Global carbon isotopic events associated with mass extinction and glaciation in the late Ordovician

Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 195–210

Global carbon isotopic events associated with mass extinction andglaciation in the late Ordovician

James D. Marshall a, Patrick J. Brenchley a, Paul Mason a, George A. Wolff a,Ricardo A. Astini b, Linda Hints c, Tonu Meidla d

a Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool, L69 3BX, UKb Catedra de Estratigrafıa y Geologıa Historica, Universidad Nacional de Cordoba. Av. Velez Sarsfield 299, CC 395,

5000 Cordoba, Argentinac Institute of Geology, Estonian Academy of Sciences, 7 Estonia Avenue, EE0100 Tallinn, Estonia

d Geology Department, University of Tartu, EE2400 Tartu, Estonia

Received 7 March 1995; accepted 19 March 1997

Abstract

Mass extinctions and glacioeustatic sea-level changes in the lower part of the Hirnantian (final stage of the Ashgill )are accompanied by shifts in marine stable-isotope compositions. Previously published stable-isotope changes havebeen used to identify the onset and demise of the Gondwana glaciation and to suggest relationships between bioticchanges and carbon cycling within the oceans. However, the existing isotopic data set had limitations because it wasderived from Ordovician low-latitude settings and from carbonates or organic carbon in separate areas. We reportnew data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate d13Cfrom Baltica.

Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated d13C values similar tothose described previously from low-latitude sites. The new data demonstrate that the positive Hirnantian d13Cexcursion, previously only recognised from low-palaeolatitude areas, was widespread and probably global in extent.The poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygenisotopic value from the same material.

Preliminary carbon isotopic data from thermally immature organic matter from Estonia provide the first indicationof a synchronous shift in organic and inorganic d13C in sediments from the same basin. This work provides new dataof critical importance for constraining models of end-Ordovician palaeoceanography and climate change. © 1997Elsevier Science B.V.

Keywords: stable carbon isotopes; isotope stratigraphy; late Ordovician

1. Introduction 1988). Our previous isotopic studies, based largelyon the analysis of brachiopods from Baltica(Sweden and Estonia) and Laurentia (NorthThe end Ordovician was marked by two phasesAmerica), have identified a positive carbon andof extinction that coincided with the major sea-oxygen isotope excursion associated with thelevel changes associated with the development and

demise of Gondwanan glaciation (Brenchley, Hirnantian glacioeustatic regressive interval lasting

0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.PII S0031-0182 ( 97 ) 00063-1

196 J.D. Marshall et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 195–210

an estimated 1 million years or less (Marshall and and Latvia demonstrate a marked positive shift inorganic carbon d13C that parallels shifts in brachio-Middleton, 1990; Brenchley et al., 1994, 1995).

Typical late Ordovician, pre-Hirnantian marine pod compositions from the same basin.carbonates have d13C values in the range −1to +3‰ (PDB) whilst lower Hirnantian(extraordinarius/bohemicus Zone) brachiopods 2. Late Ordovician stratigraphyfrom Sweden and Estonia and marine cementsfrom Sweden are characterised by d13C values > The Hirnantian Stage is the uppermost stage of

the Ordovician and until 1985, its lower boundary+4‰ with some as high as +7‰. A marked butless pronounced positive shift in brachiopod calcite coincided with environmental changes associated

with the onset of global regression and its uppercomposition was also found in the Anticosti Islandsuccession in Canada (Long, 1993; Brenchley et al., boundary coincided with a marked transgression

(Brenchley, 1988). The boundaries also coincided1994). Wang et al. (1993, 1997-) have recentlydocumented a +2 to +7‰ positive excursion in with the limits of the isotope excursion described

in this paper. The repositioning of thethe carbon isotopic composition of Hirnantianorganic matter from relatively deep-water Chinese Ordovician–Silurian boundary in 1985, one zone

higher than previously, to the base of the acumina-black shale sections that apparently coincides withthe events that we have recognised in the carbon- tus Zone (Cocks, 1985), accordingly raised the top

of the Hirnantian Stage (Fig. 2). The consequenceates. d18O values in well-preserved brachiopodsfrom Baltica and Laurentia shift from pre- is that both the Ordovician–Silurian boundary and

the upper boundary of the Hirnantian are oneHirnantian values around −4‰ to values between−2 and 0‰ in the lower Hirnantian (Brenchley zone above the pronounced environmental and

isotopic changes. There is now some danger ofet al., 1994). The shift in d18O values are thoughtto correspond to a combination of ice-volume confusion when reference is made to the

Hirnantian. In most literature prior to the lateeffects and temperature decrease during the short-lived glacial event whilst the changes in carbon 1980s, the late Ordovician environmental changes

were referred to as Hirnantian, whereas now theyisotope values have been related to major changesin carbon cycling in the world’s oceans (Brenchley should be regarded as falling in the lower part of

the Hirnantian. In this paper we informally useet al., 1995). Although the published data comefrom three disparate geographical areas, suggesting the ‘‘lower Hirnantian’’ to refer to the ‘‘old’’

