Microfossils deposited in deep ocean sediments through time, reflect changes of past ocean circulation and represent one of the most important archives of past climate variability. In a recent article in the renowned scientific journal Science, Are Olsen and Ulysses Ninnemann from the Uni Bjerknes Centre and University of Bergen find that the modern isotopic ratio of carbon in the ocean, which is used when interpreting marine sediment cores, is strongly influenced by antropogenic CO2. This has limited our ability to explain the observed changes in sediments and hence our understanding of how ocean currents have varied in in the past.
Deep ocean climate archives
|
Changes in the ocean is archived in small unicellular animals, foraminifera, which live on the seafloor. The relationship between light and heavy carbon isotopes is incorporated into the calcareous shells of these animals and gives a distinct chemical signal, reflecting changes in the waters in which they grow. his provides information on the rate and pathways of deep water renewal in the ocean. Newly formed deep water is well ventilated and has a high 13C to 12C ratio, whereas it is lower in older, less ventilated water. Changes in the 13C to 12C values provide the primary evidence that the pattern of ocean circulation in the North Atlantic has changed with climate in the past, for example during ice ages and rapid climate events. |
Are Olsen
|
Correcting for anthropogenic influences
In the Science article, Olsen and Ninnemann study the modern ocean 13C to 12C relationship, and show how this is affected by anthropogenic carbon that has particularly low 13C to 12C values, this is known as the Suess effect. The results are startling and reveal that the Suess effect has removed crucial differences between North Atlantic water masses with respect to their carbon isotopic composition. This changes our frame of reference for interpreting sedimentary carbon isotopic data.
 |
Upper panel displays the distribution of δ13C, a measure of the 13C vs. 12C relationship in the modern - anthropocene- North Atlantic ocean, while the lower shows the Suess effect corrected - preindustrial- distribution. The Suess effect has erased the δ13C signature of important, recently ventilated, water masses such as the Labrador Sea Water (LSW), the Subpolar Mode Water (SPMW) and the Iceland-Scotland Overflow Water (ISOW), while older intermediate and bottom waters have not been affected to the same extent. |
Of course, one should use the preindustrial oceans carbon isotope distributions as a reference when interpreting past ocean changes, as opposed to the modern ocean signal, which altered by anthropogenic CO2, no longer faithfully reflects ocean processes. This is now in place, and we will get a far better understanding of what has driven changes in the 13C to 12C ratio in the past. For example, it has been difficult to explain the occurrence of very high 13C to 12C ratios in sediments from the ice age because no similar waters exist in the North Atlantic today. Olsen and Ninnemann clearly shows that this is a consequence of the Suess effect, by correcting for the Suess effect they identify water masses of today with such high value.
|
Olsen and Ninnemann are therefore able to determine exactly how the distribution of water masses was significantly different during the last ice age, and believe that this will set the terms for a clearer understanding of the relationship between climate change and ocean circulation in the future. This research shows the importance of interdisciplinary meeting points at the Bjerknes Centre. By co-locating the chemical oceanography and paleooceanography groups at the Bjerknes Centre we have established new professional arenas that make such research possible. |
 Ulysses Ninnemann |
Referenced article: