Peter J Müller

Calibration of the alkenone paleotemperature index U37K′ based on core-tops from the eastern South Atlantic and the global ocean (60°N-60°S)

Abstract We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K′ paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST’s) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K′ values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K′ and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K′ = 0.033 T + 0.069, r2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K′ reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K′ vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K′ data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0–29°C) from 0 m water depth. The resulting relationship (U37K′ = 0.033 T + 0.044, r2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of Prahl and Wakeham (1987) and Prahl et al. (1988) attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K′ is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K′ in open ocean environments.

Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulationduring the last deglaciation

Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records. Rapid reorganization of ocean circulation is considered to exert some control over these changes, as are shifts in the concentrations of atmospheric greenhouse gases. The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic, whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes. So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900–15,400 calendar years bp) and the Younger Dryas event (12,900–11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

An automated leaching method for the determination of opal in sediments and particulate matter

An automated leaching method for the analysis of biogenic silica (opal) in sediments and particulate matter is described. The opaline material is extracted with 1 M NaOH at 85°C in a stainless steel vessel under constant stirring, and the increase in dissolved silica is continuously monitored. For this purpose, a minor portion of the leaching solution is cycled to an autoanalyzer and analyzed for dissolved silicon by molybdate-blue spectrophotometry. The resulting absorbance versus time plot is then evaluated according to the extrapolation procedure of DeMaster (1981). The method has been tested on sponge spicules, radiolarian tests. Recent and Pliocene diatomaceous ooze samples, clay minerals and quartz, artificial sediment mixtures, and on various plankton, sediment trap and sediment samples. The results show that the relevant forms of biogenic opal in Quaternary sediments are quantitatively recovered. The time required for an analysis is dependent on the sample type, ranging from 10 to 20 min for plankton and sediment trap material and up to 40–60 min for Quaternary sediments. The silica co-extracted from silicate minerals is largely compensated for by the applied extrapolation technique. The remaining degree of uncertainty is on the order of 0.4 wt% SiO2 or less, depending on the clay mineral composition and content.

Late Quaternary surface circulation in the east equatorial South Atlantic: Evidence from Alkenone sea surface temperatures

Angola Basin and Walvis Ridge records of past sea surface temperatures (SST) derived from the alkenone Uk37 index are used to reconstruct the surface circulation in the east equatorial South Atlantic for the last 200,000 years. Comparison of SST estimates from surface sediments between 5° and 20°S with modern SST data suggests that the alkenone temperatures represent annual mean values of the surface mixed layer. Alkenone-derived temperatures for the warm climatic maxima of the Holocene and the penultimate interglacial are 1 to 4°C higher than latest Holocene values. All records show glacial to interglacial differences of about 3.5°C in annual mean SST, which is about 1.5°C greater than the difference estimated by CLIMAP (1981) for the eastern Angola Basin. At the Walvis Ridge, significant SST variance is observed at all of the Earths orbital periodicities. SST records from the Angola Basin vary predominantly at 23- and 100-kyr periodicities. For the precessional cycle, SST changes at the Walvis Ridge correspond to variations of boreal summer insolation over Africa and lead ice volume changes, suggesting that the east equatorial South Atlantic is sensitive to African monsoon intensity via trade-wind zonality. Angola Basin SST records lag those from the Walvis Ridge and the equatorial Atlantic by about 3 kyr. The comparison of Angola Basin and Walvis Ridge SST records implies that the Angola-Benguela Front (ABF) (currently at about 14–16°S) has remained fairly stationary between 12° and 20°S (the limits of our cores) during the last two glacial-interglacial cycles. The temperature contrast associated with the ABF exhibits a periodic 23-kyr variability which is coherent with changes in boreal summer insolation over Africa. These observations suggest that surface waters north of the present ABF have not directly responded to monsoon-modulated changes in the trade-wind vector, that the central field of zonally directed trades in the southern hemisphere was not shifted or extended northward by several degrees of latitude during glacials, and that a cyclonic gyre circulation has existed in the east equatorial South Atlantic over the last 200,000 years. This scenario contradicts former assumptions of glacial intensification of the Benguela Current into the eastern Angola Basin and increased coastal upwelling off Angola.

