Reconciling down core anticorrelation of diatom carbon and nitrogen isotopic ratios from the Southern Ocean

Abstract

Oceanic nutrient cycling in the Southern Ocean is supposed to have an important impact on glacial-interglacial atmospheric CO2 changes and global climate. In order to characterize such nutrient cycling over the last two climatic cycles we investigated carbon and nitrogen isotopic ratios of diatom-bound organic matter (δ13Cdiat and δ15Ndiat, respectively) in two cores retrieved form the Atlantic and Indian sectors of the Antarctic Ocean. The two cores show the same isotopic patterns. The δ13Cdiat values are depleted during glacial periods and enriched during interglacial periods, indicating lower productivity during cold times. The δ15Ndiat values are enriched during glacial periods and depleted during interglacial periods, arguing for greater nitrate utilization during cold times. Taken at face value, this apparent contradiction leads to opposite conclusions on the role of the Southern Ocean biological pump on the atmospheric CO2 changes. However, the two sets of data can be reconciled by a "sea ice plus mixing rate scenario" that calls upon a balance between the effect of cutting off gas transfer at the ocean-atmosphere boundary and the effect of reducing vertical transport of nutrients through the pycnocline.