Faraoni Level, Latest Hauterivian, Early Cretaceous)

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Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 www.elsevier.com/locate/palaeo

Biotic changes linked to a minor anoxic event (Faraoni Level, latest Hauterivian, Early Cretaceous)

Miguel Companya,*, Roque Aguadob, Jose´ Sandovala, Jose´ M. Taveraa, Concepcio´n Jime´nez de Cisnerosc, Juan A. Veraa

aDepartamento de Estratigrafıá y Paleontologıá, Facultad de Ciencias, Universidad de Granada, 18002 Granada, Spain bDepartamento de Geologıá, Escuela Universitaria Politećnica de Linares, Universidad de Jaeń, 23700 Linares, Spain cEstacioń Experimental del Zaidıń (CSIC), C/ Pedro Albareda, 1. 18008 Granada, Spain Received 19 January 2004; received in revised form 22 October 2004; accepted 23 March 2005

Abstract

A conspicuous renewal in the ammonite faunas of the Mediterranean Tethys occurred in the latest Hauterivian. This faunal turnover took place following a stepwise pattern. The first step occurred at the boundary between the Pseudothurmannia ohmi Subzone and the Pseudothurmannia mortilleti Subzone, coinciding with the base of the so-called Faraoni Level. This is a Corg- rich interval that has been recognised in several basins of the Mediterranean Tethys and seems to be the expression of a short- lived oxygen-deficient event. It correlates with a well-documented second-order peak transgression. The oxygen depletion preferentially affected the deep nektic ammonites, which would explain the extinctions within this group around the Faraoni Level. On the contrary, an increase in the trophic resources in the photic zone favoured the diversification of epipelagic ammonites. Concurrently, an abrupt change took place at this level in the nannoconid assemblage composition. A minor second event, located at the base of the Pseudothurmannia picteti Subzone, was marked by the replacement of a few planktic ammonite species by closely related forms, and the structure of the ammonite assemblage was not substantially altered. The coincidence of this event with a further restructuring of the calcareous nannofossil assemblage suggests that some changes had to occur in the planktic ecosystem during the sea-level highstand subsequent to the peak transgression. The third and last stage of the renewal process started in the upper part of the P. picteti Subzone, coinciding with a drastic sea-level fall. It is characterised by the extinction of many of the species that had appeared in the two previous events, resulting in an extensive modification of the assemblage structure. The regression would probably cause a drop in the primary productivity and, consequently, an improvement in the oxygenation level of the sea bottom. This would explain the extinction of several planktic ammonite species and the appearance of new nektic and nektobenthic lineages. D 2005 Elsevier B.V. All rights reserved.

Keywords: Biotic changes; Anoxic event; Ammonites; Calcareous nannofossils; Hauterivian; Mediterranean Tethys

* Corresponding author. Tel.: +34 958243201; fax: +34 958248528. E-mail address: [email protected] (M. Company).

1. Introduction in sea level would have caused a severe telescoping of ammonite biotopes over the shelf edge, hence enhanc- The ammonite faunas of the Mediterranean Tethys ing selection pressure and extinction. underwent a remarkable turnover during the latest The start of this renewal process coincides with the Hauterivian and earliest Barremian. More than 90% so-called Faraoni Level, a Corg-rich stratigraphic in- of the species present in this interval were involved in terval, which has been recognised in several western the renewal, and taxa that had been major components Mediterranean basins and has been interpreted as the of the Hauterivian assemblages (like the Criocera- sedimentary record of a short-lived oxygen-deficient tites–Pseudothurmannia lineage or the genera Ple- event (Cecca et al., 1994; Baudin et al., 1999, siospitidiscus and Neolissoceras) disappeared at that 2002a,b). Moreover, marked changes in the microflo- time and were replaced by typical Barremian groups ral and microfaunal assemblages have also been (Barremites and the first representatives of Silesitidae, recorded within this interval (Coccioni et al., 1998). Holcodiscinae, and Leptoceratoidinae). Hoedemaeker In this paper, we analyse the extent and signifi- (1995a,b) previously documented this turnover, which cance of this ammonite faunal turnover, which we he related to a rapid eustatic sea-level fall. Such drop reinterpret in the context of the Faraoni event.

