Changes in ocean circulation and carbonate chemistry in the Australian sector of the Southern Ocean during the last 500,000 years
Stable oxygen and carbon isotope records and carbonate dissolution proxy records were used to reconstruct glacial/interglacial changes in ocean circulation, deep-water ventilation and carbonate chemistry in the Australian sector of the Southern Ocean for the last 500 kyr. Carbonate preservation records indicated a longitudinal gradient in Southern Ocean deep-water alkalinity at 110°E that persisted through glacial/interglacial stages. In contrast to previous studies suggesting the “Atlantic-type” carbonate preservation pattern in the entire Southern Ocean with enhanced carbonate preservation during interglacials and increased carbonate dissolution during glacials, this study, however, revealed that the reversed “Pacific-type” pattern of glacial/interglacial carbonate preservation expanded from the Pacific into the Australian sector of the Southern Ocean up to 110°E. This result implies that deep-waters in the Australian sector flow westwards against the predominant eastward drift in the Southern Ocean. Epibenthic stable carbon isotope records depicted a detailed variability of Pleistocene deep-water circulation and ventilation with new implications for glacial deep-water circulation. The major problem in understanding the glacial deep-water circulation and isotope signatures below 2,500 m derives from the extremely low delta13C values in the Atlantic-Indian sector of the Southern Ocean contrasting relatively high delta13C values in the central Pacific. The glacial delta13C signatures of deep-waters in the Australian sector represent a mixture of both endmembers and thus link the isotope signatures of both regions. This suggests a westward deep-water flow to the south of Australia, which is consistent with the “Pacific-type” pattern of carbonate preservation in this area. These findings suggest the SW Pacific, in particular the Ross Sea, as a source region for nutrient-depleted Glacial Antarctic Bottom Water, which ventilated both, the Australian sector of the Southern Ocean and the deep Pacific. Considering the intermediate- to deep-water gradient in delta13C and eliminating the regional glacial/interglacial background, carbonate dissolution proxies and the net depth-related change in delta13C imply, however, that increased carbonate preservation was associated with a relative increase in deep-water ventilation. Planktonic oxygen isotope data suggested a glacial/interglacial latitudinal variability of the Subtropical Front to the south of Tasmania on the order of 2° in latitude with a northward, glacial position between 44.2° to 46.5°S. Comparisons of delta18O records from shallow- and deep-dwelling planktonic foraminifers suggested a long-term change in salinity at a water depth of >250 m. Intermediate-water salinities were apparently lower between 420 to 170 ka than during the last 170 kyr. This shift was most likely associated with a long-term southward displacement of surface water frontal systems. Another prominent feature in upper-ocean water mass signatures are extreme delta13C minima occurring at glacial terminations. Such minima in planktonic delta13C are already known from other areas of the Southern Ocean and even from the upwelling regions of the tropical Pacific. Recent studies thus assume that Antarctic Intermediate Water acted as a conduit for transferring delta13C variability to low latitudes. An epibenthic delta13C record from the Campbell Plateau at 600 m water depth displayed these minima too during Termination II. This would corroborate the proposed link between high- and low-latitude surface waters via Antarctic Intermediate Water.