INFLUENCE OF GAS PRODUCTION AND FILAMENT ORIENTATION ON STROMATOLITE MICROFABRIC

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1 PALAIOS, 2012, v. 27, p Research Article DOI: /palo.2011.p11-088r INFLUENCE OF GAS PRODUCTION AND FILAMENT ORIENTATION ON STROMATOLITE MICROFABRIC SCOTT A. MATA, 1 * CARA L. HARWOOD, 2 FRANK A. CORSETTI, 1 NATALIE J. STORK, 3 KATHRYN EILERS, 4 WILLIAM M. BERELSON, 1 and JOHN R. SPEAR 5 1 Department of Earth Sciences, University of Southern California, Los Angeles, California , USA, scottmat@usc.edu, fcorsett@usc.edu, berelson@usc.edu; 2 Geology Department, University of California, Davis, Davis, California 95616, USA, clharwood@ucdavis.edu; 3 Center for Integrative Geosciences, University of Connecticut, Storrs, Connecticut 06269, USA, njstork@gmail.com; 4 Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, Boulder, Colorado , USA, kateilers@gmail.com; 5 Environmental Science and Engineering Division, Colorado School of Mines, Golden, Colorado 80401, USA, jspear@mines.edu ABSTRACT Alternating light-dark laminae within stromatolites have been attributed to a phototactic response of the constituent microbial communities, whereby filaments orient vertically during the day and recline at night. This study examines the orientation of cyanobacterial filaments within a laminated siliceous stromatolite from a Yellowstone National Park hot spring to identify the controls on microfabric development and whether phototaxis plays a role. Results indicate that filament orientation is predominantly perpendicular or parallel to lamination, even when laminae are steeply inclined. Thus, phototaxis is not a significant control of microfabric development in these stromatolites. Vertical aspects of the fabric are dominated by hourglass-shaped filament bundles (hourglass structures) adjacent to rounded pores, rather than being defined by individual filaments. The rounded pores likely represent oxygen-rich bubbles generated during photosynthesis. Upon stabilization by filaments, upward buoyancy of the bubbles rotated the bundles toward a vertical orientation. Thus, vertical aspects of the fabric in this stromatolite result from buoyancy forces rather than phototaxis. Examples from the Neoproterozoic Beck Spring Dolomite reveal that similar subvertical hourglass structures are present in the rock record and may be better preserved than individual filaments. The presence of rounded pores (fenestrae) and hourglass structures in ancient microbialites, here termed the hourglass-associated fenestral fabric, can serve as an indication of biogenic influence in stromatolites, especially in the absence of preserved filaments, and may be an indication of oxygenic photosynthesis. INTRODUCTION The notion that photosynthetic filamentous microbes (e.g., cyanobacteria) orient toward the light is a basic tenet of microbialite studies (Monty, 1967; Gebelein, 1969; Golubic, 1973; Walter et al., 1972, 1976; Golubic and Focke, 1978) and finds support in the microstructure of some stromatolites. Modern hot spring and marine stromatolites can exhibit alternations of vertically and horizontally oriented filaments, with the interpretation that the filaments are oriented vertically under daylight conditions and horizontally at night (Monty, 1967; Gebelein, 1969; Golubic, 1973; Walter et al., 1972, 1976; Golubic and Focke, 1978). The vertical orientation of microbial filaments is attributed to a phototactic response in which the filaments orient themselves toward a light source, with the long axis of the filament orienting with the incident angle of incoming light (Fig. 1, Golubic, 1973; Kim et al., 2005). At the meso-scale, the alternation of filament orientation appears as alternating light-dark laminae, with the vertical filaments forming thicker light laminae and the horizontal filaments forming thinner dark laminae (Fig. 1, Golubic, 1973; Golubic and Focke, 1978). *Correspondingauthor. Published Online: April 2012 Copyright G 2012, SEPM (Society for Sedimentary Geology) /12/ /$3.00 This repeated pattern of thick light laminae and thin dark laminae is preserved in many ancient stromatolites, although in many instances filaments are typically not preserved (e.g., Grotzinger and Knoll, 1999). In rare instances, filamentous microfossils are preserved in silicified stromatolites (e.g., Barghoorn and Tyler, 1965; Cloud, 1965; Awramik, 1976; Awramik and Semikhatov, 1979), although a clear pattern of alternating vertical-horizontal filament microstructures is not always apparent (e.g., Zhongying, 1986; Seong-Joo and Golubic, 1998, 1999). Alternatively, some ancient stromatolites exhibit alternations of light laminae containing filamentous microfossils and dark laminae containing coccoid forms (Hofmann, 1975). Although some ancient stromatolites do preserve microfossils, the majority do not. With varying degrees of microfossil preservation in the rock record, interpreting the activity and organization of the microbial communities from ancient stromatolites requires a modern analogue that can provide insight into what features will actually be preserved. Modern marine stromatolites have limited utility as analogues for ancient stromatolites, especially for Precambrian stromatolites (e.g., Awramik and Riding, 1988; Fairchild, 1991; Grotzinger and Knoll, 1999) because most modern marine stromatolites form through the trapping, binding, and cementation of sand-sized sediment grains and have a crude, coarse lamination (Logan, 1961; Dravis, 1983; Dill et al., 1986; Macintyre et al., 2000; Reid et al., 2000; Dupraz and Visscher, 2005). Ancient marine forms, however, show a larger preference for in situ precipitation of minerals with lesser contributions from the accretion of sediment. This in situ precipitation results in a fine-scale, well-defined lamination that contrasts with that found in modern forms (Fairchild, 1991; Grotzinger and Knoll, 1999). While it would seem that modern marine stromatolites should form the base for the interpretation of ancient marine stromatolites, it appears that stromatolites from modern lake and hot spring environments may actually be better textural analogues for these ancient forms, even though they are not of a marine origin. This is because they form primarily by in situ precipitation of minerals, being calcareous or siliceous, and contain low levels of detrital material. They commonly exhibit a submillimeter-scale lamination, and stromatolite microstructure is mediated by mineral precipitation mechanisms (e.g., Walter et al., 1976; Osborne et al., 1982; Jones et al., 2000; Petryshyn and Corsetti, 2011). Stromatolites from a silica hot spring, Obsidian Pool Prime (Spear et al., 2005; Shock et al., 2005), in Yellowstone National Park, Wyoming (Fig. 2) exhibit light-dark lamina couplets (Fig. 3) that occur beneath a surface mat consisting of cyanobacteria (Pepe-Ranney et al., 2010, 2012). These couplets are characterized by the alternation of open networks of filaments within light laminae and dense networks of filaments within dark laminae (Fig. 4). These structures are excellent microstructural analogues for many ancient stromatolites, despite differences in mineralogy, because of their fine-scale lamination and lack of trapped and bound sediment grains. Rounded pores occur

2 PALAIOS STROMATOLITE FILAMENT ORIENTATION 207 FIGURE 1 Proposed relationship between filament orientation and lamination in stromatolites for microbial communities that are orienting with light (phototaxis). All filaments are vertically oriented, even on steeply dipping laminae (modified from Golubic, 1973). within open filament networks and resemble fenestrae similar to those generated from gas production during breakdown of organic matter (e.g., Choquette and Pray, 1970) or photosynthesis (Tebbutt et al., 1965; Von der Borch et al., 1977), and these rounded pores are bounded by hourglass-shaped filament bundles. Exceptional preservation and mineralization of microbial filaments within the stromatolites, in the absence of significant diagenesis or detrital input, allow for highresolution petrographic analysis. This study quantitatively characterizes the orientation of filament fabrics at the microfacies scale within these stromatolites to examine FIGURE 2 Map showing the location of Obsidian Pool Prime within Yellowstone National Park in Wyoming, United States, and the location of the Beck Spring Dolomite in southeastern California (specific localities of Harwood and Sumner, 2011). whether filaments within the laminae show a preferred vertical or subvertical orientation, which would be predicted if the fabric dominantly reflects a photic response by the surface cyanobacterial mat, or if the filament orientation is responding to other factors (e.g., nutrient diffusion, Petroff et al., 2010). In addition, this study examines the unique fenestral fabric of the stromatolite consisting of hourglass-shaped FIGURE 3 Thin section of the stromatolite from Obsidian Pool Prime, Yellowstone National Park, United States, used in this study. The six boxes mark portions of the stromatolite that were examined in detail for fabric and filament orientation. Numbers in boxes correspond to numbers utilized in the text when referring to portions of the stromatolite.

3 208 MATA ET AL. PALAIOS FIGURE 4 Photomicrographs of the microfacies and structural components of the stromatolite from Obsidian Pool Prime, Yellowstone National Park, United States. Filament network types are labeled as I, II, and III. A) Two successive laminae within the rounded pore microfacies. The upper lamina consists of an open filament network that surrounds rounded pores. Between adjacent pores are found hourglass-shaped filament bundles. Individual laminae can show an upward increase in filament density (bottom of photomicrograph), and rounded pores are typically concentrated near the base of laminae where filament density is the lowest. B) Scanning electron micrograph of the rounded pore microfacies showing that the rounded pore spaces are three-dimensional spherical features that are contoured by filaments. C) The grading laminae microfacies is defined by the gradual upward increase in filament density within each lamina. Filament density increases from bottom-left to top-right within each dipping lamina in this example. D) The nongrading laminae microfacies showing no distinct changes in filament density within the light laminae consisting of an open filament network. E) The ultraporous laminae microfacies containing a poorly silicified open filament network in which dark laminae are weakly defined by faint increases in filament density. F) The dense laminae microfacies showing a general lack of an open filament network and consisting primarily of densely packed filaments alternating with heavily silica-encrusted horizons.

4 PALAIOS STROMATOLITE FILAMENT ORIENTATION 209 filament bundles and rounded pores and compares it to similar fabrics from the Neoproterozoic Beck Spring Dolomite to explore its utility for the interpretation of phototaxis and oxygenic photosynthesis in the rock record. OBSIDIAN POOL PRIME STROMATOLITE LOCALITY Obsidian Pool Prime (OPP) is a siliceous hot spring located within Yellowstone National Park, Wyoming, in the Hayden Valley area (Fig. 2). It derives its name from the adjacent Obsidian Pool Hot Spring (Hugenholtz et al., 1998; Spear et al., 2005; Shock et al., 2005), and receives a portion of inflow from it. The pool waters cool from,75 uc near the vent to,45 uc near the outflow of the pool(shock et al.,2005; Berelson et al., 2011). The dissolved silica in OPP waters averages 5.7 mm, which exceeds the 2 mm concentration found in the adjacent Obsidian Pool, and the ph is 5.7 (Berelson et al., 2011; Pepe-Ranney et al., 2012). Stromatolites sampled from OPP are located along the northern and western rim of the pool. The growth rate and morphology (Berelson et al., 2012), as well as the constructing microbial communities are well documented (Pepe-Ranney et al., 2012). Surface communities of OPP stromatolites consist primarily of cyanobacteria (Pepe-Ranney et al., 2012). The stromatolites grow through the upward accretion of cyanobacterial filaments, which then become encrusted with silica and preserved as a microstructure of silicified tubes (Berelson et al., 2011). The mats generate laminae on the scale of days (Berelson et al., 2011). Asinglecoupletoflightanddarklaminaeformsapproximatelyevery 1.75 days (Berelson et al., 2011). This is consistent with previous estimates from stromatolites found at Column Spouter Spring, Fairy Creek Meadows, Yellowstone National Park, that suggest that lamina couplets form on the order of 1 2 days (Walter et al., 1976). MATERIALS AND METHODS Stromatolites were collected from Obsidian Pool Prime in Yellowstone National Park, United States (Fig. 2). Samples were slabbed, impregnated with blue epoxy to highlight porosity, and thin-sectioned to examine the meso- and microstructure. Multiple stromatolite samples were examined to define microfacies, although the same microfacies were recognized to varying degrees in all examined stromatolites. One representative stromatolite was chosen to examine in detail with regard to filament orientation. Five microfacies are recognized within these siliceous stromatolites: (1) rounded pore; (2) grading laminae; (3) nongrading laminae; (4) ultraporous laminae; and (5) dense laminae. These microfacies are based on filament density, filament structure, and porosity, and are defined below. Six regions of the stromatolite (,3 3 2 mm) were chosen that represent different growth periods of the stromatolite, as well as anterior (i.e., facing the center of the pool), central, and posterior (i.e., facing the pool rim) positions (Fig. 3). Image processing software (ImageJ, was used to measure the orientation of filaments and filament bundles within these six regions. Each region of the stromatolite was divided into a mm grid of boxes to examine filament fabrics in detail. Within light laminae, filament fabrics were examined in a total of 648 squares in the six selected FIGURE 5 Examples of filament orientation scores, with a score of 1 representing the highest possible degree of orientation and 5 representing no orientation. Orientation score 1 (not shown): All filaments in the same orientation. This is an idealized orientation that was not observed in any samples, so is not pictured. A) Orientation score 2: Almost all filaments in the same orientation with few filaments in different orientations. B) Orientation score 3: Most filaments in the same orientation with more than a few in different orientations; orientation still easily recognizable. C) Orientation score 4: Dominant orientation just recognizable, with many other filaments in different orientations. D) Orientation score 5: No recognizable orientation.

