0.0024 M) was used for the analysis. The samples were then returned to the GRG where they were analysed for nitrogen (as nitrate and nitrite), phosphate and silicate by Autoanalyser using the techniques of Glibert and Loder (1977). The remaining half of the sectioned cores were taken to the Cold Regions Researchand Engineering Laboratory (CRREL) where horizontal and vertical thin sections(0.5 mm or less) were prepared. These sections were photographed in both reflected light and between crossedpolarizers in order to determine ice types, sizes,shapesand orientation of crystals and ice/entrapped sediment relationships. Results and discussion Growth and structure of ice The 1983-84 freezing seasonin Great Bay was relatively mild compared to the previous 10 years and was characterized by a considerable amount of rain. December and January were within 2 “C of the normal temperature (basedon meansfrom 195I-80). However, the mean temperature for February was 7.4 “C above normal. Precipitation in December was 5 cm greater than normal and January precipitation was 3.8 cm lower than normal, but February precipitation was 3.8 cm higher. As a result the ice cover never extended beyond Adams Point, but it did remain beyond the sampling area until nearly total ablation occurred in February. Ice-sheet formation in the Bay was very similar to that documented for the arctic regions. The first stage of the freezing process was characterized by growth of frazil crystals that began forming on the surface of the water on 17 December 1983, giving it a *Any ment
use of trade names are for descriptive by the U.S. Geological Survey.
purposes
only and does not imply
endorse-
836
D. A. Meese et al.
Figure
2. Conrinuous
sheet of ice showing
pancake
structure.
greasy appearance. These crystals, up to 2 cm in diameter, coalescedto form the initial ice sheet. Two days later, wave action caused this sheet to break up into circular-shaped masses(pancakes) that developed raised rims due to collisions with other masses.Frazil continued to grow between the pancakesuntil, on 22 December, a continuous sheet of ice had formed (Figure 2). By 28 December the ice had thickened to 12 cm and the sheet extended to the edgeof the cove. Beneath the frazil ice a structural transition wasobserved that was characterized by a change in the crystalline texture of the ice that led ultimately to the formation of relatively large columnar-shaped crystals exhibiting brine lamellalice plate substructure, the trademark of typical congelation seaice. Locally, snow ice was found, formed by the flooding of a snow-covered ice surface by seawater. This results in the formation of slush which subsequently freezes to form the top layer of the ice sheet. The crystals in this layer are generally fine-grained and have a random orientation. Mixing of snow with rain or melt water followed by freezing produces a similar result. The changes in structure of the ice with increasing depth are very similar to those observed in polar areaswhere frazil ice generally constitutes the top layer and is followed by a transition zone leading into congelation ice composing W-900, of the thickness of a mature ice sheet. In Great Bay the top 2-4 cm of the ice wasfine-grained frazil ice overlain by variable amounts of generally fine-grained snow ice. The frazil ice was followed by a fairly rapid l-2 cm transition into congelation ice (Figure 3). The fraziI crystals, as observed in thin section, are short and fine grained, ranging in diameter from 0.1 to 0.5 cm. In congelation ice, the crystals are long, up to 19 cm in a 23-cm core. Crystal cross-sections increasedprogressively with depth in the congelation ice and grain diameters of 1 cm were observed near the bottom of the ice sheet. Concentrations of the elementsanalyzed did not
831
Properties of estuarine ice
Figure 3. Photographs ice-crystal structure.
of a thin section
of core 13 showing:
(a) sediment
bands and (b)
appear to be affected by ice type. Equal concentrations of the chemical speciesanalysed existed in both frazil and congelation ice. However, in all of the cores, a slight increaseof all elements except silicate exists in the transition zone. It also appears that the finer grained cores had slightly higher concentrations than those cores where the crystal size was larger. Chemical
composition
of ice
Newly formed ice hashigher chemical concentrations than older ice becausebrine drainage has not yet occurred (Weeks & Ackley, 1982). A correlation does exist between longterm (5-10 day) temperature extremes and chemical concentrations in the bottom layer of the ice core. As stated earlier, there was an abnormally cold period between 12 and 23 January. By 1’7January, the concentrations of all chemical speciesin the bottom layer of the core began to increase(Figure 4). After 2 February, the concentrations in the bottom layer began to decreaseagain indicating a lossof brine from this part of the core. However,
838
D. A. Meese
et
al.
