δ18O δ13C Research Articles

Petrography and stable oxygen and carbon isotopic composition of the Antalo Limestone, Mekelle Basin, Northern Ethiopia: implications for marine environment and deep-burial diagenesis

The depositional environment of the Antalo Limestone was previously interpreted from regional field-based studies and microfacies. However, stable oxygen and carbon isotopes are also invaluable proxies of paleo-environmental conditions, depositional, and diagenetic environments. Hence, the analyses of stable oxygen and carbon isotopic composition of this unit were examined from bulk limestone samples and integrated with a detailed petrographic study to interpret depositional environments and diagenetic evolution. The samples were collected from three outcrop sections, which together represent the full thickness of the unit. The microfacies identified from these samples indicate shallow–deep marine depositional as well as meteoric phreatic, marine phreatic, and deep burial diagenetic environments. The limits of variations of the oxygen isotopic ratios range from − 10.46 to − 3.56‰, with − 6.07‰ mean value. The limits of variations of the carbon isotopic ratios range from − 0.02 to 2.56‰, with 1.4‰ mean value. The calculated z values range from 123.08 to129.7 and the paleo-temperatures are estimated to be 32.4–73.52 °C, with an average temperature of 46.5 °C. The carbon isotope ratios indicate marine-water sources. This was confirmed from z values (> 120), indicating that the samples are marine carbonates. The depleted oxygen isotope ratios and high-temperature values, on the other hand, represent deep-burial diagenesis, which indicates the isotopic composition and the temperature conditions of the cement precipitating diagenetic fluids. The compositional fields of the δ18O–δ13C cross plots, also indicate marine carbonates subjected to mixing zone–deep-burial diagenesis. Therefore, both the petrographic and stable oxygen and carbon isotopic evidences are mutually in agreement, indicating related depositional and diagenetic environments.

Differential response at the seafloor during Palaeocene and Eocene ocean warming events at Walvis Ridge, Atlantic Ocean (ODP Site 1262)

AbstractThe Latest Danian Event (LDE, c. 62.1 Ma) is an early Palaeogene hyperthermal or transient (<200 ka) ocean warming event. We present the first deep‐sea benthic foraminiferal faunal record to study deep‐sea biotic changes together with new benthic (Nuttallides truempyi) stable isotope data from Walvis Ridge Site 1262 (Atlantic Ocean) to evaluate whether the LDE was controlled by similar processes as the minor early Eocene hyperthermals. The spacing of the double negative δ13C and δ18O excursion and the slope of the δ18O–δ13C regression are comparable, strongly suggesting a similar orbital control and pacing of eccentricity maxima as well as a rather homogeneous carbon pool. However, in contrast to early Eocene hyperthermals, the LDE exhibits a remarkable stability of the benthic foraminiferal fauna. This lack of benthic response could be related to the absence of threshold‐related circulation changes or better pre‐adaptation to elevated deep‐sea temperatures, as the LDE was superimposed on a cooling trend, in contrast to early Eocene warming.

Diagenetic Origins of the Calcite Microcrystals That Host Microporosity In Limestone Reservoirs

Abstract Micron-size pores are common in limestones, and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide. Previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron-size calcite crystals that are present in nearly all depositional environments and burial depths throughout the Phanerozoic. In an effort to elucidate the genetic history of these calcite microcrystals, a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies. Bulk elemental analyses indicate that the vast majority of microcrystals are composed of low-Mg calcite (average 14 mmol/mol, range 1.6–90, Mg/Ca). Microcrystal Sr/Mg ratios are consistent with abiotic calcite cements, though overall Mg/Ca and Sr/Ca are lower than modern abiotic calcite cements reported in the literature. Higher Sr/Ca observed in depositional chalks implies a biotic, though probably not aragonitic, precursor. Most calcite microcrystals have oxygen isotope compositions that are 0 to 4‰ more negative, and carbon isotope compositions just slightly more negative (< 1‰), than contemporaneous abiotic marine calcite. Most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age-equivalent marine-calcite composition or parallel to that trend with an offset in δ13C. Assuming that δ13C offsets are due to marine carbon isotopic excursions, the isotopic data are consistent with shallow-burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals. For samples with information on porosity and depth, more negative δ18O (ca. 1–4‰) is associated with increased depth and reduced porosity, implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis. A meteoric calcite line was evident only in two datasets. This demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals, but that its occurrence is rare in comparison with evidence for diagenesis during shallow-burial. Geochemical data from calcite microcrystals are bulk samples, i.e., measurements averaged over many, many calcite microcrystals which themselves may have zoned chemistries. As such, microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on, for example, δ18O–δ13C plots. Bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends. With this model in mind, the assembled database suggests that most samples are mixes of primary marine and burial calcite.

