Global record of climate change, Quaternary, Cainozoic of north-east Scotland
|Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
Global record of climate change
In the last two decades significant advances have been made in the understanding of Quaternary environmental change in Scotland. This is largely because the timing and pace of climatic change in Scotland can be viewed now as part of the broader pattern affecting the whole North Atlantic region (Boulton et al., 1991). Evidence of global and regional events is found in deep-sea sediment cores, in cores of ice through the Greenland ice sheet and from extended sequences of interbedded loess and organic deposits from continental Europe. The fluctuations in climate appear to be driven by minor orbitally controlled variations in solar radiation that are amplified through complex interactions between the atmosphere, oceans, ice sheets and global tectonics.
The onset of glaciation in the Northern Hemisphere probably began in the late Miocene with a significant build-up of ice over southern Greenland, although progressive intensification did not begin until 3.5 to 3 Ma when that ice sheet expanded into northern Greenland (Maslin et al., 1998). This suggests that the Scottish climate had begun to deteriorate long before the beginning of the Quaternary Period. The Quaternary is presently defined as beginning at about 1.77 Ma (Shackleton et al., 1990), but studies of ice-rafted debris in ocean floor sediments in the North Atlantic and elsewhere have shown that significant climatic deterioration occurred some 600 ka earlier when the mid-latitude continents of the Northern Hemisphere first became glaciated (Shackleton et al., 1984; Ruddiman and Raymo, 1988). Many now agree that the beginning of the period should be placed either at 2.4 Ma, as accepted here (P915253), or at 2.6 Ma (Funnell, 1995) in order to take into account the above evidence and recent advances in micropalaeontology, magnetostratigraphy and climatostratigraphy (Mauz, 1998). Although no direct evidence of glaciation at this time has been found on the Scottish mainland or neighbouring offshore shelves, the relatively minor decline in temperature needed for glaciers to develop, suggests that they did so, at least in the western Highlands.
Deep ocean record
The frequency, rapidity and intensity of climatic change are a key feature of the Quaternary. Studies of the isotope geochemistry and micropalaeontology of deep ocean-floor sediments have revealed that climatic conditions have fluctuated continuously throughout the period and that climate systems have switched rapidly between interglacial and glacial modes (P915276). At least 50 significant ‘cold–warm–cold’ oscillations have been recognised. Many theories have been put forward to explain the initiation of Northern Hemisphere glaciation (Maslin et al., 1998), most involving changes in atmospheric composition (such as caused by increased volcanism) or in total solar radiation. The initial cooling that began during the Neogene has been attributed to the slow changes in the global configuration of the continents as a consequence of sea-floor spreading. These include the emergence of the Panama Isthmus and the deepening of the Bering Straits, both of which had pronounced affects on the ocean circulation patterns (Raymo, 1994). The cooling has also been attributed to the uplift of high mountain ranges such as the Tibetan Himalayas and the Sierra Nevadan and Coloradan mountains of North America, causing perturbations of the upper atmosphere and subsequent climatic changes (Ruddiman and Kutzbach, 1991). Uplift of the Himalayas also may have resulted in a massive increase in chemical weathering during the late Cainozoic, leading to increased sedimentation of calcium carbonate and atmospheric depletion of carbon dioxide. Global cooling, the inverse of the ‘greenhouse effect’ would thus occur (Raymo and Ruddiman, 1992; Raymo, 1994). However, none of these mechanisms can wholly explain the rapid intensification of glaciation observed in the deep ocean record at about 2.7 to 2.5 Ma (Maslin et al., 1998).
