Early Cretaceous (Valanginian � Hauterivian) calcareous

nannofossils and isotopes of the northern hemisphere: proxies

for the understanding of Cretaceous climate

KAI KESSELS, JORG MUTTERLOSE AND DIETER MICHALZIK

LETHAIA Kessels, K., Mutterlose, J. & Michalzik, D. 2006 06 20: Early Cretaceous (Valanginian �Hauterivian) calcareous nannofossils and isotopes of the northern hemisphere: proxiesfor the understanding of Cretaceous climate. Lethaia , Vol. 39, pp. 157�172. Oslo. ISSN0024-1164.

From three boreholes (DSDP Site 535; ODP Site 638; BGS borehole 81/43) of the CentralAtlantic and the North Sea Basin 379 samples of early Cretaceous age (Valanginian�Hauterivian) were examined. The localities cover a S�N transect of approximately 3000km stretching from 178N to 408N palaeolatitude. The distribution of calcareousnannofossils and fluctuations of the stable isotopes (d13C, d18O) have been recordedand were compared with results of recent studies. We differentiate between high nutrientindicators and oligotrophic taxa and propose a four step scheme to characterize thetrophic level of the surface water. (1) High abundances of the fertility group (Biscutumconstans/Zeugrhabdotus spp.) combined with a high dominance of B. constans and lowabundances of Watznaueria barnesae/W. fossacincta represent a high nutrient environ-ment (eutrophic setting). (2) High abundances of the fertility group combined with ahigh dominance of Zeugrhabdotus spp. and low abundances of W. barnesae/W.fossacincta reflect enhanced nutrient contents of the surface water (mesotrophic setting).(3) Enhanced abundances of the fertility group combined with high abundances of W.barnesae/W. fossacincta indicate slightly increased nutrient contents of the surface water(meso- to oligotrophic setting). (4) Low abundances of the fertility group and highabundances of W. barnesae/W. fossacincta are of low nutrient affinities (oligotrophicsetting). Our estimations of seawater palaeotemperatures in combination with literaturedata show a distinctive trend for the Valanginian to Hauterivian interval. A generaldecrease of water temperature from the Valanginian to the early Hauterivian is obvious.This decrease of temperature coincides with the southward migration of the highlatitudinal cold water species Crucibiscutum salebrosum to lower latitudes. Our findingsshed new light on the evolution of the earliest Cretaceous climate, which may becharacterized as a warm greenhouse world with interludes of short cooling. I Calcareousnannofossils, cooling phase, Cretaceous, palaeoclimate, palaeoecology, stable isotopes.

Kai Kessels, corresponding author [kai.kessels@ruhr-uni-bochum.de], Jorg Mutterlose[joerg.mutterlose@ruhr-uni-bochum.de] & Dieter Michalzik [dieter.michalzik@ruhr-uni-bochum.de], Institut fur Geologie, Mineralogie & Geophysik, Ruhr-Universitat Bochum,Universitatsstr. 150, 44780 Bochum, Germany; received 22 July 2004, revised 6 April 2006.

The early Cretaceous was a period of global change. A

global sea-level lowstand characterized the Berriasian

(Haq et al . 1988; Ziegler 1988; Hardenbol et al . 1998)

causing widespread deposition of non marine sediments

and the development of restricted epicontinental basins

especially in the northern hemisphere. Transgressions in

the Valanginian to Hauterivian led to more open

oceanic conditions (Rawson & Riley 1982; Haq et al .

1988; Ziegler 1988) and favoured the exchange of

marine floras and faunas between the high and low

latitudes (Kemper et al . 1981; Mutterlose 1991). This

period was marked by an onset of diversification and the

evolution of new taxa of calcareous nannofossils

(Mutterlose et al . 2005). The regressive period of the

Barremian (e.g. Ruffell 1991) was followed by a global

2nd order sea-level rise in the Aptian and Albian, which

was probably initiated by worldwide increased volcanic

activity and enhanced ocean crust production (e.g. Haq

et al . 1988; Larson 1991a, b; Hallam 1992). During the

Aptian to Albian interval the development of marine

nannofossils was marked by a significant turnover along

with extinctions and first appearances of new taxa

followed by a homogenization of the assemblages (e.g.

Roth 1986; Mutterlose & Bockel 1998).

Calcareous nannofossils are an adequate tool for

reconstructing the past depositional environments of

Mesozoic and Cenozoic oceans. The palaeoecologic,

-oceanographic and �climatic preferences of calcareous

nannofossils are still under discussion and many studies

have been published throughout the last 20 years (e.g.

McIntyre & Be 1967; McIntyre et al . 1970; Roth &

Bowdler 1981; Roth & Krumbach 1986; Roth 1986,

DOI 10.1080/00241160600763925 # 2006 Taylor & Francis

1989; Premoli-Silva et al . 1989; Watkins 1989; Coccioni

et al . 1992; Erba et al . 1992; Brand 1994; Eshet &

Almogi-Labin 1996; Mutterlose & Kessels 2000, Street &

Bown 2000; Bersezio et al . 2002; Pittet & Mattioli 2002).

Variations in the composition and abundances of

calcareous nannofossils are thought to reflect autecolo-

gical changes, especially of surface water temperature,

nutrients, salinity and detrital input from the conti-

nents. Due to their planktonic lifestyle in the upper

water column calcareous nannofossils are widely dis-

tributed and are thus often used to indicate short- and

long-term palaeoceanographic and palaeoclimatic

changes in the worlds oceans (e.g. Erba 1994; Melinte

& Mutterlose 2001, Erba & Tremolada 2004).

In order to improve the understanding of the early

Cretaceous climate three cores from the Central Atlantic

(DSDP 535, ODP 638) and the North Sea Basin (BGS

81/43) have been examined for their calcareous nanno-

fossil content and the results were compared with recent

studies. Additionally bulk rock stable isotopes have been

measured and palaeotemperature trends for the Valan-

ginian to Hauterivian interval were estimated.

Lower Cretaceous palaeogeographyand palaeoclimate

Palaeogeographic changes within the Mesozoic were

marked by the break up of the Pangaea landmass and

the opening of the Atlantic ocean. In the early Cretac-

eous the opening of the Central Atlantic led to a direct

sea-way between the Arctic Sea and the low latitudinal

areas in the south (Fig. 1). Throughout the Valanginian

to Hauterivian interval, however, the South Atlantic

remained closed and only episodic sea-ways to the

Pacific Ocean existed via the Strait of Panama (Berggren

& Hollister 1977). The oceanic system was still char-

acterized by a broad east-west stretching Tethys.

A number of recent studies argue for a more

differentiated and variable climate than previously

thought (e.g. Kemper 1987; Weissert & Lini 1991; Stoll

& Schrag 1996). Ice ages or at least ice-house conditions

or cooling events during certain parts of the Cretaceous

(e.g. Valanginian) evidenced by the findings of glendo-

nites (Kemper 1987), varied composition of marine

floras and faunas (Kemper 1987), the existence of ice-

rafted deposits (Frakes & Francis, 1988) and palaeotem-

perature calculations from oxygen isotopes (e.g.

Weissert & Lini 1991; Podlaha et al . 1998; Price et al .

2000; Price & Mutterlose 2004) have been reported. The

bipolar distribution of certain high latitude restricted

nannofossil species during the Valanginian to Hauter-

ivian may indicate distinctive temperature gradients in

the oceans (e.g. Mutterlose & Kessels 2000; Street &

Bown 2000).

Location and material

Sediments of Valanginian to Hauterivian age have been

analysed for their calcareous nannofossil content and

oxygen and carbon isotope ratios. A total of 379 samples

from three boreholes (DSDP 535, ODP 638, BGS 81/43)

which were drilled in the Central Atlantic and the North

Sea Basin, were examined. The cores were located at

palaeolatitudes of 178N to 408N, covering a palaeo-

distance of ca. 3000 km. For age assignments and

stratigraphic correlation of the cores standard zonation

schemes for the Tethys (NC/NK zonation after Bralower

et al . 1989) and for the Boreal Realm (BC zonation after

Bown et al . 1998) were used (Fig. 2). Absolute ages of

the observed nannofossil events were taken from Hard-

enbol et al . (1998).

DSDP Site 535

Grid reference. � 23842.48?N, 84830.97?W; southeastern

Gulf of Mexico, western Straits of Florida.

Palaeolatitude. � 178N.

