Nuclear Instruments and Methods in Physics ... - Science Direct

331

Nuclear Instruments and Methods in Physics Research 85 (1984) 331-339 North-Holland, Amsterdam

Section K Applications: (b) Geophysics RADIOCARBON MEASUREMENTS ON COEXISTING BENTHIC AND PLANKTIC FORAMINIFERA SHELLS: POTENTIAL FOR RECONSTRUCTING OCEAN VENTILATION TIMES OVER THE PAST 20000 YEARS Wallace

BROECKER

and Alan MIX

Lamunr - Doherty ~eo~ogicaf ~bse~vato~,

Michael

ANDREE

Col~mbja University, PaIi~ades, New York, USA

and Hans OESCHGER

Dept. of Physics, University of Beme, Switzerland

In this paper the potential of AMS i4C dating of shells handpicked from deep sea sediments is explored. We show that while the age difference between planktonic (surface dwelling) and benthic (bottom dwelling) shells must carry information regarding paleoeirculation rates, this message is likely obscured by effects associated with the coupling between bioturbation and dissolution and between bioturbation and abundance change. It is also possible that the “C/‘2C ratio in planktonic shells was initially not identical to that in surface water and that the “C/‘2C ratio in benthic shells was initially not identical to that in bottom water. These and other biases will plague all attempts to extract the desired information regarding circulation rate changes over the last 2OooO years. However in sorting them out, much will be learned about the origin and history of the calcite particles found in deep sea sediments.

1. Introduction

Current efforts to develop models of the large scale operation of the oceanic system assume it to be at steady state. To some extent this assumption is made by those attempting to model the ocean simply to ease the entry into a difficult area. To some extent it is justified by historic observations of the sea’s hydrography and by studies of the record in marine sediments. Few workers in the field would, however, deny the possibility that the system is not at steady state. Most would agree that the mode of operation of this system was perturbed by the great warming which accompanied deglaciation ten thousand years ago. Most would also agree that the current buildup of CO, and other greenhouse gases in the atmosphere could change the sea’s mode of operation during the next one hundred or so years. In this paper we discuss how the radiocarbon method, which serves as the anchor for our thinking regarding the current rates of deep sea ventilation, can be extended back in time. A record of t4C/t2C ratio difference between surface and bottom water is kept by the calcite shells of for~nifera stored in marine sediments. This is the case because some of the organisms forming these shells live on the sea floor (benthic foraminifera) while others live in the near surface waters (planktonic foraminifera). The development of the tandem accelerator atom counting technique for i4C measurements opens up this realm to us. The reason is that it permits measurements 0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

to be made on samples 1000 times smaller than those required for the conventional decay counting methods. Accurate measurements can now be made on 8 mg of shell material from deep sea sediments (yielding about 1 mg of carbon). Thus individual species rather than bulk sediment can now be run.

2. Distribution of 14C/C

ratios in today’s ocean

To understand the potential of the radiocarbon record stored in marine sediments, it is first necessary to look at the distribution of 14C/12C ratios in the present day ocean (or more precisely in the ocean prior to the onset of anthropogenic modifications of the carbon cycle). The combination of the GEOSECS data set for waters free of tritium (and hence also of bomb i4C) and of measurements made on surface waters collected prior to 1957 and on shells and corals grown prior to 1957, provides us with a fairly good 3D picture of the preanthropogenic i4C/“C distribution in the ocean (see Broecker and Peng [1] for summary). Between 40°N and 40 OS the surface waters of the ocean have nearly uniform A14C values * averaging close to -50%. The * The accepted convention for the presentation of radiocarbon measurements on samples from the contemporary active carbon reservoirs (i.e. oceans, atmosphere, living biosphere, . ) is to give the 14C/12C ratio as per mil difference from the ratio measured on a standard reference material. The V(b). GEOPHYSICS