0 0 ALBACETE 1 0 Oliva 9 IBERIAN PENINSULA 8 Alcoy Elda 7

ALICANTE Elche Caravaca 3 5 4 0 38 6 MURCIA JAEN Huéscar 1 2 Lorca Cartagena Baza Lucena Iberian Massif

Guadalquivir Bassin GRANADA Prebetic 370 ALMERIA Subbetic

Internal Zones MALAGA Mediterranean Sea 0 50 100 150 Km

Fig. 1. Simplified geological map of the Betic Cordillera and location of the studied Hauterivian/Barremian boundary sections: (1) Barranco de la Aguzadera (sections X.G and X.G1); (2) Ermita de Cuadros (section X.EC); (3) Rı´o Argos (sections X.Ag1, X.Ag4 and X.Ag5); (4) Barranco de Cavila (section X.Kv3); (5) Arroyo de Gilico (section X.V1); (6) Cerro del Tornajo (sections X.Tj1 and X.Tj2); (7) Sierra del Cid (sections X.A1 and X.A2); (8) Barranco de la Querola (section X.Q); (9) Cantera de l’Almuixic (section X.O). 188 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199

2. Biostratigraphic framework et al., 2003) by establishing a subdivision of the zones classically admitted within this interval. Thus, the C. 2.1. Ammonite biostratigraphy balearis Zone is divided into four subzones defined by the consecutive appearance of four species belong- This study is based on the stratigraphic distribu- ing to the same Crioceratites lineage: C. balearis, C. tion of more than 5000 ammonites collected bed-by- binelli, C. krenkeli and C. angulicostatus. The P. ohmi bed from 14 sections located in different palaeogeo- Zone comprises three subzones characterised by three graphic domains of the Betic Cordillera, southeastern successive species of the genus Pseudothurmannia: P. Spain (Fig. 1). Widespread lithostratigraphic and ohmi, P. mortilleti and P. picteti. The lowermost biostratigraphic markers have enabled a precise cor- Barremian T. hugii Zone can in turn be divided into relation between those sections and the construction a lower T. hugii Subzone and an upper Psilotissotia of accurate composite ranges for the ammonite spe- colombiana Subzone. cies (Fig. 2). Published data from other areas (mainly Ammonites are scarce in the lower part of the C. SE France and Italy) have also been taken into balearis Zone. For this reason, we have limited our account. study to the interval between the upper part of that The stratigraphic interval analysed corresponds to zone and the lower part of the T. hugii Zone, where the uppermost Hauterivian and the lowermost Barre- ammonites are much more abundant and data are mian, including the Crioceratites balearis, Pseu- more consistent. dothurmannia ohmi and Taveraidiscus hugii Zones. The biostratigraphic scheme used in this paper is that 2.2. Calcareous nannofossil biostratigraphy proposed by Company et al. (2002, 2003), which improves the resolution of the current standard zona- The stratigraphic distribution of calcareous nan- tion (Hoedemaeker and Rawson, 2000; Hoedemaeker nofossils has been investigated in four selected sec-

latum

bulum

ficilis

dius

anum

pseudomalbosi

ohmi

winkleri

morloti

guerini

subgrasianum

ras infundi

nnia picteti

ras

nnia

nnia mortilleti

nnia

hugii

interme

subcylindrica

jourdani

fumisugina

uhligi

munieri

thetys

favrei

vermeuleni

neumayri

mallada

” boutini

dimboviciorensi

tabarelli

meriani

obliquestrangu

densifimbriatum

spp.

ceras thiollierei

ceras koechlini

sotia mazuca

oceras

pitidiscus subdif

atites angulicostatus

atites binelli

atites majoricensis atites krenke Sequence stratigraphy Subzone (adapted from

Barremites

Zone

Anahamulina

Originations

Arnaudiella

Psilotis

Anahamulina

Phyllopachyce Acrioceras ramkrishnai

Acrioceras

Pseudothurma

Emerici

Paraspinocera

Emerici

Acrioceras Discoidellia

Phyllopachyce

Plesios Lytoceras subfimbriatum

Lytoceras

Pseudothurma

Hamulinites nicklesi

Pseudothurma

Silesite

Abrytusites Discoidellia

Pseudothurma

Phylloceras

Hamulinites

Neoliss

Lytoceras

Barremites

Paraspiticeras

Criocer

Extincions

Criocer

Taveraidiscus

Criocer

Taveraidiscus

“ “Barremites” Hardenbolet al. 1998)

STAGE

. ) TS T. hugii My

(p.p.

