Microfossils are the remains of tiny animals and plants found in rocks and sediment. They are very small and can be measured in millimetres (most are smaller than a pinhead). Scientists use microscopes to study them. Most of the palaeontologists (geologists who study fossils) working in New Zealand study microfossils, the most common fossils found in marine sediments. The organisms that grow these tiny skeletons evolve rapidly, are incredibly prolific, and are widespread. Scientists use these fossils to establish where oil and gas may be found, and to study past environments and climates.
There are different groups of microfossils. Pollen and spores are examples of terrestrial microfossils. Common marine microfossils include foraminifera, dinoflagellates and radiolarians.
Foraminifera are single-celled animals (protozoa) composed of a jelly-like mass encased in a calcareous shell. They live in diverse environments, from the sea floor to the surface waters. Those that live in surface waters are most abundant in the open ocean. As they and other tiny organisms die, their minute shells slowly sink to the ocean floor. Near to land, sediment washing into the sea from the land mixes with the tiny shells. The resulting sandstones and mudstones contain microfossils. Far away from land, the sea-floor sediments are composed almost entirely of these tiny skeletons.
Foraminifera have evolved rapidly and so can be used to determine the relative age of rocks. Any core of rock drilled from the sea floor contains a sequential record. As they are small and numerous, microfossils can easily be extracted from rock cores and used to identify which sediment layer the drill is passing through. This makes them important for geologists mapping geological structures and identifying different layers of sedimentary rock, some of which may contain oil and gas. Changes in the types of microfossils found in sedimentary rocks indicate past changes in conditions.
More recent microfossils, such as modern foraminifera, deposited in sands and mud, are being used by researchers as indicators of changes in the coastal environment in Auckland’s Waitematā Harbour.
Radiolarians are single-celled organisms with silica shells that float about in the world’s oceans. They have an evolutionary history dating back 500 million years. Scientists studying radiolarians look at when individual species first appeared and then became extinct. They have used this information to date some of New Zealand’s oldest rocks. The evolution of radiolarians also helps scientists to understand changes in ocean conditions over millions of years.
Pollen and spores
Seed-bearing plants propagate themselves by releasing massive amounts of pollen, while lower plants such as ferns release spores. Pollen and spores are widely broadcast in the air and settle onto the earth everywhere. They are often well preserved in swamp deposits and lake sediments. Fossil pollen and spores are used to study past climates and vegetation patterns. This is possible as different plants have distinct types of pollen and spores. The relative abundance of different pollen and spores reveals what types of vegetation grew at a particular time and also what the climate was like.
By comparing the quantity of different pollen types preserved in swamp sediments, scientists have concluded that widespread burning of forests first occurred in New Zealand around 1300 CE. This is one of the strands of evidence pointing to Polynesian settlement of New Zealand occurring about 1250–1300 CE.