Isotopic Equilibrium Conditions Research Articles

Climate monitoring in the Caumont cave and quarry system (northern France) reveal near oxygen isotopic equilibrium conditions for carbonate deposition

The study of modern cave deposits forming under near isotopic equilibrium conditions can potentially help disentangle the processes influencing the oxygen isotope system and suitability of stalagmites as archives of past hydrological or thermal changes. We used cave monitoring to evaluate the impact of kinetic isotope fractionation and assess the conditions under which modern cave carbonates form in the Caumont cave and quarry system, located in Normandy, northwest France. Over 20 months, we collected climatological data, dripwater, and modern carbonate samples at 2–4-week intervals at three different stations inside the Caumont cave and quarry system. We find highly stable (10.4 ± 0.3°C – 11.3 ± 0.1°C) temperature in the deeper sections of the Caumont cave and quarry system. The temporal dynamics of δ18Odrip indicates that the drip water composition in Caumont reflects the original (though subdued) signal of precipitation, rather than the impact the seasonal to interannual cave air temperature has on isotopic fractionation. The monitoring reveals that δ13C of modern carbonate is influenced by prior carbonate precipitation that occurs during the summer season when evapotranspiration can minimize effective infiltration. Comparison of δ18O from dripwater and modern calcite, precipitated on glass plates and collected every two to four weeks, reveals that modern calcite forms near oxygen isotope equilibrium. A Hendy test on modern carbonate deposited on a stalagmite-shaped glass flask over 20 months confirms this finding because neither does δ13C increase with distance from the apex, nor are δ13C and δ18O positively correlated. We conclude that the δ13C signal in speleothems reflect summer (and longer-term) prior carbonate precipitation in response to effective infiltration dynamics, and that the δ18O signal likely reflects annual to multi-annual changes in the composition of precipitation above the cave.

Delineation of water flow paths in a tropical Andean headwater catchment with deep soils and permeable bedrock

AbstractTraditional hydrometric data combined with environmental tracers such as water stable isotopes contributes to improve the understanding of catchment hydrology. Nevertheless, the application of isotopic tracers in headwater catchments of the tropical Andes with deep soils and permeable parent material influenced by recent volcanism remains limited. In this study, the stable isotopic composition of precipitation, soil water, wetlands, and streamflow was studied to provide insights into the hydrology of a small tropical Andean catchment with deep and permeable volcanic soils, ash layers, and fractured bedrock resulting from Holocene volcanic activity. Although local precipitation forms under isotopic equilibrium conditions, the stable isotopic composition of precipitation is influenced by atmospheric moisture recycling processes. The spatial and temporal variability of isotopic signals and the analysis of inverse transit time proxies (ITTPs) of surface (streamflow) and subsurface (soil and wetlands) waters indicate that vertical flow paths through the deep volcanic ash soils are dominant across the catchment. The strongly damped isotopic composition of these waters points to high soil and wetlands water storage, increasing the transit time or age of stream water in the hydrological system. These findings indicate that water mobilizing through subsurface flow paths–that is, volcanic soils, ash layers, and cracks in the fractured bedrock resulting from Holocene volcanism–is the main contributor to streamflow generation. Comparison with previously published work from Andean catchments and other volcanic areas shows the diversity of hydrological conditions that can be found as a result of pedological and lithological differences shaped by volcanic activity. Therefore, site‐specific strategies may be needed to improve water resources management.

Geographic variations in the slope of the δ 2 H – δ 18 O meteoric water line over Europe: a record of increasing continentality

Abstract δ 2 H and δ 18 O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ 2 H and δ 18 O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ 2 H– δ 18 O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures ( c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds.

Geographical variations in the slope of the δ2H–δ18O meteoric water line over Europe: a record of increasing continentality

δ2H and δ18O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ2H and δ18O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ2H–δ18O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures (c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds.

Variation in stable isotope ratios of monthly rainfall in the Douala and Yaounde cities, Cameroon: local meteoric lines and relationship to regional precipitation cycle

Hydrogen (D) and oxygen (18O) stable isotopes in precipitation are useful tools in groundwater recharge and climatological investigations. This study investigated the isotopes in rainfall during the 2013 and 2014 hydrological years in the Douala and Yaounde urban cities. The objectives were to generate local meteoric water lines (LMWLs), define the spatial–temporal variations of the isotopes in rainwater and their relationship to the regional precipitation cycle, and determine the factors controlling the isotopic variation. The LWMLs in Douala and Yaounde were δD = 7.92δ18O + 12.99 and δD = 8.35δ18O + 15.29, respectively. The slopes indicate isotopic equilibrium conditions during rain formation and negligible evaporation effect during rainfall. Precipitation showed similar wide ranges in δ18O values from −5.26 to −0.75 ‰ in Douala and −5.8 to +1.81 ‰ in Yaounde suggesting a common moisture source from the Atlantic Ocean. Enriched weighted mean δ18O (wδ18O) values during the low pre- and post-monsoon showers coincided with low convective activity across the entire region. Enriched isotopic signatures also marked the West African monsoon transition phase during each hydrological year. Abrupt wδ18O depletion after the transition coincided with the monsoon onset in the region. Peak periods of monsoonal rainfall, associated with high convective activities, were characterised by the most depleted wδ18O values. Controls on isotopic variations are the amount effect and moisture recycling. The stable isotope data provide a tool for groundwater recharge studies while the isotopic correlation with regional rainfall cycle demonstrate their use as markers of moisture circulation and detecting climatic changes in precipitation.

Stalagmite Inferred High Resolution Climatic Changes through Pleistocene-Holocene Transition in Northwest Indian Himalaya

Investigated for d18O and d13C isotopes, mineralogy and growth rate, a 20 cm long and 230Th-dated calcite stalagmite from Kalakot (Jammu and Kashmir Himalaya), has recorded high resolution precipitation variability during the Pleistocene-Holocene transition. At present, the study area is influenced by both the Indian Summer Monsoon (ISM) and Westerlies. The StalAge model indicates that the stalagmite grew between 16.3 ka to 9.5 ka BP under the ideal isotopic equilibrium conditions as revealed by the Hendy test results. The d18O and d13C values range from -5.41 to -8.82% and -7.09 to -10.84% respectively. Although the U/Th chronology is poor due to low Uranium content in the samples resulting in relatively large errors, the first stalagmite inferred precipitation variability reconstructed from NW Indian Himalaya makes this study significant. The near footprints of three global events, e.g., Older Dryas (OD), Allerod period and Younger Drays (YD) can be noticed at ~14.3-13.9, 13.9-12.7 and 12.7-12.2 ka BP. The precipitation strength was weaker during the OD and YD, but was stronger during the Allerod interstadial. By the termination of YD interval, the climate seems fluctuating in the NW Himalaya. There seems variation in commencement, duration and termination of the above mentioned events in different parts of the globe due to latitude location and response time.

High-resolution speleothem record of precipitation from the Yucatan Peninsula spanning the Maya Preclassic Period

