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Where did ancient carbonate mounds grow – in bathyal depths or in shallow shelf waters?
Dierk Hebbeln, Elias Samankassou
PII: S0012-8252(15)00043-4 DOI: doi: 10.1016/j.earscirev.2015.03.001 Reference: EARTH 2094
To appear in: Earth Science Reviews
Received date: 15 September 2014 Revised date: 3 March 2015 Accepted date: 6 March 2015
Please cite this article as: Hebbeln, Dierk, Samankassou, Elias, Where did ancient car- bonate mounds grow – in bathyal depths or in shallow shelf waters?, Earth Science Reviews (2015), doi: 10.1016/j.earscirev.2015.03.001
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Where did ancient carbonate mounds grow – in bathyal depths or in shallow shelf waters?
Dierk Hebbelna & Elias Samankassoub a MARUM – Center for Marine Environmental Sciences, University of Bremen, Leobener Straße,
28359 Bremen, Germany b Section of Earth and Environmental Sciences, University of Geneva, Rue des Maraîchers 13, 1205
Geneva, Switzerland
corresponding author:
Dierk Hebbeln
Tel. +49-421-21865650
Fax +49-421-21865654 email: [email protected]
Abstract
Carbonate mounds are important and ubiquitous components of the sedimentary rock record throughout the PhanerozoicACCEPTED. Nevertheless, factors MANUSCRIPT controlling their occurrence and growth remain enigmatic, in particular because the depositional depths of such mounds are poorly constrained and it is assumed that fossil examples lack modern analogues. Factors currently used to constrain the water depth ancient mounds grew in include water energy, occurrence of specific organisms that perform photosynthesis (particularly calcareous algae) and distinct non-biogenic components. In reviewing the main criteria used to delineate water depth of formation of fossil carbonate mounds, most (if not all) criteria traditionally used to assume shallow-water settings appear to be no longer substantiated in the light of current knowledge gained from modern carbonate mounds. High energy, assumed to indicate shallow-water conditions, is ongoing in water as deep as > 1000 m. The
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occurrence of peloids and processes such as micritization are not diagnostic of water depth.
Furthermore, virtually none of the calcareous algae assumed to indicate the photic zone have a taxonomically granted affiliation.
Over the last years, research in the deep ocean, in particular thanks to the deployment of new devices and techniques, provided a good database revealing a wide distribution of modern carbonate mounds. Reviewing the characteristics of ancient carbonate mounds, with emphasis on depositional settings, and confronting the criteria used with current knowledge from sub-recent and modern carbonate mounds, we conclude that for many ancient mounds a bathyal origin is (at least) as likely as a shallow-water origin. This conclusion is well supported by the sedimentary records left by these modern mounds that are astonishingly similar to those seen in Palaeozoic and Mesozoic carbonate mounds.
Keywords: Carbonate mounds – Waulsortian mounds – Bathymetry –Palaeozoic – Modern
1. Introduction
Carbonate mounds are common sedimentary features. They have been formed by a variety of organisms since the early Palaeozoic (Monty et al., 1995). Framework-building organisms such as corals, crinoids and bryozoans commonly represent important components in these mounds, ACCEPTED MANUSCRIPT although pure microbial mud mounds are also widely reported (Pratt, 1995). The structure and genesis of fossil carbonate mounds are a matter of debate, basically because true modern analogues are thought to be scarce (Wendt et al., 2001). In analogy to modern tropical coral reefs, most ancient carbonate mounds have widely been linked with ramp/shelf settings in tropical environments (Lees and Miller, 1995), although other depositional environments have been suggested (cf. review in
Pratt, 1995).
A shelf origin was supported by the assumption that high carbonate production in oligotrophic settings, common for modern coral reefs, has to rely on photoautotrophic symbionts producing
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energy for the framework builders (e.g., zooxanthellate scleractinian corals) (Stanton, 2006; Pomar and Hallock, 2008). However, Stanton (2006) pointed out that for ancient mounds the rate of carbonate production could not be determined confidently. Reviewing the evidence for an origin of carbonate mounds in the photic zone, Stanton (2006) concluded that for much of the Phanerozoic
(that is, at least prior to the Jurassic and, perhaps, Late Triassic) a direct photosynthetically mediated carbonate production in mounds cannot be proven and appears to be unlikely.
While the presence of photoautotrophic organisms in the fossil record of an ancient mound indeed would be a distinctive indicator for an origin within the photic zone, other criteria commonly used to infer a shallow-water origin are rather equivocal. Basically these criteria can be grouped into two lines of thought: (a) high-energy environments often invoked for ancient carbonate mounds only appear in shallow waters linked to fair weather or storm wave bases (Tsien et al., 1980; Brachert et al., 1992; Kirby and Hunt, 1996; Boulvain, 2001; Boulvain et al., 2004; Pomar and Hallock, 2008), and
(b) specific organisms or non-bioclastic components are (mostly) limited in their ecological range to shallow waters (Lees and Miller, 1985; Madi et al., 1996; cf. discussion in Monty, 1995).
