Question

nautilids are marine mollusks that begin growing their shells before emerging from their eggs and continue to add shell segments throughout their lifetimes. the walls between their shells chambers are called septa, and the concentration of the isotope oxygen - 18 they contain exactly reflects the isotopes concentration in the water at the depth at which the septa formed. paleontologist amane tajika and colleagues examined each of the septa in two nautilid shells; finding that sample f13 had a higher concentration of the isotope than sample m03 did, the researchers concluded that f13 must have formed at a significantly lower temperature than m03 did. which finding, if true, would most directly support the researchers conclusion? a) as the concentration of oxygen - 18 in water decreases, water temperature decreases and nautilid shell growth slows. b) as water depth increases, both the concentration of oxygen - 18 and the water temperature decrease. c) as water depth increases, the concentration of oxygen - 18 increases and water temperature decreases. d) as water temperature increases, nautilids are able to concentrate more oxygen - 18 into their shells.

Explanation:

The researchers concluded that F13 must have formed at a higher temperature than M03 based on the concentration of isotope oxygen - 18 in the septa of nautilid shells. If as water temperature decreases, the concentration of oxygen - 18 in water decreases and nautilid shell growth slows, and as water depth increases, the water temperature decreases and the concentration of oxygen - 18 increases and water temperature decreases, then a lower oxygen - 18 concentration in the shell (as in M03) implies a higher water temperature during its formation compared to F13 with a higher oxygen - 18 concentration.

Answer:

A. As the concentration of oxygen - 18 in water decreases, water temperature decreases and nautilid shell growth slows.