Pliensbachian-Toarcian biotic turnover in north Siberia and the Arctic region

399

ISSN 0869-5938, Stratigraphy and Geological Correlation, 2006, Vol. 14, No. 4, pp. 399417.

I Nauka

/Interperiodica (Russia), 2006.Original Russian Text V.A. Zakharov, B.N. Shurygin, V.I. Ilina, B.L. Nikitenko, 2006, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2006, Vol. 14, No. 4, pp. 6180.

INTRODUCTION

Raup and Sepkoski (1982, 1984; Sepkoski andRoup, 1986) were first to establish a global biotic turnoveracross the PliensbachianToarcian transition (183 Maago) based on changes in taxonomic diversity of marineinvertebrate families during the Phanerozoic. Accord-ing to Sepkowski, 10 of 12 extinction peaks in thePhanerozoic took place in the last 250 Ma. The Pliens-bachianToarcian extinction that followed the terminalPermian and terminal Triassic events was the third oneduring this period. Despite a substantially lower magni-tude as compared with preceding extinctions, theextinction peak at the PliensbachianToarcian bound-ary was permanently noted in all the subsequent works.Little and Benton (1995) established five extinction lev-els for ammonite families during the late Pliensba-chianearly Toarcian period 7.5 m.y. long and demon-strated that these levels were diachronic in the Boreal,Tethyan, and Austral realms. Behavior of invertebratemacrofauna was studied most carefully in completesedimentary successions of northwestern Europe,where is established that most extinction events of spe-cies took place in the early Toarcian due to the regionalanoxic event. Contrary to assumption of Sepkoski(1990), no evidence of simultaneous extinction of fam-

ilies at the end of the Pliensbachian has been foundhere. Actually, five zonal phases of extinction have beenestablished in the period from the late Pliensbachian toearly Toarcian.

At the regional extent (marine sections in northernWest Europe), the biotic turnover was first described byHallam (1986) first based on bivalves and then on othergroups of marine invertebrates: ammonites, brachio-pods, foraminifers, ostracodes (Hallam, 1987). Lateron, Zakharov, Shurygin, Ilina, and Nikitenko pre-sented evidence for a substantial biotic turnover amongbivalves, ostracodes, foraminifers, dinocysts, spores,and pollen in northern Siberia at the same stratigraphiclevel (Zakharov et al., 1993, 1994; Ilina et al., 1994;Nikitenko and Shurygin, 1994; Zakharov, 1994).Recently, Aberhan and

F

rsich

(1997) discovered reor-ganization in communities of marine bivalves from theAndean basin (South America). They claimed that dis-covery of the second region provides grounds to con-clude that the biotic turnover at the PliensbachianToarcian transition is of the global scale. References toRussian publications appeared in their later work(Aberhan and Frsich, 2000).

PliensbachianToarcian Biotic Turnover in North Siberia and the Arctic Region

V. A. Zakharov

a

, B. N. Shurygin

b

, V. I. Ilina

b

, and B. L. Nikitenko

b

a

Geological Institute, Russian Academy of Sciences, Pyzhevskii per 7, Moscow, 119017 Russia

b

United Institute of Geology, Geophysics, and Mineralogy, Siberian Division, Russian Academy of Sciences, ul. Akademika Koptyuga 3, Novosibirsk, 630090 Russia

Received January 31, 2006

Abstract

The biotic turnover in the PliensbachianToarcian transition and changes in assemblages ofbivalves, ostracodes, foraminifers, dinocysts, spores, and pollen are described. Only five of 24 bivalve generaand two of four ostracode genera cross the PliensbachianToarcian boundary so that composition of genera andfamilies to be entirely renewed at the base of the

Harpoceras falciferum

Zone. In the interval of three ammonitezones, diversity of foraminifers is reducing from 27 genera in the

Amaltheus margaritatus

Zone (upper Pliens-bachian) to 17 and then to 15 genera in the

Tiltoniceras antiquum

(lower Toarcian) and


zones, respectively. Single dinocysts of the Pliensbachian are replaced by their abundant specimens at the baseof the Toarcian, and substantial changes in composition of palynological assemblages are simultaneously estab-lished. Factors responsible for mass extinctions of marine invertebrates are suggested to be the paleogeo-graphic reorganization, anoxic events, eustatic sea-level changes, and climatic fluctuations. The biotic turnoverin the Arctic region is interrelated mainly with thermal changes, which caused the southward displacements oftaxa distribution areas during a rapid cooling and their gradual return to former habitat areas in the period ofwarming, rather than with extinction events.

DOI:

10.1134/S0869593806040046

Key words

: Pliensbachian, Toarcian, biotic turnover, North Siberia, bivalves, foraminifers, ostracodes,dinocysts, spores, pollen, paleoclimate, eustatic fluctuations, anoxia.

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STRATIGRAPHY AND PALEOGEOGRAPHIC SITUATION

The lower Toarcian clays occupy in Siberia an areasized several millions square kilometers. These sedi-ments overlie the upper Pliensbachian sandy and siltyrocks with sedimentary unconformity, though usuallywithout biostratigraphic hiatus. These sediments of arelatively uniform lithology and geochemical charac-teristics span the interval of the


and


zones coupled with a lowerpart of the

Dactylioceras commune

Zone (Knyazev etal., 1991). The interval corresponds to the KiterbyutHorizon in northern East Siberia, Kiterbyut and Togurformations in the Vilyui hemicyneclise, and to SuntarFormation (lower part) in the Verkhoyansk region. Thetotal area occupied by lower Toarcian clayeysiltysequences is over 10

6

km

2

, and their thickness isremarkably persistent in general (2040 m). Owing tothe well-coordinated biostratigraphic zonations, all theToarcian marine sequences are confidently correlatedbetween each other at the level of ammonite, bivalve,and ostracode zones (Shurygin et al., 2000). Based onthe parallel biostratigraphic scales, the PliensbachianToarcian transition defined in North Siberia is reliablycorrelated with coeval sequences through the entireArctic region, Northeastern Asia, and northern EastEurope. That is why instead of a particular section, we

consider in this work the materials on many localities ofPliensbachianToarcian boundary layers in North Sibe-ria and the Arctic region, including Svalbard and FrantzJosef Land and North Alaska (Fig. 1).

The upper Pliensbachian and lower Toarcian sec-tions of North Siberia are substantially variable withrespect of lithology. In sections of the Ust-Yenisei area,YeniseiKhatanga trough (Figs. 1A, 1B), easternTaimyr Peninsula (Fig. 1, 3B), and the Anabar River(Fig. 1, 2B), the upper Pliensbachian base correspondsto the widespread Zimnyaya Formation (Hettangianbasal upper Pliensbachian) composed of marine tocoastal-marine sandstones with argillite and siltstoneintercalations and with gravelstone and conglomerateinterbeds at different levels (Fig. 2). In northeasternEast Siberia (Olenek River basin) (Fig. 1, 4B), the Zim-nyaya Formation is correlated with a lower part of themarine Kyra Formation (HettangianPliensbachian)composed of clays with silty and sandy interbeds. In theUst-Yenisei area and western YeniseiKhatangatrough, the Zimnyaya Formation is conformably over-lain by marine dark gray argillites and siltstones of theLevinskii Formation (middle part of the upper Pliens-bachian), sediments of which contain scattered pebblesof quartz, cherts, and volcanics (Fig. 2). In northernWest Siberia, the Levinskii Formation (Fig. 1, 1A) restsupon Paleozoic rocks. In sections of the eastern Taimyr

100 km

200 km 100 km

A

C B

60 8070

60

170 19070

90 110 130 75

65

13

2 4

5

6

Ob

R.

Pur R

.

Taz R. Yeni

sei R

.

Yana

R.

Kolym

a R.

Om

olon

R.

Lena R.

Vilyu

i R.

Yana R.Ana

bar

R.

Kheta

R.

LAPTEVSEA

Fig. 1.