Study interval. � 643.93 mbsf (core 71-5) to 465.01 mbsf

(core 51-1).

Stratigraphic range of the studied interval. � NK3a (lower

Valanginian) to NC4b (lower Hauterivian). 197 samples

have been examined from this site. The lithology of the

core was described in detail by Buffler et al . (1984).

North America

TethysODP 638

DSDP 535

BGS 81/43

Africa

Central Atlantic

Arctic Sea

fo sumhtsI

amanaP

30°

0°

0°30°60°

1000 km

Fig. 1. Palaeogeographic map of the Valanginian. Modified afterOSDN Plate Tectonic Reconstruction service, 2003; www.odsn.de.

158 K. Kessels et al. LETHAIA 39 (2006)

Fig. 2. Biostratigraphic correlation of the examined boreholes. *combined nannofossil zonation after Roth (1983), Bralower et al . (1989), Bralower et al . (1993), Bown et al . in Bown (1998); Absolute Agesof the nannofossil events after Hardenbol et al . (1998).

LE

TH

AIA

39

(20

06

)E

arly

Creta

ceous

calca

reous

na

nn

ofossilsa

nd

isotopes

ofth

en

orthern

hem

isph

ere1

59

ODP Site 638 B/C

Grid reference. � 42809.2?N, 12811.8?W (Hole 638 C 30

m south of 638 B); northwest of the Iberian Peninsula,

Galicia margin.

Palaeolatitude. � 258N.

Study interval. � 539.35 mbsf (Hole 638 C, core 14-2) to

214.35 mbsf (Hole 638 B, core 23-4).

Stratigraphic range of the studied interval. � NK3a (lower

Valanginian) to NC4b (lower Hauterivian). A total of

115 samples has been analysed from this site. A detailed

core description is given by Boillot et al . (1987).

BGS borehole 81/43

Grid reference. � 54838.92?N, 0814.51?E; southern North

Sea, 80 km ENE of Speeton (UK).

Palaeolatitude. � 408N.

Study interval. � 66.80 mbsf to 35.00 mbsf. Stratigraphic

range of the studied interval: BC4a (lower Valanginian)

to BC9 (upper Hauterivian). 67 samples have been

investigated from this core. The lithostratigraphy of the

borehole is summarized by Lott et al . (1986).

Methods

Calcareous nannofossils

For the preparation of calcareous nannofossils two

standard techniques were used. The investigation of

quantitative nannofossil distribution and calculation of

absolute abundances is followed by the use of the

random settling technique (e.g. Geisen et al . 1999).

For additional biostratigraphic examinations standard

smear slide preparation after Bown (1998) was applied.

At least 300 specimens per sample were counted.

Bibliographic references for the calcareous nannofossils

are given in Perch-Nielsen (1985) and Bown (1998).

Nannofossil preparations are housed in the Institut fur

Geologie, Mineralogie und Geophysik at the Ruhr

University of Bochum.

Diversity indices have been calculated from every

sample, i.e. species richness (S) and Shannon index (H).

Equitability (E) was calculated from the Shannon index.

Low values for both the Shannon index and equitability,

caused by the dominance of only a few species in an

assemblage, indicate unstable meso- to eutrophic con-

ditions (following r-selection), whereas high values

represent more stable and oligotrophic conditions,

following k-selection (e.g. Watkins 1989; Dodd &

Stanton 1990).

The state of preservation is described following the

outline of Roth & Thierstein (1972): E0 � very good

preservation, no etching, E1 � slightly etched, E2 �moderately etched, E3 � heavily etched, E4 � no

coccoliths preserved, O1 � overgrowth.

Stable isotopes

A total of 182 bulk samples from cores ODP 638 (115

samples) and BGS 81/43 (67 samples) were analysed for

stable oxygen and carbon isotopes. Each sample has

been prepared using standard techniques (e.g. Hoefs

1987) and was measured using a Finnigan MAT 251

mass spectrometer at the Leibniz-Labor fur Altersbes-

timmung und Isotopenforschung in Kiel. The reprodu-

cibility of replicate samples was better than 0.03� for

oxygen isotopes and 0.02� for carbon isotopes. The

isotope data are expressed as relative differences in

isotopic ratios (18O/16O, 13C/12C) between a sample and

the Vienna-PDB standard. Isotope data are given using

the usual delta-notation �. For Site 535 stable isotope

data have been taken from Cottilion & Rio (1984). The

isotope data have been smoothed (weighted harmonic

mean method) in order to show the general trend.

Results

Calcareous nannofossils

Nannofossil preservation in the samples from DSDP Site

535 indicates a broad distribution ranging from well

preserved (E0) to strongly etched (E3). Only 46 of the

examined 197 samples yielded well preserved nanno-

fossils and have been assigned to the categories E0 and

E1. These samples are restricted to marly limestones and

can be used to record ecologically driven changes in the

nannofossil assemblages. Most of the remaining 151

samples have been derived from limestones and are

characterized by moderately dissolved nannofossils.

These were taken solely for biostratigraphic investiga-

tions and thus provide a high resolution biostratigraphic

framework for this core. Calcareous nannofossils in

most samples from ODP Site 638 are well preserved and

fall into category E0. No etching or overgrowth has been

observed. From a total of 115 samples, 38 samples

comprised moderately to strongly etched coccoliths and

yielded exceptionally high percentages of dissolution-

resistant placoliths. These samples were not included in

our interpretations. A similar trend has been observed in

the BGS borehole 81/43. Only 9 out of 67 samples are

affected by dissolution, 58 samples contain an extra-

ordinarily well preserved nannoflora (category E0)

160 K. Kessels et al. LETHAIA 39 (2006)

without any evidence of etching or overgrowth. Higher

abundances of small and delicate dissolution susceptible

species are typical for most of these well preserved

samples.

At Site 535 (Fig. 3) the number of species generally

varies in the well preserved samples from 28 to 40

without significant variations in the Valanginian and

early Hauterivian. The best preserved samples have a

maximum of 47 species, overall 75 species have been

differentiated. The average of 2.5 for the Shannon index

and 0.7 for Equitability, show relativley high values and

remain fairly stable over the studied intervals. Calcula-

tions of total absolute abundance give relatively constant

values around 1E�/9 Ind/g Sed. with only minor

fluctuations.

Species richness in the well preserved samples of Site

638 (Fig. 4) is more differentiated and increases from 35

to 40 species per sample in the lower Valanginian up to

40 to 50 species in the lower Hauterivian. Altogether 87

species were identified. Calculations of diversity indices

5 1E+8 2E+8 2E+8150 4E+8 1E+82E+8 0 10 010 3020 40 60

.H

L.

nainig

nalaV

reppU

L. V

.

4020 1E+9 2E+81E+8 5 2E+91E+910 300 0

DSDP 535

[%] [Ind./gSed.] [%] [Ind./gSed.]

B. constans/Zeugrhabdotus spp.W. barnesae/W. fossacinctaspecies richnessAge

[%] [Ind./gSed.] [%] [Ind./gSed.] [Ind./gSed.]

Total abundancesC. margereliiD. lehmanii

[%] [Ind./gSed.]

Nannoconus spp.

Fig. 3. Distribution curves and species richness for selected calcareous nannofossil taxa from DSDP 535. For each taxon relative abundances (leftcurve, white dots) and absolute abundances (right curve, grey dots) have been plotted.

1E+80 10 20 0 1E+9 2E+8 4E+8 0 1E+80 3E+820 4010 20 30 1 2 2 42 4 6 8E+94E+9

Upp

er V

alan

gini

anL

ower

Val

angi

nian

4020 2E+91E+9 2E+91E+90 0

ODP 638

[%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [Ind./gSed.]

Total abundancesNannoconus spp.Micrantholitus spp.D. lehmaniiC. salebrosumB. constans/Zeugrhabdotus spp.W. barnesae/W. fossacinctaspecies richnessAge

Low

er H

aute

rivi

an

Fig. 4. Distribution curves and species richness for selected calcareous nannofossil taxa from ODP 638. For each taxon relative abundance (left curve,white dots) and absolute abundance (right curve, grey dots) have been plotted.

