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USGS2001

Salt diapirs in the Dead Sea basin and their relationship to Quaternary extensional tectonics

Regional extension of a brittle overburden and underlying salt causes differential loading that is thought to initiate the rise of reactive diapirs below and through regions of thin overburden. We present a modern example of a large salt diapir in the Dead Sea pull-apart basin, the Lisan diapir, which we believe was formed during the Quaternary due to basin transtension and subsidence. Using newly released seismic data that are correlated to several deep wells, we determine the size of the diapir to be 13 x 10 km. its maximum depth 7.2 km. and its roof 125 m below the surface. From seismic stratigraphy, we infer that the diapir started rising during the early to middle Pleistocene as this section of the basin underwater rapid subsidence and significant extension of the overburden. During the middle to late Pleistocene, the diapir pierced through the extensionally thinned overburden, as indicated by rim synclines, which attest to rapid salt withdrawal from the surrounding regions. Slight positive topography above the diapir and shallow folded horizons indicate that it is still rising intermittently. The smaller Sedom diapir, exposed along the western bounding fault of the basin is presently rising and forms a 200 m-high ridge. Its initiation is explained by localized E-W extension due monoclinal draping over the edge of a rapidly subsiding basin during the early to middle Pleistocene, and its continued rise by lateral squeezing due to continued rotation of the Amazyahu diagonal fault.

Marine and Petroleum Geology↗

USGS2002

CPT site characterization for seismic hazards in the New Madrid seismic zone

A series of cone penetration tests (CPTs) were conducted in the vicinity of the New Madrid seismic zone in central USA for quantifying seismic hazards, obtaining geotechnical soil properties, and conducting studies at liquefaction sites related to the 1811–1812 and prehistoric New Madrid earthquakes. The seismic piezocone provides four independent measurements for delineating the stratigraphy, liquefaction potential, and site amplification parameters. At the same location, two independent assessments of soil liquefaction susceptibility can be made using both the normalized tip resistance ( q c1N ) and shear wave velocity ( V s1 ). In lieu of traditional deterministic approaches, the CPT data can be processed using probability curves to assess the level and likelihood of future liquefaction occurrence.

Arkansas, Missouri, Tennessee↗

USGS2000

Holocene and recent sediment accumulation rates in southern Lake Michigan

Rates of sediment accumulation in Lake Michigan are a key component of its geologic history and provide important data related to societal concerns such as shoreline erosion and the fate of anthropogenic pollutants. Previous attempts to reconstruct Holocene rates of sediment accumulation in Lake Michigan, as well as in the other Laurentian Great Lakes, have been bedeviled by the effect of refractory terrestrial material on radiocarbon ages from total organic carbon samples of lake sediments. AMS radiocarbon ages on small samples of biogenic carbonate (ostracodes and mollusks) in Lake Michigan provide accurate Holocene ages. The present bicarbonate reservoir effect is estimated from shells of mollusks collected live before atmospheric nuclear testing to be 250 yr. From paired samples of biogenic carbonate and terrestrial macrofossils, the past reservoir effect is thought to be less than 500 yr. The radiocarbon ages indicate a distinct decrease in sediment accumulation rates throughout the southern basin of Lake Michigan at about 5 ka, about the time when lake level stabilized at the Nipissing level after rising rapidly for several thousand years. Average rates of sediment accumulation for the historic period (the last 150 yr) can be estimated from radioisotopes ( 210 Pb and 137 Cs), pollen stratigraphy, and changes in sediment properties associated with human activity. Multiple methods are necessary because at any given site, problems arise in the assumptions or applicability of one or more methods. In general, the mass accumulation calculations suggest that sediments were deposited 4 to 11 times faster in the historic period than before human settlement. The character of the sediment did not change in a dramatic way, but sediment magnetic properties suggest shifts in the sources of sediment. The data suggest that some of the changes in sources and (or) character of the sediment occurred just before human settlement and were probably related to climatic changes associated with the Little Ice Age.

