ALBERT

Description: The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Paleozoic orogen in western Scandinavia and its comparison with modern analogues. The transport and emplacement of subduction-related highgrade continent-ocean transition (COT) complexes onto the Baltoscandian platform and their influence on the underlying allochthons and basement is being studied in a section provided by two fully cored 2.5 km deep drill holes. These operational data sets concern the second drill site, COSC-2 (boreholes ICDP 5054-2-A and 5054-2-B), drilled from mid April to early August 2020. COSC-2 is located approximately 20 km eastsoutheast of COSC-1, close to the southern shore of Lake Liten between Järpen and Mörsil in Jämtland, Sweden. COSC-2 drilling started at a tectonostratigraphic level slightly below that at COSC-1’s total depth. It has sampled the Lower Allochthon, the main Caledonian décollement and the underlying basement of the Fennoscandian Shield, including its Neoproterozoic and possibly older sedimentary cover. COSC-2 A reached 2276 m driller's depth with nearly 100 % core recovery between 100 m and total depth. COSC-2 B, with a driller’s depth of 116 m, covers the uppermost part of the section that was not cored in COSC-2 A. The operational data sets include the drill core documentation from the drilling information system (mDIS), full round core scans, MSCL data sets, a preliminary core description and the geophysical downhole logging data that were acquired during and subsequent to the drilling operations. All downhole logs and core depth were subject to depth correction to a common depth master (cf. operational report for detailed information). The COSC-2 drill core is archived at the Core Repository for Scientific Drilling at the Federal Institute for Geosciences and Natural Resources (BGR), Wilhelmstr. 25–30, 13593 Berlin (Spandau), Germany.

Description: The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to characterise the structure and orogenic processes involved in a major collisional mountain belt by multidisciplinary geoscientific research. Located in western central Sweden, the project has drilled two fully cored deep boreholes into the bedrock of the deeply eroded Early Paleozoic Caledonide Orogen. COSC-1 (2014) drilled a subduction-related allochthon and the associated thrust zone. COSC-2 (2020, this paper) extends this section deeper through the underlying nappes (Lower Allochthon), the main Caledonian décollement, and the upper kilometre of basement rocks. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early Paleozoic palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, and the characterisation of the microbial community in the present hard rock environment of the relict mountain belt. COSC-2 successfully, and within budget, recovered a continuous drill core to 2276 m depth. The retrieved geological section is partially different from the expected geological section with respect to the depth to the main décollement and the expected rock types. Although the intensity of synsedimentary deformation in the rocks in the upper part of the drill core might impede the analysis of the Early Paleozoic palaeoenvironment, the superb quality of the drill core and the borehole will facilitate research on the remaining targets and beyond. Protocols for sampling in the hard rock environment and subsequent sample preservation were established for geomicrobiological research and rock mechanical testing. For the former, a sparse sample series along the entire drill core was taken, while the target of the latter was the décollement. COSC-2 was surveyed by a comprehensive post-drilling downhole logging campaign and a combined borehole/land seismic survey in autumn 2021. This paper provides an overview of the COSC-2 (International Continental Scientific Drilling Project – ICDP 5054_2_A and 5054_2_B boreholes) operations and preliminary results. It will be complemented by a detailed operational report and data publication.

Description: The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project characterises the structure and orogenic processes involved in a major collisional mountain belt by multi-disciplinary geoscientific research. Located in western central Sweden, the project has drilled its second fully cored borehole COSC-2 in 2020. It extends the COSC composite geological section through the nappes of the Caledonian Lower Allochthon, the main décollement and the upper kilometre of basement rocks. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early Cambrian palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, and characterisation of the microbial community in the present hard rock environment of the relict mountain belt. COSC-2 successfully and within budget recovered a continuous drill core to 2276 m depth. The retrieved geological section is partially different from the expected geological section with respect to the depth to the main décollement and the expected rock types. The intensity of deformation in the rocks in the upper part of the drill core might impede the analysis of the Early Cambrian palaeoenvironment. However, the superb quality of the drill core and the borehole will facilitate research on the remaining targets and beyond. Although on-site science was reduced due to Covid-19 related restrictions, COSC-2 drilling was complemented by extensive downhole surveys. However, the geological description of the drill core and the sampling party were severely delayed, with the later being held about two years after drilling, concluding the operational phase of the project.

Description: Scientific drilling expeditions offer a unique op- portunity to characterize microbial communities in the sub- surface that have long been isolated from the surface. With subsurface microbial biomass being low in general, biologi- cal contamination from the drilling fluid, sample processing, or molecular work is a major concern. To address this, char- acterization of the contaminant populations in the drilling fluid and negative extraction controls are essential for assess- ing and evaluating such sequencing data. Here, rock cores down to 2250 m depth, groundwater-bearing fractures, and the drilling fluid were sampled for DNA to characterize the microbial communities using a broad genomic approach. However, even after removing potential contaminant popu- lations present in the drilling fluid, notorious contaminants were abundant and mainly affiliated with the bacterial order Burkholderiales. These contaminant microorganisms likely originated from the reagents used for isolating DNA despite stringent quality standards during the molecular work. The detection of strictly anaerobic sulfate reducers such as Candi- datus Desulforudis audaxviator suggested the presence of au- tochthonous deep biosphere taxa in the sequenced libraries, yet these clades represented only a minor fraction of the se- quence counts (〈 0.1 %), hindering further ecological inter- pretations. The described methods and findings emphasize the importance of sequencing extraction controls and can support experimental design for future microbiological stud- ies in conjunction with continental drilling operations.