Chronologies for Late Quaternary marine sediment records are usually based on radiocarbon ages of planktonic foraminifera. Signals carried by other sedimentary components measured in parallel can provide complementary paleoclimate information. A key premise is that microfossils and other indicators within a given sediment horizon are of equal age. We show here that haptophyte-derived alkenones isolated from Bermuda Rise drift sediments are up to 7000 years older than coexisting planktonic foraminifera. This temporal offset, which is apparently due to lateral transport of alkenones on fine-grained particles from the Nova Scotian margin, markedly influences molecular estimates of sea surface temperatures. More broadly, the observation raises questions about both the temporal and the geographic fidelity of paleoenvironmental records encoded by readily transported components of sediments.

[1] The stable carbon isotopic compositions of alkenones have been used to interpret the long-term history of the partial pressure of atmospheric carbon dioxide ( pCO 2 ). Although extensive water column and culture studies document the potential utility and limitations of this approach, to date the accuracy of pCO 2 values derived from sedimentary alkenones remains untested. For this study we establish Holocene-aged, alkenonebased CO 2aq estimates ([CO 2aq ] alk ) from 20 sites along a central Pacific Ocean transect and compare them against both observed modern water column CO 2aq and estimated preindustrial concentrations at the depth of alkenone production at each site. Although the [CO 2aq ] alk track measured water column values, they are conspicuously lower than modern values across the subtropics. This offset likely reflects the contributions of anthropogenic CO 2 in modern surface waters relative to preindustrial concentrations at the time of alkenone production. When a model-based estimate of anthropogenic CO 2 is removed from the modern observed values, a majority (84%) of [CO 2aq ] alk falls within 20% of modeled preindustrial values. Consistency between the modeled and alkenone-based estimates of preindustrial CO 2 levels points to the relative accuracy of the alkenone-CO 2 method across a wide range of ocean and biogeographic regimes, provided that phosphate concentrations, at the depth of haptophyte production, are reasonably constrained. It further suggests that lightlimited growth and/or active carbon uptake, if they occur, have a negligible effect on reconstructed [CO 2aq ].

Abstract. We report analytical results of long-chain (C37-C39) alkenones in the surface sediments taken along a latitudinal transect at 175øE from 48øN to 15øS. Unsaturation degree of C37 alkenones (U•37) indicated that alkenone temperature is the highest (about 28.3øC) in the tropical Pacific and decreases toward 48øN (about 10.1øC). 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 (35ø-19øN). 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 (48ø-40øN) and low (10øN-2øS) 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 155øW transect in the central Pacific. The latitudinal distribution of the alkenone production layers estimated from U•37 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 43ø-27øN with a maximum at 30øN, which may be explained by high productivity of haptophyte algae in Kuroshio Extension.

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[1] Abstract: The alkenone unsaturation index, U 37 K 0 , has now proven its worth as a tool for paleothermometry. U 37 K 0 measured in most modern sediments throughout the World Ocean can be translated using established calibrations into realistic, seemingly reliable estimates of mean annual temperature at the sea surface (SST). However, it remains mysterious why water temperature estimates based on this biotic index correspond to``mean annual'' SST and whether such estimates apply back in geological time. Solving these mysteries is imperative and will require thoughtful, concerted research effort by the biological, chemical, and geological oceanographic community. This report summarizes what is now known about alkenones, in particular, their use in estimating the growth temperature of specific haptophyte algae that synthesize and export these compounds to the marine sediment record. It highlights future field and laboratory research directions that should be taken to clarify and bolster utility of alkenones as a generally valued paleoceanographic tool.