6

The memory of ice

Some say the world will end in fire,

Some say in ice.

From what I’ve tasted of desire

I hold with those who favor fire.

But if it had to perish twice,

I think I know enough of hate

To say that for destruction ice

Is also great

And would suffice.

Robert Frost, ‘Fire and Ice’, Harper’s Magazine, 1920

the world is spun between two giant hands of ice …

Douglas Stewart, The Fire on the Snow, 1944

The diary

Sites 267 and 267A Site 4 (59^o15.74′S; 104^o29.30′E) Water depth 4,564 m. Occupied 6–7 January 1973

Sites 267B Site 4 (59^o14.55′S; 104^o29.94′E) Water depth 4,539 m. Occupied 7–8 January 1973

Friday 5 January 1973

I woke up this morning in time to see the first iceberg. So super white it’s unbelievable—it’s big, but still a long way away. The sea is the smoothest since we left—just a long swell, and not a whitecap anywhere.

We spotted several bergs throughout the day—one with 3 peaks; I made a quick sketch before my hands got too cold. And there were growlers all in a patch, one only 50 yards to port was an intense blue.

Monday 8 January 1973

No chance much since scribbling the above notes to fill out details. Site 266 took up about 3 days of time solidly, with only time for quick meals and a few hours sleep—managed to get my site summary in by the first day of our fourth site (267) and feeling quite pleased with it—it will probably get shredded editorially.

This site was an iceberg saga. When we came on site the sea was dead calm, but there was a cluster of bergs lurking to port—one enormous one on the horizon some sixteen miles away was the chief hazard, as it seemed to be on a slow but steady track towards our spot. Our situation must be a unique one in navigational history—our being stationary on account of being tied to the sea floor by a long steel string, and thus in danger of having an iceberg run into us, rather than vice-versa—how very different from the Titanic!

Anyway, it was decided that we should drill, but that we should make it as swift as possible, meanwhile keeping a close watch on those beauties to port, and a close plot on their paths on the radar screen; with calculations of their speed and course changes. Apparently they average a travel speed of around ½ a knot an hour, but their tracks are erratic.

We pushed the first hole down pretty quickly—in about half a day, by dint of not doing much coring. Geologically this was a disaster—couldn’t get core recovery, first we cored water, then we lost a whole barrel because the sock at the bottom came out, then we recovered half a core—then the captain decreed pull out and shift, as big billy had put on a spurt of speed and was now only 3 miles to port; looking like a ragged blue-white chunk of the Matterhorn with a broad gullied base. This was about 4am, so we pulled out of the hole (at least we went to bed, and the crew did the rest).

We steamed about a mile and a half out of the berg’s calculated track, dragging nearly 3 miles of steel pipe under us, with its bottom just a bit above the ocean floor. Then we stopped and spudded in again, to try and salvage something from the area—this would be hole 267B. When we got up about 10am we were right behind the iceberg—at least it lay less than half a mile away on our port side, headed away from us. It was the most incredibly beautiful thing—I made a quick sketch. It was double-peaked, green and blue in the deep crevassed places, and with a smooth side which looked snow-covered, but which was deeply and smoothly grooved (or stratified?). Down its other side was a great rust-coloured streak marring its purity—guano? Or sediment? Can’t think.

Anyway we drilled again, after a somewhat heated split in the party as to where we should take core—the palaeontologists wanting it at the bottom of the hole, where at least we could get a good date; the sedimentologists preferred a section near the top. Compromise was reached.

We had good core recovery, but, incredibly, the section is totally different from that at our first site, just over a mile away, which is strange out here in the deep part of the ocean, where the strata should go for hundreds of miles without changing their nature or their thickness—a salutary lesson. We worked hard all day splitting and sampling core.

There was a marvellous sunset around 10.30, with a brassy cloudy sky silhouetting a berg in front of the sun—it was all too much, and much too much photographed. Someone said that sunrise and sunset overtake each other down here, and it’s nearly true, as we were treated to a paler version of the same thing around 2.30 am—but even then the interval between didn’t really darken, and the sky was still pale and translucent, with the stars hard to see. Amazingly, it’s not really cold; there has been no wind at all for 3 days, and the sea has only had a long swell, with barely a ruffle on its surface. The temperature was up to 40^oF during the day and the sun moderately bright, although there always seems to be a cloudy haze.

