An approach to estimate Lower Jurassic seawater oxygen-isotope composition using δ 18 O and Mg/Ca...

An approach to estimate Lower Jurassic seawater oxygen-isotope composition using d18O and Mg/Ca ratios of belemnitecalcites (Early Pliensbachian, northern Spain)

Maider Armend�ariz,1 Idoia Rosales,1 Beatriz B�adenas,2 Laura Pi~nuela,3,4 Marc Aurell2 andJos�e Carlos Garc�ıa-Ramos31Instituto Geol�ogico y Minero de Espa~na, R�ıos Rosas 23, Madrid 28003, Spain; 2Departamento Ciencias de la Tierra, Universidad de

Zaragoza, Zaragoza 50009, Spain; 3Museo del Jur�asico de Asturias (MUJA), 33328 Colunga, Asturias, Spain; 4Departamento de

Geolog�ıa, Universidad de Oviedo, Oviedo 33005, Spain

ABSTRACT

Palaeotemperature estimates from the oxygen-isotope compo-

sitions of belemnites have been hampered by not knowing

ancient seawater isotope compositions well enough. We have

tackled this problem using Mg/Ca as a proxy for temperature

and here, we present a ~2 Ma record of paired Mg/Ca and

d18O measurements of Jurassic (Early Pliensbachian) belem-

nites from the Asturian basin as a palaeo-proxy of seawater

oxygen-isotope composition. From the combined use of the

two approaches, we suggest a d18Ow composition of about

�0.1& for the Jamesoni–Ibex zones. This value may have

been increased by about 0.6& during the Davoei Zone due to

the effect of waters with a different d18Ow composition.

These findings illustrate the inaccuracy of using a globally

homogeneous ice-free value of d18Ow = �1& for d18Ocarb-

based palaeotemperature reconstructions. Our data suggest

that previous palaeotemperatures calculated in the region

from d18O values of belemnites may have been underesti-

mated as the seawater oxygen isotopic composition could

have been higher.

Terra Nova, 25, 439–445, 2013

Introduction

The use of oxygen-isotope ratios ofbelemnites for palaeotemperature cal-culations has been widely used in thelast six decades. However, thesepalaeotemperature calculations havebeen used with some uncertaintiesdue to the poor knowledge of theancient seawater oxygen isotopiccomposition (d18Ow), and theunknown species-specific biologicalfractionation effects and belemnitelife habitat and habits (e.g. Sælenet al., 1996; Bailey et al., 2003;Rosales et al., 2004a; Dutton et al.,2007; McArthur et al., 2007a; Mut-terlose et al., 2010). In more recentyears, the Mg/Ca palaeotemperatureproxy has been used in belemnites asan alternative palaeothermometerdue to the existence of reasonablygood correlation between Mg/Ca andd18Obel (e.g. Bailey et al., 2003; Ro-sales et al., 2004b; Nunn and Price,2010; Price et al., 2011; Armend�arizet al., 2012). Moreover, the combina-tion of these two palaeo-proxies hasthe potential to be used to estimate

changes in sea-water oxygen isotopiccomposition and polar ice volume,although its efficacy has been ques-tioned by some authors due to signif-icant differences in the Mg/Caconcentration and a lack of Mg/Caand d18O correlation between somebelemnite species of the Toarcian(McArthur et al., 2007a; Li et al.,2012).To shed light on this issue, we

examine the potential and consis-tency of d18O and Mg/Ca values ofwell-preserved Jurassic belemnites ofthe Early Pliensbachian Boreal Realmof the Asturian basin (Figs 1 and 2) aspalaeoclimatic and palaeoenviron-mental proxies. To obtain more con-sistent data avoiding, as much aspossible, the noise introduced by inter-specimen variability, we used chemo-stratigraphic records based on curvesobtained from running average values,which are commonly used in time ser-ies data to filter short-term fluctua-tions and highlight the long-termtrends. The use of these curves has pro-vided a long-term record (~2 Maaccording to Gradstein et al., 2012) ofMg/Ca ratios of the investigated be-lemnites that parallel the d18Ocarb val-ues. Assuming a temperaturedependence of Mg/Ca in belemniteswithin the range observed in most

modern biogenic calcites, the signifi-cance of combining the two palaeo-proxies is that it provides an opportu-nity to calibrate the local oxygen isoto-pic composition of seawater for theLower Jurassic.

