Lipids and Hydrogen Isotopes

Aridity and vegetation composition are important determinants of leaf-wax dD values in southeastern Mexico and Central America

Peter M.J. Douglas, Mark Pagani, Mark Brenner, David A. Hodell, Jason H. Curtis

Leaf-wax hydrogen isotope composition (dDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling dDwax in the tropics remain poorly understood. We measured dDwax and the stable carbon isotope composition of leaf-waxes (d13Cwax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on dDwax. In this region the estimated hydrogen isotope composition of meteoric water (dDw) varies by only 25&, and variability in dDw does not explain the relatively large variance in dDwax. Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (ewax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in ewax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from d13Cwax, provides a secondary control on ewax/w for n-alkanoic acids (en-acid/w). Low d13Cn-acid values are associated with high en-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between d13Cn-alkane and en-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in dDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.


Environmental control on eastern broadleaf forest species’ leaf wax distributions and D/H ratios 

Brett J. Tipple, Mark Pagani

Local climate and environment broadly affect the deuterium/hydrogen (D/H)ratios of plant materials,however the degree to which an individual plant’s leaf waxes D/H ratios are affected by these parameters remains in question.Understanding these issues is particularly important in order to reconstruct past floral transitions and changes in the paleohydrologic cycle. For this study, we sampled five co-occurring tree species, Acer rubrum , Platanus occidentalis , Juniperus virginiana , Pinus taeda , and Pinus strobus and soils at forty sites along the East Coast of the US,from Florida to Maine. Hydrogen isotopic compositions of leaf wax n-alkanes, stem and surface waters were analyzed and compared against high-resolution temperature, precipitation, relative humidity,and vapor pressure deficit data to determine environmental controls on isotopic composition. Our results demonstrate that each tree species produce aunique distribution of n-alkanes with distinct chain length pattern. Average n-alkane chain lengths recovered from soils, A. rubrum , and J. virginiana leaves show significant correlations with mean annual temperature. dD values of A. rubrum leaf n-alkanes were strongly correlated to modeled mean annual precipitation dD values and other climate parameters related to latitude (i.e.temperature,relative humidity,vapor pressure deficit), while the dD values of J. virginiana n-alkanes were not. Differences in correspondence may reflect the timing of leaf wax synthesis between the two species.Further,soil n-alkane D/H compositions were strongly correlated to modeled mean annual precipitation dD values,while the apparent hydrogen isotopic fractionation was not.These findings indicate that the isotope ratio of n-alkanes from soils in Eastern North American forests and similar ecosystems likely represents atime-averaged value that smooth out the environmental influence any one plant experiences.


Leaf wax stable isotopes from Northern Tibetan Plateau: Implications for uplift and climate since 15Ma

Guangsheng Zhuang, Mark T. Brandon, Mark Pagani, Srinath Krishnan

The growth of Tibetan Plateau is considered to have played a key role during the evolution of Asian climate. Our understanding of the relationship between the plateau growth and Asian climate changes is limited, however, due to the scarcity of well-dated sedimentary sequences that could provide parallel information of the evolution of elevation and climate. Here, we report a high-resolution time series record of the stable hydrogen isotopic composition of leaf-wax n-alkanes from a continuous Neogene stratigraphic sequence (15–1.8Ma) from the Qaidam basin on the northern Tibetan Plateau. These data are used to reconstruct the isotopic composition of meteoric waters (δDm) and subsequently applied to interpret the history of paleotopography and climate in Qaidam.


Hydrogen isotope fractionation during lipid biosynthesis by Haloarcula marismortui

Sitindra S. Dirghangi, Mark Pagani

We studied the controls on the fractionation of hydrogen isotopes during lipid biosynthesis by Haloarcula marismortui, a halophilic archaea, in pure culture experiments by varying organic substrate, the hydrogen isotope composition (D/H) of water, temperature, and salinity. Cultures were grown on three substrates: succinate, pyruvate and glycerol with known hydrogen isotope compositions, and in water with different hydrogen isotopic compositions. All culture series grown on a particular substrate show strong correlations between dDarchaeol and dDwater. However, correlations are distinctly different for cultures grown on different substrates. Our results indicate that the metabolic pathway of substrate exerts a fundamental influence on the dD value of lipids, likely by influencing the D/H composition of NADPH (nicotinamide adenine dinucleotide phosphate), the reducing agent that contributes hydrogen to carbon atoms during lipid biosynthesis. Temperature and salinity have smaller, but similar effects on dDlipid, primarily due to the way temperature and salinity influence growth rate, as well as temperature effects on the activity of enzymes.


