Preservation of seawater Sr and Nd isotopes in fossil ¢shteeth: bad news and good news§

E.E. Martin �, H.D. ScherDepartment of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32605, USA

Received 9 July 2003; received in revised form 5 December 2003; accepted 23 December 2003

Abstract

We analyzed 87Sr/86Sr ratios in foraminifera, pore fluids, and fish teeth for samples ranging in age from Eocene toPleistocene from four Ocean Drilling Program sites distributed around the globe: Site 1090 in the Cape Basin of theSouthern Ocean, Site 757 on the Ninetyeast Ridge in the Indian Ocean, Site 807 on the Ontong-Java Plateau in thewestern equatorial Pacific, and Site 689 on the Maud Rise in the Southern Ocean. Sr isotopic ratios for datedforaminifera consistently plot on the global seawater Sr isotope curve. For Sites 1090, 757, and 807 Sr isotopic valuesof the pore fluids are generally less radiogenic than contemporaneous seawater values, as are values for fossil fishteeth. In contrast, pore fluid 87Sr/86Sr values at Site 689 are more radiogenic than contemporaneous seawater, and thecorresponding fish teeth also record more radiogenic values. Thus, Sr isotopic values preserved in fossil fish teeth areconsistently altered in the direction of the pore fluid values; furthermore, there is a correlation between the magnitudeof the offset between the pore fluids and the seawater curve, and the associated offset between the fish teeth and theseawater curve. These data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize andexchange Sr with its surroundings during burial and diagenesis. Therefore, Sr chemostratigraphy can be used todetermine rough ages for fossil fish teeth in these cores, but cannot be used to fine-tune age models. In contrast to theSr isotopic system, our Nd concentration data, combined with published isotopic and rare earth element data, suggestthat fish teeth acquire Nd during early diagenesis while they are still in direct contact with seawater. Theconcentrations of Nd acquired at this stage are extremely high relative to the concentrations in surrounding porefluids. As a result, Nd isotopes are not altered during burial and later diagenesis. Therefore, fossil fish teeth from avariety of marine environments preserve a reliable and robust record of deep seawater Nd isotopic compositions fromthe time of deposition.; 2004 Elsevier B.V. All rights reserved.

Keywords: seawater Sr isotopes; seawater Nd isotopes; fossil ¢sh teeth; ichthyoliths

1. Introduction

Fossil ¢sh teeth, or ichthyoliths, contain highconcentrations of both Sr (V2000 ppm) and Nd(V200 ppm). As such, they are potentially valua-ble archives for isotopic studies of seawater var-iations in these two elements. In particular, Srisotope chemostratigraphy from fossil ¢sh teeth

0012-821X / 04 / $ ^ see front matter ; 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(04)00030-5

* Corresponding author. Tel. : +1-352-392-2141;Fax: +1-352-392-9294.E-mail addresses: emartin@geology.u£.edu (E.E. Martin),

hscher@u£.edu (H.D. Scher).

§ Supplementary data associated with this article can befound at doi:10.1016/S0012-821X(04)00030-5

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R

Available online at www.sciencedirect.com

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has been proposed as a dating technique for car-bonate-poor sediment, such as red clays [1,2], orterrestrial sections with altered carbonate [3]. Onthe other hand, Nd isotopic studies of ¢sh teethcan be used to track water mass circulation in thepast and study changes in continental weatheringinputs [3^6]. Other forms of biogenic apatite, suchas conodonts, have also been analyzed for Sr andNd isotopic studies on Paleozoic time scales (e.g.[7^15]). The validity of interpretations based onthese data depends on the integrity of biogenicapatite as an archive for seawater Sr and Nd iso-topic ratios, a premise that has been questionedby several authors [13,16^19], particularly for Srisotopes.

Sr and Nd are incorporated into biogenic apa-tite through di¡erent pathways, and are likely tobehave di¡erently during burial and diagenesisdue to their unique geochemical properties. Sr2þ

replaces Ca2þ in the hydroxyapatite structure dur-ing organic growth of the ¢sh teeth. Because ofthis, Sr isotopes in ¢sh teeth record the isotopicratio of the £uids in which the ¢sh live. Thus,87Sr/86Sr analyses of ¢sh teeth could potentiallybe used to study paleosalinity of ¢sh habitats[20] and to date terrestrial sections that containremains of marine migratory species [21]. Sr con-centrations in the hydroxyapatite of living ¢shrange from 1000 to 2000 ppm [20,22], while con-centrations in the hydroxy£uorapatite of fossilteeth cover a similar, but slightly wider, range of1200^5000 ppm, with most values falling between2000 and 3000 ppm [3,4,6,20].

