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Experimental paleotemperature equation for planktonic foraminifera
Grochrmw
C Pergamon
et ~~~wmrhrmrca .4rro Vd 47. pp IOX-103 Press Ltd 19R3 Pnntcd I” U S.A.
I
Experimental paleotemperature equation for planktonic foraminifera EREZ
JONATHAN
The Hebrew University
of Jerusaiem.The
l-i. Steinitz Marine Biology Laboraiory. Eilat. P.O. Box 469. israei and
BOAZ LUZ Dept. of Geology. The Hebrew University of Jerusalem. Jerusalem. Israel (Received April 22. 1982: accepted in revised,fbrm Februar;, 24. 1983) Abstract-Small live individuals of Globigerrnoides sacculj/h’ which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14’ to 30°C) and water isotopic composition. and a correction was made lo account for the isotopic composition of the original skeleton using control groups. Comparison of.the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCOz indicate that the foraminifera precipitate their CaC03 in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with CRAIG’S (I 965) equation yields: I = -0.07
+ l.Oli
(r = 0.95)
where I is the actual growth temperature and iis the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate. is equally applicable for the planktonic foraminifer G. saccu/jj?r. Second order regression of the culture temperature and the delta difference (6”Oc - 6’*Oro) yield a correlation coefficient of r = 0.95: i = 17.0 - 4.52(6’“Oc - ~‘“OW) + O.O3(b’“Oc - ~“‘OW)’
i, d”Oc and b’RO~, are the estimated temperature. the isotopic composition of the shell carbonate and the sea water respectively. A possible cause for nonequilibrium isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.
INTRODUCT’ION
ANI (1954. 1955, 1966) applied these equations for the first time to planktonic foraminifera in deep sea cores, and concluded that planktonic foraminifera deposit their skeleton in isotopic equilibrium with sea water and hence they can be used for paleoenvironmental studies provided that d”Ow can be estimated. (See also SAWN and DOUGLAS, 1973; SHACKLETON and VINCENT, 1978; and BERGER ef al., 1978.) Following EMILIANI,a considerable body of information on stable isotope distributions in planktonic foraminifera has been gathered, and as a result much insight has been gained into Pleistocene paleoenvironmental conditions (e.g., SHACKLETON and OP-
IN 1974 H. UREY suggested that stable oxygen iso-
in carbonate materials can be used to deduce paleotemtiratures. Following his suggestion EPSTEIN ef al. ( 195 1, 1953) developed a paleotemperature scale (or equation) for biogenic CaC03. They analyzed molluscs carbonate that grew at known temperatures and known water isotopic composition. The result was the following familiar equation: tope ratios
i = 16.5 - 4.3(6’RO~ - 6”Ow) + O.l4(6’8Oc - b’8Ow)? where iis the estimated temperature (or isotopic temperature) and d”Oc and 6”~ are the oxygen isotopic compositions of the carbonate and sea water. respectively. This equation was later modified by CRAIG ( 1965) to account for “0 and instrumemal inaccuracies:
i = 16.9 - 4.2(6”Oc - 6”Ow) + O.l3(6’8Oc - 6’80M’)’ These equations match well the experimental data of MCCREA (1950) and O’NEILL et al. (1969) for inorganic calcite within experimental
errors. EMILI-
DYKE 1973, 1976: SHACKLETON 1977: EMILIANI, 1955, 1966, 197i; EMILIANI and SHACKLETON, 1974; HECHT, 1973, 1974; DUPLESSY et al., 1975; VAN DONK, 1977; and many others).
Despite the vast amount of data on stable isotopic compositions in planktonic foraminifera, until recently little attention was given to the fact that the paleotemperature equation was tested for mollusca and not for foraminifera. Questions about the validity of the basic equilibrium assumption were first asked when it was found that different benthonic foraminifera have different specific isotopic fractionations (DUPLESSY et al., 1970; VINOT-BERTOUILLE and DUPLESSY, 1973). Attempts to test whether plank-
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1026
22.5 21.5 23.0 25.0 25.5
22.5
tonic foraminifera deposit their shell in isotopic equilibrium were made by VAN DONR (1970) and later by SHACKLETON et al. (1973). They compared the isotopic temperature with the actual water temperature for live planktonic foraminifera that were collected with a plankton net. It was found that the spinose species show a slight negative deviation from equilibrium (-0.5% for G. ruber and -0.4% for G. saccuftfkr). Similarly, VERGNAUDGRAZZINI ( 1976). VAN DUNK (1977) KAHN ( 1979) KAHN and WILLIAMS ( 198 l), DEUSER et al. ( 198 1), DUPLE~~Y ef al. ( 198 I), and FAIRBAND et al. ( 1980) have reported significant deviations from equilibrium, EREZ and HONJO ( 198 1) compared the. isotopic composition of planktonic foraminifera from plankton tows, sediment traps and sediments, and concluded that slight deviations from equilibrium may exist in material collected by plankton nets. However material in sediment traps at a depth range of 400 m to 5300 m. and material from bottom sediinents do not show deviations from equilibrium. It is evident that although almost every indirect approach possible has been taken, the equilibrium assumption of the isotopic paleotemperature method has not yet been proved for planktonic foraminifera. In an attempt to resolve this problem, we adopted an experimental approach to determine whether Globigennoides saccuiifer (a widely used species in pa-
* Gametogenesis is the process of sexual reproduction in planktonic foraminifera (BE et al.. 1977). It occurs at the end of the life cycle of an individual and involves multiple divisions of the protoplasm followed by production of thousands of motile garnets. These flagellated garnets burst out of the calcamous shell very rapidly and within approximately one hour a white empty shell completely free of protoplasm is left behind. Such shells are very similar to those found in sediments.
