The variation of the trace-element content of fossil biogenic apatite through eustatic sea-level...

Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 325–342

The variation of the trace-element content of fossil biogenic apatitethrough eustatic sea-level cycles

B. Laenen a,*, J. Hertogen b, N. Vandenberghe aa Afd. Historische Geologie, University of Leuven, Redingenstraat 16, B-3000 Leuven, Belgium

b Afd. Fysico-chemische Geologie, University of Leuven, Celestijnenlaan 200C, B-3001 Leuven-Heverlee, Belgium

Received 20 February 1995; accepted 19 March 1997

Abstract

Shark teeth from the Oligocene Boom Clay Formation (Belgium) have been analyzed for trace elements by neutronactivation analysis to examine whether biogenic apatite might record changes in seawater trace-element chemistrythroughout a eustatic sequence. An additional set of samples from a wide range of depositional environments hasbeen studied to shed light on the nature of factors that govern the trace-element variation of fossil apatite.

Biogenic apatite samples from a given sedimentary basin exhibit rather specific and distinctive trace-elementfingerprints, which shows that they reliably record geochemical parameters of the sedimentary environment. It appearsthat sea-level changes do not substantially affect geochemical processes in an epicontinental marine sedimentary basin.But minor chemical changes do coincide with sequence stratigraphic levels. Slightly higher La/Yb ratios correlatewith periods of higher turbulence. Positive Ce anomalies indicate that a large fraction of the REE adsorbed by theshark teeth can be derived from the upward flux of reduced pore water. This process greatly diminishes the usefulnessof the Ce anomaly as a palaeoredox probe. A factor analysis performed on trace-element data and sedimentaryenvironment parameters suggests that REE contents, REE/Sc and Th/REE ratios are related to sediment input.© 1997 Elsevier Science B.V.

Keywords: biogenic apatite; trace elements; REE; eustasy; marine geochemistry; Boom Clay

1. Introduction promising authigenic phases in this respect, as itis present in most siliciclastic deposits and isstrongly enriched in a large suite of trace elements.The trace-element content of a cratonic shale isBecause of the presumed correlation between thelargely controlled by its mineralogical components.trace-element characteristics of biogenic apatiteSince the latter are almost entirely of detritaland the depositional environment, trace elementsorigin, trace-element analysis of bulk samples pro-of fossil bones have been used in a wide range ofvides little information on the chemistry of thepalaeo-environmental studies ( Williams, 1989).depositional environment. In principle, the geo-

The enrichment of fossil bones in most tracechemistry of authigenic phases should be moreelements occurs in a postmortal stage shortly afterrevealing. Biogenic apatite is one of the morethe death of the organism (Bernat, 1975;

* Corresponding author. Research assistant of NFWO. Henderson et al., 1983; Wright et al., 1987) and is

0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.PII S0031-0182 ( 97 ) 00068-0

326 B. Laenen et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 325–342

associated with crystallographic changes of the are amplified — as compared to a pelagic environ-ment — by the relatively shallow water depth andapatite (Shemesh, 1990). Once recrystallized, fossil

apatite seems to be rather insensitive to diagenetic the impact of sea-level changes on the position ofthe mixing zone between oceanic and riverinegeochemical changes (Grandjean et al., 1987;

Wright et al., 1987). As most of the enrichment water masses within the sedimentary basin.takes place during sedimentation and early burialdiagenesis, the trace-element composition of theapatite is controlled by the chemical characteristics 2. Locationof the water at the sediment–seawater interfaceand of the pore water of the depositional environ- The Boom Clay Formation is exposed in north-

eastern Belgium and the type area is situated inment (Henderson et al., 1983; Williams andMarlow, 1987; Newesely, 1989; Williams, 1989). the vicinity of Antwerp (Fig. 1). It spans most of

the Oligocene as indicated by the presence ofThe pore-water composition of the upper fewcentimetres of a marine sediment is influenced by planktonic foraminifera zones P18 to P21

(Hooyberghs, 1983) and nannoplankton zonelithology, Eh, pH and early diagenetic reactions.It is widely assumed that the trace-element concen- NP23 (Steurbaut, 1992). It is a siliciclastic sedi-

ment deposited in a marine environmenttration patterns — in particular of the rare-earthelements (REE) — of the interface water and pore (Vandenberghe, 1974; Vandenberghe and Van

Echelpoel, 1987) at the southern border of thewater are strongly coupled to those of the overlyingseawater, because the concentration patterns are epicontinental North Sea Basin (Ziegler, 1989). It

has been inferred from sedimentological andprimarily determined by the disintegration of labileauthigenic phases (Fe–Mn-oxyhydroxides, floccu- palaeontological information that the water depth

in the type area varied from a minium of 50 m uplated organic colloids) that originally formedwithin the water column (Hoyle et al., 1984; to a maximum of 200 m. This last figure is proba-

bly too high; 100 m seems a more plausible esti-Grandjean and Albarede, 1989; Sholkovitz et al.,1989). A number of previous geochemical studies mate (Van Echelpoel, 1991). The deepening of the

environment to the NNW resulted in thicker andof fish remains have indicated that fossil biogenicapatite is indeed a potentially useful indicator of finer-grained beds in NW Belgium.

