Do Icecore And Oceansediment Data Relate To

HUMAN EXPERIENCE? One fundamental aspect of analysing climate change must be addressed from the outset. This is whether the evidence from far-flung places like Antarctica, Greenland or the depths of the great oceans provides a real measure of what was happening across the plains of Eurasia, or in Africa or Australia. Clearly the major changes associated with the waxing and waning of the ice ages were global in nature.

figure 2.7. Deep-ocean sediment core from the central North Atlantic showing the link between Heinrich events (labelled H1 to H6), which can be seen in the increase of ice-rafted debris (lithics) (white curve) and the surface water temperature as revealed in the incidence of cold-water foraminifera Neoglobigerina pachyderma (black curve). (Data archived at the World Data Center for Paleoclimatology, Boulder, Colorado, USA.)

figure 2.7. Deep-ocean sediment core from the central North Atlantic showing the link between Heinrich events (labelled H1 to H6), which can be seen in the increase of ice-rafted debris (lithics) (white curve) and the surface water temperature as revealed in the incidence of cold-water foraminifera Neoglobigerina pachyderma (black curve). (Data archived at the World Data Center for Paleoclimatology, Boulder, Colorado, USA.)

The real question is, however, how the more rapid changes affected different parts of the world. Only if we can be sure that the most important features of climate change including its short-term variability were truly part of life across the continents of the world can we be confident of using these invaluable sources to underpin our analysis.

The first stage in this comparison is to look at the differences between the Greenland ice-core data and the results obtained from ocean sediments in the North Atlantic. A typical set of results for this region is shown in Fig. 2.7 for a site roughly midway between Newfoundland and southern Ireland.1 The first thing to note is that there is much less detail than in the ice cores, as the ocean sediments have at best a resolution of several hundred years. This is because not

1 The core used for this figure is designated DSDP-609 and the data are archived at the World Data Center for Paleoclimatology, Boulder, Colorado, USA.

only is the rate of deposition of these sediments much slower than the build up of ice cores (decadal accumulation rates typically being of the order of a millimetre rather than a metre), but also burrowing creatures searching for food stir up the top layers of the ooze. Only where conditions near the seabed are anoxic is the deposition undisturbed (see below). Nevertheless, many of the major features are the same, and the differences are, in part, a reflection of the difference in what the data are recording. In the case of the cold-water foraminifera (Neoglobigerina pachyderma) the variation from 0 to 100% covers the range from warm surface waters, typical of the Holocene, to the ice-filled waters of the severest stages of the ice age. So this curve picks out the coldest periods most clearly. The truly notable feature is the additional emphasis given to the prolonged cold period from 70 to 63 kya, to the cold spell at around 36 kya and to the frequency of extreme cold from 30 to 15 kya. The top curve shows the amount of ice-rafted debris found in the sediment, which highlights the times when armadas of icebergs floated out into the North Atlantic (see Section 2.5).

The next point to consider is that where the ocean cores are taken close enough to land they provide direct insight into regional weather patterns that may extend well inland. This applies in the case of the North Atlantic, where the prevailing westerly circulation means that inferences about climate change from ocean cores can be extended across Europe and into central Asia. Equally well, cores in the northwestern Indian Ocean show laminated organic-rich bands, reflecting strong monsoon-induced biological productivity (Leuschner & Sirocko, 2000). In contrast, periods of lowered southwest monsoonal activity produce bands poor in organic carbon. This provides valuable insights into the nature of the monsoon circulation over much of southern Asia. Furthermore, there is clear evidence of a close correlation between events in Greenland and the North Atlantic from the variations in the laminations in the cores in the northwestern Indian Ocean. This supports the conclusion that the fluctuations observed at high latitudes were also a significant component of low-latitude climate change during the last ice age. Moreover, these millennial and centennial fluctuations have been reflected in the strength of the monsoonal circulation over the Indian subcontinent.

Another prolific source of information is from the Cariaco Basin, off the coast of Venezuela (Petersen et al., 2000). Here rapidly deposited (~3mm per decade) organic-rich sediments contain visible annual laminations. What is more they are devoid of preserved seabed (benthic) faunas. This means the sediments were laid down in anoxic conditions, so there were no burrowing creatures scrambling the evidence. These deposits are composed of light (plankton-rich) and dark (mineral-rich) layers. This provides insights into what was going on in the surface waters as a result of strong seasonal fluctuations in trade-wind-induced upwelling and regional precipitation. The mineral sediments come from the surrounding watersheds and provide an accurate picture of regional hydrologic conditions. Cores from the Cariaco Basin have been used to provide detailed analysis of interannual climate change over the last 15 kyr, and of decadal fluctuations over the last 500 kyr.

