Decoupled acid reactive and biologically relevant trace element concentrations during Termination 1 in the South Pole Ice Core

Abstract:

Atmospheric delivery of biologically-relevant trace metals (e.g., Fe, Mn, Co, Cu, Cd) to the Southern Ocean plays a role in modulating atmospheric CO₂ drawdown via changes in the biological pump. Antarctic ice core dust records can provide insight into the role of dust-borne metals by providing end-member estimates of fluxes and geochemistry. Previous studies have estimated changes in the trace metal fraction available to phytoplankton with low temporal resolution and/or under the assumption that total concentrations represent the bioavailable fraction. We present a new high-resolution, continuous record of three different fractions (total, dissolved, and labile) of Fe, Mn, Co, Cu, and Cd for the past 54,000 years from the South Pole Ice Core (SPICEcore; 1751 meters). Samples were collected from a continuous flow analysis (CFA) melt stream at 2 m resolution (every ~25 years in the Holocene and ~120 years in the Last Glacial Period; 866 total samples). Each sample was split into 3 fractions for analysis via sector field ICPMS: 1) acid-reactive (acidified with nitric acid to pH <1 for 5 weeks), 2) labile (buffered using ammonia and acetic acid to pH 5), and 3) dissolved (filtered then acidified with nitric acid to pH <1). We interpret the labile fraction as the most robust estimate of bioavailable concentration. During Termination I (T1; 18 – 11.5 ± 0.5 ka), we observe decreases in the acid-reactive fraction of the following elements: Fe (270%), Mn (2354%) Co (259%), Cd (23%) and Cu (29%). Among the Fe concentrations, the dissolved fraction has the largest decrease (739% from 18-11.5 ± 0.5 ka) followed by bioavailable (324%) and acid reactive (270%), suggesting that these concentrations may be decoupled during T1. Possible explanations include changes in dust properties, atmospheric processing, or dust sources. These results imply that the assumption that total concentrations are accurate estimates of bioavailability is incorrect during large climate transitions. We discuss the implications these data may have for changes in CO₂ drawdown during T1.