pliocene boundary: Present and future

Miocene Stratigraphy: An Integrated Approach A. Montanari, G.S. Odin and R. Coccioni, eds. 9 1997. Elsevier Science B.V. All rights reserved.

Chapter AlO

THE MIOCENE/PLIOCENE BOUNDARY: PRESENT AND FUTURE

J.-P. Suc, G. Clauzon and E Gautier

INTRODUCTION Until recently, the definition and the chronostratigraphic location of the Miocene/ Pliocene boundary has not been the subject of intense discussion. A complete review of its history was made by Cita (1975), who also provided the modem definition of the boundary in the light of the deep-sea drilling project (DSDP Leg 13) in the Mediterranean Sea. It corresponds to the renewal of marine conditions in the Mediterranean Sea after the Messinian Salinity Crisis. The Sphaeroidinellopsis Acme-Zone followed by the first appearance of Globorotalia margaritae in the Mediterranean marked the boundary (Fig. 1). Aside from the fact that the definition and the location of a chronostratigraphic boundary is in the first place a matter of nomenclature, one must not neglect its global importance as a reference point. In this way, the choice of a continuous boundary stratotype is a real scientific question. Nevertheless, such a choice is not devoid of a geopolitical aspect, all the Neogene stratotypes (including the boundary ones) being located in the northwestern and central Mediterranean region, and mostly in Italy. Recent progress in integrated stratigraphy, both in the Mediterranean (Hilgen and Langereis, 1993; Gautier et al., 1994) and in the Atlantic Morocco (Benson et al., 1991), requires reconsideration of the problem. However, one must bear in mind that, in dating and identifying the lowermost Pliocene deposits, the problem was touched upon in the Mediterranean only because of the Salinity Crisis. The topic has been recently discussed in detail in several papers (Hilgen and Langereis, 1993; Benson and Hodell, 1994). As a consequence, this paper does not aim to examine again all the aspects of the question, but to emphasize the main ones taking into account recent progress in chronostratigraphy in order to synthetize the debate for the emergence of the best solution. The time scale adopted here is that of Cande and Kent (1995) who have revised their earlier magnetostratigraphic chronology (Cande and Kent, 1992) to be consistent with astronomic chronology. THE PRESENT-DAY CONVENTIONAL BOUNDARY AND ITS PROBLEMS The first problem concerning the Miocene/Pliocene boundary, but not the least, is the large diachronism of the first appearance datum (FAD) of Globorotalia margaritae between the Atlantic Ocean and the Mediterranean Sea (Figs. 1 and 2). In the Atlantic Ocean, at Mediterranean latitudes (i.e., Bou Regreg section in Morocco), the FAD of Globorotalia margaritae occurs within mid Subchron CA3n.2n (Benson and Rakic-E1 Bied, 1991; Fig. 2). In the Mediterranean (i.e., Capo Rossello, Singa, Eraclea Minoa sections), the massive increase in abundance of Globorotalia margaritae occurs in mid

J.-P. Suc et al.

150

MAGNETOSTRATIGRAPHY (Cande & Kent. 1995)

MAIN MEDITERRANEAN BIOEVENTS (Cita et al.. in pt~) ~ tit

CONVENTIONAL STRATIGRAPHY

Non distinctive zone G. marfar/tae Acme

STAGES

SERIES

z

P

A

L

N

I

c

O

L E

C E

N

C3An.ln

l:

C3An.2n

l:

i C3Br.ln

Fig. 1. Conventional status of the Miocene/Pliocene boundary with respect to the Mediterranean biostratigraphy and the modem magnetostratigraphic scale.

Subchron C3n.4n (Thvera) (Hilgen and Langereis, 1988; Langereis and Hilgen, 1991; Fig. 1). This represents a diachronism of about 1.3 Ma in spite of some isolated records in the lower Messinian Tripoli in Sicily (Colalongo et al., 1979a, b; D. Violanti, R. Sprovieri, oral commun.) (Fig. 1). Thus, there is no direct possibility of correlating the Mediterranean and the Atlantic stratigraphies using this biostratigraphic event. The FAD of Ceratolithus acutus (calcareous nannoplankton) has been used also (Cita and Gartner, 1973), but the species is very rare in the Mediterranean Pliocene (Rio et al., 1984). This bioevent follows the Miocene/Pliocene boundary by about 0.1 Ma (early Subchron C3n.4n), such a difference being too large in comparison with the desired precision for the chronologic definition of the Miocene/Pliocene boundary (Figs. 1 and 2). In addition, a rare appearance of this species has been found near the Chron C3An/C3r boundary (Benson and Hodell, 1994), denoting a

The Miocene/Pliocene boundary: present and future

I

151

POSSIBLE CHRONOSTRATIGRAPHIC RESPONSES

MAGNETO-

TO THE NEW PROPOSED STATUS

STRATIGRAPHY

OF THE MIO-PLIOCENE BOUNDARY (Benin et al., 1996)

(Cande & Kent. 1995)

Salinity Crisis

1

2

zA

I

N c

IA

Z

E A

C

N

L

o .-

!

