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A Paleogene calcareous microfossil Konservat-Lagerstätte from the Kilwa Group of coastal Tanzania

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Microfossil assemblages and their shell geochemistry are widely used in paleoceanography, but they can be significantly altered by subtle variations in preservation state. Clay-rich, hemipelagic sediments of the Paleogene Kilwa Group of coastal Tanzania host calcareous microfossils that are exceptionally preserved, as evidenced by morphological, taxonomic, and geochemical data. The planktonic foraminifera are preserved as glassy, translucent tests with original microgranular wall textures that resemble well-preserved modern specimens, and they arguably yield geochemical values that are relatively unaffected by recrystallization. The calcareous nannofossils are extraordinarily diverse and represented by unique assemblage compositions that include dissolution-susceptible taxa, especially holococcoliths and rhabdoliths, and fragile and very small (<3-μm) heterococcoliths, many of which are new taxa. Notably, the extant, deep-photic-zone taxon Gladiolithus is documented for the first time in the pre-Quaternary fossil record. The Kilwa Group calcareous nannofossil diversities are consistently higher than all coeval assemblages and provide a benchmark against which to compare other Paleogene biodiversity data. Highest diversities are preserved in hemipelagic, clay-rich lithologies and the greatest losses occur in lithified, carbonate-rich sediments. Most of the lost diversity, however, is confined to distinct taxonomic groups (holococcoliths and Syracosphaerales), and in general the preservational potential of Paleogene coccolithophores was greater than in the modern oceans because a larger proportion of the biodiversity fell within the larger size fractions. For both foraminifera and coccolithophores, incorporation into impermeable clay-rich sediments that have never been deeply buried appears to have been critical in producing this Konservat-Lagerstätte preservation.
Calcareous nannofossil images from the Paleogene Kilwa Group. Scale bars are 1 µm. (A–D) Gladiolithus fl abellatus. These images (A, C, and D) are the fi rst pre-Quaternary records of this extant, deep–photic-zone coccolithophore. A modern coccosphere (B) is provided for comparison. (A) Collapsed coccosphere comprising long tube-coccoliths and basal-disc lepidoliths (late Eocene, Sample TDP12/26-2, 62 cm). (B) Modern coccosphere from Hawaiian Ocean Time series (HOTS) station. (C and D) Collapsed Gladiolithus coccospheres (late Paleocene, Sample TDP16B/12-2, 9 cm). (E) Minuscule (<1 µm), undescribed, cup-like coccoliths with tall, hollow spines (late Eocene, Sample TDP12/23-2, 79 cm). (F) Syracosphaera sp. (late Eocene, Sample TDP12/26-2, 62 cm). (G) Clathrolithus ellipticus holococcolith (late Paleocene, Sample TDP16B/12-2, 9 cm). (H) Zygrhablithus bijugatus holococcolith (late Paleocene, Sample TDP16B/12-2, 9 cm). (I–L) Collapsed coccospheres. (I) Cruciplacolithus inseadus, a species only previously known from the Danian but found in all our SEM samples (late Paleocene, Sample TDP16B/12-2, 9 cm). (J) Neochiastozygus imbriei grouping, most likely a collapsed coccosphere, but displays signifi cant morphological variation between coccoliths, with signifi cant implications for species-level taxonomy (late Paleocene, Sample TDP14/9-1, 20 cm). (K) Calciosolenia brasiliensis displaying morphological variation between coccoliths identical to that seen in modern examples (e.g., Young et al., 2003) (middle Eocene, Sample TDP20/23-1, 40 cm). (L) Undescribed, very small (<2 µm) coccolith species (late Paleocene, Sample TDP16B/12-2, 9 cm). (M) Coccolithus pelagicus with previously undescribed, gracile central-area cross bars (late Paleocene, Sample TDP16B/12-2, 9 cm). (N) Blackites defl andrei (middle Eocene, Sample TDP13/20-1, 50 cm). (O) Blackites morionum showing an intricately constructed, hollow spine and dissolution-susceptible rim architecture (late Paleocene, Sample TDP16B/12-2, 9 cm). (P) Undescribed placolith coccolith with a fragile, central-area grill (late Paleocene, Sample TDP16B/12-2, 9 cm).
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ABSTRACT
Microfossil assemblages and their shell
geochemistry are widely used in paleocean-
ography, but they can be signifi cantly altered
by subtle variations in preservation state.
Clay-rich, hemipelagic sediments of the
Paleogene Kilwa Group of coastal Tanzania
host calcareous microfossils that are excep-
tionally preserved, as evidenced by morpho-
logical, taxonomic, and geochemical data.
The planktonic foraminifera are preserved
as glassy, translucent tests with original
microgranular wall textures that resemble
well-preserved modern specimens, and they
arguably yield geochemical values that are
relatively unaffected by recrystallization.
The calcareous nannofossils are extraordi-
narily diverse and represented by unique
assemblage compositions that include dis-
solution-susceptible taxa, especially holo-
coccoliths and rhabdoliths, and fragile
and very small (<3-µm) heterococcoliths,
many of which are new taxa. Notably, the
extant, deep–photic-zone taxon Gladiolithus
is documented for the fi rst time in the pre-
Quaternary fossil record. The Kilwa Group
calcareous nannofossil diversities are con-
sistently higher than all coeval assemblages
and provide a benchmark against which to
compare other Paleogene biodiversity data.
Highest diversities are preserved in hemipe-
lagic, clay-rich lithologies and the greatest
losses occur in lithifi ed, carbonate-rich sedi-
ments. Most of the lost diversity, however,
is confi ned to distinct taxonomic groups
(holococcoliths and Syracosphaerales), and
in general the preservational potential of
Paleogene coccolithophores was greater
than in the modern oceans because a larger
proportion of the biodiversity fell within the
larger size fractions. For both foraminifera
and coccolithophores, incorporation into
impermeable clay-rich sediments that have
never been deeply buried appears to have
been critical in producing this Konservat-
Lagerstätte preservation.
Keywords: calcareous nannofossils, foraminifera,
preservation, Lagerstätte, Paleogene, diversity.
INTRODUCTION
Konservat-Lagerstätten are extraordinary fos-
sil occurrences characterized by unusual quality
of preservation (Briggs, 2001). Most commonly,
they preserve the soft parts of animals and pro-
vide rare glimpses of the biology and biodi-
versity of ancient ecosystems. Paleontologists
studying fossils of unicellular protistan organ-
isms have rarely thought of individual deposits
as Lagerstätten because they routinely work with
stratigraphically continuous data sets, compris-
ing assemblages of many hundreds to thousands
of specimens, which are considered to equate to
relatively complete fossil records. Calcareous
microfossils, in particular, are generally robust
enough to provide stratigraphically useful data
from a wide range of lithologies and depositional
settings. There is increasing evidence, however,
that microfossil assemblages and their geochem-
ical signatures may be signifi cantly altered by
subtle or cryptic variations in preservation state
(Pearson et al., 2001, 2007; Gibbs et al., 2004;
Williams et al., 2005). One approach to assess-
ing the potential magnitude of such change
is to search for sequences with exceptional
A Paleogene calcareous microfossil Konservat-Lagerstätte from
the Kilwa Group of coastal Tanzania
P.R. Bown
T. Dunkley Jones
J.A. Lees
R.D. Randell
J.A. Mizzi
Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, England, UK
P.N. Pearson
H.K. Coxall
School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, Wales, UK
J.R. Young
Department of Palaeontology, The Natural History Museum, London SW7 5BD, England, UK
C.J. Nicholas
Department of Geology, Trinity College, Dublin 2, Ireland
A. Karega
J. Singano
Tanzania Petroleum Development Corporation, PO Box 2774, Dar-es-Salaam, Tanzania
B.S. Wade
Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA
E-mail: p.bown@ucl.ac.uk
GSA Bulletin; January/February 2008; v. 120; no. 1/2; p. 3–12; doi: 10.1130/B26261.1; 5 fi gures; 1 table.
Bown et al.
4 Geological Society of America Bulletin, January/February 2008
preservation and use them as benchmarks against
which to judge the extent of taphonomic and
geochemical alteration in other sections.
