Plant-wax, alkenone and major element analyses of ODP Site 659 for the Last Glacial cycle and associated coretop data

We present a hydrologic reconstruction of the Sahara-Sahel transition, covering the complete last glacial cycle (130 ka), based on a combination of plant-wax-specific hydrogen (dD) and carbon isotopes (d13C). The dD and d13C signatures of long-chain n-alkanes from ODP Site 659 off NW Africa reveal a significant anti-correlation. Complementary to published pollen data, we infer that this plant-wax signal reflects sensitive responses of the vegetation cover to precipitation changes in the Sahel region, as well as varying contributions from biomes north of the Sahara (C3 domain) by North-East Trade Winds (NETW). During arid phases, especially the northern parts of the Sahel likely experienced crucial water stress, which resulted in a pronounced contraction of the vegetation cover, thus reducing the amount of C4 plant waxes from the region. The increase in NETW strength during dry periods further promoted a more pronounced C3-plant-wax signal derived from the North African C3 plant domain. During humid periods, the C4-dominated Sahelian environments spread northward into the Saharan realm, in association with lower NETW inputs of C3 plant waxes. Arid-humid cycles deduced from plant-wax dD are in accordance with concomitant changes in weathering intensity reflected in varying major element distributions. Environmental shifts are generally linked to periods with large fluctuations in Northern Hemisphere summer insolation. During Marine Isotope Stages 2 and 3, when insolation variability was low, coupling of the hydrologic regime to alkenone-based estimates of NE Atlantic sea-surface temperatures becomes apparent.

Magnetic properties of ODP Site 175-1075

We present sediment magnetic and chemical analysis of cyclic ocean sediments of the upwelling region of the Lower Congo Basin (equatorial Atlantic). We investigated two >100-k.y. intervals from Ocean Drilling Program Site 1075 to analyze the hysteresis properties, sources of magnetic susceptibility, anhysteretic remanent magnetizations, thermomagnetic behavior, and element concentrations of Fe, Ca, Ti, Mn, and K using an X-ray fluorescence (XRF) core scanner. The upper interval was sampled between 14 and 32 meters composite depth (mcd; 0.09-0.21 Ma) and the lower between 141 and 163 mcd (1.31-1.54 Ma) at a resolution of 20 cm, which represents a temporal resolution of 2.0 and 1.3 k.y., respectively. XRF core-scanner data were acquired at 5-cm intervals. The measurements show that ferri(o)magnetic minerals have no significant influence on the cyclicity of the magnetic susceptibility, which is dominated by paramagnetic and diamagnetic minerals and reflects changes of sediment input from the Congo River. The Fe, Ti, K, and Mn concentrations covary with the magnetic susceptibility where high concentrations of these elements correlate with intervals of high susceptibility and low concentrations with intervals of low susceptibility. The Ca counts correlate well with the calcium carbonate concentration but do not show the same cyclicity as the other elements or the susceptibility. With the exception of the Ca concentration, which is significantly higher in the upper interval, and the magnetic grain size, which indicates that less fine grained magnetite is present in the lower interval, no significant differences in the properties of the upper and the lower intervals were detected.