Geological setting, materials andmethods

The Pliensbachian succession of theAsturian basin (northern Spain)belongs to the Santa Mera Memberof the Rodiles Formation and corre-sponds to a rhythmic alternation oflimestone and marl deposited in anepeiric platform (Su�arez Vega, 1974;Valenzuela et al., 1986). The samplesfor this study were collected fromtwo well-exposed sections of theAsturian sea cliffs: Punta La Llastraand Playa de Vega (Fig. 2A). Inthese sections, the Pliensbachian suc-cession comprises ~80 m of marl/cal-careous mudstones interbedded withbioclastic tempestites and organicfacies (black shales) at certain strati-graphic horizons (Fig. 2B; Valenzuelaet al., 1986; Aurell et al., 2003;B�adenas et al., 2009, 2012; Garc�ıa-Ramos and Pi~nuela, 2010). The suc-cession contains relatively abundantpara-autochthonous and autochtho-nous fossils (ammonites, belemnites,

Correspondence: Maider Armend�ariz, C/

Calera 1, Tres Cantos, Madrid 28760,

Spain. Tel.: +34-91-7287273; fax: +34-91-

7287202; e-mail: [email protected]

© 2013 John Wiley & Sons Ltd 439

doi: 10.1111/ter.12054

brachiopods and bivalves). Accord-ing to previous studies, palaeowaterdepth inferred for these deposits unli-kely exceeded 100 m (B�adenas et al.,2012). Ammonite faunas have pro-vided a well-constrained biostrati-graphic framework of successivezones and subzones (Su�arez Vega,1974; Comas-Rengifo and Goy,2010). A detailed sampling of belem-nite rostra through the ~18-m-thickcomposite section that correspondsto the Jamesoni (upper part), Ibexand Davoei zones was made atapproximately 10-cm sampling inter-val or less (Fig. 2C). This part of thesuccession is dominated by Borealbelemnites of the genus Passaloteu-this co-occurring with the less com-mon genus Acrocoelites (Su�arezVega, 1974). All specimens werecarefully screened for post-deposi-tional diagenetic alteration. Thescreening techniques and results ofthe diagenetic study of the samplesused in this study have been docu-mented previously in detail in aprevious paper (Armend�ariz et al.,2012). For this reason they areexplained here briefly. Samples wereinspected using binocular and opticalmicroscopy and cathodolumines-cence. Only those samples showingpristine radial structure and lack ofluminescence were powered for ele-mental and isotopic analyses. Furtherdiagenetic screening was carried outusing Ca, Fe, Mn, Sr and Mg con-centrations given also in Armend�ariz

et al. (2012). In that previous paper,Armend�ariz et al. (2012) reported Cand O stable isotopes and 87Sr/86Srisotope values and provided palaeo-temperatures derived from thedata. This study represents anadvancement of the interpretationand use of the paired Mg/Ca ratiosand d18O dataset to assess temporalchanges in local seawater isotopiccomposition.Elemental determinations of Ca

and Mg were performed in the Insti-tuto Geol�ogico y Minero de Espa~nausing a Varian Vista MPX ICP-AES spectrometer after dissolvingthe carbonate powders in 10%hydrochloric acid. Oxygen stableisotope compositions were collectedfrom the CO2 gas liberated by thecarbonate powders after reactingonline with 100% orthophosphoricacid, using a SIRA-II mass spec-trometer at the Universidad de Sal-amanca (Spain).

