Geochemical analysis of Cenozoic fossil conifers at high latitudes: Implications for molecular preservation and environmental change

Fossil materials record ancient life and their adapted environment. Arctic plant fossils are critical for our understanding of the Earth’s paleoenvironment when high latitudes were under ice-free conditions. All Arctic plant fossils in this research are conifers, plants conducive for morphological and molecular study because of their incredible genetic stability. Miocene (15 million year old) and Pliocene (5 million year old) conifer leaves were collected from Banks Island, Canada (Williams et al., 2008). Samples were analyzed and compared with Paleocene (60 million year old) and Eocene (45 million year old) samples from Axel Heiberg Island, Canada and with modern equivalent species from Washington D.C., USA (William et al., 2008). This paper has three main sample analyses. First, Pyrolysis-Mass Spectrometry-Gas Chromatography technology was used to detect organic volatile compounds. The amounts and types of organic volatile compounds provide further insights into the molecular preservation of the Miocene and Pliocene fossilized samples. Molecular preservation from this research was compared to previous research that used Scanning Electron Microscope observations of Paleocene and Eocene transverse sections to indicate extraordinary morphological preservation (Yang et al., 2005; Yang et al., 2007). Second, Miocene and Pliocene bulk peat were cross-referenced with known species in the region to reconstruct Arctic environmental changes between 5 million and 15 million years ago. Third, the ratios of three stable compounds were analyzed as biomarkers, essentially benchmarks for plant fossil preservation. However, biomarkers were inconclusive because of complications including age, species type, and environmental conditions. Overall, our analyses provide the first assessments of molecular preservation for these rare Arctic fossils which offer unique material for further paleoclimate analysis

Mimicking Early Stages Of Diagenesis In Modern Metasequoia Leaves Implications For Plant Fossil Lagerstätten

Remarkably well-preserved plants from rich fossil deposits (known as lagerstätten) provide insights into paleoenvironmental and paleoclimatic reconstructions. However, it is difficult to distinguish whether proxy signals from fossil lagerstätten represent original environmental information or modified data due to degradation. To better understand molecular and morphological changes of plant leaves over time, modern Metasequoia, a well-studied “living fossil,” was degraded to mimic early stages of diagenesis and then compared with its ancient counterpart. Two sample series were used: 1) a laboratory decay experiment with samples in a closed, controlled environment and 2) a natural decay experiment with samples collected from an open lacustrine environment. Morphological and anatomical changes during early diagenesis are evident in both decay series, as seen through SEM observations and through a new quantitative pixel-count evaluation method. The molecular and isotopic results indicate that the removal of polysaccharides collapses cellulose-based primary-walled parenchymatous cells, while cells with lignin-strengthened secondary cells walls remain intact much later in the decay process. This supports previous hypotheses that polysaccharide is significant in maintaining the three-dimensional structures found in plant fossil lagerstätten. Both modern laboratory and natural decay series show that diagenesis occurs quickly, suggesting that fossil samples must have been rapidly buried in order to avoid microbial growth. Therefore, given the overall fidelity of Metasequoia fossil lagerstätten, the approximately -24‰ δ13C values recorded in arctic Cenozoic Metasequoia fossils are likely due to physiological response to different ancient atmospheric conditions, and not due to microbial-based tissue decay during early diagenesis

Tissue Decay Tested in Modern Metasequoia Leaves: Implications for Early Diagenesis of Leaves in Fossil Lagerstätten

Sedimentary deposits yielding extraordinarily-preserved fossils (known as Lagerstätten) may provide significant insights into the physiology and environments of ancient plants, particularly when the fossils represent their original characteristics with limited diagenetic modifications. To better understand molecular, isotopic, and morphological changes during the early stages of diagenesis, degradation experiments were conducted in two time series: 1) a laboratory decay series using fungi on leaves over the course of a month and 2) a natural decay series with leaves collected from different stages of leaf senescence and early diagenesis. Both experiments used modern leaves of the dawn redwood Metasequoia glyptostroboides, referred to as a “living fossil” due to the morphological stability of the genus Metasequoia over the past 100 million years. Both decay series demonstrate that microbial degradation of polysaccharides occurs on extremely short timescales and results in cell collapse, first in the exclusively cellulose-based primary cell walls and then much later in the lignin-strengthened secondary cell walls. Despite morphological and molecular changes, the stable carbon isotopic composition of bulk leaves and n-alkanes remained virtually unchanged. Together, these findings suggest that: 1) rapid burial and tissue stabilization is essential in the formation of Lagerstätte fossils, 2) polysaccharides play a key role in maintaining three-dimensional fossil leaf structures and thus polysaccharide preservation implies rapid burial and minimal microbial degradation, and 3) carbon isotope signals, including at the molecular level, altered little during diagenesis. Thus, interpretations of physiological and environmental signals from conifer leaves in Lagerstätten are not likely impacted by early diagenesis

