BIIII. Soc. glol. F"'nc~. 2000. t. 171. n03. pp. 367-378 Siance splcialiJ'i~ : SGF·APF·CFS·SG l..ondres IA limilt.' P{/lhx:~"e-Eoc~lIe ell europe Pari.l. 19-20 janl'iu 1998

Late Paleocene event chronology : unconformities, not diachrony by MARIE- PIERRE AU BRyl. BENJAM IN S. CRAME R2, KENNETH G. MILLER2. JAMES D. WRI GHT2,

DENNIS V. KENT2. 3 and RICHARD K. OLSSON2

Key words. - Biostra!igr<lphy. Di<lchrony. Isotope straligraphy. Unconformities.

AbslmCI. - The chronology of the evcnts associated with the late Paleocene lhermal maximum (LPTM. Chron C24r) has been established through Ihe construclion of a composite reference sec tion that involved chemomagnetobiostratigraphic corrclations and assumed minimum diachrony of bioslrati­graphic CI·Cl1ts. On this basis. discrepancies between correlations in different sections were explained by inferred unconformities. However. diachrony between distant sections cannot be ruled OUI. We report here on two geographically dose sections drilled onshore New Jersey thaI yield different records of chemomagnetobiostratigraphk correlations in the interval represeming Chron CNr. Because of their proximity (- 40 km apart). diachrony of biostratigraphic events bel ween the two sections can be ruled out. In contrast. lhe marked lilhologic disconformities in lhe sections explain well the different records of el·ems. We thus conclude that thc current relative chronology for Chron C24r is firmly based and lha! the upper Paleocene-lower Eocene stWl igraphie record yields multiple unconformilies. with Subzone NP9b rarely sampled. We examine the implications thaI undceiphered un­conformities may have on the idclllification of proxies for paleoceanographic reconslruction, in particular with regard 10 the identification of the carbon isowpe excursion (CIE) lhat renects a dramatic lalest Paleocene disturbance of the carbon cycle. We propose biostratigraphic means (short-lived calcareous nannoplankwn and planktonic foraminifera taxa) that permit lhe unequivocal identification of the CIE not only in the oceanic realm but also in ncri tic sClIinss.

Chronologi e des evenem ents de la fin du Paleocene discordances ver s llS diachronie

Mots ells. - Biostratigraphie. Diachronic. Stratigraphie isotopique. Discordances.

Risw"i. ~ La chronologie des I!"cnemenls qui se sont produits duranl Ie Chron C24r a I!tl! I!tablie ~ partir de la construction d 'une coupe composite fictive. I!tablic ~ur !a base de corr~laljon; entre chimio-. magn~lO- et biostratigraphie dans diverscs coupes. el en supposanl une diachronic minimale des ~vcnemcnls biostratigraphiques. Deux coupes carollces dans Ie New Jersey. USA. nous permellenl de dcmon(rer Ie r6le des diseordances sur Jes cOrrclalions entre coupes. ct d'ctablir la "allditc de la chronologie aClUel1emcnl utilisl!e pour Ie Chron C24r.

VERSION FRAN~A ISE ABREGEE

La chronologie des evenements associes avec Ie rcchauffement max imu m de la fin du Paleocene (Late Paleocene Thermal Max imum ; Chron C24r) iI ete clablie it panir d'une coupe composite fictive construite sur Iii base de relations entre stratigraphie isotopique. magnetostratigraphie et biostratigrilph ie. en supposant une diachron ie mini male des evenements biost ratigraphiques. Da ns celie perspective. les differences de correlmions entre differentes coupes stratigraphiq ues sont exp liquees par la presence implieite de discordances. Toutefo is, it est necessaire de reconnaitre qU'une diach ronie pourrai t exister entre coupes distantes. Nous analysons iei deux coupes geographiquement tres proches carollees dans Ie New Jersey, qui ont la pan icularite de prese nter des correlations differentes entre magnetostratigraphie, biostratigraphie et stratigraphie isotopique dans l'intervaI1e correspondant au Chron magnctique C24r. En raison de leur prox imite geogra­phique (- 40 km), il est possible d'exclure tout phenomene de diach ronic. Par conlre les disconformites lithologiques, bien visibles dans les coupes, expliquent clairement les differences de relations entre evencments dans les deux coupes. Nous concluons par consequent que la chronolog ie re lative utilisee pour Ie Chron C24r a etc fe rmement etablie, e t que l'enregistrement sedimentai re correspondant au Chron C24r est for tement disconti nu , la Sous-zone NP9b etant raremen t representee. Nous analysons les consequences qui rcs uitent de discordances ignorees sur I'identification d 'i nd icateurs pa leoceanographiques, en particulier par rapport a l'identification de I'excursion isotopique du carbone (C IE), excursion qui refiele une perturbation dramatique du cycle du carbone a la fin du Paleocene. Nous proposons des elements bios­tratigraph iques (especes du nannoplancton caJcaire et foraminife res pla nctoniq ues a duree de vie tres coune) pour permettre d'identifier la CI E non seulement en domaine occanique, mais egalement en milieu neritique .

INTROD UCTION

Th e latest Paleocene (- 55.5-54.5 Ma) has been identified as one the most critical times in Cenozoic history. T he rapid global deep sea benthic foraminifera extinction l BFE; Tho­mas, 1992], and simultaneous turnover among mid-bathyal ostracodes [Steineck and Thomas. 1996J. the sharp turnover among calcareous nannoplankton [A ubry. 1998aJ and rapid diversification in the planktonic fora minifera [Kelly el al ..

1996] are the recent ly discovered marine biotic events equi­valent in amp li tude and significance to the critical turnover that affected land mammals and has been known for over a centu ry [see Hooker, 1998}. Correlative with these biotic events. a characteristic 3 to 4 %0 carbon isotope excursion (C IE) occurs in the carbon isotopic composit ion of marine carbonates Le.g. , Ken nett and Sto tt, 1991; Bralower el al., 1995; Pak and Miller, 1992], in soil nodules and the tooth ename l of terrestrial mammals LKoch el af., 1992] and in

! lnstilUt dcs Scienccs de rEvolution. UniversitC: Montpcllier II. 34095 MOnlpellier cedex 05. France. 2 Dep~rl1!lent of Geological Sciences. RUlgers University. Piscataway. NJ 08854. USA. l Lamont-Dohcrty Earlh Observalory of Columbia University. Polisades. NY 10964. USA. Manuscrit dC:posC: Ie I I juin 1999: acceplC: apr~s rC:\'ision Ie 8 novcmbrc 1999.

8ull. Soc. giol. F~. 2000. n" J

368 M.P. AUBRY cr III.

terrestrial organic mailer [Slott et al .. 1996: Sinha et al .. 19961. These events occurred in connection with a sudden shorl-term warming event dubbed the Latest Paleocene Thermal Maximum (LPTM ), during which sea surface and deep-wale r temperatures warmed by 5·6°C and 4 °C, res­pectively [Zachos el al .. 1993].

