| Summary: | Changes in the degree of nutrient consumption in Antarctic Zone (AZ) of the Southern Ocean may have played a major role in controlling past carbon dioxide levels and therefore past climate. The \(^{15}\)N / \(^{14}\)N ratio (\(\delta\)\(^{15}\)N) of diatom-bound organic matter (DB \(\delta\)\(^{15}\)N) has been investigated as a proxy for the degree of nutrient consumption in the surface mixed layer. DB \(\delta\)\(^{15}\)N records for the Indian and Pacific sectors of the Southern Ocean AZ display high \(\delta\)\(^{15}\)N values during the last ice age and lower \(\delta\)\(^{15}\)N values throughout the Holocene, suggesting more complete nitrate consumption due to reduced exchange between the AZ surface and deep ocean during the ice age, which would have worked to lower atmospheric CO2 as observed. However, DB \(\delta\)\(^{15}\)N in cores from the eastern Atlantic sector AZ show with the opposite glacial-interglacial trend, with DB \(\delta\)\(^{15}\)N being abnormally high during the interglacials and lower during the ice ages. While the Atlantic DB \(\delta\)\(^{15}\)N signal was originally hypothesized to be a consequence of species assemblage change in the pennate diatom fraction, new assemblage-specific DB \(\delta\)\(^{15}\)N records from multiple Atlantic sector cores, including the two reported here, reveal that the ice age DB \(\delta\)\(^{15}\)N is not due to assemblage changes and instead must reflect an environmental signal. In this study, we report assemblage-specific DB \(\delta\)\(^{15}\)N records for Atlantic AZ sediment cores AII 107-22GGC and RC11-076. These are compared with two previously generated, unpublished DB \(\delta\)\(^{15}\)N records, TN057-13PC and RC13-259 from farther east in the Atlantic AZ. As additional support for our Holocene DB \(\delta\)\(^{15}\)N results, we also report for the first-time paired foraminifera bound (FB) and DB \(\delta\)\(^{15}\)N core top reconstructions for Atlantic sector AZ core site CHN115-4-32GGC. The compilation shows that low glacial and high interglacial diatom \(\delta\)\(^{15}\)N is a basin wide phenomenon for the Atlantic sector AZ, with important implications for potential drivers. First, high interglacial \(\delta\)\(^{15}\)N cannot be explained by a mechanism regionally specific to the eastern Atlantic AZ, such as iron fertilization by Bouvet Island. Second, low glacial Atlantic AZ \(\delta\)\(^{15}\)N is unlikely to be the result of the summer sea ice cover that has been reconstructed for the eastern Atlantic AZ during the ice age. We propose, counterintuitively, that the low Atlantic AZ \(\delta\)\(^{15}\)N during the ice age reflects complete nitrate exhaustion in the summer surface mixed layer, forcing diatoms to rely on low-\(\delta\)\(^{15}\)N ammonium as their main nitrogen source. If so, the Atlantic AZ must be characterized by more complete nitrate consumption in both the interglacial and the ice age. This might result from the lateral advection of a partially consumed, higher \(\delta\)\(^{15}\)N nitrate pool into the Atlantic sector AZ that supersedes deeply supplied nitrate as the primary source for diatom growth.
|
|---|