Naja Mikkelsen

Sea-level –and gas-hydrate–controlled catastrophic sediment failures of the Amazon Fan

The architecture and Quaternary history of the massive and highly structured Amazon Fan has been reconstructed using sediment recovered by Ocean Drilling Program Leg 155. Huge regional mass-transport deposits make up a significant component of the Amazon Fan. These massive mass-transport deposits each cover an area over 15,000 km{sup 2} (approximately the size of Jamaica), reach a maximum thickness of 200 m, and consist of {approx}50,000 Gt of sediment. Analysis of both benthic foraminiferal fauna and the sediments indicates that the mass-transport deposits originated at a water depth of between 200 and 600 m on the continental slope, which is at least 200 km lateral to and 1500 m above their present position. Each mass-failure event was formed by the catastrophic failure of the continental slope and has been dated and correlated with climate-induced changes in sea level. Two different mechanisms initiated these catastrophic slumps: (1) Rapid drops in sea level destabilized continental slope gas hydrate reservoirs, causing slope failure and the glacial mass-transport deposits, and (2) deglaciation of the Andes and the consequent flushing of Amazon River sediment to the continental slope caused over-burdening and the deglacial mass-transport deposits.

Geological records of changes in wind regime over south Greenland since the Medieval Warm Period : a tentative reconstruction

Published marine sediment core records and new information from terrestrial aeolian deposits from the surroundings of Igaliku Fjord, south Greenland, have been used for a tentative reconstruction of multi-decadal to centennial scale changes in the intensity of regional atmospheric circulation since the Medieval Warm Period. The marine data show that aeolian activity over southern Greenland was generally enhanced in the Medieval Warm Period between c. AD 900 and c. AD 1300. The preliminary data from the onshore aeolian deposits suggest that wind activity was strongest after AD 1000, reaching a peak close to AD 1300, after which atmospheric circulation intensity decreased. A comparison with the marine data shows that this decrease coincides with increased advection of Polar Water by the East Greenland Current at the beginning of the Little Ice Age. The aeolian sediment record suggests foehn wind activity displaying multi-decadal oscillations in the range of known north Atlantic climate oscillations. The intensity of erosional processes in south Greenland has previously often been attributed to farming activities initiated after AD 1000 by the Norse who disappeared a few hundred years later. Our findings suggest, however, that erosion in this area is mainly related to marked variations in wind strength.

The Norse in Greenland and late Holocene sea-level change

Norse immigrants from Europe settled in southern Greenland in around AD 985 and managed to create a farming community during the Medieval Warm Period. The Norse vanished after approximately 500 years of existence in Greenland leaving no documentary evidence concerning why their culture foundered. The flooding of fertile grassland caused by late Holocene sea-level changes may be one of the factors that affected the Norse community. Holocene sea-level changes in Greenland are closely connected with the isostatic response of the Earths crust to the behaviour of the Greenlandic ice sheet. An early Holocene regressive phase in south and west Greenland was reversed during the middle Holocene, and evidence is found for transgression and drowning of early-middle Holocene coast lines. This drowning started between 8 and 7ka BP in southern Greenland and continued during the Norse era to the present. An average late Holocene sea level rise in the order of 2-3 m/1000 years may be one of the factors that negatively affected the life of the Norse Greenlanders, and combined with other both socio-economic and environmental problems, such as increasing wind and sea ice expansion at the transition to the Little Ice Age, may eventually have led to the end of the Norse culture in Greenland.