Recent attempts to seek out exceptional fora-
minifera for geochemical paleoclimate studies
have targeted clay-rich, hemipelagic sediments
(Wilson et al., 2002; Bice et al., 2003; Pearson et
al., 2007). However, the paleobiological poten-
tial of these predominantly shelf successions
that host well-preserved microfossils remains
largely unexploited. To a large extent, this is the
result of the enormous amount of stratigraphic
and paleoceanographic work that has accompa-
nied the Deep Sea Drilling Project and Ocean
Drilling Program since the late 1960s, and the
rather uniform state of preservation that is typi-
cally associated with such deep-sea chalks and
oozes. Cenozoic nannofossil study in particular
saw a slowing of taxonomic description after the
switch from largely continental-shelf research
to deep-sea studies, but the effect is less pro-
nounced in Mesozoic research, which has con-
tinued to rely on hemipelagic successions.
The aim of this paper is to exemplify excep-
tional calcareous microfossil preservation
through a description of the Paleogene Kilwa
Group of Tanzania. These sediments represent
a Konservat-Lagerstätte for calcareous micro-
fossils and provide a benchmark against which
to highlight the signifi cant effects that preser-
vation can have on both microfossil diversity
and geochemistry.
GEOLOGICAL SETTING
The Tanzania Drilling Project (TDP) is a
paleoclimate research program that has targeted
the recovery of Cretaceous-Paleogene sedi-
ments with exceptionally preserved planktonic
foraminifera through critical stratigraphic inter-
vals (Pearson et al., 2004, 2006; Nicholas et al.,
2006). Surface-exposure mapping and subsur-
face coring have focused on the Kilwa Group
(Santonian-Oligocene), which crops out along
a coastal strip of southern Tanzania around the
towns of Kilwa, Lindi, and Pande (Fig. 1). The
group comprises a roughly kilometer-thick suc-
cession of homogeneous, dark claystones, with
variably developed secondary lithologies of
siltstones, limestones, and sandstones. The clay-
stones are typically uncemented and unlithifi ed,
and have never been deeply buried. Based on
paleontological and lithological considerations,
the succession is thought to have been depos-
ited in an outer-shelf to upper-slope environ-
ment at water depths of 300–500 m (Nicholas
et al., 2006). The passive margin continental
shelf was located at around 19° S paleolatitude
in the Eocene. The shelf was relatively narrow,
and therefore the sediments also incorporate
shallower, inner-shelf components, such as
intermittent carbonate beds with larger benthic
foraminifera and terrestrial organic matter (van
Dongen et al., 2006; Nicholas et al., 2006).
The rich planktonic foraminifera and calcare-
ous nannoplankton assemblages indicate fully
marine, open-ocean conditions. Age determina-
tion of the sediments was achieved by integrated
micropaleontological analysis (nannofossils,
palynology, and foraminifers), and initial bio-
stratigraphic results are presented in Pearson et
al. (2004, 2006) and Nicholas et al. (2006).
METHODS
Calcareous microfossils were studied using
standard micropaleontological methods. Nan-
nofossils were viewed as simple smear slides
(Bown and Young, 1998), using transmitted-
light microscopy in cross-polarized and phase-
contrast light (507 samples), and on broken rock
surfaces using scanning electron microscopy
(SEM). Assemblage data were collected from
smear slides using semiquantitative and quanti-
tative count methods (Bown and Young, 1998).
Nannofossil species richness is a tally of all taxa
occurring in one nannofossil zone. Size data
were collected from digital light microscope
images sourced from all the TDP sites (e.g.,
Bown, 2005a; Bown and Dunkley Jones, 2006).
Foraminifera were prepared by gently disaggre-
gating the sediment in tap water and washing
over a 63-µm sieve. Residues were viewed using
light microscopy and SEM.
KILWA GROUP MICROFOSSILS
The Paleogene Kilwa Group sediments con-
tain planktonic foraminifera that in refl ected
light have a translucent and refl ective, glassy
appearance, comparable to well-preserved mod-
ern specimens (Fig. 2A). High-resolution SEM
1
6
23
10
9
11
12
13
14a
14b
15a
15b
15c
16
17
18
19
20
21
22
6
7
8
29
31
30
NP Nf. Zones
Early Oligocene
TDP cores
Age
Ma
O3
O2
O4
P5
E2
E1
E3
E4
E5
E6
E7
E8
E9
E10
E11
E12
E13
E14
E15
E16
O1
P4c
P4b
55
54
53
56
52
49
51
47
48
50
46
44
45
43
42
41
10
8
7
3
2
6
39
38
40
37
Middle EoceneEarly EoceneL. Paleocene
36
34
35
33
32
Late Eocene
PF Zones
12
11
14
57
58
20
16
19
17
pars
4
13
18
24
pars
N
KENYA
Pemba Is.
Zanzibar Is.
Mafia
Is.
Dar es
Salaam
Kilwa Masoko
Pande
Lindi
MOZAM-
BIQUE
TANZANIA
Main
N-S
road
Kilwa
Group
outcrop
Indian
Ocean
0 100km
0 20km
Mkazambo
A
B
Figure 1. (A) Stratigraphic extent of the
TDP boreholes. Nannofossil zonation (NP
Nf. Zones) from Martini (1971) and plank-
tonic foraminifer zonation (PF Zones) from
Berggren and Pearson (2005). Correlation of
the two plankton zonation schemes and time
scale are from the latter. (B) Location of the
Kilwa Group in coastal Tanzania. Detailed
location is given in Nicholas et al. (2006).
Tanzania microfossil Konservat-Lagerstätte
Geological Society of America Bulletin, January/February 2008 5
A
C
JI
E
H
B
D
FG
K
Figure 2. Paleogene Kilwa Group foraminifera and nannofossils exhibiting aspects of the exceptional preservational quality. Scale bars for
foraminifera are 100 µm (A, B, and E) and 10 µm (C and D) and for nannofossil images are 1 µm (F–K). (A–D) Specimens of Subbotina
velascoensis from Site TDP 7A (early Eocene), (A) in refl ected light, showing the glassy appearance and lack of any carbonate infi lling or
overgrowth. (C–D) High-resolution scanning electron micrographs, showing microgranular textures (early Eocene, Sample TDP7A/64-1,
50–65 cm). (E) Tubulogenerina sp., a previously undescribed benthic foraminifera species from Site TDP 12 (late Eocene, Sample TDP12/16-
3, 43–51 cm) exhibiting exquisite preservation of the ornate test wall. (F) Chiasmolithus bidens (late Paleocene, Sample TDP16B/12-2, 9 cm)
with a proximal perforate plate. A minute (~1-µm), undescribed, Calciosolenia coccolith (diamond-shaped) lies on the upper-right shield
surface. (G) Semihololithus biscayae (late Paleocene, Sample TDP14/9-1, 20 cm) with pristine holococcolith preservation and unfi lled cavate
structure. (H) Campylosphaera dela (late Paleocene, Sample TDP16B/12-2, 9 cm). (I) Coccosphere of an undescribed placolith coccolith
species (late Paleocene, Sample TDP14/9-1, 20 cm) with fragile, central-area grills. (J) Ellipsolithus andoluensis (late Paleocene, Sample
TDP16B/12-2, 9 cm), not previously seen in SEM and showing exquisite preservation of a perforate grill. (K) Braarudosphaera bigelowii
(late Paleocene, Sample TDP14/9-1, 20 cm) coccosphere showing laminated ultrastructure.
Bown et al.
6 Geological Society of America Bulletin, January/February 2008
reveals well-preserved microgranular wall tex-
tures, hollow chambers, and large, well-defi ned
pores (Figs. 2B–D), and specimens display
morphological features that have not previously
been observed (Fig. 2E). The exceptional pres-
ervation of the tests is also indicated by their
oxygen and carbon stable isotope values that are
signifi cantly different than those from coeval
sediments deposited at comparable latitudes,
but which are less well preserved (Pearson et
al., 2001, 2007; Stewart et al., 2004; Sexton et
al., 2006). However, the taxonomy and diver-
sity of these coeval foraminifera assemblages
are broadly comparable, indicating that the
robust shells conserve the overall composition
of assemblages despite the differences in shell
appearance, ultrastructure, and geochemistry.
