Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea
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fmars-06-00677 November 9, 2019 Time: 14:41 # 1 ORIGINAL RESEARCH published: 12 November 2019 doi: 10.3389/fmars.2019.00677 Natural and Anthropogenic Sediment Mixing Processes in the South-Western Baltic Sea Dennis Bunke1*†, Thomas Leipe1, Matthias Moros1, Claudia Morys2†, Franz Tauber1, Joonas J. Virtasalo3, Stefan Forster2 and Helge W. Arz1 1 Department of Marine Geology, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Rostock, Germany, 2 Department of Marine Biology, Institute of Biological Sciences, University of Rostock, Rostock, Germany, 3 Marine Geology, Geological Survey of Finland (GTK), Espoo, Finland Edited by: Anas Ghadouani, The University of Western Australia, Natural and anthropogenic sediment mixing can significantly impact the fidelity of Australia sedimentary records of climate and environmental variability and human impact. This Reviewed by: can lead to incorrect interpretations of the previous state(s) of a given ecosystem, Andrzej Osadczuk, its forcing mechanisms, and its future development. Here, natural and anthropogenic University of Szczecin, Poland Shari L. Gallop, sediment mixing processes (i.e., bioturbation, hydroturbation and direct anthropogenic University of Waikato, New Zealand impact) are investigated in the south-western Baltic Sea by sedimentological, *Correspondence: ichnological, geochemical, and radionuclide analyses to assess their impact on time- Dennis Bunke [email protected] marker profiles and sediment deposition. Depth profiles of mercury and caesium- †Present address: 137 display a varyingly strong disturbance down to 5–25 cm. The deviations from Dennis Bunke, undisturbed profiles can be used to estimate the relative degree of sediment mixing. Institute of Geophysics and Geology, Sedimentary fabric analysis of high-resolution X-radiographs provides further insight into Leipzig University, Leipzig, Germany Claudia Morys, bioturbation. Ichnofossils identified in short sediment cores suggest that the primary Department of Ecology, Environment sedimentary structure is partly overprinted by burrowing organisms living in the upper and Plant Sciences, Stockholm University, Stockholm, Sweden 5–7 cm of sediment. Meteorological and hydrological data from automated measuring stations combined with results from resuspension experiments show that hydrodynamic Specialty section: events, such as storms and saline water inflows from the North Sea, have the potential This article was submitted to Coastal Ocean Processes, to resuspend and laterally transport sediment particles in the south-western Baltic Sea. a section of the journal Partially graded layers in the scale of mm to cm in the investigated cores are likely Frontiers in Marine Science associated with such hydrodynamic events. Multiple generations of linear traces on Received: 30 April 2018 Accepted: 17 October 2019 the seafloor are made visible through sidescan-sonar mapping and document a strong Published: 12 November 2019 anthroturbation in the muddy sediments of the Mecklenburg Bight and the Arkona Basin, Citation: e.g., by bottom trawling. Depending on the core location and assumed mixing depth, Bunke D, Leipe T, Moros M, determined mean net linear sedimentation rates range between 0 and 3.5 mm/a and Morys C, Tauber F, Virtasalo JJ, 2 Forster S and Arz HW (2019) Natural reconstructed net mass accumulation rates range between 0 and 1.86 kg/m /a in the and Anthropogenic Sediment Mixing mud basins. The calculable inventory of anthropogenic contaminants in the sediments Processes in the South-Western Baltic Sea. Front. Mar. Sci. 6:677. illustrates the important, at least temporary storage function of the mud basins and of doi: 10.3389/fmars.2019.00677 the adjacent sandy areas over industrial times. The findings of this study help identify the Frontiers in Marine Science| www.frontiersin.org 1 November 2019| Volume 6| Article 677 fmars-06-00677 November 9, 2019 Time: 14:41 # 2 Bunke et al. Natural and Anthropogenic Sediment Mixing influence of natural and anthropogenic mixing on sediment profiles, and demonstrates the use of mixed sediment profiles for inferring information about past environmental conditions and anthropogenic impact. Keywords: sediment mixing, Baltic Sea, mercury, caesium-137, sedimentary fabric INTRODUCTION south-western Baltic Sea have compiled and analysed the present organisms and their bioturbation potential (Schiele et al., 2015; Shallow-water coastal and marginal seas – such as the south- Gogina et al., 2017; Morys et al., 2017). Hydrodynamic mixing western Baltic Sea – serve as natural reaction sites for the turnover or hydroturbation refers to all sediment mixing and transport and accumulation of land-derived inputs. Accordingly, one processes that are