Novel hydrocarbon-utilizing soil mycobacteria synthesize unique mycocerosic acids at a Sicilian everlasting fire

Soil bacteria rank among the most diverse groups of organisms on Earth and actively impact global processes of carbon cycling, especially in the emission of greenhouse gases like methane, CO2 and higher gaseous hydrocarbons. An abundant group of soil bacteria are the mycobacteria, which colonize various terrestrial, marine and anthropogenic environments due to their impermeable cell envelope that contains remarkable lipids. These bacteria have been found to be highly abundant at petroleum and gas seep areas, where they might utilize the released hydrocarbons. However, the function and the lipid biomarker inventory of these soil mycobacteria are poorly studied. Here, soils from the Fuoco di Censo seep, an everlasting fire (gas seep) in Sicily, Italy, were investigated for the presence of mycobacteria via 16S rRNA gene sequencing and fatty acid profiling. The soils contained high relative abundances (up to 34% of reads assigned) of mycobacteria, phylogenetically close to the Mycobacterium simiae complex and more distant from the wellstudied M. tuberculosis and hydrocarbon-utilizing M. paraffinicum. The soils showed decreasing abundances of mycocerosic acids (MAs), fatty acids unique for mycobacteria, with increasing distance from the seep. The major MAs at this seep were tentatively identified as 2,4,6,8-tetramethyl tetracosanoic acid and 2,4,6,8,10-pentamethyl hexacosanoic acid. Unusual MAs with mid-chain methyl branches at positions C-12 and C-16 (i.e., 2,12-dimethyl eicosanoic acid and 2,4,6,8,16-pentamethyl tetracosanoic acid) were also present. The molecular structures of the Fuoco di Censo MAs are different from those of the well-studied mycobacteria like M. tuberculosis or M. bovis and have relatively 13C-depleted values (38a to48), suggesting a direct or indirect utilization of the released seep gases like methane or ethane. The structurally unique MAs in combination with their depleted-13C values identified at the Fuoco di Censo seep offer a new tool to study the role of soil mycobacteria as hydrocarbon gas consumers in the carbon cycle

Limits of oxygen isotope palaeoaltimetry in Tibet

Measurements of stable water isotopes (oxygen and hydrogen) are commonly used to estimate palaeoelevation and quantify past changes in surface height across Tibet. Isotope palaeoaltimetry is often based on simple Rayleigh fractionation of a “parcel of air”, but must make a considerable number of approximations and assumptions. In this paper, we elaborate on the practicability of oxygen water isotopes in palaeoaltimetry, and evaluate a recent challenge to the palaeoaltimetry community. First, we examine the isotopic composition of oxygen (ẟ18O) versus altitude relationship in a set of five topographic realisations of Tibet using an isotope-enabled palaeoclimate model for the mid-Eocene, a period where a variety of topographic ‘uplift’ models have been proposed, and compare it to modern relationships. Second, we investigate whether isotopic composition is a good predictor of more modest changes in topography, such as the introduction of a valley system or uplift of only part of the Tibetan region. The aim of the paper is not to perform a direct comparison to data, but to use the model to further refine knowledge of the strengths and limitations of using oxygen isotopes in palaeoaltimetry. We find that oxygen isotope palaeoaltimetry works surprisingly well, with the exception that it could not identify low elevation valley systems bounded by high elevations because the isotopic composition of the water in the air becomes depleted at the first high elevation that an air parcel passes over and does not recover when it descends into the valley. Hence, isotope-based elevations are biased towards mountain range peaks. Overall, the application of oxygen isotope palaeoaltimetry does have value, but would be further strengthened when employed together with isotope-enabled models. In conjunction with other techniques such as terrestrial thermal lapse rates and energy conservation approaches, over a wide spatial region, a more accurate and fully three-dimension view of complex palaeo-topography is increasingly possible, which will in turn improve the precision of these palaeoaltimeters

Algal biomarkers as a proxy for pCO2: Constraints from late Quaternary sapropels in the eastern Mediterranean

