Bulk Sediment Analyses Research Articles

Postglacial environmental change inferred from carbonate‐ and organic‐rich sediments of groundwater‐fed Kelly Lake, Kenai Peninsula, south‐central Alaska

ABSTRACTMajor shifts in hydroclimate have been documented during the last deglacial period and the Holocene in south‐central Alaska. Rare freshwater calcium carbonate (marl) deposits in lakes on the Kenai Peninsula can be used to reconstruct past changes in hydroclimate, including the influence of groundwater inflow to lakes. Here, the postglacial sediment sequence from groundwater‐fed Kelly Lake (60.514°N, 150.374°W) was analyzed for multiple proxies including isotopes of carbon and oxygen in marl calcite (δ13Cmarl and δ18Omarl), and isotopes of carbon (ẟ13COM) and abundances of C and N in organic matter. Bulk sediment analyses include organic matter and calcium carbonate (CaCO3) contents, visual stratigraphy and sediment flux. These analyses extend those of a previous paleoenvironmental reconstruction from Kelly Lake, which focused on sedimentary diatom oxygen isotopes and mass balance modeling over the past 10 000 years. Here, we show that Kelly Lake was deglaciated prior to 14.6 ka, and that by 14.0 ka marl dominated the sediments, with CaCO3 precipitation probably driven by groundwater input and mediated by shallow‐water charophytes. Marl accumulation decreased as organic and clastic inputs increased between ~12.2 and 11.5 ka. This shift, together with an increase in both δ13Cmarl and δ18Omarl values and a decrease in CaCO3 content, indicates an increase in the influence of meteoric water on the hydrologic budget under wet conditions, possibly driven by a strengthened Aleutian Low atmospheric pressure cell. A shift to lower δ13Cmarl and δ18Omarl values at ~11.5 ka is interpreted as an increase in the proportion of groundwater relative to meteoric water in the lake. Beginning around 9 ka, the proportion of meteoric water input continued to increase, the surrounding coniferous forest was established, and by 8 ka, CaCO3 accumulation ended. Our results elucidate the environmental conditions under which marl was deposited during the Lateglacial and early Holocene in this part of Alaska, and demonstrate how a variety of synoptic‐ and local‐scale climatic variables can converge to influence sedimentation in a groundwater‐fed lake.

Assessing the chemical anthropocene – Development of the legacy pollution fingerprint in the North Sea during the last century

The North Sea and its coastal zones are heavily impacted by anthropogenic activities, which has resulted in significant chemical pollution ever since the beginning of the industrialization in Europe during the 19th century. In order to assess the chemical Anthropocene, natural archives, such as sediment cores, can serve as a valuable data source to reconstruct historical emission trends and to verify the effectiveness of changing environmental legislation. In this study, we investigated 90 contaminants covering inorganic and organic pollutant groups analyzed in a set of sediment cores taken in the North Seas' main sedimentation area (Skagerrak). We thereby develop a chemical pollution fingerprint that records the constant input of pollutants over time and illustrates their continued great relevance for the present. Additionally, samples were radiometrically dated and PAH and PCB levels in porewater were determined using equilibrium passive sampling. Furthermore, we elucidated the origin of lead (Pb) contamination utilizing non-traditional stable isotopic analysis. Our results reveal three main findings: 1. for all organic contaminant groups covered (PAHs, OCPs, PCBs, PBDEs and PFASs) as well as the elements lead (Pb) and titanium (Ti), determined concentrations decreased towards more recent deposited sediment. These decreasing trends could be linked to the time of introductions of restrictions and bans and therefor our results confirm, amongst possible other factors, the effectiveness of environmental legislation by revealing a successive change in contamination levels over the decades. 2. concentration trends for ΣPAH and ΣPCB measured in porewater correspond well with the ones found in sediment which suggests that this method can be a useful expansion to traditional bulk sediment analysis to determine the biologically available pollutant fraction. 3. Arsenic (As) concentrations were higher in younger sediment layers, potentially caused by emissions of corroded warfare material disposed in the study area after WW II.