Comparison of oxygen-isotopeand Mg/Ca-derivedpalaeotemperatures

Palaeotemperatures were estimatedindependently from oxygen-isotopevalues and Mg/Ca molar ratios(Fig. 3A) to compare the dataderived from both palaeothermome-ters. Oxygen-isotope records of well-preserved belemnite calcites(d18Ocarb) show values ranging from�0.1& V-PDB in the Jamesoni Zone

to �3.1& V-PDB in the Ibex Zone,with a relatively constant trend alongthe Jamesoni Zone, a negative shiftduring the Ibex Zone, and a returnto more positive values during theDavoei Zone (Fig. 2C). Palaeotem-peratures were calculated using theequation modified by Anderson andArthur (1983): T °C = 16.0�4.14(dcarb�dw) + 0.13 (dcarb�dw)

2, assum-ing an initial value of �1& VSMOWfor the dw, as in many other studiesfor Mesozoic times. Running-averagepalaeotemperatures calculated viad18Ocarb show a temperature changethrough the succession (DT) of~7.5 °C, with extreme mean valuesranging from 14.5 °C (JamesoniZone) to 22 °C (Ibex Zone) (Fig. 3A).The running-average palaeotemper-ature data indicate a relatively con-stant trend along the Jamesoni Zone,a rapid warming interval along theIbex Zone and a trend to coolingalong the uppermost Ibex and Davoeizones.The Mg/Ca data are mostly paral-

leling the oxygen-isotope trend(Fig. 2C). They also delineate a flattemporal trend along the JamesoniZone of about 9.7 mmol/mol, fol-lowed by a pronounced increase (upto 20.9 mmol/mol) along the IbexZone and the lowermost DavoeiZone, and finally a return tolower values during the DavoeiZone (Fig. 2C). Scatter plot of bothgeochemical parameters (d18O vs.Mg/Ca ratios) shows a relatively

Tethys

Pangea

Arctic

40º N

Borealarea

Hispanic corridor

30º N

Tethyanocean

Asturianbasin

Iberianmassif

Exposedlandmasses

Shallow marine andopen platformenvironments

Deep marine andoceanic basins

Fig. 1 Palaeogeography of NW Europe during the Early Jurassic indicating the location of the Asturian basin (modified fromThierry et al., 2000; with palaeolatitude adapted from Osete et al., 2011).

440 © 2013 John Wiley & Sons Ltd

Estimates of d18O Jurassic seawater • M. Armend�ariz et al. Terra Nova, Vol 25, No. 6, 439–445

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good inverse covariance of d18O withMg/Ca (correlation coefficientr = �0.61; see Armend�ariz et al.,2012), which is similar to thoseobserved by Rosales et al. (2004b) inbelemnites of the same age from theBasque-Cantabrian basin.

The underlying basis for the Mg/Capalaeothermometry is that the substi-tution of Mg by Ca in the calcite isfavoured at higher temperatures. Thisrelationship is exponential, but is con-sidered to be about 10% per °C formost biogenic calcites (e.g. Rosenthal

et al., 1997; Lear et al., 2002; Baileyet al., 2003). At present, the tempera-ture dependence of Mg/Ca for mostcalcitic organisms fits with equationsof the form: Mg/Ca (mmol/mol)carb = BeaT (where ‘B’ and ‘a’ areconstants and T is the calcification

..... .... ... ....... ........

Asturian basin

...

Platform facies

Sandy limestone

Oolitic limestoneOrganic rich marl& black shale

Nodular limestone

Skeletal limestone

STRATIGRAPHICGAP

.....

W E

naignatteH

n air umeniS

naihcabsneil PnaicraoT

LOW

ER JU

RA

SSIC

JamesoniIbex

Davoei

Margaritatus

Spinatum

Upper

Middle

Lower

rewoL

reppU

Upper

Lower

inosemaJ

xebIieova

D

Correlatedstratigraphicsection

δ18O ‰ (V–PDB)–3.5 –2.5 –1.5 –0.5

Mg/Ca (mmol/mol)8101214161820

0 m

2

4

6

8

10

12

14

16

Marl

Black shale

Limestone

Marl & limestonewith ammonites

Collapse breccia& dolostone

Villaviciosa

Lastres

Colunga Ribadesella

Punta La Llastra

Playa de Vega

0 5 10 kmN

Cantabric Sea

Gijón

Oviedo Bilbao

Madrid

Barcelona

4º8º 0º

SPAIN

FRANCE

PO

RTU

GA

L

Evaporite limestone& marl

Algal laminatedlimestone

Peritidal facies

(A)

