Enriched δ18O Values Research Articles

Variations of stable isotope in Typhoon Lupit (2021) rainfall and its response to upstream convective processes

AbstractTyphoons produce a large amount of rainfall with a highly depleted isotopic composition. However, the mechanisms of isotope fractionation during typhoons remain poorly understood. Here, we present the hourly resolution isotope composition of rainfall during Typhoon Lupit (2021) in Fuzhou, on the southeast coast of China, to better understand these processes. Our results showed that the rainfall of Typhoon Lupit had significant fluctuations in δ18O values, ranging from −20.1‰ to −6.3‰, and exhibited a three‐stage variation with distinctly negative δ18O values in the second stage. In the first stage of Lupit, water vapour originated primarily from the nearby sea area, which did not undergo sustained convective activity, resulting in relatively enriched δ18O values. In the second stage, the δ18O values were extremely negative mainly because of the accumulated convective activity and continuous precipitation as they travelled towards the centre of the typhoon. In the third stage, the mixing of more localized and nearby sea water vapour caused an increase in the δ18O values. In summary, the three‐stage characteristics of Typhoon Lupit were largely caused by the integration of accumulated convective activity and moisture sources. Our findings reveal how upstream rainout affects the isotopic behaviour of rainfall and may have implications for understanding the dynamics of typhoon rainfall isotopes.

Seasonal variations in surface processes and hydroclimate on an alpine lake, NE Tibetan Plateau

Koucha Lake, located in Qinghai Province, north-eastern Tibetan Plateau (NETP) at an elevation of 4530 m a.s.l. Occupies an ideal position to investigate the seasonal hydroclimatic variations and related surface processes during the Holocene. We re-investigated the lake and analysed a new sediment core from the deepest part of the lake. Grain size measurements and related endmember modelling analyses (EMMA) in combination with ostracod associations together with geochemical parameters and mineral compositions from core sediments were used as relevant proxies for hydro-climatic reconstruction.A comparison of δ18O/δD from various waters on the NETP indicates a clear depleted isotopic signature of non-monsoon moisture supply in comparison to summer rainfalls, highlighting the summer evaporation for δ18O enrichment, as a reference for interpreting δ18O values from lake sediments for various situations of hydrological balance. Based on the individual core chronology the multi-proxy data indicate a very low sedimentation rate over the entire period in accordance with the catchment topography and the absence of well-developed and constant river systems, limiting the flux of sediment supply to the lake. Thus, Koucha Lake remains almost a closed system since the Lateglacial, except for the early mid-Holocene part of the sequence. The coarser particle components and their representative endmembers for spring flood and permafrost thaw between 11.9 and 10.1 cal Ka BP and from 4.5 to ca. 1.0 cal Ka BP point to lower lake levels, influenced by long winter duration with dry summer interruptions, and lighter oxygen isotopes were preserved at low evaporations. Fine-grained sediments supplied by permafrost thaw associated with activating summer surface erosion led to lake level rise between 10.1 and 4.5 cal Ka BP and during the last 1000 years, with a maximum during 7.6–7.1 cal Ka BP. Their enriched δ18O values indicate extended summer duration and concomitant evaporation enhancement, albeit a positive hydrological balance. We suggest that the interannual latitudinal shifts of the intertropical convergence zone (ITCZ) and westerly Jet and attendant varying monsoon rain belt locations are responsible tracers for the variation of the hydrological budget of the lake and the variable spatio-temporal precipitation patterns in humid and (semi) arid regions in China.

Formation of authigenic carbonates contributed by sulfate- and metal-driven anaerobic oxidation of methane in the northern Okinawa Trough, East China Sea

