The geologic time period covered by the 30-million-year sequence represents the Late Cretaceous. Studies have shown it was a period of dramatic change in climate, beginning with one of the warmest periods on Earth, then starting to transition to cooler climates, Strganac said.
Determining carbon ratios allowed comparison with global geologic events
To discover the age of the sediments, Strganac tested 55 fossil shells of ancient oysters and clams from 40 different rock layers on the coast. Testing determined the ratio of stable carbon isotopes, carbon-13 and carbon-12, in each shell. Because these isotopes do not decay with time, the relative abundance of each relates to the ocean when the shells formed. These isotope ratios can be compiled as a sequence with the rock layers, producing a pattern of carbon isotope change in the ancient oceans through millions of years. To accurately date the rocks, Strganac matched the pattern in isotope ratios in the shell record at Angola with the pattern known from ancient geologic events that occurred elsewhere in the world.
Specifically, the red rift-valley layers at Bentiaba were deposited as Africa and South America began to split. Also observed in the layers are a reversal in the Earth's magnetic polarity at 71.4 million to 71.64 million years to delimit the age of marine fossils; rocks deposited in the South Atlantic Ocean 93.9 million years ago during an oceanic anoxic event; and rocks south of Bentiaba that bracket the mass extinction of dinosaurs at 66 million years.
Besides comparing the stable carbon isotopes, other measuring techniques included: magnetostratigraphy, which measures the ancient polarity of the Earth's magnetic field when various sedimentar
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Southern Methodist University