Obituary

Geological sections are still best drawn at natural scale. Sections with vertical exaggerations are rarely drawn correctly, and even when computed carefully give a false notion of the structural features. Many schematic sections which are not drawn to scale and are supposed to explain a proposed new tectonic interpretation give such a distorted and frequently absurd picture of the structural features that they cannot be regarded as valid.     

Sakmarian geography

The known palaeontological and stratigraphical evidence is used as a basis for the construction of maps of the continents showing the extent of their inundation by the sea in Sakmarian time in the Upper Palaeozoic. In the northern hemisphere apart from India the evidence is sufficiently reliable to give reasonable maps, and the great extent of the inundations suggests that the climate would be considerably modified from that of today; no undoubted Sakmarian glacials occur there. In Southern continents and India, the Gondwana biogeographical province has made correlation with the northern continents controversial, but reasons are given for assuming that Gondwana glacial deposits were at least in part Sakmarian; the resultant maps show that the Gondwana land surfaces were but little reduced in area, and that the main glacials (except for India) lie within a belt between 40 S and 20 S. Present lack of knowledge of Sakmarian conditions in Antarctica makes reconstructions of climatic belts too hazardous for possible use in enunciating or checking hypotheses of continental drift and polar wandering.     

Relative contributions of sulfate- and iron(III) reduction to organic matter mineralization and process controls in contrasting habitats of the Georgia saltmarsh

The objectives of this study were to partition out the predominant anaerobic respiration pathways coupled to C oxidation and to further elucidate the controls of anaerobic C respiration in three major saltmarsh habitats at Skidaway Island, GA; the short form of Spartina alterniflora (SS), the tall form of S. alterniflora (TS), and unvegetated, bioturbated creekbank (CB). Geochemical analysis of pore water and solid phase constituents revealed that the SS site experienced highly reducing conditions with two orders of magnitude higher pore water sulfide inventories (1.884 mmol m⁻²) than TS (0.003 mmol m⁻²) and CB (0.005 mmol m⁻²), respectively. Conversely, reactive Fe(III) inventories at TS (2208 mmol m⁻²) and CB (2881 mmol m⁻²) were up to 7–9 times higher than at SS (338 mmol m⁻²). Incubations and intact core experiments indicated that reduction accounted for 95% (SS), 37% (TS) and 66% (CB) of total anaerobic respiration. There was no detectable Fe(III) reduction at SS, while Fe(III) reduction accounted for up to 70% of C oxidation in the 3–6 cm depth interval at TS and 0–3 cm depth of CB, and on average, approximately 55% of C oxidation over two-thirds of marsh surface area. Laboratory manipulations provided further evidence for the importance of Fe(III) reduction as the accumulation rates of fermentation products were high when Fe(III) reduction was inhibited by removing the Fe(III) minerals from highly bioturbated CB sediments with higher Fe(III) mineral contents. Anaerobic C oxidation, - and Fe(III)-reduction rates appeared to be highest at the TS site during active plant growth in summer. Overall results suggest that bioturbation by macrofauna is the overriding factor in modulating the pathway of C mineralization in the saltmarsh, whereas availability of organic substrates from plants is a key factor in controlling the C oxidation rate.     

Quantitative analysis of variations in depositional sequence thickness from submarine channel levees

Abstract Thickness variations across‐levee and downchannel in acoustically defined depositional sequences from six submarine channel‐levee systems show consistent and quantifiable patterns. The thickness of depositional sequences perpendicular to the channel trend, i.e. across the levee, decreases exponentially, as characterized by a spatial decay constant, k. Similarly, the thickness of sediment at the levee crest decreases exponentially down the upper reaches of submarine channels and can be characterized by a second spatial decay constant, λ. The inverse of these decay constants has units of length and defines depositional length scales such that k^?1 is a measure of levee width and λ^?1 is a measure of levee length. Quantification of levee architecture in this way allowed investigation of relationships between levee architecture and channel dimensions. It was found that these measures of levee e‐folding width and levee e‐folding length are directly related to channel width and relief. The dimensions of channels and levees are thus intimately related, thereby limiting the range of potential channel‐levee morphologies, regardless of allocyclic forcing. A simple sediment budget model relates the product of the levee e‐folding width and e‐folding length to through‐channel volume discharge. A classification system based on the quantitative downchannel behaviour of levee architecture allows identification of a ‘mid‐channel’ reach, where sediment is passively transferred from the through‐channel flow to the levees as an overspilling flow. Downstream from this reach, the channel gradually looses its control on guiding turbidity currents, and the resulting flow can be considered as an unconfined or spreading flow.     