Estimating molecular preservation of the intestinal microbiome via metagenomic analyses of latrine sediments from two medieval cities.

Ancient latrine sediments, which contain the concentrated collective biological waste of past whole human communities, have the potential to be excellent proxies for human gastrointestinal health on the population level. A rich body of literature explores their use to detect the presence of gut-associated eukaryotic parasites through microscopy, immunoassays and genetics. Despite this interest, a lack of studies have explored the whole genetic content of ancient latrine sediments through consideration not only of gut-associated parasites, but also of core community gut microbiome signals that remain from the group that used the latrine. Here, we present a metagenomic analysis of bulk sediment from medieval latrines in Riga (Latvia) and Jerusalem. Our analyses reveal survival of microbial DNA representative of intestinal flora as well as numerous parasites. These data are compared against parasite taxon identifications obtained via microscopy and ELISA techniques. Together, these findings provide a first glimpse into the rich prokaryotic and eukaryotic intestinal flora of pre-industrial agricultural populations, which may give a better context for interpreting the health of modern microbiomes.This article is part of the theme issue ‘Insights into health and disease from ancient biomolecules’.

Mixing depths and sediment accumulation rates on an arid tropical shelf based on fine-fraction 210Pb analysis

Continental shelves in arid tropical settings present particular challenges to 210Pb-based analysis of sedimentation rates and surface mixing owing to the combination of coarse sediment, deep and year-round bioturbation, and lower atmospheric flux of 210Pb. The modern continental shelf at the northern terminus of the hyper-arid Gulf of Eilat/Aqaba (GOE) receives flood-runoff of siliciclastics, producing a mixed seabed with ~20% carbonate that is dominated by larger benthic foraminifera and mollusks. Focusing analysis on only the fine-fraction of deep-penetrating cores (≥70 cm) from 15 to 40 m water depths yields reliable 210Pb profiles, in contrast to analysis of bulk sediment (graphical abstract). Sedimentation rates increase offshore five-fold, from 0.01–0.04 cm/y in 15 m water depth to 0.21–0.27 cm/y in 30–40 m depths, reflecting offshore redistribution of flood-delivered siliciclastic sediments away from the wadi mouth, a result also supported by an offshore increase in the inventory of excess 210Pb. In contrast, the thickness of the surface mixed layer (SML) decreases from >30 cm to ~20 cm with proximity to anthropogenic stressors (channelized flood runoff, historic release of sewage and operation of fish cages), which we attribute to the suppression of macrobenthic burrowers. The rate of sedimentation on the Gulf shelf –away from dynamic bypassing in the 15 m shoreface – is comparable to other tropical carbonate shelves rich in large benthic foraminifera, and is higher than rates documented on the adjacent slope, increasing confidence in this approach to 210Pb analysis using only the fine fraction. Analysis of the fine fraction rather than bulk sediment would be a useful adjustment to 210Pb methodology in any area with scarce fine-grained sediment.

The chromium isotope composition of reducing and oxic marine sediments

The chromium (Cr) isotope composition of marine sediments has the potential to provide new insights into the evolution of Earth-surface redox conditions. There are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport, the major source of seawater Cr, and with partial Cr reduction during burial in marine sediments, the major sink for seawater Cr. A more comprehensive understanding of these processes is needed to establish global Cr isotope mass balance and to gauge the utility of Cr isotopes as a paleoredox proxy. For these purposes, we investigated the Cr isotope composition of reducing sediments from the upwelling zone of the Peru Margin and the deep Cariaco Basin. Chromium is present in marine sediments in both detrital and authigenic phases, and to estimate the isotopic composition of the authigenic fraction, we measured δ53Cr on a weakly acid-leached fraction in addition to the bulk sediment. In an effort to examine potential variability in the Cr isotope composition of the detrital fraction, we also measured δ53Cr on a variety of oxic marine sediments that contain minimal authigenic Cr. The average δ53Cr value of the oxic sediments examined here is −0.05±0.10‰ (2σ, n=25), which is within the range of δ53Cr values characteristic of the bulk silicate Earth. This implies that uncertainty in estimates of authigenic δ53Cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of Cr in detrital versus authigenic phases, rather than to the Cr-isotopic composition of the detrital pool. Leaches of Cariaco Basin sediments have an average δ53Cr value of +0.38±0.10‰ (2σ, n=7), which shows no dependency on sample location within the basin and is close to that of Atlantic deepwater Cr (∼+0.5‰). This suggests that authigenic Cr in anoxic sediments may reliably reflect the first-order Cr isotope composition of deepwaters. For Peru Margin samples, the average δ53Cr values of bulk sediments (+0.59±0.06‰; 2σ) and leach fractions (+0.61±0.06‰; 2σ) are also comparable with those of Cariaco Basin samples and modern deepwater values. Finally, we found that the δ53Cr of Peru Margin samples correlates with δ15N on glacial–interglacial timescales, which we attribute to secular variation in basinal or global-ocean redox conditions. Thus, the δ53Cr stratigraphic record of Peru Margin sediments indicates that Cr isotopes may be suited to tracking geologically short-term changes in ocean oxygenation.