Localities of the main Lower Jurassic sections (dots) examined in (A) West Siberia, (B) East Siberia, and (C) NortheasternRussia; ovals with numbers denote areas of standard Lower Jurassic sections: (1) northern West Siberia (boreholes Novopor-tovskaya, Bovanenkovskaya, Kharasaveiskaya, Arkticheskaya, and others fields), (2) AnabarNodvik area (outcrops, Suolem andVostochnaya boreholes), (3) eastern Taimyr, (4) OlenekElimyar area, (5) Vilyui syneclise (sections and boreholes along the Tyung,Markha, and Vilyui rivers), (6) Northeastern Russia (Levyi Kedon River basin).


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PLIENSBACHIANTOARCIAN BIOTIC TURNOVER 401

Peninsula (Fig. 1, 3B) and Anabar River (Fig. 1, 2B),the Levinskii Formation corresponds to the lower partof the Airkat Formation (middle part of the upperPliensbachian) composed largely of clays and argilliteswith siltstone interbeds and rare scattered pebbles.Higher layers of the Airkat Formation (upper half of theupper Pliensbachian) are represented by cyclic mem-bers of coarse-grained siltstones with clay and fine-grained sandstone interbeds. In northern West Siberiaand western YeniseiKhatanga trough, this level corre-sponds to the Sharapovo Formation of shallow-water tocoastal-marine siltstones, argillites, and sandstones withthin conglomerate and gravelstone interbeds (Fig. 2).

In northeastern Russia (Omolon massif), the upperPliensbachian siltstones and sandy siltstones with sand-stone and clay interbeds are discriminated into theNalednaya Formation (Fig. 2).

The Toarcian sediments are readily distinguishablein sections owing to a uniform lithology of the lowerpart composed of fine clay that is enriched in organicmatter, being bituminous sometimes. In western andcentral areas of East Siberia (Figs. 1, A, 2B, 3B), thebasal portion of the Toarcian section corresponds to theKiterbyut Formation (lower part of the lower Toarcian)of uniform argillites and fine clays with bituminousinterlayers (Fig. 2). Eastward (Fig. 1, 4B), the Kiterbyut

Subs

tage

Borealammonoidstandard

(Zakharov et al.,1997)

Lithostratigraphy

Northern East Siberia NortheasternRussia

1A 3B 2B 4B 6C

Low

er T

oarc

ian

Braunianus

Commune

Nad

oyak

h Fo

rmat

ion

Kor

otki

i Fm

.

Ere

n Fm

.

Kel

imya

r Fm

.

Kite

byur

t Fm

.

Kite

rbyu

t Fm

.

Mra

chna

ya F

m.

Ast

rono

mic

hesk

aya

Fm.

??

Upp

er P

liens

bach

ian

Falciferum

Antiquum

Viligaensis

Shar

apov

o Fm

.

Air

kat F

m.

Lev

insk

ii Fm

.

Kyr

a Fm

.

Nal

edna

ya F

m.

Margaritatus

Stokesi

Zim

.

1 2 3 4 5

Fig. 2.

Lithostratigraphic columns of the upper Pliensbachianlower Toarcian deposits in North Asia: (1) high-carbonaceous claysand siltstones; (2) clays and argillites; (3) sands and sandstones; (4) sandy silts and siltstones; (5) sands and sandstones; (Zim) Zim-nyaya Formation (other symbols as in Fig. 1).

402


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ZAKHAROV et al.

Stage

Substage

Ammonite zonation

Aa.

l.

P.m.

Toracian

lowerupper

Pliensbachian

P.f. P.w. P.c. Z.b. D.c. H.f. T.a. A.v. A.m. A.s.

Biv

alve

gen

era

Bor

eal-

Arc

ticL

ow B

orea

lSu

btet

hyan

Harpax

Homomya

Kolymonectes

Lima

Meleagrinella

Ochotochlamya

Tancredia

Anradulonectites

Malletia

Astarte

Dacryomya

Entolium

Modiolus

Nuculoma

Oxytoma

Panopea

Janaja

Liostrea

Pleuromya

Pseudomytiloiders

Taimyrodon

Arctotis

Arctica

Jupiteria

Mclearnia

Retroceramus

Chlamys

Liotrigonia

Velata

Aguilerella

Camptochlamys

Glyptoleda

Gresslya

Neocrassina

Radulonectites

Corbulomima

Cucullaea

Protocardia

Propeamussium

Camptonectes

Luciniola

Sowerbya

Goniomya

Kalentera

Mytiella

Pholadomya

Schofhaeutlia

Vaugonia

Trigonia

Am

mon

ite z

ones

: A.s

.

Am

alth

eus

stok

esi

, A.m

.

A. m

arga

rita

tus

, A.v

.

A. v

iliga

ensi

s

, T.a

.

Tilto

nice

ras

antiq

uum

, H.f

.

Har

pose

ras

falc

iferu

m

, D.c

.

Dac

tylio

cera

s co

mm

une

, Z.b

.

Zugo

dact

ylite

s br

auni

anus

, P.c

.

Pse

udol

ioce

ras

com

pact

ile

,P.

w.

P. w

urtte

nber

geri

, P.f

.

P. f

alco

disc

us

, P.m

.

P. m

aclin

tock

i.

Fig.

3.

St

ratig

raph

ic ra

nges

of b

ival

ve m

ollu

sks i

n th

e Pl

iens

bach

ian

Toar

cian

bou

ndar

y be

ds o

f Nor

th S

iber

ia a

nd N

orth

easte

rn R

ussia

; abb

revia

tions

den

ote

the A

alen

ian

Stag

e(A

a) an

d its

lower

Sub

stage

(l.).


Vol. 14

No. 4

2006


Formation is replaced by fine clays of the lower Kelim-yar Formation (Toarcianbasal Bathonian) thatincludes the thin Kurung Member (lowerbasal upperToarcian) with black foliated bituminous clays at thebase. In sections of Northeastern Russia (Fig. 1, 6B),the Kiterbyut clays correspond approximately to theAstronomicheskaya Formation of fine clays with silt-stone interbeds (Fig. 2). In northern West Siberia, theUst-Yenisei region, and western YeniseiKhatangatrough, the Kiterbyut Formation is overlain by alternat-ing sandstones, siltstones, and rare argillites of theNadoyakh Formation lower part (upper lower Toar-cianbasal lower Aalenian). In eastern Taimyr sections(Fig. 1, 3B), coeval layers are composed of clays andargillites of the Korotkii Formation (Fig. 2). In the Ana-bar River sections (Fig. 1, 2B), this part of the Toarcianis represented by the Eren Formation of cyclic sandysilty deposits (terminal lowerbasal upper Toarcian).In the Vilyui syncline (Fig. 1, 5B), sediments of thelower Toarcian are represented by fine, locally, bitumi-nous clays in the lower part of the section and by siltyclays with siltstone and sandstone interbeds of the Sun-tar Formation (Toarcianbasal lower Aalenian). InNortheastern Russia (Fig. 1, 6B), the upper part of thelower Toarcian is composed of sandysilty deposits ofthe Mrachnaya Formation (upper lowerupper Toar-cian) (Fig. 2).

Thus, the upper portion of the Pliensbachian succes-sion is composed in East Siberia of sandysilty rockslocally with conglomerates at the base, while its lowerpart is of clayey to siltyclayey lithology (Fig. 2). It ispossible to suggest therefore a substantial paleogeo-graphic rearrangement across the PliensbachianToar-cian transition. Most likely, the coastal landscapes ofPliensbachian seas were mountainous and hilly in thenorth (upper Pliensbachian conglomerates are knownin the northeastern Taimyr Peninsula and Lena Rivermouth area) and in the south, from the side of CentralSiberian land. The facies analysis implies developmentof a low land (peneplain) in the initial Toarcian time(

Paleogeography of the Northern

, 1983).