Constraints on paleofluid sources using the clumped-isotope thermometry of carbonate veins from the SAFOD (San Andreas Fault Observatory at Depth) borehole

The San Andreas Fault Observatory at Depth (SAFOD), near Parkfield, California, is a borehole drilled through two active deforming zones of the San Andreas fault, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). These zones accommodate displacement by seismic slip and aseismic creep. Elevated fluid pressures and fluid–rock interactions have been proposed to explain the low apparent strength and aseismic creep observed, but the origin of the fluids and existence of high fluid pressures remains uncertain. We use clumped-isotope thermometry and δ18O–δ13C compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern-day pore fluids from the SAFOD borehole and nearby areas. We observe that: (1) calcite vein temperatures vary from 81 to 134°C, which overlaps the current ambient borehole temperatures of 110–115 °C at sampled depths; (2) vein calcite is not in carbon isotope equilibrium with modern-day pore fluids; (3) the δ18O values of paleofluids close to the SDZ and CDZ, calculated from vein δ18O and temperature data, are not in equilibrium with local modern-day pore waters but approach equilibrium with modern pore waters far from these zones; and (4) syntectonic vein calcite is only in C- and O-isotopic equilibrium with their host rocks within the SDZ and CDZ. Spatial patterns of δ18O and δ13C show little evidence for across-fault fluid-flow. Clumped isotope temperatures are consistent with locally-derived fluid sources, but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources. Our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the SDZ and CDZ, which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones.

Diamond from recycled crustal carbon documented by coupled δ18O–δ13C measurements of diamonds and theirinclusions

The origin of variability in δ13C values of cratonic diamonds is controversial, particularly for diamonds associated with eclogitic source rocks. The variability may be due to distinct primordial reservoirs, high-temperature isotope fractionation, recycling of crustal carbon, or any combination of the three. In contrast, the interpretation of variability in δ18O values of silicate minerals in eclogite xenoliths and eclogitic inclusions in diamonds is less contentious—the hypothesis that they represent rocks that were exposed to weathering and hydrothermal alteration at the surface of Earth is broadly, but not universally, accepted. Here, we report high-precision SIMS oxygen isotope measurements of 15 eclogitic garnet inclusions in diamond from the Damtshaa mine, which comprises four kimberlites within the Orapa cluster of kimberlites. The results demonstrate a link between δ18O values of inclusions and δ13C values of their host diamonds. The δ18OVSMOW values range from +4.7‰ to +8.8‰ and have a median value of +5.7‰, similar to the distribution exhibited by cratonic eclogite xenoliths worldwide, at the nearby Orapa Mine, and of oceanic crust. Contrary to previous suggestions, there is no evidence for a unique oxygen isotope distribution for inclusions in diamond. The oxygen isotope ratios do not correlate with garnet compositions; they have, however, a strong negative correlation with the δ13C values of their host diamonds. We interpret this correlation to have geochemical significance. It cannot be due to primordial heterogeneities or high-temperature isotope fractionation, but is most likely due to an association between recycled near-surface crustal rocks and recycled carbon. We suggest that protoliths to the eclogites at Damtshaa that were more strongly affected by low-temperature seafloor weathering (recorded by δ18O≥∼6‰) also have concentrations of primary organic carbon, and, therefore, low-δ13C Damtshaa diamonds are associated with recycling of crustal carbon.

