Pathways of carbon and water through proglacial lake sediments during the Last Termination

Graham H. Edwards1, Gavin Piccione2, Julio C. Melara Jr.1

1 Earth & Environmental Geosciences, Trinity University, San Antonio, TX, U.S.A.

2 Geology & Environmental Science, The University of Pittsburgh, Pittsburgh, PA, U.S.A.

GSA Connects 2026

Modern ice sheet retreat

IPCC, AR6, WG1, Fig. 9.17

Modern ice sheet retreat

IPCC, AR6, WG1, Fig. 9.18

Proglacial lakes & wetlands are/will be emergent, dynamic environments in the modern warming climate.

Proglacial lakes and wetlands are sensitive intersections of Earth systems
Systems diagram indicating the impacts of proglacial lakes on climate, glacial, terrestrial, and marine systems.
Adapted from Carrivick+Tweed 2013

Role of (proglacial) lakes in the carbon cycle

  • Rapid sedimentation rates sequester organic carbon from bio-/atmosphere (re)cycling. Sobek+ 2009, Yan+ 2026
  • Probable (minor) methane source to atmosphere during Last Termination. Brosius+ 2024
  • Lacustrine carbon source/sink systematics are complicated, in general. Grasset+ 2025
How long do (deglacial) proglacial sediments sequester carbon?

How does carbon move through proglacial lacustrine systems over time?


Is there a representative system for a case study?
Glacial Lake Hitchcock (GLH)
Connecticut River Valley
G.H. Edwards

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

Carbonate concretions

Non-displacive, formed through oxidation of local org-C Wu+ 2021

Carbonate concretions

Non-displacive, formed through oxidation of local org-C Wu+ 2021

Fossil leaf casts indicate C source

Carbonate concretions

Widespread, associated with proglacial ice sheet environments
Region Lat Lon Ice sheet Reference
E Canada 67°N 135°W Laurentide Dionne 1972
NW Norway 66°N 14°E Eurasian Theakstone 1981
SE Finland 61°N 28°E Eurasian Parrot 1840
Baltic Sea 58°N 23°E Eurasian Kiipli+ 1995
Scotland 56°N 3°W Eurasian Duck 1990
Ireland 54°N 7°W Eurasian Knight 1998
CT Riv Vly 43°N 72°W Laurentide Tarr 1935
Patagonia 47°S 71°W Patagonian Rabassa 2006

Isotopic tracers of carbon and water

Isotopic tracers of carbon and water

Isotopic tracers of carbon and water

Approach & Methods

  1. Coupled radioisotopic systems:
    • Uranium-series (geochronology)
    • Radiogenic strontium
  2. Coupled carbonate stable isotopes:
    • Carbon
    • Oxygen
  3. Radiocarbon from Levy 1998

Two sites from the former GLH basin

Uranium-series geochronology

Element Solubility Distribution
U High Incorporated into carbonate
Th Low Adsorbs to sediment surfaces

Uranium-series geochronology

Mill Brook (n=3)

Uranium-series geochronology

River Road (n=2)

Detrital Th correction

$\left( \frac{^{230}\text{Th}}{^{232}\text{Th}} \right)_d = 0.898 ± 0.256 \qquad \chi^2_\nu=1.43$
$\left( \frac{^{230}\text{Th}}{^{232}\text{Th}} \right)_d = 1.005 ± 0.047 \qquad \chi^2_\nu= 0.46 $
e.g. Edwards+ 2022

Detrital Th-corrected U-Th ages

Initial (234U/238U) and 87Sr/86Sr

U and Sr record rock-water interaction
  • (234U/238U) ≫ 1 reflects deeply sourced subglacial groundwaters.
    • Shallow short-lived groundwater pathways
    • Chemical weathering of fine sediments
  • U-Sr records mixtures of groundwater sources along groundwater pathways.
    • No clear mixing: locally sourced Sr
    Edwards+ 2022

Stable isotopes

Stable isotopes

Stable isotopes

Carbon isotopes

Consistent with heterogenous organic carbon sources, fractionated by microbially-mediated oxidation.

Wu+ 2021

Stable isotopes

↓T → ↓ δ18O

Stable isotopes

Oxygen isotopes

Carbonate

  • Modern precipitation: −10 ≤ δ18O ≤ -7 ‰ Terzer-Wassmuth+2021
    • Mid-Holocene precipitation?

Stable isotopes

Oxygen isotopes

Clay

  • Clay formation in (sub)glacial waters
  • δ18O ≤ −19 ‰ for T = 0°C
  • Basal LIS δ18O composition?
    • Barnes Ice Cap basal ice: δ18O≈−25 ‰
    • Fossil groundwaters: −25 ≤ δ18O ≤ −12 ‰
Zdanowicz+2002, Ferguson & Jasechko 2015

A reprise of carbon sources: radiocarbon

Where did the (oxidized) organic matter come from?

Radiocarbon

Levy 1998

Radiocarbon

Levy 1998
  • Younging 14C signature during growth
  • Mixture of pre-, syn-(?), and post-lake C
Timeframe Carbon source
Pre-GLH Subglacially derived organic C
Syn-GLH Littoral and lacustrine
Post-GLH Holocene groundwater DIC

Conclusions

Transient proglacial lakes may provide millenial-scale storage of carbon from sub- and periglacial sources
  • Buried organic carbon oxidizes, remobilizes after lake drainage.
  • Lowered local water tables… or not? Levy 1998, Wu+ 2021

Future work

  • Quantify carbon sources and degree of remobilization
  • How does this scale across the GLH basin?

Questions?

More questions: gedward1@trinity.edu
Acknowledgements:

Jack Ridge, Al Werner, Laura Levy, E.R. Garrett, L. Moreton, S. Shipman, and assorted (Northeast) Friends of the Pleistocene.

Appendix Figures: more GLH sites