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A luminescence method for dating ‘dirty’ pedogenic carbonates for paleoenvironmental reconstruction
EPSL ELSEVIER
Earth and Planetary Science Letters 139 (1996) 321-332
A luminescence method for dating ‘dirty’ pedogenic carbonates for paleoenvironmental reconstruction A.K. Singhvi a’*, D. Banerjee a, R. Ramesh a, S.N. Rajaguru b, V. Gogte b a Physical Research Laboratory, Earth Science Division, Navrangpura. Ahmedabad 380 009, India b Deccan College, Pune 411 006, India
Received 5 May 1995;revised 8 August 1995; accepted 28 November
199.5
Abstract Pedogenic carbonates are sensitive paleoclimatic and paleomonsoonal indicators, stratigraphic markers and important sources and sinks in the global CO budget. Establishing a chronology of such carbonates has been difficult due to their open-system behaviour with respect to the radioisotopes used for dating. A luminescence method for chronometric dating of carbonates is proposed which is less susceptible to post-depositional changes and uses subtle changes in the radiation flux to a mineral grain on being carbonated. This change is estimated by measuring luminescence signals of carbonated and uncarbonated mineral separates from the sediment strata and can be related to the age of the carbonate formation. Such ages of pedogenic carbonates from Thar Desert, India conform with paleoclimatic and stable isotopic data and show that they formed in Thar ca. 17 ka, 5 ka and 2 ka ago, since the last glacial epoch. Keywords: luminescence;
carbonate
sediments;
paleoenvironment
1. Introduction
Pedogenic carbonates occur as Kankar nodules, rhizo-concretions, pedotubules, etc., in a variety of sedimentary sequences and soil profiles. These have long been used as important stratigraphic markers and paleoenvironmental/paleomonsoonal indicators [l-3]. Thus, in Arizona, for example, the depths of occurrence of carbonates have been correlated to the annual precipitation [4]. These carbonates also constitute a significant part of the global carbon budget. Present estimates suggest that carbonate profiles over the globe in arid zones can account for up to 50% of total soil carbon [2]. This makes it imperative to understand the time scales of carbon fixation (as carbonates) in the arid zones of the world and examine their possible correlation with global climatic events [51. Isotopic dating of pedogenic carbonates has been controversial because, geochemically, they behave as open systems. For example, radiocarbon dating of carbonates is often suspect because of perturbation(s) caused either by ‘modern’ carbon brought in by percolating HCO; with soil water or by ‘dead’ carbon derived from ancient limestone rocks (the limestone dilution effect) [6,7]. Efforts to date dirty carbonates by U-series
* Corresponding
author.
0012-82 1X/95/$12.00 0 96 Elsevier Science B.V. All rights reserved PII 0012-821X(95)00227-8
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disequilibrium methods necessitate significant corrections for the detrital contribution, and also for post-depositional diagenesis and cementation [S-10]. Some attempts to date directly the carbonates via radiation damage methods such as thermoluminescence [ 111, electron spin resonance [ 121 and even lyoluminescence [13] have been made, but complications arising out of a phase transition (aragonite/calcite) on heating, isolation of carbonate luminescence from those of associated minerals and diagenetic changes make reliable dating difficult. More recently, an attempt has been made to use the in situ production of 36Cl by cosmic rays for dating pedogenic carbonate nodules. Such methods, however, at best provide model ages, because they crucially depend on several factors, such as the estimates of sediment overburden and constancy of cosmic ray fluxes through time [ 141. We describe here a new possibility of dating ‘dirty’ pedogenic carbonates based on the luminescence dating methodology, which (unlike other radiometric methods discussed above) is less susceptible to post-depositional changes (Appendix 1A). This approach exploits subtle changes in the natural radiation field of a luminescent mineral after being trapped in a carbonate precipitate. Thus, the proposed method uses the ‘dirt’ of a nodule to date a carbonate precipitation event. The methodology should allow us to apply it to a large variety of terrestrial carbonate deposits ranging from root casts, kankar nodules, loess dolls and even to aeolianites. It can also be used for determination of long-term rates of formation of pedogenic carbonates and thus has implications for studies dealing with Global Change. The method is also immune to the variability of cosmic ray fluxes. 2. The method The proposed method is based on the same premise as used in luminescence dating of sunlight bleached sediments [l&16]. Sunlight exposure of the minerals constituting the sediment stratum during their pre-depositional transport optically bleaches the geological luminescence to a residual value (I,). On burial, a fresh acquisition of luminescence is initiated, due to irradiation arising from the decay of 238U, 232Th and @K in the ambient strata. This re-acquisition of luminescence continues unabated until the sample is excavated and a laboratory analysis of this sample provides a signal, Inat, such that: I”,, = I, + I, (1) where I, is the ‘IL, acquired since sedimentation. In terms of equivalent radiation doses, the above equation can be functionally expressed as: p, = P, + Q, (2) where P, is the total paleodose in a sample. P, is the initial dose, corresponding to TL intensity I,,, and Q, is the dose acquired since burial and corresponds to I,. The luminescence age equation can be written as:
(3) t, =
Q,
aDa,,+q3.s+ Dy.5+ Q0s.s
(4)
where t, is the age of the sedimentation episode of the host stratum (e.g. a sand dune); D, is the total annual radiation dose, and D,,, Da,$ and DYTsare the components of a, j3, y dose rates provided by the decays of 238 232Th and 40K. D,,, is the cosmic ray dose rate and a is the sample-dependent alpha efficiency parameter. U, In the case of carbonate formation, some of the mineral grains that are trapped in the carbonate matrix suffer a change in their dose rate, due to dilution (or enrichment) of the radioactivity of the sediment matrix by carbonate. If the dose rate experienced by a grain in a carbonated matrix is D,, then the total paleodose, P,, of a mineral grain trapped in the carbonate matrix can be expressed simply as (Fig. 11: P, = P, + Q,
(5)
323
A.K. Singhui et al./ Earth and Planetary Science Letters 139 (1996) 321-332
QUARTZ
IN SEDIMENT
EVENT 1
.
l
Sediment Deposition
a
time
Sediment
deposition
:
Paleodose
Dose
1=0
t =tg
PO
Ps = Po+Qs
QUARTZ
IN CALCRETE
I
I
_ Sediment Deposition
l
w
Dose rote = Ds ,
tc
l
‘.