leotemperature studies) deposits its shell in isotopic equilibrium or not. EXPERIMENTAL
AND ANALYTICAL METHODS
Experimental Live planktonic foraminifera of the species Globigerinoides sacculifer were collected in the northern Gulf of Eilat using 200 cm plankton net. Plankton net tows were made in surface water at depths of 2-5 m. The collection area was 2-3 km off the Hebrew University’s Marine Biology Laboratory in Eilat. where water depth is roughly 400 m. Within one hour of collection. the samples were brought to the laboratory and the planktonic foraminifera were separated from the other plankton using pasteur pipettes (see Bt? er al.. 1977. 1978, I98 I for detatls). Young, healthy individuals of roughly I50 sm diameter and skeleton weight of around 4 fig were separated and their size measured. All these individuals contained symbiotic algae and had long spines. Each individual was placed in a 100 ml erlenmeyer flask tilled with 80 ml tiltered (0.45 pm) sea water from the site of the plankton collection. The erlenmeyers were placed in thermostated baths where temperatures were maintained at a precision of +O. I “C of the desired temperatures. During the entire experiment. temperatures were recorded on a strip chart to ensure that there were no significant deviations from the desired temperatures (see Table I). The experiments were carried out in natural sunlight near the window. and with artificial fluorescent cool white light. We did not observe any change in the symbiont activity between these two environments. Each individual was fed daily with one newly hatched brine shrimp (Anemia salina), and growth was manitored optically daily. For isotopic analysis we selected only those individuals that grew continuously and went through gametogenesis at the end of their life (i.e.. approximately 80% of the total individuals cultured).* Usually experiments lasted from ten to fifteen days. Fifteen to twenty individuals were grown at each temperature, and in each run there were two or three different temperatures. Experimental data are given in Table 1. Ana/_vticai Water isotoptc analysis was carried out according to the method of EPSTEINand MAYEDA (1953). and 6’*Owwas
Foraminifera paleotemperatures determined at the beginning and the end of each experiment. Salinities were monitored daily using an optical salinometer. In several experiments the fomminifera were cultured in 100 mi beakers that were covered with petri dishes. Under these conditions when the temperature was above 25”C, evaporation was significant and salimties rose with time. Evaporation was minimal in experiments m which we used erlenmeyer flasks. Isotopic compositton of the shells was carried out after crushing the individuals in ethanol, drying and vacuum roasting at 450°C for 30 minutes. The calcium carbonate was dissolved in - 100% H3P04 at 50°C according to the method of SHACKLETON(1974), and the isotopic composition of the evolved CO? was measured usmg a Micromass VG 602 mass spectrometer. fnstrumental corrections of the results were done according to CRAIG (1957). The results are reported in pet-mill deviations relative to the PDB standard (calibration to PDB was done with the Cal Tech Standard PDB IV).
=mple _ po = (01s/O’6) (0'8/0'6) std
1
The cultured fotaminifera reached, on the average, a weight of -40 pg per individual. Because they went through gametogenesis, they were all completely clean of protoplasm. For each plankton collection we picked a control group that was comprised of 25-50 individuals having the same size distribution as the original experimental groups. The foraminifera in the control group were soaked for 30 minutes in distilled water to which ammonia was added to give pH value -9 and part of their plasma was leached. They were then dried on a filter paper overnight. dessicated and weighed to get the average weight per individual. -411weighmg procedures were made using a Kahn 25 electrobalance having a precision of fl.2 fig. The control group samples were analysed for their isotopic composition and a correction for the initial isotopic composition was applied to the final samples according to the following equation: 6180 controlled =
fS’*Ofinal - X- 6% initial
I-x
where x’ is the weight fraction of the initial foraminifera relative to the final foraminifera weight. The correction in
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most cases was small because the bulk of the carbonate was deposited under the controlled experimental conditions (see Table I). RESULTS
Oxygen-isotopes The results are summarized in Tables 1 and 2 and Fig. 1. 6180 values of the carbonate shells (6180c) varied between - 1.40 and +2.06. The difference between 6’80c and 6”Ow (the isotopic composition of the water in which the foraminifera were cultured), varied between -3.00 and +.46 (Table 1). Temperature estimates were calculated using earlier equations taken from the literature and by using the best fit to the data (Table 2). In all cases these are 2nd order equations of temperature (PC) as a function of (6’80c - 6180w), where i = u + b(dc - 8~) + c(dc - 6~)~. The different coefficients (b and c) and intercepts (a) are given in Table 2. The differences between the calculated temperatures (i) and the experimental temperatures (t) were squared and averaged. The square root of this average represents the average deviation between the calculated and experimental temperatures. In addition we calculated our own 2nd order best fit to the data. The different coefficients of all the equations are quite similar, and the average deviations are all small and close to that of our own best fit (see Table 2). The scatter in our data is large enough to accommodate all the coefficients of the different equations within the 95% confidence limits on the coefficients of our best fit. (The sources of the scatter are discussed below). Hence, we cannot select one equation a$ the “best” equation to describe the oxygen isotope fractionation in biogenic or inorganic calcite. The equa-
i Erer
1028 10,
*.i
.
t
** . . l .
”
.
.
a
.*
and
B
i.uz
li .:O.l %I vanabthty In the determmatton ot ?“O ~)t.the carbonate which can cause a temperature devtation of 10.5”C ;) The correctron applied for the ongmal rsotoptc composmon. especially In those experiments where the correction was large. This correction could have introduced an error of -I 1“C, 4) 50. I ‘SOin the determination of the water ISOtopic composition which can cause -irO.S”C.
The devtattons from the expected isotoptc corn position seem to be random and therefore we cannot SIrnit data points that show large deviations. We should emphasize that our experiments were i.urned out over a temperature range. which is larger than the natural range thr this species. and that over ~hts range there are large variations in growth rate compared with the optimum at 23-24°C‘. The rquiFIG. I The cultunng temperature Y.) the difference (~‘“OC librium precipitatton of skeletal calcite is indepen-. 5’“Ow) (i.