Naohiko Ohkouchi

Asynchronous alkenone and foraminifera records from the Benguela Upwelling System

Abstract Radiocarbon stratigraphy is an essential tool for high resolution paleoceanographic studies. Age models based on radiocarbon ages of foraminifera are commonly applied to a wide range of geochemical studies, including the investigation of temporal leads and lags. The critical assumption is that temporal coupling between foraminifera and other sediment constituents, including specific molecular organic compounds (biomarkers) of marine phytoplankton, e.g. alkenones, is maintained in the sediments. To test this critical assumption in the Benguela upwelling area, we have determined radiocarbon ages of total C37-C39 alkenones in 20 samples from two gravity cores and three multicorer cores. The cores were retrieved from the continental shelf and slope off Namibia, and samples were taken from Holocene, deglacial and Last Glacial Maximum core sections. The alkenone radiocarbon ages were compared to those of planktic foraminifera, total organic carbon, fatty acids and fine grained carbonates from the same samples. Interestingly, the ages of alkenones were 1000 to 4500 yr older than those of foraminifera in all samples. Such age differences may be the result of different processes: Bioturbation associated with grain size effects, lateral advection of (recycled) material and redeposition of sediment on upper continental slopes due to currents or tidal movement are examples for such processes. Based on the results of this study, the age offsets between foraminifera and alkenones in sediments from the upper continental slope off Namibia most probably do not result from particle-selective bioturbation processes. Resuspension of organic particles in response to tidal movement of bottom waters with velocities up to 25 cm/s recorded near the core sites is the more likely explanation. Our results imply that age control established using radiocarbon measurements of foraminifera may be inadequate for the interpretation of alkenone-based proxy data. Observed temporal leads and lags between foraminifera based data and data derived from alkenone measurements may therefore be secondary signals, i.e. the result of processes associated with particle settling and biological activity.

Innovative methods for determining alkenone unsaturation indices

Abstract Standards, total lipid extracts and crudely fractionated samples as well as extensively purified alkenone references were analyzed by conventional single column gas chromatography (GC), selective two-dimensional GC (GC–GC), comprehensive two-dimensional GC (GC×GC) and high resolution mass spectrometry (HRMS). U 37 K′ indices, defined as [C 37:2 ]/[C 37:2 +C 37:3 ], for independently analyzed standards showed GC–GC average accuracy of 0.014 and precision ±0.008 and HRMS average accuracy of 0.001 and precision ±0.007. The HRMS limit of precision is approximately ±0.008 for “real world samples” applied to the MS probe at greater than a few nanograms of the lesser alkenone component. Both of the two-dimensional GC techniques and HRMS indicate the presence of coeluting compounds in typical alkenone fractions that are not resolved by single dimensional GC. U 37 K′ indices measured by GC–GC, GC×GC and HRMS for total lipid extracts were generally observed to have lower U 37 K′ indices than their purified alkenone fractions.