Chronological studies of the Quaternary marine sediments of northern Spencer Gulf, South Australia

Marine Geology, 61 (1984) 265--296

265

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

CHRONOLOGICAL STUDIES OF THE QUATERNARY MARINE SEDIMENTS OF NORTHERN SPENCER GULF, SOUTH AUSTRALIA

A.P. BELPERIO'*, B.W. S M I T H 2, H.A. P O L A C H 3, C.A. N I T T R O U E R * , D.J. D e M A S T E R ' , J.R. P R E S C O T T 2, J.R. H A I L S s** and V.A. G O S T I N I

1Department of Geology, University of Adelaide, Adelaide, S.A. 5001 (Australia) 2Department of Physics~ University of Adelaide, Adelaide, S.A. 5001 (Australia) 3Research School of Pacific Studies, Australian National University, Canberra, A.C.T. 2600 (Australia) "Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27650 (U.S.A.) SCentre for Environmental Studies, University of Adelaide, Adelaide, S.A. 5001 (Australia) (Accepted for publication March 19, 1984)

ABSTRACT Belperio, A.P., Smith, B.W., Polach, H.A., Nittrouer, C.A., DeMaster, D.J., Prescott, J.R., Hails, J.R. and Gostin, V.A., 1984. Chronological studies of the Quaternary marine sediments of northern Spencer Gulf, South Australia. In: J.R. Hails and V.A. Gostin (Editors), The Spencer Gulf Region. Mar. Geol., 61 : 265--296. Four dating methods have been used to obtain chronological information about Holocene and Pleistocene coastal marine sediments in South Australia, and to check the stratigraphic reliability of each dating technique. Estimates of accumulation rates for Holocene seagrass bank deposits vary according to the time period over which they are averaged. The estimates vary from 2.0 to 2.7 m m yr-~ for measurements by the Pb-210 method (averaged over about 100 years) to 1.4--0.2 m m yr-' for C-14 derived measurements (averaged over a period of 1000--7000 yrs). Radiocarbon age determinations on carbonate and organic fractions do not date the true sediment age. Shells, shell fragments and calcareous fines predate the sediment whereas organic and seagrass detritus postdate it. By dating sample pairs, their age can be adequately defined. Thermoluminescence age estimates correlate well with available C-14 ages where the sediment grains have experienced good exposure to sunlight prior to deposition. A m i n o acid racemization and thermoluminescence measurements indicate a late Pleistocene age of ca. 110,000 yrs for the M a m b r a y Formation of Spencer Gulf. A m i n o acid racemization data also support a correlation with the late Pleistocene Glanville Formation of Adelaide.

Present addresses: *Department of Mines and Energy, P.O. Box 151, Eastwood, S.A. 5063, Australia. **Environmental Services, C.S.R. Ltd., G.P.O. Box 483, Sydney, N.S.W. 2001, Australia.

0025-3227/84/$03.00

© 1984 Elsevier Science Publishers B.V.

266 INTRODUCTION Quaternary stratigraphic studies require reliable quantitative dating techniques in order to interpret depositional history accurately. Although numerous dating methods are available, many are applicable only to specific time intervals and to particular mineralogical components of sediments (e.g., Cullingford et al., 1980). In addition, many basic assumptions fundamental to each dating method may n o t be upheld in their geological context (e.g., Rutter et al., 1979; Nielsen and Roy, 1981). Consequently, although many techniques have excellent precision (reproducibility of measurements), their accuracy (deviation of determined age from "real" age) may be questionable. In practice, therefore, it is c o m m o n for several dating methods to be combined to cover either the time interval or the variety of sedimentary facies encountered and to provide a check on the validity of the age estimates. The Quaternary marine sediments of northern Spencer Gulf (Fig.l) have been studied by shallow seismic reflection and vibrocoring and various subsurface stratigraphic horizons have been defined (Hails et al., 1984, this volume). The sediments are predominantly skeletal, carbonate-rich seagrass bank deposits (Holocene) and pedogenically altered, mixed terrigenouscarbonate estuarine deposits (Pleistocene). In this paper, four dating methods (Pb-210, C-14, amino-acid racemization and thermoluminescence), used on these marine sediments are discussed and compared. METHODS To enhance the interpretation of the Quaternary geological evolution of Spencer Gulf, various quantitative dating methods were considered. Several were rejected (K-Ar, U-series, palaeomagnetism) because their requirements were not adequately fulfilled in the Spencer Gulf sediments. Pb-210, C-14, amino acid racemization and thermoluminescence methods were selected to cover the likely time range and the sediments encountered. The Pb-210 m e t h o d has been used to calculate sediment accumulation rates in modern Posidonia seagrass meadows. C-14 data from different sedimentary components have been used to delimit the age of sedimentary strata, to provide a chronological framework for Holocene stratigraphy and for estimates of longer-term accumulation rates. Thermoluminescence of sediment grains has been used to extend chronological control to Pleistocene deposits, for which measurements of amino acid racemization in Anadara trapezia were also made. Unfortunately, the reliability of these dating methods cannot be accepted unequivocally, because they all have some degree of error or uncertainty associated with them. Estimates of the rate of short-term accumulation (e.g., Pb-210 method) are often ambiguous because of bioturbation. Despite its widespread use, radiocarbon dating is often misleading because of sample contamination, post-depositional alteration and mis-association of material

267

_ i IPORT' . AUGULTA

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.7 Lt. ~ ~-.,\t '-/ ",~

MangroveP ' ~ . . .

j

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Thermoluminescence

/,,'

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,f

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+ Pb-210 a

~, -~

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SAMPLE LOCALITIES •

('.

'

)0

~----Red52 o-.. RC5 • ' Mt Grainger Graungerbr

~\.

TwoHummockAl SG167 I

- -

Red48 ////Red2 / ~ ".--Red ,.Red50 ;58

S.G171~ '\\\\

t ~'~ ~,~=~oo

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1

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,,

~.oo ~

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,38.00,

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Fig.i. Locality map of cores and other dated samples. .Red and $ 0 prefixes are intertidal and subtidal vibrocore sitesrespectively,and b.r.are beach ridge localities.

268 dated with strata age (Nielsen and Roy, 1981; Long and Muller, 1981; Otvos, 1981). Amino acid racemization involves major assumptions about palaeoenvironments (Rutter et al., 1979; Wehmiller, 1982) so that age estimates are often inaccurate. Although thermoluminescence dating of archaeological ceramic samples is consistently capable of age estimates accurate to better than 7%, its application to sediment dating is still quite new and caution dictates conservative error estimates (Prescott, 1982). To overcome some of these difficulties, the reliability of age estimates of different sedim e n t fractions and different techniques have been examined. The total number of measurements reported are 4 Pb-210 profiles, 43 C-14 measurements, 5 amino acid racemization measurements, and 10 thermoluminescence age estimates. The cores from which the dated samples were extracted are shown in Fig.1. The methodology, results and interpretations of each dating technique are presented separately. Pb-210 MEASUREMENTS

Description of Pb-210 geochronology Pb-210 is a naturally occurring radioisotope (half-life 22.3 yrs) which has been used to study marine sedimentation rates over a time span of about 100 yrs (Koide et al., 1972; Nittrouer et al., 1979). Unsupported Pb-210 is supplied to Spencer Gulf by atmospheric precipitation, is irreversibly adsorbed to the surface of particles in the water column, and arrives at the seabed with radioactivity in excess of its effective parent, Ra-226. With net accumulation of sediment at a locality, the surface of the seabed accretes upward and older particles are progressively buried. Owing to decay, the activity of excess Pb-210 (i.e., Pb-210 activity exceeding that supported by Ra-226) decreases with depth in the seabed. Assuming that the excess Pb-210 activity (per mass of sediment) reaching the surface of the seabed has remained constant through time, the history of sedimentation can be evaluated from Pb-210 profiles. Further, if the accumulation rate has remained constant, excess Pb-210 activity will decrease logarithmically with depth in the seabed. An accumulation rate can be calculated from the slope of the Pb-210 profile according to the equation: A =

0.693z

(1)

T1/2 ln(Co/Cz) where A = sediment accumulation rate (cm yr -1); TI/~ = half-life of Pb-210 = 22.3 yrs; z = depth below sediment surface (cm); Cz = excess Pb-210 activity at depth z (dpm g-1 ; dpm = disintegrations per minute); Co = excess Pb-210 activity at surface of seabed (dpm g-l). A Pb-210 profile within the seabed can also be affected by biological mixing (Benninger et al., 1979). Intense mixing is often present in the upper 5--10 cm of the seabed and causes uniform Pb-210 activities in this region (surface mixed layer). Less intense mixing changes the slope of the Pb-210

269 profile to indicate an apparent accumulation rate greater than the actual rate. The presence of mixing can be investigated by obtaining profiles of an independent radioisotope. Cs-137 is a man-made radioisotope produced by b o m b testing, and has been present in the marine environment since a b o u t 1954 (Robbins and Edgington, 1975; Thomas and Perkins, 1975). Without mixing, Cs-137 should be found no deeper within the seabed than the depth calculated from the product of accumulation rate and time since 1954.

