Population trends in the Slavonian grebe <Emphasis Type="Italic">Podiceps auritus</Emphasis> (L.) and Chironomidae (Diptera) at a Scottish loch

Loch Ruthven holds the largest British population of the rare water-bird Podiceps auritus, the Slavonian or horned grebe. The breeding success of this bird has fluctuated annually since records began in 1970. To investigate whether these trends are linked to the abundance of chironomid midges, which are an important food-source for the grebe chicks, we analysed a sediment core from the lake, which was sliced at 2.5-mm intervals and provided near-annual sampling resolution. We also analysed diatoms and algal pigments in the lake sediments and inferred changes in total phosphorus from the diatom assemblage to determine whether changes in lake productivity have influenced the abundance of chironomids. Trends in grebe productivity, chironomid abundance and algal assemblages were compared against climate data to determine whether climate, specifically, the North Atlantic Oscillation, was the ultimate driver of the trends we recorded. Our results show that grebe breeding success is positively correlated with chironomid abundance and chironomid abundance is positively correlated with diatom-inferred total phosphorus. Lake productivity and chironomid abundance began to rise early in the twentieth century and continued to rise on a steeper trajectory from the mid-twentieth century to the present. Since the mid-1960s, chironomid abundance began to fluctuate erratically and since 1970 was in phase with grebe productivity, with the grebe trends most plausibly lagging by 1 year. These trends appear to correlate with inter-annual fluctuations in diatom-inferred total phosphorus. No correlation was found between grebe productivity or chironomid abundance and climate variables, suggesting that the size of the chironomid population and breeding success of Podiceps auritus at Loch Ruthven is resource-linked.     

Mud Tank Zircon: Long‐Term Evaluation of a Reference Material for U‐Pb Dating,Hf‐Isotope Analysis and Trace Element Analysis

Zircon megacrysts from the Mud Tank carbonatite, Australia, are being used in many laboratories as a reference material for LA‐ICP‐MS U‐Pb dating and trace element measurement, and LA‐MC‐ICP‐MS determination of Hf isotopes. We summarise a database of > 10000 analyses of Mud Tank zircon (MTZ), collected from 2000 to 2018 during its use as a secondary reference material for simultaneous U‐Pb and trace element analysis, and for Hf‐isotope analysis. Trace element mass fractions are highest in dark red‐brown stones and lowest in colourless and gem‐quality ones. Individual unzoned grains can be chemically homogeneous, while significant variations in trace element mass fraction are associated with oscillatory zoning. Chondrite‐normalised trace element patterns are essentially parallel over large mass fraction ranges. A Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) is taken as the age of crystallisation. Some grains show lower concordant to mildly discordant ages, probably reflecting minor Pb loss associated with cooling and the Alice Springs Orogeny (450–300 Ma). Our weighted mean ¹⁷⁶Hf/¹⁷⁷Hf is 0.282523 ± 10 (2s, n = 9350); the uncertainties on this ratio reflect some heterogeneity, mainly between grains. A few analyses suggest that colourless grains have generally lower ¹⁷⁶Hf/¹⁷⁷Hf. MTZ is a useful secondary reference material for U‐Pb and Hf‐isotope analysis, but individual grains need to be carefully selected using CL imaging and tested for homogeneity, and ideally should be standardised by solution analysis.     

A late Pleistocene long pollen record from Lake Urmia, NW Iran

A palynological study based on two 100-m long cores from Lake Urmia in northwestern Iran provides a vegetation record spanning 200 ka, the longest pollen record for the continental interior of the Near East. During both penultimate and last glaciations, a steppe of Artemisia and Poaceae dominated the upland vegetation with a high proportion of Chenopodiaceae in both upland and lowland saline ecosystems. While Juniperus and deciduous Quercus trees were extremely rare and restricted to some refugia, Hippophaë rhamnoides constituted an important phanerophyte, particularly during the late last glacial period. A pronounced expansion in Ephedra shrub-steppe occurred at the end of the penultimate late-glacial period but was followed by extreme aridity that favoured an Artemisia steppe. Very high lake levels, registered by both pollen and sedimentary markers, occurred during the middle of the last glaciation and late part of the penultimate glaciation. The late-glacial to early Holocene transition is represented by a succession of Hippophaë, Ephedra, Betula, Pistacia and finally Juniperus and Quercus. The last interglacial period (Eemian), slightly warmer and moister than the Holocene, was followed by two interstadial phases similar in pattern to those recorded in the marine isotope record and southern European pollen sequences.     

Lithosphere mapping beneath the North American plate

Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections.

In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan.

Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts.

SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System.

A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm.