Important crustal growth in the Phanerozoic: Isotopic evidence of granitoids from east-central Asia
BOR-MING JAHN¹ , FUYUAN WU^1,2 and DAWEI HONG^3
1 G  eosciences Rennes, Universit  e de Rennes 1, 35042 Rennes Cedex, France
email: jahn@univ-rennes1.fr
² Department of Geology, Changchun University of Science and Technology, Changchun 130 061, China
³ Lithosphere Research Centre, CAGS, Beijing 100 037, China

                The growth of the continental crust is generally believed to have been essentially completed in the Precambrian, and the amount of juvenile crust produced in the Phanerozoic is considered insignificant. Such idea of negligible growth in the Phanerozoic is now challenged by the revelation of very large volume of juvenile crust produced in the period of 500 to 100 Ma in several orogenic belts. While appreciable volumes of juvenile terranes in North America (Canadian Cordillera, Sierra Nevada and Peninsular Range, Appalachians) have been documented based on Nd isotopic data, the mass of new crust formed in the East-Central Asian Orogenic Belt (ECAOB), eastern part of the Altaid Tectonic Collage, appears to be much greater than the above terranes combined. New and published Nd-Sr isotope data indicate that the Phanerozoic granitoids from the southern belt of the ECAOB (Xinjiang-West Mongolia-Inner Mongolia-NE China) as well as from Mongolia and Transbaikalia were generated from sources dominated by a depleted mantle component. These granitoids represent a significant growth of juvenile crust in the Phanerozoic.
                Although most plutons in this huge orogenic belt belong to the calc-alkaline series, the ECAOB is also characterized by the emplacement of voluminous A-type granites. The origin of these rocks is probably multiple and is still widely debated. However, the isotopic data (Sr-Nd-O) and trace element abundance patterns of A-type granites from the ECAOB clearly indicate their mantle origin.
                The evolution of the ECAOB and the entire Altaid Collage is most likely related to successive accretion of arc complexes. However, the emplacement of a large volume of post-tectonic A-type granites requires another mechanism, probably through a series of processes including underplating of massive basaltic magma, partial melting of these basic rocks to produce granitic liquids, followed by extensive fractional crystallization. The proportion of juvenile to recycled, as well as that of arc-related to plume-generated, continental crust remains to be evaluated by more systematic dating and isotope tracer studies.

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Anasagar gneiss : A folded granitoid pluton in the Proterozoic South Delhi Fold Belt, central Rajasthan
DHRUBA MUKHOPADHYAY ¹ , TAPAS BHATTACHARYYA ¹ , NANDINI CHATTOPADHYAY¹, ROBERT LOPEZ² , OTHMAR T TOBISCH^2
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Calcutta 700 019, India.
² Earth Sciences Department, A232 E. and M.S. Building, University of California, Santa Cruz, California, 95064, U.S.A.
email: dhruba@cucc.ernet.in

The Anasagar gneiss was emplaced as a concordant sheet like body along the contact of quartzite and pelitic/semipelitic schist horizons in the northern part of the South Delhi Fold Belt. It is typically a granite gneiss containing megacrysts of K-feldspar set in a recrystallised foliated matrix. The megacrysts are in general converted to granular aggregates, often retaining their crystal outline. Garnet, sillimanite (fibrolite) and rarely staurolite are the metamorphic minerals in the gneiss; these are also present in the enveloping supracrustal rocks. Both the gneiss and the supracrustal rocks are involved in polyphase deformation. F₁ isoclinal folds are present only on minor scale in the supracrustal rocks. F₂ major and minor folding have affected both the gneiss and the supracrustal rocks. These are asymmetrical folds with alternate flat and steep, locally overturned, limbs and have consistent easterly vergence. F₃ folds are upright and coaxial with F₂. F₄ puckers and large scale warps have E-W to ESE-WNW subvertical axial planes. The gneiss is exposed in the core of an F₃ arch on the flat limb of a major F₂ antiform whose axial trace is bent by an F₄ fold. The intrusion was pre-F₂ and late-tectonic with F₁. U-Pb zircon dating suggests a crystallization age of 1849 ± 8 Ma. Hence the Anasagar gneiss is older than the late- to post-tectonic ``Erinpura-type'' granite in the South Delhi Fold Belt.

