Basal Ice

The surface of the underlying ice is not conformable with the salt beds for there are often small irregularities in its surface from 2 to 6 cm high. These control the thickness of the overlying sediment layer.

Sediment Layer (0-8 cm)

The upper surface of the sediment layer forms a sharp conformable contact with the overlying salt bed, while the lower surface contours slight irregularities of the underlying ice surface. The sediment layer, devoid of internal bedding, consists of 80% glacial flour mixed with sand and small pebbles. The sand comprises volcanic rock fragments, angular grains of quartz and plagioclase, and shards of volcanic glass. One bryzoan fragment (<1 cm long) was recovered. The rare pebbles are basaltic; the largest measures 1.4 cm along its longest axis.

The sediment contains many fragments of marine diatoms and sponge spicules, <10 µm. No species are identifiable, although most fragments come from large centric planktonic marine diatoms and some fragments belong to the Thalassiothrix /Thalassionema genera. The sponge spicules are badly preserved, often with iron-stained and weathered central canals. Some fragments are similar to those produced by one of the authors in the laboratory by repeated ice fracturing (paper in preparation by H. T. Brady). Non-marine diatom frustules and algal spores are absent from the four sediment samples inspected.

Salt Layer

The salt bed is massive, consisting of coarse crystals of mirabilite; sometimes these crystals form large granules up to 95 mm across which are coated with anhydrous sodium sulphate powder when exposed to the air. Although there is no obvious bedding in the salt bed, small pods and stringers of pebbly sand occur. These are usually parallel or sub-parallel to the salt bed itself and vary in thickness from 0 to 12 cm. Broken shell fragments occur as rare isolated individual fragments.

When the salt is dissolved in distilled water, some fine mud and rare sand grains can be recovered. This mud, in three salt samples inspected, contains a perfectly preserved flora of non-marine diatoms. The five recovered species are: Navicula cymatopleura West and West, Navicula shackletoni West and West, Nitzschia antarctica (West and West) Baker, Navicula seminulum Grun, and Tropidoneis laevissima West and West. Some small marine diatom fragments also occur but these are poorly preserved; the only identifiable fragment was from Nitzschia kerguelensis (O’Meara) Hasle.

Boulder lag (0-30 cm)

The salt bed is covered by a lag of sediment, pebbles, cobbles, and boulders. The majority of these erratics, some of which are striated, come from the McMurdo alkaline volcanic province but there are some erratics of gneiss, granite, and sandstone from continental suites. This lag is mostly overlain by a non-marine algal mat but, in some places, the mat underlies or is mixed with the boulder lag.

Algal mat (0-26 cm)

The algal mat, which overlies the deposit, forms a compact and often continuous layer. One sample 4 m above the level of the pool-and-channel systems on pressure ridge C, site 1, yielded a radiocarbon age of 870±70 years B.P. (Sydney University Radiocarbon Laboratory, sample SUA 842). The one radiocarbon age so far obtained cannot be used to date the whole algal mat, since algae are still growing in pools which lie on the salt deposit in small depressions. The algal mat contains non-marine diatoms but these are not as numerous as in nearby pools on the present-day ice shelf. A high pH which re-cycles the silica of diatom frustules is consistent with massive non-marine algal production and may explain the scarcity of non-marine diatom fossils; pH values as high as 10.2 have been recorded in non-marine pools in the McMurdo Sound area (Reference Armitage and HouseArmitage and House, 1962; Reference SpurrSpurr, 1975).

INterprelation

Reference DebenhamDebenham (1920) suggested that the mirabilite he had observed on the Ross Ice Shelf was formed under the ice shelf by precipitation from brines. Anchor ice incorporated the salt into the ice shelf and it rose to the surface gradually as the surface ablated. Some small irregular pockets of mirabilite on the ice-shelf surface in this area could well be explained by Debenham’s hypothesis but not the large beds near Black Island. It does not seem possible that such large linear beds of friable salt could be brought directly to the surface by anchor ice; furthermore, the salt beds contain non-marine diatoms which indicate surface precipitation. If mirabilite reaching the surface from basal anchor ice was re-worked and recrystallized in large beds on the ice-shelf surface, then one would expect the basal sediment layer which has fallen through the settling brines to contain non-marine diatoms from ice-shelf pools. One would also expect more re-worked surface sediment to be bedded with the salt. But the basal sediment contains only marine diatom and sponge-spicule fragments (< 10 µm), which could be carried in suspension in a sub-ice-shelf brine. Since non-marine diatoms have only settled in the salt itself, it would seem that the basal sediment layer was formed immediately after the injection of a sub-ice-shelf brine and prior to non-marine algal production in the brine pools.