To bed around 3am. I am amused at the way ‘Midnight Lunch’ is looked forward to during the long night working sessions. In part it’s a break from the seemingly unending business of splitting, sampling and logging core as it’s brought into the lab from the deck. But the hamburger and pizzas or whatever seem to taste awfully good—have successfully resisted the pies and cakes and brownies … The drillers are muscular young men mostly from the Deep South of the US—the platters of food they take away from the mess are astounding—whatever the dinner is, it’s then heaped high on top with great hunks of corn bread—there’s a clear southern influence on the cuisine on board this cruise—but all that energy is necessary as the twelve hour shifts are long, and the deck is cold and wet.

I spent most of the day sleeping, which felt great. We finished Site 267B this morning—how we will explain the results of this saga is a big query at present.

Figure 6.1. Iceberg with rusty streak, approach to Site 267.

Source: Elizabeth Truswell.

Sighting the first iceberg

The operations report for this site records that we saw our first iceberg at 58^o20′S latitude. Disappointingly for all those on deck, it was given a wide berth as the Glomar Challenger is not ice strengthened. Site 267 turned out to be near a field of icebergs, although it had been selected as an area with a minimum of icebergs! The movement of all icebergs, bergy bits (ice fragments 1–5 m above the waterline) and the ominously named growlers (up to a metre above the waterline) that were visible within a 10-mile (16 km) radius were plotted from the radar screen—when we departed the site there were some 49 icebergs counted on the screen.

On any southward voyage the first sight of icebergs causes great excitement, with far too many photographs taken of far too distant icebergs. So it was with us. Historical records suggest it was ever so. An unknown photographer on HMS Challenger was the first to photograph icebergs in the Southern Ocean. Henry Moseley, the British naturalist on that vessel reported on their first iceberg sightings as they drove southwards towards the Great Ice Barrier—now referred to as the Ross Ice Shelf—their ultimate destination before turning northwards toward Melbourne. They sighted their first iceberg on 10 February 1874, in a latitude Moseley noted as corresponding to that of Christiania in Norway. Christiania, now Oslo, is at 59^o55′N latitude. Moseley, in his popular account of 1879, reported their initial excitement and their haste to set down what they saw:

At first, all the icebergs seen were numbered each day and their positions noted down; but when we came to have 40 in sight at once this plan was abandoned, and subsequently we had more than 100 in sight on several occasions. (Moseley 1879, p.233)

Adding further:

The distant flat topped icebergs showed out black and sharp, with rectangular outlines against the bright band, and some of their dark bodies joined the dark cloud line to the dark horizon line, bridging over the band of light. The whole effect was very curious, and drew all on deck to gaze at it. (Moseley 1879, p.247)

Also on HMS Challenger, Sub-Lieutenant Herbert Swire wrote with youthful enthusiasm of a passing berg that:

the only way which I can describe it being by likening it to one of those gorgeous Christmas transformation scenes at a first class London theatre, which, glowing in dazzling whiteness and heavenly blue, call up visions of fairyland such as I thought then I would never see realized till now. (Swire 1938, p.157)

Figure 6.2. Ice floes on choppy water. Watercolour by Herbert Swire.

Source: Courtesy of State Library of Victoria.

Moseley and other commentators on the Challenger may have been unaware at the time that the icebergs they were describing were distinctively Antarctic in their character, their flat-topped, blocky nature reflecting their calving from ice shelves. But the distinctive nature of these southern icebergs had been recognised much earlier. The astronomer, navigator and all-round polymath Edmond Halley—subsequently to become Astronomer Royal—sailed from England in the pink Paramore (‘pink’ is a term for a small three-masted vessel, square-rigged and with a narrow stern) on his second Atlantic voyage. His aims were to establish the longitudes of ports in South America using the moons of Jupiter as a guide. He reported sighting tabular icebergs at 52^o24′S, close to the southernmost point of the voyage. On 1 February 1700, Halley saw what appeared to be three islands: ‘Flatt on the top and covered with Snow … milk white, with perpendicular cliffs around them … the great hight of them made us conclude them land, but there was no appearance of any tree or green thing on them’ (quoted in Thrower 1981, pp.162–65). He sketched the islands into the Paramore’s logbook—the first known drawing of an Antarctic tabular iceberg. The track of the Paramore is shown by the dotted lines passing through the cluster of icebergs in Figure 6.3.

Figure 6.3. Tabular icebergs in the South Atlantic; diagonals are isogonal lines, connecting points of equal magnetic declination. From Halley’s Atlantic chart.

Source: Courtesy of the Royal Geographic Society.