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Emplacement of Amba Dongar carbonatite-alkaline complex at Cretaceous/Tertiary boundary: Evidence from 40 Ar- 39 Ar chronology
JYOTIRANJAN S RAY, KANCHAN PANDE   and T R VENKATESAN
Physical Research Laboratory, Ahmedabad 380 009, India.
email: kanchan@prl.ernet.in

40 Ar- 39 Ar analyses of three fresh alkaline rock samples and a phlogopite separate from a carbonatite from Amba Dongar carbonatite-alkaline complex of the Deccan Flood Basalt Province, India, yield indistinguishable precise plateau ages of 64.8 ± 0.6, 64.7 ± 0.5, 65.5 ± 0.8 and 65.3 ± 0.6 Ma, giving a mean plateau age of 65.0 ± 0.3 Ma, which is the age of emplacement of this complex. This age implies contemporaneity of Amba Dongar with several other carbonatite-alkaline activities of Chhota Udaipur subprovince and is consistent with their Reunion-Deccan plume origin hypothesis. The emplacement of these complexes at 65 Ma makes them very significant in the ongoing debate on the K/T extinctions owing to their capacity to rapidly inject a substantial amount of CO₂and SO₂ into the atmosphere.

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Sr isotopic evidence on the spilitic degradation of the Deccan basalt
K V SUBBARAO
Department of Earth Sciences, Indian Institute of Technology, Mumbai (Bombay) 400 076, India.
email: subbu@geos.iitb.ernet.in

Similar Sr isotopic ratios ( ~ 0:7055) for the tholeiite-spilite flow unit and the associated mineral phases, of Bombay (Deccan Traps) provide a direct evidence for the spilitic degradation of tholeiite. In contrast, a dramatic increase in the rare earth elements (REE) from basalt to spilite is rather puzzling as rare earths are considered to be relatively immobile. The geochemistry thus suggests that the process of spilitization is due to the reaction with a complex fluid having identical Sr-isotopic composition as that of the basaltic magma - thereby masking the details of the mixing process.

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Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: Constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton
Y J BHASKAR RAO¹ , ANIL KUMAR¹ , A B VREVSKY² , R SRINIVASAN¹ ANd G V ANANTHA IYER^3
1National Geophysical Research Institute, Hyderabad 500 007, India;
email: postmast@csngri.ren.nic.in
²Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia.
³Department of Inorganic and Physical Chemistry, Indian Institute of Sciences, Bangalore 560 012, India

Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3:285± 0.17Ga, ε_Nd=0:82± 0.78 for the Honnavalli meta- anorthosite complex from a supracrustal enclave in the low-strain zone, and 2:495±0.033 Ga, ε_Nd =2.2± 0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5 Ga. Our results set a younger age limit at c.3.29 Ga for the supracrustal rocks of the HSB in the Dharwar craton.

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Compilation of radiogenic isotope data in Mexico and their petrogenetic implications
IGNACIO S TORRES-ALVARADO¹ , SURENDRA P VERMA¹ and GERARDO CARRASCO-NÚÑEZ^2
1 Centro de Investigacion en Energia, UNAM, Priv. Xochicalco s/n, Col. Centro, Apdo. Postal 34, 62580 Temixco, Morelos, Mexico.
email: ita@mazatl.cie.unam.mx
² Instituto de Geologia, UNAM, Circuito Exterior, Cd. Universitaria, 04510 Mexico, D.F., Mexico.

            Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows: ⁸⁷Sr/ ⁸⁶Sr 0.703003-0.70841; ¹⁴³ Nd/¹⁴⁴Nd 0.512496-0.513098; ²⁰⁶Pb/²⁰⁴Pb 18.567-19.580; ²⁰⁷Pb/²⁰⁴Pb 15.466-15.647; ²⁰⁸Pb/²⁰⁴Pb 38.065-38.632. The SMO shows a large variation in ⁸⁷Sr/⁸⁶Sr ranging from ~ 0.7033 to 0.71387. ¹⁴³Nd/¹⁴⁴Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows: ²⁰⁶Pb/²⁰⁴Pb 18.060-18.860; ²⁰⁷Pb/²⁰⁴Pb 15.558- 15.636; ²⁰⁸ Pb/²⁰⁴Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range: ²⁰⁶Pb/²⁰⁴Pb 18.049-19.910; ²⁰⁷Pb/²⁰⁴Pb 15.425-15.734; ²⁰⁸Pb/²⁰⁴Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc.
            Most samples with reported Sr and Nd isotopic data are spread within and around the ``mantle array''. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces andofNdisotopes in BC,AP, SMS, and SMOr.

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Error propagation in equations for geochemical modeling of radiogenic isotopes in two-component mixing
SURENDRA P VERMA
Centro de Investigacion en Energia, UNAM, Priv. Xochicalco S/N, Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, Mexico.
email: spv@mazatl.cie.unam.mx

This paper presents error propagation equations for modeling of radiogenic isotopes during mixing of two components or end-members. These equations can be used to estimate errors on an isotopic ratio in the mixture of two components, as a function of the analytical errors or the total errors of geological field sampling and analytical errors. Two typical cases (``Small errors'' and ``Large errors'') are illustrated for mixing of Sr isotopes. Similar examples can be formulated for the other radiogenic isotopic ratios. Actual isotopic data for sediment and basalt samples from the Cocos plate are also included to further illustrate the use of these equations. The isotopic compositions of the predicted mixtures can be used to constrain the origin of magmas in the central part of the Mexican Volcanic Belt. These examples show the need of high quality experimental data for them to be useful in geochemical modeling of magmatic processes.

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Factors controlling the groundwater transport of U, Th, Ra, and Rn
A TRICCA, D PORCELLI and G J WASSERBURG
The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA
email: tricca@gps.caltech.edu

            A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the ²³⁸U and ²³²Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay,  α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the ²³⁸U and ²³²Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance x_i , characteristic of each nuclide. Where   x_i is much longer than the aquifer length, (for ²³⁸U, ²³⁴U, and ²³²Th), the activities grow linearly with distance. Where  $\bar{x_i}  is short compared to the aquifer length, (for ²³⁴Th, ²³⁰Th, ²²⁸Th, ²²⁸Ra, and ²²⁴Ra), the activities rapidly reach a constant or quasi-constant activity value. For ²²⁶Ra and ²²²Rn, the limiting activity is reached after 1 km.
            High   δ²³⁴U values (proportional to the ratio &espsilon;_²³⁴Th /W_²³⁸U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios ²³⁰Th/²³²Th, ²²⁸Ra/²²⁶Ra and ²²⁴Ra/²²⁶Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for ²²⁸Ra/²²⁶Ra and ²²⁴Ra/²²⁶Ra activity ratios less than unity. From the model, the highest ²²²Rn emanation equals 2ε. This is in agreement with the hypothesis that ²²²Rn activity can be used as a first approximation for input by recoil (Krishnaswami et al 1982). However, high ²²²Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.

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Isotopic composition of xenon in petroleum from the Shell Bullwinkle Field
J NUZZO*, M HYMAN and M W ROWE**
Department of Chemistry, Texas A&M University, College Station, TX 77843.
* Present address: Barrens Consulting Co., 273 Pepe's Farm Road, Milford, CT 06470.
**email: MWRowe@tamu.edu
M N RAO⁺ and R L PALMA
Department of Physics, Sam Houston State University, Huntsville, TX 77341.
⁺ Present address: SN4, NASA-JSC, Houston, TX 77858. J WESTRICH
Shell Development Corporation, P.O. Box 481, Houston, Texas 77001.

