Geological Survey of Denmark and Greenland Bulletin 20, 2010, 83–86 83 The Greenland ice sheet is one of the most significant con- tributors to the rising global sea level with a contribution of 0.5 mm per year (Rignot & Kanagaratnam 2006). Evidence is emerging that rising temperatures of subsurface ocean cur- rents play a vital role in the recent acceleration of large fast f lowing glaciers such as Jakobshavn Isbræ in West Greenland (Holland et al. 2008) and Helheimgletscher in South-East Greenland (Straneo et al. 2010). Important questions are whether these incursions of warmer water are part of a re- current phenomenon and indeed exactly how they inf luence the glaciers. The Geocenter Denmark project SEDIMICE (Linking sediments with ice-sheet response and glacier retreat in Greenland) investigates past ice f luctuations in the Hel- heimgletscher region in South-East Greenland with regard to magnitude, possible causes and effects. One of the main tasks in this project is to analyse sedimentary deposits in the main fjord Sermilik (Fig. 1), which is inf luenced by the tidally af- fected Helheimgletscher that has a short f loating tongue. By combining sediment studies with modern climate studies we aim to extrapolate meteorological data back in time. In August 2009 the Geological Survey of Denmark and Greenland collected short sediment cores in Sermilik near Tasiilaq (Fig. 1). To select core sites and to understand the sedimentary processes, we also acquired data on the bathym- etry and conducted shallow seismic profiling. This paper presents some results of the seismic survey, preliminary sedi- ment core data and bathymetrical data from the fjord. Apart from a few isolated depth values, the bathymetry of Sermilik was unknown before the 2009 survey. Setting Sermilik is about 80 km long and 7–13 km wide. The ter- rain around the fjord is alpine with elevations of 300–600 m near the coast and up to 1000 m inland. Frequent glacial and geologically controlled fissure valleys dissect the area in a criss-cross pattern. The northern end of Sermilik branches into three fjords with calving glaciers. The westernmost – Helheimgletscher – is a fast f lowing glacier and the third most prolific iceberg producer in Greenland (Rignot & Kan- agaratnam 2006). The climate of the region is low arctic and the weather con- ditions are inf luenced by lows moving north along the coast. The fjord is covered by sea ice from December to May. The hydrographic conditions in the fjord are inf luenced by a 10– 20 m thick layer of glacial water, underlain by 100–150 m of Bathymetry, shallow seismic profiling and sediment coring in Sermilik near Helheimgletscher, South-East Greenland Camilla Snowman Andresen, Niels Nørgaard-Pedersen, Jørn Bo Jensen and Birger Larsen Fig. 1. Study area and bathymetrical data from Sermilik based on data col- lected in 2009. Depth data on the shelf south of the fjord mouth are from Clausen (1998) and this survey. The positions of the seismic lines A, B and C are indicated by black lines and sediment core sample sites are shown. 37°0´W Toqqulertivit Imiat Shelf trough Tasiilaq Sill ER07 C B A ER15 ER13 ER11 10 km Bank 700 m 200 m Helheimgletscher 38°W 66°N Se rm ili k Green- land 3–100 101–200 201–300 301–400 401–500 501–600 601–700 701–800 801–900 901–1000 1001–1100 Sample site Bathymetry Depth (m) Depotø © GEUS, 2010. Geological Survey of Denmark and Greenland Bulletin 20, 83–86. Open Access: www.geus.dk/publications/bull 8484 polar water. Below the polar water towards the bottom warm- er Atlantic water of subtropical origin with temperatures of 3.5–4°C is found (Straneo et al. 2010). The inf low of warmer waters into Sermilik takes place via deep troughs in the shelf. Glacial history of the region A study from the Toqqulertivit Imiat valley (Fig. 1) shows that a glacier f lowed through this valley and most likely coalesced with a glacier f lowing south in Sermilik and out over the continental shelf during the Last Glacial Maximum (Roberts et al. 2008). Exposure ages of 11.8–9.9 ka (kilo-an- num, 103 years BP) from bedrock surfaces at high elevations (683–740 m a.s.l.) provide minimum ages for the last degla- ciation (Roberts et al. 2008). These ages from Toqqulertivit Imiat support the ‘maximum’ model of a large Last Glacial Maximum ice sheet extending to the shelf break in South- East Greenland (Stein et al. 1996; Kuijpers et al. 2003). Evidence from the shelf south of Sermilik indicates that the ice margin retreated to the inner shelf around 15.7–14.6 cali- brated (cal.) ka (Kuijpers et al. 2003). In the Kangerlussuaq region farther north the ice-sheet margin retreated from the outer shelf around 15.5 cal. ka and reached the present outer coast around 13.6–10 cal. ka ( Jennings et al. 2006). This is in accordance with surface exposure ages