The marine biotic. isotopic and sedimentologic changes that occurred during Chron C24r have been documented mainly through the study of numerous DSDP/G OP holes and outcrop sections deposited at bathyal depths. with the notable exceptions of two Tethyan nerilic sections in Egypt [Charis; and Schmitz. 1995: Speijcr ef (If" 1996], and in New Je rsey lThomas er al .. 1997: Cramer et (/1 .. 1999J. Whereas these changes are now well described qualitatively, they are di fficult to describe quantil<ltively because there is as yet no firm numerical chronology established for Chron C24r [Berggren and Aubry, 19981. In fac\. the rela­live chronology of events in the chron remains controver­sial. and the amplitude of events as critical as the carbon isolOpe excursion varies by a factor of three between loca­lities. Aubry el al. 11996J and Aubry [1998bl have proposed that inconsisten t chemomagnetobiostratigraphic correl:ttions between sect ions are best explained by the presence of mul­tiple unconformi ties in different sec tion s. Accordingly. es­tablishment of the chronology of even IS during Chron C24r (Berggren and Aubry. 1996. 1998] has relied on the cons­truction of a composite reference seclion using two disjunct stratigraphic records. one from the Southern Ocean (O DP Hole 690). the other from the North Atlantic Ocean (DSDP Hole 550). However. because these two records are geogm­phically so distarll. there was no definitive evidence that diachrony was not responsible, at least in part. for the in­consistent chemomagnetobiostratigraphic correlations be­tween sections.

We discuss here two adjacent strat igraphic records of Chron C24r drilled on the New Jersey margin as part of the New Jersey Coastal Plain Drilling Project (Legs 150X. 174AX). Both records yield excellen t biostratigraphy. a good magnetostratigraphy and a sharp carbon isotope ex­cursion. and are thu s readily correlated with deep sea sec­tion s. In addition. they are amenable to sequence strat igraphic analysis. and a number of eros ional contacts (seq uence boundaries) occur. Because of geographic proxi­mity (the distance between the two section s is 40 km). dia­chrony of biostratig raphic events bel ween the two records can be ruled out. Yet. the chemomagnetobiostrat igraphic correlations between them differ considerably and the dif­ference is clearly related to the erosional Contacts. We de­monstrate below that together. the two section s constitute an almost complete record of Chron C24r. and bring defi­nitive evidence that the composite reference section of Au­bry el al. Ll996] - and thu s the relati ve ch ronology of events in the chron IBerggren and Aubry. 1996; Aubry el 01 .• 1996j - are firmly based and rel itlb le. We then explain that an apparent carbon isotopic "excursion" does not ne­cessarily represent a true geochemical even t and offer means of identifying the CIE with confidence. We conclude in arguing for the urgent need to recogni ze unconformities as an integral component of the stratigraphic record in order to read correctly the hi storical message embedded in it.

MA G N ETOC H E]\.'IO B I OSTRATI G RA PH IC FRA ME WORK AN D CO RR E LAT IONS

Stra tigra phi c framewo rk

. A high biostratigraphic resolution is available for the inter­val that represents Chron C24r and which essentially cor-

/JIll/. Soc. glQI. Fr.. 2000. nO 3

responds to calcareous nannofossil Zones NP9 and NPIO and planktonic foraminifera Zone P5 to Subzone P6b (see Berggren er o/.. 19951. Zone NPIO may be subdiv ided into 4 subzones (N PIOa-d) based on the Tribrachiaflls line:lge. in purticulur the total range of T (/ig iloli.r and T comor/ lls [Aubry, 19961. Developed in North At lantic Deep Sea Dril ­ling Project (DSDP) Hole 550. this subzonal scheme has now been s uccessfu lly applied to deep water seclion s from the Pacific [Aubry. 1998b] and neritic sections from the Tethys rSchmitz el af .. 1996; Faris and Strougo, 1998J and the U. S. Coastal Plain (Miller er ,,/.. 1994: Bybell lind Self-Trail. 1997]. Zone NP9 has also been subdivided into two subzones [Bukry, 19731, bUI one of the two cd leria uscd to delineate un NP9a1b subzonal boundary is unreliable [Bralower and Muttcrlose. 1995; Aubry. 19991. Although this will require rclinement when suitable sect ions permit it. delineation of the NP9a1b subzonal boundary herein is based on the simu ltaneous lowest occurrences of several taxa. i.e .. Rhomboasler c(lieilrapa. R. spillelu. Disco(lster art/llellS and D. (Illar/ias. A thin stratigraphic interval char­acte rized by the common occurrence of these taxa has been identified in several neritic and deep sections; its base is correla tive wit h the LO of Aearillill(l a/rical/a. A . . sibaiyaen­.f iJ and M. a/fis()lIel1sis [see Cramer el tI/.. 1999] in Zone P5. Zone P5 has been recently subdi vided into 2 or 3 subzo­nes by different authors. but because planktonic foramini­fera do not pro vide a means as powerful us calcareous nannofossi ls for the stratigraphy of neritic deposits, these subdi visions will not be considered here. The interested rea­der may refer to Berggren el (II. [2000 [ for discussion. As defined. the NP9a/b su bzonal boundary is also correlative with the base of the LPTM [see Cramer ef al" 1999j. The prominent CIE constitutes a powerful mean s for correlation within magnetozone C24r. synchronous with the HO of the Siellsioilla beccariiformi.r assemblage in deep sea setti ngs.

It is imporlant to recognize that the NP9alb subzona l boundary as defined above is equivalent to the NP9/NP IO zonal boundary of Bybell and Self-Trail [1995. 1997] and Angori and Monec hi [1996]. Different concepts of the spe­cies Tribraehi(l/lis brtlllllellei. the _mlrker of the base of Zone NPIO (Martini. 19711 is at the origin of the differen­ces in the strat igraphic delineation of the NP9/NPIO lonal boundary [Aubry. 1996: Aubry et al .. 2000]. This has no consequence in this study because the calcareous nannofos­sil data used in this demonstration were generated by the same author (MPA). but shou ld be taken into accoun t if the correlations presented here ltre to be compared with pub­lished material in wh ich a different taxonomic concept of T bramlellei is used. The unfortunate consequence of this taxonomic disagreement is that depending on whose taxo­nomi c framework is followed. the CIE lies at the NP9/NPIO zonal boundary. or at the NP9a1b subzonal boundary Icom­pltre Schmitz el a/.. 1996 with Schmitz ef al., 1997J. An important po int is that, as characterized herein, the NP9/NPI0 zonal boundary can be reliabil ity approximated based on the 1-1 0 of Fascic:ufirlllls IYIII/)(lIIi/ormis. which is essentially contiguous with the LO of T braflllellei [e.g .. Aubry el til .. 1996: Faris and Strougo. 1998J. Thus. inde­pendently of their nomenclatural assignments. the interval from the C IE to the LO of T (!igilllli,r can be subdivided into two interva ls: from the C IE to the HO of I~ lymlJa­lIi/ormis (= LO of T bramlellei. i.e .. nannol iths with an hexaradiate symmet ry) and fro m this latter to the LO of T digitalis.

