Oxygen Isotope Signatures Research Articles

The oxygen isotope enrichment of leaf-exported assimilates--does it always reflect lamina leaf water enrichment?

The oxygen stable isotope composition of plant organic matter (OM) (particularly of wood and cellulose in the tree ring archive) is valuable in studies of plant–climate interaction, but there is a lack of information on the transfer of the isotope signal from the leaf to heterotrophic tissues.We studied the oxygen isotopic composition and its enrichment above source water of leaf water over diel courses in five tree species covering a broad range of life forms. We tracked the transfer of the isotopic signal to leaf water-soluble OM and further to phloem-transported OM.Observed leaf water evaporative enrichment was consistent with values predicted from mechanistic models taking into account nonsteady-state conditions. While leaf water-soluble OM showed the expected 18O enrichment in all species, phloem sugars were less enriched than expected from leaf water enrichment in Scots pine (Pinus sylvestris), European larch (Larix decidua) and Alpine ash (Eucalyptus delegatensis).Oxygen atom exchange with nonenriched water during phloem loading and transport, as well as a significant contribution of assimilates from bark photosynthesis, can explain these phloem 18O enrichment patterns. Our results indicate species-specific uncoupling between the leaf water and the OM oxygen isotope signal, which is important for the interpretation of tree ring data.

Isotopic evidence of the pivotal role of sulfite oxidation in shaping the oxygen isotope signature of sulfate

The oxygen isotope composition of sulfate serves as an archive of oxidative sulfur cycling. Studies on the aerobic oxidation of reduced sulfur compounds showed discrepancies in the relative incorporation of oxygen from dissolved molecular oxygen (O2) and water (H2O) into newly formed sulfate, which likely result from slight differences in the production and consumption rate of sulfoxy intermediates that exchange oxygen isotopes with water. Sulfite is often considered the final sulfoxy intermediate in the oxidation of reduced sulfur compounds to sulfate and its residence time strongly affects the oxygen isotope signature of produced sulfate. However, data on the oxygen isotope signature of sulfate derived from sulfite oxidation are scarce.We determined the oxygen isotope effects of abiotic oxidation of sulfite with O2 or ferric iron (Fe3+) under different pH conditions (pH1, 4.9 and 13.3). These parameters impact the relative contribution of oxygen from H2O and O2 to the produced sulfate, and control the competition between the rates of oxygen isotope exchange between sulfite and water and the sulfite oxidation. There is a striking overlap in the range of oxygen isotope offsets between sulfate and water from our experiments at different chemical conditions (Δ18OSO4−H2O from 5.9‰ for anaerobic oxidation with Fe3+ up to 17.6‰ for oxidation at low pH with O2 as sole oxidant, respectively) with the variations in the oxygen isotope composition of sulfate derived from oxidative processes in the environment. This implies that oxygen isotope effects during sulfite oxidation largely control the isotope signature of sulfate derived from the oxidation of sulfur compounds. However, our results also show that preexisting non-equilibrium isotope signatures of sulfite are likely partially preserved in the final sulfate product under most environmental conditions. Our study furthermore provides a mechanistic explanation for positive isotope offsets between the oxygen isotope composition of sulfate and water observed in anoxic pyrite oxidation experiments with Fe3+ as the sole oxidizing agent. This apparently inverse isotope effect is caused by the interplay between sulfite-water oxygen exchange and normal kinetic isotope fractionation effects during sulfite oxidation, the former driving the isotope composition towards the isotope equilibrium fractionation between sulfite and water, inducing a positive offset whereas the latter induces a negative offset.

High-pressure aragonite phenocrysts in carbonatite and carbonated syenite xenoliths within an alkali basalt

