NW Indian Ocean Research Articles

Quaternary floodings in the Zanzibar Channel (NW Indian Ocean, Tanzania) – Identifying palaeoceanographic patterns and palaeoenvironment using a multiproxy study

Multiproxy palaeoenvironmental analyses represent a promising tool for complex reconstructions of continent-sea or air-sea interactions in shallow marine environments. This is in particular illustrated by the case of strong climatic variations in the late Pleistocene to mid-Holocene. A unique combination of geochemical proxies (143Nd/144Nd, 87Sr/86Sr, δ18O and δ13C isotope analyses, n-alkane analysis and biomarker study) together with micropalaeontological data (foraminifera, calcareous nannoplankton and diatoms) allowed to precisely characterise two marine floodings (mid-Holocene and late Pleistocene) in the Zanzibar Channel (NW Indian Ocean, Tanzania). The climate of the late Pleistocene interval was arid compared to the Holocene. The fully oligotrophic environment characteristic of the late Pleistocene flooding (∼115–130 ka) was episodically interrupted by intervals of intense rainfall with episodes of increased nutrient input during the mid-Holocene (∼5–10 ka). The dominance of seaweed meadows in the Holocene and late Pleistocene contrasts with the modern environment dominated by seagrass ecosystems. Relatively non-radiogenic εNd signatures in the Zanzibar Channel during the Holocene and late Pleistocene indicate a strong influence of riverine input draining the Precambrian basement on the African mainland. The main inflow of seawater was from the south, consistent with the flow direction of the East African Coastal Current and the directions of the March to October monsoon winds. A promising tool for future applications as indicators of seagrass/seaweed meadow type ecosystems may be the presence of specific diatom taxa.

The remarkable parallels between the North East Atlantic and Arctic regions

Geological understanding of the NE Atlantic and Arctic regions has increased greatly over the last two decades, revealing remarkable similarities. Continental extension in both regions onset during pausing or cessation of adjacent orogenies – the Verkhoyansk-Chukotka and Alpine orogenies – which relaxed compressional stresses and permitted extension. Severe extension accompanied by high-volume magmatism did not onset abruptly but was the culmination of long-term, regional tectonic and magmatic unrest. It was complex and piecemeal, with extension and volcanism occurring along multiple axes simultaneously, a process still ongoing in Iceland today. Large Igneous Province emplacement did not drive breakup but occurred as a consequence of continental extension. The High Arctic Large Igneous Province did not progress to breakup. As a result, much of the Arctic Ocean is floored by hyper-extended continental crust overlain by seaward-dipping reflectors. This crust resembles the passive margins of the NE Atlantic. The Alpha-Mendeleev Rise in the Arctic Ocean is a double-sided passive margin underlain by hyper-extended continental crust draped with seaward-dipping reflectors. A similar structure has been proposed for the Greenland-Iceland-Faroe Ridge. If correct, present-day volcanism in Iceland is building SDRs today. Up to ∼50% of the NE Atlantic Ocean oceanward of the continental shelves is likely underlain by at least some continental crust. For the Arctic Ocean this figure is ∼80%. Similar structures are found elsewhere, including the Madagascar Channel, the NW Indian Ocean, and the South Atlantic. These new ideas suggest promising directions of future study including assessing the true extent of continental crust in the oceans globally.

Revision of the family Sargassaceae (Fucales, Phaeophyceae) from Kuwait (Arabian / Persian Gulf, NW Indian Ocean) based on analysis of morphology and ITS2 sequences

This article investigates the species of the family Sargassaceae present in Kuwait, analyzing nuclear ribosomal internal transcribed spacer 2 (ITS2) sequences of 67 individuals collected in 2018–2020. Morphological characterization and DNA barcoding confirmed the presence of eight taxa that belong to Sargassaceae. Five species belong to the genus Sargassum, while three other taxa, namely Sirophysalis trinodis, Hormophysa cuneiformis, and Polycladia myrica, belong to other genera. For the latter species, there was no ITS2 sequence available so far. As for Sargassum species, previous records based on phenotypic identifications were only confirmed for a minority of species. Based on 100% sequence identity with records from other regions, we recognize S. aquifolium, S. ilicifolium var. acaraeocarpum, S. yinggehaiense, and one still unclear species (Sargassum sp.). According to the sequences, the fifth and most common Sargassum species in Kuwait, previously identified as S. asperifolium, is S. carpophyllum. However, the ITS2 sequence of this entity did not show a perfect match with any S. carpophyllum specimen so far sequenced. The comparatively limited diversity of Sargassum in Kuwait may be due to the prevailing extreme environmental conditions, in particular large temperature amplitude between +14 °C in winter and +37 °C in summer.