MATERIALS AND METHODS

Main sections, which yielded materials for thisstudy (observation results and collections of fossils),are located in northern East Siberia (Fig. 1). In additionto East Siberian section, we also considered data onsections of Northeast Asia, numerous boreholes drilledin West Siberia, and original materials and collectionsfrom boreholes in northern Alaska. The extent andtrend of the PliensbachianToarcian biotic turnoverhave been estimated based on the following fossilgroups: bivalves, foraminifers, ostracodes, dinocysts,and sporepollen assemblages. These fossils are mostabundant and diverse. The other, less common fossilstaken into consideration are ammonites, belemnites,gastropods, and scaphopods.

A genus is considered as a main taxonomic unit,when composition and distribution of mollusks, fora-minifers, and ostracodes have been analyzed. Thechoice is made to avoid, as far as possible, subjectivismin understanding an elementary operative taxonomicunit. A species is used as a main operative taxonomicunit by palynological analysis. In order to determinethe dominant genera of benthic bivalve communities,the occurrence frequency of each genus is estimated inthree relative categories: rare, common, and abundant.The occurrence frequency of palynological taxa is esti-mated in four relative categories: rare, few, common,abundant. To assess the biogeographic selectivity oftaxa in the course of biotic reorganization, all the fossil

Subs

tage

AmmoniteBoreal

standard(Zakharov et al.,

1997)

Low

er T

oarc

ian

Spinatum

Commune

Upp

er P

liens

bach

ian

Falciferum

Antiquum

Viligaensis

Margaritatus

Stokesi

Monestieri

Ostracode genera

Boreal-Arctic Boreal

Ogm

ocon

cha

Ogm

ocon

chel

la

Kin

kelin

ella

Pol

ycop

e

Mon

ocer

atin

a

Man

dels

tam

ia

Cam

ptoc

yher

e

Nan

acyt

here

Trac

hycy

ther

e

Fig. 4.

Stratigraphic ranges of ostracodes in the Pliensba-chianToarcian boundary beds of North Siberia and North-eastern Russia.

404


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ZAKHAROV et al.

groups are classed in terms of their affiliation with par-ticular biogeographic zone or biochore that presumablydetermines their tolerance to different thermal regimes.Bivalves are divided into BorealArctic (or HighBoreal), Low Boreal, and Subtethyan genera; foramin-ifers into the BorealArctic, Boreal, and BorealSubt-ethyan genera; ostracodes into the BorealArctic andBoreal genera; dinoflagellates into the BorealArcticand BorealAtlantic species; spores and pollen intopredominantly Siberian, cosmopolitan, and Euro-Sin-ian taxa.

EVIDENCE OF BIOTIC TURNOVER

The dynamics of turnover (crisis) in geological timeis usually analyzed based on taxonomic diversity,sometimes, on the ecological structure of biotic com-munities (a change of dominants, taxonomic homoge-nization, destruction of former nutrition chains). Thebiogeographic structure of communities is a rare caseof analysis, but precisely this issue is of particular inter-est in this work.

Judging from taxonomic reorganizations recordedin the examined fossil groups (bivalves, ostracodes, for-aminifers, dinoflagellates, spores and pollen), principalevents in the sea and on land took place in northernSiberia at the beginning of the Toarcian, in the

Til-toniceras antiquum

phase. Biota experienced the mainimpact at the very beginning of the Toarcian Age.Subsequently, dynamics of reorganizations progressedslower, being largely reduced to restoration of taxapositions lost at the early stage.

As is mentioned, we consider primarily data on theNorth Siberian sections. with attraction of materials onthe other, mostly Arctic regions, e.g., on the King KarlLand, Frantz Josef Land, Pechora River basin, andnorthern Alaska, which are considered in some speci-fied cases, when they are necessary for inferences.

Taxonomic diversity.

The taxonomic diversitydecrease in the PliensbachianToarcian transition isobservable in all the examined groups of marine inver-tebrates. A sharp reduction of diversity among bivalvemollusks is registered at the base of the Toarcian, in the


Zone (Fig. 3). In North Siberia,

Subs

tage

AmmoniteBoreal

standard(Zakharov et al.

1997)

Low

er T

oarc

ian

Spinatum

Commune

Upp

er P

liens

bach

ian

Falciferum

Antiquum

Viligaensis

Margaritatus

Stokesi

Monestieri

Foraminiferal genera

Boreal-Arctic Boreal

Am

mob

acul

ites

Boreal-Subte-thyan

Reo

phax

Am

mod

iscu

s

Glo

mos

pira

Troc

ham

min

a

Ast

acol

us

Lent

icul

ina

Hyp

eram

min

aSa

ccam

min

a

Den

talin

aSp

irop

lect

amm

ina

Gei

nitz

inita

Icht

hyol

aria

Nod

osar

ia

Mar

ginu

lina

Gau

dryi

na Mar

ginu

linop

sis

Jacu

lella

Hyp

pocr

epin

a

Fro

ndic

ulin

ita

Pse

udon

odos

aria

Glo

bulin

a

Am

mog

lobi

geri

na

Kut

seve

lla

Cith

arin

aC

onor

boid

es

Gri

gelis

Neo

bulim

ina

Sara

cena

ria

Pyr

ulin

oide

sA

nmar

ginu

lina

Rec

urvo

ides

Rei

nhol

della Evo

lutin

ella

Cor

nusp

ira

Invo

lutin

a

Trip

lasi

a

Pal

mul

a

Bul

boba

culit

es

Fig. 5.

Stratigraphic ranges of foraminifers in the PliensbachianToarcian boundary beds of North Siberia and Northeastern Russia.


Vol. 14

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2006


Stage

Substage

Zone

Toarcian

lowerupper

Pliensbachian


Din

ofla

gella

te s

peci

es

Bor

eal-

Arc

ticT

axa:

Zon

es: A

.s.

Am

alth

eus

stok

esi

, A.m

.

A. m

arga

rita

tus

, A.v

.

A. v

iliga

ensi

s,

T.a

.

Tilto

nice

ras

antiq

uum

, H.f

.

Har

pose

ras

falc

iferu

m

, D.c

.

Dac

tylio

cera

s co

mm

une

, Z.b

.

Zugo

dact

ylite

s br

auni

anus

, P.c

.

Pse

udol

ioce

ras

com

pact

ile

, P.w

.

P. w

urtte

nber

geri

, P.f

.

P. f

alco

disc

us

.

upper

No

d

in

oc

ys

ts

Bor

eal-

Atla

ntic

12

34

65

78

910

1112

1314

1516

171.

Pha

llocy

sta

eum

ekes

2.

Pha

llocy

sta

min

uta

3. ?

Ros

swan

gia

holo

tabu

lata

4.

Susa

dini

um s

crof

oide

s

5.

Dod

ekov

ia s

yzyg

ia

6.

Dod

ekov

ia ta

bula

ta

7.

Val

vaeo

dini

um a

quilo

noum

8.

Val

vaeo

dini

um p

unct

atus

9.

Reu

tling

ia fa

usta

10.

Reu

tling

ia n

asut

a

11.

Nan

noce

rato

psis

ana

bare

nsis

12.

Nan

noce

rato

psis

sen

ex

13.

Nan

noce

rato

psis

def

land

rei

14.

Nan

noce

rato

psis

gra

cilis

15.

Nan

noce

rato

psis

tria

ngul

ata

16.

Nan

noce

rato

psis

dic

tyam

boni

s

17.

Man

codi

nium

sem

itabu

lata

Fig.

6.

St

ratig

raph

ic ra

nges

of d

inofl

agel

late

s in

the

uppe

r Plie

nsba

chia

nTo

arci

an se

dim

ents

of N

orth

Sib

eria

.

