Anthony M. Faiia

Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau

The distribution of C3 and C4 plants changed in regionally contrasting ways during the last glacial period. C4 plant expansion in low-latitude Africa and America coincided with C4 plant decreases in Mesoamerica and the US Great Plains. This C4 plant expansion has been attributed to lower pCO2 and increased aridity and the decline in C4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO2, temperature, or aridity was mainly responsible for C3 vs. C4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C3 vs. C4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C4 expansion and lower temperatures favoring C3 expansion. Here, we present two high-resolution compound-specific carbon isotope records of n-alkanes from the CLP as vegetation biomarkers to estimate the relative abundance of C4 plants during the last two glacial/interglacial transitions. More negative δ13C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C4 plants declined. This decrease in C4 abundance was caused by lower temperature, despite the lower pCO2 and increased aridity on the CLP during glacial time.

The Changes in North American atmospheric circulation patterns indicated by wood cellulose

General circulation model simulations suggest that during the Last Glacial Maximum, the northern circumpolar vortex intensifi ed and enlarged, a glacial anticyclone developed over the Laurentide Ice Sheet, and the position of the jet stream was shifted southward. However, observations directly related to shifts in wind patterns across the North American continent have not yet been reported. We examined tree-ring cellulose from the Holocene and the last glacial period for: (1) covariation between precipitation δ 18 O (and δD) and relative humidity, and (2) variation of cellulose δ 18 O and δD with longitude. Holocene isotopic features are consistent with modern moisture trajectories. The isotopic features during the last glaciation are dissimilar to those in the Holocene, and constitute direct evidence for an expansion of the polar easterlies to latitudes as low as 40°N. This is the fi rst time that moisture transport patterns have been inferred from covariation between isotopic composition in precipitation and relative humidity, a technique that holds much promise for future studies of atmospheric circulation.

Altitudinal trends of leaf δ13C follow different patterns across a mountainous terrain in north China characterized by a temperate semi‐humid climate

Many studies have documented that the delta(13)C values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta(13)C values of C(3) species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta(13)C with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta(13)C-altitude variations in C(3) plants. The delta(13)C of the leaves of C(4) plants is found to increase with altitude with a mean gradient of 0.9 per thousand/km. The altitudinal trend of C(4) plants is attributed to the combined influences of water availability and other factors rather than temperature.

Experimental measurements of leaf carbon isotope discrimination and gas exchange in the progenies of Plantago depressa and Setaria viridis collected from a wide altitudinal range

Significant correlations between leaf carbon isotope discrimination (Delta) and altitude and between gas exchange and altitude have been reported in previous studies, raising the question of whether the altitudinal variations in discrimination and gas exchange can be attributed to genetic differences among populations from different altitudes. Studies that focus on in situ analysis cannot distinguish the effects of genetic variation from environmental variation. This article describes an experiment in which seeds of Plantago depressa (C3 species) and Setaria viridis (C4 species) collected from a wide altitudinal range were grown in the same environment. Carbon isotopic ratios (delta(13)C) and gas exchange of the seedlings were measured. The progenies of P. depressa and S. viridis no longer display any significant Delta decreases with the altitude of origin as seen in situ. Furthermore, photosynthetic rate, stomatal conductance, the ratio of intercellular to ambient CO(2) and intrinsic water use efficiency for P. depressa and S. viridis grown in the greenhouse are also not significantly related to the altitude of origin. The observations suggest that altitudinal variations in Delta and gas exchange are not because of genotypic differences, independent of photosynthetic type.