~, the carbonate minus the water oxygen isodent of the growth rate. We also tested the depentopic composition). The line is the best fit for the-data and dence of the skeletal tsotopic composition on the isoyields the following paleotemperature equation: I = 17.00 topic composition of the sea water. In experiments - 4SZ(b’“Oc - 6”Ow) + O.O3(d’“Oc - ~‘“OM,)-‘ wtth r = 0.95 carried out above 15°C in beakers. water evaporated rather rapidly and the final isotopic composttion ot the water and the carbonate were indeed heavier. tton of O’NEIL cf (I/. ( 1969) is the closest to our own However the difference (6’“Oc -- 6”Ow*) showed the equation (see (r values in Table 2). This equation also expected dependence on temperature (Tables I fits well with the data of SHACKLETON (1974)for and 1). deep benthomc foraminifera covering the lower temThe control groups that were analyzed contamed perature range (7°C to 1°C). We suggest that this only small individuals that were picked from natural equation should be used to derive oceanic paleotempopulations of surface water in the Gulf of Eilat. peratures. Their average size matched the average size of the A very good correlation exists between our experexperimental groups. They were typically I00 Grn to tmental data and the original data of EPSTEIN er al. 180 +m In diameter and weighed 2 gg to X pg per f 1953). A linear regression was calculated between Individual. The oxygen isotope composition. the amthe actual temperatures in which the foraminifera blent water temperature and the isotopic temperature grew 1’s the isotopic temperature calculated from the calculated according to EPSTEIN’S modified equatmn Isotopic composition using the equation of EF-STEIN ICRslC;. 1965) for the control groups are shown 111 d ui (1953) as modified by CRAIG (1065). If these rable 1. .The tsotoptc temperatures follow the seatemperatures were identical, the regression line would tonal trend of the temperature but in all the samples have had an intercept of 0.0 and a slope of ! .O The rhe tsotoptc temperature is higher than the ambtent actual linear regression yielded an intercept of -0.07 water temperature at the time of collection” The a~and a slope of t 1.OI :rage ditference IS 2..38”c‘ which is equivalent to 0.:; i) 6% negattve devumon from equilibrium in terms h,t ;) notation. This Jeviatton is similar to that &ierved by earlier researchers and will be discussed where I IS the actual culture temperature and I IS the heiow Isotopic temperature. This indicates that wtthm the variability and range of our data there is almost J perfect fit between the two sets of temperatures. The (. dwrr :~cuc~pt~c main conclusion is that there is no difference between C’arbon rsotoptc ;omposmon of X0: In sea water the temperature fractionation of oxygen isotopes in was not controlled
Foraminifera paleotemperatures tions and sea water was filtered before the experiment through 0.45 pm filters. The carbon isotopic compositions are shown in Table 2 and are plotted against temperature in Fig. 2. The data as a whole show a general trend of decrease in 613Cwith increasing temperatures. This trend is apparent in a few individual experiments (PT2, PT3, PT7. PT9). However the opposite trend is also shown (PT8, PT4). Experiments PT5, PT6, PTI did not show any trend. The entire range of 613C changes is roughly 2.5% and it is larger than the range of - IL shown by 613C in the XOz of sea water in the Gulf of Eilat (SHEMESH, 1980). The range within one experiment was - I k and at this stage we have no explanation for it. The foraminifera were fed daily with identical food. IIlumination, which could influence the symbionts was uniform. Currently we are investigating the factors that may control carbon isotope ratios in planktonic foraminifera. DISCUSSION
Ken-Equilibrium
L’alues
The nonequilibrium values that were reported b) other workers and those shown by the control groups
i
0 i’
a
O
FIG. 2. The carbon isotopic composition of G saccul$er grown IS culture v: the temperature of culture. Different symbols represent different experiments listed in Table 1. Note the general trend of decrease in 6’jC with increase of temperature.
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(Table 1) are surprising in view of the present experimental results. Most of the disequilibrium values were reported for foraminifera collected in plankton net tows (SHACF~_ETON et al., 1973; KAHN, 1979; VAN DONK. 1977: FAIRBANKS el al., 1980; KAHN and WILLIAMS, 198 1: EREZ and HONJO, 198 1; DuPLESSY ef al., 198 I ). EREZ and HONJO ( 198 I), found that in sediment traps almost ail samples were in equilibrium and the few exceptions would be accommodated into equilibrium using certain logical assumptions about seasonality and settling rates. Similarly, DEUSER et al., (198 1) found that in their sediment traps most species deposit their skeleton in equilibrium except G. bulloides which clearly is exceptional. Most of the analyses that were carried out on material from sediments show equilibrium values (EMILIANI, 1954, 1955; CURRY and MATTHEWS, 198 I: BERGER et al.. 1978; etc.). EREZ (1978a) and EREZ and HONJO ( 198 1) have discussed in detail the changes that occur in the isotopic composition of a single species during its descent in the water column and its sedimentation on the ocean floor. Their conclusion is that two processes, namely, skeleton deposition at depth and dissolution on the ocean floor tend to make the isotopic composition heavier and therefore can mask the light non-equilibrium isotopic composition observed in plankton material. A similar explanation was also suggested by DUPPLESSY et al. ( 198 I ). emphasizing the importance of gametogenic calcification in this process. However. our present experiment demonstrates that under laboratory conditions. all the skeletons show equilibrium fractionation. We therefore still need to explain the non-equilibrium values reported for plankton samples. There are several possible kxplanaiions: 1. incorrect assumptions by the authors with regard to water temperature and water isotopic composition. The temperature measured in surface water during the plankton collection time can be signilicantly different from that in which the bulk of the skeleton was deposited. Usually the foraminifera analyzed in these studies were about 2-3 weeks old and therefore their skeleton should represent the integrated temperature of that time. Incorrect assump tion on the water isotopic composition is also possible. Most authors used the linear relationship between 6’*Oc(> of sea water and its salinity as determined by EPSTEIN and MAYEDA (1953) and CRAIG and GORDON ( 1965). There are however large local and temporal deviations from these relationships which are not suitable for the calculation of isotopic composition at any locality and time of surface water, especially in the areas where precipitation and evap oration effects are large and seasonal. These two parameters (i.e. 6’60~. and t) can cause large enough negative deviations from equilibrium, as observed by most authors (in the range of 0.5%). Obviously. there are also positive deviations but they are not detected because it has always been assumed that in these cases
!030
J trez
and B. Lui
