Extraction of Nd isotopes from bulk deep sea sediments for paleoceanographic studies on Cenozoic time scales

Nd isotopes preserved in fossil fish teeth and ferromanganese crusts have become a common tool for tracking variations in water mass composition and circulation through time. Studies of Nd isotopes extracted from Pleistocene to Holocene bulk sediments using hydroxylamine hydrochloride (HH) solution yield high resolution records of Nd isotopes that can be interpreted in terms of deep water circulation, but concerns about diagenesis and potential contamination of the seawater signal limit application of this technique to geologically young samples.In this study we demonstrate that Nd extracted from the > 63 μm, decarbonated fraction of older Ocean Drilling Program (ODP) sediments using a 0.02 M HH solution produces Nd isotopic ratios that are within error of values from cleaned fossil fish teeth collected from the same samples, indicating that the HH-extractions are robust recorders of deep sea Nd isotopes. This excellent correlation was achieved for 94 paired fish teeth and HH-extraction samples ranging in age from the Miocene to Cretaceous, distributed throughout the north, tropical and south Atlantic, and composed of a range of lithologies including carbonate-rich oozes/chalks and black shales. The strong Nd signal recovered from Cretaceous anoxic black shale sequences is unlikely to be associated with ferromanganese oxide coatings, but may be derived from abundant phosphatic fish teeth and debris or organic matter in these samples.In contrast to the deep water Nd isotopic signal, Sr isotopes from HH-extractions are often offset from seawater values, suggesting that evaluation of Sr isotopes is a conservative test for the integrity of Nd isotopes in the HH fraction. However, rare earth elements (REE) from the HH-extractions and fish teeth produce distinctive middle REE bulge patterns that may prove useful for evaluating whether the Nd isotopic signal represents uncontaminated seawater. Alternatively, a few paired HH-extraction and cleaned fish teeth samples from each site of interest can be used to verify the seawater composition of the HH-extractions. The similarity between isotopic values for the HH-extraction and fish teeth illustrates that the extensive cleaning protocol applied to fish teeth samples is not necessary in typical, carbonate-rich, deep sea sediments.     

Charged Particle Monitor on the Astrosat Mission

Charged Particle Monitor (CPM) on-board the Astrosat satellite is an instrument designed to detect the flux of charged particles at the satellite location. A Cesium Iodide Thallium (CsI(Tl)) crystal is used with a Kapton window to detect protons with energies greater than 1 MeV. The ground calibration of CPM was done using gamma-rays from radioactive sources and protons from particle accelerators. Based on the ground calibration results, energy deposition above 1 MeV are accepted and particle counts are recorded. It is found that CPM counts are steady and the signal for the onset and exit of South Atlantic Anomaly (SAA) region are generated in a very reliable and stable manner.     

Metalliferous sediments from the H.M.S. Challenger voyage (1872-1876)

The legendary cruise of H.M.S. Challenger (1872-1876) around the globe must always occupy an eminent place in the annals of oceanography, as being the first systematic attempt made on a global scale to explore the ocean. This expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise. Sediment with high content of metals (later called “metalliferous”) was among the enigmatic findings taken onboard. Although the nature of metalliferous sediments is well known today, the very first sampled sediments of this type have not been studied to date. Motivated by the historical value of Challenger’s metalliferous sediment collection we undertook an investigation addressing two questions: (1) the composition of sediments from seafloor for which we have very limited data; (2) Sr-Nd-Pb-Fe-Zn-isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century.The SE Pacific metalliferous sediments sampled by the Challenger’s explorers are of 2 types: (1) metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth (CCD); and (2) stripped of CaCO₃ metalliferous sediments located beneath the CCD in the deeps near the mid-ocean ridges. The abiogenic part of these sediments is composed mainly of poorly-crystalline to X-ray amorphous Fe-Mn-oxyhydroxides, and an amorphous silicate phase. These sediments have geochemical features similar to those of all the other metalliferous sediments: very high Fe and Mn content (on abiogenic basis), very low Al/(Al + Fe + Mn), and high content (on abiogenic basis) of As, Ba, Be, Bi, Cd, Co, Cu, Mo, Ni, Pb, Sb, Th, Tl, U, V, W, Y, Zn and Zr. Their REE distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint (weak positive or no Eu anomaly).Seawater and/or terrigenous input from South America control the Sr-Nd-Pb-isotope signature of the Challenger metalliferous sediments and have almost completely obliterated any original MORB-derived hydrothermal signal. Zn isotopes are mainly contributed from seawater although other Zn sources (hydrothermal fluid and detrital aluminosilicates, barite and volcanic glass) are necessary to fully explain Zn-isotope ratios. Fe isotopes indicate relatively slow Fe²⁺ to Fe³⁺ oxidation in the non-buoyant plume, thus producing relatively lighter Fe-isotope signature of the FeOOH particles that formed the studied metalliferous sediments.     

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. VLF signal amplitude perturbation (ΔA) and amplitude time delay (Δt) (vis-á-vis corresponding X-ray light curve as measured by GOES-15) of NWC/19.8 kHz signal have been computed for solar flares which is detected by us during Jan–Sep 2011. The signal is recorded by SoftPAL facility of IERC/ICSP, Sitapur (22^° 27′N, 87^° 45′E), West Bengal, India. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (α _eff ) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay (Δt). For the C-class flares we find that there is a direct correspondence between Δt of a solar flare and the average solar zenith angle Z over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the Δt for different local diurnal time slots DT. We find that while the time delay is anti-correlated with the flare peak energy flux ? _max independent of these time slots, the goodness of fit, as measured by reduced-χ ², actually worsens as the day progresses. The variation of the Z dependence of reduced-χ ² seems to follow the variation of standard deviation of Z along the T _x -R _x propagation path. In other words, for the flares having almost constant Z over the path a tighter anti-correlation between Δt and ? _max was observed.