OXYGEN ISOTOPE STUDIES OF ILLITE/SMECTITE AND CLINOPTILOLITE FROM YUCCA MOUNTAIN : IMPLICATIONS FOR PALEOHYDROLOGIC CONDITIONS

Abstract Illite/smectite (I/S) and clinoptilolite mineral separates from drill holes in 11 Ma old altered volcanic tuffs of Yucca Mountain, Nevada were analyzed for δ 18 O values. We reconstruct the diagenetic and paleohydrologic conditions using the isotopic composition of clay minerals combined with other independent geological observations. Our isotopic data show that the δ 18 O values of clay minerals preserve paleohydrologic information from a Middle to Late Miocene episode of hydrothermal alteration at Yucca Mountain. We provide additional evidence for a dual decoupled groundwater circulation system at Yucca Mountain ca. 11 Ma ago. A near-surface system dominated by downward percolation of groundwater is separated from a deeper hydrothermal system by profound thermal and isotopic discontinuities near the R0–R1 I/S transition. Given the set of inferred formation temperatures and the isotopic composition of formation water, we show that the current set of isotopic compositions of clinoptilolite is significantly lower than those at the time of formation, and are not significantly different from the equilibrium values defined by the current groundwater δ 18 O and geothermal gradient. We conclude that, unlike I/S, clinoptilolite has not preserved its original isotopic composition over the past 11 million years, but we do not know how long is necessary to reset the isotopic signature of clinoptilolite under near-surface conditions. The current groundwater at Yucca Mountain is 4%. more enriched in 18 O than the paleogroundwater 10–11 Ma ago as inferred from our reconstruction. This shift may be attributed to a change in the pattern of atmospheric circulation, such as by surficial uplift of the Sierra Nevada, which caused it to become an orographic barrier to moisture penetrating inland from the west.

Kinetics and mechanism of oxygen isotope exchange between analcime and water vapor and assessment of isotopic preservation of analcime in geological formations

The purpose of this study is to delineate the mechanism of isotopic exchange between zeolites and fluid. Two size fractions of analcime were exchanged with water vapor at 400°C and the isotopic change of framework oxygen with time was measured. The rate of exchange was faster for the smaller size fraction yet neither fraction reached equilibrium. SEM results indicate that dissolution plays no significant role in isotopic exchange between analcime and water vapor. The difference in rate with grain size indicates that isotopic exchange is limited by diffusion. The fact that equilibrium is not reached may be explained by structural changes upon dehydration which cause Na cations to migrate into former water sites. The isotopic exchange results for both size fractions can be fit by a diffusive-reactive model, which yields a diffusion coefficient of 2.8 ± .3 × 10−9 cm2/s and a combined rate/effective surface area constant of 0.011 ± .002 cm3/mol/s. The model predicts that the oxygen isotopic composition of most geologic analcime will have been changed since formation unless it is found in a young deposit (<10 MY) or has only been exposed to low temperatures (<30°C).

Influence of sample preparation on estuarine macrofauna stable isotope signatures in the context of contaminant bioaccumulation studies

The ratios of stable isotopes of carbon and nitrogen provide important information on food sources of aquatic organisms and trophic structure of aquatic food webs. For many studies, trophic position and food source are linked to bioaccumulation and trophic transfer of contaminants from prey to predators. In these cases, it is useful to use measurements on whole organisms to make direct comparisons of contaminant bioaccumulation and food web attributes. There is a great deal of variation in methods used for stable isotope analysis, particularly in the selection of tissue type and sample preparation prior to stable isotope analysis. While there have been aquatic studies that examined methodological differences, few have focused on estuarine organisms. In this study, the effects of depuration and tissue dissection on the stable isotope enrichment of common estuarine invertebrates and fish were examined. Homogenized tissues of non-depurated whole organisms were compared to dissected muscle tissue or depurated whole organisms. A 24 h depuration did not change the mean δ15N and δ13C values for most species examined. Additionally, as expected, significant differences in carbon and nitrogen signatures were found when muscle tissues were compared to whole organisms. However, differences were small enough that food source as inferred by δ13C or trophic level as inferred from δ15N would not be inaccurately represented (differences of <1.9‰ for δ13C and <1.2‰ for δ15N). The results of this study suggest that for these common estuarine fish and macroinvertebrates, stable isotopes ratios of samples can be analyzed without depuration in the same way as samples for contaminant analysis, but differences in tissue types must be taken into account when combining data from different sources.