406

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 14 No. 4 2006

ZAKHAROV et al.

only four of 35 bivalve genera typical of the upperPliensbachian and two of four ostracode genera crossthe PliensbachianToarcian boundary; both fossilgroups experienced a complete renewal in their genericand family composition at the base of the Harpocerasfalciferum Zone (Fig. 4). Diversity of foraminifersdecrease substantially within the interval of threeammonite zones from 27 genera in the Amaltheus mar-garitatus Zone to 17 and then to 15 genera in the Til-toniceras antiquum and Harpoceras falciferum zonesrespectively (Fig. 5). Occasional species of Pliensba-chian dinocysts give place to abundant assemblages ofthese fossils at the base of the Toarcian (Fig. 6), andsubstantial simultaneous reorganizations (Classopolisfluctuations, Fig. 7) are established in sporepollenassemblages (SPA).

Structure of benthic communities. Above thePliensbachianToarcian boundary, benthic bivalvecommunities differ sharply from their older counter-parts by dominant genera, feeding chains, food captur-ing, feeding levels, homogenization level, and popula-tion density of taxa. Changes in structure of benthiccommunities are evident primarily from taxonomicdiversity, which decreased by almost an order of mag-nitude. At the onset of the Toarcian, the communitiesbecame less homogeneous due to disappearance ofdominant taxa. The communities became completelydeprived of high-level detritovore genera Malletia andTaimyrodon, the subdominants of some former assem-blages. Although high-level sestonophages constitute50% of all genera preserved in biocoenoses, they canhardly be attributed to most viable, being representedonly by scarce finds of two genera only. The only genusTancredia is a low-level sestonophage. This genus wasa dominant taxon in the late Pliensbachian and retainedthis position in the Toarcian as well. The high-leveldetritovore genus Dacryomya that was rare in thePliensbachian and survived the boundary crisisbecomes again dominant soon after the survival (Har-poceras falciferum Zone). During the early Toarcian,bivalves regenerated gradually to be, nevertheless,more than twice less diverse at the generic level in thelate Toarcian as compared with the late Pliensbachian.

Analysis of microfossils from the PliensbachianToarcian boundary sediments of North Siberia impliesa discontinuity in development of PliensbachianToar-cian foraminiferal and ostracode communities. Theearly Toarcian crisis resulted in a sharply reduced dif-ferentiation of benthic communities in affiliation to bio-nomic zones (two- or three-member catenas instead ofthree- to four-member catenas of the late Pliensba-chian), and diversity of viable forms in individual cat-ena links decreased. Habitat conditions that existed atthe beginning of the Kiterbyut time (the Falciferuminitial Commune phases) were unfavorable for microb-enthos. Geochemical data indicate that sedimentationin the shallow coastal part of the North Siberian sea atthe beginning of the Falciferum phase progressed insettings with unstable salinity (Levchuk, 1985). Fora-

miniferal tests in all the bionomic zones of the Siberianpaleosea were dwarfish and thin-walled, suggestingunfavorable environments (stagnation) in the basin atthe beginning of the Toarcian. It should be noted thatthe section interval under consideration is characterizedby alternation of sedimentary laminae enriched ineither foraminifers or ostracodes that is indicative ofunstable bottom-water environments.

Biogeographic selectivity of taxa eliminationfrom marine communities and sporepollen com-plexes. Among bivalves divided according to their bio-geographic affinity into three the BorealArctic or HighBoreal (26 genera), Low Boreal (16), and Subtethyan(7) groups, only 20% of High Boreal genera survivedthe boundary crisis, while representatives of the LowBoreal and Subtethyan groups disappeared frombenthic communities (Fig. 3). Thus, bivalves illustratethe dramatic history of the PliensbachianToarcian cri-sis most clearly. Of three BorealArctic ostracode gen-era (four in total), only the genus Mandelstamia dis-appeared at the end of the Pliensbachian, while twoother genera (Ogmoconcha, Ogmoconchella) crossedthe boundary, although being unknown above the Anti-quum Zone. The genus Nanacythere the only one offour Boreal taxa known in the Pliensbachian disap-peared at the end of this age, and the genus Kinkinellaappeared immediately above the PliensbachianToar-cian boundary (Fig. 4). Thus, changes in ostracodecommunity took place in a short interval of the Antiq-uum phase. The BorealArctic group of foraminifersconsisted of 24 genera, and 13 of them crossed thePliensbachianToarcian boundary. Only five generadisappeared for some time immediately at the boundaryor near it. Six of 13 Boreal genera become extinct atvarious levels of the upper Pliensbachian, while fivegenera appear at the base of the Toarcian. One genus(Citharina), which disappeared in the Pliensbachian,appeared three phases later in the Toarcian, and onlygenus Recurvoides crossed the boundary betweenstages. The BorealSubtethyan foraminifers are repre-sented by two genera: Involutina in the Pliensbachianand Cornuspira in the Toarcian (Fig. 5). Thus, the mostsignificant reorganization in biogeographic structure ofthe foraminiferal community affected the relativelythermophilic taxa. Dinocysts of the BorealArctic(10 species) and BorealAtlantic (7 species) groups arepractically unknown in the Pliensbachian: only threeNannoceratopsis species have been found in uppermostlayers of this stage. One of these species (N. anabaren-sis) becomes abundant and two others common imme-diately above the boundary under consideration (Fig. 6).It is remarkable that precisely the BorealAtlantic, notBorealArctic, species appeared in abundance in theearly Toarcian. In the interval of the Amaltheus vili-gaensisTiltoniceras antiquum boundary zones, sporepollen assemblages (divided into Siberian, cosmopoli-tan, and Euro-Sinian types) lost 16 of 18 Siberian taxa(Fig. 7). All the cosmopolitan taxa (four species)crossed the boundary and several of 14 Euro-Sinian



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408


ZAKHAROV et al.

species appeared at the very base of the Toarcian (or,probably, in the uppermost Pliensbachian), where theyare found in abundance at the Antiquum Zone top andin the Falciferum Zone. Thus, main events in the terres-trial vegetation happened during the Antiquum phase ofthe initial Toarcian.

BIOTA RESTORATION AFTER CRISIS

Almost all taxa of fauna and flora experienced reor-ganization in the Antiquum phase of the initial Toarcian.In all the groups, the crisis was accompanied by moreor less essential reduction of taxonomic diversity andby destruction of community structures. The restora-tion of benthic communities begins usually with returnof thermophobic taxa (the Lazarus effect). For exam-ple, the BorealArctic taxa that became extinct in thePliensbachian appear gradually in the lower Toarcian.Five BorealArctic genera appear in benthic communi-ties one phase later (0.5 to 1.0 m.y.) to become com-pletely restored during the subsequent 23 m.y. (thesame genera and majority of former species). At thesame time, different dominant genera appeared in com-munities (Fig. 3). The Low Boreal bivalves becamerestored only in the late Toarcian largely owing toimmigrants. None of nine Low Boreal bivalve taxa (thesame genera and many species) has been found imme-diately above the PliensbachianToarcian boundary.The genus Chlamys appeared in benthic communitiesonly 3 m.y. after the crisis, and 56 m.y. later newcom-ers occupied almost all the Arctic biotopes. A similarsituation is also characteristic of Subtethyan bivalvetaxa: all of them disappeared below the Toarcian, andonly one genus (Goniomya) returned. Two new generaappeared only after three and six phases (i.e., 3 and6 m.y. later). Thus, the extinction of bivalves was muchfaster as compared with their restoration.

Except for the genus Camptocythere, all the Arcticostracodes disappeared from the early Toarcian benthiccommunity, being replaced here by more thermophilicBoreal genera (Fig. 4). The thermophilic foraminiferalcommunity experienced substantial renewal above thePliensbachianToarcian boundary. During the latePliensbachianearly Toarcian, the renewal rate amongBoreal foraminiferal taxa was higher as compared withthat in their BorealArctic communities (Fig. 5).As was mentioned, the BorealAtlantic (i.e., thermo-philic) dinoflagellate species dominated in the pelagiczone of the Early Toarcian seas of North Siberia(Fig. 6). The event of reorganization was immediatelyfollowed by outburst of the thermophilic genus Classa-polis (dominant of the SPA). Eleven of 18 Siberian taxaappear again in the Dactylioceras commune Zone,while the SPA of underlying Tiltoniceras antiquum andHarpoceras falciferum zones consists of 14 Euro-Sin-ian species, most of which occur in abundance (Fig. 7).

