Contourite drifts and cold-water coral mounds in the Atlantic Moroccan coral province. Origin, evolution and driving factors = Contourietdriften en koudwaterkoraalheuvels in de Atlantische Marokkaanse koraalprovincie. Oorsprong, evolutie en sturende factoren
Vandorpe, T. (2017). Contourite drifts and cold-water coral mounds in the Atlantic Moroccan coral province. Origin, evolution and driving factors = Contourietdriften en koudwaterkoraalheuvels in de Atlantische Marokkaanse koraalprovincie. Oorsprong, evolutie en sturende factoren. PhD Thesis. Ghent University. Faculty of Sciences: Ghent. 172 pp.
|  |
| Available in | Author |
|
Document type: Dissertation
|
| Abstract |
Bottom currents were described for the first time in 1751, but it wasn’t until the 1950’s and 60’s they were acknowledged for being able to transport and deposit sediments. Bottom currents can have a wide variety of shapes and sizes, ranging from broad, continuous, contour-following currents that are part of the global thermohaline circulation till ephemeral, episodic, semi-diurnal internal tidal currents, induced by the interface between two water masses with very different characteristics. As such, they cover a wide spectrum and can occur anywhere in the ocean, from the upper shelves till the abyssal depths. Contourites result from bottom currents and are defined nowadays as sediments deposited or at least partly reworked by the persistent action of bottom currents. They are one of the three endmembers of a continuum of deep-sea deposits besides turbidites, resulting from episodic downslope processes, and pelagites, resulting from vertical settling of sediment particles in the water column. Sediment drifts are morphological units consisting (at least partly) of contourites and occur in many different shapes and sizes, ranging from extensive sheeted drifts till small-scale patch drifts. They can occur anywhere sufficient sediment is at their disposal, bottom currents are strong enough and ample accommodation space is present. Contourites are characterized by elevated sedimentation rates and are as such preferred sites for palaeoclimatic and palaeoceanographic research. Cold-water corals are corals that live in the deep-sea in the absence of light. They occur mostly at depths between 50 and 1000 m, but specimen have been found at 4000 m. Cold-water corals have the ability to create cold-water coral mounds (hereafter called coral mounds), which are a combination of coral fragments and sediment particles. Both are delivered by bottom currents, which flush them through their framework. These coral mounds can reach heights exceeding 200 m and contain several periods of coral growth and periods of mainly (hemi-) pelagic sedimentation. The growth of cold-water corals is strongly depending upon climate and consequently, coral mounds are perfect features to study the effects of changing climate in the study area. Along the Atlantic Moroccan margin, south of the Strait of Gibraltar, several sediment drifts and a large amount of coral mounds occur. Additionally, numerous mud volcanoes, two tectonic ridges and two transform faults are present. The co-occurrence of sediment drifts and coral mounds offers an excellent opportunity to unravel the Quaternary palaeoclimate of the Atlantic Moroccan margin. Furthermore, the occurrence of several sediment drifts along the topographic obstacles also provides the opportunity to study the effect of topographic obstacles on drift deposition. The differentiation between contourites and turbidites in sediment cores is difficult due to their similar characteristics and consequently, more diagnostic criteria are required in order to discern them. The facies model of contourites mostly relies on sedimentological and lithological criteria, while the differentiation based on those criteria is problematic. X-ray computed tomography (CT) is a nondestructive method which allows to investigate sediment cores rather fast. The applications of this technique in contourite research mainly focused on sedimentological structures as X-ray radiographs yield a sharp and contrasting image of the sediment core. However, CT offers much more possibilities; quantification of the grey values, obtained from the radiographs, allows to define the contourite facies in an entirely different way and may offer a solution in determining and distinguishing this facies from turbidites and pelagites. The radiographs of 5 cores from the northern Gulf of Cádiz and the Alboran Sea were analysed to quantify and describe their contourite intervals in order to determine diagnostic criteria that can be used for their recognition in the sedimentary record. Contourites and cold-water coral mounds in the southern Gulf of Cádiz The methods (Chapter 3) used to achieve the goals mainly consisted of geophysical methods. Over 3000 km of seismic profiles were gathered, using two different source, a SIG sparker and an ATLAS parametric echosounder. The sparker can reach greater depths (up till 300 m in the subsurface), while the echosounder achieves a higher resolution (decimetre-scale). Both datasets underwent extensive processing before they were interpreted. Multibeam maps were obtained based on two datasets, one gathered in 2002 using a SIMRAD E1002 system and one in 2008 using a