Pre Quaternary glaciations

A general global temperature curve (Fig. 5.1) suggests that favourable conditions for glaciation occurred during at least five periods of

Earth's history. Most probably, however, the Earth has experienced some kind of glaciation during the entire time span, but glaciation may have varied from continental ice sheets to minor ice caps or alpine glaciers. For pre-Quaternary glaciations, it is important to pay attention to continental drift and mountain building in order to explain the timing and geographical distribution of major glaciations. Since the development and acceptance of the theory of plate tectonics in the early 1960s, several attempts have been made to apply the theory to ancient glacial deposits, most of which involved correlating glaciation on a particular continent with periods when the continent was at high palaeolatitude (Table 5.1). This was successfully applied to glacial deposits of the southern continents (Gondwanaland) during the Palaeozoic (e.g. Crowell, 1978).

Evidence for pre-Quaternary glaciations is dominated by tillites, mixtites or diamictites as sediments and lithified rocks. The oldest evidence for glaciation is from the Precambrian. The Huronian glaciation dates to the Lower Proterozoic (approx. 2000-2500 million years ago) and is represented by ca. 12,000 m thick sediments in the Lake Huron region of Canada. Evidence for Late Precambrian glaciation is available on almost all of the present continents. Extensive glaciations also occurred during the Palaeozoic (570-230 million years ago). In Africa, located over the South Pole during Ordovician times, there is reliable evidence for Ordovician glaciation. The ice sheet complex there may have been some 20 times larger than the present Antarctic ice

Mean global temperature present cold

Mean global precipitation present dry wet

Mean global temperature present cold

Mean global precipitation present dry wet

4600

Figure 5.1 General global temperature and precipitation curves - note variable timescale. (Adapted from Bradley, 1985)

4600

Figure 5.1 General global temperature and precipitation curves - note variable timescale. (Adapted from Bradley, 1985)

sheet. Ordovician glaciations have also been evidence for Devonian glaciation. The Permo-reported from southern Africa, South and Carboniferous glaciation of Gondwanaland, North America and Scotland. In South America centred over what is now southern Africa and southern Africa there is fragmentary and Antarctica, had perhaps the greatest

Table 5.1 Timing of some Cenozoic global events related to glaciation (Adapted from Herman et al., 1989)

Time (Ma)

Tectonic events

Climatic events

0.9-0

Himalayan, Alpine orogeny

Major glacial-interglacial cycles

0.9

Orogeny peak

1.6

Tibetan, Himalayan, and Sierra Nevada orogenies

2.4-2.7

First major northern hemisphere ice sheets

3.5

Uplift of Panama Isthmus

Gradual temperature decline

Opening of Bering Strait

Southern hemisphere lowland glaciation

5.5

Isolation of Mediterranean Sea

6

Strong global cooling

Expansion of Antarctic ice sheet

(2-14

Major Antarctic build-up'

16-18

Subsidence of Iceland-Faeroe ridge

14-18

Renewed Himalayan Tibetan orogeny

24

Drake Passage opens

Increased glaciation of Antarctica

Intensification of global climatic gradients

36

Greenland separates from Eurasia. Tasman seaway opens

Antarctic continental glaciation

35-37

Major Himalayan and Alpine orogenies

Cooling at high and low latitudes

Mountain glaciation in Antarctica

impact of all the pre-Quaternary glaciations. The ice sheet reached the coasts of eastern South America, southern Africa, southern Australia and southern India. During its maximum extent, the ice sheet covered about twice the area covered by the present Antarctic ice sheet.

The early development and build-up of the glaciers took place during the late Tertiary. The initial glacier formation may have been caused by tectonic uplift of the Tibetan Plateau, the Himalayas, and the western Cordillera in North America (e.g. Ruddiman et al, 1989). The development history of the Antarctic ice sheet is a matter for debate. Offshore sediments have been interpreted to indicate glaciation during the last 40 million years (Hambrey et al, 1989, 1992), while others have suggested glacier formation 5-10 million years ago (e.g. Drewry, 1978). The Antarctic ice sheet has fluctuated since its initial formation, and some scientists have even suggested that for short periods it may have melted away (e.g. Barrett et al., 1992). This view has been challenged, for example by Denton et al. (1993), presenting geomorphological, geochronological and palaeoclimatological evidence indicating that the East Antarctic ice sheet has been fairly stable during the last 4.4 million years. In the Gulf of Alaska, there is a record of late Tertiary glaciation represented by glaciomarine sedimentation from the late Miocene into the Quaternary (Eyles et al., 1991).

According to a recent study by Maslin et al. (1998), the onset of northern hemisphere glaciation began in the late Miocene with a significant build-up of ice in southern Greenland. Progressive intensification of glaciation, however, did not seem to have begun until 3.5-3 million years ago, when the Greenland ice sheet expanded to include northern Greenland. Those authors suggested that the Eurasian Arctic and Northeast Asia were glaciated at about 2.74 million years ago, Alaska at 2.7 million years ago, and North East America at 2.54 million years ago. Tectonic changes (uplift of the Himalayan and Tibetan Plateau, deepening of the Bering Strait, and the emergence of the Panama Isthmus) were suggested to have been too gradual to be responsible for the speed of northern hemisphere glaciation. The authors therefore suggested that tectonic changes brought global climate to a critical threshold and that relatively rapid variations in the Earth's orbital parameters (insolation) triggered the northern hemisphere glaciation.

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