Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating

Review article

Chemostratigraphy of Neoproterozoic carbonates:implications for `blind dating'

Victor A. Melezhik,1* Igor M. Gorokhov,2 Anton B. Kuznetsov2 and Anthony E. Fallick3

1Geological Survey of Norway, Leiv Eirikssons vei 39, N-7491 Trondheim, Norway; 2Institute of Precambrian Geology and Geochronology,

Russian Academy of Sciences, Nab. Makarova, 2, 199034 St. Petersburg, Russia; 3Scottish Universities Environmental Research Centre,

G75 0QF East Kilbride, Glasgow, Scotland

Introduction

Over the last few decades, Neoprote-rozoic sedimentary formations havebeen investigated intensively for car-bon, oxygen, sulphur and strontiumisotope ratios. These studies weremotivated by several reasons, amongwhich deciphering the composition ofancient seawater and the search forregional stratigraphic correlationswere of great importance. A substan-tial database has been created, leadingto the reconstruction of temporaltrends in the carbon and strontiumisotopic composition of Neoprotero-zoic seawater. One result suggests thatwhereas the elemental composition ofseawater appears to have changedlittle during the past 545 Myr (Hol-land, 1984), the isotopic compositionof carbon, oxygen, sulphur and stron-tium in seawater has varied signi®-cantly through this interval (Kaufmanand Knoll, 1995). The present authors®nd these two statements somewhatinternally inconsistent, as both ele-mental and isotopic compositionsshould be closely linked via, and

controlled by, the same tectonic andbiogeochemical changes. However,the elemental composition of seawaterdoes not concern us here.A second result was the demonstra-

tion that carbon and strontium iso-tope signatures in Neoproterozoiccarbonate sequences can serve as im-portant tools for both regional andglobal correlations (Table 1). Re-cently, the temporal isotopic trendsof Neoproterozoic seawater have beenapplied for age determination andcorrelations of Neoproterozoic car-bonate sequences of the Una Group,Irece Basin, Brasil (Misi and Veizer,1998) and for indirect age determina-tions of nonfossiliferous, medium- tohigh-grade marbles in the NorwegianCaledonides (Trùnnes and Sundvoll,1995; Melezhik et al., 1997). These ageassessments of the Caledonian mar-bles constituted a `blind dating' sinceno additional stratigraphic controlswere available. During these Nor-wegian studies several problems wereexperienced that are addressed furtherhere. The main objective of this articleis not to resolve these problems, nor tocontribute a signi®cant amount of newdata, but rather to highlight someobstacles in the practical applicationof carbon and strontium isotopechemostratigraphy in the Neoprotero-zoic.

Temporal trends of carbon isotopes

Since Knoll and Walter (1992) andDerry et al. (1992) constructed the®rst d13Ccarb age curve for the Neo-proterozoic±early Cambrian timeinterval, at least seven other referencecurves covering various time intervalsof the Neoproterozoic have been pre-sented (Fig. 1a). It is readily apparentthat when the different age curves areplotted together they produce a ratherconfusing pattern. Below, this compli-cation is considered in terms of (i) theage of rocks, (ii) the age of articles,and (iii) possible spatial and temporalisotopic ¯uctuations in dissolved inor-ganic carbon at any given time.

Inconsistencies related to the ageof rocks

The time interval reviewed in thisarticle, 544±850 Ma, may be separatedinto three sections characterized by: (i)no agreement (except two points ofconvergence) between d13Ccarb agecurves published by different authors(interval 675±850 Ma); (ii) a relativelyhigh degree of agreement [interval c.610±675 Ma, Walter et al. (2000) dataare excluded]; and (iii) a very low levelof agreement ( interval 544±610 Ma).It is obvious (Fig. 1) that the degree

of coincidence between di�erent

ABSTRACT

The d13Ccarb and 87Sr/86Sr secular variations in Neoproteozoicseawater have been used for the purpose of `isotope stratigra-phy' but there are a number of problems that can preclude itsroutine use. In particular, it cannot be used with con®dence for`blind dating'. The compilation of isotopic data on carbonaterocks reveals a high level of inconsistency between variouscarbon isotope age curves constructed for Neoproteozoicseawater, caused by a relatively high frequency of both globaland local d13Ccarb ¯uctuations combined with few reliable agedeterminations. Further complication is caused by the unre-solved problem as to whether two or four glaciations, and

associated negative d13Ccarb excursions, can be reliably docu-mented. Carbon isotope stratigraphy cannot be used alone forgeological correlation and `blind dating'. Strontium isotopestratigraphy is a more reliable and precise tool for stratigraphiccorrelations and indirect age determinations. Combining stron-tium and carbon isotope stratigraphy, several discrete ageswithin the 590±544 Myr interval, and two age-groups at 660±610 and 740±690 Myr can be resolved.