Alien toxic dinoflagellate Heterocapsa circularisquama from the Western Pacific in Kuwait, NW Indian Ocean

A bloom of non-indigenous dinoflagellate Heterocapsa circularisquama (Peridiniales, Dinophyceae) and bloom-associated shellfish mortality were detected for the first time in Kuwait's waters (Arabian/Persian Gulf, NW Indian Ocean) during June–July 2020. This species is endemic to the Asia Pacific region, and associated with shellfish kills, causing tremendous damage to bivalve aquaculture, and economic losses in western Japanese waters and Hong Kong. In Kuwait Bay, this dinoflagellate was found in bloom proportions up to 108 cells·l−1 and represented from 72 to 83% of the total phytoplankton population in red tide patches. Native bloom-forming small-sized diatoms, Thalassiosira delicatula, T. exigua, and Minutocellus polymorphus were also abundant at the time of the H. circularisquama bloom. Microalgal cell morphology was determined using light and electron microscopy for specimens from the field and cultures established by cell isolation from the bloom area. The molecular characterization of cultured Heterocapsa strains was based on the large subunit region of the ribosomal DNA sequences. The overall morphology and body scale structure of Heterocapsa from Kuwait Bay were very similar to that of the H. circularisquama described from Japanese waters. The sequences of the LSU rDNA of Kuwait's strains were identical to those of the H. circularisquama. This is the first record of H. circularisquama occurring at high abundance outside the Asia Pacific region, and it may be an indication this invasive and harmful species is now established in Kuwait's marine environment.

Paleoredox conditions of bottom water in the northern Indian Ocean since 39 ka

Ocean circulation has undergone significant changes during the glacial and interglacial periods. The redox condition at the sediment-water interface is mainly regulated by export productivity and ventilation of abyssal water mass. The changes in the redox condition associated with bottom water circulation in the Indian Ocean are still unclear. The present study assessed multi redox proxies (Mn/Ti, Mo/Ti, U/Ti, V/Ti, Ni/Ca, Cu/Ca, MnEF, UEF, and VEF) and Corg in the sediment core collected from the Arabian Sea (NW Indian Ocean) to investigate the past variations in bottom water redox condition. The bottom water in the Arabian Sea is presently bathed with the Lower Circumpolar Deep Water (LCDW), consisting of a fraction of both North Atlantic Deep Water (NADW) and Antarctic Bottom Waters (AABW). Our results show significant changes in oxygenation conditions in response to changes in ocean circulation. The present study revealed that bottom water in the northern Indian Ocean was more oxic during LGM than the deglacial period. We interpret the variability in the contribution of NADW and AABW influenced the redox condition at the study location. Comparing the current study results with previously reported Nd isotopes suggests that the contribution of AABW to the Indian Ocean increased during LGM, resulting in a higher oxic condition in LCDW.

Does the Laxmi Ridge continue towards the Laccadive Ridge? New insights from an integrated geophysical study

The Laxmi and Laccadive ridges are two major aseismic ridges in the NW Indian Ocean. Bathymetry, gravity and magnetic data and their derivatives are analyzed and modelled to assess interrelationship between both the ridges as well as to establish their crustal structure, nature and isostatic compensation. Boundaries of the Laxmi and Laccadive ridges are delineated by tilt derivative with constraints from available seismic-sections. Integrated gravity and magnetic models reveal that both the ridges have almost similar crustal layers and are underplated all along its length. Crustal models also depict that both the ridges are carpeted with flood basalt and heavily intruded. 3D coherence between mantle Bouguer anomaly and residual bathymetry reveals that the elastic plate thickness and subsurface to surface load ratio for both the ridges vary from 3 to 4 km and 0.7 to 0.8, respectively. Several characteristic similarities viz. crustal structure and nature, elastic plate thickness, magmatism have been observed for both the ridges. Seismic sections near the junction of the ridges suggest that the basement high corresponding to the Laxmi Ridge is further continued towards the Laccadive Ridge. In addition, the northwestern part of the Laccadive Ridge has NW-SE structural lineaments similar to that of the NW-SE segment of the Laxmi Ridge. Based on results of the present and previous studies, we infer that (i) the Laxmi Ridge is extended southeastward towards the Laccadive Ridge, (ii) both the ridges are continental slivers which are underplated as well as intruded and carpeted by volcanics, and (iii) both the ridges are locally compensated.