the foramlmfera depostted their skeleton at some depth in the thermocline. -. ’ It is possible that non-equilibrium values mdeed occur in plankton material-that is, in the earlier stages of growth ofplanktonic foramlnifera. BERGER et al. (1978) suggested that when individuals are young, their metabolic activity is more intense and the metabolic CO2 portion in the skeleton is larger than at later stages. The final stages of growth seem to be in isotopic equilibrium. and because the bulk of the skeleton is deposited in these final stages. at lower temperatures. it masks the non-equilibrium initial values. Our experiment does not exclude this possibility because we corrected for the original isotopic composition. However, the small weight fraction of this original skeleton does not support this possibility. 3. Most of the analyses on plankton samples are made on material that contain large quantities of protoplasmic matter (up to 50% of their total dry weight). Usually foraminifera samples analyzed from deep sea sediments and from sediment traps contain hardly any protoplasmic organic matter. Removal of this organic matter is one of the initial problems that were dealt with by EPSTEIN d al ( 195 I. 1953) and by EMILIANI (1955, 1966). EREZ ( 1978a) and EREZ and HONJO ( I98 I) have shown that in foraminifera samples that contain large quantities of organic matter, combustion is important and should be carried out properly. Various studies of plankton samples used different combustion techniques and therefore comparison between them is complicated. It is quite obvious that low temperature ashing. using an oxygen plasma furnace, of material rich in organic matter is not suitable (EREZ, 1978a). Also, treatment with oxldizing agents such as chlorox (sodium hyperchlorite) are not recommended (EMILIANI. 1966). All combustion techniques tend to make the isotopic composition lighter (EPSTEIN et al, 1953; EMILIAN. 1966: KAHN, 1977: EREZ and HONJO, 198 I ) and thus can cause large negative deviations m the isotopic composition of organic-rich material. The fact that most of the nonequilibrium values reported in the literature are for living foraminifera collected by plankton nets indicate that the organic matter may be the cause for the nonequilibrium values. We would like to emphasize that in the present experiment the cultured foraminifera did not contain any protoplasmic organic matter because they all went through gametogenesis. This was not so for the control groups that contained organic matter. Thus, It is possible that disequilibrium values cited m the literature and those observed in the control groups are caused by exchange reactton between the isotopically light CO2 produced during the combustion of the organic matter and the skeletal CaCO,. Implicatrons jar paieoecolog~ We have shown that the bulk of the shell m the planktonic foraminifer G. sacculifer is deposited In
Isotopic rqulhbnum uver a wide range ot temperatures that exceed its ecological range. In paleoecological applications of these results. it should be remembered that in nature planktonic foraminifera contmue to calcify dunng their entire life including the period when the population sinks through the water column. For f; racclll[ftir m particular. it has been shown that calcitication occurs at depth (EREZ and HONJQ I98 I ) and probably continues until the hnal stages of gametogenesis (St?. 198 1; DL’PLESS~ i’t 111 198 I ). It also seems that a significant part ut the total skeleton weight IS deposited at a depth at‘ a few hundred meters (up c’tl.). The shells of planktonic tbrammlfera thus include fracuons which were deposlted over a wtde range of depth and temperature i\swgnmem of one particular depth or temperature of skeleton deposition represents a weighted mean depth and temperature of the entire skeleton depositlonal histon Other environmentai and blogemc factors. such as rate ot metabolism. food quantity and isotopic composition. photosynthetlc activity of symbionts, light penetration and chemical composition of sea water (especially nutrients). may affect the stable-isotope compoatlon of carbon and oxygen in foraminiferal calcite. T’hls seems to be true for benthomc foramimfera hut may also hold for some planktonic foramlmfera. Ditferent species may have different fractionatlon factors and different dependence of the isotopic composition on the environmental factors. For (; \usL,uli,/tir. however. the shell is deposlted in equlhbnum with sea water and hence temperature is a maJor factor that detennmes Its oxygen isotopic compositlon. We safely state that earlier paleoceanographic studies based an the oxygen isotopic corn. posltion of G xxuriiftir (and most studies utilized this species, are mdeed valid and can be interpreted with higher confidence ~c,know/rd~~~mrnr.\-~V~tx~rrveyour Smcere thanks to A. Szm for technical assistance m cultunng the orgamsms: to A Shemesh for technical help in the Isotopic analysis: to 2 Reiss. 4 Matthews, S Weiner. H. Lowenstam and J. C Duplessy for fruitiul discussions and review of the manunript. Special thanks are to B. Shem-Tov for editing and tvplng the manuscript.
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Forammifera paleotemperatures leotemperatures. In Proceedings Spoleto Conference on Stable Isotopes in Oceanographic Studies and Paleotemperatures (ed. E. Tongiorgi) Piss. Vol. 3, pp. 3-24. CRAIG H. and GORDON L. I. (1965) Deuterium and oxygen18 variation in the ocean and the marine atmosphere. In Proceedings Spoleto Conference Stable Isotopes in Oceanographic Studies and Paleotemperatures (ed. E. Tongiorgi) Pisa. pp. 9- 130. CURRY W. B. and MAI-~HEWSR. K. (1981) Paleo-oceanographic utility of oxygen isotopic measurement on planktonic foraminifera: Indian Ocean core-top evidence. Paleogeog. Paleoclimat. Paleoecol. 33, 173- 19 1. DEUSER W. G., Ross E. H., HEMLJZBEN C. and SPINDLER M. [ 198 1)Seasonal changes in species composition, numbers, mass, size and isotopic composition of planktonic foraminifera settling into the deep Sargasso Sea. Paleogeog. Paleoclimat. Paleoecol. 33, IO3- 127. DUPLESSYJ. C., BE A. W. H. and BLANK P. L. (1981) Oxygen and carbon isotopic composition and biogeographic distribution of planktonic foraminifera in the Indian Ocean. Paleogeog. Paleociimat. Paleoecol. 33,9-46. DUPLESSY J. C.. CHENOUARD L. and VILA F. (1975) Weyl’s theory of glaciation supported by isotopic study of Norwegian core K 11. Science 118, 1208- 1209. DUPLE~~YJ. C., LALOU C. and VINOT A. C. (1970) Differential isotopic fractionation in benthic foraminifera and paleotemperatures reassessed. Science 168,250-25 1. EMILIANIC. (1954) Depth habitats of some species of pelagic fomminifera as indicated by oxygen isotope ratios. Amer. J. Sci. 252, 149-158. EMILIANIC. (1955) Pleistocene temperatures. J. Geol. 63, 538-578. EMILIANIC. ( 1966) Paleotemperature analysis of Caribbean cores P6304-8 and P6304-9 and a generalized temperature curve for the past 425,000 years. J. Geol. 74(2), 102126. EMILIANIC. (197 1) The amplitude of Pleistocene climatic cycles at low latitudes and the isotopic composition of glacial me. In The Lute Cenozoic Glacial Ages (ed. K. K. Turekian) Yale University Press, pp. 183-198. EMILIANIC. and SHACKLETONN. J. (1974) The Brunhes Epoch: isotopic paleotemperatures and geochronology. Science 183, 51 I-514. EPSTEINS. and MAYEDAT. ( 1953) Variation of 0” content of waters from natural sources. Geochrm. Cosmochfm. Acta 4, 2 13-224. EPSTEINS.. BUCHSBAUMR., LOWENSTAMH. A. and UREY H. C. ( I95 1) Carbonate-water isotopic temperature scale. Geol. Sot. Amer. Bull. 63, 417-426. EPSTEINS.. BUCHBAUMR.. LOWENSTAMH. A. and UREY H. C. (1953) Revised carbonate-water isotopic temperature scale. Genl. Sot. Amer Bull. 64, 1315 EREZ J. (1978a) The inlluence of differential production and dissolution on the stable isotope composition of planktonic foraminifera. Ph.D. Thesis, MIT-WHOI, Woods Hoie. Mass.. U.S.A. 1 19 pp. EREZ J. (1978b) Vital effect on stable-isotope composition seen in foraminifera and coral skeletons. Nature 273, 199-202. ERU J. and HONJO S. ( 198 1) Comparison of isotopic composition of planktonic foraminifera in plankton tows. sediment traps and sediments. Paleogeog. Paleoclimat. Paleoecol. 33. 129- 156. FAIRBANKSR. G., WIEBE P. H. and BE A. W. H. ( 1980) Vertical distribution and isotopic composition of living planktonic foraminifera in the western north Atlantic Science 207. 6 l-63. HEC~~TA. D. (1973) Fauna1 and oxygen isotopic paleotemperatures and the amplitude of glacial/interglacial temperature changes m the equatorial Atlantic, Caribbean and Gulf of Mexico. Quar ReA 3, 67 I-690. HECHT A. D. (1974) The oxygen isotopic record of foraminifera in deep-sea sediment In ForummQk (eds.