Terra Nova, 13, 1±11, 2001

*Correspondence: Victor A. Melezhik,

Geological Survey of Norway, Leiv Eiriks-

sons vei 39, N-7491 Trondheim, Norway.

Tel: +47/73 904011; fax: +47/73 921620;

e-mail: [email protected]

Ó 2001 Blackwell Science Ltd 1

Table 1 List of references to the work with contributions to carbon and strontium isotope signatures in Neoproterozoic carbonate

sequences

Author(s) Area of study Time interval

Pre-1996 studies with limited reliable age constraints

Veizer et al. (1983) Africa and Australia 1000±600 Ma

Knoll et al. (1986) Svalbard and E. Greenland 900±550 Ma

Magaritz et al. (1986) Siberian Platform PC/C*

Tucker (1986a) S. Morocco PC/C

Aharon et al. (1987) Lesser Himalaya PC/C

Lambert et al. (1987) Yangtze Platform PC/C

Magaritz (1989) S. Siberia PC/C

Derry et al. (1989) Svalbard and E. Greenland Upper Proterozoic

Fairchild et al. (1990) Mauritania Neoproterozoic

Kaufman et al. (1991) NW. Canada 780±620 Ma

Kirschvink et al. (1991) Siberia, Morocco and S. China PC/C

Asmeron et al. (1991)

Pokrovsky and Vinogradov (1991) Central Siberia Upper Proterozoic

Aharon and Liew (1992) Compilation PC/C

Brasier (1992) SE. Newfoundland and England Early Cambrian

Derry et al. (1992) Namibia, Svalbard, E. Greenland and Central Australia 900±540 Ma

Kaufman et al. (1992) NW. Canada 1000±540 Ma

Knoll and Walter (1992) Compilation 630±500 Ma

Brasier et al. (1993) Siberia PC/C

Kaufman et al. (1993) Namibia, Svalbard, East Greenland and Arctic Canada 610±540 Ma

Pokrovsky and Gertsev (1993) S. Central Siberia PC/C

Pokrovsky and Missarzhevsky (1993) Central Siberia PC/C

Wickham and Peters (1993) W. North America 770±540 Ma

Corsetti and Kaufman (1994) E. California and W. Nevada PC/C

Brasier et al. (1994a,b) Siberia Lower Cambrian

Narbonne et al. (1994) NW. Canada Neoproterozoic

Smith et al. (1994) Idaho, USA Neoproterozoic

Wang et al. (1994) NW. China PC/C

Kaufman and Knoll (1995) Compilation Neoproterozoic

Knoll et al. (1995a,b,c) NW. Siberia (a) 1600±1200 Ma, (b), (c) PC/C

Post-1996 studies

Kaufman et al. (1996) N. Siberia PC/C

Kennedy (1996) Australia Ca. 600 Ma

Banerjee et al. (1997) India PC/C

Kaufman et al. (1997 Compilation PC/C

Kimura et al. (1997) N. Iran PC/C

Shields et al. (1997) W. Mongolia 750±545 Ma

Bartley et al. (1998) NW. Siberian Platform PC/C

Calver (1998) Tasmania 650±545 Ma

Hofmann et al. (1998) Compilation Neoproterozoic

Kah et al. (1998) Canada 1270±723 Ma

Kennedy et al. (1998) Congo craton 750±570 Ma

Pelechaty (1998) Siberia 565±545 Ma

Saylor et al. (1998) Southern Namibia 548±543 Ma

Jacobsen and Kaufman (1999) Compilation Neoproterozoic

Calver (2000) Australia Neoproterozoic III

Hill and Walter (2000) Australia � 830±750 Ma

Walter et al. (2000) Canada and Australia 840±544 Ma

*PC/C ± Precambrian/Cambrian boundary.

Fig. 1 d13Ccarb reference curves for Neoproterozoic seawater. The reference curves representing time interval (a) 850±500 Ma, (b)650±500 Ma, all data, and (c) 650±500 Ma, only data published after 1996. Blue arrows indicate age positions of the early (S3),middle (S2), and late (S1) Sturtian glaciations as well as of the early (V2), and late (V1) Varangerian glaciations by Jacobsen andKaufman (1999). Pink arrows indicate age positions of the early (V2), and late (V1) Varangerian glaciations by Pelechaty (1998).Red arrows indicate age positions of the early (S2) and late (S1) Sturtian glaciations as well as of the early (V2) and late (V1)Varangerian glaciations by Brasier et al. (2000). Green arrows indicate age positions of the Sturtian±Rapitan (V) and Marinoan±Varangerian (V) glaciations by Walter et al. (2000).

