Dating Methods Involving Chemical Changes

Two general categories of dating methods are based on chemical changes within the samples being studied since they were emplaced. The first involves amino-acid analysis of organic samples, generally used to assess the age of associated inorganic deposits. The method may also be used to estimate paleotemperatures from organic samples of known age. The second category encompasses a number of methods that assess the amount of weathering that an inorganic sample has experienced. They are primarily used to assess the relative age of freshly exposed rock surfaces in episodic deposits such as moraines or till sheets. There are a number of possible approaches (Colman and Dethier, 1986) but they must be calibrated by independent dating methods to convert the relative age to a numerical age estimate. Nevertheless, even without such calibration, weathering studies have proven to be useful in distinguishing and correlating glacial deposits in many alpine areas (e.g., Rodbell, 1993). One of the most widespread and well-tested methods is obsidian hydration dating (Section 4.2.2) — an example of the general group of methods that involve measurements of weathering rinds (Colman and Pierce, 1981; Chinn, 1981).

Another method involves the chemical "fingerprinting" of volcanic ashes that often blanket wide areas after a major eruption (Section 4.2.3). Chemical analyses of tephra deposits have proved to be successful in identifying unique geochemical signatures in ashes of different ages. Where the age of a tephra layer has been independently determined, the ash may be used as a chronostratigraphic marker to date the associated deposits and to correlate events over wide areas.

4.2.1 Amino-acid Dating

As all living organisms contain amino acids, a dating method based on amino acids offers a tremendous range of possible applications. Since the first use of amino acids in estimating the age of fossil mollusc shells (Hare and Mitterer, 1968), significant advances in the use of amino acids as a geochronological tool have been made. Relative ages are generally obtained, but where an independent age calibration is possible, and the thermal history of the sample can be estimated, more quantitative age estimates can be made. The method can be applied to material ranging in age from a few thousand to a few million years old and is therefore useful in dating organic material well beyond the range of radiocarbon dating.

Amino-acid analyses use very small samples (e.g., < 2 mg in the case of molluscs and foraminifera). However, for reliable results multiple analyses on a suite of samples is recommended (Miller and Brigham-Grette, 1989). The application of amino-acid dating is thus of particular significance in dating fragmentary hominid remains, where, in many cases, a conventional radiocarbon analysis would require destruction of the entire fossil to obtain a date (Bada, 1985). Analyses have also been carried out on samples of wood, speleothems, coral, foraminifera, and marine, freshwater, and terrestrial molluscs (Schroeder and Bada, 1976; Lauritzen et al., 1994). Because amino-acid changes are both temperature and time-dependent, it is often difficult to be definitive in assigning an age to a sample. More often the method enables relative chronologies to be established and stratigraphic sequences to be checked (Miller et al., 1979; Oches and McCoy, 1995a). It is perhaps in this application (aminostratigraphy) that amino-acid analysis offers the greatest potential (Miller and Hare, 1980).

4.2.1.1 Principles of Amino-acid Dating

Amino acids are so-called because they contain in their molecular structure an amino group (-NH2) and a carboxylic acid group (-COOH). These are attached to a central carbon atom, which is also linked to a hydrogen atom (-H) and a hydrocarbon group (-R) (Fig. 4.5a). If all atoms or groups of atoms attached to the central carbon atom are different, the molecule is said to be chiral or asymmetric. The significance of this is that chiral molecules can exist in two optically different forms (stereoisomers), each being the mirror image of the other (Fig. 4.5b). These optical isomers or enantiomers have the same physical properties and differ only in the way in which they rotate plane-polarized light. The relative configuration of enantiomers is designated, by convention, D or L (dextro [right] or levo [left]) and virtually all chiral amino acids in living organisms occur in the L configuration. Interconversion to the D configuration takes place by a process known as racemization. The extent of the racemization (expressed by the enantiomeric ratio, D:L) increases with time after the death of the organism. This can be measured by gas or liquid chromatographic methods.

