Alkenone Flux Research Articles

Precessional forcing of productivity in the North Atlantic Ocean

The sedimentary C37 alkenone fluxes in core SU90/08 situated in the North Atlantic (43°N) showed that export productivity related to some Haptophyta was driven by precession (1/23 kyr−1) in the last 260 kyr during the glacial periods. In the interglacial stages these alkenones were strongly depleted. This different behaviour is attributed to the displacement of Ekman pumping to lower latitudes and to thermocline stability decreases in the glacial periods. The alkenones were coherent and nearly phased with boreal winter insolation, suggesting a link between productivity and the strength of the westerlies. They were also coherent with the precessionally driven nutriocline minima and winter sea surface temperature in the equatorial Atlantic Ocean. The combination of the measurements at these different latitudes point to a strong coupling in the dynamics of the Hadley and Ferrel cells and to a dominant North Atlantic modulation of marine productivity to precession during the glacial stages.

Seasonal variability of the long-chain alkenone flux and the effect on the U 37k′-index in the Norwegian Sea

A seasonally-varying sedimentation pattern was observed for the alkenone flux measured with sediment traps in the northern North Atlantic. In the Norwegian Sea (traps were set at 500, 1000 and 3000 m) the alkenone flux varied between 0.1 and 7.1 μg m −2 d −1 and followed the seasonal pattern of the bulk parameters. Maximum fluxes occurred from mid-October until mid-November and were also high in May. A surprising result was that considerably higher particle fluxes were observed at 3000 m. For the alkenone flux, the highest additional input of 250% was observed during the period when sediment resuspension was greatest in summer. At the Barents Sea continental margin (traps at 1840 and 1950 m) the alkenone fluxes follow the sedimentation pattern of the bulk parameters, with a less visible signal of distinct seasonality observed in the 1950 m trap. The sedimentation of total alkenones varied between 0.8 and 144 μg m −2 d −1 at 1840 m and between 0.5 and 31.0 μg m −2 d −1 at 1950 m. Resuspension and lateral advection contributed significantly to measured fluxes in the two near-bottom traps. Alkenone concentrations were determined in faecal pellets of Appendicularia, ostracods and euphausids from selected samples at the Barents Sea site. The alkenone flux in pellets (4% to 24% of total) was 5 to 6 times higher at 1950 m depth than at 1840 m and the major part (77–78%) of the total flux of C 37:3 reaching the near-bottom trap at 1950 m was associated with faecal pellets of the meso-zooplankton. Spatial and temporal variations of the U 37 k′ signals were observed, indicating that the imprint in the alkenone signal depends on the origin and transport pathway of the organic material. Strong deviations occur in areas where nepheloid layers contribute particles of long residence times to the primary flux.

Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea

Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles. Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somalia and Oman, which are characterized by large seasonal sea surface temperature (SST) and particle flux changes. By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2°C). For both cores we have reconstructed SST variations by means of the unsaturation ratio of C 37 alkenones, which is compared with the δ 18O records established on planktonic foraminifera. In general, the SST records follow the δ 18O variations, with an SST maximum during oxygen isotope stage 5.5 (the Last Interglacial at about 120–130 kyr) and a broad SST minimum during isotope stage 4 and 3.3 (approximately 40–50 kyr). The SST difference between the Holocene and the Last Glacial Maximum (LGM) is of the order of 2°C. In both cores the SSTs during isotope stage 6 are distinctly higher by 1–2°C than the cold SST minima during the last glacial cycle (LGM and stage 3). To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, together with a productivity index based on coccolith species ( Florisphaera profunda relative abundance). Within each core, there is a general agreement between the different palaeoproductivity proxies. In the southeastern Arabian Sea (core MD900963), glacial stages correspond to relatively high productivity, whereas warm interstadials coincide with low productivity. All time series of productivity proxies are dominated by a cyclicity of about 21–23 kyr, which corresponds to the insolation precessional cycle. A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone. By contrast, the records are more complicated in the upwelling region of the western Arabian Sea (core TY93-929/P). This is partly due to large changes in the sedimentation rates, which were higher during specific periods (isotope stages 6, 5.4, 5.2, 3 and 2). Unlike core MD900963, no simple relationship emerges from the comparison between the δ 18O stratigraphy and productivity records. The greater complexity observed for core TY93-929/P could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons, the SW monsoon being enhanced during interglacial periods, whereas the NE monsoon was increased during glacial intervals. A similar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics.