BAR

T.hugii 3 127.10 P. picteti SBHa7 2 P. mortilleti Faraoni Level

P. ohmi p.p . 1 MFS 127.39

() P. ohmi

C. anguli- TS costatus (p.p.) SBHa6 127.82

HAUTERIVIEN C. krenkeli

MFS 128.10 C. binelli

C. balearis ABCDE F TS 4 2 0 2 4 6

Fig. 2. Stratigraphic distribution, extinction and origination levels, and turnover steps (1,2,3) of the ammonite species throughout the Hauterivian/Barremian boundary interval in the Mediterranean Tethys. Inferred life-habit groups: (A) planktic drifters; (B) epipelagic vertical migrants; (C) epipelagic nekton; (D) mesopelagic vertical migrants; (E) mesopelagic nekton; (F) nektobenthic. M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 189 tions (X.G1, X.EC, X.Ag1 and X.V1 in Fig. 1). untreated samples in order to retain the original Sampling was restricted to the marly interbeds, sample composition unaltered, and permanently which normally provide the best preserved assem- mounted using Canadian Balsam as adhesive. Quan- blages. Smear slides were prepared directly from titative analyses were performed using a light polar-

Wide canal/total Pentaliths/total 18 13 Bed Ammonite Ammonite events Calcareous nannoconids (%) nannoflora(%) O ‰ C ‰ Lithology Zonation -4 -2 0 1 2 Zonation Calc. nan. nannofossil events 10 20 30 40 5 10 15 20 183 P. colombiana

179

178 (p.p.) 177 P. mazuca

175 T. hugii T. hugii

171 Tav. hugii b

170

Silesites spp. 166 Ab. neumayri NC5C

165 Pseudothurmannia

P. picteti

160 Barremitesspp . 158 Pl. subdifficilis Par. morloti N. circularis Pseudothurmannia ohmi Ps. picteti

154 eti a 153

151 Increase of

P. mortill 149 wide canal FARAONI LEVEL 148 D. favrei nannoconids 147 L. bollii P. ohmi Ps. mortilleti 144 Ps. ohmi 143 C.anguli- e costatus d Cr. angulicostatus (p.p.) c b a C. 142 krenkeli NC5B

Cr. krenkeli

138 C.

binelli 2m

Crioceratites balearis Cr. binelli 1

135 0 Limestones Marly-limestones Marls

Fig. 3. Section X.Ag1 (Rı´o Argos): lithology, biostratigraphy, main ammonite and calcareous nannofossil events, and isotopic data. 190 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 ising microscope at 1250Â magnification. For each life habits of the different ammonite taxa. Our inter- smear slide, at least 300 specimens were counted to pretations are based on the analysis of shell mor- perform an analysis of the assemblage composition, phology in relation to hydrodynamic properties and but investigation was extended to complete 200 fields palaeobathymetry (see Westermann, 1996, for an of view in order to report the presence of rare species. updated review), as well as on their geographical The assemblages are mainly composed of highly and lithofacial distribution. resistant cosmopolitan and Tethyan taxa, such as Phylloceratina, represented by the genera Phyllo- the genera Watznaueria, Nannoconus, Micrantho- ceras and Phyllopachyceras (Fig. 4A) and Lytocer- lithus, Lithraphidites, Rhagodiscus, and Assipetra. atina, by Lytoceras (Fig. 4B), are relatively frequent Diazomatolithus and Zeugrhabdotus spp. are also in the basin areas but are scarce or absent on the common. platform. They have usually been considered inhabi- The entire interval analysed in this paper is tants of mesopelagic, deep water environments. Phyl- characterised by the concurrence of Assipetra tereb- loceratina had well-streamlined shells and were rodentaria and Calcicalathina oblongata, whereas probably nektic, while Lytoceratina, with circular Lithraphidites bollii has its last occurrence at the sections and advolute whorls, have been interpreted base of the P. mortilleti ammonite Subzone, coin- as deep, sluggish, vertical migrants (Westermann, ciding with the lower part of the Faraoni Level. 1990, 1996). These features permit us to assign the interval Ancyloceratina, which include the uncoiled ammo- studied to the NC5B and NC5C Subzones of the nites, are a very diverse group. Their current system- zonal scheme proposed by Roth (1978), modified atic arrangements seem highly artificial (e.g., Wright by Bralower (1987) and Bralower et al. (1995).As et al., 1996) and need a thorough revision. Within our L. bollii becomes scarce near its LO in the Subbetic material, we have distinguished five morphologic Domain, Aguado (1993) proposed the use of the FO types, which likely correspond to taxonomically ho- of Nannoconus circularis, which is a highly resis- mogeneous groups: Crioceratites–Pseudothurmannia tant and common species throughout the uppermost lineage, ancylocones, hamulicones, leptoceratoids and Hauterivian and Barremian as an alternative event to the genus Paraspiticeras. subdivide the NC5 Zone. The FO of N. circularis The genera Crioceratites (Fig. 4C) and Pseu- has been recorded at the base of the P. picteti dothurmannia (Fig. 4D) constitute the main compo- ammonite Subzone (Fig. 3). nents of the uppermost Hauterivian assemblages, both in the basin and on the platform. These genera had gyroconic to advolute, compressed and well-ornamen- 3. Ammonite palaeoecology ted shells, with a restricted swimming potential, and are thought to have been passive pelagic drifters. Eight major taxonomic groups, which also corre- Ancylocones, represented by the genera Emerici- spond to major morphologic types, were present in ceras, Acrioceras (Fig. 4E) and Paraspinoceras, and the western Mediterranean Tethys during the latest the hamulicone genus Anahamulina (Fig. 4G), can be Hauterivian and earliest Barremian: Phylloceratina, grouped together. They all had long spines and a U- Lytoceratina, Ancyloceratina, Haploceratidae, Pul- shaped adult body chamber with the aperture directed chelliidae, Desmoceratidae, Holcodiscidae and Sile- upwards. Batt (1989) and Westermann (1996) consid- sitidae. In order to better understand the patterns that ered that these forms had an epipelagic mode of life governed the renewal process, we will consider in with strongly limited horizontal motility, while Seila- the following paragraphs the possible habitats and cher and Gunji (1993) suggested the possibility of a