Reference Black and BowserBlack and Bowser (1968) described 132 continental mirabilite deposits near Hobbs Glacier on the western coast of southern McMurdo Sound (Fig. 1). Radiocarbon dates of non-marine algae associated with these deposits range from 12200±100 years B.P. These authors concluded that the mirabilite evaporated in surface non-marine pools but they regarded the ultimate source of the brines as unknown. Because the mirabilite is associated with volcanic erratics deposited by continental incursions of Koettlitz Glacier from McMurdo Sound (Reference PéwéPéwé, 1960), they suggested a volcanic source for the mirabilite. However, no such salt beds have been discovered on Ross Island or near volcanic cones in southern Victoria Land which can be directly associated with volcanic activity. Since all deposits in the McMurdo Sound area can be associated with ice-shelf advances, it is logical to find the key to mirabilite deposition in ice-shelf processes themselves. The mirabilite deposits near Hobbs Glacier up to 200 m above sea-level and those near Cape Barne 24 m above sea-level are at altitudes consistent with former Ross Ice Shelf levels in McMurdo Sound Reference Denton and Borns(Denton and Borns, 1974).

Reference Dort, Dort and AdieDort and Dort (1972) reviewed the literature on the McMurdo Sound and other continental mirabilites, and concluded that the salts were precipitated from sea-water stranded during the fall of high sea-levels. These authors simply applied the work of Reference Nelson and ThompsonNelson and Thompson (1954) on the crystallization of salts from freezing sea-water to the McMurdo Sound mirabilites. It is possible to precipitate mirabilite from brines after 88% of the seawater has frozen between the temperatures of — 8.9°C and — 22.9°C. These authors agree that mirabilite is formed in this manner but disagree with Dort and Dort’s suggestions about high sea-levels and stranded sea-water. Since mirabilite can form in large quantities on an ice-shelf surface, it is not necessary to invoke high sea-levels at all.

These authors suggest that massive mirabilite beds near Black Island are formed after the direct injection of sub-ice-shelf brines to the surface. The beds are deposited on a pre-existing ice-shelf surface as the basal sediment layer contours the basal ice. This sediment layer only contains small marine diatom and sponge-spicule fragments with a size consistent with suspended particles. The salt bed contains non-marine diatoms that have settled in the crystallizing brines from subsequent non-marine algal production from large productive surface pools. Mirabilite brines (density c. 1.4 g cm^-3) could be displaced from underneath a less dense but extensive ice body. This requirement can be met by a grounded and moving ice shelf forcing pockets of basal brines through crevasses or tidal cracks to its surface. The Ross Ice Shelf has moved eastward from McMurdo Sound through continental areas that were not directly glaciated by the ice sheet further to the west. Such glaciations have been well documented by Reference PéwéPéwé (1960), Reference Bull, [B, Bull, McKelvey and WebbBull and others (1962), Nichols (Reference Nichols1964, Reference Nichols and Quam1971), Denton and others (Reference Denton and Armstrong1968, Reference Denton, Denton, Armstrong and Stuiver1970), and Reference Denton and BornsDenton and Borns (1974). During the collapse of such a glaciation, discrete blocks of mirabilite carried by the intruding ice shelf would be stranded at heights consistent with ice-shelf thickness. Stagnant remnants of previous glaciations such as the Strand Moraines in western McMurdo Sound or Lower Wright Glacier would release mirabilite spasmodically in melt waters whenever it is exposed either on their surfaces or at their termini but salt would normally be absent in these streams. It is not surprising that sulphur-isotope ratios from mirabilite and gypsum associated with Lake Vanda in Wright Valley indicate a marine origin of sulphate (Reference Nakai, Nakai, Kiyosu, Wada, Takimoto and ToriiNakai and others, 1975). Such salts do not prove high sea-levels or isostatic rebound, because they could originate in melt waters from Ross glaciations mapped in easternmost Wright Valley (Reference Bull, [B, Bull, McKelvey and WebbBull and others, 1962; Nichols, Reference Nichols1964, Reference Nichols and Quam1971).

