Download Possible Impact of Polar Climatic Perturbations

Marine biodiversity variation in the
low-latitude region
Possible Impact of Polar
Climatic Perturbations
A. D. Singh, Centre of Advanced Study in Geology
Banaras Hindu University, Varanasi-221005
In order to understand response of marine biota to changes in ocean
environment linked to global climatic fluctuations, it is essential to study how
various biotic groups were evolved in the oceans, their diversifications, and
selective extinctions during certain geological periods and its possible causes?.
Marine sediment records have been the best storehouse of history of changes in
oceans’s environments (physico-chemical) occurred in the past and its impact
on the marine life. Valuable information about the past biological changes in
oceans are extracted from the shells preserved in sediments (as fossils) that were
secreted by microscopic fauna and flora remarkably sensitive to minor changes
in oceanic environment. Studies have suggested that climatic perturbations
triggered changes in ocean environment (sea surface temperature, sea level,
ocean currents, surface and bottom circulations, nutrient condition and oxygen
level etc.) causing major changes in marine biota. The changes in physicochemical condition of ocean influence profoundly marine biotic communities
in respect of their biogeographic distribution, abundances, diversities and
morphologies. We considered foraminfera for our study as they are very sensitive
to changes in ocean environment. Our study reveals a general low diversity of
benthic foraminifera during 15 to 30, 000 years ago, a period of glacial stage
when northern Hemisphere glaciation enhanced, followed by a gradual increase
in diversity reaching to the modern values in the Holocene period which is
equivalent to the warm interglacial stage. Within the glacial stage, there have
been periods of intensified cold events as recorded from the Greenland ice core,
during which benthic diversity in the Arabian Sea increased. The diversity
fluctuated in the warm Holocene period and that also probably to be linked
with polar climate perturbations.
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Session: 2
Introduction
There is increasing concern regarding the impacts
of climate change on marine biodiversity and its
conservation and management. Although the oceans
cover more than 70 % of the Earth’s surface, our
knowledge on marine biodiversity patterns is limited
as compared to that of the terrestrial biodiversities.
In order to understand response of marine biota
to changes in ocean environment linked to global
climatic fluctuations, it is essential to study how
various biotic groups were evolved in the oceans,
their diversifications, and selective extinctions during
certain geological periods and its possible causes?
Recently, a group of scientists is of the opinion that
climate change (what we are witnessing today?) can
lead extinction of certain marine species (eg. Thomas
et al., 2004). But, such statement should be taken
into consideration with a caution, because influence
of climatic perturbations on marine biota (animals
and plants) is complex and ocean specific. In view
of this, it has become necessary to study history of
changing biotic patterns in oceans that took place in
the geological past as a consequence of major changes
in oceanographic and climatic conditions. Marine
sediment records have been the best storehouse of
history of changes in oceans’s environments (physicochemical) occurred in the past and its impact on
the marine life. Valuable information about the
past biological changes in oceans are extracted
from the shells preserved in sediments (as fossils)
that were secreted by microscopic fauna and flora
remarkably sensitive to minor changes in oceanic
environment. Recent decades have witnessed a rapid
developments in various aspects of ocean research
(both modern and past ocean environments), with
the birth of a new discipline in Earth System Science
i.e. Paleoceanography dealing with the evolution
of the ocean basins, its circulation history and
hydrographic, climatic and biotic patterns. As a result
, several International ocean drilling programmes
have been initiated that enabled to recover a large
number of sediment cores from the world oceans,
rich in microfossils of groups such as foraminifera,
pteropods, ostracodes, nannoplanktons, radiolarian
and diatoms. The oceanic microfossil records are not
only important to portray the history of changes in
biotic patterns, but also crucial to better understand
how global climate change in the past has influenced
marine biota.
Studies have broadly revealed that periods of
evolution and diversification of planktons in oceans
were coincident with the major climatically induced
oceanographic changes (Wei and Kennett, 1986).
Evidences have suggested that climate played a major
role in promoting unprecedented increase in marine
biodiversity around 460 million years ago (Trotter
et al., 2008). During this period the ocean surface
temperature was estimated as similar to that of today.
The
physico-chemical
and
biological
environments of modern oceans are closely coupled
with characteristics (waxing and waning) of polar ice
caps. Before around 60 million years ago, the global
climate was warm and geographical thermal gradients
were low (Douglas and Savin, 1975). Oceanic surface
temperature of 100C was at high latitudes and 200C
at low latitudes. Significant changes occurred during
45-60 million years ago, which was a transitional
phase between earlier thermospheric circulation
and later the psychrospheric and thermohaline
circulation. The history of present day glacial mode
of Earth was initiated during 25-30 million years ago
with the beginning of ice build up on southern pole.
Subsequently, major diversification in foraminiferal
group took place during 22-23 million years ago
(beginning of the Neogene period), when global
climatic condition and ocean circulation had changed
significantly resulting major shifts in marine trophic
conditions (Kennett and Srinivasan, 1983). In the last
60 million years of the Earth, s history, this was the
period of an overall increase in marine biodiversity
(Fig. 1).
