Sediment controls dynamic behavior of a Cordilleran Ice Stream at the Last Glacial Maximum

Ellen A Cowan,Beth E Caissie,Wesley A Clary,Maureen H Walczak,Sarah D Zellers,Juliane Müller,Lindsay L Worthington,Stewart J Fallon,Jacob W Pratt,Sean P S Gulick,Alan C Mix,John M Jaeger

doi:10.1038/s41467-020-15579-0

Abstract

The uncertain response of marine terminating outlet glaciers to climate change at time scales beyond short-term observation limits models of future sea level rise. At temperate tidewater margins, abundant subglacial meltwater forms morainal banks (marine shoals) or ice-contact deltas that reduce water depth, stabilizing grounding lines and slowing or reversing glacial retreat. Here we present a radiocarbon-dated record from Integrated Ocean Drilling Program (IODP) Site U1421 that tracks the terminus of the largest Alaskan Cordilleran Ice Sheet outlet glacier during Last Glacial Maximum climate transitions. Sedimentation rates, ice-rafted debris, and microfossil and biogeochemical proxies, show repeated abrupt collapses and slow advances typical of the tidewater glacier cycle observed in modern systems. When global sea level rise exceeded the local rate of bank building, the cycle of readvances stopped leading to irreversible retreat. These results support theory that suggests sediment dynamics can control tidewater terminus position on an open shelf under temperate conditions delaying climate-driven retreat.

Highlights

Temperate tidewater glaciers can enhance their stability by building submarine morainal banks at the terminus, which both reduce buoyancy forces and insulate the ice from warm erosive seawater[3,13]
Sea surface temperature (SST) began to steadily rise ~17 ka, exceeding those previously observed in the LGM46; Bagley Ice Stream (BIS) appears to switch from the tidewater glacier cycle (TGC) and begin its terminal retreat earlier, around 17.6 ka cal BP
This timing suggests that the trigger for terminal retreat may be related to rapid sea level rise, increased subglacial meltwater erosion (Fig. 2c, d), and morainal bank failure

Summary

Introduction

When global sea level rise exceeded the local rate of bank building, the cycle of readvances stopped leading to irreversible retreat These results support theory that suggests sediment dynamics can control tidewater terminus position on an open shelf under temperate conditions delaying climate-driven retreat. In Greenland the stability of calving outlet glaciers is not uniform but is influenced by local conditions particular to their marine termini[4,5,6], as well as by thinning and acceleration attributed to climate change[7] Both theory[8,9] and field studies[10,11] demonstrate that sediment yield from temperate glacial erosion and sediment transport can reduce water depth and restrict calving by building up morainal banks and ice-contact deltas. The morphology of the 25-kmwide, 55-km-long Bering Trough is similar to Alaskan fjords, where the TGC was first described (i.e., Taku Glacier[22] and Columbia Glacier1) except that it opens directly into the Gulf of 60°N

Methods

Results

Conclusion