Box 103 Onset Of Northern Hemisphere Glaciation

Marine sediments provide a better record of Earth's glacial history than terrestrial sediments because their rate of sedimentation is more continuous and they are subjected to fewer episodes of erosion. A glacial signature can be found in these sediments via the presence of ice-rafted material such as dropstones or pods of coarse-grained debris within very fine-grained marine silts. This type of evidence has been used recently to challenge the accepted date for the onset of northern hemisphere glaciation. The widely accepted age for the start of glaciation in the northern hemisphere is between 2 and 15 Ma. However, recent marine cores extracted from the Greenland Sea suggest that that glaciation may have started as early as 30-44 Ma in parts of Greenland, because these cores contain evidence of ice-rafted material, which can only have been derived from the calving of icebergs. For this to have happened, glacier ice must have been close to sea level at this time. Glaciation in Antarctica started at a similar time and the new evidence supports the idea that bipolar glaciation occurred in a synchronous manner, perhaps as a result of a global decrease in the greenhouse gas carbon dioxide, which was reduced at this time by increased continental weathering.

Source: Tripati, A.K., Eagle, R.A., Morton, A., et al. (2008) Evidence for glaciation in the northern hemisphere back to 44 Ma from ice-rafted debris in the Greenland Sea. Earth Planetary Science Letters, 265,112-22.

Glaciomarine sedimentation involves a similar range of processes to sedimentation in glacial lakes. However, there are a number of key differences due to the salinity of the water column and the influence of tides. Key processes of glaciomarine sedimentation include the following.

1. Direct deposition from the glacier front. The dumping or release by melting of supraglacial and englacial debris at the ice margin may be an important process. The rate of sedimentation will depend on: (i) the volume of ice that is melted; (ii) the forward velocity of the ice; and (iii) its debris content.

2. 'Rain-out' from icebergs and seasonal sea-ice. The heavily crevassed and dynamic nature of most marine-terminating calving glaciers means that iceberg calving is in general a more dominant process than in lacustrine environments. In particular the presence of tidal variation causes flexure of the margin, thereby accelerating iceberg calving. Supraglacial debris is often shaken from the upper surface of the berg as it calves or as it subsequently capsizes due to differential melting, and the melting of icebergs will ultimately release subglacial and englacial debris. The rate of sedimentation at any one point in front of the ice margin depends on: (i) the concentration of debris within the glacier and therefore the iceberg; (ii) the residence time of an iceberg in the area; (iii) the rate at which calving takes places; (iv) the rate of iceberg melting; and (v) the wave climate, which influences the frequency with which icebergs may capsize. The greater the englacial debris content of the glacier the greater the sediment an iceberg calved from it will contain. Warm-based glaciers have a relatively thin basal debris layer, whereas glaciers with cold or mixed thermal regimes may have a greater thickness of basal debris due to freezing on and to thickening caused by basal folding and thrusting. Icebergs from glaciers with cold or mixed thermal regimes are likely, therefore, not only to contain a greater volume of debris, but this will be distributed throughout a greater part of the iceberg and therefore sediment will be released for longer. It is important to note that icebergs from ice shelves will contain relatively little debris within them, because most of the debris melts from the base of the ice shelf prior to calving. The sediments produced by 'rain-out' vary from occasional dropstones to dump structures and large deposits of diamict. These diamict deposits are sometimes incorrectly referred to as waterlain tills, but since they have been disaggregated by the water column they are not tills in the strict sense of the definition (see Section 8.1). Diamicts produced by the 'rain-out' of ice rafted debris may possess crude graded bedding (size sorting) if the rate of debris supply is periodic or pulsed. When a pulse of debris is released into the water column it will become sorted as it settles; the larger clasts will settle first, followed by the finer fraction. In this way a crude graded bed may form. The next pulse of debris will form a new graded unit. If the water column is too shallow or the supply or 'rain-out' of debris continuous then no graded units will form.

Seasonal sea-ice may also transport debris within this environment. Debris is entrained by sea-ice through: (i) avalanches, which carry debris laden snow out on to the sea-ice; (ii) rockfalls and mudflows, which may carry debris on to the ice; (iii) stream flow, which in the spring may deliver debris to the shore before sea-ice has broken up depositing sediment on the ice; (iv) bottom freezing, where debris may be incorporated by freezing on from the sea floor as the ice rests in shallow water; (v) sediment capture, where suspended sediment is frozen into sea-ice as it grows; and (vi) aeolian deposition. As the sea-ice breaks up it may raft sediment, depositing it as the ice melts.

