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Ages and diagenetic temperatures of pleistocene deposits of Florida based on isoleucine epimerization inMercenaria
Earth and Planetary Science Letters, 28 (1975) 275-282 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
L_LA
AGES AND DIAGENET1C TEMPERATURES OF PLEISTOCENE DEPOSITS OF FLORIDA BASED ON ISOLEUCINE EPIMERIZATION 1N M E R C E N A R I A * RICHARD M. MITTERER Institute for Geosciences, The University of Texas at Dallas, Richardson, Texas (USA)
Received August 4, 1975 Revised version received September 25, 1975
The epimerization reaction of isoleucine in skeletal carbonates provides a basis for estimating the age and the average diagenetic temperature experienced by fossils. The kinetics of this reaction in Mercenaria, a bivalve, are established by a combination of elevated temperature heating experiments and analysis of Holocene fossils of known age and diagenetic temperature. The Arrhenius activation energy for the reaction is 29.4 kcal/mole. Application of the kinetic relations to Pleistocene marine deposits of Florida yields ages of about 134,000, 180,000, 236,000 and 324,000 years for these deposits. These ages correspond to the times of eustatic high sea levels dated radiometrically throughout the world. Average diagenetic temperatures for the Late Pleistocene in Florida were about 12°C less than the present values; however, the average temperature gradient was close to the present gradient.
1. Introduction
cine with increasing age of the fossil to an equilibrium ratio of alleu/ileu ~- 1.30. The systematic interconversion of isoleucine and alloisoleucine in fossils and in elevated temperature experiments has been noted for mollusk shells [ 1 - 5 ] , foraminifera [6 9], and bones [10]. Since the alleu/ileu ratio is a function of the age of a sample and the diagenetic temperature to which it has been exposed, it is feasible to deduce ages or paleotemperatures of fossils from a knowledge of the kinetic relationships of eq. 1. In addition, there are a number of other potential applications, including stratigraphic correlation, identification of reworked fossils, and verification of radiocarbon dates [4]. The latter application is particularly significant for carbonate fossils in the critical range of 25,000--35,000 years B.P. Many radiocarbon dates provide evidence of a high sea level in this time interval (e.g., [11,12]). Considering that a slight contamination of fossils beyond the range of the radiocarbon method will result in erroneous dates, an independent check on such dates would be useful. In order to estimate ages and paleotemperatures, the kinetics of the reaction must be known. There are, however, several basic sources of uncertainty in apply-
After burial and fossilization the original protein in mollusk shells and other skeletal structures undergoes a series of diagenetic changes, which include hydrolysis to the constituent amino acids, partial conversion of these to non-protein amino acids or other compounds, and the racemization or epimerization of the original L-amino acids to D,L compounds. One basis for quantitating the extent of amino acid diagenesis in fossils is the reversible, first-order reaction in which isoleucine (ileu) interconverts to its non-protein diastereoisomer, alloisoleucine (alleu): kl L-isoleucine ~ D-alloisoleucine k2
(1)
Alloisoleucine is initially present at trace levels or absent in living organisms; after death and burial, alloisoleucine forms and slowly increases relative to isoleu-
* Contribution No. 292, Institute for Geosciences, The University of Texas at Dallas, P. O. Box 688, Richardson, Texas 75080. 275
276 ing results of laboratory heating experiments to temperature and age determinations. These are: (1) the uncertainty in the equilibrium value of a]leu/ileu which varies from 1.25 in heated specimens to about 1.40 in fossil shells [1,2]; (2) the uncertainty in kt calculated by extrapolating rate data determined at elevated temperatures down to the temperature range of I0 25°C; and (3) the uncertainty that the elevated temperature rate law is the same as the low temperature rate law. Consequently, in the absence of any other confirming evidence, extrapolation of rate data from elevated temperatures to lower values may yield inaccurate estimates of paleotemperatures or ages. One kind of confirmatory evidence is obtained by empirically comparing independently determined ages or temperatures with those calculated from the kinetic data. Another approach is to use samples of known age as a "calibration" for dating unknown fossils [ 13]. The purpose of this study is twofold: (1) to obtain more detailed kinetic data for interconversion of isoleucine and alloisoleucine in Mercenaria; and (2) to make preliminary estimates of paleotemperatures and ages obtained by application of the rate equation to Late Pleistocene specimens.
2. Materials and methods
od the samples were processed for amino acid analyses according to previously described procedures [14]. Chips of the middle layer of fossil Mercenaria taken from the growth edge were cleaned successively by a dental tool and dilute HC1 prior to hydrolysis with 6N HCI.
3. Results and discussion 3.1. Kinetics o f isoleucine epimerization in Mercenaria The integrated first-order rate equation for the reaction described in eq. 1 can be expressed as [15]: Ae In ( ~ )
(1
- constant =
(2t
A e and A t are the mole fractions of alloisoleucine [alleu/(alleu + ileu)] at equilibrium and in the sample respectively; for an equilibrium value of alleu/fleu = 1.30, A e = 0.565. The integration constant, determined from the analysis of several modern samples which were not heated, is 0.018; t is the age of the sample (or heating time) and k2/kl = 1/1.30. By inserting known values and solving for k l , the forward rate constant, eq. 2 reduces to: k] =0.565 [1n(0.565/0.565
Mercenaria (sp.), a bivalve presently found along the eastern U.S. and Gulf of Mexico coasts, was chosen for this study because it is a common fossil in Late Cenozoic deposits of these regions and has been used in a number of previous investigations of amino acid diagenesis [1-5]. Mercenaria shells characteristically have three discrete aragonitic structural layers. Amino acid analyses of the individual layers reveal significant differences in composition [14]. To avoid the unknown effects these differences might introduce in the rate of isoleucinealloisoleucine interconversion, the middle layer, which is easily separated with a dental tool, was sampled consistently for all analyses. For the heating experiments, approximately 200-mg chips from a fresh Mercenaria shell, exclusive of muscle scars and beak area, were sealed under nitrogen in pyrex tubes containing 1 ml of deionized-distilled water and heated at various temperatures in refluxing organic liquids. At the end of the heating peri-
k2 + k~) k, t
At)-O.O18]/t
(3)
Preliminary heating experiments indicated that the isoleucine epimerization reaction follows the first-order kinetic expression (eq. 2) in Mercenaria [2,16]. 2 4 •,
/
i
i
j//./
f
/i
i
i
i
l,o /
•
o,V . . / g.-"
,
,
.
20
40
60
Heoting
.
.
.
80 I00 Time (hr)
.
/ 120
140
Fig. 1. The kinetics of eplmenzation of isoleucine in Mercenaria at 141°C. The data at 165°C are those of Hare and Mitterer [2].
