Increase In δ13C Values Research Articles

Chemical and isotopic evidence for secondary alteration of natural gases in the Hetianhe Field, Bachu Uplift of the Tarim Basin

H2S and CO2 are found in elevated concentrations in the reservoirs near the Carboniferous–Ordovician unconformity in the Hetianhe Field of the Tarim Basin, NW China. Chemical and isotopic analyses have been performed on produced gases, formation waters and reservoir rocks to determine the origin of CO2 and H2S and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids. It is unlikely that H2S and CO2 had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source. Instead, H2S and CO2 are probably the result of sulphate reduction of the light hydrocarbon gases (LHG). Increasing H2S concentrations and CO2/(CO2+ΣC1–4) values to the west of the Hetianhe Field occur commensurately with increasingly heavy hydrocarbon gas δ13C values. However, thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low, no H2S or rare pyrite was detected in deeper reservoirs (where more TSR should have occurred) and inferred δ34S values of H2S (from late-stage pyrite in the Carboniferous and Ordovician reservoirs) are as low as −24.9‰. Such low δ34S values discount the decomposition of organic matter as a major source of H2S and CO2. Bacterial sulphate reduction of the light hydrocarbon gases in the reservoir, possibly coupled indirectly with the consumption of organic acids and anions is most likely. The result is the preferential oxidation of 12C-rich alkanes (due to the kinetic isotope effect) and decreasing concentration of organic acids and anions. Modern formation water stable isotope data reveal that it is possible that sulphate-reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion-related unconformity. This may account for the apparently stronger influence of bacterial sulphate reduction to the west of the Hetianhe Field, and the consequent greatest decrease of the δ13C-CO2 values and the greatest increase in δ13C values of the alkane gases.

Effect of light and brine shrimp on skeletal δ13C in the Hawaiian coral Porites compressa: a tank experiment

Previous experimental fieldwork showed that coral skeletal δ13C values decreased when solar intensity was reduced, and increased in the absence of zooplankton. However, actual seasonal changes in solar irradiance levels are typically less pronounced than those used in the previous experiment and the effect of increases in the consumption of zooplankton in the coral diet on skeletal δ13C remains relatively unknown. In the present study, the effects of four different light and heterotrophy regimes on coral skeletal δ13C values were measured. Porites compressa corals were grown in outdoor flow-through tanks under 112%, 100%, 75%, and 50% light conditions at the Hawaii Institute of Marine Biology, Hawaii. In addition, corals were fed either zero, low, medium, or high concentrations of brine shrimp. Decreases in light from 100% resulted in significant decreases in δ13C that is most likely due to a corresponding decrease in photosynthesis. Increases in light to 112% also resulted in a decrease in δ13C values. This latter response may be a consequence of photoinhibition. The overall curved response in δ13C values was described by a significant quadratic function. Increases in brine shrimp concentrations resulted in increased skeletal δ13C levels. This unexpected outcome appears to be attributable to enhanced nitrogen supply associated with the brine shrimp diet which led to increased zooxanthellae concentrations, increased photosynthesis rates, and thus increased δ13C values. This result highlights the potential influence of nutrients from heterotrophically acquired carbon in maintaining the zooxanthellae-host symbiosis in balance. In addition, evidence is presented that suggests that coral skeletal growth and δ13C are decoupled. These results increase our knowledge of how light and heterotrophy affects the δ13C of coral skeletons.

Assessment of Bacterial Degradation of Aromatic Hydrocarbons in the Environment by Analysis of Stable Carbon Isotope Fractionation

