Abstract. 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.
Assessing human impact on Rostherne Mere, UK, using the geochemistry of organic matter