We have measured the abundance and isotopic composition of xenon in petroleum samples from the Shell Bullwinkle Field off the coast of Louisiana. We used an oxidation and purification procedure designed to insure complete extraction and clean up of xenon from the petroleum. The xenon isotopic composition was found to be similar to the atmospheric value for one petroleum sample. While the results of the second sample suggest possible enrichment of the heavier isotopes, the errors associated with these excesses preclude a definitive statement to that effect. No monoisotopic enrichment in ¹²⁹Xe was detected in either sample, the presence of which might have allowed us to deduce the petroleum age. Our results represent only the second xenon measurement from petroleum, and the concentrations are within the range of values published in the earlier report.

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Stable isotope systematics of surface water bodies in the Himalayan and Trans-Himalayan (Kashmir) region
KANCHAN PANDE⁺, J T PADIA, R RAMESH and K K SHARMA*
Physical Research Laboratory, Ahmedabad 380 009, India.
*Wadia Institute of Himalayan Geology, Dehradun 248 002, India.
⁺email: kanchan@prl.ernet.in

Stable hydrogen ( δD) and oxygen (δ¹⁸O) isotope ratios of the headwaters of the Indus and its tributaries, surface ice in glaciers, saline and fresh water lakes and thermal springs in the Himalayan and Trans- Himalayan (Kashmir) region are reported. The δD- δ¹⁸O relationship for the river samples shows a slope of 9.12 ±0.29 which agrees well with the estimate of 8.99±0.33 based on a simple Rayleigh fractionation model. The unique signature of a higher deuterium excess (d) of the `Western Disturbance' is preserved in these samples. An altitude effect of -0.9 per mil/km is observed in the   δ¹⁸O of Indus waters. At a lower altitude (Beas) the altitude effect is almost double, indicating that the altitude effect decreases with elevation in this region.

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Climatic significance of D/H and ¹³C/¹²C ratios in Irish oak cellulose
M G L BAILLIE¹ , J R PILCHER¹ , A M POLLARD² and R RAMESH^3
1 Palaeoecology Centre, Queen's University, Belfast, BT7 1NN, UK
² Department of Archaeological Sciences, University of Bradford, Bradford, BD7 1DP,UK
³Physical Research Laboratory, Ahmedabad 380 009, India.
email: ramesh@prl.ernet.in

δD and δ¹³C analyses of cellulose nitrate from two modern Irish oak trees that form part of the 7400 year long chronology were carried out, covering a period of 123 years (1861-1983 A.D.) with a 5 year resolution so as to assess the potential of this long chronology for retrieval of palaeoenvironmental data. One of the trees (Q5293) showed significant correlations of δD, δ¹³C and ring width with mean annual temperatures as recorded at the Armagh weather station nearby and the mean fall temperatures of Central England. The other tree (Q5296) did not exhibit any significant climatic correlations either because it grew utilizing a nearby permanent source of ground water or because the intra-ring isotopic variations in Irish oak are significant enough to mask the climatic signal. Whilst our results have given a positive indication of the usefulness of these trees for palaeoenvironmental information, more trees need to be analysed to confirm our findings. Even though one of the trees did not exhibit climatic correlations, both trees show a significant positive correlation of δ¹³C and a negative correlation of δD with ring width variations. Furthermore, two tree samples that grew during the 1620s B.C., when a volcano is thought to have erupted on the Aegean island of Santorini, show increased δD and decreased δ¹³C for one to two decades following the eruption, though the magnitudes of change seem to vary with site and trees. We have proposed a possible mechanism based on tree phenology to explain both the above effects.

