APFIVE Carbon Isotope Ratios and Atmospheric CO₂
A scientist measures carbon isotope ratios in atmospheric CO₂ and finds decreasing ¹³C/¹²C ratios over the past century. This observation most directly supports which conclusion about carbon cycling?
A
Oceanic carbon sequestration is accelerating
B
Fossil fuel combustion is adding ancient biological carbon to the atmosphere
C
Volcanic emissions are the primary source of new atmospheric carbon
D
Plant photosynthesis rates are declining globally
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