Successive shifts of the India-Africa transform plate boundary during the Late Cretaceous-Paleogene interval: Implications for ophiolite emplacement along transforms

The Arabian Sea in the NW Indian Ocean is a place where two major transform boundaries are currently active: the Owen Fracture Zone between India and Arabia and the Owen Transform between India and Somalia. These transform systems result from the fragmentation of the India-Africa Transform boundary, which initiated about 90 Myrs ago, when the India-Seychelles block separated from Madagascar to move towards Eurasia. Therefore, the geological record of the Arabian Sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes.Here we focus on the evolution of the India-Africa transform system during the ~47–90 Ma interval. We identify the Late Cretaceous (~90–65 Ma) transform plate boundary along Chain Ridge, in the North Somali Basin. From 65 to ~42–47 Ma, the India-Africa transform is identified at the Chain Fracture Zone, which crossed both the Owen Basin and the North East Oman margin. Finally, the transform system jumped to its present-day location in the vicinity of the Owen Ridge. These shifts of the India-Africa boundary with time provide a consistent paleogeographic framework for the emplacement of the Masirah Ophiolitic Belt, which constitutes a case of ophiolite emplaced along a transform boundary. The successive locations of the India-Africa boundary further highlight the origin of the Owen Basin lithosphere incoming into the Makran subduction zone.

Three new goatfishes of the genus Upeneus (Mullidae) from the Indo-Pacific, with a redescription of colour patterns in U. margarethae.

For the goatfish genus Upeneus Cuvier 1829 (Mullidae), a new taxonomic species group called the "margarethae group" is established which can be distinguished from the six species of the most similar "tragula group" by a combination of the following characteristics: absence of dark pigmentation in the area of the first dorsal-fin tip, 21-25 total gill rakers and 28-30 lateral-line scales. Initially, three recently-described species have been included in the margarethae group: Upeneus margarethae Uiblein Heemstra, 2010, known from the Indian Ocean including the Red Sea and from the Arafura Sea (W Pacific), U. mouthami Randall Kulbicki, 2006, from New Caledonia and Vanuatu (W Pacific), and U. randalli Uiblein Heemstra, 2011, from the Arabian/Persian Gulf and the inner Gulf of Oman (NW Indian Ocean). The present taxonomic review of the margarethae group analyses a large data set of 41 morphometric, 10 meristic and many colour characters obtained from 279 preserved goatfish specimens and 166 fresh-colour photos (plus a fresh-colour drawing). For the nominal species of the group, U. margarethae, a redescription of the colour patterns is provided and new records for Myanmar, Andaman Sea (NE Indian Ocean) and the Gulf of Carpentaria, N Australia (W Pacific), are reported. Three new species are described: U. caudofasciatus n. sp. from the area of the Great Barrier Reef to Torres Strait off NE Australia (Coral Sea, W Pacific), U. gubal n. sp. from the S Gulf of Suez (Northern Red Sea), and U. heterospinus n. sp. from S Indonesia to Singapore, the Gulf of Thailand, Vietnam, Philippines, China, Taiwan and Japan (W Pacific). A comprehensive alpha-taxonomic approach is adopted, considering population differences as well as intraspecific size-related variation in morphology and colour patterns by splitting the data set into two size classes, adults (≥ 65 mm SL) and smaller subadults. Inter- and intraspecific comparisons include statistical analyses for species and population with sufficiently large samples sizes (n ≥ 20). Colour-pattern characterization and analysis are based on photos of recently collected and deceased fish (partly associated with voucher specimens), photos obtained from active or resting fishes in situ or in a tank, and inspection of pigmentation patterns retained in preserved specimens. Species differences are elaborated under consideration of distribution patterns and the characteristics found in the closest-most population of widely distributed species such as U. margarethae, resulting in clear and consistent distinction among the six species in single or in a combination of several characteristics. Comparisons among size classes revealed species-specific patterns in morphometric, meristic and colour changes with increasing size. One species, U. heterospinus n. sp., has seven or eight spines in the dorsal fin which occur in balanced ratio across populations. This is a unique characteristic for Upeneus species which usually have either seven or eight dorsal-fin spines, respectively. The best distinction of Upeneus heterospinus n. sp. from all other congeners is reached by combined examination of dorsal-fin spines with several other characters such as barbel colour, presence of a mid-lateral body stripe, pigmentation patterns (partly retained also in preserved fish), gill-raker and pectoral-fin ray numbers, and body-shape variables. The high degree of overall morphological differentation among the three most common species U. caudofasciatus n. sp., U. heterospinus n. sp. and U. margarethae, as revealed by the statistical comparisons, strongly contrasts with a still signficant, but much lower degree of differentiation among populations. The diagnostic characteristics of the margarethae group are updated and the importance to use the results of this taxonomic review in ongoing fisheries-related and ecological research is emphasized. Requirements for future taxonomic research featuring the stunning diversity of the goatfish genus Upeneus are also discussed.