The latest major climatic-oceanographic event
was the development of an ice sheet in the northern
Hemisphere (Arctic region) during 2.5-3.0 million
years ago. Thus, prior to 3 million years, polar ice
sheets were restricted to Antarctica, and a bipolar
symmetry in global climate was developed only after
development of permanent ice sheets in the arctic
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67
region (Srinivasan, 2003).
The climatic studies based on the ice cores (polar
ice sheets and high altitude regions), marine and
terrestrial archives have revealed that the Earth has
experienced cycles of cold (glacial: ice ages) and warm
(interglacial) phases during the last 900, 000 years.
The glacial phases are characterized by the expanded
polar ice sheets, low atmospheric CO2 and low surface
temperature. Super imposed on the glacial-interglacial
cycles, there has been climate variability on shorter
time scales (decadal, centennial and millennial) [eg.
Dansgaard et al., 1993]. Studies further suggested
that climatic perturbations triggered changes in ocean
environment (sea surface temperature, sea level, ocean
currents, surface and bottom circulations, nutrient
condition and oxygen level etc.) causing major
changes in marine biota. The changes in physicochemical condition of ocean influence profoundly
marine biotic communities in respect of their
68 ❘ Science & Geopolitics of Arctic & Antarctic: SaGAA 2011
biogeographic distribution, abundances, diversities
and morphologies. The nature of biotic patterns
depends on sensitivity of biota and how do they
respond to changes in various ecological parameters.
In view of present scenario of changing climate,
studies were undertaken on microfossil records of
calcareous foraminifera from the Arabian Sea region
generated from ocean bottom sediment archives to
evaluate , if there has been any change in the diversity
patterns in the past and if yes, was it linked with the
climatic perturbations at high latitudes. Foraminfera
are very sensitive to changes in ocean environment
and this group comprises two subgroups one that lives
in surface waters and other on ocean bottom, thus
providing information about biotic responses to both
the ocean surface and bottom conditions.
On longer time scale, it has been observed that
diversities of both the planktic and benthic were
generally high with a gradual minor decline since 2.5
Session: 2
Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 ❘
69
million years ago coincident with the establishment
of northern Hemisphere glaciations (Fig. 2). A high
resolution time series of foraminiferal diversity record
for the last 30, 000 years was also generated based
on the Arabian Sea sediment core (Fig. 3). The study
reveals a general low diversity of benthic foraminifera
during 15 to 30, 000 years ago, a period of glacial
stage when northern Hemisphere glaciation enhanced,
followed by a gradual increase in diversity reaching to
the modern values in the Holocene epoch which is
equivalent to the warm interglacial stage. Within the
glacial stage, there have been periods of intensified cold
events as recorded from the Greenland ice core, during
which benthic diversity in the Arabian Sea increased.
The diversity fluctuated in the warm Holocene period
and that also probably to be linked with polar climate
perturbations. It is intriguing to record that diversity
of planktic foraminifera did not change significantly
in the last 30, 000 years as was seen in the benthic
foraminifera. To evaluate the probable reasons for
diversity variations in this region, the faunal diversity
records were compared with the records of primary
productivity and sea surface temperature that are
known to be governed by the monsoon circulation
pattern and greatly influenced by the global climatic
perturbations (Singh et al., 2006, Anand et al.,
2008). Data suggest that primary productivity and
sea surface temperature in the Arabian Sea have
significantly varied in the past in concert with the
high latitude climatic perturbations. Variation in
primary productivity also results changes in bottom
oxygen condition with oxic environment during low
productivity period and oxygen poor environment
during high productivity period. Combined effects
of these factors (primary productivity and sea bottom
oxygen environment) seem to have influenced the
benthic foraminiferal species diversity in recent past.
On the other hand changes in hydrographic conditions
do not appear to significantly influence the diversity
pattern of planktic foraminifera.
Important Issues to be addressed:
(1) Has marine diversity changed through time and
70 ❘ Science & Geopolitics of Arctic & Antarctic: SaGAA 2011
space since pre-industrial era to post-industrial period
?.
(2) How did different marine faunal and floral
species particularly in coastal ecosystems respond
to environment changes related to industrial
developments?
(3) Will it be possible to separate natural changes
in marine biotic communities from anthropogenic
changes?.
(4) Are ocean waters in the Arabian Sea and Bay of
Bengal acidifying in recent years ? if yes, how does it
affect calcifying biota in the two basins ?.
References:
Thomas, C.D. et al., 2004. Nature, 427, 145-148.
Wei, K-Y. and Kennett, J.P., 1986. Paleoceanography,
1, 67-84.
Trotter, J.A. et al., 2008. Science, 321, 550-554.
Douglas, R.G. and Savin, S.M., 1975. Initial Reports
DSDP, 32, 509-520.
Kennett, J.P. and Srinivasan M.S., 1983. Neogene
planktonic foraminifera: A
phylogenetic Atlas, Hutchinson Ross Publication,
USA, 265p.
Dansgaard, W. et al., 1993. Nature, 364, 218-220.
Singh, A.D. et al., 2006. Geological Society of India,
68, 369-377.
Anand, P et al., 2008. Paleoceanography, 23, PA4207,
doi:10.1029/2007PA001564.
Srinavasn, M.S., 2003. 32nd Prof. Birbal Sahni
Memorial Lecture, Lucknow