Description

Two outlet glaciers of Langjökull drain into Hvítárvatn; Norðurjökull flows through a narrow, steep valley, topographically constrained by Skriðufell to the south and Leggjarbrjótur to the north, whereas Suðurjökull curves around Skriðufell in a wide, relatively flat unconstrained valley. Bathymetrically, Hvítárvatn can be divided into two main regions consisting of a shallow southern basin and a deep northern basin that contains the lake's major depositional centres (Fig. 1a, b). The deepest areas in Hvítárvatn are in front of and between the two outlet glaciers with water depths exceeding 80 m. A broad shoal, 30–40 m deep, rises gently from the central deep towards the NE shore of the lake. The dominant feature in this region is the Hvítárnes delta, which forms a steep basin margin (Fig. 1a). Seismic profiles show evidence for substantial progradation of this delta during Langjökull's Neoglacial expansion in the late Holocene (Black et al. 2004).

The lake bathymetry and seismic stratigraphy fronting Norðurjökull reveal three ridges that rise approximately 10–20 m above the basin floor and vary in width from 5 to 30 m (Fig. 2a). The two innermost ridges form large arcuate moraines and the third outermost ridge is composed mainly of bedrock, an extension of an onshore hyaloclastite ridge to the south. This hyaloclastite ridge is draped by 15 m of undisturbed sediment and has a crest marked by a few recent iceberg ploughmarks (Fig. 1c). Weakly developed morainal mounds associated with a breach in Norðurjökull's outer moraine are visible along the right-lateral margin in a region of deep water close to Skriðufell (Fig. 1b). These features suggest that this area was the focus of Norðurjökull ice calving, which prevented the formation of a well-defined moraine.

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Fig. 2.

Multibeam bathymetry, bathymetric transects, seismic profile and stratigraphy of lake cores collected at Hvítárvatn (acquisition systems as in Fig. 1). Glacier and related landforms at (a) Norðurjökull and (b) Suðurjökull. Positions of two bathymetric transects for each glacier forefront are shown with dashed white lines. (c) Seismic profile (dashed white line w–w′ on Suðurjökull forefield bathymetric map in (b)) extending from older generation ice-scoured trough and crossing Hvítárvatn's central basin. Superimposed is a conceptual reconstruction of Suðurjökull's repetitive surging terminus advance (blue dashed line) and collapse into the lake. The approximate positions of two sediment cores (N1, HVT02–6) are shown; multiple lenses of ponded redistributed sediment are highlighted with red shading. A seismic reflector corresponding to the H1766 tephra layer (Larsen et al. 2011) can be traced across the undisturbed portion of the profile and is highlighted for reference. (d) Stratigraphic log and correlation of two cores collected along transect of Hvítárvatn's main basin. Core sites are located near the seismic profile shown in (b). The positions and calendar ages of eight unusually thick clay layers corresponding to Suðurjökull surges are shown along with H1766 tephra layer (blue).

The lake-floor geomorphology fronting Suðurjökull is strikingly different from that of Norðurjökull and is composed of radiating ice-scoured troughs 100–200 m wide and up to 15 m deep, that extend 0.5 to >1.0 km into the main basin. Although fan-like in general appearance, close inspection reveals cross-cutting relations and differential infilling that define multiple generations of ice expansion, and two discrete broad-scale radiating features that are mainly erosive (Figs. 1b, 2b). At the distal ends of each feature are sediment piles that extend 5–8 m above the lake floor, with steep slopes on the ice-proximal side and concave slopes distally (Fig. 2b).

The terminal moraines in front of Norðurjökull and Suðurjökull mark the boundary between heavily disturbed sediment proximal to the moraines and a comparatively undisturbed smooth floor in the rest of the basin. Although the main basin is outside the limit of recent glacial advance, this region was influenced by repeated sediment gravity flows that appear as acoustically transparent sediment wedges >2 m thick stacked in the seismic records. Otherwise, the seismic reflectors in this area are relatively flat, revealing an approximately 25 m thick sediment sequence that can be divided into four seismostratigraphic units (Black et al. 2004) (Fig. 1c).

Distal to the two outlet glacier termini, pits and randomly oriented grooves are apparent in water depths of <50 m. Grooves are up to 500 m in length, although most are <200 m long. They are commonly 5–10 m wide and most are 0.2–1 m deep, with rare grooves reaching 5 m deep. Pits are common between 30 and 50 m water depth, where they are 10–30 m in diameter and about 1 m deep. Some grooves end in pits and many pits have a ridged edge.

Interpretation

Our interpretation is based on combined records from over 100 km of seismic-reflection data, high-frequency multibeam sonar bathymetry and more than 30 sediment cores that were obtained from both in front of Norðurjökull and Suðurjökull as well as from the deep central basin (e.g. Fig. 2d). The sediment cores are varved in the uppermost approximately 10 m (encompassing at least the past 3 ka) and can confidently be correlated across the whole lake basin based on distinctive varves and tephra layers (Larsen et al. 2011). Glacial landforms visible in lake bathymetry, combined with well-preserved lateral moraines along both Norðurjökull and Suðurjökull, define Langjökull's greatest Neoglacial extent late in the LIA (Geirsdóttir et al. 2008). At this time, both outlet glaciers extended into Hvítárvatn and maintained active calving margins (Larsen et al. 2011). The duration of ice incursion into the lake is documented by the presence of ice-rafted debris (IRD) quantified in five sediment cores (Larsen et al. 2013, 2015). Individual outlet glacier chronologies are constructed using sediment cores collected from multiple locations within the eroded depressions fronting Suðurjökull, multiple cores from undisturbed sediment beyond the troughs and from Karlsdráttur, a bay north of Norðurjökull's left-lateral moraine. The two prominent moraine ridges in front of Norðurjökull (Fig. 2a) reflect at least two LIA advances and Karlsdráttur cores indicate that continuous extension past the bay mouth occurred during c. AD 1766–1905 (Fig. 1a, b). Following the advance, Norðurjökull retreated back across its overdeepened basin past the bay mouth by c. 1905 and to near the lakeshore by 1945 (Larsen et al. 2015).

