Carbon Isotope Ratios and Atmospheric CO₂
Measurements of atmospheric carbon dioxide (CO₂) reveal a decreasing ¹³C/¹²C ratio over the past century. This observation provides the most direct support for which of the following conclusions about the carbon cycle?
A
Plant photosynthesis rates are declining globally
B
Oceanic carbon sequestration is accelerating
C
Fossil fuel combustion is adding ancient biological carbon to the atmosphere
D
Volcanic emissions are the primary source of new atmospheric carbon
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