13C/12C stable carbon isotope fractionation was used to assess biodegradation in contaminated aquifers with toluene as a model compound. Different strains of anaerobic bacteria (Thauera aromatica, Geobacter metallireducens, and the sulfate-reducing strain TRM1) showed consistent 13C/12C carbon isotope fractionation with fractionation factors between αC = 1.0017 and 1.0018. In contrast, three cultures of aerobic organisms, using different mono- and dioxygenase enzyme systems to initiate toluene degradation, showed variable isotope fractionation factors of αC = 1.0027 (Pseudomonasputida strain mt-2), αC = 1.0011 (Ralstonia picketii), andαC = 1.0004 (Pseudomonas putida strain F1). The great variability of isotope fractionation between different aerobic bacterial strains suggests that interpretation of isotope data in oxic habitats can only be qualitative. A soil column was run as a model system for contaminated aquifers with toluene as the carbon source and sulfate as the electron acceptor and samples were taken at different ports along the column. Microbial toluene degradation was calculated based on the 13C/12C isotope fractionation factors of the batch culture experiments together with the observed 13C/12C isotope shifts of the residual toluene fractions. The calculated percentage of biodegradation, B, correlated well with the decreasing toluene concentrations at the sampling ports and indicated the increasing extent of biodegradation along the column. The theoretical toluene concentrations as calculated based on the isotope values matched the measured concentrations at the different sampling ports indicating that the Rayleigh equation can be used to calculate biodegradation in quasi closed systems based on measured isotope shifts. A similar attempt was performed to assess toluene degradation in a contaminated, anoxic aquifer. A transect of groundwater wells was monitored along the main direction of the groundwater flow and revealed decreasing concentrations accompanied with an increase in the 13C/12C stable carbon isotope ratio of the residual toluene. Calculation of the extent of biodegradation based on the isotope values and laboratory derived isotope fractionation factors showed that the residual toluene was degraded to more than 99% by microbial activity. Calculation of the theoretical residual toluene concentrations based on the measured isotope values described the strongly decreasing concentrations along the plume. Other aromatic hydrocarbons like benzene and naphthalene which were analysed in the same course also showed decreasing concentrations along the groundwater flow path accompanied by increasing δ13C values indicating biodegradation.

Orbitally induced climate and geochemical variability across the Oligocene/Miocene boundary

To assess the influence of orbital‐scale variations on late Oligocene to early Miocene climate and ocean chemistry, high‐resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low‐ to high‐frequency variability across a 5‐Myr interval (20.5–25.4 Ma). The records also reveal several large‐scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi‐1 and Mi‐1a events of Miller et al. [1991]), and a long‐term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower‐resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice‐volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low‐frequency cycles (∼400 kyr) and long‐term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest‐resolution records of pre‐Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.

Influence of a [CO2(aq)] dependent biological C‐isotope fractionation on glacial 13C/12C ratios in the ocean

Planktonic foraminiferal shells buried in deep ocean sediments record lower δ13C values of surface water dissolved inorganic carbon during glacial times than during Holocene. In the Southern Ocean and at high northern latitudes, a drop of between 0.3‰ and 0.9‰ is observed, whereas the mean change in low and middle latitudes is only ±0.15‰. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO2 values during glacial times would raise the surface ocean δ13C values of dissolved inorganic carbon by about 1.0‰. Here the results of a three‐dimensional ocean circulation model study are presented which demonstrate that the increase of δ13C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the δ13C values in the opposite direction. This was found by performing simulations employing the three‐dimensional Hamburg Model of the Oceanic Carbon Cycle (HAMOCC) combined with a [CO2(aq)] dependent parameterization of the biological carbon isotope fractionation. The difference in the biological carbon isotope fractionation between Glacial and Holocene is responsible for a lowering of δ13C values in surface water dissolved inorganic carbon by about 0.3‰. The additional effects of the glacially elevated CO32− concentration (0.25–0.50‰) combined with the 0.35‰ lowering of δ13C values for the whole ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean‐atmosphere reservoir also contribute to a further δ13C drop of 0.6–0.85‰. Hence a small glacial decrease of the planktonic foraminifera δ13C of the order of 0.25‰ instead of an increase is predicted.

