Gregory E. Webb

Rare earth elements in Holocene reefal microbialites: a new shallow seawater proxy

The concentration of rare earth elements and yttrium (REE + Y) was determined in Holocene Mg-calcite microbialites from shallow reef framework cavities at Heron Reef, Great Barrier Reef. Shale-normalized REE + Y patterns of 52 microbialite samples show: (1) uniform heavy REE enrichment (Nd(SN)/Yb(SN) = 0.236, SD = 0.026); (2) consistent negative Ce and positive La anomalies; (3) marine Y/Ho ratios (56.17, SD = 2.66); and (4) slightly positive Cd anomalies. All of these features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater. REE partition coefficients were calculated relative to shallow Coral Sea seawater. They are uniform (relative SD = 10.2%) across the entire mass range and almost two orders of magnitude higher than those between coral and seawater. Hence, terrigenous detritus-free, modern microbialites are a more reliable proxy for seawater REE chemistry than are skeletal carbonates. Ancient limestones have been considered largely problematic as sources for REE proxies owing to perceived problems with diagenesis, partly on the basis of relatively high REE concentrations in some limestones compared to modern skeletal carbonates. However, high REE concentrations in modem microbialites suggest that ancient limestones with relatively high REE concentrations, if not contaminated by terrigenous detritus, may reflect original seawater chemistry. Terrigenous contamination, if present, is readily detectable on the basis of co-occurring trace element concentrations (Sc, Hf, Th) and Y/Ho ratio. Hence, ancient, particularly reefal, limestones may provide reliable seawater REE proxies. The occurrence of microbialites in clean limestones as old as 3.5 Ga suggests the possibility of reconstructing shallow marine REE chemistry over most of Earth history with important implications for paleogeography and paleoredox studies. Copyright (C) 2000 Elsevier Science Ltd.

Rare Earth Element Geochemistry of Late Devonian Reefal Carbonates, Canning Basin, Western Australia: Confirmation of a Seawater REE Proxy in Ancient Limestones

Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements (n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd-SN/Yb-SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement (n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd-SN/Yb-SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modem seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans. Copyright (C) 2004 Elsevier Ltd.

The geochemistry of late Archaean microbial carbonate: implications for ocean chemistry and continental erosion history

Trace element concentrations and combined Sr- and Nd-isotope compositions were determined on stromatolitic carbonates (microbialites) from the 2.52 Ga Campbellrand carbonate platform (South Africa). Shale-normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive La and Y anomalies and in being depleted in light rare earth elements. In contrast to modern seawater (and microbialite proxies), the 2.52 Ga samples lack a negative Ce anomaly but possess a positive Eu anomaly. These latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where, unlike today, hydrothermal Fe input was not oxidised, and scavenged and rare earth elements were not coprecipitated with Fe-oxyhydroxides. The persistence of a positive Eu anomaly in relatively shallow Campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated (Archaean) to continental erosion-dominated (Phanerozoic) rare earth element flux ratio. The dominant hydrothermal input is also expressed in the initial Sr- and Nd-isotope ratios. There is collinear variation in Sr-Nd systematics, which range from primitive values (87Sr/86Sr of 0.702386 and eNd of +2.1) to more evolved crustal ratios. Mixing calculations show that the range in trace element ratios (e.g., Y/Ho) and initial isotope ratios is not a result of contamination by trapped sediment, but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed. Calculated Nd flux ratios yield a hydrothermal input into the 2.52 Ga oceans one order of magnitude larger than continental input. Such a change in flux ratio most likely required substantially reduced continental inputs, which could, in turn, reflect a plate tectonic causation (e.g., reduced topography or expansion of epicontinental seas).