Authigenic carbonates of the northern Okinawa Trough have been shown to be contributed by both sulfate- and Fe-driven anaerobic oxidation of methane (SD-AOM and Fe-AOM, respectively); however, the microorganisms involved remain poorly constrained. To better understand the types and roles of microorganisms in specific biological methane oxidation processes, authigenic carbonates collected from the northern Okinawa Trough at water depths between ∼540 m and 700 m were studied using petrographic, mineralogical, elemental, and lipid biomarker analyses. The carbonate minerals were primarily aragonite, high-magnesium calcite, low-magnesium calcite, or dolomite. The extremely low δ13C values of bulk carbonates (−59.2‰), molecular fossils of anaerobic methane oxidizing archaea (ANME; −131‰), and sulfate-reducing bacteria (SRB; −77‰) indicated that the authigenic carbonates formed as products of AOM. Biogenic methane was confirmed to be the main carbon source for all carbonates, whereas bio-degraded oil and hydrothermal input complemented carbon sources, as revealed by the occurrence of unresolved complex mixture, strongly enriched As and Sb, and their correlations with Fe/Al. Combined with enriched δ18O values (as high as +5‰), the methane fluids were suggested to derive from the dissociation of gas hydrates. ANME-2 and a mixture of ANME-1 and ANME-2 were identified for the two calcitic carbonates, respectively, as revealed by their specific biomarker patterns. The relatively higher contents of 13C-depeleted isoprenoids and SRB−related fatty acids, strong Mo enrichment, and biological debris in both calcite- and aragonite-dominated carbonates reflect precipitation driven by SD-AOM in a shallow sulfate-methane transition. For other seep carbonates, low amounts of 13C-depeleted isoprenoids and the near absence of crocetane suggest dominance of the ANME-1 assemblage. The extremely low amounts and non-13C-depleted iso-/anteiso-C15:0 fatty acids (−27‰) in the dolomite-containing carbonates indicate the decoupling of ANME and SRB. These carbonates were most likely formed below or at the bottom of the sulfate-methane transition and included contributions from Fe-AOM, as indicated by the low Mo content, occurrence of dolomite and siderite, and decreased δ56Fe values with increasing Fe/Al ratios.

Continuous monitoring of surface water vapour isotopic compositions at Neumayer Station III, East Antarctica

Abstract. In this study, the first fully continuous monitoring of water vapour isotopic composition at Neumayer Station III, Antarctica, during the 2-year period from February 2017 to January 2019 is presented. Seasonal and synoptic-scale variations in both stable water isotopes H218O and HDO are reported, and their links to variations in key meteorological variables are analysed. In addition, the diurnal cycle of isotope variations during the summer months (December and January 2017/18 and 2018/19) has been examined. Changes in local temperature and specific humidity are the main drivers for the variability in δ18O and δD in vapour at Neumayer Station III, on both seasonal and shorter timescales. In contrast to the measured δ18O and δD variations, no seasonal cycle in the Deuterium excess signal (d) in vapour is detected. However, a rather high uncertainty in measured d values especially in austral winter limits the confidence of this finding. Overall, the d signal shows a stronger inverse correlation with specific humidity than with temperature, and this inverse correlation between d and specific humidity is stronger for the cloudy-sky conditions than for clear-sky conditions during summertime. Back-trajectory simulations performed with the FLEXPART model show that seasonal and synoptic variations in δ18O and δD in vapour coincide with changes in the main sources of water vapour transported to Neumayer Station III. In general, moisture transport pathways from the east lead to higher temperatures and more enriched δ18O values in vapour, while weather situations with southerly winds lead to lower temperatures and more depleted δ18O values. However, on several occasions, δ18O variations linked to wind direction changes were observed, which were not accompanied by a corresponding temperature change. Comparing isotopic compositions of water vapour at Neumayer Station III and snow samples taken in the vicinity of the station reveals almost identical slopes, both for the δ18O–δD relation and for the temperature–δ18O relation.

Stable isotopic (δ2H, δ18O) monograms of winter precipitation events and hydro-climatic dynamics in Central Mexico

The unusual winter storm event of March 2016 brought excess rainfall in the Central part of Mexico and this study attempts to determine the moisture sources by characterizing the rainwater isotopes (δ18O, δ2H) and understanding their monthly spatial variations. Rainwater samples from six different stations (Agua Blanca, Molango, Mexico City, Pachuca, Tula and Tulancingo) of Mexico were sampled in March, April and May 2016. Enriched isotopic signatures in Tulancingo during March exhibited a mixture of moisture sources in which the predominant source was from the North followed by the Gulf of Mexico, while the depleted isotopic levels in Pachuca and Tula moisture from the recycling of inland vapour. However, the isotopic characterization in the month of April presented no much variations in δ18O values irrespective of the station (−2.31 to −2.56‰), signifying that the prime moisture source is mainly from the inland sources and migrated from the central parts to the Gulf of Mexico. In contrast, the inland migration of the vapour source from Gulf of Mexico was witnessed in May with both the continental and the amount effect. However, the enriched δ18O values in Tulancingo and Agua Blanca in May were found to be −0.84‰ and −1.44‰ respectively, with significant lesser d-excess values.

