Samuel M. Savin

Tertiary marine paleotemperatures

Oxygen isotopic compositions of the tests of planktonic foraminifera from several Deep Sea Drilling Project sites provide a general picture of low-latitude marine temperatures from Maastrichtian time to the present. Bottom temperatures determined from the isotopic compositions of benthonic foraminifera are interpreted as being indicative of high-latitude surface temperatures. Prior to the beginning of middle Miocene time, high- and low-latitude temperatures changed in parallel fashion. Following an apparently small and short-lived drop in temperature near the Tertiary-Cretaceous boundary, temperatures remained warm and relatively constant through Paleocene and early and middle Eocene time; bottom temperatures then were on the order of 12°C. A sharp temperature drop in late Eocene time was followed by a more gradual lowering of temperature, culminating in a late Oligocene high-latitude temperature minimum of about 4°C. A temperature rise through early Miocene time was followed in middle Miocene time by a sudden divergence of high- and low-latitude temperatures: high-latitude temperatures dropped dramatically, perhaps corresponding to the onset of major glaciation in Antarctica, but low-latitude temperatures remained constant or perhaps increased. This uncoupling of high-and low-latitude temperatures is postulated to be related to the establishment of a circum-Antarctic circulation similar to that of today. A further drop in high-latitude temperatures in late Pliocene time probably signaled the onset of a major increase in polar glaciation, including extensive sea-ice formation. Early Miocene, small-amplitude (1 per mil) sympathetic fluctuations in isotopic compositions of planktonic and benthonic foraminifera have been identified. These have a period of several hundred thousand years. Superimposed upon these are much more rapid and smaller fluctuations (0.2 to 0.5 per mil) with a period of about 80,000 to 90,000 yr. This is similar to the period observed for Pleistocene isotopic temperature fluctuations. In low latitudes, much smaller vertical temperature gradients seem to have existed during Maastrichtian and Paleogene time than exist at present. The absence of a sharply defined thermocline during early Tertiary time is also suggested.

Mechanism of burial metamorphism of argillaceous sediments: 3. O-isotope evidence

O-isotope analysis of shales sampled from wells drilled through sedimentary deposits in the Gulf of Mexico region indicates that the sediments and rocks are not isotopically equilibrated systems — even those that have been buried to depths where temperatures are as high as 170 °C. In comparison with the coarser fractions, the finer fractions of both clay minerals and quartz are almost always richer in O 18 . O-isotope disequilibrium among the clay fractions becomes less marked as burial temperature increases. O-isotope exchange between clay and pore water become more extensive at higher temperatures; this corresponds to more extensive diagenetic alteration of mixed-layer illite-smectite. There is no evidence for O-isotope exchange between detrital quartz and pore water. However, quartz that forms diagenetically as an accompaniment to the conversion of smectite to illite layers in the mixed-layer clay forms in equilibrium with the pore water. The usefulness of O-isotope geothermometry for determination of the maximum temperatures to which shales have been heated during burial was investigated. Temperatures were calculated from the O-isotope fractionations between coexisting fine-grained quartz and clay from three wells; these calculated temperatures progressively approached the measured well (logged) temperatures as depth of burial and temperature increased. In one well, good agreement between calculated and measured temperatures was obtained for measured temperatures between 100 and 180 °C. In two other wells, satisfactory agreement was approached but not obtained at measured temperatures as high as 120 °C. Temperatures calculated from the O-isotope fractionations of quartz and calcite or calcite and clay were not reasonable. This probably reflects isotope exchange between calcite and pore water after the silicates attained their measured isotope ratios. Consequently, calcite is not a suitable mineral for use in isotope geothermometry of diagenetically altered shales.

The oxygen and hydrogen isotope geochemistry of clay minerals

Abstract Oxygen and hydrogen isotope analyses have been made on a variety of clay minerals of sedimentary and diagenetic origins. The interlayer water of clay minerals was found to exchange rapidly with atmospheric water. Conditions under which the interlayer water could be removed from the clays without affecting the isotopic compositions of their aluminosilicate oxygen and hydrogen were therefore determined, and the interlayer water was routinely removed and discarded prior to isotopic analysis. Approximate fractionation factors for clay mineral-water systems at sedimentary temperatures, inferred from the isotopic compositions of natural samples, are: α Oxygen min - H 2 O α Hydrogen min - H 2 O Montmorillonite 1.027 0.94 Kaolinite 1.027 0.97 Glauconite 1.026 0.93 Consistency of a relationship between oxygen and hydrogen isotope ratios of kaolinites taken from a variety of areas and having different isotope ratios was demonstrated. This relationship results from exchange of clays with different meteoric waters under conditions in which the fractionation factors are relatively constant. This strongly suggests that the fractionation factors are equilibrium ones and that kaolinite forms in isotopic equilibrium with its environment. Deviations of the isotopic composition of montmorillonites from a similar relationship have been interpreted as resulting from isotopic exchange at slightly elevated temperatures. There was no clearly demonstrable case of clay minerals undergoing isotopic re-equilibration at sedimentary temperatures, although this is one possible interpretation of the data of some Upper Cretaceous glauconites.

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.

Biological fractionation of oxygen and carbon isotopes by recent benthic foraminifera

Abstract Recent deep-sea benthic foraminifera from five East Pacific Rise box core tops have been analyzed for oxygen and carbon isotopic composition. The five equatorial stations, with water depths of between 3200 and 4600 m, yielded fourteen specific and generic taxonomic groups. Of the taxa analyzed, Uvigerina spp. most closely approaches oxygen isotopic equilibrium with ambient sea water. Pyrgo spp. was next closest to isotopic equilibrium, being on the average 0.59 ‰ depleted in 18 O relative to Uvigerina spp. Oridorsalis umbonatus also has relatively high δ 18 O values. Most other taxa were depleted in 18 O by large amounts. In no taxa was the carbon in the CaCO 3 secreted in carbon isotopic equilibrium with the dissolved HCO 3 − of ambient sea water.