Chloride
Or
20,OO
(ppm) 4OpO
6700
2468z E
IO12.
g
14. 16 18 20 22 24 1 I-18-84 Sulphote 0
Core 25 (ppm)
200 I
400
I
6
246~
:: I-18-84
Core
25
Figure 4. Plots of concentration VS.depth. Increased concentration in the bottom layer is due to brine concentrating between the crystal platelets.
to be a relationship between cores and either the rate of drainage between chemical speciesor the drainage of consecutive cores. This may be due partially to the effect produced by a considerable amount of rainfall during the last week of the freezing season. Concentrations of phosphate and silicate were compared to those reported for Great Bay by Loder et al. (1983). Phosphate concentrations in the water column during the freezing seasonincreased significantly. It is believed that phosphate is releasedinto the water that is held under the ice during at least part of the freezing season.The highest values of silicate found in the ice sampleswere from the layers containing sediment. The silicate is probably derived from the clay particles contained in these layers. Concentrations of the elements analysed were compared with depth in order to determine if any trends exist. With the exception of silicate and nitrogen (asnitrate and nitrite),
there does not appear
the concentration profiles of chemical species were similar. Concentrations of silicate ranged from 0.06 to 0.6 ppm in ice layers that were free of debris or sediment. If sediment was present in the ice, the silicate concentrations were as high as 14.3 ppm. An increase in the concentration of the chemical constituents was noted in all of the sediment layers. In several of the sediment layers, the sulphate concentration decreased, which could be due to the presence of sulphate-reducing bacteria in the sediment.
Properties
of estuarine
ice
839
Several times during the season the ice cover was flooded with sea water. When this occurred, the concentrations of chloride, bromide, phosphate and silicate reached their maxima for the upper ice layer. The maximum chloride concentration after flooding was 4806 ppm (salinity equivalent to 8.7%0) whereas the concentration in the top layer after a rainfall was as low as 107 ppm. The ranges for bromide, phosphate and silicate for the same conditions were 0.04-14.29,0.003-0.05 and 0.05-2.18 ppm, respectively. Sediment entrainment Sediment layers were observed in a number of the cores. In general, the ice crystals continued to grow through the sediment uninterrupted (Figure 3), indicating that in these instances the ice was not grounded but that the sediment particles had become frozen into the ice whilst remaining freely suspended in the water column probably due to tidalcurrent resuspension under the ice. Closer examination of thin sections showed that this entrainment of sediment actually occurred within the spaces between the crystal platelets where brine is normally trapped. In shallower parts of the estuary the potential always exists for ice grounding at low tide and this is known to have occurred in Great Bay iThompson, 1977), leading to freezing on of muds to the bottom of the ice sheet. Our observations appear to be the first ever of debris entrapment in actively growing crystals of congelation ice. However, there are numerous reports of sediment entrapment in arctic sea ice, usually in the top 1 m of the ice sheet. This entrainment of sediment invariably occurs in conjunction with frazil-ice formation and not in congelation ice as observed in Great Bay. One explanation for the arctic situation is that early winter-storm action causes stirring up of bottom sediments, thus promoting turbulence in water column and hence frazil formation. Suspended debris then becomes entrained mechanically between the grains of frazil. This explanation together with the Great Bay observations strongly implies that the presence of suspended sediment in the water column is not a necessary condition for generating frazil and that sediment particles and frazil can co-exist without any cause-and-effect relationship between the two, i.e. the suspended particles do not actively nucleate growth of frazil crystals in the water column. A similar situation probably applies to frazil and sediment particles found mixed together in river ice. Conclusions Ice formation in Great Bay occurs by the same processes as those that apply in the arctic and, furthermore, the crystal structure is identical. Also, trends of the chemical constituents with depth are similar to those found by Clarke and Ackley (1984) in the Weddell Sea, in that the concentrations are lower in the ice than in,the adjacent surface water. Sectioning of the cores by ice type rather than bubble stratigraphy may show that enrichment or depletion of chemical species may vary with ice type. Sediment layers were observed in several cores. Thin-section studies showed that sediment entrainment actually occurred within the spaces between the crystal platelets of actively growing sea ice, therefore, demonstrating that this sediment was not incorporated while the ice sheet was grounded, but rather that the particles had become frozen into the ice while they were freely suspended in the water column. Acknowledgements The authors would like to thank James Kauer for conducting the HPLC analyses and Mark Twickler and Dan O’Shea for assistance in sampling. Funding for this project was provided by Sea Grant.
