Towards an event stratigraphy for Baltic Sea sediments deposited since AD 1900: approaches and challenges

Abstract

Reconstructions of environmental changes at sub-decadal to decadal resolution based on central Baltic Sea sediments rely on accurate and precise high-resolution sediment depth/age relationships. A model chronology for Baltic Sea sediments is presented here based on established historical records of anthropogenic radionuclides ( 137 Cs/ 241 Am/bomb 14 C), polychlorinated biphenyls (PCBs), lead (Pb) and stable lead isotope ( 206/207 Pb ratios), and radionuclide 210 Pb and 14 C decay dating methods. Marker horizons consisting of chemical precipitates formed by documented Major Baltic Inflow (MBIs) events and an extended diatom bloom period were also integrated into the model. The main time markers in Baltic Sea sediments that formed during the last 120 years were the following: (i) the deepest observation of 210 Pb unsupp. (marking the 210 Pb dating horizon) and departure of Hg from natural background levels at c. AD 1900; (ii) first detectable presence of PCBs at AD 1935; (iii) radionuclide production (i.e. 241 Am) due to nuclear weapons testing between AD 1954 and AD 1975, with a peak in AD 1963; (iv) maximum heavy metal and PCB concentrations in the AD 1960s/1970s; (v) the Chernobyl nuclear accident in AD 1986 as a sharp 137 Cs increase; (vi) exceptionally strong diatom blooms with a massive diatom layer found in the Eastern Gotland Basin in AD 1988-1990 and (vii) characteristic manganese-carbonate layers in the deeper central basins formed by MBIs in AD 1993 and AD 2003. A precise and accurate sediment depth/age relationship can only be achieved in restricted areas of the Baltic Sea where continuous sedimentation has prevailed and there has been limited postdepositional disturbance. We demonstrate that parallel Hg and 137 Cs measurements can be used to assess the quality of sediment sequences for high-resolution environmental reconstructions. We show examples of sediment profiles that conform to the historical record, and examples from Western Baltic Sea areas where it appears to be impossible to establish accurate geochronologies.

Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

“…(, ) and Moros et al . (). However, few studies have targeted reconstruction of inflow variability in the region by using foraminifera, although e.g.…”

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

“…All AMS 14 C samples were dated at the Poznan Radiocarbon Laboratory (Poland), and the obtained dates were calibrated using Calib 7.1.0 software (Stuiver et al, 2017), marine calibration data set 'Marine13' (Reimer et al, 2013) and a reservoir age ΔR = 0±50. The Hg-profiles from the cores confirm that the top of both cores were deposited later than the 1960 -70s (Polovodova Asteman et al, 2018), when 15 peaks in Hg values are seen in association with the anthropogenic induced pollution maximum (Moros et al, 2017). In this study we target the time interval after CE 900.…”

Late Holocene palaeoproductivity changes: a multi‐proxy study in the Norwegian Trench and the Skagerrak, North Sea

“…The overlap between the mercury (Hg) profiles of the multicores and gravity cores taken in the Skagerrak document anthropogenic Hg peaks associated with the 1960–70s pollution maximum, a robust time marker suggested by Moros et al . ().…”

“…2) are presented to verify that we have modern sediment recorded at both sites for the gravity cores and multicores. The overlap between the mercury (Hg) profiles of the multicores and gravity cores taken in the Skagerrak document anthropogenic Hg peaks associated with the 1960-70s pollution maximum, a robust time marker suggested by Moros et al (2017).…”