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R. H. Hedley and C. G. Adams) Academic Press, pp. l43. HORIBE Y. and OBA T. ( 1972) Temperature scales of aragonite-water and calcite-water systems. Fossiles 23/24, 69-79 (in Japanese). KAHN M. 1. (1977) Non-equilibrium oxygen and carbon isotopic fractionation in tests of living planktonic foraminifera from the Eastern Equatorial Atlantic Ocean. Ph.D. Thesis, Umv. Southern California. Los Angeles. 224 pp. KAHN M. 1. (1979) Non-equilibrium oxygen and carbon isotopic fractionation in tests of living planktonic foraminifera. Oceanologica Acta 2(2), 195-208. KAHN M. 1. and WILLIAMSD. F. (1981) Oxygen and carbon isotopic composition of living planktonic foraminifera from the North-East Pacific Ocean. Paleogeog. Paleoclimat. Paleoecol. 33, 173-19 1. MCCREA J. M. (1950) On the isotopic chemistry of carbonates and a paleotemperature scale. J Chem. Physrcs H(6), 849-857. O’NEIL J. R.. CLAYTONR. N. and MAYEDAT. K. (1969) Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Physics 51, 5547-5558. SAWN S. M. and DOUGLAS R. G. (1973) Stable isotope and magnesium geochemistry of recent planktonic foraminifera from the South Pacific. Geol. Sot. Amer. Bull. 84, 2327-2342. SHACKLETONN. J. (1974) Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: isotopic changes in the ocean during the last glacial. Colloques lntemationaux du C.N.R.S. No. 2 19, pp. 203-209. SHACKLETONN. J. ( 1977a) The oxygen isotope stratigraphic record of the late Pleistocene. Phil. Trans. Roy. Sot. London B 280, 169-182. SHACKLETONN. J. (1977b) Oxygen isotope stratigraphy of the middle Pleistocene. In British Quaternary Studies. Recent Advances (ed. F. W. Shotton) Clarendon Press, Oxford, pp. 1-16. SHACKLETONN. J. and OPDYKE N. D. (1973) Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: Oxygen isotope temperatures and ice volumes on a lo5 year and IO6year scale. Qua!. Res. 3, 39.55. SHAC~CLETON N. J. and OPDYKE N. D. (1976) Oxygen-isotope and paleomagnetic stratigraphy of Pacific core V28239. Late Pliocene to latest Pleistocene. Geol. Sot. Amer. Mem. 145,449-464. SHACKLETONN. J. and VINCENT E. (1978) Oxygen and carbon isotope studies in recent foraminifera from the southwest Indian Ocean. Mar. Micropal. 3, 1-13. SHACKLET~VN. J., WISEMANJ. D. and BUCKLEYH. A. ( 1973)‘Non-equilibrium isotopic fractionation between sea-water and planktonic foraminiferal tests. Nature 242, 177-179. SHEMESHA. (1980) The carbonate system in selected regions in the Northern Gulf of Eilat. MSc. Thesis, The Hebrew University of Jerusalem. (in Hebrew) 61 pp. UREY H. C. ( 1947) The thermodynamic prop-&es of isotopic substances. J. Chem. Sot. 562-581. VAN DONK J. ( 1970) The oxygen isotope record in deep sea sediments. Ph.D. Thesis. Columbia University, N.Y. **D _ *LO p. VAN DONK J. (1977) 0” as a tool for micropaleontologists. In Oceanic Micropuieontolog.v (ed. A.T.S. Ramsay) Academic Press. pp. 1345-l 370. VERGNAUD-GRAZZINIC. (1976) Non-equilibrium isotopic composition of shels of planktonic foraminifera in the Mediterranean sea. Paleogeogr. Paleoclimatol. Paleoecol. 20. 263-267. VINOT-BERTUILLEA. C. and DUPLESSYJ. C. (1973) lndividual isotopic fractionation of carbon and oxygen in benthonic foraminifera. Earth Planet. Sci. Left. 18, 247252.
C Pergamon
et ~~~wmrhrmrca .4rro Vd 47. pp IOX-103 Press Ltd 19R3 Pnntcd I” U S.A.
I
Experimental paleotemperature equation for planktonic foraminifera EREZ
JONATHAN
The Hebrew University
of Jerusaiem.The
l-i. Steinitz Marine Biology Laboraiory. Eilat. P.O. Box 469. israei and
BOAZ LUZ Dept. of Geology. The Hebrew University of Jerusalem. Jerusalem. Israel (Received April 22. 1982: accepted in revised,fbrm Februar;, 24. 1983) Abstract-Small live individuals of Globigerrnoides sacculj/h’ which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14’ to 30°C) and water isotopic composition. and a correction was made lo account for the isotopic composition of the original skeleton using control groups. Comparison of.the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCOz indicate that the foraminifera precipitate their CaC03 in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with CRAIG’S (I 965) equation yields: I = -0.07
+ l.Oli
(r = 0.95)
where I is the actual growth temperature and iis the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate. is equally applicable for the planktonic foraminifer G. saccu/jj?r. Second order regression of the culture temperature and the delta difference (6”Oc - 6’*Oro) yield a correlation coefficient of r = 0.95: i = 17.0 - 4.52(6’“Oc - ~‘“OW) + O.O3(b’“Oc - ~“‘OW)’
i, d”Oc and b’RO~, are the estimated temperature. the isotopic composition of the shell carbonate and the sea water respectively. A possible cause for nonequilibrium isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.
INTRODUCT’ION
ANI (1954. 1955, 1966) applied these equations for the first time to planktonic foraminifera in deep sea cores, and concluded that planktonic foraminifera deposit their skeleton in isotopic equilibrium with sea water and hence they can be used for paleoenvironmental studies provided that d”Ow can be estimated. (See also SAWN and DOUGLAS, 1973; SHACKLETON and VINCENT, 1978; and BERGER ef al., 1978.) Following EMILIANI,a considerable body of information on stable isotope distributions in planktonic foraminifera has been gathered, and as a result much insight has been gained into Pleistocene paleoenvironmental conditions (e.g., SHACKLETON and OP-
IN 1974 H. UREY suggested that stable oxygen iso-
in carbonate materials can be used to deduce paleotemtiratures. Following his suggestion EPSTEIN ef al. ( 195 1, 1953) developed a paleotemperature scale (or equation) for biogenic CaC03. They analyzed molluscs carbonate that grew at known temperatures and known water isotopic composition. The result was the following familiar equation: tope ratios
i = 16.5 - 4.3(6’RO~ - 6”Ow) + O.l4(6’8Oc - b’8Ow)? where iis the estimated temperature (or isotopic temperature) and d”Oc and 6”~ are the oxygen isotopic compositions of the carbonate and sea water. respectively. This equation was later modified by CRAIG ( 1965) to account for “0 and instrumemal inaccuracies:
i = 16.9 - 4.2(6”Oc - 6”Ow) + O.l3(6’8Oc - 6’80M’)’ These equations match well the experimental data of MCCREA (1950) and O’NEILL et al. (1969) for inorganic calcite within experimental
errors. EMILI-
DYKE 1973, 1976: SHACKLETON 1977: EMILIANI, 1955, 1966, 197i; EMILIANI and SHACKLETON, 1974; HECHT, 1973, 1974; DUPLESSY et al., 1975; VAN DONK, 1977; and many others).