LETHAIA 39 (2006) Early Cretaceous calcareous nannofossils and isotopes of the northern hemisphere 161

show the same trend as in Site 535 with constant values

of around 2.5 (Shannon index) and 0.7 (Equitability). In

contrast variations of total absolute abundance indicate

a distinct tendency. In the Valanginian abundances of

nannofossils are relativly low (1E�/9 Ind/g Sed.), they

steadily increase from the uppermost Valanginian to the

lower Hauterivian up to 5 to 7E�/9 Ind/g Sed.

The samples of BGS 81/43 borehole (Fig. 5) yielded a

total of 74 species. Species richness varies between 25

and 44 species per sample and exhibits some minor

variations. It decreases from the lower Valanginian

(mean of 35 species) to the lower Hauterivian (up to

25 species), increases in the lower upper Hauterivian

(up to 35 species) and shows strong fluctuations (from

25 to 44 species) in the upper Hauterivian. The Shannon

index and Equitability are stable in this core stable with

values between 2.0�2.5 and 0.6�0.8. The investigated

interval is marked by characteristic changes of absolute

abundances which vary between 1E�/9 and 6E�/9 Ind/g

Sed. Following a peak of 5E�/9 Ind/g Sed. in the lower

Valanginian it decreases to 1E�/9 Ind/g Sed. in the

Upper Valanginian. The lower and upper Hauterivian

are characterised by a continous increase to 6E�/9 Ind/g

Sed. This increase is interrupted by another decrease to

1E�/9 Ind/g Sed. in the upper Hauterivian.

Since all three cores show some general trends

throughout the Valanginian to lowermost Hauterivian

species richness was not controlled by latitude. Highest

numbers of species (up to 50) are clearly confined to the

open marine setting (ODP 638), whereas lower numbers

of species (at an average of 25�35) seem to be typical for

more restricted marginal settings such as the Gulf of

Mexico (DSDP 535) and the North Sea Basin (BGS 81/

43). The Shannon index (2.5 averaged) and Equitability

(0.7 averaged) remain rather consistent througout the

whole Valanginian in all of the three cores and indicate

stable conditions for this interval. Changes of absolute

abundance are only obvious at Sites 638 and 81/43. At

Site 638 a continuous increase from 1E�/9 Ind/g Sed. up

to 5E�/1 Ind/g Sed. was recognised in the uppermost

Valanginian. BGS core 81/43 reveal, however, an increase

to 5E�/9 Ind/g Sed. in the lower Valanginian and a

decrease to 1E�/9 Ind/g Sed. in the upper Valanginian to

lower Hauterivian.

The distribution pattern of selected nannofossil taxa

of the examined cores is given in the following:

DSDP Site 535

Calcareous nannofossils at Site 535 are dominated by

cosmopolitan taxa which make up 80% of the assem-

blage (Fig. 3). The most common species include

Watznaueria barnesae and Watznaueria fossacincta

(18�77%). Biscutum constans , Zeugrhabdotus spp. (2�28%), Diazomatolithus lehmanii (0�14%), Cyclagelo-

sphaera margerelii (1�11%) and Discorhabdus rotatorius

(0�13%). Furthermore, high abundances of nannoco-

nids have been observed through certain intervals.

However, the occurrence of nannoconids is character-

ized by strong fluctuations in their abundance (0�32%).

High abundances of B. constans and Zeugrhabdotus spp.

were only recognized in the lower upper Valanginian

(up to 28%), coinciding with high abundances of

nannoconids. Otherwise the most common genus

Watznaueria shows the lowest abundances (18�30%)

during this interval and steadily increases up to an

average of 45% in the Lower Hauterivian. W. barnesae /

W. fossacincta and B. constans /Zeugrhabdotus spp. show

a negative correlation.

ODP Site 638

The assemblage composition at Site 638 is similar to that

of Site 535, with some minor differences (Fig. 4). W.

barnesae and W. fossacincta comprise only 13�38% of

the total abundance with a decreasing trend from the

lower Valanginian to the lower Hauterivian. B. constans

and Zeugrhabdotus spp., the most common nannofossil

group at Site 638, make up between 16 and 45%. This

group is negatively correlated to W. barnesae /W. fossa-

cincta . Other common taxa, which are constantly

present, include D. lehmanii (1�22%), D. rotatorius

(1�10%), Rhagodiscus asper (2�9%) and Staurolithites

crux (1�7%). It is interesting to note, that nannoconids

are almost totally absent in the Valanginian and

suddenly occur in the uppermost Valanginian and lower

Hauterivian where they make up to 4% of all nanno-

fossils. The same distribution has been observed for

Micrantolithus spp., which never exceeds 1% in the

Valanginian, but increases up to 8% in the lower

Hauterivian. This general trend correlates well with

increases of the carbonate content and total absolute

abundance of coccoliths throughout the same interval.

Remarkable is the occurrence of Crucibiscutum sale-

brosum . It appears rarely throughout two well defined

intervals in the upper lower Valanginian and the lower

Hauterivian. Here it makes up between 1�2% of the

whole assemblage.

BGS borehole 81/43

The recorded nannoflora of the BGS 81/43 borehole

shows some similarities to the other two cores (Fig. 5).

The cosmopolitan taxa W. barnesae/W. fossacincta (12�62%) and the fertility group B. constans /Zeugrhabdotus

spp. (5�37%) are the most abundant nannofossils. W.

barnesae /W. fossacincta decreases towards the upper

Hauterivian to 10% whereas B. constans /Zeugrhabdotus

spp. remains fairly stable over the whole interval with a

slight increase in the upper Hauterivian. W. barnesae /W.

162 K. Kessels et al. LETHAIA 39 (2006)

42 2 4 5E+9

Upp

er H

aute

rivi

anL

ower

Hau

teri

v..V

.L

2 1E+94020 6 10

.V.

U

5 10402020 40 10 206 20

BGS 81/43

4E+82E+8 0 1E+8 2E+8 1E+8 2E+82E+91E+91E+9 1E+9 0 1E+92E+8

[%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [%] [Ind./gSed.] [Ind./gSed.]

Total abundancesR. asperC. geometricumA. infracretaceaMicrantholitus spp.D. lehmaniiC. salebrosumB. constans/Zeugrhabdotus spp.W. barnesae/W. fossacinctaspecies richnessAge

2E+90 0 0 4E+8 0 0 0

Fig. 5. Distribution curves and species richness for selected calcareous nannofossil taxa from BGS 81/43. For each taxon relative abundance (left curve, white dots) and absolute abundances (right curve, greydots) have been plotted.

LE

TH

AIA

39

(20

06

)E

arly

Creta

ceous

calca

reous

na

nn

ofossilsa

nd

isotopes

ofth

en

orthern

hem

isph

ere1

63

fossacincta and B. constans /Zeugrhabdotus spp. show a

negative correlation. The distribution of C. salebrosum

seems to follow the same pattern as in Site 638 with

highest abundances (max. 15%) in the upper lower

Valanginian and the lower Hauterivian. Another abun-

dant taxon is R. asper, which reaches 1�30% of the total

abundance. D. lehmanii and D. rotatorius are not as

common as at Sites 535 and 638 and range between 0�10%. Less common coccoliths with lower abundances

include Corollithion ellipticum (1�22%) and Tegula-

lithus septentrionalis (1�8%) as well as S. crux (1�10%),

Assipetra infracretacea (0�7%) and the nannolith Mi-

crantolithus spp. (0�8%). Nannoconids have not been

observed in this core.

Stable isotopes

Oxygen isotope varation (Fig. 6) at Site 535 shows

minor fluctuations in the lower Valanginian and the

lower upper Valanginian between �/4.5� and �/2.5�.

Values gradually increase in the upper Valanginian up to

�/2.0�. In contrast the lower Hauterivian is character-

ized by a decrease to �/4�. However, only three

samples have been analysed from this interval. More

consistent is the d18O record at Site 638 with values

around �/2� in the lower Valanginian and a slight

increase up to �/1� in the lowermost Hauterivian.

In the upper lower Hauterivian a decrease to �/2�was noted. The BGS borehole shows fluctuations only

in the lower Valanginian which vary between �/5� and

�/1.5�. The upper Valanginian to upper Hauterivian

was clearly marked by constant values around �/2�.

The carbon isotope composition of the three cores

differs. The late Valanginian positive d13C excursion was

clearly recognized at Site 535 exhibiting an increase from

1� to 2.5� in the lower upper Valanginian. This

excursion is less obvious in Core 638 where it is

overshadowed by a decrease in the d13C record. In

contrast carbon isotopes of the BGS borehole are fairly

constant around 2� with only two decreases in the

lower Valanginian and upper Hauterivian.