Chemostratigraphy of Neoproterozoic carbonates · V. A. Melezhik et al. Terra Nova, Vol 13, No. 1, 1±11.............................................................................................................................................................

2 Ó 2001 Blackwell Science Ltd

Terra Nova, Vol 13, No. 1, 1±11 V. A. Melezhik et al. · Chemostratigraphy of Neoproterozoic carbonates.............................................................................................................................................................


age±d13Ccarb curves depends on twomajor factors: (i) the availability ofreliable age determinations, and (ii)the frequency of d13Ccarb ¯uctuations.A high level of disagreement for thetime interval of 675±850 Ma has ap-parently been caused by a relativelyhigh frequency of d13Ccarb ¯uctuations(Fig. 1a) combined with but few reli-able U±Pb age determinations. Oneage is 746 � 2 Myr obtained on zir-cons for the Lower Naauwpoort For-mation in the northern Damara Belt,Namibia (Hoffman et al., 1996),which has been assumed to be amaximum age for the Sturtian±Rapit-an (as de®ned by Young, 1995) glacialdeposits. Another U±Pb age of723 � 3 Myr for baddeleyite fromthe rocks overlying the Shaler Group,Canada (Heaman et al., 1992) estab-lishes a minimum age constraint forone of the negative d13Ccarb excursions(Jacobsen and Kaufman, 1999). Inaddition, a U±Pb age of 777 � 7 Myrobtained from volcanites at or nearthe base of the Burra Group from theAdelaide Geosyncline in South Aus-tralia (Preiss, 2000) has been used toconstrain the age of the SkilogaleeDolomite (Hill and Walter, 2000). Thelimited number of age determinationsresults in a high degree of inconsis-

tency among different curves. Conse-quently, three(?) negative d13Ccarb

excursions reported from the timeinterval 675±850 Ma (e.g. Kaufmanand Knoll, 1995) have been assignedto different ages (Fig. 1a). As all thesenegative carbon isotope excursionshave been tightly connected to post-glacial events, it follows that their ageis completely dependent on the ageconstraints of the Sturtian glaci-ation(s). However, ongoing debates[Kaufman et al. (1997) and Hoffmanet al. (1998) vs. Kennedy et al. (1998)and Prave (1999)] have demonstratedthat it is unclear whether there wasone or three glaciations around Stur-tian time. Consequently, one shoulddemonstrate not only exact agesfor the Sturtian glaciation(s), but alsoprove that there were three negat-ive d13Ccarb excursions, and not one.Such an attempt has been made byBrasier et al. (2000), who reportedtwo new ages, 723 + 16/±10 and544 � 3.3 Myr, from Oman. The ®rstwas assigned to a Sturtian age. Brasieret al. (2000; p. 178) reported that `therevised chronology con®rms four neg-ative d13C excursions between c. 723and 543 Ma, of which the lower twoare associated with glaciations'. Thus,the date of the Sturtian glaciation

(723 + 16/±10 Ma; Brasier et al.2000) has been re®ned. The latestresearch on and correlation betweenCanadian and Australian Neoprote-rozoic sequences (Walter et al., 2000)has revealed only two major glacia-tions, namely Sturtian (Rapitan gla-cials) at about 700 Ma, and Marinoan(Ice Brook glacials) at around600 Ma; a possible minor glaciationat 570 Ma has been recognized.The section of convergence, or a

`plateau', placed between c. 610 and675 Ma, initially appears to be ®rmlyestablished, though con¯icting withthe curve reported by Walter et al.(2000). However, this ``isotopic plat-eau'' has not yet been supported byany reliable age determinations. Con-sequently, the younger and partic-ularly the older age limit for thissection of convergence still requirede®nition (Fig. 1a).The younger Neoproterozoic sec-

tion (between 544 and 610 Ma;Fig. 1b) has been supported by tworeliable U±Pb date determinations onzircons: 543.9 � 1 Ma for the Pre-cambrian/Cambrian boundary and548.8 � 1 Ma for a late Vendian,postglacial, positive d13Ccarb excursionand ®rst global appearance of Ediac-aran fossils (Bowring et al., 1993;

Fig. 2 Temporal variations of 87Sr/86Sr in Neoproterozoic carbonates. Sources of data: Asmerom et al. (1991), Burns et al. (1994),Denison et al. (1998), Derry et al. (1989, 1992, 1994), Jacobsen and Kaufman (1999), Kaufman et al. (1993), Kuznetsov (1998),Walter et al. (2000).



