Geochemical compositional mapping of Lower Jurassic trace fossils: palaeoenvironmental significance and methodological implications

Geochemical compositional mapping of Lower Jurassic trace fossils: palaeoenvironmental significance and methodological implications Reolid, J., Reolid, M. Highlights XRF compositional maps show geochemical differences between burrows and host sediment BSE images show relatively high content of pyrite and organic matter within burrows Environmental conditions for preserving OM were better within burrow galleries Burrow-water reducing conditions acted as traps for organic matter preservation Compositional maps are useful for testing geochemical heterogeneity in bioturbated rocks 1 1Geochemical compositional mapping of Lower Jurassic trace fossils: 2palaeoenvironmental significance and methodological implications 3 4Reolid, Jesúsa; Reolid, Matíasb* 5 6aDepartamento de Estratigrafía y Paleontología, Universidad de Granada, Spain. email: 7[email protected] 8bDepartamento de Geología, Universidad de Jaén, Spain. email: [email protected] 9 10 * Corresponding author 11 12 ABSTRACT 13 Analyses of X-ray microfluorescence (XRF) elemental maps of the ichnofossil 14 assemblages from the upper Pliensbachian-lower Toarcian marl-limestone rhythmite of 15 the South Iberian Palaeomargin (Betic Cordillera, SE Spain) show that the compositions 16 of burrow infills are geochemically and mineralogically different with respect to the 17 surrounding sediment. XRF elemental maps and back-scattered electron images show 18 pyrite framboids within the burrows and enrichment in Fe and S relative to the host 19 sediment. This is congruent with the position of burrows beneath the redox boundary at 20 the time of their active or passive sedimentary infilling. Organic matter that accumulated 21 at the seafloor was not preserved due to oxidation and consumption by benthic organisms. 22 However, burrows located beneath the redox boundary served as traps for organic matter 23 preservation. The composition of burrow fills reflects the original signal of the redox 24 conditions of pore-waters below the sediment-water interface, which was not preserved 25 in the surrounding sediment. The XRF elemental maps also show that some trace fossils 2 26 from limestone and marly-limestone intervals contain high concentrations of Si, Al, K, 27 Zn, Cr, and Ti with respect to the surrounding sediment. This may reflect enrichment of 28 clay minerals in the burrow fills derived from overlying marly layers. This work proposes 29 the use of elemental imaging of trace fossils prior bulk-rock geochemical analyses in 30 order to evaluate the potential for biogenic heterogeneity and elemental partitioning 31 before sampling. Geochemical analysis of trace fossils represents an additional tool that 32 may be of use in hydrocarbon exploration. 33 34 KEYWORDS: Organic matter; redox conditions; X-ray fluorescence; 35 Pliensbachian;Toarcian; ichnofossil 36 37 1. Introduction 38 39 It is widely known that burrows may be characterised by redox-related 40 microenvironmental conditions that differ from those at the seafloor (Kristensen, 2000; 41 Wetzel, 2010; Wetzel and Uchman, 2012; Gilbert et al., 2016), which can affect bacterial 42 communities and metabolism (Waslenchuk et al., 1983; Gibert et al., 2016). Sediments 43 infilling burrows may be deposited under different environmental conditions than those 44 at the sediment-water interface. In addition, most tracemakers live in oxic bottom-water 45 and sediments with oxic pore-waters, but some tracemakers are tolerant of low pore-water 46 oxygen levels. These include the tracemakers of Chondrites, interpreted to live at the 47 aerobic-anoxic interface as a chemosymbiotic organism (Ekdale and Bromley, 1984; 48 Seilacher, 1990; Fu, 1991), and Trichichnus (McBride and Picard, 1991). Moreover, the 49 burrowers can modify redox conditions within the sediment and control the mobility of 50 redox-sensitive elements (Kristensen, 2000; Harazim et al., 2015). 