Time ka BP

FIGURE 6.54 Upper panel (a): The difference between 8I3C in planktonic forams Neogloboquadrina dutertrei, and in benthic forams Uvigerina senticosa, from equatorial Pacific core VI9-30 (Shackleton and Pisias, l985).This difference (A8I3C) shows the relative increase in biological productivity of surface waters in glacial periods compared to deep water that results in a higher 8I3C gradient at those times. Increased surface water productivity would have led to a reduction in atmospheric carbon dioxide,so the A8I3C can be interpreted as a paleo-C02 index (left scale).The lower panel (b) shows the same record plotted with theVostok ice-core C02 record, although the exact temporal match may not be quite correct (Shackleton et a/, 1992).

FIGURE 6.54 Upper panel (a): The difference between 8I3C in planktonic forams Neogloboquadrina dutertrei, and in benthic forams Uvigerina senticosa, from equatorial Pacific core VI9-30 (Shackleton and Pisias, l985).This difference (A8I3C) shows the relative increase in biological productivity of surface waters in glacial periods compared to deep water that results in a higher 8I3C gradient at those times. Increased surface water productivity would have led to a reduction in atmospheric carbon dioxide,so the A8I3C can be interpreted as a paleo-C02 index (left scale).The lower panel (b) shows the same record plotted with theVostok ice-core C02 record, although the exact temporal match may not be quite correct (Shackleton et a/, 1992).

that changes in the rate at which organic carbon is sequestered in the ocean (as distinct from changes in the carbonate content, which would not affect near surface 8I3C) have been the dominant cause of atmospheric C02 changes on glacial to interglacial timescales. Furthermore, spectral analysis of this record in relation to benthic 8lsO (a proxy of continental ice volume) shows that C02 changes are strongly in phase with orbital forcing, but lead ice-volume changes at all orbital frequencies (Shackleton and Pisias, 1985). This points to C02 variations as playing a key role in the forcing of climatic changes involving ice sheet growth and decay, rather than being only a passive response to such changes. It also indicates that the phosphate mobilization idea of Broecker (1982) cannot be the main factor initiating productivity changes, as that mechanism is tied to sea-level changes (which lag by several thousand years both orbital forcing and C02 changes). It is of interest that much of the variance of the COz record is in the frequency band related to obliquity, which has its main impact on radiation receipts at higher latitudes. This suggests that the subpolar ocean areas (perhaps via switches in thermohaline circulation) play a critical role in driving C02 changes and hence amplifying orbitally forced climatic change (Wenk and Siegenthaler, 1985).

One additional point to note is that the overall 813C content of the ocean was lower (by ~0.4%o) in glacial times because of the significant decrease in biomass on the continents. Approximately 500 Gton carbon (with a 813C of -25%o) was added to the ocean-atmosphere system at such times, lowering the 13C content of the ocean accordingly (Siegenthaler, 1991). With less photosynthetic activity on land, one might expect higher pC02 levels during glaciations, but this was more than compensated for by enhanced C02 solubility (in cooler waters) and increased oceanic biological activity.

In a subject as complex as ocean geochemistry there is room for many alternative hypotheses, and in this field alternatives abound. Much attention has been paid to mechanisms that could have brought about changes in oceanic nutrient content and hence biological productivity. Martin (1990) for example points to the much higher levels of iron-rich atmospheric dust that was dispersed across the globe during the last glaciation. He suggests that this dust fertilized the sub-Antarctic Ocean, greatly increasing productivity and leading to a reduction in pC02. This idea has been controversial on biological (e.g., Dugdale and Wilker-son, 1990; Sunda et al., 1991) as well as geological grounds, because there is conflicting evidence for increased productivity in this area during glacial periods (Mortlock et al., 1991; Kumar et al., 1995). Others have suggested that the general idea may have more merit in terms of changing productivity at lower latitudes, though distinguishing between increased productivity due to Fe fertilization, from a general increase in nutrient levels due to changing ocean circulation regimes, and increased surface wind stress and upwelling associated with glacial periods is very difficult (Sarnthein et al., 1988; Berger and Wefer, 1991). Furthermore, there is evidence that in some upwelling areas, even when productivity increased, the net effect was insufficient to overcome the transfer of C02 from the ocean surface to the atmosphere. Hence, there may be some situations where higher productivity does not always equate with a reduction in atmospheric pC02

due to the even stronger influence of de-gassing from carbon-rich upwelling waters (Pedersen et al., 1991).