Grotzinger et al., 1995). A series ofstudies has demonstrated the pres-ence of three negative d13Ccarb excur-sions occurring sometime between544 and 610 Ma (Fig. 1b). However,there is a very low level of agreementon age constraints amongst variouspapers. The problem of whether therewas one or three negative d13Ccarb

excursions, separated by a series ofreturns to highly positive d13Ccarb

values, return us to the number ofglacial episodes: three (e.g. Hoffmanet al., 1998) or only one (e.g. Ken-nedy et al., 1998; Prave, 1999). Theresult of this debate is a very confu-sing age±d13Ccarb curve pattern asillustrated by Fig. 1(b).Finally, considering the number of

glaciations, it is worth pointing outthat the only evidence for multipleglaciations is from chemostratigraphy.There is no rock sequence bearingrobust evidence for more than twototal Neoproterozoic glaciations (e.g.Kennedy et al., 1998; Prave, 1999;Condon and Prave, 2000).

Inconsistencies related to the timeof investigations

Articles devoted to d13Ccarb referencecurves may be separated into twogroups. The ®rst group, including allstudies performed prior to 1996(Table 1), should be considered as lessreliable, as the reference d13Ccarb

curves were characterized by poorage constraints. The ®rst reliable ageconstraints for the time interval of544±850 Ma were obtained in 1993±96(Heaman et al., 1992; Bowring et al.,1993; Grotzinger et al., 1995; Hoff-man et al., 1996). Consequently, thesecond group of papers, publishedafter 1996 (Table 1), might be expec-ted to show more agreement. How-ever, this latter group actually showsvery few points of convergencebetween various d13Ccarb curves(Fig. 1a,c). The reference d13Ccarb

curve for 550±580 and 600±670 Ma(Fig. 1), reported by Kah et al. (1998),perfectly matches the curve appearingin Jacobsen and Kaufman (1999).However, these two curves are totallyin con¯ict between 670 and 800 Ma(Fig. 1). As they were based essen-tially on the same database, it is notpossible, at this stage, to account forthe discrepancy. Two other referenced13Ccarb curves generated for 540±

590 Ma ± one by Pelechaty (1998),and the other by Saylor et al.(1998) ± exhibit good agreement(Fig. 1c). However, both of them con-¯ict with the interpretation of Jacob-sen and Kaufman (1999) and Kahet al. (1998). This kind of discrepancyleads us again to the problem ofNeoproteozoic glaciations. It can beexempli®ed by the disagreement be-tween Jacobsen and Kaufman's (1999)interpretation, on the one hand, andthe data reported by Pelechaty (1998),on the other hand. Different ageassignments to the younger Varange-rian glaciation in these two contribu-tions (Fig. 1c) are automaticallyre¯ected in a 20-Myr-shift for one ofthe negative d13Ccarb excursions.

Inconsistencies related to spatialand temporal isotopic ¯uctuationsin dissolved inorganic carbon

Particularly problematical, and ad-dressed by Veizer (e.g. Misi and Vei-zer, 1998; Veizer et al., 1999), are thecomplex and unclear reasons for thequite large variations in d13Ccarb atany given time. In the Phanerozoic,and even today, d13Ccarb ¯uctuates bythe order of 5& (Veizer et al., 1999).This important fact has apparentlyrarely been considered as a potentialcause for the discrepancies observed inthe Neoproterozoic database. For in-stance, the section of convergence, orthe 610±670 Ma isotopic plateau, ex-hibits no internal, age-related ¯uctua-tions of d13Ccarb. However, the upperd13Ccarb limit of the section haschanged (Fig. 1a) from + 6& (Derryet al., 1992) to + 7.5& (Kaufmanand Knoll, 1995) through + 8.5&(Jacobsen and Kaufman, 1999) toc. + 12& (Brasier et al., 2000). Thiscould well be caused by a spatialisotopic variation in dissolved inor-ganic carbon in¯uenced by local fac-tors. Consequently, the globalseawater d13Ccarb value, equilibratedwith atmospheric CO2, might not yethave been unambiguously established.Another example is the remarkably

variable magnitude of both negativeand positive d13Ccarb excursions, asevident from the comparison of dif-ferent databases. For instance, thenegative d13Ccarb excursion occurringat the post-earlier Varangerian ice agehas been estimated at ±2.5& (Derryet al., 1992; Pelechaty, 1998), ±3&

