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MICROPALEONTOLOGY AND
PALEOECOLOGY (GEY 402) ASSIGNMENT
DONE BY
ARIYIBI OLAWALE EMMANUEL
(MATRIC NO: 12/SCI14/006)
OF
THE DEPARTMENT OF GEOLOGY,
COLLEGE OF SCIENCES,
AFE BABALOLA UNIVERSITY ADO, EKITI
EKITI STATE
March, 2016
LECTURER IN CHARGE:
Mr. Jide Adedipe Aladesanmi
QUESTIONS
Write short note on the following and their applications in Hydrocarbon explorations
1. Calcareous microfossils
2. Calcareous nannofossils
3. Siliceous microfossils
4. Phosphatic microfossils
QUESTION 1:
CALCAREOUS MICROFOSSILS
Calcareous microfossils have shells composed of calcite or aragonite. These organisms are
present in most marine and in some nonmarine environments. At great oceanic depths
characterized by low temperature and high hydrostatic pressure, however, calcareous remains
are largely or completely dissolved. The depth below which this occurs, which varies in
different oceanographic settings, is termed the carbonate compensation depth (CCD).
There are three principal types of calcareous microfossils:
-
calcareous foraminifera
-
ostracods, and
-
calcareous nannofossils.
1.1
Calcareous Foraminifera
Foraminifera will be discussed under the following headings:
-
General statement
-
Classification- taxonomy
-
Living Foraminifera
-
Biology
-
Tests
-
Deep-Sea species
-
Evolutionary significance
-
Uses
-
Gallery
General Statement
Foraminifera is a latin word which means ‘hole bearers’, (informally called "forams"). They
are members of a phylum or class of amoeboid protists characterized by streaming granular
ectoplasm that among other things is used for catching food, and commonly by an external
shell or "test" made of various materials and constructed in diverse forms. All but perhaps a
very few are aquatic and most are marine, the majority of which live on or within the seafloor
sediment (i.e., are benthic) while a smaller variety are floaters in the water column at various
depths (i.e., are planktonic). A few are known from freshwater or brackish conditions and
some soil species have been identified through molecular analysis of small subunit ribosomal
DNA.
Foraminifera typically produce a test, or shell, which can have either one or multiple
chambers, some becoming quite elaborate in structure. These shells are commonly made of
calcium carbonate (CaCO3) or agglutinated sediment particles. Over 50,000 species are
recognized, both living (10,000) and fossil (40,000). They are usually less than 1 mm in size,
but some are much larger, the largest species reaching up to 20 cm.
Classification- taxonomy
The taxonomic position of the Foraminifera has varied since their recognition as protozoa
(protists) by Schultze in 1854, there referred to as an order, Foraminiferida. Loeblich and
Tappan (1992) reranked Foraminifera as a class as it is now commonly regarded.
The Foraminifera have typically been included in the Protozoa, or in the similar Protoctista or
Protist kingdom. Compelling evidence, based primarily on molecular phylogenetics, exists
for their belonging to a major group within the Protozoa known as the Rhizaria. Prior to the
recognition of evolutionary relationships among the members of the Rhizaria, the
Foraminifera were generally grouped with other amoeboids as phylum Rhizopodea (or
Sarcodina) in the class Granuloreticulosa.
The Rhizaria are problematic, as they are often called a "supergroup", rather than using an
established taxonomic rank such as phylum. Cavalier-Smith defines the Rhizaria as an
infrakingdom within the kingdom Protozoa.
Some taxonomies put the Foraminifera in a phylum of their own, putting them on par with
the amoeboid Sarcodina in which they had been placed.
Although as yet unsupported by morphological correlates, molecular data strongly suggest
the Foraminifera are closely related to the Cercozoa and Radiolaria, both of which also
include amoeboids with complex shells; these three groups make up the Rhizaria. However,
the exact relationships of the forams to the other groups and to one another are still not
entirely clear.