In contrast to Sr, most of the Nd in fossil ¢shteeth is incorporated post mortem during earlydiagenesis. The hydroxyapatite of living ¢sh teethcontains parts per billion levels of Nd [23,24], butthe hydroxy£uorapatite of fossil teeth typically

contains 100^500 ppm Nd [4^6,23^26]. Recentmodeling e¡orts by Reynard et al. [27] suggestNd can be both adsorbed onto surfaces duringearly diagenesis and substituted into the crystalstructure during more extensive, late diagenesis.Studies of rare earth element (REE) patterns pre-served in fossil ¢sh teeth indicate that teeth fromdeep-sea settings record seawater compositions[25]. This supports the concept that the REEsare adsorbed or incorporated during early diagen-esis while the pore £uids are still in communica-tion with seawater. Because Nd isotopes vary withdepth in the ocean (e.g. [28^37]), the isotopic ratioof Nd incorporated in vivo should vary with thedepth habitat of the ¢sh species. Thus, an impor-tant advantage of this overwhelming diageneticaddition of Nd is that the isotopic ratio of fossilteeth will always record the composition of oceanbottom waters. As such, they represent an impor-tant paleoceanographic tool that can be appliedto reconstruct past ocean circulation patterns.Understanding changes in ocean circulationlinked to gateway events is critical for under-standing global climate change (e.g. [38^42]). Sec-ular variations in Nd isotopes can also re£ectchanges in weathering inputs from the continents(e.g. [43^45]), which can be driven by climatechange. One of the major challenges in the appli-cation of Nd isotopes to paleoceanographic stud-ies is distinguishing between these two sources ofvariation.

In order to evaluate the integrity of fossil ¢shteeth as recorders of seawater 87Sr/86Sr values, wecompare the Sr isotopic ratios preserved in fora-minifera, ¢sh teeth, and pore £uids from fourOcean Drilling Program (ODP) sites distributedaround the globe (Table 1, Fig. 1). For all sites,measured 87Sr/86Sr values of dated foraminifera

Table 1Location information for ODP sites discussed in this study

Site Location Latitude/Longitude Depth Time interval studied Average sedimentation rates(m) (Ma) (m/Myr)

ODP 1090 Agulhus Ridge 42‡54PS/8‡53PE 3702 16^40 12.5 (16^29.5 Ma), 25 (33^40.6 Ma)ODP 757 Ninetyeast Ridge 17‡01PS/88‡10PE 1652 0^37 9 (0^9 Ma), 2.5 (9^32.6 Ma),

7 (32.6^37 Ma)ODP 807 Ontong-Java Plateau 3‡36PN/156‡37PE 2804 3^13 29ODP 689 Maud Rise 64‡31PS/3‡05PE 2080 18^46 5

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plot on the global seawater Sr isotope curve de-¢ned by Hodell and Woodru¡ [46], Farrell et al.[47], Mead and Hodell [48], and Martin et al. [49],but over many intervals the 87Sr/86Sr values of the¢sh teeth are o¡set from the seawater curve andthe values of contemporaneous foraminifera inthe direction of pore £uid 87Sr/86Sr values. Thiso¡set suggests that the hydroxyapatite of the teethcontinues to recrystallize and exchange duringburial. Although this is bad news for the applica-tion of Sr chemostratigraphy using biogenic apa-tite, it does not necessarily imply that the Ndisotopic values are similarly altered. We presentseveral lines of evidence, including new Nd con-centration data and an isotopic record, that sup-port the integrity of fossil ¢sh teeth as archives forpast bottom water Nd isotopic compositions.

2. Methods

2.1. Site information and age models

Site 1090 is located on the Agulhus Ridge alongthe margin of the Cape Basin in the SouthernOcean (Table 1, Fig. 1). The interval of study atthis site is represented by 334 m of sediment rang-ing in age from the late Eocene (V40 Ma; 406mcd) to the early Miocene (V16 Ma; 72 mcd).The sediment throughout this section is composed

of mud-bearing diatom ooze, mud- and diatom-bearing nannofossil ooze and chalk, with a prom-inent chert layer at 303.4 mcd (36 Ma) [50]. Per-cent carbonate is highly variable, ranging from0.7 to 87% and averaging V25%. The top ofthis interval is bounded by a hiatus between lowerPliocene and lower Miocene sediments. Anotherhiatus from V29.5 to 33.4 Ma occurs within thelower Oligocene section. A well-de¢ned age modelfor this site is based on magneto- and biostratig-raphy, as well as Sr and O isotope chemostratig-raphy [51]. As for all sites discussed in this paper,the age model re£ects the Cande and Kent [52]time scale. Sedimentation rates are highly vari-able; they range from 5 to 38 m/Myr, with anaverage of 12.5 m/Myr below the lower Oligocenehiatus and 25 m/Myr above the hiatus. The pore£uid Sr concentration data presented for this sitewere measured on shipboard [50].

Site 757B is located on the Ninetyeast Ridge inthe Indian Ocean (Table 1, Fig. 1). All of thesamples from this site come from the upper141 m of core. They are composed of late Eocene(V37 Ma; 141 mbsf) to Pleistocene nannofossiloozes (V0.5 Ma; 6 mbsf) with V96% carbonate[53]. The initial age model for this interval basedon shipboard nannofossil biostratigraphy [53] wasconverted to Cande and Kent [52] using Mead[54] ; however, Sr isotope stratigraphy of forami-nifera and fossil ¢sh teeth revealed a previouslyunde¢ned hiatus of V5 Myr in the lower Mio-cene. Therefore, we used Sr chemostratigraphy torevise the age model for the interval from 15 to 30Myr. Sedimentation rates averaged 7 m/Myr forintervals older than 32.6 Ma, 2.5 m/Myr from8.8^32.6 Ma, and 9 m/Myr for the interval youn-ger than 8.8 Ma [53].