.
and
1s
l
EVENT 2
Carbcate leaching Precipitation
Present
rate = D,
?resent
Dose rote = Dc __j
Carbonate formation
time:
t = 1s - tc
Paleodose
P=P,+(t,-
t = tc t,)D,
Pc=P,t(t,-Ic)DStlcDc
Fig. 1. Schematic representation of the basic principles involved in luminescence dating of a carbonate. See text for explanation.
Q,, the calcrete-equivalent dose, is given by the equation:
where
(6)
Q, = (b - &ID, + Wc Here t,
is the age of carbonate formation event. Realizing that:
P, = P, + Q, = P, + tsDr
(7)
using Eq. (6) and (7) one obtains: P, - PC t,= D, - DC
(8)
Since both PC and PC have identical predepositional bleaching (i.e. P,): P, - P, = Q, - Q,
(9)
so:
t, =
Q,- Qc
(4,s+&,s
+Dv,r +%,,.s)-
@%,c +“s.c +D,,c+kc)
(‘0)
The above equation simplifies if one considers 100-150 pm quartz mineral separates which have negligible internal radioactivity. Also, the alpha dose contribution from the host matrix can be ignored by etching the outer alpha-dosed skin of the mineral. The large range of gamma rays further implies that the y-dose and cosmic ray contribution from the host matrix (5 30 cm), when compared to the typical size of 1-3 cm of a carbonate nodule, remains essentially unaltered even after carbonate precipitation. Setting: D,,, = 0 = Da,,.
DY,,= DY.c and
DC,,., = D,,,,,
(11)
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A.K. Singhvi et al./Eurth
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the dose rate difference reduces to:
Ds- “c = Dp.s- I&
(12)
Thus, only the difference in the total annual beta dose from U, Th and K in uncarbonated and carbonated mineral separates needs evaluation. The age equation in its final form can be written as:
t, =
Qs - Q, DL3.s- D&c
(13)
which suggests that the dating of carbonates requires estimation of the differences in: 1. Accumulated radiation doses of quartz mineral separates from both the carbonate matrix and the host sediment; 2. radioactivity content (i.e., the difference in the net annual dose to minerals within a host matrix and a carbonate nodule).
3. Samples and procedures For the present study, a set of carbonates and root casts were sampled from a well known geoarchaeological site at Budha Pushkar, Thar Desert [ 17,181. The absence of limestone rocks in the region implies that the calcrete radiocarbon ages are free of any limestone dilution effect. These pedogenic carbonates occur as distinct horizons and earlier workers have used a combination of their depths of occurrence and inversion of radiocarbon ages as indicators of fluctuations in the precipitation regimes [17-191. Table 2 provides a summary of the reported radiocarbon data. For TL measurements, 105-150 pm quartz mineral separates were extracted by sequential pretreament of the original sand sample by HCI, H,O, and HF. The extraction of quartz from carbonates was carried out by first dissolving (and discarding) the outer 0.5-l cm of the carbonate and then fully dissolving the remaining carbonate. The residue was then treated identically as described above. The extracted quartz was tested for purity using an 880 nm infra-red stimulated luminescence system. TL measurements were done on - 4 mg mono-layer quartz separates deposited on stainless steel discs using a photon counting setup, comprising EM1 9635QA photomultiplier tube coupled to two Schott UGll + HA3 filters. This enabled isolation of a rapidly bleaching 325°C glow peak of quartz [21]. All laboratory sun bleaching was carried out using natural sunlight. The TL analysis was done using the standard procedures, viz. the total bleach (bleaching time _ 7 h and _ 30 min) and the R-P methods (bleaching time _ 30 min) generally used for sediment dating and a weighted mean of the equivalent doses was used in estimation of age. However, the weighted mean error was not used for calculating the error coo,> in Q values since the growth curves for different bleaching methods are not independent of each other (N + /3 curve is common for all methods). Instead, the error in the equivalent dose (from a typical growth curve) at a certain plateau temperature was used as uo for the samples. The carbonate nodules kept for a-counting and y-spectrometry were first cleaned with a brush to remove loose sandy grains and then powdered in an agate pestle. For both sands and calcretes, thick source ZnS(Ag) alpha counting was used for determining thorium and uranium concentrations and y-ray spectrometry (NaI) was used for the determining a K concentration. Secular equilibrium was assumed in the calculation of annual doses. The error estimates were obtained using the equivalent doses prescription of Aitken 1151and by considering the propagation of errors in the age equation mentioned above. The details are provided in Appendix 1B. For stable isotopic measurements, about 1 mg of the carbonate sample was reacted with 100% H,PO, at 50°C in an on-line CO, extraction system attached to a VG Micromass 903D (triple collector update). The resulting CO, gas was purified by passing it through alcohol slurry kept at - 100°C. S 13C and S “0 were measured to a precision of +0.1%0 and are reported relative to PDB standard. Isobaric interferences were corrected for using standard procedures [22].
A.K. Singhvi et al./
Table I Radionuclide
data for calcretes
Table 2 Thermoluminescence
kill)
and host sands
data, 14C ages and stable isotopic values for carbonates
IJab
325
Earth and Planetary Science Letters 139 (1996) 321-332
(Gy)
(mCy/a)
from Budha Pushkar.
(Gy)
Thar Desert, India
(mCy/a)
00)
w
14C ages were calibrated using Stuiver and Reimer [20]. * These samples were collected from a different location in the same dune. +The Estimated as a weighted mean of partial bleach analysis (bleaching weighted mean of these three calcrete ages is 17 f 2.9 ka (see text). time 30 min) and total bleach analysis (bleaching time 30 min and 7 h). The bleaching was carried out under natural sunlight and both the sand and calcrete samples were exposed simultaneously. Typical bleached fractions were - lo- 18% (after 30 mitt bleaching) and reduced only marginally (a few percent) thereafter. This is attributed to the selection of an easily bleachable quartz signal using a WI I filter. ++ Includes beta dose contribution from U. Ih and K. l
Table 3 Equivalent
doses CQ) obtained
l
using R-/3 and total bleach methods for quartz extracted
from host sands and calcretes
Eqtivllcnt Dose QstGy)
Equivalent Dose Q.tCy) catcrete Sample
R-P*
TB” (55hr.)
Host Sand
TB (7 hrs.)