Michigan↗

USGS2000

Depositional history of organic contaminants on the Palos Verdes Shelf, California

During more than 60 years, sedimentation on the Palos Verdes Shelf has been dominated by time-varying inputs of municipal wastewater from the Los Angeles County Sanitation Districts (LACSD) and debris from the Portuguese Bend Landslide (PBL). The present study examines the depositional history of wastewater-derived organic contaminants at a site approximately 6-8 km downcurrent from the outfall system. Sediments at this location are impacted by contributions from both sources, but the relative influence of the sources has changed over time. Two classes of hydrophobic organic contaminants (chlorinated hydrocarbons, long-chain alkylbenzenes) were determined in sediment cores collected in 1981 and 1992. Using molecular stratigraphy, we determined average sedimentation rates (cm/year) and mass accumulation rates (g cm-2 year-1) for the following periods: 1955-1965, 1965-1971, 1971-1981 and 1981-1992. The results show that sedimentation and mass accumulation rates increased from 1955 to 1971 and decreased from 1971 to 1981. These trends are consistent with historical information on the emission of suspended solids from the outfall system, indicating that the discharge of wastes dominated sedimentation at the site during this period. In the 1980s and early 1990s, however, mass accumulation rates increased in spite of continually decreasing emissions of wastewater solids. Several lines of evidence indicate that this increase was due to mobilization of debris from the PBL during and after unusually strong winter storms in the 1980s. As a result, heavily contaminated sediments deposited during the years of greatest waste emissions (i.e. 1950-1970) have been buried to greater sub-bottom depths, thereby reducing their availability for mobilization to the overlying water column. These results highlight the dynamic nature of sedimentation in contaminated coastal ecosystems and its importance to the long-term fate and effects of toxic substances.

Marine Chemistry↗

USGS2000

Diagenetic fate of organic contaminants on the Palos Verdes Shelf, California

Municipal wastes discharged through deepwater submarine outfalls since 1937 have contaminated sediments of the Palos Verdes Shelf. A site approximately 6–8 km downcurrent from the outfall system was chosen for a study of the diagenetic fate of organic contaminants in the waste-impacted sediments. Concentrations of three classes of hydrophobic organic contaminants (DDT+metabolites, polychlorinated biphenyls (PCBs), and the long-chain alkylbenzenes) were determined in sediment cores collected at the study site in 1981 and 1992. Differences between the composition of effluent from the major source of DDT (Montrose Chemical) and that found in sediments suggests that parent DDT was transformed by hydrolytic dehydrochlorination during the earliest stages of diagenesis. As a result, p , p ′-DDE is the dominant DDT metabolite found in shelf sediments, comprising 60–70% of ΣDDT. The p , p -DDE/ p , p ′-DDMU concentration ratio decreases with increasing sub-bottom depth in sediment cores, indicating that reductive dechlorination of p , p ′-DDE is occurring. Approximately 9–23% of the DDE inventory in the sediments may have been converted to DDMU since DDT discharges began ca. 1953. At most, this is less than half of the decline in p , p ′-DDE inventory that has been observed at the study site for the period 1981–1995. Most of the observed decrease is attributable to remobilization by processes such as sediment mixing coupled to resuspension, contaminant desorption, and current advection. Existing field data suggest that the in situ rate of DDE transformation is 10 2 –10 3 times slower than rates determined in recent laboratory microcosm experiments (Quensen, J.F., Mueller, S.A., Jain, M.K., Tiedje, J.M., 1998. Reductive dechlorination of DDE to DDMU in marine sediment microcosms. Science, 280, 722–724.). This explains why the DDT composition (i.e. o , p ′-, p , p ′-isomers of DDE, DDD, DDT) of sediments from this site have not changed significantly since at least 1972. Congener-specific PCB compositions in shelf sediments are highly uniform and show no evidence of diagenetic transformation. Apparently, the agents/factors responsible for reductive dechlorination of DDE are not also effecting alteration of the PCBs. Two types of long-chain alkylbenzenes were found in the contaminated sediments. Comparison of chain length and isomer distributions of the linear alkylbenzenes in wastewater effluent and surficial sediment samples indicate that these compounds undergo biodegradation during sedimentation. Further degradation of the linear alkylbenzenes occurs after burial despite relatively invariant isomer compositions. The branched alkylbenzenes are much more persistent than the linear alkylbenzenes, presumably due to extensive branching of the alkyl side chain. Based on these results, p , p ′-DDE, PCBs, and selected branched alkylbenzenes are sufficiently persistent for use in molecular stratigraphy. The linear alkylbenzenes may also provide information on depositional processes. However, their application as quantitative molecular tracers should be approached with caution.