We describe the first observation of primary magmatic aragonite in carbonatite and carbonated syenite, occurring as xenoliths in a Pliocene basaltic diatreme located near the Hungary-Slovakia border. The aragonite-hosting matrix consists of disordered P-rich calcite, occasionally associated with trachyte glass. We interpret the aragonite growth as evidence of supra-lithostatic overpressure in the magmatic plumbing system that connected the crustal basaltic reservoir with the partial melting zone of the lithospheric mantle, and the disordered calcite ± trachyte as quenched residual, immiscible melts, generated close to the solidus of the carbonated alkali basalt differentiated in the crustal reservoir. The quenching event was a phreato-magmatic eruption within the stability field of the low-pressure calcite; this was triggered by advective overpressure, caused by expanding gas bubbles in a quasiincompressible silicate melt system. The high-pressure, pre-eruption origin of aragonite is indicated by enrichment in 13C compared to the associated calcite interpreted as a record of CO2 degassing at T > 500 °C. The oxygen (δ18O ranges of 22.1-24.5‰ V-SMOW in aragonite, 21.6-22.7‰ in calcite) and carbon (δ13C ranges of -4.4 to -5.9‰ V-PDB in aragonite, -11.9 to -12.7‰ in calcite) isotope signatures are consistent with a degassed carbonatite melt primarily derived from a subduction zone.

Phosphorus cycling in the Sargasso Sea: Investigation using the oxygen isotopic composition of phosphate, enzyme‐labeled fluorescence, and turnover times

Dissolved inorganic phosphorus (DIP) concentrations in surface water of vast areas of the ocean are extremely low (<10 nM) and phosphorus (P) availability could limit primary productivity in these regions. We explore the use of oxygen isotopic signature of dissolved phosphate (δ18OPO4) to investigate biogeochemical cycling of P in the Sargasso Sea, Atlantic Ocean. Additional techniques for studying P dynamics including 33P‐based DIP turnover time estimates and percent of cells expressing alkaline phosphatase (AP) activity as measured by enzyme‐labeling fluorescence are also used. In surface waters, δ18OPO4 values were lower than equilibrium by 3–6‰, indicative of dissolved organic phosphorous (DOP) remineralization by extracellular enzymes. An isotope mass balance model using a variety of possible combinations of enzymatic pathways and substrates indicates that DOP remineralization in the euphotic zone can account for a large proportion on P utilized by phytoplankton (as much as 82%). Relatively short DIP turnover times (4–8 h) and high expression of AP (38–77% of the cells labeled) are consistent with extensive DOP utilization and low DIP availability in the euphotoc zone. In deep water where DOP utilization rates are lower, δ18OPO4 values approach isotopic equilibrium and DIP turnover times are longer. Our data suggests that in the euphotic zone of the Sargasso Sea, DOP may be appreciably remineralized and utilized by phytoplankton and bacteria to supplement cellular requirements. A substantial fraction of photosynthesis in this region is supported by DOP uptake.

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

We present 42 dual-isotope nitrate analy- ses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured d 15 N-NO3 - and d 18 O-NO3 - values corre- late negatively, while d 18 O-NO3 - displays no nega- tive correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signa- ture in the St. Lawrence River. In addition, d 18 O- NO3 - is negatively correlated with the seasonally variable d 18 O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrifi- cation). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of d 15 N-NO3 - and d 18 O-NO3 - in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regenera- tion of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).

Oxygen and strontium isotopes from fossil shark teeth: Environmental and ecological implications for Late Palaeozoic European basins

Fossil shark remains occur in both marine and nonmarine Late Palaeozoic deposits, therefore their palaeoecology is controversial. The oxygen and strontium isotopic composition of biogenic fluorapatite in 179 teeth, scales and spines predominantly of hybodontid (Lissodus) and xenacanthiform (Orthacanthus, Xenacanthus, Bohemiacanthus, Triodus) sharks from various Late Carboniferous (Moscovian) to Early Permian (Artinskian) basins of Europe are used as ecological tracers to decipher diadromous or obligate freshwater lifestyle of the investigated taxa. The δ18OP values of the different shark teeth range from 11.7 to 20.2‰ within the different basins with mean values of 16.9±0.5‰ for the Bohemian Massif, 16.2±0.8‰ for eastern Germany, 18.2±1.0‰ for southwestern Germany, 18.5±0.7‰ for southern-central Spain, 17.6±0.4‰ for Sardinia, and 16.6±0.5‰ VSMOW for the French Massif Central. The tooth δ18OP values from the basins are mostly depleted by 1–5‰ relative to those of shark teeth from contemporaneous marine settings. Oxygen isotope signatures of co-occurring taxa do not show systematic differences excluding habitat effects for different shark groups. However, distinctly higher δ18OP values from Puertollano and Saar–Nahe can be attributed to significant evaporative enrichment in 18O of the ambient water in the ancient lacustrine environments due to a warm and dry climate and sufficient residence time in the basins. The strontium isotopic composition of the teeth varies between 0.70824 and 0.71216 with a mean value of 0.71031. These 87Sr/86Sr ratios are always more radiogenic in comparison to the 87Sr/86Sr record of seawater of their stratigraphic age. Overall, the investigated tooth samples yield low δ18OP and high 87Sr/86Sr values deviating from bioapatite values expected for contemporaneous marine vertebrates and typical for freshwater settings. This indicates a fully freshwater adapted lifestyle for a variety of fossil shark taxa in Late Palaeozoic European basins.