3 51 The different conditions between sediment infilling the burrow and the 52 surrounding sediment favour the recognition of burrows in sediments when they clearly 53 contrast with the host sediment. Different conditions within burrows are evidenced in the 54 fossil record by different textures, grain sizes, organic matter (OM) contents, and colours 55 of burrow fills compared to that of host sediment. Taphonomy must be considered 56 because erosion, compaction, and diverse diagenetic factors may have significant impacts 57 on the material preserved in the trace fossils (Hallam, 1975). Additionally, the role of the 58 bioturbation and early diagenesis in the origin of nodularity from condensed sections has 59 been widely studied (e.g. Fürsich, 1973, 1979; Eller, 1981; Reolid et al., 2015). 60 Ichnofabrics provide information on palaeoecological and depositional conditions during 61 sedimentation such as oxygenation, sedimentation rate, type of ground, and availability 62 of trophic resources. The sediment pore-water oxygenation is one of the parameters that 63 strongly controls the infaunal assemblages and the ichnofabrics in mud substrates (e.g. 64 Savrda, 2007). Ichnology has proven to be a useful tool for palaeoenvironmental 65 interpretations, basin analysis, and even reservoir characterization (Knaust, 1998; Buatois 66 and Mángano, 2011; Ekdale et al., 2012; Giraldo-Villegas et al., 2016; Rodríguez-Tovar 67 et al., 2017, Reolid and Betzler, 2019). Some studies have attempted to connect 68 ichnological attributes to geophysical and geochemical proxies such as the natural gamma 69 radiation (Reolid and Betzler, 2018; Reolid et al., 2019a). However, systematic analyses 70 of the geochemical composition of trace fossils and host rocks from marls and marly 71 limestones are scarce (Izumi, 2013; Izumi et al., 2014; Harazim et al., 2015). 72 The aim of this work is to use X-ray microfluorescence elemental maps in order 73 to recognise geochemical differences between trace fossils and their host sediments and 74 to assess their significance in palaeoenvironmental reconstructions and applied geology. 75 This work shows differences in the geochemical compositions between trace fossils and 4 76 their host sediment in an upper Pliensbachian-lower Toarcian marl-limestone rhythmite 77 sequence in the Betic Cordillera of southern Spain. 78 79 2. Geological setting 80 81 This study was focused on the La Cerradura section, located on a slope along 82 highway A-44 (37º41´47.8´´N; 3º37´57.6´´W), 15 km south of Jaén city (province of 83 Jaén) at km 57. The studied section belongs to the External Subbetic (Fig. 1; Betic 84 External Zones; Reolid et al., 2018). The Betic External Zones comprise the Prebetic and 85 Subbetic, both made up of thick successions of Triassic to Miocene strata (Vera, 2004). 86 The Prebetic sediments were deposited in more proximal settings (shallow marine shelf, 87 coastal plain, and continental environments) during the Jurassic and Cretaceous. The 88 Subbetic represents distal settings, i.e. pelagic swells and subsiding central troughs during 89 the Jurassic. The studied interval ranges from the Algovianum Zone (upper 90 Pliensbachian) to the Polymorphum Zone (lower Toarcian) and comprises hemipelagic 91 marls and marly limestones of the Zegrí Formation. The fragmentation of the south 92 Iberian palaeomargin during the late Pliensbachian and the variable subsidence of 93 different tilted blocks controlled the differences in thickness and facies during the 94 Toarcian (Reolid et al., 2018). In La Cerradura section, the main changes in lithofacies 95 are registered around the Polymorphum/Serpentinum zone boundary (Figs. 1 and 2) 96 related to decreasing carbonate content and the development of dark marls associated 97 with the Toarcian Oceanic Anoxic Event (T-OAE) and a negative carbon isotopic 98 excursion (CIE) (Reolid et al., 2014). The T-OAE has been recognised in the Zegrí 99 Formation (e.g., Jiménez et al., 1996; Rodríguez-Tovar and Reolid, 2013; Reolid, 2014; 100 Reolid et al., 2014) in the Serpentinum Zone. In La Cerradura section, the lower part of 5 101 the Serpentinum Zone is also characterised by an increase in total organic carbon (TOC), 102 a negative CIE, and an increase in redox sensitive elements (Reolid et al., 2014; Rodrigues 103 et al., 2019). This succession has low TOC contents (around 0.3 wt.