Analytical techniques Four vibrocores from northern Spencer Gulf were subsampled for Pb-210 analysis. Approximately ten samples, each 1 cm thick, were obtained in a vertical profile from the upper 50 cm of each core. Samples were extracted from the central, least disturbed portion of the 80 mm cores. Each sample (about 8 ml) was dried and analysed for Pb-210 by the Po-210 technique (Beasley, 1969; Schell et al., 1973), which assumes secular equilibrium between Pb-210 and Po-210. This technique involves: spiking each sample with man-made Po-208, as a chemical yield determinant; leaching with HNO3, HC104, and HC1; plating Po-208 and Po-210 onto a silver planchet; and measuring the activity of both polonium isotopes by alpha particle spectrometry. Of the four cores analysed, three (SG38, SG182 and SG51) were from shallow seagrass environments and one (Red 48) was from an Avicennia mangrove swamp (Fig.l). Po-208 yield determination was performed only on core SG38 (North Carolina State University). Chemical yield and counting efficiency are assumed constant within each of the other three cores (Adelaide University). Profiles of Cs-137 also were obtained from core SG38 by measuring gamma ray emission with a GeLi detector (Oak Ridge National Laboratory).

Results The Pb-210 profile for core SG38 is shown in Fig.2. The total Pb-210 activity decreases from the surface to 20 cm depth and remains constant below that depth. The constant, low activity (about 0.55 dpm g-') represents Pb-210 activity supported by Ra-226. The difference between total and supported Pb-210 is excess Pb-210 activity. The decrease of excess Pb-210 is shown also in Fig.2, and an accumulation rate of 2.3 mm yr -1 can be calculated using Equation 1. This accumulation rate suggests that Cs-137 should be f o u n d to 6 cm within the seabed (i.e., 26 yrs X 2.3 mm yr -I). Table I shows that Cs-137 is observed to a depth of over 12 cm. The difference between predicted and observed Cs-137 depth probably results from biological and physical mixing. However, intensity of mixing has been insufficient to create a Surface Mixed Layer (Nittrouer et al., 1979). The effect of mixing on the Pb-210 profile would be to change the slope and cause an apparent accumulation rate higher than the actual accumulation rate. Therefore, 2.3 mm yr-' is a maximum accumulation rate for core SG38.

270 Pb-210 0

0.1

02

2.3

I

o.3 I

Activity 0.4 I

(dpm

0.5 l

g-')

0.6 0 . 7 0 . 8 0 . 9 [ l I I

I J

2 _e_l

mm/yr

I0-

~

20-

~

o

S G - 38 ~.

30-

+ Total -"-

Excess

Pb-210 Pb-210

Activity Activity

40-

50-

Fig.2. Profiles of total and excess Pb-210 in core SG38. The accumulation rate of 2.3 mm yr~ is a maximum estimate, because comparison with the Cs-137 profile (Table I) indicates that biological mixing is present. TABLE I Profile of Cs-137 for core SG-38 Depth

in

core (cm)

Cs-137

(dpm

g-l)

0- 4

0.080 + 0.022

4- 8

0.058 + 0.013

8-12

0. I00 + 0.022

12-16

0.024 + 0.020

16-20

not detectable

20-25

not detectable

25-30

not detectable

40-45

not detectable

Core S G 3 8 is from Yatala Harbor, a seagrass-floored (Posidonia australis) basin on the eastern side o f the study area (see F i g . l ) . Core S G 1 8 2 was o b t a i n e d from Yatala Harbor within 0.5 k m o f S G 3 8 . Core S G 1 8 2 was analysed for P b - 2 1 0 in the same fashion as S G 3 8 , e x c e p t that yield determ i n a t i o n was n o t performed. The a c c u m u l a t i o n rate calculated for S G 1 8 2 is 2.7 m m yr -~, and c o m p a r e s favourably with 2.3 m m yr -~ calculated for SG38. A p p a r e n t l y the assumptions o f c o n s t a n t chemical yield and c o u n t i n g e f f i c i e n c y d o n o t affect significantly the calculated P b - 2 1 0 a c c u m u l a t i o n

271

rates from Spencer Gulf. T w o additional cores near Redcliff (see Fig.l) were examined using Pb-210 geochronology (without yield determination). Core SG51 is from a tidal sand covered seagrass bank and indicates an accumulation rate of 2.0 mm yr -1. Core Red 48 is from a mangrove swamp and indicates a rate of 1.0 mm yr -'. Without additional radioisotope data, these should probably be considered maximum accumulation rates. RADIOCARBON DATING

Introduction Radiocarbon determinations were undertaken by t w o laboratories: Australian National University Radiocarbon Dating Research Laboratory, Canberra (ANU-) and Beta Analytic Inc., Miami (B-). The detection of the radiocarbon (C-14) signal is based, in both cases, on liquid scintillation spectrometers (e.g., Polach, 1969, 1974). Both laboratories follow the internationally accepted C-14 age reporting practice (Stuiver and Polach, 1977). Thus the results are reported with respect to 95% of the C-14 activity of NBS Oxalic acid. The oxalic acid standard to sample C-14 ratio determinations ( d l 4 C ) are normalized to allow for C-13/C-12 stable isotope fractionation (513C) of sample and standard. The fractionation corrected ratio (D14C)is then the basis of the conventional age B.P. (Before Present = 1950 AD) using the Libby half-life of 5570 yr. The given errors are based on the statistical uncertainty of C-14 activity determination of standard, sample, background and the 813C fractionation error. The age uncertainty is expressed as + one standard deviation. Inherent errors such as uncertainty of C-14 half-life and past variations of C-14 concentration in the environment are n o t considered. Where applicable an environmental correction is applied [environment corrected age (B.P.*)]. This, for the oceanic environment in southern Australia, requires the subtraction of 450 + 35 yr from the conventional age (B.P.) (Gillespie and Polach, 1979; see also Bowman and Harvey, 1983). In order to validate and to assist the interpretation of the C-14 age determinations we have (1) pretreated (purified) the samples whenever possible, to ensure that ages are derived from organic or calcareous matter as originally deposited; (2) selected all available carbon bearing fractions for comparative dating; (3) cross checked results obtained from participating laboratories; (4) determined mass spectrometrically, in the majority of cases, the stable carbon isotope ratios (~13C); and (5) taken into account stratigraphic and geomorphologic evidence. Sample selection, pretreatment and results Samples were obtained b y selecting datable carbon-bearing material from core samples taken in key locations of northern Spencer Gulf (Fig.l). Available for dating were: (1) whole shell of identifiable species; (2) shell

272 hash (fragmented shell of medium sand to gravel size); (3) calcareous fines (<180 pm); (4) Posidonia seagrass leaf sheath fibres; (5) disseminated coarse and fine organic detritus; (6) mangrove and samphire peats; and (7) mixed seagrass fibres and organic detritus. These fractions were present in various combinations in the cores. Laboratory pretreatment of samples was carried out at ANU and followed standard procedure. Whole shell samples (S) were washed in dilute hydrochloric acid, scrubbed clean and rinsed. The dried shell was examined under magnification and obviously deteriorated or discoloured surfaces were removed using a dentist's drill. Sections of whole shells were examined for signs of recrystallization (changes in opacity of carbonate matrix) which, together with secondary carbonate deposits in cracks or marine borer holes, were drilled out. Identifiable mangrove or samphire peat, rich in organic carbon, were dated after a h o t acid wash as fine (F), coarse (C) or total peat (P) fractions. Where single shell species were unavailable, coarse shell hash (H) or coarse organic detritus (OC) were recovered from the core section by wet sieving. The hash was repeatedly rinsed in dilute acid until all carbonaceous clays or secondary carbonate deposits were removed. The fines (<180 pm), if present in sufficient quantity, were dated as calcareous fines (CF) and their acid insoluble residue as total organic fines (OF). In many of the cores from shallow seagrass platforms, the sediment contained abundant cellulose fibres, the residue of decaying rhizomes and leaf sheaths of the seagrass Posidonia australis (Gostin et al., 1984, this volume). If present in sufficient quantity, the acid washed fibre was dated individually (SG). In other cases, the fibre was present with dispersed fine organics which could not be separated, in which case they were dated as an acid insoluble composite sample (SG + OF). F o r t y three samples have been dated. Three of them are from stranded beach ridges and the remainder have been extracted from 30 subtidal and intertidal vibrocores (Fig.l). Sample size was generally adequate as most samples yielded the recommended a m o u n t of elemental carbon (4.5 g). Given that the counting time was generally held to 1000 min, the error of determination increases as sample size decreases. However, precision of all age determinations is adequate for the purpose of this study. All the results obtained are listed in Table II. The basis of the C-14 activity determination, the relative millesimal depletion of sample C-14 and 95% of the NBS Oxalic acid standard, is given prior to isotopic fractionation correction (dl4C) and after 513C correction (D14C). The conventional age (B.P.) is based on the D14C value. The interpretation of results is based on the environment corrected age (B.P.*) whenever applicable.