840
D. A. Meese
et al.
References Clarke, D. B. & Ackley, S. F. 1984 Sea ice structure and biological activity in the Antarctic marginal ice zone. journal oj Geophysical Research 89,2007-2095. Glibert, P. L. & Loder, T. C. Automated analysis of nutrients in seawater: A manual of techniques. Woods Hole Oceanographic insriturion Technical Report 77-47. Loder, T. C., Love, J. A., Penniman, C. E. &Neefus, C. D. 1983 Long-term environmental trends in nutrient and hydrographic data from the Great Bay Estuarine system, New Hampshire-Maine. VNH Marine Program Report No. VNH-MP-D/ TR-SG-83-6. Thompson, C. 1977 The role of ice as an agent of erosion and deposition of an estuarine tidal flat. Unpublished M.Sc. thesis, University of New Hampshire. Weeks, W. F. & Ackley, S. F. 1982 The growth, structure and properties of sea ice. VSACRREL Monograph 82-l.
Coastal
and Shelf
Science
(1987) 24,833-840
The Chemical, Physical Properties of Estuarine New Hampshire
Debra Walter
and Structural Ice in Great Bay,
A. Meese”sb, Anthony J. Gowb, Paul Ficklin’ and Theodore C. Loder”
A. Mayewski”,
“Glacier Research Group, University of New Hampshire, Durham, New Hampshire 03824, U.S.A.; biJ.S. Army Cold Regions Laboratory, Hanover, New Hampshire 03755, U.S.A.; Survey, Branch of Exploration Geochemistry, Denver, Received
2 July
1985 and in revised
Keywords: chemical properties; New Hampshire
form
Research and’lJ.S. Colorado
and Engineering Geological 80225, U.S.A.
17 FebruarJll986
physical properties;
structure;
estuaries; ice;
The purpose of this study was to provide general information on the chemical, physicat and structural properties of estuarine ice and show how it compares with sea ice found at higher latitudes in order to determine whether the ice in Great Bay can be used as an analog in the study of arctic sea ice. Ice cores and water samples were collected during the 1983-84 winter season at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulfate and silicate were determined for samples chosen on the basis of identifiable stratigraphic layers (i.e. bubble size and shape, sediment layers, etc.). Similarities between ice formation in Great Bay and those in the arctic regions include the nature of the freezing process and the ice types produced. In addition, the distribution and concentration of chemical constituents were found to be similar to those observed in arctic sea ice. Factors affecting the chemistry of the ice in Great Bay include rainfall during the freezing season, the presence of sediment layers in the ice cores, the nature of incorporation of brine into the crystal structure of the ice and the drainage of brine. Introduction
To date, research on sea ice has concentrated on the polar regions (Weeks & Ackley, 1982) and only very limited information is available on this type of ice at the lower latitudes. The purpose
of this study was to provide
general
information
on the chemical,
physical
and
structural properties of estuarine ice, mainly to seehow it compareswith seaice found at higher latitudes and to determine whether the ice in Great Bay can be usedasan analog for studying arctic seaice. During
the 198384
freezing
season (late
December-late
February)
ice cores and
surface-water sampleswere taken 3-4 times per week at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulphate and silicate of both water and ice sampleswere measured. In the case 833
0272-7714!87/0b0833+08
$03.00/O
0 1987 Academic Press Inc. (London) Limited
834
D. A. Meese et al.
MNN ‘\i4-
Greenland 0
I 4
Miles
Figure 1. Location map of Great Bay.
