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

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.

Orbital and Suborbital‐Scale Variations of Productivity and Sea Surface Conditions in the Gulf of Alaska During the Past 54,000 Years: Impact of Iron Fertilization by Icebergs and Meltwater

“…The increase in bulk biogenic MAR during the SEs may reflect enhanced preservation due to increased burial rates or the addition of terrestrially derived components. The different temporal patterns between MAR and concentration of bulk sediment components (Figure 2) suggest that these biogenic maxima were impacted by enhanced burial efficiency (promoting organic matter and silica preservation; Hartnett et al., 1998; Ragueneau et al., 2000) caused by high sediment accumulation and/or the meltwater‐related discharges from the CIS along the northern Alaska margin (Cowan et al., 2020).…”

“…Since the magnitude of iron input to high‐latitude oceans has changed over geological timescales (Martínez‐García et al., 2011; Müller et al., 2018), it is essential to reassess its possible impact on past dynamics of primary producers. Possible mechanisms responsible for iron delivery to GoA surface waters between 54 and 17 ka might have been: (a) meltwater discharge from the northern CIS (Addison et al., 2012; Cowan et al., 2020), (b) seasonal mixing of the uppermost water column (Crusius et al., 2011), (c) aeolian input, and (d) eddy‐mediated offshore transport of coastal waters (Crusius et al., 2017). Although these mechanisms might have impacted primary productivity in different ways throughout the studied interval, we assign a major role of surface water fertilization to iron delivery via iceberg and meltwater discharges from tidewater termini (Cowan et al., 2020; Crusius et al., 2017).…”

“…Possible mechanisms responsible for iron delivery to GoA surface waters between 54 and 17 ka might have been: (a) meltwater discharge from the northern CIS (Addison et al., 2012; Cowan et al., 2020), (b) seasonal mixing of the uppermost water column (Crusius et al., 2011), (c) aeolian input, and (d) eddy‐mediated offshore transport of coastal waters (Crusius et al., 2017). Although these mechanisms might have impacted primary productivity in different ways throughout the studied interval, we assign a major role of surface water fertilization to iron delivery via iceberg and meltwater discharges from tidewater termini (Cowan et al., 2020; Crusius et al., 2017). Site U1419 was drilled on the Alaskan continental shelf, approximately 75 km west of the Bering Trough mouth (Jaeger, Gulick, & LeVay, 2014).…”

“…Site U1419 was drilled on the Alaskan continental shelf, approximately 75 km west of the Bering Trough mouth (Jaeger, Gulick, & LeVay, 2014). This shelf‐crossing trough was occupied by the Bering ice stream at the Last Glacial Maximum (Cowan et al., 2020, Figure 1). Because of the study site's proximity to the Bering icestream, we propose that the occurrence of the highest bulk biogenic and microfossil concentrations between early MIS 3 (c. 51.5 ka) and late MIS 2 (17 ka, shortly before the start of the last deglaciation) were due to tidewater glacier‐mediated discharges of sediment‐laden (iron bearing) meltwater and icebergs from the northern CIS (Penkrot et al., 2018).…”

“…The abrupt decrease of MAR at Site U1419 around 17 ka (Figure 2d) signaled the early stages of glaciers' stagnation or retreat into the last deglaciation (Cowan et al, 2020). The long-term glacial retreat and sea level rise associated with the last deglacial (Figure 5) led to a flooding of the coastal plain along northern Alaska.…”