Fig. 4. Uppermost Hauterivian–Lowermost Barremian ammonites. (A) Phyllopachyceras winkleri (Uhlig), X.Ag1.142e.1, Crioceratites angulicostatus Subzone; (B) Lytoceras subfimbriatum (D’Orbigny), X.Kv3.1.41, C. krenkeli Subzone; (C) C. angulicostatus (D’Orbigny), X.V1.(-2).65, C. angulicostatus Subzone; (D) Pseudothurmannia mortilleti (Pictet and de Loriol), X.Kv3.10.1, P. mortilleti Subzone; (E) Acrioceras meriani (Ooster), X.Ag1.145.30, P. ohmi Subzone. (F) Hamulinites sp., X.V1.30.28, T. hugii Zone; (G) Anahamulina jourdani (Astier sensu Ooster), X.V1.(-2), 50, C. angulicostatus Subzone. All figures Â1. M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 191 192 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 filter-feeding habit with up-and-down movements scendantBarremites (Fig.5B).Bothgeneraaremorpho- scanning for plankton-rich layers. logically very similar, having smooth or weakly Leptoceratoids, a group of small, loosely coiled constricted, involute and compressed shells. They Ancyloceratina, with annular ribs and simple sutures, have a widespread distribution, being especially fre- comprise the genus Hamulinites (Fig. 4F) and other quent inoutershelfandslopesediments,which, together genera not included in Fig. 2 (they are mostly preserved with their well-streamlined shape, indicates a nektic as fragments which are difficult to determine). This habit in the middle–upper part of the water column. group appears in the P.mortilleti Subzone and becomes Pulchelliids are represented by the genera Discoi- very abundant, even largely dominant, in some levels dellia (Fig. 5D, E), Psilotissotia and Arnaudiella. All of the P. picteti Subzone, where they often occur con- these genera have a wide distribution, although their centrated in dnestsT. Leptoceratoids are characterised by frequency is generally low. They are characterised by progenetic growth and have been interpreted as oppor- having smooth to moderately ornamented, oxyconic tunists (Cecca, 1998). A nektic (Rieber, 1977)ora shells and a very simple suture. An epipelagic, nektic bottom-related vagrant life habit (Vasˇ´eı`ek and Wied- mode of life is assigned to this group. mann, 1994) has been inferred for these ammonites. According to the most recent interpretations (Ver- Nevertheless, in agreement with Westermann (1996) meulen and Thieuloy, 1999; Aguirre-Urreta and Raw- and Cecca (1997), we envisage a planktic or, better, son, 2003), the family Holcodiscidae is divided into pseudoplanktic mode of life for this widespread group. two subfamilies: Spitidiscinae and Holcodiscinae. The The genus Paraspiticeras (Fig. 5A) is a ubiquitous, genus Abrytusites, here considered to be the last minor component of the fauna. This genus is mark- representative of the Spitidiscinae, is an accessory edly different from the other Ancyloceratina, because member of the Late Hauterivian ammonite assem- it has advolute coiling, depressed and rapidly enlarg- blages. The genus Taveraidiscus (Fig. 5F) appears at ing whorls, and blunt ribs with lateral tubercles. These the end of the Hauterivian, and is the first Holcodis- morphological features suggest a restricted horizontal cinae. It is common to abundant in the outer shelf and motility, leading us to assign a passive planktic mode upper slope but becomes somewhat scarcer in shal- of life to this peculiar ammonite. lower environments. This group includes platyconic Ammonitina are represented in the interval studied to planorbiconic, moderately ornamented ammonites, by the families Haploceratidae, Desmoceratidae, Pul- which probably had a nektobenthic mode of life re- chelliidae, Holcodiscidae and Silesitidae. The genus lated to intermediate depths. Neolissoceras (Fig. 5C) is the last member of the long- Primitive silesitids, represented by Silesites sp. (Fig. ranging family Haploceratidae. It was abundant in the 5G) in the interval studied, are much scarcer than Valanginian and Hauterivian deep-basin environ- holcodiscids, but their morphology, distribution pat- ments, but virtually absent on the platforms. This tern, and probably also their habits, were quite similar. distribution, together with its smooth, involute compressed shell, and a moderate sutural complexity, suggests that this genus had a nektic habit in the deep part 4. Assemblage changes throughout the latest of the water column (Company, 1987), which is com- Hauterivian and earliest Barremian patible with the usual interpretations for other mem- bers of this family (Cecca et al., 1990; Cecca, 1992). 4.1. Ammonites Desmoceratids are, on the contrary, a flourishing group in the Late Hauterivian. They are represented by As can be seen in Fig. 2, the ammonite turnover the lineage constituted by Plesiospitidiscus and its de- can be traced throughout the P. mortilleti, P. picteti