Carbon isotope 13C enrichment in Upper Silurian (Whitcliffian) marine calcareous rocks in Scania, Sweden

Carbon and oxygen isotope data have been obtained from three drill cores, that together span the Öved-Ramsåsa Group (Ludlow–Pridoli) in Scania. Throughout most of the sequence the δ13C values of -1.7 to +2.7‰ correspond well to a modern marine composition. An enrichment in 13C is noted in the Lower Whitcliffian by δ13C values of between +3.6 to +11.2‰. The excursion, also noted in contemporary deposits elsewhere, coincides with a sea-level lowstand. The magnitude of the excursion implies that it cannot be explained solely by an increase in primary production and/or enhanced burial of organic carbon, but a combination of factors is suggested. Carbonates with δ13C values of +3 to +5‰ precipitated from a regionally or globally modified seawater δ13C-composition. The increasing δ13C values of +6 to +11‰ are suggested to be attributed to photosynthetic activity. The local and regional increase in photosynthesizing cyanobacteria and algae is demonstrated by the high abundance of oncoids in the Bjärsjölagård Limestone in Scania and the Eke beds of Gotland. The oxygen isotope composition ranging from -12.6 to -4.7‰ (PDB) is depleted in 18O compared to modern marine carbonates but is mainly in agreement with Silurian values. It is not excluded that the absolute δ18O values, reported here, reflect a diagenetic overprinting. It is, however, suggested that salinity fluctuations may have triggered the increase in cyanophytes. The less negative δ18O values and the unstable 18O/16O ratio, recorded during the δ13C excursion, may thus be explained by evaporation and restricted water circulation.

A 15,000-year stable isotope record from sediments of Lake Steisslingen, Southwest Germany

Stable isotope records for carbon and oxygen in bulk carbonates, carbon in bulk organic matter, and for total and chromium-reducible sulfur in a lacustrine sediment core from Lake Steisslingen (Southwest Germany) show several distinct and abrupt shifts during the last 15,000 years. Variations in the isotopic composition of authigenic carbonates indicate two major phases in the lake history. In the pre-Holocene, the hydrological budget of the lake was apparently stable. Variations of δ18O values of authigenic carbonates were, therefore, dominantly controlled by temperature changes. A decrease in the δ18Ocarb values of about 2‰ at the Allerød/Younger Dryas transition is interpreted as a drop in mean annual air temperatures of approximately 5°C. An abrupt temperature increase of a similar magnitude is inferred at the Younger Dryas/Preboreal boundary. Throughout most of the Holocene, the isotopic composition of authigenic carbonates was influenced by marked changes in the hydrological budget of the lake. A major positive excursion in the δ13Ccarb and δ18Ocarb values at the beginning of the Atlantic and a smaller one in the Preboreal were related to evaporation effects, which indicate that dry climatic conditions must have prevailed at that time. A simultaneous increase in δ13C values of bulk organic matter at the beginning of the Atlantic suggests a high level of productivity in the lake. As a consequence, aqueous sulfate became limited as indicated by variations in the δ34S values of total and chromium-reducible sedimentary sulfur. Therefore, we conclude that the beginning of the Atlantic was characterized not only by dry but also by warm climatic conditions, which triggered a higher productivity in the lake. In the Subatlantic sediments, large variations in carbon, oxygen, and sulfur isotope ratios were observed as a result of human activities, causing considerable perturbations in the biogeochemical element cycling of Lake Steisslingen. Results obtained by the study of the continuous 15 ka record of Lake Steisslingen document clearly that isotopic proxy data from lacustrine sediments can provide useful information on environmental and climatic changes of local, regional, and in the case of the Younger Dryas event, of even hemispherical significance.

Spatial and temporal differences in the diet of Great Lakes herring gulls (Larus argentatus): evidence from stable isotope analysis

Stable nitrogen (δ15N) and carbon (δ13C) isotope values were measured in lipid-free homogenates of herring gull (Larus argentatus) eggs collected from the Laurentian Great Lakes during 1974-1995. δ15N values provided an indication of adult gull trophic position during egg formation. After adjustment of egg δ15N values for interlake differences in baseline food web δ15N signatures, significant differences in gull trophic position were found among colonies. These results were attributed to differing proportions of fish in the diets of gulls from the various colonies. Aquatic foods available to gulls had greater δ15N values than terrestrial foods. δ13C measurements provided some support for our hypothesis that terrestrial foods, particularly garbage, were more enriched in 13C than aquatic foods. As the proportion of fish in the diet decreased, the fraction consisting of terrestrial food increased, resulting in an increase in δ13C values and a decline in δ15N values. Stable isotope values for gull eggs from Lake Erie changed significantly during the study period and may have reflected a decline in fish availability.

