Temporal and spatial sedimentation rate variabilities in the eastern Gotland Basin, the Baltic Sea

Abstract

The present study examines sedimentation rates in the eastern Gotland Basin using a variety of methods that reveal considerable heterogeneity in the rates, both spatially and temporally. High‐resolution seismic recordings and correlation with long sediment cores indicate increased thickness of strata and higher sedimentation rates (0.75 mm a‐1) in the eastern part of the basin than in the western part (0.23 mm a‐1) since the Littorina transgression some 8000 14C years BP. This difference is apparently a consequence of a counterclockwis e near‐bottom circulation in the basin with periodically high current speeds that cause winnowing on the steep SE slope of the basin and differential settling of sediments in areas of low current speeds. On shorter time scales, recent sediment accumulation rates based on radiometric dating (210Pb) are in general twice as high as those observed 25 years ago using the same method. The higher modern rates, compared to those of the 1970s, may partly be due to increased eutrophication, as more carbon is buried in the sediment, and partly due to increased erosion in shallow water areas. However, strong lateral variations are evident. The average sediment accumulation rates vary between 119 and 340 gm‐2 a‐1 (corresponding to sedimentation rates of 2.1–2.5 mm a‐1) in the deepest part of the basin. Very high rates (6100 g m‐2 a‐1, corresponding to sedimentation rates of 30 mm a‐1) are observed on an intraslope basin site (offshore Latvia) at a water depth of only 70 m. The radiometrically determined sediment accumulation rates are up to three times higher than those estimated from average water column concentrations of suspended matter and from sediment trap flux rates. The discrepancy suggests that sedimentation in the deep basin may have a substantial contribution from near‐bottom lateral transport.