Discussion

In order to avoid dissolution signal our data are purely

based on samples with well preserved coccoliths, which

cover the preservation states E0 and E1. Variation in the

composition of calcareous nannofossil should therefore

reflect changes of the autecological and the palaeocea-

nographic parameters.

Distribution of Watznaueria barnesae/Watznaueriafossacincta

In all three cores Watznaueria is the most common

genus in most samples, ocassionally increasing up to

70% of the total abundance. Recently, two ecological

preferences have been attributed to the genus Watz-

naueria : Mutterlose (1992) considered this genus to be a

18Obulk

[‰ vs. PDB]

13Cbulk

[‰ vs. PDB]

18Obulk

[‰ vs. PDB]

13Cbulk

[‰ vs. PDB]

13Cbulk

[‰ vs. PDB]

18Obulk

[‰ vs. PDB]

BGS 81/4340°N

DSDP 53517°N

ODP 63825°N

45

55

65

-2 0 0 2-2-4

260

310

360

410

460

510

-2 0 0 2-2-4

510

560

610

-2 0 0 2-2-4

Low

er H

aute

rivi

anU

pper

Val

angi

nian

Low

er V

alan

gini

an

L.H

aute

riv.

Upp

er V

alan

gini

anL

. Val

angi

nain

Upp

er H

aute

rivi

anL

. Hau

teri

v.L

. Val

ang.

. V.

U

Fig. 6. Isotope data (d18O, d13C) from DSDP 535, ODP 638 and BGS 81/43.

164 K. Kessels et al. LETHAIA 39 (2006)

cosmopolitan and eurytropic nannofossil with broad

ecological tolerances. The genus was shown to be

relativley independent of specific environments and

hence being one of the first nannofossils to populate

new habitats. This view is supported by Thomsen (1989)

who reported monogeneric Watznaueria assemblages,

which dominate the lowermost part of cyclic nanno-

fossil successions in the Munk Marl of the North Sea

Basin and thus apparently are the first group of

calcareous nannofossils to bloom in that habitat. Other

studies (e.g. Roth & Krumbach 1986; Premoli Silva et al .

1989 and Williams & Bralower 1995) postulate that

Watznaueria is indicative for oligotrophic conditions.

This is based on its inverse correlation to B. constans and

Zeugrhabdotus spp. Mutterlose & Kessels (2000) inves-

tigated samples of Valanginian�Barremian age from the

Barents Sea and the Norwegian shelf. Exceptionally high

abundances of Watznaueria spp. in well preserved

samples coincide with extremely low percentages (up

to 4%) of B. constans and Z. erectus . Even more

distinctive are the results from Kessels et al . (2003),

where the Watznaueria group is gradually displaced by

Z. erectus and B. constans during phases of increased

eutrophication of the Upper Jurassic Volga basin. Our

results support this view. In all three cores W. barnesae /

W. fossacincta show a negative correlation to B.

constans /Zeugrhabdotus spp.

Our results apparently give rise to the assumption

that W. barnesae/W. fossacincta generally seem to be the

most tolerant nannofossil taxa in the Mesozoic but

supposedly tend to flourish under more stable and

oligotrophic conditions, however without being re-

stricted to specific pelagic settings.

Distribution of Biscutum constans andZeugrhabdotus spp.

Higher abundances of B. constans and Zeugrhabdotus

spp., the second most common nannofossil group, have

been considered to indicate enhanced fertility condi-

tions of surface waters (e.g. Roth & Bowdler 1981; Roth

1986; Roth & Krumbach 1986; Watkins 1989; Erba 1987,

1989; Erba et al . 1992; Gale et al . 2000). For Zeugrhab-

dotus , this interpretation is mainly applied to the most

abundant species Z. erectus . Roth & Krumbach (1986)

and Herrle et al . (2003), however, show that species like

Zeugrhabdotus noeliae and Zeugrhabdotus trivectis are

also adapted to colder surface waters. We use high

abundances of B. constans and Zeugrhabdotus spp. (in

this study clearly dominated by Z. erectus) to be

indicative of enhanced nutrient content of the surface

waters.

Differences in the ecological preferences between B.

constans and Zeugrhabdotus spp. have often been

neglected and need therefore further discussion. Some

studies show that B. constans and Zeugrhabdotus spp.

respond to different trophic levels. Consecutive maxima

of these taxa were observed, representing different stages

of nutrient availability (Erba 1992; Erba et al . 1992;

Williams & Bralower 1995; Fisher & Hay 1999; Herrle

2002; Kessels et al . 2003; Erba 2004). Kessels et al .

(2003) have demonstrated that under certain conditions

B. constans is the only species which tends to flourish in

more eutrophic settings, whereas Z. erectus may perhaps

indicate slightly lower trophic levels. Furthermore Fisher

& Hay (1999) pointed out that in mid-Cretaceous

sediments of the Western Interior Seaway, which were

deposited in a high-fertility oceanic front environment,

B. constans also appears to be the dominant taxon.

Premoli Silva et al . 1989; Coccioni et al . 1992 and Herrle

et al . 2003 have shown that Discorhabdus rotatorius may

also be viewed as an indicator for a higher nutrient

content.

In order to recognize clear distribution patterns

within the fertility group (B. constans , Zeugrhabdotus

spp., Discorhabdus spp.) changes of relative and absolute

abundances have been compared (Fig. 7). At Site 535

higher abundances of the fertility group (�/10%) were

only recognised in the lower Valanginian and lower

upper Valanginian, representing enhanced nutrient

contents of the surface water during this time. Through-

out this interval the fertility group is cleary dominated

by B. constans , whereas Zeugrhabdotus spp. is less

common and D. rotatorius is very rare. The situation

changes from the Upper Valanginian onwards to the

Lower Hauterivian. Total abundances of the fertility

group decrease to below 10%, Zeugrhabdotus spp.

becomes the dominant taxon and D. rotatorius generally

increases up to a maximum of 13%. These findings may

indicate that under certain conditions higher trophic

levels of the surface water seem to be associated with an

increase in the fertility group as well as a dominance of

B. constans within the group.

At Site 638 B. constans , Zeugrhabdotus spp. and D.

rotatorius are common to abundant in the Valanginian

and Hauterivian and comprise up to 40% of the whole

assemblage, indicating a high fertility setting. In contrast

to DSDP Site 535 the composition of the fertility group

remains fairly stable over the whole interval with

abundances of B. constans and Zeugrhabdotus spp. of

10�20%. This pattern may be explained by the more

open oceanic environment of Site 638 with rather stable

conditions and a consistent nutrient content of the

water. At this site, river input did not seem to be the

major controlling factor.

Similar to Site 535 variations in B. constans , Zeugr-

habdotus spp. and D. rotatorius are also observed in the

BGS core 81/43. In the lower part (Valanginian to lower

Hauterivian) the fertility group is dominated by B.

constans . A sharp decrease at the lower/upper Hauter-

LETHAIA 39 (2006) Early Cretaceous calcareous nannofossils and isotopes of the northern hemisphere 165

ivian boundary interval marks the beginning of a change

in species dominance within the fertility group. Zeugr-

habdotus spp. becomes more prominent whereas B.

constans clearly decreases. A slight increase in abun-

dance has also been observed for D. rotatorius during

this interval. In this core enhanced abundances of the

fertility group occur parallel to high abundances of W.

barnesae /W. fossacincta indicating more oligotrophic

conditions. This situation differs from that of Site 535 in

one aspect. Whereas at Site 535 the dominance of

Zeugrhabdotus spp. was linked to low percentages of the

B. constans /Zeugrhabdotus spp. group, in BGS borehole

81/43 a strong dominance of Zeugrhabdotus spp.

coincides with high abundances (up to 40%) of the

fertility group. We believe that this composition may

reflect a slightly lower nutrient level in a generally high

fertility environment.

Our results show that both, relative abundances of the

fertility group (B. constans and Zeugrhabdotus spp.) and

the species dominances within this group may indicate

the trophic situation of specific oceanic settings. In

general B. constans seems to be adapted to higher

nutrient environments than Zeugrhabdotus spp., which

represents a slightly lower productivity level. The

occurrence of D. rotatorius is associated with higher

abundances of Zeugrhabdotus spp. (e.g. Herrle et al .

2003).

