A study in the August 30 issue of Nature provides, in unprecedented detail, the history of a crucial indicator of the relationship between the carbon cycle and climate processes over the past 55 million years. Over this time period, when the Earth is known to have transitioned from "hothouse" to "icehouse" conditions, the oceans also experienced a dramatic shift in the carbonate compensation depth, or CCD. Defined as the depth below which carbonate minerals (such as calcite) dissolve completely, the CCD is known to fluctuate over time it shallows during warm periods, and deepens when ice age conditions prevail. Now, however, scientists have a detailed and quantifiable record of just how much the CCD has shifted during recent geological history.
The study, which relies on seafloor sediment cores collected during a pair of 2009 expeditions on board the JOIDES Resolution, demonstrates that 55 million years ago, the CCD of the Pacific Ocean sat at an average of about two miles (3.3-3.6 km) below the sea surface. As the Earth cooled, however, the CCD sank reaching its deepest point of almost three miles (4.8 km) between 13 and 11 million years ago. Today, the Pacific's CCD sits just less than three miles (4.5 km) deep, and is thought to be on the rise as a result of modern, human-induced climate change.
"Long-term change in CCD and changes in Earth's atmospheric carbon dioxide concentration both result from shifts in how carbon is cycled by earth processes," says study co-author Mitchell Lyle, a geoscientist at Texas A&M University who co-led the second of the two expeditions. "Adding geologic reserves of carbon into the oceans and atmosphere by burning coal or petroleum, for example causes oceans to become more acidic and causes the atmosphere and oceans to warm. This new CCD record is an important step toward understanding how the carbon system balances out over long time frames. "
The Pacific Ocean has remained the largest oce
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Integrated Ocean Drilling Program Management International