Stable nitrogen isotope analysis of the Upper Devonian palynomorph, Tasmanites

Abstract Bulk samples are commonly used in stable (N) isotope studies to infer climate changes. However, due to uncertainties in the composition of bulk samples, the inconsistency of diagenetic alteration and the variation of dominant oceanic features, such as N-fixation and denitrification, the isolation of specific sedimentary N fractions has become a crucial step to understanding the nitrogen cycle. This paper describes the extraction techniques and the stable nitrogen isotope analysis (on a nano-EA-IRMS) of the palynomorph, Tasmanites spp. (80–200 μm) found in Upper Devonian (Famennian) black shales from Kentucky, USA. Tasmanites is a widely occurring and long-ranging prasinophyte (unicellular green algae) genus. Palynomorphs, such as Tasmanites, are highly resistant to most forms of decay. It is therefore suggested that the δ15N signature of this palynomorph may be a useful, unaltered indicator of baseline isotopic change in future palaeoclimate studies. The greatly reduced sample size accepted by the nano-EA-IRMS makes this technique a viable complement to bulk sediment analyses. The results, using the nano-EA-IRMS, demonstrate that the δ15N signature of the bulk organic samples differ significantly from that of the isolated Tasmanites spp. Three distinct isotopic relationships were observed from the three different sample localities. These include parallel changes in the δ15N values of bulk samples and extracted Tasmanites spp., variation in the δ15N of bulk samples, with no corresponding change recorded in Tasmanites spp., and opposing δ15N changes recorded between the bulk samples and the extracted Tasmanites spp. This inconsistent isotopic variation further emphasises the complexity of the nitrogen cycle. Possible explanations for the observed variation include changes in the rate of N-fixation and denitrification due to climate change, incomplete nitrification, and variation in the bulk organic content.

The influence of extraterrestrial material on the late Eocene marine Os isotope record

A reconstruction of seawater 187Os/188Os ratios during the late Eocene (∼36–34Ma), based upon bulk sediment analyses from the sub-Antarctic Southern Atlantic Ocean (Ocean Drilling Program (ODP) Site 1090), Eastern Equatorial Pacific Ocean (ODP Sites 1218 and 1219) and the uplifted (land-based) Tethyan section (Massignano, Italy), confirms that the previously reported abrupt shift to lower 187Os/188Os is a unique global feature of the marine Os isotope record that occurs in magnetochron C16n.1n. This feature is interpreted to represent the change in seawater 187Os/188Os caused by the Popigai impact event. Higher in the Massignano section, two other iridium anomalies previously proposed to represent additional impact events do not show a comparable excursion to low 187Os/188Os, suggesting that these horizons do not record multiple large impacts. Comparison of records from three different ocean basins indicates that seawater 187Os/188Os begins to decline in advance of the Popigai impact event. At Massignano this decline coincides with a previously reported episode of elevated 3He flux, suggesting that increased influx of interplanetary dust particles (IDPs) contributed to the pre-impact shift in 187Os/188Os and not to the longer-term latest Eocene 187Os/188Os decline that occurred ∼1million year after the Popigai impact event.

