Sr isotope evidence for sources of terrigenous sediment in the southeast Atlantic Ocean: Is there increased available Fe for enhanced glacial productivity?

Abstract

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 δ13C of benthic foraminifera at orbital frequencies and within periods of rapid variability. Prior studies have suggested that benthic δ13C records from the Cape Basin follow Greenland ice core variability and thus global overturning circulation. Other studies suggest that these benthic δ13C 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 δ13C 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.