Jørn Bo Jensen

Late Quaternary sedimentary processes and ocean circulation changes at the Southeast Greenland margin

A study has been made of late Quaternary depositional processes and bottom current activity on the Southeast Greenland margin, using seismic, sub-bottom profiling and deep-tow side-scan sonar data as well as sediment core information. The seabed data demonstrate the occurrence of strong, southerly bottom currents prevailing on the slope and rise. Well-defined longitudinal bedforms indicate maximum mean near-bottom current velocities of up to at least 1.0 m/s at the depth stratum of Labrador Sea water (800–1500 m). Similarly strong currents occur in Denmark Strait overflow water (DSOW) at the base of the slope, whereas more basinward the maximum DSOW flow speed is lower. Iceberg plow marks were found down to about 700 m water depth. Both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition, which is concluded to be a typically glacial feature. After generally more sluggish deep-water circulation during the last glacial maximum, DSOW basin ventilation was re-established shortly before 13.3 ka. On the shelf, in front of the retreating Greenland ice margin, permanent or semi-permanent sea ice conditions prevailed until about 12.5 ka. At that time increased Irminger Current activity had resulted in warming, and East Greenland Current (EGC)-controlled iceberg drift increased. No evidence was found for a return to extreme glacial conditions or a ceasing of DSOW flow during the Younger Dryas. Abundant coarse IRD collected at greater water depth from shallow sub-seabed strata has a provenance mainly in the northern part of East Greenland (68–73°N), which demonstrates the existence of a pre-Holocene EGC system initially extending to beyond the shelf edge.

Late Quaternary records of Najas spp. (Najadaceae) from the southwestern Baltic region.

Seeds of the submerged vascular plants Najas marina, Najas minor and Najas flexilis are reported from submarine Holocene deposits from the southwestern part of the Baltic Sea, and we also report on a find of Najas minor from an Eemian deposit in Jutland, which is the first record of this species from the Eemian of Denmark. The common and widespread occurrence of especially the southern extralimital N. minor is indicative of higher than present summer temperatures during the period from 10300 to 8000cal.yearsBP.

Fauna and flora in submarine early Holocene lake-marl deposits from the southwestern Baltic Sea

Studies of plant and animal macrofossils have been carried out on early-Holocene lake marls from the southwestern Baltic. The records have been dated to the interval from c. 8500 to 7800 14C years BP by correlating pollen assemblages to an onshore radiocarbon dated pollen diagram. The sediments contain from 5 to 81% carbonate, and only few macroscopic remains of wetland and terrestrial taxa. The plant and animal (mainly mollusc) macrofossils reflect deposition at the outer edge of the belt with submerged macro-limnophytes, in the lower littoral, perhaps at water depths at 4–7 m. The ratio between the shells and opercula of the snail Bithynia tentaculata points to some post-mortem disturbance, but it can be concluded that the water was highly alkaline and mesotrophic.

Remote quantification of methane fluxes in gassy marine sediments through seismic survey

A novel methodology for predicting upward diffusive fluxes of dissolved methane in gassy marine sediments is presented. The predicted fluxes are derived from a set of theoretical simulation data gener ated using a diagenetic reaction-transport model. The model calculates the upward methane flux for a given free gas depth (FGD) below the seafloor and a given in situ gas solubility, which together define the methane concentration gradient. Fluxes can thus be extracted from a nomogram of FGD and solubility parameter space. Because, in general, microorganisms anaerobically oxidize all dissolved methane before it can escape the sediment, the estimated fluxes are equivalent to the amount of methane trapped by this subsurface microbial barrier. A test of the approach using measured methane fluxes from Aarhus Bay, Denmark, reveals a statistically significant correlation between the observed and predicted fluxes. The predicted fluxes further show a low sensitivity toward enhanced sediment mixing by faunal activity, as well as the deposition flux and reactivity of organic matter. Therefore, only a limited amount of data at strategic coring sites is required to constrain the major physical and geochemical forcings for a particular study area in order to extrapolate fluxes at a regional scale. Because the FGD can be mapped over large areas of the seafloor from shipboard seismic survey, the new approach represents a means to estimate regional methane flux budgets for gassy sediments in a cost-efficient manner.

A Baltic Ice Lake transgression in the southwestern Baltic: Evidence from Fakse Bugt, Denmark

Abstract Marine geological investigations including shallow seismic surveys and corings have been carried out in the shallow waters of Fakse Bugt (Bay) and Hjelm Bugt near to the island of Mon, Denmark. A combined sequence- and lithostratigraphy has been established and supplemented with biostratigraphic data and 14C datings. The data show that a sequence of varved glaciolacustrine sediments mapped east of Mon and in Hjelm Bugt, can be followed into the Arkona Basin. Correlation of these glaciolacustrine deposits with Baltic Ice Lake deposits in Blekinge, southern Sweden indicates the existence of a major lake basin: the initial Baltic Ice Lake. In Sweden the initial phase of the Baltic Ice Lake is dated to about 12,500 14C years BP. In the Mon area transgression of the Baltic Ice Lake is indicated in the central part of Fakse Bugt by the development of a transgressive freshwater barrier beach ridge-lagoon system following the initial onlapping sedimentation of varved deposits. The freshwater coastal system was initiated during the Allerod stage and the final maximum transgression level (13 m below present sea level) was reached in the upper part of Younger Dryas just prior to the final drainage of the Baltic Ice Lake.

