"Rock of the Month # 278, posted for August 2024" ---

Ice

- as we have noted before (20 years ago this month: ice, from Antarctica down to the humble snowflake) - is a mineral. We might go further and say "water ice", since other volatile phases that are commonly found on Earth in either liquid or vapour phases can be rendered crystalline at sufficiently low temperatures. Water ice has hexagonal crystal form, as famously seen in the symmetry of snow flakes in their myriad forms (crystal habits, you might say). As we know, ice also falls from the sky in heavier masses, variously referred to as sleet, hail or graupel. Pioneering meteorite researcher Ernst Chladni (1756-1827) compiled a catalogue of strange things seen to fall from the skies. Amongst the various undoubted meteorites, he refers to an observation by the mineral dealer Philip Rashleigh at Menabilly (Cornwall, S.W.England) which involved a fall of ice on 20 October 1791 (no meteorite was recovered: Chladni, 1826).

The MINLIB bibliographic database lists 1,222 references on ice, 1826 onwards, or 1,276 if the terms icefield and iceberg are included. Note that terms in glacial geology apply, so not all these references are about ice, as such: ice flow directions, recording behaviour of vanished ice sheets, are important in mineral exploration and landscape reconstructions. In contrast, there are just 22 records on clathrates (see below), 1976 onwards. We will look briefly at two aspects of the chilly world of ice.

Ice sheets, sampled as ice cores

The distribution of ice has waxed and waned over much of Earth history, and prior to the Cambrian explosion (the rise of diverse life forms, actually somewhat before the base of the Cambrian) there is evidence around the globe for an episode known as Snowball Earth (Hoffman et al., 1998) where virtually the entire globe was covered in a layer of ice. It has recently been suggested (Macdonald and Swanson-Hysell, 2023) that the volcanic eruptions associated with the vast Franklin large igneous province, found today from northwest Greenland across Arctic Canada to Alaska, dated at 719 Ma, preceded the Sturtian glacial phase of Snowball Earth glaciation by <2 Ma. Global cooling associated with unusually intense eruptions can cause devastating, if geologically short-lived, changes in climate.

In the last two million years, the Quaternary era, there have been a series of Ice Ages, wherein many of the familiar temperate-latitude landscapes of today were covered in ice sheets which in places were several km thick. In recent decades, ice sheets, and especially mountain glaciers, have been on the retreat. The British Isles, unlike Norway and Switzerland, lack mountain glaciers today. It was not always thus. As an example, the Scottish island of Rhum has a rugged layered igneous complex in its southern half, with the peaks Barkeval, Hallival and Askival. Hallival (723 m) is surmounted by a peridotite blockfield. The peak of Askival (812 m), highest summit on the island, comprises shattered blocks of allivalite (troctolite). The ice surface probably rose to about 700 m in the mountains of central and southern Rhum, leaving only the highest peaks as protruding nunataks (Ballantyne and McCarroll, 1997).

Isotopic and trace element analysis of ice cores, drilled from ice sheets in Antarctica, Greenland and elsewhere, can reveal episodes in past history for at least hundreds of thousands of years into the past (e.g., Petit et al., 1999; White, 2009), leading to insights concerning the current high levels of greenhouse gases CO₂ and CH₄ in the atmosphere. Much more recently, the trace radionuclides found in ice cores mark the period of atmospheric nuclear weapons tests following World War II (Elmore et al., 1982). Mountain glaciers as well as ice sheets can yield valuable ice core data. An ice core record from Dasuopu in Tibet records droughts, and chloride and dust concentrations rise in response to increased human activity in India and Nepal (Thompson et al., 2000). In east Africa, the glaciers on Mount Kilimanjaro provide an environmental record for the past 11,700 years. Spikes in elements such as fluorine and sodium may indicate peaks in erosion, while aerosol species suggest drying trends (Thompson et al., 2002). Other analyses of Greenland ice cores have indicated a 20th-century rise in non-sea-salt sulphate and nitrate (Mayewski et al., 1986), while at Mount Logan, in the St. Elias Mountains of the southwest Yukon, the level of non-sea-salt sulphate in the ice core has remained nearly constant over the past century (Monaghan and Holdsworth, 1990). Anthropogenic emissions aside, sulphate records in Antarctic ice may serve as proxies of global volcanism (Thamban et al., 2011).

