Arctic drainage of Laurentide Ice Sheet meltwater throughout the past 14,700 years

The last deglaciation saw substantial meltwater from the decaying Laurentide Ice Sheet (LIS) flowing into the Arctic, Gulf of Mexico, and North Atlantic via various routes, sometimes catastrophically. These events potentially influenced global climate, with suggestions of a link to the Younger Dryas cold period. While the Mississippi River's role as a major southern drainage route is established, the dispersal of drainage routes after the Bølling-Allerød remains debated. The formation and growth of proglacial lakes, notably Lake Agassiz, are central to this debate, particularly its potential impact on the Atlantic Meridional Overturning Circulation via a massive freshwater outburst at the Younger Dryas onset. Terrestrial evidence regarding northwestern outflow around the Younger Dryas is inconclusive due to factors such as erosion, slow vegetation regrowth, and undiscovered evidence. Marine records, less sensitive to small runoff changes but offering continuous, well-dated information, provide crucial complementary evidence. Previous studies using planktonic δ¹⁸O in the Gulf of Mexico and Beaufort Sea showed major freshwater events, but these methods have limitations in sensitivity and source identification. The study leverages Pb isotopes in authigenic sedimentary phases as a sensitive tracer for continental runoff changes, offering information about both source area and weathering intensity. This approach has proven successful in identifying the St. Lawrence River as a major freshwater route during the late deglaciation. This research focuses on analyzing the Pb isotopic composition of authigenic and detrital phases from Beaufort Sea sediments to detail the freshwater routing into the Arctic Ocean since the Bølling-Allerød, aiming to constrain Lake Agassiz’s role in Arctic Ocean freshwater input shortly before the Younger Dryas.

Previous research identified major drainage routes for LIS meltwater: the Mississippi River (south), St. Lawrence River (east), Hudson Bay/Strait (north), and Mackenzie River (northwest). While the Mississippi's dominance after the Last Glacial Maximum (LGM) is agreed upon, the distribution of drainage routes after the Bølling-Allerød is debated. Broecker et al. (1989) proposed that a Lake Agassiz outburst through the St. Lawrence River triggered the Younger Dryas. Subsequent studies aimed to reconstruct the LIS deglacial evolution and meltwater routing, but terrestrial evidence, particularly regarding northwestern outflow during the Younger Dryas, is incomplete or contested. Marine proxies, such as planktonic δ¹⁸O in the Gulf of Mexico and Beaufort Sea, revealed major freshwater events linked to the Younger Dryas, but lacked source specificity. Authigenic Pb isotopes have emerged as a more sensitive and informative proxy for continental runoff, offering insight into both source and weathering conditions. Prior work using this method identified the St. Lawrence River as a significant route during the late deglaciation.

The study analyzed the Pb isotopic composition of both the authigenic and detrital phases in three sediment cores (JPC-9, JPC-15/27, JPC-19) from the Beaufort Sea margin and Amundsen Gulf, covering the late Heinrich Stadial 1 to the present. High-resolution records allow detailed reconstruction of freshwater routing. The authigenic Fe-Mn-oxyhydroxide fraction was extracted using a weak reductive leaching solution, while the detrital phase was analyzed after removal of remaining authigenic material. Pb isotopic composition was analyzed using a Thermo Scientific Neptune Plus MC-ICP-MS, with mass bias correction performed by Tl-doping. Elemental concentrations (Li, Al, P, Ca, Ti, Mn, Fe, Sr, Ce, Nd, Pb, Th, U) were measured using an Agilent Series 7500 ICP-MS. Anhysteretic remanent magnetization (ARM) data were generated for u-channel samples from core JPC-15/27, providing information on magnetic mineral concentration and grain size. The cores were dated using radiocarbon ages on planktonic and benthic foraminifera. The age models incorporated previously published and newly obtained radiocarbon dates, calibrated using the Marine20 curve. Detailed lithological descriptions of the cores are provided, drawing on previous research. The combined analysis of Pb isotopes, elemental concentrations, ARM data, and sedimentological information allowed detailed reconstruction of the changing sources and volumes of freshwater discharged into the Arctic Ocean.