In addition to tropical coral reefs, modern, actively growing carbonate mounds that usually occur in aphotic depths have a great potential also to serve as modern analogues for many ancient carbonate mounds (Mullins et al., 1981; Cairns and Stanley, 1981). Although these modern carbonate mounds have been found in inner and outer shelf settings, their vast majority has been reported from depths ACCEPTED MANUSCRIPT between 200 m and 1000 m, thus from well below the shelf break (Roberts et al., 2006). Over the last decades the application of state-of-the-art marine technologies including remotely operated vehicles
(ROVs), autonomous underwater vehicles (AUVs), and multibeam echosounders helped to detect carbonate mounds in increasingly important numbers, e.g., along the NW European margin (Fosså et al., 2005; Wheeler et al., 2007), some Mediterranean margins (Freiwald et al. 2009; Fink et al., 2013), the NW African margin (Colman et al., 2005; Foubert et al., 2008), the Florida-Hatteras margin (Paull et al., 2000; Grasmück et al., 2006), the Brazilian (Viana et al., 1998), Mexican (Hebbeln et al., 2014), and Angolan margins (Le Guilloux et al., 2009), the Canadian Pacific margin (Conway et al., 2005), and
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in the Great Australian Bight (James et al., 2000). All these mounds depend on skeletal material produced by heterozoans with the most important components being azooxanthellate scleractinian corals (in the following referred to as cold-water corals, CWC, e.g., Roberts et al., 2006), bryozoans
(James et al., 2000) and sponges (Conway et al., 2005). Although reaching partly back with their origins to >2.5 Ma (Kano et al., 2007), most of the CWC carbonate mounds have been hosting active coral communities in recent and/or sub-recent (i.e. <50 kyr old) times (Frank et al., 2011; Fink et al.
2013).
In contrast to such heterozoan mounds, adequate modern analogues for pure microbial mud mounds are still waiting to be discovered. However, assuming that such microbial mud mounds also form today in the deeper ocean, their presumably low-relief features might only be discovered following a further refinement of seafloor mapping technologies.
The aim of this review paper is to show that these modern carbonate mounds from well below the shelf break can serve as equally good analogues to ancient heterozoan carbonate mounds as modern shallow-water carbonate mounds do. Already in the early days of research on modern carbonate mounds, their potential role as analogues for ancient carbonate mounds has been acknowledged
(Mullins et al., 1981; Cairns and Stanley, 1981). However, whereas in the early 1980s modern carbonate mounds growing in several 100s of meters water depth might have been seen as peculiar and rare features, now their well-documented widespread occurrence (see above; Fig. 1) justifies a ACCEPTED MANUSCRIPT new approach linking the research on modern and ancient heterozoan carbonate mounds, with the latter also showing a wide geographical distribution (Fig. 1).
Whereas the dominant organisms and, consequently, the biotic interactions on such mounds have been variable over the last several hundreds of million years resulting in a large variety of individual mound types (Kiessling et al., 2002; James and Wood, 2010), all these types share a common carbonate mound appearance. This calls for a common set of processes being involved in their formation affecting both, ancient and modern carbonate mounds. Indeed, a closer look at the modern carbonate mounds occurring beyond the shelf break reveals that they share many of the
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characteristic features, which are commonly used to infer a shallow-water origin of ancient carbonate mounds. Consequently, the sedimentary records of modern (e.g., Dorschel et al., 2005) as well as of ancient (Brachert et al., 1992; Wendt, 1993; Pratt, 1995) carbonate mounds are astonishingly similar in size and composition (Figs. 2, 3). Demonstrating the similarities (1) of their appearance, (2) of the processes involved in their formation, and (3) of the resulting sedimentary records, this paper might stimulate an open discussion on the origin of the ancient carbonate mounds and trigger a more intense dialogue between scientists studying either ancient or modern mound systems (see also Henriet et al., 2014). By acknowledging the potential of these modern mounds, forming in the deep sea to serve as analogues for ancient mounds, such a discussion finally may result in the re-assessment of the formation depth of many ancient carbonate mounds.
Fig. 1: Distribution of cold-water corals (dots and lines in light blue, after Roberts et al., 2006) in the
Atlantic Ocean. Superimposed are examples of proven occurrences of recent (stars, mostly representing clusters of mounds - compiled from Colman et al., 2005; Fink et al., 2013; Foubert et al.,
2008, Grasmueck et al., 2006; Hebbeln et al., 2014; Le Guilloux et al., 2009, Mienis et al. 2012; 2014;
Mullins et al., 1981; Remia and Taviani, 2005; Reveillaud et al., 2008; Reyes et al., 2005; Taviani et al.,
2005; Viana et al., 1998; Wheeler et al. 2007) and of ancient carbonate mounds (triangles, compiled from compiled from Wilson, 1975; Geldsetzer et al., 1989; Flügel and Flügel-Kahler, 1992; Bourque et ACCEPTED MANUSCRIPT al., 1995; Pratt, 1995; Gutteridge et al., 1995; Lees and Miller, 1995; Madi et al., 1996; Strogen et al., 1996; Boulvain, 2001; Wendt et al., 2001, Riding, 2002; Webb, 2002). At many continental margin sites for which the occurrence of cold-water corals has been described, modern carbonate mounds still might be discovered, as for example recently along the Angolan margin (Le Guilloux et al., 2009), the Moroccan Mediterranean margin (Fink et al., 2013), and the Mexican Campeche Bank (Hebbeln et al., 2014).
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Fig. 2: Examples for the shape and dimensions of ancient and modern carbonate mounds. Upper panel: Province of exhumed mounds (Kess Kess mounds, Devonian) from the central area of Hamar
Laghdad Ridge, Anti-Atlas, Morocco (details on composition of these mounds are provided in
Brachert et al., 1992). Individual mounds on the image are in average 30 m high. Lower panel:
Multibeam echosounder map of cold-water coral carbonate mounds at the Moroccan Atlantic continental margin with dimensions indicated. Data have been obtained in 2014 during expedition
MSM-36 with the German RV Maria S. Merian (Hebbeln et al., unpubl. data).