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Late Glacial and Holocene Paleolimnology of two temperate lakes inferred from sediment organic δ¹³C chronology
N A LOVAN and R V KRISHNAMURTHY
Department of Geology, Western Michigan University, Kalamazoo, Michigan, USA.
email: lovan1NA@mail.cmich.edu

The stable carbon isotope (δ¹³C) and elemental C/N ratios in Total Organic Carbon (TOC) extracted from radiometrically dated cores from two Midwestern USA lakes were determined to investigate the factors that control these values in temperate lakes. The range of δ¹³C values ( -26 to -32%) and C/N ratios (mean value ~ 10.8) are typical of values reported for other temperate lake organic matter in this region. In the core from Lake Winnebago, Wisconsin, a negative correlation was seen between the TOC and δ¹³C, which can be interpreted in terms of a re-mixing and consumption of sedimented organic carbon along with rapid equilibration throughout the water column. No correlation was seen between the TOC and δ¹³C in the record from Ladd Lake, Ohio, implying that in this latter lake productivity alone was not a singular process controlling the isotope ratio. Here, it is suggested that equilibrium conditions are maintained such that the DIC of the water is never depleted of aqueous CO₂ during high organic production and the resulting δ¹³C of the organic carbon lacks correlation with the TOC. Further, in this lake a fine resolution analysis was carried out which indicated a possible anthropogenic influence on the isotope ratio around times when human settlement ( ~ 300 yrs ago) and enhanced agricultural practices ( ~ 80 yrs ago) were significant. The study shows that carbon isotope studies are useful in paleolimnologic investigations.

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Is there a stable isotope evidence for the CO2 fertilization effect?
R V KRISHNAMURTHY and M MACHAVARAM
Department of Geology, Western Michigan University, Kalamazoo, MI 49008, USA
email: r.v.krishnamurthy@wmich.edu

It has been suggested that part of the so-called ''missing sink'' of carbon dioxide introduced into the atmosphere by anthropogenic activities, that is the imbalance between estimated anthropogenic carbon dioxide emissions and oceanic uptake, may be stored in the vegetation in midlatitudes. Precise mecha- nisms of abstraction of additional carbon dioxide by vegetation, also known as the ``fertilization effect'', are poorly understood. Stable carbon and hydrogen isotope ratios of cellulose extracted from annual growth rings (covering the time period 1980-1993) in an oak tree from Kalamazoo, SW Michigan provide a basis to investigate at a physiological level how the fertilization effect may operate. The carbon isotope ratios show that the intercellular concentration of carbon dioxide increased due to an increase in stomatal opening. Although increased intercellular concentration of carbon dioxide translated to increased Water Use Efficiency and assimilation rates, it also resulted in increased transpiration rate as shown by higher D/H of the fixed carbon. The two-fold significance of the isotope data are: first, they provide the first field evidence based on isotope studies for excess CO₂ induced biomass production and second, they suggest that this mechanism is likely to operate only in limited environments. Vegetation in regions where mois- ture availability is not restricted so that there can be a gain in water use efficiency despite increased leaf evaporation are best suited to sequester excess carbon from the atmosphere.

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Effect of intraband variability on stable isotope and density time series obtained from banded corals
S CHAKRABORTY¹ , R RAMESH¹* and J M LOUGH^2
1Physical Research Laboratory, Ahmedabad 380 009, India.
²Australian Institute of Marine Science, Townsville, Queensland, Australia.
*email: ramesh@prl.ernet.in

Density,δ¹⁸O and δ¹³C were measured along two tracks, one close to the central growth axis and the other, ~20° off the axis, in a coral (Porites lutea) collected from the Stanley Reef, Central Great Barrier Reef, Australia. The δ¹⁸O variations in the coral are well correlated with sea surface temperature changes. The common variances between the two tracks were about 60% in the δ¹⁸O, δ¹³C and the skeletal density variations. Part of the noise (40%) could be due to the difficulty of sampling exactly time contemporaneous parts of each band along the two tracks and part of it could be due to genuine intraband variability. In spite of the intraband variability, the time series obtained from the two tracks are similar, indicating that the dominant causative factor for the isotopic variations is external, i.e., the environmental conditions that prevail during the growth of the coral; density band formation does not appear to be directly controlled by the sea surface temperature.

