Kurze Originalmitteilungen Increase of Carbon Dioxide in the Atmosphere During Deglaciation: the Coral Reef Hypothesis

W.H. Berger

Scripps Institution of Oceanography, La Jolla, California 92093

Analyses of ice cores from Greenland and Antarctica [1, 2] have shown that the CO2 content of the atmosphere increased rather suddenly from a glacial low near 180 ppm to a postglacial high near 350 ppm early during deglaciation some 13 000 years ago. Here I present a hypothesis accounting for this observation. First, a look at the data to be explained. The various series of measurements on CO2 content exhibit considerable scatter (Fig. 1A). However, the most recent re- sults have confirmed that a deglacial CO2 pulse exists [3]. The Dome 10 data (D10 in Fig. 1) of Delmas et al. [2] show the least scatter, and agree well with the new Camp Century profile of Neftel et al. [3]. I take the D 10 data as being representative, therefore (filled circles in Fig. 1A and B). A sharp step appears in this profile. What mechanisms can produce such a CO2 step? The CO2 reservoirs which are able

to yield substantial quantities of CO2 to the atmosphere on short notice are (l) the biosphere (mainly forests), (2) the ocean, (3) sediments (carbonate and organic car- bon). The biosphere was growing during degla- cial warming of the globe [4]. Thus, it was not a source but a sink for COg. The ocean was a potential source of COa for two rea- sons : (1) warming the sea reduces its ability to hold C02 in solution, and (2) a drop in fertility, as indicated in de- creasing deep-sea sedimentation rates and changing plankton composition [5], also re- duces the COz content of the deep sea [6]. Warming apparently was unimportant, be- cause from isotopic evidence and the com- position of benthic faunas we know that the temperature of deep waters stayed about the same from glacial to postglacial

0t80 0180 0 t80

o, t Dome C 1~o 1%~o

5 C02 Dome C-.~ / \ \ \ / 0 0 2 Byrd

o. / ' \ , c0z Comp .... r \t%.~ l\ Century

. - . / ~ _ # 10 11

"---CO 2 ~ .'e 12 13 o,~ /o \/11 14

. . . . . . , .. : ?

5"" _._ 2 5

( /,i z io, 30 A k-. S i%o B I I l p I I I

001 0 0 2 0.03 0.04 0.05 0 ;1 ~ 0 0 02 03 0 04 0 05

002 ?4 C02 E o]

Fig. l. Changes in atmospheric CO2 content as seen in ice cores (A) and attempts to model the CO2 change during deglaciation, based on the coral reef hypothesis (B). The ice core data (A) were plotted using 6 ~80 curves as a means of correlation. The Camp Century results are now in doubt [3]. The model results (B) are plotted on the shaded area representing the data scatter. The black dots are the D10 data [2]. The time scale in (A) is that of [2], the one in (B) is based on correlation of their 5~sO curve with a similar curve from box cores in the Pacific [14]

10

time [7]. The fertility effect probably is im- portant, but from analysis of planktonic faunas it appears that the drop followed rather than preceded deglaciation [5]. As far as sediments, we must decrease or- ganic matter or increase carbonate to ob- tain CO2, according to the following equa- tions:

CH20 (org. matter)+O2-,CO2+H20 (1)

Ca2++2HCO~ CaCO 3 (carbonate) + CO 2 + H20 (2)

