Using Environmental DNA to search for Crocodiles in Madagascar.

The world is currently suffering from multiple interconnected environmental crises, including a rapid loss of biodiversity in many parts of the world. Madagascar, a global biodiversity hotspot with a large number of species found nowhere else, is considered to be particularly at risk, due to unprecedented rates of habitat loss. However, tracking this loss is challenging, as monitoring species loss involves a large amount of labour intensive work by specialist taxonomic experts, often involving access to expensive equipment.

Environmental DNA offers a potential way to reduce this workload, allowing for monitoring to be carried out by non-specialist staff collecting samples of sediment, water or even air. Since such methods to not require direct observation of threatened species, they are far less environmentally intrusive.

Nile Crocodiles, Crocodylus niloticus, were persecuted across their range by hunters seeking their skins, as well as harvesting of their eggs, until the mid-twentieth century. Since this time, the population has recovered in many parts of Africa due to conservation efforts. In Madagascar, the species is monitored by the Malagasy Crocodile Management Unit, which aims not just to establish the recovery of the species, but to minimise Human-Crocodile conflicts by helping people and livestock to avoid potentially lethal Crocodile encounters.

Previous work using environmental DNA has succeeded in detecting Crocodiles kept under laboratory conditions, but struggled to locate them in the wild, although methods such as water sweeps have proven more successful.

In a paper published in the journal Biodiversity and Conservation on 23 October 2025, Mai Fahmy of the Department of Undergraduate Biololgy at Stony Brook University, and the Division of Invertebrate Zoology at the American Museum of Natural History, Soja Manjakamanana Zafimanaoela of the Université d’Antananarivo, Njakamamapiadana Mamenofahasoavana Rinah and Jerison William Ranaivosolo of Stony Brook University's Centre ValBio in Madagascar, Noel Rowe of Primate Conservation Inc., Patricia Wright, also of the Centre ValBio, and of the Department of Anthropology and Interdepartmental Doctoral Program in Anthropological Sciences at Stony Brook University, and Evon Hekkala, also of the Department of Undergraduate Biololgy at Stony Brook University, and the Division of Vertebrate Zoology at the American Museum of Natural History, present the results of a study, in which they attempted to detect Crocodiles in Madagascar using sediment samples as well as specialist environmental DNA filters and Coffee filters, in an attempt to find a cost-effective method of detecting the Animals. 

Anivorano lake in northern Madagascar, traditionally held to be sacred. Here the local population periodically sacrifices Zebu Cattle to the Crocodiles, which are believed to contain the spirits of pas chiefs, in a ceremony that also involves dancing and singing. On such occasions, the Crocodiles are fed beef from a beach about 30 m long near a sacred tree, and subsequently typically remain in the area for about eight hours, basking on the beach or in the water.

Map of collection localities across Madagascar. TSP, Tsimanampetsotse National Park. Fahmy et al. (2025).

Fahmy et al. collected sediment from a belly print left on the beach by a Crocodile, Crocodile faecal matter, and water from an area close to where Crocodiles were basking. The water and faecal samples were subsequently passed through Coffee filters, which were retained. 

At the Ankarana Special Reserve, also in northern Madagascar, Fahmy et al. collected sediment samples from the river mouth leading into the Ankarana Cave System, with two samples collected from the footprints of Crocodiles, one adult and one juvenile. Water was also collected from a flooded cave entrance, and again filtered through a coffee filter.

Water samples were also collected from the Ihosy, Mananantanana, and Matitanana rivers in central and southeastern Madagascar, and filtered through specialist environmental DNA filters. These are wide, fast flowing rivers, used exclusively by the local population for transport, hygiene, and watering livestock. Samples were taken following sightings of Crocodiles, and only with the explicit permission of local communities.

Nile Crocodile, Crocodylus niloticus, spotted in the Matitanana River while sampling. Mai Fahmy in Fahmy et al. (2025).

Finally, four samples were collected from a submerged cave located in Tsimanampesotse National Park in southwest Madagascar. Nile Crocodiles have never been observed in these caves, although they are known to inhabit the nearby Onalahy River. The caves were, however, utilised by the extinct Madagascan Horned Crocodile, Voay robustus, which is thought to have died out about 1250 years ago. These samples were again filtered through Coffee filters. A sediment sample was also collected from beneath a subfossil Horned Crocodile in the caves.