In contrast to the planktonic foraminifera,
the Kilwa Group calcareous nannofossil assem-
blages are extraordinarily diverse (around 360
species in total) and have distinct taxonomic
compositions (Figs. 2 and 3). Comparative nan-
nofossil species richness data are provided in
Table 1 and Figure 4. More than a quarter of
the Paleogene nannofossil diversity is due to the
presence of holococcoliths (48 species, Figs. 2G
and 3G–H) and several diverse coccolith groups,
most notably representatives of the living fam-
ily Rhabdosphaeraceae (informally known as
rhabdoliths) (42 species, Figs. 3N–O). The
assemblages also include small and/or fragile
coccoliths (Figs. 2F and 3E–F), as well as larger
forms with delicate central-area structures,
which are usually only discernible using SEM
(Figs. 2F, 2I–J, 3M, and 3P). The small, delicate
forms include the extant genus Gladiolithus,
documented in this paper for the fi rst time in
the pre-Quaternary fossil record (Figs. 3A–D),
and the extant genera Calciosolenia and Syra-
cosphaera, which also have very poor fossil
records (Figs. 2F, 3F, and 3K).
Gladiolithus was not identifi ed until SEM
observation of rock surfaces revealed recurrent
assemblages of abundant lath-like elements
within coccolith concentrations. Almost com-
plete, collapsed coccospheres were observed,
along with entire tube coccoliths and the dis-
tinctive lepidolith coccoliths (Figs. 3A–D). The
Gladiolithus coccoliths and disaggregated ele-
ments are extremely thin, and the calcite c-axis
is oriented nearly perpendicular to the surface
of the laths, so they show very low birefrin-
gence and are thus virtually invisible in the light
microscope. Gladiolithus and Calciosolenia
are consistently present throughout the Paleo-
gene succession and are often abundant. Syra-
cosphaera is only documented in the middle
Eocene to early Oligocene part of the Kilwa
Group. Several samples have yielded concentra-
tions of Calciosolenia that appear to represent
collapsed coccospheres (Fig. 3K) showing vari-
morphism (changes in shape across the cocco-
sphere), similar to that seen in modern species
(Young et al., 2003). Coccospheres of placolith
and non-placolith taxa are relatively common in
smear slides and on rock surfaces (Figs. 2I, 2K,
3A, 3C, 3E, and 3I–L).
Other common, small coccoliths include
minuscule (<1-µm) spinose forms that are, as
yet, undescribed (Fig. 3E). They are not eas-
ily classifi ed in existing fossil groups but are
comparable to the extant Papposphaeraceae
and “narrow-rimmed muroliths” (Young et al.,
2003, p. 78), which have no previously docu-
mented fossil record. Larger coccoliths with
fragile central-area structures include new taxa
that are diffi cult to place within existing fossil
classifi cations (Figs. 2I and 3P). Well-known
species with delicate structures that have not
been previously observed (Figs. 2F, 2J, and 3M)
are also preserved. Coccolithus pelagicus speci-
mens, for example, are frequently seen with
gracile, axial cross bars (Fig. 3M), demonstrat-
ing a subtle morphological difference compared
to modern populations, where single transverse
bars are common (Young et al., 2003). Delicate
central grills are occasionally reported in other
coccolith groups, but are routinely observed in
the Tanzania material, most notably in Cyclicar-
golithus, Reticulofenestra, Chiasmolithus, and
Cruciplacolithus (Figs. 2F and 3I).
DISCUSSION
Signifi cance of Kilwa Group Microfossil
Assemblage Components
The geochemistry and visual appearance of
the planktonic foraminifera shells indicate the
exceptional quality of the larger Kilwa Group
microfossils, but it is the extraordinary diversity
and preservation of calcareous nannofossils that
substantiates the unique Konservat-Lagerstätte
status (Figs. 2–4). A component of the high nan-
nofossil diversity can be explained by the pres-
ence of broadly shelf-dwelling taxa (11%), such
as Braarudosphaera, and some diversity is the
result of updated taxonomic concepts based on
the Kilwa Group research itself (Bown, 2005a;
Bown and Dunkley Jones, 2006). However, most
of the high diversity is unequivocally related to
the quality of preservation and, in particular,
the presence of holococcoliths, rhabdoliths, and
small and fragile taxa.
Holococcoliths and Rhabdoliths
Holococcoliths and small rhabdoliths are
not routinely preserved in modern deep-sea
sediments (Roth and Berger, 1975), and they are
considered to be the most prone to dissolution
(Roth and Thierstein, 1972). Holococcoliths are
constructed from minute, equidimensional cal-
cite crystallites and formed during the haploid
phase of the haplo-diplontic coccolithophore
life cycle. They typically alternate with a dip-
loid phase that produces the more commonly
observed and robustly constructed heterococ-
coliths (compare Figs. 2G and 2H). Over 90
holococcolith morphologies have been docu-
mented in the modern ocean (32% of the total
morphological diversity), but all of them are
small (<3 µm) and none typically preserve in the
sedimentary record (Young et al., 2003). Liv-
ing holococcolith-bearing coccolithophores are
widely distributed (Kleijne, 1991) and show no
particular affi nity for shelf environments. Their
absence from seafl oor sediments is simply the
result of a preservational fi lter, which removes
these highly dissolution-prone coccoliths. There
are several larger, extinct holococcoliths (e.g.,
Zygrhablithus; Fig. 3H) that do consistently
preserve in the fossil record, but even these are
absent in sediments deposited in deeper waters
(~3000 m Bown, 2005b). In general, fossil
holococcolith preservation is patchy and largely
restricted to hemipelagic, clay-rich lithologies.
For the Paleogene and Late Cretaceous time
intervals, the new taxa described from the Kilwa
Group have effectively doubled the known holo-
coccolith fossil diversity, with the addition of
25 Paleogene and 23 Late Cretaceous species
(Bown, 2005a; Bown and Dunkley Jones, 2006;
Lees, 2007).
Rhabdoliths are also widely distributed in
the modern ocean (e.g., Boeckel et al., 2006)
but have a patchy and inconsistent fossil record.
They are typically spinose and can be large,
but minimal dissolution leads to fragmenta-
tion of the coccoliths and disarticulation of
the spines. Again, their abundance and diver-
sity in the Kilwa Group sediments is the result
of exceptional preservation, as can be seen in
the delicate rim and spine structures shown in
Figures 3N and 3O.
Gladiolithus
Gladiolithus is the most surprising compo-
nent of the new diversity preserved in the Kilwa
Group (Figs. 3A–D). It is one of a small number
of living coccolithophores specifi cally adapted
to life in the deep photic zone (100–200 m),
and it produces highly modifi ed coccoliths and
coccospheres that are suspected to be morpho-
logical adaptations related to the low-light con-
ditions (Young, 1994). Despite being abundant
in the water column, the liths are rarely found
at the seafl oor (Roth and Berger, 1975), and
they have not been previously documented in
sediments older than late Quaternary (Okada
and Matsuoka, 1996). In fact, there has been
Tanzania microfossil Konservat-Lagerstätte
Geological Society of America Bulletin, January/February 2008 7
FRAGILE STRUCTURES
COCCOSPHERES
HOLOCOCCOLITHS
GLADIOLITHUS
RHABDOLITHS/MINUTE COCCOLITHS/
Coccolithus pelagicus
Blackites deflandrei Blackites morionum undescribed coccolith
Cruciplacolithus inseadus Neochiastozygus imbriei Calciosolenia brasiliensis undescribed coccolith
minute spinose coccolith
Syracosphaera sp.