There is reason to believe that during the buildup of the biosphere the organic car- bon pool increased. Thus, Eq. (1) ran backward, and the process was a sink for CO2. On the other hand, buildup of car- bonate on shelves did proceed during the rise of sea level [8] and this process was indeed a source of CO2. My hypothesis calls for shelf carbonate buildup during transgression, which re- leases CO2 to the upper ocean and the atmosphere. The CO2 excess is subse- quently mixed into the deep ocean and is neutralized on the deep-sea floor, by the dissolution of carbonate. In essence, then, the hypothesis uses the concept of basin- to-shelf transfer of carbonate during trans- gression [9]. A lag between production and neutralization of the evolved CO2 is re- sponsible for the step-like increase. The proposed mechanism yields a Pco2 change of the right magnitude, as can be shown by numerical simulation using a simple box model. The model consists of a source of excess CO2 (the "coral reef"), a sink (the deep-sea floor), and three reser- voirs: atmosphere, upper waters and deep waters. Partitioning between atmosphere and ocean is based on a two-step equilibra- tion: (1) lowering of atmospheric Pco2 dur- ing carbonate precipitation (because of de- crease in Y~CO2), through (pretended) re- moval of evolved CO2 from the system, (2) increase of atmospheric Pco2 by the re- turn of the CO2 to the system (upper waters). The two-step procedure gives a partitioning which is distinctly less in favor of the atmosphere than it would be, were the excess CO2 entirely new to the system. A partitioning of 72/28 between upper waters and atmosphere (initial carbon mass ratios 15:1) was deemed adequate for present purposes, based on published calculations [10, 11]. A warming of surface waters during deglaciation changes the fac- tor in favor of the atmosphere but the in-

Naturwissenschaften 69 (1982) �9 Springer-Verlag 1982 87

troduction of CO2-poor meltwater op- poses this effect. The two effects are here neglected. The initial relative sizes of the boxes (at- mosphere: upper waters: deep waters) are taken as 1 : 15:45. The crucial variables are the rate of coral reef growth, the time lag in mixing upper waters with deep waters, and the rate of dissolution on the deep-sea floor. Under the assumption of basin-to-shelf transfer, the total coral reef buildup can be estimated from comparing carbonate sedimentation rates for glacial and inter- glacial periods in the deep sea. The available evidence [12-14] suggests that a factor of 2 is a reasonable guess for the difference in rates. Assume that the postglacial deep- sea deficit is due to basin-shelf transfer and lasts on the order of 10000 years. This is a typical duration for warm and cold peaks within the Pleistocene deep-sea re- cord and this period also accommodates the various leads and lags proposed for different proxy signals within the record [15, 16]. The interglacial carbonate deficit then translates into a release of carbon dioxide, by shelf carbonate buildup, cor- responding to about 3 atmospheric carbon masses (t ACM=6.2 • 1017 gC -~- 300 ppm). Newly released CO2 will eventu- ally be titrated against deep-sea carbonate according to the hypothesis. However, to begin with, it is shared only by the upper waters of the ocean and the atmosphere. Here "upper waters" means the upper 1 km [17]. The lag in the mixing of upper and deep waters is taken as 1000 years [18]. The rate of carbonate buildup on the tropi- cal shelves can be constrained from the estimate of the total, Coral reefs and asso- ciated carbonates can grow upward very rapidly [8, 19], and there seems to be no problem in achieving the postulated total by linear growth during transgression. Sea- level rise is assumed complete after 5000 years following initiation of deglaciation. Also, I assume the maximum buildup rate is only reached when sea level has risen some portion of the total, which is taken as 20%. The rate at which excess CO2 in deep waters is neutralized on the seafloor is poorly known. I have postulated one- third neutralization after 500 years [20], but this rate may be too high. Results of a numerical experiment based on the stated assumptions appear in Fig. 1 B as Curve 1. The model achieves a rapid rise of Pco~, before mixing leads to a near-balance between CO2 introduced

and CO2 mixed downward and consumed. The rise is about one-half of the target. Doubling the mixing lag (to 2000 years) yields an overshoot (Curve 2). Doubling the estimated rate of carbonate buildup yields a satisfactory fit (Curve 3). Doubling both mixing lag and carbonate buildup rate (Curve 4) produces a marked over- shoot. The overshoot peaks occur at the time of the Aller6d period. The results show that a reasonable fit to the type of COz record provided by the ice core data could readily be produced by suitably adjusting coral reef growth, mixing rate, and dissolution rate. A combi- nation of an increasing mixing lag due to the meltwater effect [21] and to warming, and a dissolution rate lower than that as- sumed, would improve the fit without call- ing for excessive carbonate buildup. The problem is to find the limits which con- strain such arbitrary tuning of the model. The deep-sea record with its data on car- bonate dissolution, stable isotopes, and ocean fertility [7] holds the key to this problem. I am indebted to R. Bacastow, J. Gieskes, M. Kastner and J. Killingley for helpful discussions.