Probabilistic Landslide-Generated Tsunamis in the Indus Canyon, NW Indian Ocean, Using Statistical Emulation

The Indus Canyon in the northwestern Indian Ocean has been reported to be the site of numerous submarine mass failures in the past. This study is the first to investigate potential tsunami hazards associated with such mass failures in this region. We employed statistical emulation, i.e. surrogate modelling, to efficiently quantify uncertainties associated with slump-generated tsunamis at the slopes of the canyon. We simulated 60 slump scenarios with thickness of 100–300 m, width of 6–10.5 km, travel distances of 500–2000 m and submergence depth of 250–450 m. These scenarios were then used to train the emulator and predict 500,000 trial scenarios in order to study probabilistically the tsunami hazard over the near field. Due to narrow–deep canyon walls and the shallow continental shelf in the adjacent regions (<100 m water depth), the tsunami propagation has a unique pattern as an ellipse stretched in the NE–SW direction. The results show that the most likely tsunami amplitudes and velocities are approximately 0.2–1.0 m and 2.5–13 m/s, respectively, which can potentially impact vessels and maritime facilities. We demonstrate that the emulator-based approach is an important tool for probabilistic hazard analysis since it can generate thousands of tsunami scenarios in few seconds, compared to days of computations on High Performance Computing facilities for a single run of the dispersive tsunami solver that we use here.

Seascape genetics of the spiny lobster Panulirus homarus in the Western Indian Ocean: Understanding how oceanographic features shape the genetic structure of species with high larval dispersal potential.

This study examines the fine‐scale population genetic structure and phylogeography of the spiny lobster Panulirus homarus in the Western Indian Ocean. A seascape genetics approach was used to relate the observed genetic structure based on 21 microsatellite loci to ocean circulation patterns, and to determine the influence of latitude, sea surface temperature (SST), and ocean turbidity (KD490) on population‐level processes. At a geospatial level, the genetic clusters recovered corresponded to three putative subspecies, P. h. rubellus from the SW Indian Ocean, P. h. megasculptus from the NW Indian Ocean, and P. h. homarus from the tropical region in‐between. Virtual passive Lagrangian particles advected using satellite‐derived ocean surface currents were used to simulate larval dispersal. In the SW Indian Ocean, the dispersion of particles tracked over a 4‐month period provided insight into a steep genetic gradient observed at the Delagoa Bight, which separates P. h. rubellus and P. h. homarus. South of the contact zone, particles were advected southwestwards by prevailing boundary currents or were retained in nearshore eddies close to release locations. Some particles released in southeast Madagascar dispersed across the Mozambique Channel and reached the African shelf. Dispersal was characterized by high seasonal and inter‐annual variability, and a large proportion of particles were dispersed far offshore and presumably lost. In the NW Indian Ocean, particles were retained within the Arabian Sea. Larval retention and self‐recruitment in the Arabian Sea could explain the recent genetic divergence between P. h. megasculptus and P. h. homarus. Geographic distance and minimum SST were significantly associated with genetic differentiation in multivariate analysis, suggesting that larval tolerance to SST plays a role in shaping the population structure of P. homarus.

