Oxygen isotopes, upper-ocean salinity, and precipitation sources in the eastern tropical Pacific

Westward transport of freshwater across the Panama Isthmus helps to maintain the salinity (S) contrast between the Atlantic and Pacific Oceans, important in global thermohaline circulation and climate. Relatively low sea-surface salinities in the Panama Bight reflect high net precipitation. We infer sources of this freshwater input to the region based on oxygen isotopes (delta(18)O) of precipitation (delta(P)) and seawater (delta(SW)). The primary trend of the surface water delta(SW)-S relationship implies a freshwater endmember delta(18)O of -8.5 +/- 0.3parts per thousand significantly lower than the mean delta(P) of rainwater we measured in the region (-4.8 +/- 1.8parts per thousand), mixing with waters upwelled from the subsurface (S = 34.9-35.1 PSU, delta(SW) = 0.3-0.4parts per thousand). Based on limited isotope and salinity data, these upwelling waters are a mixture of similar to60% tropical waters and similar to40% subtropical mode waters. Three secondary trends in the surface water delta(SW)-S distribution point to similar freshwater sources with delta(18)O of -2.5 +/- 1.2parts per thousand, -4.7 +/- 0.5parts per thousand, and -4.8 +/- 0.6parts per thousand, consistent with local precipitation sources. Accounting for the primary surface water trend, intermittent cross-isthmus transport of moist air masses from the Caribbean may introduce relatively delta(18)O-depleted rainfall due to a combination of recycling and orographic distillation. In addition, relatively infrequent but large tropical storms, which may contain freshwater of both Pacific and Caribbean origin, deliver rainfall with delta(P) of -10parts per thousand to -14parts per thousand. The primary trend of regional surface seawater data, which integrates the different sources over time, suggests that about half of the total precipitation reflects local sources, and half reflects a combination of cross-isthmus transport and large storms. The regional delta(SW)-S distribution is sensitive to the relative contributions of these freshwater sources, which may change under different climate regimes. (C) 2004 Elsevier B.V. All rights reserved.