SCENARIO OF BIOTIC TURNOVERAccording to materials discussed above, the follow-

ing, most reasonable scenario can be proposed forbiotic turnover recorded in the PliensbachianToarciantransition of North Siberia:

late Pliensbachian (late Stokesiinitial Margari-tatus phase) transgression warming immigration of Low Boreal and Peritethyan inverte-brate taxa to the Arctic basin;

late Pliensbachian (late Margaritatusearly Vili-gaensis phase) to early Toarcian (early Antiquumphase) regression formation of geographicbarriers changes in bottom topography and circu-lation system in the second half of the Viligaensisphase rapid cooling decline of the taxonomicdiversity of marine invertebrates (?reduction of north-ern distribution areas);

early Toarcian (terminal Antiquum phase) commencement of eustatic sea-level rise warm-ing slight freshening of surface waters onsetof the phytoplankton bloom and immigration of terres-trial Euro-Sinian plant taxa development of firstdisoxic settings extinction of marine invertebrates;

early Toarcian (Falciferum phase) furthersea-level rise with the maximum in the terminal Fal-ciferum phase changes in bottom topography andcirculation system substantial warming on landand in seas intermittent immigrations of LowBoreal invertebrate taxa to the Arctic basin migra-tion of Arcto-Boreal microbenthos to Low Boreal seas(Falciferuminitial Commune phases) sharpincrease in diversity of Euro-Sinian terrestrial vegeta-tion (abundance of Classopolis) phytoplanktonbloom development of disoxic to, locally, anoxicenvironments in the entire water column termina-tion of the invertebrate extinction at the beginning ofthe Falciferum phase;

early Toarcian (Commune phase) high sea-level stand reduction of disoxic settings warmwater masses and atmosphere immigration of LowBoreal and Peritethyan taxa and increased diversity ofBorealArctic marine invertebrate genera increase in diversity of phytoplankton and terrestrialplants owing to invasion of Siberian forms;

early Toarcian (Braunianus phase) sea-levelfall stability of taxonomic diversity among marineinvertebrates onset of cooling sharplydecreased species diversity of Euro-Sinian terrestrialvegetation (disappearance of Classopolis) decreased abundance of phytoplankton.

FACTORS RESPONSIBLEFOR BIOTIC TURNOVER

Paleogeography. It is reasonable to assume that sig-nificant paleogeographic events that resulted in sub-stantial renewal of marine settings could cause pertur-bations in biotic communities as well. In connection



with the PliensbachianToarcian biotic crisis, worth ofmentioning is large paleogeographic reorganization inthe North Atlantic that coincides with the transitionbetween these ages. The paleobiogeographic analysisof marine invertebrates reveals that the southern bound-ary of the TethysPanthalassa superbiochore waslocated at the latitude of southern England and north-western France in the late Pliensbachian. The paleogeo-graphic reorganization that occurred in the NorthAtlantic at the beginning of the Toarcian strengthenedthe influence of cold water masses that triggered south-ward migration of thermophilic mollusks and simulta-neous northward displacement of boundary betweenthe TethysPanthalassa and Panboreal superbiochoresfor several hundreds kilometers (Damborenea, 2002,fig. 7).

Macchioni and Cecca (2002) defined in sections ofnorthwestern and southern Europe and northwesternAfrica two distinct levels of the ammonite diversity fallat the beginning of the Toarcian (Dactyliocerastenuicostatum Zone). The first of these events coincideswith the base of the Dactylioceras mirabile Subzoneand reflects destruction of the BorealTethyan provin-ciality due to rapid extinction of the Boreal endemicfamily Amaltheidae, which populated seas of north-western Europe, on the one hand, and disappearance ofthe late Domerian endemic ammonites in the Mediter-ranean realm, on the other. The second drastic fall of theammonoid diversity in the upper part of the Dactylio-ceras semicelatum Subzone corresponds to the anoxia(OAE) attack. This is reflected in epioceanic ammonoidclades Phylloceratacea and Lytoceratacea. We considerboth extinctions as autonomous, not representing epi-sodes of a single stepwise extinction event.

It can be thought that influence of the above paleo-geographic reorganizations should be restricted to areasadjacent to the North Atlantic having distant relation tobiotic events in the Andean and North Siberian basinslocated many thousands kilometers away. Nevertheless,this is not the case. According to recent data, the earlyToarcian intensification of water exchange betweenNorth Atlantic and Arctic seas via the so-called VikingCorridor was accompanied by enhanced water andbiota exchange between the western Pacific and CentralAtlantic via Hispanic Corridor (Zakharov, 2005 andreferences therein). The widened seaways activated thefaunal exchange between paleobasins. Aberhan (2002)assumed that extinction in South America can be partlyexplained by immigration of bivalves via the HispanicCorridor and their subsequent selective migration.Although the calculated rates of molluscan migrationsfrom the Atlantic to the Pacific and back demonstratedtheir insignificant mutual influence, the effect of thisfactor cannot be ruled out completely. The North Sibe-rian biota appeared to be dependent of the North Atlan-tic biota more than of the Andean one. For example,extinction of the late Pliensbachian ammonite familyAmaltheidae in the Arctic region (and globally as well)was synchronous with the enhanced water exchange

between the Arctic and North Atlantic basins. Subse-quent colonization of Siberian and North Asian basinsby ammonites of the family Dactylioceratidae, theimmigrants from North Atlantic seas, occurred at thebeginning of the Toarcian so quickly that this event can-not be estimated by the traditional biochronostrati-graphic method. Thus, the principal renewal of ammo-nite communities in North Siberia at the time of Pliens-bachianToarcian transition and in the initial Toarcianwas caused by paleogeographic reorganization in theNorth Atlantic. The related influence of Atlantic watermasses stimulated invasion of West European Toarcianammonites from the family Dactylioceratidae andbelemnites from the family Passaloteuthide in the Arc-tic basin (Saks et al., 1971).

Eustatic factors and anoxia. A wide developmentof anoxic environments in response to sea-level riseduring the Early Toarcian transgression (Hallam, 1987;Hallam and Wignall, 1999; Aberhan and Baumiller,2003) is most popular explanation for the mass extinc-tions of bivalve mollusks. As is assumed this transgres-sion of eustatic origin most likely stimulated formationof black shale facies. Shales and clays of the early Toar-cian age are known in many regions of the NorthernHemisphere, i.e., in northwestern Europe, AlpineMediterranean domain, Canada, Arctic region, andJapan. These sediments have been deposited in differ-ent paleooceanographic settings, but the common view-point of researchers, who studied these facies and rele-vant fossils, is that the lower Toarcian high-carbon-aceous black shales have been deposited in basins withoxygen-deficient bottom waters. The worldwide distri-bution of these sediments impelled Jenkins (1988) toassume the oceanic anoxic event (OAE) that took placein the initial early Toarcian (Falciferum phase). Theoxygen-deficient environments in deep basins and onshoals (Tremto Plateau in the Southern Alps) areinferred from sedimentological and geochemical data(Jenkins et al., 1985; Jenkins and Clayton, 1997). Bel-lanca et al., (1999) who studied productivity of Toar-cian black shales in the Belluno basin of northern Italyconcluded that shales enriched in organic carbon andrhythmically alternating with bioturbated limestonesand gray marls were deposited in anoxic settings of theTethys during the early Toarcian. The negative 13excursion established in the middle of the examinedinterval is correlative with the peak content of totalorganic carbon (TOC) and high V/Rb and Ba/Rb valuesare indicators, as they believe, of the Toarcian anoxicevent, i.e., of the near-bottom anoxia coupled with ahigh productivity of surface waters. In this respect, thelower Toarcian Posidonia shales of southwestern Ger-many can be considered as a classical standard of high-carbonaceous black shale sequences. In opinion ofmost researchers who studied them, these facies formedin a shallow semiclosed sea basin. Rhl et al. (2001)who investigated Posidonia shales in sections of theSchwbische (in southwest) to Franconian Alb (in thenortheast) arrived at the conclusion that anoxic condi-

410


ZAKHAROV et al.

tions in bottom waters of the Posidonia basin existedpermanently during its development. Moreover, themaximal oxygen deficiency and extreme negativeexcursion of 13org value (34) corresponds to theinitial Falciferum phase. It should be noted also thatsome researchers (Aberhan, 2002; Aberhan and Fr-sich, 1997) consider the high sea-level stand and widedevelopment of anoxic environments as factors respon-sible, among others, for reorganization in bivalve com-munities of the Andean basin (Chile) at the time ofPliensbachianToarcian transition. The cited examplesshow that anoxic conditions in the bottom water layerwere characteristic of different-type basins in theNorthern and Southern hemispheres.