332

W. Broecker et al. / Radiocarbon measurements

deep water values everywhere have lower A14C values than those for warm surface water. As can be seen from the map in fig. 1, they decrease systematically from the northern Atlantic to the Antarctic and from the Antarctic to the northern Indian and Pacific Oceans. The trend is closely related to the path followed by deep waters formed in the northern Atlantic. An approximate budget for 14C in the deep sea is given in table 1 based on Broecker and Peng [l]. Assuming steady state, the decay of 14C in the deep sea appears to be largely matched by entry of radiocarbon with the new water from the northern Atlantic. Although an equal or perhaps even larger amount of deep water originates in the Antarctic, the carbon in this new water has a A14C value very close to that for average deep water. Hence, it does little to replenish the radiocarbon decaying in the deep sea. Because of this the average ventilation time for carbon isotopes in the deep sea (= 1000 years) is likely roughly at least twice the ventilation time for water. This circumstance will haunt the interpretation of the foram based paleoradiocarbon distributions. The relationship between 14C age of deep water and the ventilation rate of deep water depends on the extent to which the carbon dissolved in the waters descending to the deep sea has isotopically equilibrated with the CO, in the atmosphere. In today’s sea, this equilibration is more nearly complete in the northern Atlantic source regions than in the Antarctic source regions. The situation may have been different in the past. For the purposes of our discussion it proves convenient to convert the differences between the 14C/12C ratios for the surface and bottom water at any given point in the ocean to radiocarbon ages. While these ages lack the physical meaning attached, for example, to the radiocarbon age for a mummy, they provide a measure of the isolation time for the carbon in bottom water; the older its radiocarbon age, the greater the isolation time.

ratio for this standard reference material is multiplied by a constant chosen to yield a near match to the ratio for 19th century atmospheric carbon (this value has been reconstructed from measurements on tree ring samples). In order to remove the small i4C/‘*C ratio differences created by isotope fractionation, these differences are all normalized to a constant “C/i2C ratio. Thus a A14C value of - 225% is assigned to a sample with a fractionation normalized i4C/12C ratio 0.775 that for the 19th century atmosphere. The equations used are as follows. A14C=~‘4C-2(613C+25)(1+814C/1000), where

on foraminifera

shells

Table 1 The budget of i4C in the deep sea (i.e., below 1.5 km) LOSS

Via radioactive decay Volume of water Mean ZC02 Mean d4C Mean I4 C/C Amount of I4 C Amount 14C decaying in deep sea: Gain Via northern component Flux

water

zco, A14C

‘4c/cnw- ‘4C/Cmean deep

sea

Flux of “C: Via southern component Flux

A14C

deep sea

b)

14C/C

articles

-

=6X10’“m3/a 2.1 mol/m3 - 67% 0.13 x lo-‘* 165 mol/a

water

zco, d4C I4 c/c,w - I4 c/c,,, Flux of 14C: Via particulate rain Carbon flux ‘)

=8X10”m3 2.3 mol/m3 - 175% 1.00x10-‘* 1.8 x lo6 mol 220 mol/a

“C/Gcandeep sea

Flux of I%: Total flux of new 14C to deep sea:

=6x10’4m3/a 2.2 mol/m3 -154% 0.025 x lo-i2 33 mol/a 1.8 X lOI mol/ - 80% 0.12x lo-i2 22 mol/a 220 mol/a

‘) The mean initial PO, content of northern component water is 0.7 pm/kg and that of southern component water is 1.3 pm/kg. If these two water types are fed to the deep sea in nearly the same amount then the preformed PO, content of new deep water averages 1.0 pm/kg. The PO, content of deep water averages about 2.3 pm/kg. Thus about 1.3 pm/kg was received by particles. If the C/P ratio for these particles is 125 then about 160 pm/kg or about 7% of the carbon in average deep sea water was supplied by particles. The input of carbon to the deep sea is 12X 1014m3/a~2.15 mol C/m3 or 2.6~10’~ mol/a. Seven percent of this is 1.8X1O14 moles/a. The area of the sea at 1.5 km depth is about 3.2 X 10’4m2. Hence the flux is 0.56 mol/m2a. b, Because of the 20% depletion in 13C relative to ‘*C during photosynthesis there must be a 40% depletion in r4C relative to ‘*C. For CaCO, there is no depletion in 13C. Hence the mean A14C value for the raining organic matter should be -90% and that in the raining CaCO, - 506. The composite should have a Ai4C value close to - 80%

As can be seen from fig. 2 in today’s ocean, this age generally follows the deep water d4C pattern. The major exception is the Antarctic Ocean where the age differences are much smaller than expected from the deep water Ar4C values. The reason is that the surface waters of the Antarctic have low A14C values.