Clathrates

A clathrate is a lattice structure of one molecule that effectively forms about a molecule of another substance, trapping it in a cage-like setting. Many are clathrate hydrates, in which a water (ice) host entraps other simple molecules such as methane, carbon dioxide, nitrogen or hydrogen. These can be found in permafrost, and in marine sediments. The formation and subsequent decomposition of clathrates may be traced by stable isotope analyses (Davidson et al., 1983). Clathrates on the sea floor, found largely in marine continental margins, have been viewed as both a threat (release of greenhouse gases trapped in clathrates, due to warming seas and permafrost regions), and as a potential energy source (Ruppel, 2007). Clathrates are also relevant to the study of fluid inclusions in minerals, which can provide information of temperature, pressure and salinity of the ore-bearing hydrothermal fluids (see, e.g., Thomas et al., 1990; Simmons et al., 2007). The presence of CO₂ / SO₂ / CH₄ clathrates elsewhere in the solar system is also anticipated, as on Mars (Chassefiere et al., 2016) and on asteroid Ceres (Castillo-Rogez et al., 2018).

REFERENCES

Ballantyne,CK and McCarroll,D (1997) Maximum altitude of the late Devensian ice sheet on the Isle of Rhum. Scot.J.Geol. 33, 183-186.

Castillo-Rogez,J, Neveu,M, McSween,HY, Fu,RR, Toplis,MJ and Prettyman (2018) Insights into Ceres' evolution from surface composition. Meteoritics & Planetary Science 53, 1820-1843.

Chassefiere,E, Lasue,J, Langlais,B and Quesnel,Y (2016) Early Mars serpentinization-derived CH₄ reservoirs, H₂-induced warming and paleopressure evolution. Meteoritics & Planetary Science 51, 2234-2245.

Chladni,EFF (1826) A new catalogue of the fall of stones, iron, dust, and soft substances, dry or moist, in chronological order. Annals of Philosophy new series, volume XII, London, 83-96.

Davidson,DW, Leaist,DG and Hesse,R (1983) Oxygen-18 enrichment in the water of a clathrate hydrate. Geochim. Cosmochim. Acta 47, 2293-2295.

Elmore,D, Tubbs,LE, Newman,D, Ma,XZ, Finkel,R, Nishiizumi,K, Beer,J, Oeschger,H and Andree,M (1982) ³⁶Cl bomb pulse measured in a shallow ice core from Dye 3, Greenland. Nature 300, 735-737, 23 December.

Hoffman,PF, Kaufman,AJ, Halverson,GP and Schrag,DP (1998) A Neoproterozoic snowball Earth. Science 281, 1342-1346, 28 August.

Macdonald,FA and Swanson-Hysell,NL (2023) The Franklin large igneous province and Snowball Earth initiation. Elements 19 no.5, 296-301, October.

Mayewski,PA, Lyons,WB, Spencer,MJ, Twickler,M, Dansgaard,W, Koci,B, Davidson,CI and Honrath,RE (1986) Sulfate and nitrate concentrations from a South Greenland ice core. Science 232, 975-977, 23 May.

Monaghan,MC and Holdsworth,G (1990) The origin of non-sea-salt sulphate in the Mount Logan ice core. Nature 343, 245-248, 18 January.

Petit,JR plus 18 (1999) Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399, 429-436, 03 June.

Ruppel,C (2007) Tapping methane hydrates for unconventional natural gas. Elements 3 no.3, 193-199, June.

Simmons,SF, Simpson,MP and Reynolds,TJ (2007) The significance of clathrates in fluid inclusions and the evidence for overpressuring in the Broadlands-Ohaaki geothermal system, New Zealand. Econ.Geol. 102, 127-135.

Thamban,M, Laluraj,CM, Naik,SS and Chaturvedi,A (2011) Reconstruction of Antarctic climate change using ice core proxy records from the coastal Dronning Maud Land, east Antarctica. J.Geol.Soc.India 78, 19-29.

Thomas,AV, Pasteris,JD, Bray,CJ and Spooner,ETC (1990) H₂O- CH₄ -NaCl- CO₂ inclusions from the footwall contact of the Tanco granitic pegmatite: estimates of internal pressure and composition from microthermometry, laser Raman spectroscopy, and gas chromatography. Geochim. Cosmochim. Acta 54, 559-573.

Thompson,LG, Yao,T, Mosley-Thompson,E, Davis,ME, Henderson,KA and Lin,P-N (2000) A high-resolution millennial record of the South Asian monsoon from Himalayan ice cores. Science 289, 1916-1919, 15 September.

Thompson,LG plus 10 (2002) Kilimanjaro ice core records: evidence of Holocene climate change in tropical Africa. Science 298, 589-593, 18 October.

Thordarson,T and Hoskuldsson,A (2014) Classic Geology in Europe, 3. Iceland. Dunedin, Edinburgh, 2nd edition, xv+256pp.

White,JWC (2009) Geoscience of climate and energy 5. Ice cores, greenhouse gases and climate change. Geoscience Canada 36, 78-80.