California↗

USGS2000

10,000 Years of explosive eruptions of Merapi Volcano, Central Java: archaeological and modern implications

Stratigraphy and radiocarbon dating of pyroclastic deposits at Merapi Volcano, Central Java, reveals ~10,000 years of explosive eruptions. Highlights include: (1) Construction of an Old Merapi stratovolcano to the height of the present cone or slightly higher. Our oldest age for an explosive eruption is 9630±60 14 C y B.P.; construction of Old Merapi certainly began earlier. (2) Collapse(s) of Old Merapi that left a somma rim high on its eastern slope and sent one or more debris avalanche(s) down its southern and western flanks. Impoundment of Kali Progo to form an early Lake Borobudur at ~3400 14 C y B.P. hints at a possible early collapse of Merapi. The latest somma-forming collapse occurred ~1900 14 C y B.P. The current cone, New Merapi, began to grow soon thereafter. (3) Several large and many small Buddhist and Hindu temples were constructed in Central Java between 732 and ~900 A.D. (roughly, 1400-1000 14 C y B.P.). Explosive Merapi eruptions occurred before, during and after temple construction. Some temples were destroyed and (or) buried soon after their construction, and we suspect that this destruction contributed to an abrupt shift of power and organized society to East Java in 928 A.D. Other temples sites, though, were occupied by "caretakers" for several centuries longer. (4) A partial collapse of New Merapi occurred <1130±50 14 C y B.P. Eruptions ~700-800 14 C y B.P. (12-14th century A.D.) deposited ash on the floors of (still-occupied?) Candi Sambisari and Candi Kedulan. We speculate but cannot prove that these eruptions were triggered by (the same?) partial collapse of New Merapi, and that the eruptions, in turn, ended "caretaker" occupation at Candi Sambisari and Candi Kedulan. A new or raised Lake Borobudur also existed during part or all of the 12-14th centuries, probably impounded by deposits from Merapi. (5) Relatively benign lava-dome extrusion and dome-collapse pyroclastic flows have dominated activity of the 20th century, but explosive eruptions much larger than any of this century have occurred many times during Merapi's history, most recently during the 19th century. Are the relatively small eruptions of the 20th century a new style of open-vent, less hazardous activity that will persist for the foreseeable future? Or, alternatively, are they merely low-level "background" activity that could be interrupted upon relatively short notice by much larger explosive eruptions? The geologic record suggests the latter, which would place several hundred thousand people at risk. We know of no reliable method to forecast when an explosive eruption will interrupt the present interval of low-level activity. This conclusion has important implications for hazard evaluation.

Journal of Volcanology and Geothermal Research↗

USGS2000

Historical eruptions of Merapi Volcano, Central Java, Indonesia, 1768-1998

Information on Merapi eruptive activity is scattered and much is remotely located. A concise and well-documented summary of this activity has been long needed to assist researchers and hazard-mitigation efforts, and the aim of this paper is to synthesize information from the mid-1700s to the present. A descriptive chronology is given, with an abbreviated chronology in a table that summarizes events by year, assigns preliminary Volcanic Explosivity Index (VEI) ratings and Hartmann classifications, and provides key references. The history of volcano monitoring is also outlined. The study reveals that a major difference in eruption style exists between the twentieth and nineteenth centuries, although the periodicity between larger events seems about the same. During the twentieth century, activity has comprised mainly the effusive growth of viscous lava domes and lava tongues, with occasional gravitational collapses of parts of oversteepened domes to produce the nue??es ardentes - commonly defined as "Merapi-type". In the 1800s, however, explosive eruptions of relatively large size occurred (to VEI 4), and some associated "fountain-collapse" nue??es ardentes were larger and farther reaching than any produced in the twentieth century. These events may also be regarded as typical eruptions for Merapi. The nineteenth century activity is consistent with the long-term pattern of one relatively large event every one or two centuries, based on the long-term eruptive record deduced by others from volcanic stratigraphy. It is uncertain whether or not a "recurrence-time" model continues to apply to Merapi, but if so, Merapi could soon be due for another large event and its occurrence with only modest (or inadequately appreciated) precursors could lead to a disaster unprecedented in Merapi's history because the area around the volcano is now much more densely populated.

