Description: Previous work showed that South Atlantic sediments have lower glacial than Holocene 231Pa/230Th, which was attributed to a switch in the flow direction of Atlantic deep-water. Debate exists, however as to the degree to which two processes - circulation and scavenging - determine sedimentary 231Pa/230Th, making this interpretation contentious. Here we address this issue using 145-kyr records of paleocirculation proxies. Benthic foraminiferal d13C, neodymium isotopes (ENd) and sedimentary 231Pa/230Th were all measured in a single sediment core from the South Atlantic subtropical gyre. This site largely excludes the influence of local productivity changes on 231Pa/230Th records. Measured 231Pa/230Th ranges between ~0.041 during glacials to ~0.055 during interglacial periods and are consistently lower than the production ratio, indicating export of 231Pa from the central South Atlantic for the entire duration of the record. The lower glacial 231Pa/230Th is regionally consistent, suggesting that basin-scale oceanographic processes cause the decrease. In turn, less radiogenic ENd and lower benthic d13C confirm the classical picture of an increase in Southern Component Water (SCW) influence in the Atlantic during glacial periods and point to a circulation control on the observed 231Pa/230Th decrease rather than a local productivity change. We suggest that associated with this change in water mass distribution the dominant sink for 231Pa shifted from the margins of the South Atlantic and/or the Southern Ocean during interglacials, to the North Atlantic during glacial periods. Indeed, elevated 231Pa/230Th in the deep North Atlantic during glacials supports this mechanism of northward transport of 231Pa by SCW.

Description: Fluctuations in the 87Sr/86Sr ratio of seawater accompanying glacial/interglacial climate changes have been reported in recent studies but remain highly controversial. To investigate these potential fluctuations we present very high precision (13 ppm) 87Sr/86Sr measurements on planktonic foraminifera from Indian Ocean and Pacific cores. 87Sr/86Sr ratios from three different foraminiferal species are indistinguishable from one another in both core-top and 50 ka samples, demonstrating that changes due to diagenesis or contamination do not influence the measurements. Average 87Sr/86Sr ratios for Pacific Ocean and Indian Ocean samples are also indistinguishable at three intervals (core-top, 50 ka and 300-370 ka), demonstrating that the oceans have remained well mixed with respect to Sr. Also, 87Sr/86Sr ratios are not affected by changes in the precleaning of samples. Measurements from Pacific core V28-238, that used in the study of Dia et al. (1992, doi:10.1038/356786a0), do not reproduce the cycles seen in the previous study and are statistically well explained by a linear increase in the seawater 87Sr/86Sr ratio. It seems likely that an analytical artefact caused the cycles in the previous study. Measurements from two key sections of core from ODP site 758, that used in the study of Clemens et al. (1993, doi:10.1038/363607a0), do not reproduce the largest shifts seen in the previous study, despite the considerably better precision reported here. The apparent cyclicity in the Clemens et al. (1993) data is also suggested to be due to an unknown analytical artefact. The 48 measurements presented here are a statistically good fit to a straight line defined by Delta 87Sr (ppm) = -0.0613 * age (ka). At the 13 ppm level of precision no evidence for a glacial/interglacial variation in the seawater 87Sr/86Sr ratio is seen. Glacial/interglacial variation may still exist but this data constraints its maximum amplitude to 6-9 ppm, which would correspond to changes in the riverine Sr flux of c. 30%.

Description: Calcium-isotope ratios (d44/42Ca) were measured in carbonate-rich sedimentary sections deposited during Oceanic Anoxic Events 1a (Early Aptian) and 2 (Cenomanian-Turonian). In sections from Resolution Guyot, Mid-Pacific Mountains; Coppitella, Italy; and the English Chalk at Eastbourne and South Ferriby, UK, a negative excursion in d44/42Ca of ~0.20 per mil and ~0.10 per mil is observed for the two events. These d44/42Ca excursions occur at the same stratigraphic level as the carbon-isotope excursions that define the events, but do not correlate with evidence for carbonate dissolution or lithological changes. Diagenetic and temperature effects on the calcium-isotope ratios can be discounted, leaving changes in global seawater composition as the most probable explanation for d44/42Ca changes in four different carbonate sections. An oceanic box model with coupled strontium- and calcium-isotope systems indicates that a global weathering increase is likely to be the dominant driver of transient excursions in calcium-isotope ratios. The model suggests that contributions from hydrothermal activity and carbonate dissolution are too small and short-lived to affect the oceanic calcium reservoir measurably. A modelled increase in weathering flux, on the order of three times the modern flux, combined with increased hydrothermal activity due to formation of the Ontong-Java Plateau (OAE1a) and Caribbean Plateau (OAE2), can produce trends in both calcium and strontium isotopes that match the signals recorded in the carbonate sections. This study presents the first major-element record of a weathering response to Oceanic Anoxic Events.