Stable isotope and Mg/Ca record of IODP Site 339-U1385

Although millennial-scale climate variability (<10 ka) has been well studied during the last glacial cycles, little is known about this important aspect of climate in the early Pleistocene, prior to the Middle Pleistocene Transition. Here we present an early Pleistocene climate record at centennial resolution for two representative glacials (marine isotope stages (MIS) 37-41 from approximately 1235 to 1320 ka) during the "41 ka world" at Integrated Ocean Drilling Program Site U1385 (the "Shackleton Site") on the southwest Iberian margin. Millennial-scale climate variability was suppressed during interglacial periods (MIS 37, MIS 39, and MIS 41) and activated during glacial inceptions when benthic d18O exceeded 3.2 per mil. Millennial variability during glacials MIS 38 and MIS 40 closely resembled Dansgaard-Oeschger events from the last glacial (MIS 3) in amplitude, shape, and pacing. The phasing of oxygen and carbon isotope variability is consistent with an active oceanic thermal bipolar see-saw between the Northern and Southern Hemispheres during most of the prominent stadials. Surface cooling was associated with systematic decreases in benthic carbon isotopes, indicating concomitant changes in the meridional overturning circulation. A comparison to other North Atlantic records of ice rafting during the early Pleistocene suggests that freshwater forcing, as proposed for the late Pleistocene, was involved in triggering or amplifying perturbations of the North Atlantic circulation that elicited a bipolar see-saw response. Our findings support similarities in the operation of the climate system occurring on millennial time scales before and after the Middle Pleistocene Transition despite the increases in global ice volume and duration of the glacial cycles.

Model simulation output data (CCSM3) related to uplift of Africa and Himalaya/Tibet Plateau and the influence on tropical African vegetation

Hominid evolution in the late Miocene has long been hypothesized to be linked to the retreat of the tropical rainforest in Africa. One cause for the climatic and vegetation change often considered was uplift of Africa, but also uplift of the Himalaya and the Tibetan Plateau was suggested to have impacted rainfall distribution over Africa. Recent proxy data suggest that in East Africa open grassland habitats were available to the common ancestors of hominins and apes long before their divergence and do not find evidence for a closed rainforest in the late Miocene. We used the coupled global general circulation model CCSM3 including an interactively coupled dynamic vegetation module to investigate the impact of topography on African hydro-climate and vegetation. We performed sensitivity experiments altering elevations of the Himalaya and the Tibetan Plateau as well as of East and Southern Africa. The simulations confirm the dominant impact of African topography for climate and vegetation development of the African tropics. Only a weak influence of prescribed Asian uplift on African climate could be detected. The model simulations show that rainforest coverage of Central Africa is strongly determined by the presence of elevated African topography. In East Africa, despite wetter conditions with lowered African topography, the conditions were not favorable enough to maintain a closed rainforest. A discussion of the results with respect to other model studies indicates a minor importance of vegetation-atmosphere or ocean-atmosphere feedbacks and a large dependence of the simulated vegetation response on the land surface/vegetation model.

Pb and Ba concentrations and Pb isotope ratios in EPICA Dome C ice 2-217 ky BP

A record of Pb isotopic compositions and Pb and Ba concentrations are presented for the EPICA Dome C ice core covering the past 220 ky, indicating the characteristics of dust and volcanic Pb deposition in central East Antarctica. Lead isotopic compositions are also reported in a suite of soil and loess samples from the Southern Hemisphere (Australia, Southern Africa, Southern South America, New Zealand, Antarctica) in order to evaluate the provenance of dust present in Antarctic ice. Lead isotopic compositions in Dome C ice support the contention that Southern South America was an important source of dust in Antarctica during the last two glacial maxima, and furthermore suggest occasional dust contributions from local Antarctic sources. The isotopic signature of Pb in Antarctic ice is altered by the presence of volcanic Pb, inhibiting the evaluation of glacial-interglacial changes in dust sources and the evaluation of Australia as a source of dust to Antarctica. Consequently, an accurate evaluation of the predominant source(s) of Antarctic dust can only be obtained from glacial maxima, when dust-Pb concentrations were greatest. These data confirm that volcanic Pb is present throughout Antarctica and is emitted in a physical phase that is free from Ba, while dust Pb is transported within a matrix containing Ba and other crustal elements.

Geochemistry of tetrapyrrole, carotenoid, and perylene pigments in sediments at DSDP Holes 63-467 and 63-471

Twenty-one core samples from DSDP/IPOD Leg 63 were analyzed for products of chlorophyll diagenesis. In addition to the tetrapyrrole pigments, perylene and carotenoid pigments were isolated and identified. The 16 core samples from the San Miguel Gap site (467) and the five from the Baja California borderland location (471) afforded the unique opportunity of examining tetrapyrrole diagenesis in clay-rich marine sediments that are very high in total organic matter. The chelation reaction, whereby free-base porphyrins give rise to metalloporphyrins (viz., nickel), is well documented within the downhole sequence of sediments from the San Miguel Gap (Site 467). Recognition of unique arrays of highly dealkylated copper and nickel ETIO-porphyrins, exhibiting nearly identical carbonnumber homologies (viz., C-23 to C-30; mode = C-26), enabled subtraction of this component (thought to be derived from an allochthonous source) and thus permitted description of the actual in situ diagenesis of autochthonous chlorophyll derivatives.

Carbonate sedimentation beneath the Carlifornia Current system

A north-south transect of 17 cores was constructed along the eastern boundary of the California Current system from 33° to 42°N to investigate the changes in biogenic sedimentation over the past 30 kyr. Percentages and mass accumulation rates of CaCO3, Corg, and biogenic opal were assembled at 500 to 1000 years/sample to provide relatively high resolution. Time-space maps reveal a complex pattern of changes that do not follow a simple glacial-interglacial two-mode model. Biogenic sedimentation shows responses that are sometimes time-transgressive and sometimes coeval, and most of the responses show more consistency within a limited geographic area than any temporal consistency. Reconstructed conditions during late oxygen isotope stage 3 were more like early Holocene conditions than any other time during the last 30 kyr. Coastal upwelling and productivity during oxygen isotope stage 3 were relatively strong along the central California margin but were weak along the northern California margin. Precipitation increased during the last glacial interval in the central California region, and the waters of the southern California margin had relatively low productivity. Productivity on the southern Oregon margin was relatively low at the beginning of the last glacial interval, but by about 20 ka, productivity in this area significantly increased. This change suggests that the center of the divergence of the West Wind Drift shifted south at this time. The end of the last glacial interval was characterized by increased productivity in the southern California margin and increased upwelling along the central California margin but upwelling remained weak along the northern California margin. A sudden (<300 years) decrease in CaCO3, Corg, and biogenic opal occurred at 13 ka. The changes suggest a major reorientation of the atmospheric circulation in the North Pacific and western North America and the establishment of a strong seasonality in the central California region. A carbonate preservation event occurred at 10 ka that appears to reflect the uptake of CO2 by the terrestrial biosphere as the northern latitudes were reforested following retreat of the glaciers. The Holocene has been a period of relatively high productivity in the southern California margin, relatively strong coastal upwelling along the central California margin, relatively weak upwelling along the northern California margin, and the northward migration of the divergence zone of the West Wind Drift.