Site 807A is located on the Ontong-Java Pla-teau in the western equatorial Paci¢c (Table 1,Fig. 1). Samples represent the middle Miocene(V14 Ma; 389 mbsf) to the Pleistocene (V1Ma; 26.2 mbsf). This entire 363 m section consistsof nannofossil ooze, which transitions into a nan-nofossil chalk at V293 mbsf around the late/mid-dle Miocene boundary. Carbonate percentages arehigh, V92^93%, throughout the interval of inter-est. The pore £uid Sr concentration data pre-sented for this site were measured on shipboard

Fig. 1. Location map for the four ODP sites included in thestudy and Site 357, which is discussed in the text.

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[55]. Because of low remnant magnetic intensities,the age model for the Neogene at this site is basedon nannofossil datums [56] and a few foraminif-era datums [55]. Sedimentation rates range from21 to 37 m/Myr with an average of 29 m/Myr[55].

Site 689B is located on Maud Rise in the At-lantic sector of the Southern Ocean (Table 1, Fig.1). Samples from this site span from the mid Eo-cene (35.5 Ma, 132 mbsf) to the early Miocene (20Ma, 67 mbsf). This interval is composed of 65 mof diatom nannofossil ooze and nannofossil dia-tom ooze with varying amounts of carbonate thataverage 82% [57]. The pore £uid Sr concentra-tions and isotopes reported in this paper arefrom Egeberg et al. [58], who interpreted thedata to indicate that carbonate sediments at thissite have experienced unusually low rates of re-crystallization. Foraminifera 87Sr/86Sr values arefrom Mead and Hodell [48]. The age model forSite 689 is based on magnetostratigraphy [59] andplanktonic foraminifera stratigraphy [60] as pre-sented by Mead and Hodell [48]. Sedimentationrates for the interval of interest range from 1.4 to31 m/Myr, but most values fall between 1.4 and7.5 m/Myr. The average sedimentation rate is5 m/Myr [57].

2.2. Analytical methods

Foraminifera and fossil ¢sh teeth were hand-picked from the s 125 Wm fraction of sedimentthat was sonicated and sieved in deionized water.Most ¢sh teeth samples were composed of one tothree teeth, which were cleaned using the oxida-tive/reductive procedure established by Boyle [61],Boyle and Keigwin, [62], and Boyle (personalcommunication, 1993) to chemically remove fer-romanganese (Fe^Mn) coatings. Individual sam-ples of cleaned teeth and foraminifera wereweighed, dissolved in 1.8 N HCl, spiked for Srconcentration measurements, and dried. Aliquotsof pore £uids extracted from glass vials for Site757 were spiked for Sr isotopic and concentrationanalyses and dried along with unspiked aliquotsfrom Sites 1090 and 807, which were only ana-lyzed for Sr isotopic ratios at the University ofFlorida.

Sr was separated from foraminifera and pore£uid samples using Sr selective crown ether resin(Sr Specz) following a technique modi¢ed fromPin and Bassin [63]. Our Sr blank for this tech-nique is 6 100 pg. In contrast, the ¢sh teeth wereprocessed through a standard cation exchangeprocess that allowed us to isolate the Sr as wellas REEs. Sr blanks for this technique are 90 pg.

Sr from all three types of samples was loadedonto tungsten ¢laments with tantalum oxide andanalyzed for 87Sr/86Sr on a Micromass Sector 54in dynamic mode at the University of Florida.Two hundred ratios were collected at 1.5 V 88Sr.Mass fractionation was corrected to 86Sr/88Sr =0.1194. The measured 87Sr/86Sr value and the ex-ternal reproducibility of NIST-987 at the Univer-sity of Florida is 0.710245R 0.000023. This exter-nal precision is the 2c uncertainty based onreplicate analyses of NIST-987 over a period ofseveral years, and it represents the minimum un-certainty assigned to any individual sample,although within-run uncertainties were consider-ably lower (typically 0.000010^0.000018). Twenty-one replicates analyzed during the study (tables1^41 ) yielded an average di¡erence of 0.000015.

Nd concentration data were determined by iso-tope dilution thermal ionization mass spectrome-try. Cleaned, dissolved teeth were spiked for Ndconcentrations along with Sr. REEs as a wholewere isolated on the ¢rst cation exchange columnand then processed through an additional cationexchange column using methylacetic acid as aneluent to isolate the Nd and Sm. The total Ndprocedural blank is 6 pg. Samples were loadedonto Re ¢laments with silica gel and analyzed asNdOþ. Two hundred ratios were collected at 500mV 142Nd16O. Mass fractionation was correctedto 146NdO/144NdO=0.722254.

3. Results

3.1. Site 1090 Sr isotopes

Site 1090 can be divided into three sections

1 see online version of this article.

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based on Sr geochemistry (Fig. 2a, table 11) : theinterval below the chert layer in the late Eocene(section I), the section between the chert layer andthe early Oligocene hiatus (section II), and thesection between the early Oligocene and middleMiocene hiatuses (section III). The chert layerappears to create a di¡usional barrier for pore£uid Sr, such that the pore £uids above and belowthis barrier have evolved separately. High Sr con-centrations [50] below the chert layer (s 850Wmol) coupled with relatively low values abovethe layer (6 400 Wmol) (Fig. 2a) indicate thatSr2þ sourced through carbonate diagenesis atdepth has not di¡used into the overlying sedi-ment.