(B) (C)

Rhaetian

40º

44º

Fig. 2 (A) Location of the Punta La Llastra and Playa de Vega stratigraphic sections in the Asturias coastline, in northernSpain. (B) Stratigraphy and biostratigraphy of the Lower Jurassic of the Asturian basin adapted from Aurell et al. (2003) andComas-Rengifo and Goy (2010). The studied Lower Pliensbachian marl–limestone platform succession at Punta La Llastraand Playa de Vega (shown with dashed line) encompass the upper part of the Jamesoni Zone and the Ibex and Davoei zones.Ammonite zones after Comas-Rengifo and Goy (2010) and M.J. Comas-Rengifo and A. Goy (pers. comm., 2012). (C) Corre-lated stratigraphic section of Punta La Llastra and Playa de Vega and the obtained chemostratigraphy (d18Ocarb and Mg/Camolar ratios) of the studied Pliensbachian interval.


Terra Nova, Vol 25, No. 6, 439–445 M. Armend�ariz et al. • Estimates of d18O Jurassic seawater

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temperature in °C). According to pre-vious studies, for belemnites, values of1.2 and 0.105 can be assumed for ‘B’and ‘a’ respectively (Bailey et al.,2003; McArthur et al., 2007b; Nunnand Price, 2010; Armend�ariz et al.,2012). Being aware that these valuescannot be tested for belemnitesbecause they are extinct, they havebeen proved, however, to be useful inother previous studies. Running-aver-age palaeotemperature derived fromMg/Ca ratios delineate a near flattemporal trend around a mean of~19.8 °C for the Jamesoni Zone, a

rapid temperature increase (DT) of~7.2 °C along the Ibex Zone and theIbex-Davoei boundary zone (up to26 °C), and finally, a slight tempera-ture decrease to ~24 °C during theDavoei Zone (Fig. 3A). It should benoted that these values may be sub-jected to some degree of error due tothe degree of uncertainty in the exactrelationship between elemental ratiosand temperatures. The equationabove has been originally calibratedfor other organisms dissimilar tobelemnite calcite, which may haveother partition coefficients due to

differences in metabolic rates. There-fore, the accuracy of the method willdepend on the species-specific calibra-tion of the equation to belemnites. Asthis problem cannot be achievedbecause belemnites are extinct, theassignment of the absolute Mg/Capalaeotemperatures given here shouldbe taken with caution (McArthuret al., 2007b; Price et al., 2011).

Regional d18Ow estimation

Our data show that for most of theEarly Pliensbachian, mean palaeo-

Jamesoni

Ibex

Davoei

Temperature (ºC)

Mg/Caδ O (δ O = –1)18 18

w

28 24 20 16 12

Temperature (ºC)28 24 20 16 12

Ammonitechronozones

egatS

Mg/Caδ O (18 δ O = –0.14)18

w

Minordeviation

Minordeviation

Maindeviation

T3

R3

T4

T–R Faciescycles

Low

er p

liens

bach

ian

(A) (B)

Fig. 3 Palaeotemperature curves calculated via d18Ocarb values (dots) and Mg/Ca ratios (squares) from well-preserved belemniterostra from Punta La Llastra and Playa de Vega sections. Long-term transgressive–regressive (T–R) facies cycles after Quesadaet al. (2005). The curves have been obtained from the 9-point running average values. The shaded areas represent detecteddeviations between the d18Ocarb and Mg/Ca trends. A) The Mg/Ca palaeotemperatures have been calculated using the exponen-tial equation of Rosenthal et al. (1997), which was developed using benthic foraminiferal shells. Oxygen-isotope values wereconverted into palaeotemperatures using the equation formulated by Anderson and Arthur (1983), and assuming a seawaterd18O composition equal to �1& for an ice-free world. B) Comparison of palaeotemperatures estimated from Mg/Ca valuesand the oxygen-isotope compositions of calcite using a palaeoceanic water d18O value of �0.1&.



