Aerosol Iron Delivery and Geochemistry across Termination I: A New Record from the South Pole Ice Core
Abstract
Assessing changes in CO 2 downdraw via the biological pump during the Last Glacial Maximum (LGM) into the Holocene requires high resolution, continuous records of iron (Fe) transport, deposition, and bioavailability (the fractional amount of trace elements thought to be available to phytoplankton). We present a new record of fractional Fe concentrations and physical dust properties in the South Pole Ice Core (SPICEcore; 1751m; ~54ka) to quantify the change in Fe delivery to the South Pole, and by extension the Atlantic sector of the Southern Ocean, during Termination I. Particle concentrations and size distributions (30 bins; 1.0-12.0μm) were measured using a continuous flow analysis (CFA) Abakus laser particle sensor, with discrete samples analyzed on a Beckman Coulter Multisizer 3. Trace element samples were collected from the CFA melt stream at 2m resolution (every ~25 yr in the Holocene and ~120 yr in the LGM) and analyzed by SF-ICPMS. To assess changes in Fe geochemistry across the deglaciation, samples were split into: 1) total (HF-nitric acid digestion), 2) acid-reactive (acidified with nitric acid to pH <1 for 5 weeks), 3) labile (buffered using ammonia and acetic acid to pH 5), and 4) dissolved (filtered then acidified with nitric acid to pH < 1). We interpret the labile fraction as the most robust estimate of bioavailable Fe, based on the use of established oceanographic analytical procedures. LGM Fe concentrations are significantly higher in all fractions (1380 ng/L, 570 ng/L, 440 ng/L; p <0.01), representing an increase of about 530%, 635%, and 775% for acid-reactive, labile, and dissolved fractions, respectively, relative to the Holocene. Average Holocene labile Fe flux is 0.66 ± 0.41 ×10 -2 (mg/m 2 /a). Consistent with previous work, acid-reactive Fe concentrations represent only 2-3% of total digested concentrations, suggesting that existing trace metal deposition estimates from ice cores under-represent true fluxes. Proportions of dissolved, labile, and acid-reactive Fe vary across the deglaciation, possibly suggesting changes in dust sources and/or atmospheric processing during this interval. These results provide an important constraint on the delivery of bioavailable Fe to the Southern Ocean across the Termination.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.C11C1302C
- Keywords:
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- 0724 Ice cores;
- CRYOSPHERE;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE;
- 1616 Climate variability;
- GLOBAL CHANGE;
- 4994 Instruments and techniques;
- PALEOCEANOGRAPHY