Mascarene Plateau Research Articles

Mascarene basin asthenosphere reservoir source and composition revealed by Central Indian Ridge basalt geochemistry

Along the Central Indian Ridge (CIR), the geochemical and isotopic signature of mid-ocean ridge basalts (MORB) from three segments between 8° and 12°S show a FOZO-like enrichment with elevated 3He/4He (R/RA) (> 10 RA), suggesting small-scale upwelling with a deep primordial mantle component. Based on complementary major and trace elements analysis as well as Sr, Nd and Pb radiogenic isotope compositions of MORB samples from the same location, we confirm the presence of a FOZO/C-like enriched signature characterized by high Pb isotope ratios (206Pb/204Pb = 18.1134–19.1481; 207Pb/204Pb = 15.4710–15.6146 and 208Pb/204Pb = 37.8625–39.0332), relatively low 87Sr/86Sr (0.702767–0.702974) and high 143Nd/144Nd (0.512989–0.513118). In agreement with recent seismological studies highlighting an asthenospheric anomaly centered under the Mascarene Basin and flowing beneath the CIR, we propose that this plume-like anomaly named Mascarene Basin Asthenosphere Reservoir (MBAR) is the source of the enriched FOZO/C-like signature observed along this portion of the CIR. Furthermore, our analyses reveal that the source of the MBAR anomaly may have a unique geochemical signature in the Indian Ocean, distinct from other Indo-African plumes such as the Réunion plume. Moreover, the primordial characteristics, and the location of the MBAR (next to the margin of the African LLSVP) suggest that this plume-like anomaly may be a disconnected blob/proto-plume of a broad mantle structure formed by different Indo-African plumes/upwelling (e.g., Afar and Bouvet plumes) and anchored in the lower mantle.

Cretaceous–Paleocene extension at the southwestern continental margin of India and opening of the Laccadive basin: constraints from geophysical data

Abstract. Previous geophysical investigations of the western continental margin of India (WCMI) confirm the two-phase breakup history of the margin with the first breakup taking place between India and Madagascar that created the Mascarene Basin in the Late Cretaceous and the second breakup event in Early Paleocene with Seychelles separating from India. Despite numerous geoscientific studies along the WCMI, the opening of the Laccadive basin, situated along the southern part of the margin, remains poorly constrained. In this study, we evaluate the multi-channel seismic reflection and gravity anomalies at the margin to identify the early rift signatures in conjunction with the magnetic anomaly identifications in the Mascarene Basin. The analysis led to the identification of two trends of extensional structures, a NNW–SSE-oriented structure over the Laccadive Ridge north of Tellicherry Arch, interpreted to result from ENE–WSW extension, and a SSW–NNE-oriented structure in the Laccadive basin region towards the south, interpreted to result from NW–SE extension. Previous plate reconstruction models of the Mascarene Basin using marine magnetic lineations suggest that the ENE–WSW extension observed over the Laccadive Ridge could be related to the India–Madagascar separation. We associate the pattern of sediment deposition and the presence of a Paleocene trap volcanics, linked with the NW–SE grabens observed in the Laccadive basin region, to the extension between the Laccadive Ridge and the western coast of India after the separation of Madagascar from India. We further propose that the anticlockwise rotation of India and the passage of the Réunion plume have facilitated the opening of the Laccadive basin.

Changes in mesophotic carbonate‐platform export across the end of the last glacial cycle (Saya de Malha Bank, western Indian Ocean)

AbstractThe export of neritic material from the top of carbonate platforms is a key process in the construction of their slopes. However, our knowledge of the supply pattern of materials from platforms is dominantly based on platforms lying in the euphotic zone during the present sea‐level highstand. This is a somewhat biased perspective as through geological time not all platforms were euphotic. The Saya de Malha Bank in the Mascarene Plateau is an example of a modern mesophotic carbonate platform, and as such, its flooding and export patterns differ from those of euphotic ones. Using cores collected on the western slope of the Saya de Malha Bank, the export patterns of the platform since the last glacial maximum were explored. Material on the platform edge is winnowed and transported to the slope by multiple possible processes. The material on the platform is a combination of high and low magnesium calcite as well as high and low strontium aragonite, integrating pelagic and neritic sources. The ratio of these constituents varies over time with changes in the platform production capability as it was flooded and drowned during the Holocene transgression. The material from the platform is transported in both confined flows, mainly during lowstands, and unconfined flows, mainly during late transgression and early highstand. In the present state of the highstand, supply may have diminished, leading to erosion of the canyon shoulders.

