Evidence of a significant wind‐driven circulation in Akaroa Harbour. Part 1: Data obtained during the September‐November, 1998 field survey

Part 1 (P1) of a two‐part study of Akaroa Harbour, Bank's Peninsula, New Zealand, presents observational evidence of the development of a significant wind‐driven circulation in association with strong northerly and southerly wind events. These are common throughout the year though during the 3‐month field campaign (during the Southern Hemisphere spring of 1998) northerly wind events dominated. This wind‐driven circulation resulted in the weakening of density stratification within the Harbour, mixing the water column to depth over a significant portion of the study area.     

Mars high resolution gravity fields from MRO, Mars seasonal gravity, and other dynamical parameters

With 2 years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k₂ = 0.164 ± 0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4 years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGM_Sun/dt < 1.6 × 10⁻¹³ GM_Sun year⁻¹).     

Comparative petrology of silicates in the Udei Station (IAB) and Miles (IIE) iron meteorites: Implications for the origin of silicate-bearing irons

The textures and mineral chemistries of silicate inclusions in the Udei Station (IAB) and Miles (fractionated IIE) iron meteorites were studied using optical and electron microscopy, SEM, EMPA, and LA-ICP-MS techniques to better understand the origin of silicate-bearing irons. Inclusions in Udei Station include near-chondritic, basaltic/gabbroic, feldspathic orthopyroxenitic, and harzburgitic lithologies. In Miles, most inclusions can be described as feldspathic pyroxenite or pyroxene-enriched basalt/gabbro. The trace-element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from Udei Station; whereas in different inclusions from Miles, the compositions of orthopyroxene grains are similar, while those of clinopyroxene, plagioclase, and especially Cl-apatite are variable. Orthopyroxene in Miles tends to be enriched in REE compared to that in Udei Station, but the reverse is true for plagioclase and clinopyroxene.The data can be explained by models involving partial melting of chondritic protoliths, silicate melt migration, and redox reactions between silicate and metal components to form phosphate. The extent of heating, melt migration, and phosphate formation were all greater in Miles. Silicates in Miles were formed from liquids produced by ∼30% partial melting of a chondritic precursor brought to a peak temperature of ∼1250 °C. This silicate melt crystallized in two stages. During Stage 1, crystallizing minerals (orthopyroxene, clinopyroxene, chromite, and olivine) were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling, with a residence time of ∼20 ka at ∼1150 °C. During Stage 2, following probable re-melting of feldspathic materials, and after the silicate “mush” was mixed with molten metal, plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus. In Udei Station, despite a lower peak temperature (<1180 °C) and degree of silicate partial melting (∼3-10%), silicate melt was able to efficiently separate from silicate solid to produce melt residues (harzburgite) and liquids or cumulates (basalt/gabbro, feldspathic orthopyroxenite) prior to final metal emplacement. Olivine was generally out of equilibrium with other minerals, but orthopyroxene and plagioclase largely equilibrated under magmatic conditions, and clinopyroxene in basalt/gabbro crystallized from a more evolved silicate melt.We suggest that a model involving major collisional disruption and mixing of partly molten, endogenically heated planetesimals can best explain the data for IAB and fractionated IIE silicate-bearing irons. The extent of endogenic heating was different (less for the IABs), and the amount of parent body disruption was different (scrambling with collisional unroofing for the IAB/IIICD/winonaite body, more complete destruction for the fractionated IIE body), but both bodies were partly molten and incompletely differentiated at the time of impact. We suggest that the post-impact secondary body for IAB/IIICD/winonaite meteorites was mineralogically zoned with Ni-poor metal in the center, and that the secondary body for fractionated IIE meteorites was a relatively small melt-rich body that had separated from olivine during collisional break-up.     

Controls on the D/H ratios of plant leaf waxes in an arid ecosystem

The extent to which leaf water D-enrichment (transpiration) and soil water D-enrichment (evaporation) affect the D/H ratio of plant leaf waxes remains a contentious issue, with important implications for paleohydrologic reconstructions. In this study we measure δD values of precipitation (δD_p), groundwater (δD_gw), plant xylem water (δD_xw) and leaf water (δD_lw) to understand their impact on the δD values of plant leaf wax n-alkanes (δD_wax) in an arid ecosystem. Our survey includes multiple species at four sites across an aridity gradient (80-30% relative humidity) in southern California.We find that many species take up groundwater or precipitation without significant fractionation. D-enriched soil water is a minor source even in species known to perform and utilize waters from hydraulic lift, such as Larrea tridentata (+10‰). Measurements of leaf water isotopic composition demonstrate that transpiration is an important mechanism for D-enrichment of leaf waters (+74 ± 20‰, 1σ), resulting in the smallest net fractionation yet reported between source water and leaf waxes (L. tridentata −41‰; multi-species mean value is −94 ± 21‰, 1σ). We find little change in leaf water D-enrichment or net fractionation across the climatic gradient sampled by our study, suggesting that a net fractionation of ca. −90‰ may be appropriate for paleohydrologic reconstructions in semi-arid to arid environments. Large interspecies offsets in net fractionations (1σ = 21‰) are potentially troublesome, given the observed floristic diversity and the likelihood of species assemblage changes with climate shifts.     

绿色撒哈拉

占非洲面积将近三分之一的撒哈拉沙漠一度是热带稀树大草原,野生动植物种类丰富,充满鱼类的湖泊星罗棋布,而且有许多古人类生活在那里。“在约5 550~10 500年以前,撒哈拉是理想的生存地带,”斯特凡·克洛普林这样说过。克洛普林是德国科隆大学的地质考古学家,在过去的30年里,他