277 TABLE I Kinetics of epimerization of isoleucine in heated Mercenaria Temp. (°C)
Heating time (hr)
alleu/ileu
kl (hr -1 )
165
10
0.575
5.77 X 10 -2
154
8 14
0.175 0.290
2.03 X 10 -2 1.97 × 10 .2
144
47.5 67
0.479 0.595
9.91 X 10 -3 8.95 X 10 .3
6 12 24 48 96 144
0.058 0.101 0.197 0.367 0.613 0.875
7.92 7.49 7.68 7.38 6.47 6.79
132
97.5 197
0.287 0.552
2.80 × 10 -3 2.80 X 10 -3
127.5
287 364
0.624 0.667
2.21 × 10 -3 1.88 × 10 -3
117.5
308.5 373
0.333 0.340
1.04 8.76
141
X 10 .3 X 10 -3 × 10 -3 x 10 -3 X 10 . 3
× 10 .3
X 10 -3 × 10 -4
The results of the present study confirm the previous data and demonstrate that the reaction follows firstorder kinetics to an alleu/ileu ratio o f ~ 0 . 9 0 (Fig. 1). The kinetics for this reaction in Mercenaria thus differ somewhat from the kinetics in foraminiferal sediment, which is also a carbonate matrix. In the sediment, the reaction approximates first-order kinetics only up to an alleu/ileu ratio of ~ 0 . 3 5 [8]. The alleu/ileu ratios were measured for Mercenaria subjected to elevated temperatures from 117.5 to 165°C for various lengths of time. Values O f k l , calculated from the alleu/ileu ratio and eq. 3, are given in Table 1. In order to estimate diagenetic temperatures or ages of fossils at the lower in-situ temperatures of natural environments, a large extrapolation of the Arrhenius plot is necessary. An empirical approach to obviate this extrapolation is to employ natural samples having precisely known ages and diagenetic temperatures to calculate values of k~ at the lower temperatures. Although Pleistocene fossils as old as ~ 4 0 0 , 0 0 0 years
can be dated by various radiometric methods, the thermal history of the earth has varied considerably during this time and it would be difficult to estimate an average diagenetic temperature for such specimens. Holocene fossils, particularly those with ages less than 5000 years from subtropical latitudes, have not been subjected to glacial-interglacial climatic extremes. Although there are known temperature fluctuations in historical times, these have been relatively minor and long-term averages of mean temperature in subtropical latitudes probably represent reasonable approximations o f the average diagenetic temperature which prevailed for the Late Holocene. One possible problem with this approach is that ground temperatures are not as readily available as mean air temperatures and, depending on the albedo, the two values may differ slightly. A series of seven radiocarbon-dated Mercenaria from southeastern United States and Mexico was employed to obtain an empirical relationship between in-situ diagenetic temperature (as approximated by mean air temperature) and k l , the forward rate constant of eq. 3. Mean air temperatures in the collecting localities range from 19.5 to 23.5°C. Values for kl and mean temperatures are given in Table 2 and included in Fig. 2 together with the data from the elevated temperature heating experiments. If the line through the high temperature points is extended to lower temperature values it passes through the data points obtained from the natural samples. A least squares line fitted to all data has the equation: Log k~ (yr - l ) = 17.29 - 6417/T(°K)
(4)
Based on this fit there is no significant difference in the temperature dependence of the rate of isoleucine epimerization as determined from heated samples and natural samples. In addition, the inclusion of the natural samples in the Arrhenius plot removes the considerable uncertainty inherent in the extrapolation of data obtained solely at higher experimental temperatures. Eqs. 3 and 4 may be used to calculate diagenetic temperatures from independently dated Mercenaria, or conversely, to determine approximate ages if independent estimates of diagenetic temperatures are available. The Arrhenius activation energy (E a = slope X 2.303R) for the isoleucine-alloisoleucine intercon-
278 TABLE 2 Kinetics of epimerization of isoleucine in fossil Mercenaria Sample
Age (yr)
Mean temp. (°C) e
alleu/ileu
k I (yr -1 )
FLA-05 FLA-08 MEX-10 FLA-16 GA-09 FLA-18 FLA-I 7
1160 + 120 a 1450 + 100 b 2070 c 3440 _+ 110 b 3468 c 4500 +_250 d 4550 + 275 d
22.0 23.5 23.0 23.5 19.6 22.0 22.0
0.05 0.08 0.10 0.16 0.09 0.18 0.17
3.41 4.77 4.30 4.30 2.28 3.73 3.47
X X × X X X X
10 -s 10 -s 10 -s 10 -s 10 -s 10 -s 10-s
a Undated sample; age is that of associated shell (from Long and Mielke [ l 7 ]). b Isotopes-Teledyne (I-4027 and 1-4028). c Undated sample; age is average of three dates from deposit. d Geochron Laboratories (GX 2921 and GX 2922). e From U.S. Weather Bureau [18].
Temperature (*C) 50 75 I 0 0 125
25 --I!
r
I
I
I
version in Mercenaria, c a l c u l a t e d f r o m eq. 4, is 29.4 k c a l / m o l e . O t h e r r e p o r t e d values (in k c a l / m o l e ) a n d t h e i r respective sample m a t r i x are: deep-sea foraminiferal s e d i m e n t - 27.5 [8]; b u f f e r e d s o l u t i o n o f free isoleucine ( p H = 7.6) - 31.5 [19]; b o n e - 33.4 [20]. The a c t i v a t i o n e n e r g y for the t w o samples w i t h a carb o n a t e m a t r i x (Mercenaria a n d f o r m i n i f e r a ) differ slightly suggesting t h a t the overall m e c h a n i s m s o f int e r c o n v e r s i o n differ. Because o f t h e diversity o f carb o n a t e shell s t r u c t u r e s , t h e i r organic c o n t e n t a n d amin o acid c o m p o s i t i o n , t h e r e a p p a r e n t l y is n o single act i v a t i o n e n e r g y for all c a r b o n a t e s y s t e m s , so it is necessary to e s t a b l i s h t h e k i n e t i c r e l a t i o n s for e a c h carbonate structure independently.
150
I
I
-2
-3
A
7~
-4
e-
D
D~
O
_1 - 6
3.2. Absolute chronology of Pleistocene deposits in Florida
-7
-8
-9 ~
3.4.
3.2
3.0 2.8 2.6 I ~" X 10 3 ( * K - I )
2.4
2.2
Fig. 2. Log k I vs. 1/T (°K) for temperatures from 19.6 to 165°C. The high temperature data are from laboratory experiments (Table 1); the low temperature data are from natural specimens (Table 2). The line is a least squares fit to all data. The least squares fit to the subsets alone are: (1) heated specimens: l o g k l (yr -1 ) = 17.29 - 6415/T(°K); (2) natural specimens: log k I (yr- 1) = 17.99 - 6625/T(°K).
