Written by Margit Simon, researcher at the Bjerknes Centre and NORCE.
In a new study published in PLoS ONE we show that marine sediments from the ocean floor off Iceland overlap with the historical era providing a record of oceanographic changes and the carbon cycle in unprecedented temporal resolution.
The CO2 emitted from fossil fuel burning has a distinct carbon isotope ratio compared to the preindustrial background level. Since the industrial revolution, nearly 30 percent of CO2 emissions have been taken up by the ocean. This absorption is not uniform; therefore, understanding local CO2 uptake rates is essential for assessing the strength and climate sensitivity of the ocean carbon sink, as well as the risk for future ocean acidification.
We find evidence that the imprint of this fossil fuel‐derived CO2 in the waters of the NW Icelandic Shelf became detectable from ~1950 CE (±8 years) onwards. These new results are based on the carbon isotope (δ13C) signature in planktic foraminifera, marine zooplankton that forms a calcium carbonate shell in the water they live. Their carbon isotope time series reveals a negative excursion driven by anthropogenic CO2 penetration into the ocean, the so called “Suess effect” signal.
A trend in productivity?
However, this decline in the carbon isotope ratio over time is smaller than what we would expect at this location in the Iceland Sea. Our results imply a reduced Suess effect. The reason for that is, that the Suess effect is counteracted by a concurrent increase in surface ocean productivity, since the 1940/50´s (causing a positive shift in the foraminiferal δ13C).
This mechanism agrees with a recent study on the same core material that finds that enhanced freshwater discharge from melting Arctic Ocean drift sea ice and the eastern Greenland Ice Sheet has contributed to nutrient-driven fertilization of the upper ocean and consequently increased the marine primary productivity since the 1940s/50s on the North Icelandic Shelf.
Reconstructed ocean properties
Unfortunately, missing detailed instrumental observations before the 1950s limits our understanding of how the ocean–atmosphere–ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing.
The sedimentary material on the Iceland Shelf has allowed us to produce a multi-proxy record of ocean variability in the region. Proxies are substitute measurements that reflect ocean properties in the past. By doing so, we extend the oceanographic observational record ~100 years back in time for this area.
Our results show natural multi-decadal variability related to the Iceland Sea 's response to easterly wind forcing. Moreover, we are able to trace the poleward propagation of warm water moving with the currents from the south, the subtropics, into the Nordic Seas, via the Icelandic inflow branch.
Simon MH, Muschitiello F, Tisserand A, Olsen A, Moros M, Perner K, et al. A multi-decadal record of oceanographic changes of the past ~165 years (1850-2015 AD) from Northwest of Iceland. PLoS ONE. 2020. doi: 10.1371/journal.pone.0239373.