Graham A. Shields

Diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies: An isotopic and REE study of Cambrian phosphorites

Seawater rare-earth element (REE) distributions show heavy REE enrichment and cerium depletion but these characteristics are only rarely found in ancient, marine authigenic minerals such as carbonate fluorapatite and calcite. This enigma may be due to changing REE distributions in seawater through time or to post-depositional redistribution of REE. Differentiating between these two factors is, however, far from straightforward as their relative importance is likely to vary and evidence is often ambiguous. In this article, we present REE as well as strontium and sulphur isotope data for basal Cambrian phosphorites of South China, which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm, and tend to undergo, therefore, less diagenetic alteration of REE spectra than skeletal apatites and other phosphate-types. This study reveals stratigraphic trends in Ce anomalies (Ce/Ce*) that could be taken to imply the preservation of a palaeoenvironmental signal. However, further investigation of our data shows that such trends in phosphorites can also be caused by post-depositional alteration, which tends to (1) increase REE contents, (2) reverse Ce depletion, (3) alter Eu anomalies, and (4) concentrate preferentially the middle REE (MREE). Further alteration, which may be due to weathering, has led to HREE depletion, and positive La and Y anomalies (tetrad effect). This is the first study where Ceanom. and phosphate-bound sulphate S-isotope values could be compared for one particular stratigraphic section, which provides us with an additional tool for the interpretation of diagenetic redox conditions. Least altered phosphorites from Meishucun that yield a strongly negative Ceanom. constrain early Cambrian seawater δ34S to about +33‰. We conclude that phosphorite Ceanom. primarily records variations in the local conditions of diagenesis, while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom-water redox conditions.

Integrated chemo- and biostratigraphic calibration of early animal evolution; Neoproterozoic-Early Cambrian of Southwest Mongolia

Five overlapping sections from the thick Neoproterozoic to early Cambrian sediments of western Mongolia were analysed to yield a remarkable carbon-isotope, strontium-isotope and small shellyfossil (SSF) record. Chemostratigraphy suggests that barren limestones of sequences 3 and 4, which lie above the two Maikhan Uul diamictites, are post-Sturtian but pre-Varangerian in age. Limestones and dolomites of sequence 5, with Boxonia grumulosa , have geochemical signatures consistent with a post-Varangerian (Ediacarian) age. A major negative δ 13 C anomaly (feature ‘W’) in sequence 6 lies a shortdistance above an Anabarites trisulcatus Zone SSF asemblage with hexactinellid sponges, of probable late Ediacarian age. Anomaly ‘W’ provides an anchor point for cross-correlation charts of carbon isotopes and small shelly fossils. Trace fossil assemblages with a distinctly Cambrian character first appear in sequence 8( Purella Zone), at the level of carbon isotopic feature ‘B’, provisionally correlated with the upper part of cycle Z in Siberia. A paradox is found from sequence 10 to 12 in Mongolia: Tommotian-type SSFs continue to appear, accompanied by Nemakit-Daldynian/Tommotian-type 87 Sr/ 86 Sr ratios but by increasingly heavyδ 13 C values that cannot be matched in the Tommotian of eastern Siberia. The steady rate of generic diversification in Mongolia also contrasts markedly with the Tommotian ‘diversity explosion’ in eastern Siberia, which occurs just above a major karstic emergence surface. One explanation is that sequences 10 to 12 in Mongolia preserve a pre-Tommotian portion of the fossil record that was missing or removed in easternSiberia. The Mongolian sections certainly deserve an important place in tracing the true course and timing of the ‘Cambrian radiation’.

New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman

The Huqf Supergroup, Oman, contains volcanic horizons with the potential to calibrate Neoproterozoic events and chemostratigraphy using U-Pb zircon geochronology. A tuffaceous bed near the base, within the lower (Ghubrah) diamictite and beneath a lower cap carbonate, provides the first U-Pb zircon date obtained from within a Neoproterozoic glacial deposit. This date of 723 +16/−10 Ma suggests a Sturtian age. Diamictites about 1 km higher in the section are overlain by the Hadash cap dolomite, which compares with cap dolomites above Marinoan glacial units elsewhere. A U-Pb zircon age of 544.5 ± 3.3 Ma from ignimbrites in the Fara Formation, near the top of the supergroup, is just beneath that of the Precambrian-Cambrian boundary (ca. 543 Ma), consistent with the presence of the Neoproterozoic skeletal-fossil Cloudina in correlative subsurface rocks. This revised chronology confirms the existence of four negative δ13C excursions between ca. 723 and 543 Ma, the lower two of which are clearly associated with glaciations.

Ediacarian sponge spicule clusters from southwestern Mongolia and the origins of the Cambrian fauna

Carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments (with Anabarites ) from southwestern Mongolia are of Ediacarian age. Spicule morphologies include bundles of oxeas arranged in three-dimensional quadrules, linked together at junctions by tetracts, pentacts, hexacts, or polyactines. All are referred to the Phylum Porifera, Class Hexactinellida. These sponge spicules provide the oldest remains that can be assigned without question to an extant phylum, and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record. It is suggested that siliceous and phosphatic members of the “Cambrian fauna” may have had their origins in eutrophic and outer shelf facies of the Late Proterozoic.

Sr, C, and O isotope geochemistry of Ordovician brachiopods: A major isotopic event around the Middle-Late Ordovician transition

Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater 87Sr/86Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle–Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ18O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ18O in some samples, particularly those from the Lower Ordovician, maximum δ18O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ18O records the effect of decreasing tropical seawater temperatures across the Middle–Late Ordovician transition superimposed on seawater δ18O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ13C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ13C excursion up to +7‰. Nevertheless, an underlying trend in mean δ13C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater 87Sr/86Sr and δ18O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle–Late Ordovician event.

