Beth Christensen

Accumulation of organic and inorganic carbon in Pliocene–Pleistocene sediments along the SW African margin

The Ocean Drilling Program Leg 175 recovered a unique series of stratigraphically continuous sedimentary sections along the SW African margin, an area which is presently affected by active coastal upwelling. The accumulation rates of organic and inorganic carbon are a major component of this record. Four Leg 175 sites (1082, 1084, 1085, 1087) are chosen as part of a latitudinal transect from the present northern to southern boundaries of the Benguela Current upwelling system, to decipher the Pliocene–Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate. The pattern of CaCO3 and Corg mass accumulation rates (MARs) over 0.25-Myr intervals indicates that the evolution of carbon burial is highly variable between the northern and the southern Benguela regions, as well as between sites that have similar hydrological conditions. This, as well as the presence over most locations of high-amplitude, rapid changes of carbon burial, reflect the partitioning of biogenic production and patterns of sedimentation into local compartments over the Benguela margin. The combined mapping of CaCO3 and Corg MARs at the study locations suggests four distinct evolutionary periods, which are essentially linked with major steps in global climate change: the early Pliocene, the mid-Pliocene warm event, a late Pliocene intensification of northern hemisphere glaciation and the Pleistocene. The early Pliocene spatially heterogeneous patterns of carbon burial are thought to reflect the occurrence of mass-gravitational movements over the Benguela slope which resulted in disruption of the recorded biogenic production. This was followed (3.5–3 Ma) by an episode of peak carbonate accumulation over the whole margin and, subsequently, by the onset of Benguela provincialism into a northern and a southern sedimentary regime near 2 Ma. This mid and late Pliocene evolution is interpreted as a direct response to changes in the ventilation of bottom and intermediate waters, as well as to dynamics of the subtropical gyral circulation and associated wind stress.

Paleoclimatic changes in southern Africa during the intensification of Northern Hemisphere glaciation: evidence from ODP Leg 175 Site 1085

Abstract Pliocene age sediments from Ocean Drilling Program Leg 175, Site 1085-A and B in the Cape Basin were analyzed to investigate the impact of the intensification of Northern Hemisphere glaciation (INHG) on the South Atlantic Benguela Current system from 4 to 2 Ma. Proxies for productivity (concentrations and mass accumulation rates of total organic carbon, carbon to nitrogen ratios, percent calcium carbonate, and percent biogenic silica) as well as weight percent sand (a proxy for preservation or winnowing) peak at 3.2, 3.0, 2.4, and 2.25 Ma. Normative calculations of allied trace and major elemental determinations indicate synchronous increases in productivity peaks, as well as high concentrations and accumulations of terrigenous sediments. Coeval increases in hemipelagic sedimentation and productivity indicators could be the result of enhanced eolian sedimentation resulting from strengthened winds, leading to elevated rates of upwelling and enhanced productivity. However, rapid burial, as indicated by high sedimentation rates, could also enhance preservation. The very high concentrations (>30%) and accumulations (up to 60 g/cm 2 /kyr) limit the likelihood that eolian sedimentation was the only transport mechanism, invoking an additional fluvial source. Rapid burial by either eolian or fluvial transport links these intervals of enhanced preservation and productivity with continental climate changes resulting from (1) increased winds and/or dust availability due to higher aridity in the Namibia/northern South Africa region; (2) lowered sea-level related to increased ice volume; (3) increased sediment load due to wetter conditions in the continental interior; or (4) some combination. Peaks at 3.2, 2.4 and 2.25 Ma are coincident with maximum precession, suggesting a link between hemipelagic sedimentation and enhanced monsoonal circulation over southern Africa. The Site 1085 sedimentary record during the INHG seems to be controlled by low-latitude processes linked to precession rather than hig-latitude processes.

Chirp seismic reflection data on the New Jersey middle and outer shelf: The geologic response to 40,000 years of sea level change

Chirp seismic reflection data have been collected extensively on the New Jersey continental shelf in support of Office of Naval Research (ONR)‐sponsored research. This composite data set includes surveys in 2001 and 2002 for the Geoclutter program, and in 2006 for the Shallow Water 06 program. Chirp uses a swept‐frequency source (1–15 kHz), matched filtered to provide decimeter vertical accuracy and penetration up to 30 m in coarse‐grained sediments. These data have been used to map out the complex shallow stratigraphy of this region, which includes: (1) a regional reflector formed about 40,000 years ago; (2) a layered shelf‐edge wedge deposited during sea level fall; (3) fluvial channels formed at lowstand, and later filled with an estuarine sequence during sea level rise; (4) a ravinement surface created by shoreface erosion, which forms a lag of coarse material; and (5) the Holocene sand sheet, formed into sand ridges up to 10 m thick. Modern erosion has exposed many of the older units at or near the seafloor. Seabed sediments along acoustic propagation pathways are therefore highly heterogeneous. Vibracoring is planned for August, 2007. Analysis will include geoacoustic logging and geologic characterization. Preliminary results from these cores will also be presented.

Pliocene export production and terrigenous provenance of the Southern Cape Basin, southwest African margin

Abstract We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola–Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being ‘basaltic’ in composition and the other similar to an ‘average shale’. The basaltic end-member comprises approximately 10–15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.

Earth Core: Enhancing Delivery of Geoscience Content in a Diverse School System During Times of Changing State Standards

The Earth Core program provided a one week earth science content workshop to urban public school sixth grade teachers who lacked geoscience training with the goal of increasing participants pedagogical content knowledge (PCK) and confidence in teaching earth science. The workshop evolved from a concern that teachers would not have the training or experience required to engage students with earth science material due to new state standards switching geoscience instruction from 8th to 6th grade. The program, aligned to Cochrans (1992) five suggestions for enhancing teachers PCK, was designed to encourage participants to reflect by sharing teaching ideas, discuss what we found to be often naïve earth science conceptions, share lesson plan ideas, and conduct action research. Field trips facilitated a bonding experience that encouraged collaboration by participants, and interactive lesson activities helped participants recognize their limited knowledge of earth science. Both qualitative and quantitative data collection methods were used in a mixed methods approach to this study. Participant confidence in teaching earth science was slightly elevated after the workshop. The study suggested that teachers realized how little earth science understanding they have after being confronted with advanced level geoscience concepts.