(Kaufman and Knoll, 1995), ±4&(Saylor et al., 1998), and ±5& (Jacob-sen and Kaufman, 1999). Pokrovskyand Gertsev (1993) have even reportedd13Ccarb as low as ±8 to ±10&. Thelatter values were obtained from a250±1000 m-thick, well-preserved car-bonate formation developed over adistance of 1000 km. All these incon-sistencies might well be caused bylocal isotopic variations in dissolvedinorganic carbon, and therefore therepresentative global d13C values re-main unclear. Moreover, the problemof stratigraphic applications of strong,basin-wide 13C depletion recorded inmarine carbonates has been discussedin detail by Calver (2000). Based on astudy of the Adelaide Rift Complex inAustralia, he concluded that `thestrong overprint of strati®cation onboth organic and carbonate d13C stra-tigraphy needs to be recognized beforeglobal correlation¼ can be attempted'(Calver, 2000, p. 140). He also arguedthat d13Ccarb values (ranging from ±3to ±10&) obtained from the WonokaFormation do not mirror a globalseawater signal. Yet, d13Ccarb of)10.5& appeared in a compilationby Walter et al. (2000; Fig. 29, p. 414)as a marine d13Ccarb at 545 Ma.A series of d13Ccarb negative shifts

could be advantageous in carbon iso-tope stratigraphy, providing a poten-tial tool for detailed stratigraphiccorrelations. However, all puzzlingaspects of the Neoproterozoic negat-ive d13C excursions are compoundedby the enigmatic nature of postglacial`cap carbonates' (Walter and Bauld,1983; Tucker, 1986b; Singh, 1987;Fairchild, 1993). With the existinglack of consensus on the nature ofthe Neoproterozoic cap carbonates,the stratigraphic implication ofd13Ccarb negative shifts is largely amatter of debate.Grotzinger and Knoll (1995) sug-

gested an interesting interpretation:the postglacial cap dolostones andlimestones in the Neoproterozoicmight well be related to an early stageof overturn of the strati®ed ocean.They suggested further that the car-bonates were associated with the up-welling of deep, anoxic, 13C-depleted,HCO3

±-saturated water supplementedby that from perpetually Ca- and Mg-supersaturated shallow water. Severalconsequences follow. First, althoughupwelling of deep water was a



large-scale phenomenon it was not aglobal one. There must have been vastshelf zones, inter- and intracontinentalseas which were not affected by thedeep, anoxic, 13C-depleted, upwelledwater. Secondly, it is unlikely thatcarbonates precipitated from upwelledwater were in isotopic equilibrium withthe coeval atmosphere and surfacewater. Thirdly, the isotopic composi-tion of such carbonates should exhibita wide range depending on deep water/surface water ratios (e.g. Kennedy,1996). Consequently, the correlationof postglacial carbonates by means ofcarbon isotope chemostratigraphy,which is solely based on negative d13Cvalues, could be totally misleading.

Neoproterozoic trends of strontiumisotopes

Veizer and Compston (1976) reporteda ®rst seawater 87Sr/86Sr curve for thewhole of the Precambrian and dem-onstrated a sharp increase in the87Sr/86Sr ratio of seawater between2.5 and 2.0 Ga. Later, more detaileddata obtained for the Neoproterozoic(Veizer et al., 1983) exhibited low87Sr/86Sr values at around 900 Ma.This was assigned to the ocean'sresponse to a major hydrothermalevent. A substantial amount of newisotopic data has been generatedthrough the years by Derry et al.(1989, 1992, 1994), Asmerom et al.(1991) and Kaufman et al. (1993,1996). This database has recently beenscreened for postsedimentary alter-ation, and age constraints which `sig-ni®cantly improves the resolution ofthe 87Sr/86Sr record for the late Neo-proterozoic to Early Cambrian' (Jac-obsen and Kaufman, 1999; p. 44).However, the Neoproterozoic±earlyCambrian time interval can usefullybe divided into two sections. Theyounger section demonstrates amarked rise in the 87Sr/86Sr from0.7066 at 590 Ma up to 0.7085 at thePrecambrian±Cambrian boundary(Fig. 2), which seems to be well estab-lished. In contrast, the 87Sr/86Sr tem-poral trend for the older section,spanning from 850 to 590 Ma, hasno consensus (Fig. 2). Jacobsen andKaufman (1999) have already dis-cussed the inconsistency between theirdatabase and that reported from theKaratau Group which terminatesthe Riphean stratotype section on the

western slope of the southern Urals(e.g. Kuznetsov et al., 1997; Kuznet-sov, 1998). The 87Sr/86Sr ratios ob-tained from 890 to 670 Myr-oldcarbonate formations of the KaratauGroup are uniquely low, ranging be-tween 0.7053 and 0.7062, whereas thedata compiled by Jacobsen and Kauf-man (1999) exhibit far more radiogen-ic values (Fig. 2).Below, these discrepancies are

considered in terms of (i) the ageconstraints, (ii) the depositionalenvironments, (iii) the screening forpostdepositional alteration, and (iv)the laboratory techniques.

