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Seasonal variability in the salinity and oxygen isotopic composition of seawater from the Cariaco Basin, Venezuela: Implications for paleosalinity reconstructions

Oxygen isotope measurements (δ18Ow) were made on seawater samples collected monthly between May 1996 and February 1997 and between December 2005 and May 2006 at various water depths at the Cariaco Basin ocean time series station (10°30N, 64°40W). The δ18Ow values are compared with concurrent salinity measurements to assess the δ18Ow:salinity relationship in this tropical region and to determine if significant seasonal variability exists in the relationship. The δ18Ow values range from 0.88 to 1.19‰ SMOW in the upper 250 m. Our results indicate that the strongest positive linear correlation between δ18Ow and salinity in the upper 250 m occurred during the February and April 2006 upwelling season (R2 = 0.95 and 0.94, respectively). The salinity: δ18Ow relationship displays significant seasonal variability which is attributed to seasonal changes in freshwater input from the Tuy, Neverí, and Unare rivers into the Cariaco Basin. Specifically, an inverse correlation (R2 = 0.77) exists between monthly Neverí River discharge and sea surface salinity. Our results demonstrate that significant seasonal changes in the δ18Ow:salinity relationship occur in the tropics. The data also show a distinct difference between the surface water δ18Ow:salinity relationship during the upwelling season (R2 = 0.96) and the nonupwelling season (R2 = 0.93) revealing zero‐salinity end‐members of −28.53 (SE ± 3.04) and −8.77 (SE ± 1.33), respectively. The seasonal mixing lines are an important consideration when utilizing the salinity:oxygen isotope relationship for paleosalinity reconstructions. The oxygen isotope composition (δ18Oc) was also measured in two surface‐dwelling planktonic foraminiferal species, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in the Cariaco Basin between November 1996 and February 1997 and May 2003 through May 2006. The large range in δ18Oc during the study period, 1.4‰ for G. ruber and 1.5‰ for G. bulloides, is attributed to changes in calcification depths of the species from 1996–1997 to 2005–2006. Using the surface water δ18Ow:salinity equations generated for the upwelling and nonwelling seasons in the Cariaco Basin, we compare measured seawater salinity with the calculated seawater salinity at various depths of calcification. The δ18Ow:salinity equation generated from surface waters during the upwelling months yields salinity estimates that best agree with measured salinities.