ALBERT

Description: The Agulhas Leakage represents a significant portion of the warm, surface return flow of the global overturning circulation and thus may be an important feedback in the ocean climate system. Models indicate that reduced leakage could be caused by a stronger Agulhas Current and/or a more upstream (eastward) Agulhas Retroflection, while a weaker Agulhas Current would result in a more westward retroflection and increased leakage. However, data for the Last Glacial Maximum support both a weaker Agulhas Current and less leakage, implying a possible displacement of the retroflection. We present new 87Sr/86Sr results for modern sediments within this region, confirming that the modern pathway of the Agulhas Current, Retroflection, and Leakage can be traced by terrigenous sediment provenance using Sr isotopes. New 87Sr/86Sr data from sediments deposited during the Last Glacial Maximum suggest that the glacial Agulhas Current and Retroflection followed nearly their modern trajectory. The provenance data appear to rule out both a stronger Agulhas Current and a more upstream Agulhas Retroflection. We conclude that the reduced glacial leakage was caused by the weakened Agulhas Current, with no significant change in the retroflection position. This is inconsistent with the model predictions and thus emphasizes the need for further work in this region.

Description: The neodymium isotopic composition of marine precipitates is increasingly recognized as a powerful tool for identifying changes in ocean circulation and mixing on million year to millennial timescales. Unlike nutrient proxies such as d13C or Cd/Ca, Nd isotopes are not thought to be altered in any significant way by biological processes, and thus they can serve as a quasi-conservative water mass tracer. However, the application of Nd isotopes in understanding the role of thermohaline circulation in rapid climate change is currently hindered by the lack of direct constraints on the signature of the North Atlantic end-member through time. Here we present the first results of Nd isotopes measured in U-Th-dated deep-sea corals from the New England seamounts in the northwest Atlantic Ocean. Our data are consistent with the conclusion that the Nd isotopic composition of North Atlantic deep and intermediate water has remained nearly constant through the last glacial cycle. The results address long-standing concerns that there may have been significant changes in the Nd isotopic composition of the North Atlantic end-member during this interval and substantiate the applicability of this novel tracer on millennial timescales for paleoceanography research.

Description: Benthic foraminiferal delta13C suggests that there was a net shift of isotopically light metabolic CO2 from the upper ocean into the deep ocean during the last glacial period. According to the 'CaCO3 compensation' hypothesis, this should have caused a transient drop in deep ocean CO3[2-] that was eventually reversed by seafloor dissolution of CaCO3. The resulting increase in whole-ocean pH may have had a significant impact on atmospheric CO2, compounding any decrease that was due to the initial vertical CO2 shift. The opposite hypothetically occurred during deglaciation, when CO2 was returned to the upper ocean (and atmosphere) and deep ocean CO3[2-] temporarily increased, followed by excess burial of CaCO3 and a drop in whole-ocean pH. The deep sea record of CaCO3 preservation appears to reflect these processes, with the largest excursion during deglaciation (as expected), but various factors make quantification of deep sea paleo-CO3[2-] difficult. Here we reconstruct deep equatorial Pacific CO3[2-] over the last glacial-interglacial cycle using benthic foraminiferal Zn/Ca, which is strongly affected by saturation state during calcite precipitation. Our data are in agreement with the CaCO3 compensation theory, including glacial CO3[2-] concentrations similar to (or slightly lower than) today, and a Termination I CO3[2-] peak of ~25-30 µmol kg**-1. The deglacial CO3[2-] rise precedes ice sheet melting, consistent with the timing of the atmospheric CO2 rise. A later portion of the peak could reflect removal of CO2 from the atmosphere-ocean system due to boreal forest regrowth. CaCO3 compensation alone may explain more than one third of the atmospheric CO2 lowering during glacial times.

Description: Records of sortable silt (SS) percentages and means from the spliced record of IODP Site U1476 at the Davie Ridge, Mozambique Channel. The age model of this spliced record is based on benthic oxygen isotopes and biostratigraphy.

Description: Sr isotope ratios of the terrigenous sediments from the Cape Basin (southeast Atlantic Ocean) exhibit a systematic pattern of climate-related variability from the Holocene through the last glacial period. Values are high during warm climate intervals (marine isotope stages (MISs) 1 and 3) and lower during full glacial periods (MISs 2 and 4). The variability is large (87Sr/86Sr = 0.717-0.723), and the rapid changes correspond temporally to abrupt climate change during the MIS 5a/4 and 2/1 transitions and through MIS 3. The Sr isotope variability corresponds to changes in d13C of benthic foraminifera at orbital frequencies and within periods of rapid variability. Prior studies have suggested that benthic d13C records from the Cape Basin follow Greenland ice core variability and thus global overturning circulation. Other studies suggest that these benthic d13C records contain a strong overprint from isotopically light carbon, possibly associated with high fluxes of organic matter to the seabed. We explore the scenario that the relationship between lower terrigenous 87Sr/86Sr and lighter benthic d13C may reflect high productivity during cold climatic intervals as a result of iron fertilization of the southern Atlantic Ocean. Increased supply of iron during cold periods may be associated with greater terrigenous sediment fluxes from South America, characterized by a less-radiogenic Sr isotopic signature.