The evolution of Mediterranean wetlands in the first millennium AD: The case of Les Arenes floodplain (Tortosa, NE Spain)

Mediterranean wetlands are characterised by two factors: (1) a remarkable seasonal fluctuation in the water table and (2) a long history of anthropisation. Being exceptional archives for palaeoenvironmental and archaeological research, humid areas have been thoroughly studied to assess the interplay between people and the environment during the Holocene. We present here the study of a sedimentary sequence from a Mediterranean wetland: the Les Arenes floodplain (Tortosa, NE Spain). Fully dry to date, Les Arenes is documented as a partially drained floodplain in the 12th century records. With the aim of assessing the climatic and anthropic contribution to its desiccation, we report here the results of bulk sediment analyses, environmental magnetism, soil micromorphology and AMS dating of this sedimentary sequence, plus a review of the available historical records. The outcome is a clearer understanding of the evolution of this Mediterranean floodplain during the 1st millennium AD, which implications may be extrapolated to other floodplain wetlands in the Iberian Peninsula and the Mediterranean. Wet conditions and a low to medium-energy depositional environment are attested in Les Arenes between the 1st and 7th centuries coinciding with the Iberian-Roman Humid Period (IRHP) and the Dark Age Humid Period (DAHP). Drier conditions between the 7th and 10th centuries concur with enhanced fluvial dynamics, the Medieval Climate Anomaly (MCA) and the first arrival of Arabic-Berber populations in the Iberian Peninsula (al-Andalus, starting in AD 711). Results suggest that although the shift from wet to dry conditions in Les Arenes complies with major climatic trends, the construction of the first drainage canals by Arab-Berber groups likely contributed to the enhancement of on-going climate-related environmental processes.

Identifying recent sources of organic matter enrichment and eutrophication trends at coastal sites using stable nitrogen and carbon isotope ratios in sediment cores

We studied the potential for using stable carbon and nitrogen isotope ratios in sediment profiles to trace external nutrient sources and eutrophication at four coastal sites in the Baltic Sea. The sites are characterized by various present and past activities in their catchments, including residential development, sugar processing, agriculture and fish farming. Radiometrically dated sediment cores were analysed for nutrient isotope ratios, organic carbon and total nitrogen. Background information was collected from historical sources, literature and water monitoring data. Despite the multiple organic enrichment sources, it was possible to identify individual sources and processes in the sediment profiles using stable isotope analysis of bulk sediment. The largest changes in δ15N values were seen at sites receiving urban wastewaters. The site that received effluents from a sugar cane (C4-plant) refinery in the past showed a clear effect on δ13C values compared to the site that received wastewater from a sugar beet (C3-plant) factory. Fish farming produced detectable, albeit minor changes in the sediment profile. Slightly lower δ13C values reflected the influence of fish feed and fish metabolism, and higher δ15N values likely indicated the influence of increased sediment denitrification. The land-sea connection via river discharge was observable in the overall δ13C levels of the sediment cores. Our results suggest that temporal changes in sources of organic matter enrichment can be detected in well-dated coastal sediment cores using nutrient stable isotope analyses, even at sites subjected to multiple impacts. There is not, however, a simple relationship between sediment stable isotope profiles and the eutrophication history of our study sites.

Upwelling conditions in the Early Miocene Central Paratethys Sea

Upwelling conditions in the Early Miocene Central Paratethys SeaEvidence for regional upwelling conditions in the Central Paratethys Sea is presented for mid-Burdigalian (early Ottnangian) times. The oceanographic phenomenon is detected in clay-diatomite successions along the steep escarpment of the Bohemian Massif in the eastern North Alpine Foreland Basin. Interpretations are based on a multiproxy data-set including published sedimentological and paleontological data, newly performed stable isotope measurements (δ18O, δ13C) of foraminifers and bulk sediment samples, and analyses of dinoflagellate cyst assemblages. The revealed stable isotope values of planktonic foraminifers point to upwelling: low δ13C values indicate strong mixing of surface waters with rising nutrient-rich waters, high δ18O values reflect cool sea surface temperatures (SST). Temperature calculations give SSTs ranging from 10-14 °C. Cool SSTs and high productivity are additionally supported by bulk sediment analyses. Assemblages of dinoflagellate cysts indicate a distal-shelf environment with nutrient-rich waters. Westerly winds and tidal currents are discussed as potential driving forces behind the local upwelling event. As mid-Burdigalian geography favoured strong current patterns in the Central Paratethys as documented in the sedimentary record from the Rhône Basin to Hungary upwelling might have been a more common phenomenon in this epicontinental sea than currently known.

