Paleoclimatic Information From Inorganic Material In Ocean Cores

Weathering and erosion processes in different climatic zones may result in characteristic inorganic products. When these are carried to the oceans (by wind, rivers, or floating ice) and deposited in offshore sediments, they convey information about the climate of adjacent continental regions, or about the oceanic and/or atmospheric circulation, at the time of deposition (McManus, 1970; Kolla et al., 1979). On continental margins, the bulk of sediment is deposited by rivers, but in remote areas of the ocean, far from land areas and the influence of floating ice, very fine wind-blown material washed out of the atmosphere may form a significant proportion of the total sediment accumulation (Windom, 1975). Modern observations show that total dust flux thousands of kilometers downwind of arid regions is mainly a function of conditions in the source region, whereas variations in grain size are more related to changes in (upper level) wind speed (Rea, 1994). Thus, by examining variations in the eolian fraction of marine sediment cores, an important index of continental aridity, modulated by changes in airflow patterns, can be obtained. For example, in a sediment core 2500 km east of the Chinese Loess Plateau, Hovan et al. (1989, 1991) found large variations in eolian accumulation rates that correspond to changes in the environment of the Loess Plateau. During interglacials, when loess accumulation slowed and soils formed on the plateau, eolian flux rates were low, but during glacial periods (defined by the benthic 8lsO record of the same core) eolian flux rates were many times higher (Fig. 6.36). By correlating the periods of high eolian flux in the marine sediments with episodes of loess accumulation and low magnetic susceptibility, improvements in the chronology of loess deposition could be made by taking advantage of the SPECMAP 8lsO chronological template.

Numerous studies of inorganic material in cores from off the coast of West Africa have enabled climatic fluctuations of the adjacent land mass to be deduced. In this area today, vast quantities of silt and clay-sized particles (>25 million tons per year) are transported from the Sahara desert westwards across the Atlantic by the northeast trade winds (Chester and Johnson, 1971). During late Quaternary glacial epochs, an even higher proportion of terrigenous material accumulated in the equatorial and tropical Atlantic off West Africa due to stronger tradewinds and a more extensive arid zone (Fig. 6.37) (Sarnthein et al., 1981; Matthewson et al., 1995). Further support for this scenario of drier conditions during glacial episodes is provided by studies of biogenic detritus in ocean cores. The concentration of freshwater diatoms (Melosira) and opal phytoliths (minute silica bodies derived from epidermal cells of land plants, particularly grasses) increased during glacial

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FIGURE 6.36 Eolian flux recorded in North Pacific coreV2l-l46 (from -38° N, 163° E) and benthic isotope record in the same core (center graphs) compared to the magnetic susceptibility of loess in a stacked set of records from the Chinese Loess plateau (right graph).The chronology of loess and paleosol units described by Kukla (1987a) is shown on the extreme left (column I). Revised ages, based on correlations between the marine record and the loess susceptibility are shown in the second (column II) (Hovan et at, 1991).

periods in cores south of 20° N off the coast of West Africa. It is suggested that this results from deflation of lacustrine sediments by stronger tradewinds in relatively dry glacial times, following more humid interglacial periods when vegetation (grassland) was extensive and lakes were more common (Parmenter and Folger, 1974; Pokras and Mix, 1985).

Another region of major eolian dust flux to the oceans is off the coast of the Arabian Peninsula. Today, strong northwesterly winds carrying fine-grained sediments to the Indian Ocean sweep over low-level southwesterly (monsoon) winds from Somalia. In modern sediments, the 30% isoline of siliciclastic grains (>6 pm) approximates the location of the main convergence zone between the northwesterly and southwesterly airflows. Reconstructions of the position of this isoline for various time-slices in the past thus track variations in this convergence zone, showing that it was farther to the southeast during the LGM (i.e., stronger northwesterly

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FIGURE 6.37 The lithogenic component of marine core CDS3-30 (~20° N,2I° W.from off the northwest coast of Africa) (bottom panel) compared to the SPECMAP marine isotope record (upper panel) and the combined northern hemisphere record of precession, obliquity, and eccentricity forcing (middle panel, see Fig. 2.16). Glacial periods (shaded) correspond to higher levels of eolian dust flux from the Sahara to the adjacent ocean. The rapid increases in dust are generally associated with decreasing northern hemisphere radiation, driven by changes in precession, though the abrupt shifts in dust content suggest a nonlinear response to orbital forcing (Mathewson et a/, 1995).

Age (kaj

FIGURE 6.37 The lithogenic component of marine core CDS3-30 (~20° N,2I° W.from off the northwest coast of Africa) (bottom panel) compared to the SPECMAP marine isotope record (upper panel) and the combined northern hemisphere record of precession, obliquity, and eccentricity forcing (middle panel, see Fig. 2.16). Glacial periods (shaded) correspond to higher levels of eolian dust flux from the Sahara to the adjacent ocean. The rapid increases in dust are generally associated with decreasing northern hemisphere radiation, driven by changes in precession, though the abrupt shifts in dust content suggest a nonlinear response to orbital forcing (Mathewson et a/, 1995).

airflow) but much closer to the coast 6-9 ka B.P. (Fig. 6.38) (Sirocko and Sarnthein, 1989; Sirocko et al, 1991).