TB (M hr.1
R-8
I 37.4 *
CAL-O
35.4 * 1.4
I 33.4 * 4.0
CAL-I
40.2 * 3.8
34.4 * 1.6 28.9 * 1.2
31.4 * 2.5
FSH 92-1
45.8 2 3.5
37.4 * 0.75
53 * 1.6
41 * 3.3
45 2 6.1
FSH 92-2
77.5 * 4.3
65 * 1.3
49.8 f 1.3
45 * 5.0
54.5 : 1.7
PSH 92-1
45.8 f 3.5
FSH 92-1
45.8 + 3.5
1
CAL-3 CAL-Z CAL-4
36.4 * 0.9
For each method these values were computed by taking * TB = total bleach. + R-P = partila bleach (30 min).
I
an average
FSH 92-3
,
66.4 k 1.1
of the equivalent
doses which
TB (7 hrs.1
0.75
37.4 * 0.75
I
56.2 f 1.1
,
73.7 * 3.9 ~~~
61.85 f 0.8
span the equivalent
dose plateau.
326
A.K. Singhvi et al./ Earth and Planetary Science Letters 139 (19%) 321-332
Glow Curves
for PSH 92-2(Sand)
20000 SL=700 min sun SS=30 min sun
100
(a)
200
300
400
500
Temperature (Y) R-b GROWTH CURVES FOR PSH 92-2
20000
(W1
320% t
-80
-40
0
40
Dose (Cy) TOTAL BLEACHGROWTH 20000
3
CURVE FOR PSH 92-2 (c)l
320%
15000
5 x 2 %
10000
2 2
-100
-50
0 Dose (Cy)
50
100
A.K. Singhui et al./
Earth and Planetary
Science Letters 139 (1996) 32 I-332
327
X-ray diffraction analysis of the samples were carried out at the Deccan College, Pune, using a RIGAKU D Max II VC XRD system, operated at 50 KV, 25mA, at a scan speed of l’/sec. The peak intensities, peak positions and half widths were computed using standard software.
4. Results and discussion Table 1 provides data on the radionuclide contents of the calcrete and the host strata. Table 2 provides the luminescence ages, the carbon and oxygen isotopic values along with previously reported radiocarbon ages [ 181. Table 3 provides the values of the individual equivalent doses for the different methods used in age estimation. Figs. 2 and 3, provide typical TL glow curves and growth curves for - 105-150 pm quartz extracted from host sands and calcretes. With the exception of CAL- 1, the luminescence ages are significantly higher compared to their radiocarbon counterparts. This is to be expected in view of the likelihood of post-depositional contamination of the radiocarbon samples by modem carbon. Some clues on the plausibility of luminescence ages can be found from climatological aspects. 4.1. Calcrete chronology: climatic consideration Characteristically, pedogenic carbonates form in regions of net annual moisture deficit with distinct seasonality and a precipitation upper limit of < 400 mm/a. At precipitation > 400 mm, the carbonate can be completely removed from the host strata [23]. Oxygen isotopic ratios of the Pushkar carbonates ranging from - 4.82%0 to - 6.57%0 (relative to PDB) can be compared with the oxygen isotopic composition of - 5 to - 6%0 (relative to SMOW) of modem meteoric waters [24]. This suggests that the carbonates analyzed were formed primarily from leaching and thus are related to monsoonal precipitation. The carbon isotopic values range from - 7.02 to - 8.57%~ which [25] implies a contribution of - 28-35% from C, type plants and up to 65% from C, type, indicating somewhat wetter conditions. Marine records from the Indian Ocean have reasonably established that during the last glacial maximum (LGM) at 21.5 ka (calender years), the monsoon circulation over the Indian subcontinent weakened significantly, resulting in marked aridity. The re-establishment of the monsoon began at - 17.8 ka [26,27] reaching its full vigour by - 11 ka. Similarly, on land, palynological studies and radiocarbon dating of the organic fraction of the sediments of a saline lake (Lake Didwana) in the Thar desert (with a climatic regime similar to that at Pushkar) [28,29] indicate hyper-arid conditions during 21.5 ka (LGM) to - 16.2 ka. In the period 16.2-11 ka, the monsoon circulation re-established itself and eventually reached twice its present value (i.e., > 600 mm/a) by 6.8 ka. Subsequently, during 6.8-4.8 ka the region witnessed the most humid phase of high rainfall followed by a phase of - 700 a of extended aridity, beginning at - 4.8 ka [28-301. Since pedogenic carbonates can only be formed when precipitation is limited, the paleomonsoonal reconstruction in Rajasthan [30] suggests that the periods when they could have developed at Pushkar should be - 17-10 ka, 6.8-4.8 ka and < 2.5 ka. The luminescence age brackets of 17 ka (Table 2) and 5 ka thus agree well with these estimates. 4.2. Methodological considerations: Future refinements An important methodological aspect involving dating is the mobility of potassium. The X-ray diffraction analysis indicates that the potassium-bearing phases are microcline, richterite, albite (patassian) and muscovite.
Fig. 2. (a) Glow curves for quartz separates from a typical host sand, PSH 92-2. SL and SS represent bleaching times of 700 and 30 min under the natural sun. (b) R-p
growth curve for quartz separates from PSH 92-2.
growth curve for quartz separates from PSH 92-2. Q = the equivalent dose at 320°C.
Q = the equivalent dose at 320°C.
(c) Total Bleach
TL Intensity
(a.~.)
TL Intensity
(a.~.)
TL Intensity
(a.~.)