Java, Merapi Volcano↗

USGS2000

Nuées ardentes of 22 November 1994 at Merapi volcano, Java, Indonesia

Nuées ardentes associated with dome collapse on 22 November 1994, at Merapi volcano traveled to the south–southwest as far as 6.5 km, and collectively accumulated roughly 2.5–3 million cubic meters of deposits. The damaged area comprises 9.5 km 2 and is covered by two nuée ardente facies, a conventional “Merapi-type”, valley-fill block-and-ash flow facies and a pyroclastic surge facies. The proximal deposits reflect the accumulation of dozens of nuées ardentes, with many subsidiary flow units. The distal deposits are more simply organized, as only a few individual events reached to distances >3.5 km. The stratigraphic relationships north of Turgo hill indicate that the surge deposits are a facies of particularly mobile nuées ardentes that also deposited channeled block-and-ash flow facies. They further suggest that the surge facies beyond the channel margins correlate laterally with a finer-grained sublayer locally developed at the base of the block-and-ash flow facies. Eyewitness reports suggest that the emplacement of the block-and-ash flow facies in the distal part of the Boyong river may have followed, by a short time interval, the destruction and deposition of the surge facies at Turgo village. The stratigraphy is in accord with the eyewitness reports. The surge facies was emplaced by a dilute surge current, detached from the same dome-collapse nuée ardente that, as a separate flow unit, subsequently emplaced the distal block-and-ash deposit in the Boyong valley. The detachment occurred at higher elevations, likely at or above the slope break at about 2000 m elevation. This flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block-and-ash flow continued its tortuous southward movement in the Boyong channel. Dome-collapse nuée ardente activity formed the bulk of the eruption, which was accompanied by virtually no significant vertical summit explosive activity.

Journal of Volcanology and Geothermal Research↗

USGS2001

Density of basalt core from Hilo drill hole, Hawaii

Density measurements of 1600 samples of core from 889 to 3097 m depth below sea level in the Hawaii Scientific Drilling Program hole near Hilo, Hawaii show marked differences between the basaltic rock types and help define stratigraphy in the hole. Water-saturated densities of subaerial lava flows (occurring above 1079 m depth) have the broadest range because of the large density variation within a single lava flow. Water-saturated densities commonly range from 2.0 to 3.0 with an average of 2.55 ?? 0.24 g/cc. Dikes and sills range from 2.8 to 3.1 g/cc). Densities of hyaloclastite commonly range from 2.3 to 2.7, with an overall average of about 2.5 g/cc. The low-density of most hyaloclastite is due primarily to palagonitization of abundant glass and presence of secondary minerals in the interstices between fragments. Four principal zones of pillow lava, separated by hyaloclastite, occur in the drill core. The shallowest (1983-2136 m) is paradoxically the densest, averaging 3.01 ?? 0.10 g/cc. The second (2234-2470 m) is decidedly the lightest, averaging 2.67 ?? 0.13 g/cc. The third (2640-2790 m) and fourth (2918-bottom at 3097 m) are high, averaging 2.89 ?? 0.17 and 2.97 ?? 0.08 g/cc, respectively. The first pillow zone includes degassed pillows i.e. lava erupted on land that flowed into the sea. These pillows are poor in vesicles, because the subaerial, one-atmosphere vesicles were compressed when the flow descended to deeper water and higher pressure. The second (low-density, non-degassed) pillow zone is the most vesicle-rich, apparently because it was erupted subaqueously at a shallow depth. The higher densities of the third and fourth zones result from a low vesicularity of only a few percent and an olivine content averaging more than 5% for the third zone and about 10% for the fourth zone. The uppermost hyaloclastite extending about 400 m below the bottom of the subaerial basalt is poorly cemented and absorbs up to 6 wt% of water when immersed. Progressing downward the hyaloclastite absorbs less water and becomes better cemented. This change is apparently due to palagonitization of glass and addition of secondary minerals in the deeper older hyaloclastite, a process favored by the increase of temperature with depth. The cementation is largely complete at 1800 m depth where the temperature attains about 20??C. The zone of freshest, uncemented hyaloclastite represents the weakest rock in the drill hole and is a likely level for tectonic or landslide disruption. ?? 2001 Published by Elsevier Science B.V.