Hydrogen isotope fractionation during lipid biosynthesis by Tetrahymena thermophila marismortui

Sitindra S. Dirghangi, Mark Pagani

Hydrogen isotope ratio values of lipids are increasingly used to reconstruct past variation in hydrological conditions. However, apart from recording the hydrogen isotope composition of ambient water, dD values of lipids also depend on specific biosynthetic pathways and growth conditions. We have evaluated the hydrogen isotope fractionation by the ciliated protozoan, Tetrahymena thermophila, grown in pure culture at three temperatures (24 C, 30 C and 36 C) and in water with a range of hydrogen isotope composition. T. thermophila synthesizes tetrahymanol, a pentacyclic triterpenoid alcohol and the diagenetic precursor of the biomarker gammacerane. We focused our attention on the isotopic controls on tetrahymanol and various fatty acids (FAs). The dD values of FAs and tetrahymanol correlated linearly with the hydrogen isotope composition of water, but growth temperature was also clearly an important factor controlling lipid D/H composition. Hydrogen isotope fractionation during tetrahymanol biosynthesis changed with higher growth temperatures, resulting in D-depleted signatures relative to water at 24 C and 30 C and D-enriched composition at 36 C. T. thermophila grown at 36 C – a temperature above opitmum growth conditions – showed a significant change in lipid composition, with the abundance of tetrahymanol increasing relative to total FAs. We suggest the change is a response to temperature stress and a decrease in the stability of the cell membrane. The temperature effect is also presumed to impact dDlipid by altering the hydrogen isotopic composition of NADPH and potentially intracellular water.


Compound-Specific Stable Isotopes of Organic Compounds from Lake Sediments Track Recent Environmental Changes in an Alpine Ecosystem, Rocky Mountain National Park, Colorado

Sara K. Enders, Mark Pagani, Silvio Pantoja, Jill S. Baron, Alexander P. Wolfe, Nikolai Pedentchouk, Lilian Nunez

Compound-specific nitrogen, carbon, and hydrogen isotope records from sediments of Sky Pond, an alpine lake in Rocky Mountain National Park (Colorado, United States of America), were used to evaluate factors contributing to changes in diatom assemblages and bulk organic nitrogen isotope records identified in lake sediments across Colorado, Wyoming, and southern Montana. Nitrogen isotopic records of purified algal chlorins indicate a substantial shift in nitrogen cycling in the region over the past 60 years. Temporal changes in the growth characteristics of algae, captured in carbon isotope records in and around Sky Pond, as well as a 60 permil excursion in the hydrogen isotope composition of algal-derived palmitic acid, are coincident with changes in nitrogen cycling. The confluence of these trends is attributed to an increase in biologically available nitrogenous compounds caused by an expansion of anthropogenic influences and temporal changes in catchment hydrology and nutrient delivery associated with meltwater dynamics.


Carbon and Hydrogen Isotope Fractionation Under Continuous Light: Implications for Paleoenvironmental Interpretations of the High Arctic During Paleogene Warming

Hong Yang, Mark Pagani, Derek E.G. Briggs, M. A. Equiza, Richard Jagels, Qin Leng Ben A. LePage

The effect of low intensity continuous light, e.g., in the High Arctic summer, on plant carbon and hydrogen isotope fractionations is unknown. We conducted greenhouse experiments to test the impact of light quantity and duration on both carbon and hydrogen isotope compositions of three deciduous conifers whose fossil counterparts were components of Paleogene Arctic floras: Metasequoia glyptostroboides, Taxodium distichum, and Larix laricina. We found that plant leaf bulk carbon isotopic values of the examined species were 1.75 to 4.63% more negative under continuous light (CL) than under diurnal light (DL). Hydrogen isotope values of leaf n-alkanes under continuous light conditions revealed a D-enriched hydrogen isotope composition of up to 40% higher than in diurnal light conditions. The isotope offsets between the two light regimes is explained by a higher ratio of intercellular to atmospheric CO2 concentration (Ci/Ca) and more water loss for plants under continuous light conditions during a 24-h transpiration cycle. Apparent hydrogen isotope fractionations between source water and individual lipids (elipid-water) range from -62% (Metasequoia C27 and C29) to -87% (Larix C29) in leaves under continuous light.