Sample pair comparison and interpretation of results The geomorphological setting and sample pair identification are given in Table III, and Table IV gives a summary of ~13C mass spectrometric determinations. Detailed evaluation of sample pair ages is based on the observed

273 difference of age and its associated error. The ratio of these two terms, z, gives a measure of statistical significance of the ages thus compared (Polach, 1972). Table V makes this sample pair age assessment, and the grouping of z values for most often occurring sample pairs is given in Fig.3. Generally speaking, interpretation of C-14 age determinations can be undertaken only when overall limits of accuracy of the radiocarbon dates, as opposed to the precision of individual age determinations can be assessed. Polach (1976) has evaluated sources of error which affect radiocarbon age determinations and which are n o t incorporated in the conventional age + error term. These are: (1) Inherent. (a) C-14 half-life; (b) C-13/C-12 fractionation; (c) C-14 modern standard; (d) variation in past production rates; (e) distribution of C-14 in nature; and (f) anthropogenic changes of C-14 in nature. (2) Contamination. (3) Biological age of material to be dated. (4) Association of sample and event. Because, in this study, only the relative ages are of primary concern (as opposed to an absolute chronology based on sidereal years) the value of the C-14 half-life (1.a) and variation of past production rates (1.d) need n o t concern us. We have normalized the results for isotopic fractionation (1.b) and have defined the modern standard value (1.c). As the modern standard applies to the terrestrial environment only, an environmental correction has been applied to samples of marine origin. As anthropogenic changes of C-14 concentration in nature (1.f) t o o k place primarily since the industrial revolution (turn of 19th century) their effect also does n o t concern us. Thus, all factors stemming from the inherent sources of error have been accounted for. The effect of contamination on radiocarbon ages is ubiquitous, significant, and often everlasting. This is particularly evident in the oceanic sedim e n t environment which gives maximum mobility to ionized carbon bearing molecules. Thus, n o t only redeposition of dissolved carbonates, but recrystallization of the aragonitic matrix to calcite, as well as calcite recrystallization from originally deposited to secondary calcites are possible (e.g., Chappell and Polach, 1972, 1976; Chappell et al., 1974). The organic carbon c o n t e n t does n o t fare better. Organic debris cannot only be intrusive (i.e., post depositional) b u t the mobility of organic c o m p o u n d s (such as humic and fulvic acids) in oceanic sediments could be closely correlated to their mobility in soil environments (e.g., Polach and Costin, 1971; Polach, 1973). Thus possible contamination of samples needs to be taken into account. Biological age of material relates to the mean age of an organism during its life-span. Because shells and seagrasses have a short life-span, their biological age at time of deposition is n o t significant (in terms of C-14 and its determination precision). Peaty organic matter also accumulates relatively quickly and its true age is preserved in successive layers (Polach and Singh, 1980). Thus it can be said that the determined C-14 age is directly related to the biological death of these samples. However, the ages of these samples are n o t directly associated with the sedimentation event to be dated.

-994.5

A n a d a r a Trapezia

Seagrass + Organics

Oyster shell

Seagrass

Organics F.

Seagrass + Organics

Seagrass

Seagrass

Seagrass + Organics

Seagrass + Organics

ANU-2915

ANU-2904

ANU-2905

ANU-2849A

ANU-2849B

ANU-2903

ANU-2850

ANU-2851

ANU-2901

ANU-2902

-568.5

-299.1

-562.1

-246.8

-545.2

-347.4

-373.4

-384.9

-215.6

-543.9

Seagrass + Organics

ANU-2914

-487.1

Seagrass + Organics

-161.0

-147.1

-154.2

-241.7

-185.9

-206.8

-242.4

-579.4

-541.5

-475.3

dl4C

ANU-2913

Mangrove peat

ANU-2700

B-2369

Mangrove peat F.

ANU-2704

fines

Calcareous

Mangrove peat C.

ANU-2699A

Shell hash

Shell hash

B-2368

ANU-2701

Shell hash

ANU-2709A

Seagrass

Organics F.

ANU-2709B

ANU-2705

Oyster shell

TYPE OF MATERIAL

B-2367

SAMPLE CODE #i

TABLE II C a r b o n i s o t o p e r e s u l t s a n d C - 1 4 ag es , S p e n c e r G u l f

-~ 5.8

8.7

5.0

5.6

~- 5.0

z

±

z

• 10.4

~* ~.4

~+ 6.3

~ 0.3

-7.1 *~ 0.3

-8.0 z 2.0 ~

-4.8 a 0.3

-5.0 -~ 2.0

-8.8 -~ 0.3

-9.0 ± 2.0 t

-4.6 ~ 0.3

+3.2 *~ 0.3

-8.7

+2.8 ± 0.3

2.6

~- 13.2

~ 0.3

~ 0.3

± 2.0 t

± 0.3

~ 0.3

. -

-7.1

-20.0

-20.5

-19.2

-6.7

5.3

6.3

6.1

~ 0.3

± 2.0 t

~-0.3

t 0.3

~+ 0.3

-+ 0.3

± o (%.)

+1.7 ~ 0.3

+1.3

-5.0

+2.1

+1.6

-9.9

+1.8

~13C

~- 5.1

±

±

±

_~ B.2

t 6.5

*_ ~.5

*_ 14.9

z 5.7

~- 5.0

~- 7.1

-~ 4.6

_+ o (%~)

-584.0

-323.0

-579.8

-276.9

-560.0

-368.3

-398.9

-419.6

-241.2

-994.8

-560.6

-505.5

-169.4

-154.8

-164.1

-282.2

-228.7

-238.5

-283.7

-601.8

-555.4

-503.8

DI4C

~ 4.8

z 8.9

± 4.8

~ 7.0

-~I0. I

t 8.5

• 6.0

t 5.5

-~ 12.7

t 2.5

~ 4.9

~ 5.1

-~ 7.1

~ 6.1

+~ 8.1

± 6.1

-* 8.1

-~ 14.7

~- 5.4

! 4.7

-~ 6.9

~ 5.0

_+ o (7~,)

7050

3130

6970

2610

6590

3690

4090

4370

2220

42250

6610

5660

1490

1350

1440

2660

2090

2190

2680

7400

6510

5630

80

90

90

70

60

80

70

90

160

60

100

110

80

80

140

~

•

~

•

90

110

90

80

-~ 190

~

Z

~

•

~ ~1~0 ~I~

•

•

~

~

~-

±

~_

~

t

~

+~ 130

-~

CONVENTIONAL AGE, y BP #2

~

~

x

t

!

~

N.A.

95

95

80

95

170

70

105

135

85

115

85

85

90 100

6600 Z

100

2680 -* 115

6520 ~

2160 -~

6140 -* 195

3240 •

3640 ~

3920 ~

1770 -~ 145

41800 ± ~ I~ ~i~o

6160

5210

N.A.

N.A.

2210 ~

1640 ~

1740 ~

2230

6950

6060

5180

ENVIRONMENT CORRECTED AGE BP* #3

bo --~ g~

-561.9 z 5.5

Oyster shell

B-2371

Katelysia shells

-448.5 z 13.3 -353.2 z

Organics F.

Katelyaia shells

Shell mixture

Shell mixture

ANU-2180D

ANU-2334

ANU-2335

ANU-2336

~ 2.6

~ 16.7

~ 6.9

t 6.9

t 12.9

~ 4.8

~ 6.6

t 4.0

~ 8.0

~ 8.6

~ 7.6

~ 4.0

~ 5.0

~ 26.7 ~

90

~ 100

a 100

z

± 200

~ 100

Z 130

80

• 140

a 190

N.A.

f

Not applicable

Estimated value only

335

105

105

95

115

5060 z

3990 i

3500 ~

4580 ~

7350 ~

6330 %

6880 %

N.A.

7550 ~

7300 x

7900 z

4120 z

MODERN

4790 !

105

105

95

210

105

135

85

195

165

95

80

85

34100 ~ 1700

6520 ~

6600 z

5850 Z

Environmental correction factor of -450 z 35 years for marine samples (Gillespie and Polach 1979)

5510

4440

3950

5030

7800

6780

90

70

~ 160

t

7330 Z

6200

8000

7750

8350

4570

102.7 ~ 2 . 7 ~ M

80

~1700

5240 I

34550

~ 330

I 1OO

Z 1OO

MODERN

5730 z

5230 ~

#3

B = Beta Analytic

7050 6970

90

~ 1.5~

I

! 110

Prefix:ANU : Australian National University;

-496.2 z 6.1

-424.4

-388.1

-465.6

-621.2

-569.9

-698.5

-537.6

-630.8

-619.O

-646.4

-433.9

+27.2

-479.2 ~ 5.2

-986.5

-579.9

6300

101.5

6180

5680

As per Stuiver and Polach (1977)

+2.0 t 2.0 f

+2.0 a 2.0 ~

3 5.6

~ 15.1

~ 4.9

t 6.7

-584.2 ~ 5.2

-543.8

+15.0

-636.6

-507.1

#I

6.0

6.8

+2.0 ~ 2.05

-9.5 ± 0.3

+1.3 z 0.3

+1.7 z 0.3

+0.5 ~ 0.3

-17.6 ~ 0.3

-4.8 z 0.3

-8.9 Z 0.3

+0.7 ~ 0.3

+2.2 ~ 0.3

-5.O ~ 2.O*

+1.7 I 0.3

+1.7 Z 0.3

-11.3 t 0.3

+0.4 % 0.3

+2.4 ~ 0.3

-13.0 Z 0.3

+2.0 z 0.3

-8.9 ~ 0.3

#2

-467.4 •

-391.5 z

6.7

-600.2 z 5.1

Calcareous

ANU-2180C

7.0

4.2

Shell hash

-545.6 •

8.1

-615.2 z 9.0

7.8

4.2

4.9

ANU-2180B

fines

-576.9 Z

Samphire organics

ANU-2906

B-2373

-530.6 x

Organics F.