Turning to land-based measurements, there is a huge range of independent observations that can be used to draw parallels with ice-core and ocean-sediment data. One of the most relevant sources of detailed climatic analysis has been obtained from lakes where layers of silt have been deposited for over 100 kyr. In these instances, and in many other cases of measurements on land, some of the most important indicators of changing environmental conditions are the amount and types of pollen found in sediments. If these sediments have been laid down in a regular manner, the abundance of pollen from different species of trees and shrubs provides details of climate change. Because different species have distinct climatic ranges, it is possible to interpret their relative abundance in terms of shifts in the local climate. So pollen records from many parts of the world have the potential to fill the geographical gaps that ice cores and ocean-sediment records cannot cover.

Pollen records are also of great historical importance in studying climatic change because of the early work charting the emergence of northerly latitudes from the last glaciation. Most pollens (from flowering trees and plants) and spores (principally ferns and mosses) are tiny. Few exceed 100 p.m (0.1 mm) in diameter and the majority are around 30 p.m. They have a waxy coat that protects them from decay. The size and shape of this outer wall, along with the number and distribution of apertures in it, are specific to different species and can be readily identified under a microscope.

Early work on pollen records focused on alterations in regional vegetation since the end of the last ice age. This work has established that these shifts were controlled by broad global patterns of climate change. For changes extending back into the last ice age there was until recently greater uncertainty as to whether other factors involving the migration and competition of species might have been more significant. There are two principal explanations for this uncertainty. First, almost all the longer pollen records related to northern Europe, so there were doubts about whether the results were representative of global changes. Second, many of the cores contained high-frequency variations during the last glaciation. These raised doubts about the dating of the strata in the cores, some of which had substantial gaps. It was not until more recent ice-core and ocean-sediment data became available that it was possible to check the timescales of these different records.

Recent pollen records obtained from cores drilled as far apart as the Massif Central in France (Thouveny et al., 1994), southern Italy (Allen et al., 1999) and Carp Lake in the Cascade Range in northwest USA (Whitlock & Bartlein, 1997) have enabled comparisons to be made across the northern hemisphere. These have confirmed that pollen records can be accurately dated back to the last interglacial, some 125 kya, and provide an extraordinary amount of additional information about how the climate has changed. The correlation between these results is remarkable. So, variations observed in pollen records can be combined with data from ice cores and ocean sediments to form a more detailed picture of global patterns of climate change during the last ice age and since then. In addition, measurements of the magnetic susceptibility of the cores in France and Italy also provide confirmation of the climate changes observed in the pollen records. This is because under cold climate conditions the freeze-thaw alternations caused the erosion of local rocks and the deposition of clastic sediments. Under temperate climatic conditions vegetation and soil development enhanced the organic content of the sediment, thereby diluting the magnetic fraction of the sediments. So magnetic susceptibility is strongly sensitive to local climate and provides an independent check on the conditions.

Another increasingly fruitful source of climatic data is beetle assemblages (Coope et al., 1998). Beetles form roughly three-quarters of all animals found in terrestrial and freshwater-brackish environments. Their habitat is dependent on climatic factors, rather than being plant-specific, and when the climate changes they swiftly migrate to more congenial localities. Furthermore, their remains are well preserved in sediments. So drawing on present-day data of the distribution of beetle species it is possible to reconstruct the probable temperature regime represented by a fossil sample. Using information on several species can often refine this analysis. The temperature information usually combines the mean summer maximum and the range between summer and winter to draw detailed inferences about past climates on the basis of the distribution of different beetle species in the fossil record.

Rather different climatic information is recorded in the formation of stalactites and stalagmites. The deposition of calcium carbonate by running water in caves forms encrustations (speleothems) that can accumulate at approximately constant rate. Similar to the way in which the properties of foraminifera are affected by the isotope ratio of the water they grow in, speleothems are affected by the changes in the isotopic ratios in precipitation. Where such encrustations build up over a long time, they have the potential to provide useful climatic information. They also have the advantage that, because rainwater dissolves uranium and its daughter products, speleothems can be dated absolutely by measuring their uranium/thorium ratio (see

Appendix). Their disadvantage is that where the amount of rainfall falls below a critical level, or in the case of the depth of the ice age turns to snow and becomes locked in ice and permafrost, the encrustation ceases. So breaks in the record require careful calibration, which is possible by measuring the uranium/thorium ratio throughout the speleothem. This provides an absolute dating of when different layers were deposited. In recent years an increasing number of measurements of speleothems from across the continents of the world have been published. These provide valuable confirmation of the coherent picture of the detailed fluctuations in the climate throughout the last ice age.

The breadth of these measurements provides the adequate opportunity to crosscheck whether the various aspects of climate change observed in the Greenland ice cores and the northern Atlantic sediment records do indeed extend to the rest of the northern hemisphere. Furthermore, the recent pollen and speleothem records have sufficient resolution to identify rapid changes in the climate that can be correlated with the sudden shifts in Greenland and the North Atlantic. This combination allows us to build up a comprehensive picture of the fluctuations that were so much a part of the ice age.

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