The oyxgen and hydrogen isotope geochemistry of ocean sediments and shales

Oxygen and hydrogen isotope analyses were made on the carbonate-free fractions of twenty-seven ocean core samples from the Atlantic, Pacific, and Indian Oceans. δO18 values ranged from +11.5 to +28.5 permil, and δD values ranged from −5.5 to −8.7 per cent relative to SMOW. On the basis of chemical analyses the cores were divided into two groups, those with Fe + Mn between 7 and 15 wt. % and those with Fe + Mn between 2 and 5 wt. %. On the basis of mineralogic analyses by Goldberg and Griffin (1963, 1967) and isotopic and chemical analyses, the ferromanganese-rich samples appear to have a large authigenic component while the ferromanganese-poor samples appear to be primarily detrital. The oxygen isotopic compositions of the clay minerals in the detrital group of samples reflect the provenance and mode of origin of these minerals. There is no evidence for oxygen isotopic exchange, and inconclusive evidence for hydrogen isotopic exchange between these samples and sea water, even though some have been dated by the ionium-thorium method as being as old as 250,000 years (Goldberg and Griffin, 1963). The range of published δO18 values of shales, supplemented by two new analyses, is similar to that of the ocean core samples of detrital origin. Therefore, as far as the isotopic record is concerned it is impossible to distinguish between marine and non-marine shales. Material balance calculations suggest that the effect of weathering and sedimentary processes has been to deplete the hydrosphere of O18 by about 3 permil and to enrich the hydrosphere in deuterium by about 0.03 per cent during the earths history.

Stable Isotope and Magnesium Geochemistry of Recent Planktonic Foraminifera from the South Pacific

O 18 /O 16 , C 13 /C 12 , and magnesium analyses were performed on a large number of Recent planktonic Foraminifera from South Pacific Ocean sediments. Results show that oxygen isotopic temperatures of Foraminifera tests may be used to locate ocean currents and to define the orientation of large crustal plates relative to the earth9s rotational poles. Selective solution effects may cause isotopic temperatures of some species to become progressively colder with increasing water depth of the sediments from which they are taken. Where this is not taken into account, erroneous conclusions may result from the comparison of isotopic temperatures of samples from different locations. Depths at which Foraminifera secrete their tests appear to be determined by density and ultimately by osmotic equilibration with surrounding sea water. Susceptibility of Foraminifera tests to selective solution after death increases with magnesium content. Carbon isotope ratios correlate crudely with both temperature and salinity. The C 13 /C 12 ratio of dissolved or particulate carbon in the oceans is probably the most important factor in determining the C 13 /C 12 ratio of the test.

Oxygen isotopic composition of soil water : Quantifying evaporation and transpiration

Abstract The oxygen isotopic composition of soil water provides an extra quantitative dimension in water balance analysis which allows separation of evaporation from transpiration. Spatial and temporal variations in water content and oxygen isotopic composition in soils along an arid to humid transect in Hawaii reflect the processes of recharge by rain, mixing with antecedent moisture, and evapotranspiration. Rainwater is always more depleted in 18O than is the soil water with which it mixes. Input of 18O-depleted rain increases volumetric water content while lowering the soil-water δ18O value. Evapotranspiration occurs continuously, leading to a decrease in the volumetric water content and an increase in the soil-water δ18O value. These effects are most pronounced at the soil surface and decrease in a downward direction. The frequency of recharge determines temporal variability of these values within a given depth interval, while differences along the transect are due to climatic parameters. Results of a material balance model indicate that evaporation decreases, transpiration increases, and the ratio of evaporation to transpiration decreases with increasing annual rainfall and decreasing temperature.

Glucose Production in Pregnant Women at Term Gestation: SOURCES OF GLUCOSE FOR HUMAN FETUS

The effects of pregnancy and diabetes on systemic glucose production rates and the sources of glucose for the human fetus in utero were evaluated in five normal, four gestationally diabetic, and one insulin-dependent diabetic subject undergoing elective caesarean section at term gestation. Five normal nonpregnant women were studied for comparison. Systemic glucose production rates were measured with stable tracer [1-(13)C]glucose according to the prime-constant rate infusion technique. Even though the plasma glucose concentration during normal pregnancy had declined as compared with the nonpregnant subjects (P < 0.0005), the systemic glucose production rate was 16% greater, a rate sufficient to provide the glucose requirement of the fetus at term gestation. The decline in glucose concentration could be the result of an increase in apparent volume of distribution of glucose. Systemic glucose production rates in well-controlled, gestationally diabetic subjects were similar to those in normal pregnant subjects (2.07+/-0.53 vs. 2.42+/-0.51 mg/kg.min). The sources of glucose for the human fetus at term gestation were evaluated by comparing (a) natural variation in (13)C:(12)C ratio of plasma glucose and (b) enriched (13)C:(12)C ratio of plasma glucose during [1-(13)C]glucose infusion in maternal and fetal blood at delivery in both normal and diabetic subjects. These data showed that the fetal glucose pool was in equilibrium with the maternal glucose pool in both normal and diabetic subjects, indicating that a brief maternal fast did not initiate systemic glucose production in human fetus. A materno-fetal gradient was observed for betahydroxybutyrate.

Evidence for thermohaline-circulation reversals controlled by sea-level change in the latest Cretaceous

Fluctuations in oxygen (δ 18 O) and carbon (δ 13 C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71‐70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal δ 18 O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal δ 13 C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal δ 18 O values in the Indo-Pacific decreased, and benthic and planktic δ 13 C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic δ 13 C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.