Living Foraminifera
Modern Foraminifera are primarily marine organisms, but living individuals have been found
in brackish, freshwater and even terrestrial habitats. The majority of the species are benthic,
and a further 40 morphospecies are planktonic. This count may, however, represent only a
fraction of actual diversity, since many genetically discrepant species may be
morphologically indistinguishable.
A number of forams have unicellular algae as endosymbionts, from diverse lineages such as
the green algae, red algae, golden algae, diatoms, and dinoflagellates. Some forams are
kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.
Biology
The foraminiferal cell is divided into granular endoplasm and transparent ectoplasm from
which a pseudopodial net may emerge through a single opening or through many perforations
in the test. Individual pseudopods characteristically have small granules streaming in both
directions. The pseudopods are used for locomotion, anchoring, and in capturing food, which
consists of small organisms such as diatoms or bacteria.
The foraminiferal life-cycle involves an alternation between haploid and diploid generations,
although they are mostly similar in form. The haploid or gamont initially has a single
nucleus, and divides to produce numerous gametes, which typically have two flagella. The
diploid or schizont is multinucleate, and after meiosis fragments to produce new gamonts.
Multiple rounds of asexual reproduction between sexual generations is not uncommon in
benthic forms.
Abundance of certain Foraminifera is sometimes used by researchers as an indicator of the
completeness of vertical mixing in certain seas such as the Celtic Sea.
Tests
The form and composition of their tests are the primary means by which forams are identified
and classified. Most have calcareous tests, composed of calcium carbonate. In other forams,
the tests may be composed of organic material, made from small pieces of sediment
cemented together (agglutinated), and in one genus, of silica. Openings in the test, including
those that allow cytoplasm to flow between chambers, are called apertures. The test contains
an organic matrix, which can sometimes be recovered from fossil samples.
Tests are known as fossils as far back as the Cambrian period, and many marine sediments
are composed primarily of them. For instance, the limestone that makes up the pyramids of
Egypt is composed almost entirely of nummulitic benthic Foraminifera. Production estimates
indicate that reef Foraminifera annually generate about 43 million tons of calcium carbonate
per year, thus play an essential role in the production of reef carbonates.
Genetic studies have identified the naked amoeba "Reticulomyxa" and the peculiar
xenophyophores as foraminiferans without tests. A few other amoeboids produce reticulose
pseudopods, and were formerly classified with the forams as the Granuloreticulosa, but this is
no longer considered a natural group, and most are now placed among the Cercozoa.
Foraminiferan tests (ventral view)
Deep-sea species
Foraminifera are found in the deepest parts of the ocean such as the Mariana Trench,
including the Challenger Deep, the deepest part known. At these depths, below the carbonate
compensation depth, the calcium carbonate of the tests is soluble in water due to the extreme
pressure. The Foraminifera found in the Challenger Deep thus have no carbonate test, but
instead have one of organic material.
Four species found in the Challenger Deep are unknown from any other place in the oceans,
one of which is representative of an endemic genus unique to the region. They are Resigella
laevis and R. bilocularis, Nodellum aculeata, and Conicotheca nigrans (the unique genus).
All have tests that are mainly of transparent organic material which have small (about 100
nm) plates that appear to be clay.
Evolutionary significance
Dying planktonic Foraminifera continuously rain down on the sea floor in vast numbers, their
mineralized tests preserved as fossils in the accumulating sediment. Beginning in the 1960s,
and largely under the auspices of the Deep Sea Drilling, Ocean Drilling, and International
Ocean Drilling Programmes, as well as for the purposes of oil exploration, advanced deep-sea
drilling techniques have been bringing up sediment cores bearing Foraminifera fossils. The
effectively unlimited supply of these fossil tests and the relatively high-precision age-control
models available for cores has produced an exceptionally high-quality planktonic
Foraminifera fossil record dating back to the mid-Jurassic, and presents an unparalleled
record for scientists testing and documenting the evolutionary process. The exceptional
quality of the fossil record has allowed an impressively detailed picture of species interrelationships to be developed on the basis of fossils, in many cases subsequently validated
independently through molecular genetic studies on extant specimens Larger benthic
Foraminifera with complex shell structure react in a highly specific manner to the different
benthic environments and, therefore, the composition of the assemblages and the distribution
patterns of particular species reflect simultaneously bottom types and the light gradient. In the
course of Earth history, larger Foraminifera are replaced frequently. In particular,
associations of Foraminifera characterizing particular shallow water facies types are dying
out and are replaced after a certain time interval by new associations with the same structure
of shell morphology, emerging from a new evolutionary process of adaptation. These
evolutionary processes make the larger Foraminifera useful as index fossils for the Permian,
Jurassic, Cretaceous and Cenozoic.
