An evaluation of ¹⁴C age relationships between co‐occurring foraminifera, alkenones, and total organic carbon in continental margin sediments

Abstract

Radiocarbon age relationships between co‐occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site‐specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported to be 1000–4500 years older than co‐occurring foraminifera, alkenones were significantly (∼1000 years) older than co‐occurring foraminifera in the Chilean margin sediments. In contrast, alkenones and foraminifera were of similar age (within 2σ error or better) in the NW African and South China Sea sediments. Total organic matter and alkenone ages were similar off Namibia (age difference TOC alkenones: 200–700 years), Chile (100–450 years), and NW Africa (360–770 years), suggesting minor contributions of preaged terrigenous material. In the South China Sea, total organic carbon is significantly (2000–3000 years) older owing to greater inputs of preaged terrigenous material. Age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter. Physical characteristics of the depositional setting, such as seafloor morphology, shelf width, and sediment composition, may control the age of co‐occurring sediment components. In particular, offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions. Aging of organic matter is promoted by transport. Age offsets are correlated with organic richness, suggesting that formation of organic aggregates is a key process.

Rapid lateral particle transport in the Argentine Basin: Molecular 14C and 230Thxs evidence

“…Keigwin & Jones, 1994;Ohkouchi et al, 2002). Significant age differences between organic matter and foraminifera have also been reported from continental margins where high sedimentation rates are observed in local depo-centers (Mollenhauer et al, 2003(Mollenhauer et al, , 2005. Other studies have found independent evidence of lateral displacement of fine-grained particles based on sedimentological considerations (Thomsen et al, 1998;Benthien & Müller, 2000;Freudenthal et al, 2001;Englebrecht & Sachs, 2005).…”

“…Potential temporal decoupling of organic and inorganic proxies, associated with the fine and coarse sediment fractions, respectively, resulting from lateral sediment transport has also been investigated by molecular-level radiocarbon dating (Ohkouchi et al, 2002;Mollenhauer et al, 2003Mollenhauer et al, , 2005. Significant age offsets between a suite of marine organic biomarkers (alkenones) associated with the fine fraction and coarse grained calcareous foraminiferal tests have been found at sites of high local sedimentation rates.…”

Developments in Radiocarbon Technologies: From the Libby Counter to Compound-Specific AMS Analyses

“…The origin of fatty acids in sediments has also been studied (Pearson et al 2001;Hwang and Druffel 2003;Ohkouchi et al 2003). Alkenones in sediments co-occurring with foraminifera were investigated by (Mollenhauer et al 2003(Mollenhauer et al , 2005a. Discussion of carbon remineralization effects on sediments and transport of terrestrial carbon to the oceans can be found in Blair et al (2003), Blair (2004 2006), and Drenzek et al (2007).…”

Late Pliocene changes in the North Atlantic Current

“…The input of allochthonous alkenones likely did not significantly affect our biomarker records from the mid-latitude North Atlantic since this is more important in the higher latitudes such as the Nordic Seas (Bendle et al, 2005) or at sites located on or close to continental margins (e.g., Mollenhauer et al, 2005).…”