.............................................................................................................................................................

temperatures estimated from d18O aresystematically 2–4 °C cooler thanthose recorded by Mg/Ca molar ratios,although the overall trend and themean palaeotemperature rise (DT)derived from both palaeothermome-ters are basically the same (Fig. 3A).This finding supports a temperatureinfluence upon both signals and pro-vides strong confidence on the Mg/Caapproach, at least for the studied be-lemnites. Assuming the appropriate-ness of Mg/Ca palaeotemperaturesobtained from the equation above, thedifference of temperature foundbetween the d18O and Mg/Ca palae-othermometers can be attributed to aninappropriate assumption of a globallyhomogenous value of d18Ow of �1&VSMOW for past non-glacial seawater(Shackleton and Kennett, 1975).Although many authors have assumedthe single value �1& VSMOW for theMesozoic ocean seawater, it is danger-ous to extend this assumption to muchshallower sea environments and epeiricseas potentially influenced by run-offand evaporation.If we combine palaeotemperatures

from Mg/Ca ratios with d18Ocarb

data in the equation formulated byAnderson and Arthur (1983), we canestimate a mean d18Ow compositionof about �0.1& � 0.5 (2r) in theAsturian basin for the studied per-iod. Applying this estimation ofd18Ow, palaeotemperatures derivedfrom d18Ocarb values raise a mean of3.7 °C, ranging extreme mean valuesbetween 18 and 26 °C (Fig. 3B). Thisseawater isotopic composition seemsto be a good approach given that thenew d18Ocarb palaeotemperaturesoverlap most of the Mg/Ca palaeo-temperature estimates (Fig. 3B).This inference agrees with recent

reconstructions of Mesozoic andCenozoic d18Ow values based on gen-eral circulation models, which predictlatitudinal gradients in the d18Ow

composition between �1.2& and1.4& VSMOW due to latitudinalchanges in the flux of evaporationand precipitation (e.g. Roche et al.,2006; Tindall et al., 2010). Thesemodels show that the d18Ow compo-sition might have been higher intropical and subtropical latitudes,lowering 2–4 °C the isotope tempera-tures estimates at these latitudes(Roche et al., 2006), a value that cor-responds with our results.

Further research on clumped-iso-tope geothermometry (e.g. Eiler andSchauble, 2004; Wang et al., 2004;Schauble et al., 2006; Eiler, 2007;Schmid and Bernasconi, 2010) may,in the future, provide better con-strained palaeotemperatures from be-lemnites to calculate the d18Ocomposition of the Early Jurassicseawater. At present, Bernasconiet al. (2011) and Schmid (2011) havereconstructed the d18O compositionof the Early Cretaceous seawater inthe Boreal Realm by combiningclumped-isotope temperatures andthe d18O values of belemnite calcites.They obtained a range of d18Ow val-ues between �0.1& and 1.2&, whichare in accordance with our results.

Deviations of the d18O and Mg/Carecords

Figure 3B shows the overlapping oftemperatures from Mg/Ca ratios withthose obtained from d18Ocarb using ad18Ow = �0.1& for the regional EarlyJurassic seawater. The diagram showsthe good matching of the data exceptfor two minor deviations (i.e. upperJamesoni Zone and Jamesoni-Ibextransition) and a major deviationfrom the upper Ibex to Davoei zones.Owing to the sensitivity of the

d18Ocarb not only to temperature butalso to salinity, these divergences canbe better explained by periods ofdeviations of the local seawater d18Ofrom the calculated initial value dueto salinity effects and/or water massexchanges. Although evaporationincreases seawater salinity and con-centrates the heavy O-isotope, theinfluence of continental run-off and/or precipitation (depleted withrespect to the heavy isotope) tends tolower the d18Ow seawater composi-tion. This was probably the case forthe two minor deviations observedaround the upper Jamesoni Zoneand the Jamesoni-Ibex transition,characterized by almost constantmean Mg/Ca palaeotemperatures,but small fluctuations of d18O beforerapidly being in phase again with theMg/Ca palaeotemperatures (Fig. 3B).A periodic influence of freshwaterrun-off during the Jamesoni Zonehas been recently proposed toexplain the origin of high-frequencysedimentary cycles in the studiedsuccession (B�adenas et al., 2012).