Clathrolithus ellipticus
Zygrhablithus bijugatus
Gladiolithus flabellatus Gladiolithus flabellatus Gladiolithus flabellatus Gladiolithus flabellatus
BASAL PLATES
BASAL PLATES
ELONGATE ELEMENTS
ELONGATE ELEMENTS
BASAL PLATES
ELONGATE ELEMENTS
A
D
C
E
I
MP
H
F
J
L
O
B
K
N
G
Figure 3. Calcareous nannofossil images from the Paleogene Kilwa Group. Scale bars are 1 µm. (A–D) Gladiolithus fl abellatus. These
images (A, C, and D) are the fi rst pre-Quaternary records of this extant, deep–photic-zone coccolithophore. A modern coccosphere (B)
is provided for comparison. (A) Collapsed coccosphere comprising long tube-coccoliths and basal-disc lepidoliths (late Eocene, Sample
TDP12/26-2, 62 cm). (B) Modern coccosphere from Hawaiian Ocean Time series (HOTS) station. (C and D) Collapsed Gladiolithus coc-
cospheres (late Paleocene, Sample TDP16B/12-2, 9 cm). (E) Minuscule (<1 µm), undescribed, cup-like coccoliths with tall, hollow spines
(late Eocene, Sample TDP12/23-2, 79 cm). (F) Syracosphaera sp. (late Eocene, Sample TDP12/26-2, 62 cm). (G) Clathrolithus ellipticus holo-
coccolith (late Paleocene, Sample TDP16B/12-2, 9 cm). (H) Zygrhablithus bijugatus holococcolith (late Paleocene, Sample TDP16B/12-2,
9 cm). (I–L) Collapsed coccospheres. (I) Cruciplacolithus inseadus, a species only previously known from the Danian but found in all our
SEM samples (late Paleocene, Sample TDP16B/12-2, 9 cm). (J) Neochiastozygus imbriei grouping, most likely a collapsed coccosphere, but
displays signifi cant morphological variation between coccoliths, with signifi cant implications for species-level taxonomy (late Paleocene,
Sample TDP14/9-1, 20 cm). (K) Calciosolenia brasiliensis displaying morphological variation between coccoliths identical to that seen in
modern examples (e.g., Young et al., 2003) (middle Eocene, Sample TDP20/23-1, 40 cm). (L) Undescribed, very small (<2 µm) coccolith spe-
cies (late Paleocene, Sample TDP16B/12-2, 9 cm). (M) Coccolithus pelagicus with previously undescribed, gracile central-area cross bars
(late Paleocene, Sample TDP16B/12-2, 9 cm). (N) Blackites defl andrei (middle Eocene, Sample TDP13/20-1, 50 cm). (O) Blackites morionum
showing an intricately constructed, hollow spine and dissolution-susceptible rim architecture (late Paleocene, Sample TDP16B/12-2, 9 cm).
(P) Undescribed placolith coccolith with a fragile, central-area grill (late Paleocene, Sample TDP16B/12-2, 9 cm).
Bown et al.
8 Geological Society of America Bulletin, January/February 2008
no unequivocal documentation of deep-dwell-
ing nannoplankton prior to the Neogene, and of
the modern assemblage, only Florisphaera has
a fossil record, stretching back to the late Mio-
cene (Young, 1998). We have not observed Flo-
risphaera in the Kilwa Group, suggesting that it
evolved in the late Oligocene or Miocene.
The presence of abundant Gladiolithus is sig-
nifi cant, not only because it confi rms the unique
quality of preservation in the Kilwa Group but
also because it indicates that a deep–photic-
zone niche was exploited by the same group
at least back to the late Paleocene (56 Ma). It
also lends strong support to the interpretation of
these depositional environments as being open
ocean and deep water. Modern Gladiolithus are
uncommon in shelf seas, and the deep–photic-
zone community abundance is strongly corre-
lated with water depth and excluded from mar-
ginal basins (Okada, 1983).
Small Taxa, Delicate Structures, and
Coccospheres
The occurrence of small coccoliths and frag-
ile, central-area structures, such as those seen
in Calciosolenia and Syracosphaera (Figs. 3F
and 3K), represents preservation that resembles
well-preserved modern coccolithophore mate-
rial. The presence of coccospheres provides
additional paleobiological information that is
lost when coccoliths are disaggregated. Gen-
erally, only placolith coccoliths that mechani-
cally interlock are found preserved as intact
coccospheres in the fossil record (Figs. 2I, 3I,
and 3L), while all other fossil taxa are virtu-
ally unknown in this state. The Kilwa Group
has yielded the only Cenozoic examples of
undisturbed, collapsed coccospheres of non-
placolith taxa, providing indications of origi-
nal cell size, coccolith production per cell, and
ranges of intraspecifi c morphological variabil-
ity (Figs. 3A, 3C, 3E, 3J, and 3K).
Preservation of the Kilwa Group
Microfossils
Preservation of the principal calcareous
microfossil groups (planktonic foraminifera,
benthic foraminifera, and calcareous nanno-
plankton) can be affected by the initial degree of
shell calcifi cation and postmortem taphonomic
and diagenetic processes, including bioturba-
tion, erosion, dissolution, recrystallization, and
overgrowth. The planktonic groups live high in
the water column (0–200 m) and are exported
to the seafl oor by simple sinking, in the case of
foraminifera (Berger, 1971), or within marine
snow aggregates and zooplanktonic fecal pel-
lets, in the case of the smaller nannoplankton
(Steinmetz, 1994). Much is known about the
dissolution of planktonic foraminifera as they
approach the lysocline and sink beneath the cal-
cite compensation depth (Thunell and Honjo,
1981; Schmuker and Schiebel, 2002), but in
shallower settings, the death assemblages of
planktonic foraminifera are relatively faithful
TABLE 1. NANNOFOSSIL DIVERSITY COMPARISON
enecoelaP reppU noitceS
Zone NP9
Middle Eocene
Zone NP15
Upper Eocene
Zone NP19/20
References
Tanzania
Kilwa Group—total nannofossils 126 145 79 1
Kilwa Group—holococcoliths 16 20 15 1, 2, 3
Kilwa Group—new species 30 (24%) 44 (30%) 17 (22%) 1, 2, 3
Global compilation
88 (70%) 106 (73%) 67 (85%) 4
Pacific Ocean
Shatsky Rise (Site 1209) 48 (38%) 48 (33%) 26 (33%) 5
Shatsky Rise (Site 1211) 27 (21%) 31 (21%) 17 (22%) 5
Allison Guyot (Hole 865B) 57 (45%) 61 (42%) 32 (41%) 7
Atlantic Ocean
Demerara Rise (Holes 1259B,
1260A)
38 (30%) 45 (31%) 8, 9
Blake Nose (Hole 1051A) 34 (27%) 62 (43%) 10
Iberia Abyssal Plain (Hole 900A) 51 (35%) 46 (58%) 11
Tethys Ocean
21 )%42( 91 )%41( 12 )%51( 91 ylatI ,enoicattoB
21 )%61( 31 )%9( 31 )%11( 41 ylatI ,assetnoC
Southern Ocean
Maud Rise (Site 690) 43 (34%) 6
Agulhas Ridge (Hole 1090B) 41 (52%) 13
41 )%23( 64 )%73( 74 )A5311 eloH( uaetalP neleugreK
Shelf sites
51 )%73( 92 )%44( 46 dnalgnE nrehtuoS
61 )%86( 45 )%92( 24 )%22( 82 ASU ,tsaoC fluG
Bass River, New Jersey 63 (50%) 6
Clayton Core, New Jersey 54 (43%) 17
81 )%15(
04 avaJ ,eroc nalugnaN
91 )%83( 84 abuC
02 )%73( 45 yekruT ,eliS
Note: References, as follows: 1—this work; 2—Bown (2005a); 3—Bown and Dunkley Jones (2006); 4—
Bown et al. (2004); 5—Bralower (2005); 6—Gibbs et al. (2006); 7—Bralower and Mutterlose (1995); 8—Jiang
and Wise (2006); 9—Lupi and Wise (2006); 10—Mita (2001); 11—Liu (1996); 12—Cresta et al. (1989); 13—
Morino and Flores (2002); 14—Arney and Wise (2003); 15—Aubry (1983); 16—Siesser (1983); 17—Bybell
and Self-Trail (1995); 18—Dunkley Jones (personal observation, 2006); 19—Aubry (1999); 20—Varol (1989).