Geochemical, sedimentological and paleontological i n v e s t i g a t i o n s o f deep-sea s e d i m e n t s have e x p a n d e d o u r k n o w l e d g e o f Pleistocene c h r o n o l o g y , p a l e o t e m p e r a t u r e s a n d glacial-interglacial cycles. In t r o p i c a l env i r o n m e n t s r a d i o m e t r i c ages as old as a b o u t 5 0 0 , 0 0 0 years have b e e n o b t a i n e d f r o m coral reefs d e p o s i t e d d u r i n g h i g h sea levels [21 ]. O t h e r c a r b o n a t e s generally are n o t as well s u i t e d for r a d i o m e t r i c age m e a s u r e m e n t s in t h e t i m e span > 2 5 , 0 0 0 years B.P. Conseq u e n t l y , t h e h i s t o r y o f Pleistocene d e p o s i t s in t e m p e r ate coastal areas - t h e i r c h r o n o l o g y , n u m b e r o f units,
279 and correlation - is less complete because of the difficulty in correlating the frequently discontinuous and poorly exposed sediments. The amino acid method offers a new approach to decipher stratigraphic deposits generally unsuitable for radiometric dating and marginally suitable for physical and biostratigraphic techniques. The age and correlation of the fossiliferous, postMiocene, marine deposits of Florida have been subjects of considerable controversy and debate (see DuBar [22] for a recent summary). On the basis of alleu/ileu ratios, Mitterer [4] demonstrated the presence of at least five post-Miocene, probably Pleistocene, marine units apparently deposited during interglacial high sea levels in southern Florida and proposed a tentative correlation with similar deposits in central and northern Florida. A detailed stratigraphic study in southern Florida by Perkins [23] has corroborated the presence of five marine units deposited during five separate transgressive-regressive cycles. The absolute chronology and average diagenetic temperature of some of these Pleistocene units can be deduced from eq. 3 and 4. The only limitation is that the alleu/ileu ratio of datable units must fall on the linear portion of the curve in Fig. 1, i.e., alleu/ileu < ~0.90. The dating approach is the "calibration" technique of Bada and Protsch [13]; the age of unknown specimens can be estimated from a knowledge of the rate constant at the in situ diagenetic temperature, which is calculated from specimens of known age subjected to the same diagenetic temperature. Several inferences strongly point to an age of ~125,000 years for the youngest marine Pleistocene deposit, the Coffee Mill Hammock Member of the Fort Thompson Formation (Unit V of Mitterer [4]): (1) Radiometric ages of ~130,000 years B.P. are obtained for the Miami and Key Largo Formations of southernmost Florida [24,25], both of which are considered to be time-equivalents of the Coffee Mill Hammock Member [26]. (2) Radiometric dating of fossil corals throughout the world has yielded a number of dates in the 120,000-130,000-year range indicating a significant interglacial interval at this time [ 12,27]. (3) This interglacial, based on sea level curves for the past 250,000 years, constructed from a variety of data, represents the last time sea level was significantly higher than present and probably the last time the
Florida platform was significantly submerged [12,28]. It is unlikely, therefore, that the southern Florida peninsula has received any younger marine deposits of significant extent. The evidence that Unit V (Coffee Mill Hammock Member) was deposited during the 120,000-130,000-year B.P. interglacial high stand of sea level is strong. The underlying deposits also were apt to be deposited during higher-than-present sea level stands and the ages of these deposits should represent the times of these high sea levels. The ages of the underlying deposits can be deduced from the degree of isoleucine epimerization in Mercenaria by applying the rate constant calculated from fossils of the Coffee Mill Hammock Member with an assumed age of -125,000 years. Although most dates in this time span range from 120,000 years to 130,000 years, those from southernmost Florida are slightly greater and bracket 130,000 years [24,25]. Accordingly the rate constant is calculated by assuming an age of 125,000 years for the lowest alleu/ileu ratio in Unit V. Because of the regional difference in temperature, the rate constant is calculated separately for each region. The calculated ages are given in Table 3. The deduced ages for the Pleistocene units with alleu/ileu <0.90 are approximately: Unit V 134,000, Unit IV - 180,000, Unit III - 236,000 and Unit II - 324,000 years old. The latter data is probably slightly low as the alleu/ileu ratio for this unit falls on the non-linear portion of Fig. 1. Several arguments reflect on the accuracy of these dates. First, the dates from each region are concordant. This correspondence supports the stratigraphic correlation previously proposed [4]. Second, the diagenetic temperature calculated for each region decreases systematically from south to north as expected. (This is discussed more fully in the next section.) Third, all ages correspond to the ages of high sea levels dated radiometrically at many localities worldwide [ 12,21,23]. This close correspondence between radiometrically dated and amino acid dated high sea levels is striking and gives confidence in the amino acid dating method. The ages deduced above, however, are dependent on the original assumption that deposition of Unit V ceased ~125,000 years B.P. If Unit V was deposited during another high stand of the sea then the ages in Table 3 are in error; e.g., if Unit V is younger than 125,000 years the ages in Table 3 are too high. But,
280 TABLE 3 Ages of Pleistocene deposits in Florida deduced from the isoleucine epimerization reaction a Unit
V IV lII II
South kl = 5.52 × 10 .6
Central kl =4.12× 10 -6
North kl = 3.10X 10 6
alleu/ileu
age (103 yrs)
alteu/ileu
age (103 yrs)
alleu/ileu
age (103 yrs)
0.67-0.75 0.85-0.92 0.98-1.01
125-144 170--191 212-223 b
0.52 0.60 0.70-0.71 0.92 c
125 147 176-180 256
0.40 0.53 0.67 0.85
125-142 170--195 222-251 302 346
0.45 0.60 0.74 0.93
a Deduced ages are based on an assumed age of 125,000 years for the lowest alleu/ileu ratio obtained from Unit V in each region. b Probably a minimum age; alleu/ileu exceeds -0.90. c One specimen.