Carbon isotope evidence for deep-water invasion at the Marjumiid-Pterocephaliid biomere boundary, Black Hills, USA: A common origin for biotic crises on Late Cambrian shelves

Research Article| May 01, 1999 Carbon isotope evidence for deep-water invasion at the Marjumiid-Pterocephaliid biomere boundary, Black Hills, USA: A common origin for biotic crises on Late Cambrian shelves Patrick J. Perfetta; Patrick J. Perfetta 1Phillips Petroleum Company, 6330 West Loop South, Box 1967, Bellaire, Texas 77251-1967, USA Search for other works by this author on: GSW Google Scholar Kevin L. Shelton; Kevin L. Shelton 2Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar James H. Stitt James H. Stitt 2Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Patrick J. Perfetta 1Phillips Petroleum Company, 6330 West Loop South, Box 1967, Bellaire, Texas 77251-1967, USA Kevin L. Shelton 2Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA James H. Stitt 2Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (5): 403–406. https://doi.org/10.1130/0091-7613(1999)027<0403:CIEFDW>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Patrick J. Perfetta, Kevin L. Shelton, James H. Stitt; Carbon isotope evidence for deep-water invasion at the Marjumiid-Pterocephaliid biomere boundary, Black Hills, USA: A common origin for biotic crises on Late Cambrian shelves. Geology 1999;; 27 (5): 403–406. doi: https://doi.org/10.1130/0091-7613(1999)027<0403:CIEFDW>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An integrated carbon isotope, sedimentologic, and biostratigraphic study across a Late Cambrian mass-extinction boundary (Marjumiid-Pterocephaliid biomere boundary) in the Deadwood Formation, Black Hills, South Dakota, indicates invasion of the carbonate shelf by deep, cold, marine waters. Carbonate samples stratigraphically below the extinction boundary show an apparent trend of upwardly increasing δ13C values, from −0.4‰ to +0.1‰ and relate well to secular δ13C trends of Late Cambrian limestones. Coincident with the extinction horizon is a negative shift in δ13C values of 0.4‰ to 0.7‰. This shift can be attributed to introduction of cold, deep, 12C-enriched seawater accompanying invasion of opportunistic, cold-water trilobites onto depopulated shallow-water continental shelves. A similar abrupt negative shift in δ13C values has been observed at the suprajacent Pterocephaliid-Ptychaspid biomere boundary. Others have found that decreases in 87Sr/86Sr values synchronous with increases in δ13C values accompany renewed sea-level rise in this part of the Late Cambrian. Abrupt negative shifts in δ13C values at biomere boundaries require a companion process responsible for the extinction events. This process may be impingement of a shallow thermocline on the shelf or overturning of a stratified Cambrian ocean. Coupling of sea-level rise with location of the thermocline and shelf geometry may create conditions responsible for recurrent extinctions of shallow-shelf communities throughout the Late Cambrian. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotope hydrology of voluminous cold springs in fractured rock from an active volcanic region, northeastern California