Seychelles Plateau's oil spill vulnerability

Small Island Developing States, such as Seychelles, are highly susceptible to oil pollution incidents, with limited infrastructure for detection and mitigation. While an oil spill could significantly impact Seychelle's tourism industry, contributing to ~40% of its GDP, the archipelago's vulnerability remains largely unknown. Here, we developed a high-resolution ocean circulation model for Seychelles Plateau, simulating currents over three years (2018–2020) to model oil spill dispersal to six ecologically and economically significant coastal areas. Our findings reveal distinct seasonality in offshore risk distribution, driven by seasonal fluctuations in atmospheric and oceanic circulations. We show that an oil spill originating from any part of the plateau could potentially impact a sensitive coastal site in less than five days. By identifying high-risk areas, including the major north-south shipping route, we emphasize the importance of close satellite and airborne monitoring for early warnings to protect Seychelles' coastal ecosystems and tourism industry.

Middle Devonian Evandale porphyry Cu-Mo (Au) deposit, southwestern New Brunswick, Canada: Analysis of petrogenesis to potential as a source for distal intrusion-related epithermal gold mineralization

The 391.2 ± 3.2 Ma (U-Pb on zircon) Evandale Granodiorite of southwestern New Brunswick is quite a unique intrusive rock in the southern part of the northeastern Appalachians. This research examines the relationship between the porphyry Cu-Mo-(Au) and epithermal systems with Evandale Granodiorite and environs. This Middle Devonian multi-phase I-type granite intrudes along the northern boundary of the deformed Silurian sedimentary and mafic volcanic rocks of the Mascarene Basin. This granitoid is identified as an oxidized I-type system with adakitic geochemical signatures and associated porphyry Cu-Mo-(Au) mineralization, possibly linked to slab failure sourced from decompression melting of the lower crust caused by post-collisional uplift, similar to the Magaguadavic Granite to the southwest. The Evandale adakitic intrusion is divided into two phases that consist of (1) a calc-alkalic coarser phase ranging from medium- to coarse-grained, seriate to porphyritic granodiorite to monzogranite with metaluminous to peraluminous features, and (2) the later stage alkali-calcic fine-grained (to aplitic) locally porphyritic phase ranging from a monzogranite to syeno-granite with peraluminous characteristics. The primary mineralogy of the intrusive rocks consists of K-feldspar + albite + plagioclase + quartz + biotite + hornblende + apatite ± muscovite ± zircon, along with several alteration phases, such as biotite, chlorite, sericite, and calcite. Geochemical analyses (using INAA, ICP-MS, and XRF) indicate that both phases of the Evandale granodiorite show a high concentration of Cu (108 ppm) and Au (33 ppb) associated with the presence of pyrite, chalcopyrite, and arsenopyrite in the fine-grained dyke samples. Up to 6 ppm Mo was contained within the granitic phase, whereas lesser amounts of Mo were confined to the fine grained to aplite phases. Microprobe analyses of primary biotite as both phenocrysts and groundmass shows 0.55 avg. wt.% Cl and 0.21 avg. wt.% F, which are relatively concentrated compared to other high-grade porphyry deposits. Hydrothermally altered rocks, veins, and sulfides associated with the porphyritic suite result from buoyant magmatic-hydrothermal fluids from a deeper intrusion exsolving fluids (quenched porphyry related), depressurizing, and reacting with the host coarser grained granodiorite and even some of the dykes locally. Based on the geothermometry and geobarometry of amphibole (actinolite-magnesiohornblende), the crystallization temperature and pressure of the Evandale coarse-grained granite and aplite are 674–755 °C, 0.2–0.7 kbar (20–70 MPa) and 838 °C, 0.42–1.4 kbar (42–140 MPa), respectively. This is consistent with biotite formation at temperatures ranging from 670° to 760 °C, based on Ti-in-biotite thermometry. Also, zircon saturation temperatures of the earlier coarse-grained porphyritic granites and later fine-grained porphyry to aplitic dykes are 736 °C and 787 °C, respectively. Spatially and temporally associated gold deposits beside Evandale and within this region of southwestern New Brunswick suggests that these oxidized I-type intrusions are a potential source for distal, low sulfidation epithermal gold deposits in the region.

