Oceanic Thermometers: Foraminifera Paleoclimate Quiz

Foraminifera are single-celled organisms that typically produce a shell or test made of calcium carbonate. Because they are so abundant and their shells are preserved easily in seafloor sediment they provide a continuous record of ocean history.

It is the opposite. Planktonic forams float in the upper water column and reflect surface conditions while Benthic forams live on or in the seafloor sediment providing data about the deep ocean environment. Comparing the two helps scientists understand vertical temperature gradients in ancient oceans.

During cold periods like ice ages the lighter isotope O-16 is trapped in glaciers leaving the ocean enriched in the heavier isotope O-18. Forams incorporate this ratio into their shells. A higher ratio of O-18 to O-16 in a fossil shell usually indicates a colder global climate.

Forams are biological sensors. The chemical fingerprint of their shells changes based on the waters temperature chemistry and the amount of water locked up in polar ice caps. While they show sea-level trends they cannot measure specific hourly tides from millions of years ago.

Some species change the way their shell spirals based on temperature. Left-coiling shells dominate in freezing polar waters while right-coiling shells appear in warmer water. Finding a layer of left-coiling shells in a normally warm area indicates a past cooling event.

In the deep cold high-pressure parts of the ocean seawater becomes corrosive to calcium carbonate. If the ocean becomes more acidic the CCD rises and fewer foram fossils are preserved serving as a warning sign of past ocean acidification events.

Deep-sea cores provide a vertical timeline. The top of the core is the modern day and as you go deeper you travel back millions of years. Scientists wash the mud away to find the sand-sized foram shells hiding within.

While dinosaurs are famous their fossils are rare and patchy. Forams are everywhere. A single gram of seafloor mud can contain thousands of shells allowing for precise statistical analysis of climate trends that land-based fossils simply cannot provide.

Tropical waters are typically more diverse hosting a wide variety of foram shapes and sizes. Polar waters are dominated by just a few specialized species. By tracking where tropical species appear in the fossil record scientists can map the movement of warm ocean currents over geologic time.

Because their shells are made of carbon-bearing calcium carbonate they lock in the carbon isotopes of the water they lived in. For relatively recent climate events like the end of the last Ice Age radiocarbon dating of forams provides a highly accurate calendar of events.

Large portions of the deep ocean floor are covered in this biological snow. Over millions of years this ooze can be compressed into chalk or limestone. The White Cliffs of Dover are actually made of billions of microscopic marine fossils.

Since benthic forams live on the bottom they are unaffected by surface winds. However they are the first to react if deep-ocean currents stop bringing oxygen to the seafloor. A sudden disappearance of benthic forams often signals a loss of oxygen in the deep sea.

In many extinction events or periods of rapid warming the average size of foraminifera shells decreases significantly. This shrinking is a survival strategy as smaller organisms require fewer nutrients and can reproduce faster in unstable conditions.

The amount of Magnesium that a foram incorporates into its shell is directly related to the temperature of the water. By measuring this ratio with a mass spectrometer scientists can calculate the exact temperature of the ocean millions of years ago.

By lining up foram data from various oceans scientists can see if a climate change event was local or truly global. This global cross-referencing is what allowed us to discover the patterns of the Great Ice Ages.