Not all amino acids have only one chiral carbon atom. Some amino acids (e.g., isoleucine) contain two chiral carbon atoms, which means that they can exist as four stereoisomers — a set of mirror image isomers (enantiomers) and a set of non-mirror image isomers (diastereomers) (Fig. 4.5b). Interconversion of L-isoleucine

(a) Enantiomers H2N

,C00H

,C00H

CH2 COOH

D-aspartic acid

(b) Diastereomers H2N

COOH

(b) Diastereomers H2N

COOH

Isomers Ch3 Ch2 Ch3

CH2 CH3

CH2 CH3

L-isoleucine y

(c) Relative racemization rates e.g. ISOLEUCINE

COOH

COOH

Racemization Amino Acid
CH2 COOH L-aspartic acid

CH2 CH3

D-alloisoleucine

CH2 CH3

D-alloisoleucine

NH2-terminal > COOH-terminal » internally J free amino bound acid fast-

FIGURE 4.5 (a) An example of enantiomers (D-aspartic acid and L-aspartic acid), (b) An example of diastereomers (L-isoleucine and D-alloisoleucine). (c) Relative rates of racemization depending on whether the amino acid is internally bound, terminally bound, or free.

could thus theoretically produce all four stereoisomers. However, in diagenetic processes only one of the two chiral atoms undergoes interconversion, thereby producing only one other isomer (D-alloisoleucine, a diastereomer) by a process known as epimerization9 (Schroeder and Bada, 1976; Rutter and Blackwell, 1995).

9 For our purposes racemization and epimerization of amino acids can be considered as essentially equivalent processes.

The ratio of D-alloisoleucine to L-isoleucine (abbreviated as alle/Ile, or D/L) increases on the death of an organism from near zero to -1.3 at equilibrium. The time it takes to reach equilibrium varies with temperature (see in what follows) and may range from 150-300 ka in the Tropics, to -2 Ma at mid-latitudes to >10 Ma in polar regions (Miller and Brigham-Grette, 1989). Diastereomers have physical properties sufficiently different enough that they can be separated by ion-exchange chromatography. Several different amino acids have been used to assess the age of a sample, particularly aspartic acid, leucine, and isoleucine. Epimerization of L-isoleucine to D-alloisoleucine is an order of magnitude slower than aspartic acid racemization so it is potentially of more value in dating older samples or those from warmer climates where epimerization and/or racemization rates are faster. Conversely, aspartic acid can be used in resolving differences between Arctic molluscs from the last glacial cycle, the temperature history of which is too low for them to have undergone significant isoleucine epimerization (Goodfriend et al., 1996). Aspartic acid racemization has also provided excellent results in dating recent fossils, such as banded corals (Fig. 4.6) and land snails of Holocene age (Goodfriend, 1991, 1992; Goodfriend et al., 1992).

Unlike radionuclide decay rates, racemization and epimerization rates are sensitive to a number of environmental factors, particularly temperature. In addition, racemization and epimerization rates vary depending on the type of matrix in which the amino acids are found (shell, wood, bone, etc.). In carbonate fossils, rates vary from one genus to another (Fig. 4.7) so it is important to compare amino-acid ratios derived from analyses on similar genera (Miller and Hare, 1975; King and Neville, 1977). Racemization rates also depend on how amino acids are bound to each other, or if they are free (unbound). When an amino acid is bound together with others, its position in the molecule may be internal or terminal (Fig. 4.5c). If it is terminally bound it may be attached to other amino acids by either a carbon or a nitrogen atom. Racemization rates are fastest when the amino acid is terminally bound and slowest when the individual amino acid is free, having been separated

Aspartic Acid Racemization Pictures

FIGURE 4.6 Racemization (D:L) values of total aspartic acid (hydrolyzed samples) in relation to the counted age of annual bands in the coral Pontes australiensis, from the Great Barrier Reef, Australia.The linear regression for the period 1632-1985 is shown (Goodfriend et al., 1992).

