H. Christian Hass

Northern Europe climate variations during late Holocene: evidence from marine Skagerrak

Abstract Granulometric and stable oxygen isotope analyses of four sediment cores from two high accumulation areas in the Skagerrak (NE North Sea) were carried out in order to reconstruct climate fluctuations and to evaluate climate impact during the upper Holocene. Extremely high sedimentation rates, especially in the eastern Skagerrak, are explained by increased current activity which is responsible for the transport and deposition of high quantities of suspension load during periods of stormy zonal atmospheric circulation patterns. These were most frequent during colder periods, while warmer phases are characterized by calmer meridional to zonal atmospheric circulation patterns. While the Subatlantic climate deterioration and the Subboreal climate optimum left only indistinct traces in the sediments, the Roman climate optimum and a colder period between ca. 400 and 700 AD are well documented. The following Medieval Warm period is characterized by a clear temperature increase of the waterbody in connection with less frequent advances of Atlantic water masses into the Skagerak deep and a decrease in bottom current strength. A mode of sedimentation prevails, similar to that of recent summer conditions, suggesting short and mild winters during that period. The onset of the Little Ice Age (around 1350 AD), however, shows an intensified bottom current circulation most probably due to amplifying westerly winds and a decrease in water temperatures in connection with more frequent advances of higher saline Atlantic waters. The Little Ice Age can be divided into 3 phases: a stormy “zonal” onset, a calm “meridional” maximum and a stormy “zonal” end. The stormy phases are characterized by a sedimentation mode similar to that of recent winter conditions while the Little Ice Age Maximum shows conditions comparable to exceptional cold modern winters. From 1900 AD, at the onset of the Modern Climate Optimum, the winter type sedimentation decreases and conditions change again to a level comparable to the Medieval Warm Period.

A geochemical record of late Holocene palaeoenvironmental changes at King George Island (maritime Antarctica)

Abstract During RV Polarstern cruise ANT-XXIII/4 in 2006, a gravity core (PS 69/335-2) and a giant box core (PS 69/335-1) were retrieved from Maxwell Bay off King George Island (KGI). Comprehensive geochemical (bulk parameters, quantitative XRF, Inductively Coupled Plasma Mass Spectrometry) and radiometric dating analyses (14C, 210Pb) were performed on both cores. A comparison with geochemical data from local bedrock demonstrates a mostly detrital origin for the sediments, but also points to an overprint from changing bioproductivity in the overlying water column in addition to early diagenetic processes. Furthermore, ten tephra layers that were most probably derived from volcanic activity on Deception Island were identified. Variations in the vertical distribution of selected elements in Maxwell Bay sediments further indicate a shift in source rock provenance as a result of changing glacier extents during the past c. 1750 years that may be linked to the Little Ice Age and the Medieval Warm Period. Whereas no evidence for a significant increase in chemical weathering rates was found, 210Pb data revealed that mass accumulation rates in Maxwell Bay have almost tripled since the 1940s (0.66 g cm-2 yr-1 in ad 2006), which is probably linked to rapid glacier retreat in this region due to recent warming.

Distribution and characteristics of marine habitats in a subpolar bay based on hydroacoustics and bed shear stress estimates - Potter Cove, King George Island, Antarctica

Marine habitats worldwide are increasingly pressurized by climate change, especially along the Antarctic Peninsula. Well-studied areas in front of rapidly retreating tidewater glaciers like Potter Cove are representative for similar coastal environments and, therefore, shed light on habitat formation and development on not only a local but also regional scale. The objective of this study was to provide insights into habitat distribution in Potter Cove, King George Island, Antarctica, and to evaluate the associated environmental processes. Furthermore, an assessment concerning the future development of the habitats is provided. To describe the seafloor habitats in Potter Cove, an acoustic seabed discrimination system (RoxAnn) was used in combination with underwater video images and sediment samples. Due to the absence of wave and current measurements in the study area, bed shear stress estimates served to delineate zones prone to sediment erosion. On the basis of the investigations, two habitat classes were identified in Potter Cove, namely soft-sediment and stone habitats that, besides influences from sediment supply and coastal morphology, are controlled by sediment erosion. A future expansion of the stone habitat is predicted if recent environmental change trends continue. Possible implications for the Potter Cove environment, and other coastal ecosystems under similar pressure, include changes in biomass and species composition.