Production pattern of alkenones in the Mediterranean Sea

Time‐series of downward alkenone fluxes have been investigated at 200 m depth over a one year sediment trap experiment, in the Northwestern Mediterranean Sea. Alkenone flux maxima occurred in autumn and to a lesser extent in May, during the spring bloom. Temperature estimates calculated from the UK′37 index revealed that alkenone producers preferentially develop in subsurface waters (at about 50 m) in spring, whereas the autumn alkenone production occurred upper in the water column (around 30 m). Examination of the core‐top UK′37 index values at various sites of the Northwestern Mediterranean basin, suggested that the spring bloom period do not significantly imprint the temperatures recorded in the sediments. The sedimentary temperature estimates would rather reflect annually integrated SST, with a major influence of the autumnal post‐bloom development of the coccolithophores in the euphotic zone.

Response of South Atlantic Surface Circulation to African Monsoon Variability: Evidence from Late Quaternary Records of Geochemical Tracers for Sea-Surface Temperatures and Productivity

Paleoclimate reconstructions and results from atmospheric general circulation models have revealed that during the Late Quaternary the zonal strength of trade winds over the South Atlantic has significantly responded to precessional variations of boreal summer insolation because zonality of trades is tightly coupled to African Monsoon intensity. The most prominent indicator for this trade-wind response to precessional forcing via the monsoon system is the corresponding predominance of periodic 19 to 23kyr variations between paleorecords of proxies for continental climate and those for sea-surface temperatures (SST) and biological productivity in equatorial Atlantic surface waters (Pokras & Mix, 1985; Prell & Kutzbach, 1987; Mclntyre et al. 1989; De Menocal et al. 1993). In order to reconstruct Late Quaternary surface circulation in the eastern South Atlantic (10~ to 10~ 0 to 20~ we evaluated SST and paleoproductivity changes for the last 200,000 years. For this purpose records of alkenone, organic carbon and opal fluxes, as well as time series of planktonic foraminifera and organic carbon 613C were produced from Guinea and Angola Basin and Walvis Ridge sediments (Schneider 1991; Miiller et aL, 1994; Schneider et al., 1993; Schneider et al., 1994). Comparable to the results from the equatorial Atlantic mentioned before, all these records of geochemical tracers for productivity and SST are characterized by a significant 23-kyr periodicity which is coherent to precessional forcing. But phase relationships of periodic 23 kyr variations in SST and productivity estimates from the eastern South Atlantic with respect to boreal summer insolation changes document that surface waters north and south of about 15~ have responded in different way to precessional insolation changes. Generally, periods of warm SST's and low productivity cooccured with intervals of increased North African insolation, but there is a distinct phase lag of maximum SST and minimum productivity between the two areas. While periodic SST and paleoproductivity changes related to the 23 kyr frequency band from the central equatorial divergence zone and from the Walvis Ridge are in phase with changes in July insolation at 10~ those from the African continental margin in the Angola Basin lag insolation changes by about 4 kyr and are in fairly phase with SST changes in the Cape Basin. Since the equatorial upwelling and the Walvis Ridge area are located beneath the centre of the modern zonal trade-wind field, we suggest from the phase pattern described above that surface water oceanography under the field of northwestdirected trades was predominantly driven by monsoonal modulation of their zonal intensity. Low productivity and warm surface waters within this zone are in phase with July insolation maxima which indicates direct reponse to orbital forcing and maximum monsoon strength at periods of minimum Earth-Sun distance during boreal summer. The 4 kyr lag of precessional SST and paleoproductivity changes in the eastern Angola Basin suggests that this part of the surface waters in the South Atlantic was not directly effected by trade-wind variations and that the zonal-directed trade-wind zone in the southern hemisphere did not migrate northward by several degrees of latidude during glacials. Instead, the close correspondence of Angola Basin SST and productivity phasing with that from the Cape Basin SST's can be taken as evidence that surface water oceanography in the most eastern part of the South Atlantic was additionally influenced by periodic 23 kyr changes in the advection of cold surface waters from the Southern Ocean.