Conelat ions

Whereas the relationships between the LO of Di.scoasler mullirtu!iallU and HO of Global/Vlllalilla psell(!omelltlrt!ii

LATE PALEOCENE EVENT CHRONOLOGY 369

77 76 75

-- Ew9009MCS

- _. other MCS

• ODPLeg 150, 1SOX

o OOP Leg INA. 174AX

o USGS BERG boreholes

74 73

, ,

, , ,

40

39

a Future plattormlseml·submerslble drilling ,

~~~~z=~~~ ____ ~1-~2-~'~~~ 38

FIG. 1. - Location map.

FIG. 1. - C<ldre geografJil iI,,,e.

and the Chron C25n/C24r reversal boundary arc well es­tablished , sirongly inconsistent chemomagnetobiostratigra­phic correlations occur between sections representing Chron C24r. For instance. among sections with a good magne­tostratigraphic record. Ihe CIE (and the BFE) occurs at Ihe level of Ihe NP9/NP10 zonal boundary in DS DP Holes 550 and 577 , but it is separated from the zonal boundary by less than a meter in DOP Holes 8658 , 2.6 In in DSDP Hole 549 and - 12 m in aDP Hole 6908. In addit ion. the CIE may occur as low as 23.5 % of the way up in Magnetozone C24r or as high as 41.3 % (ODP Hole 690B). whereas the NP9/N P I0 zonal boundary may occur as low as 23.5 % (DSD P Hole 549) or as high as 76 % (ODP Hole 6908). In OD P Hole 865, which yields no polarity record, the thickness of Magnetozone C24r can be approximated (- 24.60 m) based on biostratigraphic datums. In this hole. the NP9/NPl0 zonal boundary is 64 cm above the CIE and located - 55 % of the way up in the interval seen to rep­resent Chron C24r.

There arc essentially two ways of ex:plaining these dis­crepancies [Aubry. 1995, 1998b]. One is through the (over­stated) diachrony of biostratigraphic events. The other is through the incompleteness of stratigraphic sections. The bl urring effect that unconform ities have on strllligraphic correlations has bee n demonstrated for at least some DSDP/O DP sections [Aubry et al. , 1996: Aubry. 1998b). However, some authors have argued for a combined effect of the twO mechanisms [e. g .. Fly nn and Tauxe. 19981, and diachrony between high and low latitude stratigraphies can­not be ruled out a priori. The only means to eliminate the role of diachrony would thus be the recovery of sections from similar latitudinal and water depth sett ings yielding

con trasted magnctoche mobiostratigraphic correlations. Such sections were recently recovered as part of the New Jersey Coastal Plain Drilling Project , and provide the opportunity to test the rel iability of the chronology of events in Chron C24r.

MAGNETO BIOSTRATIG RAPHI C CORRELATIONS BETWEEN T HE TWO SECTIONS

The Bass River and Island Beach Boreholes drilled on the New Jersey margin less than 60 km apart have yielded two shallow water (neritic) upper Paleocene and lower Eocene sections with ex:ceptional records of Chron C24r (fig. I). These are easily correlated on the basis of lithostratigraphy, and both yield a clear unconformity at the lithologic contact between the Vincentow n and Manasquan Formations. In addition. bot h yield a carbon isotope ex:c ursio n and the calcareous nannofossil preservation and diversity are re mar­kable, permitting firm chemobiostratigraphic correlations between the two sections and with deep sea sections. We first correlate the two sections on the basis of litho-, magneto­and biostratigraphy. We then use this framework to compare the isotopic stratigraphies of the two sections.

Bass River borehole.

[n the Bass River Borehole (B RB : 39° 36' 42" N. 74° 26' 12" W: elevation of 8.53 Ill; Miller el af. [ 1998], Magne­tozone C24r, precisely delineated between the magnetic re­versa ls at - 1192' (363.40 m) and - 1136' (346.25 m) [Cramer et (il. , 19991 is 56' (27 m) thick (fig. 2). It spans

BII/I. Soc. gi ol. Fr.. 2000. nO 3

370 M.I'. AU BRY t'I al.

CALCAREOUS

EVENTS

I •

,

Cibicidoides spp.

BASS RIVER BOREHOLE

FIG. 2. - The stratigraphie interval representing Cluon C24r in Ihe Bass River section: lithology. magnCIQSlraligruphy. biostratigraphy (PF '" planktonic foraminifera; eN '" calcareous nannoplankton) and isalop<! stratigraphy. Low isotopic resolution in Ihe interval belween 1080' and t 110' reflects Ihe scarcilY of benthic foraminifera. Magnetostratigraphy and isotope stratigraphy : Cramer el a/ .• (19991: Ii thoSlratigraphy. biostratigraphy: Miller ell!I .. [1998 and hereinl. Tic marks indicate locations of samples in Ihe cores. + indicates reworking; --- indicatcs iTrregular occurrences. Magnetostratigraphy: Black: normal polari lY: while : rever~d polarity: vertical lines: unintcrprctable signal. Note Illat unconformities associated willl subs tantial Ili~tuscs (:> 0.6 m.y. and:> 0.2 m.y .. respectivcly) occur at 1138.0' and 1138.2'. For figure clarity tlley are rcprcsented as a single feature. FlO. 2. - L'illlcn'alic Sira/Igmpilique reprisCII/III1/lc Cllroll C24r tlWIS III coupc de BliSS Ril·cr. Lilli%gie. mllgl1filOSlraligmpltie, bioslrilligraphit' (PF: /ommi,tijeres p/(mclolliques .. eN : 1II111110p/("'CIOII cn/caire) el S(r(uigropiJie iSOIopiqlle. 1..11 /(lib/e. Ilellsili ele ",tsl/res isolopiqw:<s tlilIIS 1'llIlen'al/c elltn 1080' CI 1110' rbllirc dc III rarele dcs /oramillifircs belllitiques. MlIglI!lOslrluigfllpltie 1'1 slrarigfllpltie IsolOpiqllC: emmer et 31. (1999}; 1illlOslraligraphie. bim"lrmigraphic; Miller el 31. 1/998 el ce pIWier}. us lirers (c%nnes 5 " 7 tl pnrtir de 1(1 gnuche) I/Olem la pO.\·ilion (Ies echamil/oll $" + I/Ole I"CmalllemCIII .. ••• 11011' presellce disconlinue . Mugllbostr(lligraphie: lIoir : polurile 1I0rm(lle.- bhmc .. po/urili illverse. liglles I'crtiL'u/es. Slgll11l 11011 illlCfpreluble. NOIez que les discordances associees Ii d'importallls /11(l/IIS (> 0.6 Mil el > 0.2 Ma. respeclil"emclII) SOli/ presellles () fl38,6' el 1138.2'. Par J'ouci de cla r le, elles som cOIi/olldues S(lr la figl/re.

LATE PALEOCENE EVENT CHRONOLOGY 371