Hirnantian and ‘‘upper Hirnantian’’ to refer to thea global event, all three come from relatively low-palaeolatitude sites (Fig. 1). An understanding of ‘‘new’’ part of the Hirnantian which is approxi-

mately equivalent to the persculptus Zone (Fig. 2).ocean carbon cycling may be constrained by paral-lel investigation of carbonate and organic carbonisotopes from the same section. Although thepublished data include analyses of both carbonates 3. Sampling and analysisand organics, they are from widely separatedlocalities. 3.1. Aims and strategy

The aim of this study was to redress shortcom-ings in the published data by: (1) analysing shell Sampling for this study was carried out solely

to determine primary marine isotopic signatures:carbonate from a high-latitude site; and (2) investi-gating organic matter from a basin where the from brachiopods in the Argentinian samples and

from organic carbon in the Baltic material.isotopic excursion had already been documentedin carbonates. This paper presents the first account Brachiopods are composed of low-Mg calcite

and are thought to precipitate in isotopic equilib-of elevated brachiopod carbon isotopic composi-tions from a high-palaeolatitude, periglacial setting rium with seawater. Diagenetic alteration of iso-

topic signals in carbonates can be detected by(Argentina) and lead us to conclude that changeswere truly global. Preliminary data from Estonia petrographic methods and by examining patterns

197J.D. Marshall et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 195–210

Fig. 1. Palaeocontinental reconstruction for the Ashgill (after Scotese and McKerrow, 1990) showing distribution of sites where highd13C have been documented. 1=central Sweden (Marshall and Middleton, 1990); 2=Baltic states (Brenchley et al., 1994 and thisstudy); 3=Anticosti Island, Canada (Long, 1993; Brenchley et al., 1994); 4=Argentina (this study); 5=Yichang, Hubei province,China (data reported in Wang et al., 1993).

of covariation of isotopic values and trace-element carbonate from the basin (Brenchley et al., 1994)was also accompanied by a change in the isotopicconcentrations. However, even if material has been

altered by diagenetic cementation or recrystallisa- composition of the organic fraction. The Balticshelf succession only contains sufficient organiction it may be possible to ‘‘backstrip’’ the effects

of alteration and derive a primary or least-altered carbon for analysis at a relatively restricted numberof localities and stratigraphic intervals. Samplingcarbonate composition (see review by Marshall,

1992). The brachiopods sampled come from a was accordingly targeted on four key stratigraphicintervals, which together form a compositesingle stratigraphic horizon which was sampled at

two localities. stratigraphic succession.Stratigraphic variation in the carbon stable-

isotopic composition of bulk organic matter can 3.2. Sampling brachiopods — Argentinabe used to indicate changes in the composition ofprimary organic accumulation. In ancient sedi- Thin sections of rock samples were examined in

transmitted light and using a cold cathodolumines-ments, the kerogenous (or non-extractable) frac-tion is more likely to preserve a primary record cence microscope with the aim of visually assessing

the general preservation state of the brachiopods.than the bituminous (or extractable) fraction whichmay contain migrated hydrocarbons. d13C deter- Samples for geochemical analysis of brachiopod

calcite were taken from valves of Hirnantiaminations on organic carbon from the Baltic suc-cessions in Estonia and Latvia were carried out in sagittifera and a dalmanellid (probably Dalmanella

testudinaria). These were removed from the cleanedorder to determine whether a positive carbonexcursion previously documented in brachiopod surfaces of rock samples using a needle. Nine


Fig. 2. Late Ordovician and Early Silurian stratigraphy showing the relationship between the standard graptolite zonal scheme, theposition of the Ordovician–Silurian boundary and the postulated relationships between bathymetric changes and isotope values (datasummarised from Brenchley et al., 1994).

microsamples (~2 mg) were initially taken for 3.4. Trace-element analysisisotopic analysis. Following the result of theiranalysis and the wide range of compositions Carbonate samples for trace-element analysis

were completely digested in 1 N HCl and analysedencountered, ten larger samples (~15 mg) weretaken for combined isotopic and elemental analy- on a Jobin-Yvon JY24 sequential ICP OES instru-

ment. Results for Mg, Mn, Sr, and Fe were cal-sis. Approximately 2 mg of the resulting powderwere used for isotopic analysis; the remainder ibrated using matrix-matched standard solutions;

analysis of duplicates suggests precisions of better(typically 5–10 mg) was used for trace-elementanalysis. than ±5% for the reported values.

3.5. Sampling and analysis — Organics3.3. Carbonate stable-isotopic analysis

Powdered carbonate samples were completely Bulk-rock samples (~50–100 g) for organicanalysis were selected from core material. Thereacted in vacuum at 25°C with 100% phosphoric

acid. The evolved CO2 was analysed on a modified results of the analysis of 26 relatively organic-richsamples are presented in Table 2. Samples wereSIRA-12 stable-isotope ratio mass-spectrometer at

Liverpool University. Results corrected using stan- powdered and ~2 g was reacted with an excess ofconcentrated (8 M ) HCl to remove carbonatedard procedures are reported in relation to the

PDB international standard. Analytical precision carbon. Approximately 1 mg of the residue wasanalysed in a Carlo Erba 1106 CHN elementalis better than ±0.1‰ for both d13C and d18O.