60 80 10020 40 120
55
50
65
60
35
30
45
40
Species richness
global compilation
Tanz ani a
Shatsky Rise
Allison Guyot
Age (Ma)
Paleocene E. Eocene M. Eocene L. Eo.
Oligo.
Figure 4. Comparative nannofossil diversity
(species richness) data for the Paleogene
interval. The values represent all nannofossil
species, excluding holococcoliths, recorded
within each nannofossil zone on the Grad-
stein et al. (2004) time scale. The aggregate
global diversity data are from Bown et al.
(2004), and deep-sea data are from Bralower
(2005—Site 1209, Shatsky Rise, NW Pacifi c)
and Bralower and Mutterlose (1995—Hole
865B, Allison Guyot, Central Pacifi c).
Tanzania microfossil Konservat-Lagerstätte
Geological Society of America Bulletin, January/February 2008 9
recorders of the overlying living plankton (Bé,
1977). However, in the modern ocean, calcar-
eous nannoplankton are subject to far stronger
taphonomic biases that signifi cantly reduce the
exported and preserved diversity. This bias is
highly correlated with coccolith size, and there
appears to be a threshold in preservation poten-
tial at 3 µm: 90% of the species with coccoliths
>3 µm are found as fossils, compared with only
20% of those with coccoliths <3 µm (Young
et al., 2005). This is not direct size selection,
but rather the result of small coccoliths having
higher surface-area-to-volume ratios, which
increases their vulnerability to dissolution. Sed-
iment trap and seafl oor samples show that the
loss of small coccoliths takes place largely in the
water column and is the result of grazing and/or
dissolution while sinking, even well above the
lysocline (Roth, 1994; Andruleit et al., 2004).
Further selective dissolution and fragmenta-
tion occurs within the sediment through inges-
tion by sediment grazers and early diagenesis.
With burial, diagenetic processes continue, and
it is commonplace to observe deterioration of
preservation with increasing depth in deep-sea
cores. In carbonate-rich oozes, this involves
increase in crystal size at the micron scale, with
small crystals being selectively dissolved and
larger ones overgrown (Wise, 1977). The net
result of the various processes occurring in the
water column, at the sediment surface, and dur-
ing burial, is that even soft oozes are increas-
ingly dominated by larger coccoliths. When the
modern, global nannoplankton diversity is com-
pared with the Holocene fossil record, the esti-
mated preserved diversity is, at best, 54% but
more typically around 30%. Preserved diversity
is even less, if holococcolith morphologies are
considered (20%–36%) (Young et al., 2003,
2005). These are signifi cantly high diversity
losses that have serious implications for paleon-
tological studies.
The exceptional preservation of calcareous
nannofossils in the Paleogene Kilwa Group has
resulted in assemblages that contain extraordi-
narily high species richness, comprising new
diversity in well-known families, alongside pres-
ervation of small and delicate forms for which
we have had no previous fossil record. The
majority of this enhanced diversity is explained
by preservation rather than paleoecology, and
demonstrates the signifi cant effect of favorable
taphonomic conditions. The same sediments
host planktonic foraminifera assemblages
that are not exceptionally diverse but which
yield stable isotope values that are considered
relatively unaffected by diagenesis. The glassy
foraminifera tests, absence of infi lling, and pri-
mary wall fabrics contrast with deep-sea ooze
taphonomy, which is characterized by frosty or
white and chalky shells that are considered to
result from recrystallization that includes both
replacement and overgrowth/infi lling (Pearson
et al., 2001; Sexton et al., 2006). Post-deposi-
tional recrystallization arguably shifts the iso-
topic values toward early diagenetic calcite and
inferred seafl oor-environment values that are
both colder, in terms of estimated paleotemper-
atures, and more homogeneous (Pearson et al.,
2001, 2007). The remarkable preservation of the
calcareous nannofossils strongly corroborates
this interpretation of minimal diagenetic modi-
cation. The contrasting diversity records of the
two microfossil groups, however, highlights the
greater sensitivity to preservational modifi cation
shown by the smaller-sized nannoplankton.
The quality of preservation is best explained
by the clay-rich lithologies that have not been
deeply buried. The clays isolate the calcite micro-
fossils tests within an impermeable medium,
preventing or inhibiting diagenetic recrystalliza-
tion. This explanation is supported by excellent
organic biomarker preservation that indicates
thermal immaturity and low sediment perme-
ability, which has inhibited organic matter bio-
degradation (van Dongen et al., 2006). There is
some variability in the preservation, from sample
to sample and even across single SEM samples at
the micron scale. Most likely, this refl ects hetero-
geneous microenvironments within the sedimen-
tary fabric, controlled by variations in grain size,
porosity, permeability, and sediment chemistry.
Quantifying the Effects of Calcareous
Nannofossil Preservation
Although the potential for preservational
modifi cation of microfossil assemblages is
universally acknowledged, the documenta-
tion of preservation is inconsistent. Two main
approaches have been used to record microfossil
preservation—fi rst, qualitative, visual observa-
tions, and second, indices based on indirect evi-
dence, such as fragmentation, dissolution-sus-
ceptibility rankings, and abundance comparisons
(Berger, 1968; Roth and Thierstein, 1972; Roth
and Krumbach, 1986; Le and Shackleton, 1992;
Boeckel et al., 2006). More recently, geochemi-
cal comparisons between different preservational
classes of planktonic foraminifera have been
attempted (Sexton et al., 2006). Visual assess-
ment is largely subjective and greatly infl uenced
by worker experience. Dissolution indices have
been successful in foraminiferal studies but are
not universally applied, and are rarely used in
nannofossil studies (Matsuoka, 1990; Gibbs et
al., 2004). Furthermore, both approaches may
still fail to discriminate cryptic preservational
effects that nevertheless signifi cantly alter both
the taxonomic and geochemical composition of
a microfossil assemblage (Gibbs et al., 2004;
Williams et al., 2005; Pearson et al., 2007).
To assess the preserved diversity of Paleogene
nannofossils, we have used the Kilwa Group
data as a benchmark against which to compare
recorded diversities from coeval sections, repre-
senting a range of preservation states, for three
time slices (Table 1). It is striking that the global
compilation returns lower diversities than the
Kilwa Group (70%–85%), but the values are
broadly comparable, given the uncertainties
associated with composite literature surveys
(Bown et al., 2004). The individual sections
yield diversity values that are, in all cases, con-
siderably less (9%–68%) than the Kilwa Group
for each of the three time slices, but there are
systematic discrepancies corresponding to sec-
tion type. The Paleogene shelf sites, with report-
edly good preservation (Clayton core—Bybell
and Self-Trail, 1995; Bass River—Gibbs et al.,
2006; Yazoo Clay, Gulf Coast—Siesser, 1983),
host diversities ranging from 43%–68% and
compare most favorably with the Kilwa Group
values. Deep-sea sections, where carbonate-rich
oozes and chalks dominate, yield values ranging
from 21%–58%, while the lowest values (9%–
24%) come from lithifi ed deepwater limestone
sequences (Contessa, Italy). These sections are
not all directly comparable, in particular those
from higher latitudes (e.g., the Southern Ocean);
however, the Eocene was a time of relatively low
nannoplankton biogeographic differentiation,
and its effects do not greatly bias the data (e.g.,
only nine species were absent from the Kilwa
Group succession due to biogeography). Indeed,
the Southern Ocean sites return relatively high
diversity values, most likely refl ecting better
preservation in more clay-rich lithologies.
The negative aspects of carbonate-rich lithol-
ogies on the taphonomy of calcareous nannofos-
sils are reasonably well known. However, there
has been no serious attempt to quantify these
effects, and the degree of taxonomic modifi -
cation, highlighted in this paper, is probably
greater than is generally perceived. This does
not usually impact on the stratigraphic applica-
tion of the group, which generally utilizes taxa
selected for size and robustness, but it does have
serious implications when assemblage abun-
dance and diversity data are considered. That
the least favorable diversity comparisons come
from the carbonate-rich successions of tropical
and subtropical latitudes is comparable to the
morphological- and geochemical-based tapho-
nomic observations from the study of planktonic
foraminifera (Sexton et al., 2006).