Uses
Because of their diversity, abundance, and complex morphology, fossil foraminiferal
assemblages are useful for biostratigraphy, and can accurately give relative dates to
sedimentary rocks. The oil industry relies heavily on microfossils such as forams to find
potential hydrocarbon deposits.
Calcareous fossil Foraminifera are formed from elements found in the ancient seas where
they lived. Thus, they are very useful in paleoclimatology and paleoceanography. They can
be used to reconstruct past climate by examining the stable isotope ratios and trace element
content of the shells (tests). Global temperature and ice volume can be revealed by the
isotopes of oxygen, and the history of the carbon cycle and oceanic productivity by
examining the stable isotope ratios of carbon; see δ18O and δ13C. The concentration of trace
elements, like magnesium (Mg), lithium (Li) and boron (B), also hold a wealth of information
about global temperature cycles, continental weathering, and the role of the ocean in the
global carbon cycle. Geographic patterns seen in the fossil records of planktonic forams are
also used to reconstruct ancient ocean currents. Because certain types of Foraminifera are
found only in certain environments, they can be used to figure out the kind of environment
under which ancient marine sediments were deposited.
For the same reasons they make useful biostratigraphic markers, living foraminiferal
assemblages have been used as bioindicators in coastal environments, including indicators of
coral reef health. Because calcium carbonate is susceptible to dissolution in acidic conditions,
Foraminifera may be particularly affected by changing climate and ocean acidification.
Foraminifera have many uses in petroleum exploration and are used routinely to interpret the
ages and paleoenvironments of sedimentary strata in oil wells. Agglutinated fossil
Foraminifera buried deeply in sedimentary basins can be used to estimate thermal maturity,
which is a key factor for petroleum generation. The Foraminiferal Colouration Index (FCI) is
used to quantify colour changes and estimate burial temperature. FCI data is particularly
useful in the early stages of petroleum generation (about 100 °C).
Foraminifera can also be used in archaeology in the provenancing of some stone raw material
types. Some stone types, such as limestone, are commonly found to contain fossilised
Foraminifera. The types and concentrations of these fossils within a sample of stone can be
used to match that sample to a source known to contain the same "fossil signature".
Gallery
Fossil nummulitid foraminiferans showing microspheric and megalospheric individuals;
Eocene of the United Arab Emirates; scale in mm.
The miliolid foraminiferan Quinqueloculina from the Belgian part of the North Sea.
Thin section of a peneroplid foraminiferan from Holocene lagoonal sediment in Rice Bay,
San Salvador Island, Bahamas. Scale bar 100 micrometres.
Ammonia beccarii, a benthic foram from the North Sea.
Typical Calcareous Foraminifera
1.2
Ostracod
Ostracods will be discussed under the following headings:
-
General statement
-
Fossils
-
Description
-
Paleoclimatic reconstruction
-
Ecology
General Statement
Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as
seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified,
grouped into several orders. They are small crustaceans, typically around 1 mm (0.039 in) in
size, but varying from 0.2 to 30 mm (0.0079 to 1.1811 in) in the case of Gigantocypris. Their
bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous
valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body.
Ostracods are grouped together based on gross morphology, but the group may not be
monophyletic; their molecular phylogeny remains ambiguous.
Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of
the benthos, living on or inside the upper layer of the sea floor. Many ostracods, especially
the Podocopida, are also found in fresh water, and terrestrial species of Mesocypris are
known from humid forest soils of South Africa, Australia, New Zealand, and Tasmania. They
have a wide range of diets, and the group includes carnivores, herbivores, scavengers and
filter feeders.