Thus, anoxia is referred to as one of principal factorsresponsible for the taxonomic diversity reduction inmarine benthic groups. In the oceanographic scenario,the expansion of anoxic settings is explained by sea-level changes viewed as main factors. The triggeringfactor of a rapid, instant in fact from geological stand-point (one phase shorter than 1 m.y.), global sea-levelrise used to be omitted in that scenario.

The North Siberian sections spaced for hundredsand even thousands kilometers from each other demon-strate a rapid (sometimes, within the interval severaldecimeters thick) replacement of sandysiltyclayeysediments by clayey and clayeysilty facies locallyenriched in org. This is accompanied by a sharp declinein the taxonomic diversity of macro- and microfossils.Several bivalve genera belonging to three trophicgroups continue to exist however: sestonophages of low(Tancredia) and high (Kolymonectes, Oxytoma) levels,and gathering detritovore genus Dacryomya. They alldwelt in oxygen-enriched bottom settings and, natu-rally, could not survive even disoxic environment, leav-ing aside the anoxic one. Moreover, Dacryomya fedbelow the water/sediments interface, and consequentlynot only H2S contamination, but even a significant oxy-gen deficiency is inadmissible below this level. Rela-tively diverse benthic foraminifers (17 genera), whichrequired aerated environments, populated the bottomduring the initial Toarcian Antiquum phase as well. Nat-urally, it cannot be ruled out that necessary conditionsexisted in the entire spacious epicontinental sea ofNorth Siberia between the Yamal and Chukotka penin-sulas. Undoubtedly, oxygen deficiency of bottomwaters could be characteristic of some areas. By logic,a wide distribution of lower Toarcian fine-grained sedi-ments, which are locally enriched in organic matter, canbe explained by a high sea-level stand and relativelyleveled low-mountainous relief around the relevantbasins (Jenkins, 1988). In opinion of some experts, thehighest sea-level stand during the entire early Jurassicand initial Middle Jurassic was characteristic preciselyof the early Toarcian (Hallam, 1992, figs. 4, 5).

Climate. All the aforesaid is correct, but manyresearchers relate episodes of general climatic warmingwith eustatic transgressions. As for biota, it was directly

influenced by climate changes precisely. As is demon-strated below, it is more logical to explain manydynamic events in transformation of invertebrate andvegetation communities by climatic fluctuations. Weconsider this factor as principal one for the biota reor-ganization in North Siberia, where warm environmentsof the late Pliensbachian were followed by cooling atthe stage termination (at the end of the Viligaensisphase) and in the initial Toarcian (the beginning of theAntiquum phase), and then by a gradual warming (seescenario). This inference is based on disappearance ofmarine thermophilic fossils from the PliensbachianToarcian boundary beds, and this event was followed inthe North Siberian seas by gradual return first of HighBoreal and then of thermophilic fauna and floral taxa.Moreover, some of the former thermophilic generawere replaced by newcomers. Such dynamics in biotarestoration points to a gradual warming. It should benoted however that a relatively early outburst of Euro-Sinian taxa in vegetation and abundance of Classopolisin the SPA imply a relatively rapid warming on land.This inference is consistent with the phytoplanktonbloom against the background of highly impoverishedbenthic invertebrate communities. The taxonomicimpoverishment of filtering organisms can be explainedby substantial changes in the nutritional chain, becauseoutburst of dinoflagellates changed it qualitatively.Indeed, prior to the Toarcian, dinoflagellates were prac-tically missing from the diet of filtering organisms. It isconceivable that precisely a long adaptation to newfood explains the fact that the post-crisis period of col-onization of North Siberian seas by the early Toarcianbivalves lasted several millions years. Nevertheless, thetemperature should be considered as a principal factor.Actually, the Subtethyan and Low Boreal bivalves,which populated the epicontinental seas northward oflatitude 68 N in the terminal Pliensbachian, migratedsouthward to the latitude 55 N (to the BorealTethyanecotone in northern Primore) at the beginning of theToarcian (Antiquum and Falciferum phases); theyreturned back to North Siberia only in the late Toarcian(Fig. 8). To some extent, this inference is consistentwith data on foraminifers and ostracodes. During thegreater part of the late Pliensbachian, microfauna dweltat the latitude 73 N. At the end of the Viligaensisphase, foraminifers and ostracodes migrated almostdown to 55 N. Later on at the beginning of the Toar-cian, both groups of fauna returned relatively soon fromlower to higher latitudes and adapted to new environ-ments during a relatively short period, probably, shorterthan 1 m.y. (Fig. 9). A similar climatic regime is recon-structed for northern Siberia based on palynologicaldata. The early Toarcian warming was maximal onethrough the entire Jurassic (Fig. 10). In opinion of Ilinaet al. (1994), the peak warming corresponds to the Fal-ciferum phase. The terminal Pliensbachian was a timeof cooling that probably progressed in the initial Toar-cian as well, although the exact period of cooling isunknown because of the boundary sediments are dated



uncertainty to some extent (at the subzone level). It isconceivable that the terminal Pliensbachian(?)initialToarcian cooling was induced by growing humidity ofNorth Siberian climate. Geochemical signals (Levchuk,1985) and specific composition of microfossil assem-blages point to water freshening in the southeasternNorth Siberian sea. The Toarcian Age of enhancing cli-mate aridity explains absence of coarse-grained sedi-ments even in coastal shallow-water Toarcian suc-cessins in North and West Siberia (Paleogeography ofthe Northern, 1983).

DYNAMICS OF BIOTIC REORGANIZATIONS

The first quantitative curve illustrating the extinc-tion dynamics of marine invertebrate families duringthe Phanerozoic (Raup and Sepkoski, 1982, 1984; Sep-koski and Raup, 1986) gives an impression that peaksof taxonomic diversity decline coincide with bound-aries between stages and substages. Subsequent, moreprecise data showed however that critical events ofbiota reorganization do not coincide frequently withboundaries of stratigraphic units. For example, Sepko-ski estimated that 22 and 11 ammonite families becameextinct in the world during the Pliensbachian and Toar-cian, respectively. According to Little and Benton(1995), 28 ammonite families became extinct in theNorthern and Southern hemispheres in the Early Juras-sic beginning from the late Pliensbachian Margaritatusphase to the early Toarcian Bifrons phase (from 191.51

to 184.00 Ma) that was a period of most intenseammonoid extinction: as they estimated 8, 7, 4, 8, and6 families disappeared respectively during the Margar-itatus, Spinatum, Teniocostatum, Falciferum, andBifrons phases. Two maximums of extinct families cor-responded to the terminal Pliensbachian (Spinatumphase) and initial Toarcian (Tenuicostatum phase).

