Tracy D. Frank

Low marine sulphate and protracted oxygenation of the Proterozoic biosphere

Progressive oxygenation of the Earths early biosphere is thought to have resulted in increased sulphide oxidation during continental weathering, leading to a corresponding increase in marine sulphate concentration. Accurate reconstruction of marine sulphate reservoir size is therefore important for interpreting the oxygenation history of early Earth environments. Few data, however, specifically constrain how sulphate concentrations may have changed during the Proterozoic era (2.5–0.54 Gyr ago). Prior to 2.2 Gyr ago, when oxygen began to accumulate in the Earths atmosphere, sulphate concentrations are inferred to have been <1 mM and possibly <200 µM, on the basis of limited isotopic variability preserved in sedimentary sulphides and experimental data showing suppressed isotopic fractionation at extremely low sulphate concentrations. By 0.8 Gyr ago, oxygen and thus sulphate levels may have risen significantly. Here we report large stratigraphic variations in the sulphur isotope composition of marine carbonate-associated sulphate, and use a rate-dependent model for sulphur isotope change that allows us to track changes in marine sulphate concentrations throughout the Proterozoic. Our calculations indicate sulphate levels between 1.5 and 4.5 mM, or 5–15 per cent of modern values, for more than 1 Gyr after initial oxygenation of the Earths biosphere. Persistence of low oceanic sulphate demonstrates the protracted nature of Earths oxygenation. It links biospheric evolution to temporal patterns in the depositional behaviour of marine iron- and sulphur-bearing minerals, biological cycling of redox-sensitive elements and availability of trace metals essential to eukaryotic development.

Stratigraphic imprint of the Late Palaeozoic Ice Age in eastern Australia: a record of alternating glacial and nonglacial climate regime

Stratigraphic and sedimentological data from New South Wales and Queensland, eastern Australia, indicate that the Late Palaeozoic Ice Age comprised at least eight discrete glacial intervals (each 1–8 Ma in duration, here termed ‘glaciations’), separated by nonglacial intervals of comparable duration. These events spanned an interval from mid-Carboniferous (c. 327 Ma) to the early Late Permian (c. 260 Ma), and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late Early Permian, followed by an opposite trend towards the final demise of glaciation in the Late Permian. The alternating glacial–nonglacial motif suggests that the Late Palaeozoic Ice Age was considerably more dynamic than previously thought. These patterns are remarkably consistent with recent interpretations of palaeofloral change, eustatic sea-level fluctuations and CO2–climate–glaciation relationships for this interval of time. The detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere, notably in far-field, ice-distal, northern hemisphere successions.

Tectonic forcings of Maastrichtian ocean-climate evolution

A global compilation of deep-sea isotopic records suggests that Maastrichtian ocean-climate evolution tvas tectonically driven. During the early Maastrichtian the Atlantic intermediate-deep ocean was isolated from the Pacific, Indian, and Southern Oceans; deep water formed in the high-latitude North Atlantic and North Pacific. At the early/late Maastrichtian boundary a major reorganization of oceanic circulation patterns occurred, resulting in the development of a thermohaline circulation system similar to that of the modem oceans. A combination of isotopic and plate kinematic data suggests that this event was triggered by the final breaching of tectonic sills in the South Atlantic and the initiation of north-south flow of intermediate and deep water in the Atlantic. The onset of Laramide tectonism during the mid Maastrichtian led to the concurrent draining of major epicontinental seaways. Together, these events caused cooling, increased latitudinal temperature gradients, increased ventilation of the deep ocean, and affected a range of marine biota.

Cold water coral mounds revealed

The discovery of mounds and reefs hosting cold-water coral ecosystems along the northeastern Atlantic continental margins has propelled a vigorous effort over the past decade to study the distribution of the mounds, surface sediments, the ecosystems they host, and their environments [Hovland et al., 1994; Freiwald and Roberts, 2005].This effort has involved swath bathymetry, remotely operated vehicle deployments, shallow coring, and seismic surveys. Global coverage is difficult to gauge, but studies indicate that cold-water corals may cover as large an area as the better known warm-water corals that form shallow reefs (284,300 square kilometers) [Freiwald et al., 2005]. Cold-water corals occur in a variety of forms and settings, from small isolated colonies or patch reefs to giant mound structures such as those found west of Ireland.

