Evidence of intense suboxia and anoxia in a tropical bay of the Gulf of California

The expansion of the upper boundary of the oxygen minimum zone (OMZ) has been associated with the low solubility of oxygen in the surface ocean due to global warming in tropical regions. However, sedimentary records of δ15N of sinking particulate organic matter and sediments suggest that oxygen solubility is controlled by oceanographic forcing in the tropical equatorial region. To determine the intensification of the OMZ within its upper border, δ15N analysis of the organic matter in superficial sediments of La Paz Bay was carried out. In the present work, 115 samples of superficial sediment were collected in the interval of 2-407 m depth in La Paz Bay. The surface sediments were analysed for the nitrogen isotopic composition of their organic matter. Hypoxia conditions (<60 μM) at 100 m to depth suggest an intense denitrification of the water column that it was reflected in the sedimentary δ15N. The δ15N of the organic matter is 1.5‰ greater in the bay than in the mouth of the Gulf of California, indicating greater denitrification towards the interior of the bay with respect to the gulf mouth. The intense suboxia/anoxia conditions have been maintained due to a well-mixed water column that promotes exported productivity in La Paz Bay.