The Geological Evolution of the Aden‐Owen‐Carlsberg Triple Junction (NW Indian Ocean) Since the Late Miocene

AbstractThe Aden‐Owen‐Carlsberg triple junction is the place where Arabia, Somalia, and India meet in the Arabian Sea (NW Indian Ocean). Here we present a new seismic data set crossing a key structure of the triple junction, namely, the Beautemps‐Beaupré pull‐apart basin at the southern end of the Owen Fracture Zone. The seismic data set is tied to Ocean Drilling Program Leg 117 Sites at the top of the Owen Ridge, which provides a detailed tectono‐stratigraphic framework since the Late Miocene. We show that the triple junction configuration has been disturbed by a major kinematic change at ~8 Ma and since then experienced a series of transient structural adjustments. A major structural episode is recorded at 2.4 Ma, expressed by the opening of the Beautemps‐Beaupré Basin and the uplift of its southern flank (the Beautemps‐Beaupré Ridge). This episode is coeval with the formation of the present‐day Owen Fracture Zone and must be considered as a part of a major structural reorganization of the entire India‐Arabia plate boundary up to the Makran subduction zone. This 2.4‐Myr‐old geological episode is unrelated to any significant kinematic change, leaving questions over its driving mechanism.

Elemental variability in the coralline alga Lithophyllum yemenense as an archive of past climate in the Gulf of Aden (NW Indian Ocean).

This study presents the first algal thallus (skeleton) archive of Asian monsoon strength and Red Sea influence in the Gulf of Aden. Mg/Ca, Li/Ca, and Ba/Ca were measured in Lithophyllum yemenense from Balhaf (Gulf of Aden) using laserablation inductively coupled plasma mass spectrometry, and Mg/Ca ratio oscillation was used to reconstruct the chronology (34y). Oscillations of element rates corresponding to the algal growth between 1974 and 2008 were compared with recorded climate and oceanographic variability. During this period, sea surface temperatures (SST) in Balhaf recorded a warming trend of 0.55°C, corresponding to an increase in Mg and Li content in the algal thallus of 2.1mol-% and 1.87μmol-%, respectively. Lithophyllum yemenense recorded decadal SST variability by Li/Ca, and the influence of the Pacific El-Niño Southern Oscillation on the NWIndian Ocean climate system by Ba/Ca. Additionally, algal Mg/Ca, Li/Ca, and Ba/Ca showed strong and significant correlations with All Indian Rainfall in the decadal range indicating that these proxies can be useful for tracking variability in the Indian monsoon system, possibly due to changes of the surface wind system, with deep water upwelling in summer, and a distinct seasonality.

Second Summer in NW Indian Ocean

Two independent SST atlases have confirmed that there is a double seasonal signal in the surface layer of the northwestern Indian Ocean. The area enclosed by the 80F isotherm increases from January to a maximum in May followed by a decrease, a cool-down, to August, which has been explained recently. Then there is a second maximum of warm surface area in October/November, called a “second summer” for convenience. A hypothesis is proposed to explain this unusual second summer feature by extrapolating from available data. During July, August and September, the sea level slopes downward from the equator to the north providing a horizontal force to drive the warm surface water accumulated in the equatorial region back into the NW Indian Ocean basin. New observations are needed to check up on the hypothesis.

NW Indian Ocean’s SSTs Are Bi-Seasonal

Two independent atlases agree that in the northwest Indian Ocean the sea surface temperatures cool down markedly in the spring and also that they have a significant double seasonal cycle, unlike any other ocean in both cases. Horizontal advection is proposed to play an important part in causing these unusual features to occur.