-606.3 ±

Organics F.

ANU-2916

-627.3 z

-401.2 ±

ANU-2710B

fines

+70.O z 27.5

t 5.5

-985.7 Z 2.7 -449.8

Calcareous

Oyster shell

5.9

-568.0 t 17.2

ANU-2710A

B-2372

Seagrass

Oyster shell

Oyster shell

ANU-2702B

ANU-2707

Seagrass + Organics

ANU-2702A

B-2370

-517.3 ±

Shell hash

ANU-2706

+40.0 z 15.5

5.2

Organics C.

-510.1 t

Kotelysia shells

ANU-2703A

ANU-2703B

-490.7 z 6.9

Seagrass + Organics

ANU-2900

0.50-0.70

3.34-3.44

Red 29

Red 31

Red 31

Red 31

Red 31

Red 38

Red 38

Red 38

Red 45

Red 45

Red 46

Red 46

Red 46

Red 48

Red 48

Red 50

Red 50

Red 52

ANU-2699A

ANU-2704

ANU-2700

B-2369

ANU-2913

ANU-2914

ANU-2915

ANU-2904

ANU-2905

ANU-2849A

ANU-2849B

ANU-2903

ANU-2850

ANU-2851

ANU-2901

ANU-2902

ANU-2900

0.55-0.80

3.80-4.20

0.83-1.13

3.58-3.90

1.0 -1.53

3.20-3.47

0.57-0.90

0.57-0.90

3.20-3.30

2.50-2.65

1.17-1.30

0.45-0.47

0.45-0.47

0.45-0.47

0.27-0.42

0.9 -1.0

1.27-1.70

ANU-27OI

1.35-1.60

1.27-1.70

Red 13

2

1.35-1.60

3.05

ANU-2705

Red

ANU-2709A

I

2

Red 13

Red

CORE DEPTH (m)

B-2368

Red

B-2367

CORE

ANU-2709B

SAMPLE CODE

)

I

O.0 )

(+2.511-(+2.241

(-0.8)-(-1.24)

(+2.4)-(+2.08)

(-I.07)-(-1.41)

(+1.72)-(+1.15)

(-1.8)-(-2.08)

(+1.0)-(+0.65)

(+1.0)-(+0.65)

(-3.31)-(-3.43)

(+0.18)-(-0.06)

(+0.1)-(

(+0.8)-(+0.65)

(+2.13)-(+2.0)

(+3.05)-(+3.07)

(+3.05)-(+3.07)

(+3.05)-(+3.07)

(+3.23)-(+3.08)

(+1.9)-(+1.8

(+1.131-(+0.7)

(+1.131-(+0.7

(-0.35)-(-0.6)

(-0.35)-(-0.6)

-I.0

REDUCED LEVEL (m)

G e o m o r p h o l o g i c a l setting and sample pair i d e n t i f i c a t i o n

T A B L E Ill

Seagrass + Organics

Seagrass + Organics

Seagrass + Organics

Seagrass

Seagrass

Seagrass + Organics

Organics F.

Seagrass

Oyster shell

Seagrass + Organics

Anadara. __ shells

Seagrass + Organics

Seagrass + Organics

Mangrove peat

Mangrove peat F.

Mangrove peat C.

Calcareous clay

Shell hash

Seagrass

Shell hash

Shell hash

Organics F.

Oyster shell

MATERIAL DATED

flat

Seagrass bank

Seagrass bank

Tidal flat

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Pleistocene sand flat

Seagrass bank

Seagrass bank

Mangrove peat

Mangrove peat

Mangrove peat

Supratldal

Tidal flat

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

S.%~[PLE ENVIRONMENT

ENVIRONMENT

5230

6600

2680

6520

2160

6140

3240

3640

3920

1770

41800

6160

5210

14;0

1350

1440

2210

1640

1740

2230

6950

6060

5180

85

70

105

135

95

95

70

6C

80

80

95

Pair No. 11

Pair No. 10

Pair No. 2

Pair No. I

COMMENT

85 85

90

: 115

~ 100

~ 115

~ 100

~

~ 195

~ 115

~

2

~ 14~

Pair No. 3

z ~f~0~f~ Dead carbon

Z

~

~

I

Z

~

•

Z 170

:

Z

t

~

CORRECTED AGE BP

Do -.3 m

0.55-0.60

1.35-1.39

SG51

$GI03

SGI03

SG167

SG182

SG182

SG182

SG182

SG267

SG268

SG279

SGTO

SGTO

B-2370

ANU-2702A

ANU-2702B

B-2371

ANU-2707

B-2372

ANU-2710A

ANU-2?IOB

ANU-2916

ANU-2906

B-2373

ANU-2180B

ANU-2180C

1.00-1.90

Mt. Grainger Beach ridge

ANU-2336

N.D.

N.D.

shells

Shell mixture

Shell mixture

Katelysia shells

Organics F.

Calcareous clay

Shell hash

t e ~

Samphire organics

Organics F.

Organics F.

Calcareous clay

Oyster shell

Seagrass

Oyster shell

Oyster shell

Seagrass + Organics

Oyster shell

Shell hash

Organics C.

Katelysia shells

Beach ridge

Beach ridge

Beach ridge

Seagrass bank

Seagrass bank

Seagrass bank

Sandflat O.G.

Sandflat O.G.

Sand bank

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass bank

Offshore bank

Pleistocene sandflat O.G.

Seagrass bank O.G.

Seagrass bank

Seagrass bank

Seagrass bank

Seagrass hank

Reduced Level is relative to low water datum (zero on Whyalla tide gauge).

N.D. : Not Defined

O.G. : Open Gulf

Blanche Harbour Beach ridge

ANU-2335

N.D.

(-2.1)-(-2.17)

SG70

Whyalla Beach ridge

(-2.1)-(-2.17)

(-2.1)-(-2.17)

(-8.68)-(-8.75)

ANU-2334

2.20-2.27

)

)

)

(-7.73)-(-7.93)

(-3.2)-(-3.3

(-2.1)-(-2.2

(-2.1)-(-2.2

(-1.75)-(-1.79)

(-0.95)-(-1.0)

(-10.86)-(-10.93)

(-5.46)-(-5.52)

)

)

)

)

(-4.??)-(-4.86)

(-2.6)-(-2.?

(-2.8)-(-2.9

(+3.0)-(+2.1

(+3.0)-(+2.1

ANU-2180D

2.20-2.27

2.20-2.27

O. 98-I. 05

3.13-3.33

3.30-3.40

1.68-1.74

1.68-1.74

1.46-1.53

2.56-2.62

1.87-1.96

3.04-3.06

3.10-3.20

RC5

SG51

ANU-2706

1.00-1.90

ANU-2703B

RC5

ANU-2703A

5060

3990

3500

4580

7350

6330

6880

6200

7550

7300

7900

4120

MODERN

4790

34100

6520

6600

5850

MODERN

5730

95

95

80

85

85

95

I 105

z 105

Z

~ 210

Z 105

~ 135

t

: 140

z 195

~ 165

~

~

~

~1700

C 335

Z 105

Z 105

z

Pair No. 13

Pair No. 12

Pair No. 9

Pair No. 8

Pair No. 7

Pair No. 6

Dead carbon

Pair No. 5

Pair No. 4

bO ~a

278 T A B L E IV S u m m a r y o f 513C m e a s u r e m e n t s Sample

61BC value

Mean + SD

Shell hash

+1.6, +2.1, +1.3, +2.4, +1.7

+1.8 + 0.4

Whole shell

+1.8, +2.8, +3.2, +2.0, +0.4, +1.7, +1.7, +2.2, +0.5

+1.8 + 1.0

+1.7, +0.7, +1.3

+1.2 + 0.5

Calc.

fines

Peat, mangrove and samphire

-19.2, -20.5, -17.6

Organics F. or C.

-9.9, -8.9, -4.8 I, -9.5, -13.02

-9.4 + 0.5

Seagrass + organics

-7.1, -6.7, -8.7, -8.8, -7.1, -8.9, -11.3

-8.4 + 1.6

Seagrass

-4.6, -4.8

-4.7 + 0.1

1

2

-19.1 + 1.5

Presumably seagrass detritus. Contaminated with terrestrial organic carbon.

In all cases, C-14 ages based on shell and shell hash pre-date the time of formation of the deposits in which they are found, b u t by how much is uncertain. The extent to which shells predate a deposit will vary from site to site and will depend, among other things, on the degree of incorporation of older material into more recent sediments. Indicators of this are reworked shells of Anadara trapezia (Tate, 1882; Gostin et al., 1981; Burne, 1982) and the foraminifer Marginopora vertebralis. Both of these were abundant in the Late Pleistocene, b u t are not now living in South Australia (Howchin, 1888, 1909, 1923; Gill, 1977; Ludbrook, 1978). Together, they characterise the fauna of the Late Pleistocene Glanville Formation near Adelaide (Ludbrook, 1976, 1978; Cann, 1978). It can thus be expected that shells and/or shell hash from one site will yield C-14 results which are not in statistical agreement. Although shells generally pre-date the deposits in which they are found, organic matter and seagrass fibres, through the growth of roots and rhizomes, intrude the formations after their establishment and thus post~date them. As in situ seagrass deposits are clearly recognizable in the Spencer Gulf cores, the reworking of seagrass strata and the redistribution of significantly " o l d e r " organic material is n o t a factor that needs to be considered. The extent to which the organic fraction postdates the strata therefore depends on the rate of sedimentation, or the time required to isolate the strata from the intrusive effect of younger biological growth. The fine carbonate fraction, considering the time taken to complete shell attrition and the various secondary sources of "old", fine carbonate (e.g., aeolian), may well be the oldest within a deposit.