Records of carbon dioxide concentrations (partial pressure expressed as pCO2) over Earth’s history provide trends that are critical to understand our changing world. To better constrain pCO2 estimations, here we test organic pCO2 proxies against the direct measurements of pCO2 recorded in ice cores. Based on the concept of stable carbon isotopic fractionation due to photosynthetic CO2 fixation (Ɛp), we use the stable carbon isotopic composition (δ13C) of the recently proposed biomarker phytol (from all photoautotrophs), as well as the conventionally used alkenone biomarkers (from specific species) for comparison, to reconstruct pCO2 over several Quaternary sapropel formation periods (S1, S3, S4, and S5) in the eastern Mediterranean Sea. The reconstructed pCO2 values are within error of the ice core values but consistently exceed the ice core values by ca. 100 µatm. This offset corresponds with atmospheric disequilibrium of present day CO2[aq] concentrations in the Mediterranean Sea from global pCO2, equivalent to ca. 100 µatm, although pCO2 estimates derived from individual horizons within each sapropel do not covary with the ice core values. This may possibly be due to greater variability in local CO2[aq] concentration changes in the Mediterranean, as compared with the global average pCO2, or possibly due to biases in the proxy, such as variable growth rate or carbon-concentrating mechanisms. Thus, the offset is likely a combination of physiological or environmental factors. Nevertheless, our results demonstrate that alkenone- and phytol-based pCO2 proxies yield statistically similar estimations (P-value = 0.02, Pearson’s r-value = 0.56), and yield reasonable absolute estimations although with relatively large uncertainties (± 100 µatm)

Reconstructing past atmospheric CO2 levels from the stable carbon isotopic composition of general algal biomarkers

Because pCO2 will likely continue to rise with increasing energy needs from a growing world population, it is essential for the future to understand the precise relationship between pCO2 and climate (i.e. climate sensitivity). Although tremendous strides have been made over the past several decades, it remains a challenge to accurately reconstruct pCO2 over geologic time. This thesis aims to improve pCO2 reconstructions by developing a new proxy. This is achieved through taking new approaches to the pCO2 proxy based on the stable carbon isotopic composition (δ13C) of characteristic organic compounds produced by algae and can stored in sediments (known as biomarkers). Instead of using the more traditional approach of using species-specific biomarkers, in this thesis, so-called general algal biomarkers produced by a multitude of species are instead used, and are thus ubiquitously found, both geographically (location) and geologically (time). This thesis is divided in two parts. First, the feasibility of general algal biomarkers as a pCO2 proxy are developed. This is achieved by measuring the δ13C values of general algal biomarkers over a large pCO2 gradient created by marine volcanic CO2 seeps. Second, the application of the δ13C of general algal biomarkers for reconstructing pCO2 was evaluated over different geologic periods: glacial-interglacial cycles (past 200 thousand years), the mid-Miocene Climatic Optimum towards today (past 16 million years), and the Phanerozoic (past 500 million years). Overall, the findings of this thesis show that δ13C of general algal biomarkers has great potential for reconstructing pCO2 for much of Earth’s geologic history. Of these general algal biomarkers, the use of phytane goes furthest back in time and seems to be generally applicable for the reconstruction of pCO2. This proxy works best in well-mixed, open oceans where the source of the general compounds can be better constrained, i.e. originating from phytoplankton. The proxy should be used with caution during periods of pCO2 stress when the proxy is less sensitive to CO2 changes. Despite that much remains to be understood, pCO2 estimates based on the δ13C of general algal biomarkers are robust and consistent, and a useful addition to the existing collection of pCO2 proxies

Reconstructing past atmospheric CO2 levels from the stable carbon isotopic composition of general algal biomarkers

Because pCO2 will likely continue to rise with increasing energy needs from a growing world population, it is essential for the future to understand the precise relationship between pCO2 and climate (i.e. climate sensitivity). Although tremendous strides have been made over the past several decades, it remains a challenge to accurately reconstruct pCO2 over geologic time. This thesis aims to improve pCO2 reconstructions by developing a new proxy. This is achieved through taking new approaches to the pCO2 proxy based on the stable carbon isotopic composition (δ13C) of characteristic organic compounds produced by algae and can stored in sediments (known as biomarkers). Instead of using the more traditional approach of using species-specific biomarkers, in this thesis, so-called general algal biomarkers produced by a multitude of species are instead used, and are thus ubiquitously found, both geographically (location) and geologically (time). This thesis is divided in two parts. First, the feasibility of general algal biomarkers as a pCO2 proxy are developed. This is achieved by measuring the δ13C values of general algal biomarkers over a large pCO2 gradient created by marine volcanic CO2 seeps. Second, the application of the δ13C of general algal biomarkers for reconstructing pCO2 was evaluated over different geologic periods: glacial-interglacial cycles (past 200 thousand years), the mid-Miocene Climatic Optimum towards today (past 16 million years), and the Phanerozoic (past 500 million years). Overall, the findings of this thesis show that δ13C of general algal biomarkers has great potential for reconstructing pCO2 for much of Earth’s geologic history. Of these general algal biomarkers, the use of phytane goes furthest back in time and seems to be generally applicable for the reconstruction of pCO2. This proxy works best in well-mixed, open oceans where the source of the general compounds can be better constrained, i.e. originating from phytoplankton. The proxy should be used with caution during periods of pCO2 stress when the proxy is less sensitive to CO2 changes. Despite that much remains to be understood, pCO2 estimates based on the δ13C of general algal biomarkers are robust and consistent, and a useful addition to the existing collection of pCO2 proxies