Fig. 3: Comparison of the lithology of ancient and modern carbonate mounds. (a) Vertical cut-surface of sediment core GeoB 14904-2 from a Mauritanian margin cold-water coral mound (to the left) compared to a vertical outcrop surface (to the right) of a Visean carbonate mound, Moroccan Anti-
Atlas (for the setting see Wendt et al., 2001). Scale bar is 15 cm. The white rectangle indicates the zoom-in displayed in (b).
2. Some definitions
Since many years there is an ongoing discussion about a classification of ancient carbonate mounds involving, among many others, terms ranging from microbial mud mounds via skeletal carbonate build-ups to reefs (James and Bourque, 1992; Bosence and Bridges, 1995; Wood, 2001; Riding, 2002; ACCEPTED MANUSCRIPT Kiessling et al., 2002; James and Wood, 2010). The commonly used term “Waulsortian mounds” essentially refers to early Mississippian mud-rich mounds (Lees et al., 1985). As this paper is interested in the processes of mound formation, the general term “carbonate mounds” is used, in order to avoid any controversial discussions in relation to the various classification schemes.
Most of the modern carbonate mounds referred to above occur beyond the shelf break in water depths down to >1000 m (White and Dorschel, 2010), although in temperate and high latitudes they also can occur in inner and outer shelf settings (Freiwald et al., 1997; Lavaleye et al., 2009). By increasing the range of mound occurrence to beyond the shelf edge, terms such as “shallow-water
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mounds” and “deep-water mounds”, which are commonly used in relation to ancient carbonate mounds, might create some confusion as especially the latter term often still refers to shelf settings.
Definitions for the differentiation of such palaeo-depth estimates are manifold and often related to either the photic zone, with “deep water” still referring to within photic zone (e.g., Pratt 1995) or to hydrodynamic energy and with “deep water” still referring to above the storm-wave base (Brachert et al., 1992; Pratt, 1995). Thus, the lower limits given for the bathymetry for ancient deep-water carbonate mounds mostly range between ~30 m and ~280 m (Lees et al., 1985; Lees and Miller,
1985; Bridges and Chapman, 1988; Monty et al., 1995; Pratt, 1995; Bourque et al., 1995; Jeffery and
Stanton, 1996). To differentiate these ancient deep-water mounds from the modern carbonate mounds occurring in much greater water depths, the latter will be assigned as “bathyal mounds” in the following, and this term also is recommended for future work.
3. Shelf versus bathyal origin of carbonate mounds
As pointed out above, indicators to allocate a shallow-water origin to ancient carbonate mounds in absence of photosynthetic organisms in their fossil record are often equivocal. In the following, the two main arguments, namely (a) indications for high-energy settings hinting to a formation above the storm/fair-weather wave base (Tsien et al, 1980; Brachert et al., 1992; Kirby and Hunt, 1996;
Boulvain, 2001; Pomar and Hallock, 2008) and (b) specific mound organisms being restricted to ACCEPTED MANUSCRIPT shallow waters (e.g., Lees and Miller, 1985, 1995; Madi et al., 1996) are critically assessed and compared to modern bathyal carbonate mound settings.
Of course, when comparing ancient with modern carbonate mounds (e.g., Fig. 3), the impact of diagenesis - primarily on the fossil record - has to be kept in mind. In his ‘diagenetic sieve’ approach,
Dullo (1990) pointed out that especially selective dissolution of biogenic components might cause a significant quantitative modification of the biotic composition preserved in the rock record. Selective dissolution (e.g., of aragonitic material) potentially might even affect the quantitative composition of a mound, but probably with no significant impact on the overall appearance of a mound. For
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example, over the Holocene the sedimentary record on Propeller Mound at the Irish margin contains up to 15% aragonitic coral material (Dorschel et al., 2007) and even the complete dissolution of this material would hardly affect the dimensions of this ~120 m high mound. And actually, for most of the
Paleozoic carbonate mounds referred to in the present study, dissolution is overall a minor issue
(Bathurst, 1971). However, as for the reconstruction of the palaeoenvironmental setting of mound formation predominantly the preserved, i.e. still readable (fossil) record is interpreted, diagenesis would have hardly an impact on comparisons like such done here. Nevertheless, still there is the chance/risk that diagenesis might have (irretrievably) erased clear indicators for specific settings – a common threat to all geological records.
3.1. High-energy environments
As most of the common heterozoans contributing to ancient and modern carbonate mounds do not have photoautotrophic symbionts, they have to rely on external food supply that most likely originates from primary production in the surface waters. For carbonate mounds largely formed by immobile suspension feeders as, e.g., cold-water corals, bryozoans, crinoids and sponges, the food particles raining down from the surface can also be transported laterally close to the sea floor, thereby significantly increasing the chances of these organisms to catch the food. Thus, in addition to a sufficiently high surface water production, also a vigorous bottom current regime is needed, ACCEPTED MANUSCRIPT powered either by geostrophic currents or by internal tides. In fact, all modern bathyal carbonate mounds characterized by a thriving mound ecosystem are intrinsically linked to a dynamic bottom water regime (see Roberts et al., 2006, for cold-water coral mounds). In situ current measurements at such sites revealed maximum current velocities of >50 cm s-1 (Dorschel et al., 2007; Mienis et al.,
2009a). Such strong currents can in various ways and at various scales interact with the mounds, from creating small sedimentary structures (e.g., ripples) to controlling the shape of a mound (e.g., elongated mounds surrounded by moats). The occurrence of such sedimentary structures in the rock
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record (Tosolini et al., 2012) should therefore be interpreted considering also the existence of strong bottom currents at bathyal depths.
