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

Abstract

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 ( δ 18O) of precipitation ( δ P) and seawater ( δ SW). The primary trend of the surface water δ SW– S relationship implies a freshwater endmember δ 18O of −8.5±0.3‰, significantly lower than the mean δ P of rainwater we measured in the region (−4.8±1.8‰), mixing with waters upwelled from the subsurface ( S=34.9–35.1 PSU, δ SW=0.3–0.4‰). Based on limited isotope and salinity data, these upwelling waters are a mixture of ∼60% tropical waters and ∼40% subtropical mode waters. Three secondary trends in the surface water δ SW– S distribution point to similar freshwater sources with δ 18O of −2.5±1.2‰, −4.7±0.5‰, and −4.8±0.6‰, 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 18O-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 δ P of −10‰ to −14‰. 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 δ SW– S distribution is sensitive to the relative contributions of these freshwater sources, which may change under different climate regimes.