Key sedimentological features of the Boom Claythe palaeochemistry of the marine environment(Shaw and Wasserburg, 1985; Grandjean et al., Formation are summarized in Fig. 2. Cyclicities in

grain size, total organic material content and car-1987, 1993; Wright et al., 1987; Grandjean andAlbarede, 1989; Schmitz et al., 1991). bonate content gave rise to a very characteristic

banding. The grain-size variations caused anThe present geochemical study of shark teethfrom the Oligocene Boom Clay Formation alternation of silt-rich and clay-rich beds which

can be correlated over the entire type area. The(Belgium) was undertaken to investigate whetherfossil biogenic apatite might record changes in the deposition of silt-rich layers is attributed to

increased turbulence ( Vandenberghe, 1974).seawater trace-element chemistry resulting fromeustatic sea-level fluctuations and any attending Fourier and Walsh spectral analyses of grain-size

and organic material cyclic data revealed regularclimatic/erosional changes. The Boom ClayFormation is well suited for this purpose, because periodicities of approximately 100 cm (146–98 cm)

and 46 cm (47–43 cm). The estimated periods, theit been the subject of high-resolution mineralogicaland cyclostratigraphic investigations (Vanden- regularity of the cyclicities and the sedimentologic

properties of the deposit suggest a link with theberghe, 1974; Van Echelpoel, 1991) and was usedto test relative sea-level theory (Stover and 100- and 41-ka fourth- and higher-order

Milankovitch cycles, respectively (Van Echelpoel,Hardenbol, 1993). The Boom Clay was depositedin an epicontinental marine basin. One might 1991). The short-term periodicities are superim-

posed on a third-order eustasy trend with a period-expect that in such an environment the effects ofeustasy changes on the water column chemistry icity of about 2 Ma (shown in Fig. 2 as the

327B. Laenen et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 325–342

Fig. 1. Location and outcrop map of the Boom Clay Formation.

envelope of the higher-order cycles). The coarsest even outside the type area of the Boom Clay(Vandenberghe and Laga, 1986).zone within the clay corresponds to a Type-2

sequence boundary (SB2; Van Wagoner et al.,1988). The zones with minima in grain size corre-spond with maximum flooding surfaces (mfs1, 3. Samplesmfs2). A smooth variation in the thickness of thebeds occurs throughout the section. It is interpre- 3.1. Boom Clay samplested as a change in the rate of sediment supply andprobably is of tectonic origin (Van Echelpoel, A set of 54 shark teeth samples was taken from

four quarries in the type area of the Boom Clay:1991). Early diagenetic remobilisation of carbon-ate gave rise to the cyclic occurrence of septaria Kruibeke (sample prefix Kb), Terhagen (prefixes:

Th and He), Steendorp (prefix St) and Sint-layers (marked S10, S20, etc., in Fig. 2). Theseptaria layers are very useful correlation tools Niklaas (prefix Sn) (Figs. 1 and 3). From all the


Fig. 2. Composite sedimentological log of the Boom Clay Formation. The first log represents the grain-size variations which reflectfourth- and higher-order Milankovitch periodicities. The envelope of the grain-size variation log reflects the third- order relative sea-level signal. The position of the maximum flooding surfaces of the first and second third-order eustasy cycles (mfs1 and mfs2) andof the Type-2 sequence boundary (SB2) is indicated. The second log shows the thickness of each silt–clay couple, which is a measureof the sedimentation rate because each couple has been deposited in an approximately equal interval of time. The corrected thicknessincludes a correction for the amount of clay removed during the deposition of a silt bed (Van Echelpoel, 1991). The silt/clay ratio,represented in the third log, is the ratio of the thickness of the silt bed and the clay bed in each couple. The carbonate and totalorganic material content data are taken from Vandenberghe (1974).

beds exposed in these quarries, 30 kg of sediment collection 40 samples of 1 to 3 teeth of Squalusalsaticus Andreae (1892) were used for analysis.were disintegrated in water containing H2O2 and

sieved on 0.5 mm. The teeth were determined and An additional set of 14 teeth of Squalus alsaticusAndreae (1892) was taken from a collection fromdescribed at the Royal Belgian Institute of Natural

Sciences (Steurbaut and Herman, 1978). From this the Museum of Geology and Mineralogy of

Fig. 3. Stratigraphic distribution of analysed shark teeth samples from the four different quarries. Sample positions are measuredrelative to the maximum flooding surface of the first third-order sequence (0m=reference position on the graph). The insets showthe NASC normalized (Gromet et al., 1984) REE patterns of the different quarries. The upper inset combines the REE patterns ofthe Terhagen and Kruibeke quarry, the one in the middle those of the Steendorp quarry, and the lowermost compiles the Sint-Niklaas samples.


Leiden, The Netherlands. These teeth samples were sedimentologic data. The shelf position (SP) is ameasure of the distance towards the coastline andseparated in much the same way; except that no

H2O2 was used to disintegrate the samples. is entirely based on the palaeogeographic positionof the facies. The sediment input (SI ) is a measureThe stratigraphic distribution of the samples is

indicated in Fig. 3 and Table 2. The sample posi- of continental influence deduced from sedimenta-tion rates and percentage of detrital material.tion is measured in metres relative to the base of

Bed 21, which is the position of the maximumflooding surface of the first third-order eustasycycle of the Boom Clay and which serves as a 4. Analytical methodclear correlation horizon in the four quarries. Inorder to eliminate potential biologic or morpho- The shark teeth have been analyzed by instru-

mental neutron activation analysis (INAA).logic influences on the trace-element content ofthe teeth only monospecific samples were used. Before irradiation the teeth were ultrasonically

cleaned in distilled water and inspected microscopi-Due to this restriction and due to the unevenstratigraphic distribution of fish remains no cally to check for possible impurities. Finally the

samples were cleaned with methanol to removesamples were available from the upper part of thesection. The low sample density in the lower part any contamination due to sample manipulation.

