Combined measurements of salinity and the oxygen/hydrogen stable isotope composition of marine
waters can characterise processes such as freshwater mixing, evaporation, precipitation and sea-ice formation.
However, stable isotope data with high spatial and temporal resolution are necessary for a detailed understanding
of mixed water bodies with multiple inputs. So far analysis of d18O and dD values in water has been a relatively
expensive, laboratory-based technique requiring collection of discrete samples. This has greatly limited the scope and
scale of field research that can be undertaken using stable isotope analysis. Here, we report the first continuous
shipboard measurements of d18O and dD values in water by diffusion sampling-cavity ring-down spectrometry. Combined
with continuous salinity recordings, a data set of nearly 6,000 measurements was made at 30-s intervals
during a 3-day voyage through the Great Barrier Reef Lagoon. Our results show that continuous shipboard measurement
of d18O/dD values provides additional discriminatory power for assessing water mass formation processes
and histories. Precise identification of river plumes within the Great Barrier Reef Lagoon was only possible because
unique d18O/dD–salinity relationships of individual plumes were measured at high spatial and temporal resolution. The
main advantage of this new technique is the ability to collect continuous, real-time isotope data at a small fraction
of the cost of traditional isotope analysi s of discrete samples. Water d18O and dD values measured by diffusion
sampling-cavity ring-down spectrometry and laboratorybased isotope ratio mass spectrometry have similar accuracy
and precision.