We produced a new high-resolution absolute U–Th dated stalagmite oxygen isotope record (δ18O) from Río Secreto, Playa del Carmen, Yucatan Peninsula (YP). This new 1434-year stalagmite record (named Itzamna after the Maya god of creation) spans the time interval between BCE 1037 and CE 397 with an average resolution of 8±2years. It provides a novel view of climate evolution over the Preclassic and early Classic periods in Maya history. To understand the controls of regional precipitation δ18O on seasonal time scales, we characterized the amount effect between precipitation amount (P) and precipitation δ18O (δP). We found that precipitation δ18O in the Yucatan Peninsula is controlled by the amount effect on seasonal scales (δP/ΔP=−0.0137±0.0031‰ per mm, r=0.9), as suspected but never before demonstrated. Cave drip δ18O is consistent with the annual amount-weighted δ18O composition of precipitation. Multiple lines of evidence suggest that stalagmite δ18O reflects isotopic equilibrium conditions and thus stalagmite δ18O changes are interpreted to reflect precipitation amount. We determined quantitative precipitation changes from the stalagmite δ18O record following previous methods (Medina-Elizalde and Rohling, 2012). The stalagmite precipitation record suggests twelve periods of anomalous precipitation reductions ranging between about 30 and 70% below mean conditions at the time and with durations from 6years to 31years. Between BCE 520 and 166, the speleothem precipitation record suggests that the YP experienced an interval of high precipitation labeled the Late Preclassic Humid Period (LPHP) with precipitation maxima of up to +86±20%. Preclassic Maya cultural expansion in El Mirador Basin, located in northern Guatemala, took place while the peninsula transitioned from the LPHP to an interval with below average precipitation. We find that the Preclassic abandonment of major centers in the Mirador Basin and others around the Maya Lowlands was synchronous with two unprecedented multi-decadal events of severe precipitation reduction with magnitudes of −55±13% and −49±12 and centered at CE 186 and 234, respectively. We also find evidence that centennial scale precipitation variability in the YP during the Preclassic Period may have been associated with shifts in rainfall fluxes from Atlantic tropical cyclones.

Controls on dissolved inorganic carbon and δ 13C in cave waters from DeSoto Caverns: Implications for speleothem δ 13C assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ 13C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ 13C DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems. [DIC] and δ 13C DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ 13C DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ 13C DIC = −2.49[DIC] + 0.64, r 2 = 0.84). A shallow pool fed by multiple drips shows a bimodal δ 13C DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO 2 fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ 13C DIC variability. Measured values of high [DIC] and low δ 13C DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ 13C DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of HCO 3 - partitioned between degassed CO 2 and precipitated CaCO 3 ( 1000 ln α [ ( CO 2 - HCO 3 ) + ( CaCO 3 (AR) - HCO 3 ) ] / 2 ) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰). On the basis of 14C mass balance and paired 14C–U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from 14C-dead dolomite cap while the remainder of ∼77% is derived from 14C-live biomass. δ 13C measurements of aragonite ( n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ 13C values calculated using the shallow pool summer/early fall data thus confirming the δ 13C seasonality in both drips and coeval aragonite. δ 13C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ ( n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ 13C records of stalagmites at DeSoto Caverns with resolving power >10 −1 year will likely carry the imprints of drip annual means that record climate-driven δ 13C seasonal biases.

Modern C, O, and H isotope composition of speleothem and dripwater from Modrič Cave, eastern Adriatic coast (Croatia)

Modrič Cave is a shallow horizontal cave situated in the middle of the eastern Adriatic coast (Croatia). The cave entrance is located 120 m from the coast at an altitude of 32 m above sea level, and due to its position on the SW slope of the Dinaridic mountain range, a Mediterranean climatic influence is dominant. Due to the stable environmental conditions [(15.6 ± 0.1) °C] Modrič Cave was recognized as a potential site for detailed palaeoclimatic studies. Isotope analyses of modern carbonate speleothems, rain and dripwater were conducted in order to evaluate the isotopic equilibrium conditions. The δ18O composition of rain and cave seepage waters shows an absence of kinetic isotopic fractionation within the epikarst zone, whereas the relation between δ13C and δ18O in modern carbonate samples and dripwater suggests the isotopic equilibrium conditions during the carbonate deposition. These results contribute to a better understanding of the present-day isotopic composition and provide a basis for interpretation of speleothem-derived palaeoclimatic records.