the upper part of the Vincentown Formation (up to 117 1' (356,92 ml: silty clay), a lithologic unit consisting of gray clay, also known from OIher subsurface cores. unnamed (e.g., Cramer et al .• 1999J or assigned alternatively to the Vincentown [e.g .. Owens et al .. 19971 or the Manasquan le.g .. Gibson et (II .. 19931 Formation, and the lowermost part (glauconitic clays) of the Manasquan Formation. Whe­reas the contact between the typical Vi ncentown silty clays and the overlying gray clays is apparenlly conformable, the prominenl contact between the gmy clay and the Manasquan Formation, at 1 [38.6' (347.05 m). is erosional, and located at - 96 % up in the reversa1.

Biozonal subdivision of MagnelOzone C24r on the basis of planktonic fo rami nifera is tentative [Miller et af .• 19981 but calcareous nannofossils afford an excellent biostratigra­phic cOnlrol. The deposits between 1210' (368.80 m) and the erosional contact at 1138.6' (347.04 m) belong to Zone NP9. There is a large contrast between the composition of the assemblages at I 171.1' (356.95 m) and at 1170.9' (356.89 m) where the simultaneous LOs of Rhomboaster ca/citrtll)(I, Discoasler oral/ellS, and D. (Illari ios. among o­thers, arc recorded. The NP9a/b subzonal boundary is thu s drawn between these two leve ls. No nannolith assignable to the ge nus Tl"ibrachial!ls. wh ich wou ld indicate Zone NPIO. was recorded be low the unconform ity al 1138.6' (347.04 m).

The silty clays between 1138.6' (347.04") and - 1135.5' (346. 10 m) belong to Zone NPIO. Subzone NPIOb is well characterized at level 1138.4' (346.98 m) immediately abo­ve the unconformity. Subzone NPIOd extends between I 138.1' (346.89 m) and I 136.2' (346.31 m). Thi s impl ies a stratigraphi c gap at abou t 113S.2' (346.92 m), with Subzone NPIOc missing. The juxtapos ition of the magnetic reversa l;lt 1[36' (346. 25 Ill) and the NPI O/NPII (and pos­sibly the P6a/P6b) zonal boundary, is indicative of an un­conformity at - 11 35.5" (346.10 m).

Is land Beach borehole

In the Is land Beach Borehole ( IBB. 390 48'N, 740 05' W: elevation of 3.7 m; Miller et lIl. 11994]) Magnetozone C24r is not precisely delineated. but it is at least 30 m thick. This is based on the identification of the Chron C24 n.3 n/C24r magnetic reversal boundary al - lOIS'S (310.53 m) and the LO of Discollster 1/I1111irlllliatilS (corre­lative wilh mid Chron C25n; Berggren et al. [1995]) at -1112' (-338.93 m: Miller et lIl. [1994[: Van Fossein [19971) (fig. 3). The magnetozone spans the upper part of the Vincentown (outer-middle neri tic clays) and lower part of the Manasquan (ou ter neritic si lty clays) Formations se­parated by a distinct erosional contact at 1075'5 (327.81 m) [Miller el (I/.. 1994: Brow ning et al.. 1997: Liu et 01 .• 1997al, thus approximately located at mid-point within the reversal. The gray clays intercalated between the typical Vincentown and Manasquan Formations in the BRB do not occur in the IBB .

Whereas the delineat ion of the planktonic foraminifera zonallsubzollal boundari es is uncertai n [Li u et (/1 .. 1997bl. the calca reous nannofoss il s prov ide an excellent stratigra­phic control. The upper part of the Vincentown Formation (between 1220' 1371.85 m] up to 1075.5" [327.S1 m]) be­longs to Zone NP9. Calcareous nann ofossil assemblages are monotonous between 1112' (338.93 m) and 1077' (328.26 m). but a cha nge occurs between 1077' and 1076' (327.96 m). with in particular the HO of DiscO(lster me­gastyptlS at 1077' and the LOs of D. aralleliS. D. Gllflrtios and RllOmbOllster calcitrapa at 1076'. On this basis, an ex­tremely thin sl iver of sediments. delineated between

T"bLI! I. - Location of magneto- el bio5trdtigraphic events in BRB and IBS. Numerical chronology From Beligr.:n ~I 01. [ 19951. T"8L. I. - PositiOI! du hi"~,,,~ms til' ChfOl' C24r sur I~squds SOli/ fOlI­diu 1f!1 c:orrilations "lIIg"ifO- .. , bios/rOligf(lphiqu<'$. Chrrmologif! "umi· rique 1If! 8erggrf!u el at /l99S{.

Datum, Age of B~55 River Island Beuh event. borehole borehOle (M~)

Chron C24n/C24r 53..347 1135' - 1137" 1017.60' -1020.40"

HO T. ,01l10r,.15 53.61 tl 36.2' - 1135.1' 1022' - 1020"

LO T. or/hOJ/lIlu, 53." 1136.2'-\135.1' 1020' -1019"

HO T. brnrnltllei 53." 1136.5' - 1136.2' 1020' - 1019"

LO T. ,on/or/us 53.93 1138.4' - 1138.1' 1022' - 1020""

HO T. dlgltalrs 54.l7 1138.4' - 1138.1' 1065' - 1062'

LO T. digit.lis S;l.J7 1138.7 - 1138 ... \' 1076' - 1075'

LO r. b,.mlrlld 55.00 1138.7-1138.4' 1016' - 1075'

LO R. t.tcr/,.,. 55;0 117l.1' - 1170.9' 101T - 1076'

LO D. a",ntU$ 3550 1171.1' - 1110.9" 1077 - 1076'

lillC excursion 55.52 1173' to 117\.} , begins at 1077

Chron 05n 55."" 1191.6' - 1193' not delineated

LO D. muhrlldillru~ 56. 1210.6' - 1211.4' 1138' - 1112'

- 1076.5" (328.11 m) and the unconformity al 1075.5' (327.8 1 m) is assigned to Subzone NP9b.

Zone NPIO is well characterized above the unconformity and up to 1020' (3 10.S9 m). Subzones NPIOb to NPIOd are clearly delinealed between 1075 ' and \065" (327.66 and 324.61 m) and between - 1021' and [020' (311.20 and 3 10.S9 m), respectively. TribrlU.:hiatlls digitalis is common in al l samples between 1075' (327.66 m) and 1065' (324.61 m), absen t at leve l [060 ' (323.0S Ill) and scarce between 1062' (323.69 m) and 1048" (3 19.43 Ill). We be lieve Ihat it is reworked above 1065' (324.61 m) as a result of the e­rosion (during Subchron NPIOc) of sediments that were de­posited updip of this location. This is well supported by the high sedimentation rales (44' for 0.2 m.y.: see table I) for the NPIOc subzonal interva l in the IBB. Reworking of older sedi ments is indicated by Ihe occurrences of upper Paleocene taxa (Discoasler aUll/ellS, Fasciculi/hus sclulilbi. F. tympllllijul"mis and Rhomboasler c(lfcilrapa ) th roughout the NPIOc zonal interval. We thus believe that the sporadic occurrences of Slellsioilla hecco/"i!ormis between 1073.8' (327.29 m) and 1055" (321.56 Ill) reported by Pak el al. 11997] and Browning et al. 119971 simp ly reflect the re­working of Paleocene sedi ments from even further updip. In this context, we point out Ihal in the Clayton Borehole. silu aled much further updip than the BRB and IBB, middle Eocene deposits rest unconformably on uppermost Paleoce· ne clays (Subzone NP9b, MPA, unpublished. cOllfra Bybell and Se lf-Trai l (1995] (see above» which exhibit a good re­cord of the LPTM, with calcareous nannofossil assemblages characteristic of Subzone NP9b (wi th D. (/r(/lIeIiS and R. calcitra/)o in particular) and the CIE [Thomas et al., 19971.