Regarding the upper Ibex to Dav-oei main deviation, the trendobserved is somewhat different: afterthe rapid warming recorded in theIbex Zone with both palaeother-mometers, the d18O palaeotempera-tures show a gradual decrease duringthe upper Ibex and Davoei zones,whereas Mg/Ca palaeotemperaturesshow still warming up to the Ibex-Davoei boundary and then adecrease also during the DavoeiZone, in phase with the d18O temper-atures, but 2–4 °C warmer (Fig. 3B).This divergence suggests a change inthe initial d18Ow composition ofabout 0.6& towards higher valuesduring the Davoei Zone. A regionalchange in the d18Ow compositioncould be supported by the hypothesisof an incursion of Tethyan or Panth-alassan water masses into the BorealRealm during the Davoei Zonereported by Dera et al. (2009) on thebasis of a change in the neodymiumisotopic signature of the Boreal sea.This palaeoceanic incursion of saltierwaters from lower latitudes, whichhas been related to intermittentopening of the Hispanic Corridorduring the Pliensbachian (Dera et al.,2009), could certainly have intro-duced waters with a higher d18Ocomposition, accounting for the off-set observed between the Mg/Ca andd18O curves at the upper Ibex andDavoei zones. However, other alter-native mechanisms to explain thechange in the d18Ow compositionshould be also considered. The tem-perature decrease observed throughthe Davoei Zone is supported alsoby oxygen-isotope data from theBasque-Cantabrian basin (Rosaleset al., 2004a). It is associated with asea-level fall documented for theEuro-boreal domain (Hallam, 1981)and recorded nearly synchronouslyin the Asturian, Basque-Cantabrianand Iberian basins (G�omez and Goy,2005; Quesada et al., 2005; Rosaleset al., 2006; Garc�ıa-Ramos andPi~nuela, 2010), in the UK (vanBuchem and McCave, 1989; Hes-selbo and Jenkyns, 1998; Hesselbo,2008), as well as in some Tethyanbasins of southern Spain (Betic Cor-dillera; O′Dogherty et al., 2000). Thissea-level fall might have caused a rel-ative isolation, affecting the waterexchange through a system of straitsand shallows. Isolation may have



............................................................................................................................................................

increased regional evaporation ratesand could explain the rise of seawa-ter d18O, but fails to explain theassociated cooling of seawater.

Conclusions

This study highlights the possibleerrors associated with reconstructingpalaeotemperatures during the Juras-sic by assuming a globally homoge-neous value of d18Ow. Theinappropriate assumption of aninvariable d18Ow composition of�1& VSMOW for ice-free ages hassystematically biased previous regio-nal palaeotemperature estimates forthe Early Jurassic, underestimatingthe isotopic palaeotemperatures in~2–4 °C. Assuming a temperaturedependence of Mg/Ca ratios for thestudied belemnites similar to moderncalcitic organisms, combination ofthe Mg/Ca and d18Ocarb palaeo-temperature approaches allows us tocalculate an initial d18Ow composi-tion of about �0.1& for the EarlyPliensbachian of the Asturian basin(Jamesoni–Ibex zones), which seemsto be in good agreement with valuespredicted with climate models. Thisvalue would have been increased inabout 0.6& during the Davoei Zonedue to a change in the d18Ow compo-sition. This event coincides with aregional sea-level fall.

Acknowledgements

The authors acknowledge contribution tothe project CGL2011-24546 (MINECO)and the Collaborative Protocol CN-04-226 between the Principado de AsturiasGovernment and the University of Ovie-do. M.J. Comas-Rengifo and A. Goy(UCM) are thanked for providing anupdated biostratigraphic framework. Theconstructive comments of four anony-mous reviewers and Associate Editorgreatly improved the original manuscript.

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Received 16 October 2012; revised version

accepted 26 April 2013



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An approach to estimate Lower Jurassic seawater oxygen-isotope composition using δ 18 O and Mg/Ca...

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