with no other assumed age (e.g., the 82,000- or 105,000-year B.P. high sea levels [27]) can such a close correspondence be obtained between radiometric dates and amino acid dates for earlier high sea levels. Since the discovery of vertebrate fossils in the Caloosahatchee Formation (Units I and II) there has been considerable controversy whether these deposits are Pleistocene or Pliocene in age [29]. The calculated ages of the deposits from Table 3 bear on this controversy. The deduced ages are in agreement with the mid-Pleistocene age assignment for the Upper Caloosahatchee Formation (Unit II) based on a variety of vertebrate remains [22]. However, this age conflicts with a long-established Pliocene age assignment based on invertebrate evidence (see Brooks [26] for a summary). The latter age is supported by H e - U dates on corals [30]. Thus, the controversy continues; invertebrate fauna and H e - U dates indicate a Pliocene age and vertebrate fauna and now amino acid dates support a mid-Pleistocene age. Were it not for the H e - U dates, the amino acid dates would be taken as confirming a mid-Pleistocene age. Lesser reliance is placed on the invertebrate evidence because the detailed amino acid survey of these deposits has shown significant reworking of older fossils into younger units [4]. As the Caloosahatchee Formation overlies highly fossiliferous Miocene units there is a strong probability that many of the extinct species reported from the Caloosahatchee Formation were derived from the underlying deposits. In view of the agreement between radiometrically dated and amino acid dated high sea levels, the internal consistency o f the amino acid dates for re-
gions with different diagenetic temperatures, and the vertebrate data, the weight of evidence supports a midPleistocene age for the Upper Caloosahatchee Formation.
3. 3. Late Pleistocene diagenetic temperatures in Florida Pleistocene fossils have experienced lower diagenetic temperatures for portions of their burial history; consequently, average diagenetic temperatures of dated Pleistocene samples calculated from eq. 4 will be lower than present mean temperatures for the region. A single calculated temperature cannot accurately express the diagenetic temperature history of a fossil which has been subjected to wide variations in thermal regimes. The calculated value will only depict the "average" thermal history of a fossil and thus will fall between the extremes of glacial and interglacial climate. In addition, the computed value is not a true average because k~ does not vary linearly with temperature over an extended temperature range;consequently the "average" diagenetic temperature calculated from eq. 4 may be slightly higher than a true average. Approximate as it is, however, the calculated diagenetic temperature will represent an upper limit to average glacial temperatures and will yield a minimum amplitude for glacial-interglacial temperature variation in a particular region. The first-order rate constant for each region, given in Table 3, can be inserted in eq. 4 to calculate average diagenetic temperatures in these regions. The values are given in Table 4 together with the
281 TABLE 4 "Average" diagenetic temperatures in Florida during the Late Pleistocene calculated from the isoleucine epimerization reactiona kl
North Central South
3.10 X 10 -6 4.12 X 10 -6 5.52 X 10 -6
Present av. temp. (°C)
Av. Pleistocene temp. (°C)
£xT(°C)b
20.9 22 23.5
8.3 9.8 11.4
12.6 12.2 12.1
age temperatures were significantly lower than present values, the average temperature gradient between northern and southern Florida was close to (perhaps slightly greater) than the present gradient.
4. Conclusions
a Deduced average Pleistocene temperatures are based on an assumed age of 125,000 years for the lowest alleu/ileu ratio obtained from Unit V in each region. b Difference between present and Pleistocene "average" temperatures.
glacial-interglacial temperature amplitude. The average diagenetic temperatures are from 8.3 to 11.4°C which is a decrease of about 12°C from present mean temperatures. These average diagenetic temperatures are in accord with climatic estimates for the southeastern U.S. based on independent criteria. Pollen diagrams from Georgia and the Carolinas indicate a predominantly boreal character to the vegetation [31 ]. Similar flora is now found in northern New England and the northern Great Lakes Region, a displacement of at least 1000 km northward. Emiliani [32] has summarized evidence from a number of sources indicating that the amplitude of glacial-interglacial temperature variations was as much as 10°C in continental mid-latitudes. Thus, the average diagenetic temperatures deduced from the isoleucine epimerization reaction are in general agreement with climatic estimates obtained from independent studies. More estimates are necessary, however, to firmly establish the thermal history of this region. An "averaged" Pleistocene temperature gradient may also be calculated from the diagenetic temperatures. The average temperature difference over the last 125,000 years between northern and southern Florida is 3.1°C (Table 4). This value is not dependent on the absolute age of the deposits, as long as they are contemporaneous. Historical weather records give a gradient between these localities of about 2.6°C [18]. Therefore, during the Late Pleistocene, although aver-
Because of the temperature dependence of k l , the forward rate constant, the time for alleu/ileu to reach ~ 0 . 9 0 increases with decreasing temperatures and thus the time range available for datingMercenaria by the isoleucine epimerization reaction is greater at lower temperatures. At an average temperature of 5°C the time to reach alleu/ileu = 0.90 is about 0.6 × 106 years; at 0°C the time is 1.6 X 106 years. At the lower diagenetic temperatures which prevailed in the Atlantic Coastal Plain north of Florida it should be possible to date all Pleistocene deposits. Based on the present results, at least five Pleistocene marine cycles can be anticipated throughout the Atlantic Coastal Plain. The amino acid dating method offers the potential to establish the absolute chronology of the entire Pleistocene sequence in the Atlantic Coastal Plain, and a precise stratigraphic correlation of these deposits from New England to Florida. In addition, an approximation to the average Pleistocene temperature for various parts of the U.S. east coast can be calculated.
Acknowledgements I thank H.K. Brooks, V.J. Henry and E.W. Behrens for providing samples. This work was supported by Grant GA-30695 from the Oceanography Section of the National Science Foundation.
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30 31
32
Adv. in Chemistry Series (American Chemical Society, Washington, D.C., 1971) 309 pp. J.L. Bada, K.A. Kvenvolden and E. Peterson, Racemization of amino acids in bones, Nature 245 (1973) 308. M.L. Bender, F.T. Taylor and R.K. Matthews, Heliumuranium dating of corals from Middle Pleistocene Barbado~ reef tracts, Quatern. Res. 3 (1973) 142. J.R. DuBar, Summary of the Neogene stratigraphy of southern Florida, in: Post-Miocene Stratigraphy, Central and Southern Atlantic Coastal Plain, R.Q. Oaks, Jr. and J.R. DuBar, eds. (Utah State Univ. Press, Logan, 1974) 206. R.D. Perkins, Discontinuity surfaces as a stratigraphic tool: the Pleistocene of South Florida, Abstracts Geol. Soc. Am. Annual Meet. 6 (1974) 908. J.K. Osmond, J.R. Carpenter and tI.L. Windom, 23°Th/ 234U age of the Pleistocene corals and oolites of Florida, J. Geophy. Res. 70 (1965) 1843. W.S. Broecker and D.L. Thurber, Uranium-series dating of corals and oolites from Bahamian and Florida Key limestones, Sciencc 149 (1965) 58. H.K. Brooks, The Plio-Pleistocene of Florida, with special reference to the strata outcropping on the Caloosahatchee River, in: Late Cenozoic Stratigraphy of Southern Florida A Reappraisal, R.D. Perkins, ed. (Second Ann. Field Trip of Miami Geol. Soc. Guidebook, Miami 1968) 3. K.J. Mesolella, R.K. Matthews, W.S. Broecker and D.L. Thurber, The astronomical theory of climatic change: Barbados data, J. Geol. 77 (1969) 250. R.K. Matthews, Relative elevation of Late Pleistocene high sea level stands: Barbados uplift rates and their implications, Quatern. Res. 3 (1973) 147. J.R. DuBar, Stratigraphy and paleontology of the late Neogenc strata of the Caloosahatchee River of southern Florida, Fla. Geol. Surv. Bull. 40 (1958). M.L. Bender, ttelium-uranium dating of corals, Geochim. Cosmochim. Acta 37 (1973) 1229. D.R. Whitehead, Late-Wisconsin vegetational changes in unglaciated eastern North America, Quatern. Res. 3 (1973) 621. C. Emiliani, Pleistocene paleotemperatures, Science 168 (1970) 822.