5 210 MATA ET AL. PALAIOS TABLE 1 Microfacies of the stromatolites from Obsidian Pool Prime, Yellowstone National Park, Wyoming, United States. Microfacies Laminae couplet thickness (mm) Components Description Porosity Lateral continuity Rounded pore I) Open filament network; II) dense filament network; III) brown siliceous cap Grading laminae I) Open filament network; II) dense filament network; III) brown siliceous cap Non-grading laminae I) Open filament network; II) dense filament network; III) brown siliceous cap Ultraporous I) Open filament network; II) dense filament network Dense II) Dense filament network; III) brown siliceous cap Thick light laminae and thin dark laminae couplet; filaments are arranged into hourglass-shaped bundles that contour rounded pore spaces. Thick light laminae and thin dark laminae couplet; couplet shows a general upwarddensening succession defined by increasing filament density and contact between light and dark laminae is gradational. Thick light laminae and thin dark laminae couplet; couplet shows no distinct trend for filament density within the couplet and contact between light and dark laminae is sharp. Thick light laminae with weakly developed dark laminae couplet; dark laminae generally defined by deflections and crooks in filaments that part light laminae; filaments are poorly silicified. Defined by the general absence of an open filament network; laminae are predominantly dark and alternate with brown, more silicified, horizons. High within pores; low within bundles High at base of couplet; low at top Moderate Very high Very low Low Moderate Moderate-high Low-moderate High regions of the stromatolite. A semi-quantitative score was assigned to each box depending upon the degree to which filaments showed a preferred orientation. A score of 1 represents the highest degree of orientation, while a score of 5 represents no recognizable orientation (Fig. 5). These scores were then averaged for each microfacies. For boxes in which a preferred filament fabric orientation could be readily recognized (orientation score 1 3), the angle of this orientation was measured relative to horizontal and relative to the lamination. No orientation data is presented for the dense laminae microfacies because only a small number of boxes were recorded with a high degree of orientation. Orientation data were eliminated for laminae dipping +/2 10u from horizontal or +/2 10u from vertical to avoid redundancy. This is because surface normal (i.e., perpendicular to lamination) and vertical filaments would generate similar values for low-angle laminae, and surface parallel filaments would be indistinguishable from vertical filaments for steep laminae angles. Of the 648 squares generated, 217 met these criteria. Orientation measurements were placed into 10u bins. Resulting graphs highlight zones that reflect +/2 10u from absolute horizontal and absolute vertical filament orientations, as well as zones +/2 10u from surface parallel and surface normal orientations. Surface parallel refers to orientations that parallel the angle of the underlying lamination, while surface normal refers to orientations that are perpendicular to the angle of the underlying lamination. The orientation of filament bundles and the individual filaments that comprise them were measured relative to horizontal and relative to lamination within the rounded pore microfacies for regions 1, 3, and 5. Regions 2, 4, and 6 lacked hourglass-shaped bundles in the rounded pore microfacies. Within regions 1, 3, and 5, the orientation of every bundle that was hourglass-shaped and spanned the vertical extent of the light laminae was measured. For each bundle orientation that was measured, 40 individual filaments were measured. Measurements were taken only from portions of the microfacies containing an open filament network and were focused on filament bundles that surround pore spaces. Due to alimiteddataset,nolaminationangleswereexcludedfromthisanalysis. Orientation measurements were then placed into 10u bins. Examples of similar textures and features found within the OPP stromatolite occur within stromatolitic facies of the Neoproterozoic Beck Spring Dolomite of southeastern California, United States (Harwood and Sumner, 2011). Silicified stromatolites within the formation yield filamentous and coccoid microfossils, suggesting a probable biogenic origin for these stromatolites (Licari, 1978). Samples were slabbed and examined in hand sample and in polished thick sections (,80 mm) for comparison with the Obsidian Pool Prime stromatolite. Folk s white card technique was used in some cases to highlight dark pigmented areas in the carbonate (Folk, 1987). All samples are petrographically well-preserved with minimal recrystallization, despite pervasive mimetic dolomitization (e.g., Zempolich et al., 1988, 1989). Recrystallization was evaluated based on standard interpretations of crystal size and shape distributions, crystal boundary geometries, and the distribution of pigments relative to crystals. RESULTS Stromatolite Microfacies Microfacies, and their constituent laminae within the Obsidian Pool Prime stromatolite, are comprised of three distinct filament networks: (I) open networks of filaments with a high porosity ( mm thick); (II) dense networks of filaments with low porosity (30 50 mm thick); and (III) brown, densely packed silica-encrusted filament networks, with essentially no porosity, that mark the uppermost surface of II (20 60 mm thick).thepresence,absence,orcombinationofthesefilament networks, in addition to the volume and distribution of internal porosity, define the microfacies found within the stromatolites (Table 1): (1) rounded pore (I, II, and III); (2) grading laminae (I, II, and III); (3) nongrading laminae (I and III); (4) ultraporous laminae (I and II); and (5) dense laminae (II and III). Filaments that comprise each microfacies are observed to be 3 5 mm indiameterandupto 230 mm inlengthintheobservedsample. The rounded pore microfacies consists of thick lamina couplets ( mm thick)comprisedofathickerlightlaminaandathinnerdark lamina (Fig. 4A). The internal structure of light laminae consists of open filament networks with bundled filaments that contour around circular, horizontal ellipsoid, and vertically elongate pore spaces that are mm in diameter in their longest dimension.pore spaces typically span the entire thickness of the light laminae. Filament bundles pinch upward around flat-topped pores and form symmetrical hourglass shapes, termed hourglass structures, around circular pore spaces. The central taper of these hourglass-shaped bundles may be as