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Glacial-interglacial changes in the surface water characteristics of the Andaman Sea: Evidence from stable isotopic ratios of planktonic foraminifera
S M AHMAD¹ , D J PATIL¹ , P S RAO², B N NATH², B R RAO² and G RAJAGOPALAN^3
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India;
email: postmast@csngri.ren.nic.in
² National Institute of Oceanography, Dona Paula, Goa 403 004, India.
³ Birbal Sahni Institute of Palaeobotany, Lucknow 226 007, India.

Stable carbon and oxygen isotopic analyses of the planktonic foraminifera (Globigerinoides ruber) from a deep sea sediment core (GC-1) in the Andaman Sea show high glacial-to-Holocene δ¹⁸0 amplitude of 2.1% which is consistent with previously published records from this marginal basin and suggest increased salinity and/or decreased temperature in the glacial surface waters of this region.
            A pulse of ¹⁸O enrichment during the last deglaciation can be attributed to a Younger Dryas cooling event and/or to a sudden decrease of fresh water influx from the Irrawady and Salween rivers into the Andaman Sea. High δ¹³C values observed during the isotopic stages 2 and 4 are probably due to the enhanced productivity during glacial times in the Andaman Sea.

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Palaeomonsoon and palaeoproductivity records of   18 O,   13 C and CaCO3 variations in the northern Indian Ocean sediments
A SARKAR^1,* , R RAMESH¹, S K BHATTACHARYA¹ and N B PRICE^2
1 Physical Research Laboratory, Ahmedabad 380009, India.
² Grant Institute of Geology, University of Edinburgh, Edinburgh, EH9 3JW, UK
*Current address: Indian School of Mines, Dhanbad.
email: ramesh@prl.ernet.in

δ¹⁸O and δ¹³C of G. sacculifer have been measured in five cores from the northern Indian Ocean. In addition, high resolution analysis (1 to 2 cm) was performed on one core (SK-20-185) for both δ¹⁸O and δ¹³C in five species of planktonic foraminifera. CaCO₃ variation was measured in two cores. The results, presented here, show that

• the summer monsoon was weaker during 18 ka and was stronger during 9 ka, relative to modern conditions;

δ¹³C variations are consistent with independent evidence that shows that during the last glacial maximum (LGM; 18 ka) the upwelling was reduced while during 9 ka it was vigorous;
• calculation of CaCO₃ flux shows that the LGM was characterized by low biogenic productivity in the Arabian Sea while during the Holocene productivity increased by ~65%, as a direct consequence of the changes in upwelling. Similar changes (of lesser magnitude) are also seen in the equatorial Indian Ocean. The amount of terrigenous input into the Arabian Sea doubled during LGM possibly due to the higher erosion rate along the west coast;
• δ¹⁸O values indicate that the Arabian Sea was saltier by 1 to 2% during LGM. The northern part was dominated by evaporation while in the equatorial part there was an increased precipitation.

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Long-term records of erosional change from marine ferromanganese crusts
R KEITH O'NIONS   and MARTIN FRANK
Department of Earth Sciences, University of Oxford, Parks Rd., Oxford OX1 3PR, UK
*email: keith.onions@earth.ox.ac.uk

            Ferromanganese crusts from the Atlantic, Indian and Pacific Oceans record the Nd and Pb isotope compositions of the water masses from which they form as hydrogenous precipitates. The ¹⁰Be/⁹Be- calibrated time series for crusts are compared to estimates based on Co-contents, from which the equatorial Pacific crusts studied are inferred to have recorded ca. 60 Ma of Pacific deep water history.
            Time series of ε_Nd show that the oceans have maintained a strong provinciality in Nd isotopic composition, determined by terrigenous inputs, over periods of up to 60 Ma. Superimposed on the distinct basin-specific signatures are variations in Nd and Pb isotope time series which have been particularly marked over the last 5 Ma.
            It is shown that changes in erosional inputs, particularly associated with Himalayan uplift and the northern hemisphere glaciation have influenced Indian and Atlantic Ocean deep water isotopic composi- tions respectively. There is no evidence so far for an imprint of the final closure of the Panama Isthmus on the Pb and Nd isotopic composition in either Atlantic or Pacific deep water masses.

