Reconstructing past flow rates of southern component water masses using sedimentary 231Pa/230Th

Thesis

This thesis uses the paleoceancirculation proxy ²³¹Pa/²³⁰Th, coupled with water mass tracers δ¹³C and εNd, to reconstruct circulation histories for southern source waters masses in the South Atlantic, in addition to North Atlantic Deep Water (NADW) far from its source, for the last ~25 kyrs.

Downcore ²³¹Pa/²³⁰Th records from a suite of cores along a depth transect in the Argentine Basin show distinct differences with depth, suggesting that ²³¹Pa/²³⁰Th ratios in sediments are reflective of conditions in only the bottom most waters. This indicates the importance of consideration of changes in water mass distribution when interpreting ²³¹Pa/²³⁰Th records. Opal and particle flux data from these cores show little correlation with ²³¹Pa/²³⁰Th values meaning that changes in ²³¹Pa/²³⁰Th cannot be explained by a local composition or particle flux effect and are instead likely to be reflecting changes in circulation.

A core bathed by Antarctic Intermediate Water (AAIW) throughout the last 25 kyrs (GeoB 2107, 1045 m), has relatively high ²³¹Pa/²³⁰Th values (0.075) during the Holocene and distincly lower values (0.055) at the LGM suggesting faster AAIW transport during the last glacial. At greater depths, ²³¹Pa/²³⁰Th, δ¹³C and εNd data in core GeoB 2109 (2504 m) indicate a change in both circulation and water mass distribution on glacial-interglacial timescales, with moderate flow of Antarctic Bottom Water (AABW) at the LGM being replaced by more vigorous flow of NADW during the Holocene.

On millenial timescales, ²³¹Pa/²³⁰Th values in deep cores GeoB 2109 and GeoB 2112 (4010 m) indicate enhanced production of AABW during northern hemisphere stadials, when variations in ²³¹Pa/²³⁰Th records are of opposite sign between hemispheres, supporting a possible bipolar seesaw relationship in deep water formation between hemispheres. These data indicate that the ²³¹Pa/²³⁰Th proxy can be used to reconstruct past flow rates of multiple water masses in the Argentine Basin and provide evidence that southern source water masses play a dynamic counterpart to NADW formation on abrupt as well as glacial-interglacial timescales.

Authors: Hickey, B

• Publication date: 2010
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: 231Pa/230Th; paleoceanography; Argentine basin
• Subjects: Geochemistry