As of 2008, around 2000 species and 200 genera of nonmarine ostracods are found. However,
a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots
of temporary habitats in Africa and Australia. Of the known specific and generic diversity of
nonmarine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families),
Cyprididae. Many Cyprididae occur in temporary water bodies and have drought-resistant
eggs, mixed/parthenogenetic reproduction, and the ability to swim. These biological
attributes preadapt them to form successful radiations in these habitats.
Fossils
Ostracods are "by far the most common arthropods in the fossil record" with fossils being
found from the early Ordovician to the present. An outline microfaunal zonal scheme based
on both Foraminifera and Ostracoda was compiled by M. B. Hart. Freshwater ostracods have
even been found in Baltic amber of Eocene age, having presumably been washed onto trees
during floods.
Ostracods have been particularly useful for the biozonation of marine strata on a local or
regional scale, and they are invaluable indicators of paleoenvironments because of their
widespread occurrence, small size, easily preservable, generally moulted, calcified bivalve
carapaces; the valves are a commonly found microfossil.
A find in Queensland, Australia in 2013, announced in May 2014, at the Bicentennary Site in
the Riversleigh World Heritage area, revealed both male and female specimens with very
well preserved soft tissue. This set the Guinness World Record for the oldest penis. Males
had observable sperm that is the oldest yet seen and, when analysed, showed internal
structures and has been assessed as being the largest sperm (per body size) of any animal
recorded. It was assessed that the fossilisation was achieved within several days, due to
phosphorus in the bat droppings of the cave where the ostracods were living.
Description
The body of an ostracod is encased by two valves, superficially resembling the shell of a
clam. A distinction is made between the valve (hard parts) and the body with its appendages
(soft parts).
Soft parts- The body consists of a head and thorax, separated by a slight constriction. Unlike
many other crustaceans, the body is not clearly divided into segments. The abdomen is
regressed or absent, whereas the adult gonads are relatively large.
The head is the largest part of the body, and bears most of the appendages. Two pairs of welldeveloped antennae are used to swim through the water. In addition, there is a pair of
mandibles and two pairs of maxillae. The thorax typically has two pairs of appendages, but
these are reduced to a single pair, or entirely absent, in many species. The two "rami", or
projections, from the tip of the tail, point downwards and slightly forward from the rear of the
shell.
Ostracods typically have no gills, instead taking in oxygen through branchial plates on the
body surface. Most ostracods have no heart or circulatory system, and blood simply circulates
between the valves of the shell. Nitrogenous waste is excreted through glands on the
maxillae, antennae, or both.
The primary sense of ostracods is likely touch, as they have several sensitive hairs on their
bodies and appendages. However, they do possess a single naupliar eye, and, in some cases, a
pair of compound eyes, as well.
Paleoclimatic reconstruction
A new method is in development called mutual ostracod temperature range (MOTR), similar
to the mutual climatic range (MCR) used for beetles, which can be used to infer
palaeotemperatures. The ratio of oxygen-18 to oxygen-16 (δ18O) and the ratio of magnesium
to calcium (Mg/Ca) in the calcite of ostracod valves can be used to infer information about
past hydrological regimes, global ice volume and water temperatures.
Ecology
Lifecycle- Male ostracods have two penes, corresponding to two genital openings, or
gonopores on the female. The individual sperm are often large, and are coiled up within the
testis prior to mating; in some cases, the uncoiled sperm can be up to six times the length of
the male ostracod itself. Mating typically occurs during swarming, with large numbers of
females swimming to join the males. Some species are partially or wholly parthenogenetic.
In most ostracods, eggs are either laid directly into the water as plankton, or are attached to
vegetation or the substratum. However, in some species, the eggs are brooded inside the shell,
giving them a greater degree of protection. The eggs hatch into nauplius larvae, which
already have a hard shell.