Below the early Oligocene hiatus (sections I andII) 87Sr/86Sr values for the pore £uids plot within0.00006^0.00010 of the seawater curve (Fig. 2a).However, there is a subtle distinction between thesections. Pore £uid Sr isotopic values in section Iare slightly less radiogenic than the seawatercurve, while the values in section II are slightlymore radiogenic. For both of these sections 87Sr/86Sr ratios of foraminifera and ¢sh teeth generallyplot within error of, or on, the seawater curve;yet, like the pore £uids, the teeth in section I tendto be slightly less radiogenic, and the teeth insection II tend to be slightly more radiogenic,than contemporaneous foraminifera.

Above the early Oligocene hiatus (section III)

Fig. 2. Sr isotopes and pore £uid concentrations versus age for (a) Site 1090, (b) Site 757, (c) Site 807, and (d) Site 689. Pore £u-id Sr concentrations were measured on shipboard for Site 1090 [50] and Site 807 [55]. For Site 689 pore £uid Sr concentrationsand isotopes are from Egeberg et al. [58] and isotopic values for foraminifera are from Mead and Hodell [48]. The double solidlines represent the error window for the seawater 87Sr/87Sr curve. These lines were drawn through values 0.000023 higher andlower than the seawater 87Sr/87Sr curve de¢ned by ¢tting a ninth order polynomial to the isotopic data from Hodell and Wood-ru¡ [46], Farrell et al. [47], Mead and Hodell [48], and Martin et al. [49]. Open squares represent ¢sh teeth 87Sr/87Sr, ¢lled circlesrepresent foraminifera 87Sr/87Sr, ¢lled triangles represent pore £uid 87Sr/87Sr, and ¢lled diamonds represent pore £uid Sr concen-trations. Note there are di¡erent scales for 87Sr/87Sr, Sr concentration, and age for individual plots.

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pore £uid 87Sr/86Sr values are less radiogenic thancontemporaneous seawater by 0.00010^0.00030(Fig. 2a). The largest o¡set is observed in theshallowest sample, and the o¡set progressively de-creases with depth. Foraminifera plot directly onthe seawater curve, while fossil ¢sh teeth plot0.00003^0.00010 below the seawater curve andbelow foraminifera from the same sample. Sr iso-tope chemostratigraphy of these teeth would yieldage estimates that are V1^2 Myr too old.

3.2. Site 757 Sr isotopes

The pore £uid Sr data from Site 757 produce atypical pro¢le for a system dominated by carbon-ate diagenesis [64,65], with a concentration max-imum at V12 Ma (Fig. 2b, table 21). Below theearly to middle Miocene hiatus, foraminifera and¢sh teeth samples tend to plot within error of theseawater curve. For this older interval foraminif-era samples have been ¢t to the seawater curve aspart of the Sr chemostratigraphic age model dis-cussed above. There is only one pore £uid 87Sr/86Sr value below the hiatus, and this plots 0.00010above the seawater curve. In contrast, pore £uidvalues above the hiatus plot below the seawatercurve by as much as 0.00045, although the o¡setin the youngest sample is only 0.00012 (Fig. 2b).Throughout most of the Pliocene/Miocene inter-val 87Sr/86Sr values of foraminifera plot on theseawater curve, but values for ¢sh teeth areV0.00015 lower than those for contemporaneousforaminifera and the seawater curve. The magni-tude of this o¡set decreases moving up the core asthe di¡erence between the pore £uids and the sea-water curve decreases. The maximum o¡set in thePliocene/Miocene translates to Sr chemostrati-graphic ages as much as V4.5 Myr too old.

3.3. Site 807 Sr isotopes

Pore £uid Sr concentration data for Site 807[55] also exhibit a typical carbonate diageneticpattern (Fig. 2c, table 31). Although we do nothave as many isotopic data for this site, the87Sr/86Sr patterns are similar to those reportedfor Sites 1090 and 757. Pore £uid 87Sr/86Sr valuesplot V0.00008 below the seawater curve, the fo-

raminifera plot on the curve, and ¢sh teeth valuesplot V0.00005 below contemporaneous forami-nifera, although the error bars generally overlap.In the worst case, ages derived from Sr isotopechemostratigraphy of these teeth would be 2 Myrtoo old.

3.4. Site 689 Sr isotopes

Site 689 is an important diagnostic site becausethe pore £uid Sr concentrations and isotopes [58]are distinct from the other sites (Fig. 2d, table 41).Due to very low rates of carbonate recrystalliza-tion, pore £uid concentrations of Sr are 6 200Wmol. Unlike the previous sites, the pore £uidsystem at this site appears to be dominated byreactions with siliceous detrital material [58],which has an isotopic ratio highly distinct fromseawater. Thus, the contribution of Sr to the pore£uids is small, but it has a major impact on theisotopic ratio. Pore £uid 87Sr/86Sr values belowthe hiatus are more radiogenic than the seawatercurve by 0.00050^0.00060. Throughout this inter-val the foraminifera [48] plot on the seawatercurve, but ¢sh teeth from the same samples plotabove the curve by V0.00015 (Fig. 2d). As aresult, ages estimated from Sr chemostratigraphyof the teeth would be V4 Myr too young.