FIGURE 4.6 Racemization (D:L) values of total aspartic acid (hydrolyzed samples) in relation to the counted age of annual bands in the coral Pontes australiensis, from the Great Barrier Reef, Australia.The linear regression for the period 1632-1985 is shown (Goodfriend et al., 1992).

Hiatella Arctica
FIGURE 4.7 Some fossil gastropods commonly found in European loess deposits,grouped according to the relative racemization rates of isoleucine in the shell matrix. Note the variable scale (Oches and McCoy, 1995b).

from the rest of the molecule by hydrolysis. Racemization rates of internally bound amino acids are intermediate between rates of terminally bound and free amino acids (Fig. 4.5c). What this means is that, as the peptide is hydrolyzed, at some point each amino acid will become terminally bound before being eventually split off (free). In the terminally bound position, racemization rates are greatest, so the probability is relatively high that the free amino acid, when released, will already be in the D-form. Consequently the D:L ratios in the free fraction are higher than in the bound fraction. It is thus important to note whether analyses reported in the literature are based on free fraction or the total acid hydrolysate (free and bound), as the resulting ratios can vary by an order of magnitude (Table 4.1). Commonly, the D:L ratios from the free fraction and the total acid hydrolysate will be plotted against each other to help differentiate units of different age (Fig. 4.8). Recent studies have involved isolating the high molecular weight (HMW) polypeptides in a sample, which are considered to be less contaminated (by post-depositional bacterial degradation of the amino acids) (Kaufman and Sejrup, 1995). Analysis of the HMW fraction produces more consistent results (i.e., a lower standard deviation) and a much slower rate of racemization because of the presence of fewer terminally bound amino acids. This approach could extend the potential range of dating

TABLE 4.1 Temperature Sensitivity of Amino-acid Reactions in Dated Early Post-glacial Mollusc Samples

l4C age

Alio: lsoc

Location

( years)

MAT (° C)a

Speciesb

Total

Free

Washington

13,010

+10

H.a.

0.078

0.27

Denmark

13,000

+7.7

H.a.

0.053

0.21

Maine

12,230

+7

H.a.

0.050

0.21

New Brunswick

12,500

+5

H.a.

0.043

0.18

Southeastern Alaska

10,640

H.a.

0.040

0.15

Anchorage

14,160

+2.1

M.t.

0.034

0.16

Southern Greenland

13,380

-1

H.a.

0.027

<0.09

Southern Baffin Island

10,740

-7

M.t.

0.024

<0.1

Spitsbergen

11,000

-8

M.t.

0.022

<0.1

Northern Baffin Island

10,095

-12

H.a.

0.020

ND

Somerset Island

9000

-16

H.a.

0.018

ND

Modern

0

H.a.

0.018

ND

From Miller and Hare (1980).

" Mean annual temperature of the past one to five decades based on records of the nearest representative weather station.

b H.a. = Hiatella arctica; M.t. = Mya truncata. Hydrolysis rates in Mya are not directly comparable with those in Hiatella. For most localities, three or more separate values were analyzed; ratios given here are mean values.

c Ratio of D-alloisoleucine to L-isoleucine; ND = no detectable alloisoleucine.

From Miller and Hare (1980).

" Mean annual temperature of the past one to five decades based on records of the nearest representative weather station.

b H.a. = Hiatella arctica; M.t. = Mya truncata. Hydrolysis rates in Mya are not directly comparable with those in Hiatella. For most localities, three or more separate values were analyzed; ratios given here are mean values.

c Ratio of D-alloisoleucine to L-isoleucine; ND = no detectable alloisoleucine.

n=SHYD 5 FREE

©

CYCLE B

A

CYCLE C

<>

CYCLE D

V

CYCLE E

FIGURE 4.8 Total vs free alle/lle for Succinea snails in loess units from central Europe.The sample means (shown by different symbols) fall into distinct clusters, which correspond to loess units of different age. Number of subsample preparations are indicated (Oches and McCoy, 1995b).

(which would be useful in some situations), but could be problematic in resolving age differences between samples from colder regions.

