2:45 PM - 3:00 PM
[MIS15-17] Distribution of neodymium isotopic compositions in seawater off Cape Darnley and characteristics of Antarctic bottom waters by formation area
Keywords:Antarctic bottom water, Nd isotopic composition, Southern Ocean, Cape Darnley
The isotopic composition of dissolved neodymium (Nd) in seawater is one of the chemical tracers. The Nd isotopic composition of seawater is characterized by records in fossil fish teeth/debris and ferromanganese oxides in sediments in contact with seawater. Therefore, Nd isotopic composition is used as a paleoceanographic proxy to reconstruct changes in ocean circulation not only in the present but also in the past. To use Nd isotopic composition for paleoceanographic studies, it is necessary to understand the current Nd isotopic distribution of seawater in the target area and the factors that control it.
Nd isotopic composition studies of seawater in the Southern Ocean have been conducted in various areas over the past two decades, with only the Indian Ocean sector of the Southern Ocean being a data vacuum. In this study, we focused on the area off Cape Darnley in the Indian Ocean sector of the Southern Ocean to determine the distribution of Nd isotopic compositions. The area off Cape Darnley is the fourth Antarctic bottom water (AABW) formation area, and it is known that dense shelf water is generated and subducted by high-salinity water discharged during the sea ice formation process in winter in coastal polynyas where large amounts of sea ice are formed. By clarifying the Nd isotope distribution in this important void of data, we discuss the systems that control Nd isotope distribution of the entire Southern Ocean.
The Nd isotopic composition of the AABW formed off Cape Darnley is characterized by a less radiogenic value compared to surrounding seawater. The deep water at the site closest to Prydz Bay had the lowest εNd value. The Nd isotopic composition is negatively correlated with the CFC partial pressure of seawater obtained during the same cruise, which has already been published, indicating that the AABW formed off Cape Darnley is an important source of less radiogenic Nd. The AABW formed off Cape Darnley and in Adélie Land is transported in the same direction as the Antarctic slope current (ASC) and is transported to lower latitudes via the Weddell Gyre. Studies in the Weddell Sea and South Atlantic have suggested that the average Nd isotopic composition of the AABW is -9 to -8 εNd, although lower values of -10 εNd have been reported in the Atlantic section of the Southern Ocean. This indicates that the contribution of AABW characterized by less radiogenic Nd formed off Cape Darnley and Adélie Land to the AABW flowing out of the Weddell Sea is not negligible.
The Nd isotopic compositions of AABW throughout the Southern Ocean has wide variations, as does the Nd isotopic distribution of Antarctic coastal sediments. The distribution of the AABW, with characteristic Nd isotopic compositions for each formation area, corresponds very well to the shelf systems controlled by the average ASC flow path and ASC strength inferred from observations over the last two decades. The ASC is known to vary in strength under the influence of surface wind stress, El Niño/La Niña, and the Southern Hemisphere Circumpolar Mode, and is expected to change significantly with more long-term changes in climatic conditions, such as glacial-interglacial periods. Therefore, the Nd isotopic composition of the AABW as an end-member in paleoceanographic studies should also be considered as potentially variable.
Nd isotopic composition studies of seawater in the Southern Ocean have been conducted in various areas over the past two decades, with only the Indian Ocean sector of the Southern Ocean being a data vacuum. In this study, we focused on the area off Cape Darnley in the Indian Ocean sector of the Southern Ocean to determine the distribution of Nd isotopic compositions. The area off Cape Darnley is the fourth Antarctic bottom water (AABW) formation area, and it is known that dense shelf water is generated and subducted by high-salinity water discharged during the sea ice formation process in winter in coastal polynyas where large amounts of sea ice are formed. By clarifying the Nd isotope distribution in this important void of data, we discuss the systems that control Nd isotope distribution of the entire Southern Ocean.
The Nd isotopic composition of the AABW formed off Cape Darnley is characterized by a less radiogenic value compared to surrounding seawater. The deep water at the site closest to Prydz Bay had the lowest εNd value. The Nd isotopic composition is negatively correlated with the CFC partial pressure of seawater obtained during the same cruise, which has already been published, indicating that the AABW formed off Cape Darnley is an important source of less radiogenic Nd. The AABW formed off Cape Darnley and in Adélie Land is transported in the same direction as the Antarctic slope current (ASC) and is transported to lower latitudes via the Weddell Gyre. Studies in the Weddell Sea and South Atlantic have suggested that the average Nd isotopic composition of the AABW is -9 to -8 εNd, although lower values of -10 εNd have been reported in the Atlantic section of the Southern Ocean. This indicates that the contribution of AABW characterized by less radiogenic Nd formed off Cape Darnley and Adélie Land to the AABW flowing out of the Weddell Sea is not negligible.
The Nd isotopic compositions of AABW throughout the Southern Ocean has wide variations, as does the Nd isotopic distribution of Antarctic coastal sediments. The distribution of the AABW, with characteristic Nd isotopic compositions for each formation area, corresponds very well to the shelf systems controlled by the average ASC flow path and ASC strength inferred from observations over the last two decades. The ASC is known to vary in strength under the influence of surface wind stress, El Niño/La Niña, and the Southern Hemisphere Circumpolar Mode, and is expected to change significantly with more long-term changes in climatic conditions, such as glacial-interglacial periods. Therefore, the Nd isotopic composition of the AABW as an end-member in paleoceanographic studies should also be considered as potentially variable.