279 TABLE V Sample pair C-14 comparison TYPE OF

PAIR NO.

MATERIAL

i

2

3

4

5

6

7

8

9

I0

II

12

13

H

RESERVOIR CORRECTED AGE

DIFFERENCE

z

H OF

ANU-2709A ANU-2709B

6950 + 105 6060 + 135

890 + 171

5.2

H SG

B-2368 ANU-2705

2230 + 70 1740 + 170

490 + 184

2.7

SG OF

ANU-2849A ANU-2849B

3640 + 85 3240 + 115

400 + 143

2.8

S OC

ANU-2703A ANU-2703B

5730 + MODERN

H S

~J-2706 B-2370

5850 + 105 6600 ~ 105

750 + 150

5.0

CF OF

ANU-2710A ANU-2710B

7900 + 95 7300 + 165

600 + 190

3.2

H CF

ANU-2180B ANU-2180C

6330 + 135 7350 ~ 105

1020 + 171

6.0

H OF

ANU-2180B ANU-2180D

6330 + 135 4580 + 210

1750 + 250

7.0

CF OF

ANU-2180C ANU-2180D

7350 + 105 4580 ~ 210

2770 + 235

11.8

P1 P2

ANU-2704 ANU-2700

1440 + 1350 ~

80 60

90 + I00

0.9

P1 P3

ANU-2704 B-2369

1440 + 1490 ~

80 70

50 + 106

0.5

$I $2

ANU-2335 ANU-2334

3500 + 95 3990 T 105

490 + 142

3.5

S1 $3

ANU-2334 ANU-2336

3500 + 95 5060 + 105

1560 + 142

Ii.0

= Shell hash;

OF = Organic fine; P

LAB. CODE

S = Whole shell;

95 >5000

>I00

SG = Seagrass

OC = Organic coarse;

CF = Calcareous fines;

= Peats

i/

Arithmentic difference between ages; error of difference is calculated as square root of sum of variance,

2/

z = Difference/Error of difference

(Polach,

1972).

280 •

>IOC -

•

•

e

•

•

•

e

®OC ®OF

II

II

9

rr

,,, o z ::3 o >-

7-

OF ®

5-

I 3 1 N

3 5 7

H

®

s%

OF

®

Organic or seagrass

1 e:: r"' m

OF CF ®

P3 PI®

Hash or shell

Ca~c.fines

H®®SG®c F

Shell

®P'I P2 $2 ®

®H

H® ®H

9 II

>tO0

®S3

®CF

• ,,~S

I1

• •

Sample poir number and fractions being compared

Fig.3. Plot o f z values (based o n difference in

ages) in relation t o sample pairs.

The above general statements are upheld in the data listed here (Tables II--V and Fig.3). Contamination by modern carbonate is apparent for ANU-2915 (Anadara) and A N U - 2 7 0 2 8 (Oyster) which, from their geological context, are of Late Pleistocene origin and should be beyond the C-14 dating range. The finite ages they give are indicative of recrystallization incorporating soluble carbonate of the same C-13/C-12 ratio as oceanic carbonates (confirmed by their 613C value) but different C-14/C-12 ratio, hence their apparent age. The degree of contamination is equivalent to the incorporation of 0.5 to 1% of modern carbon. This value then sets the upper limit to the expected contamination of other shell samples. However, the effect of 1% contamination on a 5 0 0 0 yr old sample is a reduction in age of 50 yrs, whereas samples 10,000 yrs old would appear to be only 200 yrs younger. We therefore consider that contamination of other shell and shell hash samples listed in above Tables by young carbonate is not a factor that needs to be allowed for. Contamination of organic matter is evident in two samples, ANU-2703B (organic) and A N U - 2 7 0 7 (seagrass). Both give m o d e m values, indicating that contemporary organics were deposited or mixed into those levels recently. On the other hand, the general trend of other organic matter age determinations, taken in their stratigraphic context, as well as in the sample pair relationship, is indicative that their contamination is low. The association of sample and event presents the greatest problem in

281

age interpretation. Sample pair age differences, expressed in terms of the z value, i.e. the ratio of absolute age difference of selected pairs and their error (square r o o t of sum of respective variances) (Polach, 1972), are given in Fig.3. Taking group one, organics (fine and coarse) or seagrass as the base line it is apparent that associated shell hash is always older, with calcareous clays the oldest; of course, the converse is also true. Although individual specimens of Anadara trapezia in Holocene sediments clearly indicate the presence of reworked or " o l d " carbonate, other contaminants, particularly fine carbonate, may n o t be so evident (Gostin et al., 1981; Hails et al., 1983). Sample ANU-2699A, calcareous clay from the modern samphire flat, is an extreme example. The radiocarbon age of 2210 + 80 yr predates underlying mangrove peat (Table III). Sediment on these flats is predominantly fine carbonate detritus and has a varied source. It includes the winnowed fines from offshore seagrass banks and tidal channels, as well as an aeolian continental input. The age obtained represents an "average" age of the calcareous constituents and n o t the strata age. Where more than one carbonate date is available from the same level, the time of formation of the deposit will be more closely related to the youngest of these (e.g., sample pairs 5, 7, 12). Only the peat samples form a consistent group of ages in statistical agreement and more closely parallel the true sediment age. Seagrass rhizomes, roots and leaf sheaths grew within the sediment and their fibrous organic residues thus post~date the sediment. The precise pre- or post-dating age correction factor cannot be defined from this study alone. However, age of sediment formation must lie between the age of the seagrass and organic fractions (which post-date it) and the age of the carbonate fractions (which pre-date it). Fig.4, where z values and age differences of dated pairs are plotted against each other, indicates a grouping of results within the specified limits. If a median value was to be

H

,3,/

J2

/

9

N5 4

12, 6 / i

2 t

o 0

a t ~ 2 4

i 6

i 8

i i i IO 12 14 16

~ i i , l J 18 20 22 24 26 z8

D i f f e r e n c e in oge of group pairs x t 0 0

Fig.4. Grouping of sample pairs based on ratio of z and ' d i f f e r e n c e ' of respective ages.

282 taken, then this would correspond to z = 4 -+ 2. Whilst the z value is uniquely defined, the age difference it represents has to be individually assessed and ranges from 300 to 1000 yrs. Thus, whilst the age obtained from any one fraction is not the true age of the formation, by dating sample pairs originating in the same strata, it can be adequately bracketed. Further interpretation then requires stratigraphic input (see Belperio et al., 1984, this volume). Accumulation rates from radiocarbon data In uniform sedimentary sequences (e.g., deep-sea strata), contiguous radiocarbon data may be used, with appropriate porosity normalization (Behrens, 1980) and a suitable mixing model, to calculate incremental mass sedimentation rates (e.g. Berger and Killingiey, 1982). The Spencer Gulf sedimentary sequences, like most shallow water deposits, are not uniform, and our data do not allow for fine-scale interpretation other than to make overall linear accumulation rate averages. Seagrass meadows are known, both morphologically and biologically, as areas of prolific skeletal carbonate production and rapid accumulation (Davies, 1970; Burrell and Schubel, 1977; Gostin et al., 1984, this volume). The Pb-210 measurements support this view, with suggested maximum values of 2.0--2.7 mm yr- '. Estimates of accumulation rates from C-14 age bracketed seagrass strata (top and bottom) also support this, though the rates are lower at between 0.6 and 1.4 mm yr-1 (Table VI). Pb-210 estimates are averages over about TABLEVI A c c u m u l a t i o n r a t e e s t i m a t e s for Posidonia seagrass facies s e d i m e n t

Core Interval

Core

(cm)

Time Interval*

(years)

Accumulation Rate

(mm y r " )

SG 182

171-

0

7500

0.2

SG 182

137-

0

4000

0.3

SG 182

30-

0

I00

2.7

SG 38

30-

0

I00

2.3

SG 51

30-

0

100

2.0

SG 51

315-

0

5800

0.5

SGT0

223-

0

5500

0.4

Red 38

257-123

950

1.4

Red 48

373-126

4400

0.6

Red 46

332- 73

2500

1.0

Red 45

339- 60

2100

1.3

*

Interpreted from C-14 and Pb-210 data.

