Reply to comment on the paper by Davies et al. “Resolving MISS conceptions and misconceptions: A geological approach to sedimentary surface textures generated by microbial and abiotic processes”

We thank Noffke (2017) for her comment and for providing an opportunity to clarify our classification of “sedimentary surface textures”. We accord great credit to Dr. Noffke and other dedicated researchers whose detailed work has brought microbially induced sedimentary structures (MISS) to the widespread attention of geoscientists. However, we stand by our assertion that attributing structures observed in practical field and laboratory studies to processes of formation is much more problematic than Noffke (2017) indicates. Indeed, points in the Comment confirm the need for a classification system that categorises the degree of certainty attributed to a given interpretation. We stress that our paper was not designed as a critique of previous studies of MISS but rather was designed to encourage a reasonable assessment of uncertainty in assigning sedimentary surface textures to physical processes or to MISS

Resolving MISS conceptions and misconceptions: A geological approach to sedimentary surface textures generated by microbial and abiotic processes

The rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone, siltstone and mudstone bedding planes. In recent years, an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition, resulting in their classification as microbially induced sedimentary structures, or MISS. Research into MISS has developed at a rapid rate, resulting in a number of misconceptions in the literature. Here, we attempt to rectify these MISS misunderstandings. The first part of this paper surveys the stratigraphic and environmental range of reported MISS, revealing that contrary to popular belief there are more reported MISS-bearing rock units of Phanerozoic than Precambrian age. Furthermore, MISS exhibit a pan-environmental and almost continuous record since the Archean. Claims for the stratigraphic restriction of MISS to intervals prior to the evolution of grazing organisms or after mass extinction events, as well as claims for the environmental restriction of MISS, appear to result from sampling bias. In the second part of the paper we suggest that raised awareness of MISS has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features. By introducing the umbrella term ‘sedimentary surface textures’, of which MISS are one subset, we suggest a practical methodology for classifying such structures in the geological record. We illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata. Resultant interpretations, microbial or non-microbial, should be couched within a reasonable degree of uncertainty. This approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin, and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current MISS lexicon. It is hoped that this new terminology will reduce the number of overly sensational and misleading claims of MISS occurrence, and permit the means to practically separate initial observation from interpretation. Furthermore, this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone, informing the search for true MISS in Earth's geological record and potentially on other planetary bodies such as Mars

Dendrochronological dating of coal mine workings at the Joggins Fossil Cliffs, Nova Scotia, Canada

Joggins, Nova Scotia was one of the first places in North America where coal was mined. In this paper we employ dendrochronology to date timber pit props preserved within relic coal mine workings on the closely adjacent Fundy and Dirty seams. These remains comprise a system of adits created through ‘room and pillar’ mining. Of the seventy-three samples collected, forty-eight were successfully cross-dated against a local red spruce (Picea rubens Sarg.) master chronology thereby establishing the year in which each individual sample was cut as a live tree. Results indicate cut dates of 1849-1875 which are generally consistent with written archival records of mining activity on these coal seams. Our analysis of fourteen separate adits allows us to distinguish two phases of mining. Most adits (numbers 1-9 and 11-12 with cut dates of 1849-1868) are relics of an initial operation by the General Mining Association (1865-1871), which opened a mine entered at beach level. Dendrochronological dates preceding the opening of this mine may suggest that timber stockpiled from the nearby Joggins Mine (opened 1847) was used in its construction. The remaining adits (numbers 10 and 13-14 with cut dates of 1873-1875) are probably relics of a later mine opened by the Joggins Coal Mining Company (1872-1877). Although this mine was centered ~500 m inland, its western peripheral workings passed through the earlier workings to the shore. Findings improve knowledge of the industrial archaeology of the UNESCO World Heritage Site and help refine the regional master red spruce chronology for future dendrochronological studies

The role of discharge variability in the formation and preservation of alluvial sediment bodies

Extant, planform-based facies models for alluvial deposits are not fully fit for purpose, because they over-emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform, and because the planform of individual rivers vary in both time and space. Accordingly, existing facies models have limited predictive capability. In this paper, we explore the role of inter-annual peak discharge variability as a possible control on the character of the preserved alluvial record. Data from a suite of modern rivers, for which long-term gauging records are available, and for which there are published descriptions of subsurface sedimentary architecture, are analysed. The selected rivers are categorized according to their variance in peak discharge or the coefficient of variation (CVQp = standard deviation of the annual peak flood discharge over the mean annual peak flood discharge). This parameter ranges over the rivers studied between 0.18 and 1.22, allowing classification of rivers as having very low ( 0.90) annual peak discharge variance. Deposits of rivers with very low and low peak discharge variability are dominated by cross-bedding on various scales and preserve macroform bedding structure, allowing the interpretation of bar construction processes. Rivers with moderate values preserve mostly cross-bedding, but records of macroform processes are in places muted and considerably modified by reworking. Rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures, abundant in situ trees and transported large, woody debris, and their deposits contain pedogenically modified mud partings and generally lack macroform structure. Such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits. This high-peak-variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation. We propose that discharge variability, both inter-annual peak variation and “flashiness” may be a more reliable basis for classifying the alluvial rock record than planform, and we provide some examples of three classes of alluvial sediment bodies (representing low, intermediate, and high/very high discharge variability) from the rock record that illustrate this point

River Systems and the Anthropocene: A Late Pleistocene and Holocene Timeline for Human Influence

Rivers are central to debate about the Anthropocene because many human activities from antiquity focused on channels and floodplains. A literature compilation for the onset of human modification of rivers identifies six stages that represent key innovations focused in the Near East and adjoining areas: (1) minimal effects before about 15,000 cal yr BP, with the use of fire and gathering of plants and aquatic resources; (2) minor effects from increased cultivation after about 15,000 cal yr BP, with plant and animal domestication after about 10,700 cal yr BP; (3) agricultural era after about 9800 cal yr BP, with legacy sediments, widespread fire use, the first dams and irrigation, and mud-brick manufacture; (4) irrigation era from about 6500 cal yr BP, with large-scale irrigation, major cities, the first large dam, urban water supplies, expanded groundwater use, river fleets, and alluvial mining; (5) engineering era with embankments, dams, and watermills after about 3000 cal yr BP, especially in the Chinese and Roman empires; and (6) technological era after about 1800 CE. Anthropogenic river effects were more varied and intense than commonly has been recognised, and they should be considered routinely in interpreting Late Pleistocene and Holocene fluvial archives

Evolution of fixed-channel alluvial plains in response to Carboniferous vegetation

The establishment of terrestrial plants during the Palaeozoic era was one of the most significant changes to the Earth system during the Phanerozoic eon. The continuing evolution and expansion of land plants irrevocably altered the alluvial landscape: the broad, unconfined and unconsolidated blankets of coarse sediment found at the start of the Cambrian period were replaced by a diverse array of braided and meandering channel styles and stable floodplains by the end of the Devonian period. Here we show that the first appearance of a sedimentary facies suite attributed to low-energy, organic-rich river systems with multiple channels and stable alluvial islands, known as anabranching or anastomosing rivers, occurred during the Carboniferous period. Our field studies and literature review demonstrate that the appearance of these rivers coincides with the continuing evolution of tree-like plants. We suggest that increased floodplain stability and new triggers for channel avulsion were provided by the increase in complexity and diversity of root assemblages, density of floodplain forests and production of woody debris associated with expansion of arborescence. We conclude that the expansion of tree habitats led to the crossing of a threshold in vegetative control of floodplain and river morphology during the Carboniferous