329
A.K. Singhvi et al./ Earth and Planetary Science Letrers 139 (1996) 321-332
Table 4 p dose fraction from potassium for host sands and calcretes
Calcrete
I
Fraction of beta dose from potassium
Host sand
Fraction of beta dose from potassium
CAL-O
0.71
PSH 92-1
0.83
CAL-l
0.76
I’SH 92-1
0.83
CAL-3
0.80
PSH 91-l
0.83
CAl-2
0.82
I’SH 92-2
0.70
CAL-4
0.78
PSH 92-3
0.83
The presence of minerals such as microcline, the semi-arid nature of the region and low water-exchangeable potassium (5 50 ppm) suggest that K mobility is low and the potassium largely remains locked up in silicate phases. Consequently, the dose rate estimates can be taken as realistic. The dominance of the beta dose from potassium (Table 4) further indicates that the effect of a possible mobility of uranium also gets substantially diluted. The p range of - 2 mm further suggests that, with respect to /3 doses, the luminescence dating system closes over scale lengths of a few millimetres, an aspect that could allow examination of growth rates of carbonate nodules based on sequential leaching of grains from the core to its centre. Another important methodological aspect of this new approach is the total error in age, which gets compounded since the age is computed as a ratio of two differences. Two possible ways can be suggested, both of which aim at improving the accuracy of paleodose determination. The first possible approach is to eliminate the sun bleaching step in the estimation of Q, and Q, and use directly a difference of P, and P,. This can be done since the quartz mineral separates from both the sand and carbonate matrices have identical predepositional solar bleaching history. Computations using the unbleached paleodose differences P, - P, in Eq. (8) gave calcrete ages for four samples (CAL- 1, CAL-2, CAL-3 and CAL-4) which were comparable to ages calculated using R-/3 and total bleach methods within experimental errors. An alternative approach would be to use Optically Stimulated Luminescence (OSL) where, by definition, only the optically sensitive traps are probed [31]; that is, the OSL method does not sample the I, related signal. This, coupled to a luminescence normalisation procedure (termed short-shine normalisation) is also expected to yield lower experimental errors on P, and P, values. The advent of new approaches, such as the single aliquot method [32], also augurs well for this application as it would permit examination of small diffused nodules often present in arid zone sequences. We thus conclude with the optimism that in the coming years the luminescence methods for dating carbonates might develop further and perhaps be able to provide a major contribution to Global Change research by providing time scales of CO, fixation and release in terrestrial carbonates.
Fig. 3. (a) Glow curves for quartz separates from a typical calcrete, CAL-4. SL and SS represent bleaching times of 700 and 30 min under the natural sun. (b) R- /3 growth curve for quartz separates from CAL-4. Q = the equivalent dose at 350°C. (c) Total Bleach growth curve for quarts separates from CAL-4. Q = the equivalent dose at 350°C.
A.K. Singhvi et al/Earth
330
and Planetmy
Science Letters 139 (1996) 321-332
Acknowledgements The authors thank the Ford Foundation, India for financial support towards upgrading the TL Laboratory and Professors R.P. Dhir, R.J. Wasson and AS. Murray for suggestions. We thank J.T. Padia for the stable isotopic analyses. We are indebted to the two reviewers for their criticism, which helped in improving the scientific content and presentation of this manuscript. This research is a contribution to IGCP-349 on Paleomonsoons from Desert Margins. fUC1
Appendix A. Dosimetric changes due to loss of ?J
and/or
u2 Th
The calcrete age equation can be written as:
Q, - Qc Age
=
p~j,,~
+
&,s
+
&.s
-
PTh.c
-
kJ.e
change to&h.c
Now if PT~,~ad &
-
(A’)
&.c
and p;,, after precipitation of the nodule, the expression for the new
age is given by:
Q, - Q; pm,$ + p”,s + PK.S - &h,c
Age’ =
-
&J,c
-
W)
kc
Using the two equations above and assuming Q, - Q, (a reasonable approximation since - 80% of the p dose comes from K), the relative change in age becomes: A Age -= Age
Age - Age’ (A31
Age
given by the expression:
&h.c + P”,c- ( &h,c (
&,,s
+
p&s
+
&,s
-
PTh.c
-
&J.c
-
&c)
+
((
&J,c) PTh.c
+&kc>
-
(
%‘h.c
+&C))
& c &) is the original annual beta dose difference between the host sand ( &h,s + P”.s + &s PTh,c and the calcrete. For CAL-3 this difference is 0.56 mGy/a whereas &,,e and & are - 0.086 mGy/a and 0.084 mGy/a, respectively. Calculations using these values show that: (1) if 50% loss of either 238U or 232’lh occurs, the change in the age will be 7%; while (2) if 50% loss of both 238U and 232’Ih occurs, the age changes by 13%.
Appendix B B.I. Estimation of errors
If f is a function dependent on n parameters, xi, each with a standard deviation vi, then the variance in f, a-* is defined as: n
t?f
*=
u* -Sf ’ 6Xi
*
4 )
i-1
P1)
A.K. Singhui ef al. / Earth and Plunetary Science Letters 139 (1996) 321-332
331
The calcrete age is given by:
If UQ,, uQ,v uD,, CD,. and u&e are the standard deviations for Q,, Q,, and DC, D, and the calcrete age respectively then, using Eq. (2) and simplifying, we get: 2 aA$
=
Age’
4,
(Q,-a,>’
4
+ (Qs-Q,>’
2
on,
+ tJ%‘k,2
+ (D, - De)’
WI
Thus an evaluation of the standard deviation in age involves use of five equantities oQ,, oQ,, oD,, oD, and the calcrete age. Using Eq. (31, it can be shown that, if Q, - Q, is < 5 Gy and D, - DC< 0.5 mGy/a, measurement errors of Q and D values must be less than 5% for achieving a reasonable error limit ( < 20%) on the calcrete age. Another practical implication of Eq. (3) is that, with this luminescence method, optimum accuracy will be achieved in dating pedogenic carbonates where: (1) carbonate precipitation occured at least a few ka after the depositional event of the host matrix and (2) there exists a large annual beta dose difference between the host sand and the carbonate , - D,I > 2 mGy/a).
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and Planetary Science Lerters
I39 (1996) 321-332
[ZO] M. Quiver and P.J. Reimer, Extended “C data base and revised calib.-3.0 ‘%Zage calibration program, Radiocarbon 35, 215-230, 1993. [21] J.R. Prescott and R.A. Purvinskis, Zero thermoluminescence for zero age, Ancient TL 9, 19-20, 1991. [22] A. Sarkar, R. Ramesh and S.K. Bhattacharya, Effect of size and pretreatment on the S 13C, S180 of foraminifera from the Arabzin Sea sediments, Terra Nova 2, 489-493, 1990. [23] R. Cooke, A. Warren and A.S. Goudie, Desert Geomorphology, 525 pp., University College of London Press, 1993. [24] R. Ramesh. R.A. Jani and R. Bhushan, Stable isotopic evidence for the origin of salt lakes in the Thar Desert, J. Arid. Environ. 25, 1, 1993. [25] T-E. Cerling, The stable isotopic composition of modem soil carbonate and its relationship to climate, Earth Planet. Sci. Lett. 71, 229-240, 1984. [26] E.V. Campo, Monsoon fluctuations in the 20,OCOyr B.P. oxygen- isotope/pollen records off southwestern India, Quat. Res. 26, 376-388, 1986. [27] F. Sirocko, M. Samthein, H. Erlenkeusu, H. Lange, M. Arnold and J.C. Duplessey, Century-scale events in monsoonal climate over the past 24,000 years, Nature 364, 322-324, 1993. [28] G. Singh, R.J. Wasson and D.P. Agrawal. Vegetational and seasonal climatic changes since the last full glacial in the Thar Desert, northwestern India, Rev. Palaeobot. Palynol. 64, 351-358, 1990. [29] G. Singh, R.D. Joshi, S.K. Chopra and A.B. Singh, Late Quatemary history of vegetation and climate of the Rajasthan Desert, India, Philos. Trans. R. Sot. London Ser. B 267, 467-501, 1974. [30] R.A. Bryson and A.M. Swain, Holocene variations of monsoon rainfall in Rajasthan, Quat. Res. 16, 135-145, 1981. [31] D.J. Huntley, D.J. Godfrey-Smith and M.L.W. Thewalt, Optical dating of sediments, Nature, 313. 105-107, 1985. [32] G.A.T. Duller, Equivalent dose determination using single aliquots, Nucl. Tracks Radiat. Measure. 18(4), 371-378, 1991.
