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.

PIES time series in the North Atlantic at the Mid-Atlantic Ridge (2006-2010)

In the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four inverted echo sounders with high precision pressure sensors (PIES) were moored between 47°40' N and 52°30' N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central, and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid-Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21 years is 27 ± 5 Sv, consisting of 60% warm water of subtropical origin and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4-9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the southern segment.