Glacial/interglacial variations in production and nitrogen fixation in the Cariaco Basin during the last 580 kyr

The effect of sea level change on nutrient supply to the anoxic Cariaco demonstrates the fundamental importance of nitrogen (N₂) fixation and phosphate to oceanic production. As N₂ fixation produces biomass of low δ¹⁵N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it contributes to the light interglacial δ ¹⁵N (~2‰ - 3‰) values observed in the Ocean Drilling Program (ODP) site 1002 sediment record. During the glacials the sediments are bioturbated (oxic conditions) with low total organic carbon (TOC) contents and sedimentary δ¹⁵N values of ~5‰, suggesting that nitrogen (N₂) fixation contributed little to the N nutrition of Cariaco surface waters. The most plausible explanation for the inferred glacial/interglacial changes in N₂ fixation in the Cariaco is that they have occurred in response to variations in the N/P ratio of the nutrient supply, driven by changes in denitrification.