Monsoon control over erosion patterns in the Western Himalaya: possible feed-back into the tectonic evolution

Abstract The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative ε Nd and higher 87 Sr/ 86 Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ 13 C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. Supplementary material: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412

Palaeoenvironmental and Archaeological Implications of a Sediment Core from Polje Čepić, Istria, Croatia

Palaeoenvironmental and archaeological records provide an invaluableframework for land and water management in karstic areas of theMediterranean realm. We present the results from analyses carried outon three segments of a sediment core extracted in 2004 from a portionof Polje Čepić (Istria, Croatia), a tectono-karstic depression coveredin water until artificial drainage took place in 1932. We used bulksediment analyses, pollen analysis and radiocarbon dating to assessthe possible contribution of people to the progressive siltation of thelake, and compared our results with recent archaeological discoveriesmade on the polje margins. The dating of the sequence points tothe presence of a predominantly wet landscape at the coring locationsince at least 7000 years cal BP. The coincidence of cereal type pollengrains with several open-air archaeological sites indicates that smallscaleagricultural practices possibly developed around the polje inNeolithic times, ca 6500 years cal BP. Concentrations of charcoal inthe sediment core at this time suggest the use of fire clearance as partof the agricultural practice.During the Bronze and Iron Ages, the possible intensificationof forest clearance and agricultural practices is indicated by: (1) anincrease in the number and type of archaeological sites found aroundthe polje; (2) a more open plant landscape in the upper part of the analysed sequence; and (3) a three-fold increase in the sedimentation rate, possibly starting as early as 4000 years cal BP. Wetter climate conditions and higher erosion of the surrounding slopes probably led to the progressive siltation of the lake. Finally, in 1932 the lake was artificially drained and Istria lost its largest natural basin of fresh water.

Sources of REE in sediment cores from the Rainbow vent site (36°14′N, MAR)

A geochemical investigation was carried out on two sediment cores collected at 2 and 5 km from the Rainbow hydrothermal vent site. Bulk sediment compositions indicate that these cores record clear enrichments in Fe, Cu, Mn, V, P and As from hydrothermal plume fallout ( Cave et al., 2002) [Cave, R.R., German, C.R., Thomson, J., Nesbitt, R.W., 2002. Fluxes to sediments underlying the Rainbow hydrothermal plume at 36°14′N on the Mid-Atlantic Ridge. Geochim. Cosmochim. Acta 66 (11), 1905–1923]. Sequential dissolution of the bulk sediments has been used to discriminate between a leach (biogenic and oxy-hydroxide) component and a residual phase (detrital and sulphide/sulphate fractions). Major element data (Al, Fe, Ti, Mn, Mg, Ca, Si and index%) reveal that the hydrothermal input, as recorded in the leach phase, is much stronger than apparent from bulk sediment analyses alone. REE patterns for the leach phase record contributions from both biogenic carbonate (mimicking seawater REE patterns) and hydrothermal oxy-hydroxides, with the latter exhibiting positive Eu anomalies (hydrothermal derived) and negative Ce anomalies (seawater derived). Based on major element and REE data, the residue contains contributions from aeolian dust input, local MORB material and a hydrothermal component. Ternary REE mixing calculations indicate that most of the REE within the residual fraction (∼80%) is derived from hydrothermal material, while detrital contributions to the REE budget, as deep-sea clay and volcanic debris, are <20%. By combining bulk and REE data for the various end-member components of the residue, we calculate that the chemical composition of the residue hydrothermal end-member is high in Ca (6–15%) and with a Nd/Sr ratio of 0.004. These characteristics indicate the presence of low-solubility hydrothermal sulphate (rather than sulphide) material within the residue component of Rainbow hydrothermal sediments.