6 PALAIOS STROMATOLITE FILAMENT ORIENTATION 211 thin as a few filaments, and is usually mm wide.bundlelengthis mm, roughly the same scale of adjacent pores. Scanning electron microscopy (SEM) of circular pore spaces reveals that they are spherical three-dimensional features (Fig. 4B). Laminae within the rounded pore microfacies exhibit limited lateral continuity and have high levels of porosity. Unlike all other microfacies, which occur at nearly all lamina dip angles, the rounded pore microfacies is rare on laminae dipping more than approximately 60u. The grading laminae microfacies is comprised of thin to thick lamina couplets ( mm thick)that show a gradual upward increase(i.e., grading) in filament density. This is capped by a brown silica-encrusted horizon (Fig. 4C). A sharp contact exists between underlying dense filament networks and overlying open networks, while the transition between underlying open networks and overlying dense networks is gradational. Lamina couplets exhibit moderate levels of lateral continuity and have high porosity at the base of a couplet and low porosity at the top of a couplet. The nongrading laminae microfacies is defined as thin to thick lamina couplets ( mm thick)in which the thicker light laminae are characterized by an open filament network that shows a density structure that does not change from its base to top (i.e., nongrading) (Fig. 4D). Dark, thinner laminae are commonly more highly silicified than lighter laminae and usually contain a dense network of filaments. Laminae show moderate to high levels of lateral continuity and have high porosity in the open filament networks and low porosity in the dense, silica-encrusted networks. The ultraporous laminae microfacies consists of very loosely packed filamentous laminae with an open filament network coupled with thinner, slightly denser laminae to comprise light-dark couplets ( mm thick)(fig.4e).thinner,denserlaminaeappeartobedefined by minor surface-parallel crooks and deflections in surface normal filaments. These laminae appear short-lived and are poorly established. Dense, silica-encrusted laminae as found in all other microfacies are conspicuously absent, and the majority of filaments within this microfacies are poorly silicified and therefore poorly preserved. This is the most porous of all the microfacies. Lamina couplets exhibit limited to moderate lateral continuity. The dense laminae microfacies is defined by the general absence of an open filament network, with a concomitant decrease in porosity and increase in density relative to other microfacies (Fig. 4F). Lamina couplets within this microfacies consist of a dense network of filaments capped by a more silicified horizon and range in thickness from 60 to 170 mm. Subtle changes in filament density and silicification define light and dark laminae. Filament Orientation Score of Microfacies Orientation score results for each microfacies reveal that the rounded pore and ultraporous microfacies have the lowest orientation scores, which indicates that filaments in these microfacies have the highest degree of orientation (Fig. 6). The average orientation score for the rounded pore microfacies is 2.61, while the ultraporous laminae microfacies has an average score of Higher average scores belong to the grading laminae, nongrading laminae, and dense laminae, with scores of 3.54, 3.75, and 4.44, respectively. At a first approximation, this data indicates that the rounded pore and ultraporous laminae microfacies show the highest degree of orientation. These two microfacies are notably the most porous of the microfacies, and the open filament networks that comprise them are more loosely packed than in the other microfacies. A strong contrast to this is the dense microfacies, which generally lacks an open filament network, has the lowest porosity, and has the highest average orientation score, reflecting a very low degree of orientation. While this proxy does address the degree to which each microfacies shows a preferred orientation, it does not show what that orientation is. FIGURE 6 Orientation score for the microfacies: rounded pore, ultraporous, grading, nongrading, and dense. Scores are arranged from lowest to highest, with the lowest score marking the highest degree of orientation and the highest score marking the lowest degree of orientation. Note that the highest degree of orientation is found in the rounded pore and ultraporous laminae microfacies, which also have the highest porosity. Filament Fabric Orientation Orientations of filaments relative to horizontal, and relative to lamination, show variability between microfacies (Fig. 7). In all data, a measurement of 90u to horizontal corresponds to vertical filaments, while 90u to lamination refers to filaments oriented perpendicular to the lamination (i.e., surface normal). The rounded pore microfacies shows a normal distribution with respect to horizontal with a peak in the 90u 100u bin; with respect to lamination, a prominent surface normal zone exists between 80u and 100u (Fig. 7A). The grading laminae microfacies shows no clear trend with respect to horizontal; however, with respect to the lamination there are elevated values in the zones of surface normal and surface parallel (Fig. 7B). The orientation of filaments with respect to horizontal in the nongrading laminae microfacies shows a normal distribution with a very prominent peak in the 70u 80u bin (Fig. 7C). Filaments in the nongrading laminae microfacies have two distinct bell-shaped distributions with respect to lamination centered around 80u 90u and 10u 20u, which correspond to surface normal and surface parallel, respectively. The ultraporous laminae microfacies shows similar bell-shaped orientation distributions relative to horizontal and relative to lamination, centered around 80u 100u (Fig. 7D). When the filament orientation data is compiled for all microfacies (except the dense microfacies, see Materials and Methods for why this was removed), the most prominent zone for filament orientation relative to horizontal occurs within 70u 100u (Fig. 8). The most prominent zone for filament orientation relative to the lamination is from 80u to 100u. Considering the peaks for the bins +/2 10u from 90u (80u 90u and 90u 100u), 26.2% of all boxes fell within these values when examining filament orientation relative to horizontal. By contrast, 38.0% of filament fabrics are +/2 10u from 90u relative to the lamination, indicating that a higher percentage of filaments are oriented perpendicular to the lamination than absolutely vertical. When expanded to +/2 20u from 90u (70u 90u and 90u 110u), 50.9% of all measured filament fabric angles fell within these values when examined relative to horizontal. Filament fabric relative to the lamination occurs within these values for % of all boxes. Therefore, there is a higher preference for filaments to be oriented perpendicular to the lamination, rather than oriented vertically. Rounded Pore Microfacies Filaments and Filament Bundles The orientation of the long axes of individual filaments within hourglass-shaped bundles of the rounded pore microfacies are normally distributed with respect to horizontal and to the lamination, and both