Inconsistencies related to the ageconstraints

It is apparent that the discrepanciesassociated with the older time intervalof 850±590 Ma, which is representedby the Shaler, Akademikerbreen, Po-larisbreen, and Karatau groups, mightwell be caused by the lack of reliableage determinations.The Shaler Group, from Victoria

Island in Canada (Asmerom et al.,1991) lies unconformably on a sequ-ence containing 1.2-Gyr-old lavas. Itis overlain unconformably by ba-saltic lavas dated by the U±Pb methodon baddeleyite to be 723 � 3 Myrold. The palaeomagnetic data for theMackenzie Supergroup, correlatedwith the Shaler Group, indicate thatthe latter is younger than 880 Myr(Asmerom et al., 1991 and referencestherein). More recent isotopic workhas demonstrated that the maximumage of Shaler Group carbonate rocksis not more than 802 � 10 Myr (Jac-obsen and Kaufman, 1999 and refer-ences therein). Although Jacobsen andKaufman (1999) have re®ned the ageof Shaler carbonate rocks and as-signed it to 740 Myr, they largely usedcircular arguments involving the car-bon and strontium isotopic trendsfor the correlation. Moreover, in therecent publication by Walter et al.(2000) the age of the Shaler Groupcarbonate rocks (840±834 Myr,table 7, p. 407) was adopted fromAsmerom et al. (1991). As far as theAkademikerbreen and Polarisbreengroups are concerned, there has beenno isotopic age published yet. It isonly known that the Akademikerb-reen Group rocks contain acritarchsof latest Late Riphean age, and the

shales at the base of the PolarisbreenGroup might be a correlative of theVaranger tillites in northern Norway(e.g. Knoll et al., 1986). However, theage estimate of the two-stage Varan-gerian glaciation determined by theRb±Sr method on ®ne clay mineralsfrom the Norwegian tillites exhibits awide range of ages from 630 to560 Myr (Gorokhov et al., 1996).The geochronological baseline for

the 850±670 Ma interval of Kuznet-sov's 87Sr/86Sr curve (Fig. 2) is basedon the Pb±Pb and Rb±Sr dates for thecarbonate and siliciclastic rocks of theKaratau Group consisting of (bottomto top) the Inzer, Minyar and Ukformations. Two Pb±Pb dates of836 � 25 and 780 � 80 Ma on lowerInzer Formation limestone (Ovchin-nikova et al., 1998) and middleMinyar Formation dolostone (Ovchi-nnikova et al., 2001) provide deposi-tional age constraints for the lowerpart of the Karatau Group. Both theages and the 87Sr/86Sr values areroughly consistent with the ShalerGroup data (Jacobsen and Kaufman,1999).However, the upper portion of the

Karatau Group, with an inferred ageof 760±670 Myr, speci®cally demon-strates low 87Sr/86Sr values (Fig. 2),thus con¯icting with the previouslyreported data (see Jacobsen andKaufman, 1999). The age of the UkFormation based on Rb±Sr and K±Artechniques on glauconites has beenconstrained to 688 � 10 and 670 �10 Myr, respectively (Gorozhanin andKutyavin, 1986). Podkovyrov et al.(1998), however, inferred severalstratigraphic hiatuses of unknownduration in the Karatau carbonatesuccession and thus opened the possi-bility that the geological record of theKaratau succession is not complete.At the same time, there is a paucity ofstrontium isotopic data from the timeinterval 690±600 Ma (Fig. 2). Jacob-sen and Kaufman (1999) have notreported any 87Sr/86Sr data frombetween 690 and 650 Ma, and thusKuznetsov's data (0.07054±0.07060)are the only isotopic record availablefor this period of time.

Inconsistencies related to depositionalenvironments

Sr isotopic ratios measured fromsedimentary carbonates re¯ect mixing



of strontium delivered by continentalruno� and that derived from sea-water±basalt interaction (e.g. Veizer,1989). Therefore, transitions fromopen marine to restricted marine tolacustrine environments exhibit regu-lar changes in 87Sr/86Sr: restricted andlacustrine carbonates become enrichedin 87Sr. There are some exceptions: ina certain type of rift basin ¯oored bybasalts (e.g. Red Sea, Friedman, 1996)shore-associated carbonates may bedepleted in 87Sr as compared to coevalcarbonates precipitated from well-mixed seawater. However, examplesof unambiguous resolution betweenmarine vs. nonmarine environmentsin transgressive/regressive carbonatesequences have been reported fromboth the Cenozoic (McCulloch et al.,1989) and the Palaeoproterozoic(Gorokhov et al., 1998; Kuznetsovet al., 1999). It is not entirely clearwhether all Neoproterozoic carbonateformations studied for strontium iso-topes were deposited in open marinebasins. As far as the Karatau Group isconcerned, the carbonate rocks of theInzer, Minyar and Uk formationswere accumulated in relatively deep-water marine and shallow-marinesettings. No evidence of evaporiticsediments has been found in theKaratavian carbonate sequences(Maslov, 1997). In contrast, the lowerpart of the Shaler Group includesseveral horizons of deltaic sediments,and the upper part contains evaporiticintercalations, restricted coastal car-bonates and red beds (Asmerom et al.,1991 and references therein). Akadem-ikerbreen Group carbonate sedimentsalso might have been precipitated in apartly restricted basin, as they areenriched in Mn relative to the highbackground Sr content, and includecalcian dolomites (Derry et al., 1989,1992). Based on these facts, theKaratau carbonate sequences are byfar the best match with deposition inan open marine setting.