Journal of Volcanology and Geothermal Research↗

USGS2002

The oligocene Lund Tuff, Great Basin, USA: A very large volume monotonous intermediate

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (> 1000 km3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite-dacite-andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff - one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province - provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km3. It was emplaced 29.02 ?? 0.04 Ma in and around the coeval White Rock caldera which has an unextended north-south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase > quartz ??? hornblende > biotite > Fe-Ti oxides ??? sanidine > titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63-71 wt% SiO2) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere. ?? 2002 Elsevier Science B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗

USGS1992

Lake trout spawning habitat in the Six Fathom Bank-Yankee Reef lake trout sanctuary, Lake Huron

Attempts to reestablish self-sustaining stocks of lake trout ( Salvelinus namaycush ) in the lower four Great Lakes, where the species was extinguished in the 1950s and 1960s, have been largely unsuccessful. To avoid many of the problems believed to be contributing to this failure, the fishery management community recently established several sanctuaries in the offshore waters of the Great Lakes where the development and protection of self-sustaining stocks of lake trout would be a primary management objective. One of these, the Six Fathom Bank-Yankee Reef sanctuary, was created in the south-central portion of Lake Huron. This sanctuary covers 168,000 ha and includes the shallower portions of the Six Fathom and Ipperwash scarps, which are major bathymetric features in the southern half of the lake. Historical accounts describe Six Fathom Bank as the most important lake trout spawning ground in the lake. Here we present the results of lake bed surveys conducted in the sanctuary with side-scan sonar, underwater videocamera systems, and a small research submarine. Our observations of the lake bed are consistent with what is known of the bedrock stratigraphy, glacial history, and karst geomorphology of the Lake Huron basin. Most of the loose rock we found seemed to be derived from local carbonate bedrock formations, although non-carbonate rock probably from Precambrian sources to the north was also present in some areas. Much of the bedrock and loose rock displayed karst solution features described for the Bruce Peninsula on the Ontario shoreline. Our surveys revealed substantial areas of lake bed at water depths of 20–36 m that resembled suitable spawning and fry production habitat for the shallow-water strains of lake trout that are the focus of the rehabilitation effort. Low mid-lake nutrient levels documented recently by others and the extremely high abundance of Mysis relicta (an important item in the diet of young lake trout) that we documented on Yankee Reef also contributed to our evaluation of the sanctuary as a site with high potential to support a self-sustaining population of lake trout.

