The Late Quaternary sedimentary record in Scoresby Sund, East Greenland

Abstract

Until recently, little was known about the Quaternary marine sedimentary record in East Greenland. Geophysical and geological investigations in Scoresby Sund were undertaken to characterize the nature and chronology of this record. Seismic records show that almost 70% of the outer fjord system is covered by about 10 m of unlithified sediments, making direct correlation with the Quaternary records on land and the adjacent continental margin difficult. These acoustically unstratified sediments are scoured by icebergs above 550 m water depth. Almost 90% of core material is massive diamicton of Holocene age, deposited mainly from iceberg rafting and turbid meltwater. Sedimentation rates are 0.1 ‐0.3 m 1000 yr‐1. Thicker accumulations of unlithified Quaternary sediments in Scoresby Sund occur as sediment ridges and in two other major depocentres. A low sediment ridge runs across the mouth of Scoresby Sund, and is interpreted as an end moraine of Late Weichselian Flakkerhuk stadial age. The very restricted sediment thickness suggests that grounded ice filled the fjord during the Flakkerhuk and an ice shelf was not present. High inputs of ice rafted debris to the continental margin at about 18 000 BP indicate this as a probable age for the moraine. During the Allerød Interstadial, ice probably retreated from the outer fjord system, since massive diamictons similar to those of Holocene age are present at the base of most cores. A major depocentre of acoustically stratified sediments at the head of Hall Bredning is interpreted to represent ice proximal deposits from a glacier margin extending across the fjord. It is adjacent to dated moraines on land and is inferred to be of Milne Land stadial age (about 10 000 BP). A similar age is interpreted for acoustically laminated sediments and a moraine at the entrance of Vikingebugt, on the south side of Scoresby Sund. Dated kame terraces in the inner fjord system indicate that ice retreated to its present position 6–7000 years ago.

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

“…The debris included within icebergs is assumed to originate from sequences of debris-rich ice formed at the base of outlet glaciers that calve icebergs into the fjord system (Dowdeswell et al, 1994a,b). Such basal debris-rich ice layers (a few centimetres to a few metres in thickness) have been reported from icebergs (exposed after overturn) in Antarctica (Anderson et al, 1980), Svalbard (Dowdeswell and Dowdeswell, 1989) and Scoresby Sund (Dowdeswell et al, 1994b). Subsequently, as icebergs drift through the fjord system they undergo melting, which releases debris included within the ice (Dowdeswell et al, 1994a,b).…”

“…Certainly, radiocarbon dates show that glacial ice was absent from the outer fjord throughout the Holocene when this diamicton was deposited (Dowdeswell et al, 1994a,b). Moreover, the outer fjord is wide and relatively flat (Ó Cofaigh et al, 2001) with little evidence (on acoustic records) of sub-aqueous mass-wasting, and turbid melt-water plumes are restricted to within a few kilometres of fjord shores (Dowdeswell et al, 1994b). Therefore, diamicton is unrelated entirely to subglacial processes and is highly unlikely to be related to, or affected by, processes of mass-wasting (including turbidity currents and debris flows).…”

“…The Holocene section is therefore made up almost entirely of macroscopically massive diamicton facies, with scattered clast-rich beds/lenses embedded within it (Dowdeswell et al, 1994a,b). Below the laminated facies, undated diamicton (similar to the diamicton deposited during the Holocene) is also thought to originate from iceberg rafting and scouring, although this is still open to debate (Dowdeswell et al, 1994b). Note that the current investigation is only concerned with examining the macroscopically massive diamicton.…”

“…Large vertical/horizontal pebbles and clasts (particularly those in association with depressed/draped laminae and enhanced birefringence where they have come to settle), horizontal gravel beds/lenses and coarse sand lenses, discontinuous crudely stratified sandy beds, concentrated patches of clasts, and grain dumps -all within a fine-grained matrix -are indicative of rainout and dumping of IRD (Dowdeswell et al, 1994a;Linch et al, 2012;Linch and van der Meer, 2014). In addition, reworking by iceberg scouring will contribute to lack of stratification in diamicton (Dowdeswell et al, 1994b).…”

“…It is thought that during the Last Glacial Maximum (LGM), a glacier filled the fjords and extended onto the East Greenland shelf (Håkansson et al, 2007), eroding most or all Quaternary sediments from the outer fjord, indicating that the current thin sedimentary record comprises almost exclusively the postglacial period (Marienfield, 1991a(Marienfield, ,b, 1992aDowdeswell et al, 1994b;Ó Cofaigh et al, 2001). Ice retreat up the inner fjord is recorded by radiocarbon dates indicating that ice reached approximately its present extent by 6-7 ka BP (Funder, 1978;Marienfield, 1991b;Dowdeswell et al, 1994a,b).…”