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A revised Pliocene record for marine-87Sr/86Sr used to date an interglacial event recorded in the Cockburn Island Formation, Antarctic Peninsula

A revised Pliocene record for marine-87Sr/86Sr used to date an interglacial event recorded in the Cockburn Island Formation, Antarctic Peninsula
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  A revised Pliocene record for marine- 87 Sr/  86 Sr used to date aninterglacial event recorded in the Cockburn IslandFormation, Antarctic Peninsula J.M. McArthur   a, ⁎ , D. Rio  b , F. Massari  b , D. Castradori  c , T.R. Bailey  a,d ,M. Thirlwall  d , S. Houghton  a  a   Department of Earth Sciences, UCL, Gower Street, London WC1E 6BT, UK   b  Department of Geology Palaeontology and Geophysics, University of Padova, Italy c  AGIP S.p.A.-STIG, Via Maritano 26, 20097 S. Donato Milanese (MI), Italy d  Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK  Received 18 July 2005; received in revised form 25 May 2006; accepted 7 June 2006 Abstract We present a revised  87 Sr/  86 Sr-curve for marine-Sr in the interval 2.5 – 4.5 Ma using analysis of   Orbulina universa  from theMiddle Pliocene type section in Sicily (Punta Piccola section) and a mixed assemblage of planktonic foraminifera from ODP Site758A and B. The new calibration is used, together with new  87 Sr/  86 Sr analysis of pectenid calcite from Cockburn Island,Antarctica, to confirm an age of 4.7 Ma for the pectenids of the Cockburn Island Formation (formerly the Pecten Conglomerate), aunit important in marking a period of warmth and possible glacial retreat in Antarctica. Finally, to aid interlaboratory comparison of  87 Sr/  86 Sr data, we calibrate a value of 0.709174 for EN-1 against a value of NIST 987 of 0.709248.© 2006 Published by Elsevier B.V.  Keywords:  Pliocene; Interglacial; Antarctic Peninsula; Sr-isotopes;  87 Sr/  86 Sr; Punta Piccola 1. Introduction Marine sediments can be dated by measurement of  87 Sr/  86 Sr in well-preserved biogenic carbonate (Miller et al., 1991; Hodell et al., 1991; Veizer et al., 1999;McArthur and Howarth, 2004, and references therein).Themethodprovidesaccuratedatesonlyifthecalibrationcurve of   87 Sr/  86 Sr against time is accurate. For thePliocene period, the  87 Sr/  86 Sr-calibration usually used is based on the data of  Farrell et al. (1995; hereinafter termed FCG95) for ODP site 758, or its derivative (e.g.,McArthur and Howarth, 2004), but they have not beenused much for dating.The basis of the  87 Sr/  86 Sr calibration of FCG95 is the plot of   87 Sr/  86 Sr against depth that is shown in Fig. 1. Inthe interval 42 – 85 mbsf, the shape of the trend is pseudo-cyclic with a quasi-periodicity around 0.5 my. Suchvariations are not expected on theoretical grounds(Richter and Turekian, 1993), given a residence time for  Sr in seawater of around 2 my, so we suspected thevariationswerenot real.The interval2.5 – 4.5iswhere theslope of the  87 Sr/  86 Sr-curve is lowest, yet, if accurately Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 126 – ⁎ Corresponding author.  E-mail address:  (J.M. McArthur).0031-0182/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.palaeo.2006.06.004  defined, it might be useful for Pliocene stratigraphy. Toimprove the accuracy of the curve, and to try to improveits precision, we have determined a new calibration of  87 Sr/  86 Sr against time for the interval of time between2.5 Ma and 4.5 Ma. The new calibration may be useful indatingsedimentsincarbonate-poorenvironments,suchasthose on the continental margins of Antarctica, particu-larly in the Pliocene warm interval, which peaked around3.0 – 3.5 Ma.We haveusedthe new calibration,and new analysisof  pectenids, to date the Cockburn Island Formation(Jonkers, 1998; previously the Pecten Conglomerate,termed the CIF hereinafter) on Cockburn Island,Antarctica. The formation is important in that it mayreflect a minor interglacial event and glacier retreat, so anaccurate age for the unit is pertinent to the debate on thestability of Antarctic climate and ice sheets. A date of 4.7 Ma was determined for the CIF by Dingle et al.(1997). An age of 2.5 – 3.0 Ma was yielded by diatom biostratigraphy, so the  87 Sr/  86 Sr-age was questioned byJonkers and Kelley (1998). Our new analysis of   87 Sr/  86 Sr in pectenids addresses that discrepancy.A final objective is to focus attention on the potentialfor dating with SIS, even for times when the slope of thecalibration curve is low. We do so by minimising twofurtheruncertaintiesunconnectedwiththeprecisionofthecalibration line. The first is in the measurement of  87 Sr/  86 Sr, which is minimized by replicate analysis. Thesecond,interlaboratorybias,weminimize,for otherswhomight wish to compare their data to ours, by replicatemeasurement and comparison of   87 Sr/  86 Sr in EN-1 and NIST 987 (formerly NBS987), the two standards incommon use. 2. Samples and their source For our recalibration of the marine- 87 Sr/  86 Sr curve,we analysed samples from Sicily and from ODP Site758 (Figs. 2 and 3; Table 1). For Site 758, we obtained  plugs from the ODP Core Repository at College Station,TX, from the same levels as sampled by FCG95;sedimentological details of the site are given by FCG95.In Sicily, we sampled the Punta Piccola section of theRossello Composite Section in SW Sicily: the section islocated 4 km east Capo Rossello and 3 km to the west-northwest of Porto Empedocle, Agrigento province,Sicily (Fig. 2).The Punta Piccola section is a reference standard for the Pliocene Series and so for Pliocene time. The sectionconstitutes that part of the Rossello Composite Sectionthatliesbetweeninsolationcycles352at3.65Maand250at 2.55 Ma (Fig. 3) and so spans 1.1 my of time in asequenceinwhichthecycliclimestone-marlperiodicityistightly linked to astrochronology (Langereis and Hilgen,1991; Hilgen, 1991a,b; Lourens et al., 1996, 1997), biostratigraphy (Rio et al., 1990; Sprovieri, 1992, 1993;Castradori et al.,1998) and magnetostratigraphy (Zachar-iasse et al., 1989, 1990; Zijderveld et al., 1991; Langereisand Hilgen, 1991). The sediments comprise 100 m of alternating limestones and marls of the Trubi Formation,whichgradeupwardsintothemoremarlyMonteNarboneFormation, which is characterized by the cyclical Fig. 1. Sr-isotope data of  Farrell et al. (1995) plotted against metres below sea floor and numerical age. Arrows show peaks and troughs of mainepisodic variations these occur only at levels below 42 mbsf.127  J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 126   –  136   occurrence of laminated (sapropelic) layers. The transi-tion from the Trubi to the Monte Narbone formationsoccurs in the Punta Piccola segment of the RosselloComposite at about 20 m from its base.Cockburn Island lies between Seymour Island andKing George Island at the northern end of the Antarctic peninsula (Fig. 2). The island and its pectenids aredescribedindetailinGazdzickiandWebb(1996),Jonkers (1998), and Jonkers and Kelley (1998). Our samples are large (10 cm size)  Zygochlamys  (now  Australochlamys ) anderssoni  that have suffered little abrasion (Fig. 2), theintact nature of which lead Jonkers (1998) to presume thespecimens were essentially  in situ  although disturbed by periglacial processes. Consequently, the age of the pectenids and the age of the CIF should be identical. 3. Methods and analytical results 3.1. Sample preparation Specimens of   Orbulina universa  from Sicily wereseparatedfrombulksedimentbydisaggregationinsodiumcarbonate solution, wet sieving to retain the >250  μ mfraction and hand-picking under a microscope. Pickedspecimens were glassy under the microscope (Fig. 1) andhad open pores, no noticeable overgrowths, and nointernalfillings:theywerecrushedunderultra-purewater,washed ultrasonically in water, then methanol/water, thenleached for several minutes in 10 ml of water into whichonedropof5%aceticacidwasadded,beforebeingrinsedwith ultrapure water and dried for analysis. For samples Fig. 2. Location and view of the studied sections: Punta Piccola in southern Sicily, Cockburn Island' in the Antarctic Peninsula, and ODP Site 758 inthe Indian Ocean. Insets show specimens of   Zygochlamys anderssoni  from the Cockburn Island Formation and cleaned, fragmented  Orbulinauniversa  from Sicily.128  