* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Radiocarbon dating of late Quaternary sediments: reservoir
Survey
Document related concepts
Marine life wikipedia , lookup
Sea in culture wikipedia , lookup
Arctic Ocean wikipedia , lookup
Anoxic event wikipedia , lookup
Ocean acidification wikipedia , lookup
Marine debris wikipedia , lookup
Physical oceanography wikipedia , lookup
Marine habitats wikipedia , lookup
Blue carbon wikipedia , lookup
Raised beach wikipedia , lookup
Ecosystem of the North Pacific Subtropical Gyre wikipedia , lookup
The Marine Mammal Center wikipedia , lookup
Marine pollution wikipedia , lookup
Transcript
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 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. 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. 28 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 29 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. 30 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 31 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. 32 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. References Awasthi, N., Ray, J.S., Laskar, A.H., Kumar, A., Sudhakar, M., Bhutani, R., Sheth, H.C., Yadava, M.G. (2010). Major ash eruptions of Barren Island volcano (Andaman Sea) during the past 72 kyr: clues from a sediment core record. Bulletin of Volcanology 72, 1131-1136. Bradley, R.S. (1999). Paleoclimatology: Reconstructing Climates of the Quaternary. Academic Press, San Diego. Buck, C.E., Christen, J.A., and James, G.N. (1999). BCal: an online Bayesian radiocarbon calibration tool. Internet Archaeology 7 (http://intarch.ac.uk/journal/issue7/buck/)) Butzin, M., Prange, M. and Lohmann, G. (2005). Radiocarbon simulations for the glacial ocean: the effects of wind stress, Southern Ocean sea ice and Heinrich events. Earth and Planet. Sci. Lett. 235, 45-61. Cao, L., Fairbanks, R. G., Mortlock, R. A. and Risk, M. (2007). Radiocarbon reservoir age of high latitude North Atlantic surface water during the last deglacial. Quat. Sci. Rev. 26, 732-742. Colin, C., Kissel, C., Blamart, D. and Turpin, L. (1998). Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon. Earth and Planet. Sci. Lett. 160, 623-635. Dutta, K., Bhushan, R. and Somayajulu, B.L.K. (2001). 14C reservoir ages and ΔR correction values for the northern Indian Ocean, Radiocarbon 43, 483-488. 34 A. V. Sijinkumar, B. Nagender Nath /GCK Science Letters 2-2 (2013) 26-34 Hajdas, I. (2006). Radiocarbon dating for Quaternary studies - New developments and perspectives. Quat. Int. 154, 160-163. Prins, M.A., Postma, G., Weltje, G.J. (2000). Controls on terrigenous sediment supply to the Arabian Sea during the Late Quaternary: the Makran continental slope. Mar. Geol. 169, 351-371. Ramsey, B.C. (1995). Radiocarbon calibration and analysis of stratigraphy: The OxCal program. Radiocarbon, 37(2), 425-430. Rashid, H., Flower, B.P., Poore, R.Z. and Quinn, T.M. 2007. A ~25 ka Indian Ocean monsoon variability record from the Andaman Sea. Quat. Sci. Rev. 26, 2586-2597. Reimer, P. J., Brown, T. A. and Reimer, R.W. (2004). Discussion reporting and calibration of post-bomb 14C data. Radiocarbon 46(3), 1299-1304. Fairbanks, R.G., Mortlock, R.A., Chiu, T.C., Cao, L., Kaplan, A., Guilderson, T.P., Fairbanks, T.W. and Bloom, A.L. (2005). Marine Radiocarbon Calibration Curve Spanning 0 to 50,000 Years B.P. Based on Paired 230Th, 234 U, 238U and 14C Dates on Pristine Corals. Quat. Sci. Rev. 24, 1781-1796. Sijinkumar, A.V., Nath, B.N., Guptha, M.V.S. (2010). Late Quaternary record of pteropod preservation from the Andaman Sea. Mar. Geol. 275, 221-229, doi:10.1016/ j.margeo.2010.06.003. Stuiver, M., Braziunas, T. (1993). Modeling atmospheric 14C influences and ages of marine samples to 10,000 BC. Radiocarbon 35, 137-189. 14 C Stuiver, M., Reimer, P. J., and Reimer, R. W. (2005). CALIB 5.0. [WWW program and documentation]. Stuiver, M., Reimer, P. (1993). Extended 14C data base and revised CALIB 3.0 14 C age calibration program. Radiocarbon 35, 215-230. von Rad, U., Schulz, H., Riech, V., den Dulk, M., Berner, U., Sirocko, F. (1999). Multiple monsoon-controlled breakdown of oxygen-minimum conditions during the past 30,000 years documented in laminated sediments off Pakistan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 152, 129-161. Weninger, B., Jöris, O. and Danzeglocke, U. (2007). CalPal-2007 Cologne Radiocarbon Calibration & Palaeoclimate Research Package, http://www.calpal.de