Download Radiocarbon dating of late Quaternary sediments: reservoir

GCK SCIENCE LETTERS
Vol. 2 (2), July 2013, pp. 26-34
Radiocarbon dating of late Quaternary sediments: reservoir
correction and calibration
A.V. Sijinkumar1,*, B. Nagender Nath2
1
Department of Post Graduate Studies & Research in Geology, Govt. College
Kasaragod, Kerala, 671123, India
2
National Institute of Oceanography (Council of Scientific and Industrial
Research), Dona Paula, Goa, 403004, India
Abstract
Accurate dating is of fundamental importance to late Quaternary
paleoclimatic and paleoceanographic studies. According to the principle of
superposition, younger sediments superimpose on the older ones resulting in a
sedimentary record that provides a direct record of geological events in
chronological order. To place the geological changes in a time perspective, one
needs to have knowledge of absolute dates of such events recorded in a
sedimentary sequence. The major problem in the present Quaternary studies is
that each of the chronostratigraphic method used for dating the sediments have
either time or material limitations. Without reliable estimates on the age of events
in the past it is impossible to investigate the synchrony of the events. The most
common method for determining ages of marine sediments is radiocarbon dating
of fossil calcareous tests of surface dwelling foraminifera and is frequently
employed in high-resolution paleoclimate studies of the late Quaternary period.
At the same time, radiocarbon dating has the capability to date only the records of
age < 50,000 years. In this study, an attempt is made to understand different
aspects of radiocarbon dating of carbonate shells, their calibration methods and
application in estimating the sedimentation rate. We have used surface dwelling
mixed planktic foraminifera for the radiocarbon dating of a gravity core collected
from the Andaman Sea. The dating of a core from the Andaman Sea has yielded
an average sedimentation rate of 8 cm/ka which is compared with other published
records.
Keywords: Radiocarbon; late Quaternary; Reservoir correction; planktic
foraminifera; Calibration
__________________________________________________________________
* Corresponding author: Tel. +91 9020495237; E-mail: [email protected]
Introduction
Radiocarbon dating (also known as carbon dating) is a radiometric dating
method that uses the naturally occurring radioisotope carbon-14 (14C) to estimate
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
the age of carbon-bearing materials of younger age. The radiocarbon method was
developed by a team of scientists led by the late Professor Willard F. Libby of the
University of Chicago in 1949. 14C or radiocarbon dating has proved to be by far
the most useful method (Stuiver and Reimer, 1993). Because of the ubiquitous
distribution of 14C the technique can be used throughout the world and has been
used to date samples. 14C is continuously producing in the upper atmosphere by
the reaction of atmospheric nitrogen with neutrons that are produced from cosmic
ray spallation reaction on other atmospheric components (14N7 + 1n0 = 14C6 + 1H1
+ energy) (Bradley, 1999).
The energetic 14C atoms freshly formed in the atmosphere are soon
oxidized to 14CO2 and enters the earth's plant and animal life ways through
photosynthesis and the food chain. Radioactive carbon dioxide 14CO2 being
indistinguishable from other forms of CO2. 14C activities of most terrestrial living
organisms are therefore in equilibrium with that in the atmosphere, through
continuous exchange of 14C by photosynthesis or food intake and respiration (Fig.
1). When an organism dies the 14C exchange halts, and the 14C in the dead tissues
start to decrease exponentially through radioactive decay. 14C forms stable
nitrogen through beta decay and half-life of 5730±40 years and the concentration
can be measured by the use of atomic mass spectrometer. Radiocarbon dating
underwent a technological revolution in the late 1970s and early 1980s when a
method for dating very small organic samples was developed, using an accelerator
coupled to a mass spectrometer (AMS dating).
The most common method for determining ages of marine sediments is
radiocarbon dating of fossil calcareous tests of surface dwelling foraminifers and
is frequently employed in high-resolution paleoclimate studies of the late
Quaternary period. Because of the ubiquitous distribution of 14C, the technique
can be used throughout the world and has been used to date samples. Major
limitation of radiocarbon dating is the capability to date only the records of age <
50 ka. Accurate dating is also important to decipher the lead and lag of events
recorded in different sequences and regions. At present, dating of the Quaternary
sediments is mainly carried out using radiometric methods. For the radiocarbon
dating, widely used carbonate shells is of planktic foraminifera (Fig. 2). Oxygen
isotope stratigraphy also contributed greatly to Quaternary studies and provides
another reliable means for correlation and age assignment. Biological methods,
which are mainly based on index fossils, first and last appearance datum and acme
zone of a species in a sedimentary formation.