283 100 yrs whereas C-14 derived values are averages over a b o u t 1000--4000 yrs. Overall sedimentation rates from basal Holocene radiocarbon dates are even lower (0.2--0.5 mm yr-l). These estimates are averages over 4000--7000 yrs and probably reflect an average of time-varying accumulation rates. AMINO ACID RACEMIZATION DATING

In troduc tion The application of time-dependent protein diagenesis to geochronology is now well-known and widely used (e.g., Schroeder and Bada, 1976). Fossil shells contain preserved organic material within their crystallized carbonate structure. The extent of chemical racemization of the amino acids within this material may be used to estimate the time elapsed since the death of the organism. Hence the m e t h o d has been used for both correlation and quantitative age dating of shell bearing Quaternary sediments (e.g., Mitterer, 1975; Masters and Bada, 1977; Kvenvolden et al., 1 9 7 9 ; A t w a t e r et al., 1981). Although the extent of amino acid racemization (D/L enantiomer ratio) can be measured quite precisely (Rutter et al., 1979), the derivation of an age estimate also requires a knowledge of the temperature history of the fossil and of the racemization rate of the relevant amino acid. Both of these have most likely varied with time and are usually only poorly known. The uncertainty in these parameters may result in large errors in the age estimate. Other errors may be introduced by contamination and by diagenetic alteration (see Schroeder and Bada, 1976; R u t t e r et al., 1979).

Samples and techniques The estuarine cockle Anadara trapezia is a diagnostic fossil in Late Pleistocene marine sediments of South Australia. Near Adelaide, Anadara are abundant in the Late Pleistocene Glanville Formation (Ludbrook, 1976, 1978; Cann, 1978). As there are no recorded living species in South Australian waters, Howchin (1923) considered its presence to indicate a warmer climate during the last Pleistocene high stand of sea level and attributed its subsequent extinction to a fall in water temperature. In northern Spencer Gulf, Anadara is a c o m m o n fossil in the Late Pleistocene Mambray Formation (Hails et al., 1984, this volume). Its occurrence in modern sediments indicates reworking of older deposits (Gostin et al., 1981; Burne, 1982; Halls et al., 1983). Five Anadara shells were analyzed for leucine D/L values. T w o samples were taken from the subsurface Mambray Formation (Vibrocores SG182 and SG171) and a third from a stranded gravel beach ridge (Gostin et al., 1981) in northern Spencer Gulf (Fig.l). A fourth sample was collected from the recently exposed section of the Glanville Formation near Adelaide (Cann, 1978}. A modern sample of the bivalve from Queensland was obtained for comparative analysis. Sample preparation, amino acid extraction and analysis were carried o u t by E. Keenan (Geology Dept., University of

284

Delaware, U.S.A.) following the procedure of Wehmiller and Emerson (1980) and Atwater et al. (1981). Results and discussion

Leucine D/L ratios for the five samples are listed in Table VII. The modern sample has a characteristically low D/L value (0.05) whereas the other four have higher values (0.42--0.63) consistent with their greater age expectation. The three subsurface samples (1, 2 and 4) show good agreement and, if they underwent similar diagenetic histories, can be considered of equivalent age (Rutter et al., 1979). The beach ridge sample (3) shows a significantly higher D/L ratio and is either older or has suffered higher ambient temperatures and, therefore, faster racemization. Considering that this sample was collected from an exposed shingle beach ridge, it is not unreasonable to attribute it to higher ground temperatures. For the purpose of arriving at an age estimate, only the three subsurface samples are considered together. The average leucine D / L value of samples 1, 2 and 4 is 0.48. To convert this to an age estimate requires a knowledge of the temperature history of the fossils (Wehmiller and Belknap, 1978) and of the time-varying rate of racemization (Wehmiller and Hare, 1971; Wehmiller, 1981). The present day mean annual air and water temperatures in northern Spencer Gulf are 18.5 ° and 18°C, respectively. During the last glacial maxima, sea temperatures in southern Australia were about 3°C cooler (Prell et al., 1980). Leucine racemization rates for Anadara are poorly known, but may be estimated from measurements made on North American samples where they are found in conjunction with a more intensively studied species, Mercenaria. North American data on aspartic acid/leucine ratios indicate that Anadara racemizes at a rate about 1.25 times that of Mercenaria (E. Keenan, personal communication, 1981). Using the above estimates yields an age of 110,000 + 19,000 yrs for the average of the three samples. The error range quoted --17,000 TABLE VII Leucine D/L values for Anadara trapezia

Sample

Location

Leuclne D/L

1

SG182 - 220 cm

0.48

2

SGITI - I00 cm

0.54

3

Cultana beach ridge

0.63

4

Glanville Formation

0.42

5

Queensland (Modern)

0.05

Average of I, 2 and 4

0.48

285

is for a 1°C uncertainty in the estimate of effective temperature. Larger temperature uncertainties (ca. 3°) could result in 60% age uncertainties. Therefore, the age q u o t e d should be taken only as a preliminary indication of the true age of the sediment. Irrespective of the error in the age estimate, the similarity inD/L ratios supports a lithological correlation of the Mambray and Glanville Formations. Von der Botch et al. (1980) report a similar leucine D/L value (0.46) for Katelysia from Late Pleistocene coastal marine sediments in the southeast of South Australia. An equivalent age is also indicated for the stranded shingle beach ridge only if it experienced a higher (+3°C) temperature history. THERMOLUMINESCENCE DATING

Introduction Thermoluminescence (TL) has been used extensively as a method for dating p o t t e r y and other materials that have previously been heated above 500°C (Fleming, 1979; Wintle, 1980). Such samples contain minerals with a crystalline structure. The technique is based on the premise that the heating removes all the electrons that are trapped in irregularities in the crystal lattice. Ionizing radiation from the environment then causes electrons to be retrapped at a rate which is uniform with time. If the total number of trapped electrons can be measured, and the rate at which they accumulate is known, then the time which has elapsed since the last heating can be determined. This is done by measuring the light given o u t when the sample is subjected to a further heating; hence "thermo-luminescence". The observed TL is proportional to the total number of trapped electrons. The total radiation dose that has been required to produce the measured TL is called the natural dose. It is f o u n d by comparison with a known dose from a laboratory radioactive source by measuring the TL produced by the latter. Radioactive analysis of the sample and surrounding material gives a value for the annual dose to the sample. The age of the sample is then the natural dose divided by the annual dose. In practice there are m a n y complications and some of these are mentioned below. Exposure of the sample to sunlight produces a result similar to heating. The solar ultra-violet (u.v.) light removes electrons from traps, and results in a reduction in TL. This provides a basis for dating wind- and water-borne sediments which have been exposed to sunlight during transportation, prior to deposition. However, even long exposures to sunlight m a y n o t remove all of the electrons, resulting in a "residual" level of TL. The m o s t recently deposited grains at the top of a range of sediments will have only this residual TL b u t once they are covered b y subsequent opaque layers, the TL will increase almost constantly with time. Hence a knowledge of the residual TL, together with the rate at which TL increases, can give an indication of the time period since the sediment was last exposed to sunlight. The first major attempts to date sediments were made by several U.S.S.R.

286 researchers (for a review see Dreimanis et al., 1978). Wintle and Huntley (1979, 1980) independently described several techniques which are based on the established procedures for dating heated samples. They dated samples from two deep-sea sediment cores using fine grains (of 4--11 pm) extracted from the sediments. The Estonian group (Hiitt et al., 1978) use a coarser fraction in the size range 7 0 - 1 2 0 pm. We have dated cores from the shallower waters of Spencer Gulf using both fine (4--11 pm) and coarse ( 9 0 - 1 2 5 pm) grains. The cores selected for TL dating were SG100 and SG182 (Fig.l). SG182 was selected in particular as it also contained organic materials suitable for carbon-14 and amino acid dating. Method All sample extractions and preparations were carried o u t in a dark room. Samples were extracted, at several intervals, from each of the two (80 mm diameter) cores. Each sample was digested in warm dilute HCI to remove all carbonate. After washing and drying, the samples were sieved into different size fractions. The 90--125 #m fraction was then given a 40 min etch in 40% HF to remove the outer layer of the grains. Etching removes that portion of the grains that would have been exposed to alpha particles and simplifies the calculation of radiation dose. This was followed by magnetic separation to produce a fairly pure quartz sample which was used for coarse grain dating. About 3 mg of the coarse grains were then sprinkled onto 1 cm diameter stainless steel discs. Depending on the regime of measurements used, between 20 and 100 discs were prepared for each sample. The fraction less than 45 p m was treated with M/100 sodium oxalate, which has the effect of breaking up aggregates of clay and other finely divided minerals. The fraction 4--11 pm was recovered by settling from suspension in M/100 sodium oxalate (fraction between 0.5 and 4 h settling time at a depth of 20 cm). After washing to remove the oxalate, the residue was resuspended in methanol, aliquots of which were transferred to small settling tubes where the fine grains were deposited on 1 cm aluminium discs and the methanol allowed to evaporate. The samples on these discs were the material for fine grain dating. The principal advantage of fine over coarse grain dating in the present context is that saturation of TL is less likely to occur, as the mineral fraction is of a different composition. For analysis, the discs were placed on a nichrome heating plate in a nitrogen atmosphere (combined 02 and H20 less than 7 ppm) and heated at a constant rate (2.8 K s-~ for core SG182 and 10 K s-I for SG100) to a temperature of 470°C (the difference in heating rates is not significant in this context). The form of the TL versus temperature curve obtained for SG182-175 cm is shown in Fig.5. Curve a shows the natural TL obtained by heating the sample; curve b shows the TL after a further 4000 tad from a laboratory beta-source; curve c shows the bleaching effect of 20 min under a sunlamp, while curve d shows the effect of prolonged bleaching by natural

287 3.0

2.5

2.0

h5

LO

0.5

{b} (d)