A total of seventeen samples were collected and processed. Four of the samples were subsequently discarded because they were found to contain only Human DNA, and one because it contained Sheep DNA. In the remaining samples, the greatest number of species were recovered from the Coffee filter samples; although this did not represent the greatest diversity. Sediment and environmental DNA samples produced equal numbers of species, although greater overall diversity was recorded in the sediment samples. DNA associated with members of the Family Podicipedidae (Grebes) was recovered exclusively in the environmental DNA filters, while DNA associates with the Family Naididae (Tubifex Worms) was recovered exclusively from sediment samples.

Crocodile DNA was recovered from both sediment samples and Coffee filters, but not from specialist environmental DNA filters. However, Fahmy et al. do not suggest that this is because Coffee filters are superior to environmental DNA filters, as the later were only used to filter water samples from fast flowing rivers, where recovering Crocodile DNA is known to be difficult. 

Zebu Cattle drinking from Matitanana River. Mia Fahmy in Fahmy et al. (2025).

Three samples yielded Crocodile DNA, all from Lake Anivorano. These included the sediment sample taken from the Crocodile belly print, and the Coffee filters through which the faecal sample and water from close to Crocodiles basking in the lake were filtered. The analysis recovered these as belonging to the genus Crocodylus, and most likely as coming from Nile Crocodiles, Crocodylus niloticus, with a lower chance of coming from Orinoco Crocodiles, Crocodylus intermedius, of American Crocodiles, Crocodylus acutus, neither of which are found in Madagascar. 

The three most abundant species in the results were Cattle, Bos taurus, Chickens, Gallus gallus, and Domestic Pigs, Sus scrofa. Also detected were Ring-tailed Lemur, Lemur catta, and an unknown species of Grebe (Podicepepidae). This could not be identified to species level, although several Grebes are known from Madagascar, including the Little Grebe, Tachybaptus ruficollis, and the Madagascar Grebe, Tachybaptus pelzelnii. Interestingly, the Madagascar Grebe does not have any genetic material recorded in the GenBank database, against which the samples were compared.

The method developed by Fahmy et al. did not establish the presence of Crocodiles in anywhere they were not known to occur, but did establish that it was possible to detect Crocodiles using environmental DNA in Madagascar, and that it was possible to use Coffee filters as a means of recovering environmental DNA, a much cheaper option than specialist filters.

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Thonningia alba: A new species of Parasitic Plant from Madagascar.

The Balanophoraceae is a family of obligate root-parasitic plants (which is to say, Plants which tap into the root systems of other Plants for nutrients, and carry out little-or-no photosynthesis for themselves) in the order Santalales, which also includes Sandalwoods and Mistletoes. Six species of these plants, in five genera, are known from Africa, while three are found in Madagascar, Ditepalanthus malagasicus and Thonningia malagasica (sometimes known as Langsdorfa malagasica), which are endemic to the island, and  Balanophora abbreviata, which is also found in Southeast Asia.

In a paper published in the Kew Bulletin on 25 January 2024, Leandro Jorge Telles Cardoso and João Marcelo Alvarenga Braga of the Instituto de Pesquisas Jardim Botânico do Rio de Janeiro, describe a new species of Balanophoraceaen Parasitic Plant from the Masoala Peninsula of northeastern Madagascar.

Cardoso and Braga first came across this species as specimens in the collections of the Missouri Botanical Garden and Muséum National d'Histoire Naturelle, which were identified as belonging to Thonningia malagasica, but which appeared more similar to Thonningia sanguinea, a species known from West and Southern Africa, but never previously recorded in Madagascar. Closer examination of this material and comparison to specimens of all known species of Thonningia and Langsdorfa led Cardoso and Braga to conclude that the material did in fact represent a new species.

The new species is named Thonningia alba, where 'alba' means 'white' in reference to the colour of the flowers. Known specimens of the species have a single branch with 1-3 clusters of inflorescences. Leaves are reduced to a few 3-5 whorls of scales around the base of these inflorescences. Inflorescences can be male or female, and reach a maximum diameter of about 3 cm in diameter. Flowers are tubular, with male flowers reaching 12-16 mm in length, while female flowers are 9-12 mm long.