Cetaceans of the Saya de Malha bank region, Indian Ocean: A candidate Important Marine Mammal Area

The banks of Saya de Malha and the surrounding Mascarene Plateau in the Indian Ocean are among the least studied shallow sea water regions in the world. The steep sloping bathymetry of this region is thought to drive high levels of primary productivity and support a diverse range of marine species. Until now, no surveys for cetaceans have been conducted in the waters surrounding Saya de Malha, and so the diversity of cetaceans is unknown. Opportunistic visual and acoustic surveys for cetaceans were conducted onboard a platform of opportunity, the MY Arctic Sunrise, which was involved in a wider project to document the marine life in the region. Survey effort was conducted over 7,700 km, with twelve species of cetacean encountered, including Bryde’s, sperm, beaked and killer whales, along with spinner, striped, pantropical spotted and bottlenose dolphins. A match of a sperm whale coda vocalisation to another, well-studied population off the coast of Mauritius suggests possible connectivity between these regions although further data would be required to confirm this. The banks of Saya de Malha appear to support a diverse range of cetacean species and further systematic surveys are required to increase our understanding of how different species utilise the banks. We provide whistle contours from visually confirmed acoustic detections to contribute towards building a region-specific whistle classifier. Given the diversity of species detected in the region, we suggest that the Saya de Malha bank area be designated, either as an Important Marine Mammal Area or as a marine protected area.

Chemical fractionation in benthic fauna from Saya de Malha Bank (Mascarene Plateau, western Indian Ocean): Vital and habitat effects

The temperature, salinity, nutrient availability and dissolved oxygen characterizing ocean water-masses and circulation patterns are essential environmental controlling parameters of marine benthic assemblages. This work presents for the first time the geochemical composition of different taxonomic groups of benthic assemblages from the Saya de Malha Bank, Mascarene Plateau (Indian Ocean). The studied stations, located in water masses at different water depths, are perfect spots for studying biogeochemical proxies of diverse taxonomic groups to elucidate the impact of the vital factor as opposed to environmental or oceanographic factors. Forty-two specimens were studied, including scleractinian corals, bryozoans, brachiopods, bivalves, gastropods, scaphopods, and echinoids. Specimens were taphonomically analyzed in order to discard those with a poor preservation that might affect the geochemical assays. Geochemical analyses of the specimens involved high resolution inductively coupled plasma-mass spectrometry for main and trace elements. According to the results, (1) the content of trace elements in the shells is primarily controlled by their mineralogy: i.e. aragonitic, low-Mg calcite, and high-Mg calcite. (2) Vital effects related to metabolism play a major role in the co-precipitation and adsorption of trace elements in the shells, especially for Ca, Mg, Sr, Ba, and Zn. (3) The environmental parameters (habitat effect) such as oxygenation, primary productivity, salinity, but mainly temperature, at different water depths were found to be related to the growth rate of the specimens and the incorporation of trace elements. In the shallower station, the Li/Mg ratio is lower and the content in U is higher among the selected taxa, which is related to an increased growth rate in warmer waters.

Revision of the Western Indian Ocean Angel Sharks, Genus Squatina (Squatiniformes, Squatinidae), with Description of a New Species and Redescription of the African Angel Shark Squatina africana Regan, 1908.

Sampling efforts on the Saya de Malha Bank (part of the Mascarene Plateau, western Indian Ocean) unveiled three unusual small juvenile angel shark specimens, that were a much paler color than the only known western Indian Ocean species, Squatina africana Regan, 1908. However, it took many years before further specimens, including adults of both sexes, and tissue samples were collected. The present manuscript contains a redescription of S. africana based on the holotype and additional material, as well as the formal description of the new species of Squatina. All specimens of the new species, hereafter referred to as Squatina leae sp. nov., were collected in the western Indian Ocean off southwestern India and on the Mascarene Plateau at depths of 100-500 m. The new species differs from S. africana in a number of characteristics including its coloration when fresh, smaller size at birth, size at maturity, and adult size, genetic composition, and distribution. Taxonomic characteristics include differences in the morphology of the pectoral skeleton and posterior nasal flap, denticle arrangement and morphology, vertebral counts, trunk width, pectoral-pelvic space, and clasper size. A key to the species of Squatina in the Indian Ocean is provided.