%) with the highest 104 values (0.46 wt.%) around the Polymorphum-Serpentinum zone boundary (Rodrigues et 105 al., 2019). The main contribution to the organic matter content in La Cerradura section is 106 from terrestrial input with a minor marine contribution (see Rodrigues et al., 2019). Trace- 107 fossil assemblage diversity is low in the Serpentinum Zone and, locally, trace fossils are 108 absent in the interval of maximum values of TOC and the negative CIE. 109 110 3. Materials and methods 111 112 La Cerradura section is 81.6 m thick including upper Pliensbachian and lower 113 Toarcian. Strata with the Algovianum Zone (upper Pliensbachian) through the 114 Polymorphum Zone (lower Toarcian) are composed of alternating marls and marly 115 limestones (65.1 m), whereas the Serpentinum Zone is represented by dark marls (16.5 116 m). Marly layers of the rhythmite are composed mainly of calcite (average 71%), 117 phyllosilicates (19%), quartz (6%), and minor amount of K feldspar, gypsum, and 118 celestine. Phyllosilicates correspond to illite and illite/smectite mixed layers (average 119 86%), kaolinite, and chlorite. The amount of calcite in the dark marls of the Serpentinum 120 Zone is lower (53%) than in the rhythmite, whereas phyllosilicates content is higher 121 (35%). This mineralogical composition was also reported by Palomo (1987) from other 122 Pliensbachian and Toarcian sections of the Subbetic. 123 For this study, a total of 21 samples were collected from the marly limestone beds 124 of the Algovianum (14) and Polymorphum (7) zones. Polished slabs with 1 cm thick were 125 prepared and scanned at the Universidad de Jaén using a Bruker XR-microfluorescence 6 126 M4 Tornado equipped with a rhodium target X-ray tube with a high voltage of 50 kV, a 127 current of 600 µA and pressure of 20 mbar. The spotsize of the X-ray optics is 25 µm. 128 The maximum penetration depth from which fluorescence X-rays can still reach the 129 detector is less than 20 µm, which allows for the comparison of the different polished 130 slabs independently of their thickness. This low penetration does not allow deep analysis 131 inside the rock, it does permit analyses of vertical and lateral compositional changes, 132 especially the contrast between the host sediment and the infills of trace fossils. 133 The geochemical compositional maps obtained for each element are represented 134 by a range of colour intensity that indicates the relative concentration of each element. 135 The microfacies and the burrow infillings were characterised in thin section using a 136 stereographic petrographic microscope Leica M205 C. In addition, carbon-coated 137 polished thin sections were examined by scanning electron microscopy (SEM) using 138 back-scattered electron (BSE) images and energy dispersive X-ray spectroscopy (EDX) 139 with a Merlin Carl Zeiss instrument housed in the Centro de Instrumentación Científico- 140 Técnica of the Universidad de Jaén (Spain). This technique was employed to obtain 141 textural data from BSE imaging based on the atomic-number contrast mode and semi- 142 quantitative chemical data by EDX analysis. Sieved samples were prepared for the 143 Polymorphum Zone for determining the presence of phytodetritus in the < 500 µm 144 fraction. The retrieved small wood fragments were photographed in the SEM with 145 secondary electrons images. 