The more than 1550 km2 (600 mi2) Hat Creek Basin in northeastern California is host to several first magnitude cold springs that emanate from Quaternary basaltic rocks with individual discharge rates ranging from 1.7 to 8.5 m3 s−1 (60–300 ft3 s−1). Stable isotope (δ18O, δD, δ13C) and 14C measurements of surface and groundwater samples were used to identify recharge areas, and to evaluate aquifer residence times and flow paths. Recharge locations were constrained from the regional decrement in meteoric water δ18O values as a function of elevation, determined to be −0.23‰ per 100 m for small springs and creek waters collected along the western Cascade slope of this region. In general, the large-volume springs are lower in (δ18O than surrounding meteoric waters, and are inferred to originate in high-elevation, high-precipitation regions up to 50 km away from their discharge points. Large spring 14C abundances range from 99 to 41 % modern carbon (pmc), and most show evidence of interaction with three distinct carbon isotope reservoirs. These reservoirs are tentatively identified as (1) soil CO2 gas equilibrated under open system conditions with groundwater in the recharge zone [δ13CDIC ≈ −18‰, 14C > 100 pmc], (2) dissolved carbon equilibrated with atmospheric CO2 gas [δ13CDIC ≈ +1‰, 14C > 100 pmc], and (3) dissolved carbon derived from volcanic CO2 gas emissions [δ13CDIC≈0‰, 14C=0 pmc]. Many regional waters show a decrease in 14C abundance with increasing δ13C values, a pattern indicative of interaction with dead carbon originating from volcanic CO2 gas. Several lines of evidence suggest that actual groundwater residence times are too short (⩽ 200 years) to apply radiocarbon dating corrections. In particular, water temperatures measured at springs show that deep groundwater circulation does not occur, which implies an insufficient aquifer volume to account for both the high discharge rates and long residence times suggested by 14C apparent ages. The large springs also exhibit rapid decreases in flow during periods of drought that suggests a high level of aquifer interconnectivity to the recharge area. The estimated amount of volcanic CO2 dissolved in surface and groundwater originating from the Lassen highlands is consistent with the conversion of approximately 10% of the geothermal CO2 flux into dissolved inorganic carbon.

A δ13C record of Upper North Atlantic Deep Water during the past 2.6 million years

Benthic foraminiferal δ13C data from site 502 in the Caribbean Sea (sill depth ∼1800 m) indicate that throughout the past 2.6 m.y., glacial δ13C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10 % higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma‐present) δ13C values in the Caribbean Sea were approximately 0.2‰ higher than they were from 2.6 to 2.0 Ma. The δ13C rise is not due to an increase in the mean ocean δ13C value, nor can it be entirely attributed to an increase in the proportion of high‐δ13C source waters. An increase in the δ13C value of the surface source waters must have contributed to the δ13C rise.

Variations in the strontium isotopic ratio of seawater during the Miocene: Stratigraphic and geochemical implications

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid‐Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM‐O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in 87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of 87Sr/86Sr growth slowed and 87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87Sr/86Sr resumed and continued until ∼9 Ma, when the rate of 87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high‐order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87Sr/86Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan‐Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ18O values), which did not begin until ∼14.5 Ma.

Effect of altitude on the carbon‐isotope composition of forest and grassland soils from Papua New Guinea

The carbon‐isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3 vegetation (forests and alpine grasslands above 4000 m) show an increase in δ13C value with altitude paralleling the increase in δ13C value observed in plant leaves by Körner et al. [1988]. Grassland soils show a decrease in δ13C value above about 1000 m, from maximum values which are close to pure C4 values (−12 to −13‰ vs. PDB) to minimum values which are indistinguishable from pure C3 values at 3500–4000 m (∼−26‰). Within this general framework, several factors can influence the soil δ13C value at individual locations. In local forest settings, soil δ13C values will be influenced by the degree to which respired CO2 is re‐utilized during photosynthesis, the proportions of leaf and wood litter, and the degree of decomposition. In grassland settings the primary factor controlling the observed δ13C variability at any specific altitude is the amount of nongrass C3 carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3 and C4 grasses at any given altitude, although further work would be required to substantiate such a link. The results provide a framework within which to more accurately constrain the carbon‐isotope composition of terrestrial carbon pools and to interpret variations in the isotopic composition of riverine particulate organic carbon.