Stable isotope and chemical composition of pearls: Biomineralization in cultured pearl oysters in ago bay, Japan

The δ18O, δ13C and trace element composition of pearls collected from Ago Bay, Japan, were investigated in order to evaluate biomineralization in the cultured pearl oyster (Pinctada fucata martensii). The oxygen isotopic data suggest that the pearls were produced around 23–24°C, mainly in June to early July, which is consistent with their occurrence in the field. Therefore the pearls were produced under or close to isotopic equilibrium conditions, although they showed high calcification rates (higher than 0.2–1.0 g cm− 2yr−1) under which, for example, coral skeletons (calcification rate ∼0.28 g cm− 2yr−1) often show non-equilibrium isotope partitioning. The δ13C values were ∼− 2.9‰ lower than those calculated for offshore waters under equilibrium conditions. This may be due to low-δ13C bottom waters resulting from the degradation of organic matter (OM) or to a contribution of low-δ 13C food. In the latter case, a simple mass balance calculation gives a respiration component of 14%. Twelve trace elements of bulk pearl samples were classified into four groups on the basis of their enrichment/depletion patterns relative to seawater and inter-element relationships: group 1, Co, Cr, Pb; group 2, Ba, Cs, U; group 3, Cu, Sn, V, and group 4, Mn, Rb, Mo. Comparison with coral skeletons suggests that Ba and Mn (groups 2 and 4) were definitely much enriched in proteinaceous OM relative to aragonite crystals in pearls and that V (group 3) in pearls showed only slight enrichment in the organicrich layer. By contrast, the other elements showed small differences between both layers (enrichment factor of <3), suggesting that these elements occur largely in aragonite crystals.

Halloysite as a kinetically controlled end product of arid-zone basalt weathering

Mineralogical and isotope results paired with field observations suggest that halloysite is the favored, albeit metastable, aluminosilicate end product of arid-zone basalt weathering on Kohala, Hawai'i, and that the formation of smectite has been inhibited by kinetic factors. Soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance, thus allowing us to interpret chemical and mineralogical changes as the result of a time-dependent process. The halloysite content of the soil increases with increasing soil age; its growth is at the expense of allophane, which, in turn, forms at the expense of primary minerals. These mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil. Smectite, the thermodynamically stable phyllosilicate phase predicted by soil solution composition, is not found in these arid soils. We determined that our soil system is controlled by kinetic factors, and that, therefore, thermodynamic predictions do not reflect reality. The main factors favoring kinetic control of halloysite formation are intense, but short wet periods followed by prolonged extremely dry seasons, and microenvironmental conditions leading to immediate uptake of released Al by the halloysite-precursor mineral allophane. The δ 18O relationship between present-day soil water and halloysite, formed over the last 170 ka, was used as a tracer of past soil conditions. Results from a reconstruction of paleo-climate and -soil conditions, combined with δ 18O data, observed mineral relationships along the 350 ka chronosequence, and field evidence for long-term aridity rule out the possibility that the arid side of Kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and, therefore, altered mineralogical stabilities. We conclude that pedogenic halloysite has been continually forming from the early stage of soil formation, and that it has consistently formed under isotopic equilibrium conditions, indicating that halloysite δ 18O compositions imply paleoclimatic conditions over the time of its formation that are similar to today's.

δ 18O(PO 43−) and δ 18O(CO 32−) from belemnite guards from Eastern Europe: implications for palaeoceanographic reconstructions and for the preservation of pristine isotopic values