Based on distribution of eight fossil groups(bivalves, gastropods, scaphopods, ammonites, belem-nites, brachiopods, serpulids, crinoids) in the northernYorkshire section (England) of the PliensbachianToar-cian transition, it was established that the highest rate ofspecies extinction was characteristic of the Semicela-tum Subzone, the uppermost one of four subzones ofthe Dactylioceras tenuicostatum Zone (Little and Ben-ton, 1995). Hallam (1987) demonstrated that taxo-nomic diversity of ammonite genera and species ofbivalves, brachiopods, foraminifers, and ostracodesincreased progressively in the Early Jurassic seas ofnorthwestern Europe during the Hettangian, Sine-murian, and Pliensbachian to drop at the Pliensba-chianToarcian boundary (except for foraminifers innorthwestern France), especially in bivalve communi-ties. In all these groups, except for foraminifers andostracodes in England, the peak appearance of new-comers corresponds to the uppermost Pliensbachianzone, and exactly these taxa new for the NorthwestEuropean sea disappeared in majority during the biotaturnover. The gradual growth of taxonomic diversity isobservable practically in all the invertebrate groups, but

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Ammonitezonation

Region,latitude Primore Okhotsk region Taimyr

7268646056

Pseudolioceras mclintocki

Pseudolioceras falcodiscius

Pseudolioceras wurtenbergeri

Pseudolioceras compactile

Zugodactylites braunianus

Dactylioceras commune


Tiltoniceras antiqum

Amaltheus viligaensis

Amaltheus margaritatus

Amaltheus stokesi

Subtethyan bivalves

Low Boreal bivalves

Fig. 8. Migration of Subtethyan and Low Boreal marine bivalve genera in the Arctic paleobiogeographic region during the latePliensbachian and early Toarcian from high to low latitudes and backward, a principal model; abbreviations denote Aalenian Stage(Aa) and its lower substage (l.).

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it has never reached the diversity level of the terminalPliensbachian even near the Aalenian base.

Dynamics of biota reorganizations was not identicalin different paleobiochores in terms of rate, successionof events, and time, as is evident from correlation ofsections at the zonal (subzonal) level. For example, twoextinction levels of ammonoids are defined in theTethyan and Boreal superbiochores at the base and inthe upper part of the Tenuicostatum Zone (Macchioniand Cecca, 2002). Little and Benton (1995) demon-strated however that a minimal number of extinct fam-ilies corresponds to this zone in northwestern Europe.Consequently, the extinction peaks in question aredetermined by Mediterranean ammonoids.

The maximal taxonomic diversity of benthic macro-fossils (bivalves and brachiopods) is confined in theWest European Boreal province to the Falciferum Zone(Exaratum Subzone). In the Andean basin, where zona-tion has not been described, the extinction interval cor-responds to the terminal PliensbachianEarly Toarcian,

as is evident from diagrams published by Aberhan andFrsich (1997, 2000). Hallam (1987), who considersanoxia as the main factor responsible for extinction,believes that this phenomenon suppresses nekton andbenthos but not plankton. As it follows from analysis ofextinctions in particular zones, diversity of ammonitesin the northern West European basin dropped signifi-cantly earlier than that of benthic fossils (Hallam, 1987,fig. 1). This event coincided with the onset of formationof high-carbonaceous black shales close to the Fal-ciferum phase, the initial part of which is marked by theoxygen deficiency maximum and anomalous negative13org (34) excursion established in the South Ger-man basin (Rhl et al., 2001). Bailey et al. (2003) con-firmed this inference based on the data from the York-shire basin, where they registered the OAE signals atthe base of the Falciferum Zone (Exaratum Subzone).

The extinction rates detectable in different fossilgroups are of particular interest. It is well known thatdiversification rate of Mesozoic ammonoids was higherthan of other marine invertebrates. The relative bio-chronological scale based on the ammonite successionis unsuitable however for an accurate assessment ofrates characterizing their morphogenesis. On the otherhand, the absolute dates characterizing intervals even ofthe Mesozoic ages, not to mention the zonal phases, arestill far from being estimated precisely (Gradstein et al.,1994). Nevertheless, the dates estimated by interpola-tion of 87Sr/86Sr values for the upper Pliensbachianlower Toarcian boundary subzones (McArthur et al.,2000) deserve attention. The mentioned authors calcu-lated time spans of the lower Toarcian ammonite sub-zones and demonstrated that one can be 30 times longerthan the other one, e.g., the Clevelandicum Subzone0.036 m.y. long versus the Exaratum Subzone 1.08 m.y.long. The early Toarcian OAE lasted 0.52 m.y. If equalrelative deviations (systematic calculation errors) butnot the above absolute values are taken into consider-ation, one can figure out the transformation rate ofammonite community was at least an order of magni-tude higher than the transformation of benthic commu-nities. In fact, the integral duration of four subzones ofthe Tenuicostatum Zone is 0.302 m.y. (reorganizationperiod of ammonite assemblages), while that of theExaratum Subzone (reorganization period of benthiccommunities) is 1.080 m.y. It should be noted onceagain that reorganization in ammonite communities ofthe Boreal and Tethyan basins commenced prior to theanoxic event.

The scenario proposed above for the PliensbachianToarcian turnover in benthic communities of the Arcticbasin is similar in many respects to that peculiar ofNorthwest European and even Andean basins. On theother hand, there were substantial differences indynamics of taxa extinction. First, the Late Pliensba-chian nekton was represented in the Arctic basin by thegenus Amaltheus only. This genus was sole representa-tive of the ammonite family Amaltheidae that becameextinct worldwide, in the Arctic basin as well, at the

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Fig. 9. Migration of Subtethyan and Low Boreal foramin-iferal and ostracode genera in the Arctic paleobiogeo-graphic region during the late Pliensbachian and early Toar-cian (a principal model).



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time of PliensbachianToarcian transition. Almost allthe Toarcian ammonites of the Arctic region originatefrom the North Atlantic. Evolution of these forms inArctic seas was identical to their development in origi-nal habitats, i.e., in West European seas. Second, incontrast to the EnglandParisian and Andean basins theshare of extinct taxa among benthic groups was negli-gible in the Arctic region. Dynamics of taxonomicdiversity was controlled here largely by changes inboundaries of distribution areas rather than by extinc-tion and diversification (Figs. 7, 8). At the end of thePliensbachian, the only genus Harpax became extinctamong Arctic bivalves. Most of the BorealArctic gen-era migrated at the beginning of the Toarcian southwardto return subsequently to their former biotopes (Fig. 8).More thermophilic genera leaved the Arctic seas, notdied off. Microfauna followed in general the scenariosuggested for bivalves (Fig. 9). Thus, there was no massextinction of marine invertebrates in the Arctic basins atthe time of PliensbachianToarcian transition. Thebiotic crisis was mostly connected here with a rapidsouthward migration of Arctic marine invertebratesduring a sharp climatic cooling, which graduallyreturned afterward to former biotopes under conditionsof subsequent warming (the Lazarus effect).

CORRELATION OF EVENTS IN DIFFERENT REGIONS

Two regions, where a sharp decrease in speciesdiversity of bivalves is established in the Pliensba-chianToarcian transition, are the southern England(Yorkshire) to southern Germany (epicontinental seasof northwestern Europe) and South America (Andeanbasin). Hallam (1986, 1987) considered this phenome-non as regional, but after discovery of turnover inbivalve community of the Andean basin (Aberhan andFrsich, 1997, 2000), the event gained a global status.The inferred global character of biotic crisis at the timeof PliensbachianToarcian transition is consistent withdata obtained in North Siberia and Arctic regions as awhole (Shurygin and Nikitenko, 1996; Nikitenko andMickey, 2004). Dynamics of taxonomic diversity inthree remote regions (northwestern Europe, Andeanbasin, Arctic region) is similar: the diversity growingrapidly in the Sinemurian and Pliensbachian decreasedsharply afterward, in particular at the beginning of theToarcian, and was increasing gradually again up to theformer (pre-crisis) level during the terminal Toarcian orin the Aalenian. Like in epicontinental seas of north-western Europe and in the Andean basin, the period ofbiota decline in the North Siberian basin was severaltimes shorter than the restoration period. The Arcticbenthic communities acquired former diversity in thelate Toarcian, while in West Europe and the Andeanbasin the bivalve communities reached their pre-crisisstate in the Aalenian only. This is thought to be a con-sequence of bivalve larvae migration via the HispanicCorridor. As was suggested, the diversity decline dur-

ing the crisis is related to immigration of taxa from theAndean basin to West European seas in the terminalPliensbachianinitial Toarcian, while the restoration ofbiota is caused by their backward migration from WestEuropean to East Pacific basins, because bivalvesacquired their pre-crisis diversity already in the Toar-cian. Calculations by Aberhan (2002) show, however,that the migration rate was low in both regions duringthe early PliensbachianAalenian, and this fact dis-proves both hypotheses. On the contrary, diversificationwas much more important than the immigration, con-trolling the divergence in both regions. Aberhan arguedthat the global PliensbachianToarcian crisis in biotaevolution is better explainable by a combination ofphysicochemical (intense volcanism, high sea-levelstand, widespread anoxia environments) and biologicalfactors. The mass extinction was followed by restora-tion, when generation rate of new taxa increased againduring the Aalenian in the Andean basin and during thelate Toarcian in West European basins.