Predators- A variety of fauna prey upon ostracods in both aquatic and terrestrial
environments. An example of predation in the marine environment is the action of certain
Cuspidariidae in detecting ostracods with cilia protruding from inhalant structures, thence
drawing the ostracod prey in by a violent suction action.[16] Predation from higher animals also
occurs; for example, amphibians such as the rough-skinned newt prey upon certain ostracods.
Bioluminescence- Some ostracods have a light organ in which they produce luminescent
chemicals. Most use the light as predation defense, while some use the light for mating (only
in the Caribbean). These ostracods are called "blue sand" or "blue tears" and glow blue in the
dark at night.
1.3
Calcareous Nannofossil
The term calcareous nannofossils includes both fossil coccoliths and nannoliths. Coccoliths
are minute (<25μm) calcite objects produced by unicellular marine plants (golden-brown
algae). The origin of nannoliths is uncertain, but these calcite bodies are associated with fossil
coccoliths assemblages in marine sediments and are also organically derived.
Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution
and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well
as their varied and distinct morphologies. The oldest known calcareous nannofossils are Late
Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine
record. Relatively little has been published about the paleogeographic distributions of
calcareous nannofossils; less is known about their exact paleoenvironmental preferences,
although they have been shown occasionally to penetrate into shallow marine environments.
Their main industrial application is their calibration to published time scales and sequence
stratigraphic records, especially the association of high abundance with condensed marine
sections.
Typical calcareous nannofossils
Application of Calcareous Microfossil in Hydrocarbon Exploation
1. Benthic Foraminifera provides information about the environment of deposition of
sediments suspected for Hydrocarbon.
2. Planktonic Foraminifera are useful in time and space correlation across a depositional
basin or even across whole oceans.
N.B: Other uses of Microfossils in general in hydrocarbon exploration include:
- Biostratigraphy
- Paleoenvironmental analysis
- Paleoclimatology
- Biogeography
- Thermal maturation
QUESTION TWO: CALCAREOUS NANNOFOSSIL
Introduction
Calcareous nannofossils include the coccoliths and coccospheres of haptophyte algae and the
associated nannoliths which are of unknown provenance. The organism which creates the
coccosphere is called a coccolithophore, they are phytoplankton (autotrophs that contain
chloroplasts and photosynthesise). Their calcareous skeletons are found in marine deposits
often in vast numbers, sometimes making up the major component of a particular rock, such
as the chalk of England. One freshwater species has been reported. Formally
coccolithophores are separated from other phytoplankton such as diatoms by the presence of
a third flagella-like appendage called a haptonema, although the flagella bearing stage is
often only one of a multi-stage life cycle.
A coccolith is a single disc-like plate which is secreted by the algal organism and held in
combination with several other, sometimes varying shaped plates by an organic coating to
form the coccosphere. On death the individual coccoliths invariably become separated and it
is these that are most commonly preserved in the sedimentary record. Occasionally complete
coccospheres are preserved and provide valuable information, particularly regarding
coccospheres which possess two or more morphologicaly different coccoliths. There are two
forms of coccoliths, the holococcoliths which are formed from calcite crystals which are
essentially identical in shape and size and the heterococcoliths which are formed from larger
calcite crystals which vary in size and shape. Most living forms are known to produce only
heterococcoliths and then only during the non-motile stage of their life cycle. Those that do
produce holococcoliths do so only during their motile stage.
History of Study
The first recorded use of the term "coccoliths" is from Ehrenberg's 1836 study of the chalk
from the island of Rugen in the Baltic Sea. Ehrenberg and other early workers beleived
coccoliths to have an inorganic origin. It was not untill the second half of the nineteenth
century when Wallich found coccoliths joined to form coccospheres that an organic origin
was suggested. Even after the publication of Sorby's 1861 paper, following which the organic
origin of coccoliths was generally accepted, Ehrenberg remained unconvinced. The 1872
HMS Challenger expedition recovered coccospheres from the upper water layers and
correctly concluded that they were the skeletons of calcareous algae. The term nannoplankton
was coined by Lohmann in 1902. The study of coccolithophores has flourished since the
1960's, with much ground breaking work done on their biology as well as on the systematics
of fossil and living forms. The Deep Sea Drilling Project (DSDP), now the Ocean Drilling
Program (ODP), brought the stratigraphic value of calcareous nannofossils to the attention of
industry as well as the scientific community. Today, due to the speed of preparation,
calcareous nannofossils have bec ome the preferred tool for quick accurate stratigraphic age
determination in post-Palaeozoic calcareous sequences.