Carbonate analyses of the brachiopods from analyser run at 1050°C: total organic carbon(TOC) for the rock was calculated after correctingArgentina are reported in Table 1.


Table 1Stable-isotope and trace-element composition of brachiopod calcite shells from the La Pola and Don Braulio sections, Argentina

Sample Spec. Brachiopod d13CPDB d18OPDB Mn Fe Mg Sr(‰, PDB) (‰, PDB) (ppm) (ppm) (ppm) (ppm)

ARGORD/10 lp1 Hirnantia +4.1 −4.5 536 815 2504 1145ARGORD/11 lp1 Hirnantia +3.8 −4.9 613 1408 3455 1188ARGORD/12 lp1 Hirnantia +0.9 −6.3 2229 4959 3619 814ARGORD/13 lp1 Hirnantia +3.5 −5.1 735 1280 2759 1040ARGORD/14 lp3 Hirnantia +3.7 −5.1 361 686 2612 1181ARGORD/15 lp2 Hirnantia +4.3 −4.3 404 744 2449 1160ARGORD/16 lp2 Hirnantia +3.9 −6.2 444 760 2521 1171ARGORD/17 lp2 Hirnantia +3.3 −5.0 373ARGORD/19 lp4 Hirnantia +4.8 −3.2 184 262 2299 1180ARGORD1 lp1 Hirnantia +4.0 −5.6ARGORD2 lp1 Hirnantia +3.9 −5.6ARGORD3 lp1 Hirnantia +5.0 −3.5ARGORD4 lp1 Hirnantia +3.9 −6.6ARGORD5 lp2 Hirnantia +1.4 −8.1ARGORD6 lp2 Hirnantia +3.1 −6.2ARGORD7 db1 dalmanellid +4.3 −4.8ARGORD8 db1 dalmanellid +3.5 −5.9ARGORD9 db1 dalmanellid +3.9 −5.0ARGORD/18 db1 dalmanellid +3.9 −5.6 355 1090 3781 1150

lp=sample from La Pola; db=sample from Don Braulio.

for carbonate content. The remaining carbonate- sion of Cambrian to Devonian marine rocks over-lain by Carboniferous and Triassic continentalfree powder was refluxed in solvents (dichloro-

methane with 10% methanol ) in a Soxhlet appara- strata (Sanchez et al., 1991). Ordovician rockshave been buried to a depth of several hundredstus for 24 h to extract soluble organic matter

(bitumen). Biomarker analyses of the bitumen of metres and have been folded and thrust, but inthe eastern tectofacies (see Astini, 1991, 1992a),fraction (work in progress and not reported in

detail here) indicate that the samples are thermally where the glacial sediments outcrop, they are notstrongly deformed. Lower Palaeozoic glacial rocksvery immature. The insoluble kerogen-containing

samples were used for d13Corg measurement in were first recognised in South America bySchlagintweit (1943) in northwest Argentina.order to best determine the composition of primary

organic carbon in the sediments. CO2 for analysis Subsequently, glacial diamictites have been recog-nised in a discontinuous belt of lower Palaeozoicwas derived from the thermal decomposition of

samples containing 0.5 mg organic carbon at 850°C rocks that crop out in the foothills of the AndeanChain stretching from Ecuador to Argentinawith copper oxide and silver wire. Analytical

precisions on duplicates (>90% of the samples (Crowell et al., 1981; Hambrey, 1985; Astini,1993). Although originally assigned to the earlywere duplicated) were better than ±0.05‰.Silurian on general stratigraphic criteria, the glacialhorizons are poorly constrained on fossil evidence;it has been suggested that they range from late4. Hirnantian marine carbonates, ArgentinaOrdovician (Ashgill ) to mid-Silurian ( Wenlock)(Hambrey, 1985). The discovery of a varied4.1. Stratigraphic settingHirnantia fauna (Benedetto, 1986) in sedimentsimmediately overlying the glacial sediments inThe late Ordovician succession in the

Precordillera of Argentina is part of a thick succes- Argentina indicates that, there at least, the diamic-


Fig. 3. Location map of the Sierra de Villicum in the Eastern Precordillera of western Argentina. PE=Precambrian; E=Cambrian;O=Ordovician; S=Silurian; T=Tertiary.

tites are pre-Silurian and very probably of localities [the Quebrada Don Braulio (31°14∞S,68°30∞W ) and the La Pola section (31°15∞S,Hirnantian age (Sanchez et al., 1991; Astini and

Benedetto, 1992; Buggisch and Astini, 1993; 68°30∞W )] in the Sierra de Villicum of thePrecordillera of San Juan Province (Fig. 3). LyingAstini, 1993).