The driving force of climatic change during the Quaternary appears to be the long-term cyclical variation in solar energy. Cyclical changes in solar insolation appear to be associated with the Earth’s orbital periodicities, rather than on long-term changes in energy output of the Sun, although the latter is difficult to disprove (Shackleton and Opdyke, 1973). There are three significant orbital (‘Milankovich’) cycles, caused by:
- precession (‘wobble’) of the planet’s axis
- the tilt of the Earth’s axis
- the eccentricity of the Earth’s orbit (Imbrie et al., 1984)
The periodicities of these cycles are roughly 23 ka, 41 ka and 100 ka, respectively. The climatic fluctuations that occurred during each major glacial–interglacial cycle have been attributed to the first two cycles (Imbrie and Imbrie, 1979), but now it is generally agreed that they alone could not have produced the magnitude or rapidity of the documented changes. Maslin et al. (1995) suggested that the increase in the amplitude of orbital obliquity cycles deduced from the deep ocean record from 3.2 Ma onwards may have increased the seasonality of the Northern Hemisphere, thus initiating the long term cooling trend. The subsequent sharp rise in the amplitude of precession between 2.8 and 2.55 Ma may have forced the rapid intensification of glaciations that began then.
The primary Milankovich fluctuations of solar insolation must have been amplified substantially by additional factors involving physical, biological and chemical interactions and ‘feedback loops’ between the atmosphere, oceans and ice sheets. For example, complex changes in the surface and deep water circulation patterns of the oceans, and the concentrations of carbon dioxide and other ‘greenhouse’ gases in the atmosphere all play a crucial role (Broecker and Denton, 1990). Of particular importance to the British Isles are changes in the position of the Gulf Stream. This northward-flowing current of warm surface water is compensated by the return southwards of cold, dense water at depth. Sudden changes in this circulation pattern, the so-called ‘North Atlantic conveyor’, may have had a major impact on climate (Skinner and Porter, 1995; P915277). For example, large volumes of fresh water released during rapid warming events may have temporarily ‘switched off’ the conveyor leading to cooling in north-west Europe (Lagerklint and Wright, 1999).
The ‘SPECMAP’ deep ocean oxygen isotope record (P915253) indicates that during the early Quaternary, up until about 760 ka, each dominant warm–cold cycle lasted about 40 ka (Ruddiman et al., 1989). Ice caps in Greenland, Alaska, Iceland and Scandinavia expanded to the coast and glaciers probably developed in the western Scottish Highlands at high elevations (Clapperton, 1997). Iceberg drop-stones found in sediment cores taken offshore from northwest Europe provide indirect evidence of these early glaciations (Holmes, 1997; Sejrup et al., 2000). Following a major change at about 760 ka, the build-up of much larger ice sheets in the Northern Hemisphere occurred, and to date there have been seven major glacial–interglacial cycles (P915276). Each ‘glacial’ episode lasted between 80 and 120 ka and was followed abruptly by an ‘interglacial’ lasting 10 to 15 ka. The rapid deglaciations are defined as ‘Terminations I–VII’ (Broecker, 1984; P915276). The glacial periods included long, cold intervals, termed stadials, and less cold, and even warm, intervals lasting for a few thousand years termed interstadials. Most terrestrial evidence preserved in Scotland relates to the last glacial–interglacial cycle (Holocene, Devensian and Ipswichian stages), but older deposits are preserved locally (P915254).
The SPECMAP timescale of Imbrie et al. (1984) has been the cornerstone of Quaternary palaeoclimate studies. It assumes that the climatic cycles observed in deep ocean sediment records were driven by Milankovitch orbital parameters. Although this assumption has been challenged (Schrag, 2000), the validity of the timescale has been confirmed independently by Raymo (1997). She hypothesises that the general intensification of the major glaciations during the Quaternary results from long-term reduction of atmospheric carbon dioxide levels by tectonic processes (as shown by the gently inclined straight line on P915276). This gradual weakening of the ‘greenhouse effect’ has caused cold periods to become colder, allowing ice sheets to expand in regions that were previously too warm. Raymo’s (1997) model shows that major glaciations (coloured areas on the insolation curve of P915276) only occurred when both ‘obliquity’ and ‘eccentricity’ Milankovitch cycles reinforced themselves, causing long intervals of low summer insolation in northern lattitudes. Furthermore, her model
Greenland ice core record
New, high resolution evidence derived from cores taken through the Greenland ice sheet near its summit (P915278), demonstrate that the Earth’s climate has been much more variable during the past 250 ka than previously thought (Dansgaard et al., 1993). Dramatic climatic changes have occurred, both on the millennial scale and over just a few years. Some 24 interstadial intervals have been identified in the last (Devensian) glacial stage alone, compared with the five or six that were recognised previously from the pollen record and formalised in the north-west European and British stratigraphy (Mitchell et al., 1973; P915254). Each of the 24 interstadials within the Devensian started with abrupt warming and typically cooled over a period of 1 to 3 ka, ending in a cold event (Broecker, 1994). These periods, known as Dansgaard–Oeschger (D-O) cycles, are sometimes grouped together within longer, Bond cycles, during which temperatures also declined gradually (Bond et al., 1993; P915279). It is important to note that even the milder interstadials in Greenland had a climate in which average temperatures were five to six degrees colder than on the summit of the Greenland ice sheet today (Dansgaard et al., 1993). Intervals when glaciers were probably present in Scotland during the past 25 ka are indicated in P915278).
