It has long been known that the North Sea was once dry land, known as Doggerland. Since the nineteenth century, and perhaps even earlier, tons of mammoth bones, other fossils, archaeological remains, tree trunks, and peat have been found. Some of the drowned peat layers were dated and used for sea-level research, but the sites were too scattered to obtain a coherent picture.
This paper brings together legacy data (existing data points gathered in isolation, but of good enough quality to have value when combined with other data) and many new data points from our research campaigns aimed at obtaining new sea-level data. The North Sea is a vast area, so how did we find the drowned peat layers?
Example of a seismic line with identified peat layers and vibrocores
For the most recent campaigns (2017, 2018), we used the extensive Dutch boreholes database to find offshore peat layers that likely formed in the early Holocene. Using interpolation techniques, we mapped out patches within the coring reach of our vibrocore system (see video). With the research vessel Pelagia from the Royal Netherlands Institute for Sea Research (NIOZ), we went to those patches and deployed geophysical equipment to find the optimal locations for drilling. In the evening, we analyzed the geophysical data, picked xy-coordinates, and provided them to the captain. In the morning, we would be at the first coordinate on the list to start drilling. By the end of the summer of 2018, we had stored more than one hundred boreholes in the fridge.
Going to the captain with coordinates.
It took us nearly seven years to publish our work. A crucial step in this long process was combining the Pelagia data with unpublished work from the BGR, who conducted similar research campaigns in the German part of the North Sea in the 2000s. Freek Busschers and I drove to Hannover one day to discuss joining forces, and by the end of the day, the size of the new sea-level database of the North Sea had greatly increased!
Plot of all sea-level data.
But with the resulting database, many directions can be explored. When no Ph.D. candidates or post-docs are involved, everyone has to put in time and energy alongside other responsibilities. A few years ago, the definitive outline of the work, transitioning from regional North Sea data to global insights about sea-level changes and ice-sheet volumes, began to take shape, and we steadily moved towards the end result.
So, what's next? Our paper discusses global sea-level rise in the early Holocene, but our oldest seea-level index point is about 11,000 years old and the early Holocene started 700 years earlier. That 700-year period is very interesting because it directly follows the cold Younger Dryas, and hence massive ice-sheet melt and a shift in rates of sea-level change can be expected. The good news is that during our research campaigns, we observed deeper lying peat layers than our oldest samples. Our drilling system couldn't reach them now, but we hope to return to the North Sea and finish the work.
Apart from NIOZ and BGR, the research was done by Deltares, TNO-Geological Survey of the Netherlands, Utrecht University, Delft University of Technology, LIAG, Leeds University, Wageningen University and Research, Institute for Biodiversity and Ecosystem Dynamics, and the University of Sheffield.