of the seaice, sampleswere chosen using identifiable stratigraphic layers. Results of this study together with data on the crystalline structure of the seaice and entrained debris are presented in this paper. Sampling area The sampling site chosenfor this study wasAdams Point in Great Bay, south-eastern New Hampshire (Figure 1). This site was chosenbecauseof its accessibility and assurancesof a seasonalice cover. In Great Bay, ice usually begins to form in late December and may persist into March. The extent of ice cover varies throughout the freezing seasonand from seasonto season depending on weather conditions. During a mild seasonice may be limited to small inlets and remain relatively thin (l-2 cm), whereas during a harsh winter ice may cover the entire Bay and reach thicknessesof up to 30 cm. Although there appears to be a large inflw of freshwater into the Bay, which would create a brackish water situation, Loder et al. (1983) found that the combined freshwater discharge of the major tributaries entering Great Bay contributes lessthan 2% of the total water volume exchanged during each tidal cycle. Accordingly, mixing in the Estuary is predominately tidal. The tidal current and the turbulence associatedwith it produce a well-mixed water column throughout most of the Bay (Loder et al., 1983). Average January salinity at Furber Strait is 23%, which is lessthan that of the Gulf of Maine (32%0) (Loder et al., 1983) but normal for estuarine conditions.
Properties ojestuarine
835
ice
Methods Sampleswere taken 2-4 times per week during the 9-week sampling period. During each visit two ice cores and two water sampleswere collected. In addition, weather data, ice thickness, ice temperature and water temperature were measured. A total of 45 ice cores and 74 water sampleswere collected. Cores were taken using a t&on-coated SIPRE ice auger. Cores were double bagged in polyethylene bags, transported in a cooler and stored in a freezer at - 20 “C in the Glacier Research Group (GRG) Laboratory, University of New Hampshire. Water sampleswere collected in 30-ml precleaned Nalgene containers and kept frozen until analysis. Containers were precleaned by rinsing 3-4 times with double distilled-deionized water. Each ice core was described structurally (including observations of bubble size and density, brine pocket characteristics and sediment layers). Cores were cut in half lengthwise and were then sectioned for chemical analysis basedon stratigraphy. The outer 1 cm of half of the core wasmelted off using double distilled-deionized water in order to remove any contaminants introduced during coring and preliminary handling. Each section was then placed in a precleaned polystyrene jar. The jars were precleaned by rinsing them 3-4 times with double distilled-deionized water, filled and soakedovernight. The following day, the jars were rinsed 2-3 times and sealedwith plastic tape until use. At the end of the seasonthe sampleswere sent frozen to the Water Laboratory, Branch of Exploration Geochemistry, United States Geological Survey, Denver, where they were analysed for chloride, bromide, phosphate and sulphate using a Dionex Model 212Oi* Ion Chromatograph. A standard eluent of sodiumbicarbonate (0.003 M) and sodium carbonate (0.0024 M) was used for the analysis. The samples were then returned to the GRG where they were analysed for nitrogen (as nitrate and nitrite), phosphate and silicate by Autoanalyser using the techniques of Glibert and Loder (1977). The remaining half of the sectioned cores were taken to the Cold Regions Researchand Engineering Laboratory (CRREL) where horizontal and vertical thin sections(0.5 mm or less) were prepared. These sections were photographed in both reflected light and between crossedpolarizers in order to determine ice types, sizes,shapesand orientation of crystals and ice/entrapped sediment relationships. Results and discussion Growth and structure of ice The 1983-84 freezing seasonin Great Bay was relatively mild compared to the previous 10 years and was characterized by a considerable amount of rain. December and January were within 2 “C of the normal temperature (basedon meansfrom 195I-80). However, the mean temperature for February was 7.4 “C above normal. Precipitation in December was 5 cm greater than normal and January precipitation was 3.8 cm lower than normal, but February precipitation was 3.8 cm higher. As a result the ice cover never extended beyond Adams Point, but it did remain beyond the sampling area until nearly total ablation occurred in February. Ice-sheet formation in the Bay was very similar to that documented for the arctic regions. The first stage of the freezing process was characterized by growth of frazil crystals that began forming on the surface of the water on 17 December 1983, giving it a *Any ment
use of trade names are for descriptive by the U.S. Geological Survey.
purposes
only and does not imply
endorse-
836
D. A. Meese et al.
Figure
2. Conrinuous
sheet of ice showing
pancake
structure.