3.2. Coarse layers and cross-bedding
Layers with coarse sediments indicative of reworking and/or local bioclast accumulations in fossil mounds are thought to record major storm events (e.g., event stones in the Devonian Kess-Kess mounds of Brachert et al., 1992) and, thus, a deposition above the storm wave base. Tosolini et al.
(2012) used, among other criteria, the texture indicative of high energy (grainstone) to infer a shallow depositional setting for Devonian mounds (see also Wendt et al., 2001). Furthermore, cross- stratification in a crinoidal grainstone found close to Carboniferous carbonate mounds in Derbyshire led Gutteridge (1995) to infer a deposition above the fair-weather wave base. In contrast, for greater depths generally rather quiet conditions are assumed (Pray, 1958).
The strong currents of often several tens of centimeters per second observed at modern bathyal carbonate mounds (Dorschel et al., 2007; Mienis et al., 2009a; Hebbeln et al., 2014) also trigger sediment reworking and bedform formation. Thus, ripples (see Fig. 4) and coarse layers indicative for reworking and/or erosion in the mound record (Dorschel et al., 2005) are common features in and around recent carbonate mounds also in several hundreds of meters water depth, well below the storm wave base. ACCEPTED MANUSCRIPT
Fig. 4: Rippled sea floor with coral fragments from the Porcupine Seabight at the Irish margin in a water depth of ~600 m. The two dots are 20 cm apart from each other. The photograph has been taken in 2010 during expedition POS400 with the German RV Poseidon with the ROV CHEROKEE
(MARUM, University of Bremen, Germany).
3.3. Fossils in growth position
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The scarcity of larger fossils such as corals, bryozoans and crinoids in growth position in ancient mound records often has been interpreted as a clear indicator for strong hydrodynamics linked to shallow waters (Bourque et al., 1995; Aretz and Herbig, 2003). As pointed out above, strong bottom currents also shape bathyal mounds and, especially as the involved organisms do not form massive carbonate structures (Fig. 5a), can contribute to their disintegration (possibly favoured by bioerosion that is also common in modern bathyal carbonate mounds; Beuck et al., 2010) and move (parts of) these organisms. On many CWC carbonate mounds this process results in the widespread so-called coral rubble/debris facies (Huvenne et al., 2005; Wienberg et al., 2008; see Fig. 5b). As this facies is finally also preserved within the sediments, the sedimentary record found on the bathyal mounds usually reveals abundant coral fragments in a matrix of hemipelagic sediments (Figs. 5c, 3) with only rare large fossil fragments that might be interpreted as still being in growth position. Actually, the appearance of these modern sedimentary records is astonishingly similar to those of many ancient mounds (Fig. 3).
Fig. 5: Development from the living coral thicket towards the sedimentary record. (a) Living Lopehlia pertusa framework (in pink) growing on dead, but still intact framework (grey to white) in ~640 m water depth at Rockall Bank, NE Atlantic. The entire thicket reaches ca. 1 m in height. The photograph has been taken in 2004 during expedition M61-3 with the German RV Meteor with the ACCEPTED MANUSCRIPT ROV QUEST (MARUM, University of Bremen, Germany). (b) Coral rubble facies in the Porcupine Seabight at the Irish margin in ~750 m water depth (width of the image: ~ 30 cm). The photograph has been taken in 2010 during expedition POS400 with the German RV Poseidon with the ROV
CHEROKEE (MARUM, University of Bremen, Germany). (c) Computer-tomograph (CT) image of sediment core GeoB 11569-2 collected from a Mauritanian margin cold-water coral mound (width:
12 cm) showing that generally disintegrated coral fragments (visualized here by CT technique without surrounding sediment) form the mound facies.
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Thus, considering the composition of modern mounds consisting mainly of fragmented fossils (e.g., coral debris), the discussion as to which degree organisms occurring within mounds (particularly
Waulsortian mounds) are in growth position (cf. Ahr and Stanton, 1994) is not straightforward.
Crinoids and bryozoans, the main skeletal components in early to mid-Palaeozoic mounds (Pray,
1958; Lees and Conil, 1980; Lees et al., 1985; cf. Wood, 1998), can be even more delicate than the scleractinian corals living on modern bathyal mounds. Thus, crinoids and bryozoans often are even more prone to disintegration resulting in various degrees of preservation and probably in even less specimens having the potential to be preserved in growth position.
3.4. Episodic mound growth
Vertical changes in facies and grain assemblages are phenomena widely reported from, and often used to characterize, Waulsortian mounds (Lees, 1964; Lees et al., 1985; Lees and Miller, 1985;
Murphy, 1988; Bourque et al., 1995; Jeffery and Stanton, 1996; Madi et al., 1996). Kirby and Hunt
(1996) interpreted hiatal surfaces found within the Lower Carboniferous Muleshoe Mound in New
Mexico, USA, to indicate episodic growth. This development was interpreted to reflect the alternation of relatively well-oxygenated and high-energy conditions representative of shallow water conditions and periods of mound crises, triggered by anoxic bottom waters (Kirby and Hunt, 1996;
Stanton et al., 2000). Such an episodic growth has also been inferred for Carboniferous carbonate ACCEPTED MANUSCRIPT mounds in northern Africa based on the presence of recurring, internal packages (growth phases?) interpreted as having been induced by sea-level fluctuations (Bourque et al., 1995; Madi et al., 1996).
