Radiocarbon dating of late Quaternary sediments: reservoir correction and calibration

GCK Science Letters July 2013 ISSN: 2320-2572 Volume 2

Issue 2

A Science Journal Published from

Government College Kasaragod

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

1Department of Post Graduate Studies & Research in Geology, Govt. College

Kasaragod, Kerala, 671123, India

2National 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 (14

C) to estimate

27 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. 14

C or radiocarbon dating has proved to be by far

the most useful method (Stuiver and Reimer, 1993). Because of the ubiquitous

distribution of 14

C the technique can be used throughout the world and has been

used to date samples. 14

C 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 (14

N7 + 1n0 =

14C6 +

1H1

+ energy) (Bradley, 1999).

The energetic 14

C atoms freshly formed in the atmosphere are soon

oxidized to 14

CO2 and enters the earth's plant and animal life ways through

photosynthesis and the food chain. Radioactive carbon dioxide 14

CO2 being

indistinguishable from other forms of CO2. 14

C activities of most terrestrial living

organisms are therefore in equilibrium with that in the atmosphere, through

continuous exchange of 14

C by photosynthesis or food intake and respiration (Fig.

1). When an organism dies the 14

C exchange halts, and the 14

C in the dead tissues

start to decrease exponentially through radioactive decay. 14

C 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 14

C, 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.


Fig. 1 Schematic representation of 14

C production in the atmosphere and its

interaction with other reservoirs i.e., ocean, biosphere, soils and

sediments. The radioactive 14

C 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


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 14

C 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.


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 14

C 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 14

C/12

C ratios compared to the atmosphere.

This is mainly due to the mixing with deeper 14

C depleted water (Stuiver and

Braziunas, 1993). Considerable spatial variability is seen in the apparent 14

C 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 14

C (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 14

C is

long compared to the atmosphere. In the Indian Ocean, von Rad et al. (1999)

measured 14

C 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 14

C

content that complicate the conversion of conventional 14

C 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


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.

Name of the

Program

Developed by Website Address Reference

1 Oxcal Oxford University http://c14.arch.ox.a

c.uk/

Ramsey, 1995

2 CALIB Queen's University

Belfast

http://calib.qub.ac.

uk/calib/

Stuiver et al.,

2005

3 BCal University of

Sheffield

http://bcal.shef.ac.u

k/

Buck et al.,

1999

4 CALPAL Cologne

Radiocarbon

Calibration

http://www.calpal-

online.de/

Weninger et

al., 2007

5 CaliBomb Queen's University

Belfast

http://calib.qub.ac.

uk/CALIBomb

Reimer et al.

2004

6 Fairbanks

Radiocarbon

Calibration

Columbia University http://radiocarbon.l

deo.columbia.edu

Fairbanks et

al., 2005

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.


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 12- 344; 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


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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Radiocarbon dating of late Quaternary sediments: reservoir correction and calibration

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