Thonningia alba. Male inflorescence, Masoala Peninsula, Madagascar, 15°41'43.55"S, 49°57'50.63"E, 270 m above sealevel, September 2018. Neek Helme in Cardoso & Braga (2024).

Thonningia alba has been found growing in tropical rainforests on the Masoala Peninsula at altitudes of between 110 and 650 m above sealevel, and appears to flower between July and November, with fruit being produced in November; although the limited number of observations of the species in the wild mean that this cannot be stated with absolute confidence. It is clearly parasitic in nature, but the identity of its host Plant is unclear. 

The Masoala Peninsula is considered to be a mega-diverse region of global biodiversity importance, and is covered by a mosaic of protected areas, the largest of which is the Masoala National Park, which includes large expanses of rainforests and coastal landscapes. Specimens of Thonningia alba were collected in 1986, 1993, and 1994, and the species was observed in the wild in September 2018. All observations were within the Masoala National Park, at two locations separated by about 16 km. While the park itself is a protected area, the surrounding area is home to a growing Human population, which is at least partially dependent on collecting wild resources from the area's forests. For this reason, Cardoso and Braga recommend that the species be treated as Endangered under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

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Zorotypus komatsui: A new species of Ground Louse from Cameroon.

Ground Lice, Zoraptera, are small (less than 3 mm) Insects typically found under bark. They are one of the smallest orders of Insects, with only 46 described species, although this may be a severe underestimation of their true diversity, as they are both easily overlooked and morphologically conservative. Ground Lice have a fossil record dating back to the Palaeozoic, with the last common ancestor of all living species thought to have lived about 270 million years ago. They are mostly wingless, and were once thought to be so throughout their life-cycle (the name Zoraptera means 'pure wingless') but winged forms are produced at some times of year, helping them colonise new areas.

In a paper published in the journal ZooKeys on 1 September 2023, Yoko Matsumura of the Department of Ecology and Systematics at Hokkaido University, Munetoshi Maruyama of the Kyushu University Museum, Nelson Ntonifor of the Department of Agronomic and Applied Molecular Sciences at the University of Buea, and Rolf Beutel of the Entomology Group at Friedrich-Schiller-Universität Jena, describe a new species of Ground Louse from Cameroon.

The new species is described from a single wingless male specimen discovered under a 30 cm long rock half embedded in soil close to the village of Nyasoso in Southwest Region, Cameroon, by Takashi Komatsu and Munetoshi Maruyama. A subsequent search of the surrounding forest yielded no further specimens. It is placed in the genus Zorotypus, and given the specific name komatsui; no explanation is given for this name, but it presumably honours Takashi Komatsu. The single known specimen of Zorotypus komatsui is a wingless male, 2.42 mm in length, with a head 0.47 mm long and 0.50 mm wide. It is light brown in colour, lacking any markings, even eye spots. 

Digitalmicroscopic images of the holotype of Zorotypus komatsui (ventral view) (A) habitus, (B) head and prothorax, (C) hindleg and abdomen. Matsumura et al. (2023).

Matsumura et al. also report the presence of Zorotypus vinsoni, a species previously known only from Mauritius, on Madagascar. This is the second species found to be present on both islands, following Zorotypus delamarei, which was discovered initially on Madagascar and subsequently on Mauritius. Mauritius was formed by volcanic activity about 8.9 million years ago, and probably largely resurfaced by another volcanic episode about 1 million years ago. This being the case, it is likely that both species of Zorotypus found on Mauritius have colonised the island from Madagascar in the recent past.

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Cyclone Freddy makes landfall in Mozambique after killing seven people in Madagascar.

Cyclone Freddy made landfall in the coastal town of Vilanculos in southern Mozambique on Friday 24 February 2023, brining with it windspeeds of 113 km per hour, as well as high rainfall levels. The rain is predicted to be the bigger part of the problem for the country, which is already suffering from widespread flooding following exceptionally high seasonal rains, and may also cause problems in neighbouring Malawi, Zimbabwe, and northern South Africa. While the storm is likely to bring severe problems to the country, it is hoped that the number of casualties will be kept low, with thousands of people having been evacuated from the path of the cyclone as part of the World Meteorological Organization's Early Warnings for All Programme. The storm has already claimed seven lives in Madagascar, as well as causing damage to property on the islands of Mauritius and La Reunion.

