Andrew D. Czaja

Laser-Raman imagery of Earth's earliest fossils

Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil ‘lookalikes’ can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of lifes history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser–Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.

Iron Isotope Composition of Particles Produced by UV-Femtosecond Laser Ablation of Natural Oxides, Sulfides, and Carbonates

The need for femtosecond laser ablation (fs-LA) systems coupled to MC-ICP-MS to accurately perform in situ stable isotope analyses remains an open question, because of the lack of knowledge concerning ablation-related isotopic fractionation in this regime. We report the first iron isotope analysis of size-resolved, laser-induced particles of natural magnetite, siderite, pyrrhotite, and pyrite, collected through cascade impaction, followed by analysis by solution nebulization MC-ICP-MS, as well as imaging using electron microscopy. Iron mass distributions are independent of mineralogy, and particle morphology includes both spheres and agglomerates for all ablated phases. X-ray spectroscopy shows elemental fractionation in siderite (C-rich agglomerates) and pyrrhotite/pyrite (S-rich spheres). We find an increase in (56)Fe/(54)Fe ratios of +2‰, +1.2‰, and +0.8‰ with increasing particle size for magnetite, siderite, and pyrrhotite, respectively. Fe isotope differences in size-sorted aerosols from pyrite ablation are not analytically resolvable. Experimental data are discussed using models of particles generation by Hergenröder and elemental/isotopic fractionation by Richter. We interpret the isotopic fractionation to be related to the iron condensation time scale, dependent on its saturation in the gas phase, as a function of mineral composition. Despite the isotopic variations across aerosol size fractions, total aerosol composition, as calculated from mass balance, confirms that fs-LA produces a stoichiometric sampling in terms of isotopic composition. Specifically, both elemental and isotopic fractionation are produced by particle generation processes and not by femtosecond laser-matter interactions. These results provide critical insights into the analytical requirements for laser-ablation-based stable isotope measurements of high-precision and accuracy in geological samples, including the importance of quantitative aerosol transport to the ICP.

Comment on “Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation”

Guilbaud et al. (Reports, 24 June 2011, p. 1548) suggest that the geologic record of Fe isotope fractionation can be explained by abiological precipitation of pyrite. We argue that a detailed understanding of the depositional setting, mineralogy, and geologic history of Precambrian sedimentary rocks indicates that the Fe isotope record dominantly reflects biological fractionations and Fe redox processes.

A phylogenetic view of low-level CAM in Pelargonium (Geraniaceae)

Crassulacean acid metabolism (CAM) is common in several plant families and is often associated with succulence. Few studies have examined the occurrence of CAM from a phylogenetic perspective. The genus Pelargonium is promising for such a study because members are characterized by dramatic variation in growth form (including geophytes, shrubs, and stem succulents) and because growth form diversity is expressed to the greatest extent in a monophyletic group comprising 80% of Pelargonium species. This clade, predominantly from the winter rainfall region of southern Africa, likely proliferated in response to Miocene or Pliocene aridification. We present a survey for CAM across Pelargonium, emphasizing the winter rainfall clade. Dawn/dusk fluctuations in titratable acidity were examined in 41 species, with detailed measurements of carbon uptake and stomatal conductance under progressive water stress in four species. No species exhibited obligate CAM. When well-watered, most species exhibited stomatal conductances and acid fluctuations characteristic of C(3) photosynthesis, though some exhibited more pronounced increases in nocturnal acidity, suggesting CAM cycling. In four species examined during dry-down, water stress led to increased nighttime acid levels and decreased daytime stomatal conductance. Ultimately, stomata closed and external carbon uptake ceased, consistent with CAM idling. These results are discussed from the perspective of the evolution of CAM flexibility.

