Edouard Bard (Aix-en-Provence, France)
With 2 Figures
Radiocarbon ages measured on shells of biological organisms growing on shallow coastlines or living among the plankton must be corrected for the difference in 14C composition between the atmosphere and the sea surface. This problem has been clearly identified since the mid-1950s (e.g. Craig 1954), but fortuitously, the usual amplitude of the 14C shift to be corrected for is on the order of 400 years, which, for marine carbonates, cancels out the isotopic frac-tionation correction embedded in the calculation of a conventional 14C age (the δ13C of the dated sample being normalized to –25 ‰).
Subsequent surveys of the sea surface reservoir age (SSRA) have demonstrated that its value is not always equal to 400 years, but that it varies between 300 and 1200 years in the modern ocean (see Fig. 1 and compilations published by Bard 1988 and Reimer and Reimer 2001). From the point of view of 14C chronometry, the SSRA spatial variability is thus viewed as a problem which limits the accuracy of radiocarbon ages.
Fig. 1 Sea surface reservoir ages plotted versus the latitude based on direct 14C measurements of the dissolved TCO2 in surface water from the World Ocean (green labels stand for samples from the Atlantic Ocean and blue labels for those from the Indian and Pacific Oceans). These samples were collected in the 1950s before the thermonuclear bomb tests in the atmosphere (Bard 1988 and references therein).
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In parallel, numerical modelling has allowed to quantify the residence time of carbon atoms in the different reservoirs of the global carbon cycle, notably in the different water masses of the World Ocean. The ocean mixed layer is a complex reservoir, sandwiched as it is between the atmosphere and the intermediate and deep ocean with its slow circulation accompanied with a significant radioactive decay of 14C atoms. At steady state, the SSRA is thus determined by the mixing of young carbon from the atmosphere with older carbon from advected waters.
Bard (1988) performed the first systematic and quantitative study of the various causes of SSRA variations by using the box-diffusion model of Oeschger et al. (1975). It was shown that the SSRA depends on a combination of multiple parameters such as atmospheric pCO2, the free ocean surface, the piston velocity for gas exchange, the solubility of CO2, kinetic fractionation factors, the mixed layer depth, the mixing of the deep ocean, upwelling inten-sity and even transient changes of the 14C production in the upper atmosphere. Several of these factors are directly related to climate parameters such as sea-surface temperature, wind strength, sea-ice cover.
This led Bard (1988) to propose that the SSRA should be viewed as a new paleoceano-graphic proxy, rather than being considered solely as an obstacle to accurate chronometry. He went further by proposing two methods to reconstruct paleo-SSRA by measuring 14C in coeval contemporaneous marine and terrestrial organic matter which are found in association at the same site or can be linked stratigraphically by the same precise time marker, instantaneous at the geological scale. The first technique is restricted to shallow coastal sediments enabling the physical association of continental and marine material, while the second technique can be used over long distances with marine samples from the open ocean. Volcanic eruptions were identified as the best instantaneous time marker since they can be found as tephra layers in ma-rine and lake sediments. The Vedde Ash eruption which occurred about 12,000 years ago in the middle of the Younger Dryas cold event was proposed as an ideal test case for this new proxy.
By dating planktonic foraminifera mixed with Vedde tephras at several sites from the North Atlantic, Bard et al. (1994) reported the first results using this technique and document-ed a significant increase of the SSRA in the middle of the Younger Dryas event (800 years as compared with a modern value of 400 years). In the wake of this initial work, several other papers have been published on SSRA variations based on tephra layers of various ages from the North-Atlantic (Austin et al. 1995, Haflidason et al. 1995, Bondevik et al. 1999, 2001, 2006, Eiriksson et al. 2000, 2004, 2011, Knudsen and Eiriksson 2002, Larsen et al. 2002, Thornalley et al. 2011b) and other locations from the World Ocean (Sikes et al. 2000, Hutchinson et al. 2004, Siani et al. 2001, 2013, Ikehara et al. 2011, Skinner et al. 2015).
Based on the tephra approach, large SSRA values of up to 2000 years have been reported for specific cold periods such as the Younger Dryas and Heinrich event 1 in the North Atlantic and Mediterranean Sea (Siani et al. 2001, Thornalley et al. 2011b). Similarly, large SSRA values in the subtropical Southern Pacific have been reconstructed for the last glacial peri-od (Sikes et al. 2000, Skinner et al. 2015). These studies point to the possibility of SSRA change by more than 1000 years at a specific location.
Unfortunately, suitable volcanic eruptions are not that frequent and tephra layers are unevenly distributed in the sediments of the World Ocean. This has led several authors to apply usual stratigraphic techniques to date oceanic records by correlation of paleoceano-graphic records with other paleoclimatic records that have been dated accurately. This tech-nique has allowed to calculate SSRA changes that are similar to or sometimes even larger than those reconstructed with the tephra method. For example, SSRA values on the order of
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2000 –2500 years have been reported for the Northeast Atlantic during the last deglaciation period (Waelbroeck et al. 2001, Peck et al. 2006, Thornalley et al. 2011a, Skinner et al. 2014), but a recent compilation of North-Atlantic data led to smaller values of up to 1300 years (Stern and Lisiecki 2013).