One interesting argument in favor of increased high latitude biological activity was suggested by Kumar et al. (1995), who examined "excess" 231Pa/230Th and 10Be/230Th in sub-Antarctic sediments as a measure of past biological productivity. They point out that because most of the biomass initially deposited as sediment is not preserved, the measured sediment accumulation rate is not a reliable index of former productivity. The radionuclides protactinium-231 and thorium-230 (dispersed throughout the ocean from the decay of uranium in seawater) are removed from the water column by attachment to particles. However, 231Pa is less easily removed than 230Th, so as particle flux (productivity) increases the disparity in the scavenging rate of the two radionuclides increases (i.e, the 231Pa/230Th ratio increases). A similar index is provided by 10Be/230Th; 10Be is a cosmogenic isotope that is widely dispersed across ocean basins, but like 231Pa, it has a longer residence time than 230Th. Thus an increase in 10Be/230Th also provides a measure of particle flux. Both 231Pa/230Th and 10Be/230Th accumulate in the sediments even though the particles that transported them to the sea floor may no longer be present. An "excess" of these radionuclides (over that expected from a steady sedimentation rate) thus documents formerly high productivity in the overlying water column. Studies of these and other productivity indices reveal that sub-Antarctic waters were much more productive in glacial times, but this was not the case in the Southern Ocean, closer to the Antarctic continent (perhaps because of more extensive sea ice). The sediments provide evidence that perhaps 30-50% of the observed reduction in C02 can be explained by a more efficient biological pump in the cold waters surrounding Antarctica during glacial episodes.

Studies of nitrogen isotopes point to another mechanism with important implications for atmospheric C02 changes (Altabet et al., 1995; Ganeshram et al., 1995). As already noted, nitrate (N03-) is a key nutrient limiting biological productivity in many parts of the ocean. Oceanic nitrate concentration is the result of the balance between upwelling of nitrate-rich waters and processes of denitrification carried out by bacteria in low oxygenated zones of the ocean. Denitrification produces gaseous nitrogen and nitrous oxide, which then escapes from the water column, thereby reducing the supply of fixed nitrogen available for plant growth, which in turn influences atmospheric C02 levels. During denitrification, fractionation results in the preferential loss of 14N, enriching the waters in 15N, which is then incorporated into whatever organic material is forming and settling to the sea floor. Consequently, sediments from times of reduced denitrification have a lower 815N, as is evident during marine isotope stages 2, 4, and 6 in cores from off the northwestern coast of Mexico (Fig. 6.55) and from the Arabian Sea. Together with the eastern equatorial South Pacific, these areas are especially important for denitrification in the modern ocean, accounting for almost all of the water column denitrification going on today. The evidence for greatly reduced denitrification in glacial periods, plus the fact that lower sea level would have reduced the contribution of denitrification processes in continental shelf sediments, suggests that overall ocean nitrate levels would have been considerably higher at such times. This would have led to increased biological

Calendar age [ka)

FIGURE 6.55 The record of 8I80 in benthic forams and 8I5N in bulk sediments from off the northwestern Mexican continental margin, spanning the last 140,000 yr. Principal glacial stages are shaded.The benthic record shows the familiar index of continental ice growth and decay; the 8I5N records denitrification processes occurring in the water column. Low values of 815 indicate relatively low rates of denitrification, which imply higher levels of productivity and a reduction in atmospheric C02 levels (Ganeshram et a/., 1995).

Calendar age [ka)

FIGURE 6.55 The record of 8I80 in benthic forams and 8I5N in bulk sediments from off the northwestern Mexican continental margin, spanning the last 140,000 yr. Principal glacial stages are shaded.The benthic record shows the familiar index of continental ice growth and decay; the 8I5N records denitrification processes occurring in the water column. Low values of 815 indicate relatively low rates of denitrification, which imply higher levels of productivity and a reduction in atmospheric C02 levels (Ganeshram et a/., 1995).

activity in areas of the ocean that are today relatively unproductive (oligotrophic), causing a general decline in atmospheric pC02. Altabet et al. (1995) also point out that the reduced level of greenhouse gas NzO, observed in glacial periods in the Vostok ice core (see Fig. 5.35) may be a direct consequence of reduced oceanic denitrification at those times.

