Alkenone analyses of sediment cores from the western Arabian Sea

Records of total organic carbon (TOC) and C37 alkenones were used as indicators for past primary productivity in the western and eastern Arabian Sea. Data from GeoB 3005, an open ocean site in the western Arabian Sea upwelling area, are compared with similar records of GeoB 3007 from the Owen Ridge, Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the eastern Arabian Sea. TOC/C37 alkenone records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing all over the Arabian Sea. Based on their phase-relationship to variations in boreal summer insolation they can be divided into three groups. In the western Arabian Sea the precession-related phasing is different between productivity proxies and those for summer monsoon wind strength and upwelling intensity. TOC and C37 alkenone records from the western Arabian Sea lag the other monsoonal indicators by about 5 kyr, but lead productivity indicators from the eastern Arabian Sea by 3 kyr. Based on the differences in phase relationships associated with the precessional cycling between productivity and monsoonal proxies in the western Arabian Sea it is proposed that the TOC/C37 alkenone signal in the western Arabian Sea document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds. In the eastern Arabian Sea the phasing hints to coincidence between maximum productivity and stronger NE monsoon winds associated with precession-related maxima in ice volume. In contrast, variations in paleoproductivity at site GeoB 3007 from the Owen Ridge indicate productivity maxima during glacial substages 8.2, 6.2 and 2.2, whereas precessionrelated changes are of only minor importance at this location. The results of frequency analyses confirm that productivity at site GeoB 3007 responds predominantly to glacialinterglacial climate changes, while site GeoB 3005 from the open ocean upwelling region near the Gulf of Aden is dominated by precessional insolation. A possible explanation for the pattern revealed at the Owen Ridge is the periodic NW-SE displacement of the Findlater Jet axis, which separates the region of open ocean upwelling to the northwest from downwelling to the southeast ofthe jet. The carbon isotopes of planktic foraminifera reflect nutrient related d13C variations of dissolved inorganic carbon. The difference between the planktic foraminifera Globigerinoides ruber (w), living in the upper 50 m of the water column, and the deeper Iiving Neogloboquadrina dutertrei (Delta d13Cr-d) of core GeoB 3005 displays nutrient variations in the upwelling area near the Gulf of Aden. The results of cross-spectral analyses between Deltad13Cr-d of GeoB 3005 and proxies for SW monsoon intensity indicate, too, a dissociation of productivity from monsoonal upwelling intensity. Instead, productivity depends mainly on the availability of nutrients, while upwelling intensity of sub-surface water masses seems to be of only secondary importance. Additionally, sea surface temperatures (SSTs) were reconstructed using the unsaturation ratio of C37 alkenones. Alkenone SSTs reflect annual mean temperatures rather than explicitly the season of upwelling. This is evident from alkenone SSTs in a transect of surface sediments extending from the inner Gulf of Aden into the western Arabian Sea. The alkenone-derived SST records of GeoB 3005 from the open ocean upwelling region near the Gulf of Aden and GeoB 3007 from the Owen Ridge reveal similar variations with high SSTs during interglacial and low SSTs during glacial periods. The glacial marine oxygen isotope stage (MIS) 6 remains relatively warm and was not as cold as MIS 3 to 4 and 8 according to the alkenone SST. Similar variation-patterns were reconstructed in the coastal upwelling area off Oman for ODP Site 723 as weIl as in the eastern Arabian Sea for MD 900963, where upwelling is not as pronounced as in the western Arabian Sea. Spectral-analyses indicate that SST changes are in good agreement with the modulation of low-latitude precessional insolation changes by eccentricity. However, they do not show the pronounced cydicity in the precessional frequency band, which is characteristic for variations in paleoproductivity. Although the overall variation pattern is very similar, a dose comparison between the western (GeoB 3005) and the eastern Arabian Sea (MD 900963) shows larger differences between both sites during cold intervals than during periods of warm SSTs. This is attributed to a more effective cooling of surface waters in the western Arabian Sea by prolonged NE monsoon winds during times of expanded Northern Hemisphere ice-sheets, thereby lowering the annual mean SSTs stronger than in the eastern Arabian Sea.

