Basin-Scale 3D Sedimentary Modelling: An Approach to Subdivide Baltic Sea Onshore Sediments for Land use and Construction

Geotechnical and Geological Engineering - Understanding the local stratigraphy and geometry of sediment units is necessary for successful 3D modelling and the prediction of ground behaviour and...     

Toward a standard stratigraphical classification practice for the Baltic Sea sediments: the CUAL approach

The Late Pleistocene and Holocene glacial and postglacial sediments of the Baltic Sea basin are conventionally classified into units according to the so‐called Baltic Sea stages: Baltic Ice Lake, Yoldia Sea, Ancylus Lake and Litorina Sea. The Baltic Sea stages have been identified in offshore sediment cores by fundamentally different criteria, precluding detailed comparisons of the sediment units amongst different sea areas and studies. Here, long sediment cores and reflection seismic and pinger sub‐bottom profiles were studied from an offshore area in the Gulf of Finland, northern Baltic Sea. The strata are divided on the basis of sedimentological criteria into three allostratigraphical formations with subordinate allostratigraphical members and lithostratigraphical formations, following the combined allostratigraphical and lithostratigraphical (CUAL) approach. Sedimentological features are recommended as the primary stratigraphical classification criteria because they do not require the palaeoenvironmental inferences of salinity and water level that are inherent in the conventional classification practice. The presented stratigraphical division is proposed as a flexible template for future stratigraphical work on the Baltic Sea basin, whereby lower‐rank allounits and lithounits can be included and removed locally, while the alloformations will remain at the highest hierarchical level and guarantee regional correlatability. The stratigraphical division is compatible with international guidelines, facilitating communication to the wider scientific community and comparison with other similar basins.     

Particulate organic carbon (POC) in surface sediments of the Baltic Sea

In this study, particulate organic carbon (POC) contents and their distribution pattern in surficial sediments of the Baltic Sea are presented for 1,471 sampling stations. POC contents range from approx. 0.1% in shallow sandy areas up to 16% in deep muddy basins (e.g. Gotland Basin). Some novel relationships were identified between sediment mass physical properties (dry bulk density (DBD), grain size) and POC levels. Notably, the highest POC concentrations (about 10–17 mg cm^–3) occur in sandy mud to mud (60–100% mud content) with intermediate POC contents of about 3–7% and DBDs of 0.1–0.4 g cm^–3. Areas with this range in values seem to represent the optimum conditions for POC accumulation in the Baltic Sea. The maximum POC contents (8–16%) are found in fluid mud of the central Baltic Sea characterized by extremely low DBDs (<0.1 g cm^–3) and moderate POC concentrations (4–7 mg cm^–3). Furthermore, sediment mass accumulation rates (MAR), based on ²¹⁰Pb and ¹³⁷Cs measurements and available for 303 sites of the Baltic Sea, were used for assessing the spatial distribution of POC burial rates. Overall, these vary between 14 and 35 g m^–2 year^–1 in the mud depositional areas and, in total, at least 3.5 (±2.9) Mt POC are buried annually. Distribution patterns of POC contents and burial rates are not identical for the central Baltic Sea because of the low MAR in this area. The presented data characterize Baltic Sea sediments as an important sink for organic carbon. Regional differences in organic carbon deposition can be explained by the origin and transport pathways of POC, as well as the environmental conditions prevailing at the seafloor (morphology, currents, redox conditions). These findings can serve to improve budget calculations and modelling of the carbon cycle in this large brackish-water marginal sea.     

Assessment of recent eutrophication and climate influence in the Archipelago Sea based on the subfossil diatom record

Two short sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study recent eutrophication in the area. The diatom record was divided into diatom assemblage zones. Locally weighted weighted averaging regression and calibration was applied for the quantitative reconstruction of past TN concentrations. The cores were dated using radiometric (²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs) methods. The diatom assemblages in the topmost zones in both cores indicate eutrophication during the last decade. TN reconstructions partially fail to trace the actual measured TN concentrations. Especially from the late 1980s to the mid 1990s, the dominating diatom species are more influenced by other factors, such as the length of the ice season, than by nutrient concentrations. It seems that in the Archipelago Sea, diatom assemblages are principally governed by climatic fluctuations with a moderate influence of eutrophication in recent years. The peak in sedimentation in the early 1990s coincides with the short ice seasons indicating that sedimentation may increase with decreasing ice-cover extent due to warming climate.     

Holocene diatom stratigraphy in the Archipelago Sea,northern Baltic Sea

Two sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study the Holocene palaeoenvironmental history of the area. The diatom record was divided into local diatom assemblage zones (LDAZ). An age model was constructed using independent palaeomagnetic and AMS-¹⁴C methods. The early history of the Archipelago Sea was freshwater. Initial brackish-water influence is observed at 7,950 ± 80 cal. BP (LDAZ4), but fully brackish conditions were established at 7,700 ± 80 cal. BP (LDAZ5). Diatom assemblages indicate increasing salinity, warming climate, and possible increasing trophic state during the transition from lacustrine to brackish-water conditions. The decreasing abundance of Pseudosolenia calcar-avis (Schultze) Sundström and the increasing abundance of the ice-cover indicator species Pauliella taeniata (Grunow) Round and Basson indicate reduced salinity and climatic cooling after ～5,000 cal. BP. LDAZ boundaries do not always correlate with changes in the sediment appearance, which underlines the importance of defining biostratigraphic boundaries independently to the sediment visual character, in contrast with the conventional practice for classifying the Baltic Sea sediments.     

Geochemistry of surface sediments in the Archipelago Sea,SW Finland: a multiparameter and multivariate study

This study investigates the geochemistry of soft, organic-rich brackish-water surface sediments in the Archipelago Sea (SW Finland). The area is one of the world’s largest archipelagos, and, although it is scarcely populated, frequent ship and boat traffic along with fish farming occurs. From 76 sites, 47 chemical elements were determined after aqua regia digestion. Other parameters determined were grain size, water, organic and nitrogen contents. At sampling, the sediments were visually classified into groups representing oxic, anoxic and fluctuating (intermediate) oxic seafloor conditions. The results show that the element concentrations are quite similar to other studies in the region but with clear signs of lower anthropogenic loading. Point source pollution was identified in one sample taken nearby a fish farm and a cable ferry. This site was strongly enriched in Cu, Zn and Sn derived from chemicals used in either, or both, activities at the site. Overall, the sediments can be divided into two groups that separate chemically and geographically. One group contains more fine-grained sediments that occur near shore, while the other group contains more anoxic, organic and sulfur-rich sediments occurring more offshore. The second group is interpreted to reflect material reworked from previous deposits as a result of shoaling of the area due to glacio-isostatic land uplift. The visual classes were included in both partial least squares regression (PLSR) and PLS discriminant analysis. These analyses showed that the oxic and anoxic seafloor sediments can be predicted from the chemical variables and grain-size distribution.