From this brief summary of various lines of evidence, it is apparent that several factors were probably operating simultaneously to lower atmospheric carbon dioxide levels during glacial periods. Collectively, these factors increased ocean productivity to the point where there was a net flux of C02 from the atmosphere to the ocean, eventually leading to atmospheric pC02 levels -100 p.p.m.v. lower than pre-industrial levels. The initial driving force for such changes seems to be orbital forcing, though no doubt additional feedbacks (involving SST changes, thermohaline circulation, surface wind stress, etc.) then amplified these changes. In view of the importance of fully understanding all the factors affecting atmospheric C02 levels (as well as other greenhouse gases such as N20) there will no doubt be a great deal more research on this topic in the future that may considerably revise our current understanding of this complex topic.

6.12 ORBITAL FORCING: EVIDENCE FROM THE MARINE RECORD

The availability of continuous paleoclimatic records from the ocean floor, spanning several hundred thousand years, has enabled hypotheses about the causes of climatic change to be tested, and has facilitated the development of new models. One of the most important hypotheses is that propounded by Milankovitch (1941), who argued that glaciations in the past were principally a function of variations in the

Earth's orbital parameters, and the resulting redistribution of solar radiation reaching the Earth (see Section 2.6 for a complete discussion). Emiliani (1955, 1966) was the first to note that 8lsO maxima in Caribbean and equatorial Atlantic cores closely matched summer insolation minima at 65° N, which was the latitude that Milankovitch had considered critical for the growth of continental ice sheets. Subsequently, Broecker and van Donk (1970) suggested revisions of Emiliani's timescale, but still concluded that insolation changes were a primary factor in continental glaciation. In addition, dates of coral terrace formation, indicative of a formerly higher sea level (lower global ice volume), were shown to be closely related to times of insolation maxima, again supporting the ideas of Milankovitch (Broecker et al., 1968; Mesolella et al., 1969; Veeh and Chappell, 1970).

The first rigorous attempt to assess the evidence for orbital changes in paleocli-matic data was made by Hays et al. (1976) using two ocean core records from the southern Indian Ocean (43° and 46° S). Three parameters were studied: 8180 values in the foraminifera Globigerina bulloides (an index of global, but primarily northern hemisphere ice volume); summer sea-surface temperature (TJ derived from radiolaria-based transfer functions (an index of sub-Antarctic temperatures); and abundance variations of the radiolaria Cycladophora davisiana (considered to be an index of Antarctic surface water structure). Using the ~450,000-yr record available, Hays et al. (1976) showed that much of the variance in these proxy records was concentrated at frequencies corresponding closely to those expected from an orbital forcing function. Specifically, spectral peaks were found at periods of -100,000 yr, 40,000-43,000 yr, and 19,500-24,000 yr. Such periodicities closely match spectral peaks in orbital data (at -100,000, -41,000, and 19,000-23,000 yr, associated with variations in eccentricity, obliquity, and precession, respectively). Furthermore, not only are the proxy and orbital series closely matched in the frequency domain, but an examination of the time domain of each periodic component showed fairly consistent phase relationships (back to 300,000 yr) between orbital parameters and the "resultant" climatic signal. Such results are very improbable by chance and the work of Hays et al. thus provided the first really strong evidence that changes in the Earth's orbital geometry played an important role in causing glacial-interglacial variations over the past 300,000-400,000 yr.

Numerous other studies have subsequently confirmed these pioneering results and it is now quite clear that orbital forcing played a key role in pacing glaciations during the Quaternary period (Berger, 1990). However, the mechanisms involved in linking changes in insolation to changes in the climate system are not so clear. As noted in Section 2.6, the principal periodicity in the 8lsO marine sediment record lies in the 100 ka band, but this has relatively little power in the insolation record (see Fig. 2.22). Imbrie et al. (1992, 1993b) have examined this matter in great detail and propose a comprehensive model to account for this enigma. Their model builds on the earlier ideas of Weyl (1968), Broecker et al. (1985b), and Broecker and Denton (1989), different aspects of which were discussed in Sections 6.9 and 6.10. However, by focusing on how different parts of the climate system respond at the three distinct orbital frequencies (23 ka, 41 ka, and 100 ka), Imbrie et al. were able to demonstrate a recurrent geographical sequence of orbitally driven changes at all frequencies during the course of a glacial-interglacial cycle. By looking at the phase relationships between insolation, global ice volume, and other climate proxies, it was clear that certain parts of the climate system have a consistent early response to northern hemisphere high latitude insolation changes, whereas others respond later. By mapping the geographic distribution of these responses a mechanistic model of glacial-interglacial changes was constructed. This revealed that four key subsystems control the rate at which radiation changes are propagated through the climate system, each having a different level of inertia. Near-surface processes on the land and in the upper ocean, and in the deep waters of the southern ocean respond quickly (in < lka) whereas changes involving ice sheets, displaced wind systems, and deep ocean chemistry take longer (3-5 ka). Through the interaction of these different subsystems, the sequence of events leading to glaciation and déglaciation slowly unfold.

