Temporal and environmental significance of microbial lamination: insights from Recent fluvial stromatolites in the River Piedra, Spain CONCHA ARENAS, BRIAN JONES &# $"$"!' !"$$ "$#&"#$' " ( " (! !"$$ "$ $ #!"#&"#$' "$ $ "$ "# %("# ------------------------------------ 1 1 2 Temporal and environmental significance of microbial lamination: insights 3 from Recent fluvial stromatolites in the River Piedra, Spain 4 5 CONCHA ARENAS1*, BRIAN JONES2 6 7 1 Division of Stratigraphy, Department of Earth Sciences, University of Zaragoza. 50009 8 Zaragoza, Spain 9 2 Department of Earth and Atmospheric Sciences, University of Alberta. Edmonton, Alberta 10 T6G 2E3, Canada 11 12 *Corresponding author. E-mail address: [email protected]. 13 14 Associate Editor - Cathy Hollis 15 Short Title – Temporal and environmental significance of microbial lamination 16 ------------------------------------ 17 18 19 20 21 22 23 24 25 26 27 28 2 29 30 31 ABSTRACT 32 Despite extensive research, the environmental and temporal significance of microbial 33 lamination is still ambiguous because of the complexity of the parameters that control its 34 development. A 13 year monitored record of modern fast-accreting calcite stromatolites 35 (mean 14 mm/year) from artificial substrates installed in rapid-flow in the River Piedra (NE 36 Spain) allows comparison of the sedimentological attributes of successive six-month 37 depositional packages with the known climatic, hydrophysical, and hydrochemical 38 parameters of the depositional system. The stromatolites are formed of dense, porous and 39 macrocrystalline composite laminae. The dense and porous composite laminae, which are 40 composed of two to eight laminae consisting largely of calcified cyanobacteria, are 41 characterized by: (i) dense composite laminae, up to 15 mm thick, mostly with successive 42 dense laminae and minor alternating dense and porous laminae, and (ii) porous composite 43 laminae, up to 12 mm thick, consisting mainly of porous laminae alternating with thinner 44 dense laminae. Most of the dense composite laminae formed during the warm periods (April 45 to September), whereas most of the porous composite laminae developed in the cool periods 46 (October to March). Each dense and porous composite lamina represents up to or slightly 47 longer than six months. The alternation of these two types of composite laminae parallels 48 seasonal changes in temperature. The dense and porous laminae result from shorter (e.g., 49 intraseasonal) variations in temperature, insolation and hydrological conditions. The 50 macrocrystalline laminae, with crystals >100 µm long, occur isolated and grouped into 51 composite laminae up to 1.7 mm thick. Their occurrence suggests the absence or poor 52 development of microbial mats over periods of weeks to several months. Thus, stromatolite 53 lamination can record different-order, periodic and non-periodic changes in magnitude of 3 54 environmental parameters over a single year. These results hold important implications for 55 the temporal and environmental interpretation of lamination in microbial structures. 56 57 Keywords: Environmental parameters, fluvial carbonate deposits, microbial and non- 58 microbial laminae, seasonal and intraseasonal variations, stromatolite lamination, textural 59 cyclicity 60 61 INTRODUCTION 62 The analysis of laminations in ancient microbialites has been the focus of many studies 63 (Monty, 1976; Casanova, 1994; Zamarreño et al., 1997; Riding, 2000; Suárez-González et al., 64 2014). Interpretation of the environmental and temporal significance of these laminations is, 65 however, commonly ambiguous because of the complexity of the physical, chemical and 66 biological parameters that collectively control their development (Hofmann, 1973; Seong-Joo 67 et al., 2000; Storrie-Lombardi & Awramik, 2006; Petryshyn et al., 2012). The situation is 68 further complicated by diagenetic processes that may modify the original textural (Park, 69 1976; Golubić et al., 2008). 70 Hofmann (1973) noted that stromatolite lamination could be related to several cycles of 71 different duration (e.g., daily, short tidal cycles, monthly or seasonal), the origin of which 72 could be astronomic (gravitational and climatic), geologic, or biologic. Stable-isotope (13C 73 and 18O) records from laminated microbialites commonly reveal climatic and associated 74 hydrological changes on various time scales (i.e. seasonal and pluriseasonal; Andrews & 75 Brasier, 2005; Kano et al., 2007; Kremer et al., 2008; Osácar et al., 2013; Tang et al., 2014; 76 Arenas et al., 2015). Although the recognition of such changes based on textural attributes 77 and/or thicknesses of the laminae is not always straightforward (e.g. Kremer et al., 2008; 4 78 Brasier et al., 2011), daily or nyctohemeral laminations have been identified in some 79 stromatolites on the basis of textural variations (Gebelein, 1969; Monty, 1978; Wright & 80 Wright, 1985). 