Analysis of core JPC-15/27 revealed a shift towards more radiogenic (higher) ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values at the Bølling-Allerød onset (~14.7 ka BP), reflecting a change in sediment and runoff sourcing. The authigenic and detrital Pb isotopic compositions were very similar during this period. The strongest radiogenic excursion in authigenic Pb isotopes occurred just before the Younger Dryas (~13.1 ka BP), with subsequent smaller excursions during the Younger Dryas and early Holocene. Comparison with the Amundsen Gulf core (JPC-19) indicated that pre-Bølling-Allerød sediments at JPC-15/27 were not exclusively from the Amundsen Gulf, highlighting changes in sediment transport patterns with deglaciation. The Bølling-Allerød onset showed a sharp change in sediment provenance, with a shift towards more radiogenic Pb isotope ratios correlating with an increase in regional sedimentation rates. The strong radiogenic excursion at the Bølling-Allerød/Younger Dryas transition likely represents Lake Agassiz outflow via the Athabasca spillway, predating the δ¹⁸O freshwater event in the core. Subsequent smaller excursions during the Younger Dryas may also be linked to Lake Agassiz overflows or overflows from other northern glacial lakes. Detrital Pb isotope signatures traced continuous changes in sediment sourcing, with a return to more radiogenic values around 8.5 ka BP, possibly linked to the final drainage of Lake Agassiz into Hudson Bay. Core JPC-9 showed similar deglacial trends, with less intense radiogenic excursions, suggesting preferential freshwater transport to the east and central Beaufort Sea. The study combines the findings with data from other locations (Gulf of Mexico, Laurentian Fan, Hudson Strait) to establish a detailed deglacial drainage chronology for the LIS.

The findings provide crucial new constraints on LIS meltwater routing, particularly concerning the controversial northwestern route during the Younger Dryas. The strong authigenic Pb isotope excursion at the Bølling-Allerød/Younger Dryas transition provides direct evidence for Lake Agassiz outflow via the Mackenzie River, supporting the hypothesis of Arctic Ocean freshening as a trigger for Younger Dryas cooling. The study expands upon previous research by providing a high-resolution, multi-proxy record from the Arctic Ocean, which complements existing data from other drainage routes. The decoupling of authigenic and detrital Pb isotope signals during the Younger Dryas suggests independent controls on freshwater and sediment transport. The decreasing amplitude of Pb isotopic excursions throughout the Younger Dryas could indicate decreasing outflow intensity or varying source contributions. The changing composition of Lake Agassiz itself over time, including the influence of different lithologies as the ice sheet retreated, also likely influenced the Pb isotopic signature of the outflow. Future studies should investigate Lake Agassiz sediments directly to resolve these complexities.

This study provides a comprehensive reconstruction of Laurentide Ice Sheet meltwater drainage into the Arctic Ocean using high-resolution Pb isotopic data. The strongest Lake Agassiz outflow event occurred at the end of the Bølling-Allerød, supporting the hypothesis of Arctic Ocean freshening as a potential trigger for the Younger Dryas cooling. The presented chronology clarifies the roles of different drainage routes during deglaciation and highlights the power of authigenic Pb isotopes in reconstructing past hydrological changes. Further research should focus on refining the understanding of the relative contributions from different source regions during meltwater events, and investigating the eastern drainage route in more detail to fully constrain the chronology of Lake Agassiz outflow events.

The study primarily focuses on the Mackenzie River drainage basin, and does not directly analyze Lake Agassiz sediments. While the study strongly suggests Lake Agassiz as a primary source for several of the observed events, direct evidence from within the lake itself would strengthen the conclusions. The complexity of the Pb isotope signal, influenced by factors including bedrock composition and weathering processes, requires careful consideration in interpretation. The potential for other minor sources of freshwater to contribute to the isotopic signal cannot be fully excluded.