Despite the vast amount of data on stable isotopic compositions in planktonic foraminifera, until recently little attention was given to the fact that the paleotemperature equation was tested for mollusca and not for foraminifera. Questions about the validity of the basic equilibrium assumption were first asked when it was found that different benthonic foraminifera have different specific isotopic fractionations (DUPLESSY et al., 1970; VINOT-BERTOUILLE and DUPLESSY, 1973). Attempts to test whether plank-
1025
1026
22.5 21.5 23.0 25.0 25.5
22.5
tonic foraminifera deposit their shell in isotopic equilibrium were made by VAN DONR (1970) and later by SHACKLETON et al. (1973). They compared the isotopic temperature with the actual water temperature for live planktonic foraminifera that were collected with a plankton net. It was found that the spinose species show a slight negative deviation from equilibrium (-0.5% for G. ruber and -0.4% for G. saccuftfkr). Similarly, VERGNAUDGRAZZINI ( 1976). VAN DUNK (1977) KAHN ( 1979) KAHN and WILLIAMS ( 198 l), DEUSER et al. ( 198 1), DUPLE~~Y ef al. ( 198 I), and FAIRBAND et al. ( 1980) have reported significant deviations from equilibrium, EREZ and HONJO ( 198 1) compared the. isotopic composition of planktonic foraminifera from plankton tows, sediment traps and sediments, and concluded that slight deviations from equilibrium may exist in material collected by plankton nets. However material in sediment traps at a depth range of 400 m to 5300 m. and material from bottom sediinents do not show deviations from equilibrium. It is evident that although almost every indirect approach possible has been taken, the equilibrium assumption of the isotopic paleotemperature method has not yet been proved for planktonic foraminifera. In an attempt to resolve this problem, we adopted an experimental approach to determine whether Globigennoides saccuiifer (a widely used species in pa-
* Gametogenesis is the process of sexual reproduction in planktonic foraminifera (BE et al.. 1977). It occurs at the end of the life cycle of an individual and involves multiple divisions of the protoplasm followed by production of thousands of motile garnets. These flagellated garnets burst out of the calcamous shell very rapidly and within approximately one hour a white empty shell completely free of protoplasm is left behind. Such shells are very similar to those found in sediments.
leotemperature studies) deposits its shell in isotopic equilibrium or not. EXPERIMENTAL
AND ANALYTICAL METHODS
Experimental Live planktonic foraminifera of the species Globigerinoides sacculifer were collected in the northern Gulf of Eilat using 200 cm plankton net. Plankton net tows were made in surface water at depths of 2-5 m. The collection area was 2-3 km off the Hebrew University’s Marine Biology Laboratory in Eilat. where water depth is roughly 400 m. Within one hour of collection. the samples were brought to the laboratory and the planktonic foraminifera were separated from the other plankton using pasteur pipettes (see Bt? er al.. 1977. 1978, I98 I for detatls). Young, healthy individuals of roughly I50 sm diameter and skeleton weight of around 4 fig were separated and their size measured. All these individuals contained symbiotic algae and had long spines. Each individual was placed in a 100 ml erlenmeyer flask tilled with 80 ml tiltered (0.45 pm) sea water from the site of the plankton collection. The erlenmeyers were placed in thermostated baths where temperatures were maintained at a precision of +O. I “C of the desired temperatures. During the entire experiment. temperatures were recorded on a strip chart to ensure that there were no significant deviations from the desired temperatures (see Table I). The experiments were carried out in natural sunlight near the window. and with artificial fluorescent cool white light. We did not observe any change in the symbiont activity between these two environments. Each individual was fed daily with one newly hatched brine shrimp (Anemia salina), and growth was manitored optically daily. For isotopic analysis we selected only those individuals that grew continuously and went through gametogenesis at the end of their life (i.e.. approximately 80% of the total individuals cultured).* Usually experiments lasted from ten to fifteen days. Fifteen to twenty individuals were grown at each temperature, and in each run there were two or three different temperatures. Experimental data are given in Table 1. Ana/_vticai Water isotoptc analysis was carried out according to the method of EPSTEINand MAYEDA (1953). and 6’*Owwas
Foraminifera paleotemperatures determined at the beginning and the end of each experiment. Salinities were monitored daily using an optical salinometer. In several experiments the fomminifera were cultured in 100 mi beakers that were covered with petri dishes. Under these conditions when the temperature was above 25”C, evaporation was significant and salimties rose with time. Evaporation was minimal in experiments m which we used erlenmeyer flasks. Isotopic compositton of the shells was carried out after crushing the individuals in ethanol, drying and vacuum roasting at 450°C for 30 minutes. The calcium carbonate was dissolved in - 100% H3P04 at 50°C according to the method of SHACKLETON(1974), and the isotopic composition of the evolved CO? was measured usmg a Micromass VG 602 mass spectrometer. fnstrumental corrections of the results were done according to CRAIG (1957). The results are reported in pet-mill deviations relative to the PDB standard (calibration to PDB was done with the Cal Tech Standard PDB IV).
