Biogenic silica

Siliceous sponges were the main biological secretors of silica during the Cambrian, when they were mainly confined to shallow water. They formed the dominant biotic flux for this important geochemical cycle. However, since the Cambrian, two key factors have moved the site of this important flux from shallow water to deep water. These are the change in habitat of siliceous sponges and the evolution of plankton-building siliceous skeletons, the radiolarians and diatoms.

The geological record of siliceous sponges is poor, and may miss important evolutionary events. However, during the Cretaceous, siliceous sponges formed an important component of the chalk seas, and their silica often reprecipitated during burial to form flint. Flint nodules are most common in shallow water chalk, deposited in less than 100 m of water. Most modern hexactinellids, or glass sponges, are found in deep water, between 200 and 600 m, on the continental slope (Fig. 3.3). They have also been dredged from abyssal depths. This suggests that the major move into deep water occurred in the Cenozoic.

In the modern oceans, the combination of siliceous plankton and deep water siliceous sponges means that almost all

Fig. 3.3 A modern glass sponge found in deep water (height 15 cm).

biogenic silica is preserved in deep water sediments. In contrast to the Cambrian, the shelves are relatively starved of opaline silica, emphasizing the importance of evolution on major biogeochemical cycles.

Demosponge Mesozoic

A tulip-shaped sponge with a bulbous cup developed on a long stalk, held firmly to the substrate by a root system (height approximately 10 cm). Commonly found within flint nodules in chalk. The most commonly preserved elements of this sponge are irregularly shaped, siliceous spicules. There is a leucon grade of organization, with a series of canals within the thick wall of the tulip head that would have accommodated a complicated series of filtering cells.

Calcarea

Triassic- Cretaceous

An irregularly shaped, roughly vase-shaped sponge composed of simple, calcarous spicules (diameter approximately 10 cm). A poorly developed internal canal system suggests a leucon grade of organization. The skeleton was robust and survives well in the fossil record. Specimens are often found in great abundance, suggesting that, in common with modern sponges, this species was gregarious and tended to live in large groups on appropriate, high energy substrates.

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Responses

  • nea
    What sponge is found 100 m deep and is vase shaped?
    8 years ago

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