Vertical changes in the composition of fossil carbonate mounds are often interpreted to be indicative of sea-level fluctuations e.g., by affecting the position of the fair-weather/storm wave base (Wendt,
1993; Lees et al., 1995; Bourque et al., 1995; Boulvain et al., 2004).
Episodic growth is also common in modern bathyal carbonate mounds, e.g., triggered by changing hydrodynamics induced by climate change (Dorschel et al., 2005), changed productivity conditions
(Wienberg et al., 2010), or by temporary sub-oxic bottom waters (Fink et al., 2012). As a result of this
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episodic growth, modern mounds are often characterized by internal hiatal surfaces (Dorschel et al.,
2005; Eisele et al., 2008; Mienis et al., 2009b; van der Land et al., 2014), which resemble, for example, those described for the Muleshoe Mound (Kirby and Hunt, 1996).
3.5. Mound shape
According to the differentiation between mounds having a flat top and cone-shaped mounds introduced by Schlager (2003) and as recently applied by Berra et al. (2012), flattened mound tops are caused by erosion inevitably indicating that the mounds reached at least the storm wave base, whereas cone-shaped mounds are common in quiet and deep waters. In addition, morphologic features such as elongated mound shapes and current scours around them have been interpreted to reflect high-energy, rather shallow settings near the fair-weather wave base (Kirby and Hunt, 1996).
However, investigations of modern CWC carbonate mounds off Ireland reveal that these mounds underwent significant cyclic erosion at the turn from glacial to interglacial conditions when bottom currents in approx. 800 m water depth picked up from sluggish glacial to erosive interglacial levels
(Dorschel et al., 2005). Due to such erosive settings, also bathyal carbonate mounds often have flattened tops (Fig. 6).
Fig. 6: Detailed bathymetric 3D-map for Propeller Mound, an actively growing cold-water coral ACCEPTED MANUSCRIPT carbonate mound in ~800 m water depth at the Irish continental margin (Huvenne et al., 2005). The map clearly shows the flattened top of the mound (map courtesy by B. Dorschel).
Furthermore, as the bathyal carbonate mounds interact with a dynamic bottom water regime, moats commonly form around these mounds features (De Mol et al., 2002; Grasmueck et al., 2007; Fink et al., 2013, see also Fig. 6). In places where modern bathyal mounds are located in unidirectional flow regimes, elongated mound shapes are typical features most likely resulting from the preferred growth of the main suspension feeders towards the currents (Mullins et al. 1981).
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3.6. Organisms and non-biogenic components as bathymetry indicators
Crinoids and fenestellid bryozoans are the main bioclastic components in Waulsortian mounds in
Belgium as originally defined by Lees and Miller (1985). Sponges (mostly hexactinellids) were reported as important components in comparable mounds elsewhere (e.g., Madi et al., 1996). The proportion of the latter, having a lower preservation potential, was thought to be underestimated in earlier descriptions (Bourque and Gignac, 1983, 1986; Bourque and Boulvain, 1993). Other, volumetrically less important bioclastic components include corals, brachiopods, trilobites, ostracodes, various foraminifers and “problematic algae” (Bridges and Chapman, 1988; Brachert,
1991; Brachert et al., 1992; Jeffery and Stanton, 1996). Some of these organisms (in particular green algae) are used for bathymetric estimates for mound depositional environments, along with non- bioclastic components such as peloids and post-depositional features such as micritization (Lees and
Miller, 1985, 1995; Bridges and Chapman, 1988).
Lees et al. (1985) and Lees and Miller (1985, 1995) distinguished four component assemblages (A at the bottom to D at the top) in Waulsortian mounds. These assemblages were interpreted to represent stratigraphic phases tied to water depth. A key factor hereby is the delineation of the photic zone. For instance, Lees and Miller (1985, 1995) linked the lower limit of the photic zone to the deepest occurrence of micritization, which is assumed to be caused by algae and to occur down ACCEPTED MANUSCRIPT to a maximum water depth of 220 m. This depth limit is subsequently used to estimate the bathymetric range of other organisms. As in this case study multilocular foraminifers occur stratigraphically 30 m below the assumed lower limit of micritization, a maximum depth for the occurrence of multilocular foraminifers at 250 m water depth has been inferred. In the same line of reasoning, sponge spicules, found stratigraphically 60 m below the lowest level of micritization, are used to delineate the 280 m isobath.
This method appears questionable in many aspects. Micritization is not exclusively caused by algae as pointed out in Jeffery and Stanton (1996) and can hardly be a reliable criterion for bathymetry.
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Indeed, micritization is reported from settings as deep as 871m (Hook et al., 1984). Relative water depth deduced from components relative to the occurrence of micritization is thus arbitrary and, moreover, does not take into account possible displacement of components and post-depositional processes (see discussion in Jeffery and Stanton, 1996; Flügel, 2004).
Other critical components having been used for bathymetric reconstructions are peloids in geopetal fillings of voids, with the lower limit of occurrence estimated at depths around 250 m (Lees and
Miller, 1985, 1995). However, peloids can occur in virtually all settings and, thus, are not indicative for water depth (see review in Flügel, 2004). For instance, peloids including those occurring in geopetal infillings are widespread in slope rocks in the northern Bahamas, at depths ranging from
200 to 800 m (Wilber and Neumann, 1993).
Furthermore, and possibly most important, none of the presumed red and green algae occurring in
Palaeozoic mounds (problematic forms such as Fasciella, Aphralysia, Rothpletzella, Wetherella;
Wendt et al., 2001) used as indicative of a setting in the photic zone are unequivocally identified as algae, i.e. the position and taxonomic affinity of these fossils are uncertain (see Madi et al., 1996).