Determination of nitrate isotopic signature in waters of different sources by analysing the nitrogen and oxygen isotopic ratio

A reference study on both the nitrogen content in waters and nitrogen and oxygen isotopic signatures characterising nitrate from different sources was conducted within the San River catchment area. Three kinds of catchments were studied: (1) forested and uncultivated; (2) artificially fertilised with nitrate; and (3) fertilised with manure and sewage. Moreover, atmospheric water was studied. The obtained values were found to be similar to others in the literature, with the exception of nitrate from the atmosphere, in regard to which influence reflecting the local conditions was to be noted. The isotopic signature of nitrate in the studied water results from the biogeochemical transformation of nitrogen compounds rather than from the mixing of different sources. The obtained results were statistically distinct and can be used as end-member values in further modelling studies connected with the management of nitrate in river waters, especially under middle-eastern European conditions.

A comparison of pretreatment methods for the analysis of phosphate oxygen isotope ratios in bioapatite

The integrity of the biological phosphate oxygen isotope (δ(18)O(p) ) signal is thought to be contingent upon the complete removal of competing sources of oxygen such as associated organic matter. A range of pretreatment methods to purify phosphate material from competing sources of oxygen has been reported, with contradictory evidence on the usefulness and efficiency of one or another. Yet, a systematic comparison of these techniques for bioapatite phosphate has not been conducted. Chemical and thermal pretreatment techniques were tested for their effectiveness at removing organic matter and the likelihood that they modify original δ(18)O values. The test was performed in inorganic (synthetic apatite and a phosphorite rock) and organic (bone and tooth tissues) phosphate materials for which we had an expectation of the actual original δ(18)O(p) value. Analysis of nitrogen content (wt.%), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy were employed. We detected variable efficiency at removing organic matter between pretreatment methods with no correlation to any specific structural change. The δ(18)O(p) results showed considerable variation between samples pretreated with the different methods and the untreated samples, with a compositional range of up to 4.5 ‰ in the bone samples. Variations of the δ(18)O(p) values within error were found for tooth enamel, phosphorite rock and inorganic apatite. We recommend a cautious approach when interpreting and comparing δ(18)O(p) data from bone samples treated with different pretreatment protocols. In general, the untreated samples seem to show δ(18)O(p) values closer to the expected ones. According to our results, pretreatment is completely unnecessary in highly mineralized tissues.

Micromilieu-controlled glauconitization in fecal pellets at Oker (Central Germany)