J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 126   –  136   Fig. 3. Astrochronometric calibration of the Punta Piccola section showing the cycles sampled for this work (arrowed). From Castradori et al. (1998),with numerical ages updated from Gradstein et al. (2004). Samples span cycles 76 to A5; details are in Table 1.129  J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 126   –  136   from Site 758, mixed foraminifera were separated from bulk sediment by the methods given above, but omittingtheacidleach. Foranalysisofpectenids, we sampledonlylarge (10 cm), relatively intact, shells of   Z. anderssoni . Inthe laboratory, they were broken into pieces and the smallareas of visually altered shell were removed usingdiamond cutting tools and acid leaches. Samples werethen fragmented to sub-millimeter size, acid washed, thenwashed withultrapurewater, before being dried ina cleanenvironment. When dry, small pieces were picked for analysis under the microscope.Pickedsamplesofalltypesweredissolvedin5%aceticacid, any residue was separated bycentrifugation, andtheremaining solution was evaporated to dryness with 2.5 MHCl. The resulting solid residue was dissolved in 2.5 MHCl for separation of Sr by standard methods of ion-exchange.Formicrofossils,isotopicanalysiswasdoneonrhenium filaments using the multi-dynamic peak-switch-ing routine SrSQ (Thirlwall, 1991) with five collectors.For   Z. anderssoni , analysis was done on tantalum fila-ments using a multi-dynamic peak-switching routineSrSLL (Thirlwall, 1991) with four collectors. Data aregiven in Table 1, adjusted to a value for NIST 987 of 0.710248. During these runs, internal precision was ≤ 0.000015 (2 S.E.) and mostly <0.0000012. External precision, as judged by the 2 S.D. on the long-term meanof NIST 987, was ±0.000015.In order to assess the effect of incomplete cleaning on 87 Sr/  86 Sr of   Orbulina , specimens of one sample (85) wasalso analysed in duplicate without being cleaned. Theuncleaned specimens had  87 Sr/  86 Sr that was lower by0.000020 (Table 1) than for cleaned specimens. 3.2. Standards comparison We usedEN-1,a standardpreparedbythe USGSfromtheshellofamoderngiantclamfromEniwetokAtoll;and NIST 987, a synthetic carbonate (formerly NBS987) ob-tained from the National Bureau of Standards (now the National Institute of Standards Technology). MultiplesamplesofEN-1werepreparedthroughthefullprocedureof sample dissolution and ion-exchange separation and Table 1Values of   87 Sr/  86 Sr in  Orbulina universa  from Punta Piccola, Sicily and ODP Site 758Bed Number Age (Ma)  87 Sr/  86 Sr Means Sicily: cleaned O. universa A4/5b 2.580 0.709065 0.709067 0.709063 0.709066A4b 2.620 0.709068 0.709068A2b 2.670 0.709069 0.709069A1b 2.695 0.709069 0.709061 0.709057 0.709064 0.709055 0.709061112b 2.770 0.709071 0.709057 0.709060 0.709052 0.709062111b 2.815 0.709061 0.709066 0.709063 0.709070 0.709065110c 2.850 0.709063 0.709063 0.709063105b 2.980 0.709066 0.709061 0.709060 0.709053 0.709060100b 3.100 0.709062 0.709051 0.70905795 3.220 0.709061 0.709058 0.70906090b 3.320 0.709055 0.709063 0.70905985b 3.420 0.709063 0.709059 0.709062 0.709054 0.709046 0.709057c81g 3.531 0.709055 0.709055c80W 3.542 0.709056 0.709053 0.709055c80g 3.552 0.709054 0.709054 0.709054c77g 3.608 0.709056 0.709048 0.709051 0.709052c76g 3.638 0.709055 0.709051 0.709053 Sicily: uncleaned O. universa 85b bulk 3.420 0.70904385b bulk 3.420 0.709044 ODP Site 758A/B A: 5H-3,111 3.003 0.709062 0.709056 0.709059B: 5H-5,061 3.509 0.709057 0.709054 0.709060 0.709057A: 6H-5,071 4.014 0.709061 0.709045 0.709056 0.709051 0.709053A: 7H-2,111 4.503 0.709048 0.709051 0.709050 0.709050B: 7H-5,031 5.002 0.709030 0.709037 0.709040 0.709036A: 9H-4,042 5.999 0.708989 0.708975 0.708982Values are adjusted to NIST 987 of 0.710248. Bed numbers, numerical ages, and astrochronology from Castradori et al. (1998). Ages in Castradori et al. (1998) and Gradstein et al. (2004) for the levels reported here differ by 0.001 or less. Ages for Site 758 are those of  Farrell et al. (1995). 130  J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 126   –  136 
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