Miocene benthic foraminiferal Cd/Ca records: South Atlantic and western equatorial Pacific

Abstract

Cadmium‐to‐calcium records in benthic foraminifera, interpreted in conjunction with carbon isotope records, reflect oceanic circulation patterns and oceanic inventories of Cd, carbon 13/carbon 12, and indirectly, phosphorus. Cd/Ca ratios have been determined in benthic foraminifera Cibicidoides spp. for the late, middle, and early Miocene from a site in the South Atlantic (Deep Sea Drilling Project (DSDP) site 525, leg 74, ∼2500 m, Walvis Ridge) and from a site in the western equatorial Pacific (DSDP site 289, leg 30, ∼2200 m, Ontong‐Java Plateau). Both sites have well‐established carbon isotope records for benthic foraminifera typical of the global record dominated by inventory changes. Cd/Ca ratios and carbon isotope values were averaged for the Miocene in time intervals substantially longer than estimated oceanic residence times of Cd, C, or P. Oceanic mean Cd/Ca and carbon isotopes were estimated as weighted means of the values from both sites using weighting factors based on replicating modern oceanic means; this may tend to underestimate oceanic inventories earlier than the late Miocene because of changing water mass dominance at the South Atlantic site. From these means, the oceanic Cd inventory appears relatively stable during the Miocene, while there are large changes in carbon isotope inventories. Miocene Cd inventory estimates are at least 20% lower than published estimates for Quaternary stages 1 and 2. Increasing oceanic Cd inventories over the past 10 m.y. are consistent with hypotheses of increased river fluxes in the recent geologic past. Cd/Ca differences between sites may be indicative of the extent of basin‐basin fractionation from bottom water circulation, although the validity of this comparison depends on how representative each of these two sites is through time for its respective ocean basin. The Pacific‐Atlantic Cd/Ca difference based on these two sites was larger in the late Miocene compared to the middle and early Miocene. The relationship between Cd/Ca and carbon isotope inventories and intersite gradients for these sites suggests that either oceanic phosphorus inventories or parameters related to marine organic matter composition must have been different in the Miocene than at present. Large changes in the carbon isotope balance, reflected as both positive and negative excursions in the carbon isotope inventories, occurred with little effect on Cd inventories. Changes in phosphorus fluxes associated with these carbon isotope flux changes may have been compensated for by changing deposition fluxes to other phosphorus sinks. Phosphorus inventories may have increased from the middle to late Miocene with increasingly negative oceanic carbon isotope inventories. However, confirmation of these suggestions depend on collecting more Cd/Ca (and δ13C) data throughout the Miocene oceans and on developing the application of other constraints for these interpretations.