Fig. 5. Upper Hauterivian–Lower Barremian ammonites (cont.): (A) Paraspiticeras guerinianum (D’Orbigny), X.O.9.6, P. mortilleti Subzone, Â1; (B) Barremites dimboviciorensis Breskovski, X.V1.24.10, T. hugii Zone, Â1; (C) Neolissoceras subgrasianum (Drushchits), X.V1.2.40, P. mortilleti Subzone, Â2; (D) Discoidellia vermeuleni Cecca, Faraoni and Marini, X.O.6.30, P. ohmi Subzone, Â1; (E) Discoidellia favrei (Ooster), X.Ag1.158.12, P. picteti Subzone, Â1; (F) Taveraidiscus intermedius (D’Orbigny), X.Ag4.37.11, T. hugii Zone, Â1; (G) Silesites sp., X.Ag4.31.6, T. hugii Zone, Â1. M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 193 194 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199

Epipelagic vertical Epipelagic Mesopelagic and SUBZONES migrant and planktic nektic nektobenthic Total diversity drifter ammonites ammonites ammonites

STAGE

ZONES

T. hugii C

BARR.

T. hugii 3 P. picteti B 2

P. mortilleti Faraoni

P. ohmi Level 1 P. ohmi

HAUTERIVIAN C. angulicos- A tatus

C. krenkeli

C. balearis

246 8246 2 4 6 246810121416

Fig. 6. Ammonite diversity (species number) curves for the different life-habit groups and total diversity (A: species appearing before the Faraoni Level; B: species appearing at steps 1 and 2; C: species appearing at step 3). and T. hugii Subzones, where extinction and origina- The diversity (number of species) curves for tion rates increased markedly as compared to the ammonites as a whole, and for each of the life-habit background conditions of the C. balearis Zone. groups are represented in Fig. 6. Epipelagic vertical

