Autotrophic response to lake age,conductivity and temperature in two West Greenland lakes

Predicted changes in future climate necessitate a better understanding of climate impacts on lake biota, and the role of within-lake processes in modifying biotic response. Therefore we examined two climate-related variables (lake-water conductivity and GRIP temperatures) and lake ontogeny (lake age), to determine their influence on lake autotrophic communities in two neighbouring closed-basin lakes from West Greenland spanning the past 8,000 years. Using sedimentary pigments as proxies for lake autotrophic communities, we used synchrony and variance partitioning analyses (VPA) to test three specific hypotheses (a) that lake primary production would increase with lake age, (b) that climate would be the dominant process controlling autotrophic communities in these pristine lakes and (c) that the response of autotrophs to conductivity and temperature would vary depending upon the age of the lake. The results supported our first hypothesis, showing that lakes changed significantly with age, exhibiting an increase and decline in production in the first millennium of their existence, followed by a steady increase in production and increasingly frequent abrupt switches between mixed and meromictic states. The highly synchronous detrended response (r = 0.769) of lake autotrophs in the two study lakes, supported our hypothesis that climate was the dominant factor controlling lake autotrophs. However, VPA revealed that our climate-related variables (temperature and conductivity) explained only small amounts of variance alone (<12.9%) because covariance among them hindered efficient partitioning. In support of our third hypothesis, autotrophs responded significantly to temperature and conductivity in interaction with lake age (>50% variance explained) and with each other (>28% variance explained), such that autotrophic response changed as lakes aged. In spite of this, lakes sometimes responded independently, as a result of differences in the relative proportion of benthic to pelagic production and because of differences in lake morphometry. Together these results show that long-term control of lake autotrophs by climate and lake age is modified on shorter timescales by non-linear responses related to within-lake processes, and by the interaction of different climate variables with each other and with lake age.     

Six comments on the Bangladesh Flood Action Plan

Comments

Six comments on the Bangladesh Flood Action Plan     

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland,USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (～1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e₁ and e₂). The better-developed e₁ sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e₂ lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e₁ and e₂ strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.     

Ancient mantle lithosphere beneath the Khanka massif in the Russian Far East: in situ Re–Os evidence

The Os‐isotope compositions of sulphides in mantle xenoliths hosted by Late Miocene alkali basalts from the Sviyaginsky volcano, Russian Far East, reveal the presence of Archaean–Proterozoic subcontinental lithospheric mantle beneath the Khanka massif. Their T_MA and T_RD model ages reveal similar peaks at 1.1 and 0.8 Ga suggesting later thermotectonic events in the subcontinental lithospheric mantle, whereas T_RD model ages range back to 2.8 ± 0.5 (2σ) Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those recorded in the crust of the Khanka massif. The sulphide Os‐isotope data show that the subcontinental lithospheric mantle beneath the Khanka massif had formed at least by the Mesoproterozoic, and was subsequently metasomatized by juvenile crustal‐growth events related to the evolution of the Altaids. The Khanka massif is further proposed to have tectonic affinity to the Siberia Craton and should originate from it accordingly.     

Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban,southern Baikal region,Russia

 Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K₂O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K₂O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H₂O-poor fluid into spinel peridotites. Received: 20 March 1995 / Accepted: 26 June 1995     

Comment on the postulate of three plane strain mechanisms

This note discusses the inconsistencies that are inherent in the postulate of three plane strain mechanisms. It is shown that this postulate violates the principle of invariance and one obtains different results depending on the choice of the reference axes. If formulated in the principal stress space, this postulate requires that the principal stress and principal plastic strain increment directions be coaxial. Constitutive models based on this postulate cannot be used for general loading situations involving principal stress rotation where significant non-coaxiality is obtained.     

Reunion de la Commission Gravimetrique Internationale

Sans résumé Bureau Gravimétrique International     

Geochronological (U–Pb,U–Th–total Pb,Sm–Nd) and geochemical (REE, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>18</Superscript>O, δ<Superscript>13</Superscript>C) tracing of intraplate tectonism and associated fluid flow in the Warburton Basin,Australia

The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ¹⁸O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.