Biology
Culture techniques have resulted in great advances in the study of coccolithophore life cycles.
The existence of a haploid and diploid phase has been proved by the extraction of DNA, with
mitotic reproduction occurring in both stages. Syngamy (sexual reproduction) has not been
observed but is assumed to occur, the recent discovery of combination coccospheres (where
coccoliths of two distinct forms occur on the same coccosphere) has meant the traditional
classification will have to be radically revised and updated.
The defining feature of the haptophytes is the flagella-like haptonema which is generally
coiled. It differs from the flagella proper in its internal structure and its basal attachment.
During the non-motile phase the flagella disappear but the haptonema often remains, the
exact function of the haptonema is not fully understood. The algal cell contains a nucleus and
two golden-brown chloroplasts which may be moved around the cell to optimise collection of
available light. The cell also contains mitochondria which contain enzymes which produce
the energy for cell function, vacuoles which deal with waste products and the Golgi body
which is the site of coccolith secretion in many species. In many species overlapping oval
organic scales coat the outer cell membrane. These have concentric ridges on their distal
faces and radiating ridges on their proximal faces. It seems the organic scales act as bases for
the precipitation of the calcite coccoliths. A variety of coccolith secretion strategies have
been observed in different species, however it is probably true of all coccolithophores that the
production of coccoliths is controlled by light. Emiliania huxleyi has been observed to start
coccolith production within half an hour of being introduced to light, and produce an
individual coccolith in one hour and a complete coccosphere in about thirty hours.
Above diagram from Bown,P.(Ed.), 1998, Calcareous Nannofossil Biostratigraphy.
Chapman and Hall.
Range
First recorded occurrences of calcareous nannofossils (nannoliths) are from the late Triassic
(Carnian). The locations from which the earliest nannofossils are found include; the Northern
and Southern Calcareous Alps, Timor, North-West Australia and Queen Charlotte Islands
(Canada), all low latitude sites at the time. There are many claims for earlier occurrences but
a lack of substantiated evidence means these must be excluded. One consequence of the first
occurrence of calcareous nannofossils in the late Triassic lies in the fact that this was the first
time open ocean planktonic organisms utilised calcareous skeletons and exported calcium
carbonate into the deep oceans. This has important repercussions in terms of biogeochemical
cycles. Today coccolithophores are one of the most important forms of phytoplankton found
in the oceans, and may be described as the grass of the sea.
Lifecycle
Reproduction of coccolithophores is by single or double fission sometimes accompanied by a
swarm-spore stage. The information we have on coccolithophore reproduction is based on
only a few species so care must be taken when making generalisations, however, it is thought
the coccolith-bearing phase is diploid and capable of asexual (mitotic) reproduction. This
allows rapid population growth during periods of optimum conditions, producing what are
known as "blooms". Motile naked haploid gametes may be produced by meiosis and nonmotile benthic stages are also known to be produced. Sexual fusion has rarely been observed
but is inferred by the variation of DNA found within coccolithophpores.
Mineralogy of Coccoliths
The calcium carbonate in coccoliths normally crystallizes as calcite and to a lesser degree as
aragonite. In laboratory cultures traces of third polymorph of lime, vaterite have also been
found (Wilber and Watabe, 1963). In fossil state only calcite is found.
