E10 Physical Biogeochemistry

Topic

Physical biogeochemistry describes how physical ocean processes—circulation, mixing, stratification, and waves—control the distribution of nutrients, carbon, oxygen, and other elements that determine both the sea's biological productivity and the ocean's role in the global carbon cycle. It is precisely this capacity of ocean physics to transport elements from one place to another that allows the ocean to function simultaneously as the engine and the reservoir for much of the chemistry that sustains marine life.

The ocean absorbs approximately 25 to 30 percent of all human-generated carbon dioxide emitted into the atmosphere, making it the planet's largest long-term carbon sink. This absorption is driven by the so-called biological carbon pump: phytoplankton fix dissolved carbon dioxide into organic carbon within the ocean's sunlit zone, and when these organisms die or are consumed, a portion of that carbon is exported to deep waters before decomposing; this process ensures that carbon captured near the surface ends up sequestered—far from contact with the atmosphere—for extended periods.

Nutrient availability within the ocean's sunlit zone depends on three supply pathways: upwelling, vertical turbulent mixing, and atmospheric deposition. In the planet's most intense upwelling zones—off the coasts of Peru, Benguela, and California—the continuous upwelling of nutrient-rich deep water enables these regions to support some of the most productive marine ecosystems in the world.

Oxygen minimum zones form precisely where much of the organic carbon exported from the surface by the biological pump sinks; it is the decomposition of this carbon—occurring within the ocean interior beneath regions of high surface productivity—that causes biological oxygen consumption to outpace the supply delivered by circulation, leaving these waters virtually devoid of dissolved oxygen. Ocean acidification is the direct chemical consequence of the very same carbon dioxide absorption with which this document began: the carbon dioxide absorbed by seawater reacts to form carbonic acid, which in turn dissociates to release hydrogen and bicarbonate ions; it is precisely this increase in hydrogen ions that causes ocean pH to drop and reduces aragonite and calcite saturation. Ocean pH has fallen by one-tenth of a unit since the pre-industrial era—equivalent to a twenty-six percent increase in hydrogen ion concentration—and projected effects include both a reduction in the depth at which water ceases to be saturated with carbonate and a direct impact on calcifying organisms.

Thus, from the physical processes that distribute nutrients and carbon throughout the ocean to the chemical acidification with which this document concludes—encompassing the biological pump that sinks carbon and the oxygen-minimum zones left in its wake—physical biogeochemistry ensures that ocean circulation, mixing, and stratification ultimately determine both the ocean's capacity to capture carbon and its own internal chemistry.

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