Katja Lindhorst

Active basins and neotectonics: morphotectonics of the Lake Ohrid Basin (FYROM and Albania) [Aktive Becken und Neotektonik: die Morphotektonik des Ohridbeckens (FYROM und Albanien).]

The Lake Ohrid Basin in FYROM (Former Yugoslav Republic of Macedonia) and Albania meets all criteria of an active, seismic landscape: linear step-like fault scarps in the landscape and under water within the lake. Post-glacial (or Late Pleistocene) bedrock fault scarps at Lake Ohrid are long-lived expressions of repeated surface faulting in tectonically active regions, where erosion cannot outpace the fault slip. Other morphotectonic features are wind gaps, wineglass-shaped valleys and triangular facets, which are well preserved. Generally, the faults and fault scarps are getting younger towards the basin centre, as depicted on seismic and hydroacoustic profiles. Additionally, mass movement bodies within the lake and also onshore (rockfalls, landslides, sub-aquatic slides, homogenites, turbidites) are likely to have been seismically triggered. These morphotectonic observations are in line with focal mechanisms of earthquakes in the greater Lake Ohrid area. We investigated the neotectonic history and tectonic geomorphology of the Lake Ohrid Basin by means of an integrated multidisciplinary approach, using tectonic geomorphology and a variety of geophysical and remote sensing methods.

Hydroacoustic Analysis of Mass Wasting Deposits in Lake Ohrid (FYR Macedonia/Albania)

Lake Ohrid is a tectonically formed basin on the Balkan Peninsula. The Udenisht slide complex (USC), in the southwestern part of the lake, is by far the largest mass failure event within the basin. The slide deposits cover an area of ∼27 km², are up to 50 m thick, and sum up to a volume of ∼0.11 km³. The USC extends for up to 10 km into the central basin. First age estimations suggest that the USC is less than 1,500 years old. The volume of the USC is well within the range of landslide volumes capable to trigger tsunamis. However, since the slide event was retrogressive with at least two sub-events and sudden failures of major blocks are not supported by available data, no major tsunami was triggered by the USC. In contrast, subsurface and morphological indications for lake floor instabilities north of the USC suggest rotational slumping processes occurring in this area with higher possibility for tsunami generation in the future.

Modeling Submarine Landslide-Generated Waves in Lake Ohrid, Macedonia/Albania

We study potential tsunami hazards associated with submarine landslides in Lake Ohrid, Macedonia/Albania. The transboundary Lake Ohrid located on the Balkan Peninsula shared by Macedonia and Albania is considered to be the oldest- continuously existing lake in Europe (2–5 Ma), though the age and the origin are not completely unraveled to date. Previous studies by means of hydroacoustic methods have shown that the western margin of Lake Ohrid has a long history of mass wasting. Based on seismic data, slide deposits are found in different stratigraphic levels as well as on the lake floor where they have affected a large area. This study is focused on the well-studied Udenisht Slide Complex covering an area of 27 km2 within the southwestern part of Lake Ohrid. The Udenisht slide is by far the largest mass movement with an average thickness of 30–40 m and an estimated volume of about 0.11 km3. It is therefore well within the limits of submarine landslides that are known to be capable of triggering tsunamis. Using numerical modeling, the propagation of a landslide-generated tsunami with an initial wave height of more than 5 m has been calculated. Run-up heights estimated for coastal communities around the lake are moderate in the north (2–3 m) can reach up to 10 m directly at the site where the slide initiated. This study is a first generation of landslide tsunami hazard assessment for Lake Ohrid and further detailed modeling is recommended for the region.

Reconstructing the sediment concentration of a giant submarine gravity flow

Submarine gravity flows are responsible for the largest sediment accumulations on the planet, but are notoriously difficult to measure in action. Giant flows transport 100s of km3 of sediment with run-out distances over 2000 km. Sediment concentration is a first order control on flow dynamics and deposit character. It has never been measured directly nor convincingly estimated in large submarine flows. Here we reconstruct the sediment concentration of a historic giant submarine flow, the 1929 “Grand Banks” event, using two independent approaches, each validated by estimates of flow speed from cable breaks. The calculated average bulk sediment concentration of the flow was 2.7–5.4% by volume. This is orders of magnitude higher than directly-measured smaller-volume flows in river deltas and submarine canyons. The new concentration estimate provides a test case for scaled experiments and numerical simulations, and a major step towards a quantitative understanding of these prodigious flows.Giant submarine gravity flows are a key mechanism in global sediment transport, yet their properties remain enigmatic. Here, the authors reconstruct the properties of a historic giant submarine gravity flow from deposits across the seafloor.

Linear and non-linear responses of vegetation and soils to glacial-interglacial climate change in a Mediterranean refuge

The impact of past global climate change on local terrestrial ecosystems and their vegetation and soil organic matter (OM) pools is often non-linear and poorly constrained. To address this, we investigated the response of a temperate habitat influenced by global climate change in a key glacial refuge, Lake Ohrid (Albania, Macedonia). We applied independent geochemical and palynological proxies to a sedimentary archive from the lake over the penultimate glacial-interglacial transition (MIS 6–5) and the following interglacial (MIS 5e-c), targeting lake surface temperature as an indicator of regional climatic development and the supply of pollen and biomarkers from the vegetation and soil OM pools to determine local habitat response. Climate fluctuations strongly influenced the ecosystem, however, lake level controls the extent of terrace surfaces between the shoreline and mountain slopes and hence local vegetation, soil development and OM export to the lake sediments. There were two phases of transgressional soil erosion from terrace surfaces during lake-level rise in the MIS 6–5 transition that led to habitat loss for the locally dominant pine vegetation as the terraces drowned. Our observations confirm that catchment morphology plays a key role in providing refuges with low groundwater depth and stable soils during variable climate.

Mass Wasting History Within Lake Ohrid Basin (Albania/Macedonia) Over the Last 600 ka

Lake Ohrid (LO), a transboundary lake shared by Macedonia and Albania on the Balkan Peninsula, is not only considered to be the oldest lake in Europe (~2 Ma) but has a long and continuous sedimentary history. An advantage at LO is the availability of hydroacoustic data sets of good quality covering the entire lake basin. The tectonically formed basin is filled with thick undisturbed sediments. However, the overall internal structure of LO is characterized by numerous faults, clinoform structures, and several Mass Transport Deposits (MTDs). By using a seismic chronology model (SCM) correlating seismic reflector packages with Marine Isotope Stages (MIS) we estimate the occurrence of the deepest MTD detected in the southern basin at the transition of MIS9 to MIS8 (~300 ka) defining the onset of the sliding history in LO that is still ongoing today. In general, MTDs are widespread within the basin but they do cluster at active faults. Two large MTDs occurred in the early MIS7 (~230 ka, ~220 ka) and after a quiesence period of about ~70 ka two additional large MTDs have been detected in the late penultimate glacial period MIS6 (~150 ka, 130 ka). MIS5 seemed to be another quiet period with respect to mass wasting. In the younger sedimentary history mass movement is a common process with several large and mid-sized deposits mapped at all stratigraphic levels. The youngest slide deposits are estimated to occur within the last 2,000 years. The main outcome of this paper is a model for the spatial and temporal distribution of mass wasting for Lake Ohrid.