Fig. 7. Cross-polarized light (XPL) and polarized light (PL) micrographs of selected calcareous nannofossil taxa from the latest Hauterivian to earliest Barremian interval. (1–4) Nannoconus steinmannii Kamptner: 1, XPL, sample X.V1.25, T. hugii Zone; 2, XPL, sample X.V1.-1, C. angulicostatus Subzone; 3, XPL, sample XV1.1, P. mortilleti Subzone; 4, XPL, sample X.V1.28, T. hugii Zone. (5) Nannoconus bermudezii Bro¨nnimann, XPL, sample X.V1.16, T. hugii Zone. (6,7) Conusphaera rothii (Thierstein), XPL, sample X.V1.11, P. picteti Subzone: 6, high focus; 7, low focus. (8) Zeugrhabdotus embergeri (Noe¨l), XPL, large-sized specimen, sample X.Ag1.155, P. mortilleti Subzone. (9) Nannoconus wassallii (Bralower and Thierstein), XPL, sample X.V1.25, T. hugii Zone. (10) Nannoconus globulus Bro¨nnimann, PL, sample X.V1.16, T. hugii Zone. (11–13) Nannoconus bucheri Bro¨nnimann: 11,12, PL, sample X.V1.-1, C. angulicostatus Subzone; 13, XPL, sample X.V1.8, P. picteti Subzone. (14) Calcicalathina oblongata (Worsley), XPL, sample X.V1.11, P. picteti Subzone., XPL, sample X.V1.6, lateral view. (15–19) Nannoconus circularis De´res and Ache´riteguy: 15, XPL, apical view, sample X.V1.11, P. picteti Subzone; 16, PL, same specimen as 15; 17, XPL, apical view, sample X.V1.16, T. hugii Zone; 18, PL, same specimen as 17; 19, PL, lateral view, sample X.V1.8, P. picteti Subzone. (20) C. oblongata (Worsley), XPL, sample X.V1.21, T. hugii Zone. (21–25) N. circularis De´res and Ache´riteguy: 21, XPL, apical view, sample X.V1.20, T. hugii Zone; 22, PL, same specimen as 21; 23, PL, lateral view, sample X.V1.20, T. hugii Zone; 24, XPL, same specimen as 23; 25, PL, lateral view, sample X.V1.8, P. picteti Subzone. (26) C. oblongata (Worsley), XPL, lateral view, sample X.V1.6, P. mortilleti Subzone. (27–29) Lithraphidites bollii (Thierstein): 27, XPL, sample X.V1.1, P. mortilleti Subzone; 28, XPL, sample X.Ag1.137, C. binelli Subzone; 29, XPL, sample X.V1.-12, C. krenkeli Subzone. (30–33) Assipetra terebrodentaria (Applegate, Bralower, Covington and Wise): 30,31, XPL, normal-sized specimens, sample X.V1.25, T. hugii Zone; 32, XPL, large sized specimen, sample X.V1.-1, C. angulicostatus Subzone; 33, XPL, large sized specimen sample X.V1.-16, C. binelli Subzone. (34) Micrantholithus obtusus Stradner, XPL, sample X.V1.24, T. hugii Zone. (35) Micrantholithus hoschulzii (Reinhardt), XPL, sample X.V1.28, T. hugii Zone. (36) Braarudosphaera sp. cf. B. bigelowii (Gran and Braarud), XPL, sample X.V1.20, T. hugii Zone. (37–39) Micrantholithus sp. 1: 37, XPL, incomplete specimen showing only two elements, sample X.V1.15, P. picteti Subzone; 38, XPL, complete specimen sample X.Ag1.138, C. binelli Subzone; 39, XPL, single element of a very large and fragmented specimen, sample X.Ag1.149, P. mortilleti Subzone. Scale bar=10 Am. M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 195 migrants and planktic drifters have been plotted to- the ammonite assemblages from the P. mortilleti Sub- gether due to the uncertainty in interpreting the mode zone onwards, obviously coinciding with the taxo- of life of some genera. Mesopelagic and nektobenthic nomic renewal. forms have also been grouped. Likewise, the diversity The faunal turnover takes place following a step- curves reveal conspicuous changes in the structure of wise pattern, and we can recognise three distinct 196 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 extinction and origination events, separated by inter- nids with respect to total