Bioturbation

Bioturbation is the reworking of soils and sediments by animals or plants.^[1]

Its effects include changing the texture of sediments (diagenesis), bioirrigation, and displacement of microorganisms and non-living particles. Common bioturbators include annelids (ringed worms) such as oligochaetes and Spirobranchus giganteus, bivalves like mussels and clams, gastropods, holothurians, or any other infaunal or epifaunal organisms. Faunal activities, such as burrowing, ingestion and defecation of sediment grains, construction and maintenance of galleries, and infilling of abandoned dwellings, all displace sediment grains and mix the sediment matrix.

Aquatic

The process leads to an increase in sediment-water interface, which facilitates particle exchange between the sediment and water column. In coral reefs sea cucumbers are one of the important bioturbators, capable of reworking approximately 4600 kg(dry weight) of sediment 1000 m² per year.^[2]

Terrestrial

In soil science, pedology, geomorphology, and archaeology, bioturbation is the physical rearrangement of the soil profile by soil life. Plants and animals exploit the solum for food and shelter and, in the process, disturb the fabric of the soil and the underlying parent material.^[3][4] Burrowing animals and insects, and plant root systems create passageways for air and water movement, changing soil morphology. A passageway created by an animal that becomes backfilled with soil is known as a krotovina. Invertebrates that burrow and mound soil tend to produce a biomantle topsoil (soil biomantle), and as such are primary agents of horizonation.^{[3][4][5][6][7][8]} Uprooted trees break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and disrupt soil horizonation.^{[9][10][11][12]}

Bioturbation was initially recognized as a pedogenic force by Charles Darwin,^[5] and developed further by Nathaniel Shaler.^[4] Although recognized as an important soil forming process since the last half of the 19th century, the importance of Darwin's mixing–sorting-biomechanical principles eluded Vasily Dokuchaev, the highly influential founder of modern pedology. Further development of the concept languished as a result. The term bioturbation did not exist before 1952, when it was coined to aid in ichnological assessments. Bioturbation appeared in the soil and geomorphic literature in the early 1980s,^[13] and remains a key term of the pedogenic lexicon.^[14] Bioturbation is central to the soil biomantle concept formulated in 1990.^[7] The biomantle is the upper part of soil produced largely by biota, dominantly by bioturbation. Biomantles tend to be one-layered when formed in homogeneous materials, and two-layered when formed in mixed fine-and-coarse materials. Bioturbation by burrowing animals results in soil landscapes that are both polygenetic (having many causes) and polytemporal (having been created at many times).

Mathematical models

Mathematical models are often used to describe sediment biogeochemistry. Commonly, these models take the form of ordinary differential equations or partial differential equations in which bioturbation appears as a diffusive term completed or not with advective terms. A diffusive description is often adopted to avoid quantifying the plethora of mixing modes resulting from faunal activities. The diffusion coefficient describing the intensity of bioturbation is usually determined by fitting mathematical models to vertical distributions of natural radioactive tracers, radioisotopes resulting from nuclear weapon testing, or introduced particles, such as glass beads tagged with radionuclides or inert fluorescent particles.

Evolutionary significance

Bioturbation's importance for soil processes and geomorphology was first realised by Charles Darwin, who devoted his last scientific book to the subject (The Formation of Vegetable Mould through the Action of Worms, 1881). Modern research has provided further insight into the evolutionary and ecological role of bioturbation.^[15] In modern ecological theory, bioturbation is recognised as an archetypal example of 'ecosystem engineering', modifying geochemical gradients, redistributing food resources, viruses, bacteria, resting stages and eggs. From an evolutionary perspective, recent investigations provide evidence that bioturbation had a key role in the evolution of metazoan life at the end of the Precambrian Era.^[15]

Geology

Section of hand sample of dolomitic siltstone showing a bioturbation. From I-71, exit 28, Kentucky. Probably Upper Ordovician Saluda Dolomite.

Patterns of bioturbation may be preserved in lithified rock, and the study of such patterns is called ichnology, or the study of "trace fossils". In some cases bioturbation is so pervasive that it completely obliterates sedimentary structures, such as cross-bedding. It thus affects the disciplines of sedimentology and stratigraphy within geology.

See also

• Zoophycos
• Nereis Park (the World of Bioturbation)

References