Earth and Planetary Science Letters 139 (1996) 321-332
A luminescence method for dating ‘dirty’ pedogenic carbonates for paleoenvironmental reconstruction A.K. Singhvi a’*, D. Banerjee a, R. Ramesh a, S.N. Rajaguru b, V. Gogte b a Physical Research Laboratory, Earth Science Division, Navrangpura. Ahmedabad 380 009, India b Deccan College, Pune 411 006, India
Received 5 May 1995;revised 8 August 1995; accepted 28 November
199.5
Abstract Pedogenic carbonates are sensitive paleoclimatic and paleomonsoonal indicators, stratigraphic markers and important sources and sinks in the global CO budget. Establishing a chronology of such carbonates has been difficult due to their open-system behaviour with respect to the radioisotopes used for dating. A luminescence method for chronometric dating of carbonates is proposed which is less susceptible to post-depositional changes and uses subtle changes in the radiation flux to a mineral grain on being carbonated. This change is estimated by measuring luminescence signals of carbonated and uncarbonated mineral separates from the sediment strata and can be related to the age of the carbonate formation. Such ages of pedogenic carbonates from Thar Desert, India conform with paleoclimatic and stable isotopic data and show that they formed in Thar ca. 17 ka, 5 ka and 2 ka ago, since the last glacial epoch. Keywords: luminescence;
carbonate
sediments;
paleoenvironment
1. Introduction
Pedogenic carbonates occur as Kankar nodules, rhizo-concretions, pedotubules, etc., in a variety of sedimentary sequences and soil profiles. These have long been used as important stratigraphic markers and paleoenvironmental/paleomonsoonal indicators [l-3]. Thus, in Arizona, for example, the depths of occurrence of carbonates have been correlated to the annual precipitation [4]. These carbonates also constitute a significant part of the global carbon budget. Present estimates suggest that carbonate profiles over the globe in arid zones can account for up to 50% of total soil carbon [2]. This makes it imperative to understand the time scales of carbon fixation (as carbonates) in the arid zones of the world and examine their possible correlation with global climatic events [51. Isotopic dating of pedogenic carbonates has been controversial because, geochemically, they behave as open systems. For example, radiocarbon dating of carbonates is often suspect because of perturbation(s) caused either by ‘modern’ carbon brought in by percolating HCO; with soil water or by ‘dead’ carbon derived from ancient limestone rocks (the limestone dilution effect) [6,7]. Efforts to date dirty carbonates by U-series
* Corresponding
author.
0012-82 1X/95/$12.00 0 96 Elsevier Science B.V. All rights reserved PII 0012-821X(95)00227-8
322
A.K. Singhvi et al./
Earth and Planetary
Science Letters I39 (1996) 32/-332
disequilibrium methods necessitate significant corrections for the detrital contribution, and also for post-depositional diagenesis and cementation [S-10]. Some attempts to date directly the carbonates via radiation damage methods such as thermoluminescence [ 111, electron spin resonance [ 121 and even lyoluminescence [13] have been made, but complications arising out of a phase transition (aragonite/calcite) on heating, isolation of carbonate luminescence from those of associated minerals and diagenetic changes make reliable dating difficult. More recently, an attempt has been made to use the in situ production of 36Cl by cosmic rays for dating pedogenic carbonate nodules. Such methods, however, at best provide model ages, because they crucially depend on several factors, such as the estimates of sediment overburden and constancy of cosmic ray fluxes through time [ 141. We describe here a new possibility of dating ‘dirty’ pedogenic carbonates based on the luminescence dating methodology, which (unlike other radiometric methods discussed above) is less susceptible to post-depositional changes (Appendix 1A). This approach exploits subtle changes in the natural radiation field of a luminescent mineral after being trapped in a carbonate precipitate. Thus, the proposed method uses the ‘dirt’ of a nodule to date a carbonate precipitation event. The methodology should allow us to apply it to a large variety of terrestrial carbonate deposits ranging from root casts, kankar nodules, loess dolls and even to aeolianites. It can also be used for determination of long-term rates of formation of pedogenic carbonates and thus has implications for studies dealing with Global Change. The method is also immune to the variability of cosmic ray fluxes. 2. The method The proposed method is based on the same premise as used in luminescence dating of sunlight bleached sediments [l&16]. Sunlight exposure of the minerals constituting the sediment stratum during their pre-depositional transport optically bleaches the geological luminescence to a residual value (I,). On burial, a fresh acquisition of luminescence is initiated, due to irradiation arising from the decay of 238U, 232Th and @K in the ambient strata. This re-acquisition of luminescence continues unabated until the sample is excavated and a laboratory analysis of this sample provides a signal, Inat, such that: I”,, = I, + I, (1) where I, is the ‘IL, acquired since sedimentation. In terms of equivalent radiation doses, the above equation can be functionally expressed as: p, = P, + Q, (2) where P, is the total paleodose in a sample. P, is the initial dose, corresponding to TL intensity I,,, and Q, is the dose acquired since burial and corresponds to I,. The luminescence age equation can be written as:
(3) t, =
Q,
aDa,,+q3.s+ Dy.5+ Q0s.s
(4)
where t, is the age of the sedimentation episode of the host stratum (e.g. a sand dune); D, is the total annual radiation dose, and D,,, Da,$ and DYTsare the components of a, j3, y dose rates provided by the decays of 238 232Th and 40K. D,,, is the cosmic ray dose rate and a is the sample-dependent alpha efficiency parameter. U, In the case of carbonate formation, some of the mineral grains that are trapped in the carbonate matrix suffer a change in their dose rate, due to dilution (or enrichment) of the radioactivity of the sediment matrix by carbonate. If the dose rate experienced by a grain in a carbonated matrix is D,, then the total paleodose, P,, of a mineral grain trapped in the carbonate matrix can be expressed simply as (Fig. 11: P, = P, + Q,
(5)
323
A.K. Singhui et al./ Earth and Planetary Science Letters 139 (1996) 321-332
QUARTZ
IN SEDIMENT
EVENT 1
.