Like earlier researchers, Imbrie et al. (1992) determined that a critical factor driving glacial-interglacial changes is salinity-controlled convection in the Icelandic, Norwegian, and Greenland ("Nordic") Seas and in the Labrador Sea, which they refer to as the Nordic and Boreal heat pumps, respectively. During interglacials, both areas produced deepwater, which drove the thermohaline circulation of the Atlantic and carried heat to the Southern Ocean, thereby restricting sea ice around Antarctica. With both heat pumps operating, ventilation of the deep ocean was at its maximum. As summer insolation decreased at high northern latitudes the atmosphere and ocean surface cooled, reducing evaporation and increasing snowfall and sea-ice cover. Eventually, salinity in the Nordic Seas decreased, at first slowing and then entirely eliminating convective overturning via the Nordic heat pump, drastically curtailing warm water flux to the southern hemisphere. However, the Boreal heat pump continued to operate, producing intermediate water, but the net result was for a dramatic reduction in thermohaline circulation and, hence, in the return flow of warm water to the North Atlantic. As Antarctic sea ice expanded, Antarctic Bottom Water flux to the north increased and an equatorward shift in the southern Westerlies led to ocean circulation changes that sequestered carbon in the Southern Ocean. This led to a reduction in atmospheric C02 levels, which further reinforced the downward insolation trend. Further cooling in the northern hemisphere resulted in ice growth on land and a fall in sea level, allowing extensive marine-based ice sheets to develop in locations that were vulnerable to later sea-level increases. Ice sheet growth eventually disrupted the westerly wind system causing sea ice to form over large areas of the North Atlantic which increased the production of Intermediate Water and caused a slight increase in NADW, leading to a minor recession in Antarctic sea-ice extent. As the insolation cycle returned once more to higher northern hemisphere summer energy receipts, the northern oceans slowly warmed, and on the continents, snow melted and the ice margins retreated. This allowed the zone of maximum westerly winds to shift northward, and warmer waters and subtropical air masses could then be advected from the south, causing the ice sheets to rapidly melt, and sea level to rise very quickly with catastrophic collapse of marine-based ice sheets. At the same time, sea ice rapidly receded and this, together with the advected waters from the south, resulted in a sharp increase in the salinity of the

North Atlantic and a resumption of the Nordic heat pump. Large-scale melting events due to the rapid melting of continental and marine-based ice sheets may have resulted in short-term reversals of these trends (as discussed in Section 6.10.2) but eventually the primary sequence of events driven by orbital forcing prevailed, leading once more to a strong thermohaline circulation involving both the Boreal and Nordic heat pump systems.

Although this sequence of events appears to be causally linked to the 23 ka and 41 ka precession and obliquity cycles in a simple linear relationship, most of the variance in the ice volume record is actually in the 100 ka eccentricity cycle. As variations in radiation due to eccentricity changes are an order of magnitude smaller than those due to precession and obliquity changes, it is difficult to understand why changes at this frequency are so large. Several models have been proposed to explain this problem. One set of models generally views the climate system as having the ability to develop internal (free) oscillations or resonance in response to some external forcing, possibly unconnected to orbital forcing. Another set views the climate system as following the same sequence of responses noted for the 23 ka and 41 ka cycles, but in a non-linear manner; at some point a threshold is crossed that then gives rise to a much larger response (Imbrie et al., 1993b). The critical factor driving this non-linear response appears to be the size of the continental ice sheets (primarily the Laurentide ice sheet). Large ice sheets significantly disrupt the westerlies, leading to strong meridional circulation, which then greatly amplifies the sequence of climate system changes associated with 23 and 41 ka cycles. Thus, when some combination of forcing at the 23 ka and 41 ka bands leads to the ice sheets exceeding some critical size, they then overtake the "normal" system response to orbital forcing. In effect the ice sheets themselves become the principal agents driving the climate system, producing a cycle of changes with a longer (100 ka) periodicity due to the large amount of inertia associated with ice-sheet growth and decay. This model provides an elegant explanation for the changes observed over the last few hundred thousand years, but why this pattern became more significant in the last million years remains to be fully explained.

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