Depth ranges of alkenone production in the central Pacific Ocean

We report analytical results of long-chain (C-37-C-39) alkenones in the surface sediments taken along a latitudinal transect at 175 degrees E from 48 degrees N to 15 degrees S. Unsaturation degree of C-37 alkenones (U-37(K')) indicated that alkenone temperature is the highest (about 28.3 degrees C) in the tropical Pacific and decreases toward 48 degrees N (about 10.1 degrees C). Although the latitudinal trend of the alkenone temperature is generally similar to that of the observed sea surface temperature of the surface mixed layer, the former exhibits markedly lower values than the latter does in the midlatitude (35 degrees-19 degrees N). On the basis of the comparisons between water column temperatures and alkenone temperatures in the surface sediments, we estimated that in the midlatitudes alkenones are produced in the thermocline waters, whereas in the high (48 degrees-40 degrees N) and low (10 degrees N-2 degrees S) latitudes they are produced in the surface mixed layer. This result is supported by the depth distribution of Emiliania huxleyi and Gephyrocapsa oceanica along a 155 degrees W transect in the central Pacific. The latitudinal distribution of the alkenone production layers estimated from U-37(K') is found to be consistent with that of the nitrate, suggesting that the depth of alkenone production is mainly controlled by the nutrient supply from deeper waters. Alkenones in the sediments exhibited relatively large abundances in 43 degrees-27 degrees N with a maximum at 30 degrees N, which may be explained by high productivity of haptophyte algae in Kuroshio Extension.