Submergence and Herbivory as Divergent Causes of Marsh Loss in Long Island Sound

Tidal marsh degradation has been attributed to a number of different causes, but few studies have examined multiple potential factors at the same sites. Differentiating the diverse drivers of marsh loss is critical to prescribing successful interventions for conservation and restoration of this important habitat. We evaluated two hypotheses for vegetation loss at two marshes in Long Island Sound (LIS): (1) marsh submergence, caused by an imbalance between sea-level rise and marsh accretion, and (2) defoliation associated with herbivory by the purple marsh crab, Sesarma reticulatum. At our western LIS site, we found no evidence of herbivory: crabs were scarce, and crab-exclusion cages provided no benefit. We attribute degradation at that site to submergence, a conclusion supported by topographic and hydrologic data showing that loss of vegetation occurred only in wetter parts of the marsh. In contrast, at our central LIS site, our observations were consistent with herbivory as a driving force: There were substantial populations of Sesarma, crab-exclusion cages allowed plants to thrive, and vegetation loss took place across a variety of elevations. We also analyzed soil conditions at both sites, in order to determine the signatures of different degradation processes and assess the potential for restoration. At the submergence site, unvegetated soils exhibited high bulk density, low organic content, and low soil strength, posing significant biogeochemical challenges to re-colonization by vegetation. At the herbivory site, unvegetated soils had a characteristic “riddled-peat” appearance, resulting from expansion and erosion of Sesarma burrow networks. The high redox potential and organic content of those soils suggested that revegetation at the herbivory site would be likely if Sesarma populations could be controlled before erosion leads to elevation loss.     

Inferring the mass fraction of floc-deposited mud: application to fine-grained turbidites

Fine sediment deposition in the ocean is complicated by the cohesive nature of muds and their tendency to flocculate. The result is disaggregated inorganic grain size (DIGS) distributions of bottom sediment that are influenced by single‐grain and floc deposition. This study outlines a parametric model that characterizes bottom sediment DIGS distributions. Modelled parameters are then used to infer depositional conditions that account for the regional variation in the grain sizes deposited by turbidity currents on the Laurentian Fan–Sohm Abyssal Plain, offshore south‐eastern Canada. Results indicate that, on the channellized Laurentian Fan, the mass fraction of floc‐deposited mud increases only slightly downslope. The small evolution in this fraction arises because sediment concentration and turbulent energy are associated in turbidity currents. On the Sohm Abyssal Plain, however, the mass fraction of floc‐deposited mud decreases, probably as a result of lower sediment concentration at this source‐distal site. Estimates of the mass fraction of mud deposited as flocs suggest that floc deposition is the dominant mode by which sediment is lost from suspension, although single‐grain deposition contributes more to the depositional flux in proximal areas where high energy breaks flocs and in distal areas where low sediment concentration limits floc formation. It is concluded that, throughout the dispersal system, changes in the fraction of flocculated mud deposited from turbidity currents reflect changes in sediment concentration and energy downslope.     

Laboratory investigation of the effects of mineral size and concentration on the formation of oil-mineral aggregates

Controlled laboratory studies of the formation of oil-mineral aggregates (OMA) in seawater demonstrate that sediment concentration and sediment size are key variables for determining the quantity of oil droplets stabilised by OMA formation. Experiments with a single sediment size and a range of sediment concentrations show that as sediment concentration increases, the quantity of oil trapped in OMA increases abruptly. In experiments with a single sediment concentration and a range of sediment sizes, the quantity of oil trapped in OMA decreases as sediment size increases. These results provide direct support to the hypothesis that there is a critical sediment concentration for OMA formation. Below this concentration, stabilisation of oil droplets by OMA decreases rapidly, while above this concentration, stabilisation is extensive. The results also support simple geometric models of OMA formation that predict that the critical sediment mass concentration increases linearly with sediment particle diameter. These results will help to place quantitative constraint on predictions of where and when OMA formation will be a factor in the natural dispersal of oil accidentally spilled into the ocean.     

Three-dimensional modelling of a Holocene tufa system in the Lathkill Valley, north Derbyshire, using ground-penetrating radar

Unlithified and partly lithified carbonate sequences are ideally suited to the application of ground‐penetrating radar (GPR), augmented by percussion augering and shallow seismic techniques, all tied to present‐day topography using global positioning system (GPS) methods. This methodology provides the first clear information on the distribution and geometry of lithofacies within buried tufa complexes. The approach has been applied to a thick succession of Holocene tufas filling a gorge site along a 3·5‐km length of the River Lathkill, north Derbyshire. Earlier studies have demonstrated the presence of up to 16 m of tufas and sapropels associated with two transverse tufa dams (barrages). These strata have been accumulating throughout the Holocene, although tufa developments at present are of minor extent. Internal tufa morphologies are recorded by GPR as ‘bright’, laterally continuous reflections for lithified, concretionary and lithoclast‐rich horizons. The ‘brightest’ reflectors occur within well‐cemented barrages and delineate core areas and prograding buttress zones. In contrast, unlithified lime muds and sapropels produce low‐contrast reflections. Lithostratigraphic control and depth calibration of the GPR profiles was provided by percussion augering at selected sites. Six distinct lithofacies and four secondary barrages are identified in the study. Constructional and destructional events can be identified and correlated within the GPR profile network, and the internal growth morphologies of the barrages are apparent. GPR profiles also clearly define the evolution of the facies geometries. Three phases of tufa development can be recognized within the GPR data and greatly extend our understanding of Holocene tufa‐forming processes in valley sites: (a) Early Holocene barrage build‐ups but with limited paludal deposition; (b) Middle Holocene ponding and sapropel accumulation under ‘warm’ conditions; and (c) Late Holocene barrage termination and valley levelling, probably coincidental with anthropogenic activity. This type of multidisciplinary approach should be considered as an essential prerequisite to all biostratigraphic and geochemical studies of Holocene freshwater carbonate sites.