Journal of Great Lakes Research↗

USGS2002

Boninites: Characteristics and tectonic constraints, northeastern Appalachians

Boninites are high Mg andesites that are thought to form in suprasubduction zone tectonic environments as primary melts from refractory mantle. Boninites provide a potential constraint on tectonic models for ancient terranes that contain boninites because the only unequivocal tectonic setting in which "modern" boninites have been recognized is a fore-arc setting. Tectonic models for "modern" boninite genesis include subduction initiation ("infant arc"), fore-arc spreading, and the forearc side of intra-arc rifting (spreading). These models can be differentiated by the relative age of the boninites and to a lesser degree, geochemistry. The distinctive geochemistry of boninites promotes their recognition in ancient terranes. As detailed in this report, several mafic terranes in the northeastern Appalachians contain boninites; these terranes were situated on both sides of Iapetus. The characteristics of these boninites can be used to constrain tectonic models of the evolution of the northeastern Appalachians. On the Laurentian side of Iapetus, "infant arc" boninites were not produced ubiquitously during the Cambrian subduction initiation, unless sampling problems or minimum age dates obscure a more widespread boninite "infant arc". The Cambrian subduction initiation on the Laurentian side was probably characterized by both "infant arc" boninitic arc construction (perhaps the >496 Ma Hawley Formation and the >488 Ma Betts Cove Ophiolite) and "normal" arc construction (Mt. Orford). This duality is consistent with the suggestion that the pre-collisional geometry of the Laurentian margin was complex. The Bay of Islands Complex and Thetford Mines ophiolite boninites are likely associated with forearc/intra-arc spreading during the protracted evolution of the Cambrian arc system. The relatively young boninites in the Bronson Hill Arc suggest that the Taconic continuous eastward subduction tectonic model is less tenable than other models. On the Gondwana side of Iapetus, the Tea Arm boninites of the Exploits Group stratigraphically rest on arc and MORB volcanics. This stratigraphy, and the relatively young age of the boninites (486 Ma), compared to assumed subduction initiation age (>513 Ma), suggest that the boninites may be more consistent with fore-arc spreading/intra-arc spreading. However, an "infant arc" model cannot be dismissed, and is commonly proposed for the nearby boninites in the Wild Bight Group. ?? 2002 Elsevier Science Ltd. All rights reserved.

Physics and Chemistry of the Earth↗

USGS2008

Cenozoic climate history from seismic reflection and drilling studies on the Antarctic continental margin

Seismic stratigraphic studies and scientific drilling of the Antarctic continental margin have yielded clues to the evolution of Cenozoic climates, depositional paleoenvironments and paleoceanographic conditions. This paper draws on studies of the former Antarctic Offshore Stratigraphy Project and others to review the geomorphic and lithostratigraphic offshore features that give insights into the long-duration (m.y.) and short-term (k.y.) changes that document the great variability of Cenozoic Antarctic paleoenvironments. The lithologic drilling record documents non-glacial (pre-early Eocene) to full-glacial (late Pliocene to Holocene) times, and documents times of cyclic ice-sheet fluctuations at k.y. scales (early Miocene to Pliocene and Holocene). Times of significant change in types and/or amounts of glaciation are also seen in the offshore lithologic record (early Oligocene, mid-Miocene, early Pliocene). Seismic data illustrate large-scale geomorphic features that point to massive sediment erosion and dispersal by ice sheets and paleoceanographic processes (e.g. cross-shelf troughs, slope-fans, rise-drifts). The commonality of these features to East and West Antarctica since late Eocene time points to a continent that has been intermittently covered, partially to completely, by glaciers and ice sheets. The greatest advances in our understanding of paleoenvironments and the processes that control them have been achieved from scientific drilling, and future progress depends on a continuation of such drilling.

Book chapter↗

USGS2004

Molecular markers and their use in environmental organic geochemistry

Molecular markers are organic substances that carry information about sources of organic matter or contamination. The source/marker relation can be used to indicate the presence of a given source material (qualitative), or, under appropriate conditions, to estimate the amount of a source material (quantitative source apportionment) in the environment. Assemblages of markers can also be used as process probes. In this instance, systematic differences and/or similarities in the physical-chemical properties of markers are coupled with compositional changes in marker composition to infer the operation of natural processes. This paper provides an overview of what molecular markers are, what types of markers are present in the environment, the requirements for the use of markers, and some common applications. To illustrate how molecular markers can answer specific environmental questions, three case studies are presented. The first case study examines the impact of municipal waste on a large urban harbor (Boston Harbor). Linear alkylbenzenes (unreacted residues of linear alkylbenzenesulfonate surfactants) and coprostanol (a fecal indicator) provide information on the sources and likely transport pathways of municipal wastes in a complex hydrologic system. The marker data are also used to estimate the proportion of sewage-derived poly chlorinated biphenyls (PCBs) in polluted harbor sediments. The second case study concerns a portion of the continental shelf off southern California (Palos Verdes) where discharge of municipal wastewaters has led to extensive contamination of sediments and biota. Long-chain alkylbenzenes (surfactant residues), PCBs and the pesticide, DDT (dichlorodiphenyltrichloroethane), are used to develop sedimentation rate estimates for several time periods by molecular stratigraphy. These results, when combined with other information, allow conclusions to be drawn about the most likely transport pathway of sediments at the study site and to predict the fate of historically deposited contaminants. Finally, an investigation of a crude-oil spill in Bemidji, MN illustrates how monoaromatic hydrocarbons can be exploited as process probes, providing insights into the relative importance of different attenuation processes in a contaminated aquifer. The results show that natural attenuation of the monoaromatic hydrocarbons is occurring at this site and is dominated, not by physical and/or chemical processes, but by biodegradation.