Brachiopod faunas were collected from two with a sharp disconformable contact on the Lower

Fig. 4. Schematic representation of the Hirnantian succession in the Don Braulio and La Pola sections with the location of thebrachiopod samples. The Don Braulio Formation extends from the disconformity at the base of the diamictites to the base of theMogotes Negros Formation, several metres above the top of the section shown. O=Ordovician; S=Silurian.


Caradoc shales and turbidites of the La Cantera cement to a fossiliferous silty limestone with pack-Formation are about 30 m of glacial diamictites of stone to grainstone texture. The limestone matrixthe Don Braulio Formation (Fig. 4) (Astini, 1991). and cements were invariably more luminescentThe diamictites are sharply overlain by a discontin- than the brachiopods. Luminescence increases inuous lenticular conglomerate, <20 cm thick, con- intensity from brachiopod shells to micrite and totaining fragments of brachiopods, bryozoa and spar cements. Some echinoderm fragments showedcrinoid columnals. This unit is interpreted as a the brightest luminescence.shallow-marine transgressive-lag deposit (Astini,1991; Sanchez et al., 1991, 1993). The overlyingbioturbated muddy siltstones (5–8 m thick) con- 4.3. Geochemistrytain a varied Hirnantia fauna, which is especiallyconcentrated in calcareous siltstone beds in the The stable-isotopic and trace-element data forlower part. These yield abundant examples of brachiopod shells are shown in Table 1. d13C valuesHirnantia and Dalmanellid brachiopods, many of range from +0.9 to +5.0‰, d18O values rangewhich are articulated, together with modiolopsid from −8.1 to −3.2‰, Mn varies from 184 tobivalves. This association of the Hirnantia fauna 2229 ppm, Fe from 262 to 4959 ppm, Mg fromis regarded as a shallow shelf community (Sanchez 2229 to 3781 ppm, and Sr from 814 to 1188 ppm.et al., 1991). Samples from these horizons have There is an apparent pattern of positive covariationbeen collected in both the Don Braulio and La between d13C and d18O (Fig. 5), and a negativePola sections. Overlying the siltstones with the relationship between both carbon and oxygen iso-brachiopods are yellowish mudstones which have topic values and Fe and Mn concentrationsyielded the zonal graptolite Glyptograptus persculp- (Fig. 6) Analyses of the Dalmanella brachiopodtus in their lower part, indicating a late Hirnantian calcite fall within the range of values for theage (Sanchez et al., 1991). Several metres higher Hirnantia shells and the values fall on similara diverse chitinozoan fauna of early Silurian trends on the cross plots.age is present (Volkheimer et al., 1980). TheOrdovician–Silurian boundary, therefore, appearsto lie within the shales of the Don BraulioFormation (Sanchez et al., 1991; Astini, 1992b)

4.2. Petrography

In hand specimen, the foliate structure ofthe brachiopods appears to be well preserved.Transmitted light petrography suggests thatfibrous secondary calcite layer of the brachiopodshell structure is well preserved, although minorspar-filled fractures cross cut some shells.Cathodoluminescence investigation, however,revealed significant diagenetic alteration. The shellcalcite fibres were generally non-luminescentalthough some small patches, cross-cutting theprimary fabric, appeared to have a pervasive dullorange luminescence. In many shells brighter lumi-nescent calcite occurred as thin veins parallel toand cross cutting the foliation and filling thepunctae in the primary shell structure. The host Fig. 5. Stable isotope cross-plot for brachiopod samples from

the La Pola and Don Braulio sections.sediment varied from a siltstone with a calcareous


Fig. 6. Trace-element and stable-isotope cross-plots for the brachiopod stable isotopic data, Argentina.

4.4. Interpretation cence study indicates that virtually all areas of theshells are contaminated by luminescent calcitecement and that some areas appear to haveThe preservation of shell structures and fairly

high magnesium and strontium concentrations undergone more pervasive alteration.The decrease of both isotopic values withseemingly indicate that the shell calcites are reason-

ably well preserved. However, the variable stable- increasing iron and manganese concentrations iscompatible with diagenetic alteration in reducingisotope values and elevated iron and manganese

concentrations suggest that the brachiopod com- conditions, probably in a shallow burial environ-ment. The decrease in d18O indicates either anpositions have been altered. The cathodolumines-


increase in temperature or precipitation from which are close to the most positive measuredvalue of +5.0‰. The oxygen–Fe and oxygen–Mn(meteoric) fluids with d18O lower than marine,

whilst the decrease in carbon most likely reflects a relationships although apparently showing a gene-ral decrease in d18O values with increase in Fe andchange in dissolved carbonate composition, proba-

bly reflecting minor addition of organogenic Mn do not have a statistically significant correla-tion and cannot be used to calculate a least-alteredcarbon to the diagenetic pore fluids.