A new species of Chicomurex (Gastropoda, Muricidae) from the Saya de Malha Bank, Western Indian Ocean

Chicomurex kozlovi sp. nov., collected from the Saya de Malha Bank in the Mascarene Plateau, Western Indian Ocean, is described and illustrated. The holotype was sampled at a depth of 200 m during the expedition of R/V Gordy in 1989, while paratypes 1 and 2 were trawled from a depth of 100-200 m by research-fishery boats during the eighties. Chicomurex kozlovi sp. nov. is compared with morphologically similar congeneric species.

Heterogeneous Fossil Réunion Plume Component in the Source Region of Enriched MORB Along the Central Indian Ridge Between 12° and 17°S

AbstractMid‐ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12° and 17°S show a wide range of geochemical and isotopic variations. Particularly, MORB from a segment between 14° and 15°S are more enriched in incompatible trace elements with more radiogenic Sr and Pb isotope and unradiogenic Nd isotope values than the lavas between 15° and 16°S with geochemical features of normal MORB. However, the causes for the enrichment between 14° and 15°S are poorly constrained. In this study, we re‐examined the CIR MORB from 12° to 17°S with new geochemical data obtained based on high spatial resolution sampling to better understand the nature of the enriched mantle source. Our new geochemical data show that the MORB between 14° and 15°S, with maximum values for (La/Sm)N = 1.95, 87Sr/86Sr = 0.703526 and 206Pb/204Pb = 18.7558, are more enriched than those from the southern segments (16° to 20°S) known to be influenced by the Réunion mantle component. The new trace element and isotopic compositions of MORB suggest that three mantle end‐members are required to explain the geochemical variations shown between 14° and 16°S: the depleted Indian‐type MORB mantle, Réunion Plume (RP), and Seychelles/Madagascar‐like continental crust components. Moreover, our mixing model suggests that the differences in enriched MORB signature from 14° to 20°S are due to variable proportions of continental material previously mixed with the RP. Our study implies that a continental component interacted with the plume into the asthenosphere, possibly beneath Madagascar or below Mauritius Island and the Mascarene plateau.

Satellite Altimetry and Gravimetry Data for Mapping Marine Geodetic and Geophysical Setting of the Seychelles and the Somali Sea, Indian Ocean

AbstractEvaluation of the representative cartographic techniques demonstrated that there are still considerable challenges facing the methods of marine geodetic, geophysical and bathymetric data visualisation. In an oceanic seafloor formation, the interaction between the geological structural elements and topographical relief can be analysed by advanced mapping. In present study, a correlation between geodesy, geophysics and topography has been examined including the following variables: geological structure, coastal topography and bathymetry, geophysical fields, free-air gravity anomalies and geoid undulation, sediment thickness, bathymetric patterns, and extension of the transform faults. The variables were visualised on the high-resolution raster grids using Generic Mapping Tools (GMT) scripting toolset. The study area is located in the Seychelles and the Somali Sea segment of the Indian Ocean. The data incorporates satellite-derived gravity grid, EGM-2008, geological structures, topography from GEBCO grid and GlobSed sediment thickness, processed by GMT scripts. The results demonstrated that western continental slope of Somalia is wide, gently declining to the seafloor at depths exceeding -5000 m. Kenya and Tanzania present a wide continental foot with depths ranging from -3500 to -5000 m. The Somali Sea basin shows low sedimentation lower than 500 m, while ridges and island chains have higher sediment influx (1,000-2,000 m). The Mozambique Channel has dominating values at 2,500-3,500 m. Higher values are noted near the Reunion and Mauritius islands until the Seychelles via the Mascarene Plateau (500-1,000 m) against the <500 m in the areas of the Mid-Indian Ridge, Carlsberg Ridge and open water.

