A precisely‐dated lake‐level rise marked by diatomite formation in northeastern Ireland

The intercorrelation of palaeoclimate events from various studies is often hindered by a lack of precise chronological control. Tephra isochrons can overcome this problem by providing direct site linkages. This paper outlines a study of Holocene peat and diatomite deposits that accumulated within the floodplain of Lough Neagh, Northern Ireland. The Icelandic Hekla 4 tephra has been identified at the base of diatomite deposits at a number of sites and provides firm dating evidence for a widespread flooding event in the area at ca. 2300 BC . The evidence is consistent with other studies in Ireland and elsewhere for increased wetness at this time. The results demonstrate that the terrestrial deposits around Lough Neagh contain an important record of Holocene lake‐level change. Dendrochronological evidence from the Lough Neagh area provides additional information about lake‐level fluctuations over the past two millennia. Copyright © 2004 John Wiley & Sons, Ltd.     

Riverine influence determines nearshore heterogeneity of nutrient (C,N, P) content and stoichiometry in the KwaZulu-Natal Bight,South Africa

Riverine influences on nearshore oceanic habitats often have detrimental consequences leading to algal blooms and hypoxia. In oligo- to mesotrophic systems, however, nutrient delivery via rivers may stimulate production and even be a vital source of nutrients, as may nutrient supplements from upwelling. We investigated the nutrient content (C, N, P) and stoichiometry of sediment, and several pelagic, benthopelagic and benthic species in the KwaZulu-Natal (KZN) Bight, a narrow shelf area on the south-east coast of South Africa, bordering the Agulhas Current. Three suggested nutrient sources to the bight are the Thukela River in the central region of the bight, upwelling in the northern part and a semi-permanent eddy (Durban Eddy) in the southern part. Elemental content of the various groups studied showed significantly higher values for most groups at the site near the Thukela River. C:P and N:P were highest in the southern part of the bight, and lowest near the Thukela Mouth or at Richards Bay in the north, indicating the latter were the P-richer sites. Sediment organic matter showed lowest elemental content, as expected, and zooplankton stoichiometry was highest compared to all other biotic groups. Environmental heterogeneity played a greater role in organismal C, N and P content and stoichiometry compared to phylogeny, with the exception of the differences in C:P and N:P of zooplankton. From this bight-wide study, the higher elemental content and lower ratios at the Thukela Mouth site supported previous findings of the importance of coastal nutrient sources to the bight ecosystem. Reductions in river flow for water use in the catchment areas may therefore have negative consequences for the productivity of the entire ecosystem.     

Turbulent mixing in a stratified estuarine tidal channel: Hikapu Reach,Pelorus Sound,New Zealand

Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s^?1 and the buoyancy frequency squared was in the range 10^?5 to 10^?3?s^?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10^?6?m²?s^?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact.     

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ¹⁸O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sr_i and δ¹⁸O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ¹⁸O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.     

The effects of dissolved organic matter on the aqueous partitioning of polynuclear aromatic hydrocarbons

The partitioning of six polynuclear aromatic hydrocarbons into aqueous solutions containing dissolved organic matter from various coastal environments was investigated by liquid chromatography. Specific hydrocarbon-organic matter interactions were observed. The greatest interactions occurred with the very low solubility hydrocarbons and with organic matter of terrestrial origin. Ultrafiltration experiments suggested that it was the relatively high (greater than approximately 500) molecular weight organic matter that was interacting with the hydrocarbons.     

Seasonal variations of the clear-sky greenhouse effect: the role of changes in atmospheric temperatures and humidities

We present an analysis of the factors which control the seasonal variations of the clear-sky greenhouse effect, based on satellite observations and radiative transfer simulations. The satellite observations include the radiation budget at the top of the atmosphere from the Earth Radiation Budget Experiment and the total column moisture content derived from the Special Sensor Microwave/Imager. The simulations were performed with the SAMSON system described in an earlier paper, using atmospheric temperatures and humidities from operational analyses produced by the European Centre for Medium Range Weather Forecasts. At low latitudes, the magnitude of the clear-sky greenhouse effect is dominated by the strong thermodynamic link between the total column moisture content of the atmosphere and sea surface temperatures, with minimal seasonal variations. In contrast, at middle to high latitudes there are strong seasonal variations, the clear-sky greenhouse effect being largest in winter and smallest in summer. These variations cannot be explained by the seasonal cycle in the total column moisture content, as this is largest in summer and smallest in winter. The variations are controlled instead by the seasonal changes in atmospheric temperatures. The colder atmosphere in winter enhances the temperature differential between the atmosphere and the sea surface, leading to a larger greenhouse effect despite the lower moisture contents. The magnitude of the clear-sky greenhouse effect is thus controlled by atmospheric humidity at low latitudes, but by atmospheric temperature at middle and high latitudes. These controls are illustrated by results from sensitivity experiments with SAMSON and are interpreted in terms of a simple model.     