3.5. Nd concentrations in ¢sh teeth

Concentrations of Nd in the ¢sh teeth arehighly variable, ranging from 50 to 1000 ppm,although most of the teeth contain 100^500 ppmNd (Fig. 3). When all the sites are plotted togeth-er there is an apparent trend of increasing Ndconcentration with age, but this trend is largelyan artifact of the distribution of ages representedby sites with distinct concentrations. Examinationof each site individually reveals that there is notrend in concentration with age for three of thesites (1090, 757, and 807). For these sites therange in Nd concentrations for the youngest sam-ples is equivalent to the range for the oldest sam-ples. The highest concentrations overall are ob-served in the youngest samples from Site 689,and there is a general decreasing pattern with in-creasing age for this site (Fig. 3).

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4. Discussion

4.1. Sr isotopes: the bad news

For all four sites 87Sr/86Sr data for foraminifera(Fig. 2a^d) consistently plot on the seawater Srisotope curve [46^49]. For most of the intervalsstudied at Sites 1090, 757, and 807 pore £uid 87Sr/86Sr values are less radiogenic than seawater val-ues by substantial amounts (0.00008^0.00045).Over these intervals, the values for fossil ¢sh teethare also less radiogenic than contemporaneousseawater (Fig. 2a^c). In contrast, pore £uid 87Sr/86Sr values are signi¢cantly more radiogenic thanseawater at Site 689 (Fig. 2d), as are the values for¢sh teeth. Even over the intervals with less dra-matic o¡sets between the pore £uid and seawater(9 0.00010), 87Sr/86Sr values of the ¢sh teeth areconsistently o¡set in the direction of the pore £u-id, although the Sr isotopic values of the forami-nifera and the teeth tend to agree within error.These data imply that the teeth continue to re-crystallize during burial and their Sr isotopic val-ues are altered through exchange with pore £uidSr2þ. Barrat et al. [19] reached a similar conclu-sion in a study of Neogene biogenic phosphatesexposed on land.

Below the pore £uid Sr2þ concentration maxi-ma at Sites 757 and 807, the pore £uid 87Sr/86Srvalues and associated ¢sh teeth values tend to plotcloser to the seawater curve (Fig. 2b,c). In thesecarbonate-rich sites, the Sr2þ maximum coincideswith maximum rates of carbonate recrystallization[64,65]. Below the concentration maximum thepore £uid Sr2þ system is dominated by carbonatereactions rather than di¡usion, thus most of theSr2þ introduced to the pore £uid comes from al-teration of in situ foraminifera, which have theisotopic compositions of contemporaneous sea-water. In contrast, above the concentration Sr2þ

maximum most of the Sr2þ is introduced to thepore £uids as an upward di¡usive £ux, which de-livers Sr2þ with older, less radiogenic 87Sr/86Srvalues than the surrounding carbonate sediment.

Shemesh [66] suggested that the Sr content ofbiogenic apatite should decrease during recrystal-lization based on a negative correlation betweenhis crystallinity index and Sr concentrations forfossil ¢sh teeth. We have not quanti¢ed the extentof recrystallization of the ¢sh teeth in this study,but our results clearly suggest that the hydroxy-£uorapatite of the fossil ¢sh teeth can have higherSr concentrations (up to 5500 ppm) than the hy-droxyapatite of living specimens (up to 2100 ppm)(Fig. 4), although the two populations overlapand there are a limited number of modern speci-

Fig. 3. Concentrations of Nd (ppm) in fossil ¢sh teeth versusage for Sites 1090, 757, 807, and 689. Within the individualsites there is no consistent trend with age. The oldest samplesfor each site tend to have similar or slightly lower concentra-tions than the youngest samples, suggesting that Nd fromthe pore £uids is not being added to the hydroxy£uorapatiteduring burial.

Fig. 4. Concentrations of Sr (ppm) in the ¢sh teeth versusage for all four sites. Sr concentrations tend to be a bit high-er in the fossil specimens compared to the modern speci-mens. Data for modern specimens from Table 2 and Venne-mann et al. [22].

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mens (n=15). This suggests that Sr2þ continues tobe incorporated into the ¢sh tooth structure dur-ing diagenetic exchange with pore £uids.

Teeth from Site 807 have Sr concentrations thatare higher than, and beyond the range of, all oth-er sites (Fig. 4). Yet, the pore £uid Sr2þ concen-trations at Site 807 are not any higher than thoseobserved in parts of Site 1090, and are onlyslightly higher than those observed at Site 757.Martin and Haley [6] suggested that the di¡eren-ces in Sr concentrations of fossil teeth betweensites could re£ect the initial concentrations forteeth from di¡erent regions or for di¡erent typesof ¢sh. However, data on teeth from living ¢sh donot suggest any distinct populations; similar con-centrations are reported for teeth from a widerange of locations, such as o¡ South Africa, inVictor Bay (Canada), Prince William Sound(Alaska) [22], and the Gulf of Mexico and Atlan-tic regions o¡ of Florida (Table 2). Also, measure-ments from di¡erent types of ¢sh including shark([22] and Table 2), mackerel, £ounder, grouper,and snapper (Table 2) do not indicate any distinctpopulations at the family level.