146 147 4. Results 148 149 Reolid et al. (2014) previously analysed the trace fossil assemblages from the 150 Emaciatum Zone (upper Pliensbachian) to Serpentinum Zone (lower Toarcian) and 7 151 identified six ichnogenera. These include common Planolites, Thalassinoides, 152 Teichichnus, and Chondrites, and rarer Palaeophycus and Trichichnus. In addition to 153 these ichnogenera, Taenidium and Lamellaeichnus were also documented in the 154 Algovianum Zone in the current study (Fig. 3). Most of the ichnogenera corresponds to 155 dwelling structures with passive infilling (Thalassinoides, Teichichnus, and 156 Palaeophycus) and burrows of deposit-feeders with active infilling (Planolites, 157 Lamellaeichnus, and Taenidium) (see Gerard and Bromley, 2008). Chondrites has been 158 interpreted as a unknown deposit-feeder tracemaker adapted to oxygen depleted 159 conditions (Savrda and Bottjer, 1986; Fu, 1991) as a chemosymbiotic organism involving 160 sulphide microorganisms (e.g. Ekdale and Bromley, 1984) 161 Field observations show that the infill of the trace fossils are darker in colour than 162 the host rock both in marls and marly limestone beds (Fig. 3). Such ichnofabrics are 163 referred to as dark-on-light zones by Savrda (2007). In the analysis under the 164 stereographic microscope, the fills of trace fossils also contain higher concentrations of 165 small (< 5 mm) wood fragments (preserved as coal) and pyrite framboids than the host 166 mudstones do (Figs. 4 and 5). Coal fragments are particularly more evident in the samples 167 coming from the Polymorphum Zone (Figs. 3E, F). The semi-quantitative analysis by 168 EDX confirmed that black grains observed in BSE images are organic matter. Moreover, 169 wood remains are identified after the analysis of sieved samples (Fig. 6), and their 170 presence was also reported by Reolid et al. (2019b). 171 The BSE images of the thin sections (Fig. 5) comprising the burrow fills show the 172 presence of pyrite framboids as previously reported by Gallego-Torres et al. (2015) in the 173 Fuente Vidriera section, also located in the External Subbetic. Pyrite associated to trace 174 fossils may appear as single framboids but commonly as lenticular-shape and dumbbell- 8 175 shape polyframboid aggregates. Only locally, pyrite concretions have been identified in 176 areas with high pyrite framboid content. 177 The compositional maps obtained with the XR-microfluorescence show chemical 178 differences in the content of some elements between the burrow fills and the host 179 sediment (Figs. 7-10). These compositional differences are not uniform within a sample 180 and they can change even within a single trace fossil. 181 The compositional maps of trace fossils of the Algovianum Zone display a clear 182 enrichment in pyrite framboids as deduced by higher concentrations in Fe and S (Figs. 7 183 and 8). Pyrite framboids are mainly concentrated in medium- and large-diameter trace 184 fossils such as Planolites, Thalassinoides, and Lamellaeichnus (Figs. 7 and 8). The 185 enrichment in pyrite framboids is less common in smaller trace fossils such as Chondrites 186 (Fig. 7). However, the enrichment in Fe and S is less evident in other samples from the 187 Algovianum Zone with the same ichnoassemblage (Fig. 9). Some trace fossil fills also 188 contain higher concentrations of Si, Al, K, Ba, and Cr compared to the host sediment (Fig. 189 9). However, within a single sample other trace fossils show depletion in Si, Al, and K 190 (Fig. 9). 