Neoproterozoic chemostratigraphy and correlation of the Port Askaig glaciation, Dalradian Supergroup of Scotland

The δ13C of preglacial carbonates beneath the glaciogenic Port Askaig Formation falls from +5‰ to negative values, while least-altered 87Sr/86Sr values are close to 0.7067. Postglacial ‘cap carbonates’ of the Bonahaven Formation begin with negative δ13Ccarb values, jumping sharply to +11.7‰ near the top, which we correlate with excursions above the Elbobreen–Wilsonbreen glaciations of Svalbard and putative Sturtian (not Varanger) glaciations elsewhere, dated at c. 720 Ma. Much of the succeeding Terminal Neoproterozoic is missing in NE Svalbard and East Greenland, raising doubts about their use for global Sr and C isotope curves in the Neoproterozoic.

The Ediacaran Period

This book is an essential reference for all geoscientists, including researchers, students, and petroleum and mining professionals. The presentation is non-technical and illustrated with numerous colour charts, maps and photographs.

Metabolism controls Sr/Ca ratios in fossil aragonitic mollusks

Sr/Ca and Mg/Ca ratios in biomineral CaCO3 have recently been regarded as more reliable than δ18O values as proxy for paleotemperature because they are less affected by salinity or polar ice volume. We argue, however, that vital effects can exert a greater control than paleotemperature over fossil Sr/Ca. Seasonal perturbations in isotopic data from the Eocene bivalve Venericardia planicosta reveal a gradually decreasing annual growth rate through ontogeny. High-resolution Sr/Ca ratios, analyzed with a new proton-microbe technique, increase markedly through ontogeny, however, suggesting that more Sr was incorporated as growth rate slowed rather than as a result of changing paleotemperature. Comparative δ18O and Sr/Ca data from the broadly coeval marine gastropod Clavilithes macrospira, which exhibits a linear growth rate through ontogeny, also shows a significant increase in Sr concentration with age as well as seasonal, possibly temperature-related variations. Our observations show that neither growth, calcification rate, nor temperature can be the sole factor controlling Sr incorporation into molluscan aragonite. Metabolic activity, related to factors such as temperature, salinity, age, and growth rate, is likely to exert the primary control over Sr/Ca ratios in aragonitic mollusks.

The Monterey Event in the Mediterranean: A record from shelf sediments of Malta

Oligo-Miocene carbonate platform and shelf sediments outcropping on the Maltese Islands provide an excellent archive of the paleoceanography of the central Mediterranean. A sequence of shallow water limestones, than shelf limestones, and marls, followed again by shallow water limestones, reflects drowning of a carbonate platform, the establishment of a shelf environment and, in the late Miocene, renewed progradation and aggradation of shallow water carbonates. The sequence recording the deepening of the Maltese platform contains several phosphorite hardgrounds and phosphorite pebble beds. These phosphorites were dated with strontium isotopes. Major episodes of phosphogenesis occurred between 25 and 16 Ma, and they are coeval with those phosphorite events reported from Florida and North Carolina. A Miocene carbon isotope and oxygen isotope stratigraphy was established on planktic and benthic foraminifera and on bulk samples. A major carbon isotope excursion with an amplitude of up to +l‰ between 18 and 12.5 Ma can be correlated with the globally recognized Monterey carbon isotope excursion. This is the first record of this event both in shallow water sediments and in the Mediterranean. The carbon isotope excursion precedes an oxygen isotope excursion which also was recognized in deep-sea records. Major episodes of phosphogenesis and platform drowning preceded the carbon isotope excursion by up to millions of years.

Sulphur isotope compositions of sedimentary phosphorites from the basal Cambrian of China: implications for Neoproterozoic-Cambrian biogeochemical cycling

The Meishucun Section (Yunnan Province, South China) is considered to be an important Precambrian–Cambrian boundary section, primarily because of its rich small shelly fossil record. In this article, we report the results of a sulphur isotope study of phosphate-bound sulphate from the Meishucun Section and several correlative sections in South China. Forty clastic, granular phosphorites from Meishucun yield tightly grouped δ34S values averaging 33‰ (CDT), which agree well with published evaporite data for the lower Cambrian of Siberia and elsewhere. We argue that these strongly positive values reflect the sulphur isotopic composition of ambient seawater, confirming further the existence of uniquely high δ34S values in the earliest Cambrian oceans. This novel use of trace-sulphate in phosphate to constrain seawater δ34S represents the first time that sulphate δ34S data for this period have been given precise biostratigraphic assignments. Superimposed on the overall trend are short-term, stratigraphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published data, indicate that seawater sulphate δ34S rose from low values (15–20‰) during the pre-750 Ma Proterozoic to possibly all-time high values (>32‰) by the earliest Cambrian. We argue that this rise may, in part, relate to increases in the amount of sulphur isotopic discrimination during microbially mediated sulphate reduction as a result of increased sulphide reoxi-dation. On the other hand, the Neoproterozoic trend to high δ34S values appears to mirror a trend to decreasing seawater δ13C towards the Proterozoic–Phanerozoic transition, implying progressive increases in the efficiency of organic carbon recycling, which would normally be coupled with real increases in sulphate reduction on the global scale. We consider that both these changes in biogeochemical cycling derive ultimately from the introduction of macrofauna around this time and, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic require additional trace sulphate studies.