On the basis of these findings four different levels of

nutrification are suggested (Fig. 8):

(1) High abundance of the fertility group combined

with high dominance of B. constans and low

abundances of W. barnesae /W. fossacincta reflect

the highest nutrient content.

(2) High abundance of the fertility group combined

with high dominance of Zeugrhabdotus spp. and

low abundances of W. barnesae /W. fossacincta

represent an enhanced nutrient content of the

surface water. The nutrient content is, however,

lower than in level 1.

(3) Enhanced abundance of the fertility group com-

bined with high abundances of W. barnesae /W.

fossacincta indicates a setting with slightly in-

creased nutrient content of the surface water.

(4) Low abundance of the fertility group combined

with high abundances of W. barnesae /W. fossa-

cincta hint toward a low nutrient environment.

The isotopic signal

The carbonate fraction of all three investigated cores

mainly consists of calcareous nannofossils (�/70�80%).

In addition to predominantly well preserved complete

coccoliths most of the samples contain many accessory

fragments of broken nannofossil debris. Secondary

constituents were occasionally identified and comprise

callpionellids, pteropod fragments and planktonic for-

aminifera. Micrite and cements are rare in almost all of

the samples.

foec na ni

moD

.ppssutodbahrgue

Zfo

e cna nimo

Dsnatsnoc.

B

foec nan i

moD

. pp ss utodbah rgue

Zfo

ecn anim o

Ds na tsnoc.

B

3E+9 20 40

BGS borehole 81/43

0 2E+9

[%] [Ind./gSed.] [Ind./gSed.] [Ind./gSed.]

2E+9 5E+9 1E+93E+920 40

DSDP Site 535

3E+8 3E+9

B.constans, Zeugrhabdotus spp.,D. rotatorius

[%] [Ind./gSed.] [Ind./gSed.]

Total abundancesof all taxa

W. barnesae,W. fossacincta

[Ind./gSed.]

1E+8 1E+9 1E+9 20 40

ODP Site 638

2E+9 5E+9

[%] [Ind./gSed.] [Ind./gSed.] [Ind./gSed.]

1E+9 9E+9 1E+9 2E+91E+9

17°N 25°N 40°NB.constans, Zeugrhabdotus spp.,

D. rotatoriusTotal abundances

of all taxaW. barnesae,W. fossacincta

B.constans, Zeugrhabdotus spp.,D. rotatorius

Total abundancesof all taxa

W. barnesae,W. fossacincta

= B.constans

= Zeugrhabdotus spp.

= Discorhabdus spp.

Fig. 7. Distribution curves of B. constans , Zeugrhabdotus spp. and D. rotatorius for DSDP 535, ODP 638 and BGS 81/43. For each taxon relativeabundance (left curve) and absolute abundance (right curve) have been plotted.

166 K. Kessels et al. LETHAIA 39 (2006)

The d18O records of the Valanginian to lower

Hauterivian interval display the same trends for Sites

535 and 638. Both cores are marked by a slight increase

of d18O up to the Hauterivian. In the northern Site 638

the d18O values are in general 1� more positive

(suggesting slightly colder water temperatures) than

the equatorial Site 535. These findings in combination

with a lack of correlation between d18O and d13C and

the overall good preservation of calcareous nannofossils

let us assume that diagenetic alteration did not change

the isotopic composition at Sites 535 and 638 signifi-

cantly. The d18O signature of the northernmost BGS

borehole 81/43 shows, however, a positive correlation of

d18O and d13C. These data indicate strong fluctuations

in the lower Valanginian and fairly constant values

around �/2� for the upper Valanginian to upper

Hauterivian. Due to the good preservation of calcareous

nannofossils in this core, diagenetic alteration of the

samples is unlikely. It is more probable that fluctuations

of salinity due to freshwater influx (increase of 16O) in

the marginal North Sea basin throughout certain

intervals were responsible for an increase in d18O in

the BGS borehole.

According to our results it does not seem that

secondary diagenetic processes considerably changed

the stable isotope composition of all three cores.

General trends in the d18O variation

In order to obtain a broader picture of sea-water

temperatures in the Valanginian�Hauterivian of the

northern hemisphere we have compared our results with

datasets of other recent studies (Price et al . 2000; Van de

Schootbrugge et al . 2000; Price & Mutterlose 2004;

McArthur et al . 2004). These are all attributed to stable

isotope measurements of well preserved belemnites (Fig.

9). The isotopic composition of belemnites reflects the

isotope composition of their life habitat, i.e. a specific

water depth. Bulk rock samples yield a mixed isotope

signal of different depths. This fact, however, does not

play a significant role in the interpretation of the relative

trends.

Van de Schootbrugge et al . (2000) calculated tem-

peratures of 158C for the early Valanginian of the

Vocontian Basin (SE France) with a cooling to 118C in

the early Hauterivian. The same trend was observed by

Price et al . (2000) giving palaeotemperatures of 168C for

the early Valanginian of the Speeton section (UK) and a

decrease to 128C in the early Hauterivian. McArthur

et al . (2004), who also examined belemnites from

Speeton, confirm this cooling trend, giving palaeotem-

peratures of 118C for the base of the Hauterivian. Price

& Mutterlose (2004), who investigated belemnites from

high latitude outcrops of the Yatria River (Russia)

present temperatures of 13�158C for the early Valangi-

nian with a decrease (11�138C) to the early Hauterivian.

All these results coincide with the findings of our work.

In all three cores (DSDP 535, ODP 638, BGS 81/43) the

same trend has been observed: warmer palaeotempera-

tures for the Valanginian with a trend to lower

temperatures in the early Hauterivian. The early Cretac-

eous of the northern hemisphere was apparently

characterized by a widespread cooling phase culminat-

ing in the early Hauterivan.

Southward migration of Crucibiscutum salebrosum

A conspicuous feature of the two northern cores (Site

638, BGS borehole 81/43) is the distinctive occurrence of

C. salebrosum (Fig. 10), a species which is considered to

be adapted to cooler surface waters and being more

abundant in the high latitudes (Mutterlose 1992;

Mutterlose & Kessels 2000; Street & Bown 2000). Bipolar

distributions of C. salebrosum were observed and

possibly represent the existence of latitudinal restricted

floral belts during certain parts of the early Cretaceous,

which were presumably controlled by different surface

water temperatures. In two of the three cores, Site 638

trophicsituation

percentages ofB. constans /

Zeugrhabdotus spp.dominance of B. constans

within the fertility groupproductivity

level

4

3

2

1

percentages ofW. barnesae/W. fossacincta

cihportogilocihportue

+ +

+

Fig. 8. A proposed four-step scheme for the characterization of different fertility stages using changes in nannofossil composition.

LETHAIA 39 (2006) Early Cretaceous calcareous nannofossils and isotopes of the northern hemisphere 167

and BGS 81/43, C. salebrosum makes up to 15% of the

assemblages. In Site 535 it was not identified.

At Site 638 the occurrence of C. salebrosum is

restricted to two intervals. In the lower Valanginian to

lowermost upper Valanginian it comprises up to 1% and

in the lower Hauterivian it reaches 2.5% of the whole

assemblages. The same distribution pattern has been

observed in the BGS borehole 81/43. Two distinctive

maxima of C. salebrosum (up to 15%) are obvious for

the lower to upper Valanginian and the lower to upper

Hauterivian boundary interval. By comparing these

results with the general distribution of C. salebrosum

in the northern hemisphere during the Valanginian and

Hauterivian, some trends can be described (Fig. 11). In

the high latitudes of the Barents Sea Mutterlose &

Kessels (2000) noticed the highest abundances of C.

salebrosum (up to 48%) in the lower Valanginian and

Hauterivian. Jeremiah (2001) has examined various

boreholes of the Central North Sea and showed that

C. salebrosum reaches up to 14% in the lower Valangi-

nian and 30% in the lower Hauterivian. Our results

from BGS borehole 81/43 confirm these findings, with

slightly lower percentages for both intervals, up to 8% in

the lower Valanginian and up to 15% in the lower

Hauterivian. At Site 638, C. salebrosum was very rare

except for two intervals in the lower Valanginian (up to

1%) and the lower Hauterivian (up to 2.5%).

