ALBERT

Description: Obtaining geochemical profiles using X-ray fluorescent (XRF) techniques has become a standard procedure in many sediment core studies. The resulting datasets are not only important tools for palaeoclimatic and palaeoceanographic reconstructions, but also for stratigraphic correlation. The International Ocean Discovery Program (IODP) has therefore recently introduced shipboard application of a handheld XRF device, making geochemical data directly available to the science party. In all XRF scanning techniques, the physical properties of wet core halves cause substantial analytical deviations. In order to obtain estimates of element concentrations (e.g. for quantitative analyses of fluxes or mass-balance calculations), a calibration of the scanning data is required. We test whether results from the handheld XRF analysis on discrete samples are suitable for calibrating scanning data. Log-ratios with Ca as a common denominator were calculated. The comparison between the handheld device and conventional measurements show that the latter provide high-quality data describing Al, Si, K, Ca, Ti, Mn, Fe, Zn, Rb and Sr content (R2 compared with conventional measurements: ln(Al/Ca) = 0.99, ln(Si/Ca) = 0.98, ln(K/Ca) = 0.99, ln(Ti/Ca) = 0.99, ln(Mn/Ca) = 0.99, ln(Fe/Ca) = 0.99, ln(Zn/Ca) = 0.99 and ln(Sr/Ca) = 0.99). Our results imply that discrete measurements using the shipboard handheld analyser are suitable for the calibration of XRF scanning data. Our test was performed on downcore sediments from IODP Expedition 355 that display a wide variety of lithologies of both terrestrial and marine origin. The implication is that our findings are valid on a general scale and that shipboard handheld XRF analysis on discrete samples should be used for calibrating XRF scanning data.

Description: A multidisciplinary mineralogical, geochemical and biomarker study of Indus Fan sediments cored during International Ocean Discovery Program (IODP) Expedition 355 to the Laxmi Basin was carried out to define the different compositional signatures of sand, silt and clay. Upper Pliocene – lower Pleistocene turbidites from sites U1456 and U1457 were selected as the best candidates for this study. The integrated dataset presented here was obtained by coupling traditional and innovative bulk-sediment and single-mineral techniques on the same samples. Turbiditic deposits mostly consist of medium to fine silt, including rich and diverse heavy-mineral assemblages. Such a fine grain size forced us to push the limits of high-resolution quantitative heavy-mineral analysis down to as low as 5 μm. Heavy-mineral analysis allowed us to establish a Himalayan origin of the detritus in the studied turbidites. Heavy-mineral concentrations are higher in channel-fill than in overbank deposits. Mineralogical and geochemical data concur in revealing that fast-settling ultradense minerals such as zircon are preferentially concentrated in channel-fill deposits, whereas the top of overbank deposits are notably enriched with slow-settling platy phyllosilicates. Biomarker analysis represents a most suitable complementary technique that is able to investigate the provenance signature of the finer sediment fraction, largely consisting of clay. This technique allowed us to identify a largely terrigenous origin of organic matter at Site U1456 and an open marine origin at Site U1457. The latter site lies closer to the Laxmi Ridge, where thermal maturity increases with depth to reach the early oil window (127°C at c. 320 m below the seafloor).