Patterns of isotopic and elemental variation can d18O value. Visible diagenetic alteration is evi-dently accompanied by decrease in d18O values sobe used to interpret diagenetic processes. Variable

amounts of a single cement generation will result the most positive d18O value measured (−3.2‰)provides our best estimate for a primary composi-in linear patterns of covariation between

d18O, d13C and Fe and Mn concentrations whilst tion although the true ‘‘marine’’ value may havebeen significantly more positive than this.more complicated patterns of cementation and

recrystallisation, the most common form of In summary, Hirnantian brachiopods fromArgentina have positive d13C values that are com-water–rock interaction, may produce non-linear

distributions [see Marshall (1992) and references parable with those documented from contempora-neous low-latitude sites and are much higher thantherein]. The general patterns of covariance

between d18O and d13C and Fe and Mn in the those from pre- or post-Hirnantian faunas(Brenchley et al., 1994). The d18O values are lowerbrachiopods most likely reflect variable intraskele-

tal cementation in primary voids (punctae), than the most positive values documented else-where but this is likely an artifact of the pervasivebetween shell fibres and in microfractures in the

different subsamples. Such patterns of alteration diagenetic alteration of the samples.and geochemical variation can be used to unravelthe effects of diagenetic alteration and derive aprimary or least-altered compositon (Veizer, 1983; 5. Carbon isotopic composition of organic matter:

Baltic StatesMarshall, 1992).Iron and manganese are virtually absent from

marine carbonates precipitated in oxic environ- 5.1. Geological settingments and are only compatible with the calcitelattice under reducing conditions more typical of The end-Ordovician and lower Silurian succes-

sion in the Baltic States has been described bypost-depositional diagenetic environments. Thepatterns of covariation of d13C, d18O, Fe and Mn Kaljo et al. (1988), Mannil (1990) and Mannil

and Meidla (1994). Sediments were deposited ondocumented above are compatible with the simplemixture of primary marine calcite (with relatively a southerly dipping shelf (Fig. 7). Fig. 8 shows the

stratigraphy of the area. Upper Caradoc, Ashgillpositive isotopic signature and low Fe and Mn)with one or more later diagenetic calcites with and Llandovery sediments are typically argilla-

ceous limestones and marls intercalated with bio-lower isotopic ratios and higher Fe and Mn con-centrations. The covariant trend in isotopic values micritic nodular limestones and rare shales. The

end Ordovician sediments assigned to the regionalalone is not enough to determine end-membercompositions but the combination of isotopic and Porkuni Stage are very variable. In the northern,

shallower areas (northern Estonia) they are repre-elemental variation may provide an indicator ofprimary, or at worst ‘‘least-altered’’, isotopic com- sented by high-energy oolitic and coral-bearing

biohermal limestones, with lower-energy, micriticposition (Veizer, 1983; Marshall, 1992). If it isassumed that the primary shell carbonate con- deposits. In the very shallowest areas reef carbon-

ates are truncated by a palaeokarstic surface; evi-tained no iron or manganese and that the trendsreflect simple mixing, linear regression analysis of dence that Hirnantian sea-level fall resulted in

local sub-aerial exposure. In the deepest part ofthe carbon–Fe and carbon–Mn relationships pro-duce statistically valid ( p<0.05) ‘‘primary’’ d13C the basin, in south Estonia and Latvia, the Porkuni

is represented by argillaceous limestones overlainvalues of +4.7 and +4.6‰, respectively; values


Fig. 7. Baltic states: location of boreholes sampled. Facies belts from Kaljo et al. (1988) generally reflect deepening from Estonianlocations in the north to Latvian boreholes in the south.

by interbedded grey marls and calcarenitic lime- Mossen and Fjacka shales, lagoonal limestonesfrom the Siuge and Torevere Members of thestones (with elements of a Hirnantia fauna) which

become sandy and oolitic in the upper part. Faunal Porkuni, and lowermost Juuru sediments from thebasinal cores contained sufficient organic carbonevidence (Kaljo et al., 1988) suggests that the

Porkuni regressive interval correlates with the for analysis (Table 2). The pre-Porkuni samplescome from boreholes, situated at different (thoughHirnantian period of low sea-level. The shallow-

water Porkuni sediments are overlain, often southerly) positions on the former shelf (Fig. 7):they have remarkably consistent d13Ckerogen com-sharply, by deeper-water nodular limestone facies

which are assigned to the Juuru regional stage. positions: 9 analyses, including both of the shaleintervals from 3 wells, have d13C ranging fromThese locally contain lower Llandovery fossils but

the precise age of the flooding is impossible to −30.8 to −29.7‰ (Table 2). The only organic-rich facies available in Porkuni (Hirnantian) sedi-determine.