Inconsistencies related to thescreening of carbonate material

Conventional geochemical assessmentfor postdepositional alteration of car-bonate is based largely on the relativeabundances of Mn, Fe, Rb and Sr(e.g. Brand and Veizer, 1980; Gorok-hov, 1996). Elemental ratios, such asMn/Sr, Fe/Sr, Ca/Sr andRb/Sr, as well

as carbon and oxygen isotopes, arewidely used as geochemical criteria forrevealing the least disturbed Rb±Srsystems. Di�erent authors, however,use not only dissimilar values of thesame ratios, but also dissimilar combi-nations of these ratios. Asmerom et al.(1991) used Mn/Sr < 1.5 and Rb/Sr < 0.004 to identify altered samples,whereas Derry et al. (1992) suggestedfor Mn/Sr < 1, Rb/Sr < 0.002, andCa/Sr < 1000. Kaufman et al. (1993)suggested Mn/Sr < 1.5 and Rb/Sr < 0.0005, whereas Kuznetsov et al.(1997) and Semikhatov et al. (1998)advocated Mn/Sr £ 0.2, Fe/Sr £ 5.0and Rb/Sr £ 0.0010. In all cases thechoices of the elemental ratios and theirvalues are empirical and arbitrary.However, the elemental ratios dependnot only on the extent of postsedimen-tary alteration, but also on the initialcarbonate composition. The conse-quences of different criteria appliedfor the recognition of altered samplescan be exempli®ed by the following. IfMn/Sr £ 0.2 (as suggested by Kuznet-sov et al., 1997) is applied for ®ve 609±610 Myr-old limestone samples fromSvalbard andEastGreenland (Table 1,p. 416, Kaufman et al., 1993), onlytwo, each having 87Sr/86Sr of 0.7066,would pass this test. Yet the `bestpreserved' 610 Myr-old samples (asbased on other criteria) shown byJacobsen and Kaufman (1999) have87Sr/86Sr of 0.7074. Thus, it is believedthat at least some of the inconsistenciespresented in Fig. 2 might well be rela-ted to different approaches applied togeochemical screening.The work by MontanÄ ez et al. (1996)

documenting an elaborate screeningapproach is considered to representstate-of-the-art in isotopic studies ofancient carbonates. Another relatedand serious problem is that only a verylimited number of samples per timeunit (very often a single sample) hasbeen used in most studies of recon-struction of 87Sr/86Sr ratios in Neopro-terozoic seawater. Evidently, a limitednumber of samples is seldom suf®cienteven to recognize alteration trends andtherefore should not be used for thereconstruction of 87Sr/86Sr ratios.

Inconsistencies related to the laborat-ory processing of carbonate material

Many authors who measured stron-tium isotopes on marine carbonate

rocks have used a preliminary labor-atory leaching of carbonate materialfor removal of late, recrystallized car-bonate phases and/or contaminatingconstituents (Gao, 1990; Kupecz andLand, 1991; Ohde and Elder®eld,1992; Gorokhov et al., 1995; Kuznet-sov et al., 1997; Semikhatov et al.,1998; McArthur et al., 1993a, 1993b).Acetic acid, ammonium acetate andchloride are the reagents commonlyused. Details of the technique havebeen discussed by McArthur, 1994).Although some scientists (Gao, 1990;Kupecz and Land, 1991) use leachingstrictly for removal of the Sr bound toclay components, others (Gorokhovet al., 1995) suggest this procedure forremoval of both clay-bound Sr andlate, 87Sr-rich, calcite cement. The 1 NN

ammonium acetate procedure appearsto involve the most appropriate leach-ing reagent: it removes not only theclay-bound Sr (Morton, 1985; Gorok-hov et al., 1994), but also late calcitephases (Wada and Furumura, 1994).An experimental study has shown that87Sr/86Sr values, obtained from Riph-ean nonmetamorphosed limestones byselective leaching, are lower by0.0002±0.0019 than values measuredfrom bulk carbonate componentswithout leaching (e.g. Gorokhovet al., 1995). Thus, Sr isotopic valuesobtained with the leaching procedureare less enriched in 87Sr and likely thebest proxy for initial seawater compo-sition. Therefore, the inconsistenciesexposed in Fig. 2 should also beconsidered in terms of various labor-atory techniques.

