The Bornholm Basin, southern Scandinavia: a complex history from Late Cretaceous structural developments to recent sedimentation

Abstract

The Bornholm Basin has experienced a complex history linked closely to the structural development of the Tornquist Zone strike‐slip fault system. The latest major tectonic development was late Cretaceous and early Tertiary Rønne Graben inversion caused by dextral transpressional strike‐slip movements focused around rigid blocks such as Bornholm. By applying the strike‐slip concept, it is possible to show that the Rønne Graben inversion push‐up structure was created by restraining left step‐over. The fault system continues northwest and north of Bornholm where the strike‐slip movement resulted in right step‐over and the formation of a pull‐apart basin: the northern part of the Bornholm Basin. Detailed seismo‐acoustic and sediment core studies show that, despite a rather uniform present bathymetry in the central part of the Bornholm Basin, the Quaternary deposits mirror the pull‐apart basin synsedimentary depositional pattern with depocentres close to the major normal faults, bounding the Christiansø Ridge. IODP Site M0065 penetrated a secondary fault zone and provided detailed information on neotectonic events and on environmental changes during the Holocene. The generally rather homogeneous sediments in the cores indicate a dominance of oxic conditions in the Bornholm Basin during the Holocene. However, strong internal seismic reflectors, indirectly reflecting variations in the sediment density and the reflection coefficient, were caused by a shift to stronger laminated, organic‐rich sediments that were deposited during the Medieval Warm Period (MwP). The variable depth positions of the MwP‐related strong reflectors indicate large differences in the spatial and temporal sedimentation pattern in the Bornholm Basin. These differences in the depositional environment may have been caused by local neotectonic events in the basin.

Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

“…We also use the already published LOI data for the core 303770 (Jensen et al . ) and present the unpublished TOC data from this core (M. Moros, unpublished data) for comparison with our data.…”

“…For comparison, we also use the data from core 303770 retrieved at 55°15.67′ N; 15°28.15′ E (91 m water depth) with R/V ‘Poseidon’ in 2005 (Harff ; Jensen et al . ). On both sampling occasions, sediment was collected by using a gravity corer.…”

“…The age model for core 303770 is based on 26 AMS 14 C dates of mostly mollusc shells ( Macoma balthica ; Jensen et al . ). All 14 C dates from cores M86‐1/24/3 and 303770 were calibrated by OxCal v4.2 software (Ramsey ), marine calibration data set ‘Marine13’ (Reimer et al .…”

“…The dates of core 303770 were re‐calibrated from those published in Jensen et al . () for the purpose of this study.…”

Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

“…After the termination of bottom-water euxinia, which we set at 7000 years BP, bottom waters were assumed to remain anoxic. Even though periods of oxygenation have occurred since then (Jensen et al, 2017;Zillén et al, 2008), the average bottom-water oxygen concentration was likely low.…”