Biomarker Reconstruction of the Early Eocene Paleotopography and Paleoclimate of the Northern Sierra Nevada

Michael T. Hren, Mark Pagani, Diane M. Erwin, and Mark Brandon

We reconstruct ancient temperature and elevation gradients across the early Eocene (52-49 Ma) northern Sierra Nevada (California, United States) using organic molecular proxies that record atmospheric and ground-level effects of topography. Paleoelevation was determined by reconstructing the change in the hydrogen isotopic composition of precipitation and mean annual temperature from the isotopic composition of fossil angiosperm leaf n-alkanes and the distribution of microbially produced soil tetraethers preserved in leaf-bearing sediments. Organic molecular data produce equivalent range-scale and channel paleoelevation estimates that show the northern Sierra Nevada was a warm (6 to 8 degrees C warmer than modern), high-elevation (>2km), and moderate- to low-relief landscape at the Eocene Climatic Optimum. Modern northern Sierra Nevada topography likely reflects post-Paleocene reduction of mean surface elevation and late Cenozoic increases in relief.


Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation

Carlos Jaramillo, Diana Ochoa, Lineth Contreras, Mark Pagani, Humberto Carvajal-Ortiz, Lisa M. Pratt, Srinath Krishnan, and others

Temperatures in tropical regions are estimated to have increased by 3 to 5 degrees C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.


A 35 Myr North American leaf-wax compound-specific carbon and hydrogen isotope record: Implications for C4 grasslands and hydrologic cycle dynamics

Brett J. Tipple and Mark Pagani

Hydrology, source region, and timing of precipitation are important controls on the climate of the Great Plains of North America and the composition of terrestrial ecosystems. Moisture delivered to the Great Plains varies seasonally and predominately derives from the Gulf of Mexico/Atlantic Ocean with minor contributions from the Pacific Ocean and Arctic region. For this work, we evaluate long-term relationships for the past ~35 million years between North American hydrology, climate, and floral change, using isotopic records and average carbon chain lengths of higher plant n-alkanes from Gulf of Mexico sediments. We posit that hydrological changes were linked to tectonic and oceanographic processes including the shoaling and closure of the Panamanian Seaway, amplification of North Atlantic Deep Water Production and an associated increase of meridional winds. The southerly movement of the Intertropical Convergence Zone near 4 Ma allowed for the development of a near-modern pressure/storm track system, driving increased aridity and changes in seasonality within the North American interior.


δ13C and δD Compositions of n-Alkanes From Modern Angiosperms and Conifers: An Experimental Set Up in central Washington State, USA

Nikolai Pedentchouk, William Sumner, Brett Tipple, and Mark Pagani

 This study investigates stable carbon and hydrogen isotope compositions of leaf wax n-alkanes from 3 angiosperm and 2 conifer species from a semi-desert natural environment. The δ13C and δD n-C27 alkane data show a clear distinction between Populus tremuloides/Syringa vulgaris (31.9 to 32.7 permil and 168 to 186 permil, respectively) and Pinus sylvestris/Picea pungens (28.8 to 30.6 permil and 190 to 212 permil) throughout the 2005 growing season. Like the other angiosperm species, Betula pendula was D-enriched (162 to 178 permil) relative to the conifer species. However, its δ13C values were more similar to those of Pinus sylvestris and Picea pungens in May, but had a 4 permil positive shift later in the season. We suggest that the observed isotopic variations derive mainly from lower stomatal conductance for COand H2O vapor in conifers relative to angiosperms. Betula pendula experienced severe environmental stress that affected its carbon metabolism for most of the season. Our results suggest that shifts in δD and δ13C values of sedimentary leaf waxes may result not only from shifts in the paleoclimatic variables but also from temporal shifts in the distribution of angiosperms and conifers as well as from changes in the isotopic signatures of higher plants undergoing metabolic changes.


Arctic Hydrology During Global Warming at the Palaeocene-Eocene Thermal Maximum

Mark Pagani, Nikolai Pedentchouk, Matthew Huber, Appy Sluijs, Stefan Schouten, Henk Brinkhuis, Jaap S. Sinninghe Damste, Gerald R. Dickens, and the IODP Expedition 302 Expedition Scientists

 

Sediments were recovered from the central Arctic Ocean, providing the first opportunity to evaluate the environmental response at the North Pole during the PETM. Here we present stable hydrogen and carbon isotope measurements of terrestrial-plant and aquatic-derived n-alkanes that record changes in hydrology, including surface water salinity and precipitation. Records are interpreted as documenting decreased rainout during hemispheric moisture transport and increased moisture delivery to the Arctic at the onset of the PETM, consistent with predictions of poleward storm track migrations during global warming. The terrestrial-plant carbon isotope excursion is substantially larger than those of marine carbonates. Previously, this offset was explained by a plant’s physiological response to increases in surface humidity. However, this mechanism is not an effective explanation in this wet Arctic setting, leading us to hypothesize that the true magnitude of the carbon isotope excursion and associated carbon input was greater than originally surmised. Greater carbon release and strong hydrological cycle feedbacks help explain the maintenance of this unprecedented PETM warmth.