The potential for large-magnitude nanno-
fossil diversity loss is a signifi cant factor for
those using fossil data in paleobiological or
paleoceanographic interpretation. These losses
Bown et al.
10 Geological Society of America Bulletin, January/February 2008
can be signifi cantly large when comparing liv-
ing and Holocene assemblages, as highlighted
earlier, and are highly variable in the Paleogene
comparisons, described above. However, as in
all considerations of the fossil record, there are
important caveats to these data that must be
considered before such information is dismissed
as potentially fallacious, and in the case of the
calcareous nannofossils, we are convinced that
these explanations justify the long-established
value of these paleontological data.
First, the size-range distribution of living
coccoliths is strongly skewed toward small sizes
(<3 µm; Fig. 5), but this may be an anomalous
situation, having followed the sequential evolu-
tionary loss of large taxa through the Pliocene
and Pleistocene (Gibbs et al., 2005; Schmidt et
al., 2006). Qualitative reviews of pre-Neogene
coccolith size suggest that far higher propor-
tions of the total diversity were concentrated
in the larger size ranges for much of the last
200 m.y., and this would have signifi cantly
increased the proportion of taxa with fossiliza-
tion potential. The Kilwa Group coccolith-size
data support this view and show a very different
distribution spectrum to that of modern taxa,
with a broader range of sizes, a signifi cantly
higher mean length value (8.5 µm versus 3 µm),
and higher frequencies throughout the larger
size-classes (i.e., >10 µm). Young et al. (2005)
argued that subtle changes in coccolith size-fre-
quency through time could result in signifi cant
changes in observed diversity, independent of
any change in actual diversity, especially if
large numbers of coccoliths shifted above or
below the 3-µm preservation-potential thresh-
old (Fig. 5). Although we cannot unequivocally
prove the fi delity of the Kilwa Group Paleo-
gene fossil record in the <3-µm size range, the
observation of abundant coccoliths of this size
(e.g., Gladiolithus), with only limited diversity,
is strongly suggestive that the skewing seen in
the modern group was not as signifi cant in the
Paleogene coccolith record.
Second, the modern to Holocene diversity
loss is highly selective, taxonomically, because
of low preservation potential in several groups
that produce small and/or fragile coccoliths,
most importantly the holococcoliths and Syra-
cosphaerales (including rhabdoliths), which
represent 32% and 27% of total modern diver-
sity, respectively (Young et al., 2005). Holo-
coccolith loss can be discounted, if we assume
conservation of this diversity in the preservable
heterococcolith life-cycle phases. The Syraco-
sphaerales diversity loss is largely a Cenozoic
phenomenon, because most of its modern diver-
sity has only appeared since the Cretaceous-Ter-
tiary boundary taxonomic turnover (Bown et al.,
2004). Indeed, Cretaceous and Jurassic cocco-
liths, in general, were more robustly constructed
across the diversity of the group. Moreover,
exceptionally preserved Cretaceous assemblages
do not yield such extreme diversity values when
compared to more typical preservation states,
but they do contain high abundances of small,
fragile coccoliths (e.g., Corollithion and Strad-
nerlithus) and, in common with the Cenozoic
examples, Calciosolenia and holococcoliths
(Covington, 1985; Lambert, 1987; Lees, 2007).
The Late Cretaceous sediments of the Kilwa
Group yield diversities that are comparable to,
or rather less than, the global compilation, but
still, for the Turonian at least, host the highest
diversities yet recorded from a single site.
Given these critical caveats, we have reason
for confi dence in the documented fossil record
of coccolithophores. Preservation potential may
well have been far greater for much of the pre-
Quaternary time interval, when coccolith sizes
were not as strongly skewed toward the smaller
size frequencies, and, excepting the small and
fragile coccoliths of the Syracosphaerales,
many of the extant groups have good preserva-
tion potential, as evidenced by their long and
abundant fossil records. However, there remains
20 300.511.522.5345678910111213 1514
0
10
20
30
40
50
60
Maximum coccolith length
Number of species
20 300.511.522.5345678910111213 1514
0
10
20
30
40
50
60
Maximum coccolith length
Number of species
8.5
3
PLANKTON/HOLOCENE RECORD
KILWA GROUP PALEOGENE RECORD
plankton record only
plankton and fossil record
holococcoliths
holococcoliths
A
B
Figure 5. Size-frequency histograms for (A) Paleogene coccoliths from the Kilwa Group, and
(B) modern coccolith species, as observed in the plankton and Holocene fossil record. The
data are an estimate of maximum coccolith size and for the Paleogene were based on mea-
surements chosen from many thousands of light micrographs. The modern data are from
Young et al. (2005) and are based on measured light-micrograph and published images.
Tanzania microfossil Konservat-Lagerstätte
Geological Society of America Bulletin, January/February 2008 11
much to learn about the preservation of cocco-
liths, and fi ne-fraction carbonate in general, and
a need to develop protocols that allow for the
adequate description, quantifi cation, and com-
munication of this essential information. These
issues are being addressed in the foraminifera
and geochemistry communities, which, by and
large, accept that qualitative, descriptive meth-
ods of conveying preservation quality are no
longer adequate (Pearson et al., 2001; Sexton
et al., 2006). Instead, strict criteria that require
high-resolution morphological analysis, or indi-
rect geochemical methods, are being used to
ensure effective documentation of preservation.
CONCLUSIONS
The Paleogene Kilwa Group sediments of
coastal Tanzania host calcareous microfossils
that are exceptionally preserved. The quality
of preservation is demonstrated by the glassy
appearance, wall ultrastructure, and stable
isotope geochemistry of the planktonic fora-
minifera, and high diversity of the calcareous
nannofossils, which includes very small and
fragile taxa. For both groups, this fossil material
resembles well-preserved modern specimens.
The glassy foraminifera shells provide stable
isotope values that have been relatively unaf-
fected by diagenesis and are providing valu-
able new paleoclimate proxy records (Pearson
et al., 2007). The taxonomic composition of
these foraminiferal assemblages, however, is
comparable to those from different taphonomic
settings. By contrast, the calcareous nannofos-
sils are remarkably diverse and have distinct
assemblage compositions that are primarily
the result of the exceptional preservation. They
are characterized by the presence of dissolu-
tion-susceptible and fragile taxa, in particular
holococcoliths and rhabdoliths. The uniqueness
of this Konservat-Lagerstätte is especially well
demonstrated by the abundant occurrence of
Gladiolithus, which is a delicate extant taxon
that, until now, has had no documented fossil
record prior to the Pleistocene.
For both foraminifera and coccolithophores,
incorporation into impermeable, clay-rich
sediments that have never been deeply buried
appears to have been critical in producing the
exceptional preservation. The enhanced diversity
seen in the calcareous nannofossils highlights
the different sensitivities of these two fossil
groups to preservational modifi cation. The inte-
grated taphonomic observations from both fossil
groups, however, provide the maximum amount
of information in support of the interpretation of
both geochemical and paleontological proxies.
The Kilwa Group calcareous nannofossil
diversities are consistently higher than all coeval
assemblages, and even slightly higher than com-
posite global estimates. These comparisons dem-
onstrate the degree of taxonomic modifi cation
that can result from varying preservation states.
The highest diversities are preserved in hemipe-
lagic, clay-rich lithologies and the greatest losses
occur in lithifi ed, carbonate-rich sediments. The
majority of the lost diversity, however, is con-
ned to distinct taxonomic groups, and espe-
cially the holococcoliths and rhabdoliths (Syra-
cosphaerales). The preservational potential of
Paleogene coccolithophores may well have been
signifi cantly greater than in the modern oceans
because a larger proportion of the biodiversity
fell within the larger size fractions.
Study of the Kilwa Group hemipelagic sedi-
ments has highlighted the signifi cant effects that
preservation can have on both the diversity and
geochemistry of calcareous microfossils. These
exceptionally preserved fossils are providing
high-quality paleontological and geochemical
paleoclimate proxy information, and, for the
calcareous nannoplankton, this includes paleo-
biological and biodiversity data that are cur-
rently unique for this fossil group.