This distinctive distribution pattern of C. salebrosum

(increase in abundance during two well defined inter-

vals) cannot be explained by a global sea-level rise

during the Valanginian and lower Hauterivian. A sea-

level rise alone should cause a general homogenization

of the composition of calcareous nannofossils, it should

not lead to the migration of one species only. Cooling

phases must have led to a southward migration of C.

salebrosum into lower latitudes. Our idea of an expan-

sion of high latitudinal nannofossil taxa during cool

periods has been supported by the results of Melinte &

Mutterlose (2001). These authors, who observed a

‘nannofossil excursion’ (including rare C. salebrosum

-1 0 1-1 0 1

-2 -1 0

-4 -3 -2

-3 -2 -1

DSDP 535

Tethys

BGS81/43

Africa

Europe

Speeton**

Vocontian Basin°

ODP 638

NC4B

NC4A

NK3B

NK3A

NN

AIIG

NAL

AV

.LN

AINI

GN

ALA

VR

EPPU

NAI

VIR

ET

UA

HR

EW

OLH

U

NAI

NIG

NA L

AV

RE

WOL

NAI

VIR

ET

UA

HR

EW

OL

18Obulk rock

[‰ vs. PDB]

Temperature [°C]

18Obelemnites

[‰ vs. PDB] Temperature [°C]

18Obelemnites

[‰ vs. PDB]

18Obulk rock

[‰ vs. PDB]

NC4B

NC4A

NK3B

NK3A

NAI

NIG

NAL

AV

.LN

AiNI

GN

ALA

VR

E PPU

.TU

AH

. L

.N

A LA

V.L.V I

RE

TU

AH

.LN

A IVI

RE

TU

AH

REPP

U.V .

U

18Obulk rock

[‰ vs. PDB]

Ytria River°°

17°N

25°N

40°N

40°N

55°N

relative temperature

cool warm

relative temperature

cool warm

relative temperature

cool warm

8 10 12 14 16

NAI

NIG

NAL

AV

RE

WOL

NAi

NIG

NAL

AV

REP P

U.T

UA

H.L

6 10 14 18

Fig. 9. Temperature curves for the Valanginian to Hauterivian interval of the northern hemisphere. 8Van de Schootbrugge et al . 2000: EarlyValanginian: 158C, Early Hauterivian: 118C; **Isotope data Speeton from Price et al . 2000; Additional isotope data from Speeton give 118C for theearly Hauterivian (McArthur et al . 2004); Isotope data DSDP 535 from Cottilion & Rio 1984; 88Isotope data Yatria River section from Price &Mutterlose (subm.); Palaeogeography modified after ODSN, 2003; Nannofossil Zones after Roth (1983), Bralower et al . (1989), Bralower et al . (1993);relative temperature trends calculated after Epstein et al . (1953), Craig (1965) and Anderson & Arthur (1983); the range of temperatures (48C) wasfollowed by the use of two different V-SMOW values: �/0.2� (today’s ocean) and �/1.0� (ice-free world).

168 K. Kessels et al. LETHAIA 39 (2006)

and other boreal restricted taxa) from the Boreal Realm

into the Tethys during the early late Valanginian, explain

this migration by a combination of climatic control and

sea-level change.

Evidently calcareous nannofossils seem to have had

the ability to fluctuate spatially in response to changes of

water temperature or other autecological parameters.

Therefore we assume a cooling phase for the early

Valanginian and the early Hauterivian indicated by the

southward migration of C. salebrosum . This was inter-

rupted by a warmer episode in the late Valanginian.

Further evidence for this early Valanginian and early

Hauterivian cooling is supported by findings of glendo-

nites in early Cretaceous sediments (Kaplan 1978;

Kemper 1987), the occurrence of ice-rifted deposits in

Siberia, Australia and Spitzbergen (Frakes & Francis

1988; Frakes et al . 1992; Alley & Frakes 2003) and

evidences from isotopic studies (Weissert & Lini 1991;

Podlaha et al . 1998; Price et al . 2000; Puceat et al . 2003;

Price & Mutterlose 2004).

We believe that the bipolar distribution of C. sale-

brosum and the temporarity limited extension of its

habitats during certain intervals are a result of a more

variable climate of the early Cretaceous in particular by

cooling phases.

Conclusions

The observed changes in the calcareous nannofossil

record provide new information about the ecological

strategies and preferences of some taxa in the Valangi-

nian and Hauterivian. Both the distribution of the cold

water species C. salebrosum and the isotopic signature

support the idea of a more differentiated climate for

parts of the early Cretaceous:

(1) The most abundant species of the Cretaceous

period, W. barnesae and W. fossacincta seem to be

adapted to more oligotrophic environments. In

all of the examined boreholes a clear negative

correlation to the fertility indicators B. constans /

Zeugrhabdotus spp. is indicated.

(2) We believe that both total abundance of the

fertility group (B. constans / Zeugrhabdotus spp.)

and species dominance within this group may

indicate the trophic level of an oceanic environ-

9E+98E+7

210

310

410

510

31 4E+7 5E+9

C. salebrosum absolute abundances

[%] [Ind./gSed.] [Ind./gSed.]

35

45

65

5E+92E+9

C. salebrosum absolute abundances

[%] [Ind./gSed.] [Ind./gSed.]

ODP 638: BGS 81/43:

.viretuaH-.

O. viretua

H-.U

.nignalaV-.

O.nignala

V-.U

sairreB

FO L. bolli

FO E. striatus

FO E. windii

FO T. septentrionalis/ P. plethotretus

FO E. striatus

FO E. windii

16 1E+98 0.5E+9

m m

Fig. 10. Correlation of increased abundance of C. salebrosum in cores ODP 638 and BGS 81/43.

LETHAIA 39 (2006) Early Cretaceous calcareous nannofossils and isotopes of the northern hemisphere 169

ment. On the basis of our findings four different

settings can be distinguished representing a high

eutrophic, enhanced eutrophic, mesotrophic and

oligotrophic environment.

(3) The calculation of palaeotemperatures from

stable isotope measurements of bulk rock (this

work) and belemnite samples (literature data)

show a distinctive trend. A general decrease of

temperatures towards the lower Hauterivian is

obvious both from temperatures calculated from

belemnites and those calculated from bulk rock

samples, reflecting a possible cooling phase at this

time.

(4) A distinctive southward migration of the en-

demic cold water species C. salebrosum into

lower latitudes in the late early Valanginian and

the early Hauterivian apparently indicates peri-

ods of climatic cooling within the northern

hemisphere.

Acknowledgements. � Financial support by the Deutsche Forschungs-

gemeinschaft (Mu 667/19-1) is gratefully acknowledged. Stable

isotopes were kindly measured by H. Erlenkeuser at the Leibniz-Labor

fur Altersbestimmung und Isotopenforschung in Kiel. Helpful com-

ments were given by J. O. Herrle (Southampton) and T. Steuber

(Bochum). G. Esmay (Lamont Doherty Earth Observatory) and G.

Tulloch (British Geological Survey) are thanked for their help in

sample procuration. E. Sheldon and an anonymous reviewer improvedan earlier version of this study by useful discussions.

References

Alley, N.F. & Frakes, L.A. 2003: First known Cretaceous glaciation:Livingston Tillite Member of the Cadna-owie Formation, SouthAustralia. Australian Journal of Earth Sciences 50 , 139�144.

Anderson, T.F. & Arthur, M.A. 1983: Stable isotopes of oxygen andcarbon and their application to sedimentologic and environmentalproblems. In Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J. &Land, L.S. (eds): Stable isotopes in Sedimentary Geology. SEPMShort Course Notes 10 , 1.1�1.151.

Berggren, W.A. & Hollister, C.D. 1977: Plate tectonics and Paleocircu-lation � Commotion in the Oceanographic. Tectonophysics 3, 11�48.

Bersezio, R., Erba, E., Gorza, M. & Riva, A. 2002: Berriasian�Aptianblack shales of the Maiolica formation (Lombardian Basin, SouthernAlps, Northern Italy): local to global events. Palaeogeography,Palaeoclimatography, Palaeoecology 180 , 253�275.

Boillot, G., Winterer, E.L., Meyer, A.W. et al . 1987: Proceedings of theOcean Drilling Program, Initial Reports 103 , 663 pp.

Bown, P.R. (ed.) 1998: Calcareous Nannofossil Biostratigraphy . 314 pp.Chapman & Hall, London.

Bown, P.R., Rutledge, D.C., Crux, J.A. & Gallagher, L.T. 1998: LowerCretaceous. In Bown, P.R. (ed.): Calcareous Nannofossil Biostrati-graphy, 86�131. Chapman & Hall, London.