James Cook, during his epic second voyage, first encountered icebergs as he sailed south from Cape Town. They first met the icebergs, which Cook habitually referred to as ‘ice islands’, on 10 December 1772, at 50^o57′S. The ice islands became more abundant as they voyaged to the southeast, until eventually they were stopped by impenetrable sea-ice. Cook reported the encounters in his journal thus, with a noteworthy interest in the bird life:

Thursday 10 December 1772 at 50^o57′S. at 6 saw an Island of Ice to the Westward.

Friday 11th … at 1pm saw an Island of Ice right a head distant 1 mile, which the Adventure took for land, till I made signal for her to come in under our stern. A little before noon pass’d two Islands of Ice one on each side. Saw some birds which were about the size of Land pigions, shaped like Fulmers, Plumage White as Snow, with blackish bills and feet. I believe them to be intirely new as I never saw such Sea Birds before and Mr Forster [Johann Reinhold Forster, naturalist to the expedition] has no knowledge of them.

Saturday 12th Passed Six Islands of ice this 24 hours, some of which were near two miles in circuit and about 200 feet high, on the weather side of them the sea brake very high, some gentlemen on deck saw some Penguin.

Monday 14th – 54.55′S. … At half past six we were stoped by an immense field of Ice to which we could see no end, over it to the SWBS (South West by South) we thought we saw high land, but can by no means assert it.

Figure 6.4. Large ice island. Pen and ink by William Hodges, January 1773.

Source: Courtesy of Mitchell Library, State Library of New South Wales, Sydney.

They then proceeded along the edge of the ice to the southeast, eventually reaching clear seas, before again being surrounded by ice. They penetrated south of the Antarctic Circle on 17 January 1773, reaching a latitude of 66^o36½′S. In the course of the three-year voyage Cook made two further excursions beyond the Antarctic Circle, both of these in the south Pacific sector of their route.

The particular shape of these southern icebergs was also recorded by James Clark Ross, during the voyage of the Erebus and Terror of 1839–43. Sailing south from Sydney toward the sea that now bears his name, Ross on 16 December 1841, reported the first iceberg at a latitude of 58^o36′S:

The height of this berg was one hundred and thirty feet, and its circumference three quarters of a mile. It was one of the table-topped, or barrier kind, and deep caverns had been worn into its vertical sides by the action of the sea: a long line of loose pieces extended several miles to leeward of it, and many large masses appeared ready to fall from it, to continue the line of fragments, as the others drifted away before the wind. (Ross 1847, vol. 2, p.142)

Icebergs in the Southern Ocean

When snow falls on the present ice sheet of Antarctica, its ultimate fate is to be returned to the ocean as fresh water. This happens through its transformation into ice, occurring when the amount of snow that falls in summer is less than the amount that falls or melts in winter. The gradual build-up of the resulting ice forms glaciers that push out to sea. The melting out of ice through the basal part of the floating ice shelves of the coast gives rise to icebergs that break, or calve, from the seaward edges of the ice shelves. There is an important difference between this continent-derived ice and sea-ice, which forms when ocean water freezes and which varies annually in extent.

The drifting icebergs calved from the ice shelves melt as they move into the Southern Ocean; this may be the most important source for returning freshwater to the oceans. Large icebergs are sometimes described as moving sources of freshwater. They break up into smaller ones, and this speeds up the return of freshwater. Both the ice shelves and the floating icebergs are important in another way. They transport nutrients—such as iron—into the oceans, enriching the surface waters and encouraging the growth of algae and organisms that feed on it. It is even possible that this effect, that of fertilising the oceans, may increase the capacity of the Southern Ocean to absorb CO₂. Satellite measurements of giant icebergs—those with one side exceeding 18 kilometres—show increased levels of chlorophyll trailing in their wake; some traces of these heightened levels persist for as long as a month after the berg has passed. Such bergs have been dubbed ‘big friendly giants’. Just how much this increase in biological activity affects the drawing down of carbon is difficult to measure. However, it has been suggested that global warming may well increase iceberg discharge in the future, particularly from the West Antarctic Ice Sheet, and this may increase the sequestration of carbon in the Southern Ocean, and thus counteract climate change to some degree.

Figure 6.5. Satellite image of giant iceberg in the Weddell Sea, showing enhanced levels of chlorophyll (yellow and red patches) trailing in its wake. Nature Geoscience, 2016.

Source: Courtesy of Grant R. Bigg.