191 Trace fossils at the top of the Polymorphum Zone (Fig. 10) are characterised by 192 the same compositional features of those of Algovianun Zone, with an enrichment in 193 pyrite framboids (Fe and S) and minor trace elements such as Cr, Zn, and Ti. The record 194 of large Chondrites corresponding to the ichnospecies Chondrites affinis requires special 195 consideration. These are characterised by a branched trace fossil with large size (burrow 196 width ranges from 4 to 5 mm) of almost horizontal, flat, straight to slightly curved tunnels 197 (see more details in Uchman et al., 2012). These burrow systems are enriched in organic 198 matter (OM, Fig. 3E) and, locally, iron oxides (Fig. 3F). In the compositional maps, Ch. 15 347 conditions than those at the sediment-water interface. Chemical gradients from pore- 348 water to burrow-water favoured the preservation of OM trapped within the burrow. 349 The XRF elemental maps also show that some trace fossils from limestone and 350 marly-limestone intervals contain higher concentrations of Si, Al, and K, and other minor 351 components as Zn, Cr, and Ti, with respect to the surrounding host sediment. This is 352 related to the enrichment of clay minerals inside the burrows resulting from infilling from 353 overlying (clastic-richer) marly layer of the rhythmite. Enrichment in Ba within some 354 trace fossils describes the same distribution and may be related to fine detrital plagioclase 355 or barite crystals. 356 Bulk-rock geochemical analysis should be complemented with elemental imaging 357 of trace fossils in order to evaluate the potential for biogenic heterogeneity and elemental 358 partitioning. Geochemical analysis on trace fossils presented in this work is an implement 359 with respect to classical ichnological work. The compositional differences observed 360 between trace fossils and host rock have relevance for sampling methods employed on 361 bioturbated sediments, particularly for studies directed towards predicting reservoir 362 quality and analysing stratigraphic fluctuations of redox geochemical proxies in 363 palaeoenvironmental studies 364 365 366 Acknowledgments 367 The MINECO (Spain) is thanked for personal funding to J. Reolid through the Juan de la 368 Cierva Program. This study was supported by the research groups RNM-190 and RNM- 369 200. We wish to thank Prof. Vladimir Simo for identification of the ichnogenus 370 Lamellaeichnus and technicians Antonio Piedra and Baltasar Deutor (Universidad de 371 Jaén) for preparing polish slabs and thin sections. We would like to thank Editor Thomas 16 372 Algeo and two reviewers for their careful supervision of the manuscript. This is a 373 contribution of the IGCP-655 project Toarcian Oceanic Anoxic Event: Impact on marine 374 carbon cycle and ecosystems (IUGS-UNESCO). 375 376 References 377 378 Baeza-Carratalá, J.F., Reolid, M., García Joral, F., 2017. New deep-water brachiopod resilient 379 assemblage from the South-Iberian Palaeomargin (Western Tethys) and its significance for 380 the brachiopod adative strategies around the Early Toarcian Mass Extinction Event. Bulletin 381 of Geosciences 92, 233–256. 382 Bockelie, J.F., 1991. 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Geochimica et Cosmochimica Acta 60, 537 3897–3912. 538 539 Figure caption 540 541 Fig. 1. Location of the studied section. A. Geological setting of the La Cerradura section 542 (star) in the External Subbetic (External Zones of the Betic Cordillera). B. Field view of 543 the outcrop including marl and marly limestone rhythms of the Pliensbachian 544 (Algovianum and Emaciatum zones) and lower Toarcian (Polymorphum Zone), and dark 545 marls of the lower Toarcian (Serpentinum Zone). 546 Fig. 2. Stratigraphic log of La Cerradura section with location of the studied samples and 547 the interval representing the Toarcian Oceanic Anoxic Event and the negative carbon 548 isotopic excursion (Reolid et al., 2014; Rodrigues et al., 2019). 549 Fig. 3. Field and polished slab photos of trace fossils. A-D. Ichnoassemblage from 550 Algovianum Zone (upper Pliensbachian). Note: Ch., Chondrites; Th., Thalassinoides, Pl., 551 Planolites, Ta., Taenidium. E and F. Detail of Chondrites affinis from the top of 552 Polymorphum Zone (lower Toarcian) with red circles indicating the presence of organic 553 matter (OM) rich areas. Note the presence of a tunnel composed of iron oxides, probably 554 after pyrite (Py.) in F. Scale bars = 1 cm. 555 Fig. 4. Trace fossils in thin section. A-D Photomicrographs of Planolites under 556 transmitted light (A and C), and reflected light (B and D). Dark particles in A and C are 557 phytodetritus (OM, yellow arrows and circles) and pyrite framboids (Py, white arrows). 