Age discrepancy between molecular biomarkers and calcareous foraminifera isolated from the same horizons of Northwest Pacific sediments

Abstract

Recent observations show that establishment of molecular-level radiocarbon stratigraphy is increasingly critical for paleoceanographic studies, especially to enable comparable discussion about past ocean and climate changes inferred from multiple proxy records based on lipid biomarkers and calcareous foraminiferal microfossils. So far, although we have interesting evidence that there are temporal and spatial age offsets between the algal “alkenone” biomarker used for SST (sea surface temperature) estimates and planktonic foraminifera in regions with high sedimentation rates, less is known about such age offsets in the settings of lower sedimentation. In this study, to investigate the potential of biomarkers as an alternative dating proxy to foraminifera in a low sedimentation rate setting, we measured radiocarbon ages of 17 different lipid biomarkers [fatty acids (FAs), n-alkanes, and alkenones], bulk total organic matter (TOC), and foraminifera from two sections (12–15 cm and 21–24 cm depth) of a surface sediment core in the Northwest Pacific. The sedimentation rate, estimated from the TOC, ranged from 10.5 cm/kyr in the core top to 0.9 cm/kyr in the lower part of the core. The ages of the FAs detected at 12–15 cm depth ranged from 530 yr BP (C 18) to 3250 yr BP (C 28). The 14C analysis of the FAs could be divided into two groups: FAs (C 16, C 18) derived from marine organisms and those (C 24, C 26, C 28) derived from terrestrial higher plants. The high molecular weight (HMW) FAs' ages were older [2550 yr BP (C 24) to 3250 yr BP (C 28)] than those of the low molecular weight (LMW) FAs [530 yr BP (C 18) to 1820 yr BP (C 16)]. At 21–24 cm depth, the alkenone ages were 7100–7300 yr younger than those of the planktonic foraminifera. In this horizon, the alkenone 14C age (7500 yr BP) and the alkenone-estimated SST (15.3 °C) suggest that these alkenones were produced in this region during the Holocene. The alkenone 14C age was also in good agreement with those of the LMW FAs (C 14, iC 15, aiC 15, C 16, and C 18:1) derived from marine plankton and bacteria. Similarly, the TOC age was 5700 yr younger than that of the planktonic foraminifera. These age differences are large compared with those estimated for the horizon at 12–15 cm depth, which was characterized by a relatively higher sedimentation rate. These trends between ages of alkenones, TOC, and planktonic foraminifera are not comparable to the results from sites with high sedimentation rates, such as the Benguella upwelling system [Mollenhauer, G., Eglinton, T.I., Ohkouchi, N., Schneider, R.R., Muller, P.J., Grootes, P.M., et al., 2003. Asynchronous alkenone and foraminifera records from the Benguela upwelling system. Geochim. Cosmochim. Acta 67, 2157–2171] or the Bermuda Rise drift deposit [Ohkouchi, N., Eglinton, T.I., Keigwin, L.D., Hayes, J.M., 2002. Spatial temporal offsets between proxy records in a sediment drift. Science 289, 1224–1227], which are significantly different sedimentary settings from our site. The radiocarbon results from these high sedimentation rate regions suggest that the alkenones are several thousand years older than the coexisting planktonic foraminifera. On the other hand, the age offsets between alkenones and planktonic foraminifera at our site are larger than those from the other sites, and the alkenones are younger than the planktonic foraminifera.