EXPERIMENTAL LEACHING OF BEDROCK AND FLUVIAL SEDIMENTS: IMPLICATIONS FOR CONTINENTAL RUNOFF

Past continental weathering conditions are preserved in the Pb isotopes of Fe-Mn oxyhydroxides in marine sediments in the North Atlantic. These records reflect an excursion toward more radiogenic Pb values at the start of Holocene warming. Leaching experiments of moraine chronosequences indicate that younger, more chemically reactive soils yield more radiogenic weathering solutions than older soils or whole rock. These experiments suggest the Holocene marine records reflect enhanced chemical weathering of rock flour produced by retreating glaciers in a warmer climate. As glaciers retreat and the fluvial system reorganizes, older deglacial watersheds are likely to contribute a greater flux of water and less radiogenic dissolved Pb relative to younger proglacial watersheds. This transition should be recorded in marine records; however, little work has been carried out in terrestrial fluvial systems to evaluate how distinct fluvial weathering environments impact the signature of continental runoff.

For this study a series of dilute acid leaching experiments were designed to represent weathering conditions on land and subsequent isotope fluxes to the ocean. Powdered, homogenized samples of bedrock and fluvial sediments from Greenland were reacted with 0.1-1M HCl for 24-120 hours, and the leachates and residues analyzed for ^208,207,206Pb/²⁰⁴Pb ratios. Results indicate the leachate is consistently more radiogenic than the residual solid, but the offset between the two does not vary with the strength of the acid. Repeated leaching produced a general trend toward less radiogenic leachate and residual values, supporting the idea that younger proglacial watersheds should produce a more radiogenic Pb isotopic signal than older deglacial watersheds. Therefore, as the proportion of deglacial to proglacial watersheds increases with glacial retreat, we expect a trend toward less radiogenic Pb values in the dissolved and particulate flux to the ocean as observed in late Holocene Pb isotopic records from the North Atlantic. In fact, the same order of magnitude of change is observed in our leaching experiment and the late Holocene record. Comparison of the leaching data to the isotopic composition of pro- and deglacial waters will help refine our interpretation of high resolution deglacial marine records.