L_LA
AGES AND DIAGENET1C TEMPERATURES OF PLEISTOCENE DEPOSITS OF FLORIDA BASED ON ISOLEUCINE EPIMERIZATION 1N M E R C E N A R I A * RICHARD M. MITTERER Institute for Geosciences, The University of Texas at Dallas, Richardson, Texas (USA)
Received August 4, 1975 Revised version received September 25, 1975
The epimerization reaction of isoleucine in skeletal carbonates provides a basis for estimating the age and the average diagenetic temperature experienced by fossils. The kinetics of this reaction in Mercenaria, a bivalve, are established by a combination of elevated temperature heating experiments and analysis of Holocene fossils of known age and diagenetic temperature. The Arrhenius activation energy for the reaction is 29.4 kcal/mole. Application of the kinetic relations to Pleistocene marine deposits of Florida yields ages of about 134,000, 180,000, 236,000 and 324,000 years for these deposits. These ages correspond to the times of eustatic high sea levels dated radiometrically throughout the world. Average diagenetic temperatures for the Late Pleistocene in Florida were about 12°C less than the present values; however, the average temperature gradient was close to the present gradient.
1. Introduction
cine with increasing age of the fossil to an equilibrium ratio of alleu/ileu ~- 1.30. The systematic interconversion of isoleucine and alloisoleucine in fossils and in elevated temperature experiments has been noted for mollusk shells [ 1 - 5 ] , foraminifera [6 9], and bones [10]. Since the alleu/ileu ratio is a function of the age of a sample and the diagenetic temperature to which it has been exposed, it is feasible to deduce ages or paleotemperatures of fossils from a knowledge of the kinetic relationships of eq. 1. In addition, there are a number of other potential applications, including stratigraphic correlation, identification of reworked fossils, and verification of radiocarbon dates [4]. The latter application is particularly significant for carbonate fossils in the critical range of 25,000--35,000 years B.P. Many radiocarbon dates provide evidence of a high sea level in this time interval (e.g., [11,12]). Considering that a slight contamination of fossils beyond the range of the radiocarbon method will result in erroneous dates, an independent check on such dates would be useful. In order to estimate ages and paleotemperatures, the kinetics of the reaction must be known. There are, however, several basic sources of uncertainty in apply-
After burial and fossilization the original protein in mollusk shells and other skeletal structures undergoes a series of diagenetic changes, which include hydrolysis to the constituent amino acids, partial conversion of these to non-protein amino acids or other compounds, and the racemization or epimerization of the original L-amino acids to D,L compounds. One basis for quantitating the extent of amino acid diagenesis in fossils is the reversible, first-order reaction in which isoleucine (ileu) interconverts to its non-protein diastereoisomer, alloisoleucine (alleu): kl L-isoleucine ~ D-alloisoleucine k2
(1)
Alloisoleucine is initially present at trace levels or absent in living organisms; after death and burial, alloisoleucine forms and slowly increases relative to isoleu-
* Contribution No. 292, Institute for Geosciences, The University of Texas at Dallas, P. O. Box 688, Richardson, Texas 75080. 275
276 ing results of laboratory heating experiments to temperature and age determinations. These are: (1) the uncertainty in the equilibrium value of a]leu/ileu which varies from 1.25 in heated specimens to about 1.40 in fossil shells [1,2]; (2) the uncertainty in kt calculated by extrapolating rate data determined at elevated temperatures down to the temperature range of I0 25°C; and (3) the uncertainty that the elevated temperature rate law is the same as the low temperature rate law. Consequently, in the absence of any other confirming evidence, extrapolation of rate data from elevated temperatures to lower values may yield inaccurate estimates of paleotemperatures or ages. One kind of confirmatory evidence is obtained by empirically comparing independently determined ages or temperatures with those calculated from the kinetic data. Another approach is to use samples of known age as a "calibration" for dating unknown fossils [ 13]. The purpose of this study is twofold: (1) to obtain more detailed kinetic data for interconversion of isoleucine and alloisoleucine in Mercenaria; and (2) to make preliminary estimates of paleotemperatures and ages obtained by application of the rate equation to Late Pleistocene specimens.
2. Materials and methods
od the samples were processed for amino acid analyses according to previously described procedures [14]. Chips of the middle layer of fossil Mercenaria taken from the growth edge were cleaned successively by a dental tool and dilute HC1 prior to hydrolysis with 6N HCI.
3. Results and discussion 3.1. Kinetics o f isoleucine epimerization in Mercenaria The integrated first-order rate equation for the reaction described in eq. 1 can be expressed as [15]: Ae In ( ~ )
(1
- constant =
(2t
A e and A t are the mole fractions of alloisoleucine [alleu/(alleu + ileu)] at equilibrium and in the sample respectively; for an equilibrium value of alleu/fleu = 1.30, A e = 0.565. The integration constant, determined from the analysis of several modern samples which were not heated, is 0.018; t is the age of the sample (or heating time) and k2/kl = 1/1.30. By inserting known values and solving for k l , the forward rate constant, eq. 2 reduces to: k] =0.565 [1n(0.565/0.565
Mercenaria (sp.), a bivalve presently found along the eastern U.S. and Gulf of Mexico coasts, was chosen for this study because it is a common fossil in Late Cenozoic deposits of these regions and has been used in a number of previous investigations of amino acid diagenesis [1-5]. Mercenaria shells characteristically have three discrete aragonitic structural layers. Amino acid analyses of the individual layers reveal significant differences in composition [14]. To avoid the unknown effects these differences might introduce in the rate of isoleucinealloisoleucine interconversion, the middle layer, which is easily separated with a dental tool, was sampled consistently for all analyses. For the heating experiments, approximately 200-mg chips from a fresh Mercenaria shell, exclusive of muscle scars and beak area, were sealed under nitrogen in pyrex tubes containing 1 ml of deionized-distilled water and heated at various temperatures in refluxing organic liquids. At the end of the heating peri-
k2 + k~) k, t
At)-O.O18]/t
(3)
Preliminary heating experiments indicated that the isoleucine epimerization reaction follows the first-order kinetic expression (eq. 2) in Mercenaria [2,16]. 2 4 •,
/
i
i
j//./
f
/i
i
i
i
l,o /
•
o,V . . / g.-"
,
,
.