Climate change frames debate over the extinction of megafauna in Sahul (Pleistocene Australia-New Guinea)

Around 88 large vertebrate taxa disappeared from Sahul sometime during the Pleistocene, with the majority of losses (54 taxa) clearly taking place within the last 400,000 years. The largest was the 2.8-ton browsing Diprotodon optatum, whereas the ∼100- to 130-kg marsupial lion, Thylacoleo carnifex, the world’s most specialized mammalian carnivore, and Varanus priscus, the largest lizard known, were formidable predators. Explanations for these extinctions have centered on climatic change or human activities. Here, we review the evidence and arguments for both. Human involvement in the disappearance of some species remains possible but unproven. Mounting evidence points to the loss of most species before the peopling of Sahul (circa 50–45 ka) and a significant role for climate change in the disappearance of the continent’s megafauna.

Inferred syngenetic textural evolution in Holocene cryptic reefal microbialites, Heron Reef, Great Barrier Reef, Australia

Cryptic microbialites in the Heron Reef framework occur as crusts of fingerlike microcolumns or branching dendrolites, rarely more than 1 cm long. Microstructure of the most recently growing microbialite surfaces consists of coalesced, 3 µm scalenohedra that are indistinguishable from previously described Mg-calcite “abiotic” cement. The transformation from submicrometer, anhedral crystallites to >3 µm scalenohedra is inferred to have occurred only during active microbialite accretion beneath a biofilm. This syngenetic change from primary, biologically induced microstructures to microstructures that are indistinguishable from abiotic cement has important implications for the recognition and interpretation of early marine microcrystalline carbonates and cements.

Earliest known Carboniferous shallow-water reefs, Gudman Formation (Tn1b), Queensland, Australia: Implications for Late Devonian reef collapse and recovery

The Phanerozoic history of reefs extensively has been considered a direct reflection of the history of skeletal reef-building organisms. However, such a relationship does not characterize global mid-Paleozoic reef history. The extinction of most reef-building stromatoporoids and corals at the Frasnian-Famennian boundary correlates with the collapse of North American and European stromatoporoid-dominated reefs, but Western Australian, Russian, and Chinese reefs were much less severely affected until the late Famennian, when algae, calcimicrobes, and nonskeletal microbialites (i.e., stromatolites, thrombolites) declined globally. Additionally, reef recovery was more rapid than previously thought. Small, early Tournaisian (Tn1b) shallow-water reefs in the Gudman Formation of eastern-central Queensland substantially reduce the duration of the “reefless lag time” following Late Devonian reef decline, essentially confining it to the Strunian. Gudman reefs are dominated by microbialite, but contain a diverse, although volumetrically insignificant, skeletal fauna and flora, including large colonial corals, bryozoans, crinoids, brachiopods, and calcareous algae. Hence, mid-Paleozoic reef collapse and recovery reflect an amalgam of more-or-less independent histories of skeletal organisms, calcimicrobes, and nonskeletal microbialites, in response to regional and global environmental parameters. A better understanding of mid-Paleozoic reef history will require detailed local- and regional-scale studies to isolate global from nonglobal signals.

Biologically Induced Carbonate Precipitation in Reefs through Time

Modern reefs are constructed largely by scleractinian corals and coralline red algae. However, through geological time, reef-building communities have varied in terms of biotic composition, community structure, and the mechanisms of reef construction. Entire groups of organisms that do not build reefs today were prominent reef builders in the past. Most studies of reef history have emphasized the role of skeletal organisms in reef building (Newell, 1972; James, 1983; Fagerstrom, 1987; James and Bourque, 1992; Kauffman and Fagerstrom, 1993), but for most of geological time (i.e., Precambrian time) reefs lacked skeletal organisms altogether (Grotzinger, 1989c), and even Phanerozoic reefs that contained skeletal reef builders typically also contained nonskeletal constructional fabrics (Heckel, 1974; Pratt, 1982a Pratt, 1982b; Webb, 1996). Nonskeletal (nonenzymatic sensu Webb, 1996) constructional reef fabrics result predominantly from biologically induced [sensu Lowenstam, 1981) carbonate precipitation and include microbialites and biologically localized marine cements.