l
Sediment Deposition
a
time
Sediment
deposition
:
Paleodose
Dose
1=0
t =tg
PO
Ps = Po+Qs
QUARTZ
IN CALCRETE
I
I
_ Sediment Deposition
l
w
Dose rote = Ds ,
tc
l
‘.
.
and
1s
l
EVENT 2
Carbcate leaching Precipitation
Present
rate = D,
?resent
Dose rote = Dc __j
Carbonate formation
time:
t = 1s - tc
Paleodose
P=P,+(t,-
t = tc t,)D,
Pc=P,t(t,-Ic)DStlcDc
Fig. 1. Schematic representation of the basic principles involved in luminescence dating of a carbonate. See text for explanation.
Q,, the calcrete-equivalent dose, is given by the equation:
where
(6)
Q, = (b - &ID, + Wc Here t,
is the age of carbonate formation event. Realizing that:
P, = P, + Q, = P, + tsDr
(7)
using Eq. (6) and (7) one obtains: P, - PC t,= D, - DC
(8)
Since both PC and PC have identical predepositional bleaching (i.e. P,): P, - P, = Q, - Q,
(9)
so:
t, =
Q,- Qc
(4,s+&,s
+Dv,r +%,,.s)-
@%,c +“s.c +D,,c+kc)
(‘0)
The above equation simplifies if one considers 100-150 pm quartz mineral separates which have negligible internal radioactivity. Also, the alpha dose contribution from the host matrix can be ignored by etching the outer alpha-dosed skin of the mineral. The large range of gamma rays further implies that the y-dose and cosmic ray contribution from the host matrix (5 30 cm), when compared to the typical size of 1-3 cm of a carbonate nodule, remains essentially unaltered even after carbonate precipitation. Setting: D,,, = 0 = Da,,.
DY,,= DY.c and
DC,,., = D,,,,,
(11)
324
A.K. Singhvi et al./Eurth
and Planetary
Science Letters 139 (1996) 32/-332
the dose rate difference reduces to:
Ds- “c = Dp.s- I&
(12)
Thus, only the difference in the total annual beta dose from U, Th and K in uncarbonated and carbonated mineral separates needs evaluation. The age equation in its final form can be written as:
t, =
Qs - Q, DL3.s- D&c
(13)
which suggests that the dating of carbonates requires estimation of the differences in: 1. Accumulated radiation doses of quartz mineral separates from both the carbonate matrix and the host sediment; 2. radioactivity content (i.e., the difference in the net annual dose to minerals within a host matrix and a carbonate nodule).
3. Samples and procedures For the present study, a set of carbonates and root casts were sampled from a well known geoarchaeological site at Budha Pushkar, Thar Desert [ 17,181. The absence of limestone rocks in the region implies that the calcrete radiocarbon ages are free of any limestone dilution effect. These pedogenic carbonates occur as distinct horizons and earlier workers have used a combination of their depths of occurrence and inversion of radiocarbon ages as indicators of fluctuations in the precipitation regimes [17-191. Table 2 provides a summary of the reported radiocarbon data. For TL measurements, 105-150 pm quartz mineral separates were extracted by sequential pretreament of the original sand sample by HCI, H,O, and HF. The extraction of quartz from carbonates was carried out by first dissolving (and discarding) the outer 0.5-l cm of the carbonate and then fully dissolving the remaining carbonate. The residue was then treated identically as described above. The extracted quartz was tested for purity using an 880 nm infra-red stimulated luminescence system. TL measurements were done on - 4 mg mono-layer quartz separates deposited on stainless steel discs using a photon counting setup, comprising EM1 9635QA photomultiplier tube coupled to two Schott UGll + HA3 filters. This enabled isolation of a rapidly bleaching 325°C glow peak of quartz [21]. All laboratory sun bleaching was carried out using natural sunlight. The TL analysis was done using the standard procedures, viz. the total bleach (bleaching time _ 7 h and _ 30 min) and the R-P methods (bleaching time _ 30 min) generally used for sediment dating and a weighted mean of the equivalent doses was used in estimation of age. However, the weighted mean error was not used for calculating the error coo,> in Q values since the growth curves for different bleaching methods are not independent of each other (N + /3 curve is common for all methods). Instead, the error in the equivalent dose (from a typical growth curve) at a certain plateau temperature was used as uo for the samples. The carbonate nodules kept for a-counting and y-spectrometry were first cleaned with a brush to remove loose sandy grains and then powdered in an agate pestle. For both sands and calcretes, thick source ZnS(Ag) alpha counting was used for determining thorium and uranium concentrations and y-ray spectrometry (NaI) was used for the determining a K concentration. Secular equilibrium was assumed in the calculation of annual doses. The error estimates were obtained using the equivalent doses prescription of Aitken 1151and by considering the propagation of errors in the age equation mentioned above. The details are provided in Appendix 1B. For stable isotopic measurements, about 1 mg of the carbonate sample was reacted with 100% H,PO, at 50°C in an on-line CO, extraction system attached to a VG Micromass 903D (triple collector update). The resulting CO, gas was purified by passing it through alcohol slurry kept at - 100°C. S 13C and S “0 were measured to a precision of +0.1%0 and are reported relative to PDB standard. Isobaric interferences were corrected for using standard procedures [22].
A.K. Singhvi et al./
Table I Radionuclide
data for calcretes
Table 2 Thermoluminescence
kill)
and host sands
data, 14C ages and stable isotopic values for carbonates
IJab
325
Earth and Planetary Science Letters 139 (1996) 321-332
(Gy)
(mCy/a)
from Budha Pushkar.