G

40 °

80°

120°

160°

200°

240"

280°

320°

360°

400 °

440 °

480"

TEMPERATURE °C F i g . 5. T L

glow

curves for

sample

SG182

-- 175 cm

showing

(a) natural

TL,

(b) TL

after

exposure to a further 4000 rad from a laboratory beta source, (c) TL after a 20 min sunlamp exposure, and (d) TL after prolonged bleaching by natural sunlight. sunlight. For coarse grain dating, the TL levels measured were normalized by later giving each disc a standard beta-dose and comparing the resulting TL. It was f o u n d t h a t this gave a normalization reproducibility of better than 5% no matter what irradiation, u.vo exposure or heating history the disc had received. For fine grain discs, the m e t h o d of preparation ensured a reproducibility of about 2%. In order to obtain accurate dates, it is necessary to k n o w the radioactive dose the grains have received throughout their history. The present-day c o n t e n t of uranium and its daughters, thorium and its daughters, and potassium-40, provides an approximate guide to this. For the Spencer Gulf samples the potassium-40 c o n t e n t was f o u n d by atomic absorption analysis. The uranium and thorium contents were measured using two different methods. The first was neutron activation analysis, which measures the uranium and thorium parent concentrations directly. The second was thick source alpha-counting which measures the uranium and thorium parent concentrations by looking at decays of them and their daughter products. Because of TL saturation in some samples, several variations in natural dose estimation were used. These are referred to as methods 1, 2 and 3.

Results, coarse grain dating Core SG100: The first m e t h o d used for dating SG100 (method 1) was to add several different additional radiation doses on top of the natural dose before heating and to look at the resulting TL. In ideal circumstances, the TL should increase linearly with dose so that an extrapolation back to the residual level of TL leads to a measure of the natural dose. Unfortunately

288 with both samples from SG100, the T L had begun to saturate and the growth curve with dose was no longer linear. In this case, the data were fitted by an exponential instead of a linear fit. As a result, the errors became quite large although a minimum natural dose could be found for each sample. The residual T L was f o u n d by exposing samples for up to 200 h of sunlight. After this exposure, the T L level is only a small fraction of the natural TL (as in Fig.5d). In order to overcome the large errors induced by exponential fitting, a technique similar to m e t h o d b described by Wintle and Huntley (1980) was a d o p t e d (method 2). A n o t h e r set of discs was prepared and given additional beta-doses as before. After the irradiations, they were exposed to a m e rcu r y vapour u.v. lamp for six h at a distance of 27 cm. T hey were then re-irradiated with the beta-source until the T L was the same as for those discs that had n o t received the u.v. light exposure. The beta-dose required to do this is proportional to the total dose the sample has received, so extrapolating the required beta-dose to zero gives a measure of the natural dose. This m e t h o d was used for SG100-75 cm, but for SG100-201 cm, the T L was too saturated. The results for core SG100 are shown in Table VIII. Core SG182: Core SG182 showed little or no coarse grain T L saturation (except at 220 cm) so only method 1 was used for m ost samples. Natural sunlight was used to determine the residual TL. Sample SG182-75 cm was also dated using a technique based on m e t h o d (c) of Wintle and H unt l ey (1980) (method 3). A set of discs was prepared with several different beta-doses added to the natural dose. These were then given a 20 min exposure at a distance of 26 cm from a Philips sunlamp TABLE VIII TL doses and ages for cores SG100 and SG182 Natural Oose (kraq) Depth (cm)

Core

~thod (I)

,SG

75 201 Core

Method (2)

Method (3)

Age (ka)

Last 8 ka

Before Last 8 ka

0.37 + C.05 0.31 ~ 0.03

0.43 + 0.05 0.35 ~ 0.03

Geologically interpreted age (ka)

I00 TL (coarse gr.) TL. (coarse gr. )

SG 5 5

#~nual Oose rate (rad/yr)

Dating Method

182 TL (fine gr.) TL (coarse gr.]

25

PU-210

58 ~C

S~ia (ANU-2707) h (coarse g r . )

Z37 13Z i~4

TL (coarse g r . ) C-14 (0-2372) TL ( f i n e g r , )

171 175

C-14 (&NU~2710) TL (coarse gr.)

220 220

iL (fine gr. quartz) TL (coarse g r . )

220

Pmino a c i d

M = Modern

>6.2 >16

7.8 + 2.3 -

5.2 ~ 2.0 12 6

- 0.2 -0,15

19 + ~, >~4

~:I~ 740 - J.~

20 Z 6

0.2;' Z L.]I

7.0 Z 0.8

0.2~ Z C.08

3.4 Z 0.8 47 7~ 32 -:

P = Pleistocene stage 5e

M * %

8 - 100 100

M m O.] ~.s u.',

1.22 + O.1

0.2a Z 0.02

27 ÷ 9 4.} ~ 0,1 ,,.~ ~ [i,7

,'~.7 a.7 :4.1

L9

0.5

~ + 1.2 ,~[ + 3,B

8.2 8.7

Z i ,3

.i 0.19

0.44 . 0.03 0.29 Z- 0,03

0.53 + 0.04 0.34 ~ 0.04

90 ~ Ib 96 _+ 24 llG

• 19 }7

125 (P) 125 (P) 125 (P)

289 (MLU300W). The difference in TL between those discs that did n o t have the u.v. exposure and those that did is proportional to the total dose the sample has received, the assumption being that a standardised bleaching procedure reduces the accumulated TL always by the same fraction. Hence it is possible to determine the natural dose by extrapolating back to where the TL levels w o u l d be the same for both u.v. exposed and unexposed discs. The results for core SG182 are also shown in Table VIII.

Results, fine grain dating Fine grain dating was carried out on three samples from SG182, partly to provide a cross-check between the coarse and fine grain techniques and partly to provide a cross-check between the Adelaide laboratory and Huntley's laboratory at Simon Fraser University. The m e t h o d was described earlier. The SG182-5 cm sample showed far more TL than expected for a zero-age sample so only a lower limit for the age was determined using method 1. The fine grain fraction extracted in the normal w a y from the 220 cm sample exhibited anomalous fading. This is n o t u n c o m m o n with fine grain samples and leads to underestimates of age. The sample was therefore reprocessed to extract only the quartz from the fine grains by treatment with hexafluorsilicic acid (Berger et al., 1980). There was no anomalous fading after this treatment. The 220 cm sample showed signs of TL saturation so an exponential was fitted to the TL measurements. The results appear in Table VIII.

Annual dose rate estimates The radioactive content of the sediment throughout its history determines the dose rate the grains have received. Measurements of present day radioactive c o n t e n t may n o t necessarily give a true measure of past radioactive dose rates. For Spencer Gulf samples neutron activation analysis gave in all cases a higher uranium c o n t e n t than did alpha-counting. This means that the parent uranium is present in greater quantities than would be needed to support its daughters in equilibrium. The implication is that the concentration of some of the radionuclides has changed in the past. The annual dose delivered to the sediment therefore has an error associated with this uncertainty. The other major error is uncertainty in the water content, which also would have varied in the past. Water shields the grains for the surrounding radiation. For the last 7000--8000 yrs the sediments have been under water, b u t before that {since a b o u t 110,000 yrs B.P. by geological evidence) they were in a dry environment. Our calculations therefore assumed total saturation for the last 8000 yrs with zero water content before that. The annual dose rates for both of these periods are included in Table VIII.

290 Discussion

The TL ages of the sediment samples from cores SG100 and SG182 are given in Table VIII. For core SG100, no other dates are available for comparison with the TL results. From geological evidence (Hails et al., 1984, this volume), the sediments in core SG100 {False Bay Formation) were originally deposited during a glacio-eustatic high sea level phase at about 100,000 yrs B.P. (Stage 5c of Shackleton, 1969). The strata represented by the sample at 75 cm were later reworked by aeolian activity {clay dune environment) so the sample grains were probably exposed to sunlight and had their TL re-set. The TL date obtained corresponds to the time of this last exposure. Geologically, this could be anytime between 8000 and 100,000 yrs ago. The TL age estimate places this event at about 20,000 yrs B.P. The sample at 201 cm is from the undisturbed part of the Pleistocene marine strata, but the saturation in TL for this sample allows only a minimum age estimate (> 44,000 yrs) to be made. Core SG182 was also intensively dated by radiocarbon, a m i n o acid and Pb-210 techniques. The comparative results are included in Table VIII. Note that the radiocarbon dates are environment corrected ages (B.P.*) and are n o t corrected for the true C-14 half life or past variations of C-14 concentration in the environment. They are therefore likely to be several hundred years too young. The coarse grain TL dates from the upper half of the Holocene sedimentary section are much older than corresponding C-14 derived ages. In the lower part of the Holocene strata, quartz TL dates agree very well with C-14 derived ages. This discrepancy may have a sedimentological explanation. One of the major sources of terrigenous quartz grains in the Gulf today is from erosion of the older sediment along the deeper channel floor. Very little, if any, quartz originates from rivers or streams. As a result, the grains are rarely exposed to u.v. light during deposition and consequently the TL measured will n o t correspond to the latest depositional event. This interpretation is supported by the fact that the natural dose measured at 300°C is much less than at 400°C where the TL needs a longer u.v. exposure to reach the residual level. For the mid-Holocene (4000-7300 B.P.) where coherent TL ages are obtained, the source of quartz, or the depositional environment, must have differed sufficiently so as to allow the TL signal to be re-set. This source may have been an aeolian blanket which mantled the Gulf floor prior to marine inundation. In the Pleistocene section of core SG182, the coarse grain TL age estimate of 96 + 24 ka and the fine grain quartz age of 90 -+ 15 ka are comparable with the amino acid racemization age estimate of about 110,000 yrs. This Pleistocene marine stratum (Mambray Formation) is correlated with the Stage 5e high sea level phase of Shackleton (1969).