A final example comes from the southern hemisphere where eolian sediments in the Tasman Sea record variations in aridity in southeastern Australia (Hesse, 1994). During glacial periods, dust flux increased by 50-300% and the northern boundary of the main dust plume shifted equatorward by -350 km. This cyclical pattern of increased dust flux during glacial periods and reduced dust flux in interglacials is superimposed on a longer-term increase in overall eolian sediment which began -350-500 ka ago, reflecting the increasing aridification of southeastern Australia.

In each of these cases, there is strong evidence that eolian dust flux to the oceans was much greater during the last glaciation, as well as during earlier glacial events. Part of this increase was probably related to a stronger Pole-Equator temperature gradient and higher wind speeds (Wilson and Hendy, 1971) but there were also much larger arid areas in the intertropical zone during glacial times (Sarnthein, 1978). Both factors led to far higher levels of atmospheric turbidity during glacial periods, and this

FIGURE 6.38 Percentages of siliciclastic grains >6p.m in the siliciclastic fraction of cores off the Arabian Peninsula, at 3000-yr intervals from 21-24,000 yr B.P (bottom right) to 0-3000 yr B.P (top left).The 30% isoline corresponds to the main convergence zone between northwesterly airflow carrying dust from the Arabian Peninsula and southwesterly monsoon airflow from the horn of Africa (Somalia) (Sirocko et al„ 1991).

FIGURE 6.38 Percentages of siliciclastic grains >6p.m in the siliciclastic fraction of cores off the Arabian Peninsula, at 3000-yr intervals from 21-24,000 yr B.P (bottom right) to 0-3000 yr B.P (top left).The 30% isoline corresponds to the main convergence zone between northwesterly airflow carrying dust from the Arabian Peninsula and southwesterly monsoon airflow from the horn of Africa (Somalia) (Sirocko et al„ 1991).

is well recorded in remote polar (and high-elevation) ice cores as a pronounced increase in particulate matter (Petit et al., 1990). There has also been speculation that the higher amounts of eolian material may have played a role in controlling carbon dioxide levels in the atmosphere during glacial periods. Because biological activity (particularly in the southern ocean) is limited by a lack of iron, it has been suggested that the additional iron deposited in the oceans during glacials may have increased oceanic photosynthetic activity to the point that carbon dioxide levels were reduced (Martin et al., 1990). Recent experiments to "seed" large ocean areas with iron have indeed shown that productivity increases significantly when this limiting factor is removed (Price et al., 1991; Coale et al., 1996) but whether this effect can explain the lower carbon dioxide levels of glacial times remains controversial.

Coarse-grained sediments found in sediment cores in remote parts of the ocean provide evidence of former ice-rafting episodes (either from icebergs or formerly land-fast sea ice). One of the most compelling lines of evidence that continental glaciation began in late Pliocene time (-2.4 M yr ago) is ice-transported coarse sediment in cores from both the North Atlantic and the North Pacific (Shackleton et al., 1984). In the late Quaternary, there were quasi-periodic episodes of major ice-rafting in the North Atlantic (above ambient background levels); these are now termed Heinrich events (Heinrich, 1988) (see Section 6.10.1). They appear to be correlated in some way with abrupt changes in 8lsO seen in ice cores from the Greenland ice sheet (Dansgaard-Oeschger events — see Section 5.42). At least some of these coarse layers contain an abundance of detrital carbonates, with a geographical distribution that indicates a source region in Foxe Basin (west of Baffin Island) or Hudson Bay. It appears that the material was transported to the North Atlantic through Hudson Strait, via icebergs that calved from a major ice stream of the Laurentide ice sheet (Andrews et al., 1994; Dowdeswell et al., 1995) (see Fig. 6.47).

Isotopic studies of individual mineral grains have been carried out in an attempt to pinpoint the provenance of material in different Heinrich events. Two approaches, one using lead isotopes, the other using neodynium and strontium isotope ratios, and strontium concentrations, to characterize the sediments and their potential source rocks have led to conflicting interpretations. Lead isotopic ratios point quite specifically to the Churchill Province of the Canadian Shield (northwest of Hudson Bay, Hudson Strait, and Baffin Island) as the source of debris in Heinrich event 2 (-21,000 yr B.P.) (Gwiazda et al., 1996a). By contrast, Sr and Nd isotopes allow for the possibility of multiple sources of detrital material during this and other Heinrich events (including the Icelandic, Fennoscandian, and British Isles ice sheets) (Grousset et al., 1993; Revel et al., 1996). These different interpretations have important implications for identifying the forcing mechanisms that led to Heinrich events. If all Heinrich events involve only material from the Laurentide ice sheet (via Hudson Strait) this points to some sort of internal mechanism controlling ice discharge, such as the binge-purge model of MacAyeal (1993). However, if the Heinrich events result from ice being discharged from ice sheets all around the North Atlantic, this suggests a more pervasive climatic forcing mechanism that can influence both small (Icelandic) and large (Laurentide) ice sheets more or less simultaneously (Bond and Lotti, 1995). This is discussed further in Section 6.10.1.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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