3. Deposition from meltwater flows. The influx of subglacial meltwater is an important process. This sediment-laden water is fresh and is therefore usually less dense than sea water and consequently it will enter as an overflow plume. It rises quickly from the base of the glacier, where it spreads out over the surface of the sea water. Deposition from this plume is rapid and the point of meltwater exit is usually marked by a fan of sand and gravel. Underflow may occur occasionally where the fresh water is particularly dense due to very high sediment concentrations. The importance of meltwater is again a function of basal thermal regime; it is more important for warm-based than cold-based glaciers (see Section 3.4).

4. Settling from suspension. Suspended sediment introduced into the sea will gradually settle out. This is accelerated in sea water, however, due to a number of processes: (i) flocculation, fine clay particles, which normally carry small electrical charges that repulse one another, attract each other in sea water because salt neutralises the electrical charges; (ii) agglomeration, particles may be attached to one another by organic matter; and (iii) pellitisation, some planktons and other small organism ingest fine sediment and bind it into large faecal pellets, which then sink rapidly.

5. Subaqueous resedimentation by gravity flows. Sediment may become unstable on steep slopes. Slumping or flow of this sediment may give rise to a range of diamicts. This process may also generate subaqueous sediment gravity flows such as turbidity currents, which further redistribute sediment towards the basin centre.

6. Subaerial rock fall and mass flow. In fjords, material may be deposited by rock fall and mass flow directly from the valley sides into the water body and subaqueous talus cones may develop.

7. Re-mobilisation by iceberg scour. The keels of large icebergs may ground in shallow water and remobilise sediment, returning it to suspension.

8. Current reworking. Sediment reworking by wave-induced currents close to the shore and by tides, particularly in fjords, is an important process in redistributing sediments.

9. Shoreline sedimentation. The action of waves on shorelines may modify material collected here. The addition of material from surrounding slopes may also be important in building up shoreline deposits. Streams entering the lake may build small shoreline deltas.

10. Biological sedimentation. This is an important component of the marine environment. The skeletal remains of micro-organisms such as diatoms, foraminifera and radiolaria may add to the sediment in these environments, and larger organisms may be present depending on the proximity of the glacier and the nature of the environment. Organisms have an important role in mixing (bio-turbating) sediment that has already been deposited.

11. Coriolis Force. Sedimentation in fjords is partly controlled by the effect of the Earth's rotation on the water body. In the northern hemisphere, sediment plumes are commonly deflected towards the right-hand side of the fjord and to the left-hand side in the southern hemisphere. This may cause asymmetry in sediment accumulation on the fjord floor.

Laminated muds with dropstones £ j Suspension settling ft Iceberg rain-out ■ttv* Sediment gravity flows i.-W

Talus cone

Laminated muds with dropstones £ j Suspension settling ft Iceberg rain-out ■ttv* Sediment gravity flows i.-W

Talus cone

Settling from suspension

_Rain-out from icebergs

Settling from suspension

_Rain-out from icebergs

Outwash_

Sediment flows Valley-side debris

Subglacial lodgement^ Non-glacial river input

_Bioturbation_

Biological sedimentation

Figure 10.6 Sediment sources and the distribution of processes within a glacially influenced fjord.

[Modified from: Hambrey (1994) Glacial Environments, UCL Press, figure 7.3, p. 191]

The distribution of these processes within a glacial fjord is illustrated in Figure 10.6. In general the distribution of glaciomarine environments, and therefore the sedimentary facies associated with them, can be viewed as a sequence of environments away from the ice front: (i) ice contact; (ii) the proximal zone - inner shelf or inner fjord; and (iii) the distal zone - outer shelf or outer fjord.

The ice-contact environment comprises the subglacial environment, which will be progressively exposed on the sea floor during glacier retreat and the immediate proglacial area. Subglacial processes will be similar to those of glaciers that terminate on land. It has been noted frequently that greater thicknesses of tills are produced by marine-based glaciers. For example, marine-based glaciers may typically deposit a layer of till between 5 and 20 m thick, whereas most tills deposited by terrestrial glaciers are only 1-2 m thick. This is probably due to the availability of soft deformable sediment beneath the glacier. A glacier advancing over saturated marine muds can remould substantial thicknesses of sediment through subglacial deformation, producing large till thicknesses. On land, by contrast, a glacier will

Supraglacial debris Ice

Basal ice debris Lodgement till Coarse sands and gravels Bedded sands and silts predominantly override coarse proglacial fluvial sands and gravels, which are better drained and therefore less easily deformed.