Famennian mud-mounds in the proximal fore-reef slope, Canning Basin, Western Australia

Abstract Famennian (Late Devonian) carbonate buildups and, in particular, mud-mounds, are poorly known, in general, and few have been documented in detail. Relatively small Famennian mud-mounds occur in proximal fore-reef slope settings in the Canning Basin, Western Australia. The Famennian platform margin facies passes from typical shoaling carbonate facies in the back reef, through massive, calcimicrobial, cement-rich reef-margin facies, to relatively steeply dipping (20–30°), well-bedded fore-reef slope facies containing shelf-derived, winnowed grainy sediments and extremely coarse reef-block debris. Isolated or coalescing mounds occur in the proximal slope, immediately adjacent to and, in some cases, possibly grading into the margin facies. Mounds are elongate perpendicular to the margin and some had synoptic relief greater than 2 m. Mounds are lithologically variable and consist of varying proportions of micrite, multiple generations of marine cement, abundant Rothpletzella , Renalcis , poorly preserved sparry microbial crusts and sporadically distributed laminar stromatoporoids. Surrounding grainy slope facies abut and slope off of mound flanks. Mound facies are very similar to nearby reef-margin facies, with the exceptions that stromatoporoids have not been observed in margin facies and solenoporoid algae, which occur in the margin, have not been observed in the mounds. Stromatolites are conspicuously absent from both facies. Mound facies appear to be more closely related to Frasnian and Famennian calcimicrobe cement-dominated reef-margin facies than to Famennian deep-water stromatolite–sponge-mound facies, such as those that occur elsewhere in the Canning Basin. The observed Canning Famennian reef and mound frameworks were constructed by communities that appear to be very similar to earlier Frasnian communities, despite the Frasnian–Famennian extinction event, and provide good examples of microbial reef framework construction in a high energy setting.

Brucite microbialites in living coral skeletons: Indicators of extreme microenvironments in shallow-marine settings

Brucite [Mg(OH)2] microbialites occur in vacated interseptal spaces of living scleractinian coral colonies (Acropora, Pocillopora, Porites) from subtidal and intertidal settings in the Great Barrier Reef, Australia, and subtidal Montastraea from the Florida Keys, United States. Brucite encrusts microbial filaments of endobionts (i.e., fungi, green algae, cyanobacteria) growing under organic biofilms; the brucite distribution is patchy both within interseptal spaces and within coralla. Although brucite is undersaturated in seawater, its precipitation was apparently induced in the corals by lowered pCO2 and increased pH within microenvironments protected by microbial biofilms. The occurrence of brucite in shallow-marine settings highlights the importance of microenvironments in the formation and early diagenesis of marine carbonates. Significantly, the brucite precipitates discovered in microenvironments in these corals show that early diagenetic products do not necessarily reflect ambient seawater chemistry. Errors in environmental interpretation may arise where unidentified precipitates occur in microenvironments in skeletal carbonates that are subsequently utilized as geochemical seawater proxies.

Cryptic meteoric diagenesis in freshwater bivalves: Implications for radiocarbon dating

Shells of freshwater bivalves are commonly used for radiocarbon dating late Pleistocene archaeological and vertebrate fossil sites, thus providing important constraints on late Pleistocene human dispersal and megafauna extinction hypotheses. The reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized; typically, wherein original shell aragonite is replaced by calcite in meteoric environments. Here we document late Pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals. Aragonite cement was identified in situ using Raman microspectroscopy and formed rather than calcite as a result of unusually high Mg:Ca ratios in local groundwaters. Thus, altered shells contain diagenetic 14C, rendering their dates unreliable, but they may slip past common vetting techniques because (1) epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals; (2) X-ray diffraction indicates that no calcite is present; (3) alteration is not apparent in cathodoluminescence studies; and (4) stable isotopes of C and O are difficult to interpret in shells that originate in terrestrial-meteoric environments. Hence, although freshwater with a high Mg:Ca ratio is not common, groundwater chemistry should be considered before accepting bivalve-based radiocarbon dates uncritically. More broadly, meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite. Our data show that such is not invariably the case, even in fully terrestrial, freshwater systems.