Late pleistocene palaeoceanographic and geochemical evolution of the South Tasman Rise

Global ocean circulation affects climate through its influence on transport and the carbon cycle. Past changes in global ocean circulation may have played an important role in past glacial-interglacial cycles. This study examines changes in deep-water circulation and carbonate chemistry in the Southwest Pacific sector of the Southern Ocean over the past 160 kyrs using benthic foraminiferal stable isotopes and carbonate dissolution proxy records. Two sediment cores (located at 3310 m and 4002 m water depths) are the focus of this research and are positioned south of the Subtropical Front south of Australia in modern Circumpolar Deep Water. The chronostratigraphy of the cores is established by accelerator mass spectrometry (AMS) 14C dates, benthic and planktonic foraminiferal δ180 and lithostratigraphy. Results of this study provide new, deep-water Southern Ocean δ13C values from sites unlikely to be impacted by productivity effects and establishes that glacial (Last Glacial Maximum (LGM); 18-24 kyrs and Stage 6.2; 134-137 kyrs) benthic foraminifera (Cibicidoides spp.) δ13C values are depleted compared to interglacial (Holocene; 0-10 kyrs and Stage 5.5; 118-124 kyrs) values. Previously, depleted Southern Ocean glacial δ13C values have implied a reduced contribution of northern source deep-water input into the Southern Ocean [Oppo and Fairbanks, 1987; Oppo et al., 1990; Charles and Fairbanks, 1992; Ninnemann and Charles, 2002]. However, the global δ13C reconstruction in this study, suggests Southern Ocean δ13C values were lower than northern source deep-water values and greater than or equal to Pacific δ13C values, implying there was no 'shut down' of deep-water circulation patterns during the glacial intervals over the past 160 kyrs. Comparisons between LGM δ13C records from the South Tasman Rise (STR) and published records from Atlantic, Pacific, Indian and Southern Oceans suggests: ( 1) horizontal δ13C gradients were maintained in the LGM between the Atlantic, Pacific and Southern Oceans above ,. ,3500 m, (2) LGM Southern Ocean δ13C values resemble LGM Pacific b13C values at water depths below ,. ,3500 m, and (3) LGM bottom waters in the Atlantic, Pacific and Southern Ocean converge to similar δ13C values below ,. ,4000 m. A criterion to exclude benthic δ13C records possibly affected by productivity overprints (Δδ13CcIb-DIc> -0.2 %0) enabled this interpretation. Planktonic foraminiferal shell weights for Globorotalia injlata (355-425 μm and 300-355 μm) and Globerigina bulloides (355-425 μm) are calibrated to depthnormalised carbonate ion concentrations. LGM shell weights estimate the deep-water carbonate ion concentration ([C03=]) at the STR was ,. ,10-13 μmol/kg greater than during the Holocene. Together with a revised shell weight calibration in the equatorial Atlantic and Pacific, the LGM deep-water [C03 =] reconstruction suggests: (1) Southern Ocean [C03=] were intermediate to Atlantic and Pacific [C03=] above ,. ,4000 m and, (2) LGM bottom waters in the Atlantic, Pacific and Southern Ocean converge to similar [C03=] below ,. ,4000 m, suggesting a similar deep-water source. Carbonate preservation indices (%CaC03 and %whole planktonic foraminifera) at the STR indicate three dissolution maxima at ,_,159 kyrs, ,. ,66 kyrs (Stage 4), and ,. ,27 kyrs. The dissolution records display similar timing to those recorded in the deep equatorial Pacific [Le and Shackleton, 1992] and deep South Atlantic [Hodell et al. 2001]. The synchroneity between the carbonate dissolution events at the STR, the equatorial Pacific and the deep South Atlantic 'lndo-Pacific' carbonate stratigraphy [Hodell et al. 2001], along with a similar deep-water [C03=] for these basins below ,_,4000 m, suggest a similar deep-water source at these locations during glacial intervals. The oxygen and carbon isotopic records were obtained from Globigerina bulloides (300-355 μm) at MD972106 and GC34. The two cores are located close to the modem calcite saturation horizon in modern Circumpolar Deep Water (CPDW), providing a test of the effects of carbonate dissolution on G. bulloides stable isotopic composition. There are differences between the stable isotopic compositions (δ180 and δ13C) between the two cores, particularly during interglacial intervals. G. bulloides δ180 and shell weights are tightly correlated to each other and to atmospheric pC02 , and are most likely due to the combined effects of the selective dissolution of foraminiferal shell calcite and calcification rates of G. bulloides. Isotopic and shell weight gradients between the two cores show large changes at glacial-interglacial transitions. The complex relationship between post-depositional dissolution and calcification rates of G. bulloides suggest both influence its stable isotopic composition. It is difficult to isolate the effects of each process on G. bulloides δ180 and δ13C records at both sites.