Diversity of Siphonaria Sowerby I, 1823 (Gastropoda, Siphonariidae) in the Seychelles Bank and beyond

AbstractMolecular phylogenetic studies have shown that the characters of the reduced shell of the false limpets of the genus Siphonaria Sowerby I, 1823 are highly variable and often insufficient for species delimitation. The taxonomy and distribution of Siphonaria in the Indian Ocean are poorly known. We sampled Siphonaria in the Seychelles Bank to check the occurrence of recorded species using DNA sequences and to study the paths through which Siphonaria species have colonised the Seychelles Bank by reconstructing their phylogenetic relationships. Analyses of a dataset comprising 16 S rRNA gene sequences of 33 specimens from the Seychelles Bank and 300 additional Siphonaria sequences from other regions from GenBank with various methods for species delimitation resulted in 19–102 primary species hypotheses. Assemble Species by Automatic Partitioning provided a conservative estimate of the species number (42) in which several indisputable species were lumped. The results of Automatic Barcode Gap Discovery depended strongly on the assumed prior maximum intraspecific divergence, whereas the tree‐based methods Generalised Mixed Yule Coalescent and Poisson Tree Processes resulted in high overestimates. The specimens from the Seychelles Bank represent three clades, belonging to the Siphonaria ‘atra’ group, the Siphonaria ‘normalis’ group and a possibly undescribed species recorded previously only from Hainan. At least two of the three species recorded from the Seychelles Bank came from the east, i.e., from the Coral Triangle in the Indo‐Australian Archipelago, the region with the highest marine biodiversity worldwide. A major transport mechanism across the Indian Ocean was probably the South Equatorial Current.

Variable photo-physiological performance of macroalgae and seagrasses from Saya de Malha and Nazareth Banks, Mascarene Plateau

The photosynthetic performance of macroalgae and seagrasses related to their body parts, depth and colours from the poorly-studied Saya de Malha and Nazareth Banks on the Mascarene Plateau was investigated in this study. Two seagrass (Thalassodendron ciliatum and Halophila decipiens) and seven macroalgae species (Caulerpa cupressoides, Acrosorium ciliolatum, Dictyosphaeria cavernosa, Halimeda opuntia, Ulva sp., Udotea orientalis and Udotea palmetta) were collected using the five Van Veen grabs attached to the Video-Assisted Multi-Sampler (VAMS) from 29-79 m depths in May during the FAO EAF-Nansen Research Programme 2018. The photosynthetic performance was measured using a Diving-Pulse-Amplitude-Modulated (D-PAM) fluorometer and the parameters included effective quantum yield at photosystem II (PSII) (ΦPSII), relative maximum electron transport rate (rETRm), photosynthetic efficiency (α), photoinhibition (β), saturating light level (Ek), and maximum non-photochemical quenching (NPQmax). All photo-physiological parameters varied significantly in T. ciliatum and C. cupressoides across their body parts. However, variation with seawater depths was not significant for NPQmax and β in the seagrass, and ΦPSII, rETRm and β in the macroalgae. Photo-physiological functioning of the leaf of T. ciliatum was optimal at 40 m. The photosynthetic performance of the frond and stolon of C. cupressoides decreased and remained unchanged, respectively, at 79 m when compared to that at 29 m. The whitish lobes of H. opuntia at 31 m exhibited significantly lower photosynthetic performance, in terms of ΦPSII, rETRm,α and Ek, than the greenish lobes. These findings provide a first insight of seaplant body parts-, depth- and colour-related photo-physiological performance from the Mascarene Plateau.

Spatial distribution of surface chlorophyll a and micro-phytoplankton density and diversity around two islands and at two banks of the Mascarene region