AbstractAlthough numerous models for the formation of glauconite have been presented, the precise process and micro-environment of glauconitization are still poorly constrained. We characterize the special micromilieu of glauconitization developed during early diagenesis and present a model for glauconite formation in fecal pellets.Glauconitization at Oker (Central Germany) occurred predominantly in fecal pellets deposited in a shallow marine-lagoonal environment during the Kimmeridgian. Within the fecal pellets, rapid oxidation of organic matter provides the post-depositional, physicochemical conditions favourable for glauconitization. Replacements of matrix calcite, dissolution of detrital quartz, K-feldspar, and clay minerals, and Fe redox reactions were observed within the early micro-environment, followed by the precipitation of euhedral pyrite, matrix-replacive dolomite, and megaquartz accompanied by I-S formation as thin section analyses and SEM observations show. Carbonate geochemical compositions based on ICP-OES and stable oxygen and carbon isotope signatures demonstrate that glauconite formation started in a suboxic environment at a pH of 7–8 and a temperature of 22±3°C to 37±2°C at maximum.TEM-EDX-SAED and XRD analyses on separated glauconite fecal pellets and on the <2 μm clay mineral fraction reveal the predominance of authigenic 1Md-glauconite, 1Md-glauconite-smectite, and 1Mdcis-vacant I-S, besides accessory detrital 2M1-illite and montmorillonite. Kinetic modelling of the glauconite (93–94% Fe-illite layers and 6–7% Fe-smectite layers, R3) and of I-S (66–68% Al-illite layers and 32–34% Al-smectite layers, R1) leads us to conclude that the I-S formed solely by slow burial diagenesis, whereas the glauconite formed close to the seafloor, suggesting significantly faster kinetics of the glauconitization reaction compared with smectite-illitization related to burial diagenesis. Thermodynamically, the substitution of octahedral Al3+ for Fe3+ and Mg2+ during the Fe-Mg-smectite to glauconite reaction via the formation of glauconite-smectite mixed-layered clay minerals may have resulted in a higher reaction rate for this low-temperature glauconitization process.

EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE - PEGMATITE SUITE AT RED CROSS LAKE, MANITOBA, CANADA. II. PETROLOGY OF THE LEUCOGRANITES AND PEGMATITES

Spatial relations and bulk chemical compositions of three bodies of multiphase leucogranites and two categories of granitic pegmatites in the Archean Red Cross Lake greenstone belt, Sachigo Subprovince of the Superior Province in the Canadian Shield, indicate a comagmatic origin and an enormous range of fractionation. The leucogranite + pegmatite assemblages, probably emplaced at ~2.65 Ma, represent an upward fingering-out system presumably rooted in a deeper-seated granitic parent, which was derived by anatexis of felsic lithologies of the greenstone belt, with possible contribution from granitoid components of the flanking plutonic terranes. The leucogranitic melts so derived underwent extensive fractionation, culminating in the lepidolite pegmatites that solidified within the relatively low-P – high-T stability field of petalite. The most primitive fine-grained NGR leucogranite shows a K/Rb of 303. In contrast, the most evolved LPG lepidolite-type pegmatites [with albite >> K-feldspar, K-, Rb- and Cs-dominant lepidolite, spodumene + quartz after petalite, elbaite, beryl, pollucite, amblygonite, and (Ta,Sn)- oxide minerals] average K/Rb 1.8, and Cs 4199, Be 236, Ga 161, Nb 14, and Ta 349 ppm. Crystal–liquid and gravity-assisted liquid–liquid fractionation and differential solubilities of HFSE in the melts could have cooperated with selective complexing of elements and mass-controlled diffusion rates to attain the extreme enrichment and fractionation observed at the scale of pegmatite dikes down to local crystal-matrix levels. The bulk chemical composition of the dikes was not affected by mylonitization. The oxygen-isotope signature of the pegmatites also suggests relatively dry conditions during shearing, with very limited exchange with the host rocks. In contrast, the Rb–Sr isotopes are extensively disturbed in both the leucogranites and LPG dikes.

Reconstructing Late Pleistocene climate and environment in East Central Europe using the oxygen and carbon isotopic signal of tooth in fossil horse, mammoth, reindeer and rhinoceros remains

Reconstructing Late Pleistocene climate and environment in East Central Europe using the oxygen and carbon isotopic signal of tooth in fossil horse, mammoth, reindeer and rhinoceros remains

Comment on article “Kovács, J., et al. Reconstructing the paleoenvironment of East Central Europe in the Late Pleistocene using the oxygen and carbon isotopic signal of tooth in large mammal remains” (doi:10.1016/j.quaint.2012.04.009)

This forum communication is written to call attention to a missing reference in the article of “Kovács, J., et al. Reconstructing the paleoenvironment of East Central Europe in the Late Pleistocene using the oxygen and carbon isotopic signal of tooth in large mammal remains” (http://dx.doi.org/10.1016/j.quaint.2012.04.009).In this article we present the excavation conditions of the faunal finds of Kozármisleny.