By far the most significant factor affecting the rate of racemization is temperature, specifically the effective diagenetic temperature (EDT), which is an integrated temperature of the sample since deposition. Racemization rates more or less double for a 4-5 °C increase in temperature, so the thermal history of a sample becomes of critical importance to the apparent age. An uncertainty of only ±2 °C is equivalent to an age uncertainty of ±50%, so this is clearly a major source of error in assessing the numerical age of a sample (McCoy, 1987a). Thermal histories are rarely known to within ±2 °C, even in isolated environments, such as in caves or in the deep oceans. However, the temperature dependence of racemization rates may be put to advantage if the sample age is known independently (e.g., by 14C dating). In such cases, the relative amount of racemization can indicate the EDT of the sample since deposition (Bada et al., 1973) or the extent of a step-change in temperature (Schroeder and Bada, 1973; see Section 4.2.1.4). It is important to note that because racemization rates increase exponentially with temperature, the amount of time a sample experiences high temperatures is much more significant than the time spent at low temperatures (Fig. 4.9). Thus EDT is not simply the long-term mean temperature of the site, and it will always be higher than the actual long-term mean. This means that at mid- to high-latitude sites, samples from the beginning or end of the last glacial period may be indistinguishable in terms of their D:L ratios, but they would be distinctly different from samples of the last interglacial, or preceding glacial period (which had passed through the last interglacial). This strong temperature influence on racemization also imposes strict sampling criteria, as samples

High

Chanine Mangerud

FIGURE 4.9 Schematic diagram illustrating the increase in D/L ratio in a fossil mollusc over the course of the last glacial-interglacial cycle. Most of the change in D:L values occurs during the warm interglacial episodes (Miller and Mangerud, 1985).

0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time (ka BP)

FIGURE 4.9 Schematic diagram illustrating the increase in D/L ratio in a fossil mollusc over the course of the last glacial-interglacial cycle. Most of the change in D:L values occurs during the warm interglacial episodes (Miller and Mangerud, 1985).

which have been close to the surface for a prolonged period (either the modern surface or a paleosurface) may give erroneous results because they were exposed to high temperatures, significantly raising their effective diagenetic temperature. Experiments suggest that samples should be deeply buried (>2m) to avoid such problems (Miller and Brigham-Grette, 1989).

There are basically three approaches used in amino-acid geochronological work. Two of these aim at producing an estimate of numerical age and the third uses enantiomeric ratios as simply a stratigraphic tool, to establish the relative age of two or more samples.

4.2.1.2 Numerical Age Estimates Based on Amino-acid Ratios

Numerical sample ages are estimated by either calibrated or uncalibrated methods (Williams and Smith, 1977). The uncalibrated method is based on high-temperature laboratory experiments, which attempt to simulate in a short period of time the much slower processes that occur in samples at the lower temperatures typical in nature. The general amino-acid racemization reaction is as follows:

where k1 and k2 are rate constants for the forward and reverse reactions. In the high-temperature studies, racemization rates are determined by sealing in a tube a sample of the same species as the fossil under investigation, and heating it for known lengths of time in a constant temperature bath. Providing the initial ratio of the sample is known, genus-specific rate constants for the amino acid in question can be determined in this way, at different (elevated) temperatures. These are then

L-amino acid kJk-,D-amino acid plotted on an Arrhenius plot in which the log of the rate constant forms the ordinate and the reciprocal of the absolute temperature the abscissa (Fig. 4.10). If the calculated rate constants fall on a straight line, extrapolation is made (beyond the experimental results) to obtain the rate constants applicable at lower temperatures (Miller and Hare, 1980). Providing that the EDT of the sample since its deposition is known (or can be closely estimated), racemization rate constants can be obtained for that temperature from the Arrhenius plot; sample age can then be calculated from the measured D:L ratio (for the appropriate equations, see Williams and Smith, 1977, p. 102). One might question whether such high-temperature, short-term laboratory kinetic studies accurately reflect the low-temperature, long-term diagenetic changes that occur in fossils. However, there is an increasing body of evidence that this is not a problem and that the high-temperature results can be extrapolated to

