Mm

Paleosols of "Paks Loess"

Including:

Brown forest paleosol (Phe)

Hydromorphous paleosol (Mtp)

Red-brown dry forest paleosols of "Mediterranean type" (PD1. PD2. PDK, DV1)

Lithology (composite section modified from Pesci. 1987a)

PD 25

m?QKm

Glacial Cycles (Kukla. 1977} (Pesci. 1992)

(Oches & McCoy 1995a)

-128

-423

-52412 -620

-790

-790

-245

-339

-423

FIGURE 4.1 2 The composite loess-paleosol stratigraphy characteristic of sites in Hungary is shown schematically in the center column (generally the sections are incomplete in any one area). Columns to the right show previous interpretations of the stratigraphy, subdivided into the main interglacial/glacial cycles (designated A to K), according to Kukla (1977) and Pecsi (1992).Their proposed correlations with the marine isotope stages are indicated by the age of glacial stage Terminations (based on SPECMAP dated records of Imbrie et a/., 1984). Amino-acid relative age dating on snails in the loess suggests that the "correct" interpretation is that shown in the right-hand column (from Oches and McCoy, l995a;Zoller et a/., 1994).

in an individual paleotemperature estimate cancel out when temperature differences are computed (McCoy, 1987a). Using this approach McCoy (1987b) cast new light on the controversy of "cooler or wetter" conditions in the Great Basin during the late Wisconsin. Mean annual temperatures from 16,000 to 11,000 yr B.P. were estimated to have been 9 °C or more below the post-11,000 yr B.P. averages. If correct, no increase in regional precipitation would be required to explain the high levels of Lake Bonneville during the Late Glacial, with the maximum lake level phase occurring during a relatively cold, dry climate compared to the present (see Section 8.2.3). In another interesting application, Oches et al. (1996) used alle/Ile ratios in gastropods from Mississippi valley Peoria loess (dating from the last glacial maximum) to estimate effective diagenetic temperature gradients from the Gulf of Mexico (30° N) to inland sites at 43° N. Today, the gradient is 0.9 °C/degree of latitude, whereas the amino-acid data point to gradients of only 0.3-0.6 °C/degree of latitude, and overall temperatures at least 7-13 °C lower than today. This suggests that SSTs in the Gulf of Mexico were significantly lower than today, otherwise the temperature depression would have been less in the lower Mississippi valley, and the overall temperature gradient would probably have been stronger, not weaker.

4.2.2 Obsidian Hydration Dating

Obsidian is one of the glassy products of volcanic activity, formed by the rapid cooling of silica-rich lava. Although its precise chemical composition varies from one extrusion to another, it always contains >70% silica by weight. Obsidian hydration dating is based on the fact that a fresh surface of obsidian will react with water from the air or surrounding soil, forming a hydration rind. The thickness of the hydration rind can be identified in thin sections cut normal to the surface; a distinct diffusion front can be recognized by an abrupt change in refractive index at the inner edge of the hydration rind. Hydration begins after any event that exposes a fresh surface (e.g., cracking of the lava flow on cooling, manufacture of an obsidian artifact, or glacial abrasion of an obsidian pebble); thus, providing one can identify the type of surface or crack in the rock, it is possible to date the event in question.

As one might expect, hydration rind thickness is a (non-linear) function of time; hydration rate is primarily a function of temperature, though chemical composition of the sample is also an important factor. For this reason, it is necessary to calibrate the samples within a limited geographical area against a sample of known age and similar chemical composition. These are difficult criteria to meet in a paleoclimatic context but are somewhat easier in archeological studies, where obsidian hydration dating has been most widely applied (Michels and Bebrich, 1971). Obsidian was widely traded in prehistoric time and often the precise source of the material can be identified and its diffusion throughout a geographical area can be traced. If samples can be found in a 14C-dated stratigraphic sequence, hydration rinds can be calibrated, providing an empirically derived hydration scale for the site. This can then be used to clarify stratigraphy elsewhere, where radiocarbon-dated samples are unavailable. Alternatively, the hydration rate can be calibrated in the laboratory by heating experiments; if the effective hydration temperature of the sample can be estimated (i.e., its integrated temperature history), age can then be calculated (Lynch and Stevenson, 1992).