Fasciella is considered a red alga by Madi et al. (1996) and as Calcifolliida and, thus, of uncertain affinity (Phylum, Class and Order incertae sedis) by Vachard et al. (2004). The systematic position of
Aphralysia is controversial, ranging from foraminifer (Belka, 1981) to cyanobacterium (Vachard and
Aretz, 2004). Rothpletzella is considered as a cyanobacterium (cf. Riding, 1991) or as a calcimicrobe ACCEPTED MANUSCRIPT (Webb, 1996, 2001; Nose et al., 2006); thus, its occurrence is not necessarily diagnostic for the photic zone. The occurrence of Rothpletzella in deep-water slopes (Webb, 2001) compromises their attribution to green algae (Wray, 1972), and even to cyanobactria. The taxonomic position of
Wetherella is also uncertain, regarded as foraminifer (Wood, 1948), alga, cyanobacterium or microproblematicum (cf. Riding and Soja, 1993).
Forms ascribed to green algae by Jeffery and Stanton (1996), based on similarities in morphology, are poorly preserved and lack unequivocal taxonomic determination. All specimens illustrated are strongly fragmented. Sphaerinvia, a taxon identified as charophyte (Mamet, 1991; Mamet and Preat,
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2013), was found in situ in a deep, aphotic marine environment, precluding an algal affinity (Jeffery and Stanton, 1996).
4. Further aspects of carbonate mound formation
4.1. Mound and off-mound stratigraphic relationships
Often a generic link between off-mound sediments and mound facies is drawn, to increase the information about the palaeoenvironmental setting favoring mound growth (Lees et al., 1985;
Stanton et al. 2000) and to argue for the autochthonous production of carbonate on mounds embedded in siliciclastic off-mound sediments (Bosence and Bridges, 1995; Monty et al., 1995).
Although the resolution provided by biostratigraphical studies of ancient mounds is usually too low to allow any distinction of age differences between both facies, the “mound-concept”, presuming the mound rising above the surrounding seafloor (Pray, 1958; Lees et al., 1985; Meyer et al., 1995), requires a chronostratigraphic offset between on- and off-mound facies being preserved side-by- side. For the modern bathyal mounds, such offsets can be determined. The best-studied example is
Challenger Mound at the Irish margin that has been drilled during IODP Leg 307 (Ferdelman et al.,
2006). This mound grew for more than one million years to a height of ~130 m, while non-deposition or even erosion prevailed in surrounding areas, highlighting the potential of a carbonate mound to create its own depositional setting, to baffle background sediment effectively, and to create large ACCEPTED MANUSCRIPT stratigraphic offsets between mound facies and adjoining off-mound facies (Titschack et al., 2009). Furthermore, on- and off-mound facies can document very different environmental settings caused by cyclic switches of the main deposition between both settings. For instance, at and around
Propeller Mound at the Irish margin a predominant preservation of glacial sediments in the off- mound facies and of interstadial to interglacial sediments in the mound facies has been described
(Dorschel et al., 2005). Due to the autochthonous carbonate production but also to the enhanced allochthonous, pelagic carbonate sedimentation during interglacials, the mound facies contains on average 50% carbonate compared to <25% in the glacial off-mound facies (Dorschel et al., 2007).
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Thus, by revealing the linkages between on- and off-mound facies, especially in a stratigraphic sense
(ranging from a principal chronostratigraphic offset to alternating deposition reflecting different environmental forcings), studies on modern, bathyal carbonate mounds might trigger new approaches in interpreting ancient carbonate mounds in relation to their surrounding sediments.
The higher volume of mud in cores of fossil mounds compared to intermound/off-mound facies
(Samankassou et al., 2013) remained enigmatic, because the former was supposed to rise topographically higher (assumedly into the zone of high energy where mud should be removed) than the latter (assumedly lower energy where mud can be accumulated) (Pray, 1958). Also this apparent contradiction might become understandable when, for the interpretation of on- vs. off-mound sediments, the possible impacts of (a) (large) chronostratigraphic offsets, (b) different environmental settings, and (c) microenvironments on the mounds where fine sediments might be baffled within a biogenic structural framework are considered. Such factors would affect shallow and bathyal mounds in a very similar way.
Furthermore, especially in a shallow-water setting, bathymetric changes induced by mound growth are expected to be recorded in vertically superimposed facies that are also observed next to each other spatially, following Walther’s Law (Walther, 1894). However, facies architecture in examples of fossil sequences of mounds, e.g., in and around the Devonian mounds of Morocco, fails to show prograding sequences (Fig. 7; Wendt, 1993) that would be indicative of a progressive shift of ACCEPTED MANUSCRIPT environment through time. This observation is in agreement with results of Brachert et al. (1992) who pointed out the absence of obvious vertical change in the depositional types of sediment also for another Devonian mound setting in Morocco. These observations are in line with those of recent settings along the Irish continental margin, where the facies on top of the self-sustained CWC carbonate mounds (basically forming micro-environments) also cannot be traced spatially to shallower or deeper settings.
Figure 7
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(a) Sketch of a Middle Devonian sequence at Jebel el Oftal, Morocco, including carbonate mounds and the surrounding bedded limestone. Redrawn from Wendt (1993).
(b) Sequence of Devonian Kess Kess mounds and the surrounding bedded limestone of Hamar
Laghdad Ridge, Anti-Atlas, Morocco. Mounds seem to be amalgamated and/or aligned. As in the examples reported by Wendt (1993), the facies architecture lacks vertically superimposed facies
(mounds versus bedded limestone) indicative of prograding and/or shallowing-upwards sequences.