The single teeth samples were weighed on a analyti-of the section is also due to low abundances offish teeth in this stratigraphic interval. It is evident cal micro-balance and sealed in high-purity poly-

ethylene tubes. Sample weights varied from 0.27that we could not attain a sufficiently high sam-pling density to study fourth- and higher-order to 72.8 mg, with most samples lying in the

2.0–10-mg range (Table 2).sedimentary cycles.The samples were irradiated in the ‘‘Thetis

Reactor’’ of the Institute of Nuclear Sciences at3.2. Reference sample setthe University of Gent for 7-h radiation in aneutron flux of ca. 2×1012n cm−2 s−1. NeutronTo further examine the relationship between the

sedimentary environment and the trace-element flux gradients were calculated from the inducedspecific 59Fe activity of high-purity Fe foilscontent of fish remains, additional samples were

analyzed from sedimentary environments ranging attached to each sample vial. After 1 and 3 weeksthe induced gamma-ray activity of the samplesfrom lagoonal to continental slope locations. In

order to minimize the risk of examining reworked was measured with a 75-cm3 coaxial Ge(Li) detec-tor and small planar extra-high resolution Ge(Li)biogenic apatite, all samples are teeth from species

which lived in a habitat consistent with the sedi- or HPGe Low Energy Photon Detectors (LEPD).Each sample was hence counted four times, formentary facies, and have a stratigraphic range

comprising the time of deposition. The samples counting times which varied from 1 to 24 h.Concentrations were calculated from the activitiesare from a collection of the Royal Belgian Institute

of Natural Sciences. of the samples and of a simultaneously irradiatedsecondary ‘‘in-house’’ silicate rock standardTable 1 summarizes the palaeogeographic loca-

tions, nature of the host sediments, and ages of powder.Attention has to be drawn to the benefits of thethe samples. Also listed are three parameters which

express some properties of the depositional envi- use of the LEPD detectors in the present study. Itgreatly improved the sensitivity, precision andronment in numeric form. The parameters are

based on palaeontologic, sedimentologic and accuracy of the Ce, Nd, Sm, Tb, Yb, Th and Udata (Hertogen and Gijbels, 1971). An accuratepalaeogeograhic information and are normalized

on a relative scale ranging from 0 to 10. The first U analysis is often essential in NAA of biogenicapatites, because one needs to correct for an oftenparameter is a measure of the deposition depth

(DD) derived from fish assemblages (Nolf and substantial uranium-fission interference to the141Ce and 147Nd activities (about 0.28 ppm appar-Cappetta, 1989; Nolf and Brzobohaty, 1992) and


Table 1Reference set of nine shark teeth from different marine environments

Locality Age Environment SI SP DD Species Reference(s)

1t Termo Fora Sciolze, Italy Burdigalian t; cs 4 7 8 Squaliform Bonsignore et al. (1969)2t Arenarie de Ranzano, Italy Rupelian cl; ms 6 5 7 Squaliform Gelati (1977), Nolf and

Brzobohaty (1994)3t Marnes de Huıtres, France Rupelian a; l 10 0 0 Dasyatis sp.4t Kiscell Clay, Noszvaj, Hongary Oligocene cl; os 4 6 10 Squaliform Baldi (1986); Nolf and

(NP24) Brzobohaty (1994)5t Pietra da Cantoni, Italy Burdigalian cc; os 2 6 8 Soymnodon sp. Montrasio et al. (1968)6t Sidi Daoui, Morocco Thanetian p; os 0 4 3 Striatolamia striata Arambourg (1952)7t Sidi Daoui, Morocco Ypresian p; os 0 4 3 Striatolamia macrata Arambourg (1952)8t Boom Clay, Belgium Rupelian s, cl; is 7 2 1 Squalus alsaticus Vandenberghe and

Van Echelpoel (1987)9t Boom Clay, Belgium Rupelian cl; ms 5 3 3 Squalus alsaticus Vandenberghe and

Van Echelpoel (1987)

Environment description: a=anhydrite and clay; s=sand; cl=clay; p=phosphorite; t=turbidite; l=lagoonal; is=inner shelf; ms=mid-shelf; os=outer shelf; cs=continental slope. DD=deposition depth; SP=position on shelf; SI=sediment input. These valuesare based on palaeontologic, sedimentologic and palaeogeographic data from the references cited in the right column. See text fordetails of the method of calculation. Samples 8t and 9t are identical to Boom Clay samples Sn2L and St24 listed in Table 2.

ent Ce and 0.25 ppm apparent Nd for 1 ppm U ). 8000 ppm. The elements Cr, Hf and Ta wereFailure to make the U-fission correction would systematically searched for, but signals were belowhave resulted in spurious positive Ce anomalies in the detection limits for all samples. Approximatequite a number of the analyzed samples. upper limits to the concentrations are 5 ppm forFurthermore, in LEPD spectra the 106-keV peak Cr, and 0.5 ppm for Hf and Ta. Most of theof 239Np (from 238U ) is well resolved from the samples have Fe contents between 0.4 and 1 wt%.103-keV peak of 153Sm, and an accurate correction The occasionally high Fe values presumably reflectcan be made for the contribution of Np/Pu K the precipitation of secondary pyrite inside theX-rays to the 103-keV peak multiplet. INAA using porous teeth during recrystallisation. The Na con-only conventional Ge(Li) spectrometry, tends to centrations (0.4–0.8 wt% for most of the samples)produce systematically high Sm results in samples scatter around the value of 0.5 wt% expected forwith high U/Sm ratios if this spectrometric prob- the approximate structural formula of biogeniclem is not properly handled. apatite (Pietzner et al., 1968).