Meteosat-9 image of Cyclone Freddy making landfall in Mozambique on Friday 24 February 2023. NOAA/AP.

Tropical storms, called Cyclones in the Indian Ocean and South Pacific, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it..

The formation of a tropical cyclone. Natural Disaster Management.

Cyclone Freddy is particularly unusual in that it formed off the coast of Australia, tracking its way across the entire southern Indian Ocean before hitting Madagascar and Mozambique. It is only the fourth time a storm has been recorded doing this, with the previous examples being cyclones Litanne in 1994 and Leon–Eline and Hudah in 2000. This long journey allowed Freddy to gain considerable energy, hitting Madagascar as a Category 5 Cyclone (i.e. a storm with sustained winds in excess of 252 km per hour). The storm lost most of its energy passing over the island, but regained some of this passing over the Mozambique Channel.

Track map of Severe Tropical Cyclone Freddy The points show the location of the storm at 6-hour intervals. The colour represents the storm's maximum sustained wind speeds as classified in the Saffir–Simpson scale, with warmer colours representing higher wind speeds. Wikimedia Commons.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms. 

The formation and impact of a storm surge. eSchoolToday.

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Tropical Storm Ana kills at least 46 people in Madagascar, Mozambique, and Malawi.

Tropical Storm Ana is now thought to have killed 39 people on Madagascar, which it swept across on the weekend off 22-23 January 2022. The storm brought with it heavy rains, causing many rivers to burst their banks, with flooding being particularly severe around the island's capital, Antanarivo, where around 55 000 people are thought to have lost their homes. 

 

Flooding in the 67 Hectares neighbourhood of Antananarivo on 24 January 2022. AFP.

The storm made landfall on the African mainland on Monday 24 January, in Nampula Province, Mozambique, and has caused severe flooding in the Zambezi River Basin in Mozambique, where at least three people have died in Zambezia Province, with 66 people injured and about half a million displaced by flooding across Zambezia, Nampula and Sofala provinces. In neighbouring Malawi, four people are reported to have died, with around 30 injured and many more displaced by flooding. Affected areas of both countries are suffering widespread power outages and are likely to run short on food and drinkable water rapidly.

 

Storm damage at Topuito in Nampula Province, Mozambique, on 24 January 2022. Songo9Dades.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 

The formation of a tropical cyclone. Natural Disaster Management.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.

 

The formation and impact of a storm surge. eSchoolToday.

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Modelling turbidites off the East Coast of Africa.

As the terminal part of sedimentary source-to-sink systems, deep-sea deposits have been used to reconstruct past climates, sediment and carbon budgets, and the distribution of anthropogenic pollution. Bottom currents, i.e. density-driven circulation in the deep ocean, and sediment gravity flows are the main processes that form and modify these deposits. While the influence of bottom currents on submarine channel architecture has been recognised, interpretation has been hampered by a lack of direct monitoring data. Modern, integrated data sets, including direct measurements and monitoring of bottom currents, are of great importance in understanding the complexity of oceanographic processes, and the preservation of strata within submarine channel complexes.

In a paper published in the journal Geology on 18 March 2020, Arne Furhmann and Ian Kane of the Department of Earth and Environmental Sciences at the University of Manchester, Mike Clare of the National Oceanography Centre, Ross Ferguson, also of the Department of Earth and Environmental Sciences at the University of Manchester, Edwin Schomacker of Equinor, Enrico Bonamini of Eni Upstream and Technical Services, and Fabio Contreras of Eni Rovuma Basin, present the results of a study which combines an integrated subsurface study (three-dimensional seismic, core, and well-log data) with modern seafloor geomorphology and near-bed bottom-current measurements to develop a process-product–based sedimentological model for bottom current–influenced submarine channel complexes.

