Pieter M. Grootes and Michael Sarnthein ML (Kiel)
With 2 Figures
Although the fact that the surface ocean is depleted in 14C relative to the atmosphere is well known, it has not been easy to determine the natural degree of depletion due to the distur-bance of the atmospheric 14C content by atmospheric nuclear weapons tests started in 1954.
Initial information, largely derived from paired marine/terrestrial samples providing mostly near-coastal coverage, indicated a globally averaged 14C depletion of the surface ocean by about 5 % with some local deviations. This depletion is similar to the one reached after 400 years of decay from atmospheric concentration, and customarily 14C depletions are reported as apparent decay ages, the “reservoir ages”. Since transport of 14C from its production at the stratosphere/troposphere transition to its decay in the (deep) ocean leads to differences in 14C concentration between the well-mixed atmosphere and different oceanic reservoirs, this description as an apparent age is misleading, especially for the surface ocean where the 14C concentration reflects the balance between ocean-atmosphere exchange and oceanic mixing instead of decay. Considering the paucity of data, the 14C depletion/reservoir age was assumed to be constant over time, at least for the Holocene. The analytical capabilities of accelerator mass spectrometry (AMS) have, however, increasingly documented oceanic 14C depletions changing with location and time. These changes reflect the atmospheric produc-tion of 14C as well as the dynamics of the carbon cycle in the ocean and atmosphere. The 14C record of planktonic and benthic foraminifera in sediments thus holds important information on the behaviour of these systems in the past.
As the ocean reservoirs, especially the deep ocean, contain large amounts of carbon (>90 % of the active carbon of the global carbon cycle resides as DIC in the ocean), their reservoir ages are considered to be fairly stable. Thus, indications of large, rapid changes in ocean reservoir age are often seen as problematic. Yet, the difference in 14C concentration between an oceanic reservoir and the atmosphere, that is the apparent oceanic reservoir age, may vary significantly with changes in the 14C concentration in the atmosphere – that contains less than 2 % of the active carbon – without a significant change in the 14C concentration of the oceanic reservoir (Fig. 1). Such atmospheric changes may be due to 14C production and/
or oceanic outgassing. Thus the 14C concentration of a surface ocean reservoir cannot be used as a stand-alone indicator of ocean dynamics, but requires complementary information on atmospheric and oceanic mixing for its interpretation (Fig. 2).
To develop radiocarbon calibration beyond the range of tree rings, it was assumed that 14C concentrations in the surface ocean closely follow those in the atmosphere with a constant, though locally different, offset expressed as reservoir age. This approach is still used for
Pieter M. Grootes and Michael Sarnthein
82 Nova Acta Leopoldina NF 121, Nr. 408, 81– 84 (2015)
14C depletion ~ reservoir age Atmosphere
Surface ocean Thermocline
Deep ocean
cal BP
10000 8000 6000 4000 2000 0
8000 6000 4000 2000 [0]
cal BC cal AD
10000 200 150 100 50 0 -50 -100 -150 -200 Δ14C (‰)
Atmospheric Δ14C and modelled ocean Δ14C values (Stuiver and Braziunas, 1993)
Fig. 1 Atmospheric Δ14C (bidecadal values) used as input for the model calculations and calculated surface ocean (0 –75 m), thermocline (75 –1000 m), and deep ocean (1000 –3800 m) Δ14C values. The oceanic 14C depletion relative to the atmosphere is often reported as an apparent reservoir age.
IntCal 2013, where the meanwhile documented variability in reservoir ages is treated as extra uncertainty in the data or, in some cases, leads to the exclusion of the data from the calibration data set (Reimer et al. 2013). Following this reasoning, one may assume that the fine structure of the atmospheric 14C record over time – with periodic increases and decreases in atmospher-ic 14C concentration reflected by steep parts (“jumps”) and “flat” parts (“plateaus”) in the age calibration curve – can also be observed in the 14C record of the undisturbed surface ocean.
Thus, tuning a suite of 14C age plateaus and jumps in the planktonic 14C record of a sediment core to the corresponding suite of age plateaus in the atmosphere can provide both absolute ages and reservoir ages (Sarnthein et al. 2007). By now, the deglacial sections of 14 sedi-ment records from key sites in the ocean circulation have been tuned (Sarnthein et al. 2015, and unpublished data) to the varve counted Suigetsu record of past atmospheric 14C concen-trations (Bronk Ramsey et al. 2012). For the sediment records concerned, the varve counted timescale appears preferable to the modelled Suigetsu timescale that is generally used. Yet, the varve counted Suigetsu timescale creates in the deglacial some, as yet unresolved, prob-lems with calibration data derived from carbonates. The tuning has revealed surprisingly high and variable reservoir ages for the surface as well as the deep ocean, ranging from 100 to
Oceanic Reservoir Ages, 14C Concentrations, and Carbon Dynamics
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2500 years. To obtain the 14C concentration of the local surface or deep ocean, the atmos-pheric 14C concentration of the time needs to be multiplied by the fraction of atmospheric 14C indicated by the reservoir age. As the atmospheric 14C concentration during the LGM was quite high and the deglacial showed a variable and strongly decreasing concentration, the 14C concentration changes calculated for the oceanic reservoirs are generally far more modest and are easy to reconcile with the general understanding of carbon dynamics in the ocean.