558 Pyrite framboids appear as white masses in B. E-F Transmitted light and reflected light 559 images of a Chondrites affinis. Pyrite framboids (Py, white arrows) correspond to dark 560 areas in E and to white masses in F. 23 561 Fig. 5. BSE images of trace fossils in thin section of the Algovianum Zone (upper 562 Pliensbachian). A and B. Lamination within the infilling of Lamellaeichnus reflected by 563 the concentration of dark organic particles (OM). Pyrite framboids (Py) appear as intense 564 brightness masses. C. Pyritized section of Chondrites. D. Section of partially pyritized 565 unidentified trace fossil. 566 Fig. 6. SEM images of wood fragments preserved as coal retrieved from sieved sample 567 in the Polymorphum Zone. 568 Fig. 7. Polished slab of a burrowed limestone bed from Algovianum Zone (upper 569 Pliensbachian) showing trace fossils (A; yellow areas are weathered parts) and 570 compositional map highlighting the distribution of Fe (B). Abbreviations: Ch., 571 Chondrites; La., Lamellaeichnus; Ta., Taenidium; Th., Thalassinoides. 572 Fig. 8. Two polished slabs of bioturbated limestones (A, B) from Algovianum Zone 573 (upper Pliensbachian) and corresponding maps of Fe (C, D) and S (E, F). Note the pyrite 574 concretion in A with 1 cm in diameter. Abbreviations: Ch. Chondrites; Pl., Planolites; 575 Te., Teichichnus; Th., Thalassinoides; Py., pyrite. 576 Fig. 9. Polished slab of bioturbated limestone (A) from Algovianum Zone (upper 577 Pliensbachian) and corresponding compositional maps for Al, Ba, Cr, Fe, K, S and Si. 578 Some trace fossils observed in A such as Planolites (Pl.) present enrichment in all 579 elements, others such as Thalassinoides (Th.) show relative enrichment just for Fe and S 580 but not always, and Palaeophycus (Pa.) are similar to host sediment. 581 Fig. 10. Bioturbated limestone sample from the top of the Polymorphum Zone (lower 582 Toarcian) and corresponding compositional maps for Si, Ca, Fe, Al, K, Ba, Cr, Ti and Zn. 583 The surface of the sample is parallel to the bedding. 584 Fig. 11 Location of food resources with respect to the sediment-water interface and the 585 idealized vertical distribution of oxic, suboxic, and the reduced zones of Kristensen 24 586 (2000) compared with the trace-fossil tiering for La Cerradura section. Note: 1) The 587 highest concentrations of OM occur at sediment-water interface; 2) Labile OM is 588 consumed (aerobic respiration and OM oxidation) in the oxic zone within the sediment; 589 3) OM content decreases with depth in the suboxic zone as a result of microbial 590 decomposition by anaerobic bacteria; 4) The microbial activity (mainly SRB, see text) 591 and the labile OM are low in the reduced zone (anoxic conditions) and most of the OM is 592 refractory not a food resource. 593 Fig. 12. Model of OM and oxygen distribution in a transect without burrow (left) and a 594 transect through a burrow (right). Note the vertical distribution of the OM within the 595 sediment presents a gradient with higher content close to the sediment-water interface. 596 OM contents in the seafloor decrease with depth due to consumption by deposit and 597 detritus feeder and aerobic and anaerobic microbial decomposition. However, the burrow 598 contains a comparatively higher OM concentration with respect to the host sediment 599 because it is infilled by material coming from the seafloor. The oxygen-poor, sulfide-rich, 600 burrow-water led to enhanced OM preservation compared with the oxic bottom-waters. 601 In this sense, the burrows served as traps for OM preservation.