Long-term changes of intertidal and subtidal sediment compositions in a tidal basin in the northern Wadden Sea (SE North Sea)

Wadden Sea tidal flats are highly dynamic regarding the spatial distribution and the grain size composition of their sediments. From 2003 to 2006 surface sediments have been surveyed in an intertidal and a subtidal area within the tidal inlet Königshafen (south-eastern North Sea, northern Wadden Sea, island of Sylt) with the goal to gain information on short-term development trends in the grain size composition. The average grain size (Mean) becomes finer in the sheltered part of the intertidal survey area whereas a coarsening tendency can be observed in the more exposed part of the intertidal and especially in the subtidal survey area. The trend of the most frequent grain size (first Mode) shows the same spatial distribution pattern but is far less distinct. Thus, the changing Mean must be related to an increase in the deposition of fines in the sheltered part of the intertidal Königshafen as well as a general removal of fine-grained material in the exposed intertidal and subtidal Königshafen. In order to see long-term trends the surveys of 2003–2006 were compared to earlier studies conducted in 1932/1933, 1981 and 1989. A significant depletion of mud can be observed in the entire survey area. It is concluded that primarily changed hydrodynamics that may accompany ongoing climate change are responsible for this. However, the loss of fine-grained sediments is additionally amplified by a reduced vegetation cover and coastal protection measures.

Submarine landforms related to glacier retreat in a shallow Antarctic fjord

Abstract Since the Last Glacial Maximum, ice has retreated through the fjords of the South Shetland Islands leaving a valuable record of submarine landforms behind. In this study, glacial landforms and sub-bottom characteristics have been mapped to investigate the late Holocene retreat behaviour of the Fourcade Glacier and to delineate past environmental processes in Potter Cove, King George Island. The comprehensive datasets include high-resolution swath bathymetry, shallow seismic profiling and one sediment core. Moraines, moraine incisions and glacial lineations were mapped on the sea floor in the inner part of the cove, whereas pockmarks, ice scour marks and channel structures were identified in the outer part. Sub-bottom characteristics have been assigned to different acoustic facies types indicating different depositional settings. The results reveal glacial recessions as well as stillstands and potential readvances during the late Holocene. Furthermore, the sediment record indicates that the Fourcade Glacier was situated inside the inner cove during the Little Ice Age (500–100 cal yr bp).

Glacial melting: an overlooked threat to Antarctic krill

Strandings of marine animals are relatively common in marine systems. However, the underlying mechanisms are poorly understood. We observed mass strandings of krill in Antarctica that appeared to be linked to the presence of glacial meltwater. Climate-induced glacial meltwater leads to an increased occurrence of suspended particles in the sea, which is known to affect the physiology of aquatic organisms. Here, we study the effect of suspended inorganic particles on krill in relation to krill mortality events observed in Potter Cove, Antarctica, between 2003 and 2012. The experimental results showed that large quantities of lithogenic particles affected krill feeding, absorption capacity and performance after only 24 h of exposure. Negative effects were related to both the threshold concentrations and the size of the suspended particles. Analysis of the stomach contents of stranded krill showed large quantities of large particles ( > 106 μm3), which were most likely mobilized by glacial meltwater. Ongoing climate-induced glacial melting may impact the coastal ecosystems of Antarctica that rely on krill.