Reconstructing the paleoenvironment of East Central Europe in the Late Pleistocene using the oxygen and carbon isotopic signal of tooth in large mammal remains: Reply to comments of Radvánszky and Varga

Reconstructing the paleoenvironment of East Central Europe in the Late Pleistocene using the oxygen and carbon isotopic signal of tooth in large mammal remains: Reply to comments of Radvánszky and Varga

Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–10 μm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk‐rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re‐equilibration in a Si‐undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite‐analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.

Inter- and intra-annual stable carbon and oxygen isotope signals in response to drought in Mediterranean pines

Interpreting the isotopic tree-ring responses of pines to drought provides insight into the causes of tree mortality. For this reason, we examine such responses for low elevation Pinus halepensis subsp. brutia trees that neighbor recently desiccated pine stands. A strong correlation between 13C discrimination (Δ) signals recorded in tree-rings and concurrent drought indicates a rapid transfer of newly synthesized assimilates to wood formation. However, in dry years the limited moisture allows only early- to mid-spring and mid- to late-autumn as productive periods. Thus, isotopic signals for summer drought may be missing in tree-rings. Yet, over a 30year observation period, drought clearly reduced both, tree-ring width and Δ13C. Intra-annual microtome slices (1/10mm) indicate the highest δ13C signals in autumn (after growth resumed in response to rainfall) as a likely result of autumn wood (late-wood) incorporating carbon fixed and stored during the summer drought related growth cessation. Thus, non structural carbon reserves accumulating under drought are likely to leave a δ13C fingerprint when complete tree-rings (early-plus late-wood) are analyzed. Both inter- and intra-annually, δ18O in tree-rings from these pines declined following precipitation and in contrast to the rise in temperature. Any evaporative enrichment of leaf water in the heavier isotope under drought appears to be masked by the source water utilized for tree growth. Source water from deeper moisture pools is less enriched in 18O than surface water. Therefore, as inter- or intra-annual drought intensifies, pines appear to use this deep water, which accumulates from precipitation over a series of years. This was also confirmed by Δ13C and tree growth data, as both were best correlated with multiple years of past precipitation rather than with seasonal precipitation. Consequently, the most likely cause of drought induced mortality for such plants is chronic depletion of deeper moisture pools and hydraulic failure rather than C starvation.

Isotopic events in the Early/Middle Badenian (Miocene) of the Upper Silesia Basin (Central Paratethys)

Middle Miocene foraminifera from the Skawina Formation of the Upper Silesia Basin have been analysed. The stable oxygen and carbon isotope signatures in Globigerinoides quadrilobatus (shallow pelagic), Globigerina bulloides (deeper pelagic) and Uvigerina spp. (benthic) tests show a temporal pattern of changes in the sections studied. The foraminiferal assemblage biozones of the Badenian that were identified earlier, on the basis of taxonomic composition, correspond to changes in the foraminiferal d 18 O and d 13 C values. The CPN7/CPN8 transition – as on the Moravian/Wielician border – is marked by distinct isotopic events: a d 18 O increase (IIAB/IIC boundary) followed by a d 13 C decrease (IIC/IID boundary)

Fault-related fluid flow history in shallow marine sediments from carbonate concretions, Crotone basin, south Italy

We present the results of a multidisciplinary study on the relationships between the structural architecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry, and isotopic signature, carbonate concretions are grouped into (1) tabular (encompassing also nodular and lens-shaped) concretions developed within the fault zones, and (2) elongate and coalescent strata-bound concretions formed adjacent and mostly parallel to fault zones. Cement chemistry and morphology, as well as stable carbon–oxygen isotopic signatures, indicate that tabular concretions formed during early diagenesis in the vadose mixing marine–meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, as a result of slow capillary suction along the low-permeability fault zones. On the other hand, elongate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones, which acted as a hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids, which were fundamentally driven by fault system propagation and resulting fault architecture during two major diagenetic stages.