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5 m length indicates a maximum age of about 60,000–80,000 years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0 < δ13C ‰ V-PDB < –40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4 < δ18O ‰ V-PDB < +6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.

Les spéléothèmes de la grotte Chauvet-Pont-d'Arc : apports chronologiques et paléoclimatiques. Synthèse des travaux publiés

The study of the Chauvet Cave (Ardèche, South-France) speleothems, initiated in 1999, brought results about the chronology of the history of the cave and allows paleoclimatic reconstructions of the last climatic cycle. We present here a synthesis of already published results that were obtained with the help of numerous collaborators. Fiveteen speleothems were analysed : 11 stalagmites, 3 flowstones and one stalactite. Thanks to 90 uranium-thorium ages, three main phases of speleothem growing have been defined : (i) 124.2-99.8 ka coinciding with M1S5 ; (ii) 32.8-22.3 ka that covers MIS3 and 2 and (iii) 15.2-0 ka covering the Lateglacial and Holocene periods and in which most speleothems developed. A growth hiatus that coincides with the climatic deterioration of the last glacial maximum was precisely dated on a single stalagmite between 22.1 ka ± 0.4 and 15.3 ka ±0.25. These U-Th datings also permitted to constrain the age of a collapse that happened far inside the cave (Salle Hillaire) after 7.7 ka ± 1. 6. Ln the same area, the first calcite layers that grew on the archaeological soil were dated at ~ 27.8 ka±1.9 confirming the old 14C ages made on prehistoric charcoals found on this soil (33. 7 to 35.3 cal. ka). Finally, the oldest ages found on the first calcite layers of several stalagmites that grew on the top part of the entrance collapse demonstrate that the Chauvet Cave entrance was closed before 13.1 ka ±0.1, which is in agreement with other dating methods that dated this collapse at about 22 ka. Parallel to the U-Th dating, more than 600 isotopic analyses, performed on the speleothem calcite (δ18O δ13C), permitted to reconstruct detailed paleoclimatic changes since 15.3 ka. These results highlight similarities but also differences with Greenland ice core records and gave precise ages to climatic events that occurred during the Bølling-Allerød and Younger-Dryas periods.

Negative δ18O–δ13C relationship of pedogenic carbonate from northern China indicates a strong response of C3/C4 biomass to the seasonality of Asian monsoon precipitation

Evaluating how future climate changes may impact C3/C4 biomass in East Asia depends largely on the understanding of the relationship between past C3/C4 variations and Asian monsoon circulation. The glacial–interglacial variations in C3/C4 biomass have been readily ascribed to the summer monsoon. However, the internal processes governing the link of C3/C4 vegetation to the Asian monsoon have not been clearly described. Here we present isotopic results of pedogenic carbonate from northern China for the Holocene, the last and penultimate interglacial periods. Comparison of the observed and predicted δ18O values of modern soil carbonate from gravelly soils suggests that pedogenic carbonate forms mainly in warm, rainy season. Carbonate nodules from the Chinese loess demonstrate a distinct negative δ18O–δ13C relationship and a wider scatter of isotopic values in the southern Loess Plateau than in the northern part. By combining rainfall, δ18O of precipitation and the peak of C3 and C4 plant metabolism, we develop a conceptual model to explain the isotopic signatures of the carbonate nodules. Our model shows that a narrowly-focused season of monsoon precipitation at a specific site produces low δ18O values of soil water and simultaneously favors C4 over C3 plants. Our model further suggests that the scattered isotopic values of soil carbonate reflect a strong summer monsoon while the focused values indicate a weak summer monsoon. δ13C values of soil carbonate indicate a striking pattern of northward-increasing C4 vegetation for the last interglacial, while a flat spatial pattern is seen for the penultimate interglacial and the Holocene. It is inferred that the summer monsoon was stronger during the last interglacial than during the penultimate interglacial and the Holocene, leading to a northward displacement of C3 forest ecosystems into the southern Loess Plateau. In addition, an in-phase relationship between intra-nodule δ13C and δ18O values suggests a strong and possibly rapid response of C3/C4 biomass to the seasonality of Asian monsoon precipitation. In this context, if the last interglacial period (MIS 5) is taken as an analog for the projected near future, then C3 plants may be favored in the south while C4 plants may be efficient in the north.