greasy appearance. These crystals, up to 2 cm in diameter, coalescedto form the initial ice sheet. Two days later, wave action caused this sheet to break up into circular-shaped masses(pancakes) that developed raised rims due to collisions with other masses.Frazil continued to grow between the pancakesuntil, on 22 December, a continuous sheet of ice had formed (Figure 2). By 28 December the ice had thickened to 12 cm and the sheet extended to the edgeof the cove. Beneath the frazil ice a structural transition wasobserved that was characterized by a change in the crystalline texture of the ice that led ultimately to the formation of relatively large columnar-shaped crystals exhibiting brine lamellalice plate substructure, the trademark of typical congelation seaice. Locally, snow ice was found, formed by the flooding of a snow-covered ice surface by seawater. This results in the formation of slush which subsequently freezes to form the top layer of the ice sheet. The crystals in this layer are generally fine-grained and have a random orientation. Mixing of snow with rain or melt water followed by freezing produces a similar result. The changes in structure of the ice with increasing depth are very similar to those observed in polar areaswhere frazil ice generally constitutes the top layer and is followed by a transition zone leading into congelation ice composing W-900, of the thickness of a mature ice sheet. In Great Bay the top 2-4 cm of the ice wasfine-grained frazil ice overlain by variable amounts of generally fine-grained snow ice. The frazil ice was followed by a fairly rapid l-2 cm transition into congelation ice (Figure 3). The fraziI crystals, as observed in thin section, are short and fine grained, ranging in diameter from 0.1 to 0.5 cm. In congelation ice, the crystals are long, up to 19 cm in a 23-cm core. Crystal cross-sections increasedprogressively with depth in the congelation ice and grain diameters of 1 cm were observed near the bottom of the ice sheet. Concentrations of the elementsanalyzed did not
831
Properties of estuarine ice
Figure 3. Photographs ice-crystal structure.
of a thin section
of core 13 showing:
(a) sediment
bands and (b)
appear to be affected by ice type. Equal concentrations of the chemical speciesanalysed existed in both frazil and congelation ice. However, in all of the cores, a slight increaseof all elements except silicate exists in the transition zone. It also appears that the finer grained cores had slightly higher concentrations than those cores where the crystal size was larger. Chemical
composition
of ice
Newly formed ice hashigher chemical concentrations than older ice becausebrine drainage has not yet occurred (Weeks & Ackley, 1982). A correlation does exist between longterm (5-10 day) temperature extremes and chemical concentrations in the bottom layer of the ice core. As stated earlier, there was an abnormally cold period between 12 and 23 January. By 1’7January, the concentrations of all chemical speciesin the bottom layer of the core began to increase(Figure 4). After 2 February, the concentrations in the bottom layer began to decreaseagain indicating a lossof brine from this part of the core. However,
838
D. A. Meese
et
al.
Chloride
Or
20,OO
(ppm) 4OpO
6700
2468z E
IO12.
g
14. 16 18 20 22 24 1 I-18-84 Sulphote 0
Core 25 (ppm)
200 I
400
I
6
246~
:: I-18-84
Core
25
Figure 4. Plots of concentration VS.depth. Increased concentration in the bottom layer is due to brine concentrating between the crystal platelets.
to be a relationship between cores and either the rate of drainage between chemical speciesor the drainage of consecutive cores. This may be due partially to the effect produced by a considerable amount of rainfall during the last week of the freezing season. Concentrations of phosphate and silicate were compared to those reported for Great Bay by Loder et al. (1983). Phosphate concentrations in the water column during the freezing seasonincreased significantly. It is believed that phosphate is releasedinto the water that is held under the ice during at least part of the freezing season.The highest values of silicate found in the ice sampleswere from the layers containing sediment. The silicate is probably derived from the clay particles contained in these layers. Concentrations of the elements analysed were compared with depth in order to determine if any trends exist. With the exception of silicate and nitrogen (asnitrate and nitrite),
there does not appear
the concentration profiles of chemical species were similar. Concentrations of silicate ranged from 0.06 to 0.6 ppm in ice layers that were free of debris or sediment. If sediment was present in the ice, the silicate concentrations were as high as 14.3 ppm. An increase in the concentration of the chemical constituents was noted in all of the sediment layers. In several of the sediment layers, the sulphate concentration decreased, which could be due to the presence of sulphate-reducing bacteria in the sediment.