Cryogenic origin for brine in the subsurface of southern McMurdo Sound, Antarctica

Sampling of interstitial fluids during deep coring in southern McMurdo Sound, Antarctica, revealed the presence of seawater-sourced, hypersaline brine at depths >200 m below the seafloor. Na-Cl-Br and SO 4 -Cl-Br relationships are consistent with a concentration mechanism that involves the removal of pure H 2 O as ice and precipitation of mirabilite (Na 2 SO 4 ·10H 2 O) during progressive freezing of seawater. The brine is in Neogene subglacial, glacimarine, and marine facies that record advance and retreat of glaciers through the Ross Sea embayment. In this environment, sea ice formation in semi-isolated marine basins that occupied flexural troughs along ice sheet margins produced dense brines that sank and infiltrated the permeable subglacial sediment. Repeated cycles of glacial advance and retreat provided multiple opportunities for batches of seawater to be transformed into brine that now is in the subsurface of southern McMurdo Sound. Results demonstrate the feasibility of brine formation via seawater freezing and attest to the potential of a cryogenic origin for subsurface brines in high-latitude regions of the Northern Hemisphere, as proposed by some workers.

Marine origin for Precambrian, carbonate-hosted magnesite?

Large-scale, carbonate-hosted magnesite (MgCO 3 ) deposits, although rare, occur mainly in Precambrian strata. Although many occurrences have characteristics consistent with penecontemporaneous formation in an evaporative marine setting, the general absence of CaSO 4 minerals has precluded the adoption of evaporative marine depositional models. In modern seawater, excess Ca 2+ and Mg 2+ relative to \(CO^{2{-}}_{3}\) and \(HCO^{{-}}_{3}\) as well as abundant \(SO^{2{-}}_{4}\) require that, upon evaporation, MgCO 3 precipitation is accompanied by substantial deposition of CaSO 4 minerals. Here we use evidence from a Neoproterozoic magnesite deposit to suggest that differences in Precambrian seawater geochemistry enabled MgCO 3 to form in isolation under evaporative conditions. During the Precambrian, precipitation of CaSO 4 evaporites was hindered by (1) elevated dissolved inorganic carbon and enhanced precipitation of CaCO 3 , which limited the availability of Ca 2+ , and (2) a small marine sulfate reservoir. Because sulfate is an inhibitor to dolomitization, low sulfate concentrations increased the potential for penecontemporaneous dolomitization in marine settings. By utilizing Ca 2+ , dolomitization served to increase fluid Mg/Ca ratios. In this \(HCO^{{-}}_{3}\) -rich but \(SO^{2{-}}_{4}\) -poor system, dolomitization coupled with significant evaporative concentration resulted in magnesite formation without coprecipitation of CaSO 4 minerals. Decreasing carbonate saturation, progressive oxygenation, and a concomitant increase in sulfate availability during the Proterozoic ultimately led to the development of the more familiar conditions of the Phanerozoic, in which dolomitization was restricted to environments where elevated Mg/Ca ratios could overcome the inhibitory effects of sulfate and significant magnesite deposition was restricted to sabkhas and alkaline lakes.

Lithostratigraphy of the late Early Permian (Kungurian) Wandrawandian Siltstone, New South Wales: Record of glaciation?

The late Early Permian (273 – 271 Ma) Wandrawandian Siltstone in the southern Sydney Basin of New South Wales represents a marine highstand that can be correlated over 2000 km. A mainly fine-grained terrigenous clastic succession, the Wandrawandian Siltstone contains evidence for cold, possibly glacial conditions based on the presence of outsized clasts and glendonites, mineral pseudomorphs after ikaite, a mineral that forms in cold (0 – 7°C) marine sediments. A lithostratigraphic and facies analysis of the unit was conducted, based on extensive coastal outcrops and continuous drillcores. Eight facies associations were identified: (i) siltstone; (ii) siltstone with minor interbedded sandstone; (iii) interbedded tabular sandstone and siltstone; (iv) admixed sandstone and siltstone to medium-grained sandstone; (v) discrete, discontinuous sandstone intervals; (vi) chaotic conglomerate and sandstone in large channel forms; (vii) chaotically bedded and pervasively soft-sediment-deformed intervals; and (viii) tuffaceous siltstone and claystone. Using lithology and ichnology, relative water depths were ascribed to each facies association. Based on these associations, the unit was divided into five informal members that reveal a history of significant relative sea-level fluctuations throughout the formation: member I, interbedded/admixed sandstone and siltstone; member II, siltstone; member III, slumped masses of members I and II; member IV, siltstone and erosionally based lensoid sandstone beds and channel bodies; and member V, interbedded/admixed sandstone and siltstone with abundant tuffs. Member I marks an initial marine transgression from shoreface to offshore depths. Member II records the maximum water depth of the shelf. Member III is interpreted to be a slump sheet; plausible mechanisms for its emplacement include seismicity produced by tectonism or glacio-isostatic rebound, changes in pore-water pressures due to sea-level fluctuations, or an increase in sedimentation rates. Members IV and V record minor fluctuations in depositional environments from offshore to shoreface water depths. Member IV includes regionally extensive, large channel bodies, with composite fills that are interpreted as storm-influenced mass-flow deposits. Member V includes a greater abundance of volcanic ash. Glacial controls (isostasy, eustasy) and tectonic affects may have worked in concert to produce the changes in depositional environments observed in the Wandrawandian Siltstone.