ACKNOWLEDGMENTS
We thank the Tanzania Commission for Science
and Technology for permission to conduct this study,
the Tanzania Petroleum Development Corporation
for fi eldwork support, and the Natural Environment
Research Council (Grant NE/C510508/1) and Uni-
versity College London Graduate School for funding
the research.
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... Most of the material examined here is from the Kilwa Group of Tanzania (Nicholas et al., 2006), which is known for the exceptional "glassy" preservation of microfossils such as foraminifera (e.g. Pearson et al., 2001Pearson et al., , 2007Sexton et al., 2006b;Pearson and Wade, 2015) and coccolithophores (Bown et al., 2008;Dunkley Jones et al., 2009). This feature is attributed to the relatively impermeable clay-rich facies (Pearson et al., 2001;Nicholas et al., 2006) and shallow maximum burial depths (van Dongen et al., 2006;Nicholas et al., 2007;Bown et al., 2008), which are thought to have hindered low-temperature recrystallization. ...
... Pearson et al., 2001Pearson et al., , 2007Sexton et al., 2006b;Pearson and Wade, 2015) and coccolithophores (Bown et al., 2008;Dunkley Jones et al., 2009). This feature is attributed to the relatively impermeable clay-rich facies (Pearson et al., 2001;Nicholas et al., 2006) and shallow maximum burial depths (van Dongen et al., 2006;Nicholas et al., 2007;Bown et al., 2008), which are thought to have hindered low-temperature recrystallization. This material is from Tanzania Drilling Project (TDP) Sites 2 (Pearson et al., 2004), 11, 13, 14, 17 and 20 (Nicholas et al., 2006 age model for TDP Site 20 revised in Pearson and Coxall, 2014) and ranges in age from late Paleocene to early Oligocene. ...
... Direct SEM observation of mudrock fracture surfaces (e.g. Bown et al., 2008) is a possible line of future research that might produce such evidence. ...
Article
Full-text available
Muricate planktonic foraminifera comprise an extinct clade that was diverse and abundant in the Paleogene oceans and are widely used in palaeoclimate research as geochemical proxy carriers for the upper oceans. Their characteristic wall texture has surface projections called “muricae” formed by upward deflection and mounding of successive layers of the test wall. The group is generally considered to have lacked “true spines”: that is, acicular calcite crystals embedded in and projecting from the test surface such as occur in many modern and some Paleogene groups. Here we present evidence from polished sections, surface wall scanning electron microscope images and test dissections, showing that radially orientated crystalline spine-like structures occur in the centre of muricae in various species of Acarinina and Morozovella and projected from the test wall in life. Their morphology and placement in the wall suggest that they evolved independently of true spines. Nevertheless, they may have served a similar range of functions as spines in modern species, including aiding buoyancy and predation and especially harbouring algal photosymbionts, the function for which we suggest they probably first evolved. Our observations strengthen the analogy between Paleogene mixed-layer-dwelling planktonic foraminifera and their modern spinose counterparts.
... Scanning Electron Microscopy (SEM) was done to examine finer details for some selective forms of nannofossils. SEM analysis was carried out by two types of sample preparation: one using broken rock surfaces (Lees et al., 2004;Bown et al., 2008) and the other using smear slides on circular cover slips. Observation of the rock surfaces is a particularly productive method in these sediments that allows imaging of in situ nannofossil concentrations on the surfaces as it remains undisturbed by large metazoan bioturbation, which conserve collapsed coccospheres and fragile taxa (Bown et al., 2008). ...
... SEM analysis was carried out by two types of sample preparation: one using broken rock surfaces (Lees et al., 2004;Bown et al., 2008) and the other using smear slides on circular cover slips. Observation of the rock surfaces is a particularly productive method in these sediments that allows imaging of in situ nannofossil concentrations on the surfaces as it remains undisturbed by large metazoan bioturbation, which conserve collapsed coccospheres and fragile taxa (Bown et al., 2008). The overall abundance and preservation of the nannofossils was assessed and individual species abundance was estimated semiquantitatively. ...
Thesis
The Ph.D. thesis work hovers on a first comprehensive study on the diversity and palaeoecology of various benthic and planktonic, siliceous and calcified microfossils from the Neogene outcrops at different islands of Andaman and Nicobar Group. Amongst the two islands of Ritchie’s Archipelago the three outcrops of Havelock Island yielded moderately preserved diatoms, fairly preserved radiolarians and calcareous nannofossils. The radiolarian assemblage clearly indicates late early to early Middle Miocene (Burdigalian to Langhian) age based on the presence of characteristic forms of RN 3-RN 4 zones. The age has also been reconfirmed more precisely by calcareous nannofossils of NN 4-NN 5 zones, comparable to CNM 6-CNM 7 zones. The recorded diatoms from these outcrops also denote similar age connotation. From the two outcrops of Neil Island belonging to Ritchie’s Archipelago well preserved diatoms, radiolarians and calcareous nannofossils have been recorded. Marker radiolarians (RN 8 and RN 9 zones) of late Miocene (Tortonian) have been identified. Precise age determination by calcareous nannofossils also designates the same age for both the outcrops (NN 11A subzone). Precise palaeoecological zones have been derived from SHEBI and CONISS cluster analyses. Specifically the dominance of Thalassionema nitzschioides indicates ocean upwelling during that time. The planktonic/benthic ratio of diatoms indicates a planktonic bloom owing to the increase in nutrient rich conditions, resulting from a high amount of terrigenous input in the Andaman Sea. It has been interpreted that the late Miocene sediment of Ritchie’s Archipelago might have derived from deltaic sediments of the Ayeyarwady River. Upwelling and bloom of some specific diatoms indicate an intense monsoonal system. The first record of the radiolarian taxon Acrobotrys disolenia from the late Miocene of Neil Island is a significant addition to the radiolarian database from this part of the world. It has been interpreted that A. disolenia could have migrated from the Pacific Ocean through the Indonesian Gateway with the help of South Equatorial Current, which is also known as Indonesian Through Flow (ITF). Analysis of siliceous microfossils from two outcrops of Sawai Bay Formation on Car Nicobar Island yielded well preserved siliceous microfossils along with calcareous nannofossils. An early Pliocene (Zanclean) age has been assigned for the sediments of Sawai Bay Formation of Car Nicobar Island based on the recovered marker radiolarians of RN 10 zone and calcareous nannofossils of NN 12 and NN 13 zones. It has been revealed that the biogenic bloom which was prevalent in the late Miocene remarkably reduced in the early Pliocene in the study area. The overall diversity and abundance of siliceous microfossils at ~4.24 Ma was low. However, there is evidence of a silica crash at ~6 Ma in the Andaman Sea followed by a short pulse of recovery phase. The gradual decrease and cessation of BSi (Biogenic Silica) in the Andaman Sea may be due to nutrient depletion as an impact of closure of ITF. However, the study indicates that at ~4.24 Ma the recovery of BSi took place, though it may be considered as a short pulse of recovery. So, it may be inferred that the short pulse in BSi productivity in the studied region was possibly due to riverine influx from nearby Irrawaddy or Ganges-Brahmaputra rivers that enhanced the nutrient rich water at ~4.24 Ma coupled with monsoonal activity. Benthic calcareous microfossils have been analysed in petrographic thin sections from two islands of Andaman and Nicobar Basin. Samples collected from the three outcrops of Little Andaman Island (Hut Bay) yielded moderately well preserved calcareous fossil algae, foraminifers, echinoid spines etc. All the three outcrops are assignable to Serravallian age (Middle Miocene) based on tropical planktonic foraminiferal zonations. Five different species of the green algal genus Halimeda have been identified from the outcrop of Butler Bay in this island. Based on taxonomic, growth form and taphonomic analyses, along with overall facies analysis of the studied sequence from the Butler Bay it can be interpreted that the diversified benthic forms thrived in photic/epipelagic zone in a shallow to slightly deeper bathymetry. It has been inferred that the deposition of sediments took place in a reefoidal environment having moderate to high energy condition. Another outcrop from the Little Andaman (Hut Bay Quarry No. 4) yielded very well preserved rhodoliths containing melobesioid algal forms (non-geniculate coralline red algae). The rhodolith assemblage recovered from the base of the outcrop dominated by melobesioids. On the other hand, taphonomic analysis of the algal forms from the upper part of the outcrop reveals strong wave action i.e., high energy condition. High degrees of taphonomic features have been observed in the benthic coralline algal assemblage of the New Quarry Section in Little Andaman Island. Abundance of these taphonomic features indicates an allochthonous or para autochthonous deposition that took place in a near shore environment. In the West Coast Section of Neil Island the diversity and abundance of benthic foraminifers are more in comparison to the coralline algal forms. Growth form and taphonomic analyses of coralline red algae indicate a wide variation of growth forms along with variety of taphonomic features. The coralline algae as well as foraminifera recorded here are generally found in shallow-water setting. The yellowish-grey, moderately sorted grainstone facies is characterised by foraminiferal-algal bioclastic limestones composed of broken fragments of coralline algae, foraminifers and other biogenic components. It has been interpreted that Pleistocene limestones of Neil West Coast Formation, Neil Island indicates intertidal to near shore environment of deposition with high energy condition and increased hydrodynamic activity.