Origin of carbonate cements and reservoir evolution of tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, China

The Upper Triassic Chang 8 Member, the eighth member of the Yanchang Formation, is a key reservoir interval in the Jiyuan area of the Ordos Basin. The reservoir quality of the Chang 8 Member tight sandstones is extremely heterogeneous owing to the widespread distribution of carbonate cements. The carbonate cements commonly develop near sandstone–mudstone interfaces and gradually decrease away from the interfaces to the centres of the sand bodies. However, the content of carbonate cements (≤6%) has a positive correlation with the visual porosity in the Chang 8 Member sandstone, revealing that the carbonate cements contribute to the compaction resistance and the residual primary pores of reservoirs during the diagenetic process. Three main types of carbonate cement are identified: type I (calcite), type II (calcite and ferrocalcite), and type III (dolomite and ankerite). The type I calcite is characterised by enriched δ13C (mean –3.41‰) and δ18O (mean –15.17‰) values compared with the type II (mean δ13C = –7.33‰, δ18O = –18.90‰) and type III (mean δ13C = –10.0‰, δ18O = –20.2‰) cements. Furthermore, the mean δ18O value (–4.7‰) of the type I pore fluids is 1.5‰ and 0.9‰ lower than the type II (mean –3.2‰) and type III (mean –3.8‰) pore fluids, respectively. This indicates that the evolving pore fluids experienced some relative strong water–rock interactions that provided the original materials (e.g. Ca2+, Fe3+, and Mg2+) for the carbonate cements during the diagenetic process. The highly saline lake water directly provided the primary material for the type I calcite precipitation, which also provided the material necessary for the precipitation of the type II and type III carbonate cements, causing enriched δ18O values of the pore fluids during the precipitation of the type II and type III carbonate cements. Although the earlier dissolved pores were filled with ferrocalcite, dolomite and ankerite in the middle–late diagenetic stages, some residual pores and fractures remained to become the potential reservoir storage spaces for the oil and gas exploration in the Jiyuan area.

Groundwater isotopic variations in a uranium mining site: subsidies for contamination studies

The Caetite Experimental Basin (CEB), located in the semi-arid region of Northeastern Brazil, faces not only the challenges associated with water scarcity but also the potential contamination processes due to mining activity. The only active uranium production center in Brazil (URA) is located in this watershed and the sustainability of mining and milling operations, as well as the survival of the local community, is highly dependent on the availability of groundwater resources . This paper analyzes the stable isotopes variation of Deuterium ( 2 H) and Oxygen-18 ( 18 O) in CEB’s groundwater to investigate its dynamics and mixing of water sources as part of initial efforts to characterize the hydrogeology of this area for future contamination and recharge studies. Measurements of δ 2 H, δ 18 O, total dissolved solids (TDS), pH, and electrical conductivity (EC) were carried out in water samples from 28 wells. A total of 102 groundwater samples were analyzed during the dry and wet seasons from 2012 to 2014. All the groundwater samples plotted below the local meteoric line toward more enriched δ18O values, an indicative of evaporation process . 2 H and 18 O data suggests that the main source of groundwater recharge is local precipitation and there is no mixing of infiltrating rainwater with older groundwater. These results provide evidence that the aquifer system in the CEB has a relatively fast turnover time, which contribute to the vulnerability of the aquifer to contamination. These findings are corroborated by the low TDS and EC values indicative of short time in water-rock interaction .

Provenance of polychrome and colourless 8th–4th century BC glass from Pieria, Greece: A chemical and isotopic approach

Glass objects from Pydna and Methoni in modern-day Greece, dated to the eighth to fourth century BC, were chemically investigated. The combined use of multiple analytical techniques allowed the elemental and isotopic characterization of these polychrome and colourless glass artefacts in order to examine their provenance. All fragments were found to be soda-lime-silica natron-based glass produced from a rather pure silica-rich sand containing sea shells, and mixed with natron possibly coming from more than one source. Based on the strontium and neodymium isotopic signatures, most glass artefacts likely derive from a Syro-Palestinian production site although the exact location is unknown. Also the oxygen isotopic signature of most of the samples suggests the manufacturing of the artefacts from raw materials with a primary origin along the Syro-Palestinian coast. Nevertheless, the use of particular raw materials cannot be excluded for some artefacts, as some samples show enriched δ18O values pointing to a different glassmaking tradition.