20
40
60
Heoting
.
.
.
80 I00 Time (hr)
.
/ 120
140
Fig. 1. The kinetics of eplmenzation of isoleucine in Mercenaria at 141°C. The data at 165°C are those of Hare and Mitterer [2].
277 TABLE I Kinetics of epimerization of isoleucine in heated Mercenaria Temp. (°C)
Heating time (hr)
alleu/ileu
kl (hr -1 )
165
10
0.575
5.77 X 10 -2
154
8 14
0.175 0.290
2.03 X 10 -2 1.97 × 10 .2
144
47.5 67
0.479 0.595
9.91 X 10 -3 8.95 X 10 .3
6 12 24 48 96 144
0.058 0.101 0.197 0.367 0.613 0.875
7.92 7.49 7.68 7.38 6.47 6.79
132
97.5 197
0.287 0.552
2.80 × 10 -3 2.80 X 10 -3
127.5
287 364
0.624 0.667
2.21 × 10 -3 1.88 × 10 -3
117.5
308.5 373
0.333 0.340
1.04 8.76
141
X 10 .3 X 10 -3 × 10 -3 x 10 -3 X 10 . 3
× 10 .3
X 10 -3 × 10 -4
The results of the present study confirm the previous data and demonstrate that the reaction follows firstorder kinetics to an alleu/ileu ratio o f ~ 0 . 9 0 (Fig. 1). The kinetics for this reaction in Mercenaria thus differ somewhat from the kinetics in foraminiferal sediment, which is also a carbonate matrix. In the sediment, the reaction approximates first-order kinetics only up to an alleu/ileu ratio of ~ 0 . 3 5 [8]. The alleu/ileu ratios were measured for Mercenaria subjected to elevated temperatures from 117.5 to 165°C for various lengths of time. Values O f k l , calculated from the alleu/ileu ratio and eq. 3, are given in Table 1. In order to estimate diagenetic temperatures or ages of fossils at the lower in-situ temperatures of natural environments, a large extrapolation of the Arrhenius plot is necessary. An empirical approach to obviate this extrapolation is to employ natural samples having precisely known ages and diagenetic temperatures to calculate values of k~ at the lower temperatures. Although Pleistocene fossils as old as ~ 4 0 0 , 0 0 0 years
can be dated by various radiometric methods, the thermal history of the earth has varied considerably during this time and it would be difficult to estimate an average diagenetic temperature for such specimens. Holocene fossils, particularly those with ages less than 5000 years from subtropical latitudes, have not been subjected to glacial-interglacial climatic extremes. Although there are known temperature fluctuations in historical times, these have been relatively minor and long-term averages of mean temperature in subtropical latitudes probably represent reasonable approximations o f the average diagenetic temperature which prevailed for the Late Holocene. One possible problem with this approach is that ground temperatures are not as readily available as mean air temperatures and, depending on the albedo, the two values may differ slightly. A series of seven radiocarbon-dated Mercenaria from southeastern United States and Mexico was employed to obtain an empirical relationship between in-situ diagenetic temperature (as approximated by mean air temperature) and k l , the forward rate constant of eq. 3. Mean air temperatures in the collecting localities range from 19.5 to 23.5°C. Values for kl and mean temperatures are given in Table 2 and included in Fig. 2 together with the data from the elevated temperature heating experiments. If the line through the high temperature points is extended to lower temperature values it passes through the data points obtained from the natural samples. A least squares line fitted to all data has the equation: Log k~ (yr - l ) = 17.29 - 6417/T(°K)
(4)
Based on this fit there is no significant difference in the temperature dependence of the rate of isoleucine epimerization as determined from heated samples and natural samples. In addition, the inclusion of the natural samples in the Arrhenius plot removes the considerable uncertainty inherent in the extrapolation of data obtained solely at higher experimental temperatures. Eqs. 3 and 4 may be used to calculate diagenetic temperatures from independently dated Mercenaria, or conversely, to determine approximate ages if independent estimates of diagenetic temperatures are available. The Arrhenius activation energy (E a = slope X 2.303R) for the isoleucine-alloisoleucine intercon-
278 TABLE 2 Kinetics of epimerization of isoleucine in fossil Mercenaria Sample
Age (yr)
Mean temp. (°C) e
alleu/ileu
k I (yr -1 )
FLA-05 FLA-08 MEX-10 FLA-16 GA-09 FLA-18 FLA-I 7
1160 + 120 a 1450 + 100 b 2070 c 3440 _+ 110 b 3468 c 4500 +_250 d 4550 + 275 d
22.0 23.5 23.0 23.5 19.6 22.0 22.0
0.05 0.08 0.10 0.16 0.09 0.18 0.17
3.41 4.77 4.30 4.30 2.28 3.73 3.47
X X × X X X X
10 -s 10 -s 10 -s 10 -s 10 -s 10 -s 10-s
a Undated sample; age is that of associated shell (from Long and Mielke [ l 7 ]). b Isotopes-Teledyne (I-4027 and 1-4028). c Undated sample; age is average of three dates from deposit. d Geochron Laboratories (GX 2921 and GX 2922). e From U.S. Weather Bureau [18].
Temperature (*C) 50 75 I 0 0 125
25 --I!
r
I
I
I
version in Mercenaria, c a l c u l a t e d f r o m eq. 4, is 29.4 k c a l / m o l e . O t h e r r e p o r t e d values (in k c a l / m o l e ) a n d t h e i r respective sample m a t r i x are: deep-sea foraminiferal s e d i m e n t - 27.5 [8]; b u f f e r e d s o l u t i o n o f free isoleucine ( p H = 7.6) - 31.5 [19]; b o n e - 33.4 [20]. The a c t i v a t i o n e n e r g y for the t w o samples w i t h a carb o n a t e m a t r i x (Mercenaria a n d f o r m i n i f e r a ) differ slightly suggesting t h a t the overall m e c h a n i s m s o f int e r c o n v e r s i o n differ. Because o f t h e diversity o f carb o n a t e shell s t r u c t u r e s , t h e i r organic c o n t e n t a n d amin o acid c o m p o s i t i o n , t h e r e a p p a r e n t l y is n o single act i v a t i o n e n e r g y for all c a r b o n a t e s y s t e m s , so it is necessary to e s t a b l i s h t h e k i n e t i c r e l a t i o n s for e a c h carbonate structure independently.