The variation of the concentration of selectedtrace elements in the Boom Clay stratigraphic

5. Resultscolumn is shown in Figs. 3 and 4. The overallvariation of absolute abundances is rather large:The minor- and trace-element data of the teethby factors of up to 30. The lanthanide patterns —samples from the Boom Clay and the reference setnormalized to the ‘‘North American Shalesamples are listed in Table 2. The following ele-Composite’’ (NASC ) values of Gromet et al.ments could be quantitatively determined in almost(1984) — are shown in Fig. 3. (Due to the inher-all the samples: Na, Sc, Fe, Th, U, and theently low analytical precision, Lu data have notlanthanides La, Ce, Nd, Sm, Eu, Tb, Yb and Lu.been included in the figures.) Notwithstanding theCobalt could be analyzed in about one-third oflarge range of REE contents, the shape of thethe samples. The elements Sr and Ba were detectednormalized patterns is rather uniform throughoutin almost all the samples. However, results are notthe whole formation. Many of the Boom Claylisted in Table 2 due to the poor precision of thesamples show minor Ce anomalies, defined asdata. Ba abundances appear to vary from 400 to

1000 ppm. Sr contents range from 2000 to deviations from a smooth La–Ce–Nd–Sm trend.


Table 2Chemical data for fossil biogenic apatite (shark teeth) samples

Position Weight Na Sc Fe Co La Ce Nd Sm Eu Tb Yb Lu Th U(m) (mg) (%) (%)

Boom Clay samples:

Kb63 18.89 11.42 0.41 6.6 17.6 7.3 79 231 129 27.8 8.0 5.1 12.6 3.1 22.2 14Kb55 14.66 2.02 0.72 3.5 0.74 <1.0 50 113 75 17.3 4.6 2.8 8.0 n.d. 7.4 <8Kb54 14.06 2.45 0.66 4.1 0.63 <1.0 55 202 97 26.4 6.7 4.0 10.2 n.d. 12.2 5.1Kb49/S5 11.70 6.19 0.76 2.6 0.59 <0.5 35 58.0 36 9.2 2.5 1.41 4.6 0.6 2.3 10Th49/S5 11.70 4.62 0.75 4.3 0.45 <0.7 58 116 75 16.7 4.4 3.2 8.6 1.3 5.0 11Th48 11.27 3.27 0.70 4.3 0.56 <1.0 71 178 116 25.7 6.6 3.8 9.9 7.0 12.8 6Th47 10.84 1.63 0.51 2.9 6.8 <2.0 40 83 <80 10.7 2.7 2.0 5.9 n.d. 6.9 n.d.St44 9.95 2.51 0.60 3.1 0.53 <1.0 38 89 <120 13.8 3.7 2.5 5.2 n.d. 6.2 n.d.St43t 9.80 6.19 0.66 3.6 2.4 1.1 38 78 43 11.4 3.0 1.8 4.5 1.2 4.0 11St43 9.58 1.27 0.58 4.7 0.5 <2.0 59 164 <60 25.5 6.4 4.1 12 n.d. 14.3 n.d.St42 9.09 7.64 0.64 13.6 0.6 0.48 137 323 181 47.2 12.4 8.6 21 n.d. 23 26He19 9.00 5.64 0.86 9.8 2.59 4.4 182 478 291 76 18.3 13.2 28 3.6 31 22Th41t 8.89 8.26 0.70 3.4 0.95 <0.5 43 97 64 13.9 3.6 2.5 6.3 0.8 4.9 7.9St41t 8.89 5.74 0.74 1.8 1.92 <0.6 24.0 52 18 6.6 1.8 0.98 3.1 0.5 2.0 6.9St41 8.64 3.12 0.69 6.0 0.57 <0.6 65 139 324 23 5.3 3.8 10 n.d. 12.7 26Th40 8.44 6.86 0.65 9.6 0.45 0.73 144 367 200 49.1 12.3 8.7 21 3.1 20.3 31St40 8.44 2.77 0.76 15.1 0.59 <1.0 204 525 n.d. 72 17.4 10.9 28 n.d. 32 33St39 8.19 7.11 0.70 17.7 0.54 <10 246 709 421 98 22.9 14.9 36 n.d. 42 37Th39 8.19 5.76 0.69 28.3 1.27 8 512 1230 767 168 40 27 59 7.0 74.0 78St37 7.43 9.00 0.77 6.7 0.58 <5 85 175 103 25.1 6.6 4.8 13 1.2 11 23St36 7.03 11.90 0.66 9.8 0.73 <0.5 138 387 218 55.9 12.8 8.2 22 n.d. 23 24He29 7.03 0.27 1.19 34.7 <0.7 <9 551 1616 724 232 56.1 40.8 107 15 85 72Th35 6.71 12.81 0.74 8.3 0.48 0.9 122 227 143 31.3 8 5.4 16.1 1.9 8.3 38St35 6.71 3.49 0.71 12.1 0.44 <1.0 155 346 207 46.2 11.2 7.1 23 n.d. 18.8 37St33 6.23 4.29 0.75 9.2 0.57 <1.0 115 260 194 35.7 8.0 8.3 14 n.d. 13.2 24St32 5.76 7.33 0.69 14.6 0.53 3.3 180 455 246 60 14.6 9.8 27 3 24 53Th31t 5.43 2.55 0.74 8.6 0.56 <1.0 134 342 201 46 12.3 8.5 22 2.4 17.2 20St31 5.32 2.93 0.58 24.0 1.78 1.8 347 1033 572 129 31.6 20.7 59 4.5 50 n.d.St29 4.26 5.50 0.70 7.9 0.49 <0.3 83 233 128 32 8.5 6.4 17.7 n.d. 13.6 19St28 3.84 2.04 0.97 27.0 0.70 3.9 361 1020 559 165 44.2 31 71 2.5 76 40St26 2.92 2.47 0.71 18.8 0.92 2.6 324 1028 516 141 35.3 23.5 60 n.d. 46 31St24 2.15 6.92 0.62 6.4 0.91 2.6 89 269 127 40.1 10.5 8.5 22 4.3 14.6 24St23 1.67 4.19 0.70 10.1 0.55 1.4 171 430 217 62 15.6 12.1 32 1.2 19.5 31St20 −0.54 0.47 0.81 6.8 <1.0 <6.0 167 534 128 60.5 17.4 13.5 20 0.8 22.4 <20St19 −1.23 1.45 0.60 2.8 0.93 1.6 56.0 180 <160.0 20.5 5.2 3 10.7 n.d. 12.7 n.d.St18 −1.70 7.30 0.61 3.6 0.95 0.78 69 197 107 24.6 6.1 4.4 8.4 1.2 8.3 17.0St17 −2.38 0.82 0.82 4.8 0.99 <3.0 136 474 256 53.3 12 7.7 16 n.d. 37 <7.0St15 −3.97 0.21 1.25 19.1 <0.6 <8.0 573 1723 805 194 38.9 23.9 39 n.d. 139 n.d.Sn27a −5.32 1.88 0.87 9.5 0.52 <1.0 302 749 427 95 22.4 15.1 36 4.5 46 19Sn27b −5.32 1.76 0.60 3.4 0.75 2.3 74.0 232 105 29.4 7.5 5.2 12 1.4 14.1 16Sn26 −5.58 1.13 0.63 7.6 1.81 <3.0 166 418 266 74 17.5 12 26 3 45 30Sn12 −5.79 6.21 0.64 8.0 1.19 2.8 140 408 212 55.3 14.2 10 20 2.6 21.7 16Sn25 −5.98 1.27 0.66 5.7 0.49 <2.0 117 405 133 52.4 13.2 10 21 2.2 25.4 <10.0Sn11 −6.29 1.49 0.67 6.5 0.57 <2.0 111 336 171 44.3 11.4 8 15 1.3 26.9 <10.0Sn23 −7.00 0.78 0.96 11.3 0.92 <3.0 204 758 394 94 23.2 18 38 4.4 48 <20.0Sn21 −7.78 3.79 0.67 2.4 1.01 1.9 80 233 101 27.8 6.4 7.1 8.4 1.04 18.7 7St8 −8.12 3.51 0.59 3.6 0.55 2.3 70 216 94 27.5 7 4.4 11 n.d. 14.6 <7Sn19a −8.25 3.82 0.67 10.6 3.07 3.5 233 868 389 99 25 17.6 34 4.7 65 10Sn19b −8.25 3.55 0.52 5.7 1.62 2.2 102 353 145 43.3 10.9 7.4 15.1 2 24.5 6.5Sn18b −8.40 7.93 0.48 2.3 1.77 1.0 41.0 130 64 17.8 4.6 3.8 8.4 1.2 8 7.4