0

MESSINIAN

i

i L

A

A

N

N

TORTONIAN

Fig. 2. The proposed new definition of the Miocene/Pliocene boundary and its inference on the Messinian Stage according to the recent chronologic location of the Salinity Crisis.

diachronism of about 0.7 Ma between the Atlantic and the Mediterranean. There is no possibility to use the same biostratigraphic markers both in the Mediterranean and in the Atlantic for the recognition of the Miocene/Pliocene boundary. As a consequence, the present-day definition of the boundary in the Mediterranean region cannot have a worldwide significance. Two options are to be discussed: either to change the chronologic location (and the definition) of the boundary outside the Mediterranean, or to modify its definition (without changing its chronologic location). THE PROPOSED NEW LOCATION OF THE BOUNDARY AT THE C3An/C3r CHRON BOUNDARY This solution at the topmost Chron C3An, proposed by Benson et al. (1996) would present the advantage of a global significance of the boundary (Fig. 2). Indeed, the top of

152

J.-P. Suc et aL

Chron C3An.ln is relatively easy to determine in long, continuous sedimentary records, even without a detailed biostratigraphy. Such a definition of the boundary contains two major disadvantages: (1) there is no significant biostratigraphic event at the boundary level; and (2) according to its new magnetostratigraphic assignment in the early Chron C3r (Gautier et al., 1994), the Messinian Salinity Crisis would belong to the Pliocene (Fig. 2). Three stratigraphic responses can be advanced to such a modified definition of the Miocene/Pliocene boundary (Fig. 2): (1) To retain the Messinian Stage as the last stage of the Miocene. in this way, it would have a reduced duration, from about 7.0 to ca. 5.9 Ma, and would be separated from the Salinity Crisis which formed an integral part of its origin characterization. This would raise an erroneous situation vis ~ vis accepted stratigraphic principles. (2) To transfer the Messinian Stage to the lowermost Pliocene (from ca. 5.9 to 5.32 Ma). It would then almost exactly correspond to the Salinity Crisis; this situation would give to the Messinian Stage a regional (i.e., non-global) significance. (3) To delete the Messinian Stage from the global stratigraphic scale as already proposed by Berggren (1971), to become a regional stage. In that case the Tortonian would be the terminal stage of the Miocene, and the Zanclean the first of the Pliocene. The regional Messinian Stage would have exactly the same chronologic definition as at present. It is clear that the main obstruction for solving the Miocene/Pliocene boundary question is the sedimentary hiatus in the Mediterranean caused by the Salinity Crisis. DOES AN ACCURATE CHRONOLOGY OF THE MESSINIAN SALINITY CRISIS ALLOW A NEW DEFINITION OF THE CLASSICAL BOUNDARY? An accurate chronology of the Messinian Salinity Crisis is proposed by Clauzon et al. (1996) (Fig. 3), who consider three newly gathered sets of data. (1) The Salinity Crisis occurred entirely during the magnetostratigraphic 'quiet zone' Chron C3r (Gautier et al., 1994; Fig. 2). (2) On the basis of new geomorphological observations, the evaporites can be separated into (older) marginal, including the Sicilian evaporites, and (younger) deep basin evaporites (Clauzon et al., 1996; Fig. 3). (3) The astronomically calibrated ~180 curves of ODP Site 846 (eastern equatorial Pacific; Shackleton et al., 1995) and the Sal6 core (Morocco; Hodell et al., 1994) yield a ca. 20 ka subdivision of Chron C3r (Fig. 3). Twenty two isotopic stages (TG1-TG22) ran between Subchrons C3n.4n and C3An.ln. Some peaks (glacials: TG22, TG20, TG14, TG12; interglacials: TG15, TGll, TG9, TG5), offer a very similar feature in these distant regions and can be used as chronostratigraphic datums. The Miocene/Pliocene boundary should be drawn at a stratigraphic level representative of a rise in sea-level, i.e., in correspondence with an interglacial peak. Peaks TG15, TG11 and TG9 cannot be selected because they are coeval with the Salinity Crisis and such a boundary would not have a global significance. In addition, they are devoid of any bioevent. Only peak TG5, which appears the most intense in the Sal6 section, occurred near the end of the Salinity Crisis. Its age is certainly very close to that of the lowermost Sicilian Trubi layers. The sudden Zanclean transgression

The Miocene/Pliocene boundary: present and future

153

MEDITERRANEANEVENTS Ma

P m

ODP Site

g

846

Sal~....

(Shackleton et al., 1995)

"t ti

. . . . .

E

(Clauzon el al.. 1996)

Ma

(Hodell et al.. 1994)

16

2

14

t,,,

12

1

0.8

0.6

0.4

I,..~...~...J...J..-J

Deep basin

Margins

Normal

Normal

marine

marine

IL"~

sedimentatio sedimentatio~ . . . .

.

.

.

.

.

.

. . . .

