The sizes, frequencies, and freeboards of East Greenland icebergs observed using ship radar and sextant

Abstract

The Scoresby Sund fjord system, East Greenland, contains the most productive fast‐flowing outlet glaciers draining east from the Greenland Ice Sheet, calving 18 km3 a−1 of icebergs. The sizes, frequencies, and freeboards of 1900 icebergs were measured from F.S. Polarstern, using ship X‐band radar and sextant. Radar beam spreading exaggerates iceberg width by 60 m per nautical mile of range beyond the first mile. Data sets on iceberg size (e.g., that collated for Antarctic icebergs) collected using ship radars which do not take this effect into account will overestimate iceberg dimensions significantly. The location and concentration of icebergs within the fjord complex can be explained by (1) the locations of the principal source glaciers and (2) fjord topography and bathymetry. Iceberg concentration (maximum 0.6 icebergs km−2) declines with distance from the major iceberg sources. We found that 69% of icebergs within the fjord system are <200 m in width. Only five are >1 km in length. The largest is 2.7 km long. Icebergs become spread more evenly over the range of size classes in the outer fjord and shelf. Modal iceberg keel depth, calculated from freeboard measurements, is 4–500 m in the inner fjords, shifting to lower values in the outer fjords, reflecting shallower bathymetry. Radar measurements of iceberg width cannot be used to infer keel depths accurately, because width and keel depth are only weakly correlated. Comparison between freeboards and keel depths for icebergs from East Greenland and the Barents Sea indicates that the iceberg source (i.e., floating or grounded) exerts a fundamental control on iceberg dimensions. The drift pattern of icebergs is from the head to the mouth of the fjord system, although fjord bifurcations, bathymetry, and currents provide additional complications. The trends in observed iceberg size and frequency, and in inferred keel depth, in the Scoresby Sund region are likely to be applicable to other fjords and shelves around Greenland.

Micromorphology of diamicton affected by iceberg-keel scouring, Scoresby Sund, East Greenland

“…Parasound and 3.5 kHz acoustic measurements recorded seabed irregularities up to 10 m in height (from berm crest to the base of the trough) and widths of a few metres to 20 m or more (Dowdeswell et al, 1993). These irregularities are inferred to be scours because i) they are similar in morphology to those seen on 3.5 kHz records from the Labrador Shelf and Barents Sea (Josenhans et al, 1986;Solheim et al, 1988); ii) Dowdeswell et al (1992Dowdeswell et al ( , 1993 observed a large number of ice-bergs with keels deeper than the measured water depths; and iii) the dimensions of these seabed irregularities are consistent with previous scour descriptions in terms of size and berm/trough morphology. Given sedimentation rates in Scoresby Sund over the last 10 ka are on the order of 0.2-0.3 mm yr -1 (Marienfield, 1992a), it is likely that, as well as modern scours, relict scours dating back to the Late Wiechselian are probably also present on the modern sea floor, and even older scours (Mid and Early Weichselian) are likely to exist throughout the sedimentary sequence where it has not been eroded by later glaciations (Dowdeswell et al, 1993).…”

“…More recent estimates from the period 2000-2010 measured an ice flux at the grounding-line of Daugaard-Jensen Gletscher of 25 km 3 yr -1 , although a daily submarine melt rate of about 2.5 m per day means that calving may account for only 4 km 3 yr -1 (Enderlin and Howat, 2013). These two outlet glaciers produce tabular icebergs between 100 m and 2.5 km in width, and up to 600 m in thickness (Dowdeswell et al, 1992); by contrast smaller tidewater glaciers tend to produce smaller icebergs of irregular shape (Dowdeswell et al, 1994a,b). The net drift of icebergs is to-wards the mouth of the fjord system where icebergs leave the outer fjord on its southern side (Dowdeswell et al, 1993).…”

“…During September 1990 radar and sextant observations made along two transects across Hall Bredning and Scoresby Sund by F. S Polarstern showed that a number of icebergs in the outer fjord were grounded in water depths of <300 m on the shallower northeastern side of the outer fjord and up to 520 m on the south side (Dowdeswell et al, 1992(Dowdeswell et al, , 1993. Parasound and 3.5 kHz acoustic measurements recorded seabed irregularities up to 10 m in height (from berm crest to the base of the trough) and widths of a few metres to 20 m or more (Dowdeswell et al, 1993).…”

“…Today, glacier and ice-sheet fed ice cliffs occupy 85 km of the fjord coastline of the Scoresby Sund system (Dowdeswell et al, 1992). Fast-flowing outlet glaciers entering the fjords are among the largest producers of icebergs on the eastern margin of the Greenland Ice Sheet (Koch, 1945;Reeh, 1985); accounting for approximately 18 km 3 of mass loss into the fjord system annually (Olsen and Reeh, 1969).…”

“…Marienfield, 1991aMarienfield, ,b, 1992aStein et al, 1993;Dowdeswell et al, 1992Dowdeswell et al, , 1993Dowdeswell et al, , 1994aÓ Cofaigh et al, 2001). In summary, cores retrieved from Scoresby Sund comprise: i) ~90% macroscopically massive diamicton (Dm), formed by the release of IRD and reworking by iceberg scouring; ii) ~4% lenses or beds of gravel or coarse sand with sharp contacts (within diamicton), indicating iceberg dumping of debris; iii) ~4% laminated sediments (Fl) of alternating fine sand to coarse silt and fine silt to clay with some large clasts (gravels to cobbles); and iv) ~3% crudely layered, less wellsorted sediments (Fsd) with a significant proportion of larger clasts (Dowdeswell et al, 1994a,b).…”