Arabian alluvial fans grow and disintegrate with Earth’s orbital cycles

Erosion of the land’s topography carries sediment into rivers that flow through the watershed’s mountains, canyons, and other naturally steep landscapes. These silts, sands and gravels are variably transported by suspension in the water column, bouncing along the river bed (saltation) or rolling larger pebbles (traction). With a deceleration in water velocity, and therefore carrying capacity, as the topography flattens, this material is deposited in a fan shape at the river mouth on the shallower plain of the break in slope, becoming progressively finer with distance from the source. .

Monsoons, seasonal climatic phenomena that cause extremely wet and dry conditions, can impact the development of alluvial fans by increasing water volume and velocity for watershed erosion and sediment transport, promoting thus the development of larger alluvial fans. This aggradation has been linked to forcings of Earth’s orbital cycles in new research published in Scientific reviews of the Quaternary.

Dr Sam Woor, from the University of Oxford, and colleagues studied alluvial fans in the arid Hajar Mountains of southeast Arabia, extending 0.01 km² at 10,000 km² and dated to the mid-late Quaternary (up to about 770,000 years ago). Specifically, the scientists aimed to determine the links between alluvial fan sedimentation and monsoon rain patterns affected by geological timescales of approximately 23,000 years (precession, “oscillation” of the Earth’s axis). and about 100,000 years (eccentricity, change between circular and elliptical orbits).

The research team collected samples from eight sites across mountain front cones (bajadas) and valley fills, including the Wadi Sahtan channel near Rustaq, Oman, where the river incision exposes 20 m deep river sediments, consisting of silt to large pebbles, as well as alluvial fans near Dhank and Ibri, both also in Oman, which extend across the vast foreland basin and merge.

In the laboratory, the samples were treated with acids to remove carbonates and organic materials before isolating medium-coarse quartz grains for use in a technique known as optically stimulated luminescence. This process relies on the radioactive signal from the crystal’s elements (such as uranium, thorium, and potassium) that accumulate once the quartz is buried and removed from exposure to sunlight.

Radioactivity causes electrons in quartz crystals to migrate and become trapped in the structure. When crystals are exposed to stimulated light in the laboratory, scientists can measure the light they return and estimate the number of electrons trapped, and therefore the amount of radiation to which they were exposed, thus determining the ages of alluvial aggradation. Importantly, the scientists had to keep their samples protected from sunlight exposure during collection and in the lab so as not to reset this signal, which required working in orange light conditions.

Dr Woor and colleagues determined a link between grain size and climate patterns, with coarse conglomerates of rock fragments embedded in a finer matrix forming thick floodplain deposits typical of heavy rainfall during the monsoon of summer of the Indian Ocean. These coincide with precession maxima, making seasonal contrasts more extreme in one hemisphere than the other.

Conversely, alluvial fan samples collected during times of ephemeral flow with fine silty sand, laminations, and organismal bioturbations coincided with precession minima. Sustained hydrological activity has been identified during 10 periods over the past 400,000 years, each of which broadly coincides with peaks in precession and eccentricity.

The oldest site (OM20/6) in the sampling area was dated to 300,000 to 370,000 years ago, with silty paleosols (ancient soils) indicating seasonal flooding on a floodplain interspersed with conglomerates forming braided streams during high flow events. This coincides with an interglacial period in the Northern Hemisphere, as well as records of speleothems (characteristic cave formations formed from groundwater mineral deposits) in Hoti Cave, Oman, which provide evidence of a increased precipitation.

Around 156,000 years ago, site OM20/12 preserved a conglomerate deposited from high-energy fluvial flow and records fan-shaped aggradation. The next phases of aggradation of the alluvial fan are recorded ~132,000, ~105,000, ~89,000, ~67,000, ~45,000, ~25,000, ~7,000 and ~2,000 years ago (with uncertainty relative mean of ±10,700 years). Scientists therefore suggest that arid conditions between Indian Ocean summer monsoon events occur during physical alteration of the landscape, which is then mobilized by heavy rains during climate events and deposits grained sediments coarser in constantly growing alluvial fans.

In addition to the precessional cycle above, interglacial periods over 100,000 year cycles would have increased sea surface temperatures and thus the abundance of water vapor in the atmosphere evaporating from the ocean Indian, further increasing precipitation. The superimposition of these two orbital cycles apparently had a much greater impact on Earth’s climate, with drainage networks incredibly sensitive to even small increases in precipitation.

This research provides important insight into the climate variability of an arid region during the Quaternary and reveals that much larger factors are at play to impact local environmental conditions.

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