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

A comprehensive multi-proxy study on two sediment cores from the western and central Skagerrak was performed in order to detect the variability and causes of marine primary productivity changes in the investigated region over the last 1100 years. The cores were dated by Hg pollution records and AMS ¹⁴C dating and analysed for palaeoproductivity proxies such as total organic carbon, δ¹³C, total planktonic foraminifera, benthic foraminifera (total assemblages as well as abundance of Brizalina skagerrakensis and other palaeoproductivity taxa) and palaeothermometers such as Mg∕Ca and δ¹⁸O. Our results reveal two periods with changes in productivity in the Skagerrak region: (i) a moderate productivity at ∼ CE 900–1700 and (ii) a high productivity at ∼ CE 1700–present. During ∼ CE 900–1700, moderate productivity was likely driven by the nutrients transported with the warm Atlantic water inflow associated with a tendency for a persistent positive NAO phase during the warm climate of the Medieval Climate Anomaly, which continues into the LIA until ∼ CE 1450. The following lower and more variable temperature period at ∼ CE 1450–1700 was likely caused by a reduced contribution of warm Atlantic water, but stronger deep-water renewal, due to a generally more negative NAO phase and a shift to the more variable and generally cooler climate conditions of the Little Ice Age. The productivity and fluxes of organic matter to the seafloor did not correspond to the temperature and salinity changes recorded in the benthic Melonis barleeanus shells. For the period from ∼ CE 1700 to the present day, our data point to an increased nutrient content in the Skagerrak waters. This increased nutrient content was likely caused by enhanced inflow of warm Atlantic water, increased Baltic outflow, intensified river runoff, and enhanced human impact through agricultural expansion and industrial development. Intensified human impact likely increased nutrient transport to the Skagerrak and caused changes in the oceanic carbon isotope budget, known as the Suess effect, which is clearly visible in our records as a negative shift in δ¹³C values from ∼ CE 1800. In addition, a high appearance of S. fusiformis during the last 70 years at both studied locations suggests increased decaying organic matter at the sea floor after episodes of enhanced primary production.