Late glacial and Holocene marine records from the Independence Fjord and Wandel Sea regions, North Greenland

The first marine sediment cores from the unexplored Independence Fjord system and theWandel Sea, North Greenland, have been investigated to reveal the glacial marine history of the region. Two key sites in the Independence Fjord system, and an earlier analysed site from the Wandel Sea continental slope, off the mouth of Independence Fjord, are presented. The Independence Fjord sites reveal an early Holocene record (10.0–8.9 Kya) of fine-grained reddish muds with calcareous microfossils, dominated by the benthic foraminifera Cassidulina neoteretis. We suggest that a semi-permanent fast ice cover characterized the region in the early Holocene, and that the deeper troughs in the mouth region of the Independence Fjord system were intruded by subsurface Atlantic water. A stiff diamicton, at least 1.3 m thick, with coal and sandstone clasts of mainly local origin, and a 0.5-m-thick Holocene cover, are found in one of the sites. The diamicton is assumed to represent a subglacial till predating the early Holocene sediments (> 10 Kya). Shallow seismic records off the mouth of Independence Fjord reveal kilometre-sized troughs with signs of glacial erosion, till deposition and a Holocene glaciomarine deposition. These features could indicate that glacial ice debouching from the Independence Fjord system at some time during the last glacial period extended to the mid-outer Wandel Sea shelf. Data from a high-resolution sediment core previously retrieved from the adjacent Wandel Sea slope indicate that the maximum ice sheet advance in this area culminated about 25–20 Kya.

Impact of Medieval Fjord Hydrography and Climate on the Western and Eastern Settlements in Norse Greenland

Abstract A comparison of the Medieval fjord hydrography and climate regime of the main Norse settlements in Greenland demonstrates important differences in the timing of sea-ice expansion and storminess when comparing the Western and Eastern Settlement regions. The Western Settlement, as well as the northern hunting grounds around Disko Bugt, had already experienced major climate deterioration in the first decades after AD 1200. This regime shift in West Greenland included an expansion of fjord and sea ice (“West Ice”) in coastal waters as well as a drastic atmospheric cooling and an increase in storminess, mainly in the summer season. In contrast, environmental conditions in the Eastern Settlement deteriorated notably later, i.e., around AD 1400. At that time, ice conditions became much more severe, whereas the previously prevailing strong wind activity decreased, which was coeval with a general decrease in aeolian activity in West Greenland, eastern Canada, and NW Iceland. Summer blockage of the fjord entrance by thick, multi-year sea ice (“Storisen”) is a specific feature of the Eastern Settlement area, whereas in the Western Settlement region, the West Ice would have threatened Norse sailing in late winter. We may thus conclude that by shortly after AD 1200 living conditions in the Western Settlement had already became less attractive due to adverse effects of the early, regional climate deterioration. Since then, the Western Settlement was probably increasingly dependent on supplies from the Eastern Settlement, where milder climate conditions continued to prevail for another century. Increased summer blockage of the Eastern Settlement fjords by the Storisen beginning around AD 1400 would have imposed serious limitations to sailing and pasture productivity in coastal areas and is suggested to have played a crucial role in the final demise of the Eastern Settlement a few decades later.

Fluid flow and methane occurrences in the Disko Bugt area offshore West Greenland: indications for gas hydrates?

The present study is the first to directly address the issue of gas hydrates offshore West Greenland, where numerous occurrences of shallow hydrocarbons have been documented in the vicinity of Disko Bugt (Bay). Furthermore, decomposing gas hydrate has been implied to explain seabed features in this climate-sensitive area. The study is based on archive data and new (2011, 2012) shallow seismic and sediment core data. Archive seismic records crossing an elongated depression (20×35 km large, 575 m deep) on the inner shelf west of Disko Bugt (Bay) show a bottom simulating reflector (BSR) within faulted Mesozoic strata, consistent with the occurrence of gas hydrates. Moreover, the more recently acquired shallow seismic data reveal gas/fluid-related features in the overlying sediments, and geochemical data point to methane migration from a deeper-lying petroleum system. By contrast, hydrocarbon signatures within faulted Mesozoic strata below the strait known as the Vaigat can be inferred on archive seismics, but no BSR was visible. New seismic data provide evidence of various gas/fluid-releasing features in the overlying sediments. Flares were detected by the echo-sounder in July 2012, and cores contained ikaite and showed gas-releasing cracks and bubbles, all pointing to ongoing methane seepage in the strait. Observed seabed mounds also sustain gas seepages. For areas where crystalline bedrock is covered only by Pleistocene–Holocene deposits, methane was found only in the Egedesminde Dyb (Trough). There was a strong increase in methane concentration with depth, but no free gas. This is likely due to the formation of gas hydrate and the limited thickness of the sediment infill. Seabed depressions off Ilulissat Isfjord (Icefjord) previously inferred to express ongoing gas release from decomposing gas hydrate show no evidence of gas seepage, and are more likely a result of neo-tectonism.