(Table 1) Results of alkenone analyses of ODP Hole 165-1002C

We determined alkenone concentrations (µg/g dry sediment) and unsaturation indices (Uk'37) on 280 samples from Ocean Drilling Program Hole 1002C over the last full glacial cycle (marine oxygen isotope Stages [MIS] 1-6). Alkenone concentrations vary dramatically in relation to glacial-interglacial cycles, with high concentrations typical of interglacial stages, high sea level, inferred high surface productivity, and bottom-water anoxia. Our reconstruction of low productivity during the last glacial maximum is consistent with previous reports of a sharp decline in the foraminiferal species Neogloboquadrina dutertrei, an upwelling index. Alkenone paleotemperatures show little cooling at both the last glacial maximum and MIS 6. Variations of as much as 4°C occurred during the earlier part of MIS 3 and MIS 4 as well as the latter part of MIS 5. The absence of cooling during glacial maxima determined from alkenone paleothermometry is consistent with faunal reconstructions for the western Caribbean but requires that much of the oxygen isotopic record of the planktonic foraminifer Globigerinoides ruber be influenced by salinity variations rather than temperature.

Alkenone data of sediment traps off Cape Blanc, NW Africa

We analysed long-chain alkenones in sinking particles and surface sediments from the filamentous upwelling region off Cape Blanc, NW Africa, to evaluate the transfer of surface water signals into the geological record. Our study is based on time-series sediment trap records from 730 m (1990-1991) to 2195-3562 m depth (1988-1991). Alkenone fluxes showed considerable interannual variations and no consistent seasonality. The average flux of C37 and C38 alkenones to the deep traps was 1.9 µg/m**2/d from March 1988 to October 1990 and sevenfold higher in the subsequent year. Alkenone fluxes to the shallower traps were on average twice as high and showed similar temporal variations. The alkenone unsaturation indices UK'37, UK38Me and UK38Et closely mirrored the seasonal variations in sea-surface temperature (weekly Reynolds SST). Time lags of 10-48 days between the SST and unsaturation maxima suggest particle sinking rates of about 80 and 280 m/d for the periods of low and high alkenone fluxes, respectively. The average flux-weighted UK'37 temperature for the 4-year time series of the deeper traps was 22.1°C, in perfect agreement with the mean weekly SST for the same period. This and the comparison with seasonal temperature variations in the upper 100 m of the water column suggests that UK'37 records principally the yearly average of the mixed-layer temperature in this region. A comparison between the average annual alkenone fluxes to the lower traps (2400 µg/m**2/yr) and into the underlying sediments (4 µg/m**2/yr) suggests that only about 0.2% of the alkenones reaching the deep ocean became preserved in the sediments. The flux-weighted alkenone concentrations also decreased considerably, from 2466 µg/gC in the water column to 62 µg/gC in the surface sediments. Such a low degree of alkenone preservation is typical for slowly accumulating oxygenated sediments. Despite these dramatic diagenetic alkenone losses, the UK'37 ratio was not affected. The average UK'37 value of the sediments (0.796±0.010 or 22.3±0.3°C) was identical within error limits to the 4-year average of the lower traps. The unsaturation indices for C38 alkenones and the ratio between C37 and C38 alkenones also revealed a high degree of stability. Our results do not support the hypothesis that UK'37 is biased towards higher values during oxic diagenesis.

Alkenone and foraminifera records from sediment samples off Namibia

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.

