Allan R. Chivas

Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources

Geochemical and Sr, Nd, Pb, O and C isotopic data are reported for 13 carbonatites from Africa, Australia, Brazil, Europe and the United States. The carbonatites possess generally high Ba, Th, LREE, Sr and low Cs, Rb, K and HREE abundances. Some examples have low Ti, Nb, Ta, P, Zr, Hf and U concentrations which are consistent with variable fractionation of sphene, apatite, perovskite, monazite or zircon. The samples range in age from Proterozoic to Tertiary and possess a range of initial Sr isotopic compositions between 0.7020 and 0.7054, initial ϵNd values of −0.4 to +3.8 and (with the exception of the Brazilian Jacupiranga carbonatite) generally radiogenic initial Pb isotopic compositions. δ18OSMOW compositions of the intrusive carbonatites range from +5.5 to +12.4‰ Higher δ18OSMOW values of +14 and +17%. are found in the volcanically-emplaced Proterozoic Goudini complex of South Africa, suggesting the involvement of secondary alteration processes. δ13CPDB ranges from −0.5 to −6.6‰ with samples having near-primary δ18OSMOW (between +5.5 and +8%.) possessing δ13CPDB between −2.9 to −6.6‰. On the initial Sr-Nd isotope diagram, most carbonatites plot below the mantle array and below or within the field of many ocean-island basalts. The Pb isotopic compositions of carbonatites generally lie along the array defined by oceanic basalts. n nThe characteristics of carbonatites from a number of continents and their isotopic similarity to some ocean-island basalts favour an asthenospheric mantle “plume” origin. This conclusion suggests that some ocean-island alkali basalts may have been derived from trace-element-depleted mantle sources which have been re-fertilised by low-viscosity, trace-element-rich carbonatitic melts. The common close spatial and temporal association and the overlap in trace-element geochemistry and isotopic characteristics of Group 1 (basaltic) kimberlites and carbonatites argues strongly for a genetic relationship. Although late-stage melt/vapour fractionation may play some role, the extreme LREE-enrichment typical of carbonatites requires their derivation by small degrees of melting (< ≈ 1%) from a garnet-rich eclogitic source. This source may originally have been CO2- and volatile-rich subducted oceanic lithosphere.

A high-resolution Sr/Ca and δ18O coral record from the Great Barrier Reef, Australia, and the 1982–1983 El Niño

Abstract A high-resolution (near weekly) Sr/Ca and oxygen isotopic record is presented for a coral from the Pandora Reef in the Great Barrier Reef (GBR) of Australia during the period 1978 to 1984. The records are well correlated except for periods of high rainfall when river runoff has significantly modified the δ18O value of seawater. Using the Sr/Ca temperature calibration of De Villiers et al. (1994), the Sr/Ca records exhibit seasonally controlled cyclical SST (sea surface temperature) variations of from ~21 to ~28δC. During the very strong El Nino of 1982–1983, the Sr/Ca systematics indicate a sharp drop in the winter SST to ~ 18.5δC. This represents a temperature anomaly of −3δC which is approximately twice that given by the δ18O variations, suggesting an ~×2 amplification of the anomaly by the Sr/Ca system, possibly due to the increasing dominance of inorganically controlled aragoniteseawater fractionation. The oxygen isotopic systematics show the combined effects of both temperature and changing seawater δ18O values, the latter reflecting the influx of 18O-depleted runoff during periods of high rainfall. Due to the extremely low (~10−3) Sr and Ca contents of river run off relative to seawater, it is possible to use the Sr/Ca thermometer to calculate temperatures independent of major floods and hence deconvolve the combined effects in the oxygen isotopic record of variable temperature and the δ18O value of seawater. Using this approach it is possible to quantitatively reproduce the volume of runoff from the Burdekin River during the periods of major flooding that occurred in early 1979 and 1981. The results of this study demonstrate that the combined use of high-resolution Sr/Ca and δ18O systematics in scleractinian corals is a powerful tool for providing quantitative constraints on past climate.

Biological Controls on Coral Sr/Ca and δ18O Reconstructions of Sea Surface Temperatures

Coral strontium/calcium ratios have been used to infer that the tropical sea surface temperature (SST) cooled by as much as 6�C during the last glacial maximum. In contrast, little or no change has been inferred from other marine-based proxy records. Experimental studies of the effect of growth rate and the magnitude of intraspecific differences indicate that biological controls on coral skeletal strontium/calcium uptake have been underestimated. These results call into question the reliability of strontium/calcium-based SST reconstructions.