Properties
of estuarine
ice
839
Several times during the season the ice cover was flooded with sea water. When this occurred, the concentrations of chloride, bromide, phosphate and silicate reached their maxima for the upper ice layer. The maximum chloride concentration after flooding was 4806 ppm (salinity equivalent to 8.7%0) whereas the concentration in the top layer after a rainfall was as low as 107 ppm. The ranges for bromide, phosphate and silicate for the same conditions were 0.04-14.29,0.003-0.05 and 0.05-2.18 ppm, respectively. Sediment entrainment Sediment layers were observed in a number of the cores. In general, the ice crystals continued to grow through the sediment uninterrupted (Figure 3), indicating that in these instances the ice was not grounded but that the sediment particles had become frozen into the ice whilst remaining freely suspended in the water column probably due to tidalcurrent resuspension under the ice. Closer examination of thin sections showed that this entrainment of sediment actually occurred within the spaces between the crystal platelets where brine is normally trapped. In shallower parts of the estuary the potential always exists for ice grounding at low tide and this is known to have occurred in Great Bay iThompson, 1977), leading to freezing on of muds to the bottom of the ice sheet. Our observations appear to be the first ever of debris entrapment in actively growing crystals of congelation ice. However, there are numerous reports of sediment entrapment in arctic sea ice, usually in the top 1 m of the ice sheet. This entrainment of sediment invariably occurs in conjunction with frazil-ice formation and not in congelation ice as observed in Great Bay. One explanation for the arctic situation is that early winter-storm action causes stirring up of bottom sediments, thus promoting turbulence in water column and hence frazil formation. Suspended debris then becomes entrained mechanically between the grains of frazil. This explanation together with the Great Bay observations strongly implies that the presence of suspended sediment in the water column is not a necessary condition for generating frazil and that sediment particles and frazil can co-exist without any cause-and-effect relationship between the two, i.e. the suspended particles do not actively nucleate growth of frazil crystals in the water column. A similar situation probably applies to frazil and sediment particles found mixed together in river ice. Conclusions Ice formation in Great Bay occurs by the same processes as those that apply in the arctic and, furthermore, the crystal structure is identical. Also, trends of the chemical constituents with depth are similar to those found by Clarke and Ackley (1984) in the Weddell Sea, in that the concentrations are lower in the ice than in,the adjacent surface water. Sectioning of the cores by ice type rather than bubble stratigraphy may show that enrichment or depletion of chemical species may vary with ice type. Sediment layers were observed in several cores. Thin-section studies showed that sediment entrainment actually occurred within the spaces between the crystal platelets of actively growing sea ice, therefore, demonstrating that this sediment was not incorporated while the ice sheet was grounded, but rather that the particles had become frozen into the ice while they were freely suspended in the water column. Acknowledgements The authors would like to thank James Kauer for conducting the HPLC analyses and Mark Twickler and Dan O’Shea for assistance in sampling. Funding for this project was provided by Sea Grant.
840
D. A. Meese
et al.
References Clarke, D. B. & Ackley, S. F. 1984 Sea ice structure and biological activity in the Antarctic marginal ice zone. journal oj Geophysical Research 89,2007-2095. Glibert, P. L. & Loder, T. C. Automated analysis of nutrients in seawater: A manual of techniques. Woods Hole Oceanographic insriturion Technical Report 77-47. Loder, T. C., Love, J. A., Penniman, C. E. &Neefus, C. D. 1983 Long-term environmental trends in nutrient and hydrographic data from the Great Bay Estuarine system, New Hampshire-Maine. VNH Marine Program Report No. VNH-MP-D/ TR-SG-83-6. Thompson, C. 1977 The role of ice as an agent of erosion and deposition of an estuarine tidal flat. Unpublished M.Sc. thesis, University of New Hampshire. Weeks, W. F. & Ackley, S. F. 1982 The growth, structure and properties of sea ice. VSACRREL Monograph 82-l.