Zircons, isotope ratios and element analyses from tonalite and oxide gabbro of the mid-ocean ridge from ODP Hole 176-735B

A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPazircon/DUzircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPazircon/DUzircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt (2007; doi:10.2138/am.2007.2449) and Schmitt (2011; doi:10.1146/annurev-earth-040610-133330)). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.

Seasonal cycles of monthly means of ground truth wind speed of METAR station GVAC and of area-averaged TMI wind speed at 37 GHz for the Cape Verde Island Sal

In the present paper ground truth and remotely sensed datasets were used for the investigation and quantification of the impact of Saharan dust on microwave propagation, the verification of theoretical results, and the validation of wind speeds determined by satellite microwave sensors. The influence of atmospheric dust was verified in two different study areas by investigations of single dust storms, wind statistics, wind speed scatter plots divided by the strength of Saharan dust storms, and wind speed differences in dependence of microwave frequencies and dust component of aerosol optical depth. An increase of the deviations of satellite wind speeds to ground truth wind speeds with higher microwave frequencies, with stronger dust storms, and with higher amount of coarse dust aerosols in coastal regions was obtained. Strong Saharan dust storms in coastal areas caused mean relative errors in the determination of wind speed by satellite microwave sensors of 16.3% at 10.7 GHz and of 20.3% at 37 GHz. The mean relative errors were smaller in the open sea area with 3.7% at 10.7 GHz and with 11.9% at 37 GHz.

(Table 1) Composition of DSDP Hole 37-334 clinopyroxenes

Recent studies of abyssal peridotites (Johnson et al., 1990, doi:10.1029/JB095iB03p02661), mid-ocean-ridge basalts (MORBs) (McKenzie, 1985, doi:10.1016/0012-821X(85)90001-9) and their entrained melt inclusions (Sobolev and Shimizu, 1993, doi:10.1038/363151a0; Humler and Whitechurch, 1988, doi:10.1016/0012-821X(88)90055-6) have shown that fractional melting of the upwelling sub-oceanic mantle produces magmas with a much wider range of compositions than erupted MORBs. In particular, it seems that strongly depleted primary magmas are routinely produced by melting beneath ridges (Johnson et al., 1990, doi:10.1029/JB095iB03p02661). The absence of strongly depleted melts as erupted lavas prompts the question of how long such magmas survive beneath ridges, before their distinctive compositions are concealed by mixing with more enriched magmas. Here we report mineral compositions from a unique suite of oceanic cumulates recovered from DSDP Site 334 (Aumento et al., doi:10.2973/dsdp.proc.37.1977), which indicate that the rocks crystallized from basaltic liquids that were strongly depleted in Na, Ti, Zr, Y, Sr and rare-earth elements relative to any erupted MORB. It thus appears that the magmatic plumbing system beneath the Mid-Atlantic Ridge permitted strongly depleted magmas to accumulate in a magma chamber and remain sufficiently isolated to produce cumulate rocks. Even so, spatial heterogeneity in the compositions of high-calcium pyroxenes suggests that in the later stages of solidification these rocks reacted with infiltrating enriched basaltic liquids.

A four-year record of UK'37- and TEX86-derived sea surface temperature estimates from sinking particles in the filamentous upwelling region off Cape Blanc, Mauritania

Lipid biomarker records from sinking particles collected by sediment traps are excellent tools to study the seasonality of biomarker production as well as processes of particle formation and settling, ultimately leading to the preservation of the biomarkers in sediments. Here we present records of the biomarker indices UK'37 based on alkenones and TEX86 based on isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), both used for the reconstruction of sea surface temperatures (SST). These records were obtained from sinking particles collected using a sediment trap moored in the filamentous upwelling zone off Cape Blanc, Mauritania, at approximately 1300 water depth during a four-year time interval between 2003 and 2007. Mass and lipid fluxes are highest during peak upwelling periods between October and June. The alkenone and GDGT records both display pronounced seasonal variability. Sinking velocities calculated from the time lag between measured SST maxima and minima and corresponding index maxima and minima in the trap samples are higher for particles containing alkenones (14-59 m/d) than for GDGTs (9-17 m/d). It is suggested that GDGTs are predominantly exported from shallow waters by incorporation in opal-rich particles. SST estimates based on the UK'37 index faithfully record observed fluctuations in SST during the study period. Temperature estimates based on TEX86 show smaller seasonal amplitudes, which can be explained with either predominant production of GDGTs during the warm season, or a contribution of GDGTs exported from deep waters carrying GDGTs in a distribution that translates to a high TEX86 signal.