Obsidian hydration may also be used to date glacial events if obsidian has been fortuitously incorporated into the glacial deposits. Glacial abrasion of obsidian fragments creates radial pressure cracks normal to the surface and shear cracks subparallel to the surface. The formation of such "fresh" cracks allows new hydration surfaces to develop, and these effectively "date" the time of glacial activity. Hydration rinds resulting from glacial abrasion can then be compared with rinds that have developed on microfractures produced when the lava cooled initially. This event can be dated by potassium-argon isotopic methods (see Section 3.2.2), providing independent calibration for the primary hydration rind thicknesses. Pierce et al. (1976), for example, analyzed obsidian pebbles in two major moraine systems in the mountains of western Montana. Dates on two nearby lava flows indicated ages of 114,500 ± 7300 and 179,000 ± 3000 yr B.P. Hydration rinds on cracks produced during the initial cooling of these flows averaged 12 and 16 |xm, respectively. These points enabled a graph of hydration thickness versus age to be plotted. It was then possible to estimate, by interpolation, the age of hydration rinds produced on glacially abraded cracks in the moraine samples. Two distinct clusters of hydration rind thicknesses enabled glacial events to be distinguished, at 35,000-20,000 and 155,000-130,000 yr B.P. Although the dates are by no means precise, they do at least indicate the important fact that the earlier glacial event predated the Sangamon interglacial (-125,000 yr B.P.), a point of some controversy in the glacial history of the western United States.

Obsidian hydration dating methods are limited by the problems of independent (radioisotopic) calibration, variations in sample composition, and temperature over time. Temperature effects are particularly difficult to evaluate. It is really necessary to produce a calibration curve for each area being studied, and this is not always possible. Nevertheless, where the right combination of conditions is found, obsidian hydration methods can provide a useful time-frame for events that might otherwise be impossible to date.

4.2.3 Tephrochronology

Tephra is a general term for airborne pyroclastic material ejected during the course of a volcanic eruption (Thorarinsson, 1981). Extremely explosive eruptions may produce a blanket of tephra covering vast areas, in a period which can be considered as instantaneous on a geological timescale. Tephra layers thus form regional isochronous stratigraphic markers. Tephras themselves may be dated directly, by potassium-argon or fission-track methods, or indirectly by closely bracketing radiocarbon dates on organic material above and below the tephra layer (Naeser et al., 1981). In favorable circumstances, organic material incorporated within the tephra may provide quite precise time control on the eruption event (Lerbemko et al., 1975; Blinman et al., 1979). Providing that the dated tephra layer can be uniquely identified in different areas, it can be used as a chronostratigraphic marker horizon to provide limiting dates on the sediments with which it is associated. For example, a tephra layer of known age provides a minimum date on the material over which it lies and a maximum date on material superimposed on the tephra. If a deposit is sandwiched between two identifiable tephra layers of known age, they provide bracketing dates for the intervening deposit (Fig. 4.13). A prerequisite for such tephrochronological applications is that each tephra layer be precisely identified. This has been the subject of much study both in the field and in the laboratory. In the field, stratigraphic position, thickness, color, degree of weathering, and grain size are important distinguishing characteristics. In the laboratory, a combination of petrographic studies and chemical analyses are generally used to identify a unique tephra signature (Kittleman, 1979; Westgate and Gorton, 1981; Hunt and Hill, 1993). Multivariate analysis is commonly employed on the various parameters measured to provide optimum discrimination (or correlation) between the tephras being studied (Beget et ai, 1991; Shane and Froggatt, 1994).

In many volcanic regions of the world, tephrochronology is a very important tool in paleoclimatic studies. In northwestern North America, explosive eruptions have produced dozens of widely distributed tephra layers (Table 4.3). Some, such as the Pearlette "O" ash, covered almost the entire western United States and probably had a significant impact on hemispheric albedo (Bray, 1979). Others were more local in extent; around Mt. Rainier, for example, at least ten tephra layers have been identified spanning the interval from 8000 to 2000 yr B.P. (Mullineaux, 1974). Because of the eruption frequency and widespread distribution of tephra in this area, tephrochronological studies have proved to be invaluable in understanding its glacial history (Porter, 1979).