Millennial‐scale glacial meltwater pulses and their effect on the spatiotemporal benthicδ18O variability

Ratios of oxygen isotope values obtained from foraminiferal calcite are one of the most established paleoceanographic proxies. They are used in the context of estimating variations in ice volume, ocean temperature, and salinity and provide a means to date marine sediment cores across different ocean basins. Our study addresses the question how the δ18O of the deglacial meltwater signal propagates into the interior ocean, when large‐scale millennial‐scale reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) are present. Analyzing a series of idealized tracer‐injection experiments conducted with an earth system model, we find that a substantial weakening of the AMOC leads to a massive delay in the export of the glacioeustatic oxygen isotope signal into the deep ocean, whereas the Atlantic‐Pacific lag in benthic oxygen isotope signals is not increased. Furthermore, it is shown that an AMOC cessation causes a decoupling ofδ18O propagation time and water mass age, in particular in the deep Pacific. Our results lend further support to the notion that benthic oxygen isotope records obtained from stacks are not a useful global chronostratigraphic tool during periods of millennial‐scale global ocean circulation changes. The regionally varying delay effect of the deglacial sea level signal studied here adds onto existing uncertainties in the interpretation and decomposition of benthic oxygen isotope in terms of sea level, temperature, and hydrographic variations.

Quantifying kinetic fractionation in Bunker Cave speleothems using Δ47

Isotopic signals in speleothems are used for investigating paleoclimate variability on land and are useful to constrain the dating of prominent climate events. A quantitative use, however, is limited by an incomplete understanding of parameters contributing to the carbon and oxygen isotope signals. These include external and environmental parameters such as δ18O of cave drip waters as well as internal parameters associated with speleothem formation, such as the presence of non-equilibrium effects and especially the magnitude of their isotopic shifts.We explore the use of clumped isotopes as a new tool for investigating the kinetic isotope effect in speleothems. Holocene and modern speleothems from Bunker Cave (Germany) as well as modern material from the adjacent Dechen Cave are all offset from the equilibrium relationship due to kinetic fractionation. This kinetic offset in clumped isotopes is observed in a stalagmite despite mostly negative Hendy tests, providing a sensitive indicator for kinetic fractionation in cave carbonates. The temperature dependence of the clumped isotope values (0.005‰ per °C) is low compared to the observed magnitude of kinetic offsets (between −0.021 and −0.075‰), so that the mean offsets in apparent temperatures due to kinetic isotope effects are on the order of 10 °C. As a result clumped isotopes are useful in identifying temporal variations in the kinetic fractionation in a stalagmite, when the temperatures during the speleothem growth period are either relatively constant (variations <2 °C) or can be independently constrained.The variations in the kinetic isotope fractionation in Bunker Cave are associated with changing drip water super saturation with periods of stronger prior calcite precipitation associated with lower kinetic offsets in the speleothem calcite. In contrast, stalagmite growth rates show no direct correlation with the degree of kinetic fractionation in the investigated range (13–1500 μm/a).

Geochemical and petrographical characterization of fine-grained carbonate particles along proximal to distal transects

The origin of carbonate ooze particles is often poorly understood. This is due to their polygenic origin and potential post-depositional alteration. Here, the outcome of a physical separation study with regard to different component classes of micritic carbonates is shown. The focus is on grain size and morphology, mineralogy and isotope signatures. Two contrasting proximal-to-distal transects were investigated: (1) the Miocene leeward margin of Great Bahama Bank (ODP Leg 166) and (2) the transition between the Maiella platform and the Umbria–Marche basin in central Italy near the Cenomanian–Turonian boundary. In both case settings, carbonate particles of biogenic origin include at least three groups of organisms: (i) planktonic foraminifera, (ii) calcareous nannofossils and (iii) fragments of unspecified neritic skeletal material. Two further particle types lack diagnostic structures, and based on particle size and mineralogy, are here referred to as (iv) macroparticles (5–20μm, mainly xenomorphic) and (v) microparticles (<12μm, mainly automorphic to sub-automorphic). Macro- and microparticles represent 50 to 80% of the carbonate phase in slope and toe-of-slope domains and share characteristic carbon and oxygen isotope signatures. Macro- and microparticles are considered shallow-water precipitation products subsequently exported into the slope and toe-of-slope domains. Macroparticles are probably related to the fragmentation of neritic skeletal components while microparticles point to inorganic and/or bioinduced precipitation in the water column. In some cases, macro- and microparticles may have an early diagenetic origin. The identification of the origin of fine-grained particles allows for a quantitative assessment of exported, in situ and diagenetic carbonate materials in periplatform environments. The data shown here represent an important step towards a more complete characterization of carbonate ooze and micrite.