The distribution of icebergs in the Southern Ocean is not entirely random. Mapping of iceberg distribution by French scientists Jean Tournadre and colleagues, published in 2012, shows that they enter the ocean current system at a number of well-defined geographical points along the coast. Three of these are significant. One of the maximum areas of distribution runs from Enderby Land in the west to the region of the Merz Glacier, a longitudinal span extending from 60^oE to around 120^oE, an area of ocean encompassing the major track of the Leg 28 cruise. Most icebergs remain in a drifting trajectory close to the coast, travelling westward in the coastal current. Some may be forced slightly northwards under the influence of the Coriolis effect, caused by the Earth’s rotation, and travel to around 60^oS. particularly over the Kerguelen Plateau. Such may have been the route of the first icebergs encountered by Captain Cook.

Other areas with high concentrations lie in the South Atlantic, fed by current gyres in the Weddell Sea, and a lesser one in the South Pacific.

There are databases for Antarctic icebergs, based on both satellite and ship-borne observations. Large icebergs are tracked and named, each with a letter signifying its point of origin (the broad geographic regions mentioned), followed by a running number.

The larger bergs can move into lower latitudes without totally disintegrating, provided sea surface temperatures are suitably low. For example, in 2006 a chain of icebergs was seen off Dunedin in New Zealand, at a latitude of 46^oS. Do such occurrences mean that icebergs could provide water for arid regions such as Australia and Africa? From time to time the issues surrounding such a possibility are aired. One private organisation, Iceberg Transport International, has taken the prospect very seriously. The founders, French eco-entrepreneur Georges Mougin, and business partner Saudi prince Mohammed al Faisal, at an international conference on iceberg utilisation held in landlocked Iowa in 1977, outlined an ambitious plan to tow an Antarctic iceberg, wrapped in sailcloth and plastic, to the Arabian Peninsula. They claimed that the cost, some 100 million US dollars, was less than the costs of desalinisation.

The true environmental costs have rarely been built into such proposals. There is the cost of fuel for the towing tugboats, for instance. Neither has much attention been paid to the fact that icebergs, with some 90 per cent of their bulk below the waterline, couldn’t be brought close to shore, close to where the water might be harvested. Also, once outside the protective cooler waters around Antarctica, the loss of iceberg mass would not only be by melting, but almost certainly by the berg breaking into smaller pieces. Could these bits be caught within the protective wrapping?

But ideas like these survive, and much of the problem-solving has been done by computer modelling. The Ice Dream Project, with a base in France, has theoretically considered practical issues such as an underwater curtain to slow melting; the nature of the towing cable and the iceberg ‘belt’ that would be needed to tow a Newfoundland iceberg across the Atlantic to the Canary Islands. The Ice Dream Project was, at last enquiry, talking of sea trials as recently as 2013. I can find no record of these having occurred, and it may be that the romance inherent in the idea of iceberg towing continues to outweigh the practicalities.

However, only recently the proposal to tow Antarctic icebergs north to serve as a water source has been raised again, this time with respect to the water crisis in Cape Town. This proposal, which involves wrapping the icebergs in fabric and taking advantage of the northward-flowing Benguela Current, appeared in The New York Post in May 2018.

The Great Ice Age

A key aim of our voyage was to ‘explore the long-term glacial and climatic history of the South Pole continent’. When we left port it was generally believed that the glaciation that enveloped the continents of the northern hemisphere began around 3 million years ago, or, to be more precise, at the beginning of the Quaternary Period, a point defined as 2.6 million years ago. In fact, the date of approximately 3 million years means that this most significant of climatic events—the crossing of a major climatic threshold, enabling the development of extensive ice in the northern hemisphere—occurred in the latest phase of the preceding Pliocene epoch. At the start of our voyage it seemed reasonable to expect that the Antarctic ice cap would have a similar history to that of the northern hemisphere glaciation.

During the last million years the Earth has been in a glacial phase, with large areas of continents covered with ice for much of that time. This has been most evident in the northern hemisphere, where thick ice sheets have covered northern Europe and much of North America, as well as Greenland and, in the southern hemisphere, Antarctica. Major oscillations of the northern hemisphere ice sheets have been well documented. These reflect times when ice was accumulating, the drier and colder Glacial intervals, and the warmer and wetter phases when melting is predominant, the Interglacials. We live at present in an interglacial, the Holocene, which has lasted for the last 11,000 years. It might be expected that the Earth will revert to a colder, glacial phase, beginning at an unknown future date, but the anthropogenic effect of the burning of fossil fuels may well interfere with this pattern.

This present ice age is but one of five great glaciations to which the Earth has been subject throughout geological time. Two of these are known from the Proterozoic Eon, before the obvious evolution of life on Earth. Of the others, that of the Carboniferous and Permian, between 350 and 250 million years ago, is most prominent in the Gondwana continents, including Antarctica. Rock samples from that earlier glaciation were collected by the artist and scientist Edward Wilson during the fatal expedition of Captain Robert Falcon Scott in 1912.

