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doi:10.2204/iodp.proc.347.109.2015 LithostratigraphyAt Site M0065, cores were recovered from three holes (M0065A–M0065C) at a water depth of 84 m. Hole M0065A reached a total depth of 73.9 mbsf, Hole M0065B reached 49.3 mbsf, and M0065C reached 47.9 mbsf. In Hole M0065A, core recovery was very low in the lowermost part, so core material available for onshore description only includes sediment to 46.6 mbsf. Site M0065 is located in the vicinity of a World War II ammunition dump, and special precautions were required. In Hole M0065A, the uppermost 2 mbsf was washed down to avoid contamination and additional PPE was worn for the first run (see “Operations”). Gas expansion characterized the upper few meters, whereas the lower part was in general only slightly disturbed by coring. Piston coring was carried out down to a hard sand layer at 46 mbsf before switching to a combination of open holing and hammer sampling to maximize recovery in the sandier lithologies (see “Operations”). This continued to 73.90 mbsf, where bedrock was encountered. Holes M0065B and M0065C followed the same procedure but were stopped as they entered the hard sand layer. Hole M0065C was dedicated to microbiological sampling. Lithostratigraphic divisions are based on descriptions on the cut face of the split core from Holes M0065A and M0065B, which gives the most complete composite record of Holocene and late glacial sediments, with a core recovery of ~95% for the uppermost 40 m. Supplementary information is collected from Hole M0065C in addition to smear slide studies. Site M0065 is divided into three lithostratigraphic units (Units I–III; Figs. F1, F2). Unit I (0–9 mbsf) is composed of organic-rich clays containing fragments of bivalve shells and organic remnants. Unit II (9–13 mbsf) is gray clay dominated by iron sulfide lamination in the upper part; freshwater diatoms were found in a smear slide. The lowermost Unit III is divided in three clay subunits: Subunit IIIa (13–36 mbsf) is grayish brown contorted clay, Subunit IIIb (36–36.6 mbsf) is dark gray homogeneous clay, and Subunit IIIc (36.6–49 mbsf) is interlaminated clay and silt gradually grading downward to silt and sand with a few dispersed pebbles. At the Onshore Science Party (OSP), no samples deeper than 49 mbsf were available, as samples had been utilized offshore for either optically stimulated luminescence dating or palynological and sedimentological inspection. However, offshore data reported that very well sorted sand was collected to 68 mbsf, where it changed into sandy silt and became gradually more like a diamicton. At ~74 mbsf, the Mesozoic (Cretaceous) bedrock was reached. Unit I
Unit I consists of very well sorted dark greenish gray organic-rich clay with weak lamination by color due to uncommon bioturbation (Fig. F2). The general stratification is overprinted by intervals of black bands with sharp bases. Dispersed shell fragments are found down to the lowermost transition zone with Unit II, where ~10 cm of prominent laminations at the millimeter scale and no bioturbation are found. The boundary with Unit II is gradual. Smear slide data (see “Core descriptions”) show, in general, very low silt and sand contents, remarkable contents of opaque authigenic minerals, and the existence of large centric diatoms. Organic debris is common, possibly algal or plant debris. The organic-rich clay with bioturbated weak lamination and intervals of black bands is interpreted to indicate general oxic conditions in the marine Holocene sediments of the Bornholm Basin, whereas the lowermost laminated transition zone may represent an initial anoxic phase, similar to the anoxic phases reported in the Eastern Gotland Basin (Zillén and Conley, 2010). Unit II
In Unit II, organic content diminishes and the clay is gray to dark gray (Fig. F2). The clay is laminated by color with very fine dark gray iron sulfide–rich laminae at 2–3 mm scale. Downhole, the number of laminae decreases and is substituted by black spots and specks, as well as homogeneous gray intervals. In the lowermost part, a gradual transition to brown clay is observed. Smear slide studies (see “Core descriptions”) show, as with Unit I, typical very low sand and silt contents and fragments of freshwater colonial “lake-dump” diatoms with complete valves of centric diatoms. Sulfide migration downhole from the upper organic-rich Unit I sediment is a likely example of diagenetic iron sulfidization enhancing Unit II laminations. Alternatively, this migration may be the result of breakdown of primary organic material in the laminations. The smear slide observations of lake-dump diatoms indicate freshwater lake deposition and iron sulfide–laminated clay deeper than the lacustrine Holocene clay that was previously documented as Ancylus Lake sediment (Andrén et al. 2000b). Unit IIISubunit IIIa
Subunit IIIb
Subunit IIIc
In Subunit IIIa, the clay color shifts to grayish brown (Fig. F2). The very well sorted clay shows weak lamination by color with a few silt laminae at the millimeter scale. However, the larger intervals are characterized by a massive to contorted (marble structure) appearance. In addition, the unit is characterized by numerous dispersed gray clay/silt intraclasts of millimeter to centimeter scale. Subunit IIIa has a sharp lower boundary. Smear slide studies (see “Core descriptions”) show very homogeneous clay with barely any silt and sand content, as well as traces of authigenic minerals characterized by numerous brown flakes, possibly biotite. Another color change characterizes Subunit IIIb, which consists of dark gray homogeneous clay with weak light–dark color banding and a sharp lower boundary (Fig. F2). Subunit IIIc, the lowermost subunit, consists of very well sorted grayish brown silty clay with parallel lamination (Fig. F2). Color change on a millimeter to centimeter scale defines the clay lamination, and millimeter-scale silt laminations are common. Downhole, the unit coarsens to a fine to medium sand with laminated silt interbeds and in the lowermost few meters to massive medium-grained sand with few dispersed pebbles. Smear slide studies (see “Core descriptions”) reflect the observed coarsening of grain size. Detrital carbonate is found in all grain sizes up to fine gravel, as well as angular to subrounded quartz sand grains. Reworked foraminifers, rounded oxidized ferromagnesian minerals, and glauconite grains are likewise common. Unit III is interpreted as a glacial lake deposit. The weak lamination combined with massive and contorted sedimentary structures, as well as clay intraclasts in Subunit IIIa, may be indicative of slumping in an unstable sloping environment with high sedimentation rates. Subunits IIIb and IIIc represent earlier more stable phases of the glaciolacustrine environment. The rhythmically banded clays, increased grain size, and frequency of sand laminations indicate that the lower part is deposited in a more ice-proximal setting. |
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