The present study validated the use of AquaMODIS sea surface chlorophyll a (Chla) concentrations and investigated the spatial variation in density and diversity of micro-phytoplankton around two islands and two fishing banks of the Mascarene region. The study included areas around Mauritius (MRU) and Rodrigues (ROD) Islands, at Nazareth (NZ) Bank, and in the Joint Management Area (JMA) between the Republic of Mauritius and the Republic of Seychelles, more specifically at the Saya de Malha (SM) Bank. The AquaMODIS data were based on 67 match-up data points of in-situ against satellite Chla concentrations. The micro-phytoplankton community structure was investigated by determining the density variation and using the Shannon Wiener (H’) and Evenness (Evar) diversity indices. The satellite and in-situ Chla data were significantly and positively correlated when pooled for the four sites studied (R2 = 0.441; r = 0.642, P < 0.01), and when analysed separately for islands (R2 = 0.480; r = 0.694), and banks (R2 = 0.233; r = 0.483). However, the Chla satellite values tended to be lower than the in-situ Chla data. The highest densities of micro-phytoplankton were observed in the eastern and northern regions for MRU and ROD, respectively. The most dominant genera of micro-phytoplankton were Coscinodiscus, Navicula, Chaetoceros and Ceratium. The Shannon-Wiener diversity index values for diatoms were all above 2.5 with waters around the islands having higher diversity compared to the banks. Overall, the different micro-phytoplankton around the islands, except for the group of cyanobacteria at ROD Island, were more evenly distributed (Evar > 0.6) compared to the banks. This study indicated that AquaMODIS Chla satellite data is valid and may be potentially used as a proxy for in-situ Chla concentration on the Mascarene Plateau. The results of this study also provide detailed insight into the spatial variation in micro-phytoplankton density and diversity on the Mascarene Plateau in the Western Indian Ocean. Further long-term studies are warranted to thoroughly understand the temporal (including seasonal and inter-annual) variations in Chla and micro-phytoplankton distribution for adequate and appropriate management of these ocean territories.

First in-situ observation of the endemic giant clam Tridacna rosewateri from the Nazareth Bank, Mascarene Plateau

Field note - N/A

Inorganic nutrient distribution at Saya de Malha and the eastern slope of the Nazareth Banks

The distribution of nutrients in the Mascarene Plateau is poorly studied due to limited data collected in the region. This study investigated the distribution of nutrients in the water column of the Saya de Malha and eastern slope of the Nazareth Banks. Water samples were collected at the Saya de Malha and eastern slope of Nazareth Banks during the scientific expedition on board the R/V Fridtjof Nansen in May 2018 and the vertical profiles of physical parameters were used to identify the different water masses. Phosphate, nitrate and silicate showed typical nutrient profiles along with the nutriclines. The dissolved oxygen and chlorophyll-a showed a correlation with nutrients. Most of the sampling stations on the Saya de Malha and Nazareth Banks showed high oxygen saturation above 92 % in the upper 75 m. Low chlorophyll-a values were recorded at both banks indicating an oligotrophic system. The nutrient results also reveal an increase in the concentration towards the northern part of the region due to the South Equatorial Current crossing the plateau.

First field observations of Halimeda beds at depths of 37-62 m at Saya de Malha and Nazareth banks, Mascarene Plateau

Field note - N/A

Diversity and distribution of the shallow water (23-50 m) benthic habitats on the Saya de Malha Bank, Mascarene Plateau

The Saya de Malha Bank (SMB) is one of the largest and least studied marine banks on the Mascarene Plateau. This study aimed to examine the diversity and distribution of the main benthic habitats in the shallow waters of the SMB (23 to 50 m). The survey was carried out in May 2018 during the EAF-Nansen Indian Ocean Research Expedition using a Remotely Operated Vehicle (ROV) deployed at 15 stations. Four main benthic habitats were investigated and their relative abundance determined during the survey. The 143,110 m2 surveyed area revealed proportional benthic habitat cover of 43.6 ± 22.4, 24.5 ± 21.9, 21.2 ± 27.8, and 10.5 ± 12.6 % for seaweed, abiotic substrate, seagrasses and corals, respectively. The seaweed habitat (43.6 %) was mainly composed of Halimeda spp. It represented up to 77 % of the habitats observed at SS34 (4553 m2). Even though seaweeds are considered seasonal, their dominance at all stations creates an important habitat structure for many organisms. The seagrass habitat (21.2 %) was dominated by Thalassodendron ciliatum. This habitat covered up to 93 % of the area investigated at SS38 (5950 m2) and was found mainly on the eastern side of the bank. The live hard coral habitat (10.5 %) was the highest at SS36-2 (35% of 9819 m2) and was more homogenously spread within the shallow areas. The unstable and the stable bare bottom substrate habitat (24.7 %) characterized as abiotic habitat was mainly composed of bedrock, sand, and rubble. It dominated at SS42 where it constituted 72.5 % of the 5114 m2 investigated and was recorded at all stations. Further research is warranted to better understand the diversity and the distribution of benthic habitats within the shallow waters of the SMB, along with collection of targeted benthic organisms for identification at higher taxonomic levels, to better formulate conservation and management measures and strategies.