Monsoon related variations in Zaire (Congo) sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial Atlantic during the last 200,000 years

Two sediment cores from the Zaire Fan and the Angola Margin have been investigated for their composition of terrigenous and biogenic constituents, respectively. For the late Quaternary, kaolinite/feldspar ratios and variations of terrigenous element ratios of Zr, Ti, K, Rb, and Al reveal that the composition of Zaire River sediment load has fluctuated in tune with precessional variations of boreal summer insolation. In particular, the correspondence of high kaolinite/feldspar and Al/K ratios with low-latitude insolation maxima strongly corroborates the assumption that west African monsoonal precipitation and chemical weathering was enhanced during periods of increased central African heating. The most striking feauture observed is that opal accumulation has been 2 to 10 times higher in Zaire Fan sediments than outside in continental margin sediments off Angola, although biogenic Ba and Corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the Zaire River plume and off Angola. From this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the Zaire River plume than in the oceanic upwelling regime farther to the south off Angola over the last 200,000 years. The pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the Zaire River to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions. We presume that paleoproductivity off the Zaire was controlled by the following two sources of nutrients: (1) marine nutrients including nitrate and phosphate as well as the uptake of Ba on particulate Corg caused by upwelling, and (2) silica, mainly delivered by the Zaire River dissolved silicate. Hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical Atlantic at a millenial timescale.

Asynchronous alkenone and foraminifera records from the Benguela Upwelling System

Abstract 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.

Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: A 1500‐kyr record of carbonate dissolution

Appendix Table Al Is available with entire article onmicrofiche. Order from the American Geophysical Union, 2000Florida Avenue, N.W., Washington, D.C. 20009. DocumentP94-001;

Late Quaternary Surface Temperatures and Productivity in the East-Equatorial South Atlantic: Response to Changes in Trade/Monsoon Wind Forcing and Surface Water Advection

2.50. Payment must accompany order. We examined coarse fraction contents of pelagic carbonates deposited between 2000-and 3700-m water depth in the tropical Indian Ocean using Ocean Drilling Program (ODP) sites 722 (Owen Ridge, Arabian Sea) and 758 (Ninetyeast Ridge, eastern equatorial Indian Ocean), and four giant piston cores collected by the French R/V Marion Dufresne during the SEYMAMA expedition. Over the last 1500 kyr, coarse fraction records display high-amplitude oscillations with an irregular wavelength on the order of ∼500 kyr. These oscillations can be correlated throughout the entire equatorial Indian Ocean, from the Seychelles area eastward to the Ninetyeast Ridge, and into the Arabian Sea. Changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation. The well-known “mid-Bruhes dissolution cycle”represents the last part of this irregular long-term dissolution oscillation. The origin of this long-term oscillation is still poorly understood. Our observations suggest that it is not a true cycle (it has an irregular wavelength) and we propose that it may result from long-term changes in Ca++flux to the ocean. Sites 722 and 758 δ18O records provide a high-resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr. Site 722 (2030 m) lies above the present and late Pleistocene lysocline. The lysocline shoaled to the position of site 758 (2925 m) only during the interglacial intervals that occurred between about 300 and 500 ka (Peterson and Prell, 1985a). Despite these supralysoclinal positions of the two sites, short-term changes in coarse fraction contents are correctable from one site to another and probably result from regional (or global) dissolution pulses. By stacking the normalized coarse fraction records from sites 722 and 758, we constructed a Composite Coarse Fraction Index (CCFI) curve in which most of the local signals cancelled out. The last 800 kyr of this curve appear to compare extremely well with the Composite Dissolution Index curve from core V34-53 (Ninetyeast Ridge), which unambiguously records past variations of carbonate dissolution in the equatorial Indian Ocean (Peterson and Prell, 1985a). In the late Pleistocene the CCFI variations are mainly associated with glacial-interglacial changes. They show strong 100 and 41 kyr periodicities but no clear precession-related periodicities. As proposed earlier by Peterson and Prell (1985a), the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation. We cannot totally reject the possibility, however, that low temporal resolution and/or bioturbation degrade somehow the precessional signal at ODP sites 722 and 758. In contrast, spectral density of dissolution cycles in the giant (53 m long) piston core MD900963 (Maldives area) displays clear maxima centered on the precession frequencies (23 and 19 kyr−1) as well as on the kyr−1 frequency but shows little power at the 100- kyr−1 frequency. These high-frequency changes most probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon.