The oxygen isotopic composition of coexisting carbonate and phosphate from belemnite rostra was measured according to well established techniques in 42 samples of Early and Middle Jurassic age and in five samples of oyster shells. Most of the samples come from various locations in the Western Carpathians of Slovakia and Ukraine, and from central Poland. Three samples come from the Isle of Skye. The phosphate content of belemnite rostra, though variable, is systematically very low: consistently lower than about 0.3%. However, this phosphate concentration is close to that found in shells of modern marine organisms including pelecypods, gastropods and Sepia cuttlebones which, in some way, could be considered the modern belemnite counterpart. The measured oxygen isotopic composition of carbonate is within the normal range of values obtained from these fossils ranging from about −1.3 to about +0.6‰ (PDB-1) with the exception of three samples; the δ 13C values range from about −0.8 to about +2.8‰ (PDB-1). With the single exception of one sample from the Isle of Skye, the oxygen isotopic composition of phosphate from belemnite rostra ranges from +19.8 to +24.9‰ (V-SMOW), 22 of the samples measured showing δ 18O values equal to or heavier than +23.0‰. In contrast, the oyster values are considerably lighter, in the case of both carbonate and phosphate. 18O-enriched values can hardly be related to diagenetic processes that normally cause an oxygen isotope shift towards light values. If deposition temperatures are calculated from the heavily enriched values by means of the equation of Longinelli and Nuti [Earth Planet. Sci. Lett. 19 (1973) 373–376] and assuming the δ 18O of Jurassic ocean water to be equal to −1‰ taking into account the lack of ice caps during the Jurassic, the obtained temperatures range from about 8°C to about zero. These temperatures are obviously unreliable when Mesozoic palaeoceanographic conditions and palaeoclimate are taken into account. Two different hypotheses are suggested to explain these results, other hypotheses being rejected as unreliable. (1) Phosphate derived from the decaying organic matter of belemnites might have been introduced into belemnite rostra by early diagenetic fluids. If the phosphate of belemnite organic matter was isotopically heavy as happens nowadays in the flesh of molluscs, the inflow of this phosphate into the rostra could be responsible for the very positive δ 18O values shown by many belemnite rostra (this hypothesis is suggested by H.W.); (2) previous oxygen isotope measurements on Upper Cretaceous belemnites yielded δ 18O values very close to the most positive values obtained from Lower Tertiary pelecypods and fish teeth which are known to precipitate their phosphate under isotopic equilibrium conditions with seawater. These data suggest the possibility that the phosphate in belemnite rostra is primary phosphate so that the very positive data reported here can be considered the result of good preservation of the pristine isotopic composition of primary phosphate. Consequently, the only way to explain the very positive δ 18O values is to consider the oxygen isotopic composition of Jurassic ocean water to be more positive than nowadays by at least 3‰. This hypothesis is suggested by A.L. and A.D.M.

A discontinuous climatic record from 187 to 74 ka from a speleothem of the Clamouse Cave (south of France)

This study presents a continental paleoclimatic record in the south of France, based on δ 18O, δ 13C and the 234U/ 238U activity ratio, dated by the 230Th/ 234U disequilibrium method with thermal ionization mass spectrometry. A stalagmite (Cla4) from the Clamouse Cave offers a discontinuous stable isotopic record between 189 ka and 74 ka which covers marine isotope stages (MIS) 7, 6.4, 5.5, 5.3 and 5.1. The growth phases of the Cla4 stalagmite correspond to high sea level stages, except during MIS 6.4 (169–162 ka). All the growth phases of Cla4 correspond to humid periods, corresponding to the sapropel events observed in the eastern Mediterranean basin. Thus, the influence of a strong hydrological activity in the eastern Mediterranean basin during these periods prevailed as far west as the south of France. Because the karstic system studied strongly buffers the isotopic composition of water, isotopic variations of the calcite deposited in the cave represent mainly global and large-scale environmental variations when isotopic equilibrium conditions prevailed for calcite crystallization. Sub-stage 5.3, the end of 5.5 and MIS 7 were colder by about 4–6°C (calculated temperature) compared to present-day temperature while the growth phases of sub-stages 5.1 and the beginning of 5.5 reflect environmental conditions close to present ones. The δ 18O and δ 13C values of cave deposits of the sub-stage 6.4 are: (1) strongly marked by kinetic fractionation processes such as evaporation due to moisture deficit within the cave atmosphere during the first step of this growth phase and (2) related to higher humidity due to a second period of enhanced rainfall during the second step of growth. This study shows that even if calcite has not been deposited at isotopic equilibrium, its isotopic composition can give insights into the environmental conditions at the time of deposition.