On stable isotopic variation and earliest Paleocene planktonic foraminifera

Extant planktonic foraminifera display positive covariance between δ13C signals and test size. As documented by other studies, primary causes of increased δ13C values with increased test size may include increased reliance on ambient CO2 for calcification at larger test sizes, decreased kinetic fractionation during calcification at larger test sizes, and increased photosymbiotic activity in larger symbiont‐bearing planktonic foraminifera. Planktonic foraminiferal δ18O values also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between δ18O signals and test size include changing habitat depth over ontogeny, correlations between adult test size and environmental conditions, and changing isotopic disequilibrium with size, ontogenetic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene planktonic foraminifera, we measured the stable isotopic signals of multiple size fractions of 10 earliest Paleocene species. All of these taxa exhibit a strong positive correlation between δ13C and test size. The slope and magnitude of this trend varies between species, with Woodringina claytonensis displaying the largest shift (1.1‰ over a 130 µm range in mean sieve size) and Guembelitria cretacea displaying the smallest (0.2‰ over a 38 µm range). By analogy with modern planktonic foraminifera, this general relationship between δ13C and size probably resulted from increased reliance on ambient CO2 for calcification at larger test sizes. The high magnitude of this shift in some taxa may reflect either photosymbiotic enhancement of the general trend or relatively greater changes in the proportions of metabolic and ambient CO2 used for calcification at different test sizes. Failure to account for relationships between test size and δ13C signals can lead to underestimation of early Paleocene surface ocean δ13C values by 1‰ or more. These size‐related δ13C effects provide an alternative explanation for decreases in whole‐rock δ13C values and some decreases in planktonic‐tobenthic foraminiferal δ13C gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of δ18O derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and δ18O signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene δ18O and biogeographic data are consistent with a general trend of surface‐to‐deep diversification of microperforate planktonic foraminifera following the K/T boundary. Such a trend may simply result from exploitation of a near‐surface open‐ocean habitat by the epicontinental K/T survivor G. cretacea.

Oceanographic and climatic implications of the Palaeocene carbon isotope maximum

ABSTRACTWe have compared detailed planktonic and benthonic foraminiferal carbon and oxygen isotope records from the Palaeocene and early Eocene successions at DSDP Site 577 (Shatsky Rise, North Pacific), a composite section derived from DSDP Leg 74 sites (Walvis Ridge, South Atlantic) and a composite section from ODP Leg 113 sites (Maud Rise, Weddell Sea). The δ13C records of Palaeocene and early Eocene Foraminifera at Site 577 and the Leg 74 sites show that an increase in δ13C values in surface waters at 64 Ma (end of Zone P1) resulted in increased vertical carbon isotope gradients (δ13C) between surface and deeper dwelling planktonic foraminifera, and between surface‐dwelling planktonics and benthonic foraminifera which became progressively steeper until the iniddle Late Palaeocene (Zone P4). This steepening also occurs in the latest Palaeocene of the composite Leg 113 section and can be explained by an increase in surface ocean productivity. This increase in productivity probably resulted in an expansion of the oxygen minimum zone (OMZ).Benthonic δ13C values increased during the late Palaeocene in Site 577 and the composite Leg 74 section, suggesting that the Palaeocene carbon isotope maximum was composed of both within‐ocean reservoir (increased surface water productivity) and between‐reservoir (organic carbon burial) ftactionation effects. The benthonic δ13C increase lags the surface ocean δ13C increase in the early Palaeocene (63–64 Ma) suggesting that surface water productivity increase probably led an increase in the burial rate of organic carbon relative to carbonate sedimentation. Moreover, inter‐site δ13C comparisons suggest that the locus of deep to intermediate water formation for the majority of the Palaeocene and the earliest Eocene was more likely to have been in the high southern latitudes than in the lower latitudes.Oxygen isotope data show a decline in deeper water temperatures in the early and early late Palaeocene, followed by a temperature increase in the late Palaeocene and across the PalaeoceneEocene boundary. We speculate that these changes in deeper water temperatures were related to the flux of CO2 between the oceans and the atmosphere through a mechanism operating at the high southern latitudes.