The burial history of the region has not been ments were recovered from a number of shallowborehole cores from the northern facies belt. Alldetermined in detail though thermal maturity of

the sediments is low (see below) and geological 15 samples analysed are from micritic limestonesof the Siuge and Torevere Members. These depositsreconstructions suggest that the sediments have

not been buried much more deeply than at present. are typically associated with the reef facies, andare interpreted as lagoonal deposits perhaps laiddown in inter-reef depressions. In many boreholes5.2. Samples and resultsorganic-rich carbonates both underlie and overliethin reefal limestones. The Porkuni samples allBulk sediment samples were taken from several

levels in the Upper Ordovician and lowermost have organic carbon isotopic values that aregreater than those of earlier and later sediments.Silurian section. Only samples from the black


Fig. 8. Stratigraphic variation in kerogen d13C values from Estonia and Latvia. Ordovician stratigraphy from Mannil (1990).Stratigraphic intervals sampled are highlighted in bold text and specific units are marked with an asterisk (*). Possible disconformitiesare marked with vertical stripes.

The ten Siuge samples from below the reefal later (inter reef ) ones consistently have values of~−25‰.carbonates have d13Ckerogen ranging from −28.5

to −27.1‰ (mean −28.2±0.4‰) whilst the fiveTorevere samples from inter-reef limestones have 5.3. Interpretationcompositions ranging from −26.0 to −24.6‰(mean −25.2±0.6‰). The only lower Silurian The d13C data suggest a major temporal shift in

organic carbon compositions during the(Juuru stage) samples with sufficient organiccarbon for analysis again come from a southern Hirnantian but this interpretation must be treated

with some caution as the section (Fig. 8) is alocation (Aispute borehole, northern Latvia, burialdepth 976 m). d13Ckerogen values from two samples composite of sediments from very different facies

and burial depths. Neither the apparent shift fromof organic-rich argillaceous limestones are almostidentical at −30.0 and −29.8‰ PDB. the older sediments to the Hirnantian, nor that

from the Hirnantian to younger deposits, can beIn summary: The d13C values of organic matterfrom pre-glacial late Ordovician sediments of early documented from a single stratigraphic section.

Furthermore, all the Silurian data and the pre-Ashgill and late Caradoc age (−30‰) are verysimilar to those for post-glacial, lower Llandovery, Porkuni Ordovician data come from boreholes in

the south of Estonia and northern Latvia wheresediments (Fig. 8). Organic matter from the regres-sive Porkuni interval has significantly higher these successions represent deeper-water facies and

the successions are relatively deeply buriedd13Ckerogen values than material from either thepreceding or postdating intervals and it seems that (900–1420 m) today. In contrast, the Porkuni

samples come from shallow-water facies, at shal-within the Porkuni, stratigraphically earlier (pre-reef ) sediments have values of ~−28‰ whilst low current burial depths (<60 m) in the northern


Table 2d13C of kerogen samples from the Baltic states

Core/location Depth TOC d13Corg Stage Formation Member/facies(m) (%) (‰, PDB)

Aispute 976.14 11 −30.0 JuuruAispute 976.45 4.9 −29.8 JuuruAispute 1032.3 6.9 −30.4 VormsiAispute 1034.35 13.7 −30.8 VormsiAispute 1043.1 5.3 −30.0 OanduAispute 1045.4 4.2 −29.7 OanduK-40 55.7 0.2 −24.6 Porkuni Arina TorevereK-40 56 0.2 −25.8 Porkuni Arina TorevereK-40 56.6 0.2 −28.5 Porkuni Arina SiugeK-40 56.7 0.7 −28.5 Porkuni Arina SiugeK-40 57.4 0.3 −28.0 Porkuni Arina SiugeK-23 53.9 0.2 −25.0 Porkuni Arina TorevereK-38 36.1 1.1 −27.1 Porkuni Arina SiugeK-25 58 0.4 −24.6 Porkuni Arina TorevereK-25 58.7 0.4 −26.0 Porkuni Arina TorevereK-52 36 0.6 −28.2 Porkuni Arina SiugeLassi ( K-39) 55.3 0.4 −28.2 Porkuni Arina SiugeLassi ( K-39) 55.5 0.4 −28.3 Porkuni Arina SiugeLassi ( K-39) 55.6 50.4 −28.3 Porkuni Arina SiugeLassi(K-39) 55.8 0.3 −28.1 Porkuni Arina SiugeMannamaa 52 0.9 −28.3 Porkuni Arina SiugeKandava-25 905 5.0 −30.2 VormsiKandava-25 916.8 13.4 −30.4 VormsiPriekule 1401.5 7.7 −30.8 VormsiPriekule 1416.5 4.2 −30.0 OanduPriekule 1418.5 7.3 −30.3 Oandu

part of the area. The apparent stratigraphic shift We accordingly interpret the elevatedHirnantian d13Ckerogen values as representing amay thus be an artifact of differences in facies,

preservation history, or thermal maturity. temporal change in the d13C of primary organicmatter. The shift apparently parallels that observedPreliminary biological marker work (P. Mason,

unpublished data) suggest, however, that the type for brachiopod calcite from the same region(Brenchley et al., 1994) and the shift in organicof organic matter throughout the succession is

similar. All samples show a very similar algal carbon compositions documented from black shalesuccessions in China ( Wang et al., 1993). Thesignature that is typical of marine sediments of

this age (Guthrie and Pratt, 1994). Despite differ- magnitude of the maximum d13C shifts in kerogenand brachiopod shells in the Baltic area areences in burial depths all samples are thermally

immature. Ni porphyrins and D4 and D5 sterenes approximately the same. The organic carbonvalues and shifts are similar to those from Chinaand diasterenes are commonly present and stanols,

thought to be directly derived from biological (figured but not fully documented by Wang et al.,1993): although one of the Chinese samples, withprecursors, are present in some samples