The radiating ice-eroded troughs in front of Suðurjökull glacier are proposed to have occurred through multiple surges of Suðurjökull into Hvítárvatn (Fig. 2b). During each surge of Suðurjökull, the glacier terminus advanced rapidly over a deformable bed of lacustrine sediment and fractured into a series of ice fingers that excavated narrow depressions and redistributed previously deposited sediment (Fig. 2b). We propose that the narrow fingers of ice were temporarily maintained by the buttressing effect of a dense mélange of calved icebergs on the lake surface (Geirsdóttir et al. 2008). Small push moraines (<10 m high) positioned at the end of each depression are visible in the seismic profiles as ridges of disturbed sediment (Fig. 2c). Discrete lenses of ponded sediment originating from the push moraines were identified in seismic profiles and sediment cores taken from the central basin. These lenses are interpreted as redistributed lacustrine sediment associated with Suðurjökull's dynamic advance into the lake and can be stratigraphically tied to cores from distal locations, where they are expressed as unusually thick clay layers (Fig. 2d) (Larsen et al. 2013, 2015). The clay layers are thickest in cores collected proximal to the finger-like depressions beyond modern Suðurjökull, confirming that the sediment originated from this glacier. Most Suðurjökull depression cores bottom out on a dark grey stony till unit located below typically varved sediments that continue to the core tops (Fig. 2d). The dark grey stony till and overlying varved sediment correspond to the timing of ice retreat from individual depressions.

Using the Hvítárvatn varve chronology, we are able to date this contact in each core with an estimated accuracy of ±10 years. Calendar ages of all trough cores coincide with ages of the unusually thick clay layers identified in distal cores. Together, both the trough dates and reworked clay deposits record eight surges of Suðurjökull into Hvítárvatn at AD 1828, 1838, 1855, 1866, 1885, 1905, 1917 and 1929. The two largest surges occurred at AD 1855 and 1885 when the glacier terminus advanced c. 1600 m in less than two years, amounting to advance rates of 800–1600 m a⁻¹.

Both outlet glaciers produced icebergs during their respective occupation of the lake. We interpret the many pits and grooves observed in depths below 50 m to have been produced by grounded, relatively deep-keeled icebergs calved by the two glaciers. Grooves were formed in places where icebergs intersected the lakebed and the iceberg was dragged by wind shear or, in some cases, pushed by advancing ice. Rimmed pits indicate that grounded icebergs were stable for significant periods of time, a condition most likely explained by the lake being so choked with icebergs that little wind-drift motion was possible.

Discussion

Morphological patterns revealed by the high-resolution multibeam sonar bathymetry, coupled with seismic-reflection data and a wide array of sediment cores from key localities, allow derivation of a unique explanation for the genesis of the presently subaqueous glacial landforms in Hvítárvatn. The striking patterns found around the Suðurjökull terminal complex were formed during multiple advances or surges of Suðurjökull. When the ice reached the lake floor, thinning and extensional flow allowed the ice margin to break into several ice fingers. Maintaining narrow fingers of ice <200 m wide and >1000 m long, in water depths of 50–60 m, as demonstrated by the multibeam sonar images, requires buttressing. Because the outflow channel of Hvitarvatn is less than 3 m wide and paved with large boulders, even modest icebergs would have been unable to be evacuated from the lake. As a result, we propose that, at times during the LIA, the lake became tightly packed with a mélange of icebergs produced by Suðurjökull and Norðurjökull. Once the lake was filled by icebergs, there would have been little or no surface iceberg motion. Evidence for this lake-surface state is provided in the rimmed pits on the lake floor (Fig. 2a) and the unusually long, narrow ice-eroded depressions that demand support of the advancing ice fingers of the calving margin.

Our bathymetric data suggest that ice masses terminating in closed water bodies may exhibit unusual flow behaviour. Suðurjökull advanced into a deep lacustrine basin with a thick fill of unconsolidated sediment mainly lacking bedrock ridges (Fig. 2c). Water depths must have approached those required to float the advancing ice terminus. As Suðurjökull advanced into the main basin, extensional flow led to thinning and increased ice velocity. Eventually the terminus broke up in an extensional flow regime. When the production of icebergs exceeded the combination of iceberg melt and evacuation, the basin became choked with icebergs that helped to buttress and stabilize any advancing ice margin.

The lake-floor geomorphology at the Norðurjökull terminus is strikingly different from that of Suðurjökull. Norðurjökull flows through a narrow, steep valley into the lake supported by the steep valley sides and stabilized by the morainal ridges that formed during the LIA advances, whereas Suðurjökull advanced rapidly over a deformable bed of lacustrine sediment. The lake-floor morphology beyond Norðurjökull shows a series of arcuate moraine ridges oriented transverse to ice flow that is more typical of simple LIA glacier advance and retreat (Fig. 2a).