291

CONCORDANCY OF DATING TECHNIQUES Accumulation rates of seagrass facies sediment calculated from Pb-210 measurements are apparently greater than those from C-14 measurements of basal Holocene strata. Age bracketed seagrass strata yielded intermediate accumulation rates (Table VI). The overall range of accumulation rates for seagrass facies sediment is 0.2--2.7 mm yr -1. These differences could be based on problems inherent to calculations of accumulation rate from Pb-210 as well as from C-14 measurements (e.g. biological mixing, porosity variations, variable age of carbonaceous material reaching or penetrating the sea bed). Alternatively, the accumulation rates over the past century (as measured by the Pb-210 method) may have been affected by anthropogenic activities. However, accumulation rate measurements show a negative correlation (r = --0.88) with the time period over which they were averaged. Sadler (1981) discussed the significance of a negative correlation in a comprehensive compilation of published (world-wide) accumulation rate measurements. Our data (Table VI) show a similar trend (Fig.6) and are consistent with previous accumulation rate measurements for carbonate platforms and reefs. This "average accumulation rate" is n o t a single figure b u t is a function of the time interval over which the average is taken. The range of Pb-210 and C-14 derived accumulation rates obtained for Spencer Gulf sediments m a y merely reflect this effect. The interpretation of strata ages from radiocarbon data also requires caution and a thorough understanding of the sample environment and sedimentological history. Shells, shell hash and organic debris do n o t necessarily date the age of sediments in which they are found. Carbonate C-14 results are the averages of individual shell ages at the time of their biological death which at a later stage were incorporated in the sediments. Thus they predate the sediments by a period which cannot be uniquely defined. Organic debris such as seagrass fibre post-date the sedimentary strata, as they have intruded the sediment after deposition. Calcareous fines associated with the deposits form the most unreliable samples. We have shown that by dating sample pairs originating in the same strata, the age relationship of sedimentary constituents can be factorized and the strata age can be delimited. Clearly, in Spencer Gulf this factor is variable from deposit to deposit, grouping itself around the 300--1000 yr mark. We are confident, however, that using this approach, the true age of sedimentary strata can be adequately defined. Amino acid racemization in fossil Anadara trapezia provides a useful correlation between the Mambray Formation of Spencer Gulf and the Glanville Formation of Adelaide. It also indicates a similar correlation with the uppermost c o m p o n e n t of the Woakwine Range barrier--estuarine sequence on the coastal plain of southeast South Australia (Von der Borch et al., 1980). A preliminary age estimate of 11n nnn + 19,000 yrs is derived for the Anadara , ~ v , v v v --17,000 samples from palaeotemperature and racemization rate estimates. Because

292

I0

E E E] (l: O C}

E

.~ io-1 .E "o U?

i0-z

IO

IO2

103

104

105

Time Interval (years)

Fig.6. Accumulation rates plotted against the time interval for which they were determined. Black circlesare for this study and the envelope (stippled) isfrom a compilation of carbonate platform and reef environment data by Sadler (1981). of the uncertainty in these estimates and the small number of samples analyzed, this figure should only be regarded as an approximate indication of the true age. Thermoluminescence measurements at the same level as one of the Anadara shells yielded age estimates of 96,000 -+ 24,000 and 90,000 -+ 15,000 yrs. Geologically, the M a m b r a y and Glanville Formations correspond to the m a x i m u m high sea level stand of the last interglacial (i.e.,stage 5e of Shackleton, 1969). The age of this glacio-eustatic event is variously put at between 100,000 and 140,000 yrs B.P. (Bloom et al., 1974; Marshall and Thorn, 1976; Chappell and Veeh, 1978; Szabo, 1979), with a most generally accepted age, from uranium-series dating of coral terraces, being about 125,000 yrs (Cronin et al., 1981). The T L dating method gives reasonable agreement with other methods w h e n the sediment grains have been given a good u.v. light exposure prior to or during deposition. W h e n this is not the case, T L ages will be too old. Other later sedimentological processes (e.g., aeolian reworking) m a y also imprint themselves into the T L signal. However, the method does have the

293 p o t e n t i a l t o bridge t h e gap b e t w e e n t h e u p p e r age l i m i t f o r reliable c a r b o n - 1 4 d a t e s a n d the l o w e r age limits f o r o t h e r d a t i n g t e c h n i q u e s . I t also o f f e r s a m a j o r a d v a n t a g e o v e r c a r b o n - 1 4 in t h a t it is the s e d i m e n t itself t h a t is being d a t e d r a t h e r t h a n an organic inclusion w i t h a s o m e t i m e s a m b i g u o u s association with the sediment. ACKNOWLEDGEMENTS T h i s r e s e a r c h was p r i m a r i l y s u p p o r t e d b y an A u s t r a l i a n R e s e a r c h G r a n t s S c h e m e a w a r d (E 7 5 / 1 5 4 2 4 ) . We t h a n k N. Cutshall a n d I. L a r s e n ( O a k Ridge N a t i o n a l L a b o r a t o r y , U.S.A.) f o r Cs-137 m e a s u r e m e n t s a n d E. K e e n a n ( U n i v e r s i t y o f D e l a w a r e , U.S.A.) f o r a m i n o acid r a c e m i z a t i o n m e a s u r e m e n t s (U.S.G.S. g r a n t 1 4 - 0 8 - 0 0 1 - G 5 9 2 ) . J. H e a d , S. R o b e r t s o n a n d L. Bell ( A u s t r a l i a n N a t i o n a l U n i v e r s i t y ) c o n t r i b u t e d t o the r a d i o c a r b o n dating o f s a m p l e s , a n d m a s s s p e c t r o m e t e r usage f o r stable c a r b o n i s o t o p e s was b y c o u r t e s y o f t h e R e s e a r c h S c h o o l o f Biological Sciences, A . N . U . D . B l a c k b u r n assisted w i t h p e a t i d e n t i f i c a t i o n , N. K e l s e y and F. G o r o s t i a g a w i t h s a m p l e p r e p a r a t i o n , a n d S. P r o f e r e s w i t h draughting. T h e R e s e a r c h C o m m i t t e e o f T h e U n i v e r s i t y o f A d e l a i d e and A I N S E p r o v i d e d a d d i t i o n a l s u p p o r t f o r t h e r m o l u m i n e s c e n c e m e a s u r e m e n t s a n d we g r a t e f u l l y a c k n o w l e d g e the h o s p i t a l i t y o f D. H u n t l e y a n d his colleagues o f S i m o n F r a s e r U n i v e r s i t y , w h e r e s o m e of t h e T L m e a s u r e m e n t s w e r e m a d e . Critical c o m m e n t s o n the original m a n u s c r i p t b y N. H a r v e y a n d N. L u d b r o o k are g r a t e f u l l y a c k n o w l edged. REFERENCES Atwater, B.E., Ross, B.E. and Wehmiller, J.F., 1981. Stratigraphy of Late Quaternary estuarine deposits and amino acid stereochemistry of oyster shells beneath San Francisco Bay, California. Quat. Res., 16: 181--200. Beasley, T.M., 1969. Lead-210 in Selected Marine Organisma Thesis, Oregon State University, Corvallis, Oreg., 82 pp. (unpublished). Behrens, E.W., 1980. On sedimentation rates and porosity. Mar. Geol., 35: Mll--M16. Belperio, A.P., Hails, J.R., Gostin, V.A. and Polach, H.A., 1984. The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia. In: J.R. Hails and V.A. Gostin (Editors), The Spencer Gulf Region. Mar. Geol., 61 : 297--313 (this volume). Benninger, L.K., Aller, R.C., Cochran, J.K. and Turekian, K.K., 1979. Effects of biological sediment mixing on 21°Pb chronology and trace metal distribution in a Long Island Sound sediment core. Earth Planet. Sci. Lett., 43: 241--259. Berger, G.W., Mulhern, P.J. and Huntley, D.J., 1980. Isolation of silt sized quartz from sediments. Ancient T.L., 11: 8--9. Berger, W.H. and Killingley, J.S., 1982. Box cores from the equatorial Pacific: 14C sedimentation rates and benthic mixing. Mar. Geol., 45: 93--125. Bloom, A.L., Broecker, W.S., Chappell, J., Matthews, R.S. and Mesolella, K.J., 1974. Quaternary sea level fluctuations on a tectonic coast: New Th=3°/U =34 dates from the Huon Peninsula, New Guinea. Quat. Res., 4: 185--205. Bowman, G. and Harvey, N., 1983. Radiocarbon dating marine shells in South Australia. Aust. Archaeol., 17 : 113--123. Burne, R.V., 1982. Relative fall of Holocene sea level and coastal progradation, northeastern Spencer Gulf, South Australia. BMR J. Aust. Geol. Geophys., 7 : 35--45.

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