The Nd-isotopic fingerprinting of North Atlantic water masses and its influences from local sources such as Iceland

Abstract:

The ¹⁴³Nd/¹⁴⁴Nd ratio of seawater has become a valuable tracer of north Atlantic circulation patterns, driven by continental runoff, boundary exchange and water mass advection and mixing. A region of particular interest is the overflow across the Iceland – Scotland Ridge injecting water from the Arctic Ocean into the Iceland basin. However, Iceland itself constitutes a singularity in terms of the release of radiogenic ¹⁴³Nd/¹⁴⁴Nd to seawater, due to leaching of young volcanic basalts, whereas the Atlantic is dominated by less radiogenic Nd inputs from old Canadian crustal rocks and the European continent. Hence, release of volcanic material may affect the Nd-isotopic composition of water masses recirculating throughout the sub-polar gyre. This is of major importance regarding the use of Nd isotopes in sediments and corals to trace changes of recirculation patterns in the past. Here, seawater was analysed on three transects (local to basin scale) to fingerprint advection patterns as well as local radiogenic Nd sources in the sub-polar gyre domain. The first local transect followed open ocean water on to the shelf near the outflow of the Vatnajökull glacier. It demonstrates that freshwater runoff modifies the open ocean Nd signature only very close to the shelf (<40km near the outflow) and thus reflects a very local feature unlikely possible to alter the predominant water mass composition of Northeast Atlantic water or the underlying Iceland-Scotland Overflow Water. The second transect follows the Iceland shelfbreak from north-east towards the Reykjanes ridge and includes seawater samples near the recently formed volcanic island of Surtsey. Again no major modification of the Atlantic water mass compositions is observed, besides samples in direct vicinity of Surtsey island where local release of Nd modifies seawater εNd (-14) to values as high as -6. Finally we have monitored the isotopic composition of water masses along a North-South transect from Iceland towards the Azores, which confirms that εNd clearly fingerprints water masses of the sub-polar gyre down to 50°N. Hence, Nd isotopes seem a reliable tracer of water mass mixing in the North Atlantic, with only local influences from basalt weathering in shelf areas.