=mple _ po = (01s/O’6) (0'8/0'6) std
1
The cultured fotaminifera reached, on the average, a weight of -40 pg per individual. Because they went through gametogenesis, they were all completely clean of protoplasm. For each plankton collection we picked a control group that was comprised of 25-50 individuals having the same size distribution as the original experimental groups. The foraminifera in the control group were soaked for 30 minutes in distilled water to which ammonia was added to give pH value -9 and part of their plasma was leached. They were then dried on a filter paper overnight. dessicated and weighed to get the average weight per individual. -411weighmg procedures were made using a Kahn 25 electrobalance having a precision of fl.2 fig. The control group samples were analysed for their isotopic composition and a correction for the initial isotopic composition was applied to the final samples according to the following equation: 6180 controlled =
fS’*Ofinal - X- 6% initial
I-x
where x’ is the weight fraction of the initial foraminifera relative to the final foraminifera weight. The correction in
1027
most cases was small because the bulk of the carbonate was deposited under the controlled experimental conditions (see Table I). RESULTS
Oxygen-isotopes The results are summarized in Tables 1 and 2 and Fig. 1. 6180 values of the carbonate shells (6180c) varied between - 1.40 and +2.06. The difference between 6’80c and 6”Ow (the isotopic composition of the water in which the foraminifera were cultured), varied between -3.00 and +.46 (Table 1). Temperature estimates were calculated using earlier equations taken from the literature and by using the best fit to the data (Table 2). In all cases these are 2nd order equations of temperature (PC) as a function of (6’80c - 6180w), where i = u + b(dc - 8~) + c(dc - 6~)~. The different coefficients (b and c) and intercepts (a) are given in Table 2. The differences between the calculated temperatures (i) and the experimental temperatures (t) were squared and averaged. The square root of this average represents the average deviation between the calculated and experimental temperatures. In addition we calculated our own 2nd order best fit to the data. The different coefficients of all the equations are quite similar, and the average deviations are all small and close to that of our own best fit (see Table 2). The scatter in our data is large enough to accommodate all the coefficients of the different equations within the 95% confidence limits on the coefficients of our best fit. (The sources of the scatter are discussed below). Hence, we cannot select one equation a$ the “best” equation to describe the oxygen isotope fractionation in biogenic or inorganic calcite. The equa-
i Erer
1028 10,
*.i
.
t
** . . l .
”
.
.
a
.*
and
B
i.uz
li .:O.l %I vanabthty In the determmatton ot ?“O ~)t.the carbonate which can cause a temperature devtation of 10.5”C ;) The correctron applied for the ongmal rsotoptc composmon. especially In those experiments where the correction was large. This correction could have introduced an error of -I 1“C, 4) 50. I ‘SOin the determination of the water ISOtopic composition which can cause -irO.S”C.
The devtattons from the expected isotoptc corn position seem to be random and therefore we cannot SIrnit data points that show large deviations. We should emphasize that our experiments were i.urned out over a temperature range. which is larger than the natural range thr this species. and that over ~hts range there are large variations in growth rate compared with the optimum at 23-24°C‘. The rquiFIG. I The cultunng temperature Y.) the difference (~‘“OC librium precipitatton of skeletal calcite is indepen-. 5’“Ow) (i.~, the carbonate minus the water oxygen isodent of the growth rate. We also tested the depentopic composition). The line is the best fit for the-data and dence of the skeletal tsotopic composition on the isoyields the following paleotemperature equation: I = 17.00 topic composition of the sea water. In experiments - 4SZ(b’“Oc - 6”Ow) + O.O3(d’“Oc - ~‘“OM,)-‘ wtth r = 0.95 carried out above 15°C in beakers. water evaporated rather rapidly and the final isotopic composttion ot the water and the carbonate were indeed heavier. tton of O’NEIL cf (I/. ( 1969) is the closest to our own However the difference (6’“Oc -- 6”Ow*) showed the equation (see (r values in Table 2). This equation also expected dependence on temperature (Tables I fits well with the data of SHACKLETON (1974)for and 1). deep benthomc foraminifera covering the lower temThe control groups that were analyzed contamed perature range (7°C to 1°C). We suggest that this only small individuals that were picked from natural equation should be used to derive oceanic paleotempopulations of surface water in the Gulf of Eilat. peratures. Their average size matched the average size of the A very good correlation exists between our experexperimental groups. They were typically I00 Grn to tmental data and the original data of EPSTEIN er al. 180 +m In diameter and weighed 2 gg to X pg per f 1953). A linear regression was calculated between Individual. The oxygen isotope composition. the amthe actual temperatures in which the foraminifera blent water temperature and the isotopic temperature grew 1’s the isotopic temperature calculated from the calculated according to EPSTEIN’S modified equatmn Isotopic composition using the equation of EF-STEIN ICRslC;. 1965) for the control groups are shown 111 d ui (1953) as modified by CRAIG (1065). If these rable 1. .The tsotoptc temperatures follow the seatemperatures were identical, the regression line would tonal trend of the temperature but in all the samples have had an intercept of 0.0 and a slope of ! .O The rhe tsotoptc temperature is higher than the ambtent actual linear regression yielded an intercept of -0.07 water temperature at the time of collection” The a~and a slope of t 1.OI :rage ditference IS 2..38”c‘ which is equivalent to 0.:; i) 6% negattve devumon from equilibrium in terms h,t ;) notation. This Jeviatton is similar to that &ierved by earlier researchers and will be discussed where I IS the actual culture temperature and I IS the heiow Isotopic temperature. This indicates that wtthm the variability and range of our data there is almost J perfect fit between the two sets of temperatures. The (. dwrr :~cuc~pt~c main conclusion is that there is no difference between C’arbon rsotoptc ;omposmon of X0: In sea water the temperature fractionation of oxygen isotopes in was not controlled
Foraminifera paleotemperatures tions and sea water was filtered before the experiment through 0.45 pm filters. The carbon isotopic compositions are shown in Table 2 and are plotted against temperature in Fig. 2. The data as a whole show a general trend of decrease in 613Cwith increasing temperatures. This trend is apparent in a few individual experiments (PT2, PT3, PT7. PT9). However the opposite trend is also shown (PT8, PT4). Experiments PT5, PT6, PTI did not show any trend. The entire range of 613C changes is roughly 2.5% and it is larger than the range of - IL shown by 613C in the XOz of sea water in the Gulf of Eilat (SHEMESH, 1980). The range within one experiment was - I k and at this stage we have no explanation for it. The foraminifera were fed daily with identical food. IIlumination, which could influence the symbionts was uniform. Currently we are investigating the factors that may control carbon isotope ratios in planktonic foraminifera. DISCUSSION
Ken-Equilibrium
L’alues
The nonequilibrium values that were reported b) other workers and those shown by the control groups
i
0 i’
a
O
FIG. 2. The carbon isotopic composition of G saccul$er grown IS culture v: the temperature of culture. Different symbols represent different experiments listed in Table 1. Note the general trend of decrease in 6’jC with increase of temperature.