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
Fig. 1 Schematic representation of 14C production in the atmosphere and its
interaction with other reservoirs i.e., ocean, biosphere, soils and
sediments. The radioactive 14C clock begins
to measure time when
the equilibrium between the production and the decay is broken (Hajdas,
2006).
In this study, we have used radiocarbon method for dating a sedimentary core
SK 168 collected from the Andaman Sea. The radiocarbon ages are calibrated to
calendar ages by using CalPal radiocarbon calibration program. The calendar ages
are used to calculate the sedimentation rate of this part of the Andaman Sea. The
sedimentation rate was calculated is compared with other published records.
Methodology
A sediment core SK 168/GC-1 was collected (Lat 11° 42.463′ N; Long 94°
29.606′ E, water depth = 2064 m, core length: 4.20 m) during the 168th cruise of
ORV Sagar Kanya from the Alcock Seamount Complex in the Andaman Sea
(Fig. 3). About 10 g of dried samples were soaked in distilled water overnight and
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
washed through a 63μm mesh sieve using distilled water. Later the dried filtrate
was sieved through 125μ mesh sieve. The coarse fraction (>125μm) was used for
picking of selected planktic foraminifera by using a stereo zoom binocular
microscope for radiocarbon dating. The AMS dating was carried out at NOSAMS
facility at WHOI, USA. We used mixed planktic foraminifera such as
Globigerinoides ruber and Globigerinoides sacculifer (Fig. 2). The procedures for
the analysis of Accelerator Mass Spectrometer (AMS) include: acid hydrolysis
(HY), combustion (OC), or stripping of CO2 gas from water (WS) samples.
Radiocarbon ages are calculated using 5730 years as the half-life of radiocarbon.
Atoms of 14C contained in a sample are directly counted using the AMS method
of radiocarbon analysis.
Fig. 2 Photographs of planktic foraminifera (a, b) Globigerinoides ruber and (c,
d) Globigerinoides sacculifer from the core SK 168, Andaman Sea.
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
Reservoir effect
It is now well known that radiocarbon dating of marine shell samples or
marine mammal residue is skewed by the reservoir effect of the oceans. As a
result, in most regions marine samples yield radiocarbon ages substantially older
than those yielded by terrestrial samples. The 14C ages of marine fossils collected
from the surface waters (~200 m) measure on worldwide average of 400 years
older than contemporary terrestrial wood, since the reservoir from which these
foraminifers derive carbon has lower 14C/12C ratios compared to the atmosphere.
This is mainly due to the mixing with deeper 14C depleted water (Stuiver and
Braziunas, 1993). Considerable spatial variability is seen in the apparent 14C ages
of marine calcareous shells due to variations in the regional ocean circulation
patterns. At the same time, however, the effect in the various levels/depths of the
ocean is to dampen reflections of short-term oscillations that occur in the
incidence of atmospheric 14C (Stuiver and Braziunas, 1993). This phenomenon
constitutes the oceanic reservoir effect. But it has been clearly shown that
substantial regional variation in the magnitude of this effect in surface waters
results from the degree of local upwelling, which brings deeper waters into the
upper levels (Stuiver and Braziunas, 1993). Hence, radiocarbon calibration is
essential to be carried out for an accurate final chronology.
Because the ocean is a large carbon reservoir, the residence time of 14C is
long compared to the atmosphere. In the Indian Ocean, von Rad et al. (1999)
measured 14C in known age forams from varved sediment cores recovered off
Pakistan, and Dutta et al. (2001) dated mollusks to determine reservoir ages from
several sites in the Arabian Sea and Bay of Bengal. These studies have yielded a
value which nearly matching with worldwide average of 400 years (Dutta et al.
2001, Butzin et al. 2005, Cao et al. 2007).