Mid‐Miocene isotope stratigraphy in the deep sea: High‐resolution correlations, paleoclimatic cycles, and sediment preservation

Mid‐Miocene pelagic sedimentary sections can be correlated using intermediate and high resolution oxygen and carbon isotopic records of benthic foraminifera. Precision of a few tens of thousands of years is readily achievable at sites with high sedimentation rates, for example, Deep Sea Drilling Project sites 289 and 574. The mid‐Miocene carbon isotope records are characterized by an interval of high δ13C values between 17 and 13.5 Ma (the Monterey Excursion of Vincent and Berger 1985) upon which are superimposed a series of periodic or quasi‐periodic fluctuations in δ13C values. These fluctuations have a period of approximately 440 kyr, suggestive of the 413 kyr cycle predicted by Milankovitch theory. Vincent and Berger proposed that the Monterey Excursion was the result of increased organic carbon burial in continental margins sediments. The increased δ13C values (called 13C maxima) superimposed on the generally high mid‐Miocene signal coincide with increases in δ18O values suggesting that periods of cooling and/or ice buildup were associated with exceptionally rapid burial of organic carbon and lowered atmospheric CO2 levels. It is likely that during the Monterey Excursion the ocean/atmosphere system became progressively more sensitive to small changes in insolation, ultimately leading to major cooling of deep water and expansion of continental ice. We have assigned an absolute chronology, based on biostratigraphic and magneto‐biostratigraphic datum levels, to the isotope stratigraphy and have used that chronology to correlate unconformities, seismic reflectors, carbonate minima, and dissolution intervals. Intervals of sediment containing 13C maxima are usually better preserved than the overlying and underlying sediments, indicating that the δ13C values of TCO2 in deep water and the corrosiveness of seawater are inversely correlated. This again suggests that the 13C maxima were associated with rapid burial of organic carbon and reduced levels of atmospheric CO2. The absolute chronology we have assigned to the isotopic record indicates that the major mid‐Miocene deepwater cooling/ice volume expansion took 2 m.y. and was not abrupt as had been reported previously. The cooling appears abrupt at many sites because the interval is characterized by a number of dissolution intervals. The cooling was not monotonic, and the 2 m.y. interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period. The increase in δ18O values of benthic foraminifera between 14.9 and 12.9 Ma was greatest at deeper water sites and at sites closest to Antarctica. The data suggest that the δ18O value of seawater increased by no more than about 1.1 per mil during this interval and that the remainder of the change in benthic δ18O values resulted from cooling in Antarctic regions of deepwater formation. Equatorial planktonic foraminifera from sites 237 and 289 exhibit a series of 0.4 per mil steplike increases in δ13C values. Only one of these increases in planktonic δ13C is correlated with any of the features in the mid‐Miocene benthic carbon isotope record.

Lithic geochemistry of the Claggett marine cyclothem in south-central Saskatchewan

Sediments of the Lea Park Formation in south-central Saskatchewan were deposited in the Claggett sea during the Late Cretaceous (Campanian) Claggett marine cycle. Rocks of the Lea Park Formation have remained virtually unaffected by diagenetic alteration; hence, variations in mineralogical and chemical compositions result primarily from changes in the source of detritus. Quartz, feldspar, discrete illite, kaolinite, and chlorite were derived from weathering of the western highlands, whereas illite–smectite (I–S), the dominant clay phase, originated from volcanic ash. Changes in the proportions of these minerals suggest a large-scale volcanic episode during deposition of the Lea Park sediments, an episode that affected not only the detritus delivered to the seaway but also the chemical and isotopic compositions of the sea water itself. δ13C values of organic matter increase with little change in the quantity of organic carbon preserved through both the transgressive and regressive phases of the Claggett cyclothem, unlike other cyclothems in which δ13C values generally decrease through the regressive phase. The increase in δ13C values in the Claggett cyclothem suggests a partial contribution from volcanic emanations to the 13C/12C ratio of the sea. Dissolution of Mg-rich volcanic debris from a major volcanic episode resulted in anomalously high Mg/Ca ratios in the sea and in the shells of molluscs living in the sea at that time. Rocks containing the shells with anomalously high Mg/Ca ratios are not themselves unusually Mg rich, but a general increase in Mg/SiO2 ratios in the rocks points to an increase in volcanic influx. The similarity between the mineralogical composition of the Lea Park Formation in Canada and that of the coeval lower part of the Pierre Shale in the United States indicates that volcanic ash was an important detrital source for both formations.