Pseudofaults and associated seamounts in the conjugate Arabian and Eastern Somali basins, NW Indian Ocean – New constraints from high-resolution satellite-derived gravity data

Marine gravity data derived from satellite altimeters are effective tools in mapping fine-scale tectonic features of the ocean basins such as pseudofaults, fracture zones and seamounts, particularly when the ocean basins are carpeted with thick sediments. We use high-resolution satellite-generated gravity and seismic reflection data to map boundaries of pseudofaults and transferred crust related to the Paleocene spreading ridge propagation in the Arabian and its conjugate Eastern Somali basins. The study has provided refinement in the position of previously reported pseudofaults and their spatial extensions in the conjugate basins. It is observed that the transferred crustal block bounded by inner pseudofault and failed spreading ridge is characterized by a gravity low and rugged basement. The refined satellite gravity image of the Arabian Basin also revealed three seamounts in close proximity to the pseudofaults, which were not reported earlier. In the Eastern Somali Basin, seamounts are aligned along NE-SW direction forming ∼300kmlong seamount chain. Admittance analysis and Flexural model studies indicated that the seamount chain is isostatically compensated locally with Effective Elastic Thickness (Te) of 3–4km. Based on the present results and published plate tectonic models, we interpret that the seamounts in the Arabian Basin are formed by spreading ridge propagation and are associated with pseudofaults, whereas the seamount chain in the Eastern Somali Basin might have probably originated due to melting and upwelling of upper mantle heterogeneities in advance of migrating/propagating paleo Carlsberg Ridge.

The Sawqirah contourite drift system in the Arabian Sea (NW Indian Ocean): A case study of interactions between margin reactivation and contouritic processes

The relationships between oceanic circulation in the Arabian Sea and Late Cenozoic climate changes, including variations in monsoon intensity at the million year time-scale, remain poorly investigated. Using multibeam and seismic data, we provide the first description of a contourite drift in the Arabian Sea, along the south-eastern Oman margin. This contourite drift is referred as the “Sawqirah Contourite Drift System”. Late Miocene reactivation of the south-eastern Oman margin resulted in the formation of a complex anticline system, which shaped the seafloor topography above which the Sawqirah Drift subsequently developed. The drift resulted from the circulation of bottom currents within the North Intermediate Indian Water. Major seismic unconformities identified within the Sawqirah Drift were tied to Ocean Drilling Program (ODP) drill holes, and allowed defining distinct episodes of drift construction. At least two of these unconformities record reorganizations of the oceanic circulation at ~4.5–4.8Ma and ~2.4Ma. The 4.5–4.8Ma-old unconformity is coeval with the onset of upwelling of deep and cold waters in the Owen Basin. The 2.4Ma-old unconformity records a major episode of Indian monsoon intensification (at the million year time scale) over the Arabian Sea, indicating strong coupling between oceanic and atmospheric circulation processes.

Omura's whale Balaenoptera omurai stranding on Qeshm Island, Iran, Persian Gulf:Further Evidence for a Wide (Sub) Tropical Distribution

Omura's whale Balaenoptera omurai stranding on Qeshm Island, Iran, Persian Gulf:Further Evidence for a Wide (Sub) Tropical Distribution A juvenile rorqual live-stranded on Qeshm Island, Iran, in the northern Strait of Hormuz, Persian Gulf, in September 2007. The cause of stranding remains unknown but the whale (QE22.09.2007) showed no severe traumatic injuries nor was it emaciated. Six key morphological features, diagnostic in combination, allow positive identification as Omura's whale Balaenoptera omurai, a species that has not before been reported from the NW Indian Ocean. Features include diminutive body size (397 cm); an unusually large number of ventral grooves (n=82) and grooves near the midline extended caudad of the umbilicus; a strongly falcate, pointed dorsal fin; asymmetric colouration of the head (especially lower jaws) reminiscent of fin whale, and unilateral dark (eye, ear, flipper-toflank)stripes; faint/incomplete lateral rostral ridges; and extremely low number of short, broad baleen plates (204 in right jaw). The likelihood for the existence of a local B. omurai population in the eastern Persian Gulf or Arabian Sea is deemed higher than the wandering of a small juvenile or mother/calf pair from any of the known distant distribution areas in the eastern or south-western Indian Ocean. This is the first record of Omura's whale for the Persian Gulf and Iran. Combined with two recent strandings of similar juveniles in the Atlantic (Mauritania, Brazil) it suggests that B. omurai may have a significantly wider (sub) tropical distribution than hitherto assumed.