Amino acids and amino sugars of surface particulate and sediment trap material from waters of the Scotia sea

In order to reconstruct Late Quaternary variations of surface oceanography in the east- equatorial South Atlantic, time series of sea-surface temperatures (SST) and paleoproductivity were established from cores recovered in the Guinea and Angola Basins, and at the Walvis Ridge. These records, based on sedimentary alkenone and organic carbon concentrations, reveal that during the last 350,000 years surface circulation and productivity changes in the east-equatorial South Atlantic were highly sensitive to climate forcing at 23- and 100-kyr periodicities. Covarying SST and paleoproductivity changes at the equator and at the Walvis Ridge appear to be driven by variations in zonal trade-wind intensity, which forces intensification or reduction of coastal and equatorial upwelling, as well as enhanced Benguela cold water advection from the South. Phase relationships of precessional variations in the paleoproductivity and SST records from the distinct sites were evaluated with respect to boreal summer insolation over Africa, movements of southern ocean thermal fronts, and changes in global ice volume. The 23-kyr phasing implies a sensitivity of eastern South Atlantic surface water advection and upwelling to West African monsoon intensity and to changes in the position of the subtropical high pressure cell over the South Atlantic, both phenomena which modulate zonal strength of southeasterly trades. SST and productivity changes north of 20°S lack significant variance at the 41-kyr periodicity; and at the Walvis Ridge and the equator lead changes in ice volume. This may indicate that obliquity-driven climate change, characteristic for northern high latitudes, e.g fluctuations in continental ice masses, did not substantially influence subtropical and tropical surface circulation in the South Atlantic. At the 23-kyr cycle SST and productivity changes in the eastern Angola Basin lag those in the equatorial Atlantic and at the Walvis Ridge by about 3500 years. This lag is explained by variations in cross-equatorial surface water transport and west-east countercurrent return flow modifying precessional variations of SST and productivity in the eastern Angola Basin relative to those in the mid South Atlantic area under the central field of zonal trade winds. Sea level-related shifts of upwelling cells in phase with global climate change may be also recorded in SST and productivity variability along the continental margin off Southwest Africa. They may account for the delay of the paleoceanogreaphic signal from continental margin sites with respect to that from the pelagic sites at the equator and the Walvis Ridge.

Late Quaternary PCO2 Variations in the Angola Current: Evidence from Organic Carbon δ13C and Alkenone Temperatures

Abstract Two coarse suspended matter size classes (75–150 μm, >150 μm), from subantarctic and Antarctic surface waters in the Scotia Sea, and sediment trap material from the Drake Passage were analyzed for their elemental, amino acid and amino sugar compositions. Different proportions of biogenic silica and organic matter in the particulates of both regions reflect a zonation of primary producers, with diatoms predominating in the waters south of the Polar Front. High SiO2:Corg ratios, elevated proportions of hydroxyl amino acids, and essentially identical amino acid compositions for both size classes indicate that diatoms account for a major portion of the particulate proteinaceous material from Antarctic surface waters. Of the two amino sugars, glucosamine and galactosamine, only the former was detected in significant amounts in the surface particulates. The total amino acid : glucosamine ratio was lowest in surface particulates of subantarctic waters and increased with increasing latitude in Antarctic waters, reaching the highest values in the region of the Bransfield Strait. Moreover, amino acid : glucosamine ratios suggest day-night differences in particulate matter resulting from primary productivity in conjunction with the feeding behavior of vertical migrators. Significantly different amino acid and amino sugar compositions of the surface particulate matter >75 μm in size and the sediment trap material reflect fractionation processes at shallow depths. Remineralization and digestion of organic matter appear to result in a relative enrichment of structural components (diatom cell walls, chitinaceous matter) in fecal pellets and other large aggregates. The preferential preservation of diatom cell wall material is indicated by a strong relative enrichment of glycine and hydroxyl amino acids in the sediment trap material in conjunction with high biogenic silica:organic carbon ratios. Similarly, low amino acid:glucosamine ratios in the sediment trap material from depths point to a preferential preservation of chitinaceous matter. Slight compositional differences between the material from the upper and lower trap may indicate that bacteria are acting on the rapidly sinking particles. These changes appear to be insignificant, however, when compared to the fractionations occurring at shallow depths.