Bralower, T.J., Monechi, S. & Thierstein, H.R. 1989: Calcareousnannofossil zonation of the Jurassic�Cretaceous Boundary Intervaland Correlation with the Geomagnetic Polarity Timescale. MarineMicropaleontology 14 , 153�235.

Bralower T.J., Sliter, W.V., Arthur, M.A., Lekkie, R.M., Allard, D. &Schlanger, S.O. 1993: Dysoxic/Anoxic Episodes in the Aptian-Albian(Early Cretaceous). In Pringle, M.S., Sager, W.W., Sliter, W.V. &Stein, S. (eds): The Mesozoic Pacific: Geology, Tectonics and Volcanism77, 5�37. Geophysical Monograph, American Geophysical Union.

Brand, L.E. 1994: Physiological ecology of marine coccolithophores. InWinter, A. & Siesser, W.G. (eds): Coccolithophores , 39�50. Cam-bridge University Press, Cambridge.

Buffler R.T., Schlager, W. et al . 1984: Initial Reports. DSDP, 77. 747 pp.US Government. Printing Office, Washington.

Coccioni, R., Erba, E. & Premoli-Silva, I. 1992: Barremian�Aptiancalcareous plankton biostratigraphy from the Gorgo Cerbara section(Marche, central Italy) and implications for plankton evolution.Cretaceous Research 13 , 517�537.

Cottilion, P. & Rio, M. 1984: Cyclic sedimentation in the Cretaceous ofDeep Sea Drilling project Sites 535vand 540 (Gulf of Mexico), 534(Central Atlantic), and the Vocontian Basin (France). In BufflerR.T., Schlager, W. et al . 1984: Initial Reports. DSDP, 77, 339�377.US Government. Printing Office, Washington.

Craig, H. 1965: The measurement of oxygen isotope palaeotempera-tures. In Tongiorgi, E. (ed.): Stable Isotopes in Oceanographic Studiesand Palaeotemperatures , 161�182. Consiglio Nazionale delle Ri-cherche, Laboratorio di Geologia Nucleare, Pisa.

Dodd, J.R. & Stanton, R.J. 1990: Paleoecology � Concepts andApplications . 502 pp. Wiley & Sons, New York.

Epstein, S., Buchsbaum, R., Lowenstam, H.A. & Urey, H.C. 1953:Revised carbonate-water isotopic temperature scale. GeologicalSociety of America Bulletin 64 , 1315�1326.

Erba, E. 1987: Mid-Cretaceous cyclic pelagic facies from the Umbrian-Marchean Basin: what do calcareous nannofossils suggest? INANewsletters 9 , 52�53.

Erba, E. 1989: Upper Jurassic to Lower Cretaceous Nannoconusdistribution in some sections from Northern to Central Italy.Memorie di Scienze Geologiche 41 , Universitadi Padova , 255�261.

Erba, E. 1992: Middle Cretaceous calcareous nannofossils from theWestern Pacific (Leg 129): Evidence for paleoequatorial crossings. InLarson, R.L. & Lancelot, Y. et al . (eds): Proceedings of the OceanDrilling Program, Scientific Results 129 , 189�201.

Africa

Tethys638

81/43

Boreal influx in thelower upper Valanginian(Melinte & Mutterlose, 2001)

C. salebrosum max.48% in lowerValanginian and Hauterivian (Mutterlose & Kessels, 2000)

C. salebrosum peaks up to 8% in lower upperValanginian and 16% in lower Hauterivian

C. salebrosum peaks up to 1%in lower upper Valanginian and2% in lower Hauter ivian

C. salebrosum peaks up to 14%in upper lower Valanginianand 30% in lower Hauterivian (Jeremiah, 2001)

Fig. 11. Southward migration of C. salebrosum during short periods ofclimatic cooling in the northern hemisphere (Palaeogeography mod-ified after ODSN Plate Tectonic Reconstruction Service, 2003;www.osdn.de).

170 K. Kessels et al. LETHAIA 39 (2006)

Erba, E. 1994: Nannofossils and superplumes: The early Aptian‘nannoconid crisis’. Paleoceanography 9 , 483�501.

Erba, E. 2004: Calcareous nannofossils and Mesozoic oceanic anoxicevents. Marine Micropaleontology 52 , 85�106.

Erba, E. & Tremolada, F. 2004: Nannofossil carbonate fluxes during theEarly Cretaceous: Phytoplankton response to nutrification episodes,atmospheric CO2, and anoxia. Paleoceanography 19 , 1�18.

Erba, E., Castradori, D., Guasti, G. & Ripepe, M. 1992: Calcareousnannofossils and Milankovitch cycles: the example of the AlbianGault Clay Formation (southern England). Palaeogeography, Palaeo-climatography, Palaeoecology 93 , 47�69.

Eshet, Y. & Almogi-Labin, A. 1996: Calcareous nannofossils aspalaeoproductivity indicators in Upper Cretaceous organic-richsequences in Israel. Marine Micropalaeontology 26 , 37�61.

Fisher, C. & Hay, W. 1999: Calcareous nannofossils as indicators ofmid-Cretaceous paleofertility along an ocean front, U.S. WesternInterior. In Barrera, E. & Johnson, C.C. (eds): Evolution of theCretaceous Ocean-Climate system. Geological Society of America,Special Paper 332 , 161�180.

Frakes, L.A. & Francis, J.E. 1988: A guide to Phanerozoic cold polarclimates from high latitude ice-rafting in the Cretaceous. Nature333 , 547�549.

Frakes, L.A., Francis, J.E. & Syktus, J.I. 1992: Climate Modes of thePhanerozoic . 274 pp. Cambridge University Press, Cambridge.

Gale, A. S., Smith, A. B., Monks, N. E. A., Young, J. A., Howard, A.,Wray, D.S. & Huggett, J.M. 2000: Marine biodiversity through theLate Cenomanian � Early Turonian: palaeoceanographic controlsand sequence stratigraphic biases. Journal of the Geological Society,London 157 , 745�757.

Geisen, M., Bollmann, J., Herrle, J.O., Mutterlose, J. & Young, J.R.1999: Calibration of the random settling technique for calculation ofabsolute abundances of calcareous nannoplankton. Micropalaeon-tolgy 45 , 123�138.

Hallam, A. 1992: Phanerozoic Sea-level Changes . 266 pp. ColumbiaUniversity Press, New York.

Haq, B.U., Hardenbol, J. & Vail, P.R. 1988: Mesozoic and Cenozoicchronostratigraphy and cycles of sea-level change. SEPM 42 , 71�108.

Hardenbol, J., Thierry, J., Farley, M.B., Jacquin, T., de Graciansky, P.C.& Vail, P.R. 1998: Mesozoic and Cenozoic sequence chronostrati-graphic framework of European basins. Journal of the Society ofSedimentological Geology , 7�14.

Herrle, J.O. 2002: Mid-Cretaceous paleoceanographic and paleoclima-tologic implications on black shale formation of the Vocontian Basinand Atlantic. Evidence from calcareous nannofossils and stableisotopes. Tubinger Mikropalaontologische Mitteilungen 27 , 114 pp.

Herrle, J.O., Pross, J., Friedrich, O., Koßler, P. & Hemleben, C. 2003:Forcing mechanisms for mid-Cretaceous black shale formation:evidence from the Upper Aptian and Lower Albian of the VocontianBasin (SE France). Palaeogeography, Palaeoclimatology, Palaeoecology190 , 399�426.

Hoefs, J. 1987: Stable Isotope Geochemistry. 241 pp. Springer Verlag,Berlin/Heidelberg.

Jeremiah, J. 2001: A Lower Cretaceous nannofossil zonation for theNorth Sea Basin. Journal of Micropalaeontology 20 , 45�80.

Kaplan, M.E. 1978: Calcite pseudomorphoses in Jurassic and LowerCretaceous deposits of the Northern Area of Eastern Siberia.Geologiya I Geofizika 19 , 62�70.

Kemper, E. 1987: Das Klima der Kreide-Zeit. Geologisches Jahrbuch. A96 , 5�185.

Kemper, E., Rawson, P.F. & Thieuloy, J.P. 1981: Ammonites of Tethyanancestry in the early Lower Cretaceous of northwest Europe.Palaeontology 24 , 251�311.