Deposition at the front of warm-based glaciers is dominated by the outflow of meltwater, which rises to the surface to form a plume. The flow velocity of this plume is checked dramatically as it enters the body of water, and consequently, unlike terrestrial environments, most of the fluvial sediment is deposited close to the ice margin, where submarine fans develop. These fans are known as grounding-line fans. These fans may be associated with subaqueous push-moraines and moraine banks if the glacier is advancing or if seasonal ice-marginal fluctuations occur (see Section 11.2). If the ice front is stationary these fans can grow rapidly and may emerge from the water to form ice-contact deltas. The fans grow by the direct addition of material from meltwater and extend laterally, at the front of the delta, by gravity flows. The height of these fans or deltas may be increased by pushing during ice-marginal readvances as well. This model only holds if the grounding line coincides with the ice front. If the ice front is partially floating, as will be the case for an ice shelf, then a subglacial delta or fan may form.

Beyond the ice-contact environment, the proximal zone - inner shelf or inner fjord - is dominated by sedimentation from suspension, which builds up a large thickness of sediment. This can be divided into an inner zone, where the rate of sedimentation is so fast that it inhibits benthic life (bottom dwellers), and an outer zone, where benthic life can exist. This outer zone will be defined by the presence of bioturbation within the sediment, caused by burrowing organisms. There is usually a very rapid decrease in particle size away from the glacier within this zone and the rate of sedimentation also falls away rapidly from the ice margin. However, sediment accumulation in the inner part of the proximal zone may give rise to considerable thicknesses of sea-floor sediment. This sediment may be laminated with a rhythmitic form. The laminations and rhythmites form in a number of different ways. For example, tidal currents can produce two graded couplets each day. Sand is predominantly deposited from the plume at low tide whereas finer grained sediment dominates at high tide. Diurnal variation in discharge may also affect the sediment plume and cause rhythmic lamination to develop. As a consequence two types of rhythmically laminated sediment are commonly identified based on the size fractions involved. Cyclopsams consist of graded sand-clay couplets, whereas cyclopels consist of graded silt-clay couplets.

The high rates of sedimentation in this inner proximal zone result in a sedimentary pile that frequently becomes unstable and is redistributed basinward, by sediment gravity flows, which may feed turbidity currents. In the inner part of the proximal zone suspension settling may dominate over 'rain-out' from icebergs, but in the outer part of the zone the reverse may be true (Figure 10.7). This may reintroduce coarse particles to the sea floor in the outer part of the proximal zone. The action of currents removing finer particles, winnowing, may also cause the surface sediment to become slightly coarser in this area. Streams or rivers entering the fjord within this zone may develop deltas and provide another source of coarser sediment. In fjords, coarse sediment may also be derived from the valley sides from subaqueous talus cones.

The distal sedimentary environment - outer shelf or fjord - contrasts strongly with the proximal zone. Suspended sediment concentrations in the water column are significantly less than in the proximal zone. Increased current activity, due to upwelling of deep water along continental margins, or at the mouths of fjords

320 Glacial Sedimentation in Water B

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Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 7B, p. 23. Copyright © 1990, Geological Society

Publishing House]

Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 7B, p. 23. Copyright © 1990, Geological Society

Publishing House]

ensures that there are usually strong sea-floor currents (Figure 10.7). As a consequence, sediment reworking dominates in this environment. Reworking of sediment by currents tends to remove finer particles and concentrates coarser sediment into lag horizons. Nutrient supply is good due to the current activity, and this gives high biological productivity (Figure 10.7). Benthonic and planktonic floras and faunas often contribute significantly to sedimentation. The proportion of sediment from icebergs is relatively small, although dropstones may be found.

The pattern of facies, the facies architecture that develops in glaciomarine environments can be very complex. Unlike some terrestrial glacial environments the preservation potential for complex glaciomarine facies assemblages can be quite good, especially on gently subsiding continental shelves, like many of those along the margin of the North Atlantic. Figure 10.8A shows a hypothetical facies sequence for a glacier advancing and retreating across a continental shelf. The sequence of glaciomarine environments in front of the glacier is superimposed in a vertical log as the glacier advances and retreats as explained by Walther's law (see Section 8.1.3) (Figure 10.8B). As the glacier advances there is a gradual increase in the amount of

log

Ice retreat

Marine muds

Distal glaciomarine: fine muds, decreasing number of dropstones

Proximal glaciomarine: ice-rafted diamict and mud from suspension. Diamict deposited by sediment gravity flows

Grounding-line fan vvVv.'A

Glaciation

Ice advance

Lodgement or deformation till

Proximal glaciomarine: ice-rafted diamict and mud from suspension units of diamict formed by sediment gravity flows v:

Distal glaciomarine: fine muds, decreasing number of dropstones

Marine muds

Mu,ds with dr°pstones pq Marine muds

1-1 and rain-out diamicton 1-1

Figure 10.8 (A) Illustration of Walther's law with reference to glaciomarine environments and sedimentary facies during a glacial cycle in which a grounded glacier advances and decays across a continental shelf. (B) Hypothetical vertical log deposited on the continental shelf during a single glacial cycle in which a grounded glacier advances and decays across a continental shelf. [Modified from: (B) Hambrey (1994) Glacial Environments, University College London

ice-rafted debris, which is followed by the deposition of a lodgement or deformation till as the area is covered by the advancing ice. Deglaciation is first marked by the development of a grounding-line fan and subsequently by a gradual decrease in the amount of ice-rafted debris as the glacier retreats and the glacial influence falls. In practice the pattern of sedimentary facies associated with the advance and retreat of a glacier across a continental shelf may be much more complex.

Within fjords the facies architecture depends on the dynamics and behaviour of the glacier and therefore may be even more complex than that found on the continental shelf. Work on fjord glaciers in Alaska has identified five types of glacier behaviour, each associated with its own pattern of glaciomarine facies.

1. Association I: rapid deep water retreat. Here the glacier is retreating rapidly in deep water by iceberg calving. In the ice-proximal zone the sediment facies consists of reworked subglacial till and glaciofluvial sands and gravels exposed as the glacier retreats. These are associated with supraglacial debris dumped from the ice margin. The proximal and distal glaciomarine zones contain large amounts of ice-rafted debris.

2. Association II: stabilised or slowly retreating ice margin. Here the ice recession has been retarded by a fjord constriction or pinning point (see Figure 3.2). Calving continues, however, and thick deposits of ice-rafted sediment accumulate. The ice margin is marked by the deposition of coarse-grained sediment either in a grounding-line fan, moraine bank or ice-contact delta (see Section 11.2).

3. Association III: slow retreat in shallow water. Here the glacier is either retreating or slowly advancing in shallow water. Calving is severely reduced and so there is relatively little ice-rafted sediment. An ice-contact delta may develop at the ice margin.

4. Association IV: proximal terrestrial glacier. By this point the glacier is terrestrial, producing a large outwash delta that progrades into the fjord. The resulting facies are coarse-grained sediments on the delta top, while the delta fronts comprises sand and gravel, which alternate with marine muds deposited from suspension. There is little ice-rafted debris beause only small icebergs are introduced into the fjord via meltwater streams. Sand and silt rhythmites may be deposited on the fjord floor due to the interplay of sedimentation from suspension and turbidity currents generated from the delta slope.

5. Association V: distant terrestrial glacier. By now the glacier is distant and so the facies comprise tidal-flat muds and braided stream gravels

These facies associations can be combined in a variety of different ways depending on: (i) the morphology of the fjord; and (ii) the behaviour of an individual glacier. Fjord morphology controls the location and frequency of pinning points and the depth of water in which calving will occur (Figure 3.21). The behaviour of the individual glacier is also important. Some fjord glaciers are inherently unstable and prone to cyclic episodes of advance and decay, driven not by climate but by decoupling of the glacier from the stabilising effect of moraine banks or pinning points (Box 11.4). As a result, facies architecture within fjords may be complex, as illustrated in

Figure 10.9. Here a relatively simple facies pattern produced by the retreat and subsequent advance of a glacier down a glaciomarine fjord under stable sea-level conditions. Initially the glacier margin is stabilised via a moraine bank or subaqueous outwash fan (see Section 11.2). On becoming uncoupled from this pinning point the

Figure 10.9 Hypothetical facies architecture associated with the retreat and advance of a glacier down a marine fjord. See text for facies descriptions. [Modified from: Powell (2003) in: Glacial Landsystems (ed. Evans), Arnold, figure 13.17, pp. 345-6]

glacier retreats rapidly via calving into deep water and a range of glaciomarine diamicts and dropstone deposits are produced. The margin may stabilise during this period of retreat to produce another moraine bank before becoming terrestrial as it retreats above the water line. As the terrestrial meltwater streams feed into the fjord they will build up a glaciofluvial delta. The final stage in Figure 10.9D shows what happens if the ice margin should advance down the fjord again, moving over the sediment ramp created earlier to produce a deformed cap to the underlying glacio-marine sediments. This is a highly simplified model and the level of behavioural complexity that can be added is considerable, particularly as fjord glaciers are very sensitive to mass-balance changes and can retreat and advance dramatically (see Section 3.6). Fjord glaciers are therefore highly dynamic and can generate very complex facies patterns.

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