Alkenone unsaturation index and alkenones in ODP Hole 161-977A

A number of essential elements closely related to each other are involved in the Earth's climatic system. The temporal and spatial distribution of insolation determines wind patterns and the ocean's thermohaline pump. In turn, these last two are directly linked to the extension and retreat of marine and continental ice and to the chemistry of the atmosphere and the ocean. The variability of these elements may trigger, amplify, sustain or globalize rapid climatic changes. Paleoclimatic oscillations have been identified in this thesis by using fossil organic compounds synthesized by marine and terrestrial flora. High sedimentation rate deposits at the Barents and the Iberian peninsula continental margins were chosen in order to estimate the climatic changes on centennial time resolution. At the Barents margin, the sediment recovered was up to 15,000 years old (unit ''a'', from latin ''annos'') (M23258; west of the Bjørnøya island). At the Iberian margin, the sediment cores studied covered a wide range of time spans: up to 115,000 a (MD99-2343; north of the Minorca island), up to 250,000 a (ODP-977A; Alboran basin) and up to 420,000 a (MD01-2442, MD01-2443, MD01-2444, MD01-2445; close to the Tagus abyssal plain). At the northern site, inputs containing marine, continental and ancient reworked organic matter provided a detailed reconstruction of climate history at the time of the final retreat of the Barents ice sheet. At the western Barents continental slope, warm climatic conditions were observed during the early Holocene (~from 8,650 a to 5,240 a ago); in contrast, an apparent long-term cooling trend occurred in the late Holocene (~from 5,240 a to 760 a ago), in consistence with other paleoarchives from northern and southern European latitudes. The Iberian margin sites, which were never covered with large ice sheets, preserved exceptionally complete sequences of rapid events during ice ages hitherto not studied in such great detail: during the last glacial (~from 70,900 a to 11,800 a ago), the second glacial (~from 189,300 a to 127,500 a ago), the third ice age (~from 278,600 a to 244,800 a ago) and the fourth (~from 376,300 a to 337,500 a ago). In this thesis, crucial research questions were brought up concerning the severity of different glacial periods, the intensity and rates of the recorded oscillations and the long distance connections related to rapid climate change. The data obtained provide a sound basis to further research on the mechanisms involved in this rapid climate variability. An essential point of the research was the evidence that, over the past 420,000 a, at the whole Iberian margin, warm and stable long periods similar to the Holocene always ended abruptly in few centuries after a gradual deterioration of climate conditions. The detailed estimate of past climate variability provides clues to the natural end of the present warm period. Returning to an ice age in European lands would be exacerbated by a number of factors: a lack of differential solar heating between northern and southern north Atlantic latitudes, enhanced evaporation at low latitudes, and an increase in snowfall or iceberg discharges at northern regions. It must be emphasized that all climatic oscillations observed in this thesis were caused by forces of nature, i.e. the last two centuries were not taken into consideration.

(Table 1) Alkenone concentration, UK'37 index and derived sea surface temperatures for ODP Site 167-1020

Uk'37 sea-surface temperature (SST) estimates obtained at ~2.5-k.y. resolution from Ocean Drilling Program Site 1020 show glacial-interglacial cyclicity with an amplitude of 7°-10°C over the last 780 k.y. This record shows a similar pattern of variability to another alkenone-based SST record obtained previously from the Santa Barbara Basin. Both records show that oxygen isotope Stage (OIS) 5.5 was warmer by ~3°C relative to the present and that glacial Uk'37 temperatures warm in advance of deglaciation, as inferred from benthic d18O records. The alkenone-based SST record at Site 1020 is longer than previously published work along the California margin. We show that warmer than present interglacial stages have occurred frequently during the last 800 k.y. Alkenone concentrations, a proxy for coccolithophorid productivity, indicate that sedimentary marine organic carbon content has also varied significantly over this interval, with higher contents during interglacial periods. A baseline shift to warmer SST and greater alkenone content occurs before OIS 13. We compare our results with those from previous multiproxy studies in this region and conclude that SST has increased by ~5°C since the last glacial period (21 ka). Our data show that maximum alkenone SSTs occur simultaneously with minimum ice volume at Site 1020, which is consistent with data from farther south along the margin. The presence of sea ice in the glacial northeast Pacific, the extent of which is inferred from locations of ice-rafted debris, provides further support for our notion of cold surface water within the northern California Current system, averaging 7°-8°C cooler during peak glacial conditions. The cooling of surface water during glacial stages most likely did not result from enhanced upwelling because alkenone concentrations and terrestrial redwood pollen assemblages are consistently lower during glacial periods.

Ice rafted debris, tetra-unsaturated alkenone and reconstructed sea surface temperature in sediment of ODP Hole 104-642B

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