Table 2 (continued )

Position Weight Na Sc Fe Co La Ce Nd Sm Eu Tb Yb Lu Th U(m) (mg) (%) (%)

Sn18a −8.85 1.21 0.64 12.9 0.78 <2.0 210 605 344 92 21.5 15.2 36 5.3 44 <12.0Sn2 −15.38 0.54 0.91 13.4 0.42 <3.0 306 974 547 113 25.4 15.6 24 12.7 98 n.d.Sn2L −16.00 8.67 0.80 3.0 0.083 0.4 53.0 162 74 20.3 5.4 3.8 6.1 0.7 16.1 <4SnBasis −16.75 2.06 0.97 6.1 1.35 6.9 233 696 255 63 13.9 10.4 14 2 15.6 23

Reference samples:

1t 1.36 0.70 2.6 0.36 <2.0 57.0 121 <85.0 11.3 2.5 2.1 6 <1.0 3.35 312t 1.71 0.53 14.3 1.49 5.9 238 627 246 75 15.2 11.5 22 n.d. 9.4 253t 2.97 0.82 6.9 0.29 <0.7 612 2122 1251 335 64 43.8 47 8.3 111 <7.04t 0.80 0.68 6.0 9.95 6.1 56.0 109 <130.0 21.7 5.5 3.9 5.97 <1.0 7.7 135t 0.48 0.79 63.2 <0.4 <5.0 348 583 320 73 17.4 13.3 50 <4 9.8 2116t 72.80 0.58 4.5 0.076 <0.1 57.0 24 28 6.4 2.02 1.43 6 <1.0 <1.0 1997t 68.74 <1.0 4.8 0.027 <0.1 99 26 33 6.3 2.05 1.43 5.9 <2.0 n.d. 204

Position of the Boom Clay samples in the stratigraphic column is expressed as metres relative to the maximum flooding surface ofthe first third-order sequence (mfs1; see also Figs. 2 and 3). Concentration units are weight % for Na and Fe and microgram pergram (ppm) for the other elements. See Table 1 for additional data for reference samples; reference samples 8t and 9t listed in Table 1are Boom Clay samples Sn2L and St24. n.d.=not determined; upper limits could not be reliably estimated.

Negative and positive anomalies both occur, curves for La/Yb, Th/Sc, Th/Sm and Sm/Sc ratiosthough with a preponderance of positive anoma- are shown in Fig. 5. Tooth samples from the samelies. Some of the Ce anomalies in low-weight, low- or stratigraphically equivalent layers as a rule showLREE (light REE) samples should be interpreted substantial variation of absolute trace-elementwith caution, because they might mainly reflect abundances (e.g., samples Kb49–Th49; St43t–the inherently lower analytical precision of the Ce St43; St41–St41t–Th41t).and Nd data. The figures illustrate three principal findings of

The reference samples exhibit an even larger the present study: (1) the stochastic fluctuationsvariation of absolute trace-element abundances of absolute abundances, which show no clear-cut(Table 2; Fig. 6), and show a variety of REE relationship to the high-frequency bed-to-bed vari-patterns. The REE data for samples from Morocco ations in grain-size (Fig. 4); (2) the trace-element(6t and 7t) obtained in the present study are in geochemistry of biogenic apatite does not correlateexcellent agreement with results reported by

with the third-order eustasy trend that governs theGrandjean et al. (1987) for samples from the samelonger-term variation in average grain size (Figs.or similar localities. The Moroccan shark teeth2, 4 and 5); and (3) the minor but consistenthave a pronounced negative Ce anomaly, a smallchanges in trace-element characteristics at or nearpositive Eu anomaly, and high U contents butsequence stratigraphically important levels SB2very low Th contents.and mfs1 (Figs. 4 and 5).