Concluding remarks: ef®cacyof isotope stratigraphy

The compilation presented in Fig. 1demonstrates that, in general, it is veryunlikely that carbonate formationsdeposited between 850 and 544 Macan currently be correlated unambig-uously or dated indirectly by means ofcarbon isotope stratigraphy if noother independent stratigraphic orisotopic (i.e. 87Sr/86Sr ratios) controlsare provided (`blind experiments'). If acarbonate sequence is characterizedby negative d13Ccarb (to less than)4&), this may provide an approxi-mate age restriction: it could not havebeen deposited between 670 and610 Ma, or even as long ago as700 Ma providing that the 680 Ma



assignment for the negative d13Ccarb

excursion, which appeared in Kahet al. (1998), is an artefact or error(Fig. 1a). Rather, it might haveformed either at around 740±700 Ma(Sturtian glaciation) or between 590and 540 Ma (late to post-Varangerianglaciation) (Fig. 1a). These alterna-tives may be further resolved bymeans of `strontium isotope stratigra-phy' (see discussion below). d13Ccarb

values ¯uctuating around zero mightbe found throughout the 850±544 Mainterval, whereas d13Ccarb > + 5&indicates that the carbonates mighthave been deposited between 670 and610 Ma.Although the causes of the 87Sr/86Sr

inconsistencies in Fig. 2 for 760±670 Ma are not fully understood,one should prefer the lower 87Sr/86Sr curve (e.g. Veizer and Comps-ton, 1974) unless the inconsistenciesinvolve improper age constraints(Fig. 2). Under these conditions stron-tium isotope data, even when usedalone, can clearly discriminate car-bonate formations deposited prior to670 Ma (87Sr/86Sr £ 0.7063) fromthose formed between 670 and544 Ma (87Sr/86Sr ³ 0.7063, Fig. 2).Moreover, as the result of a progres-sive and rather uniform increase of87Sr/86Sr from 0.7061 to 0.7085, sev-eral discrete age-groups of carbonateformations may be resolved within thetime interval 590±544 Ma (Fig. 2).The combined application of

strontium and carbon isotope datamay further resolve the age uncer-tainties for the low-d13Ccarb carbon-ates (see above). Low strontiumisotope ratios (< 0.7063) combinedwith low-d13Ccarb values are indicat-ive of an age > 670 Myr, whereas87Sr/86Sr > 0.7061 would place theage of the rocks between 590 and544 Myr.Thus, following an earlier statement

by Veizer (e.g. Misi and Veizer, 1998),it is concluded herein that, at thepresent time, strontium isotope strati-graphy provides a more de®nitive andprecise tool for stratigraphic correla-tions and indirect age determinationscompared to the d13Ccarb approach.

Some speci®c steps recommendedin future isotope stratigraphy studies

The current practice in isotope studiesof Precambrian carbonates rarely

includes thorough investigation anddocumentation of depositional envi-ronments. As contemporaneous openmarine, restricted marine, lagoonal,and lacustrine carbonates may becharacterized by very di�erent isotop-ic compositions, the study of deposi-tional environments and palaeofaciesanalysis are strongly suggested to bean essential part of all isotopic studiesattempting to reconstruct the compo-sition of seawater.It is already apparent that the

isotopic composition of carbonates issubject to considerable internal vari-ability, and therefore the isotopic dataobtained from any single sectionshould not be used for the reconstruc-tion of the seawater composition untilit is established, by comparison with aseries of coeval distant sections, to berepresentative for the time intervalstudied.Without exceptions, all ancient car-

bonates experienced some degree ofpostdepositional alteration and modi-®cation of carbon, oxygen and stron-tium isotope systems. Given thecondition that all necessary geochem-ical, mineralogical, and petrologicalprecautions have been undertaken toidentify the alteration trends, a limitednumber of samples and analyses areconsidered to be insu�cient for therecognition of alteration trends andthus such databases should not beused for the reconstruction of theseawater composition.

Acknowledgements

This work was supported by the GeologicalSurvey of Norway (Project 270500), theRussian Foundation of Basic Research(Grants 99±05±65181 and 00±05±64915for IG and AK), and the Scottish Univer-sities Environmental Research Centre,Glasgow. é. Nordgulen is acknowledgedfor comments on an earlier draft. M.Schidlowski and L. Derry, the o�cialreferees, are thanked for providing com-ments that improved the manuscript.

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Received 18 May 2000; revised versionaccepted 5 April 2001



Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating

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