Kongsberg EM300 system. Besides these geophysical methods, oceanographic measurements were performed as well. Several CTD (conductivity temperature depth) profiles and ADCP (acoustic Doppler current profiler) measurements were gathered. CTD profiles yield the physical properties of the water column, while ADCP measurements yield current direction and intensities. Additionally, water samples were gathered at specific depths and the nutrient content of these water samples were determined in order to define the water masses. Sediment cores were analysed using MSCL (multi-sensor core logger) and XRF (X-ray fluorescence) devices to ascertain the physical (MSCL) and chemical (XRF) composition. Grain-size analyses were performed using the Malvern Masterseizer, a device that measures grain sizes based on laser diffraction. Finally, CT scans were acquired using the medical SOMATOM scanner of the UZ Ghent hospital. The obtained resolution was 0.2x0.x0.6 mm and the images were reconstructed using the ‘J37 smooth medium’ algorithm. The seismic stratigraphy of the Pen Duick and Renard north drift systems were discussed in Chapter 4. To achieve this goal the seismic (sparker) and multibeam dataset were mainly used. Results indicate that 5 different units could be discerned, off which the 4 most recent ones were influenced by bottom currents. From the base of the Quaternary till the Early-middle Pleistocene transition (EMPT), the sedimentation mainly consisted out of sheeted drifts, while from the EMPT onwards, mounded drifts occur. Between the seafloor and the EMPT boundary, 10 subunits could be differentiated, each linked to a glacial period. The sediment drifts were substantially influenced by the uplift of the tectonic ridges (till the EMPT), indicated by the pinch-out of the reflectors on the ridges, and the extrusion of mud from the mud volcanoes, indicated by the Christmas-tree structure in the subsurface. Seven coral mounds are situated in the moat at the foot of the Pen Duick escarpment. They influence the bottom currents and induce a shift in the position of the moat. In turn, the coral mounds are influenced by the bottom currents, as they contain a sediment and a coral side. The stratigraphy of the drift systems indicates no similarities with drifts linked to the Mediterranean outflow water and consequently, the water mass that is responsible for the drifts systems originates elsewhere. The AAIW (Antarctic Intermediate Water) is a suitable candidate due to its proven presence in the southern Gulf of Cádiz. The position and the driving factors behind the different drift systems related to the prevailing water masses and bottom currents in the EAMVP (El Arraiche mud volcano province) were investigated in Chapter 5. To achieve this goal, not only the multibeam and seismic (sparker) data were used, but also the oceanographic measurements (CTD, ADCP and nutrient content). The latter indicated that both NACW (North Atlantic Central Water) and AAIW are present along the Moroccan Atlantic margin. The currents are mainly directed south-north, but along the seafloor, a more complex pattern is present, related to the deviation of bottom currents on the topographies and the proven presence of internal tides. The position of the drift systems along the tectonic ridges is determined by the steepness of the ridges, while the drift systems along the mud volcanoes (both at their northern and southern side) originate due to the asymmetric internal tidal currents. The latter are also capable of creating erosional features when forced to pass through a narrow gateway. Patch drifts may occur in those erosional features. The link between the steepness of the bounding topographic features and drift occurrence has been described for the first time, as well as sediment drifts resulting from internal tidal currents. The conclusions may contribute to our understanding of the dynamic interactions between bottom Contourites and cold-water coral mounds in the southern Gulf of Cádiz currents and topographies, which may lead to the discovery of thousands of additional drift systems all over the world. The Atlantic Moroccan coral province (AMCP) is up till now the largest discovered coral mound province in the world as well as the first coral province where several initiation horizons are evidenced on such a broad scale. Ubiquitous coral mounds were observed on top of the seafloor as well as in the subsurface and their spatial and temporal evolution is discussed in Chapter 6. To achieve this goal, the seismic (both sparker and echosounder) and multibeam dataset have been studied. The coral mounds are on average 20 m high and developed during different epochs, indicated by the presence of at least 10 initiation levels in the subsurface. Some coral mounds span several epochs, but most of them remained rather small and were buried before the successive period of coral mound initiation and aggradation started. The 10 initiation levels were tentatively linked to glacial periods from the EMPT till present, given the majority of the ages (obtained by absolute datings of coral rubble) from this area are linked to glacial stages. This link indicates the strong climatic dependence of cold-water corals along this margin. Coral mounds seem to occur grouped in the AMCP, which may be