The Jurassic to Palaeogene basins offshore of East Africa formed during the breakup of Gondwana. Following Pliensbachian to Aalenian northwest-southeast rifting, about 2000 km of continental drift took place along north-south–striking lineaments, such as the Davie Ridge fracture zone and the Sea Gap Fault, between the Kimmeridgian and Barremian. Cessation of rifting was marked by active seafloor spreading between Madagascar and India, leading to the development of the present-day East African passive continental margin. Albian transgression resulted in the development of the extensive deep-marine deposits that are the focus of this study. Major river systems drained the African continent and supplied fine-grained sediment, while additional sediment was shed from uplifted rift shoulders along the palaeocoastline.

 

(A) Location of the subsurface data (red outline I) and modern analogue (red outline II), the Sea Gap Fault (SGF) and the Davie Ridge Fracture Zone (DFZ) along the East African Margin. Dotted lines refer to regional oil/gas license boundaries. Wells were used for biotratigraphic correlation; Well A is marked by red dot. (B) Lithology and tectonic history of the deep-water basins offshore of Tanzania. Colours of seismic horizons correlate to the interpreted seismic cross sections in panels (E)–(I). Quat, Quaternary; EARS, East African Rift System. (C), (D) Contour map of mid–Campanian (C) seismic horizon and root mean square (RMS) amplitude extraction of the base Turonian to mid-Campanian (D). Coarse-grained sediment (high amplitudes) is influenced by drift-related topography (low amplitudes); white arrows mark direction of sediment gravity flows. TWT, two-way traveltime. (E) Seismic cross section showing Well A (blue line) and lateral seismic facies variation. (F) Seismic cross section showing drift-confined slope channels and sediment waves east of Sea Gap fault. (G) Seismic cross section displaying spatial distribution of drift deposits and Sea Gap fault. (H), (I) Zoom of hybrid levee-drift deposits in Upper Cretaceous and Paleogene (top) with interpretation (bottom). Note overall similar trend of coeval channel and drift migration. Furhmann et al. (2020).

Furhmann et al.'s study used high-resolution 3-D seismic reflection data (covering 4885 km²) and 14 exploration wells provided by Equinor ASA and ExxonMobil. Seismic data were tied to the biostratigraphically calibrated wells (and core data of Well A) to map seismic and stratigraphic geometries offshore Tanzania. The average vertical resolution in the Upper Cretaceous of our study area is about 20–30 m (average velocities of about 2.9–3.3 km per second, and average frequency of 35 Hz). The data have a bin spacing of 12.5 × 12.5 m and a 4 ms sampling rate, and are processed in the Society of Exploration Geophysicists normal polarity to zero phase, where a peak represents a downward increase in acoustic impedance. High-resolution modern seafloor data covering the modern analog, offshore northern Mozambique, comprises extensive (65 × 50 km) multibeam bathymetric data acquired by our study using an autonomous underwater vehicle (5 m bin size), and a focused (190-m-wide) bathymetric survey using a remotely operated vehicle (0.6 m bin size). Down-looking 1200 kHz acoustic Doppler current profilers deployed by Eni energy company on three single-point deep-water moorings measured near-bed (5.5 m above bed) current direction and velocity every 10 minutes from March 2013 to September 2014.

 

Modern bottom-current and seafloor data, offshore northern Mozambique. (A) Bathymetric map of the seafloor including location of moorings and cross sections. AUV, autonomous underwater vehicle. (B) Furrows and scours at the seafloor indicate the dominance of north-flowing bottom currents. ROV, remotely operated vehicle. (C) Cross sections X, Y, and Z showing less-steep canyon walls (drift) at the northern flank of canyons. (D) Bottom-current velocity measurements for each mooring indicate strong seasonal variability. Gray bars on time axis represent times in which data were recorded. (E) Cumulative vector plot shows dominant trend of currents flowing to north. Furhmann et al. (2020).

