Снеговой Павел Иванович
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Тема: #40659
Сообщение: #1308895
22.04.05 08:48
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Scientific Report 1995-1997
PE-04. A 45.000 YEAR VARVE CHRONOLOGY FROM JAPAN
H. Kitagawa, J. van der Plicht
Lake Suigetsu is located near the coast of the Sea of Japan. A 75-m long continuous core was taken from the center of the lake. The sediments are characterized by dark-coloured clay with white layers due to spring season diatom growth. The seasonal changes in the depositions are preserved in the clay as thin, sub-millimeter scale laminations or "varves". Based on observation of varve thickness change, we expect that the annually laminated sediment records the palaeoenvironmental changes during the past 100 ka.
This sequence of annually laminated sediments not only forms a unique continuous palaeoenvironmental record after the last interglacial but also permits us to reconstruct a complete 14C calibration extending back to at least 45 ka BP, and probably even more by means of combined isotope enrichment and AMS 14C dating1. We have performed AMS 14C measurements on more than 250 terrestrial macrofossil samples of the annual laminated sediments from lake Suigetsu.
In order to build up a calendar time scale (i.e., varve chronology) for the Suigetsu (SG) core, a total of 85 subsamples were taken in a section of SG extending from 10.43 to 30.34 m below the top sediment, each ca. 25 cm in length, including a 1.5 cm overlap with neighbouring subsamples. To allow detailed observation of the sediments, the well-cleaned surfaces of sediments were scanned with a digital camera. By means of computer image analysis of digital pictures, we found that the much less distinct varves observed in some intervals during the deglaciation and Glacial could be determined only with a relatively large error. In order to reconstruct a more precise and longer varve chronology for the laminated sediments from Lake Suigetsu, we have reassessed the varve chronology in the whole section during the deglaciation as well as the Glacial up to a depth of 30.45 m.
The uncertainty in the varve chronology comes from two sources: core sampling and varve counting. Detailed comparison with short piston cores shows that the sampling does not cause significant loss of varves - typically 0-2 cm to a maximum of 3 cm, corresponding to ca. 20-30 yr in the Holocene and ca. 50 yr in the Glacial. Since the varve ages from below 18 m (corresponding to ca. 20,000 cal BP) were estimated by varve counting of a single core, the ages quoted should be considered as minimum ages, the error increasing with depth. Based on the results of some duplicated countings of selected subsamples and independent counting of different subsamples collected from the same horizon, we estimate that the counting error is < 1.5%, corresponding to 150 yr for 10,000 varve years.
From the laminated sediments we selected terrestrial-origin macrofossils such as leaves, branches and insects for AMS 14C measurements. The samples are processed using a strong acid-alkali-acid (AAA) treatment for both samples and reference blank materials. The reference blank consists of more than 50 14C-free plant materials collected from the deep layer of the same core (corresponding to an age of ca. 90-100 ka).
The Lake Suigetsu floating varve chronology consists of 29,100 varves. The sedimentation or annual varve thickness is relatively uniform (typically 1.2 mm yr-1 during the Holocene and 0.62 mm yr-1 during the Glacial). The age below 30.45 m depth is obtained by assuming a constant sedimentation in the Glacial (0.62 mm yr-1).
In order to reconstruct the calendar time scale, we compared the Lake Suigetsu chronology with calibration curves obtained from recently revised absolute German oak and the floating German pine calibration curves2. Figure PE-4 shows the best match between the tree-ring and the Lake Suigetsu chronologies, estimated by minimizing the weighted sum of squared differences between the 14C ages of macrofossils and the tree-ring calibration curve. The features in our data overlapping the tree-ring calibration agree very well, even for "wiggles" in the 14C calibration curves. Using this match, we defined the absolute time scale for the Lake Suigetsu varves chronology. The 29,100 yr Lake Suigetsu chronology then covers the absolute age range from 8830 to 37,930 cal BP.
The combined 14C and varve chronologies from Lake Suigetsu are used to calibrate the 14C time scale beyond the range of the absolute tree-ring calibration. Figure 2 shows an atmospheric 14C calibration for the complete 14C dating range (<45 ka).The tree-ring calibration range, our calibration agrees well with the European sediments3 and generally with marine calibrations obtained by combined U/Th and 14C dating of corals4,5. The datelist with varve years vs. 14C datings (fig. PE-5) is published in ref. 6.