Recent Developments on a Nearshore, Terrigenous-Influenced Reef: Low Isles Reef, Australia

Abstract Low Isles Reef is the most southerly located of 46 coral reef platforms unique to the inner shelf of the northern Great Barrier Reef Province, Australia, which support both sea grass and mangrove growth. Such reefs develop in areas that are influenced by river flood plumes and where interreef sediments are dominated by terrigenous mud. Low Isles Reef has long been a popular tourist destination. Informal reports of decreasing visibility, a decline in scleractinian corals, and increases in soft coral and macroalgae have sparked speculation that agricultural activities in coastal catchments are affecting the reef. Comparison of the modern surface of Low Isles Reef with historical surveys and photographs dating back to 1928 allows quantification of modern sedimentary processes, rates of change, and factors influencing reef development. Results indicate that changes on Low Isles Reef are related to remobilization of coarse sediment during storm events and gradual shoreline retreat associated with rising sea level. Retreat of shingle ramparts and elongate ridges of coral debris toward the reef interior has led to the infilling of subtidal ponds on the reef top, which supported hard coral colonies in 1928. The gradual development of a composite shingle rampart along the windward margin has promoted an increase (∼150%) in the area of the reef top covered by mangroves. On the leeward margin, a decrease in hard corals since 1950 may reflect a rising contribution of organic debris from the expanding mangrove swamp. Results suggest that recent changes on Low Isles Reef can be explained in the context of natural processes. Further study is needed before the effects of agricultural activities in coastal catchments on reef health can be confirmed.

Deep-water microbialites of the Mesoproterozoic Dismal Lakes Group: microbial growth, lithification, and implications for coniform stromatolites

Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high-relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge-like vertical supports draped by concave-upward, subhorizontal elements, resulting in tent-shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late-stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice-covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non-motile communities under nutrient-limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O2 conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic--across a variety of depositional environments--may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.

Late Holocene island reef development on the inner zone of the northern Great Barrier Reef: Insights from Low Isles Reef

A sedimentological and stratigraphic study of Low Isles Reef off northern Queensland, Australia was carried out to improve understanding of factors that have governed Late Holocene carbonate deposition and reef development on the inner to middle shelf of the northern Great Barrier Reef. Low Isles Reef is one of 46 low wooded island-reefs unique to the northern Great Barrier Reef, which are situated in areas that lie in reach of river flood plumes and where inter-reef sediments are dominated by terrigenous mud. Radiocarbon ages from surface and subsurface sediment samples indicate that Low Isles Reef began to form at ca 3000 y BP, several thousand years after the Holocene sea-level stillstand, and reached sea-level soon after (within ∼500 years). Maximum reef productivity, marked by the development of mature reef flats that contributed sediment to a central lagoon, was restricted to a narrow window of time, between 3000 and 2000 y BP. This interval corresponds to: (i) a fall in relative sea-level, from ∼1 m above present at ca 5500 y BP to the current datum between 3000 and 2000 y BP; and (ii) a regional climate transition from pluvial (wetter) to the more arid conditions of today. The most recent stage of development (ca 2000–0 y BP) is characterised by extremely low rates of carbonate production and a dominance of destructive reef processes, namely storm-driven remobilisation of reef-top sediments and transport of broken coral debris from the reef front and margins to the reef top. Results of the present study enhance existing models of reef development for the Great Barrier Reef that are based on regional variations in reef-surface morphology and highlight the role of climate in controlling the timing and regional distribution of carbonate production in this classic mixed carbonate–siliciclastic environment.