explained by the deviation of bottom currents on coral mounds, resulting in more focused and intensified bottom currents around the coral mounds. These conditions are ideal for other cold-water corals, as a higher flux of food particles and sediment (both prerequisites for coral mound growth) is caused by this mechanism. As such, coral mounds exert a positive effect on their environment and induce the further development of cold-water corals in the vicinity. Unambiguous distinctive criteria to recognize contourites in sediment cores have not yet been discovered. CT may prove to be a big asset to achieve this goal, given the characteristics of contourites based on CT scans have not yet been considered in their identification. In Chapter 7, five cores were analysed using CT, XRF, MSCL and grain-size measurements after which a workflow was created, allowing to discern components of the core based on the grey values histogram. The variations of these components throughout the core were compared to known contourite intervals and plotted beside XRF, MSCL and grain size data. A direct link (based on these 5 cores) between Hounsfield unit (HU) values and more vigorous bottom currents (derived from grain sizes and elevated ratios of Zr/Al) was discovered. Also a gradual increase in HU values with increasing energetic sedimentation environments was established, further highlighting the link between bottom currents and HU values. Analysing additional sediment cores from different locations is required though before this technique could be generally applied. The results obtained from chapters 4-7 invoke several questions and mutual comparisons induce remarks regarding the current classifications and definitions. These questions and remarks are the subject of Chapter 8. The stratigraphies obtained in chapters 4 and 6 do not agree from the EMPT onwards. This may be caused by several factors, off which the two most important ones are the lack of tectonic activity (yielding possible markers) and the lack of absolute datings from the sediment drifts and coral mounds. Based on calculated sedimentation rates (resulting from both stratigraphies), the coral mound stratigraphy seems the most plausible, although one or several erratic age-depth correlations may be present nonetheless. The AMCP is the first region in the world where sediment drifts are linked directly to internal tidal currents. For the larger mounded drifts along the tectonic ridges, they are merely a helping factor, but for the patch drifts along the mud volcanoes, they are considered to be the sole cause. Moreover, the build-up of the sediment drifts along the tectonic ridges is related to two water masses containing bottom currents flowing in the same direction. Consequently, these drifts could support the hypothesis that contourite depositional systems may originate from (or be maintained by) more than one water mass. A new classification scheme for patch drifts was proposed based on the small drift systems along the mud volcanoes and coral mounds in Contourites and cold-water coral mounds in the southern Gulf of Cádiz the AMCP. Additionally, the preposition ‘tidal’ was proposed to describe sediment drifts that are (at least partially) the result of or are maintained by internal tidal currents. A maximum extent has been proposed for these tidal sediment drifts (i.e. the area where bottom current influence can be recognized in the sedimentary record) since otherwise a large part of the ocean floor would be covered with sediment drifts, which would not represent the location of bottom-current influenced deposits. As sediment drifts can result from the deviation of bottom currents on topographic obstacles surpassing a certain steepness-threshold (11° in the AMCP), a large amount of additional (small-scale) sediment drifts could be discovered in the world’s oceans, further strengthening the need of a limited extent for at least (tidal) patch drifts. Finally, the conclusions regarding CT-analyses of contourite cores are put into a broader perspective. The differentiation between contourites and turbidites remains difficult though as turbidites also originate from high-energetic depositional environments. Consequently, additional analyses are required before general diagnostic characteristics can be obtained. To achieve this goal, sediment cores are required that contain well-defined and well-studied pelagites, turbidites and contourites. In Chapter 9, the main conclusions of this thesis are summarized. The conclusions are grouped into 5 main topics: (1) the climatic growth phases of cold-water corals and coral mounds in the AMCP, (2) the spatial and temporal evolution of sediment drifts in the EAMVP, (3) the link between oceanographic processes and drift deposits, (4) the classification and extent of (mostly small-scale) sediment drifts and (5) the analysis of sediment cores using CT-analyses. Moreover, several possible future research topics are proposed, which would enable to extent and verify some of the conclusions resulting from this thesis. They consist mainly of acquiring sediment cores, conducting long-term oceanographic measurements and acquiring additional geophysical data as well as specific CT-analyses of sediment cores containing turbidites, contourites and pelagites. |
|