<450

■Glacier

>2200 <3400

<450

■Glacier

AAAAAAA

>6700

AAAAAAA

Layer

|4C age (years)

oooooo

Wn

450

■■■■■■

C

2200

AAAAÛA

Yn

3400

O

6700

FIGURE 4.13 The use of tephra to date glacial deposits. If tephra age is known and tephra can be uniquely identified, ages can be used to "bracket" timing of glacial advance (Porter, 1981 a).

FIGURE 4.13 The use of tephra to date glacial deposits. If tephra age is known and tephra can be uniquely identified, ages can be used to "bracket" timing of glacial advance (Porter, 1981 a).

^H TABLE 4.3 Some Important Tephra Layers in North America

Tephra layer

Source

Approximate age

Katmai

Mt Katmai, Alaska

A.D.1912

Mt St Helens, Set T

Mt St Helens, Washington

A.D. 1800

Mt St Helens, Set W

Mt St Helens, Washington

450"

White River East

Mt Bona, South-eastern Alaska

1250a

White River North

Mt Bona, South-eastern Alaska

1890»

Bridge River

Plinth-Meager Mt, British Columbia

2600a

Mt St Helens, Set Y

Mt St Helens, Washington

3400"

Mazama

Crater Lake, Oregon

6600»

Glacier Peak B

Glacier Peak, Washington

11,200»

Glacier Peak G

Glacier Peak, Washington

12,750-12,000»

Old Crow

Alaska Peninsula

150,000

Pearlette O

Yellowstone National Park

600,000 ± 100,000

Bishop

Long Valley, California

700,000 ± 100,000

Pearlette S

Yellowstone National Park

1,200,000 ±40,000

Pearlette B

Yellowstone National Park

2,000,000 ± 100,000

After Porter (1981b) and Westgate and Naeser (1995). " Age given in radiocarbon years.

After Porter (1981b) and Westgate and Naeser (1995). " Age given in radiocarbon years.

Tephrochronology has provided valuable time control in many paleoclimatic studies. For example, in the North Atlantic, the Icelandic Vedde ash has been identified in both lake and marine sediments (Mangerud et al., 1984) and has recently been found in ice cores from Greenland (Gronvald et al., 1995). This provides a very important chronostratigraphic marker (10,320 ± 50 14C yr B.P. or 11,980 ± 80 [ice core counted] calendar years B.P.) at a critical time for correlating the rapid environmental changes that were then taking place. Other important ash layers found in both marine sediments and ice cores are the Saksunarvatn ash (-10,300 calendar yr [ice cores] or -9000 14C years B.P.) and the Z2 ash zone, which is dated in the GISP2 ice core at -52,680 yr B.P. (Birks et al., 1996; Zielinski et al., 1997). Tephras have also been isolated from Holocene peat deposits (Pilcher et al., 1995) opening up the prospect of more widespread applications of tephrochronology in paleoclimatic studies. It should also be noted that even when tephras are not present, geochemical signals in ice cores (principally excess sulphate washed out of the atmosphere following major explosive eruptions) are very important geochronolog-ical markers for dating ice at depth and hence for correlating different records (see Section 5.4.3.1).

Expanding our understanding of the frequency and extent of explosive eruptions in the past is extremely important (Beget et al., 1996). There is abundant evidence to demonstrate that such eruptions lead to lower temperatures, at least for a limited period (Bradley, 1988; Palais and Sigurdsson, 1989). Whether periods with a high frequency of explosive eruptions in the past experienced a persistent temperature depression (possibly reinforced by additional positive feedbacks in the climate system, due to persistence of high albedo snow cover, or more extensive sea ice) remains controversial. However, there is persuasive circumstantial evidence that episodes of explosive volcanism have been associated with periods of glacier advance in the past, including those of the most recent neoglacial episodes, collectively known as the "Little Ice Age" (Bray, 1974; Porter, 1986; Grove, 1988). Numerous other examples of the importance of tephrochronological studies in paleoclimatic research are found in the two volumes edited by Sheets and Grayson (1979) and by Self and Sparks (1981).

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