References
Bronk Ramsey, C., Staff, R. A., Bryant, C. L., Brock, F., Kitagawa, H., van der Plicht, J., Schlolaut, G., Marshall, M. H., Brauer, A., Lamb, H. F., Payne, R. L., Tarasov, P. E., Haraguchi, T., Gotanda, K., Yonenobu, H., Yokoyama, Y., Tada, R., and Nakagawa, T.: A complete terrestrial radiocarbon record for 11.2 to 52.8 kyr B.P. Science 338, 370 –374 (2012)
Reimer, P. J., Bard, E., Bayliss, A., Beck, J. W., Blackwell, P. G., Bronk Ramsey, C., Buck, C. E., Cheng, H., Edwards, R. L., Friedrich, M., Grootes, P. M., Guilderson, T. P., Hajdas, I., Hatté, C., Heaton, T. J., Hoffmann, D. I., Hogg, A. G., Hughen, K. A., Kaiser, K. F., Kromer, B., Manning, S. W., Niu, M., Reimer, R. W., Richards, D. A., Scott, E. M., Southon, J. R., Staff, R. A., Turney, C. S. M., and van der Plicht, J.: INTCAL13 and MARINE13 radiocarbon age calibration curves, 0 –50,000 years cal. BP. Radiocarbon 55/4, 1869 –1887 (2013)
-72 ‰Δ14C 600 yr Res. Age
-117 ‰Δ14C 1000 yr Res. Age
-50 ‰Δ14C 410 yr Res. Age
-258 ‰Δ14C 2400 yr Res. Age 0 ‰Δ14C
SO
NP
NA TA 90 90 Gt/yr TIP
65°N
0°W 60°S
0°-360°E / W 55°N
180°W Fig. 2 Simplified scheme of the meridional overturning circulation (blue arrows) and accompanying changes in 14C concentration in the modern ocean. The 14C ages and carbon contents are subject to a delicate balance between (i) the gradual aging of preformed carbon from the northern North Atlantic (NA) via the Southern Ocean (SO) up to the subpolar North Pacific (NP) and (ii) the incremental absorption of young biogenic organic and inorganic carbon supplied by the biological pump from the sea surface. Vertical mixing (black arrows) brings intermediate and deep waters depleted in 14C to the surface. TA indicates Tropical Atlantic, TIP the Tropical Indo-Pacific.
Pieter M. Grootes and Michael Sarnthein
84 Nova Acta Leopoldina NF 121, Nr. 408, 81– 84 (2015)
Sarnthein, M., Grootes, P. M., Kennett, J. P., and Nadeau, M.-J.: 14C Reservoir ages show deglacial changes in ocean currents and carbon cycle. In: Schmittner, A., Chiang, J. C. H., and Hemming, S. R. (Eds.): Ocean Cir-culation: Mechanisms and Impacts – Past and Future Changes of Meridional Overturning. Geophys. Monograph 173, pp. 175 –196. AGU. Wash. D.C.; doi: 10.1029/173GM13 (2007)
Sarnthein, M., Balmer, S., Grootes, P. M., and Mudelsee, M.: Planktic and benthic 14C reservoir ages for three ocean basins, calibrated by a suite of 14C plateaus in the glacial-to-deglacial Suigetsu atmospheric 14C record.
Radiocarbon (2015, in press)
Stuiver, M., and Braziunas, T. F.: Modeling atmospheric C-14 influences and C-14 ages of marine samples to 10,000 BC. Radiocarbon 35/1, 137–189 (1993)
Prof. Dr. Pieter M. Grootes University of Kiel
Institute of Ecosystem Research Olshausenstraße 40
24098 Kiel Germany
Phone: +49 431 8801229 Fax: +49 431 8804083
E-Mail: pgrootes@ecology.uni-kiel.de Prof. Dr. Michael Sarnthein University of Kiel
Institute for Geosciences Ludewig-Meyn-Straße 10 24118 Kiel
Germany
Phone: +49 431 8802882 Fax: +49 431 8804376 E-Mail: ms@gpi.uni-kiel.de
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