Continental runoff lead isotopic signatures released during incongruent chemical weathering in subarctic Lake Melville associated with the retreat of the Laurentide Ice Sheet over the past 14 ka

Lead (Pb) and its isotopes are known to be released incongruently during early chemical weathering in continental settings. Incongruent weathering implies that a chemical weathering induced continental runoff trace metal isotope signature is not identical to bulk rock isotopic compositions. The incongruent release of Pb can mostly be ascribed to preferential chemical weathering of less weathering resistant accessory uranium and thorium-rich mineral phases present that are most abundant in differentiated continental crust. If this continental crust is ancient, these accessory mineral phases contain present-day Pb isotopic signatures that are in places extremely radiogenic, as well as substantially different from bulk rock Pb isotopic compositions. Several studies that investigated the Pb isotopic runoff evolution in the Labrador Sea, NW Atlantic and Arctic Beaufort Sea already reported very radiogenic Pb isotopic runoff signatures in these marine basins bordering the Laurentide Ice Sheet (LIS) during key time intervals of the last deglaciation. These earlier results require the existence of very radiogenic Pb isotopic freshwater signatures inland North America that were generated during incipient post-glacial chemical weathering reactions in response to the retreat of the LIS during the last deglaciation.

We targeted subarctic Lake Melville in central Labrador aiming to resolve how the Pb specific chemical weathering signature changed in response to deglacial warming, in an initially subglacial setting that transitioned to completely ice-free conditions in the early Holocene. Lake Melville is a fjord‑like subarctic estuary in central Labrador that receives most of its freshwater and sediment from the Churchill River and other major tributaries draining a large early to mid-Proterozoic shield. We analysed two sediment cores from central Lake Melville that together archived the ambient dissolved Pb isotope signature over the past 13 ka. Our authigenic Pb isotope records are complemented by associated bulk detrital Pb isotope compositions, enabling us to compare the dissolved Pb isotope signature in the lake with corresponding sedimentary signatures. The lake was covered by the LIS until about 10.3 ka BP, yet still located in an ice-proximal setting until 8.5 ka BP. The region Labrador-Québec was ice free after ca. 5.7 ka BP.

The most striking result of our record is the observation of (i) very radiogenic authigenic Pb isotope compositions throughout that are (ii) much elevated relative to the associated detrital compositions, which are rather unradiogenic. Very invariant Pb isotopic signatures observed until 10.5 ka BP confirm the suggested subglacial lacustrine sedimentary setting in the oldest section. The subsequent deglaciation witnessed most variable compositions, with most radiogenic compositions seen at ~8.2 ka BP. The record becomes substantially smoother after ~6 ka BP when the catchment area was no longer influenced by direct glacial runoff. While the detrital compositions suggest some geographic variability in sediment sourcing, the authigenic Pb isotopic compositions are not following these detrital signatures. Our results highlight the unique geological setting that make authigenic Pb isotopes in proximal North American sediment cores a sensitive proxy for for the detection of elevated deglacial runoff fluxes in circum-North American marine basins.