Sediment geochemistry of ODP Holes 206-1256B and 206-1256C

We measured the chemical composition of 100 samples from the 250-m sediment sequence retrieved from Ocean Drilling Program Site 1256 in the Guatemala Basin using a newly developed microwave-assisted acid digestion protocol followed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis. We compared these data gathered onshore to the results from the flux fusion prepared samples analyzed by shipboard ICP-AES during the leg and published in the Leg 206 Initial Reports volume, as well as to 35 randomly selected samples that were prepared by flux fusion at Boston University and analyzed by ICP-AES. Comparison of the newly developed acid digestion protocol to shore-based flux fusion demonstrates that the microwave-assisted acid technique yields a complete digestion, and because this procedure includes boric acid, it is safe for use with HF acid as boric acid neutralizes excess HF. The precision for nearly all elements in shore-based acid digestions is better than 3% of the measured values, including for elements such as Ni, Cr, and V, which are typically difficult to measure in biogenic-rich sediments. The shore-based flux fusions, while better than shipboard reported precision values (as expected), has precision better than 3% of their respective measured values for all major elements (Si, Al, Ti, Fe, Mn, Ca, Mg, Na, and K) and several trace elements (Ba and Sr). Results for P, Cr, Ni, V, Sc, and Zr are better than 5% of their measured values. Not only does the newly developed acid digestion provide better analytical results than the typical flux fusion method, the shore-based acid procedure also exhibits downhole lithologic and chemical characteristics similar to the shipboard flux fusion prepared results. These results confirm that the current shipboard methods are adequate for first-order geochemical interpretations and that the microwave-assisted acid digestion holds great potential to be the primary technique of preparing sediments on future Integrated Ocean Drilling Program expeditions.

(Table T1) Trace-element abundances for igneous basement at ODP Site 206-1256

In this report, I present trace element data for basement samples at Ocean Drilling Program (ODP) Site 1256. The samples analyzed represent a subset of the group ("pool") samples from ODP Leg 206, and these trace element data are part of a more comprehensive data suite for the same samples, with analyses of stable and radiogenic isotopes (e.g., Sr, Li, and O) in progress or recently completed that will be presented elsewhere. The trace element analyses were performed in the GeoAnalytical Lab at Washington State University. The following elements were analyzed: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ba, Th, Nb, Y, Hf, Ta, U, Pb, Rb, Cs, Sr, Sc, and Zr. Trace element data indicate that the igneous basement at Site 1256 is geochemically normal mid-ocean-ridge basalt. A massive ponded flow sampled in both Holes 1256C and 1256D is distinguished by higher abundances of rare earth elements (REE) and most of the other trace elements analyzed. One interval of highly altered basalt has significantly higher concentrations of Cs, Rb, and Ba and lower concentrations of Sr, Pb, Zr, Hf, Sc, and most REE than the samples of background alteration or halos. No correlation is obvious between trace element abundance and macroscopic type of alteration within the background alteration or halos.

(Table T1) Geochemistry of calcium carbonate veins from ODP Holes 206-1256C and 206-1256D

Drilling a complete deep crustal section has been a primary yet elusive goal since the inception of scientific ocean drilling. In situ ocean crustal sections would contribute enormously to our understanding of the formation and subsequent evolution of the ocean crust, in particular the interplay between magmatic, hydrothermal, and tectonic processes. Ocean Drilling Program (ODP) Leg 206 was the first of a multileg project to drill an in situ crustal section that penetrated the gabbroic rocks of the Cocos plate (6°44.2'N, 91°56.1'W), which formed ~15 m.y. ago on the East Pacific Rise during a period of superfast spreading (>200 mm/yr) (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). During Leg 206, the upper 500 m of basement was cored in Holes 1256C and 1256D with moderate to high recovery rates. The igneous rocks recovered are predominantly thin (10 cm to 3 m) basalt flows separated by chilled margins. There are also several massive flows (>3 m thick), although their abundance decreases with depth in Hole 1256D, as well as minor pillow basalts, hyaloclastites, and rare dikes. The lavas have been slightly (<10%) altered by low-temperature hydrothermal fluids, which resulted in pervasive dark gray background alteration and precipitation of saponite, pyrite, silica, celadonite, and calcium carbonate veins. Here we present a geochemical analysis of the CaCO3 recovered from cores. The compositions of ridge flank fluids within superfast spreading crust will be determined from these data, following the approach of Hart et al. (1994, doi:10.1029/93JB02035), Yatabe et al. (2000, doi:10.2973/odp.proc.sr.168.003.2000), and Coggon et al. (2004, doi:10.1016/S0012-821X(03)00697-6).

Physical oceanography from the TRACTOR project

Primary Objectives - Describe and quantify the present strength and variability of the circulation and oceanic processes of the Nordic Seas regions using primarily observations of the long term spread of a tracer purposefully released into the Greenland Sea Gyre in 1996. - Improve our understanding of ocean processes critical to the thermaholine circulation in the Nordic Seas regions so as to be able to predict how this region may respond to climate change. - Assess the role of mixing and ageing of water masses on the carbon transport and the role of the thermohaline circulation in carbon storage using water transports and mixing coefficients derived from the tracer distribution. Specific Objectives Perform annual hydrographic, chemical and SF6 tracer surveys into the Nordic regions in order to: - Measure lateral and diapycnal mixing rates in the Greenland Sea Gyre and in the surrounding regions. - Document the depth and rates of convective mixing in the Greenland Sea using the SF6 and the water masses characteristics. - Measure the transit time and transport of water from the Greenland Sea to surrounding seas and outflows. Document processes of water mass transformation and entrainment occurring to water emanating from the central Greenland Sea. - Measure diapycnal mixing rates in the bottom and margins of the Greenland Sea basin using the SF6 signal observed there. Quantify the potential role of bottom boundary-layer mixing in the ventilation of the Greenland Sea Deep Water in absence of deep convection. Monitor the variability of the entrainment of water from the Greenland Sea using time series auto-sampler moorings at strategic positions i.e., sill of the Denmark Strait, Labrador Sea, Jan Mayen fracture zone and Fram Strait. Relate the observed variability of the tracer signal in the outflows to convection events in the Greenland Sea and local wind stress events. Obtain a better description of deepwater overflow and entrainment processes in the Denmark Strait and Faeroe Bank Channel overflows and use these to improve modelling of deepwater overflows. Monitor the tracer invasion into the North Atlantic using opportunistic SF6 measurements from other cruises: we anticipate that a number of oceanographic cruises will take place in the north-east Atlantic and the Labrador Sea. It should be possible to get samples from some cruises for SF6 measurements. Use process models to describe the spread of the tracer to achieve better parameterisation for three-dimensional models. One reason that these are so resistant to prediction is that our best ocean models are as yet some distance from being good enough, to predict climate and climate change.