Brazilian mangrove forests

Mangrove forests are geosystems and ecosystems generated by rapid deposition of sediments on the coast in tropical and subtropical areas. Our scientists are studying the rapid clay mineral transformations taking place in these dynamic environments.

Mangrove sediments form near the mouths of rivers, where abundant land sediments and organic matter are deposited in areas protected from tides and high-energy waves. These sediments are rapidly colonised first by plants and then mangrove trees.

The sediments in tropical areas are usually rich in kaolinite and iron oxides, both of which accumulate in newly formed mangrove forests.

Mineral transformations

Mangrove environments are powerful mineral reactors, with some of the fastest sedimentary mineral transformations on the planet.

Mineral reactions are triggered by a combination of factors:

• Sediments are flooded by a mixture of sea and fresh water, with abundant salts.
• The large amount of organic matter carried by the sediment and generated in the mangrove forest decomposes quickly, consuming oxygen within the sediment.
• Tidal changes subject sediments to daily salinity and oxygen concentration changes, which prevent the system from reaching an equilibrium point.

As a result:

• Iron oxides are dissolved and become very reactive in the waters.
• Fine kaolinite clay reacts with both the iron and salts in solution to form montmorillonite.

Why study mangroves?

Mangrove forests are very active sedimentary environments where the original minerals are quickly transformed. This is in contrast to most other sedimentary environments, which are typically passive.

The study of these dynamic environments provides a view of the reactions at different stages, allowing us to investigate the mechanisms that produce them.

We are studying mineral transformations in eight mangrove forests along the Brazilian coast, where the composition of sediments and water regimes vary.

Ancient coastal sediments

Cretaceous rocks from central Spain contain clay minerals intriguingly similar to those found in present-day Brazilian mangrove forests. These include kaolinite, iron oxides and iron-rich clay, which appear to have been deposited in a coastal environment.

Learning to read the mangrove signature in ancient rocks can help us to define past climates and environments, because:

• the original minerals are indicative of the climate in which they formed
• mineral transformations are typical of mangrove-like environments