Chemical and isotopic composition of modern water bodies in the Lake Kopais Basin, central Greece: analogues for the interpretation of the lacustrine sedimentary sequence

Stable-isotope measurements and modern aquatic chemistry are used to aid the reconstruction of past hydrological changes recorded in a 126 ka-long sediment core from Lake Kopais, Greece. This is the most southeasterly European site with a long Late Quaternary sequence. Although the lake has been artificially drained, calcareous sediments from the centre of the basin, 38°26′16ʺN, 23°03′01ʺE, provide a continuous record from the early part of the last interglacial to the Early Holocene. Because the physiography and hydrology are complex, modern analogues for past environmental conditions are essential. The modern waters are of Ca–Mg–HCO3 or Mg–Ca–HCO3 type, reflecting the prevalence of limestones and metamorphic rocks in the catchment. They show a clear evolutionary trend in δ18O and δD values from the source areas to the modern lakes. However, the modern water bodies are less evolved than the palaeolake. A larger evaporating surface, higher values of δ18O in precipitation, decreased precipitation/evaporation ratios and hydrological closure increased the δ18O values of endogenic carbonates at different times in the past. However, Palaeolake Kopais never desiccated completely or became saline. Analyses of the organic fraction from the core show that: (i) it is mainly of algal origin; (ii) productivity and carbon cycling in the lake waters were relatively stable through time; and (iii) the bulk δ13C values vary between −29‰ and −25‰, indicating that the lake waters were never appreciably carbon-limited and that C4 plants did not invade the catchment. Although significant shifts in the δ18O and δ13C values of endogenic low-Mg calcite are attributable mainly to past variations in water balance, weak δ18O–δ13C covariance during marine isotope stages (MIS) 5e–5c suggests that the palaeolake leaked significantly through karstic sinkholes. Since it was effectively operating as an open system, δ18O may have retained the isotopic imprint of increased summer precipitation. Between MIS 5a and MIS 4, strong isotopic covariance suggests minimal outflow and drier conditions than today. After an increase in winter snowmelt during MIS 3, the climate again became cold and dry in the full-glacial (MIS 2). A fluctuating increase in moisture occurred during the late-glacial to early Holocene transition (MIS 2–1).

Late Quaternary Paleoclimate in the Eastern Mediterranean Region from Stable Isotope Analysis of Speleothems at Soreq Cave, Israel

The eastern Mediterranean continental paleoclimate during the past 25,000 years was determined by a high-resolution petrographic, stable isotopic, and age study of speleothems from Soreq Cave, Israel. δ18O–δ13C trends indicate that all speleothems older than 7000 yr formed under conditions that differ from those of today. The period from 25,000 to 17,000 yr B.P. was characterized by the highest δ18O and δ13C values, which indicate deposition at temperatures of 12°–16°C, annual rainfall of 300–450 mm, and vegetation typical of a mixed C3–C4type. From 17,000 to 10,000 yr B.P. (deglaciation in northern Europe) δ18O values dropped progressively, correlative with warming (2°–3°C) and a gradual increase in precipitation. A simultaneous decrease in δ13C gives a range expected for C3-type vegetation. This period also shows significant δ18O and δ13C “spikes” which are correlatable with global events (e.g., Heinrich events and the Younger Dryas Stade). The speleothems that grew between 10,000 and 7000 yr B.P. have a unique petrography showing irregular thin laminae of various colors and much detritus. They have the lowest δ18O (corresponding to ∼1000 mm rain) coupled with the highest δ13C (caused by flooding events which stripped the soil cover). From 7000 to 1000 yr B.P. conditions became closer to those of today. This study demonstrates that global events which were recognized in Northern Europe and North Africa are also evident in the eastern Mediterranean and are reflected principally by large changes in the rainfall rate.