Cd/Ca in late Miocene benthic foraminifera and changes in the global organic carbon budget

Abstract

The late Miocene carbon shift (∼6.2 Myr)—a 0.5–1.0‰, δ¹³C decrease in benthic and planktonic foraminifera—has been ascribed to changes in global inventory, deep-ocean circulation, and/or productivity^1–16. Cadmium, δ¹³C, and nutrients in the ocean are linked; comparison of δ¹³C and Cd/Ca yields circulation and chemical inventory information not available from either alone^17,18. We determined Cd/Ca ratios in late Miocene benthic foraminifera from DSDP Site 289. Results include: (1) late Miocene Pacific Cd/Ca values fall between those of late Quaternary Atlantic and Pacific benthic foraminifera; (2) there are no systematic Cd/Ca offsets between Cibicidoides kullenbergi, Cibicidoides wuellerstorfi and Uvigerina spp.; and (3) there is a very slight Cd/Ca change coincident with δ¹³C. Cd/Ca, slightly higher in younger, isotopically lighter samples, exhibits a smaller increase than predicted if circulation were the primary cause of the carbon shift. The carbon shift may have been due to a long-term shift in the steady-state carbon isotope input or to a change in the sedimentation of organic carbon relative to calcium carbonate.