from lower Toqqul- ertivit Imiat indicating that ice retreated to the mouth of Ser- milik between 11.1 and 9.7 ka (Roberts et al. 2008). These data are further supported by a minimum age of 11 cal. ka for the formation of the local marine limit (at 69 m) and thereby local ice retreat near Tasiilaq (Long et al. 2008). Methods We used the locally hired motor boats Erik den Røde and Pu- ite for the work. An Innomar SES-2000 Medium sub-bottom echo sounder from Innomar Tecnologies, Rostock, Germany, was used for bathymetrical and sub-bottom sediment profil- ing. This parametric device is designed for water depths down to about 2000 m and has the ability to resolve sediment lay- ers a few decimetres thick and penetrate down to about 50 m below the sea f loor. The transducer was mounted on a vertical steel tube attached to the side of the boat and a motion sen- sor was used to compensate for movements of the boat. Ad- ditional bathymetrical data were recorded in the inner fjord from the echo sounder of Erik den Røde during the sediment coring cruise. Comparisons of depth recordings obtained by the two methods showed that the results are compatible to within a few metres. Sediment coring was performed with a Rumohr lot corer with up to 1.5 m long core liners. Bathymetrical data from Sermilik The southern part of Sermilik Fjord is an up to 920 m deep and f lat basin (Fig. 1). The bathymetry of the fjord mouth can be described as terminating into a SE-directed trough and a SW-directed trough separated by a broad bank with water depths of less than 200 m. The bathymetry of the SW- directed trough is very uncertain. The shallowest part of the SE-directed trough forms a c. 550 m deep sill between the deep fjord and the 800–900 m deep trough that stretches the entire shelf towards the Irmiger Sea. The deep fjord basin extends up to 40 km northward from the fjord entrance into the middle part of Sermilik where several bathymetrical highs (400–550 m) narrow the connection to the northern part of the fjord. Towards the northern part of Sermilik, the basin f loor rises steadily to a depth of about 600–650 m just south of Depotø. The fjord bottom in the inner part is more irregular with channel sys- tems more than 100 m wide and up to 20 m deep. During the survey we could not measure water depths in the inner east– west-trending fjord arm leading up to Helheimgletscher due to semi-permanent sea ice extending tens of kilometres ER15 ER13 ER11ER07 5 cm Fig. 2. Selected examples of X-ray radiographs from core ER15 (600 m water depth), ER13 (660 m water depth), ER07 (525 m water depth) and ER11 (600 m water depth) that document different sedimentation regimes. Note the dark sand layer in core ER13 with a lower erosive boundary; this unit is interpreted to be a turbidite. Core sites are shown in Fig.1. 85 from the Helheimgletscher calving front. However, accord- ing to the skipper of Erik den Røde (Sigurdur Petursson), water depths up to 800 m are found north-west of Depotø. Sediment cores Altogether 19 cores with lengths ranging from 30 to 150 cm were retrieved during the sediment coring cruise. The full sediment core data (sedimentology, geochemistry and chro- nology) will be presented elsewhere. However, preliminary inspection of cores ER07, ER11, ER13 and ER15 documents the variable sediment regimes that characterise the fjord (Figs 1, 2), and X-ray radiography of the cores show diamic- ton facies, laminated mud facies and sand layers with erosive lower boundaries. These lithofacies are similar to lithofacies described from sediment cores from Kangerlussuaq (Smith & Andrews 2000) and Scoresby Sund (Dowdeswell et al. 1994; Ó Cofaigh & Dowdeswell 2001). For example, core ER15 consists of laminated mud with variable content of pebbles, which is interpreted as ice-proximal glaciomarine sediments mainly deposited by suspension settling from turbid over- f low plumes and turbidity currents and occasional iceberg rafting. In contrast, cores ER07 and ER11 are characterised by massive diamicton facies with abundant pebbles, which is interpreted as the result of iceberg rafting. Core ER13 has a unit of diamicton facies above a unit of laminated mud facies. This may ref lect an environmental change from a long-last- ing sea-ice cover in the fjord prohibiting iceberg passage to a period with increased passage and melting of icebergs. As also suggested by Jennings & Weiner (1996) variable inf low of Atlantic water may inf luence the melting and traversing of icebergs. 