Coccolithophore Ecology
Coccolithophores are exclusively planktonic marine organisms which are distributed from
open ocean, neritic (shelf) environment to near shore littoral and inshore lagoonal
environment. Coccolithophores rely on photosynthesis as its prime nutritional mode limiting
them to photic zone of the oceans (Fig. 2). Environmental parameters affect plankton
communities both spatially and temporally (Honjo, 1976). Temperature plays a key role and
is important in controlling species distribution in largely defined latitudinally arranged
biogeographical zones (Baumann et al., 2005). By and large coccolithophores live in open
ocean and are adapted to salinities 32-37 ppt (Baumann et al., 2005). Coccolithophorid
species diversity is highest in stratified, warm, oligotrophic environments where salinity
values are high. The coastal areas are dominated by lower salinities and thus the nannofloral
species diversity is low e.g. dominance of Emiliania huxleyi (Houghton, 1988, 1991). Only
one coccolithophorid species Hymenonas roseola is known to inhabit freshwater environment
(Baumann et al., 2005). All phytoplankton require some nutrients for their growth and
biochemical reactions. The most important nutrients are nitrate and phosphate. Nitrate is
essential for growth and calcification, whereas phosphate is a controlling agent for
calcification
(Baumann
et
al.,
2005).
In
eutrophic
(nutrient-rich)
environments
coccolithophores are outcompeted by diatoms forming relatively minor components of the
total communities. High productivity of coccolithophorids takes place in eutrophic conditions
whereas K-selected ones are adapted to oligotrophic conditions. However, coccolithophores
as a group achieve highest relative abundances within phytoplankton communities in
oligotrophic environments (Winter and Siesser, 1994). Being photosynthetic they require
light for carbon fixation. Most species thrive in upper photic waters. Some species, such as
Florisphaera profunda inhabit the lower photic zone i.e. LPZ (<1% to 4% of the surface
irradiance). The LPZ is usually a permanent feature of the subtropical gyres but may develop
in well-stratified waters in subtropical and temperate regions in summer months.
Application of Calcareous Nannofossils in Hydrocarbon Exploration
Calcareous nannofossils are extremely small objects (less than 25 microns) produced by
planktonic unicellular algae. As the name implies, they are made of calcium carbonate.
Nannofossils first appeared during the Mesozoic era and have persisted and evolved through
time. The function of the calcareous “plates”, even in living forms, is uncertain. One extant
group that produces “nannofossils” is the Coccolithophorans, Planktonic golden-brown algae
that are very abundant in the world’s oceans. The calcareous plates accumulate on the ocean
floor, become buried beneath later layers, and are preserved as nannofossils. Like the
planktonic foraminifera, the planktonic mode of life and the tremendous abundance of
calcareous nannofossils make them very useful tools for biostratigraphy.
Nannofossils are amongst the rare group of fossils which have been tied with chronologic
time through magnetostratigraphy or rarely radiometric dates, during the vast amount of data
gathered during the Deep Sea Drilling and Ocean Drilling Projects. This advantage along
with the fine chronostratigraphic resolution of nannofossil zones and events make them one
of the most potent tools for stratigraphic correlation. This is of prime importance in the
hydrocarbon industry, where fine zonations are required to decipher pay zone level
correlations. The added advantage of nannofossils is that where ever present, their
distribution is largely independent of the depositional facies in which they occur, the only
danger being diagenetic modification and destruction.
QUESTION THREE: SILICEOUS MICROFOSSILS
Siliceous microfossils are protists with shells constructed of opaline (amorphous) silica.
There is no intense dissolution of siliceous remains in the deep ocean. Sediments deposited
below the carbonate compensation depth are commonly enriched in silica by removal of the
carbonate, sometimes to the point of forming siliceous oozes. With subsequent remobilization
of the silica, deep-sea cherts may be formed. Siliceous microfossils are subject to burial
diagenesis and become rare at great well depths except when recrystallized, preserved in
nodules or concretions, or replaced by pyrite or calcite.
There are three major groups of siliceous microfossils:
-
Radiolarians
-
Diatoms and
-
Silicoflagellates.
Radiolarians
Radiolarians are planktonic protists that occur primarily in open marine, deep-water settings.
They are useful time indicators and are found in rocks of Cambrian to Holocene age. They
may be the only common microfossils in abyssal environments, commonly forming
radiolarian oozes. Radiolarian chert, the product of silica diagenesis, is fairly widespread in
the geologic record. Radiolarians are common in some marine source rocks.