The mounds reach ca. 30 m in height.
4.2. Mound growth and sedimentation
In reviewing controls on the formation of Palaeozoic and Mesozoic carbonate mounds, Schmid et al.
(2001) conclude that mound development is restricted to conditions of reduced sedimentation and that high sediment input probably results in burial of mounds as mound growth might be outcompeted by background sedimentation. However, in the first step, the concept of mound formation requires mound growth to be faster than background sedimentation. Furthermore, especially for mounds hosting branching epibenthic organisms, baffling of hemipelagic sediment is a major component contributing to mound growth. Comparing average shelf and slope sedimentation rates (0.04 to 0.5 mm/yr; Seibold and Berger, 1993) with much higher growth rates of such organisms (e.g., for the branches of the cold-water coral Lophelia pertusa: 5 to 34 mm/yr; Roberts et ACCEPTED MANUSCRIPT al., 2009) indicates that burial of a growing carbonate mound by increasing sedimentation appears highly unlikely. Actually, individual modern bathyal carbonate mounds can reach growth rates of >2 mm/yr over several centuries to millennia (Frank et al., 2009; Fink et al., 2013), demonstrating the need for external sediment supply and the efficiency of sediment baffling by mound organisms.
Considering that even such extreme records mainly consist of corals not being preserved in growth position, but of mostly reworked (i.e. compacted) corals, underlines the capacity of coral growth to contribute to mound growth. Thus, burial of carbonate mounds appears to be only a consequence of
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the cessation of active mound growth induced by the deterioration of living conditions for the respective key organisms.
4.3. Mound belts and clusters
Ancient as well as modern carbonate mounds often occur in large numbers in clearly confined regions that are referred to as mound belts/clusters/provinces. This applies, e.g., for the Palaeozoic mounds of Morocco and Algeria (>100 in 440 km2, Brachert et al., 1992; Wendt et al., 1993, 1997,
2001; Fig. 2), Belgium (Aretz and Chevalier, 2007), the UK (Bridges and Chapman, 1988) and New
Mexico, USA (Jeffrey and Stanton, 1996) and for modern mounds off Norway (Fossa et al., 2005),
Ireland (>1000 mounds in distinct clusters between 500 m and 1500 m water depth along the Irish margin; Dorschel et al., 2010), the Bahamas (along the western slope of Great Bahamas Bank; Correa et al., 2012), Mexico (Hebbeln et al., 2014), and Morocco (Foubert et al., 2008). The clustering of mounds is an issue open to discussion and various possible reasons triggering such clustering have been suggested. These range from antecedent structural topography and fault systems to regional productivity patterns and current systems (Wilson, 1975; Ahr, 1989; Brachert et al., 1992; Hovland et al., 1994; Bridges et al., 1995; Lees and Miller, 1995; Meyer et al., 1995; Bourque et al., 1995; Jeffery,
1997; Bourrouilh et al., 1998; Belka, 1998; Huvenne et al., 2007; Eisele et al., 2011; Hebbeln et al.,
2014). Thus, clustering of carbonate mounds also is a feature that is not linked to either shallow or ACCEPTED MANUSCRIPT bathyal settings.
5. Conclusions
Following some general ideas such as (1) that high carbonate production needed to create carbonate mounds has to involve photoautotrophic symbionts (Stanton, 2006; Pomar and Hallock, 2008), (2) that sedimentological indications for high energy levels reflect deposition near the fair-weather or storm wave base (Brachert et al., 1992; Tosolini et al., 2012), (3) that the occurrence of remains of photosynthetic, benthic organisms proves a deposition within the photic zone (e.g., Mamet and
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Preat, 2013), and some other indications as outlined above, the depositional environment of
Palaeozoic and Mesozoic carbonate mounds often has been linked to shallow shelf/ramp settings.
However, modern analogues for such shallow carbonate mounds are rare and do not match the wide distribution of fossil carbonate mounds.
All the commonly used criteria to allocate a shelf origin of ancient carbonate mounds, except the presence of photoautotrophic organisms, are also applicable for carbonate mounds being presently formed well below the shelf break down to water depths of ~1000 m along many continental margins in the world. This refers specifically to vigorous hydrodynamics that are even critical for the development of these modern bathyal mounds. Although the presence of benthic photoautotrophic organisms would be a clear indicator for shallow-water settings, for most of the organisms found in fossil carbonate mounds and being interpreted to be photoautotrophic, the taxonomic position and the metabolism are controversially discussed.
Interestingly, nowadays bathyal mounds appear to be much more common than shallow-water carbonate mounds. Considering the work done on modern carbonate mounds over the last two decades, it appears that for many ancient carbonate mounds a formation beyond the shelf break appears as likely, if not even more likely, as a formation in shallow shelf waters. Thus, as long as no proven photoautotrophic in-situ fossils are found, for many ancient carbonate mounds a reconsideration of the arguments used to conclude their depth of formation might be timely, ACCEPTED MANUSCRIPT especially as such conclusions can have large implications in terms of interpreting the palaeogeography and delineating the extension of former shelf seas. In addition, as modern analogues, recent carbonate mounds might serve as laboratories to gain a better process understanding to support the interpretation of ancient carbonate mounds. By comparing the substantial new knowledge about the distribution of modern carbonate mounds and about their controlling factors gained over the last ~20 years with the long history of research done on ancient carbonate mounds, this paper hopefully will stimulate an intense, open and fruitful dialogue
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between scientists studying either ancient or modern mound systems in order to advance our knowledge of carbonate mounds in general.