7 212 MATA ET AL. PALAIOS FIGURE 7 Binned orientation angles, relative to horizontal and relative to lamination, for the filament fabrics within the A) rounded pore microfacies; B) grading laminae microfacies; C) nongrading laminae microfacies; and D) ultraporous laminae microfacies. exhibit a distinct saddle located around 90u (Fig. 9A). This distribution spans all bins from 0u to 180u, withfilamentabundancedecreasing toward angles approaching parallel to horizontal or parallel to the underlying lamination. Relative to horizontal, filaments exhibit a saddle near the 80u 90u bin, with abundance peaks located around 50u 70u and 120u 140u.Relativetothelamination,thissaddleiscenteredon the 90u 100u bin and abundance peaks are located near 50u 70u and 110u 130u. The orientation of hourglass-shaped filament bundles within the rounded pore microfacies is more tightly distributed than the

8 PALAIOS STROMATOLITE FILAMENT ORIENTATION 213 FIGURE 8 Binned orientation angles, relative to horizontal and relative to lamination, for the filament fabrics for all microfacies, except the dense laminae microfacies. FIGURE 9 Binned orientation angles, relative to horizontal and relative to lamination, for A) individual filaments within hourglass-shaped filament bundles in the rounded pore microfacies; and B) filament bundles within the rounded pore microfacies.

9 214 MATA ET AL. PALAIOS FIGURE 10 Photomicrograph of filament bundles within the rounded pore microfacies. Note that the filament bundles are generally oriented vertical 5u off vertical (left bundle) and 13u off vertical (right bundle) while the lamination dips 15u to the right. orientation of individual filaments (Fig. 9B). Orientations of bundles are between 60u and 140u relative to horizontal and relative to lamination. The orientation of most filament bundles relative to horizontal is vertical to subvertical, with prominent peaks in the 70u 80u bin and the 90u 100u bin. The orientation of filament bundles relative to the lamination is more widely distributed and is centered around 80u 100u. Overall, hourglass-shaped filament bundles are oriented subvertically to vertically, but the orientation of individual filaments is much more variable (Fig. 10). DISCUSSION Factors Determining Filament Orientation The orientation scores reveal, at first approximation, that those microfacies with an open filament network and a high primary porosity show a higher degree of orientation than those with closely spaced filaments. The orientation of filament fabrics relative to horizontal and relative to lamination for each microfacies reveals that most of the microfacies do not show a preferred vertical or subvertical orientation, which would be expected for a conventionally defined phototactic response (Figs. 11A B). Rather, an orientation of 90u +/2 10u to lamination (i.e., approximately surface normal, Fig. 11C) either matches closely or exceeds the counts recorded for filament fabrics relative to horizontal. All the microfacies and filament orientation data show that there is not an overwhelming phototactic signal within the stromatolite, in contrast to previous stromatolite studies that claim filaments orient vertically toward light during the day (Monty, 1967; Gebelein, 1969; Golubic, 1973; Walter et al., 1972, 1976; Golubic and Focke, 1978). In fact, the total distributions of filament fabric relative to horizontal and relative to lamination are not easily distinguishable from each other. There may even be a slight preference for surface normal over vertical filament fabrics, which has been shown by Petroff et al. (2010) to be a response to nutrient diffusion (Fig. 11D). If filament fabric was influenced by phototaxis, a preference for vertical filaments would especially be expected in the most light-limited areas of the stromatolites, primarily the sides. However, on the sides of the stromatolite (region 2 of Fig. 3), filaments are preferentially oriented bimodally, perpendicular and parallel to the lamination (Fig. 12). This suggests that a different process, other than a photic response, may control a subvertical fabric in stromatolites, or alternatively, that the phototactic response has been overstated with respect to vertical orientation towards incident light. FIGURE 11 Possible relationships between filament orientation and lamination in stromatolites. A) and B) illustrate the expected vertical orientation for filaments with a phototactic response to a direct light source (modified from Golubic, 1973). B) and C) illustrate filaments growing normal to the lamination. Filaments in OPP stromatolites are neither strongly vertically oriented or perpendicular to the lamination direction, suggesting that they are not strongly recording a phototactic response. D) illustrates the more complex filament orientation, with a slight preference for surface normal orientation that characterizes the filament fabric of OPP stromatolites. The orientation of individual filaments within hourglass-shaped bundles versus filament bundle orientation provides insights into the processes controlling stromatolite fabric. The distribution of individual filament orientations within hourglass-shaped bundles in the rounded pore microfacies is nearly indistinguishable between those relative to horizontal and relative to lamination. This may indicate that there is no prominent process that preferentially orients the filaments relative to either horizontal or to the underlying lamination. It may also suggest that the difference between the lamina angle and horizontal is minimal, resulting in only subtle differences between the two datasets. The saddle centered around 80u 100u and peaks around 50u 70u and 110u 140u in both datasets likely reflect the tapering angles of filaments within the observed filament bundles. Filaments that comprise these hourglassshaped bundles are widely spaced at the base and top of the bundles, but converge on the central portion of the bundle. A peak on either side of 90u simply reflects the fact that filaments are converging on the central axis of the bundle from both sides. Although individual filaments in the rounded pore microfacies do not have an overall preferential orientation, hourglass-shaped bundles of filaments are preferentially oriented vertically (90u +/2 10u). This suggests that a subvertical to vertical fabric in these stromatolites is not produced by a phototactic response by individual filaments, but rather reflects the orientation of bundles of filaments adjacent to rounded pores. The Origin of Rounded Pores and Fenestral Fabrics The origin of fenestral fabrics in ancient sedimentary rocks has been highly debated, especially in regard to those associated with ancient stromatolites. Fenestral fabrics consist of primary voids and pores that are larger than could be sustained by surrounding sediment (Tebbutt et al., 1965) and are fabric selective (Choquette and Pray, 1970), as opposed to fractures and vugs that do not necessarily follow rock fabric. While it is clear that fenestrae represent the preservation of primary pore space, due to either early lithification of the surrounding sediments, or to early filling of voids with cement before compaction, the mechanisms behind the void generation are varied and may be