nannoconids) is on the aver- vals having few or no first and last occurrences (Fig. age 6.65%. This ratio suddenly changes to an average 2). The first step occurs at the boundary between the of 17.75% in the Faraoni Level, and to higher values P. ohmi Subzone and the P. mortilleti Subzone, throughout the upper part of the P. mortilleti and P. coinciding with the base of the Faraoni Level. picteti ammonite subzones. Two bioevents related to Some long-ranging species, characteristic of deep this change have been detected: 1) the increase in environments (N. subgrasianum, Ph. infundibulum abundance of a group of nannoconids close to N. and Lytoceras subfimbriatum), disappear at or near circularis, but showing a higher than wide lateral this boundary. Some epipelagic (planktic and shallow view (Nannoconus sp. cf. N. circularis), at the base nektic) species also disappear, but the originations of the P. mortilleti Subzone; and 2) the first occurrence (replacements within the same genus or appearances of true N. circularis near the base of the P. picteti of new lineages such as Leptoceratoidinae) in these Subzone. A similar event has been recorded in the groups outnumber the extinctions. As a consequence, Tethyan realm in the earliest Aptian, prior to the the specific richness of epipelagic ammonites rises bnannoconid crisisQ and the onset of the widespread remarkably, whereas that of mesopelagic forms anoxic/dysoxic conditions of Ocean Anoxic Event 1a decreases (Fig. 6). (OAE-1a; Erba, 1994; Aguado et al., 1999; Erba et al., A minor second event is located at the base of the 1999; Premoli-Silva et al., 1999; Bellanca et al., 2002). P. picteti Subzone. It is marked by the replacement of Moreover, a relative increase in the abundance of a few species by closely related forms. Most of the pentaliths has been detected slightly preceding the species involved in this event are planktic drifters or Faraoni Level. These pentaliths correspond to several shallow vertical migrants, and thus the structure of the large-sized species of the genus Micrantholitus, ammonite assemblage is not substantially altered. namely, M. obtusus, M. hoschulzii and Micrantholitus The third and last stage of the renewal process sp. 1 (see Aguado, 1993), an unnamed species that has starts in the upper part of the P. picteti Subzone and its first occurrence in the lower part of the C. balearis extends across the Hauterivian/Barremian boundary. It ammonite Zone and shows strongly negative intersu- is characterised by the extinction of many of the tural angles. Pentaliths may constitute up to 25% of species that had appeared in the two previous events. the nannofaunal assemblages in the P. ohmi ammonite These species, mainly planktic drifters and shallow Zone (Fig. 3), but it must be pointed out that counting vertical migrants, are not replaced by equivalent of pentaliths is difficult and inaccurate because their forms. On the contrary, most of the species that appear constituent elements usually appear disaggregated and at this level correspond to nektic and nektobenthic disperse in poorly preserved samples. A similar ammonites (some belonging to new lineages such as bpentalith acmeQ, has been reported to occur at the Holcodiscinae and Silesitidae), resulting in an exten- same stratigraphic position in several Italian sections sive modification of the assemblage structure. (Erba et al., 1999; Channell et al., 2000; Bellanca et al., 2002; Bersezio et al., 2002). 4.2. Calcareous nannofossils