Spatial and temporal oxygen isotope variability in northern Greenland – implications for a new climate record over the past millennium

Abstract. We present for the first time all 12 δ18O records obtained from ice cores drilled in the framework of the North Greenland Traverse (NGT) between 1993 and 1995 in northern Greenland. The cores cover an area of 680 km × 317 km, 10 % of the Greenland ice sheet. Depending on core length (100–175 m) and accumulation rate (90–200 kg m−2 a−1) the single records reflect an isotope–temperature history over the last 500–1100 years. Lowest δ18O mean values occur north of the summit and east of the main divide as a consequence of Greenland's topography. In general, ice cores drilled on the main ice divide show different results than those drilled east of the main ice divide that might be influenced by secondary regional moisture sources. A stack of all NGT records and the NGRIP record is presented with improved signal-to-noise ratio. Compared to single records, this stack represents the mean δ18O signal for northern Greenland that is interpreted as proxy for temperature. Our northern Greenland δ18O stack indicates distinctly enriched δ18O values during medieval times, about AD 1420 ± 20 and from AD 1870 onwards. The period between AD 1420 and AD 1850 has depleted δ18O values compared to the average for the entire millennium and represents the Little Ice Age. The δ18O values of the 20th century are comparable to the medieval period but are lower than that about AD 1420.

On the enigmatic similarity in Greenland δ18O between the Oldest and Younger Dryas

AbstractThe last deglaciation (20.0–10.0 kyr B.P.) was punctuated by two major cooling events affecting the Northern Hemisphere: the Oldest Dryas (OD; 18.0–14.7 kyr B.P.) and the Younger Dryas (YD; 12.8–11.5 kyr B.P.). Greenland ice core δ18O temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO2, while modeling studies suggest that the YD was approximately 4–5°C warmer than the OD. This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture‐tracing module to investigate atmospheric processes that may have been responsible for the similar δ18O values during the OD and YD. Our results show that the summer‐to‐winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the δ18O signal toward warmer (summer) temperatures (enriched δ18O values and it accounts for ~45% of the expected YD‐OD δ18O difference). A change in the inversion (cloud) temperature relationship between the two climate states further contributes (~20%) to altering the δ18O‐temperature‐relation model. Our experiments also show a 7% decrease of δ18O‐depleted precipitation from distant regions (e.g., the Pacific Ocean) in the OD, hence further contributing (15–20%) in masking the actual temperature difference. All together, these changes provide a physical explanation for the ostensible similarity in the ice core δ18O temperature reconstructions in Greenland during OD and YD.

Identifying Salinization Using Isotopes and Ion Chemistry in Semi-Arid Region of Punjab, India

The Semi-arid Southwest part of the Punjab has intense agricultural and industrial activity and this part is affected by groundwater salinity problems. To identify the groundwater salinity using isotopes (δ18O) and ion chemistry 22 random samples were collected from semi-arid region of South West Punjab i.e. Bhatinda district. The hydro-chemical analysis shows that the southern and western parts have high values of EC. Some areas in the district especially in north, central and small patches in south-western and eastern parts showing high values of fluoride exceeding the permissible limit of 1.5 mgL-1 for drinking water standards. The water belongs to the category of very hard water. The Na% in groundwater indicates that from 17 groundwater water sampling locations water can be used for irrigation leaving only 5 sampling locations which is found unsuitable for irrigation. The enriched δ18O values are found in the eastern and western parts of the district, while southern and northern parts are showing highly depleted values. The enriched values found between the canals Bhatinda branch and Kotla branch may be attributed to evaporation from shallow water table areas. The enriched δ18O values of groundwater are indicator of salinity. There is an increase in EC and enrichment in isotopic composition as observed in some samples due to the evaporation effect and in some samples the increase is EC is found but the isotopic composition remains constant indicating the increase in EC due to geogenic reasons. Therefore, the integrated data of ion chemistry and isotope is useful for identifying the salinity in semi-arid region.