210Pb dating of the upper decimetres of the cores show sed- imentation rates >1 cm/yr in ER15, 0.4 cm/yr in ER13 and 0.2 cm/yr in ER11. The decreasing sedimentation rates with increasing distance to the present front of the calving gla- ciers ref lect the decreasing inf luence from meltwater plume sedimentation. Seismic profiles The seismic profile (Fig. 3A) shows an outer f lat, deep ba- sin in Sermilik with an upper 4–6 m thick, transparent seismic unit overlying a well-stratified section (>15 m thick) that is characterised by strong, continuous, parallel ref lec- tors. There is no distinct boundary between the two seismic units, as weak, discontinuous, parallel ref lectors characterise the uppermost c. 2 m of the stratified section. On vertically extremely exaggerated sections, wide lenticular units and stratigraphical onlap structures are visible in some parts of the lower seismic unit. To the north, the transparent upper unit disappears and well-stratified sediments, with some channel features, dominate the seismic profiles (Fig. 3B). The profiles of the bathymetrical highs in the middle part of the fjord are dominated by overlapping diffraction hyper- bolae. The inner, shallower part of the fjord is characterised by large channel and levee structures, and broad f lank units showing well-stratified sediments in the north-western part of the seismic survey area (Fig. 3C). Formation of fjord-bottom sediment structures The sediment unit with a transparent pattern in the outer fjord basin indicates a uniform lithology that possibly origi- NWNW SESE NWNW SESE NW SE NW SE NN SSN S C B A 1 km10 m 1 km5 m 1 km 5 m Fig. 3. Representative seismic sections that illustrate different structures in different parts of Sermilik (see text for description and Fig. 1 for location). 8686 nates from suspension sedimentation (meltwater plumes) and ice-rafting during the main part of the Holocene (Fig. 3A). The lower stratified section is interpreted as glacioma- rine sediments consisting of turbidites and mass-transport deposits interbedded with suspension-deposited sediments. The lower unit was possibly deposited during the final degla- ciation of the main part of Sermilik at about 10 ka (cf. Rob- erts et al. 2008). Turbidite sedimentation typically creates very f lat fjord basins with ref lectors onlapping basin margins and structural highs. It is possible that the structural highs in the middle part of Sermilik could serve as anchor points for the retreating glacier, causing a stagnation of the fjord glacier front during the last deglaciation. The inner basin with its apparent active channel and levee sedimentation and over-all fill geometry (Fig. 3C) can be characterised as a progradational–aggradational wedge of sediments with possibly very high sedimentation rates from turbidites and mass f lows, as well as plume sedimentation. It is an open question whether the channel systems are directly fed by the Helheimgletscher source, or whether bedrock thresholds in the innermost fjord system prohibit bed trans- port of glaciogenic sediments. If a deeper sub-basin exists north of Depotø, we can only explain the seismic signature and large channel-levee systems of the inner basin by a very advanced position of Helheimgletscher to near Depotø dur- ing the late Holocene. Hopefully, future exposure ages from the land terrain near Depotø by our collaborators can show if the front of Helheimgletscher had a standstill near Depotø during the Little Ice Age. In conclusion, the seismic survey has revealed a rather complex pattern of sub-bottom sediment structures (down to 50 m) in Sermilik ref lecting the early Holocene retreat of Helheimgletscher, probably followed by a Holocene ice advance – perhaps during the Little Ice Age. Preliminary results from analyses of the sediment cores support the in- terpretation that the glaciomarine sediments in Sermilik is related to settling from meltwater plumes and iceberg raft- ing. Knowledge of the sediment depositional regime on the fjord bottom from seismic investigations is highly relevant for retrieval of sediment cores suitable for Holocene palaeo- climatic reconstructions. We hope to collect more and longer sediment cores in the fjord in coming years. Acknowledgements Geocenter Denmark is thanked for financial support. We would also like to thank our two skippers Bendt Josvassen of M/V Puite and Sigurdur Petursson of M/V Erik den Røde. Rineke Gieles at the Royal Netherlands Institute for Sea Research is thanked for X-ray radiography of sediment cores. References Clausen, L. 1998: The Southeast Greenland glaciated margin: 3D stratal architecture of shelf and deep sea. Geological Society Special Publica- tions (London) 129, 173–203. Dowdeswell, J.A., Whittington, R.J. & Marienfeld, P. 1994: The origin of massive diamicton facies by iceberg rafting and scouring, Scoresby Sund, East Greenland. Sedimentolog y 41, 21–35. Holland, D.M., Thomas, R.H., de Young, B., Ribergaard, M. H. & Lyberth, B. 2008: Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters. Nature Geoscience 1, 659–664. 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