Radiocarbon calibration
As already mentioned, calibration is essential for interpretation of
radiocarbon ages, especially when comparing to historical records or to other data
with a different chronological basis. The accurate radiocarbon calibration of ages
is critical for developing late Quaternary chronologies of paleoclimate and
archaeological research. There are various softwares which can be applied for
radiocarbon calibration. The major difficulty is the variations in atmospheric 14C
content that complicate the conversion of conventional 14C ages BP into real
calendar ages. The most commonly using calibration softwares are CalPal 2007
programme (Weninger et al., 2007; http://www.calpal.de) and CALIB 5.0.2
program (Stuiiver and Reimer 1993). The other programs include Oxcal, BCal,
CaliBomb, Fairbanks Radiocarbon Calibration etc. Each program has its own
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
advantages and disadvantages. Some of these radiocarbon calibration programs
are compiled in table 1.
Table 1. The available and frequently using radiocarbon calibration programs
Sl.
No.
1
Name of the
Program
Oxcal
Developed by
Website Address
Reference
Oxford University
Ramsey, 1995
2
CALIB
3
BCal
4
CALPAL
5
CaliBomb
6
Fairbanks
Radiocarbon
Calibration
Queen's University
Belfast
University of
Sheffield
Cologne
Radiocarbon
Calibration
Queen's University
Belfast
Columbia University
http://c14.arch.ox.a
c.uk/
http://calib.qub.ac.
uk/calib/
http://bcal.shef.ac.u
k/
http://www.calpalonline.de/
http://calib.qub.ac.
uk/CALIBomb
http://radiocarbon.l
deo.columbia.edu
Reimer et al.
2004
Fairbanks et
al., 2005
Stuiver et al.,
2005
Buck et al.,
1999
Weninger et
al., 2007
Linear sedimentation rates
Based on the age model constructed from the radiocarbon dating, linear
sedimentation rate is calculated for thousand years (cm/ka). The accumulation of
terrigenous materials in the marine basins is generally controlled by climate and
resultant strength of fluvial system. The rate of sedimentation varies with water
depth and distance of land to the basin. The accumulation of sediments on the
seafloor is not evenly distributed and depends basically on the bottom topography
and hydrographical conditions. The changes in marine sedimentation rates
provide preliminary clues about the past variation in fluvial erosion input, aeolian
dust and the marine productivity (Prins et al., 2000). Linear sedimentation rate is
also useful in estimating mass accumulation rates of the components at the
seafloor.
The available published records from the Andaman Sea are compared with
core SK 168 which is shown in the figure 3. The average sedimentation rate from
the north to south are 20 cm/ ka (RC 12-344; Rashid et al., 2007), 5 cm/ ka (SK 234;
Awasthi et al., 2010), 8 cm/ka (Sijinkumar et al., 2010) and 11 cm / ka (MD77-169;
Colin et al., 1998). This high spatial variability in sedimentation may be related
with the bottom topography of the Andaman Sea and the vicinity of core location
with respect river mouth.
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A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
Fig. 3 The available published records from the Andaman Sea are compared with
core SK
168. The average sedimentation rate is marked on the right
side of the figure as box. (RC 12344; Rashid et al., 2007; SK 234;
Awasthi et al., 2010; Sijinkumar et al., 2010 and MD77- 169; Colin et
al., 1998).
Conclusions
The major problem in the present Quaternary studies is that each of the
chronostratigraphic method used for dating the sediments have either time or
material limitations. The most common method for determining ages of marine
sediments is radiocarbon dating of fossil calcareous tests of surface dwelling
foraminifera and is frequently employed in high-resolution paleoclimate studies
of the late Quaternary period. In this study, we used surface dwelling mixed
33
A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34
planktic foraminifera such as Globigerinoides ruber and Globigerinoides
sacculifer. The radiocarbon ages are calibrated into calendar ages by using
radiocarbon calibration program CalPal 2007. Other most popular radiocarbon
calibration programs are also discussed along with reservoir effect. The
radiocarbon dating of core for the present study from the Andaman Sea has
yielded an average sedimentation rate of 8 cm/ka which is comparatively lower
than other published records. The higher sedimentation rate of northern core RC
12-344 is possibly due to its close proximity to the Ayeyarwady river mouth. This
high spatial variability in sedimentation may be related with the bottom
topography of the Andaman Sea and the vicinity of core location with respect
river mouth.
Acknowledgements
We thank the Principal, Government College Kasaragod and Director,
National Institute of Oceanography, Goa, for the permission to publish this paper.
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