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Cosmogenic 10 Be: A critical view on its widespread dominion in geosciences
D. LAL
Scripps Institution of Oceanography, Geosciences Research Division, La Jolla CA 92093-0220.
email: dlal@ucsd.edu

The radionuclide ¹⁰Be (half-life 9 1:5 my), produced naturally in the Earth's atmosphere by nuclear interactions of cosmic rays, was sought in ocean sediments in the late fifties, considering its potential usefulness as a radiotracer for dating sediments. ¹⁰Be was discovered independently by two groups, one in India and the other in the USA, and used only for dating marine sediments and manganese nodules until the seventies. Subsequently, as a result of a technical advance resulting in the improvement in the sensitivity of measurement of ¹⁰Be by about a factor of 10⁶, there was a global rush to measure this nuclide in most materials participating in the physical, chemical and biological processes in the dynamic geosphere. This paper outlines the reasons for this ``isotope rush'', and the lessons learned from these studies. I also present my personal views of the special attractive features of this nuclide on the one hand, and on the other, the pitfalls or the wrong message this nuclide could convey!

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Sr isotopes in the Orgueil CI meteorite: Chronology of early solar system hydrothermal activity
J D MACDOUGALL
Scripps Institution of Oceanography, La Jolla, CA 92093-0220, USA.
email: jdmacdougall@ucsd.edu

New Sr isotopic analyses and calculated formation ages of carbonates from the Orgueil CI meteorite are reported. Among the samples analyzed in this work, dolomites give the youngest formation ages and may have been deposited intermittently starting near the time of parent body formation and continuing for at least 30 Ma. The Sr isotope data also suggest that breunnerites (Fe-Mn-Mg carbonates) crystallized after dolomite formation. Leaching experiments on bulk meteorite samples provide evidence for a very mobile, water soluble Sr reservoir in Orgueil that is characterized by extremely radiogenic Sr ( ⁸⁷Sr/⁸⁶Sr ~ 0.81- 0.82). This unsupported Sr reflects recent element redistribution, possibly at the time of parent body breakup recorded by the ~10 Ma exposure age of Orgueil. The carbonate data in particular corroborate earlier indications that hydrothermal processes were among the earliest events to affect the CI parent body.

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Conceptual problems with remote element synthesis
O K MANUEL
Department of Chemistry, University of Missouri, Rolla, MO 65401, USA
email: om@umr.edu

The notion of remote element synthesis has recently been modified to explain the presence of nucleogenetic isotopic anomalies and decay products of short-lived nuclides by injection of a small amount of exotic nucleogenetic material. Even with this modification, remote element synthesis seems inconsistent with the following observations:

• Evidence of coupled variations in the chemical and isotopic compositions of the source material for meteorites.
•  Residual coupling of chemical and isotopic heterogeneities across planetary distances in the solar system today.
• The mass-fractionation relationship seen across isotopes of elements in the planetary system, in the solar wind, and in solar flares.
• Linkage of short-lived radioactivities with isotopic anomalies and with physical properties of their host grains, as expected for early condensate of fresh stellar debris.
• Temporal and spatial distributions of short-lived nuclides and their decay products.
• Mirror-image ( D and ÿ ) isotopic anomalies in meteorite grains that sum to ``normal'' isotopic ratios, as expected of unmixed products of the same nuclear reactions that produced our bulk elements.
• The lack of supporting evidence for ``presolar'' grains or nearby stars that injected exotic material into the early solar nebula.

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