Sedimentological and stratigraphic observations in El Mirón, a late Quaternary cave site in the Cantabrian Cordillera, Northern Spain

AbstractEl Mirón cave on the northern edge of the Cantabrian Cordillera is one of the first montane sites in this area to be the object of modern interdisciplinary research. Five excavation seasons have revealed an archaeological sequence attributable to the Medieval, Bronze, Chalcolithic, Neolithic, Mesolithic, Azilian, Magdalenian, and Solutrean periods, and early Upper and Middle Paleolithic levels have just been reached. Forty‐four radiocarbon ages range from 900 to 41,000 14C yr B.P. Sediments in the vestibule include sediment redeposited from an ancient colluvial fill in the inner cave, limestone rock fragments, biogenic CaCO3 generated by cryptogamic and vegetation growth on the cave ceiling, eolian silts, bird droppings, and anthropogenic lithic and faunal remains. Sedimentological analyses of the cave fill reported here document the transition from a sandy silt facies with rounded cobbles in the inner vestibule to a carbonate‐rich silt facies with angular limestone clasts near the cave mouth. There is no clear regional paleoclimatic signal in the bulk sediment analyses because of the heavy influence of local input and human disturbance. Micromorphology by M.‐A. Courty (see Courty and Vallverdu [this issue]; and also Courty et al. [1989]) reveals subtle evidence for some short‐term variations that can be correlated with regional paleoclimates, such as the Younger Dryas event. © 2001 John Wiley &amp; Sons, Inc.

Nd Isotope Evidence for Provenance of the Terrigenous Fraction of Deglacial Sediments in the Orca Basin, Gulf of Mexico

Documentation of the routing of fresh water runoff during disintegration of the Laurentide Ice Sheet is important to understanding the impact of glacial fresh water contributions to thermohaline circulation in the North Atlantic. During the Last Glacial Maximum, the Gulf of St. Lawrence drainage was blocked, and glacial lakes Agassiz and Superior were damned and filled with meltwater (Broecker et al., 1989). Oxygen isotope data from foraminifera of the Orca Basin in the Gulf of Mexico have been used to suggest that catastrophic meltwater floods from these glacial lakes were channeled down the Mississippi River, starting about 18 ka and ending by 10.5 ka (Leventer et al., 1982). After the ice sheet retreated far enough to the north, fresh water from the melting ice was sent directly into the North Atlantic via the St. Lawrence River drainage. It has also been suggested that this change in meltwater routing could have been the trigger for the Younger Dryas cooling (Rooth, 1982; Broecker et al., 1989). The suggestion that it is possible to temporarily shut down deep water formation in the North Atlantic by adding a spike of fresh water through the St. Lawrence drainage, thereby reducing the transport of heat from northward-flowing, warm surface currents has been supported by global circulation models. Work is in progress to evaluate radiogenic isotope evidence for variations in terrigenous sediment sources via the Mississippi River during late glacial and Holocene times. Previously, Hurley et al. (1961) documented that bulk, surface sediments from a variety of delta and pro-delta settings on the Mississippi River produce a relatively small range of apparent K-Ar ages of 214 to 357 Ma. In contrast, clay size fractions from normal gray sediments at various core depths within Mississippi Delta cores yield apparent ages of 145 to 206 Ma, and bulk sediment aliquots from these same samples yielded apparent ages of 280 to 390 Ma, similar to bulk sediment analyses of surface sediments. The interpretation of these results was that the coarsest potassium-rich fractions consist of minerals derived from crystalline rocks of the Appalachian Mountains with ages of c. 400 Ma. Hurley et al. (1961) also suggested that these coarser fractions additionally contained a larger proportion of illite, a stable, highK mineral that is common in Palaeozoic shale deposits such as those tapped by the Ohio River. The finer fraction contains a larger proportion of smectite derived from younger, Cretaceous and Tertiary, sedimentary and volcanic sources to the west, via the Missouri River. The truly impressive part of the Hurley et al. (1961) study was their data from a red-brown clay layer from down-core samples from two different cores. In contrast to the gray clay samples, bulk samples of this red-brown layer yielded apparent ages of 540 to 810 Ma! In order to have made such a tremendous impact on the provenance of sediment from such a mighty river as the Mississippi must have required an enormous and catastrophic flooding event. Hurley et al. (1961) reasonably interpreted the ancient age for the red-brown layer to be a product of sources from the southern Colorado Front Range via the Red River. An alternative hypothesis is that the flood might instead reflect one of the catastrophic floods known to have flowed into the Gulf of Mexico from glacial lakes pinned against the southern margin of the Laurentide Ice Sheet during its final phases. An appropriate approach to testing reconstructions of drainage history is to analyse the radiogenic isotope composition of terrigenous sediments in the Orca Basin, a very anoxic basin located near the Mississippi Delta. The highly anoxic conditions tend to prevent biological disturbance of sediment layering and thus a detailed stratigraphy is possible. Several studies have been published on oxygen isotope evidence from Orca Basin cores for the occurrence of a large meltwater event in the Gulf of Mexico that ended shortly before the Younger Dryas (Broecker et al., 1989). Due to the very reducing conditions in the Orca Basin, the red colouration noted by Hurley et al. (1961) on the Mississippi Delta would not likely be preserved in these sediments. New data collected for this study show that sediments from within the meltwater interval of core EN32 PC6