Many Bond cycles appear to culminate in a Heinrich event (P915279). Originally it was concluded that temperatures rose substantially within a decade or so following these events (Bond and Lotti, 1995), but there is growing evidence that they are actually the result of extremely rapid warming (Lagerklint and Wright, 1999). Heinrich events, as originally defined, were massive discharges (‘armadas’) of icebergs from the Laurentide ice sheet, recognised in deep ocean cores in the North Atlantic by the presence of ice-rafted debris (IRD) at intervals in the sediment sequence (Heinrich, 1988; P915278). These horizons were thought to result from the sudden mass wastage, via iceberg calving, of the Laurentide ice sheet every 11 ka or so, possibly as a result of it having grown too large to sustain itself and resulting in catastrophic collapse. Such ‘binge-purge’ cycles are thought by some to have played a pivotal role in driving world climate variability (Broecker, 1994; Alley and MacAyeal, 1994). More recently, Heinrich events have been identified in North Pacific cores (Hicock et al., 1999) and ice sheets in Iceland, Britain and Scandinavia are now known to have contributed IRD during Heinrich events (Frontal et al., 1995; Sejrup et al., 2000). It therefore seems unreasonable that all of the ice sheets ‘binged and purged’ in harmony unless there was some independent forcing mechanism (Bond and Lotti, 1995; Kotilainen and Shackleton, 1995). Furthermore, it appears that there was a synchronous deposition of ice-rafted layers in the Nordic seas and North Atlantic (Dowdeswell et al., 1999) and glacial events in north-west Europe may actually have driven events across the Atlantic, not the other way around (Grousset et al., 2000). One suggestion is that Heinrich events are triggered every 6100 years as a result of direct changes in solar radiation (Lehman, 1996), their effects being greatest during the longer cold periods when glaciers, ice sheets and tidewater glacier margins were most extensive.
Heinrich events probably led to ‘armadas’ of icebergs in the seas around the British Isles. The release of large volumes of glacial meltwater and icebergs from North America possibly ‘switched off’ the ‘North Atlantic Conveyor’ for a while leading to cooling of the north-east Atlantic and north-west Europe (Lagerklint and Wright, 2000). Minor re-advances of ice streams flowing through the major firths of the west and east coasts of Scotland are possibly due to such events (Clapperton, 1997). The Heinrich events of the North Atlantic occurred at about 10 ka BP (H0), about 14.2 ka BP (H1), 21.4 ka BP (H2), 26.7 ka BP (H3), 34.8 ka BP (H4) and 47.2 ka BP (H5) (Chapman et al., 2000; P915278).
The future challenge is to determine to what extent the various fluctuations in climate have left a recognisable signature in the geological record of sediments, landforms, and fossil floras and faunas (Alverson and Oldfield, 2000 and papers cited therein). The offshore stratigraphical record obtained over the last two decades has already significantly augmented the relatively incomplete terrestrial record, particularly for the Early and Middle Quaternary. Onshore, recent integrated studies at the few sites at which organic materials are well preserved, involving sedimentology, the analysis of pollen, beetle remains and marine molluscs, and the use of a range of dating techniques, have also provided much new information on past Quaternary environments.