The large fluctuations in composition imposeconstraints on the time scale of diagenetic inter-6. Discussionaction between biogenic apatite and pore waters.If diagenesis were to take place over a long period,6.1. Variation with stratigraphic height in the Boomas suggested by Toyoda and Tokonami (1990),Clay Formationone would expect that all samples approached acomposition governed by diffusion coefficientsFig. 4 shows the distribution with stratigraphic

height of Sm, Sc and U concentrations. Similar and pore water/apatite distribution coefficients.


Fig. 4. Stratigraphic variation of the Sm, Sc and U concentrations of the Boom Clay biogenic apatite samples. The first column isrepeated from Fig. 2 to allow easy comparison of Figs. 2–5.

Instead, the results rather support the view that The large fluctuations in absolute abundancescontrast with the much more limited variation ofthe time scale of trace-element uptake is commen-

surate with the short recrystallisation time of the trace-element ratios (Fig. 5). For example, La/Ybratios of the great majority of analyzed samplesoriginal teeth, during which uptake appears to be

a question of supply and demand on a very local falls in the range 4 to 8, which means that thisratio is effectively buffered by the sedimentary/scale. The demand could to a large extent depend

on the initial porosity of each individual sample. early diagenetic environment. It follows that onemust mainly rely on ratios to detect trends whichUnfortunately, this implies that the absolute trace-

element content of fossil apatite is a poor proxy might reflect environmental changes.An examination of Figs. 2 and 5 reveals a closefor fluctuations in supply or behaviour of trace

elements in the sedimentary environment. relationship between turbulence and La/Yb ratio.The linear correlation between these two parame-Notwithstanding this caveat about absolute

abundances, the consistently low REE abundances ters is highly significant (r=+0.614 for n=54).There is a striking resemblance of the maximaat the top of the section above SB2 are probably

not an artifact of limited sampling and most likely occurring in the two parameters at or near theSB2 boundary, at the mfs1, and at the base of thereflect a change of the sedimentation conditions at

the top of the section. The consistently higher U section. Higher turbulence presumably marksperiods of lowered wave-base or of higher stormconcentrations in the interval between mfs1 and

SB2 also seem to be geochemically significant. frequency. One could speculate that turbulence


Fig. 5. Stratigraphic variation of La/Yb, Th/Sc, Th/Sm and Sm/Sc ratios of the Boom Clay biogenic apatite samples. The first columnis repeated from Fig. 2 to allow easy comparison of Figs. 2–5.

modulated the rate at which pore waters have been part, 8.5 in lower part) over a transition interval,due to a difference in source or basin water chemis-transferred to the sediment–seawater interface.

Indeed, a substantial La/Yb fractionation in a try below and above mfs1. Another geochemicalshift at the mfs1 level is the higher average Th/Smpore-water column has been reported by

Sholkovitz et al. (1989). Additional evidence for ratio below mfs1 as compared to the average valuein the interval between mfs1 and SB2.involvement of pore waters will be presented

below.The trend of systematic changes of Sm/Sc ratios 6.2. Shape of the REE patterns

can hardly be considered as fortuitous. A case inpoint are the samples from the Steendorp quarry, The NASC-normalized REE diagrams of all the

Boom Clay samples are characterized by a maxi-which straddle the mfs1 boundary. Only one outof nineteen samples above this boundary has mum at the middle REE (MREE: Sm–Eu–Tb).

(Fig. 3), and differ from most of the patternsSm/Sc>6.5, while all six samples from below thisboundary have Sm/Sc>6.5. One could argue that reported for Cenozoic fish remains. Those apatite

samples commonly exhibit either a seawater-likethe increase of Sm/Sc ratio is a diagenetic trend,related to deposition age of samples. However, it pattern (LREE depleted and having negative Ce

anomalies) or flat river water-like patternsappears more straightforward to see this trend asa switch between two stable values (5 in upper (Grandjean et al., 1987; Wright et al., 1987; Toyoda


Fig. 6. NASC-normalized (Gromet et al., 1984) REE patterns of the 9 biogenic apatite reference samples used for factor analysis.See Table 1 for details of sample provenance and depositional environment.

and Tokonami, 1990; Fig. 6, this work). However, exceptional, because they are also observed forOligocene samples from other locations (samplesthe MREE-enriched (‘‘hat-shaped’’) patterns typi-

cal of the Boom Clay apatite cannot be greatly 3t and 4t; Table 1; Fig. 6).


Fig. 7. Comparison of NASC-normalized (Gromet et al., 1984) REE patterns of three extensive sets of samples of biogenic apatite.With a few exceptions all the teeth samples from the Boom Clay Formation fall within the dotted field. The middle field (slantedruling) covers the biogenic apatites from Holt Summit Devonian Sandstone, Missouri ( Wright et al., 1987). The vertically ruled lowerfield covers data for conodonts from the Upper Devonian Coumiac limestone, France (Grandjean et al., 1993).