The younger part of Ch ron C24 r is not represented in the IBB due to an unconformi ty at 1019' (310.59 m). well marked by the juxtaposition of the NP IOd/NP II, (approxi­mate) P6a/P6b subzonal boundaries and the magnetic re­versal boundary.

Stra tigraphic COlTclat ions

The magnetobiostratigraphic interpretations described above reveal that each section offers a partia l representation of Chron C24r (fig . 4). The main unconformity at 113S.6' (347.04 m) in the BRB corresponds 10 a stratigraph ic gap

/Jull. Soc. aMI. Fr.. 2000, nO 3

372 M.P. AUBRY ttl {II.

! > 8 CALCAREOUS oi3(;PD8

• " 0 to • EVENrS O{oo • > 0

,

ISLAND BEACH BOREHOLE

FIG, 3. - The strat igraphic inter~aJ representing Chron C24r in the Island Beach section : lithology. magnetostratigraphy. biostratigraphy (PF = plank.tonic foraminifera: eN = calcareous nannoplank.ton) and isotope ~tratigraphy. Magnetostratigraphy IVon Fossein. 19971: litho- and sequence stratigraph~ IMiller el al .. 1994: Browing el al., 1997: Liu et al .. 199701: Biostratigraphy [Liu el (II., 1997b and hereinl: isotope m:nigraphy [Pak el al .. 19971. Symbols as in figure 2.

Fto. 3. - L'itllen'ull" Slratigfllphiqlle repriUnlcml Ie Chron CUr dUlu 10 cOllpe If'lsland Beach: Lilhologie, mogll i loSlrOligraphie. lJioslrnligraphie (PF : foramillifiru IJI(lllclOlliques: CN : IlUllllOplallctoll caleaire} et stratigraphie isotopique. Magllltoslr(uigraphie (VOII Fossein, /997J: lithOSITlUigraphie el slratigrapliie siqllfmtielle (Miller el al.. 1994: Bro"'ing el al.. 1997: UrI et at.. 1997aJ: Bioslrafigmphie (UII el aI., 1997b el ce papier]: slflItigruphie isotopique (Pak et a1.. 1997J. Symboles comme porlr la figure 2.

that encompasses Subzone NP10a and the upper and lower part of, respectively, Subzones NP9b and NP10b (si nce Subzone NPIOa was not recovered). Based on the estimated ages of calcareous nannofossi l FADs and LADs [Berggren el 01., 19951 the hiatus is at least 0.65 m. y.-Iong.

The stratigraph ic gap at 1075.5' (327.8 1 m) in the ISB encompasses most of Subzone NP9b, Subzone NPlOa and part of Subzone NP10b. The hiatus is at least 1.\3 m.y.­long. The 1076' (327.96 m) level in the IBB correlates with the [170.9' (356.89 m) level in the BRB (= NP9a/b zonal boundary, both with an age of - 55.52 Ma), based on the simultaneous LOs and HOs of several nannofossil spec ies, but it is clear that the unnamed lithologic unit in the BRB

BIIII. Sm:. geol. Fr.. 2000. n" 3

is missing in the IBB , except for a few centimeters of sec· tion below the unconformity at 1075,50' (327.8 1 m). In contrast, most of the interval that constitutes the lower part of the Manasquan Formation in the IBB is missi ng in the BRB. The lower surface of the seq uence boundary at 1075.5' in the IBB is at least I m.y. older than that at 1138.6 ' in the BRB section, based on the chronology of Berggren er ai, l I995}. In contrast, the upper surfaces of the sequence bou nd ary in the twO holes have approximately the same age (w ithin Biozone NPI Ob, that has an estimated duration of 0.2 m.y.).

T hus the BRB and IBB sections complement each other for a more complete representation of Chron C24r, although

LATE PALEOCENE EVENT CHRONOLOGY 373

U I'="EHTS::' ., ..

•

•

~'''''P

ISLAND BEACH BOREHOLE

FIG. 4. _ Correlation of the intervals representing Chron C24r in the BRB and ISB scctions. FIG. 4. _ Corrllm;on rnlft Its ;,IItn'ollrs rrpristnlall/ It rhron magnlriqut CUr dUlls Its COlipts 8R8 tl 188.

neither the younger part of Ch ron C24r (late Biochron NPI 0 to early Biochon NPll) nor the latest Biochron NP9 and · earliest Biochron NPlO (Biochron NPIOa) is represented in the composite record (rig. 5). As diachrony of calcareous nann ofossil events be tween such nearby loca tio ns ca n be defin it ely excl uded, we remark appropri ately that the d ifferences in biozo nal representat ion between the two sections are sole ly explained by the occurrence of uncon­formi ties.

CARBON ISOTOPE STRATIGRAPHY OF TH E SECTIONS

Carbon isotopic records, estab lished from mixtu res of Ci­bicidoide.~/Cjbicjde.~ spp .. have been established for both sections. They show simi lar tre nds in the BRB and IB B. and are generall y comparable with upper Paleocene-lower Eocene deep-sea carbon isotopic records. Like their oceanic counterparts, the twO neritic records reg ister a negative iso­tope excursion but of differe nt amplitudes. In the BRB , the

negative excu rsion is of 4 %'" [Cramer el 01 .. 1999) whereas in the IBB it is of 1.2 ~ [Pak er 01., 1997). Eq ual maximum isotopic values are recorded j ust below the BRB and IBB excurs ions ( 1.20 %0 at 1179' [359.35 m] in BRB ; 1.22 %c at 1078' f328.57 ml in IBB ) and the decrease in isotopic values occu rs at the NP9a1b subzona l bou ndary in both sec­tions. Th is supports our interpretation that both sections have registered the begin ning of the CIE. However, only the BRB yields an essent iall y complete record of it, so that the ampl itude of the isotopic decrease in the section is eq ui­valent to that in Hole 690B [Kennett and StOll, 1991]. The IBB yields on ly the very beginni ng of the CIE, represented by the val ue of 0.9 1 %to at 1076' (327 .96 m). Concatenation of two disj unct records below and above the unconformity at 1075.5' (327 .81 rn ) has resulted in an isotop ic excursion that was thought to correspond to the ClE [Pak er al. , 1997] but which, in fact. is merely an insignificant pseudoeve nt, s im ilar to that already identified in DSDP Hole 577 [Aubry. 1998bl.

The critical element of our di scussion is now to consider the chemomagnetobiost ratigraphic correlations. Both the

81111. Sue. giol. Fr.. 2000. nU 3

374 M.P. AUBRY el <II.

MAIN eVENTS DSDP SITe

690 (Maud Rise)

DSDP SITE

'" (Porcupine

DSDP SITE

'" (Goban Spur)

BASS AIVER BOREHOLE (NJ)

ISLAND BEACH BOREHOLE (NJ)

Abyssal Plain)

I !i nr 1

I , 1 I "

,

• • '39".,..

• 'QI~I\.

(31o..e ... )

.... . ,

" -.

, ..

