Summer Research Fellowship Programme of India's Science Academies 2017
Variability of the abundance of
Globorotalia menardii and
Neogloboquadrina dutertrei in the
Eastern Indian Ocean
Madhurima Ganguly
Guided by:
Planktonic foraminifera are a group of unicellular, free-floating protozoa mostly in photic
zone but can also be found in deeper waters including tropical as well as subtropical and
polar water masses. Size of the foraminiferal test ranges from 0.05mm to 0.5mm with a
maximum diameter of 18cm. They are composed of calcite tests which after their death
settles in the ocean bottom forming “calcareous ooze” (Vincent and Berger, 1981).There are
approximately 40 planktonic species till date. Planktonic foraminifera serves as an essential
potential proxy for studying paleoclimatology/paleoceanography due to their sensitivity to
environmental conditions, their distribution through passive transport, their abundance all
over the ocean waters and due to their potential to be well preserved.
The discovery of planktonic foraminifera took place over a century ago and since then they
have been used in paleoceanography for analysis in biostratigraphy. D’Orbigny (1826,
1839a,b) explained planktonic foraminifera from beach sands of the islands of Cuba and
Canary. Brady (1884) found that the collection of planktonic foraminifera could be carried
out in plankton samples. Murray and Renard (1891) described climatic biased pattern of
distribution of planktonic foraminifera. Since then shells of planktonic foraminifera have
been used to calculate the age of terrestrial and/or marine sedimentary facies and also
decipher past climatic changes. Rhumbler (1901, 1911) determined that spinous planktonic
foraminifera are inclined towards copepod diet, whereas non-spinous species shows a
preference for diatoms and radiolarians. Schott, in 1935 published data on the patterns of
distribution of planktonic foraminifera in tropical waters and interpreted stratigraphy of
sediments of Pleistocene age. Emiliani (1954, 1955b) deduced the paleo temperature of the
surrounding environment with the help of stable isotope method where growth of
foraminifera was based on O
isotope composition of carbonate shells.
Temperature and other water mass properties served as major factors which
determinedthe distribution of planktonic foraminifera geographically and also their
evolutionary course (e.g., Frerichs, 1971; Caron and Homewood, 1983). Sea surface
temperature fluctuations have been paralleled with the evolutionary events of planktonic
foraminifera. During the cold periods, there was a drop in the diversity of species which was
identified by the occurrence of simple trochospiral tests. Warm periods followed thereafter
witnessed species radiation as the niches left vacant were invaded and utilised by new
evolving species. Variation in shell morphology coupled with adaptive evolutionary radiation
during warm climatic conditions include development of pustules and spines, accessory
aperture, clavate chambers with tubular elongations and texture of test-surface wall, as
major identification features.
Biology of planktonic foraminifera was studied extensively by two new methods:-
Blue-water SCUBA diving methods for collection of completely undisturbed
specimens for observations in laboratory and other ecological experiments (Hamner,
1975; Alldredge and Jones, 1973).
High resolution Transmission Electron Microscope and Scanning Electron Microscope
is used to deduce the events of shell deposition and to characterize foraminiferal
cellular processes such as reproduction, digestion and relationship between host and
How Are Organisms Used As Proxies?
Foraminifera are shelled organisms found in aquatic and marine environment. Their shells
are composed up of calcium carbonate which record evidence of environmental conditions
of the past. Foraminiferal remains are obtained by taking sediment cores from oceans after
their death when their shells get buried and preserved in sediment. The chemical
composition of the shells suggests the chemistry of water when the shell was formed. Past
water temperature is inferred from stable O
isotope ratios of shells. The lighter isotopes
are evaporated off by warmer water hence the shells which grow in those waters are
saturated with heavier isotopes. Organisms tend to flourish in warm weather conditions.
Since each species has a definite set of condition that is ideal for its growth, composition of
species at a particular site and time indicate environmental conditions of the past.
Why is Paleoclimatology Important?
The Quaternary period witnessed major climatic changes due to which it is considered as the
most eventful compared to all other geologic periods. There were large changes in global
climatic conditions during the Late Quaternary. In order to comprehend our present day
climate, it is necessary to understand the variation in climate and oceanographic changes during
the Quaternary period. Paleoclimatology is the science of understanding the climate of
geological past i.e. palaeoclimate. It is associated with changes in climatic condition of the past
and is also essential for present and future issues. Current ecosystems conditions can be
inferred by understanding past climate. The data provided by paleoclimatology can be used
for modelling as well as predicting the climate change scenarios of both present and future.