Acknowledgements
We thank the numerous colleagues who guided us in the field or supplied material for comparisons used in the paper, in particular the organizers of the COCARDE fieldtrip 2011 in Morocco that actually triggered the writing of this manuscript. We acknowledge funding by the Deutsche
Forschungsgemeinschaft (D.H., grant He 3412/17 - WACOM), the Swiss National Science Foundation
(E.S.) and the European Science Foundation in the frame of COCARDE. We are grateful to Gregor
Eberli, John Reijmer, André Strasser and two anonymous journal referees for commenting earlier versions of the manuscript.
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Figure captions
For the printed version of the manuscript the use of black & white figures is intended, while colour figures should be used for the online version.
Fig. 1: Distribution of cold-water corals (dots and lines in light blue, after Roberts et al., 2006) in the
Atlantic Ocean. Superimposed are examples of proven occurrences of recent (stars, mostly representing clusters of mounds - compiled - compiled from Colman et al., 2005; Fink et al., 2013;
Foubert et al., 2008, Grasmueck et al., 2006; Hebbeln et al., 2014; Le Guilloux et al., 2009, Mienis et al. 2012; 2014; Mullins et al., 1981; Remia and Taviani, 2005; Reveillaud et al., 2008; Reyes et al.,
2005; Taviani et al., 2005; Viana et al., 1998; Wheeler et al. 2007) and of ancient carbonate mounds
(triangles, compiled from compiled from Wilson, 1975; Geldsetzer et al., 1989; Flügel and Flügel-
Kahler, 1992; Bourque et al., 1995; Pratt, 1995; Gutteridge et al., 1995; Lees and Miller, 1995; Madi et al., 1996; Strogen et al., 1996; Boulvain, 2001; Wendt et al., 2001, Riding, 2002; Webb, 2002). At many continental margin sites for which the occurrence of cold-water corals has been described, modern carbonate mounds still might be discovered, as recently e.g. along the Angolan margin (Le
Guilloux et al., 2009), the Moroccan Mediterranean margin (Fink et al., 2013), and the Mexican
Campeche Bank (Hebbeln et al., 2014). ACCEPTED MANUSCRIPT
Fig. 2: Examples for the shape and dimensions of ancient and modern carbonate mounds. Upper panel: Province of exhumed mounds (Kess Kess mounds, Devonian) from the central area of Hamar
Laghdad Ridge, Anti-Atlas, Morocco (details on composition of these mounds are provided in
Brachert et al., 1992). Individual mounds on the image are in average 30 m high. Lower panel:
Multibeam echosounder map of cold water coral carbonate mounds at the Moroccan Atlantic continental margin with dimensions indicated. Data have been obtained in 2014 during expedition
MSM-36 with the German RV Maria S. Merian (Hebbeln et al., unpubl. data).
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Fig. 3: Comparison of the lithology of ancient and modern carbonate mounds. (a) Vertical cut-surface of sediment core GeoB 14904-2 from a Mauritanian margin cold-water coral mound (to the left) compared to a vertical outcrop surface (to the right) of a Visean carbonate mound, Moroccan Anti-
Atlas (for the setting see Wendt et al., 2001). Scale bar is 15 cm. The white rectangle indicates the zoom-in displayed in (b).
Fig. 4: Rippled sea floor with coral fragments from the Porcupine Seabight at the Irish margin in a water depth of ~600 m. The two dots are 20 cm apart from each other. The photograph has been taken in 2010 during expedition POS400 with the German RV Poseidon with the ROV CHEROKEE
(MARUM, University of Bremen, Germany).
Fig. 5: Development from the living coral thicket towards the sedimentary record. (a) Living Lopehlia pertusa framework (in pink) growing on dead, but still intact framework (grey to white) in ~640 m water depth at Rockall Bank, NE Atlantic. The entire thicket reaches ca. 1 m in height. The photograph has been taken in 2004 during expedition M61-3 with the German RV Meteor with the
ROV QUEST (MARUM, University of Bremen, Germany). (b) Coral rubble facies in the Porcupine
Seabight at the Irish margin in ~750 m water depth (width of the image: ~ 30 cm). The photograph ACCEPTED MANUSCRIPT has been taken in 2010 during expedition POS400 with the German RV Poseidon with the ROV CHEROKEE (MARUM, University of Bremen, Germany). (c) Computer-tomograph (CT) image of sediment core GeoB 11569-2 collected from a Mauritanian margin cold-water coral mound (width:
12 cm) showing that generally disintegrated coral fragments (visualized here by CT technique without surrounding sediment) form the mound facies.
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Fig. 6: Detailed bathymetric 3D-map for Propeller Mound, an actively growing cold-water coral carbonate mound in ~800 m water depth at the Irish continental margin (Huvenne et al., 2005). The map clearly shows the flattened top of the mound (map courtesy by B. Dorschel).
Figure 7
(a) Sketch of a Middle Devonian sequence at Jebel el Oftal, Morocco, including carbonate mounds and the surrounding bedded limestone. Redrawn from Wendt (1993).
(b) Sequence of Devonian Kess Kess mounds and the surrounding bedded limestone of Hamar
Laghdad Ridge, Anti-Atlas, Morocco. Mounds seem to be amalgamated and/or aligned. As in the examples reported by Wendt (1993), the facies architecture lacks vertically superimposed facies
(mounds versus bedded limestone) indicative of prograding and/or shallowing-upwards sequences.
The mounds reach ca. 30 m in height.
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