FIG. 5. _ Temporal interpretation of the IBB and BRB and temporal correlution wilh DS DP Holes 550. 549 und ODP Hole 690S. DSDP Hole 549 is included bccau~c ils isotopic record bclow lhe unconformity 31 335.4 mbsf was critical in correlating corrcclly the records in Holes 690B and 550. The ages of Ihe ~urfaccs :Lnd duration of lhe hiatuses for Holes 550. 549 and 690B werc established in Aubry ell/I. [1996) and Aubry t 1998b). The datums used \0 cSUlblish the temporal imeTprclations of Ihe New Jersey boreholes are given in lable I. Thiel; plain line = age of the CIE (and LPTM). Thick d"sh line = age of the Ypresian transgression as dmed by base of the London Cloy Formation Isee Aubry. 19961. WllVy lines = unconformities. Blank between wavy lines rcpresenl hi"tuse~ .

FIG. 5. - Inrerprhmioll /<'lIIporel/<, d,> /88 1'1 OR8 <'I ('orr8(11i'1II It'mp(Jre/lt (ll'tc les puils I)SDP 550. 549 el 001' 6901J. LI' PlIil)' DS{)I' 549 est int/us pllTet' que )'011 <,11rt'gil·trelllelll isO/opiqul' IW-/lesSOl/S de In discord/IIIct' 1/ JJ5 . .J mh,'f l'ppOrtll ,m i/imel1l crili,!u<, p"""ell/'III (/'flllyer fa corriialillll elllre les puils OIJI' 690lJ <'I {)SIJf' 550. /.('.1 ages (Iu surjm:e,f 1'1111 dude des /tillius litH ,mils 550, 549 el 6900 jl/rem elllhli.r t/IIIIS Aubry el 31. f 1996{ <'I Aubry f 199811/. /.es t/ol","S IIIilish pour Ilo/)/ir l'imerprlllllioll lelJJporelle t/I'S JlllilS dll New Jersey SOIlI (Jollllfs t/(ms Ie mblrou I. Lig"e ('omilltle ipoisse,' lige lie In Cl£ (III till COlJJmellcemelll (/1/ I.f'TM). !.is'le ,Jis('omim.e Iplliss<, : {IKe I/e /11 1""IS8"' $sioli ,I1)resiemll' IImle flnr /11 bllsl' iiI' /11 /omtmirm II" IAimloll CIII)' {rQir Allbr)'. 1996j. Liglics omJulin,' Discort/allces. us b/w/cs I'/Ilre CI'S /i8"<'S reprb"ltlem les iti(lllls.

BRB and IBB excursions are loc.lIed esse nti ally at mid­point in Magnetozone C24r. but th e former is located in mid Zone NP9 whereas the lalter is located across an NP9/N PIO (lowermost NP9b/NP IOb) zonal con tact. If the BRB and IBB sect ions were geographically far apart. the si mil ar location of the BRB and IBB exc ursions with res­pect to the whole extent of Magnetozone C24r could easi ly

81111. SOl'. gli()l. Fr.. 2000. nO 3

be seen as a demonstration that the excursion constitutes a reliable correlation level in both sections. with the conse­quence thlll the biostratigraphic events wo uld be seen dia­chronous. However, on the ground of three comp lementary pieces of ev idence . thi s reasoning cannot apply to our case. The two first pieces of evidence, litho- and biostratigraphic, were discussed above. The lhird is brought by chcmostra-

LATE PALEOCENE EVENT CHRONOLOGY 375

tigraphy itself. If both the BRB ;Uld IBB exc ur:>ions repre­sented the CIE, it would be difficul t to explain the dif­ference in amplitude of thc BRB and IBB excursions (- 4 %Cl and 1.2 %Cl. respecti ve ly). parlicularly in view of the large excursion in the Clayton Corehole IThomas et al .. 1997j. The difference in ampl itude between the BRB and IBB ex­cu rsio ns is easily explained if only the BRB excursion rep­resents the CIE. The BRB and IBB sections thus provide a clear demonstration that unconformities can account for the reported discrepancies regarding the variable location of the j)13C excursion with regard to the magnetobiostrati­graphy in different sections. They also provide a sound jus­tification for the differences in the ampli tude measured for excursions in different sections.

We can safely and firmly conclude that differences in chemomagnetobiostrmigraphic correlat ions between sec­tions can result solely from truncations in the stratigraphic record. We ca n also lIdd that ( I) as the Chron C24r was onc o f the warmest in terval of the Cenozoic. and (2) as a strong warm ing affected the high latitudes during the LPTM [Kennell and Stott. 1991: Zachos et al .. 1993J. it is reaso­nabl e to assume minimum diach rony of biostratigraphic e­ven ts during that chroll.

DIS CUSS IO N

Our demonstration that two nearby neritic sections through Chron C24r show (somewhat unexpectedly) substantiall y different magnetochemobiostratigraphic records have se­veral fundamental implications for deep sea stratigraphy and the study of latest Paleocene-earliest Eocene history. These are examined in turn below.

The composite refen= nce seclion and the relati\'e chronology for ChrOIl C24r

The first implication of ou r demonstration concerns the re­liability of the composite reference secti on of Chron C24r that Aubry el (If. 119961 and Berggren and Aubry [1996J constructed in order to derive a chronology for the chron . Using integrated magnetochemobiostratigraphy rather than relying on magnetochemostratigraphic correlat ions alone. these ;\U thors constructed a reference section usi ng the early and late records of Chron C24r as preserved in ODP Hole 690 and DSDP Hole 550. respectively. In so doing. they assumed min imum diachrony of hiostratigraph ic events and explained the disc repanc ies between chemomag netobiostra­tigraphi c correlations in the deep sea sections by the oc­currence of multiple unconformities [see Aubry. 1998b. cl.

Our two neritic records arc easily correlated with deep sea records through magneto-. chemo- and calcareous nan­nofossil strat igraph y. They correlate well with the records of Chron C24r in DSDP and ODP Holes 550 and 690B. The upper part of the Vincentown Formation between - 1210' (386.80 m) and 1171' (356.92 m) in the BRB is esse ntially equiva lent with the interval between - 195.94 mbsf and 170.26 mbsf in Hole 690B. and the unnamed li­thologic unil between 1171· (356.92 m) and 1138.6' (347.04 m) in the borehole equivale nt with the 170.26 mbsf to - 150 mbsf interval in the ODP hole. In turn. the lower part of the Manasquan Formation in the IBB between 1075.5" (327.81 Ill ) and 1019' (310.59 m) is esse ntially cor­rel ative with the in terval between 383.28 mbsf and 372 mbsf in DSDP Hole 550 [Aubry l'f of .. 1996. Table 3]. This allows us to establish temporal correlations between the four sect ions (fig. 5). a key to sound geological reconst ruc­ti ons [Aubry. 1995J.