The degree to which initial 87Sr/86Sr values ofthe teeth have been altered is a function of numer-ous factors including, but probably not limited to,the timing, duration, and rate of recrystallization,the composition of the pore £uids, and the slopeof the seawater curve over the interval of interest.For the sites in this study, Sr chemostratigraphicdetermination of ages using fossil ¢sh teeth intro-duced errors of 0^4.5 Myr. Thus, Sr isotopes fromfossil ¢sh teeth recovered from these deep marine

sediments can be used to de¢ne gross age esti-mates ( R 5 Myr), but cannot be used to ¢ne-tune age models.

The implications of these results for the appli-cation of Sr chemostratigraphy to red clay sec-tions [1,2] and Paleozoic studies using conodonts(e.g. [8,12^14,23]) are not clear. There are limiteddata on pore £uid geochemistry for red clay sites.Pore £uid Sr concentrations from ODP Site 886 inthe central north Paci¢c range from 87 to 99 Wmol[67], which are only slightly higher than, or equiv-alent to, the seawater value of 87 Wmol. These lowvalues suggest that in the absence of abundantcarbonate the extent of diagenetic alteration in-volving Sr may be minimal and, thus, pore £uid87Sr/86Sr values may be close to initial seawatervalues. However, there is evidence for alterationof clay material and ash at red clay sites [68,69],which would introduce 87Sr/86Sr values very dis-tinct from seawater. Our data from Site 689 sug-gest that even small amounts of silicate alterationcan have a signi¢cant impact on the 87Sr/86Sr val-ues preserved in fossil ¢sh teeth.

The magnitude of the error in the age estimatesobserved in this study is similar to the error asso-ciated with many Paleozoic radiometric datingtechniques. However, data from this study onlyinvestigated the e¡ects of burial diagenesis in adeep-sea environment; Paleozoic samples havebeen exposed to more extensive meteoric diagen-esis as well, which is likely to introduce Sr withmore terrestrial values.

4.2. Nd isotopes: the good news

Although the Sr isotopes in fossil ¢sh teethstudied here have clearly been altered in the pres-ence of pore £uids, studies of Nd isotopes andconcentrations in fossil ¢sh teeth suggest the teethpreserve an early diagenetic signal that is repre-sentative of Nd from bottom water. This dichot-omy can be attributed to the fact that Nd exhibitsdi¡erent geochemical behavior than Sr. In partic-ular, Nd is highly particle reactive and there arefew diagenetic sources to the pore £uids. As aresult, concentrations of Nd3þ in the pore £uidsare extremely low; there are no published Nd3þ

concentration data from deep-sea pore £uids be-

Table 2Sr concentration data for modern ¢sh teeth from the Gulfand Atlantic regions o¡ Florida (measured at the Universityof Florida)

Type of ¢sh Sr(ppm)

Shark 1655Snapper 1155Snapper 1602Mackerel 1316Mackerel 1448Mackerel 1573Grouper 1468Halibut 1734

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cause the concentrations and small £uid volumesyield samples that are below the detection limitsof analytical techniques. In addition, Nd is prob-ably less mobile than Sr during £uid^rock inter-actions in the deep-sea environment.

There is abundant evidence for the distinct be-havior of the two isotopic systems in studies ofigneous rocks. Both high temperature and lowtemperature studies consistently illustrate thatthe Sm^Nd system is more resistant to alterationduring post-magmatic water^rock interactionsthan the Rb^Sr system. During low temperatureconditions, Cousens et al. [70] demonstrated thatexchange between meteoric waters and silicicignimbrites altered initial 87Sr/86Sr ratios, butdid not a¡ect Nd isotopes. Under high temper-ature conditions, hydrothermal alteration of ande-sites, rhyolites [71] and granites [72] again alteredSr isotopes but did not perturb Nd isotopes. Ndwas not even mobilized during metamorphic min-eralization of Precambrian granitic gneisses [73].As mentioned, one explanation for the resistanceof Nd isotopes to alteration is the low concentra-tion of Nd3þ in circulating £uids relative to theconcentrations in the minerals themselves [71].

In the marine realm, Henderson and Burton[74] calculated di¡usion rates for several elementsused as isotopic tracers in Fe^Mn crusts, includ-ing Sr and Nd. Fe^Mn crusts are exposed to ma-rine conditions similar to those experienced byfossil ¢sh teeth. In their study they compared Uand Th isotope pro¢les to concentration pro¢lesfor Li, Os, Sr, Nd, Pb and Be in a crust. A calcu-lated U di¡usion rate was combined with data onthe distribution coe⁄cients for each element rela-tive to U in order to assess the di¡usion rates.Their results suggest that the di¡usion rate forSr is quite high relative to the growth rate ofthe crusts, and therefore the crusts will not pre-serve secular variations in the 87Sr/86Sr ratio ofseawater, as has been observed [75,76]. In con-trast, calculated Nd di¡usion rates were quitelow, implying Nd is highly immobile. The di¡u-sion rates for Fe^Mn crusts would not apply to¢sh teeth, however, the results again support theidea that Nd is geochemically less mobile andmore resistant to alteration than Sr.

For biogenic apatite, Holmden et al. [13] dem-

onstrated that plots of 87Sr/86Sr versus Sr concen-trations for Ordovician phosphatic brachiopods,conulariids, and conodonts create a linear trend,indicating post-depositional exchange of Sr. Thesesame samples yield identical initial 143Nd/144Ndvalues over a wide range of Nd concentrations,which are interpreted as unaltered, initial 143Nd/144Nd values.