The geochemistry of the Keli Mutu crater lakes, Flores, Indonesia.

Three crater lakes occur on top of the dormant stratovolcano Keli Mutu in Flores (Indonesia). The lakes contain cool (20–30°C) acid-sulfate-chloride brines and have high TDS with relatively high Zn and Pb contents. Two lakes show plume-like upwellings, probably fed by subaqueous fumaroles. A river on the mountain slope contains fluids that escaped from the lakes through seepage. The fluid compositions have varied strongly over time for S and the halogens (the volcanic-gas derived elements), whereas many of the major cations (rock-derived elements) show a net increase in the lakes over this century. Speciation and mineral saturation modeling, bulk sediment analyses, and micro-probe sediment analyses indicate that the lakes are reducing at depth, and all three lakes are close to saturation with gypsum/anhydrite. One lake is also saturated with Fe-oxides, while the other two are thought to be saturated with a suite of Cu and As minerals. The most acid lake is also saturated with native sulfur and possibly pyrite. Mass balance estimates based on fresh volcanic rocks and ashes, the dissolved element load and the lake sediment chemistry strongly suggest that the lakes are partly fed by underlying geothermal circulation systems. The lakes are thus not closed reaction vessels with dissolution of local rock debris by fluids acidified by volcanic gases, but they draw a rock-derived element flux from deeper levels. Mass transfer is large in these dynamic systems, including the transport of tonnes of ore metals over a period of centuries.

Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora.

The regulatory effect of NH4+ on nitrogen fixation in a Spartina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4+ in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4+. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the NH4+ switch-off-switch-on mechanism of nitrogenase regulation.

A 96-hour sediment bioassay of Duluth and Superior Harbor basins (Minnesota) using Hexagenia limbata, Asellus communis, Daphnia magna, and Pimephales promelas as test organisms.

The environmental effects of sediments upon water quality and the aquatic ecosystem are currently being investigated by bulk sediment analysis or elutriate procedures, with neither procedure being totally adequate. Using a sediment bioassay procedure developed at the Central Regional Laboratory, U.S. EPA, Region V by Prater and Anderson (1977), sediment bioassays were conducted on eight sediments collected from Duluth and Superior Harbors in July of 1976. The stations were chosen to reflect areas that were suspected to be heavily polluted, moderately polluted, and nonpolluted. In addition, a correlation was drawn between the percent mortality of biological test organisms and the bulk sediment chemistry data collected by Bowden (1976).