Temperature, °C

110 157

e High-temperature data ■ Dated postglacial e High-temperature data ■ Dated postglacial

Arrhenius Plot Ageing

FIGURE 4.10 Arrhenius plot of isoleucine epimerization in Hiatella arctica derived from heating experiments at 75°, 110°, 152°, and l57°C,and MC-dated early postglacial samples (Miller and Hare, 1980).

real-life situations (see Fig. 4.10; Goodfriend and Meyer, 1991). The real difficulty concerns the problem of knowing accurately the thermal history of the sample, as slight errors in this parameter lead to large errors in numerical age estimates (McCoy, 1987a). As a result, the ages derived by means of this uncalibrated method are considered to be the least reliable.

A more fruitful approach, though not entirely free of the problems already discussed here, is to derive rate constants empirically by the measurement of D:L ratios in situ, in fossil samples of known age (the calibrated method). Other samples at the same site can then be dated, if it is assumed that they have experienced essentially the same EDT as the fossil used for calibration (Bada and Schroeder, 1975). A Holocene fossil calibration sample is thus not suitable for assessing the age of older "glacial age" samples because their thermal histories will be quite different. Because of the reduction in racemization rate with sample age, and the sensitivity of the process to temperature, resolution of age becomes increasingly more uncertain in very old samples (Wehmiller, 1993). Miller and Brigham-Grette (1989) suggest that age estimates that are independently calibrated and are within the early "linear" stage of sample racemization (D:L < 0.3) should be reliable to ± 15-20%, but this is likely to increase to ± 30-40% for older samples. However, further cross-checks with independent age estimates can reduce such uncertainties.

Figure 4.11 illustrates the calibrated approach to estimating sample age. The three lines are calculated alle/Ile ratios (based on a kinetic model, from heating experiments) for EDTs of 8, 11, and 14 °C (Wehmiller, 1993). The Peruvian samples are from a series of uplifted coastal terraces; sample Ha was independently dated at 100-130 ka B.P. (i.e., last interglacial, sensu lato) and thus provides an age calibration point. Assuming a long-term EDT of 14 °C for this area, the age of the older samples (Ilb-V) can be estimated. A similar approach is also shown for samples of Mercenaria from the North Carolina and Virginia coastal plain, again using samples from the last interglacial as the single age calibration point. Greater confidence in the ages of older samples would be achieved by having more than one calibration point. Note that the estimation of EDT is critical for obtaining a "correct" age, and that its importance increases with sample age. For example, an alle/Ile value of 0.6 could indicate a sample age of 0.4 Ma with an EDT of 11 °C, or 0.8 Ma with an EDT of 8 °C. Fortunately, since for most of the last million years the Earth was in a glacial mode, the EDT is primarily controlled by the temperature during relatively brief interglacial episodes; the long-term EDT is therefore assumed to be similar to that experienced by a sample since the time of the last interglacial. Such an assumption could be drastically in error if, for example, a sample that had been submerged below sea level for a significant amount of time was compared to a sample that had been subjected to air temperature changes. Table 4.2 illustrates this point; it shows alle/Ile ratios on marine molluscs (Hiatella arctica) from Svalbard, Norway. Shells of the same 14C age had significantly different values, depending on whether they had been continuously submerged for the entire Holocene, or exposed to much lower air temperatures (therefore reducing the racemization rate). Table 4.2 also shows that shells dated >61 ka B.P. from the same area gave alle/Ile ratios identical

FIGURE 4.1 I Isoleucine epimerization model curves for different EDTs, In relation to mean Alle/lle ratios for samples from Peru (ovals) and from the U.S. Atlantic Coastal Plain (rectangles).The last interglacial samples used for calibration are shown by arrows (Wehmiller, 1993).

Time, million years

FIGURE 4.1 I Isoleucine epimerization model curves for different EDTs, In relation to mean Alle/lle ratios for samples from Peru (ovals) and from the U.S. Atlantic Coastal Plain (rectangles).The last interglacial samples used for calibration are shown by arrows (Wehmiller, 1993).

to the continuously submerged early Holocene samples, illustrating the potential for misinterpretation if the thermal history of a sample is not understood.

