Henry S. Chafetz

Travertines; depositional morphology and the bacterially constructed constituents

ABSTRACT Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced ound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 µm in diameter and less than 1.0 µm in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 µm in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminat d layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.

Bacterially induced lithification of microbial mats

Below the photic zone within live microbial mats from tidal flats, the cyanobacteria are dead and bacteria are the dominant living biota. Field data indicate that precipitation of calcium carbonate occurs predominantly within the mats in the aphotic zone. In order to determine whether bacteria could be responsible for inducing the precipitation of calcium carbonate within the microbial mats, live microbial mats and bacteria were collected from modern tidal flats. Over 50 experiments were set up in the laboratory in which live, naturally dead, and sterilized (autoclaved) dead filamentous cyanobacteria were inoculated with bacterial cultures

Marine peloids; a product of bacterially induced precipitation of calcite

High-magnesian calcite peloids are a common constituent of cemented marine carbonate accumulations. They are abundant and well developed within both isolated microcavities and surface crusts. The nuclei of marine peloids are composed of fossil bacterial clumps encased within anhedral, submicron- to micron-sized, high-magnesian calcite crystals. The marine peloids are similar to silt-sized particles found in travertine deposits that also are composed of fossil bacterial clumps and their surrounding calcite coronas. These similarities include size (20-60 mu m), a cloudy brown nucleus, and rims of clear euhedral crystals. In addition, previous studies have established the presence of fatty acids of bacterial origin within peloidal deposits, and enriched stable isotopic signatures have been interpreted as recording organic fractionation concomitant with calcite precipitation. These characteristics all indicate that the nuclei of many peloids originate as calcite precipitates within and around clumps of bacteria and that this precipitation was induced by the vital activity of bacterial colonies.

Habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy

ABSTRACT Marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature, which results in single crystals and aggregates of crystals (crystal bundles). The end-member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms, the most common of which is the dumbbell. Rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and, consequently, may serve to identify bacterially induced precipitates in the rock record. Viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted. The medium viscosity controls the ion diffusion rate and thus the rate of precipitation and, consequently, the mineral precipitated. In a liquid medium, circulation and the ion diffusion rate are both high, precipitation is rapid, and aragonite forms. In a gelatinous medium, there is little circulation, the ion diffusion rate is slow, the rate of precipitation is slow, and calcite is precipitated. The overall form of the resultant precipitate, rods or spheres, was the same regardless of the mineral composition.

Bacterial shrubs, crystal shrubs, and ray-crystal shrubs: bacterial vs. abiotic precipitation

Hot-water travertine deposits commonly contain shrub-like morphologies, on the centimeter to meter scale, that range from highly irregular forms (bacterial shrubs) to features that display regular geometric patterns (crystal shrubs and ray-crystal crusts). The bacterial shrubs have been previously recognized as the product of bacterially induced precipitation whereas the ray-crystal crusts have been described as due to dominantly abiotic precipitation. The bacterial shrubs have a highly irregular morphology, similar to woody plants, in contrast, end member crystal shrubs display distinct crystal habits and regular repeating morphologies, i.e., they commonly have attributes of noncrystallographic as well as crystallographic dendrites. There is a complete gradational relationship between the bacterial shrubs and crystal shrubs. Ray-crystal crusts are large fan-shaped features composed of subparallel, extremely coarse bladed crystals of calcite. All three types (bacterial shrubs, crystal shrubs, and ray-crystal crusts) contain either a dense tangle of bacterial body fossils and/or micron-sized pores. The micropores are bacterial molds. The immediately enveloping spar crystals around all three morphologic types are devoid of bacterial body fossils and micropores. The bacterial shrubs, crystal shrubs, and ray-crystal crusts are, to differing degrees, the product of bacterially induced precipitation as well as abiotic mineral precipitation. The differences in morphology are due to the relative contribution of bacterially induced precipitation as compared with that of abiotic mineral precipitation. Bacterially induced precipitates can form in environments too chemically harsh or otherwise inhospitable for other taxa to thrive. Epiphyton and Renalcis commonly formed in occult environments under conditions inhospitable to other taxa. Consideration of the published descriptions and interpretations concerning Epiphyton and Renalcis, and by analogy the origin of bacterial shrubs, have led to the conclusion that Epiphyton and Renalcis also are bacterially induced precipitates.

Polymeric substances and biofilms as biomarkers in terrestrial materials: Implications for extraterrestrial samples

Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment, biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the Early Archaean (3.5 b.y.). We describe FPS and biofilms from hot spring, deep-sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilms are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.

