Unlocking the Mystery: Why Foraminifera, Not Diatoms, Hold the Key to Temperature Proxies in Earth’s Climate

Temperature proxies are essential tools for understanding the Earth’s past climate. They are used to reconstruct past temperature records by analyzing the chemical and physical properties of natural archives such as corals, tree rings, and sedimentary records. Foraminifera and diatoms are two types of microorganisms that have been used as temperature proxies in sedimentary archives. However, foraminiferal shells are more commonly used than diatom shells because foraminiferal shells have some advantages over diatom shells in terms of preservation, abundance, and sensitivity to temperature changes.

Shell composition and preservation

Foraminifera and diatoms have different shell compositions and preservation potentials that affect their suitability as temperature proxies. Foraminiferal shells are composed of calcium carbonate, which is more resistant to dissolution in seawater than the silica-based shells of diatoms. As a result, foraminiferal shells are better preserved in sedimentary archives than diatom shells, which can dissolve during burial and diagenesis. The preservation potential of foraminiferal shells makes them more reliable temperature proxies than diatoms, especially in deep-sea sediments where dissolution rates are higher due to low pH and high pressure.
Another factor affecting the preservation of diatom shells is their size. Diatoms are much smaller than foraminifera, with an average size of 10-50 microns, whereas foraminifera can range from 100 to 1000 microns. The small size of diatoms makes them more susceptible to dissolution and degradation during transport and deposition. In contrast, the larger size of foraminiferal shells makes them more resistant to degradation and better preserved in sedimentary archives.

Sensitivity to temperature changes

Foraminifera and diatoms also differ in their sensitivity to temperature changes, which affects their suitability as temperature proxies. Foraminifera are more sensitive to temperature changes than diatoms, especially in high latitudes and polar regions where water temperature changes are more significant. The sensitivity of foraminifera to temperature changes is due to their symbiotic relationship with photosynthetic algae, which are sensitive to changes in water temperature and light intensity.
In contrast, diatoms are less sensitive to temperature changes than foraminifera because they are not symbiotic with algae and can survive in a wide range of temperature and light conditions. Diatoms are more influenced by nutrient availability, which can affect their growth rates and silica production, than by temperature changes. Therefore, diatoms are not as reliable as foraminifera for reconstructing past temperature records, especially in regions where nutrient availability is variable.

Ancient climate records

Foraminiferal shells have been used extensively as temperature proxies in ancient climate records, such as ice cores, sediment cores, and coral records. For example, oxygen isotopes in foraminiferal shells can be used to reconstruct past sea surface temperatures, while magnesium/calcium ratios can be used to estimate past ocean temperature and salinity. The stable carbon and oxygen isotopes in foraminiferal shells can also be used to reconstruct past climate conditions, such as El Niño events and changes in the global carbon cycle.
In contrast, diatoms have been used less frequently as temperature proxies in ancient climate records, mainly due to their preservation problems and lower sensitivity to temperature changes. However, recent studies have shown that diatoms can be used as temperature proxies in certain regions and under certain conditions, such as in the Arctic and Antarctic, where diatom assemblages are more sensitive to temperature changes than in other regions.

Conclusion

In conclusion, foraminiferal shells are more commonly used as temperature proxies than diatoms due to their better preservation potential and sensitivity to temperature changes. However, diatoms can also be used as temperature proxies in certain regions and under certain conditions. Therefore, the choice of temperature proxy depends on the research question, the location, and the preservation potential of the sedimentary archive.

FAQs

1. What are temperature proxies?

Temperature proxies are natural archives that preserve information about past temperature changes on Earth. They can be used to reconstruct past temperature records by analyzing the chemical and physical properties of natural archives, such as corals, tree rings, and sedimentary records.

2. What are foraminifera and diatoms?

Foraminifera and diatoms are two types of microorganisms that have been used as temperature proxies in sedimentary archives. Foraminifera are single-celled organisms with calcium carbonate shells, while diatoms are single-celled organisms with silica-based shells.

3. Why are foraminifera shells more commonly used as temperature proxies than diatoms?

Foraminifera shells are more commonly used as temperature proxies than diatoms because they have some advantages over diatoms in terms of their preservation, abundance, and sensitivity to temperature changes. Foraminifera shells are made of calcium carbonate, which is more resistant to dissolution in seawater than the silica-based shells of diatoms. Additionally, foraminifera are more sensitive to temperature changes than diatoms, especially in the high latitudes and polar regions.

4. What factors affect the preservation of diatom shells?

The preservation of diatom shells can be affected by their size, composition, and burial conditions. Diatoms are much smaller than foraminifera, with an average size of 10-50 microns, whereas foraminifera can range from 100 to 1000 microns. The small size of diatoms makes them more susceptible to dissolution and degradation during transport and deposition. Additionally, the silica-based shells of diatoms are more prone to dissolution than the calcium carbonate shells of foraminifera, especially in low pH and high pressure conditions.

5. What are the advantages of using foraminifera shells as temperature proxies?

The advantages of using foraminifera shells as temperature proxies include their better preservation potential, abundance, and sensitivity to temperature changes. Foraminifera shells are made of calcium carbonate, which is more resistant to dissolution in seawater than the silica-based shells of diatoms. Additionally, foraminifera are more sensitive to temperature changes than diatoms, especially in the high latitudes and polar regions. Foraminifera shells are also more abundant in sedimentary archives than diatoms, which makes them easier to analyze and compare among different samples.

6. Can diatoms be used as temperature proxies?

Although foraminifera shells are more commonly used as temperature proxies than diatoms, diatoms can also be used as temperature proxies in certain regions and under specific conditions. Recent studies have shown that diatoms can be used as temperature proxies in Arctic and Antarctic regions, where the diatom assemblages are more sensitive to temperature changes than in other regions.

7. What are some of the challenges in using microfossils as temperature proxies?

Some of the challenges in using microfossils as temperature proxies include the variability of the microfossil assemblages among different regions and environments, the potential for post-depositional changes in the microfossil shells, and the potential for contamination or alteration during laboratory analysis. Additionally, some microfossil groups, such as dinoflagellates, may have complex life cycles and ecological requirements that make it difficult to interpret their temperature signals.