Because isotope fractionation occurs during sea-ice formation, deuterium values were obtained from ice samples taken directly below the mirabilite beds and from the water of hydration trapped in the mirabilite crystal (10H₂O). The presence of marine fossils on the ice-shelf surface suggests that the ice shelf at this location now consists of frozen sea-water. Using the fractionation factor of Reference Craig and HomCraig and Horn (1968) for the concentration of deuterium in ice from frozen sea-water (αD= 1.00265), we obtained a theoretical value, assuming perfect fractionation in equilibrium conditions, of +20. Our values for ice samples under the mirabilite deposits at 128 and 141 cm, site 1 (cf. Fig. 3), are +10, +19, and +21, +14, + 27.4, respectively. These values are consistent with a sea-water origin for such ice. Furthermore, deuterium values of lake ice in the southern Victoria Land area are much lower, e.g. —210 for Lake Fryxell, — 150 for Don Juan Pond (Reference Matsubaya, Matsubaya, Sakai, Torii, Burton and KerryMatsubaya and others, 1979). If mirabilite crystallized from the saline terrestrial lakes of southern Victoria Land (even if it had originally come from sea-water) or if it was simply dissolved and crystallized from glacial melt waters, we would expect the water of hydration trapped in the salt crystal to show these very negative values. Such is not the case. Our readings of —27.8 and —32.0 δ_D (w.r.t. SMOW) indicate a direct sea-water origin after a substantial amount of sea-water is frozen.

In our hypothesis, the Cape Spirit mirabilite beds were formed after the brine beneath the ice shelf has been displaced to its surface. Because the basal growth of the ice shelf was due to sea-water, the sub-ice-shelf brine has a sea-water origin. When the ice shelf grounds over the irregular sea floor, the trapped brines are displaced to the ice-shelf surface due to the weight of a large area of the ice shelf. If these brines immediately precipitate mirabilite, prior to dilution from snow, the deuterium values of the water of hydration in mirabilite should reflect those in the original brine.

Using Reference DansgaardDansgaard’s (1964) equation for deuterium fractionation in a two-phase system, presuming immediate removal of the second phase, we obtain δ_D (w.r.t. SMOW) values of —25, —34, —45, and —65 in the sea-water brine after 20, 50, 70, and 80% of the original sea-water has been frozen. Reference Nelson and ThompsonNelson and Thompson (1954) have shown that mirabilite precipitates from sea-water after 88% has been frozen. In this case, assuming maximum fractionation, the water of hydration of the mirabilite should reflect δ_D values near —65. But, if an ice-water equilibrium is first established, then the δ_D values of the sea-water will not be as low as —65. In nature the actual values lie between those formed in perfect equilibrium and those formed by the rapid removal of the second phase from the system (Reference DansgaardDansgaard, 1964). Our δ_D values of —27.8 and —32 are therefore consistent with crystallization from a sub-ice-shelf brine in natural conditions.

The radiocarbon dates from the algal mats associated with mirabilite beds in the McMurdo Sound area range from 12200 ±1000 years B.P. near Hobbs Glacier (Reference Black and BowserBlack and Bowser, 1968) to 870±70 years B.P. near Black Island (this paper). Although the algal mats may often post-date the mirabilite beds, the existence of non-marine diatoms in the Black Island deposit indicates contemporaneous non-marine algal production. The spread of dates may indicate that mirabilite formation is episodic near ice shelves as their grounding line waxes and wanes.

The surface boulder lag covering the Cape Spirit salt beds could not have been present when the mirabilite was deposited as it is not found within the salt bed. Today the boulders, cobbles, pebbles, and sediment exposed on pressure ridges are continually collapsing into the adjacent pools and channel systems. In such a mobile system, boulders could be moved and deposited on the mirabilite or on overlying algal mats. The present model suggests that the precipitation of mirabilite is rapid due to the scarcity of algal mat within the salt, while the deposition of the overlying algal mats and boulders could occur over a long period of time (hundreds or even thousands of years). During this time the salt bed is divided laterally parallel to the coast as new tidal crack systems develop. While this lateral displacement is proceeding, boulders brought into the new channel system by anchor ice together with algae from overlying pools are gradually added to the surface.