Tools to evaluate seafloor integrity: comparison of multi-device acoustic seafloor classifications for benthic macrofauna-driven patterns in the German Bight, southern North Sea

To determine the spatial resolution of sediment properties and benthic macrofauna communities in acoustic backscatter, the suitability of four acoustic seafloor classification devices (single-beam echosounder with RoxAnn and QTC 5.5 seafloor classification system, sidescan sonar with QTC Swathview seafloor classification, and multi-beam echosounder with QTC Swathview seafloor classification) was compared in a study area of approx. 6 km2 northwest of the island of Helgoland in the German Bight, southern North Sea. This was based on a simple similarity index between simultaneous sidescan sonar, single-beam echosounder and multi-beam echosounder profiling spanning the period 2011–2014. The results show a high similarity between seafloor classifications based on sidescan sonar and RoxAnn single-beam systems, in turn associated with a lower similarity for the multi-beam echosounder system. Analyses of surface sediment samples at 39 locations along four transects (0.1 m2 Van Veen grab) revealed the presence of sandy mud (southern and western parts), coarse sand, gravel and cobbles. Rock outcrops were identified in the north-eastern and eastern parts. A typical Nucula nitidosa–Abra alba community was found in sandy muds to muddy sands in the northern part, whereas the southern part is characterised by widespread occurrence of the ophiuroid brittle star Amphiura filiformis. A transitional N. nitidosa–A. filiformis community was detected in the central part. Moreover, the southern part is characterised by a high abundance of A. filiformis and its commensal bivalve Kurtiella bidentata. The high number of A. filiformis feeding arms (up to ca. 6,800 per m2) can largely explain the gentle change of backscatter intensity along the tracks, because sediment composition and/or seafloor structures showed no significant variability.

Seafloor monitoring west of Helgoland (German Bight, North Sea) using the acoustic ground discrimination system RoxAnn

Marine habitats of shelf seas are in constant dynamic change and therefore need regular assessment particularly in areas of special interest. In this study, the single-beam acoustic ground discrimination system RoxAnn served to assess seafloor hardness and roughness, and combine these parameters into one variable expressed as RGB (red green blue) color code followed by k-means fuzzy cluster analysis (FCA). The data were collected at a monitoring site west of the island of Helgoland (German Bight, SE North Sea) in the course of four surveys between September 2011 and November 2014. The study area has complex characteristics varying from outcropping bedrock to sandy and muddy sectors with mostly gradual transitions. RoxAnn data enabled to discriminate all seafloor types that were suggested by ground-truth information (seafloor samples, video). The area appears to be quite stable overall; sediment import (including fluid mud) was detected only from the NW. Although hard substrates (boulders, bedrock) are clearly identified, the signal can be modified by inclination and biocover. Manually, six RoxAnn zones were identified; for the FCA, only three classes are suggested. The latter classification based on ‘hard’ boundaries would suffice for stakeholder issues, but the former classification based on ‘soft’ boundaries is preferred to meet state-of-the-art scientific objectives.

High-Resolution Hydroacoustic Seafloor Classification of Sandy Environments in the German Wadden Sea

ABSTRACT Mielck, F.; Hass, H.C., and Betzler, C., 2014. High-resolution hydroacoustic seafloor classification of sandy environments in the German Wadden Sea. The knowledge regarding the distribution of seabed environments in the North Sea is still fragmentary. Hydroacoustic devices can provide rapid and reliable information on the acoustic characteristics of the seafloor. Data of a typical, exclusively sandy and shallow investigation area located in the German Wadden Sea are here presented. The study area includes shallow sublittoral areas and deep channels. The acoustic ground discrimination system RoxAnn was used to measures backscatter intensities, which indicate roughness and hardness parameters of seafloor. Sidescan-sonar data provide acoustic images that are instrumental in seafloor characterization. For ground truthing, surface-sediment samples were collected. The results reveal surficial sediments that range from fine to coarse sand. Finer material is rather restricted to the shallow patches, while coarser sediments characterize the deeper tidal channels. The determined roughness and hardness parameters also strongly increase within these inlets following the change in grain size. The sidescan-sonar imagery shows flow-transverse subaqueous dunes of different sizes. Both ebb- and flood-dominated structures are present. RoxAnn reflects these zones of subaqueous dunes basically through the formation of distinct clusters of data points in hardness vs. roughness scatter plots. Further, the results show that areas characterized by similar grain-size spectra and sonar imagery occasionally reveal completely different roughness and hardness properties. This is caused by varying amounts of shell fragments on the seafloor. Both in situ and hydroacoustic methods are suitable for identifying seafloor properties, but the combination of methods yields more information at higher precision, especially about shell content and grain-size distribution.