(Mackenzie et al., 1982; Brassell et al., 1983). d13C of ~ −22‰, apparently documents a maxi-mum increase of almost 7‰. The lack of strati-Stable-isotopic analysis of only the kerogen frac-

tion rules out the possibility of contamination by graphic resolution in both the Baltic brachiopodand organic records and the difficulty in detailedmigration products.


correlation between our sections and those in by a marked positive shift in carbonate carbonand oxygen isotope values and d13Corganic.China (see below) make it difficult to prove that

the shifts are synchronous. Nevertheless, the Elevated d13C values characterise the lowerHirnantian, roughly equivalent to the extra-apparent relationship between the d13C shifts and

the, demonstrably eustatic, sea-level variation ordinarius/bohemicus zone, and lower values sim-ilar to pre-Hirnantian ones are encountered in(Brenchley et al., 1994) it make it likely that the

positive shift in carbon values corresponds to the sediments postdating the second extinction eventand sea-level rise. The new evidence presented hereonset of regression and the return to lower values

corresponds to the mid-Hirnantian transgression suggests that the Hirnantian positive isotopicexcursion was truly global. High carbonate d13C(Fig. 9).values have now been reported from centralSweden, Estonia and Latvia (Baltica); AnticostiIsland, Canada, formerly part of Laurentia; and6. Discussion — the global picturefrom Argentina which lay near the margin ofGondwanaland (Fig. 1). Synchronous excursionsFig. 9 shows our interpretation of the relation-

ships between extinction, sea-level changes and the in the composition of organic carbon have nowbeen described from China (on a small plate onisotopic compositions of brachiopods and kerogen

in the late Ordovician. The onset of glacioeustatic the opposite margin of Gondwanaland toArgentina) and the Baltic states.sea-level fall and first extinction event were marked

Fig. 9. Summary diagram showing the relationships between biological, bathymetric and stable isotopic changes in the late Ordovician(modified from Brenchley et al., 1995). The marked positive carbonate isotopic excursion in the early Hirnantian is paralleled by ashift in the isotopic composition of organic carbon (this study). New data from Argentina (this study) demonstrate elevated d13Cin brachiopods from high palaeolatitudes but the oxygen values shown should be regarded as a minimum because even the‘‘least-altered’’ sample showed signs of diagenetic alteration.


6.1. Timing to −29‰ to values around −26‰) at the base ofthe bohemicus zone with values up to −25‰, inan overlying Hirnantia–Kinella zone. A return toIn Baltica the onset of the carbon isotopic event

seems to correspond precisely with the sea-level lower values (−30 to −29‰) marks the base ofthe persculptus zone. The other stratigraphic sec-fall recorded at the base of the Hirnantian.

Brachiopods from levels postdating the first strati- tion shows values of ~ −29 to −28‰ in thebohemicus and lowermost Hirnantia zone with agraphic evidence for shallowing invariably show

elevated d13C values. Similarly the end of the sharp jump to −24‰ in the upper part of theHirnantia–Kinnella zone and a single determina-excursion is very well constrained with respect to

the sea-level change: all brachiopods in strata tion with d13C of −22‰ at the very top of thatzone. The base persculptus zone is again markedoverlying the initial flooding surface have lower

isotopic values. The precise age of the flooding is by a return to lower values (−30 to −28‰) butthere is another apparently short-lived positivedifficult to determine. The Juuru sediments overly-

ing the flooding surface have traditionally been shift to values of −26‰, in the uppermost part ofthe persculptus zone.described as lower Landovery (Kaljo et al., 1988)

but precise correlation in terms of graptolite zonalscheme is impossible. In Laurentia, there is some 6.2. Significancedoubt about the timing of the isotopic change assome stratigraphic correlations (e.g., Long, 1993) The causes of the late Ordovician geochemical

events and the relationship to the extinctions andplace the base of the Hirnantian well below thevery thin interval characterised by the high d13C glaciation have been discussed in detail elsewhere

