Читать книгу Reconstructing Earth's Climate History - Kristen St. John - Страница 22

Corals are colonial animals that primarily live in clear, shallow, warm waters of the tropical ocean. The animals themselves are small polyps with flower‐like tentacles that move with the currents to catch plankton. Corals also receive nutrients from photosynthetic algae that live within their tissue. Individual coral polyps secrete crystals of aragonite, a form of calcium carbonate (CaCO₃), which makes a hard external skeleton that is shared by the colony. This is like a shared apartment building for thousands of individual coral polyps. The skeletal structure grows as coral polyps build upon the existing structure that earlier generations secreted. Annual growth patterns can be recognized in climate‐related seasonal variations in the density of the skeletal material. These density differences show up easily on X‐ray images of corals (Figure 1.1d).

The skeletal structure of living and dead corals extends the record of climate change from modern day into prehistorical times, before instrumental observations of ocean conditions such as sea surface temperature were available. Some large living corals contain continuous records of climate and environmental change for the past 500 yr, and fossil corals give glimpses into even older time periods.

1 Corals are a marine paleoclimate archive, but they share some characteristics with tree ring archives and speleothem archives. Provide at least one similarity for each:Similarity with tree rings:Similarity with speleothems:

2  Go to the supplemental resources to watch videos and read a short article on the coral coring process. Make a list of the challenges of obtaining coral cores for paleoclimate research and the strategies scientists use to overcome these challenges.ChallengesSolutions

3 Starfish and worms can attach and burrow through the coral surface. How might this action affect the quality of the paleoclimate record that a coral can provide?

4 Go to the supplemental resources and read the USGS article on Corals as Climate Indicators. How are the coral cores prepared for sampling once the cores arrive in the lab?

5 The composition of the skeletal structure of corals can contain trace elements including magnesium, strontium, and barium (Mg, Sr, and Ba, respectively), which substitute for some of the Ca in the chemical formula of aragonite (CaCO3). Studies of modern corals show that the relative proportion of these trace elements to the proportion of Ca depends on the temperature of seawater in which the coral is living.Look at an online periodic table of the elements. Why would it make sense that Mg, Sr, and Ba can substitute for Ca in the aragonite formula? (Hint: what do Ca, Sr, Mg, and Ba all have in common?)What is the value of having coral records that can be linked to contemporaneous instrumental sea surface temperatures?How would such records be useful for deciphering past coral data?

FIGURE 1.8. Scientific research vessel, JOIDES Resolution.

Source: Credit: William Crawford, IODP/TAMU, http://iodp.tamu.edu/scienceops/gallery/exp321