The detailed record in atmospheric D14C during the deglaciation shows millennium scale fluctuations superimposed on a long-term increasing trend, resulting from a decreasing geomagnetic intensity as reconstructed from geomagnetic records. Abrupt D14C drops correspond to radiocarbon plateaus in the calibration curve. Near (a few centuries after) the onset of the Younger Dryas (YD), the D14C value drops by 80 per mil from 10,800 to 9,800 BP (12.500 to 10,000 cal BP); the drop thus extends into the Preboreal (the earliest Holocene). This radiocarbon plateau has been well known in marine and terrestrial records, and is referred to as the YD plateau. Our calibration shows apparently that the YD plateau
consists of two sub-plateaus at 10,000 and about 10,400 BP; the older one is characterized by a time of slow increase of the radiocarbon age. A similar drop in D14C of ca. 100 per mil (including magnetic effect) is observed from 12,600 to 12,100 BP (or 15,000 to 13,800 cal BP), which starts within the Oldest Dryas (OD) cold period and extends until nearly the end of the Allerød/Bolling warm period. This plateau can be related to the radiocarbon plateau recorded in (non-varved) sediment cores from Switzerland. Our data show a strong indication for a plateau around the OD cold period. It appears that the two radiocarbon plateaus in the YD and OD cold periods started a few centuries after the warm-to-cold transition. Furthermore, we also observe the millennium scale fluctuations of about 100î in D14C before the OD; maxima at 16,000, 17,500 and 19,000 cal BP and minima at 15,500, 17,000 and 19,600 cal BP. For a detailed discussion of our 14Ä data during the Late Glacial period in terms of thermohaline circulation, we refer to ref. 7.
From the last Glacial Maximum to 31,000 cal BP, the long-term trend of D14C agrees well with reconstruction of cosmogenic isotope production rate deduced by 10Be deposition reconstruction and geomagnetic field intensity reconstruction (Fig. PE-6). For this time span, we observe two pronounced peaks in D14C at 23,000 and 31,000 cal BP. The apparent D14C increases correspond to an increase in the concentration of another cosmogenic isotope, 10Be, at 23,000 and about 35,000 cal BP, respectively, observed in ice cores from the Antarctic and Greenland as well as in marine sediments8. Furthermore a 14C anomaly at these times has been observed previously in speleothems, dated by both 14C and U-series. The time gap between the 14C and 10Be enhancements can be explained by the errors in both varve and ice core chronologies, as well as the different geochemical behaviour of these different isotopes; 14C is present in gaseous form (CO2) and gradually diffuses in the earth system, while 10Be is a solid attached to aerosol particles and is deposited with precipitation.
The broad increase in cosmogenic isotopes (both 10Be and 14C) at 23,000 BP can be explained as the increase in production rate by geomagnetic effects. The sharp 14C peak we observed at ca. 31,000 BP is roughly 300 per mil in D14C after removing the long-term trend. The 10Be increases by a factor of 2 in ice cores during a period of ca. 2,000 years. This factor of 2 increase corresponds to a 14C increase by a factor 1.3 or 300 per mil, which is exactly what we observe in our data. These sharp enhancements in 10Be and 14C at the same time are too large to be explained by rearrangements of the carbon reservoirs on the earth.
Increased cosmogenic isotope production caused by a nearby supernova explosion has been suggested as a cause for the drastically increased 10Be levels at this time. Another possible explanation is a magnetic excursion with a sharp change in inclination of the geomagnetic field and the implied concomitant decrease in the geomagnetic field strength. Such events are observed as the Mono Lake and Laschamp excursions, dated at 28,000 BP and 33,000 BP (uncalibrated), respectively. The sharp D14C increase from Lake Suigetsu corresponds chronologically to the Mona Lake excursion. However, all of these explanations remain hypothetical.
Literature
Kitagawa, H. and van der Plicht, J., 1997. Enrichment of sub-milligram size carbon samples. Nuclear Instruments and Methods B 123, 218-220.
Kromer, B., Ambers, J., Baillie, M. G. L., Damon, P. E., Hesshaimer, V., Hofmann, J., Joris, O., Levin, I., Manning, W., McCormac, F. G., van der Plicht, J., Spurk, M., Stuiver, M. and Weninger, B. (1996) Report: summary of the workshop " Aspects of high-precision radiocarbon calibration". Radiocarbon 38 (3) 607-610.
Goslar, T. Arnold, M. Bard, E., Kuc, T., Pazdur, M. F., Palska-Jasiewiczowa, M., Rózanski, K., Tisnerat, N., Walanus, A., Wicik, B. and Wieckowski, K. (1995) High concentration of atmospheric 14C during the Younger Dryas cold episode. Nature 377: 414-417.
Bard, E., Arnold, M., Fairbanks, R. G. and Hamelin, B. (1993) 230Th /234Th and 14C ages obtained by mass spectroscopy on corals. In Stuiver, M., Long,, A. and Kra, R. S. eds., Calibration 1993. Radiocarbon 35: 191-199.
Edwards, R. L., Beck, J. W., Burr, G. S., Donahue, D. L., Chappel, J. M.A., Bloom, A. L., Druffel, E. R. M. and Taylor, F. W. (1993) A large drop in atmospheric 14C /12C and reduced melting in Younger Dryas, documented with 230Th ages of corals. Science 260: 962-967.
Kitagawa, H. and van der Plicht, J. (1998) Radiocarbon, in press (Groningen Conference Proceedings).
Kitagawa, H. and van der Plicht, J. (1998) Atmospheric Radiocrbon calibration to 45000 BP : Late Glacial fluctuations and Cosmogenic Isotope production. Science 279, 1187-1190.
Raisbeck, G.M. , Yiou,F., Bourles, D., Lorius, C., Jouzel, J., and Barkov, N.I., Evidence for two intervals of enhanced 10Be deposition in Antartic ice during the last glacial period, Nature 326, 273-277.
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