1029
(Table 1) are surprising in view of the present experimental results. Most of the disequilibrium values were reported for foraminifera collected in plankton net tows (SHACF~_ETON et al., 1973; KAHN, 1979; VAN DONK. 1977: FAIRBANKS el al., 1980; KAHN and WILLIAMS, 198 1: EREZ and HONJO, 198 1; DuPLESSY ef al., 198 I ). EREZ and HONJO ( 198 I), found that in sediment traps almost ail samples were in equilibrium and the few exceptions would be accommodated into equilibrium using certain logical assumptions about seasonality and settling rates. Similarly, DEUSER et al., (198 1) found that in their sediment traps most species deposit their skeleton in equilibrium except G. bulloides which clearly is exceptional. Most of the analyses that were carried out on material from sediments show equilibrium values (EMILIANI, 1954, 1955; CURRY and MATTHEWS, 198 I: BERGER et al.. 1978; etc.). EREZ (1978a) and EREZ and HONJO ( 198 1) have discussed in detail the changes that occur in the isotopic composition of a single species during its descent in the water column and its sedimentation on the ocean floor. Their conclusion is that two processes, namely, skeleton deposition at depth and dissolution on the ocean floor tend to make the isotopic composition heavier and therefore can mask the light non-equilibrium isotopic composition observed in plankton material. A similar explanation was also suggested by DUPPLESSY et al. ( 198 I ). emphasizing the importance of gametogenic calcification in this process. However. our present experiment demonstrates that under laboratory conditions. all the skeletons show equilibrium fractionation. We therefore still need to explain the non-equilibrium values reported for plankton samples. There are several possible kxplanaiions: 1. incorrect assumptions by the authors with regard to water temperature and water isotopic composition. The temperature measured in surface water during the plankton collection time can be signilicantly different from that in which the bulk of the skeleton was deposited. Usually the foraminifera analyzed in these studies were about 2-3 weeks old and therefore their skeleton should represent the integrated temperature of that time. Incorrect assump tion on the water isotopic composition is also possible. Most authors used the linear relationship between 6’*Oc(> of sea water and its salinity as determined by EPSTEIN and MAYEDA (1953) and CRAIG and GORDON ( 1965). There are however large local and temporal deviations from these relationships which are not suitable for the calculation of isotopic composition at any locality and time of surface water, especially in the areas where precipitation and evap oration effects are large and seasonal. These two parameters (i.e. 6’60~. and t) can cause large enough negative deviations from equilibrium, as observed by most authors (in the range of 0.5%). Obviously. there are also positive deviations but they are not detected because it has always been assumed that in these cases
!030
J trez
and B. Lui
the foramlmfera depostted their skeleton at some depth in the thermocline. -. ’ It is possible that non-equilibrium values mdeed occur in plankton material-that is, in the earlier stages of growth ofplanktonic foramlnifera. BERGER et al. (1978) suggested that when individuals are young, their metabolic activity is more intense and the metabolic CO2 portion in the skeleton is larger than at later stages. The final stages of growth seem to be in isotopic equilibrium. and because the bulk of the skeleton is deposited in these final stages. at lower temperatures. it masks the non-equilibrium initial values. Our experiment does not exclude this possibility because we corrected for the original isotopic composition. However, the small weight fraction of this original skeleton does not support this possibility. 3. Most of the analyses on plankton samples are made on material that contain large quantities of protoplasmic matter (up to 50% of their total dry weight). Usually foraminifera samples analyzed from deep sea sediments and from sediment traps contain hardly any protoplasmic organic matter. Removal of this organic matter is one of the initial problems that were dealt with by EPSTEIN d al ( 195 I. 1953) and by EMILIANI (1955, 1966). EREZ ( 1978a) and EREZ and HONJO ( I98 I) have shown that in foraminifera samples that contain large quantities of organic matter, combustion is important and should be carried out properly. Various studies of plankton samples used different combustion techniques and therefore comparison between them is complicated. It is quite obvious that low temperature ashing. using an oxygen plasma furnace, of material rich in organic matter is not suitable (EREZ, 1978a). Also, treatment with oxldizing agents such as chlorox (sodium hyperchlorite) are not recommended (EMILIANI. 1966). All combustion techniques tend to make the isotopic composition lighter (EPSTEIN et al, 1953; EMILIAN. 1966: KAHN, 1977: EREZ and HONJO, 198 I ) and thus can cause large negative deviations m the isotopic composition of organic-rich material. The fact that most of the nonequilibrium values reported in the literature are for living foraminifera collected by plankton nets indicate that the organic matter may be the cause for the nonequilibrium values. We would like to emphasize that in the present experiment the cultured foraminifera did not contain any protoplasmic organic matter because they all went through gametogenesis. This was not so for the control groups that contained organic matter. Thus, It is possible that disequilibrium values cited m the literature and those observed in the control groups are caused by exchange reactton between the isotopically light CO2 produced during the combustion of the organic matter and the skeletal CaCO,. Implicatrons jar paieoecolog~ We have shown that the bulk of the shell m the planktonic foraminifer G. sacculifer is deposited In
Isotopic rqulhbnum uver a wide range ot temperatures that exceed its ecological range. In paleoecological applications of these results. it should be remembered that in nature planktonic foraminifera contmue to calcify dunng their entire life including the period when the population sinks through the water column. For f; racclll[ftir m particular. it has been shown that calcitication occurs at depth (EREZ and HONJQ I98 I ) and probably continues until the hnal stages of gametogenesis (St?. 198 1; DL’PLESS~ i’t 111 198 I ). It also seems that a significant part ut the total skeleton weight IS deposited at a depth at‘ a few hundred meters (up c’tl.). The shells of planktonic tbrammlfera thus include fracuons which were deposlted over a wtde range of depth and temperature i\swgnmem of one particular depth or temperature of skeleton deposition represents a weighted mean depth and temperature of the entire skeleton depositlonal histon Other environmentai and blogemc factors. such as rate ot metabolism. food quantity and isotopic composition. photosynthetlc activity of symbionts, light penetration and chemical composition of sea water (especially nutrients). may affect the stable-isotope compoatlon of carbon and oxygen in foraminiferal calcite. T’hls seems to be true for benthomc foramimfera hut may also hold for some planktonic foramlmfera. Ditferent species may have different fractionatlon factors and different dependence of the isotopic composition on the environmental factors. For (; \usL,uli,/tir. however. the shell is deposlted in equlhbnum with sea water and hence temperature is a maJor factor that detennmes Its oxygen isotopic compositlon. We safely state that earlier paleoceanographic studies based an the oxygen isotopic corn. posltion of G xxuriiftir (and most studies utilized this species, are mdeed valid and can be interpreted with higher confidence ~c,know/rd~~~mrnr.\-~V~tx~rrveyour Smcere thanks to A. Szm for technical assistance m cultunng the orgamsms: to A Shemesh for technical help in the Isotopic analysis: to 2 Reiss. 4 Matthews, S Weiner. H. Lowenstam and J. C Duplessy for fruitiul discussions and review of the manunript. Special thanks are to B. Shem-Tov for editing and tvplng the manuscript.
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