Sulfur-oxidizing bacteria prior to the Great Oxidation Event from the 2.52 Ga Gamohaan Formation of South Africa

The first 2 b.y. of Earth’s history was an important time for life when microbes evolved and diversified into essentially all of the metabolic forms that now exist. Because of feedbacks between biology and the surface environment, understanding Earth’s biological history can help us understand the evolution of Earth itself. The morphological and geochemical evidence for this ancient biological history is sparse but is increasing. Here we report evidence for 2.52 Ga exceptionally large, organic, smooth-walled, coccoidal microfossils preserved in a deep-water black chert in the Gamohaan Formation of the Kaapvaal craton of South Africa. These fossils occur mainly as compressed solitary coccoids that range in size from 20 to 265 µm but occasionally occur in short chains of cells. Morphologically these fossils are similar to Proterozoic and Phanerozoic acritarchs and to certain Archean fossils interpreted as possible cyanobacteria. However, their exceptionally large size, simple cell wall microstructure, and paleoecological setting, as well as multiple sulfur isotope systematics of pyrite within the unit, suggest that the Gamohaan Formation fossils were sulfur-oxidizing bacteria similar to those of the modern genus Thiomargarita , organisms that live in anoxic and sulfidic deep-water settings. These are the oldest reported fossil sulfur bacteria and reveal a diversity of life and ecosystems, previously only interpreted from geochemical proxies, just prior to the Great Oxidation Event, a time of major atmospheric evolution.

New method for the microscopic, nondestructive acquisition of ultraviolet resonance Raman spectra from plant cell walls.

Raman spectroscopy has long been used for the chemical analysis of organic matter, including natural products, using excitation wavelengths in the visible, infrared, or ultraviolet portions of the spectrum. The use of ultraviolet resonance Raman spectroscopy (UVRR) to study bulk samples of plant tissue has typically been carried out by rotating homogeneous macro-samples beneath the laser beam in order to minimize the amount of UV radiation impinging on any one spot, thereby avoiding its potentially damaging effects on the organic matter analyzed. This paper extends the use of UVRR to the study on a microscopic scale of individual plant cell walls by use of the controlled micro-displacement of a sample.

Late Cretaceous marine arthropods relied on terrestrial organic matter as a food source: Geochemical evidence from the Coon Creek Lagerstätte in the Mississippi Embayment

The Upper Cretaceous Coon Creek Lagerstätte of Tennessee, USA, is known for its extremely well-preserved mollusks and decapod crustaceans. However, the depositional environment of this unit, particularly its distance to the shoreline, has long been equivocal. To better constrain the coastal proximity of the Coon Creek Formation, we carried out a multiproxy geochemical analysis of fossil decapod (crab, mud shrimp) cuticle and associated sediment from the type section. Elemental analysis and Raman spectroscopy confirmed the presence of kerogenized carbon in the crabs and mud shrimp; carbon isotope (δ13 C) analysis of bulk decapod cuticle yielded similar mean δ13 C values for both taxa (-25.1‰ and -26‰, respectively). Sedimentary biomarkers were composed of n-alkanes from C16 to C36 , with the short-chain n-alkanes dominating, as well as other biomarkers (pristane, phytane, hopanes). Raman spectra and biomarker thermal maturity indices suggest that the Coon Creek Formation sediments are immature, which supports retention of unaltered, biogenic isotopic signals in the fossil organic carbon remains. Using our isotopic results and published calcium carbonate δ13 C values, we modeled carbon isotope values of carbon sources in the Coon Creek Formation, including potential marine (phytoplankton) and terrestrial (plant) dietary sources. Coon Creek Formation decapod δ13 C values fall closer to those estimated for terrigenous plants than marine phytoplankton, indicating that these organisms were feeding primarily on terrigenous organic matter. From this model, we infer that the Coon Creek Formation experienced significant terrigenous organic matter input via a freshwater source and thus was deposited in a shallow, nearshore marine environment proximal to the shoreline. This study helps refine the paleoecology of nearshore settings in the Mississippi Embayment during the global climatic shift in the late Campanian-early Maastrichtian and demonstrates for the first time that organic δ13 C signatures in exceptionally preserved fossil marine arthropods are a viable proxy for use in paleoenvironmental reconstructions.