Seasonal evolution of oxygen isotope‐salinity relationships in high‐latitude surface waters

Abstract

The relationship between the isotopic composition of seawater and salinity and the variation of the isotopic composition of precipitation with latitude have frequently been used to identify freshwater sources and to tag water masses. Underlying these applications is the assumption that the isotopesalinity relationship is affected only by the mixing of salt and fresh waters. Simple box models are used to show how /j180-salinity (/J-S) relationships in the upper water column vary seasonally in areas where sea ice forms or melts. During melting, the/J-S relationship takes a number of different shapes, but it returns to a straight line late in the melting season. During freezing, the/J-S shape stays linear, but its slope varies. The models show that ice-related processes can produce very large changes in the 18 18 /J O value of apparent freshwater components (/J Os=o), the parameter usually used to label water masses. At the end of the melting season the /j18Os= 0 value may be 10%o more positive than at the beginning' at the end of the freezing season, /j18Os= 0 may be more than 20%o more negative than at the beginning. Thus the models provide explanations for both anomalously heavy/j18Os= 0 values in the Labrador Current (-7.6%o) and anomalously light values in the Canadian arctic archipelago and the East Greenland Current (•-50%o). They can also explain the negative slope occasionally observed in /J-S plots. Caution must be exercised in using •j18Os= 0 to identify source waters derived from the Labrador Sea, which receives significant amounts of freshwater from Baffin Bay and Hudson Strait which, in many cases, have been altered by more than a single season of melting/freezing activity.

Hydrographic Changes in Nares Strait (Canadian Arctic Archipelago) in Recent Decades Based on δ18O Profiles of Bivalve Shells

“…This chemical analysis can be used to derive insights into ocean circulation and the region's hydrographic response to climate change (e.g., Alkire et al, 2007;Yamamoto-Kawai et al, 2005Guay et al, 2009). Among the various available geochemical tracers, salinity -δ 18 O relationships have been used since the 1980s (e.g., Östlund and Hut, 1984;Strain, 1980, 1996;Strain and Tan, 1993;Macdonald et al, 1995) as a means of quantifying the seasonal icemelt and meteoric water contributions to the freshwater budget. Alkire (2010) recently constrained the net contributions of these two components by applying conservation equations to an extensive hydrographic data set collected from our study area in 1997 and 2003.…”

Paleosalinity and δ18O: A critical assessment

“…use of bx80 to assess water mass mixing assumes no sources or sinks and so is not applicable in surface waters, where precipitation, evaporation, run-off, melting, or freezing [Strain and Tan, 1993] are significant terms in the freshwater budget, except where run-off is very important and can be regarded as a separate water mass of zero salinity.…”

“…This ratio is generally less than the theoretical value of 1'0.24 [Dansgaard, 1964] but varies significantly over the world ocean' 1:0.50 in the Atlantic and Pacific [Broecker, 1989] Table 1. Mook [1982] Mook [1982] Mook [1982] shelf, the S:•180 ratio shows little seasonal variation around 1:0.63 [Fairbanks, 1982] To For example, consider the change in the near-surface S:•80 ratio a month after the melting of both 1 and 3 m thick layers of seaice using realistic melt rates and vertical exchange velocities [Strain and Tan, 1993]. This lag roughly represents the time by which a spring bloom peak in foraminiferal production would occur, providing the fossil carbonate remains employed by …”

Testing the alkenone D/H ratio as a paleo indicator of sea surface salinity in a coastal ocean margin (Mozambique Channel)

“…37 Index (Brassell et al,45 1986; Rostek et al, 1993;Nürnberg et al, 1996). However, the correlation between δ 18 O and 46 salinity is affected by mixing and evaporation processes (Craig and Gordon, 1965), seasonality 47 (Strain and Tan, 1993) and likely changes over glacial -interglacial periods (Rohling and Bigg,1998 subject to uncertainty andis difficult to calibrate (Rohling and Bigg, 1998;Rohling, 2000). 50…”