The persistence of memory: the fate of ancient sedimentary organic carbon in a modern sedimentary system

The cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales. The burial of organic carbon occurs predominantly at sea in association with clay-sized particles derived from the erosion of uplifted continental rocks. It follows that the history of the fine-grained particles on land may bear on the nature of the organic carbon buried. In this study, the evolution of clay-associated organic matter was followed from bedrock source to the seabed in the Eel River sedimentary system of northern California using natural abundance ¹³C and ¹⁴C tracers. Approximately half of the fine-grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted Mesozoic-Tertiary Franciscan Complex of the watershed. The short residence time of friable soils on steep hill slopes, coupled with rapid sediment accumulation rates on the shelf-slope, act to preserve the ancient organic carbon. A comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope. The bimodal mixture of ancient and modern C in soils and sediments may be characteristic of many short, mountainous rivers. If the Eel River chemistry is typical of such rivers, more than 40 Tg of ancient organic C may be delivered to the world’s oceans each year. A flux of that magnitude would have a significant influence on marine and global C-cycles.     

Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana

Two major epigenetic gold-forming events are recorded in the world-class gold province of southwest Ghana. A pre-Tarkwaian event was the source of the world-class Tarkwa palaeoplacers whereas post-Birimian and Tarkwaian deformation, which was related to the Eburnean orogeny, gave rise to the world-class (e.g. Prestea) to giant (e.g. Obuasi) orogenic gold deposits which have made the region famous for more than 2,500 years. A maximum age of 2133±4 Ma for Tarkwaian sedimentation is provided by 71 of 111 concordant SHRIMP II U–Pb dates from detrital zircons in Tarkwaian clastic rocks from Damang and Bippo Bin, northeast of Tarkwa. The overall data distribution broadly overlaps the relatively poorly constrained ages of Birimian volcanism and associated Dixcove-type granitoid emplacement, indicating syntectonic development of the Tarkwaian sedimentary basin. These zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the Eburnean orogeny. Instead, they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous Eburnean orogeny or ca. 2200–2100 Ma intrusion-related gold lode. The CO₂-rich fluid inclusions in associated vein-quartz pebbles are permissive of either source. At the Damang deposit, an epigenetic, orogenic lode-gold system clearly overprinted, and sulphidised low-grade palaeoplacer hematite–magnetite gold occurrences in the Banket Series conglomerate within the Tarkwaian sedimentary sequence. Gold mineralisation is demonstrably post-peak metamorphism, as gold-related alteration assemblages overprint metamorphic assemblages in host rocks. In alteration zones surrounding the dominant, subhorizontal auriferous quartz veins, there are rare occurrences of hydrothermal xenotime which give a SHRIMP U–Pb age of 2063±9 Ma for gold mineralisation. The similar structural timing of epigenetic gold mineralisation in Tarkwaian host rocks at Damang to that in mainly Birimian host rocks elsewhere in southwest Ghana, particularly at Obuasi, suggests that 2063±9 Ma is the best available age estimate for widespread orogenic gold mineralisation in the region. Argon–argon ages of 2029±4 and 2034±4 Ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 °C, as estimates of the temperature of gold mineralisation are higher, at around 400 °C. If peak metamorphism, with temperatures of about 550 °C, is assumed to have occurred at about 2100 Ma, the biotite ages, in combination with the xenotime age, suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 Ma.     

Tephra-fall deposits from the 1992 eruption of Crater Peak, Alaska: implications of clast textures for eruptive processes

 The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater. Received: 5 September 1997 / Accepted: 1 February 1998     

Long‐term growth of a valley‐bottom gully,western Iowa

Growth of a permanent, valley‐bottom gully from 1964 to 2000 was determined annually from survey and sediment‐discharge data and compared with runoff and base?ow discharges. Data were analysed to test the hypothesis that rates of gully growth decay exponentially with time in response to shrinking catchment area caused by gully enlargement. Also, monthly values of growth rates and runoff, averaged over the 36‐year record, were analysed with mass‐wasting data to determine the extent to which colluvium availability affected growth rates seasonally. From 1964 to 2000, the gully volume increased by 9200 m³, accounting for 34 per cent of sediment yield from the watershed. There were tight power‐law relationships between annual growth rates and annual runoff, with runoff exponents of 1·57 and 1·30 for headward and volumetric growth, respectively. Increases in gully length, area, and volume were ?tted successfully assuming an exponential decay in growth rate with time. Rather than being due to a decrease in catchment area, however, the decline in growth rate was caused by a 77 per cent decrease in the ratio of runoff to base?ow, which also widened the gully and reduced the mean slope of its banks. Order‐of‐magnitude seasonal changes in erosion ef?ciency, de?ned as the fraction of stream power used to evacuate sediment from the gully, were roughly correlated with colluvium availability, as indicated by seasonal changes in the number of bank mass‐wasting events. No more than 2·2 per cent of stream power was used to evacuate sediment during any month. This study demonstrates the danger of attributing declining rates of gully growth to a shrinking catchment area if corroborative runoff and base?ow data are not available. Moreover, it illustrates that stream power alone provides only a rough and physically indirect measure of erosion potential. Copyright © 2004 John Wiley & Sons, Ltd.     

A benthic chamber for intensely metabolic lotic systems

We have designed a flow-through benthic chamber that has the desirable characteristics of: simulation of tidal currents, continuous operation, ability to measure photosynthesis, respiration and nutrient exchanges, and maintenance of near-ambient oxygen conditions in a highly metabolic environment. The apparatus is based on a 1.3 m long Plexiglas tunnel, closed at bith ends, with both constant circulation and periodic flushing. The results of a 9-month study at one location in Flax Pond, a Long Island, New York, tidal pond, show large seasonal variations, influences of both sunlight intensity and seasonality, and a P/R ratio of less than one.     

Influence of rural land use on streamwater nutrients and their ecological significance

Concentrations and loads of N and P fractions were examined for lowland rivers, the Wye and Avon, draining a range of representative agricultural land-use types in two major UK river basins. Data collected over a 2-year period demonstrated important diffuse agricultural source contributions to N and P loads in these rivers. Ground water provided a major source of total dissolved nitrogen (TDN) loads, whereas near-surface sources provided a major contribution to total phosphorus (TP) loads. In terms of aquatic ecology, concentrations of nutrients, at times of eutrophication risk (spring and summer low flows) were of key environmental and management significance. Agricultural diffuse sources provided the major source of long-term P loads across the two basins. However, the results demonstrated the dominance of point-source contributions to TP and SRP concentrations at times of ecological risk. Point sources typically ‘tip the balance’ of dissolved inorganic P (soluble reactive P, SRP) above the 100 μg l⁻¹ guideline value indicative of eutrophication risk. The significance of point sources for TP and SRP concentrations was shown by (a) the strong correlations between TP, SRP and B concentrations, using B as a tracer of sewage effluent, (b) the dominant contribution of SRP to TP concentrations and (c) the predominant pattern of dilution of SRP and B with flow. The clean Chalk streams draining low intensity grassland in areas of the Avon with sparse human settlement were oligotrophic and P limited with low SRP concentrations under spring and summer baseflows attributable to groundwater sources. The data provide important insights into the ecological functioning of different lowland stream systems. There was evidence of greater SRP losses and N-limitation in a stream which drains a pond system, demonstrating the importance of longer water residence times for biological nutrient uptake.     

Large geographic and temporal extensions of the Río de la Plata Craton,South America,and its metacratonic eastern margin

Integration of existing isotopic and geological data allows a reconsideration of the distribution and age of the Río de la Plata Craton within South America. The reinterpretation increases the area of the craton to about 2,400,000 km² with implications for the tectonic map of South America and for global reconstruction of palaeocontinents. Four areas previously considered as separate cratons (Luís Alves, Curitiba, Tebicuary, and Paranapanema) are interpreted as part of the same Río de la Plata Craton. The craton is organized into six provinces and domains: Buenos Aires–Piedra Alta, Taquarembó, Tebicuary, Luís Alves, Encantadas, and Nico Pérez. The term ‘Transplatense’ is proposed to replace ‘Trans-Amazonian’ for Rhyacian events that occurred within the Río de la Plata Craton. The craton is formed not only by dominant Rhyacian rocks and local Archaean rocks, but also by Statherian and Mesoproterozoic rocks. The domains are all partially to totally covered by Phanerozoic basins (Paraná, Chacoparanense, Claromecó, Salado, Balcarce, and Colorado) which makes their investigation difficult. The Ediacaran–Cambrian collisions of the Brasilian orogen generated tectonic mixtures of orogenic and cratonic zones. This is more evident in the eastern margin of the craton, which behaved as a metacraton.     

The value of high-resolution nutrient monitoring: A case study of the River Frome, Dorset, UK

The River Frome was sampled at sub-daily sampling interval, with additional storm sampling, through an annual cycle. Samples were analysed for total phosphorus (TP), soluble reactive phosphorus (SRP), total oxidisable nitrogen (TON) and dissolved reactive silicon (Si). The resulting data set was artificially decimated to mimic sampling frequencies from 12 h to monthly time interval. Monthly sampling interval resulted in significant errors in the estimated annual TP and SRP load of up to 35% and 28% respectively, and the resulting data sets were insufficient to observe peaks in P concentration in response to storm events. Weekly sampling reduced the maximum percentage errors in annual load estimate to 15.4% and 6.5%. TON and silicon concentrations were less variable with changing river flow, and monthly sampling was sufficient to predict annual load estimates to within 10%. However, to investigate within-river nutrient dynamics and behaviour, it is suggested that a weekly sampling interval would be the minimum frequency required for TON and Si studies, and daily sampling would be a minimum requirement to adequately investigate phosphorus dynamics. The loss in nutrient-concentration signal due to reduced sampling interval is presented. Hysteresis in the nutrient concentration/flow relationships for all 32 storm events during the study period were modelled and seasonal patterns discussed to infer nutrient sources and behaviour. The high-resolution monitoring in this study identified, for the first time, major peaks in phosphorus concentration in winter that coincide with sudden falls in air temperature, and was associated with biofilm breakdown. This study has shown that to understand complex catchment nutrient processes, accurately quantify nutrient exports from catchments, and observe changes in water quality as a result of nutrient mitigation efforts over time, it is vital that the newly emerging field-based automated sampler/analyzer technologies begin to be deployed, to allow for routine high-resolution monitoring of our rivers in the future.