Generation, transport, and preservation of the alkenone‐based U37K′ sea surface temperature index in the water column and sediments of the Cariaco Basin (Venezuela)

Abstract

Alkenone fluxes in the water column of the Cariaco Basin ranged from 12 to 20 μg m−2 d−1 and were inversely related to upwelling strength. The U37K′ ratios of sinking particles varied from 0.78 to 0.96 and exhibited seasonal changes that were coherent with a 7°C variation in sea surface temperature (SST). The correlation between SST and U37K′ ratios closely overlapped with the calibration of Prahl et al. [1988]. Alkenone burial fluxes in Cariaco Basin sediments varied markedly over the past ∼6000 years, ranging from 0.2 to 5 μg m−2 d−1. The U37K′ ratios of surface sediments indicate SST was higher (26.3°C) during the last 50 years of deposition than in the previous 300 years (∼25°C), signaling an upwelling decrease in the latter part of the twentieth century. The lowest U37K′‐derived temperatures (∼25°C) were measured in sediments deposited during the little ice age (LIA). These compositions, coupled with relatively low alkenone fluxes (≤2 μg m−2 d−1), are consistent with conditions of enhanced upwelling, decreased SST and reduced haptophyte production. The highest U37K′‐derived SST estimates (over 26.5°C) were measured during the Medieval Warm Period (MWP) and suggest reduced upwelling at this time. Prior to the MWP, the alkenone record indicates temperatures of ∼26°C and burial fluxes of ∼2 μg m−2 d−1. These compositions indicate stronger upwelling conditions during the Holocene relative to the last 50 years and the MWP but annual SSTs above those estimated for the LIA.