A group of West European researchers who studiedgeochemistry and isotopic composition calcitic belem-nite rostra from the Pliensbachian and Toarcian sec-tions of the Yorkshire coast in the United Kingdom andsouthern Germany arrived at some important conclu-sions (Bailey et al., 2003). Owing to confident bio- andchemostratigraphic correlation of sections, theseresearchers demonstrated that geochemical trendsestablished for belemnite successions of both regionsare correlative to each other. The Mg/Ca, Sr/Ca, andNa/Ca ratios in calcitic material of belemnites increasefrom 1.7 to 2.0 within the interval of negative (by 3)18 excursion between the middle of the Tenuicos-tatum Zone and the lower part of the Falciferum Zone(time span 0.6 to 0.7 m.y. long) of the ammonite zona-tion determined in England. These data support theassumption that dramatic environmental changes weresynchronous to the Toarcian OAE. The Mg/Ca and18 values imply a sharp warming (by 67C) andsubstantial water freshening at that time. The globalwarming accompanied by increased hydrologicalcycles and runoff has been assumed responsible forthese changes. Data on the lower part of the Yorkshiresection suggest that these events were preceded bycooling and synchronous growth of water salinity dur-ing the period from the late Pliensbachian Margaritar-ius phase to the initial Tenuicostatum phase of the earlyToarcian.

Thus, the succession of the terminal Pliensbachianto initial Toarcian events in the Northwest Europeanepicontinental sea and Andean basin is well consistentwith scenario proposed for concurrent biota reorganiza-tions in North Siberian seas and the Arctic basin as awhole, although the dynamics of reorganization hadspecific features in different regions.



CONCLUSIONSIn North Siberia and the Arctic as a whole, there is

recorded a biotic turnover in the PliensbachianToar-cian transition. The event is evidenced by sharp reduc-tion of taxonomic diversity at the generic level ofmarine invertebrates (bivalves, ostracodes, foramini-fers) in the uppermost Pliensbachian(?)basal Toarcianboundary layers (Tiltoniceras antiquum Zone). Concur-rent sporepollen assemblages show disappearance orsubstantial decrease in abundance of many speciescharacteristic of Siberian paleoflora. Restoration ofbiota was gradual and asynchronous in different groupsof marine and terrestrial organisms. The BorealArcticbivalves and Boreal foraminifers were first (Harpo-ceras falciferum Zone) to appear in benthic communi-ties of North Siberian seas; the Low Boreal and Subt-ethyan bivalves and Boreal ostracodes appeared here inthe late Toarcian only. The BorealAtlantic dinoflagel-lates, which are represented in the terminal Pliensba-chian by single specimens of the genus Nannoceratop-sis, demonstrate an outburst at the very beginning of theToarcian, being flourishing throughout the early Toar-cian. Abundant BorealArctic dinoflagellate taxa (10species of 6 genera) appear only in the late Toarcian. Inthe basal Toarcian (Tiltoniceras antiquum Zone),sporepollen assemblages become diverse largelyowing to the Euro-Sinian taxa (14 species of 12 gen-era). Siberian taxa appear in abundance in the Dactylio-ceras commune Zone at the stratigraphic level, whereabundance of Euro-Sinian forms is sharply declined.

Based on bivalves, microfossils, and sporepollenassemblages, the biotic turnover can be logicallyexplained by climate changes: warm conditions of theterminal Pliensbachian were followed by a sharp cool-ing at the time of PliensbachianToarcian transition andthen by warming in North Siberia. This warming mightbe related to the sea-level rise that explains a wideoccurrence of clayey, locally high-carbonaceous sedi-ments in the Arctic region, West and East Siberiaincluded. The sea-level rise coupled with the water tem-perature increase was likely responsible for the outburstof thermophilic phytoplankton in the initial Toarcian.This could reduce oxygen concentration in the watercolumn (red tide phenomenon) and, locally, in bot-tom waters or beneath the sediment/water interface.The same event destroyed the former nutritional chainsand caused a sharp reduction of diversity of filteringbivalves.

The gradually grown diversity of macrofossils(bivalves, ammonites, belemnites) and, partly, ofmicrofauna indicates that warming in northern Siberiawas gradual. At the same time, the outburst of thermo-philic terrestrial vegetation and thermophilic marinemicroscopic algae points to a rapid warming at the timeof two the terminal AntiquumFalciferum and initialCommune ammonite phases. The exact duration of thiswarming episode is unknown, approximately estimatedto be 2 m.y. long. Paleobotanical data suggest the onset

of the next cold period in northern Siberia already at theearlylate Toarcian transition. This event was concur-rent to appearance of abundant and diverse BorealArctic dinoflagellates in North Siberian seas and toprosperity of taxa, which dominated in on-land vegeta-tion of the Siberian phytogeographic region. This cli-matic scenario is weakly consistent with data onbivalves and ostracodes whose communities includedrelatively thermophilic genera in the terminal Toar-cianinitial Aalenian. Should we assume differentresponses of bivalves and microscopic algae to changesin basin water temperature? The question remains with-out answer.

It should be noted that climatic changes representedprobably a decisive factor that caused reduction of dis-tribution areas of some taxa doomed to extinction in theMesozoic. For example, last conodonts disappeared inthe Tethyan realm in the Rhaetian, while their last rep-resentatives are found in Norian sediments of the Arcticregion. The last remains (vertebrae) of marine dino-saurs are known in North Siberia from the mid-Maas-trichtian. In the tropical zone, large dinosaurs existedup to the terminal Maastrichtian and, probably, to theDanian. Cephalopods became extinct in Tethyan seas inthe terminal Cretaceousinitial Paleogene, while in theArctic paleobiogeographic region, their extinction isrecorded in the mid-Maastrichtian. Bivalves of the fam-ily Inoceramidae are even more illustrative. Their lastrepresentatives in the Arctic region are known from thelowermost Campanian, while in the western Tethysthey occur in the CretaceousDanian (Paleogene)boundary layers. Thus, the additional evidence for cli-mate-induced extinctions is obtained by analysis of dis-tribution areas of doomed-to-extinction taxa: extinctionof particular groups in the Mesozoic was usually pre-ceded by reduction of their distribution areas and sub-sequent disappearance from the Boreal realm.

ACKNOWLEDGMENTSWe are grateful to M.A. Akhmetev and O.A. Kor-

chagin for comments that helped to improvement of themanuscript. The work was supported by the RussianFoundation for Basic Research, project nos. 03-05-64391, 03-05-64780, and 05-05-6494, by the Programno. 25 of the Presidium RAS and Program ONZ no. 14,and by Project 27.2.2 of the Program 27.2 of the Sibe-rian Division RAS.

Reviewers M.A. Akhmetev and O.A. Korchagin

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Pliensbachian-Toarcian biotic turnover in north Siberia and the Arctic region