Evaporites

Hiatus

5.5 TG I ~ S . 14 ' ~:~-

~,

c, ": 5.6

C3r

TG

" ~ . " ~ . --_ __

~

C3r

~"~

TG9? Normal Evaporites

5.7-

i marine

5.84 -/ 5.9

_r

1 ~

~sedimentatior

5.9 -~

Normal marine sedimentatio~

Fig. 3. Proposal for a newly defined Miocene/Pliocene boundaryat 5.32 Ma, based on isotopic stratigraphy. Pmg = palaeomagnetism.

in the Mediterranean was probably caused by a global sea-level rise connected with a change in the tectonic activity in the Alboran Sea. Such a definition based on the isotopic stratigraphy needs a new intensive biostratigraphic effort both in the Atlantic Morocco and in the Mediterranean. The detailed study of dinocysts from the Bou Regreg section and in the reference Mediterranean sections could provide an appropriate response. THE ELECTION OF THE CORRESPONDING STRATOTYPE The Bou Regreg section (including the Sal6 core) would appear to offer all the appropriate advantages to become elected as the Global Stratotype Section and Point (GSSP) for the Miocene/Pliocene boundary. Peak TG5 is easily identifiable in field (Rakic-E1 Bied, 1990; Hodell et al., 1994). This could constitute the reference section for the passage from the Messinian to the Zanclean in continuous sedimentary areas. The Zanclean stratotype, in Sicily (the Capo Rossello section or the complementary Eraclea Minoa section), should offer a very precise datation of the base of the Pliocene considering the astronomic cycles. We consider that the Carmona section (Guadalquivir basin, Spain), is inappropriate because an important unconformity seems to be present near the Miocene/Pliocene boundary in the boreholes cored in 1991 (Clauzon et al., to be published). In conclusion, this proposal, characterized by the stability of the Miocene/Pliocene boundary, offers several advantages among which the most relevant are: (1) it preserves the historical argument in the Mediterranean where the boundary has a biostratigraphic basis; and (2) it provides a modem definition (isotopic stage) as well as a global significance to the boundary.

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J.-P. Suc et aL

The question was greatly debated during the 10th RCMNS Congress (Bucharest, Sept. 1995) on the occasion of two meetings organized by Prof. M.B. Cita: the Workshop on the Miocene/Pliocene boundary and the session of the Subcommission of Neogene Stratigraphy. At the end of these meetings, large consensus appeared among the members of the SNS despite any official decision. The Miocene/Pliocene boundary should be maintained at 5.32 Ma. R.H. Benson announced his intention to abandon the proposal to place the boundary at the top of the C3An.ln Chron and to investigate new biostratigraphic markers within the Bou Regreg section near isotopic stage TG5. In this way, we can conclude that our proposal (see above), presented at Bucharest, has been considered as the best solution. SOMMAIRE m LA LIMITE MIOCI~NE/PLIOCI~NE: PRI~SENT ET FUTUR

(Manuscrit soumis: Fgvrier 1995, Mvisg: Octobre 1995; rgdacteurs responsables: AM et RC) La d6finition chronostratigraphique de la limite Mioc~ne/Plioc~ne correspond au rrtablissement des conditions marines en Mrditerranre apr~s la 'crise de salinit6 messinienne'. La zone d'abondance de Sphaeroidinellopsis suivie de l'apparition de Globorotalia margaritae en Mrditerranre marquent la limite en biostratigraphie. La definition de la limite chronostratigraphique est une convention; son application globale est un probl~me qui fut discut6 rrcemment par divers experts; les auteurs soulignent les principaux aspects de ce probl~me afin de conduire vers la meilleure solution pour le choix d'un point stratotypique dans un drp6t continu. Longtemps, le probl~me de la localisation de la limite a 6t6 circonscrit ~ la Mrditerranre ?t cause de la 'crise de Salinit6'. Mais l'6tage a une validit6 globale par sa durre et la question doit &re examinee dans cette optique globale. Des progr~s rrcents sur la stratigraphie intrgrre ~ la fois en Mrditerran6e et au Maroc atlantique requi~rent un rrexamen du probl~me. Le diachronisme des biosignaux entre l'Altantique et la Mrditerranre est une difficult6 fondamentale (par exemple on a estim6 celui du biosignal apparition de Globorotalia margaritae 7t environ 1,3 Ma). Pour s' abstraire du probl~me biostratigraphique, 2 solutions ont 6t6 proposres: (1) faire correspondre la limite avec l'inversion N/I magnrtostratigraphique C3An/C3r 'facile' reprrer; (2) faire correspondre la limite avec une 616vation du niveau matin (un pic interglaciaire) dEnotE par la courbe d'rvolution des isotopes de l'oxyg~ne. La seconde solution semble la plus approprire, elle est seule compatible avec la chronostratigraphie traditionnelle du Messinien et du Zancl6en. Le choix du niveau precis se porterait sur le pic TG5 de la courbe des isotopes de l'oxyg~ne, particuli~rement intense dans les series du Maroc o?a le PSG pourrait &re localisr. Ce niveau se situerait dans le quart suprrieur de la magnrtozone C3r. (Sommaire propos~ par les r~dacteurs, GSO) ACKNOWLEDGEMENTS The help of W.A. Berggren and M.-E Aubry is highly appreciated in reviewing and correcting the paper.