Stable isotopes in modern planktonic foraminifera: Sediment trap and plankton tow results from the South China Sea

Abstract This study reports on the stable isotopic composition of modern planktonic foraminifera tests collected from plankton tows and sediment trap moorings in the northern South China Sea. Plankton tow samplings were conducted at water depths of 50, 100, and 200 m during various seasons between December 2002 and August 2008. The sediment trap array was deployed between August 2004 and February 2005 at water depths of 121 m, 619 m, and 3451 m. Four common and widely distributed tropical/subtropical planktonic foraminifer species, including Globigerinoides ruber (white variety), Globigerinoides sacculifer (without sac), Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, were analyzed for δ18O and δ13C within narrow shell size ranges and compared with the concurrent sea surface temperature (SST) and wind stress. Our results show that foraminiferal δ18O is primarily influenced by seawater temperature, while δ13C is affected by surface water nutrients, which in this region can be discerned from wind stress data. The δ18O and δ13C values from foraminifers recovered in plankton tows generally show greater variability and more depleted δ18O than values of tests collected by sediment traps. For foraminifer tests collected by sediment trap, the δ18O of the shallow dwelling G. ruber shows the largest amplitude variation (ca. 1.6‰) among the four species. In contrast, the δ13C of P. obliquiloculata shows relatively constant values throughout the study period. A pattern of enriched δ18O values, associated with marked δ13C depletion, is common to three species collected between late October and late December 2004. This distinct isotopic signal corresponds to a decrease in SST and increase in wind stress, indicating the onset of prevailing northeast winds during the winter season.

Stable isotopes (2H, 18O and 13C) in groundwaters from the northwestern portion of the Guarani Aquifer System (Brazil)

The groundwater flow pattern of the western part of the Guarani Aquifer System (GAS), Brazil, is characterized by three regional recharge areas in the north, and a potentiometric divide in the south, which trends north–south approximately. Groundwater flow is radial from these regional recharge areas toward the center of Parana Sedimentary Basin and toward the western outcrop areas at the border of the Pantanal Matogrossense, because of the potentiometric divide. The isotopic composition of GAS groundwater leads to understanding the paleoclimatic conditions in the regional recharge areas. The δ18O and δ2H isotopic ratios of GAS groundwaters vary, respectively, from –9.1 to –4.8‰ V-SMOW and –58.4 to –21.7‰ V-SMOW. In the recharge zones, enriched δ18O values are observed, while in the confined zone lighter δ18O values are observed. These suggest that climatic conditions were 10°C cooler than the present during the recharge of these waters. The δ13C ratios in groundwater of GAS, in the study area, vary from –19.5 to –6.5‰ VPDB, increasing along the regional flow lines toward the confined zone. This variation is related to dissolution of carbonate cement in the sandstones.

Surface wetness and mire development during the late Holocene in central Sweden

The aim of this thesis was to reconstruct humidity variability in central Sweden during the late Holocene. A multi-proxy approach was used to infer humidity changes as recorded in a lake and a mire. Age-models were constructed based on radiocarbon dating and the Askja-1875 tephra. Stable isotopes (δ18O and δ13C) on Chara spp encrustations and Pisidium spp mollusc shells and carbon content were analysed in the lake record, whereas peat stratigraphy, humification, testate amoebae assemblages, C/N ratio and stable isotopes (δ13C and δ15N) were analysed in the mire record. Stable isotopes (δ2H and δ18O) on lake water showed that Lake Blektjarnen responded to changes in the balance between evaporation and input water (E/I ratio). A high E/I ratio results from a dry and probably warmer climate during which evaporation and atmospheric equilibration likely enrich lake water in 18O and 13C, respectively, and vice versa for a low E/I ratio. The relatively high Chara δ18O and δ13C values between ca 4400 and 4000 cal yr BP thus suggest relatively dry and likely warm conditions, whereas depleted values suggest wetter and probably cooler conditions between ca 4000 and 3000 cal yr BP. Again, drier and probably warmer conditions were inferred from the relatively enriched δ18O values between ca 2500 and 1000 cal yr BP, and depleted δ18O values were recorded between ca 1000 and 50 cal yr BP indicating wetter and likely cooler conditions. The results from the mire mainly indicated vegetation succession, however, the changes inferred at ca 2600 and 1000 cal yr BP could have been triggered by climate change. This study shows that the proxies responded sensitively to humidity changes in the investigated archives allowing for reconstruction of climate change in central Sweden during late Holocene.

