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

Abstract

To detect climatic linkages between the Baltic Sea, the Skagerrak and the Nordic Seas, we present multi‐proxy reconstructions covering the last 4500 years from three sediment cores taken in the Skagerrak and along the SW Norwegian margin. Foraminiferal assemblages at all three sites show a distinct change at c. 1700 years BP, associated with a transition from absence and rare occurrence of Brizalina skagerrakensis during c. 4500–2300 years BP to its subsequent abundance increase, suggesting a stronger influence of nutrient‐rich water‐masses during the last c. 1700 years. Increased nutrient availability, which probably stimulated higher primary productivity, is further supported by an increase in diatoms, total organic carbon and benthic foraminiferal species indicative of high productivity and carbon fluxes during the last c. 1700 years as compared to c. 4500–2300 years BP. The amplitude of the B. skagerrakensis signal is largest in the central Skagerrak and gradually becomes smaller towards the Norwegian Sea suggesting that the dominant source of the nutrient‐rich water was the brackish outflow from the Baltic Sea. The generally lower abundances of planktonic foraminifera since c. 1700 years BP support the hypothesis of less saline surface water conditions in the Skagerrak. These results agree with other studies, which suggest a stronger Baltic outflow over the last 1700 years coinciding with a general cooling, increased wintertime westerlies bringing more winter precipitation to northern Europe, increased river runoff and higher frequency of floods. The increase in outflow also occurs during deposition of laminated sediments in the deep Baltic Sea. Leakage of dissolved inorganic phosphorus from anoxic sediments, as well as enhanced erosion due to deforestation in combination with higher runoff from Norway, coastal upwelling and more vigorous frontal dynamics may all have contributed to higher nutrient availability within the adjacent Skagerrak during the last 1700 years BP as compared to c. 4500–2300 years BP, when low productivity prevailed in the study area.

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

“…In addition we evaluate our results in context of the long-term productivity changes described previously in Polovodova Asteman et al (2018). 20…”

“…Although the Skagerrak is a well-investigated area, retrospective studies from this region have focused on climate 30 change instead of productivity, and hence, past primary productivity changes are not well known (Polovodova Asteman et al, 2018). Previous studies suggested that among the processes driving primary productivity changes in the Skagerrak are 1) the North Atlantic Oscillation (NAO), 2) an input of anthropogenic nutrients from the Skagerrak catchment area and 3) nutrients Biogeosciences Discuss., https://doi.org /10.5194/bg-2018-94 Manuscript under review for journal Biogeosciences Discussion started: 5 March 2018 c Author(s) 2018.…”

“…Filipsson and Nordberg, 2010). In addition, eutrophication of the Baltic ecosystem and resulting transport of nutrient-rich water with the Baltic Current through surface water exchange processes represents potentially an additional nutrient source (Andersson, 1996;5 Hjalmarsson et al, 2010;Filipsson et al, 2017;Krossa et al, 2015;Polovodova Asteman et al, 2018). In its turn, a nutrientoverloaded ecosystem in the Skagerrak may further supply dissolved inorganic nitrogen (DIN) to the Kattegat bottom waters (Carstensen et al, 2006) and increase the nutrient level in the coastal waters of Norway (Rosenberg et al, 1987;Rydberg et al, 2006).…”

“…In perspective of the last 4500 years, it is seen that productivity and its variability has increased from 50 BCE towards present day (Polovodova Asteman et al, 2018). The increased productivity occurred as a response to enhanced local runoff coinciding with high winter rainfall and general cooling in Scandinavia, as well as intensified Baltic outflow, which may have significantly contributed to the nutrient supply in the Skagerrak in the past (Polovodova Asteman et al, 2018). 15…”