The origins of an ice age theory

The presence of ‘erratic boulders’—blocks of rocks that do not seem to be part of the local geology, and are far removed from their original locality—had been recognised in the Swiss Alps and in Scandinavia by local inhabitants and scholars from the middle of the eighteenth century. A Swedish mining expert, one Daniel Tilas, was first to suggest that the erratic boulders might be explained by drifting sea-ice. But it was one of the founders of geology, the Scottish farmer and naturalist James Hutton (1726–1797), who in 1795 attributed the erratics to their transport by mountain glaciers. Sedimentary deposits with a mass of poorly sorted debris, including large blocks, attracted a biblical explanation in the early years of the nineteenth century, especially in Britain, but also in the Swiss Alps. In England the colourful and eccentric William Buckland, in his Reliquiae Diluvianae of 1823, appealed to the turbulent waters of a universal flood to explain such deposits.

A less catastrophic view was advanced by Charles Lyell, another of geology’s founding fathers and friend and mentor of Charles Darwin. Lyell, in his Principles of Geology (1830–33), with his view coloured by his overriding philosophy that processes in the present provide the key to the past, suggested that drifting ice, rather than turbulent water, was responsible for larger erratic boulders. The source of the ice was likely to be polar, delivered from Scandinavia across a submerged Europe. Lyell gave a copy of Volume 1 of his Principles to the young Charles Darwin as he set out on his voyage on the Beagle in 1831; further, he asked Robert Fitzroy, the Beagle’s captain, to search for erratic boulders on his voyage. Darwin was thus made acutely aware of the importance of these deposits in unravelling the geology of the places he encountered.

But the recognition of a ‘Great Ice Age’ is forever linked with the name of the Swiss scientist of natural history Louis Agassiz, and with observations he and others made in the Swiss Alps. In 1829 a Swiss civil engineer, Ignaz Venetz, felt that the distribution of erratic boulders in the Alps and the nearby Jura Mountains—indeed much of Europe—was best explained by the action of expanded glaciers. Others linked with the glacier theory are Jean de Charpentier and the German botanist Karl Friedrich Schimper, a university friend of Agassiz. The details of the history of the ice age, the development of the theory and the people involved, were published by father and daughter John Imbrie and Katherine Palmer Imbrie in 1979 in Ice Ages: Solving the Mystery.

It was Schimper who coined the term Eiszeit or Ice Age. With Agassiz he spent the summer months of 1836 in the Alps, and together they developed the idea of a sequence of glaciations. Agassiz’s enthusiasm for the study of the alpine glaciers led him to construct a hut on the Aar Glacier in the Bernese Alps. During their ‘vacations’ in this tiny environmental observatory, Agassiz and his colleagues observed the jumble of varied rocks dumped at the glacier edge—the ‘lateral moraine’. They saw too the distinctive striations, the grooves on rock faces reflecting the dragging action of rocks embedded within the glacier—all typical of such an environment. Agassiz widened his investigations to other parts of northern Europe, including a visit to Scotland in 1840. These excursions strengthened his view that much of Europe had been covered by an ice sheet resembling that of Greenland today. He published his observations in his Études sur les Glaciers in 1840. On his emigration to the United States he was able to trace the extent of glaciation on that continent, observing there the former presence of an ice sheet comparable to that of Europe. But Agassiz’s early presentations of the theory to learned scientific societies met with resistance. It ran counter to the commonly accepted theory that the Earth had been cooling since its inception as a molten globe. The ‘ice age theory’ thus had a slow acceptance, perhaps not assisted by Agassiz’s overenthusiastic claims of comparable continent-wide glaciers in regions such as South America.

The causes of the ice ages are complex and subject to a great deal of scientific investigation and speculation. A long accepted key to the cyclic variations of climate observable over the last million years may lie in the details of the Earth’s orbit round the sun, a theory that was proposed by the Serbian astronomer Milutin Milankovic in 1920. This, the theory of orbital forcing, draws attention to the changing distance of Earth from the sun due to the elliptical form of the orbit; to the changing tilt of the Earth’s axis of rotation, and the wobble (the precession) of that axis. All of these factors redistribute the sunlight received by the Earth in the passage of its seasons. A similar theory was advanced by James Croll in the nineteenth century, but the evidence he needed to test it—such as well-dated sequences of sedimentary rocks—was not available in 1875.

