Sub-marine Continuation of Peat Deposits From a Coastal Peatland in the Southern Baltic Sea and its Holocene Development

Abstract

Coastal low-lying areas along the southern Baltic Sea provide good conditions for coastal peatland formation. At the beginning of the Holocene, the Littorina Sea transgression caused coastal flooding, submergence and erosion of ancient coastlines and former terrestrial material. The present Heiligensee and Hütelmoor peat deposits (located near Rostock in Northern Germany) were found to continue more than 90 m in front of the coastline based on on-and offshore sediment cores and geo-acoustic surveys. The seaward areal extent of the coastal peatland is estimated to be around 0.16-0.2 km 2 . The offshore boundary of the former peatland roughly coincides with the offshore limit of a dynamic coast-parallel longshore bar, with peat deposits eroded seawards. While additional organic-rich layers were found further offshore below a small sand ridge system, no connection to the former peatlands can be established based on 14 C age and C/N ratios. The preserved submerged peat deposits with organic carbon contents of 37% in front of the coastal peatland Heiligensee and Hütelmoor was radiocarbondated to 6725 ± 87 and 7024 ± 73 cal yr BP, respectively, indicating an earlier onset of the peatland formation as presently published. The formation time of the peat layers reveals information about the local sea level rise. The local sea level curve derived from our 14 C-dated organic-rich layers is in general agreement to nearby sea level reconstructions (North Rügen and Fischland, Northern Germany), with differences explained by slightly varying local isostatic movements.

Carbon release and transformation from coastal peat deposits controlled by submarine groundwater discharge: a column experiment study

“…In a recent geological assessment of the development of the seabed (Kreuzburg et al 2018), Holocene peat deposits with C org contents of 37–53% and C/N ratios of 21.3–36.4 were found more than 90 m (areal extent: 0.16–0.2 km 2 ) in front of the coastline (Fig. ).…”

“…The isotopic signature of the lowermost offshore peat seems to be purely terrestrial (δ 13 C –28.9‰) and thus no fractionation processes or contamination with marine organic carbon were detected. The outer boundary of the peat deposits roughly coincides with the offshore limit of a dynamic coast‐parallel longshore bar (Kreuzburg et al 2018). The location of shallower outcropping peat deposits in the northern coastal area coincides with temperature, salinity and bottom water CH 4 anomalies most likely originating from SGD (Jurasinski et al 2018).…”

“…Details of the geology have been presented in Fig. in Kreuzburg et al (2018). One core from each site was kept as a reference to compare geochemical parameters of initial and altered conditions.…”

“…The beach area has been subject to beach nourishments, wooden groynes were installed to retain the sand and a dyke was constructed after several wash‐over events (Kolp 1957; Voigtländer et al 1996). The shoreline is covered by permeable heterogeneous sediments except for areas with peat deposits (Hübner and Gräff 2013; Kreuzburg et al 2018), which are only low‐permeable substratum (Gosch et al 2019).…”

“…Previously observed positive correlations between the release of DOC from subsedimentary sources (e.g., buried peat) and CH 4 concentrations in the water column suggested a common carbon source (Aravena and Wassenaar 1993; Liu et al 2011). Submerged organic‐rich peat sediments can be found in shallow coastal waters offshore of adjoining peatlands (Delaune et al 1994; Taffs et al 2012; Kreuzburg et al 2018), where marine microbial communities can be supplied by peat‐derived DOC and SW‐derived electron acceptors (e.g., O 2 , , ). How these submerged peat deposits in the offshore areas contribute to coastal carbon cycling, however, are currently not well constrained.…”