Photo-physiology of healthy and bleached corals from the Mascarene Plateau

This study presents the first report of variable photo-physiology of healthy-looking and bleached corals from the upper mesophotic waters of the Mascarene Plateau. In May 2018, during the FAO EAF-Nansen research expedition cruise, coral bleaching was visually observed. Five coral species from Saya de Malha Bank, namely Heliopora coerulea, Favites sp. and Porites sp. from 27 m and Acropora sp. and Lithophyllon repanda from 30 m, and three coral species from the Nazareth Bank, namely Acropora sp. and Galaxea fascicularis from 36 m and Stylophora-like species from 58 m were studied using the Video-Assisted Multi-Sampler (VAMS) and collected using a Van Veen grab. Chlorophyll a fluorescence parameters such as effective quantum yield at photosystem II (ΦPSII), relative maximum electron transport rate (rETRm), photosynthetic efficiency (α), photoinhibition (β), saturating light level (Ek), and maximum non-photochemical quenching (NPQm) were measured using a Diving-Pulse-Amplitude-Modulated (D-PAM) fluorometer to study variable photo-physiology in bleached and non-bleached corals. All photo-physiological parameters varied significantly among coral species tested and between coral conditions, except for β. The interaction between species and coral conditions was only significant in the case of β, but generally not significant. A two-way ANOVA indicated significant effects of depth and coral conditions in Acropora sp. on almost all photo-physiological parameters, except for β, and the effect of depth on rETRmax and α, and the effect of depth along with its interaction with coral conditions on Ek. ΦPSII did not differ in bleached and healthy-looking coral parts of Porites and Lithophyllon from 27 m, Galaxea and Acropora from 36 m while it decreased significantly in Heliopora and Favites at 27 m, Acropora from 30 m, and Stylophora-like at 58 m. NPQm did not change for Porites, Acropora (30 m) and Galaxea but it tended to increase for Heliopora, Acropora (36 m), Lithophyllon, Galaxea, and decrease for Favities, Acropora (30 m) and Stylophora-like. The thermally tolerant coral Porites exhibited normal photo-physiology even in bleached conditions while the bleached parts of Favites, Acropora (30 m) and Stylophora-like corals exhibited photo-physiological dysfunctioning. This study revealed that the seven studied corals from the upper mesophotic waters of the Mascarene Plateau are not spared from the bleaching phenomenon and exhibit variable photo-physiology in bleached and non-bleached conditions. Further studies are warranted to thoroughly understand the coral bleaching patterns and severity during summer periods at the Saya de Malha and Nazareth Banks.

Grain size analysis and total organic matter and carbonate contents of sediments on Saya de Malha and Nazareth Banks

The Mascarene Plateau is one of the least studied regions of the Western Indian Ocean. The aim of this paper is to describe the qualitative grain size characteristics and the total organic matter and carbonate contents of sediments on the two underwater banks known as the Saya de Malha and Nazareth Banks. Sediment samples were collected during the R/V Dr Fridtjof Nansen expedition in 2018, using a Video-Assisted Multi-Sampler (VAMS). Sieving techniques and GRADISTAT software were used for granulometric analysis and grain size statistics, respectively. Total organic matter and carbonate contents were determined with the sequential weight loss on ignition method. Grain size parameters from 15 stations (depths ranging from 37 to 381 m) are reported in this study. The results showed that sand was the dominant grain size on both banks. Eleven of the 15 stations had a mean grain size of coarse sand, three stations had a mean size of medium sand and one station had fine sand. The average total organic matter and carbonate contents on the Banks ranged from 2.1 to 7.7 % and from 53.9 to 58.3 %, respectively. This study provides baseline information that will help to classify benthic habitats and better understand the ecosystem functioning and dynamics on of the Saya de Malha and Nazareth Banks.