Factors other than the Earth’s orbit also come into play when considering the causes of changing climates. Such may include changes in atmospheric composition with fluctuating levels of greenhouse gases, changes in the position of continents and, in a related way, changes in ocean currents. For example, the position of a continent over the pole—such as Antarctica—may block the flow of warm ocean currents into high latitudes and thus cause cooling at the poles. There can be no doubt that the interaction of a number of factors and feedbacks among the mechanisms all contribute at one time or another to rapid and dramatic changes in Earth’s climate.

Tracking the passage of ice

Our voyage was much about the history of ice. That meant the nature and timing of the first ice; it meant being aware of the footprints that ice leaves in its passage. Ice leaves its mark in the sediments of the sea floor. Close to the continent, grounded ice can scour deep grooves on the continental shelf. Given that 90 per cent of an iceberg’s mass is under water, in shallow water the keels can drag across a soft ocean floor. There, sediments on the sea floor can also be pushed into ridges and ripples by the ploughing action of icebergs. Hollows formed by ploughing in this way can be challenging for the benthic (sea floor) life forms. Underwater cameras show, however, that a wide variety of organisms, including deep sea corals, thrive in such habitats and form distinctive communities close to Antarctica where the action of grounded ice has moulded the immediate environment.

Traces of past ice can be direct, in the form of pebbles dropped or melted out from passing icebergs, or indirect, in the way the nature of sediments distant from the ice itself, distant perhaps from an ice shelf, will be much affected by the same climate that supports the ice. The presence of diatom oozes, for instance, that characterise the present Southern Oceans, suggest that sea-ice is likely to be present in a vast region offshore from Antarctica.

Within deep sea cores, material eroded from the continent, carried out into the ocean and eventually dropped from icebergs as they melt is termed ‘ice-rafted debris’—often shortened to its acronym IRD. The most obvious component of this debris takes the form of pebbles, cobbles or even boulders of continental rock. Some of these show facets—flattened surfaces—or striae, longitudinal grooves, where they have been dragged and scraped across the bedrock surface as they become embedded into the ice of an eroding glacier. This erosion is a high energy process. The transport out to sea and subsequent melting out of the pebbles is gentler, and this shows in the type of sediments encountered on the sea floor. Pebbles—sometimes evocatively called dropstones—often occur in fine-grained muds or clays that indicate deposition in relatively quiet waters.

But it is not only the presence of pebbles in fine muds that shows their carriage by glaciers. Even something as fine as sand grains can also reflect an iceberg origin. The signatures of an icy origin for sand grains can be seen with scanning electron microscopy, where three-dimensional images are produced by scanning with a focused beam of electrons. Sand grain surfaces seen in this way show rough and abraded grain edges and, again, the presence of striae or grooves, in this instance at a microscopic level. The study of sand grain surfaces in relation to their origin, including in glacial settings, was pioneered by sedimentologist David Krinsley. His Atlas of Sand Grain Surface Textures, written with John Doornkamp, appeared in 1973 but remains current with a reissue in 2011.

During our Leg 28 voyage, glacial pebbles were absent in the sections older than 25 million years (the Late Oligocene period) but began accumulating after that. This observation was the first hint that the Antarctic continent had been covered by ice much earlier than previously believed—perhaps as long as 20 or 25 million years ago—certainly long before the major ice sheets that had covered the northern hemisphere.

Figure 6.6. Leg 28 core showing large glacial clasts derived from the Antarctic continent—granite (to the right) and probably gabbro. Site 268 Core 6–1. The section of core in the foreground shows dark muds rich in diatoms.

Source: Elizabeth Truswell.

Some dramatic accumulations of crystalline rocks, almost certainly derived from the continent, were encountered in Site 268, which was some 300 kilometres from the coast of East Antarctica. There, clasts, fragments of rock reflecting transport by icebergs, were abundant at intervals through the core.

Most were hard crystalline igneous or metamorphic rocks—granites and gneisses. Most were in the size range 3–6 centimetres; but one monster was close to 30 cm long (Figure 6.6). They may have come from the adjacent coast, but could have come from as far away the Shackleton Ice Shelf far to the west.

Closer to the continent, particularly in the Ross Sea, sediments of glacial origin predominated, and the process of describing cores as ‘pebbly silty clay’ seemed endless. There, granules, pebbles and cobbles were ubiquitous in the sediments, with about 10 per cent of the pebbles showing the striations so characteristic of a glacial history. In the Ross Sea drill holes, ice-rafted debris was clearly present from the latest Oligocene, from 25 to 26 million years.