The genus Lithophyllum in the north-western Indian Ocean, with description of L. yemenense sp. nov., L. socotraense sp. nov., L. subplicatum comb. et stat. nov., and the resumed L. affine, L. kaiseri, and L. subreduncum (Rhodophyta, Corallinales)

Based on literature, the genus Lithophyllum was represented in the whole Indian Ocean by 14 taxa, mostly in need of revision in a modern context. Molecular analyses integrated with morpho-anatomical comparisons between the recently revised type material of Lithophyllum kotschyanum Unger, and the related infraspecific taxa, lead to a resumption of L. affine, L. kaiseri, and L. subreduncum, and the description of L. socotraense sp. nov., L. yemenense sp. nov., and L. subplicatum comb. et stat. nov. Detailed accounts are provided for each species, including keys, along with information on synonymy, examined collections, distribution, habitat as well as sequence data of the 5’ end of the LSU from the type specimens. The anatomical features of the tetrasporangial conceptacle that were considered collectively diagnostic for species identification are: the mean diameter of the tetrasporangial conceptacles, the length of the pore-canal in the tetrasporangial conceptacles (with the number of cells in roof filaments), the occurrence of a depression at the top of the conceptacle roof in relation to the pore opening, and the number of cells from the floor of the tetrasporangial conceptacle chamber to the thallus surface. This study provides evidence of a previously unsuspected diversity within the Red Sea and NW Indian Ocean species of Lithophyllum.

Tectonic interactions between India and Arabia since the Jurassic reconstructed from marine geophysics, ophiolite geology, and seismic tomography

AbstractGondwana breakup since the Jurassic and the northward motion of India toward Eurasia were associated with formation of ocean basins and ophiolite obduction between and onto the Indian and Arabian margins. Here we reconcile marine geophysical data from preserved oceanic basins with the age and location of ophiolites in NW India and SE Arabia and seismic tomography of the mantle below the NW Indian Ocean. The North Somali and proto‐Owen basins formed due to 160–133 Ma N‐S extension between India and Somalia. Subsequent convergence destroyed part of this crust, simultaneous with the uplift of the Masirah ophiolites. Most of the preserved crust in the Owen Basin may have formed between 84 and 74 Ma, whereas the Mascarene and the Amirante basins accommodated motion between India and Madagascar/East Africa between 85 and circa 60 Ma and 75 and circa 66 Ma, respectively. Between circa 84 and 45 Ma, oblique Arabia‐India convergence culminated in ophiolite obduction onto SE Arabia and NW India and formed the Carlsberg slab in the lower mantle below the NW Indian Ocean. The NNE‐SSW oriented slab may explain the anomalous bathymetry in the NW Indian Ocean and may be considered a paleolongitudinal constraint for absolute plate motion. NW India‐Asia collision occurred at circa 20 Ma deforming the Sulaiman ranges or at 30 Ma if the Hindu Kush slab north of the Afghan block reflects intra‐Asian subduction. Our study highlights that the NW India ophiolites have no relationship with India‐Asia motion or collision but result from relative India‐Africa/Arabia motions instead.

Tectonics of the Dalrymple Trough and uplift of the Murray Ridge (NW Indian Ocean)

The Dalrymple Trough is a 150-km-long, 30-km-wide basin located at the northern termination of the Owen Fracture Zone (OFZ), which is the present-day active India–Arabia plate boundary. The Dalrymple Trough is closely associated with the Murray Ridge, a complex of prominent bathymetric highs located on its eastern flank. Recent multibeam mapping of the connection between the Dalrymple Trough and the OFZ revealed a horsetail structure, which suggests a close relationship between geological histories of both structures. However, the 3–6Ma age of initiation of the OFZ contrasts with the commonly accepted Early Miocene emplacement of the Dalrymple Trough. Recent seismic lines document a new tectonic history of the Dalrymple Trough, involving two major episodes of deformation along the India–Arabia plate boundary at ~8–10Ma and ~1.9±0.9Ma. The 8–10Ma episode is marked by a system of folds linked to the main uplift of the southern Murray Ridge and the first uplift of the northern Murray Ridge. This episode is related to a global plate reorganization event in the Late Miocene, well expressed by intraplate deformation in the Central Indian Ocean. The Dalrymple Trough opened at ~1.9±0.9Ma subsequently to the formation of a stepover at the India–Arabia plate boundary, coeval with the regional M-unconformity in the Oman abyssal plain, which marks a structural reorganization of the Makran accretionary wedge, and the last uplift of the northern Murray Ridge.