Such ‘‘hat-shaped’’ REE patterns have been column, in which the dissolved plus authigenicparticle-bound REE fraction was still bearing apreviously reported for two extensive sample sets

of biogenic apatite from the Holt Summit strong riverine signature. The observation that inthe three locations documented above, all theDevonian Sandstone (Missouri, U.S.A.) (Wright

et al., 1987), and from the Upper Devonian samples have quite specific and distinctive REEfingerprints, is evidence of a stable supply of REECoumiac limestone (France) (Grandjean et al.,

1993). These three REE data sets are compared in from the same general source area and indicatesthat trace-element signals of biogenic apatite areFig. 7. It turns out that samples from the three

sites — though sharing several common features — rather robust.are characterized by distinctly different patterns.The Coumiac samples have generally lower REE 6.3. Ce anomalies and redox conditions.abundances, a rather low La/Yb ratio (<1 relativeto NASC) and show a very pronounced ‘‘hat Wright et al. (1987) have advocated the use of

Ce anomalies as palaeoredox indicators. In con-shape’’, with maxima at Eu–Gd. The Holt Summitconodonts have consistently higher REE contents trast to other lanthanides, Ce(III ) is readily oxi-

dized by microbial activity to Ce(IV ), in whichand La/Yb ratios (~5 relative to NASC ) and themaxima are shifted towards the LREE. In terms state it is rapidly removed from the water column

and fractionated from the trivalent REE byof absolute contents there is virtually completeoverlap of samples from the Boom Clay and those scavenging on Fe–Mn-oxyhydroxide particles

(Moffett, 1990). The negative Ce anomaly in theof Holt Summit. But the Boom Clay samples showa maximum at Eu–Tb and La/Yb ratio below or dissolved REE fractions of seawater produced by

Ce oxidation can be decreased, entirely counterbal-close to 1 (relative to NASC ).Of the three case studies summarized in Fig. 7, anced, and even overcompensated (positive Ce

anomaly) by reductive dissolution of Ce-enrichedthe Boom Clay apatite samples exhibit the leastfractionated, shale-like pattern. The REE patterns Fe–Mn-oxyhydroxides (de Baar et al., 1988).

Biogenic apatite can in principle record theof authigenic phases presumably reflect the degreeat which the total REE inventory of the water redox conditions of the seawater if REE are only

adsorbed from the dissolved pool and if the REEcolumn has been geochemically processed. Riverwater is the main contributor of REE to a sedi- are not modified by diagenetic processes. The

shark teeth from Morocco (Grandjean et al., 1987;mentary basin, and has REE patterns that areonly slightly fractionated with respect to shales. this work, Fig. 6) might represent such limiting

cases where Ce anomalies carry useful information,The LREE are removed from the water columnlargely through scavenging by Fe–Mn-oxyhydrox- because the low REE abundances seem to be

consistent with uptake from dissolved REEide particles, while heavy REE (HREE) are greatlyaffected by flocculation of organic colloids (Hoyle from slightly enriched seawater, judged from

the hydroxyapatite/seawater Kd values ofet al., 1984; Byrne and Kim, 1990). The low REEabundances and the very pronounced relative Koeppenkastrop and De Carlo (1992). However,

it also follows from the restrictions stated above,enrichment of MREE of the Coumiac samplescould be attributed to their deposition in a distal that the Ce anomaly is only a one-sided redox

probe. While negative Ce anomalies of apatite maypelagic platform environment (Grandjean et al.,1993), wherein the particles that contributed to be regarded as reliable redox indicators, it is not

correct to consider zero or positive Ce anomaliesthe REE enrichment of the sediment–seawaterinterface and the early diagenetic pore fluids origi- as unambiguous evidence for anoxic to suboxic

conditions. The latter certainly applies to the casenally precipitated from seawater that was depletedand heavily processed with respect to REE. of the Boom Clay Formation. Anoxic to suboxic

conditions would be completely incompatible withConversely, the higher REE contents and flatterpatterns of the Boom Clay apatite are consistent the diversity of benthonic fauna and extent of

bioturbation of the sediment (Vandenberghe, 1974,with formation closer to the continent in a seawater


1981; Vercauteren, 1975). The recurrent positive positive loadings on this factor are also found forabsolute REE and Th abundances, and REE/Sc,Ce anomalies are a strong indication that a sub-

stantial fraction of the REE adsorbed by the Boom Th/Sc, and Th/REE ratios. Apparently, factor 1stands for a dominant riverine influence. The highClay apatite derived from an upward benthic flux

of reduced pore water (de Baar et al., 1988; loading of the Ce anomaly [defined asCe-an=3CeN/(2LaN+NdN), where N stands forSholkovitz et al., 1989). It appears that the Ce

anomalies of the Boom Clay apatite mainly record NASC normalized ] on factor 1 indicates that highsediment input favours the development of zerothe presence of a shallow redox boundary within

the top of the sediment column. This boundary or positive Ce anomalies in fossil apatite. Thiscould be brought about by a reduced, upwardhas been inferred from sedimentological data as

well (e.g., co-occurrence of pyrite and bioturb- benthic flux, as explained above, or by the fasterrate of Ce(III ) scavenging relative to Ce(III )ations; Vandenberghe, 1974).oxidation in such a high particle flux environment(Moffett, 1990).6.4. Factor analysis of the reference set of samples

The second factor cannot readily be associatedwith a clear-cut environmental or geochemicalIn order to gain a better insight in the impact

of sedimentary parameters on the trace-element parameter. Deposition depth and position on theshelf have high loadings on this factor. Togethercontent of biogenic apatite, a factor analysis was

performed on data for nine samples from different with factor 3, factor 2 accounts for higher Sccontents and higher Sc/REE, Th ratios. The associ-environments (Tables 1 and 2). The set encom-

passes two key samples from the Boom Clay — ation of enhanced Sc with deposition depth andshelf position could be a hint that the upper lefti.e., samples Sn2L (8t) and St24 (9t) — which

represent two extremes in the depositional history quadrant of Fig. 8 represents a higher contributionto the diagenetic pore fluids of trace elementsof this formation. Samples from other locations

could not be used because analytical data for released from authigenic or altered clay particlesin deeper parts of a sedimentary basin. The separa-highly diagnostic elements (Sc, Th, U ) were not

available. The first Boom Clay reference sample tion of variables and samples in the lower leftquadrant of Fig. 8 is largely due to high uraniumcomes from the first silt layer of Bed 2. The grain

size and the presence of gullies points to deposition content. Since the reference samples 5t, 6t and 7tderive from carbonate sedimentary basins, onein a shallow marine, wave-agitated environment.

