Diatom dominated marine primary productivity in the Mexican Northeastern Pacific during the last millennium

Diatoms, the main group of siliceous plankton, contribute more than 40% of the marine primary productivity and dominate the plankton communities of the Southern Ocean, north Pacific and the upwelling regions of the equatorial and tropical Pacific. The Si(OH)(4)/NO3- > 1.0 ratio in subsurface waters of the southwestern margin of Baja California (SWM-BC) suggests that diatoms grow in optimum nutrient conditions and reach a value of Si (OH)(4)/NO3 < 3 in conditions near the limit because of Fe or P deficiency. These values are higher than the Si:N < 1 ratio for many oceans, therefore if primary productivity is diatom dominated in the last millennium, the Si: N ratio must be <3. To answer this question, a multicore was recovered from a depth of 700 meters in the SWM-BC. The core was sampled every 1 cm and freze-dried to obtain the biogenic opal content (BO) and total nitrogen (TN), from which the Si: N ratio and organic phosphorus were calculated. The BO and TN content increased in a constant manner and the Si: N ratio had an average and standard deviation of 2.8 +/- 0.1 and P-org had an average and standard deviation of 1.1 +/- 0.9 mg kg(-1). In the last millennium, primary productivity has been diatom dominated, a product of the silicic acid exported from the Gulf of California to the Magdalena margin, hypothesis that has to be validated, under a Fe-limited ocean, and not by nitrates and orthophosphates.