Stable isotope evidence for moisture sources in the asian summer monsoon under present and past climate regimes

Stable isotope compositions of present day precipitation in the 70°E–160°W longitude range reveal a strong correlation with moisture source and transport patterns and are not correlated with the amount of precipitation, contrary to existing interpretations. Spatially and temporally variable moisture sources and cyclonic circulation result in more depleted isotope values (δ18O ∼ −5 to −7‰) in the 100°E–170°E range. Oceanic moisture with minimal continental loss or input results in comparatively enriched δ18O values (∼−2 to −4‰) in the 70°E–100°E range and east of 170°E. Isotope compositions of dated groundwater archives from the Philippines, Vietnam, Thailand and Bangladesh, reflecting past isotopic composition of precipitation, indicate that this distinction in moisture regimes was preserved during the last glacial period and strongly suggest that there was no significant shift in the summer Inter‐Tropical Convergence Zone (ITCZ) in this region.

Multiple fluid flow events and the formation of saddle dolomite: case studies from the Middle Devonian of the Western Canada Sedimentary Basin

In Western Canada Sedimentary Basin (WCSB), large scale and focused fluid flow that caused hydrothermal dolomitization have been suggested with different timing and intensity. In this study, we conducted a petrographic and geochemical comparison between the Middle Devonian Sulphur Point and Slave Point carbonates from northwestern Alberta. The results demonstrate the presence of both an early fluid flow event associated with the Late Devonian to Mississippian Antler Orogeny as well as a later event coincident with the Late Cretaceous to Early Tertiary Laramide Orogeny. Early fluid flow event is characterized by high salinity fluids, and high temperatures and oxygen isotopic values of marine or slightly enriched values, as demonstrated in saddle dolomite from the Slave Point Formation. In contrast, later fluids that caused the precipitation of saddle dolomite in the Sulphur Point formation are characterized by having slightly saline values, comparable homogenization temperatures but more enriched δ18O values and slightly depleted δ13C signatures. Geochemical data indicate that this later hydrothermal fluid was mixture of Middle Devonian brines and radiogenic basement fluids.

Application of carbon isotope chemostratigraphy to the Renison dolomites, Tasmania: A Neoproterozoic age

This study uses carbon isotope chemostratigraphy to propose an age for the Success Creek Group and Crimson Creek Formation in the absence of any direct radiometric dates, palaeomagnetic or reliable palaeontological data. The δ13C values were determined for the least‐altered dolomite samples. Suitable samples were selected on the basis of grainsize, cathodoluminescence petrography, most enriched δ18O values (> ‐2%o) low Mn/Sr ratios and low Fe and Mn concentrations. The average least‐altered, most 13C‐enriched dolomicrite samples in the youngest (No. 1) dolomite horizon are + 4.6%o. This is typical of Neoproterozoic (but not Cambrian) carbonates. The δ13C values of all dolomite samples in the succession are significantly positive (up to + 7.5%o) and the excursion characteristic of the Proterozoic/Cambrian boundary has not been observed. The lack of negative δ13C values in all dolomite samples studied also suggests an absence of correlatives of Sturtian and Varanger tillites in the dolomite successions. The δ13C values in all three dolomite horizons suggest a Neoproterozoic age between about 820 to 570 Ma (Cryogenian to Neoproterozoic III) on the current global compilation carbon isotope curves. This age for the Success Creek Group and Crimson Creek Formation, inferred from carbon isotope chemostratigraphy, can be substantiated by other evidence. The age of the Renison dolomites is constrained by K‐Ar dates of 708 ± 6 Ma from detrital muscovite in the underlying Oonah Formation and 588 ± 8 and 600 ± 8 Ma from doleritic rock in a lithostratigraphic equivalent of the Crimson Creek Formation from the Smithton Basin. Furthermore, acritarchs and the stromatolite Baicalia cf. B. burra also suggest a Neoproterozoic rather than Cambrian age.