Syndepositional shallow-water precipitation of glauconitic minerals

Abstract Numerous studies have demonstrated that glauconitic minerals predominantly form in water depths of mid-shelf to upper slope in modern oceans. These areas tend to have slow sedimentation rates, another commonly cited requisite for glauconitic mineral precipitation. Cambro-Ordovician strata from the southwestern US are rich in glauconitic minerals. Stratigraphic, sedimentological, and petrographic constraints indicate that the glauconitic minerals are autochthonous. In marked contrast to the modern environments of deposition, these Cambro-Ordovician strata formed under very shallow-water to tidal-flat conditions. The trough cross-stratified deposits of the most glauconitic mineral-rich accumulations (glaucarenites) indicate a high energy environment and probably a normal to high rate of sedimentation. The presence of fibroradiated rims of glauconitic minerals on glauconitic mineral pellets, echinoderm fragments, and quartz grains demonstrates that the Cambro-Ordovician glauconitic minerals precipitated on or in close proximity to the sea floor and prior to calcite precipitation. Consequently, glauconitic minerals must have formed under markedly different conditions in the lower Paleozoic than they do today. Thus, the occurrence of glauconitic minerals in the rock record cannot be used a priori as an environmental indicator of either mid-shelf and deeper water and/or a slow rate of sedimentation.

Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA

Numerous siliceous hot spring systems in the Norris and Lower Geyser Basins of Yellowstone National Park, Wyoming, provide insights into spring geometries, depositional facies, and lithofacies associated with modern hot springs. Analyses of active (Cistern Spring, Octopus Spring, Deerbone Spring, and Spindle Geyser) and inactive (Pork Chop Geyser) siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at Yellowstone. Yellowstones siliceous springs tend to group into four broad morphological categories: siliceous spires and cones, domal mounds, terraced mounds, and ponds. Siliceous spires/cones are subconical accumulations up to 5–7 m high and about 2 m in diameter, and are common deposits in Yellowstone Lake. Domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry (2–3 m in vertical relief), discharge channels, and an areal accumulation of approximately 150 m2. In contrast, terraced mounds have a stair-step morphology, a substantial pool (∼8–10 m in diameter), “shrubby” precipitates, and occupy areas of ∼2000 m2. Siliceous ponds are variable in size, have little outflow, and exhibit low amounts of silica precipitation. Of these morphological varieties, domal mounds and terraced mounds are thought to have the best long-term preservation potential. The four spring morphotypes are composed of up to eight cumulative hot spring depositional facies: (1) vent (>95 °C), (2) proximal vent (<95 °C), (3) pool (∼80–90 °C), (4) pool margin (∼80 °C), (5) pool eddy (<80 °C), (6) discharge channel/flowpath (<80 °C to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures). This facies model based on numerous springs facilitates our ability to interpret ancient hot spring deposits and to infer depositional conditions. Precipitation of siliceous sinter is the result of abiotic and biotic processes. Abiotic precipitational processes are dominant in the vent area, whereas biotic influences on the precipitate fabric become progressively more important downflow.

Bacterially Induced Microscale and Nanoscale Carbonate Precipitates

Bacteria are able to form carbonate rocks and minerals at all scales, from deposits many meters thick, to distinctive shrubs, to minute crystal forms. They are particularly common in peloids, stromatolites, and hot-water travertines. The peculiar crystal morphologies they produce can be duplicated in the laboratory. Nanobacteria are much smaller forms, spheroids 0.03–0.1 µm in diameter. A quantitative census of nanobacterial bodies in limestones from Holocene to Proterozoic, and in micrite vs ooids vs sparry calcite show that the abundance is enormously variable. In a 4 µm2 area, most samples studied contain between one and 16 bacterial bodies; the median value is about four. Bacteria are significant producers of carbonate deposits.

Pisoliths (Pisoids) in Quaternary Travertines of Tivoli, Italy

The “type locality” for travertine is at Tivoli, about 25 km east of Roma. In fact, the name “travertine” descends from the Latin, “Lapis Tiburtinus”, referring to the stone quarried at Tibur (the ancient name for Tivoli). Quarries in the area around Tivoli have been active for two millenia, and the stone has been used for most of the famous monuments of Rome ranging from the Coliseum through St. Peter’s to the modern University. Today there are a great many active quarries, and travertine from Tivoli is exported worldwide.