Coprecipitation and isotopic fractionation of boron in modern biogenic carbonates

Abstract The abundances and isotopic composition of boron in modern, biogenic calcareous skeletons from the Gulf of Elat, Israel, the Great Barrier Reef, Australia, and in deep-sea sediments have been examined by negative thermal-ionization mass spectrometry. The selected species (Foraminifera, Pteropoda, corals, Gastropoda, and Pelecypoda) yield large variations in boron concentration that range from 1 ppm in gastropod shells to 80 ppm in corals. The boron content of the biogenic skeletons is independent of mineralogical composition and is probably related to biological (vital) effects. The δ 11 B values of the carbonates range from 14.2 to 32.2%. (relative to NBS SRM 951) and overlap with the δ 11 B values of modern deep-sea carbonate sediments ( δ 11 B = 8.9 to 26.2%.). The variations of δ 11 B may be controlled by isotopic exchange of boron species in which 10 B is preferentially partitioned into the tetrahedral species, and coprecipitation of different proportions of trigonal and tetrahedral species in the calcium carbonates. Carbonates with low δ 11 B values (~ 15%.) may indicate preferential incorporation of tetrahedral species, whereas the higher δ 11 B values (~30%.) may indicate 1. (1) uptake of both boron species assuming equilibrium with seawater 2. (2) preferential incorporation of B(OH) 4 − from in situ high-pH internal fluids of organisms that are isolated from seawater. The B content and δ 11 B values of deep-sea sediments, Foraminifera tests, and corals are used to estimate the global oceanic sink of elemental boron by calcium carbonate deposition. As a result of enrichment of B in corals, a substantially higher biogenic sink of 6.4 ± 0.9 × 10 10 g/yr is calculated for carbonates. This is only slightly lower than the sink for desorbable B in marine sediments (10 × 10 10 g/yr) and approximately half that of altered oceanic crust (14 × 10 10 g/yr). Thus, carbonates are an important sink for B in the oceans being ~20% of the total sinks. The preferential incorporation of 10 B into calcium carbonate results in oceanic 11 B-enrichment, estimated as 1.2 ± 0.3 × 10 12 per mil · g/yr. The boron-isotope composition of authigenic, well-preserved carbonate skeletons may provide a useful tool to record secular boron-isotope variations in seawater at various times in the geological record. The potential use of boron-isotope geochemistry in skeletons as a tracer for palaeoenvironments is demonstrated in Ostracoda and Foraminifera from the Gulf of Carpentaria, Australia. The δ 11 B values of glacial-age, buried skeletons (4.0 and 4.9%., respectively) are lower than that of their modern equivalents (17.6 and 13.3%., respectively). This may reflect a “terrestrial” boron-isotope signature of the water in the gulf during the Late Quaternary when it was isolated from the ocean.

High-resolution isotopic records from corals using ocean temperature and mass-spawning chronometers

Abstract We present a 6-year-long (1978–1984) δ 18 O and δ 13 C record from a Great Barrier Reef (Pandora Reef) Porites lutea coral based on near-weekly sample intervals. A sampling technique was designed to minimise any smoothing or distortion of the isotopic record due to complex coral growth, calyx architecture, and calcification at depth within the tissue layer. The arrival-time of the mid-winter minimum sea-surface temperature is very consistent (±2 weeks) near Pandora Reef and provides an annual time-marker offering more precision than the traditional density band chronometer. The improved chronology and high-resolution record demonstrate that signal distortion in Porites , due to calcification within the tissue layer and variable intra-annual coral extension, is generally negligible. Also confirmed is that the arrival-time of monsoonal floods is precisely preserved (±1 week) within the skeleton of Porites . The sensitivity of weekly sampling allows the detection of a subtle warm-to-cool sea-surface temperature anomaly which preceded, by more than one year, a similar temperature anomaly associated with the 1982–83 El Nin˜o in the east Pacific. Sharply higher δ 13 C values coincide with the time of the annual coral mass-spawning event in the Great Barrier Reef. Recognition of this mass-spawning signal should simplify the interpretation of coral δ 13 C records and provides an additional, precise time-marker with which to adjust chronologies when intra-annual coral extension is not constant.

A 16-Ma record of paleodiet using carbon and oxygen isotopes in fossil teeth from Pakistan

Abstract The Siwalik Sequence of northern Pakistan contains a 16-Ma record of paleosol carbonate and fossil teeth from which a record of paleovegetation can potentially be reconstructed and compared. The carbon isotopic composition of paleosol carbonate and organic matter from Siwalik strata reflects a major paleoecological change on the floodplains of major rivers beginning7.3 Ma ago. By 6 Ma C 3 -dominated plant communities, probably composed of mostly trees and shrubs, were displaced by nearly continuous C 4 grassland. We find that the carbon isotopic ratios in herbivore tooth enamel reflect this dramatic ecologic shift. Carbonate in enamel older than 7 Ma averages −11‰ in δ 13 C PDB , consistent with a largely C 3 diet. Enamel from the Plio-PIeistocene averages +1.9‰ in δ 13 C, similar to the value displayed by modern C 4 grazers. Analysis of post-burial carbonate cements, and the concordance with isotopic evidence from paleosols argues strongly against major isotopic alteration of the enamel, while coexisting bone may have been altered early in burial. This study confirms that enamel apatite is useful for paleodietary reconstruction much further back in the geologic record than was previously thought.