One technique that can be used to test the mo-bility of REEs speci¢cally in teeth is to comparethe REE patterns and Nd isotopes in the teeth tothose in seawater. Elder¢eld and Pagett [25] ar-gued that the heavy REE enrichments observed inichthyoliths from deep-sea environments re£ectearly diagenetic £uids that have REE patternssimilar to seawater. Grandjean et al. [5] demon-strated that ¢sh teeth taken from terrestrial sec-tions ranging in age from the Triassic to thepresent display two distinct REE patterns: onethat is typical of seawater and another with amiddle REE enrichment pattern that Reynard etal. [27] refer to as a ‘bell-shaped’ pattern. In amodeling study using REE partition coe⁄cientsextrapolated from mineral/melt partition data,Reynard et al. [27] determined that the bell shapepattern can be produced by crystal-chemicallycontrolled fractionation between apatite and sea-water or continental £uids at low temperature, orso-called late diagenesis. Preservation of seawater-like REE patterns in many of the teeth from old-er, terrestrial sections [5] suggests that in somecases even teeth exposed to more extreme condi-tions than the deep-sea environment preserve sea-water REE patterns. Evaluation of the REE pat-terns in the teeth o¡ers a potential technique fortesting the integrity of the teeth as an archive forNd isotopic studies of paleoseawater.

The correlation between Nd isotopic variationspreserved in fossil ¢sh teeth and those in Fe^Mncrusts and coatings also indicates that both ar-chives record a paleoseawater signature. Martinand Haley [6] illustrated that data from teeth fortwo sites in the eastern Paci¢c recorded the sametrends and similar isotopic ratios to data fromFe^Mn crusts from similar depths and watermasses reported by Ling et al. [77]. In addition,Scher and Martin [78] showed that a high resolu-tion record of Nd isotopes from fossil teeth from

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the Cape Basin con¢rms the secular trend re-ported by Palmer and Elder¢eld [79] based ondata from Fe^Mn coatings on foraminifera fromthe South Atlantic (Fig. 5). The fact that di¡erentarchives preserve the same record is evidence thatboth are recording a seawater signal. An unlikelyalternative explanation is that both archives havebeen altered through diagenetic processes andhave coincidentally preserved the same values indi¡erent cores with di¡erent sediment composi-tions.

The preliminary data from Site 1090 [78] alsodemonstrate that ¢sh teeth record short-lived,rapid variations in the Nd isotopic compositionof seawater (Fig. 5), which may be a response tocontinental weathering or ocean circulation.These rapid variations suggest the isotopic signalhas not been blurred by diagenetic alteration. Incontrast, Fe^Mn crusts grow very slowly, 1^10mm/Myr, and therefore tend to integrate seawaterisotopic values over long periods of time. Forexample, individual samples from cores takenfrom the Southern Ocean are believed to integratethe signal over several glacial^interglacial cycles[80].

Finally, measured Nd concentrations are simi-lar in the youngest and oldest ¢sh teeth from eachsite (Fig. 3). This suggests that the Nd is not beingsigni¢cantly altered during burial. Staudigel et al.[4], Elder¢eld and Pagett [25], and Grandjean etal. [5] also noted this lack of correlation betweenNd concentration and age or depth. Bernat [81]documented that even teeth within the top fewmillimeters of sediment have high concentrationssimilar to those at depth. These data imply thatthe teeth incorporate or adsorb Nd during veryearly diagenesis and that the concentration of theNd does not alter with time and/or burial.

Although the typical tooth in our study con-tained 100^500 ppm Nd, there are reports of con-centrations for cleaned teeth from deep-sea sedi-ments that are as high as s 1000 ppm [6]. AsStaudigel et al. [4] pointed out, these high concen-trations imply that the hydroxy£uorapatite struc-ture or surface adsorption is capable of incorpo-rating much more Nd than is found in a typicalfossil tooth. The Sr isotopic data clearly implythat the teeth continue to alter and/or recrystallize

with burial. Thus, the fact that the Nd concentra-tions do not increase with depth suggests there isno source of Nd to the pore £uids. Instead Nd isintroduced to the system from seawater at theseawater/sediment interface. During burial muchof that Nd is removed from the pore £uid by ¢shteeth and Fe^Mn oxides, but little is addedthrough diagenesis. This process could alterREE patterns, but is unlikely to a¡ect the Ndisotopic ratios. In most of the sites we studied,the pore £uid geochemistry records a historydominated by carbonate diagenesis. Because theconcentration of Nd in carbonate is extremelylow, 6 1 ppm [82], this process does not introducesigni¢cant Nd into the pore £uid. Even alterationof ash with highly distinct Nd isotopic values doesnot appear to introduce enough Nd to the pore£uid to alter the isotopic ratio of the ¢sh teeth.Sediment from Site 786 in the Izu-Bonin Arc hasa low carbonate content, a high clay content, anda persistent volcanogenic component (5^57% vit-ric debris [83]). ONd values for volcanic materialfrom this site range from +6.2 to +9.3 [84] ; how-ever, ¢sh teeth Nd isotopic values from this siteare consistent with seawater values from othersites [6].