(Gy)
Thar Desert, India
(mCy/a)
00)
w
14C ages were calibrated using Stuiver and Reimer [20]. * These samples were collected from a different location in the same dune. +The Estimated as a weighted mean of partial bleach analysis (bleaching weighted mean of these three calcrete ages is 17 f 2.9 ka (see text). time 30 min) and total bleach analysis (bleaching time 30 min and 7 h). The bleaching was carried out under natural sunlight and both the sand and calcrete samples were exposed simultaneously. Typical bleached fractions were - lo- 18% (after 30 mitt bleaching) and reduced only marginally (a few percent) thereafter. This is attributed to the selection of an easily bleachable quartz signal using a WI I filter. ++ Includes beta dose contribution from U. Ih and K. l
Table 3 Equivalent
doses CQ) obtained
l
using R-/3 and total bleach methods for quartz extracted
from host sands and calcretes
Eqtivllcnt Dose QstGy)
Equivalent Dose Q.tCy) catcrete Sample
R-P*
TB” (55hr.)
Host Sand
TB (7 hrs.)
TB (M hr.1
R-8
I 37.4 *
CAL-O
35.4 * 1.4
I 33.4 * 4.0
CAL-I
40.2 * 3.8
34.4 * 1.6 28.9 * 1.2
31.4 * 2.5
FSH 92-1
45.8 2 3.5
37.4 * 0.75
53 * 1.6
41 * 3.3
45 2 6.1
FSH 92-2
77.5 * 4.3
65 * 1.3
49.8 f 1.3
45 * 5.0
54.5 : 1.7
PSH 92-1
45.8 f 3.5
FSH 92-1
45.8 + 3.5
1
CAL-3 CAL-Z CAL-4
36.4 * 0.9
For each method these values were computed by taking * TB = total bleach. + R-P = partila bleach (30 min).
I
an average
FSH 92-3
,
66.4 k 1.1
of the equivalent
doses which
TB (7 hrs.1
0.75
37.4 * 0.75
I
56.2 f 1.1
,
73.7 * 3.9 ~~~
61.85 f 0.8
span the equivalent
dose plateau.
326
A.K. Singhvi et al./ Earth and Planetary Science Letters 139 (19%) 321-332
Glow Curves
for PSH 92-2(Sand)
20000 SL=700 min sun SS=30 min sun
100
(a)
200
300
400
500
Temperature (Y) R-b GROWTH CURVES FOR PSH 92-2
20000
(W1
320% t
-80
-40
0
40
Dose (Cy) TOTAL BLEACHGROWTH 20000
3
CURVE FOR PSH 92-2 (c)l
320%
15000
5 x 2 %
10000
2 2
-100
-50
0 Dose (Cy)
50
100
A.K. Singhui et al./
Earth and Planetary
Science Letters 139 (1996) 32 I-332
327
X-ray diffraction analysis of the samples were carried out at the Deccan College, Pune, using a RIGAKU D Max II VC XRD system, operated at 50 KV, 25mA, at a scan speed of l’/sec. The peak intensities, peak positions and half widths were computed using standard software.
4. Results and discussion Table 1 provides data on the radionuclide contents of the calcrete and the host strata. Table 2 provides the luminescence ages, the carbon and oxygen isotopic values along with previously reported radiocarbon ages [ 181. Table 3 provides the values of the individual equivalent doses for the different methods used in age estimation. Figs. 2 and 3, provide typical TL glow curves and growth curves for - 105-150 pm quartz extracted from host sands and calcretes. With the exception of CAL- 1, the luminescence ages are significantly higher compared to their radiocarbon counterparts. This is to be expected in view of the likelihood of post-depositional contamination of the radiocarbon samples by modem carbon. Some clues on the plausibility of luminescence ages can be found from climatological aspects. 4.1. Calcrete chronology: climatic consideration Characteristically, pedogenic carbonates form in regions of net annual moisture deficit with distinct seasonality and a precipitation upper limit of < 400 mm/a. At precipitation > 400 mm, the carbonate can be completely removed from the host strata [23]. Oxygen isotopic ratios of the Pushkar carbonates ranging from - 4.82%0 to - 6.57%0 (relative to PDB) can be compared with the oxygen isotopic composition of - 5 to - 6%0 (relative to SMOW) of modem meteoric waters [24]. This suggests that the carbonates analyzed were formed primarily from leaching and thus are related to monsoonal precipitation. The carbon isotopic values range from - 7.02 to - 8.57%~ which [25] implies a contribution of - 28-35% from C, type plants and up to 65% from C, type, indicating somewhat wetter conditions. Marine records from the Indian Ocean have reasonably established that during the last glacial maximum (LGM) at 21.5 ka (calender years), the monsoon circulation over the Indian subcontinent weakened significantly, resulting in marked aridity. The re-establishment of the monsoon began at - 17.8 ka [26,27] reaching its full vigour by - 11 ka. Similarly, on land, palynological studies and radiocarbon dating of the organic fraction of the sediments of a saline lake (Lake Didwana) in the Thar desert (with a climatic regime similar to that at Pushkar) [28,29] indicate hyper-arid conditions during 21.5 ka (LGM) to - 16.2 ka. In the period 16.2-11 ka, the monsoon circulation re-established itself and eventually reached twice its present value (i.e., > 600 mm/a) by 6.8 ka. Subsequently, during 6.8-4.8 ka the region witnessed the most humid phase of high rainfall followed by a phase of - 700 a of extended aridity, beginning at - 4.8 ka [28-301. Since pedogenic carbonates can only be formed when precipitation is limited, the paleomonsoonal reconstruction in Rajasthan [30] suggests that the periods when they could have developed at Pushkar should be - 17-10 ka, 6.8-4.8 ka and < 2.5 ka. The luminescence age brackets of 17 ka (Table 2) and 5 ka thus agree well with these estimates. 4.2. Methodological considerations: Future refinements An important methodological aspect involving dating is the mobility of potassium. The X-ray diffraction analysis indicates that the potassium-bearing phases are microcline, richterite, albite (patassian) and muscovite.
Fig. 2. (a) Glow curves for quartz separates from a typical host sand, PSH 92-2. SL and SS represent bleaching times of 700 and 30 min under the natural sun. (b) R-p
growth curve for quartz separates from PSH 92-2.
growth curve for quartz separates from PSH 92-2. Q = the equivalent dose at 320°C.