Sample 9t was extracted from Bed 24 which is a could speculate that the high U content and rela-tive enrichment of HREE expresses a high contri-clay layer just above the maximum flooding surface

of the first third-order eustasy cycle. The sequence bution to the pore fluids of elements released fromlabile carbonate phases.stratigraphic position of this bed, its very fine

grain-size distribution and the drop in sedimentsupply just below indicate that it represents one of 6.5. Factor analysis of shark teeth from the Boom

Claythe deepest deposits of the Boom Clay(Vandenberghe, 1974; Van Echelpoel, 1991).

A simultaneous R and Q mode factor analysis Since absolute abundances as such do notappear to carry much diagnostic information (see(Davis, 1986) was performed using absolute con-

centrations and a number of interelement ratios. above), a simultaneous R and Q mode factoranalysis of apatite samples from the Boom ClayFour factors account for 91% of the observed

variance. Eigenvalues and associated percent vari- was based only on ratios which exhibit significantvariation: Th/Yb, La/Yb, Sc/La, Yb/Sc, Th/Sc,ance (between parentheses) are respectively: 11.7

(48.8), 4.5 (18.9), 4.2 (17.3) and 1.5 (6.0). The Th/La. Three factors account for 98.2% of thevariance. The eigenvalues and associated % vari-loadings of samples and variables on the first two

factors are shown in Fig. 8. The first factor repre- ance (between parentheses) are: 3.4 (56.8), 1.5(24.6), and 1.0 (16.8). It turns out that the domina-sents the sediment input into the basin. High


Fig. 8. Factor analysis of the reference set of nine fossil apatite samples from various locations (see Table 1). The figure shows theloadings on the two main factors of the samples and their depositional and geochemical variables calculated from a simultaneous Rand Q mode factor analysis (Davis, 1986). The marked (1t to 9t) open squares represent the Q-mode loadings of the nine referencesamples. Ce-an stands for Ce anomaly. The parameters SI, SP and DD are explained in the text and their values are listed in Table 1.

ting first factor has the same physical meaning as (2) Factor analysis of trace-element data for areference set of apatite samples from verythe first factor of the reference data set. Th/REE,

Th/Sc and REE/Sc have high positive R-mode different depositional environments shed lighton the nature of the factors that govern theloadings on this factor, which by analogy implies

that the first factor expresses a higher sediment trace-element variation. High riverine input atshallow depths is reflected in higher averageinput. La/Yb, Th/Yb and Yb/Sc ratios have high

loadings on R-mode factor 2. This factor accounts REE abundances, relatively high Th/REE andlow Sc/REE.for secondary variations in these diagnostic ratios.

(3) Well-documented third-order eustatic sea-levelchanges had no substantial impact on thetrace-element chemistry of shark teeth samples7. Conclusionsfrom the epicontinental Boom ClayFormation. From which it is deduced that(1) Biogenic apatite samples from a given sedi-

mentary basin exhibit rather specific and dis- eustasy does not greatly affect the geochemicalprocesses occurring in a relatively shallowtinctive trace-element fingerprints, which show

that they reliably record geochemical parame- marine sedimentary basin. Nonetheless, minorchanges in average abundances and particu-ters of the sedimentary environment.


au Maroc, Dir. Prod. Ind. Mines, Div. Mines Geol., Noteslarly trace-element ratios do coincide withMem. No. 92, 332 pp.sequence stratigraphic levels in the Boom Clay

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(Off. Rech. Sci. Tech. Outre-mer), Ser. Geol. 7, 68–83.coincide with periods of higher turbulence dueBonsignore, G., Bortolami, G., Elter, G., Montraio, A., Pet-to lowered wave-base or higher storm

rucci, F., Ragni, U., Sacchi, R., Sturani, C., Zanella, E.,frequency.1969. Note illustrative della carta geologica d’Italia. Carta(4) While interelement ratios in the Boom Clay Geologica d’Italia alla scala 1:100,000, Folgi 56, 57

shark teeth are fairly constant, absolute trace- Byrne, R.H., Kim, K.H., 1990. Rare earth element scavengingelement abundances fluctuate greatly. in seawater. Geochim. Cosmochim. Acta 54, 2645–2656.

Davis, J.C., 1986. Statistics and Data Analysis in Geology, 2ndAbsolute concentrations appear to depended. Wiley, New York, 646 pp.mainly on properties of the individual tooth

de Baar, H.J.W., German, C.R., Elderfield, H., van Gaans, P.,samples (porosity, recrystallisation rate) and1988. Rare earth element distributions in anoxic waters ofon the local supply of trace elements from the the Cariaco Trench. Geochim. Cosmochim. Acta 52,

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(Alessandria) al margine orientale del Bacino Terziario Lig-(5) Recurrent small positive Ce anomalies in theure–Piemontese. Riv. Ital. Paleontol. 83, 103–136.Boom Clay apatite samples are an indication

Grandjean, P., Albarede, F., 1989. Ion probe measurement ofthat a substantial fraction of the REE derivedrare earth elements in biogenic phosphates. Geochim. Cos-from the upward flux of reduced pore waters.mochim. Acta 53, 3179–3183.

As a consequence, the Boom Clay data cast Grandjean, P., Cappetta, H., Michard, A., Albarede, F., 1987.serious doubts on the usefulness of Ce anoma- The assessment of REE and 143Nd/144Nd ratios in fish

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