The oxygen isotope composition of water masses within the North Sea Andrew J.P. Harwood a, * , Paul F. Dennis a , Alina D. Marca a , Graham M. Pilling b , Richard S. Millner b a Stable Isotope Laboratory, School of Environmental Science, University of East Anglia, Norwich, Norfolk NR4 7TJ, UK b Centre for Environment, Fisheries and Aquaculture Science (Cefas) Fisheries Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 OHT, UK Received 18 September 2007; accepted 21 December 2007 Available online 11 January 2008 Abstract Based on measurements of the 18 O isotope composition of 247 samples collected over a 3-year period we have assessed the oxygen isotope composition of water masses in the North Sea. This is the first d 18 O data set that covers the entire North Sea basin. The waters lie on a mixing line: d 18 O(& VSMOW ) ¼9.300 þ 0.274(S ) with North Atlantic sub-polar mode water (SPMW) and surface waters, and Baltic Sea water rep- resenting the saline and freshwater end members respectively. Patterns exhibited in surface and bottom water d 18 O distributions are represen- tative of the general circulation of the North Sea. Oxygen-18 enriched waters from the North Atlantic enter the North Sea between Scotland and Norway and to a lesser extent through the English Channel. In contrast, oxygen-18 depleted waters mainly inflow from the Baltic Sea, the rivers Rhine and Elbe, and to a lesser degree, the Norwegian Fjords and other river sources. Locally the d 18 Oesalinity relationship will be controlled by the isotopic composition of the freshwater inputs. However, the range of local freshwater compositions around the North Sea basin is too narrow to characterise the relative contributions of individual sources to the overall seawater composition. This dataset provides important information for a number of related disciplines including biogeochemical research and oceanographic studies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: seawater d 18 O; salinity; North Sea; mixing line 1. Introduction Oxygen stable isotope compositions of sea and ocean water can be used as an oceanographic tracer, similar to salinity (Frew et al., 2000). Salinity changes resulting from evaporation and precipitation, and mixing processes are coupled with changes in d 18 O. However, the two tracers are decoupled in sea ice processes (Craig and Gordon, 1965; LeGrande and Schmidt, 2006). Thus, combined isotope salinity studies of the World’s oceans allow us to characterise the relative importance of evaporation, precipitation and fresh water run-off, and sea ice processes during the formation of water masses. The work of Craig and Gordon (1965), the Geochemical Ocean Section Study (GEOSECS) program initiated in 1971 (http://ingrid. ldeo.columbia.edu/SOURCES/.GEOSECS/) and databases such as the NASA Global Seawater Oxygen-18 Database (http://data.giss.nasa.gov/o18data/) provide a large resource on the trends in global patterns of d 18 O. The work of these pro- grams has also elucidated trends in d 18 Oesalinity relationships over large spatial scales. However, there is a lack of high reso- lution, area specific data that may contribute to the understand- ing of local circulation and processes. The isotopic compositions of water masses are imprinted at their source locations and once isolated from the ocean surface the d 18 O is only subject to modification through conservative mixing with other water masses of differing composition (Frew et al., 2000). In local basins, such as the North Sea, it is possible to con- struct a relationship between d 18 O and salinity that represents a mixing line between the various contributing water masses. Thus, three-dimensional spatial and temporal analysis of the water oxygen isotope composition can help elucidate patterns of saline and freshwater input, subsequent circulation and de- tails of mixing associated with stratification and overturning. * Corresponding author. E-mail address: andrew.harwood@uea.ac.uk (A.J.P. Harwood). 0272-7714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.12.010 Available online at www.sciencedirect.com Estuarine, Coastal and Shelf Science 78 (2008) 353e359 www.elsevier.com/locate/ecss