Thus we believe that the composite reference section constructed frolll DSDP Site 550 and OD P Site 690 [Aubry el af .. 1996[ is corroborated by the data presented above and that future correlat ions between upper Paleoce ne-lower Eocene sections can rely on the existing relative chronology for the Ch ron establi shed by Aubry et (/1. ll996J and Berggren and Aubry [19961. The BRB provides conrirma­tion that the CIE occurs at abou t the mid-point in Zone NP9IAubry el af .. 1996: Schmitz et al.. 1996]. even though the NP9/NPIO zonal boundary was not recovered in that section. Thus. although variations in sedimentation rates are difficult to estimate with the current numerical chronology available. records in which the excu rsion occurs close to the NP9fNPIO zona l boundary should be considered sus­pect.

Isotopic excu rsion s and the C IE

Carbon isotope strati graphy is extensively used to correlate upper Paleocene-lower Eocene stratigraphic sections. and man y a record of Chron C24r have been "tuned" using ··the·· Cllrbon isotope excursion (assumed to represent the CIE) and magnetic reversals. and purposely avoiding biostrati­graphic datums because of their assumed diachronous nature [e.g., Pak and Mill er. 1992. Zachos el lil., 1993: Thomas and Shack leton. 1996: Pak ct af .. 19971. While it is temp­ting to do so. incorrect interpretation of isotopic featu res in sec tions results in the miscorrelation of stratigraphic re­cords. j)1lC excursions that refl ect a true disturbance of the carbon cycle are extremely powerful stratigraphic and tem­poral markers because of the short duration of the carbon cycling 16 x 10J yrs. Broecker. 1974 1. and such is the case for the CIE. However. interpreting an excursion as the CIE when in reality it constitutes a pseudoevent has several se­rious consequences. It results in incorrect stratigraphic and temporal resolution nnd correlations [see Aubry. 1998b. Fig. 3.3. and 3.4]. and ultimately in incorrect paleoceano­graphic interpretations. For instance. interpretation of a j)13C excursion in Hole 577 as corresponding to the CIE has resulted in the unwarran ted compari son between high and low lati tude isotopic records and unsubstan tiated con­clusions regarding I;He Paleocene-early Eocene deep sea cir­culation IAubry. 1998bJ.

The interpretation of chemost ratigraphic even ts and iso­topic exc ursions in particular requires a careful evaluation of data in a manner simi lar to the identification of magne­tozones. In other words. the identification of the CIE re­quires close scrutiny lind support from biostratigraphic even ts. The freque nt association of "the·' j)13C excursion wi th the 1-1 0 of the Slell.fioilll/ becclIriijormis is often cited as supporting evidence that the excurs ion represents the CIE: yet. this is utterly insufficient because the upper range of a taxon or group of taxa can easily be truncated by un­conformities. For in stance the 1-10 of the S. beccarii/ormis in DSDP Hole 577 docs not correspond to the BFE [Aubry. 1998b]. Multiple criteria are necessary to firmly identify isotopic events. Spec ifica ll y. to be unequivocally identified. an excursion must be bracketed by non iterative even ts and the amplitude of the excursion must probably not be signi ­ricantly different at different locations.

We propose that idemiricalion of the CIE should rcly on major evolu tionary planktonic events. Kelly el af. [1996J have shown that the CIE is associated with a rapid diver­sification among planktonic foraminifera. in particular with the occu rrence of AClirillill(/ (ljricll/1(1. A . . fib(liyaeluis and Moro:"OI'efla 1IIfiSOIiCIISi.f. This may serve to predict the lo­cat ion of the CIE in deep sea sections bu t also in neritic settings. The th ree t:lxa cited above occ ur in the BR B sec­tion [Miller el (11 .. 1998: Cramer et al .. 1999J two of them

HIIII. sm..·. gt'QI. Fr.. 2000. nO 3

376 M .P, AU BRY el <II.

extending up to the NP9/NPIO biozonal contact Ohey were not found in the IBB section, confirming. if necessary, the different siraligraphies in the two holes. and COl/tra Pak el fli. l1997J. More readi ly, the LO of some calcareous nan­noplankton species. includin g Disco(lsler lINI/Ie/O', D. (/I/(/r­(io.~, Rhomboaste,. calcitrapa and R. spinel/s. may be used to identify the elE lit least in neritic and some deep sea sections where these spec ies appear suddenly at the level of the excu rsion and dominate the assemblages over a thin stnltigraphi c interval representing possibly a few (? 0.2 m.y.) hundred thousand years. This has been observed in the At lantic (e.g., BRB). Indian (e.g., DSDP Site 213 ; MPA and D. Norri s. unpublished data) and Tethyan (e.g .. Alame­dilla section: Aubry et (II., submitted) oceans. We regard thef>c taxa af> the calcareous nannoplankton cou ntcrpart of the excursion planktonic foraminifera. that also evolved in response to the event reflected by the CIE. What differen­ti ates them is thei r poor symmetry. which contrasts strongly with the symmetry that all other coccoliths/nan noliths ex­hibit. At no ot her time in the Cenozoic history of calcareous nannoplankton were poorly sy mmetrical coccoliths massi­vely secreted.

Confirmation that the correlation established by Aubry et lIf. L1996] between the sections representing Chron C24r in Holes 550 and 690 are essentially correct revivef> the question regardin g the occurrence of mult iple SI 3C excur­sions or rapid shi ft s in lower Bi ozone NPI O. Aubry et (II. L1996] and Stott et al. [1996] suggested that at least one other SDC excursion may exist but this has not yet been proved. This situati on may reflect the fact that the interval of interest has not been recovered from deep sea sections. and we agree with Thomas et (i1. fl999] that only integrated high resolution isotopic and biostratigraphic studi ef> will help to determine whether the LPTM was unique or one of several short-term events.

Deep sea unconformities

Perhaps the most important impl ication of ou r demonstra­tion on a general level is the fact that unconformities should be recognized as intcgral components of the stratigraphic record , and this is true for the deep sea basins as well as for the continental shelves LAubry. 19861- While the dis­continuous nature of stratigraphi c sections deposited on e­picontinental margins is well recogni zed, the view that deep sea sec ti ons are continuous remains prevalent. This study supports the documentation that upper Pal eocene-lower Eo­cene deep sea records are discontinuous at many locations [Aubry et al., 1996, 1998b, c, and unpublished]. As Chron C24r was a time of global warmth , latitudinal diachrony must be minimal in the upper Paleocene and lower Eocene strat igraphi c record, and we can be confident that most che­momagnetobiostratigraphic discrepancies between sections most li kely reflect truncation by unconformities. We em­phasize that the hi atuses that have been described in Chron C24r are not of negligable duration. Many are I m.y. long or longer. This durati on is greater than the life span in Chron C24r of some of the marker species [e.g., T. digita­lis: - 0.2 m.y.; T. colltortllS : - 0.32 m. y .. Berggren el (II., 1995J. There may bc shorter hiatuses as we ll « 50 x 103 yrs), but the current means available to assess the comple­teness of stratigraphic section s do not pe rm it their confident identification.

BIIII. Soc. gio!. Fr.. 2000. nO 3

Considering our current understanding of the Earth Sys­tem , it is difficult to conceive of the widespread occurrence of deep sea unconformities, but their progress ive documen­tation is the fi rst step towards resolving their enigmatic ori­gin. Other widespread Paleogene deep sea unconformities have also been described. in part icular around the 10-werhniddle Eocene boundary. Recognizin g the incomplete­ness of the deep sea record will have a double benefit, ( I) to perm it the correct temporal correlatio ns between sections and thus the correct timing of the major events that have punctualed Earth history. and (2) to determine the architec­ture of the stratigraphic record in parti cular concerni ng the re lationships between marginal and deep sea deposits, in order to identify the basic forcing mechanism(s) that shaped it.

The origin of the multiple upper Paleocene-lower Eocene unconformities that seem to occur both in deep sea and llwr­ginal setti ngs rema in s unclear. and it would be prematu re to consider them related in any fashion to the LPTM. mai nl y because multiple unconformities occur at other times as well (e.g., around the lower/middle Eocene boundary). Ne­vert heless. it is clear that ignoring them can only blur the historical message embedded in the stratigraphic record and eventually delay a sound understandi ng of what really hap­pened 55.5 to 54.5 mil lion years ago.

CONCLUSIONS

This study of two neritic records of Chron C24r which offer an excellent magneto-, bio- and chemostratigraphic record has demonstrated that the current relative chronology of ma­rine events in Chron C24r is sound, and that unconformities rather than diach rony is responsible for the inconsistent che­momagnetobiost rat igraphi c correlations that have been made between geographica lly di stant sections. However, we cauti on that. for reasons that have been di scussed elsewhere [Berggren and Aubry, 1998: Aubry_ 1998b], the numerical chronology is as yet unverified, and is likely 10 be revised. The New Jersey sections do not shed new light on the lo­cation of the NP9/N P1 0 biochronal boundary in Chron C24r. a boundary that has been used as a tie-point in the cal ibration of the curren t GPTS [Can de and Kent, [992, 1995]. This will req uire a section which offers a comp lete stratigraph ic representation of Chron C24r, a situation un­likely to be found in a marginal setti ng.

We also call attention to the widespread occurrence of unconformities around the NP9/N PIO zonal boundary. and to the need to take them in to account when interpreting and comparing isotopic records from different areas. As with any other data, isotopic signatures, in particular excursions, cannot be taken at face value: rather their sign ificance must be determined based on biostrat igraphic con trol. Such con­trol demonstrates that many excursions identified in deep sea sections are pseudoevents unrelated \0 the C IE. Paleo­ceanograph ic interpretations would gain from the recogni­tion of the incomp leteness of the deep sea strati graphic record. and more accurate temporal interpretations and cor­relation s would be derived from the identification of un­conformit ies before records are fine-tuned for comparison.

Adllo..-leJgllle/lls. _ WI! :tre thankful 10 w. A. Berggren. R. O·B. Knox and B. Schmilz ror comments on Ihe manuscript. This is IS EM COlllribu­lion number 98-81. and LamOIll Doherty Eanh Observatory ContribUlion number 6050.

LATE PALEOCENE EVENT CHRONOLOGY 377

Refe r e n ces

A;';OORI E. &:: MO"IlCltI S. (1996). - High·resolution calcareous nannofossil biostratigrJpliy ac rO)$ Ihe Paleocene/Eocene boundary 0.1 Carll­"aea (southern Spain). In: M .-? Al,.BRY & C. BESJAMESI Eds .• PaleocenefEocenc boundary c"cnu in space and lime. - Isra<'l 8ull. Ear/It Sci .. 44. 197·206.

AUBRY M.-P. ( 1986). - Paleogene calcareous nanllaplDntlon biostratigra­phy of norlh"'cslcrn Europe. - Pulat'ogcogr.. Pillat'odinm/ol .. l'lllucot'cal .. 55, 267·334.

AUBRY M.- P. (1995). - From chronology 10 stratigraphy: lnlC:rprcting the lower and middle Eocene slmligraphic reeord in lhe Allantic Ocean. hI : W.A. BERGGRE, ... D.V. Kru.-r. M.-P. AURII.Y & J. H AR.

Ot:.'1BOL J. Eds .. Geochronology lime scales and global sirali ­gl':lphic corre lalion. A unified temporal framework for an historical geo logy. - St)('. Eroll. P(I/I'OIIW/. Mi"ert~/. Sp. Puhl .. 54. 213-274.

AUBRY M.-P. (1996). - Towards an upper Paleocene-IO"'er Eocene high resolution stratigraphy. I" : M.-P. AUBRY & c. BENJAMtSt Eds" Paleocene/Eocene boundary events in space and time. - IsrCle/ 81111. I:"urth Sci.. 44. 239-253.

AUIIRY M.-P. ( 1998a). - Early Paleogene caleareous nmmoplankton 1'\'0-

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