(Brenchley et al., 1995). The additional data pub-values (Brenchley et al., 1994). In Argentina theonly brachiopod-bearing part of the succession lished here confirm the global nature of the isotopic

events and enable us to develop some of theoverlies the main tillites and is accordingly likelyto be late or immediately post glacial in age. The arguments, although some important problems

remain unresolvedsingle-horizon isotopic determination presumablytherefore reflects conditions towards the end of The Hirnantian d13C events reflect major

changes in carbon cycling in the late Ordovicianthe lower Hirnantian (i.e., uppermost extraordina-rius zone). The least-altered values from Argentina oceans. The recognition of parallel shifts in organic

and brachiopod carbonate d13C compositions inare similar to the lowest values encountered infossils of similar age from Baltica (Fig. 9). the Baltic area demonstrate that the changes

affected the whole surface-ocean carbon reservoir.The poor stratigraphic resolution of thed13C kerogen data from the Baltic States cannot help This is compatible with previous ideas about burial

of organic carbon or at least its removal fromdelineate the boundaries of the event although thedata from the northern part of the area suggest surface oceans during the early Hirnantian

(Marshall and Middleton, 1990; Qing and Veizer,that there may be two stages in the organic carbonexcursion. This seems to agree with data from 1994; Brenchley et al., 1995). Elevated carbonate

d13C values are associated with lowered levels ofChina (Wang et al., 1993). To our knowledge,neither the detailed biostratigraphy or the thermal atmospheric CO2 at a number of times in the

Phanerozoic (see examples in Frakes et al., 1992)history of the Chinese sections have been docu-mented fully and the zonal scheme used differs and we have previously suggested that the

Hirnantian d13C excursion is compatible with afrom the international one. The isotopic patternsfrom China, however, potentially provide the most marked reduction in greenhouse effect during the

end-Ordovician glacial event. It has been suggestedcomplete temporal sequence for carbon isotopicchange available. The two sections figured by that the magnitude of photosynthetic fractionation

(i.e. the difference in d13C between organic matterWang et al. (1993) both show a positive carbonshift but differ in detail. One section shows a 3‰ and the d13C of dissolved inorganic carbon) is

dependent on pCO2

in the atmosphere and surfacecarbon shift (from typical Ashgill values of −30


waters (e.g., Hollander and McKenzie, 1991). In Baltica (central Sweden, Estonia and Latvia),Laurentia (Anticosti Island) and Gondwanalandthis case the difference between carbonate and

kerogen d13C (Dd13Ccarb–org), should then provide (Argentina), and in organic matter from Baltica(Estonia) and Gondwanaland (China). The widean independent monitor of carbon dioxide levels.

Our data from the Baltic basin suggest that palaeogeographical spread of the sites and theinclusion of data from high palaeolatitudes suggestDd13Ccarb–org remains roughly constant during

the glacial epoch; and thus is seemingly incompati- that this was truly a global event.There is clearly scope for additional high-reso-ble with significant draw down of CO2 levels.

However, Popp et al. (1997-this issue and pers. lution stratigraphic studies linked to detailed docu-mentation of faunal records; ideally we need acommun.) have recently questioned the link

between Dd13Ccarb–org and atmospheric CO2 and direct comparison of organic and carbonaterecords. Future investigations should aim to deter-demonstrated that the fractionation may be depen-

dent on a variety of factors including changes in mine the precise timing of the isotopic events,investigate the changes in the carbon cycle in moreproductivity.

The new carbonate d13C data from Argentina detail through detailed investigation of the typesof organic carbon buried before during and afterclearly demonstrate that elevated Hirnantian

values were very widespread and affected high- the event, and try to determine independent esti-mates of palaeotemperatures from a wide range ofand low-latitude environments. With data from a

single, post diamictite, horizon it is impossible to palaeolatitudes.make a precise age correlation with the publishedbrachiopod data from low-latitude sites or todetermine whether there were latitudinal gradients Acknowledgementsin the magnitude of the d13C excursion that mightperhaps be associated with regional differences in This work has been supported by NERC grantscarbon cycling. The highest, and presumably least- GR3/8943 and GR9/373 and by a NERC student-altered, d18O obtained (−3.2‰) is similar to those ship to P.M.from low-latitude sites and, if pristine, would David Whitehead is thanked for the ICP analy-indicate little or no latitudinal paleotemperature ses and Fraser Steele for the stable-isotope deter-gradient during the glaciation. This is clearly unre- minations on the carbonates. Denis Lavoie andalistic. Alternative explanations include: (1) that Ethan Grossman are thanked for their thoroughthe least-altered sample had still suffered significant and constructive reviews.alteration of its d18O value (a possibility supported This is a contribution to IGCP project 293.by the petrographic data), or (2) that the brachio-pods in a periglacial setting were living in waterswith a negative isotopic composition due to the Referenceslocal effects of melting polar ice. There is clearlya need to resolve this problem by attempting to Astini, R.A., 1991. Paleoambientes sedimentarios y secuencias

depositiacionales del Ordovicico de la Precordillera Argen-find well-preserved material from a range oftina. Ph.D. Thesis, Univ. Cordoba, Cordoba, Argentina.different environments and palaeolatitudes.(Unpubl.)

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tas ferruginosas en la sase del Silurico de la PrecordilleraNew data from Argentina and from the BalticArgentina. Estud. Geol. 48, 297–303.states provide further evidence for a positive iso-

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