... The term Konservat-Lagerstätte (Seilacher, 1970) is used to characterize exceptional preservation in the fossil record. The Paleogene sediments of coastal Tanzania contain remarkably well-preserved calcareous microfossils (Pearson et al., 2001(Pearson et al., , 2007Wade and Pearson, 2008), and these were described as a microfossil Konservat-Lagerstätte by Bown et al. (2008). The development of a calcareous microfossil Konservat-Lagerstätte appears to be related to clay-rich sediments that have never been deeply buried . ...
... The lack of G. semiinvoluta in Samples A and B raises the possibility that these samples could be from the short stratigraphic interval between the T M. crassatus and the B G. semiinvoluta. This interval was designated the Turborotalia pseudoampliapertura Zone by Haggag (1990) and has been recognized in several sections worldwide (Canudo and Molina, 1992;Wade, 2004;Agnini et al., 2011;Strougo et al., 2013). The B G. semiinvoluta may be diachronous, as the duration of the interval between T M. crassatus and B G. semiinvoluta is only 20 kyr at ODP Site 1052 (Wade, 2004;Wade et al., 2012), 330 kyr at Alano and 408 kyr at the Varignano section (Luciani et al., 2020). ...
Article
Full-text available
The Rashrashiyah Formation of the Sirhan Basin in northern Saudi Arabia contains diverse assemblages of planktonic foraminifera. We examined the biostratigraphy, stratigraphic range and preservation of upper Eocene planktonic foraminifera. Assemblages are well-preserved and diverse, with 40 species and 11 genera. All samples are assigned to the Priabonian Globigerinatheka semiinvoluta Highest Occurrence Zone (E14), consistent with calcareous nannofossil biostratigraphy indicating Zone CNE17. Well-preserved planktonic foraminifera assemblages from the lower part of the upper Eocene are rare worldwide. Our study provides new insights into the stratigraphic ranges of many species. We find older (Zone E14) stratigraphic occurrences of several species of Globoturborotalita previously thought to have evolved in the latest Eocene (Zone E15, E16) or Oligocene; these include G. barbula, G. cancellata, G. gnaucki, G. pseudopraebulloides, and G. paracancellata. Older stratigraphic occurrences for Dentoglobigerina taci and Subbotina projecta are also found, and Globigerinatheka kugleri occurs at a younger stratigraphic level than previously proposed. Our revisions to stratigraphic ranges indicate that the late Eocene had a higher tropical–subtropical diversity of planktonic foraminifera than hitherto reported.
... Scanning Electron Microscopy (SEM) was carried out to check preservation quality and to examine the finer details of the nannofossil taxa. SEM analysis was carried out using two methods of sample preparation: one using broken rock surfaces Bown et al., 2008) and the other using smear slides on circular cover slips. Observation of rock surfaces is particularly a productive method in some of these sediments, allowing imaging of in situ nannofossil concentrations on the surfaces, which conserve collapsed coccospheres and fragile taxa (Bown et al., 2008). ...
... SEM analysis was carried out using two methods of sample preparation: one using broken rock surfaces Bown et al., 2008) and the other using smear slides on circular cover slips. Observation of rock surfaces is particularly a productive method in some of these sediments, allowing imaging of in situ nannofossil concentrations on the surfaces, which conserve collapsed coccospheres and fragile taxa (Bown et al., 2008). Rock surface observations in SEM reveal in situ nannofossil occurrences within the sedimentary fabric, unmodified by slide preparation and disaggregation of the sediments. ...
Article
Neogene sediments of the Andaman and Nicobar Islands in northern Indian Ocean, specifically the lower Miocene (Burdigalian) to lower Pliocene (Zanclean) sequences, were analysed for calcareous nannofossils that yielded well-preserved and diverse assemblages. The majority of samples contained stratigraphic marker taxa that allowed us to identify successions within the Burdigalian-Langhian (uppermost lower Miocene to lowermost middle Miocene) on the Havelock Island, Tortonian (upper Miocene) on the Neil Island and Zanclean (lower Pliocene) on the Car Nicobar Island. The nannofossil assemblages of uppermost lower Miocene to lowermost middle Miocene sediments reflect strong preference for warm tropical climate and nutricline condition i.e., depth dependent high variation of nutrient content. The presence of relatively unusual assemblage compositions, especially the presence of abundant sphenoliths and small reticulofenestrids, in conjunction with rare discoasters and alongside common diatoms, are indicative of high-productivity surface waters and probably upwelling conditions in the late Miocene. In the Tortonian (upper Miocene), low abundance of discoasters and high abundance of small reticulofenestrids (<3 μm) indicate eutrophic condition. This may be due to the influence of intensification of monsoonal activity and terrigenous input. The high-productivity surface-water environments can be linked with the intensification of the Indian Summer Monsoon. Well-preserved calcareous nannofossils have been also been recovered from the Zanclean (lower Pliocene) sediments on Car Nicobar Island. During the Zanclean warm water condition prevailed and from marginal to deep water setting i.e., transgressive event was recognised in the northern Indian Ocean.
... Tanzania.-Site 11, 12, and 17 from the Tanzania Drilling Project have become world-renowned for their exceptional preservation of calcareous microfossils, including the preservation of aragonite (e.g., Bown et al. 2008;Pearson et al. 2008) and contain abundant LBF (Cotton and Pearson 2012). Whilst the preservation of the LBF is variable, dependent on the lithology of specific beds, many have the same exceptional preservation seen in the planktonic assemblages (Fig. 1A, 1D , 1E, 1H;). ...
Article
Nummulites were one of the most abundant and widespread larger benthic foraminifera of the Paleogene, however, confusion remains within the literature as to whether their original test mineralogy was high or low magnesium calcite. As the number of studies using proxies based on Nummulites and related nummulitid geochemistry increase, it is essential to have a firm understanding of test composition to assess preservation within potential samples, and to interpret results. Here we employ a combination of X-ray diffraction, Fourier transform infra-red spectroscopy, and laser ablation ICPMS to determine magnesium content across exceptionally preserved and poorly preserved fossil material as well as modern examples of nummulitids—showing conclusively a primary intermediate to high magnesium calcite composition. This composition appears to be closely related to fluctuating ocean chemistry through the Paleogene. Using these results as an indicator of preservation we examine variation in trace element data across a suite of samples, and introduce the concept of the preservagram, a method of quickly visualizing different styles of carbonate diagenesis. Understanding the original mineralogy of nummulitids and, therefore, the extent to which specimens have been diagenetically altered, is essential as larger foraminifera are increasingly used in geochemical studies.
... Another example came from the Upper Jurassic Kimmeridge Clay Formation (Dorset, UK) [16,17] from coccolith stone bands which occur within nannofossil subzone NJ17a [18]. Most of the best preserved fossil coccospheres have been found in clay-rich hemipelagic sediments that have not been affected by more advanced diagenesis [13,17,19,20]. Many other examples come also from laminated limestones, e.g., the Kimmeridge White Stone Band [16,17]. ...
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