The results of the quantitative study of calcareous 5. Interpretation and conclusions nannofossils in section X.Ag1 show a relatively sharp change in the nannoconid assemblage composition As discussed above, the first step in the extensive occurring within the Faraoni Level (Fig. 3). This ammonite renewal that took place during the latest change consists of a relative increase in the abundance Hauterivian corresponds to the bioevent at the base of of wide-canal nannoconids (N. circularis, Nannoco- the P. mortilleti Subzone, coinciding with the Faraoni nus sp. cf. N. circularis, N. globulus, N. kamptneri and Level (Fig. 2). This is a Corg-rich interval that has N. wassallii) with respect to narrow-canal nannoco- been recognised in several basins of the Mediterra- nids (N. steinmannii) (see Fig. 7). Below the Faraoni nean Tethys (Cecca et al., 1994, 1996; Faraoni et al., Level, the abundance of wide-canal nannoconids 1997; Baudin et al., 1999; Erba et al., 1999; Bellanca (expressed as percentage ratio of wide-canal nannoco- et al., 2002; Bersezio et al., 2002; Company et al., M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199 197

2002, 2003). A small positive excursion of d13Cin the proportion of wide-canal nannoconids) implies carbonates is recorded in association with this interval that some changes had to occur in the planktic eco- (Fig. 3), which implies an episode of increased organ- system at that time. ic carbon storage in marine sediments. These data The start of the final phase of the ammonite re- suggest, according to Baudin et al. (1999, 2002a,b), newal process coincides with a drastic sea-level fall that the Faraoni Level could be the result of a short- near the end of the Hauterivian (sequence boundary lived oxygen-deficient event in the Mediterranean Ha7 in Hardenbol et al., 1998), which led to the Tethys. exposure and erosion of large areas of the perimedi- Other major oceanic anoxic events have been com- terranean shelves (Arnaud-Vanneau and Arnaud, monly related to sea-level rises (Jenkyns, 1980; 1991; Company et al., 1992). This regression would Drzwiecki and Simo´, 1997; Jacquin and de Gra- have caused a drop in the primary productivity and, ciansky, 1998). In fact, the base of the P. mortilleti consequently, an improvement in the oxygenation Subzone (=P. catulloi Subzone of the standard zona- level of the sea bottom. An increase in the competition tion) correlates with a second-order peak transgres- for limited trophic resources in the photic zone could sion (Hardenbol et al., 1998) well documented on explain the extinction of several planktic drifters and several Mediterranean platforms (Arnaud-Vanneau shallow vertical migrants, which characterise this final and Arnaud, 1990; Company et al., 1992; Fo¨llmi et event (Fig. 2). On the other hand, the amelioration of al., 1994). Flooding of landmasses may largely in- the environmental conditions in the lower part of the crease nutrient transfer into the oceans, thus favouring water column would enable the appearance of new marine phytoplankton production, deposition of organ- nektic and nektobenthic ammonite lineages. ic-rich sediments and expansion of the oxygen-mini- mum layer in the pelagic basins. This oxygen depletion would preferentially affect the deep nektic ammonites Acknowledgements (Batt, 1989, 1993), which would explain the extinctions within this group around the Faraoni Level. On We acknowledge F. Cecca and E. Erba, as well as the contrary, a higher primary productivity implies an co-editors F. Maurrasse and M.A. Lamolda for criti- increase in the trophic resources in the photic zone, cally reviewing the manuscript. This work has been favouring the diversification of planktic ammonites. co-financed by Project BTE 2001-3020 (Spanish Min- Concurrently, the abrupt change which took place at istry of Science and Technology) and Research Group this level in the nannoconid assemblage composition RNM-178 (Junta de Andalucıá). (relative increase of wide-canal forms) might be related to this dnutrificationT event (Erba, 1994; Premoli-Silva et al., 1999). Pentalith acmes, such as that preceding the Faraoni Level (Fig. 3), have been interpreted to corre- References spond to the formation of less saline surface waters Aguado, R., 1993. Nannofo´siles del Cretaćico de la Cordillera (Bellanca et al., 2002; Bersezio et al., 2002), which Be´tica. Bioestratigrafıá. Universidad de Granada, Granada. could be a result of accelerated continental runoff that Aguado, R., Castro, J.M., Company, M., de Gea, G.A., 1999. takes place during incipient drowning phases (Fo¨llmi Aptian bioevents—an integrated biostratigraphic analysis of et al., 1994; Weissert et al., 1998). the Almadich Formation, Inner Prebetic Domain, SE Spain. The second step in the ammonite turnover is a Cretaceous Research 20, 663–683. Aguirre-Urreta, M.B., Rawson, P.F., 2003. Lower Cretaceous bioevent that occurred during the sea-level highstand ammonites from the Neuqueń Basin, Argentina: the Hauterivian subsequent to the peak transgression (Fig. 2). It entails genus Holcoptychites. Cretaceous Research 24, 589–613. only a few species-level replacements within the Arnaud-Vanneau, A., Arnaud, H., 1990. Hauterivian to Lower planktic drifter and shallow vertical migrant ammonite Aptian carbonate shelf sedimentation and sequence stratigraphy groups. It is difficult to interpret the causal mechan- in the Jura and northern Subalpine chains (southeastern France and Swiss Jura). In: Tucker, M.E., Wilson, J.L., Crevello, P.D., isms of this event, but its coincidence with a restruc- Sarg, J.R., Read, J.F. (Eds.), Carbonate Platforms, Special Pub- turing of the calcareous nannofossil assemblage lication of the International Association of Sedimentologists, (appearance of N. circularis and further increase in vol. 9, pp. 203–233. 198 M. Company et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 224 (2005) 186–199

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