Research since then has pushed the start of Antarctic glaciation back even further in time, to something like 34 million years, close to the Eocene–Oligocene boundary. This is now widely accepted as the time when a continental ice sheet first appeared on Antarctica. In the synthesis of the Ocean Drilling Program Leg 188 in Prydz Bay, off East Antarctica, scientists Alan Cooper and Philip O’Brien described glacial sediments denoting the first advance of the ice sheet on to the continental shelf around the time of that boundary. Similarly, more recent drilling in the very high latitudes of the northern hemisphere has pushed the inception of ice there further back in time, and ice-rafted debris from the Greenland Sea has yielded dates possibly as old as 40 million years, but whether these represent sea-ice or deposits from ice shelves is unknown.

The ice-rafted debris that melts out from icebergs can act as indicators of its source on land. Again, layers of sea floor sediments encountered during ODP Leg 188 are rich in pebbles that might be expected to have come from the nearby Amery Ice Shelf that feeds from the continent into Prydz Bay. But when the minerals in the pebbles were examined this was not the case. Chemically, the minerals are at odds with those known from the bedrock geology on the closer adjacent Antarctic continent. Rather, it appears that the debris in this case reflects icebergs calved into the sea from the Wilkes Land coast, some 1,500 kilometres to the east. Such bergs would have drifted westward in the Polar Current, perhaps dropping their load of debris when they were stopped by bedrock highs to the west of Prydz Bay.

Like any discipline, or subdiscipline, the study of ice and its carriage of debris has many sidetracks. One of the most intriguing is that labelled ‘iceberg armadas’, a term that carries a strong visual impression. It has been linked to massive discharges of icebergs laden with continental debris, known to have occurred in the North Atlantic, particularly during the last glacial period. These armadas of icebergs, identified by accumulations of rock debris within the floor of the North Atlantic, have been dubbed ‘Heinrich Events’, after marine geologist Hartmut Heinrich, who first recognised them, describing the phenomena in the journal Quaternary Research in 1988. The most likely explanation for these events recorded in the sea floor sediments links them to instability of the enormous Laurentide Ice Sheet which covered much of North America during the last glacial period, although other northern hemisphere ice sheets may have been involved as well. Using anthropomorphic terms to describe the disintegration of these ice sheets has added colour and clarity to the processes. The build up of ice—the accumulation of its mass, probably during colder periods—has been called the ‘binge’ phase; binge being defined as ‘a brief period of doing something excessively’. Following this, some factor causes the ice to slide seaward, in the process divulging glacial debris, and a great deal of fresh water, into the oceans. This is the ‘purge’—a ridding of what is undesirable or impure.

The causes underlying these events continue to be debated. They probably reflect the disintegration of ice shelves that would have fringed the glaciers and supported them. This would have allowed the continental ice sheets to flow into the ocean as icebergs and dump their loads of rocky debris. But was it widespread warming that destabilised the ice shelves—or was the process linked to the rises in sea level that were associated with the warming?

There are hints now that meltwater from Antarctic sources may also have contributed to this early phase of sea-level rise, and that a rise of some 15 metres might have been involved.

Loaded icebergs; comments from the past

Perhaps surprisingly, it is rare to see icebergs with an obvious load of sediment. The rusty streak we could see on the iceberg that threatened our drill site in January 1973 may have been sediment, but we were too distant to be sure.

One of the best-known reports of rocks within icebergs is that of Charles Darwin, who published a brief note in the Journal of the Royal Geographical Society of London in 1839. He recounts an instance during the voyage of the vessel Eliza Scott in Antarctic seas, captained by John Balleny and sailing under the flag of the English whaling firm Enderby Brothers. Balleny sailed south from New Zealand, along longitude 175^oE, searching for new territory and for possible new sealing grounds. In the course of his voyage he discovered the islands that now bear his name; and he may have seen the continent itself. One of the mates of the vessel, a Mr McNab, reported seeing an irregular, angular fragment of dark-coloured rock embedded in a perpendicular face of an iceberg—a piece about 12 feet in height (3.6 m) and 5–6 feet (1.5–1.8 m) across. He made a rough but lively sketch of it at the time.

Figure 6.7. Sketch of an iceberg with included rock. Included by Darwin in his note to the Journal of the Royal Geographical Society in 1839. Drawn by a Mr McNab of the Enderby expedition.

Source: Courtesy of the Royal Geographical Society.