Magnesium content of non-marine ostracod shells: A new palaeosalinometer and palaeothermometer

Abstract Ostracods are microcrustaceans with calcitic carapaces (each consisting of two valves or shells) that grow by moulting and regeneration of sequentially larger valves up to nine times before reaching maturity. The Mg content of these valves is species-dependent and is a function of their growth stage and the temperature, salinity, Mg content and Mg/Ca ratio of their host water. Newly formed, partly calcified valves of the non-marine nektic ostracod Mytilocypris henricae have high Mg contents (100,000 ppm Mg in CaCO3 at individual shell weights of 2 μg). This relative Mg content decreases as calcification proceeds, until shell weights exceed 120 μg, whereupon the Mg content is constant and useful as an indicator of the ostracods environment. Mg/Ca ratios in individual ostracod valves from several species of the genera Mytilocypris and Australocypris from culture experiments and a series of Australian lakes indicate that the valves Mg content increases as a function of increasing temperature and salinity. Within the range 11–25°C and for waters containing 24–2325 ppm Mg, the molar Mg/Ca ratio in these ostracods fully calcified valves increases by 0.0015 (or 370 ppm Mg in CaCO3) per 1°C and per change of 125 ppm Mg in solution. For most mainland Australian lakes, a change in water composition of 125 ppm Mg corresponds to a change of 3.5‰ in salinity. For a given Mg content of a lake, the Mg/Ca ratio of the water may vary within the range of common lacustrine values without significantly affecting Mg uptake in the ostracod valves. These relationships can provide palaeosalinities and/or palaeotemperatures for ostracods recovered from continental sediments. For Mg partitioning, an increase in temperature and salinity produce increased Mg content in ostracods, whereas for 18O fractionation in biogenic calcite an increase in temperature produces a decrease in δ18O of the ostracod and an increase in salinity leads to an increase in δ18O of the ostracod. Thus a combination of δ18O and Mg measurements on non-marine ostracods should enable unique resolution of palaeotemperature and palaeosalinity variations.

Oxygen isotope composition of the bone phosphate of Australian kangaroos: Potential as a palaeoenvironmental recorder

δ18O determinations of bone phosphate (δ18Op) have been performed on a suite of modern kangaroos and wallabies (known collectively as Macropods) that come from a wide variety of climatic zones within Australia. Macropod δ18Op values range from 16 to over 30%. (V-SMOW) and are found to correlate with environmental relative humidities. As Macropods are herbivorous mammals with low drinking water requirements that consume large amounts of plant leaf water, it is likely that the δ18Op signature of Macropods is reflecting leaf water fractionation processes that are controlled by relative humidity. These results indicate that δ18Op from fossil bones of Macropods and similar drought-tolerant herbivorous mammals may yield palaeohumidity estimates.

Magnesium and strontium in non-marine ostracod shells as indicators of palaeosalinity and palaeotemperature

Measurements of the Ca, Sr, and Mg contents of individual calcitic shells of non-marine ostracods and their host waters, both in lakes and controlled aquaria, permit the calculation of the distribution coefficients of Sr/Ca and Mg/Ca partitioning in ostracod shells. We report new KD[Sr] for seven genera of non-marine ostracods and KD[Mg] for Cyprideis at 25°C.Strontium partitioning is virtually temperature-independent, and is related to the Sr/Ca of the host water, and in Ca2+-saturated waters, to the salinity of the water. Magnesium partitioning is dependent on both temperature and Mg/Ca of the host water.For simple closed-basin lakes (crater lakes are ideal), the Sr content of ostracods is a sensitive indicator of salinity and thus evaporation/precipitation changes, which in turn, indicate variations in continental climate. A 10000-year continuous palaeosalinity record established by Sr and Mg contents of fossil ostracods for Lake Keilambete, southeastern Australia, is in close agreement with an independent palaeosalinity estimate based on sediment textures.We suggest rules that allow Sr and Mg analyses of suites of individual fossil ostracod shells from lacustrine sediments to be interpreted in terms of palaeosalinity and palaeotemperature variations.

The retention of primary oxygen isotope compositions of fossil elephant skeletal phosphate

Fossil elephant remains from sediments of Plio-Pleistocene age in the vicinity of Lake Turkana, Kenya, provide insights into the ability of the phosphate-oxygen system to preserve original δ18O values (δ18Op) in the depositional environment. Individual fossil specimens exhibit systematic variations in the δ18Op values of their various tooth and bone components. Differences in δ18Op values between these skeletal components can be as high as 3.6%., despite the fact that equivalent phases for individual modern elephants have δ18Op values which vary by less than 0.6%.. n nXRD traces of the different skeletal phases of modern and fossil samples, also infer differential preservation of primary apatite crystallinities among the various fossil phases. Fossil samples of dentine, cementum and bone have X-ray spectra with sharper and more intense peaks compared to their modern counterparts, but little change in crystallinity is observed for fossil enamel samples. A model based on the premise that the enamel phase preserves primary isotope signatures and that the accompanying skeletal phases are partially re-equilibrated with the fluids associated with the microbial decomposition of the elephants carcass is consistent with the features of the 18Op dataset. n nThis isotopic evidence, in conjunction with the physical and chemical attributes of enamel, has led us to postulate that enamel could be preserving primary isotope signatures while other skeletal components are affected to varying degrees by alteration processes in the burial environment.