How do ice sheets die?



Graham H. Edwards

Earth & Environmental Geosciences

My science spans two broad categories…

Quaternary climate
Isotope geochemistry
Cosmochemistry


NASA/JPL-Caltech

Reconstructing ice sheet responses to climate change

IPCC, AR6, WG1, Fig. 9.17

Laurentide Ice Sheet

(ca. 25,000 years ago)

Reconstructions of Laurentide Ice Sheet (de)glaciation to better understand ice sheet life cycles

Baffin Island

Subglacial records of ice sheet response to millenial-scale climate change
Edwards+ 2022, Science Advances

Proglacial environments, including lakes & wetlands, are emergent, dynamic environments of the modern warming climate.

Glacial Lake Hitchcock (GLH)
Dalton+2023, Ridge+2012, McGann+2024, Springston+2024
Connecticut River Valley
G.H. Edwards

Carbonate concretions / claybabies

Thanks to Sophie Shipman for polishing cross-section surfaces.

Carbonate concretions / claybabies

Non-displacive, formed through oxidation of organic carbon Wu+2021

Reconstructing the hydrochemistry of the GLH proglacial lacustrine system.

Reconstructing the hydrochemistry of the GLH proglacial lacustrine system.

Field transect of the former GLH basin

Carbonate concretions

Bulk sediment

Sediment cores

Stable isotopes

Stable isotopes

Carbonate — concretions & ostracod valves

Stable isotopes

Carbonate — concretions & ostracod valves
↓T → ↓ δ18O

Stable isotopes

Carbonate — concretions & ostracod valves
  • Modern precipitation: -7 ≤ δ18O ≤ -10 ‰ Terzer-Wassmuth+2021
  • Similar δ18O for SN ostracods and concretions Danhof 2025
  • Concretion-forming waters ≈ lakewater
  • S→N transect of ↓δ18O (MVR, MB, WR)
    • Proximity to ice sheet margin?

Trace elements

Li, Na, Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, Pb, Th, U

Elevated Mn

Elevated Mn

Redox

  • Reducing conditions → high-Mn carbonate Wittkop+2020
  • Fe-Mn oxide formation Dean+1981
    • Reducing conditions → Mn2+
    • Oxidizing conditions → MnO2

Several reduction/oxidation cycles

↑ Mn → ↓ δ13C

Emerging story

  • Concretions form shortly after sediment deposition
  • Spatiotemporal variability → environmental diversity
  • Local/near-shore processes dominate record
  • Complex biogeochemistry!
Acknowledgements
Gavin Piccione, Rose Garrett, Luke Moreton, Sophie Shipman, Jack Ridge, Al Werner, Dave Jones, Clara Danhof, and assorted (NE) Friends of the Pleistocene.

Appendix

North American Varve Chronology

  • Precisely calibrated varve chronology of GLH, spanning 18.2–12.5 ka.
  • Well-established Laurentide Ice Sheet retreat and GLH residence Antevs 1922, Stone+2025
  • Lacustrine environment less well-described.
Ridge et al (2012), https://varves.as.tufts.edu/