Export production: seasonality and intermittency, and paleoceanographic implications

Export production (XP), the portion of primary production (PP) which leaves the photic zone, depends greatly on seasonality and intermittency, that is, annual and interannual variability. A “seasonality index” is here defined in terms of the number of months which yield one half of the total production (production half-time). An equation relating the export factor (xf = XP/PP) to the level of productivity is proposed: xf = 2 × PP0.5, with a dependency on seasonality: xf = 2 × PP0.5 × (1 + (SI + 1)4/1000), where SI is the seasonality index (6 minus production half-time in months). According to this relationship (which is supported by available evidence), strongly pulsed production has a percentage output twice higher than constant production. In the glacial North Atlantic, when the polar front was relatively close to the intertropical convergence zone, severe storms would have favored pulsed and sporadic production, with a corresponding increase in export potential. Such a process would have led to more efficient downdraw of carbon from upper waters, and greater proportion of burial in continental margins. The result would have been diminished atmospheric pCO2, as seen in ice cores. Also, highly episodic upwelling would have produced nonanalog foraminiferal assemblages, making paleoceanographic reconstruction difficult.

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