81 The role of microbes and extracellular polymeric substances (EPS) on carbonate 82 precipitation in modern microbial mats has been reviewed by Dupraz et al. (2009) and 83 Spadafora et al. (2010). Under rapid CO2 degassing conditions, like those found in fast-flow 84 and agitated water conditions, carbonate precipitation is more influenced by physico- - 85 chemical processes than in still water conditions where microbial CO2- and HCO3 -uptake 86 may be more important (Merz-Preiß & Riding, 1999; Arp et al., 2001). In terms of 87 cyanobacterial calcification, sheath encrustation dominates in fast-flowing conditions, 88 whereas sheath impregnation dominates where calcification is slower (Merz-Preiß & Riding, 89 1999). Nonetheless, precipitation below the surface mat related to other bacteria that degrade 90 organic matter is also important. There, lamina formation has also been attributed to 91 degradation of cyanobacterial biomass, phototrophic sulphide oxidation and sulphate 92 reduction (Vasconcelos et al., 2013). Grain trapping and binding by microbes are also 93 important, particularly in the formation of coarse-grained, agglutinated stromatolite 94 laminations (Riding, 2000; Reid et al., 2000; Suárez-Gonzaléz et al., 2014). 95 Despite the varied factors and processes involved in the origin of laminations, the 96 “petrographic” result is quite simple and similar in most modern and fossil examples (Monty, 97 1976). Monty (1976) defined different types of laminations in stromatolites on the basis of 98 the cyclic or recurring pattern of the laminae that are defined by their colour, crystal size and 99 porosity. In most ancient and recent fine-grained microbialites, the most common 100 arrangement is an alternation of dense, dark laminae and porous, light laminae that has 101 commonly been considered a yearly record. In many cases, this yearly record reflects 102 seasonal variations in precipitation and/or temperature that have been described in continental 5 103 (Casanova, 1994; Zamarreño et al., 1997; Matsuoka et al., 2001; Ihlenfeld et al., 2003; Kano 104 et al., 2003, 2004; O'Brien et al., 2006; Brasier et al., 2011) and marine environments 105 (Kremer et al., 2008; Tang et al., 2014). These parameters can also operate over other time 106 spans (e.g. Petryshyn et al., 2012) and other astronomic factors may also influence marine 107 microbialite lamina formation (e.g., tidal cycles, Hofmann, 1973). 108 The accretion rates of oncolites and stromatolites growing in modern fluvial carbonate 109 systems are high (4 to 14 mm/year in stromatolites) with a variety of laminae commonly 110 forming within several months (Ordóñez et al., 1980; Gradziński, 2010; Vázquez-Urbez et al., 111 2010; Manzo et al., 2012; Arenas et al., 2014). The cyclicity of such laminae and their 112 temporal and environmental significance, however, have not been fully explained. 113 This paper focuses on stromatolites that form under rapid flow conditions in a fluvial 114 environment (i.e. tufa system). It is based on deposits that accumulated on artificial substrates 115 placed in the River Piedra (northeastern Spain) from 1999 to 2012 (Fig. 1A). During this 13- 116 year time span, the deposits were examined and their thicknesses measured every six months 117 (at the end of winter and at the end of summer). This information allows correlation of the 118 lamina/stromatolite development with the known climatic, hydrochemical, and hydrophysical 119 attributes of the system. 120 Using information collected from the River Piedra, this paper (i) describes the main 121 structural and textural attributes of the laminae in the stromatolites and discusses the 122 factors/parameters that controlled their formation, and (ii) relates the variations in textural 123 features and the different styles of lamina to environmental parameters that operate at 124 different time scales in response to a variety of intrinsic and extrinsic factors. Integration of 125 all the available information provides a model for the development of laminae in fluvial 126 stromatolites and allows discussion on their temporal significance. These results also carry 127 important implications for the interpretation of laminae found in other microbial structures. 6 128 LOCATION, GEOLOGICAL, CLIMATIC AND HYDROLOGICAL SETTINGS OF 129 THE STUDIED SITE 130 Geographical, geological and climatic context 131 The River Piedra is a 41 km long indirect tributary of the River Ebro that flows south to north 132 across the Iberian Range, which is located in the northeastern part of the