10 PALAIOS STROMATOLITE FILAMENT ORIENTATION 215 FIGURE 13 Scanning electron micrograph of a spherical pore in the rounded pore microfacies. Silicified filaments contouring the edges of the pore are interpreted as representing stabilization of a gas bubble produced on the surface of the stromatolite. FIGURE 12 Orientation of filaments on the base and side of the stromatolite in an area with very steeply dipping laminae (region 2 of Fig. 3). There is no preference for vertical filaments even on these steeply dipping laminae in a light-limited area of the stromatolite. A) Photomicrograph showing light-dark lamination couplets with filaments oriented nonvertically, and closely resembling the schematic of Figure 11D. B) Binned orientation angles for the filament fabrics within region 2. Filaments are preferentially oriented perpendicular and parallel to lamination. unique to each type of fenestrae (Ham, 1952; Tebbutt et al., 1965; Wolf, 1965; Shinn, 1968; Choquette and Pray, 1970; Logan, 1974; Shinn, 1983). Laminoid fenestral fabrics (sensu Tebbutt et al., 1965) in which voids are irregular, horizontally elongate, and parallel to bedding are generally regarded as forming either through shrinkage during intertidal or supratidal desiccation or through expansion due to gas produced through the breakdown and decay of a laminar microbial mat (Choquette and Pray, 1970). Highly rounded pores appear most closely allied with irregular fenestrae (Logan, 1974; Grover and Read, 1978), similar to keystone vugs (Dunham, 1970), that are generally equidimensional and do not orient with bedding. These fenestrae are commonly interpreted as the preserved remnant of bubbles generated through the evolution of gas which may be microbially derived from photosynthesis (Tebbutt et al., 1965; Von der Borch et al., 1977) or the breakdown of organic matter (Cloud, 1960; Tebbutt et al., 1965) or due to emergent conditions and desiccation (Illing, 1959; Shinn, 1968; Dunham, 1970; Bain and Kindler, 1994). Often, these more rounded and regular pore spaces have been termed birdseyes (Shinn, 1968); however, birdseyes have become more synonymous with fenestrae (e.g., Choquette and Pray, 1970). AuniquetypeoffenestralfabricwasdocumentedbyBosaketal. (2009) in which round fenestrae are concentrated at the peaks of conical stromatolites with ages ranging from the Archean through the Mesoproterozoic. These fenestrae form a linear series of stacked rounded pores that highlight the central axis of the conical stromatolite represented by the peaks of upward projecting chevrons, and are rare in portions of the laminae away from the central axis. These fenestrae are interpreted by Bosak et al. (2009) as preserved fossil oxygen-rich bubbles that formed during photosynthesis concentrated at the tips of growing conical-shaped aggregates of microbial filaments. This seems a valid interpretation as modern filamentous cyanobacteria are known to be capable of surrounding and enmeshing oxygen-rich bubbles generated during photosynthesis, and this process leads to the formation and preservation of rounded pores within a meshwork of filaments (Bosak et al., 2010). Fenestrae (i.e., rounded pores) from the stromatolites of Obsidian Pool Prime in Yellowstone National Park are most easily interpreted as fabrics created around photosynthetically derived gas bubbles. This interpretation is based on the observation that the filaments contour around these spherical features (Fig. 13). The pore-contoured filaments indicate a primary origin and a window of formation during which the filaments were still motile or at least still flexible, which is consistent with the findings of Bosak et al. (2010) that filaments are capable of producing, capturing, and stabilizing bubbles. Gas bubbles forming on, and within, laminae of OPP stromatolites significantly influenced the orientation of filaments and filament bundles during the formation of laminae within the stromatolite. While filament bundles in the rounded pore facies show

11 216 MATA ET AL. PALAIOS apreferenceforaverticalorientation,thefilamentsthatcomprisethem do not (Fig. 13). This may be due to the upward buoyancy of gas bubbles that preferentially rotate filament bundles into a vertical orientation as they exert an upward force. The general absence of the rounded pore microfacies from laminae dipping steeper than 60u may be due to the absence of significant gas production on steeply dipping limbs, the production of micro-bubbles that could be more subject to gaseous dissolution into the aqueous phase, or may be due to an inability to trap bubbles generated on surfaces steeper than this angle. Rounded pores and associated hourglass structures also provide information about environmental conditions that allowed for textural preservation. OPP stromatolite laminae lithified very rapidly, on the scale of days (Berelson et al., 2011). Extremely rapid lithification is likely required to preserve the morphology of a bubble as a rounded pore, and to prevent collapse of vertically oriented filaments and bundles. High levels of dissolved silica in Obsidian Prime Pool facilitated very rapid silica precipitation and the stabilization of microbial filaments (Berelson et al., 2011). Bosak et al. (2010) also suggest that syngenetic precipitation of crystals in the biomass surrounding bubbles is required for preservation. As in OPP stromatolites, the rounded pores generated during the trapping of gas bubbles remain open (Bosak et al., 2010) and are later filled in with cements, like in the Beck Spring stromatolites presented below. Thus, the presence of rounded pores and associated hourglass structures in ancient stromatolites requires not only motile filamentous microbial communities producing and trapping gas bubbles, but also rapid lithification in order to preserve the fabric. The