Received December 7, 1981

1. Berner, W., et al.: Radiocarbon 22, 227 (198o)

2. Delmas, R.J., et al. : Nature 284, 155 (1980) 3. Neftel, A., et at. : ibid. (in press) 4. Shackieton, N.J, in: The Fate of Fossil

Fuel COa in the Oceans, p. 401 (N.R. An- dersen, A. Malahoff, eds.). New York: Ple- num Press 1977

5. Berger, W.H. : ibid., p. 505 6. Broecker, W.S., in: CIMAS Symposium

Volume (E. Kraus, ed.). (in press) 7. Berger, W.H., in: The Sea, Vol. 7 (C. Emi-

liani, ed.), New York: Wiley-Interscience 198t

8. Milliman, J.D.: Marine Carbonates. Ber- iin-tteidelberg-New York: Springer 1974

9. Berger, W.H., Winterer, E.L.: Spec. Publ. Int. Assoc. Sedim. 1, I1 (1974)

10. Plass, G.N.: Envir. Sci. Technol. 6, 736 (i972)

11. Bacastow, R.B., Keeling, C.D., in: Work- shop on the Global Effects of Carbon Dioxide from Fossil Fuels, p. 72 (W.P. EP iiott, L. Machta, eds.). Springfield: Nat. Tech. Info. Service 1979

12. Turekian, K.K., in: Chemical Oceanogra- phy, p. 81 (J.P. Riley, G. Skirrow, eds.). London: Academic Press 1965

13. Berger, W.H. : J. Foram. Res. 8, 286 (1978) 14. Berger, W.H., Killingley, J.S.: Marine

Geol. (in press) 15. Luz, B., Shackleton, N.J.: Cushman

Found. Foram. Res. Sp. Pub. 13, 142 (i975)

16. Moore, T,C., et al., in: The Fate of Fossil Fuel COz in the Oceans, p. 145 (N.R. An- dersen, A. Malahoff, eds.). New York : Ple- num Press 1977

17. Munk, W.H.: Deep-Sea Res. 13, 707 (1966)

18. Broecker, W.S., in: The Sea, Voi. 2, p, 88 (M.N. Hill, ed.). New York: Interscience 1963

19. Hay, W.W., Southam, J.R., in: The Fate of Fossil Fuel CO2 in the Oceans, p. 569 (N.R. Andersen, A. Malahoff, eds.). New York: Plenum Press t977

20. Broecker, W.A., Takahashi, T.: ibid., p. 213

21. Berger, W.H., et al. : Naturwissenschaften 64, 634 (1977)

Metalle in kommunalen Kl irschl immen

tI. Heinrichs

Geochemisches Institut der Universit~it, D-3400 G6ttingen

KlfirschlS, mme k6nnen, sofern sic nicht keimfrei gemacht wurden, gemeinsam mit M/ill oder in weitgehend entw/isserter Form auch allein kompostiert werden. Die Kompostierung ist ein alt eingefiihrtes Ab- fallbehandlungsverfahren, bei dem durch einen biologischen UmwandlungsprozeB die organischen Stoffe im Abfall von Mi- kroorganismen zu anorganischen Stoffen zersetzt werden. Es entsteht ein Humus- ~ihnliches Endprodukt, das als Abdeckma- terial ffir Deponien, zur Landschaftsgestal-

tung und als Bodenverbesserungsmittel in Land- und Forstwirtschaft eingesetzt wird. Ein Hauptproblem der Kompostierungs- anlagen ist der Absatz des erzeugten Kom- postes. Dies hat eine Ursache in der unge- ntigenden Information /.~ber Gehalt und Bedeutung yon Schadstoffen. Die frischen und teilweise behandelten Klfirschl~tmme yon 8 Klgranlagen (Duis- burg-Emschergenossenschaft, K61n- Stammheim, Dfisseldorf-Sfid, Kassel, G6t- tingen, Hann.-Mfinden, Witzenhausen und

88 Naturwissenschaften 69 (1982) �9 Springer-Verlag 1982