By correlating the 14C stratigraphies directly with the 14C calibration curve, Sarnthein et al. (2007) calculated SSRA for several sites ranging from low latitudes (South China Sea, Santa Barbara Basin) to high latitudes (Icelandic Basin, Subarctic Northwest Pacific). They reconstructed very large SSRA values of up to 2000 –2500 years for the glacial period. Com-paring those evaluations with present day values for the same locations translates into dra-matic SSRA increases of up to greater than +1500 years for the South China Sea and of about +2000 years for the Icelandic Sea.
Kubota et al. (2014) made a recent attempt to calculate SSRA changes by compiling 14C ages in U-Th dated coral from tropical Pacific islands. Comparison with the 14C calibration curve led them to propose that SSRA increased by +400 years during the Heinrich 1 event, resulting in SSRA values on the order of 800 years in the tropical Pacific.
In parallel to reconstructing paleo-SSRA values from geological archives, numerical mod-els can be used to simulate SSRA as a response to past climate changes occurring over the
14C time range. For example, the atmospheric CO2 concentration was lower during the glacial period (190 vs. 280 ppm) which led to an increase of the reservoir age by about +200 years for the full change between the Last Glacial Maximum and Holocene periods (Bard 1988, 1998;
Fig. 2). It is generally considered that wind speeds were higher during the last glacial period as a response to a steepened temperature gradient between low and high latitudes. Increasing the wind speed velocity by 50 % on average would increase the CO2 piston velocity, thereby leading to a reduction of the reservoir age by about –250 years (Bard 1988). This first-order calculation based on a box diffusion model is certainly a maximum value, as increased wind speed also favours mixing with older water from below the surface box.
In addition to these global changes, which partly cancel out, it is important to take into account the possibility of local variations in 14C reservoir ages linked to regional paleoceano-graphic changes. For example, high latitudes are affected by sea-ice which limits air-sea gas exchange. Bard et al. (1994) used a 13-box model to calculate shifts of up to +350 years as a response to perennial sea-ice in the Nordic Seas.
Several modelling groups have gone farther in simulating the spatial variations of SSRA by using more complex models to mimic the ocean-atmosphere couple and its physical and biogeochemical responses (Stocker and Wright 1998, Delaygue et al. 2003, Butzin et al. 2005, Franke et al. 2008, Singarayer et al. 2008, Ritz et al. 2008, Hain et al. 2011).
These models have been used to calculate global maps of the SSRA under steady state climate changes representing glacial conditions. The modelled SSRA are generally larger than for the present day ocean with increases of up to a couple of centuries at low latitudes, several centu-ries in the Northern latitudes and up to a millennium in the Southern Ocean.
Numerical models have also been used to calculate rapid SSRA changes linked to tran-sient climate changes resulting from alterations of the Meridional Overturning Circulation (MOC) when it is forced with freshwater in the zones of deep-water convection. The SSRA response can be complex with transient decrease and increase, but the overall amplitude of the swings is limited to a few centuries, and even less in zones remote from the convection zones.
Overall, the SSRA changes simulated by models are smaller than the largest changes recon-structed from 14C measurements in oceanic sediments. This contrast may be due to deficiencies
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of numerical models in capturing the exchange timescales between the main carbon reservoirs, the effects of the dynamical oceanic circulation and of altered biogeochemical cycles.
Alternatively, the discrepancy may point to unrecognized biases in the 14C proxy data linked to subtle sedimentological or geochemical effects. For example, the reconstruction of SSRA by the different proposed methods is sensitive to phenomena such as sediment reworking, bioturbation, dissolution, and contamination coupled with abundance changes of the carrier of the 14C signal (e.g. Bard et al. 1987, 2001, Barker et al. 2007, Broecker and Clark 2011).
I will discuss these issues in the light of the recent literature, notably by comparing the amplitude of model outputs and by assessing the possible magnitude of biases on the SSRA reconstructions based on 14C in oceanic sediments.
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Fig. 2 Steady state calculations of the dependence between the sea surface reservoir age and the atmospheric pCO2 (updated from Bard 1988, 1998 by using calculations performed by Delaygue et al. 2003 with the Bern model).
PD stands for the present day preindustrial conditions and LGM for the Last Glacial Maximum.
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Prof. Edouard Bard CEREGE
(Aix-Marseille University CNRS, IRD, Collège de France) Le Trocadéro
Europole de l’Arbois BP80 13545 Aix-en-Provence Cedex4 France
Phone: +33 4 42507418 Fax: +33 4 42507421 E-Mail: bard@cerege.fr
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