Kessels, K., Mutterlose, J. & Ruffell, A. 2003: Calcareous nannofossilsfrom late Jurassic sediments of the Volga Basin (Russian Platform):evidence for productivity controlled black shale deposition. Inter-national Journal of Earth Science 92 , 743�757.

Larson, R.L. 1991a: Latest pulse of Earth: Evidence for a mid-Cretaceous superplume. Geology 19 , 547�550.

Larson, R.L. 1991b: Geological consequences of superplumes. Geology19 , 963�966.

Lott, G.K., Fletcher, B.N. & Wilkinson, I.P. 1986: The stratigraphy ofthe Lower Cretaceous Speeton Clay Formation in a cored borehole

off the coast of north-east England. Proceedings of the YorkshireGeological Society 46 , 39�56.

McArthur, J.M., Mutterlose, J., Price, G.D., Rawson, P.F., Ruffell, A. &Thirlwall, M.F. 2004: Belemnites of Valanginian, Hauterivian andBarremian age: Sr-isotope stratigraphy, composition (87Sr/86Sr, d13C,d18O, Na, Sr, Mg), and palaeooceanography. Palaeogeography,Palaeoclimatology, Palaeoecology 202 , 253�272.

McIntyre, A. & Be, A.W.H. 1967: Modern Pacific Coccolithophorida: Apaleontological thermometer. Transactions of New York Academy ofScience 32 , 720�731.

McIntyre, A., Allen, W.H. & Roche, N.B. 1970: Modern Coccolitho-phoridae of the Atlantic Ocean-I. Placoliths and Cyrtoliths. Deep SeaResearch 14 , 561�597.

Melinte, M. & Mutterlose, J. 2001: A Valanginian (Early Cretaceous),Boreal nannoplankton excursion in sections from Romania. MarineMicropalaeontology 43 , 1�25.

Mutterlose, J. 1991: Das Verteilungs- und Migrationsmuster deskalkigen Nannoplanktons in der borealen Unterkreide (Valangin�Apt) NW-Deutschlands. Palaeontographica B 221 , 27�152.

Mutterlose, J. 1992: Biostratigraphy and palaeobiogeography of EarlyCretaceous calcareous nannofossils. Cretaceous Research 13 , 167�189.

Mutterlose, J & Bockel, B. 1998: The Barremian�Aptian interval inNW Germany � A review. Cretaceous Research 19 , 539�568.

Mutterlose, J. & Kessels, K. 2000: Early Cretaceous calcareousnannofossils from high latitudes: implications for palaeobiogeogra-phy and palaeoclimate. Palaeogeography, Palaeoclimatology, Palaeoe-cology 160 , 347�372.

Mutterlose, J., Bornemann, A. & Herrle, J. 2005: Mesozoic CalcareousNannofossils � state of the art. Palaontologische Zeitschrift 79, 113�133.

Perch-Nielsen, K. 1985: Mesozoic calcareous nannofossils. In Bolli,H.M. Saunders, J.B. & Perch-Nielsen, K. (eds): Plankton Stratigra-phy, 329�426. Cambridge University Press, Cambridge.

Pittet, B. & Mattioli, E. 2002: The carbonate signal and calcareousnannofossil distribution in an upper Jurassic section (Balingen-Tieringen, Late Oxfordian, southern Germany). Palaeogeography,Palaeoclimatology, Palaeoecology 179 , 71�96.

Podlaha, O. G., Mutterlose, J. & Veizer, J. 1998: Preservation of d18Oand d13C in belemnite rostra from the Jurassic / Early Cretaceoussuccessions. American Journal of Science 298 , 324�347.

Premoli-Silva, I., Erba, E. & Tornaghi, M.E. 1989: Paleoenvironmentalsignals and changes in surface fertility in mid Cretaceous Corg. richpelagic facies of the Fucoid Marls (Central Italy). Geobois memoirespecial 11 , 225�236.

Price, D.G., Ruffell, A.H., Jones, C.E., Kalin, R.M. & Mutterlose, J.2000: Isotopic evidence for temperature variation during the earlyCretaceous (late Ryazanian � mid-Hauterivian. Journal of theGeological Society, London 157 , 335�343.

Price, G. D. & Mutterlose, J. 2004: Isotopic signals from late Jurassic-early Cretaceous (Volgian�Valanginian) subArtic Belemnites, YatriaRiver, Western Siberia. Journal of the Geological Society, London 161 ,959�968.

Puceat, E., Lecuyer, C., Sheppard, S.M.F., Dromart, G., Reboulet, A. &Grandjean, P. 2003: Thermal evolution of Cretaceous Tethyanmarine waters inferred from oxygen isotope composition of fishtooth enamels. Paleoceanography 18 , 1029.

Rawson, P.F. & Riley, L.A. 1982: Latest Jurassic � Early Cretaceousevents and the ‘late Cimmerian unconformity’ in North Sea area.Journal of the American Association of Petroleum Geologists 66 ,2628�2648.

Roth, P.H. 1983: Jurassic and Lower Cretaceous calcareous nannofos-sils in the western North Atlantic (Site 534): Biostratigraphy,preservation, and some observations on biogeography and paleo-ceanography, 587�621. In Initial Reports DSDP 76. US Govern-ment. Printing Office, Washington.

Roth, P.H. 1986: Mesozoic palaeoceanography of the North Atlanticand the Tethys Oceans. In Summerhayes, C.P. & Shackelton, N.J.(eds): SEPM Special Publication 32 , 517�546.

Roth, P.H. 1989: Ocean circulation and calcareous nannofossilevolution during the Jurassic and Cretaceous. Palaeogeography,Palaeoclimatology, Palaeoecology 74 , 111�126.

LETHAIA 39 (2006) Early Cretaceous calcareous nannofossils and isotopes of the northern hemisphere 171

Roth, P.H. & Bowdler, J.L. 1981: Middle Cretaceous calcareousnannoplankton biogeography and oceanography of the AtlanticOcean. SEPM Special Publication 32 , 517�546.

Roth, P.H. & Tierstein, H.R. 1972: Calcareous nannoplankton: Leg 14of the Deep Sea Drilling Project. In Hayes, D.E. (ed.): InitialReports. DSDP, 14, 421�485. US Government. Printing Office,Washington.

Roth, P.H. & Krumbach, K.R. 1986: Middle Cretaceous calcareousnannofossil biogeography and preservation in the Atlantic andIndian Oceans: Implications for paleoceanography. Marine Micro-palaeontology 10 , 235�266.

Ruffell, A. 1991: Sea-level events during the Early Cretaceous inwestern Europe. Cretaceous Research 12 , 527�551.

Stoll, H.M. & Schrag, D.P. 1996: Evidence for glacial control of rapidsea-level changes in the Early Cretaceous. Science 272 , 1771�1774.

Street, C. & Bown, P.R. 2000: Palaeobiogeography of Early Cretaceous(Berriasian�Barremian) calcareous nannoplankton. Marine Micro-paleontology 39 , 265�291.

Thomsen, E. 1989: Seasonal variation in boreal early Cretaceouscalcareous nannofossils. Marine Micropaleontology 15 , 123�152.

Van de Schootbrugge, B., Follmi, K.B., Bulot, L.G., & Burns, S.J. 2000:

Paleoceanographic changes during the early Cretaceous

(Valanginian�Hauterivian): evidence from oxygen and carbon

stable isotopes. Earth and Planetary Science Letters 181 , 15�31.Watkins, D.K. 1989: Nannoplankton productivity fluctuations and

rhythmically-bedded pelagic carbonates of the Greenhorn Limestone

(Upper Cretaceous). Palaeogeography, Palaeoclimatology, Palaeoecol-ogy 74 , 75�86.

Weissert, H. & Lini, A. 1991: Ice age interludes during the time of

Cretaceous greenhouse climate? In Mueller, D.W., Mckenzie, J.A. &

Weissert, H. (eds): Controversies in Modern Geology, 173�191.Academic Press, London.

Williams, J.R. & Bralower, T.J. 1995: Nannofossil assemblages, fine

fraction stable isotopes, and the paleoceanography of the

Valanginian�Barremian (Early Cretaceous) North Sea Basin. Paleo-ceanography 10 , 815�839.

Ziegler, P.A. 1988: Geological Atlas of Western and Central Europe .

239 pp. Shell Internationale Petroleum Maatschappij B. V., DenHaag.

172 K. Kessels et al. LETHAIA 39 (2006)