Effect of increased CO
on climate can be studied by computer models. Palaeoclimate data is
also used as a reference for climate scientists by providing important information like rates
of change of climate of the past and how such changes affect vegetation and animal
populations. Paleoceanography is the new idea which is brought into light in recent earth
sciences which deals with the past history of oceans, evolution of circulation patterns of shallow
and deep sea, water masses and also changes in productivity of biological organisms due to
change in climate. Planktonic foraminifera provide a potential proxy for study of Quaternary
climatic and oceanography.
Applications of Planktonic Foraminifera
Past Monsoon Variations:
Planktonic foraminifera serve as sensitive monsoon indicators due to their direct
response to oceanographic condition (Prell and Curry, 1981). Reconstruction of
Indian Monsoon Variability was carried out by for over 230 ka BP with the help of
summer monsoon proxies such as G. bulloides, G. glutinata, G. falconensis and
G.menardii and winter monsoon proxies such as G. ruber, N. dutertrei, N. Compacta
and G. sacculifer from NW Arabian Sea.
Past Productivity Reconstruction:
Reconstruction of paleoceanography and paleoproductivity are done by studying
planktonic foraminiferal assemblages along with their isotopic composition (e.g.,
Niebler et al., 1999; Lea et al., 2000; Meggers et al., 2002). Abundance of
microfossilhas a direct relationship with productivity. Abundance of planktonic
foraminifera are used as proxies extensively for upwelling related changes in
productivity. Thus, certain species with relative abundances are identified as high
productivity indicators (Prell and Curry, 1981). Some organisms occur in high
productivity regions whereas others do not within each group of planktonic shelled
Past Seawater Temperature Reconstruction:
Planktonic foraminifers are used for estimation of past Sea Surface Temperature
(SST) by O
isotope studies and also by transfer function of planktonic foraminiferal
shells. They provide important clues for estimating SST, and also the difference in
composition of species contains clues to the water temperatures in which they lived
(Broecker, 1986).
Past Salinity Reconstruction:
Salinity controls seawater density as well as deep water circulation along with
temperature (Henderson, 2002). Rostek et al (1993) used alkenones for calculating
temperature for Indian Ocean and further used δ
O values of G. ruber for
determining salinities. Paleo-salinity of Late Quaternary was reconstructed with the
use of planktonic foraminiferal assemblages (Cullen, 1980) coupled with O
records (Kudras et al., 2001) of Bay of Bengal.
In Determining Chemistry of Ocean Water:
Shell chemistry is essential to understand the chemistry of water in which it grew.
Ratio of stable O
isotopes is dependent on the temperature of wateras warmer
water has a tendency to evaporate off the lighter isotopes. Past surface and bottom
temperatures of the water have been employed to measure stable O
isotope in
planktonic shells from over hundreds of deep sea cores.
Sea Level Changes:
Fluctuation in the Eustatic sea level took place in accordance with waxing and
waning of continental ice sheets (Chappell and Shackleton, 1986). Reconstruction of
sea level changes as well as understanding bathymetry of variety of species is carried
out with the help of foraminifera. Paleo-depth models are constructed by the use of
barnacle fouling which are intertidal sessile crustaceans, and with change in depth,
sea- level changes can be inferred(Nigam, 1996).
Biostratigraphy is mainly dependent upon index fossils along with first and last
appearance datum and also species acme zone in sedimentary formation. Evidence
of relative ages of marine rocks are provided by foraminifera due to their fairly
continuous evolutionary development; hence at different times, a variety of
different species are found. The abundance and widespread distribution of
foraminifera in all types of marine environments, their small size and easy collection
even from deep oil wells finds a wide application in biostratigraphy.
Petroleum Exploration:
Planktonic foraminifera serve as excellent index fossils for exploration of oil and
natural gas. A particular geological period is determined when organic matters
decayed under anaerobic conditions in marine rocks. Usually marine environment is
essential for the formation of petroleum and the foraminiferal assemblages serve as
good indicators of a particular environmental condition and thus they help in
locating oil-bearing horizon. These fossils find its application in directing sideways
drilling within an oil-bearing horizon so as to increase productivity of wells.