induced by water movements caused by wind important aspect in the evaluation of their service functions is and currents. In the south-western Baltic Sea these are triggered, the role as intermediate storage or final sink of imported material, e.g., by storms and saline water inflows from the North Sea, such as nutrients and contaminants. Moreover, measured profiles and can also leave traces in the sedimentary fabric (Milkert and of various anthropogenically introduced contaminants may serve Werner, 1996; Nittrouer et al., 1998; Kersten et al., 2005). The as markers for stratigraphic purposes. In the muds of the influence of tides is negligible in the Baltic Sea due to its minor Baltic Sea, some useful time markers are (i) the exceedance connection to the world ocean. On the other hand, anthropogenic of mercury (Hg) over natural background levels at around mixing or anthroturbation in aquatic environments refers to the 1900, (ii) the maximum Hg contents at around 1970, and disturbance of sediments by human activities such as bottom (iii) the sharp caesium-137 (137Cs) activity increase due to trawling, construction, dredging, dumping and anchoring as the Chernobyl nuclear disaster in 1986 (Moros et al., 2017). well as sediment sampling (Berner, 1976; Soulsby et al., 2007). However, this event stratigraphy is only reasonable in areas with While the latter five activities have a potentially strong but local continuous sediment deposition and limited post-depositional impact, bottom trawling affects wide areas of the Baltic Sea floor disturbance like the Gotland Basin, where it is also possible (HELCOM, 2010). During the past decades, extensive studies to date the sediments via unsupported lead-210 (210Pb). The have been conducted on the diverse impacts of bottom trawling influx of old re-worked sediment dilutes the artificial signals on the seafloor (Krost et al., 1990; Werner et al., 1990; Smith and dampens the radionuclide activity (Moros et al., 2017). et al., 2003) and on benthic communities (Jennings et al., 2001; Hence, a detailed examination of the natural and anthropogenic Duplisea et al., 2002; Rosenberg et al., 2003) in the Baltic Sea. sediment mixing processes is required to assess their impact However, only a few actually dealt with effects on biogeochemical on sediment deposition and contaminant accumulation in the processes and element fluxes (Krost, 1990). south-western Baltic Sea. In this study, 21 short sediment cores have been analysed Naturally occurring processes include both biogenic from the south-western Baltic Sea, covering the most important and hydrodynamic sediment mixing. Biogenic mixing or sediment types (mud, silt, sand) in the region. First, Hg bioturbation in aquatic environments is caused by benthic fauna content and 137Cs activity were determined to examine the living on and in the surface sediment, mainly through foraging relative degree of sediment disturbance. Second, high-resolution and construction of burrows (Meysman et al., 2010; Kristensen X-radiographs were taken to investigate the sedimentary fabric et al., 2012). It comprises particle reworking (displacement that provides insight into bio- and hydroturbation. Third, or biomixing of particles) as well as burrow ventilation resuspension experiments were conducted and the results (displacement or bioirrigation of water). Biomixing occurs were compared to meteorological and hydrological data from either by frequent and random particle displacements over short automated measuring stations to assess the hydroturbation distances (cryptic bioturbation) analogous to diffusion, or over potential of storms and saline water inflows. Finally, side-scan longer distances in discrete steps (burrow-mottling) analogous sonar images of the seafloor and bottom-trawl effort data were to advection (Boudreau, 1986a,b; Boudreau and Imboden, evaluated for anthropogenic impact. The overall aim was to 1987). The same also applies to bioirrigation (Shull et al., 2009), investigate the contributions of these natural and anthropogenic but is not further investigated within this study, although it is processes to sediment mixing and how they affect time-marker clear that moving water can also move particles and vice versa profiles as well as sediment accumulation. (Kristensen et al., 2012). Cryptic bioturbation is caused by small- sized fauna, the so-called local biodiffusors, and occurs mainly within the uppermost sediment layer, where it may disturb or STUDY AREA completely erase pre-existing structures (Pemberton et al., 2008). Cryptic bioturbation leads to a more or less homogeneous or The south-western Baltic Sea consists of several basins of different biodeformed sedimentary fabric (Wetzel, 1981; Werner, 2002; depths, separated by shallower submarine sills that restrict the Virtasalo et al., 2011). Burrow-mottling