Q = the equivalent dose at 320°C.
(c) Total Bleach
TL Intensity
(a.~.)
TL Intensity
(a.~.)
TL Intensity
(a.~.)
329
A.K. Singhvi et al./ Earth and Planetary Science Letrers 139 (1996) 321-332
Table 4 p dose fraction from potassium for host sands and calcretes
Calcrete
I
Fraction of beta dose from potassium
Host sand
Fraction of beta dose from potassium
CAL-O
0.71
PSH 92-1
0.83
CAL-l
0.76
I’SH 92-1
0.83
CAL-3
0.80
PSH 91-l
0.83
CAl-2
0.82
I’SH 92-2
0.70
CAL-4
0.78
PSH 92-3
0.83
The presence of minerals such as microcline, the semi-arid nature of the region and low water-exchangeable potassium (5 50 ppm) suggest that K mobility is low and the potassium largely remains locked up in silicate phases. Consequently, the dose rate estimates can be taken as realistic. The dominance of the beta dose from potassium (Table 4) further indicates that the effect of a possible mobility of uranium also gets substantially diluted. The p range of - 2 mm further suggests that, with respect to /3 doses, the luminescence dating system closes over scale lengths of a few millimetres, an aspect that could allow examination of growth rates of carbonate nodules based on sequential leaching of grains from the core to its centre. Another important methodological aspect of this new approach is the total error in age, which gets compounded since the age is computed as a ratio of two differences. Two possible ways can be suggested, both of which aim at improving the accuracy of paleodose determination. The first possible approach is to eliminate the sun bleaching step in the estimation of Q, and Q, and use directly a difference of P, and P,. This can be done since the quartz mineral separates from both the sand and carbonate matrices have identical predepositional solar bleaching history. Computations using the unbleached paleodose differences P, - P, in Eq. (8) gave calcrete ages for four samples (CAL- 1, CAL-2, CAL-3 and CAL-4) which were comparable to ages calculated using R-/3 and total bleach methods within experimental errors. An alternative approach would be to use Optically Stimulated Luminescence (OSL) where, by definition, only the optically sensitive traps are probed [31]; that is, the OSL method does not sample the I, related signal. This, coupled to a luminescence normalisation procedure (termed short-shine normalisation) is also expected to yield lower experimental errors on P, and P, values. The advent of new approaches, such as the single aliquot method [32], also augurs well for this application as it would permit examination of small diffused nodules often present in arid zone sequences. We thus conclude with the optimism that in the coming years the luminescence methods for dating carbonates might develop further and perhaps be able to provide a major contribution to Global Change research by providing time scales of CO, fixation and release in terrestrial carbonates.
Fig. 3. (a) Glow curves for quartz separates from a typical calcrete, CAL-4. SL and SS represent bleaching times of 700 and 30 min under the natural sun. (b) R- /3 growth curve for quartz separates from CAL-4. Q = the equivalent dose at 350°C. (c) Total Bleach growth curve for quarts separates from CAL-4. Q = the equivalent dose at 350°C.
A.K. Singhvi et al/Earth
330
and Planetmy
Science Letters 139 (1996) 321-332
Acknowledgements The authors thank the Ford Foundation, India for financial support towards upgrading the TL Laboratory and Professors R.P. Dhir, R.J. Wasson and AS. Murray for suggestions. We thank J.T. Padia for the stable isotopic analyses. We are indebted to the two reviewers for their criticism, which helped in improving the scientific content and presentation of this manuscript. This research is a contribution to IGCP-349 on Paleomonsoons from Desert Margins. fUC1
Appendix A. Dosimetric changes due to loss of ?J
and/or
u2 Th
The calcrete age equation can be written as:
Q, - Qc Age
=
p~j,,~
+
&,s
+
&.s
-
PTh.c
-
kJ.e
change to&h.c
Now if PT~,~ad &
-
(A’)
&.c
and p;,, after precipitation of the nodule, the expression for the new
age is given by:
Q, - Q; pm,$ + p”,s + PK.S - &h,c
Age’ =
-
&J,c
-
W)
kc
Using the two equations above and assuming Q, - Q, (a reasonable approximation since - 80% of the p dose comes from K), the relative change in age becomes: A Age -= Age
Age - Age’ (A31
Age
given by the expression:
&h.c + P”,c- ( &h,c (
&,,s
+
p&s
+
&,s
-
PTh.c
-
&J.c
-
&c)
+
((
&J,c) PTh.c
+&kc>
-
(
%‘h.c
+&C))
& c &) is the original annual beta dose difference between the host sand ( &h,s + P”.s + &s PTh,c and the calcrete. For CAL-3 this difference is 0.56 mGy/a whereas &,,e and & are - 0.086 mGy/a and 0.084 mGy/a, respectively. Calculations using these values show that: (1) if 50% loss of either 238U or 232’lh occurs, the change in the age will be 7%; while (2) if 50% loss of both 238U and 232’Ih occurs, the age changes by 13%.
Appendix B B.I. Estimation of errors
If f is a function dependent on n parameters, xi, each with a standard deviation vi, then the variance in f, a-* is defined as: n
t?f
*=
u* -Sf ’ 6Xi
*
4 )
i-1
P1)
A.K. Singhui ef al. / Earth and Plunetary Science Letters 139 (1996) 321-332
331
The calcrete age is given by:
If UQ,, uQ,v uD,, CD,. and u&e are the standard deviations for Q,, Q,, and DC, D, and the calcrete age respectively then, using Eq. (2) and simplifying, we get: 2 aA$
=
Age’
4,
(Q,-a,>’
4
+ (Qs-Q,>’
2
on,
+ tJ%‘k,2
+ (D, - De)’
WI
Thus an evaluation of the standard deviation in age involves use of five equantities oQ,, oQ,, oD,, oD, and the calcrete age. Using Eq. (31, it can be shown that, if Q, - Q, is < 5 Gy and D, - DC< 0.5 mGy/a, measurement errors of Q and D values must be less than 5% for achieving a reasonable error limit ( < 20%) on the calcrete age. Another practical implication of Eq. (3) is that, with this luminescence method, optimum accuracy will be achieved in dating pedogenic carbonates where: (1) carbonate precipitation occured at least a few ka after the depositional event of the host matrix and (2) there exists a large annual beta dose difference between the host sand and the carbonate , - D,I > 2 mGy/a).
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