GeoForschungsZentrum Potsdam, Federal Republic of Germany

Fixed nitrogen (N) is a limiting nutrient for algae in the low-latitude ocean, and its oceanic whose inventory may have been higher during ice ages, thus helping to lower atmospheric CO (sub 2) during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the (super 15) N/ (super 14) N ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate (super 15) N/ (super 14) N 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.