150
I
I
-2
-3
A
7~
-4
e-
D
D~
O
_1 - 6
3.2. Absolute chronology of Pleistocene deposits in Florida
-7
-8
-9 ~
3.4.
3.2
3.0 2.8 2.6 I ~" X 10 3 ( * K - I )
2.4
2.2
Fig. 2. Log k I vs. 1/T (°K) for temperatures from 19.6 to 165°C. The high temperature data are from laboratory experiments (Table 1); the low temperature data are from natural specimens (Table 2). The line is a least squares fit to all data. The least squares fit to the subsets alone are: (1) heated specimens: l o g k l (yr -1 ) = 17.29 - 6415/T(°K); (2) natural specimens: log k I (yr- 1) = 17.99 - 6625/T(°K).
Geochemical, sedimentological and paleontological i n v e s t i g a t i o n s o f deep-sea s e d i m e n t s have e x p a n d e d o u r k n o w l e d g e o f Pleistocene c h r o n o l o g y , p a l e o t e m p e r a t u r e s a n d glacial-interglacial cycles. In t r o p i c a l env i r o n m e n t s r a d i o m e t r i c ages as old as a b o u t 5 0 0 , 0 0 0 years have b e e n o b t a i n e d f r o m coral reefs d e p o s i t e d d u r i n g h i g h sea levels [21 ]. O t h e r c a r b o n a t e s generally are n o t as well s u i t e d for r a d i o m e t r i c age m e a s u r e m e n t s in t h e t i m e span > 2 5 , 0 0 0 years B.P. Conseq u e n t l y , t h e h i s t o r y o f Pleistocene d e p o s i t s in t e m p e r ate coastal areas - t h e i r c h r o n o l o g y , n u m b e r o f units,
279 and correlation - is less complete because of the difficulty in correlating the frequently discontinuous and poorly exposed sediments. The amino acid method offers a new approach to decipher stratigraphic deposits generally unsuitable for radiometric dating and marginally suitable for physical and biostratigraphic techniques. The age and correlation of the fossiliferous, postMiocene, marine deposits of Florida have been subjects of considerable controversy and debate (see DuBar [22] for a recent summary). On the basis of alleu/ileu ratios, Mitterer [4] demonstrated the presence of at least five post-Miocene, probably Pleistocene, marine units apparently deposited during interglacial high sea levels in southern Florida and proposed a tentative correlation with similar deposits in central and northern Florida. A detailed stratigraphic study in southern Florida by Perkins [23] has corroborated the presence of five marine units deposited during five separate transgressive-regressive cycles. The absolute chronology and average diagenetic temperature of some of these Pleistocene units can be deduced from eq. 3 and 4. The only limitation is that the alleu/ileu ratio of datable units must fall on the linear portion of the curve in Fig. 1, i.e., alleu/ileu < ~0.90. The dating approach is the "calibration" technique of Bada and Protsch [13]; the age of unknown specimens can be estimated from a knowledge of the rate constant at the in situ diagenetic temperature, which is calculated from specimens of known age subjected to the same diagenetic temperature. Several inferences strongly point to an age of ~125,000 years for the youngest marine Pleistocene deposit, the Coffee Mill Hammock Member of the Fort Thompson Formation (Unit V of Mitterer [4]): (1) Radiometric ages of ~130,000 years B.P. are obtained for the Miami and Key Largo Formations of southernmost Florida [24,25], both of which are considered to be time-equivalents of the Coffee Mill Hammock Member [26]. (2) Radiometric dating of fossil corals throughout the world has yielded a number of dates in the 120,000-130,000-year range indicating a significant interglacial interval at this time [ 12,27]. (3) This interglacial, based on sea level curves for the past 250,000 years, constructed from a variety of data, represents the last time sea level was significantly higher than present and probably the last time the
Florida platform was significantly submerged [12,28]. It is unlikely, therefore, that the southern Florida peninsula has received any younger marine deposits of significant extent. The evidence that Unit V (Coffee Mill Hammock Member) was deposited during the 120,000-130,000-year B.P. interglacial high stand of sea level is strong. The underlying deposits also were apt to be deposited during higher-than-present sea level stands and the ages of these deposits should represent the times of these high sea levels. The ages of the underlying deposits can be deduced from the degree of isoleucine epimerization in Mercenaria by applying the rate constant calculated from fossils of the Coffee Mill Hammock Member with an assumed age of -125,000 years. Although most dates in this time span range from 120,000 years to 130,000 years, those from southernmost Florida are slightly greater and bracket 130,000 years [24,25]. Accordingly the rate constant is calculated by assuming an age of 125,000 years for the lowest alleu/ileu ratio in Unit V. Because of the regional difference in temperature, the rate constant is calculated separately for each region. The calculated ages are given in Table 3. The deduced ages for the Pleistocene units with alleu/ileu <0.90 are approximately: Unit V 134,000, Unit IV - 180,000, Unit III - 236,000 and Unit II - 324,000 years old. The latter data is probably slightly low as the alleu/ileu ratio for this unit falls on the non-linear portion of Fig. 1. Several arguments reflect on the accuracy of these dates. First, the dates from each region are concordant. This correspondence supports the stratigraphic correlation previously proposed [4]. Second, the diagenetic temperature calculated for each region decreases systematically from south to north as expected. (This is discussed more fully in the next section.) Third, all ages correspond to the ages of high sea levels dated radiometrically at many localities worldwide [ 12,21,23]. This close correspondence between radiometrically dated and amino acid dated high sea levels is striking and gives confidence in the amino acid dating method. The ages deduced above, however, are dependent on the original assumption that deposition of Unit V ceased ~125,000 years B.P. If Unit V was deposited during another high stand of the sea then the ages in Table 3 are in error; e.g., if Unit V is younger than 125,000 years the ages in Table 3 are too high. But,
280 TABLE 3 Ages of Pleistocene deposits in Florida deduced from the isoleucine epimerization reaction a Unit
V IV lII II
South kl = 5.52 × 10 .6
Central kl =4.12× 10 -6
North kl = 3.10X 10 6
alleu/ileu
age (103 yrs)
alteu/ileu
age (103 yrs)
alleu/ileu
age (103 yrs)
0.67-0.75 0.85-0.92 0.98-1.01
125-144 170--191 212-223 b
0.52 0.60 0.70-0.71 0.92 c
125 147 176-180 256
0.40 0.53 0.67 0.85
125-142 170--195 222-251 302 346
0.45 0.60 0.74 0.93
a Deduced ages are based on an assumed age of 125,000 years for the lowest alleu/ileu ratio obtained from Unit V in each region. b Probably a minimum age; alleu/ileu exceeds -0.90. c One specimen.
