Major and trace element geochemistry of ODP Leg 178 holes

Geochemical analyses have been performed on sediment samples collected during Ocean Drilling Program Leg 178 from the continental rise and outer continental shelf of the Antarctic Peninsula. A suite of 21 trace elements was measured by neutron activation analysis in 39 sediment samples, and major element oxides were determined in 67 samples by electron microprobe analyses of fused glass beads. These geochemical data, combined with the X-ray diffraction and X-ray fluorescence data from shipboard analyses, provide a reasonable estimate of the mineral and chemical composition of sediments deposited along the western margin of the Antarctic Peninsula.

Pb isotopes in individual sand-sized (>150 µm) ice-rafted feldspars deposited at subpolar North Atlantic Ocean Deep Sea Drilling Project Site 611 during the late Pliocene (Marine Isotope Stages, MIS, G6 through 100)

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39°N until 2.4 ±0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem,we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma.

Superoxide decay rates during METEOR cruise M83/1

Superoxide is an important transient reactive oxygen species (ROS) in the ocean formed as an intermediate in the redox transformation of oxygen (O2) into hydrogen peroxide (H2O2) and vice versa. This highly reactive and very short-lived radical anion can be produced both via photochemical and biological processes in the ocean. In this paper we examine the decomposition rate of O2- throughout the water column, using new data collected in the Eastern Tropical North Atlantic (ETNA) Ocean. For this approach we applied a semi factorial experimental design, to identify and quantify the pathways of the major identified sinks in the ocean. In this work we occupied 6 stations, 2 on the West African continental shelf and 4 open ocean stations, including the CVOO time series site adjacent to Cape Verde. Our results indicate that in the surface ocean, impacted by Saharan aerosols and sediment resuspension, the main decay pathways for superoxide is via reactions with Mn(||) and organic matter.