Typical radiolarians
Diatoms
Diatoms are photosynthesizing protists that occur in both marine and nonmarine
environments. Marine diatoms range from Upper Jurassic or Lower Cretaceous to Holocene
and are particularly useful for age and environmental determinations in the upper Cenozoic.
Nonmarine diatoms range from Eocene to Holocene and also are useful in the upper
Cenozoic. These microfossils can be a major rock-forming group, forming sedimentary rock
(diatomites) consisting primarily of diatoms. Diatomaceous sediments, when altered by burial
diagenesis, are converted to siliceous shale, porcellanite, and chert. Such rocks can serve as
sources and fractured reservoirs for hydrocarbons (e.g., Monterey Formation of California).
The changes in rock properties associated with silica diagenesis permit seismic definition of
silica phase transformation zones in the subsurface (e.g., bottom-simulating reflector).
Typical diatoms
Silicoflagellates
Silicoflagellates are another group of planktonic photosynthesizing marine protists; they
commonly occur with diatoms. Silicoflagellates range in age from Cretaceous to Holocene.
Although not as common as diatoms, they are useful time indicators, particularly in the upper
Cenozoic. As a group, they were much more abundant during the early and middle Cenozoic
than today. They have been used to estimate marine paleotemperatures in the late Tertiary
and Quaternary.
Typical silicoflagellates
Application of Siliceous Microfossils in Hydrocarbon Exploration
Just like other Microfossils, Siliceous Microfossils have the following application in
Hydrocarbon exploration:
-
Biostratigraphy (Biostratigraphy is the differentiation of rock units based upon the
fossils which they contain)
-
Paleoenvironmental analysis (this is the interpretation of the depositional environment
in which the rock unit formed, based upon the fossils found within the unit)
-
Paleoclimatology
-
Biogeography
-
Thermal maturation
QUESTION FOUR: PHOSPHATIC MICROFOSSILS
Phosphatic microfossils, notably conodonts, are composed of crystallites of calcium
phosphate (apatite) embedded in an organic matrix. There is one type of stratigraphically
significant phosphatic microfossils (conodonts); but fish teeth, of less practical utility, are
found in some marine strata.
Conodonts
Conodonts are extinct toothlike microfossils composed of calcium phosphate whose
biological affinities, while poorly understood, lie with chordates. Conodonts are widely
distributed in marine rocks of Cambrian through Triassic age. They are excellent indicators of
time and thermal maturity—especially in carbonates, where other methods of evaluating
organic thermal maturity are less successful. Conodonts are commonly used as zonal indices
for the latest Cambrian through Triassic because they were abundant, evolved rapidly, and
were widespread geographically. Although found in most marine rocks, conodonts are most
efficiently recovered from the insoluble residues of carbonates dissolved in weak acids or
from easily disaggregated shales.
Individual conodonts vary greatly in morphology, and taxonomy was originally based on the
morphology of these individual specimens. While conodonts are common, the preserved
remains of the soft-bodied animal that bore them are extremely rare. Based on a few
preserved whole-animal specimens discovered recently (e.g., conodonts appear to have been
located in the cephalic area and may have functioned as teeth. However, the conodont animal
apparently bore many conodonts of differing shapes and morphologies, based on the study of
the very rare whole-animal specimens and rare bedding-plane groupings of conodonts
representing individual animals. This recent information has led to more accurate
multielement species concepts.
Typical conodonts
Application of Phoshatic Microfossils in Hydrocarbon Exploration
Just like other Microfossils, Phosphatic Microfossils have the following application in
Hydrocarbon exploration:
-
Biostratigraphy (Biostratigraphy is the differentiation of rock units based upon the
fossils which they contain)
-
Paleoenvironmental analysis (this is the interpretation of the depositional environment
in which the rock unit formed, based upon the fossils found within the unit)
-
Paleoclimatology
-
Biogeography
-
Thermal maturation
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