rapid lithification required for preservation of hourglass structures and rounded pores may prevent this fabric from being widespread in ancient stromatolites. Hourglass Structures and Rounded Pores as a Biosignature The fabric and porosity generated by hourglass structures associated with rounded pores is most closely allied with fenestral fabric porosity, as it is fabric selective and follows lamination rather than cutting it. However, the rounded pore microfacies is distinctly different from typical fenestral fabrics and is herein termed hourglass-associated fenestral fabric. While most fenestral fabrics are defined by the shape and arrangement of pore spaces, the fabric of the Yellowstone stromatolites is defined uniquely by the hourglass-shaped structures formed from filament bundles that occur in association with pore spaces. Rounded pores within the stromatolite are highly regular, follow lamination, and only rarely exhibit irregular or horizontally elongate morphologies due to the lateral amalgamation of pore spaces. Hourglass structures are typically mm at the central taper and contour rounded pores that are approximately mm in diameter in their longest dimension. The hourglass-associated fenestral fabric generated by these hourglass structures and associated rounded pores can be differentiated from other fenestral fabrics based on a few unique features. Hourglassassociated fenestral fabrics are distinct from laminoid fenestral fabrics due to the paucity of laterally amalgamated fenestrae that generate horizontally elongate and irregularly shaped voids (Figs. 14A B). FIGURE 14 A comparisonoftheroundedfenestralfabriccontaininghourglass structures with similar porosity fabrics. A) Rounded fenestral fabric with hourglass structures generated within the stromatolite due to the orientation of microbial filament bundles (left) and the resulting fabric that may be found in a rock if the morphology of the fabric were preserved without the preservation of individual filaments (right). B) Laminoid fenestral fabric consisting of horizontally elongate irregularly shaped pore spaces that orient with and follow the host fabric. C) Irregular fenestral fabric comprised of equidimensional pore spaces that are round to irregular and do not orient with, but do follow, the host fabric. D) Keystone vugs are pore spaces typically formed in sandstone, are not fabric selective, and can cut across primary depositional features.

12 PALAIOS STROMATOLITE FILAMENT ORIENTATION 217 FIGURE 15 Hourglass structures (arrows) and rounded pores in stromatolites from the Neoproterozoic Beck Spring Dolomite; photomicrographs, plane-polarized light. A) Well-defined subvertical hourglass structure spans the distance between successive stromatolite laminae. Arrow points down the long axis of the hourglass structure. B) Hourglass structures with only a fragmentary portion connected with overlying laminae preserved (marked by arrows). C) Less well-preserved hourglass structures defined by dark pigments in fine-grained carbonate were likely more degraded prior to lithification (marked by arrows). D) Higher magnification of hourglass structure in (C). Hourglass-associated fenestrae can also be differentiated from irregular fenestrae in that they show a high degree of regularity in their distribution, unlike irregular fenestrae that do not orient with bedding (Fig. 14C). Highly rounded keystone vugs that resemble hourglassassociated fenestrae usually occur in clean sandstones, typically of a sandy foreshore or backshore environment, and are not fabric selective (Dunham, 1970; Bain and Kindler, 1994). More commonly, keystone vugs cut inclined stratification (Bain and Kindler, 1994), which differs from the highly regular and fabric selective nature of the hourglassassociated fenestral fabric (Fig. 14D). If the hourglass-associated fenestral fabric can be unambiguously identified in the rock record, then it can serve as a biosignature. Stromatolites in exceptionally preserved shallow marine carbonates of the Neoproterozoic Beck Spring Dolomite (e.g., Harwood and Sumner, 2011) display rounded pores associated with hourglass-shaped structures (Fig. 15). These rounded fenestrae are interpreted to result from the lithification of filaments bundled around gas bubbles based on the similar fabric of hourglass-shaped bundles and rounded pores in OPP stromatolites. Hourglass structures in Beck Spring stromatolites, defined by the presence of dark pigments in carbonate, are mmatthecentral taper, with rounded pore spaces approximately mm indiameterin their longest dimension (Fig. 15). As in the OPP stromatolites, hourglass structures in Beck Spring stromatolites are subvertical to vertical, and connect stacked laminae. Thus, they are structurally similar in size and geometry to OPP stromatolite hourglass structures. The utility of the hourglass-associated fenestral fabric is that it is not restricted solely to deposits in which distinct microfossils are found. Beck Spring Dolomite stromatolite microfabrics demonstrate that it is possible to recognize this fabric in the rock record in the absence of discrete filaments. Beck Spring stromatolites do not preserve individual filaments in isolation or in filament bundles, yet the rounded pore and hourglass structure fabric is easily recognized. Its recognition is contingent primarily on the presence of hourglass structures that appear to have a limited size distribution, being typically less than 90 mm at the central constriction and within the vertical scale of the lamination. The preservation of rounded pores without significant horizontal amalgamation is another hallmark of this fenestral fabric, which is atypical for other fenestral fabrics in which amalgamation of adjacent pores is common. The presence of the hourglass-associated fenestral fabric reflects the upward growth of microbial filaments around gas bubbles or the generation of gas bubbles within a network of filaments, resulting in filament deflection. At present, there are no porosity fabrics in the rock record or in the modern record that perfectly resemble or reproduce all the features generated from the contouring of microbial filaments around gas bubbles to form hourglass structures, and therefore it is unique. Interpreting Phototaxis and Photosynthesis from Ancient Stromatolites Microbial microfossils are not preserved in the majority of ancient stromatolites (e.g., Grotzinger and Knoll, 1999). Thus, interpreting