Evidence for a pulse of 1.45 Ga anorthosite–mangerite–charnockite–granite (AMCG) plutonism in Lithuania: implications for the Mesoproterozoic evolution of the East European Craton

Several subcropping anorthosite–mangerite–charnockite–granite (AMCG) plutonic suites are aligned along E–W trending lineaments in the Lithuanian part of the East European Craton. The Rukai quartz monzodiorite from the Nemunas suite yields a zircon U–Pb intrusion age of 1447 ± 5 Ma, and the Geluva granite an age of 1445 ± 8 Ma, both obtained using secondary ion mass spectrometry. These rocks are 50 Myr younger than the 1.53–1.50 Ga Mazury AMCG complex in southern Lithuania and northern Poland. The Nemunas and Geluva AMCG rocks correlate in age with Bornholm granitoids in the Baltic Sea and Blekinge granites in southern Sweden, and are similarly aligned along E–W trending lineaments. This regional 1.45 Ga magmatic event across the Baltic Sea may be regarded as an inboard manifestation of the accretionary 1.50 Ga Danopolonian orogeny (cf. Pol. Mineral. Soc. Spec. Publ., 2005, 26: 18) farther west.     

Archean of Greenland and Fennoscandia

The North Atlantic craton in southern West Greenland mainly consists of a tectonic collage of Mesoarchean continental crustal terranes, which were amalgamated at c. 2.7 Ga and are currently exposed at mid-crustal amphibolite to granulite facies levels. Tonalitic orthogneisses predominate, intercalated with slightly older tholeiitic to andesitic metavolcanic rocks and associated gabbro-anorthosite intrusive complexes. The North Atlantic craton also contains enclaves of Eoarchean, c. 3.86-3.6 Ga orthogneisses and supracrustal rocks including the Isua greenstone （or supracrustal） belt. This is the oldest known assemblage of rocks deposited at the surface of the Earth, comprising mafic pillow lavas, banded iron formations and metasedimentary schists with local disseminated graphite of possible biogenic origin. Eoarchean rocks have not been found in Kola and Karelia in Fennoscandia where most rocks are 2.9-2.7 Ga tonalitic-trondhjemitic-granodioritic orthogneisses with intercalated coeval greenstone belts and amphibolites. Mesoarchean 3.0-3.2 Ga rocks are found in the eastern and western parts of the Karelian province. Subduction-related rocks like the Iringora supra-subduction type ophiolite and basalt-andesite-dacite-rhyolite series volcanic rocks in many greenstone belts, as well as eclogites are found in the Archean of Fennoscandia. A clear distinction between Greenland and Fennoscandia is the abundance of 2.75-2.65 Ga igneous rocks in Fennoscandia which indicates that these two cratons had a separate evolution during the Neoarchean.     

Chemical characterization of earth’s most ancient clastic metasediments from the Isua Greenstone Belt, southern West Greenland

New and published major and trace element abundances of clastic metasediments (mainly garnet-biotite-plagioclase schists) from the ∼3.8 Ga Isua Greenstone Belt (IGB), southern West Greenland, are used in an attempt to identify the compositional characteristics of the protoliths of these sediments. Compositionally, the metasediments are heterogeneous with enrichment of LREE (La/Sm_chond = 1.1-3.9) and variable enrichment and depletion of HREE (Gd/Yb_chond = 0.8-4.3). Chondrite-normalized Eu is also variable, spanning a range from relative Eu depletion to enrichment (Eu/Eu* = 0.6-1.3). A series of geochemical and geological criteria provides conclusive evidence for a sedimentary origin, in disagreement with some previous studies that questioned the presence of genuine clastic metasediments. In particular, trace element systematics of IGB metasediments show strong resemblance to other well-documented Archaean clastic sediments, and are consistent with a provenance consisting of ultramafic, mafic and felsic igneous rocks. Two schists, identified as metasomatized mafic igneous rocks from petrographic and field evidence, show distinct compositional differences to the metasediments.Major element systematics document incipient-to-moderate source weathering in the majority of metasediments, while signs of secondary K-addition are rare. Detailed inspection of Eu/Eu*, Fe₂O₃ and CIW (chemical index of weathering) relationships reveals that elevated iron contents (when compared to averages for continental crust) and strong relative enrichment in Eu may be due to precipitation of marine Fe-oxyhydroxides during deposition or diagenesis on the seafloor.Some of the IGB metasediments have yielded anomalous ¹⁴²Nd and ¹⁸²W isotopic compositions that were respectively interpreted in terms of early mantle differentiation processes and the presence of a meteorite component. Alternatively, W and possibly Nd isotopes could have been affected by thermal neutron capture on the Hadean surface. The latter process was tested in this study by analysis of Sm isotope compositions, which serve as an effective monitor for neutron capture effects. As no anomalous variation from terrestrial values was detected, we infer that isotope systematics (including ¹⁸²W and ¹⁴²Nd) of IGB metasediments were not affected by neutron capture, but reflect decay of radioactive parent isotopes.     

CO<Subscript>2</Subscript> fluid inclusions in Jack Hills zircons

The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO₂ inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO₂ densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO₂ inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO₂ prior to analysis. This indicates a close relationship between CO₂ and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO₂-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.