Isotopic climate record in a Holocene stalagmite from Ursilor Cave (Romania)

AbstractThe PU‐2 stalagmite from Ursilor Cave provides the first dated Romanian isotope record for the Holocene. The overall growth rate of the speleothem was 3.5 cm kyr−1, corresponding to a temporal resolution of 142 y between each isotope analysis. The ‘Hendy’ tests indicate that isotopic equilibrium conditions occurred during the formation of PU‐2, and hence that it is suitable for palaeoclimatic studies. The relationship between δ18O and temperature was found to be positive. This can be interpreted either as rain‐out with distance from the west‐northwest ocean source of evaporation or shifts in air mass source with changing North Atlantic Oscillation indices. Applying five U–Th thermal ionisation mass spectrometric (TIMS) dates to a 17.5 cm isotope profile (δ18O and δ13C) along the stalagmite growth axis enabled a tentative interpretation of the palaeoclimate signal over the past 7.1 kyr. Spikes of depleted isotopic δ18O values are centred near ca. 7, ca. 5.2 and ca. 4 ka, reflecting cool conditions. The record shows two warm intervals between ca. 3.8 and ca. 3.2 ka (the maximum warmth) and from ca. 2 to ca. 1.4 ka, when the δ18O values were less negative than present. The ‘Holocene Climate Optimum’ spanning the time interval from ca. 6.8 to ca. 4.4 ka is not well expressed in the PU‐2 stalagmite. Individual spikes of lighter δ13C are interpreted as indicative of periods of heavy rainfall, at ca. 7, ca. 5.5, and ca. 3.5 ka. The overall trend to lighter δ13C in the PU‐2 stalagmite may reflect a gradual decrease in water–rock interaction. The results demonstrate that the effect of North Atlantic oceanic changes extended to the investigated area. Nevertheless, some differences in temporal correlation and intensity of stable isotopic response to these climatic events have been found, but the exact nature of these differences and the underlying mechanism is yet to be determined. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Stable isotope stratigraphy of Holocene speleothems: examples from a cave system in Rana, northern Norway

High-precision TIMS U-series dates and continuous stable oxygen and carbon isotope profiles of a 4000 year stalagmite record from Rana, northern Norway, are presented and compared with data from two other speleothems from the same cave. The dating results yield ages from 3875±34 to 296±3 years before AD2000, with 2 σ errors from 0.5 to 1%. The overall growth rate is 35 mm/ka, corresponding to a temporal resolution of 29 years/mm. The stalagmite is tested for isotopic equilibrium conditions, where all ‘Hendy’ tests, except one, indicate isotopic equilibrium or quasi equilibrium deposition. Both the stable oxygen and carbon isotope records reveal a strong and abrupt enrichment in the near-top measurements. This corresponds in time to the opening of a second cave entrance in the late 1960s, which caused changes in the cave air circulation. The stable oxygen isotope signal is enriched compared to the modern value over the last 300 years, indicating a negative response to temperature changes. Likewise, the stable carbon isotope record is enriched in this period. However, both of the stable isotope records are shown to be significantly enriched compared to the isotope ranges displayed by other stalagmites in the same cave, and this questions the reliability of the proxy records derived from the presented stalagmite. Still, a general good correspondence of large scale fluctuations is found between the three stable oxygen isotope records from this cave. The stable carbon isotope records show large variations within the cave and are believed to be governed by soil-zone conditions, percolation pathways and possibly driprates.

Late Holocene Climatic Change in the Balkans: Speleothem Isotopic Data from Serbia

A detailed profile of the stable isotopes of carbon and oxygen was obtained from a speleothem (stalagmite) from the Ceremosjna Cave in eastern Serbia. The stalagmite is a low magnesian calcite that did not show any evidence of diagenetic alteration. It was precipitated under isotopic equilibrium conditions from dripping water. The age and rate of deposition was derived from six internally consistent radiocarbon dates. The initial 14C activity was determined to be approximately 80 pMC. The stalagmite appears to preserve a continuous record of calcite deposition from approximately 2300 BP until the present.Oxygen isotopic data, based upon 100 samples, are used to derive the first paleotemperature record for Serbia. A regression analysis of the all the data indicates that over the period of time that the speleothem was deposited there was a general trend of lowering of the average temperature. Superimposed upon this are significant long-term temperature fluctuations. These can be divided into four broader climatic groupings. Going from the oldest times to the present, there are two warm periods separated by a period when the temperatures fell below the temperature trend line. However, the absolute temperatures were generally above those of the more recent period that is generally characterized by the coolest climatic conditions.

Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

This paper discusses the factors controlling the variations of heavy isotope contents of 57 rain events collected throughout the rain forest regions of Cameroon, Central Africa, during the great rainy season (August–November) of 1994. Our data display a wide range of values, from +1.69 to −10.80‰ for 18O and from +23.2 to −85.5‰ for 2H. The data follow closely the global meteoric water line suggesting that rain formation processes occurred under isotopic equilibrium conditions between both the condensate and the corresponding vapour. This also indicates that the observed heavy isotope contents have not been altered by evaporation during the descent of raindrops to the ground. The low heavy-isotope contents observed for some precipitation events are assumed to be controlled by the amount of rainfall and/or by the low condensation temperature. The former effect is related to the northward passage of the intertropical convergence zone (ITCZ) over the region under study, whereas the second effect is linked to high altitudes of condensation. Moreover, the latitudinal variations of the weighted mean 18O contents indicates that in coastal areas (0–230 km inland from the Atlantic coast) the stable isotopic compositions of precipitation are controlled by a continuous extraction from the atmospheric reservoir of vapour as rains according to the Rayleigh distillation model, whereas at distances >230 km, recycling of continental moisture is thought to have influenced the isotopic compositions of rains.

An isotopic palaeoenvironmental study of human skeletal remains from the Nile Valley

Stable isotope measurements were carried out on human skeletal remains from the Nile Valley, ranging in age from about 7000 to 1750 yr B.P. and on a set of other samples among which are archaeological food remains. Bone samples were analyzed for δ18O(PO3−4), δ18O(CO2−3) and δ13C(CO−3) of carbonate hydroxylapatite and δ13C and δ15N of collagen. Food remains and tissue samples were analyzed for their δ13C and δ15N values. The isotopic composition of bone collagen, apatite and food remains (partially representative of the diet in ancient Egypt) suggest a mixed diet including C3 plant food and, probably, animal resources (both fresh-water fish and C3 animals). A rather surprising observation is the lack of differences between isotopic composition of remains of different social classes spanning from the very poor village of Gebelein to the middle class of the rich town of Asyut to the distinguished people who underwent mummification processes after their death. From the palaeoclimatological point of view, the δ18O(PO3−4) values, ranging from 20.6 to 24.5‰, seem to reflect the isotopic composition of Nile river water (also related to relative humidity) rather than real climatic (temperature) variations through time. The oxygen isotope compositions of carbonate and phosphate of the same samples (from 30.2 to 32.5 and from 20.6 to 22.2, respectively) suggest isotopic equilibrium conditions, thus confirming the possibility of also using the carbonate measurements for palaeoclimatological studies, at least in the case of recent and well preserved samples.

Rapid remodeling of arachidonate from phosphatidylcholine to phosphatidylethanolamine pools during mast cell activation.

The objective of the present study was to better understand the remodeling of arachidonic acid (AA) in phospholipids of the mouse bone marrow-derived mast cell (BMMC) during Ag and ionophore A23187 activation. Initial studies were designed to understand the movement of AA in phospholipid classes under resting conditions. BMMC pulse labeled with AA incorporated greater than 95% of the label into the major phospholipid classes. Phosphatidylcholine (PC) subclasses, 1-acyl-2-arachidonoyl-(sn-glycero-3-phosphocholine (GPC)) in particular, initially accounted for most of the label incorporated into the cells with phosphatidylinositol/phosphatidylserine (PI/PS) and phosphatidylethanolamine (PE) subclasses containing much smaller quantities. Prolonged incubation of labeled BMMC resulted in a decrease in the radioactivity in PC with a concomitant increase in PE such that 1-alk-1-enyl-2-arachidonoyl-(sn-glycero-3-phosphoethanolamine (GPE)) became the single largest labeled AA pool by 24 h. Further experiments indicated that 24 h was the time required to reach isotopic equilibrium among AA-containing phospholipids of the BMMC. In the next series of experiments, BMMC phospholipids were labeled to different specific activities by either labeling the cells for 0.5 h or for 24 h followed by stimulation. Under isotopic equilibrium conditions (24 h), stimulation resulted in AA release from PE greater than PC much greater than PI/PS with 1-alk-1-enyl-2-arachidonoyl-GPE providing the bulk of AA released from the BMMC. By contrast, cells labeled for 0.5 h released AA from PC much greater than PI/PS, with 1-acyl-2-arachidonoyl-GPC accounting for most of the AA released from BMMC phospholipids. Label associated with PE subclasses under nonequilibrium conditions remained unchanged or slightly increased throughout a 10-min stimulation period. Finally, BMMC were double labeled with [14C]-AA for 24 h and then with [3H]-AA for 0.5 h. Cell stimulation resulted in a decrease in the [3H]/[14C] ratio in PC and PI and an increase in the ratio in PE. The decrease in [3H]/[14C] ratio in PC was mainly in 1-acyl-2-arachidonoyl-GPC, whereas the increase in PE subclasses was primarily in 1-alk-1-enyl-2-arachidonoyl-GPE. The [3H]/[14C] ratio in cellular neutral lipids and in supernatant fluid products were at values between PC and PE subclasses. Taken together, these data suggest that during Ag activation, the release of free arachidonic acid is from predominantly PE subclasses. Concomitant with the release of AA, there is a rapid remodeling of AA from PC subclasses into PE subclasses (1-alk-1-enyl-2-acyl-GPE).

Adenosine stimulates Ca2+ fluxes and increases cytosolic free Ca2+ in cultured rat mesangial cells.

Adenosine has been associated with cellular Ca2+ metabolism in some cell types. Since adenosine is able to contract glomerular mesangial cells in culture, and since Ca2+ is the main messenger mediating contractile responses, we studied the effect of adenosine on 45Ca2+ movements into and out of mesangial cells and on the cytosolic free Ca2+ concentration ([Ca2+]i). Adenosine at 0.1 mM increased 45Ca2+ uptake (basal, 9993 +/- 216; + adenosine, 14823 +/- 410 d.p.m./mg; P less than 0.01) through verapamil-sensitive Ca2+ channels. These channels seem to be of the A1-adenosine receptor subtype. Adenosine also stimulated 45Ca2+ efflux from 45Ca(2+)-loaded mesangial cells. This effect was accompanied by a net depletion of intracellular 45Ca2+ content under isotopic equilibrium conditions (basal, 24213 +/- 978; + adenosine, 18622 +/- 885 d.p.m./mg; P less than 0.05). The increase in 45Ca2+ efflux was inhibited by a Ca(2+)-free medium or in the presence of 10 microM-verapamil. However, the intracellular Ca(2+)-release blocker TMB-8 (10 microM) only partially inhibited the adenosine-stimulated 45Ca2+ efflux. In addition, adenosine induced an elevation in [Ca2+]i in mesangial cells with an initial transient peak within 15 s (basal, 113 +/- 7; adenosine, 345 +/- 46 nM), and a secondary increase which was slower (3-4 min) and of lower magnitude than the initial peak (250 +/- 21 nM). In summary, adenosine elevates [Ca2+]i and stimulates both Ca2+ uptake from the extracellular pool and Ca2+ efflux from intracellular pools in mesangial cells. The Ca2+ release from internal stores is produced by a combination of a TMB-8-inhibitable and a non-TMB-8-inhibitable mechanism, and seems to be dependent on Ca2+ influx.