13 218 MATA ET AL. PALAIOS microbial behaviors including phototaxis and microbial metabolisms in the rock record is difficult. In the absence of preserved microfossils, other morphologic and microstructural features have been used as evidence of phototaxis. One such feature is the thickening of laminae over crests within stromatolites (e.g., Gebelein, 1969; Walter et al., 1976; Buick et al., 1981; Walter, 1983). This is typically interpreted as migration of microbial filaments toward a light source, moving up inclines and concentrating near crests, which mark the highest points on the stromatolite at a given point in time (e.g., Gebelein, 1969; Walter et al., 1976; Buick et al., 1981; Walter, 1983). The other feature commonly used to interpret phototaxis is the alternation of thick light laminae and thin dark laminae, which is interpreted as the vertical orientation of filaments during the day as they reach toward the light and the horizontal orientation of filaments at night when the light stimulus is removed, respectively (Monty, 1967; Gebelein, 1969; Golubic, 1973; Walter et al., 1972, 1976; Golubic and Focke, 1978). While actual filamentous microfossils do show some support for vertical-horizontal filament fabrics in the rock record (e.g., Seong-Joo and Golubic, 1998, 1999), the majority of ancient stromatolites show the alternations of light and dark laminae, but lack the microfossils. In the absence of microfossils, largerscale fabrics are all that remain for deciphering the potential behaviors, phototactic or otherwise, of microbes within ancient stromatolites. Based on the concept that phototaxis influences stromatolite fabrics, subvertical to vertical structures in ancient stromatolites might be interpreted as microbial filaments orienting with incoming light. Alternatively, we suggest that this pattern may represent the trapping of gas bubbles generated during photosynthesis. The resulting vertical fabric is a product of the orientation of filament bundles as the gas exerts an upward force due to buoyancy. Even without the presence of individual filaments, as in Beck Spring stromatolites, the distinct and well-defined fabric of the hourglass-associated fenestral fabric allows recognition of ancient microbial influences and clarifies the potential origin of some ancient vertical-dominant fabrics in the rock record. In the case of the OPP stromatolite, vertically oriented microfabrics actually reflect filament bundles contouring around gas bubbles rather than a physical photic response of microbial communities. Herein, we have proposed that the hourglass-associated fenestral fabric represents the interaction of microbial mats with gas bubbles. In the modern examples from Yellowstone, the gas bubbles clearly originate from oxygenic photosynthesis, as we have noted throughout the text. On the one hand, it is possible that any process that produces gas (microbial degradation as well as oxygenic photosynthesis) could foster the formation of the hourglass-associated fenestral fabric. However, the very nature of the fabric suggests the gas was formed in situ within the laminae; that is, it did not form via degradation of underlying mats, as no gas escape structures are noted in the fabric. Furthermore, the degradation of biomass from the lamination itself would not provide enough gas to create the bubbles. Therefore, we cautiously suggest that an hourglass-associated fenestral fabric may specifically be the hallmark of oxygenic photosynthesis, and as such, would provide a method to track the presence of oxygenic photosynthesis in the rock record, where conical stromatolites may or may not be present (e.g., Bosak et al., 2009). CONCLUSIONS The alternation of laminae with vertically and horizontally oriented filaments within stromatolites is commonly interpreted as a phototactic response in which the microbes grew upward vertically during the day and reclined at night. Quantitative analysis of filament fabric orientation within a siliceous stromatolite from Obsidian Pool Prime, Yellowstone National Park, Wyoming, reveals that fabrics in which filaments are oriented perpendicular to the lamination match or exceed fabrics with vertically oriented filaments. This suggests that either phototaxis is not important in influencing filament fabric, or that the case for phototaxis driving vertically oriented filaments has been overstated in the past. Alternatively, hourglass-shaped bundles of filaments termed hourglass structures within the rounded pore microfacies are preferentially vertically oriented, although the constituent filaments that form the bundles show a wide distribution of orientations with no preference. Rounded pores associated with hourglass-shaped bundles are interpreted as preserving the morphology of gas bubbles that were stabilized by filamentous microbial communities. Thus, the vertical orientation of filament bundles is likely due to the filament bundles trapping oxygen-rich gas bubbles generated during photosynthesis. These bubbles would exert an upward-directed force due to buoyancy and may therefore have deflected or supported the filament bundles into a vertical orientation. Thus, vertical fabrics in the stromatolite likely reflect filament bundles associated with gas production rather than phototaxis. Rounded pores and associated hourglass structures of the rounded pore microfacies define a new style of fenestral fabric, termed the hourglass-associated fenestral fabric, which is easily distinguishable from similar fenestral fabrics and vugs. It consists of highly regular and well-defined rounded pores spaces separated by thin hourglass structures. Pore spaces are typically less than 300 mm in diameter within both modern and ancient examples, while the hourglass structures range from 10 to 90 mm attheircentralconstriction. In ancient stromatolites, the pore spaces are fabric selective and within the scale of the lamination because they reflect processes that occurred contemporaneously with stromatolite growth and lithification. Stromatolites from the Neoproterozoic Beck Spring Dolomite reveal that rounded pores and hourglass structures can be recognized in the ancient rock record and therefore may serve as an important biosignature, especially in ancient stromatolites in which microbial filaments are not preserved and biogenicity is ambiguous. ACKNOWLEDGMENTS We would like to thank and acknowledge the Yellowstone Center for Resources for all their help and assistance through the development of this project, and would like to thank the 2010 International Geobiology Course for the stimulating discussions and guidance that made it all possible. We would also like to thank T. Bosak, J.P. Zonneveld, and two anonymous reviewers for their constructive and helpful reviews that greatly strengthened this manuscript. Funding was provided by the Agouron Institute, the Gordon and Betty Moore Foundation, NASA Exobiology, and the National Science Foundation. 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