Foraminiferal isotope evidence of reduced nitrogen fixation in the ice age Atlantic ocean

Fixed nitrogen (N) is a limiting nutrient for algae in the low-latitude ocean, and its oceanic inventory may have been higher during ice ages, thus helping to lower atmospheric CO₂ during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the ¹⁵N/¹⁴N ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate ¹⁵N/¹⁴N ratio in the Caribbean thermocline. This change and other species-specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age-interglacial change in the N inventory.