with no other assumed age (e.g., the 82,000- or 105,000-year B.P. high sea levels [27]) can such a close correspondence be obtained between radiometric dates and amino acid dates for earlier high sea levels. Since the discovery of vertebrate fossils in the Caloosahatchee Formation (Units I and II) there has been considerable controversy whether these deposits are Pleistocene or Pliocene in age [29]. The calculated ages of the deposits from Table 3 bear on this controversy. The deduced ages are in agreement with the mid-Pleistocene age assignment for the Upper Caloosahatchee Formation (Unit II) based on a variety of vertebrate remains [22]. However, this age conflicts with a long-established Pliocene age assignment based on invertebrate evidence (see Brooks [26] for a summary). The latter age is supported by H e - U dates on corals [30]. Thus, the controversy continues; invertebrate fauna and H e - U dates indicate a Pliocene age and vertebrate fauna and now amino acid dates support a mid-Pleistocene age. Were it not for the H e - U dates, the amino acid dates would be taken as confirming a mid-Pleistocene age. Lesser reliance is placed on the invertebrate evidence because the detailed amino acid survey of these deposits has shown significant reworking of older fossils into younger units [4]. As the Caloosahatchee Formation overlies highly fossiliferous Miocene units there is a strong probability that many of the extinct species reported from the Caloosahatchee Formation were derived from the underlying deposits. In view of the agreement between radiometrically dated and amino acid dated high sea levels, the internal consistency o f the amino acid dates for re-
gions with different diagenetic temperatures, and the vertebrate data, the weight of evidence supports a midPleistocene age for the Upper Caloosahatchee Formation.
3. 3. Late Pleistocene diagenetic temperatures in Florida Pleistocene fossils have experienced lower diagenetic temperatures for portions of their burial history; consequently, average diagenetic temperatures of dated Pleistocene samples calculated from eq. 4 will be lower than present mean temperatures for the region. A single calculated temperature cannot accurately express the diagenetic temperature history of a fossil which has been subjected to wide variations in thermal regimes. The calculated value will only depict the "average" thermal history of a fossil and thus will fall between the extremes of glacial and interglacial climate. In addition, the computed value is not a true average because k~ does not vary linearly with temperature over an extended temperature range;consequently the "average" diagenetic temperature calculated from eq. 4 may be slightly higher than a true average. Approximate as it is, however, the calculated diagenetic temperature will represent an upper limit to average glacial temperatures and will yield a minimum amplitude for glacial-interglacial temperature variation in a particular region. The first-order rate constant for each region, given in Table 3, can be inserted in eq. 4 to calculate average diagenetic temperatures in these regions. The values are given in Table 4 together with the
281 TABLE 4 "Average" diagenetic temperatures in Florida during the Late Pleistocene calculated from the isoleucine epimerization reactiona kl
North Central South
3.10 X 10 -6 4.12 X 10 -6 5.52 X 10 -6
Present av. temp. (°C)
Av. Pleistocene temp. (°C)
£xT(°C)b
20.9 22 23.5
8.3 9.8 11.4
12.6 12.2 12.1
age temperatures were significantly lower than present values, the average temperature gradient between northern and southern Florida was close to (perhaps slightly greater) than the present gradient.
4. Conclusions
a Deduced average Pleistocene temperatures are based on an assumed age of 125,000 years for the lowest alleu/ileu ratio obtained from Unit V in each region. b Difference between present and Pleistocene "average" temperatures.
glacial-interglacial temperature amplitude. The average diagenetic temperatures are from 8.3 to 11.4°C which is a decrease of about 12°C from present mean temperatures. These average diagenetic temperatures are in accord with climatic estimates for the southeastern U.S. based on independent criteria. Pollen diagrams from Georgia and the Carolinas indicate a predominantly boreal character to the vegetation [31 ]. Similar flora is now found in northern New England and the northern Great Lakes Region, a displacement of at least 1000 km northward. Emiliani [32] has summarized evidence from a number of sources indicating that the amplitude of glacial-interglacial temperature variations was as much as 10°C in continental mid-latitudes. Thus, the average diagenetic temperatures deduced from the isoleucine epimerization reaction are in general agreement with climatic estimates obtained from independent studies. More estimates are necessary, however, to firmly establish the thermal history of this region. An "averaged" Pleistocene temperature gradient may also be calculated from the diagenetic temperatures. The average temperature difference over the last 125,000 years between northern and southern Florida is 3.1°C (Table 4). This value is not dependent on the absolute age of the deposits, as long as they are contemporaneous. Historical weather records give a gradient between these localities of about 2.6°C [18]. Therefore, during the Late Pleistocene, although aver-
Because of the temperature dependence of k l , the forward rate constant, the time for alleu/ileu to reach ~ 0 . 9 0 increases with decreasing temperatures and thus the time range available for datingMercenaria by the isoleucine epimerization reaction is greater at lower temperatures. At an average temperature of 5°C the time to reach alleu/ileu = 0.90 is about 0.6 × 106 years; at 0°C the time is 1.6 X 106 years. At the lower diagenetic temperatures which prevailed in the Atlantic Coastal Plain north of Florida it should be possible to date all Pleistocene deposits. Based on the present results, at least five Pleistocene marine cycles can be anticipated throughout the Atlantic Coastal Plain. The amino acid dating method offers the potential to establish the absolute chronology of the entire Pleistocene sequence in the Atlantic Coastal Plain, and a precise stratigraphic correlation of these deposits from New England to Florida. In addition, an approximation to the average Pleistocene temperature for various parts of the U.S. east coast can be calculated.
Acknowledgements I thank H.K. Brooks, V.J. Henry and E.W. Behrens for providing samples. This work was supported by Grant GA-30695 from the Oceanography Section of the National Science Foundation.
References 1 P.E. ttare and R.M. Mitterer, Non-protein amino acids in fossil shells, Carnegie Inst. Wash. Yearbook 65 (1967) 362. 2 P.E. Hare and R.M. Mitterer, Laboratory simulation of amino acid diagenesis in fossils, Carnegie Inst. Wash. Yearbook 67 (1969) 205. 3 R.M. Mitterer, Calcified proteins in the sedimentary environment, in: Advances in Organic Geochemistry 1971, H.R. von Gaertner and H. Wehner, eds. (Pergamon, Oxford, 1972) 441.
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