4.2.1.3 Relative Age Estimates Based on Amino-acid Ratios

In view of the numerous difficulties surrounding the assignment of numerical ages to fossil samples many investigators have found it prudent to use amino-acid ratios as relative age criteria only. By establishing a standard aminostratigraphic

TABLE 4.2 The Effect of Contrasting Thermal Histories on uC-dated Hiatella arctica from Western Spitsbergen, Svalbard, Norway

Thermal History

Current MAT (°C)

D:L ratio

l4C age

Continuously submerged

+2.2

0.031

9900

Emerged shortly after deposition

-6.0

0.018

9940

Emerged shortly after deposition

-6.0

0.031

>61,000

From Miller and Brigham-Grette (1989).

From Miller and Brigham-Grette (1989).

framework for deposits in a region (where it is reasonable to assume a similar EDT history) other units can then be fitted into that relative age chronology (Wehmiller, 1993). For example, Oches and McCoy (1995a) showed that the conventional interpretation of loess stratigraphy in Hungary was incorrect, based on D:L ratios in fossil gastropod shells (snails) associated with each deposit. In some sections, units of quite different age had been assumed to be correlative, but they were clearly di-achronous according to the aminostratigraphy. This approach has provided an independent means of testing the veracity of TL dates on loess across a wide swath of central Europe, from Germany to the Ukraine (Zoller et ah, 1994; Oches and McCoy, 1995b). Furthermore, by correlating the revised loess-paleosol sequences with marine isotope stages, as first suggested by Kukla (1977), it is possible to assign approximate ages to the D:L ratios in the snails of each loess unit (Fig. 4.12). A similar approach was taken by Miller and Mangerud (1985), who used alle/Ile ratios in shallow water marine molluscs from European interglacial deposits to correlate deposits of similar age and thermal history, and to distinguish units of last interglacial age from older deposits. The results were helpful in resolving previous uncertainties in the age of many isolated and fragmentary stratigraphic sections. Bowen et al. (1989) also found that aminostratigraphic studies of non-marine molluscs were extremely useful in reassessing the chronology of Pleistocene depositional units in Great Britain; the revised stratigraphy could then be correlated with the SPECMAP standard marine chronostratigraphy. These are all relatively simple applications, using racemization of a single amino acid to solve a stratigraphic problem. More rigorous differentiation seems possible using several enantiomeric ratios and multivariate statistical techniques such as discriminant analysis.

At the present time, using amino-acid racemization and epimerization processes for establishing relative age seems to be the most practical application of the method. There may still be problems of contamination, leaching, and possibly thermal differences among sites, but these are relatively minor problems compared to those associated with numerical age determinations.

4.2.1.4 Paleotemperature Estimates from Amino-acid Racemization and Epimerization

Although amino-acid analyses are being increasingly used in stratigraphic studies, perhaps the most significant application of amino-acid ratios is in paleotemperature reconstruction. As already noted, a major barrier to accurate age estimates from amino-acid ratios is a knowledge of the integrated thermal history of the sample. However, the "age equation" (which includes the important thermal term) can be solved for temperature if the sample age is known. In the resulting "temperature equation," the time value is of relatively minor significance; as a result, for samples of known age, quite accurate estimates of the integrated thermal history of the depositional site can be achieved. Typically, for well-dated late Wisconsin or Holocene age samples, an uncertainty of ~3 °C (~1% of the absolute temperature of the site) can be expected in the paleotemperature estimates. However, if paleotemperatures are calculated from two samples of differing age, the temperature difference between the two periods can be estimated with considerably more accuracy (typically to within ± 1 °C). This is because many of the factors causing the initial uncertainty

Explanation

Polarity

Holocene soil - variable

Loess

Chernozem-like forest-steppe paleosols

Pa ra braunerde (Brown forest paleosol)

0 0

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