Paraglacial barrier—lagoon development in the late pleistocene Baltic Ice Lake, southwestern Baltic

Abstract Marine geological investigations including shallow seismic, sedimentological and stratigraphical studies were carried out in Fakse Bay, southwestern Baltic. Seismic sequence analytical methods were used, supplemented by detailed lithological descriptions, pollen analysis and 14C dating of core material. Detailed correlations between sediment facies units and seismic sequences resulted in the establishment of four depositional sequences: I. Glacial related deposits, II. lagoon/pond and freshwater coastal deposits, III. Lake-bog-lake deposits, and IV. Marine, (sub)littoral deposits. Most attention has been paid to Sequences II and III, showing the development of a transgressive freshwater barrier beach ridge and a following regression. Pollen spectra indicate Allerod and Younger Dryas Chronozone ages (11,500-10,000 14C yrs B.P.) for the development of a sheltered lagoon/pond basin behind the transgressive freshwater barrier beach ridge. The freshwater coastal deposits are interpreted as the southwesternmost marginal deposits of the Baltic Ice Lake and represent the transgression maximum. This transgression maximum reached a level of about 13 m below the present sea level. The final drainage of the Baltic Ice Lake resulted in the development of a sheltered basin behind the former beach ridge. A fluctuating groundwater level here favoured the deposition of alternating lake gyttja and bog peat deposits in the central part of the basin. Pollen analysis and 14C dating reveal that the lake deposits correspond to high groundwater levels associated with the Ancylus Lake (9000 yrs B.P.) and the Littorina Sea (7000 yrs B.P.) transgressions, while the bog sediments represent the low water stands in the early Holocene.

Estimating the free gas content in Baltic Sea sediments using compressional wave velocity from marine seismic data

A 2-D high-resolution velocity field was obtained from marine seismic data to quantify free gas content in shallow muddy sediments at in situ pressure and temperature. The velocities were acquired applying Migration Velocity Analysis on prestack time-migrated data. Compressional wave velocities are highly sensitive to free gas as very small amounts of gas can cause a significant decrease in the medium velocity. The analyzed profile crosses a depression filled with organic-rich Holocene mud in the Bornholm Basin, Baltic Sea. The interval velocity field reveals two low-velocity patches, which extend from the reversed polarity reflections marking the top of the gassy sediment layer down to the base of the Holocene mud. Average interval velocities within the gassy mud are lower than the seafloor migration velocity by up to ∼500 m/s. This decrease, using a geoacoustic model, is caused by an average 0.046% gas volume fraction. The interval velocities in individual cells of the velocity field are reduced to ∼200 m/s predicting up to 3.4% gas content. The velocity field is limited in resolution due to velocity determination at and between reflections; however, together with the stratigraphic interpretation, geological units containing free gas could be identified. Shallow gas occurs vertically throughout most of the Holocene mud in the gassy area. Comparison with biogeochemical studies at other Baltic Sea sites suggests that the distribution of free gas is likely to be patchy in the sediment, but the gas concentration may peak below the sulfate-methane transition zone and gradually decrease below.

Evidence for a Grounded Ice Sheet in the central North Sea during the early Middle Pleistocene Donian Glaciation

Interpretation of 3D seismic data from the central North Sea yields evidence of a pre-MIS (Marine Isotope Stage) 12 grounded glaciation. The glaciotectonic complex shows buried push moraines resulting from the thrusting of multiple ice advance phases with horizontal shortening of 35 – 50%. The earliest feature observed within the complex, a hill–hole pair, represents the initial glaciation of the area. This is overlain and deformed by multiple thrust units with numerous inferred ice-flow directions. The thrust deformation observed shares characteristics with kinematic processes, push moraines and static gravity processes, seen as gravity spreading and contraction. The glaciotectonic complex in its entirety is interpreted to correlate to a pre-Elsterian glaciation, becaue of its stratigraphic position below central North Sea tunnel valleys, estimated to be Elsterian in age (MIS 12; 450 ka). The study proposes that the thrust complex correlates to the Donian glaciation in Russia (MIS 16; 600 ka) with ice sourced from Norway. The complex therefore represents a glaciation where a significant area of the central North Sea was covered by an ice sheet, 200 kyr prior to the Elsterian. This study highlights the fragmentary record of pre-Elsterian glaciations and the importance of incorporating offshore sedimentary archives and regional frameworks when reconstructing Pleistocene climate change.

Early Holocene estuary development of the Hesselø Bay area, southern Kattegat, Denmark and its implication for Ancylus Lake drainage

High-resolution shallow seismic data, sediment core information, radiocarbon dating and sequence stratigraphy have been used to interpret the late glacial to early Holocene geological evolution of Hesselø Bay in the southern Kattegat, Denmark. A reconstruction of the early Holocene coastal environment and a description of coastal processes associated with a river outlet into the bay are presented. Weichselian glacial deposits form the lowermost interpreted unit, covered by late glacial (LG) and postglacial (PG, Holocene) sediments. A funnel-shaped estuary existed at the mouth of channels in the period 10.3–9.2 cal. ka BP; the channels drained water from south to north. The early PG is characterised by estuarine and coastal deposits. The early Holocene bars that developed in the estuary are preserved as morphological features on the present-day seabed, possibly as a result of rapid relative sea-level rise. The estuary existed simultaneously with the occurrence and drainage of the Ancylus Lake. The drainage of this lake occurred through the Dana River (palaeo-Great Belt channel) into the southern Kattegat and then into the study area. The level of the Ancylus Lake in the Baltic Sea region dropped significantly at about 10.2 cal. ka BP at the same time as the estuary developed in the Kattegat region. One outcome of the present study is an enhanced understanding of the Ancylus Lake drainage path. No evidence of major erosion is seen, which indicates non-catastrophic continuous water flow from the south without major drainage events of the Ancylus Lake to the southern Kattegat. During the Littorina transgression, coastal estuarine conditions characterized the Hesselø Bay area where elongated ridges formed a bar system. As the Littorina transgression continued, back-stepping of the bar system and coastline occurred. When the transgression breached the Great Belt threshold, flooding caused major erosion throughout the study area.