Tuppiap Qeqertaa (Tobias Island): a newly discovered island off northeast Greenland

The small island of Tuppiap Qeqertaa, formerly known as Tobias O or Tobias Island, is situated 80 km off the northeast Greenland coast. The island was discovered in 1993 and is approximately 2 km long and 1.5 km wide. Most of the island is covered by an ice cap that rises to 35 m above sea level, as determined by airborne laser scanning. High- quality geodetic GPS measurements determine the position of a small cairn erected on the island to 79 ◦ 20 � 34.48 �� N, 15 ◦ 46 � 31.23 �� W. Minor parts of the island are ice free, with small hills of gravel. Late Holocene shells of marine bivalves have been found and were presumably brought up by ice floe push. Shell fragments of southern extralimital marine invertebrates point to the presence of pre-Holocene deposits. Three species of moss are the only land plants that have been found on the island.

Timing of the late Quaternary Amazon Fan Complex masstransport deposits

Abstract The Amazon Fan at the mouth of the Amazon River is a large, highly structured muddy deep-sea fan. Sediment recovered by Ocean Drilling Program Leg 155 has allowed us to reconstruct the structure of the late Quaternary Amazon Fan Complex. A significant sedimentary component of the complex are the mass-transport deposits (MTDs) formed by catastrophic failure of the continental slope. There are two sets of MTDs: the near-surface MTDs and the deep MTDs. The near-surface MTDs are divided into the Eastern and Western Debris Flows which are capped by Holocene sediments. The deep debris flows are divided into the Unit R MTD in the western Amazon Fan complex, and the Deep Eastern MTD in the eastern Amazon Fan Complex. The MTDs each cover an area of up to 25 000 km2, reach a maximum thickness of 200 m, and consist of approximately 50 000 gigatonnes of sediment. A characteristic of the MTDs is the inclusion of reworked glacial and interglacial material from both the continental slope and the surrounding fan. The relatively thin interglacial deposits below the MTDs are from the nanofossil zones CN14b and CN15a and thus younger than 460 ka. Bio-, seismic- and magnetostratigraphy and sedimentation rate constraints have been used to date the top of both the near-surface and the deep MTDs. It seems that the near-surface Western and Eastern Debris Flows were last active during Termination I. The Deep Eastern MTD was last active at about 33 ka, while the Unit R MTD was last active at about 45 ka. It is inferred that the interglacial deposits beneath these MTDs were formed during Oxygen Isotope Stages 7, 9 and 11. We speculate that the MTDs were triggered by climatically induced changes in sea level, and that the interglacial deposits may have acted as slip planes for the MTDs.

Challenges in selecting sites for Arctic Ocean drilling

Overcoming Barriers to Arctic Ocean Drilling: The Site Survey Challenge; Copenhagen, Denmark, 1–3 November 2011 The climate of the high Arctic appears to be changing faster than any other region on Earth. To place contemporary change in context, it is necessary to use scientific ocean drilling to sample the climate history stored in the sediments of the Arctic Ocean. The focus of the November 2011 workshop was to define site survey investigations for drilling campaigns based on existing proposals and preproposals; to identify themes and areas for developing new and innovative science proposals; and to discuss opportunities, technical needs, and limitations for drilling in the Arctic Ocean.