Holocene Climate Effects on the Development of a Peatland on the Tuktoyaktuk Peninsula, Northwest Territories

Six cores were collected from a small peatland on the Tuktoyaktuk Peninsula, yielding up to 1.8 m of organic material and 1.2 m of ice-rich material. Stratigraphic relationships, radiocarbon dating, and pollen, plant macrofossil, and isotopic analyses are used to reconstruct the developmental history of the peatland. Organic matter began to accumulate in the basin about 7200 yr B.P., during a period when the climate is thought to have been warmer than present. Initially the permafrost table was probably below the bottom of the basin, and an open-water mineral wetland with emergent and submergent aquatic vegetation occupied the site. Enriched δ18O values from ice samples confirm the existence of an open water body with a water balance strongly affected by evaporation. Transformation to a graminoid fen peatland was underway by 6300 yr B.P. Permafrost and ice began to form in the peatland at this time. Low-centered polygons probably began to develop by 4700 yr B.P. and subsequently evolved into the high-centered polygons which characterise the southwestern part of the peatland today. Peat accumulation in the polygons has been extremely slow for the last 4000 years. The eastern, wetter part of the basin has largely remained unchanged since its inception about 7000 yr B.P. Development of the peatland may be linked to permafrost formation that was controlled by regional climate changes during the Holocene.

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

ABSTRACTStable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar‐Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ18O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ18O of 0%0, calculation of temperature based on the maximum δ18O value of 31.4%0 yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert‐phosphate pairs. Diagenetically recrystallized microcrystalline chert‐dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.

Deep circulation changes in the South Atlantic Ocean: Response to initiation of northern hemisphere glaciation

Particle size statistics (coarsest 1 percentile and mean) of the nonbiogenic silt fraction of samples from Deep Sea Drilling Project sites 514, 516A, 517, 518, and 548 were used to infer relative changes in paleospeed of deep water masses in the Atlantic Ocean during the late Pliocene initiation of northern hemisphere glaciation. Grain size statistics were integrated with previous studies of geochemical and faunal assemblages to examine how climatic forcing regulates deepwater mass paleospeed and stratification. On the basis of between‐site comparison of particle size, the late Pliocene was divided into four intervals of deepwater activity: 3.2 to 2.9 Ma, 2.9 to 2.6 Ma, 2.6 to 2.5 Ma, and 2.5 to 2.3 Ma, with pulses of increased paleospeed occurring at 3.15–3.10, 2.85, 2.7, 2.6, and 2.4 Ma. Except for a brief increase in paleospeeds at 3.1 Ma, the first time interval is characterized by low inferred Antarctic Bottom Water (AABW) paleospeed corresponding to depleted δ13C and enriched δ18O values in all but the shallowest core. Enriched δ13C and depleted δ18O values in the shallowest core correspond to coarse grain sizes and are inferred to represent a deep and intense Antarctic Intermediate Water (AAIW). The expansion of AABW and AAIW precludes a significant North Atlantic Deep Water (NADW) incursion to 30°S at this time. The interval from 2.9 to 2.6 Ma is dominated by climatic cooling and an influx of deep waters originating in the North Atlantic. The introduction of significant volumes of Mediterranean Outflow Water (MOW), as demonstrated by increased inferred paleospeed at site 548, and an expanding glaciation beginning at 2.9 Ma, resulted in increased paleospeed and an expansion of NADW in the South Atlantic. A pulse of increased inferred paleospeed at 2.7 Ma resulted in the temporary deepening of NADW to the depth of site 518 (presently 3944 m). From 2.9 to 2.6 Ma AABW velocity remained low, but AAIW deepened and intensified. The 2.6‐to 2.5‐Ma interval represents the retreat of deep water, originating in the North Atlantic, from the depth of site 518 to the depth of site 517 (presently 2963 m). Increased inferred paleospeed at site 518 corresponds with depleted δ13C values and a northward shift in the polar front, thus suggesting the reintroduction of Circumpolar Water to the depth of site 518 and the establishment of a deepwater stratification similar to today's. A continuing trend toward a modern stratification continued into the latest time interval (2.5–2.3 Ma) as inferred deepwater paleospeed at all depths increased in response to the formation of extensive northern hemisphere ice sheets.