The deep-marine sedimentary systems of the Upper Cretaceous offshore Tanzania were strongly influenced by topographic relief associated with the Sea Gap fault and large drift deposits. These drifts are characterized by low-amplitude, parallel to wavy reflectors, which show a lateral, upslope accretion toward the southwest. A 1650-m-thick drift (central drift) with horizontal dimensions that exceed the data coverage of the 3-D seismic survey toward the north (maximum west-east width of 50 km) dominates the stratigraphy. Smaller drifts as much as 12 km wide and 30 km long, to the west of the central drift, migrated obliquely along the northwest-southeast–oriented upper slope. Located in their topographic lows are concave-up, roughly 3500-m-wide high-amplitude reflectors, interpreted as coarse-grained submarine slope channel fills. These drifts show characteristics of levees, developed on the northeastern side of the channel complexes, and migrated up dip (southwest), confining and stepping into the channels. One-sided hybrid levee-drift deposits on the northeastern sides of channel systems are recognized from the Upper Cretaceous to modern systems. East of the Sea Gap fault, large drifts, low-amplitude sediment waves (wavelengths approximately 1.5–2.5 km, about 100 m thick), and distal slope channel complexes indicate similar channel and drift interaction along the lower slope. The time-continuous migration toward the southwest, the low seismic amplitude of the drifts, and the dimming of high-amplitude channel fills to the north suggest relatively persistent, long-lived bottom-current flow.

Acoustic Doppler current profiler measurements offshore of Mozambique show a dominance of northward-directed bottom currents, which typically attain near-bed velocities between 0.2 and 0.4 metres per second, with a maximum of 1.4 metres per second. Cumulative vector plots of each deep-water mooring calculated from the current speed and duration show the northward transport of the near-bed water masses along the entire slope. These currents are likely related to the East African Coastal Current (moorings B and D) and deeper Antarctic Intermediate Water. Annual variation in current strength may relate to the seasonal occurrence of eddies along the Mozambique Channel. Bathymetric observations of north- to north-northeast–oriented obstacle-scour features and linear furrows match the measured current direction from acoustic Doppler current profilers, and indicate bottom-current velocities of about 0.2–1.0 metres per second, The rounded, less-steep northern channel flanks are interpreted as drifts stepping into the channels.

Well A intersects high-amplitude, sandstone-dominated channel fills. These high amplitudes decay toward the northeast, where they interfinger with low amplitudes of muddier drift deposits. The gamma-ray log shows repetitive low-intensity intervals that grade into spiky well-log responses, interpreted as coarse-grained sandstone packages that fine upward into mud-prone heterolithic sediments. Core facies are dominated by (1) turbidites, with lesser debrites and hybrid event beds (facies Fa1–Fa4); 2) muddy siltstones interbedded with 5–10-cm-thick, sharp-based sandstones grading into mottled siltstone with starved ripples and laminae, interpreted as reworked low-density turbidites (facies Fa5); and (3) muddy siltstones, 0.5–2 m thick, with parallel lamination, rare cross-cutting laminae, and ripples, interpreted as bottom-current deposits (facies Fa6). The well-sorted, narrow grain-size range of facies Fa6 suggests relatively weak, but long-lasting, flow. Individual cross-cutting laminae that are overlain by ripples indicate short-term increases in bottom-current strength and erosion. The limited occurrence of normal and inverse grading, well-preserved primary sedimentary structures, and relatively uniform grain size distinguish these deposits from those of other contourite facies models. The removal of organic matter in combination with high silt sedimentation rates could account for the suppression of bioturbation and the preservation of primary sedimentary structures.

 

(A) Sedimentological log of Well A, offshore Tanzania. Vfs, very fine sand; Fs, fine sand; Ms, medium sand; Cs, coarse sand. (B) Well A core photographs: (i) drift deposits interbedded with turbidites; (ii) reworked turbidites interbedded with toes of drift; (iii) drift facies transitioning into muddy turbidites. Zoom on individual facies associations (Fa): Fa6, drift deposit with parallel, cross-cutting and ripple-laminated muddy siltstone (I); Fa5, coarsening up into sharp-based thin low density turbidites (LDTs) with starved ripple lamination and bioturbated bed tops (II); Fa1, LDT, including muddy cross-lamination (III) and muddy climbing ripple lamination (II); Fa2, transitional flow deposits; Fa3, debrite; Fa4, slumped deposits. (C) Well A to seismic tie shows seismic amplitudes in the background with the correlated wiggles along the well path (red line). Seismic pick D represents a tie point between seismic and well; dotted lines are seismic horizons; blue bar represents the cored interval (A). GR, gamma ray; API, standardized unit of the American Petroleum Institute; TWT, two-way traveltime. Furhmann et al. (2020).