If the only source of Nd to the ¢sh teeth isseawater, one might expect the teeth in contact

Fig. 5. Variations in ONdðTÞ preserved in fossil ¢sh teeth fromSite 1090 [78] and Fe^Mn oxide coatings on foraminiferafrom Site 357 [79]. The excellent correlation between the twodi¡erent archives from sites in the South Atlantic argues thatboth preserve the seawater isotopic value. The high resolu-tion variations preserved in Site 1090 also indicate that therecord has not been compromised by diagenetic alteration.

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with seawater the longest (slowest sedimentationrates) to have the highest Nd concentrations, asproposed by Elder¢eld and Pagett [25]. Fig. 6 il-lustrates that lower Nd concentrations (6 500ppm) occur over a wide range of sedimentationrates; however, there is a general correlation be-tween the concentrations observed in the teeth ateach site and sedimentation rates. For example,Site 689 is characterized by the lowest sedimenta-tion rates and the highest maximum Nd concen-trations, Site 1090 has intermediate sedimentationrates and intermediate maximum concentrations,and Site 807 has the highest sedimentation ratesand the lowest maximum Nd concentrations (Ta-ble 1 and tables 1^41). Site 757, which has low tointermediate sedimentation rates, does not followthis pattern; however, within this site there isclearly a correlation between the sedimentationrate and the Nd concentration in the teeth (Fig.7). Thus, there is a general correlation betweenthe amount of time teeth were exposed directlyto seawater and their Nd concentrations. Di¡er-ences between sites and intervals within one sitemay relate to di¡erences in bottom water chem-istry or variations in the water masses occupying

the sites at a given time, since the concentration ofNd in deep water changes along the £ow path[30^33].

5. Conclusions

Sr isotopic analyses of Eocene to recent fossil¢sh teeth from four ODP sites illustrate that 87Sr/86Sr values of the hydroxy£uorapatite of fossil¢sh teeth can be altered through interactionswith pore £uid Sr2þ. For all four sites the 87Sr/86Sr values of foraminifera consistently fall on theseawater Sr isotope curve. In contrast, pore £uid87Sr/86Sr values are generally less radiogenic thanthe seawater curve for the sites dominated by car-bonate diagenesis (Sites 1090, 757, and 807) andmore radiogenic than the seawater curve for thesite dominated by alteration of detrital silicates(Site 689). The Sr isotopic values of the fossil¢sh teeth are o¡set from the seawater curve inthe direction of the pore £uids. The greatest o¡-sets coincide with intervals exhibiting the greatestdi¡erence between the seawater curve and pore£uid values. This suggests that the teeth recrystal-lize and exchange Sr during burial and diagenesis.Because of this alteration, ages determined by Srchemostratigraphy for fossil ¢sh teeth from some

Fig. 6. Concentrations of Nd (ppm) in ¢sh teeth versus sedi-mentation rates for all four sites. There is a lot of scatterwithin each site, particularly for lower concentrations, butthe maximum values for each site tend to correlate with sedi-mentation rates. Higher maximum Nd concentrations occurin sites with low sedimentation rates, implying the teeth de-rive their Nd from seawater and can incorporate or adsorbmore Nd when they are exposed to seawater for a longer pe-riod of time.

Fig. 7. A detailed comparison of the Nd concentrations(ppm) in ¢sh teeth samples and sedimentation rates for Site757. In general, higher concentrations again coincide withlower sedimentation rates.

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deep-sea environments may be up to 5 Myr tooold or too young.

Sr and Nd are incorporated into fossil ¢sh teeththrough di¡erent processes and they exhibit dis-tinct geochemical behavior; therefore, poor pres-ervation of seawater Sr isotopes does not neces-sarily imply poor preservation of Nd isotopes.Geochemically, Nd is highly particle reactive. Asa result, it is less mobile than Sr during water^rock interactions. Furthermore, the concentra-tions of Nd3þ in marine pore £uids are extremelylow. Because there is so little Nd3þ available fromthe pore £uid during burial, the teeth must incor-porate or adsorb Nd during early diagenesis whilethe pore £uids are still in direct communicationwith seawater. Evidence for this early, post-mor-tem uptake of deep seawater Nd includes: (1) fos-sil ¢sh teeth recovered from the deep sea preserveREE patterns similar to seawater patterns, (2)teeth from di¡erent locations that were exposedto similar bottom water masses, but contain dis-tinct lithologies and pore £uid geochemistries,preserve similar Nd isotopic ratios, (3) ¢sh teethand Fe^Mn oxide crusts or coatings from local-ities exposed to similar bottom water masses pre-serve similar Nd isotopic ratios, (4) teeth preservehigh concentrations of Nd in the surface sedi-ment, and concentrations do not appear to in-crease with burial depth and age, and (5) teeththat were exposed to seawater longer, or wereburied at slower sedimentation rates, tend to yieldthe highest Nd concentrations. Therefore, fossil¢sh teeth from marine sediments appear to berobust archives for deep seawater Nd isotopes.

Acknowledgements

We would like to thank Brian Haley for hiswork on Site 807 and Alisa Hasse for her workon Site 757. Ray Thomas provided valuable tech-nical support and Ann Heatherington contributedher analytical expertise. Reviews by James Glea-son and an anonymous reviewer greatly improvedthe manuscript. This research was supported byNSF Grant OCE-9629370 (E.E.M.). Sampleswere provided by the Ocean Drilling Program.[BOYLE]

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