Darwin comments further on the rarity of sightings such as this in the Southern Ocean. He cites one Captain Biscoe, who by 1833 had become the third man to circumnavigate Antarctica, but who claimed to have ‘never once seen a piece of rock in the ice’. But the apparent rarity of such observations loses it import under the influence of time, and Darwin commented further:

If then, but one iceberg in a thousand, or in ten thousand, transports its fragment, the bottom of the Antarctic Sea, and the shores of its islands, must already be scattered with masses of foreign rock—the counterpart of the ‘erratic boulders’ of the northern hemisphere. (Darwin 1839b, p.528)

Erratic boulders—those masses of rock that have been transported from their place of origin, usually by the action of glaciers, and deposited far from their source in a different terrain—intrigued Darwin. His interest goes back to his youth. He reported in the diary of his student years at Edinburgh University, remembering the dull lectures that convinced him that he would never study geology, a single earlier event that might have later predisposed him to a change of heart. The entry was included in a volume published by Darwin’s son, Francis, himself a noted botanist, and recorded an elderly gentleman:

old Mr Cotton, in Shropshire, who knew a good deal about rocks, had pointed out to me two or three years previously a well-known large erratic boulder in the town of Shrewsbury, called the ‘bell-stone’; he told me that there was no rock of the same kind nearer than Cumberland or Scotland, and he solemnly assured me that the world would come to an end before anyone would be able to explain how this stone came where it now lay. This produced a deep impression on me, and I meditated over this wonderful stone. So that I felt the greatest delight in when I first read of the action of icebergs in transporting boulders, and I gloried in the progress of Geology (F. Darwin 1887, vol. 1, p.41)

During the voyage of the Beagle among the complex coastlines of South America, Darwin’s attention was focused on the presence of large erratic boulders spread in a linear fashion, with elongate lines of rocks lying on the coastal plain of Tierra del Fuego. These boulders had clearly been transported some distance to their place of rest. In speculating on the origin of these boulder ‘trains’, Darwin proposed that ice-rafting must have been responsible for the patterns of boulders he could see. In reaching the conclusion that icebergs were the key to transport of the boulders he was much influenced by the ideas of Charles Lyell, whose Principles of Geology Darwin had with him—at least volumes 1 and 2—on the Beagle.

Lyell was inclined to believe in slow vertical movements of the Earth, with submergence allowing for the encroachment of shallow seas in which icebergs were arguably the appropriate mechanism for the transport of large rock masses. Recent studies of the boulder trains have proved such a mechanism unlikely. Rather, evidence accumulated since Darwin seems to suggest that the boulder trains in fact result from avalanches of rock spilled onto the surface of glaciers and carried by glacier flow until they were deposited in moraines, or accumulations of rock debris, along the melted edges of glaciers (see Banks 1971).

But the experience of the transport of boulders by ice in Tierra del Fuego clearly remained in Darwin’s mind throughout the later stages of the Beagle voyage. In Tasmania, during a stay of some 13 days in Hobart Town in February 1836 he enjoyed rewarding, though sometimes arduous walks around the town and its outskirts, including one very strenuous climb up Mt Wellington. As always, he had an astute eye for the local geology and kept meticulous notes of what he saw. Most of his observations remain unpublished, but details of his notebooks on the subject have been brought to light in recent years by the Tasmanian geologist Max Banks.

From these notebooks we learn that Darwin noted the presence of pebbles in ancient marine rocks we now know to be of Permian age (300–250 million years), and that he scribbled down how these reminded him of the sediment dropped from icebergs on the ‘bottom of the sea near Tierra del Fuego’. That he did not publish these observations may have been due to the fact that very old glaciations—glaciations far older than those that sculpted the present face of Europe—were not recognised until the 1850s. In On the Origin of Species (1859, p.381) Darwin did make brief reference to geological evidence for glaciations much older than the ‘last great Glacial period’, but didn’t develop these ideas further (see Darwin 1949, p.295).

Fragments of rock in deep sea deposits, presumably reflecting iceberg transport and melting, were observed by members of the HMS Challenger expedition. Between Heard Island and Melbourne, John Murray and the Belgian geologist Abbé Alphonse Renard reported the dredging of blocks, pebbles and fragments of ancient rocks—a variety of lithologies, all lying within the present limits of icebergs. The blocks they described:

are of all sizes, from several feet in diameter to the smallest dimensions; their angles are sometimes rounded or softened, at other times sharp, and the larger fragments are frequently covered on one or more surfaces by glacial striations. In their nature the fragments are very heterogeneous, being derived from almost all the varieties of rocks that crop out on the continents. This great variety in the dimensions and lithological nature of the continental debris spread over the floor of the ocean towards the polar regions of either hemisphere is exactly what we would expect to find in materials transported by floating ice. (Murray and Renard 1891, p.323)