To understand the interaction of sediment gravity flows and bottom currents, it is important to consider these processes in terms of their orientation, thickness, velocity, steadiness, and persistence over time. Furhmann et al.'s model is based on geometries and facies stacking patterns considered to be representative of strong (i.e. velocities of meters per second), short-duration (minutes to days), unsteady, episodic turbidity currents, and relatively weak (i.e., velocities of centimeters per second) but quasi-steady bottom currents (over geological time scales). For turbidity flows, velocity values were collated from recent monitoring studies, while a compilation of published bottom-current velocity data augments our own measurements from offshore Mozambique. Bottom-current velocity typically fluctuates annually, with average recorded velocities in the region of 0.4 m/s and short-lived peaks of up to 1.4 m/s; flows of these velocities are able to entrain and redistribute the silt- and sand-grade sediment deposited by turbidity currents.

 

(A) Sedimentological model of hybrid levee-drift systems: (i) with concurrent gravity-driven turbidity currents; (ii) during dominance of bottom currents. Red arrow represents turbidity current; blue arrows represent bottom current. (B) Laterally offset channel complexes after repeated intervals of bottom current– dominated and turbidity current–dominated deposition. (C) Graphic visualization of the spatial variation of turbidity currents and bottom-current flows over time. Velocity (U) values are taken from published work and bottom-current measurements offshore of Mozambique. Furhmann et al. (2020).

When sediment gravity flow systems are active, the channels undergo cycles of erosion, bypass, and deposition under unsteady flow conditions. Thick, amalgamated, high-density turbidites are deposited in the channel axis, laterally changing into individual turbidite beds with preserved bed tops in channel off-axis positions (facies Fa1 and Fa2). Direct interaction of turbidity currents and northward-flowing bottom currents is short lived, but is anticipated to result in partial flow-stripping of the super-elevated fraction of turbidity currents. For the majority of the time, sediment gravity flows are subordinate to the north-flowing bottom currents. During this time, the channel and levee deposits are reworked and redistributed, forming one-sided, hybrid levee-drift deposits to the north. Deceleration and partial deflection of bottom currents interacting with the topography of the channel cause high accretion rates on the upstream-facing channel flank (relative to the bottom current) under lee-wave conditions. Thick homogenous muddy siltstones (facies Fa6) deposited by bottom currents step into the channel and ultimately interfinger with the channel margin (facies Fa1 and Fa2) and reworked overbank and/or levee facies (facies Fa5). For this reason, there is minimal levee development on the bottom current–upstream side of the channel. Sedimentary facies and architecture of the hybrid turbidite-drift channel systems are therefore controlled by the frequency of sediment gravity flow activity and the relative persistence and strength of bottom currents. Deposit modification would mostly occur during periods when bottom currents dominate; during this time, the strength and character or direction of the bottom current system would be variable due to, for example, seasonal eddies and benthic storms. However, changes in sediment flux (i.e., frequency of sediment gravity flows) and fluctuation of bottom currents over geological time scales (10 million years) govern the large-scale architecture and stacking pattern of hybrid turbidite-drift channel complexes along the East African margin. 

Modern and ancient submarine slope channels offshore of Tanzania and Mozambique are, and were, formed by episodic, unsteady, high-energy but short-duration, east-flowing turbidity current events superimposed on long-lived, quasi-steady, northward-flowing bottom currents. The channels are bordered by hybrid levee-drift deposits on their bottom current–downstream (northern) sides, which step progressively southward. Channels have steep eroded margins on their bottom current–upstream (southern) side, and gently dipping downstream flanks where the drift-levees step into the channel. We relate the upstream-migrating levee-drifts to lee-wave conditions as bottom currents traverse the channel. The continued development of the drift-levee pins the channels to the slope for protracted time periods. Well core data indicate that the 'toes' of the drift stepping into the channel are dominantly finely laminated siltstones, and that the internal channel architecture and facies distributions are strongly controlled by turbidity- and bottom-current interaction. Furhmann et al.'s integrated study is likely applicable to many other drift systems globally, and provides new quantitative data to enable the inference of bottom-current direction from ancient sedimentary sequences, which can be applied to existing and future studies.

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