Frozen pore water from beneath nearly a mile of ice: geochemical and stable isotope analysis of Camp Century basal sediment

Geochemical and isotopic analyses of 26 samples of frozen pore water from the 3.44-meter-long Camp Century sub-ice sediment core, collected in 1966, provide information about the source, history, and reactivity of that water. From these data, we interpret changes in weathering regimes in sediment deposited during both glacial and interglacial times on a landscape now under more than a kilometer of ice.

We thawed each frozen sample at 4°C and centrifuged to extract pore water. We measured pH and conductivity using Myron L meters and filtered samples using 0.45 µm PTFE. We measured d¹⁸O and dD using a Picarro L2130i Mass Spectrometer, cation concentrations using Agilent 7700 ICP-MS, and anions using a Metrohm 883/863 Ion Chromatograph.

The core contains 5 distinct stratigraphic units; diamict at the base (unit 1, 13% ice) overlain by silty ice (unit 2, 88% ice), modified diamict (unit 3, 34% ice), and fluvial sediment (units 4 and 5, 15% ice). Na⁺, K⁺, Mg²⁺, and Ca²⁺ are the main dissolved and mobile cations (up to 100 mg/L). Cl^-, SO₄^2-, CH₃COO^-and HCOO^- are predominant anions (up to 150 mg/L). Unit 1 has the highest conductivity (1000 µS) and the lowest pH (6.84); for units 2-5, conductivity does not exceed 500 µS, except for the bottom of unit 3. Units 2-5 are enriched with immobile cations (Al and Fe) compared to unit 1. Average water stable oxygen isotope ratios vary between units: unit 1, -21; unit 2, -26 and units 3-5, -24 ‰ relative to VSMOW.

Ice elemental concentrations indicate more extended sediment/water contact time for unit 1 than the overlying units. The enrichment in immobile elements in the ice of units 2-5 under basic values of pH (>7), suggests a transport-limited weathering regime for these units with a potential aeolian contribution as opposed to a static-limited weathering regime for unit 1. d18O and dD show no sign of phase changes at the bed, and a typical GMWL slope of 8.1. All units show an isotopic signature incompatible with the presence of an ice sheet, with the exception of Unit 2 with coldest conditions, reaching values closer to the Holocene ice at Camp Century. Considered together, pore water data suggest extended weathering of glacial sediment (unit 1) during interglacial conditions, cover by glacial ice (unit 2), then deglaciation and rapid deposition of interglacial fluvial sediment and porewater (units 3-5).