Book chapter↗

USGS2017

How could a freshwater swamp produce a chemical signature characteristic of a saltmarsh?

Reduction–oxidation (redox) reaction conditions, which are of great importance for the soil chemistry of coastal marshes, can be temporally dynamic. We present a transect of cores from northwest Florida wherein radical postdepositional changes in the redox regime has created atypical geochemical profiles at the bottom of the sedimentary column. The stratigraphy is consistent along the transect, consisting of, from the bottom upward, carbonate bedrock, a gray clay, an organic mud section, a dense clay layer, and an upper organic mud unit representing the current saltwater marsh. However, the geochemical signature of the lower organic mud unit suggests pervasive redox reactions, although the interval has been identified as representing a freshwater marsh, an unlikely environment for such conditions. Analyses indicate that this discrepancy results from postdepositional diagenesis driven by millennial-scale environmental parameters. Rising sea level that led to the deposition of the capping clay layer, created anaerobic conditions in the freshwater swamp interval, and isolated it hydrologically from the rest of the sediment column. The subsequent infiltration of marine water into this organic material led to sulfate reduction, the buildup of H 2 S and FeS, and anoxic conditions. Continued sulfidation eventually resulted in euxinic conditions, as evidenced by elevated levels of Fe, S, and especially Mo, the diagnostic marker of euxinia. Because this chemical transformation occurred long after the original deposition the geochemical signature does not reflect soil chemistry at the time of deposition and cannot be used to infer syn-depositional environmental conditions, emphasizing the importance of recognizing diagenetic processes in paleoenvironmental studies.

Florida↗

USGS2008

Susceptibility of synthetic long-chain alkylbenzenes to degradation in reducing marine sediments

Long-chain alkylbenzenes (LCABs) synthesized for production of alkylbenzene sulfonate surfactants have been used as molecular markers of anthropogenic waste for 25 years. Synthetic LCABs comprise two classes, the tetrapropylene-based alkylbenzenes (TABs) and the linear alkylbenzenes (LABs). LABs supplanted TABs in the mid-1960s because of improved biodegradability of their sulfonated analogs. Use of LCABs for molecular stratigraphy depends on their preservation in sediments over decadal time scales. Most laboratory and field studies suggest that LABs degrade rapidly under aerobic conditions but are resistant to degradation when oxygen is absent. However, recent work indicates that LABs may not be as persistent under reducing conditions as previously thought. To assess the potential for degradation of LCABs in reducing sediments, box cores collected in 1992 and 2003 near a submarine wastewater outfall system were analyzed using gas chromatography/mass spectrometry. The TABs were effectively preserved; differences between whole-core inventories were within analytical error. By contrast, whole-core inventories of the LABs decreased by about 50−60% during the same time interval. Based on direct comparison of chemical inventories in coeval core sections, LAB transformation rates are estimated at 0.07 ± 0.01 yr −1 . These results indicate that caution should be exercised when using synthetic LCABs for reconstruction of depositional records.

Environmental Science & Technology↗

USGS1998

Stable carbon and oxygen isotope record of central Lake Erie sediments

Stable carbon and oxygen isotope data from mollusc aragonite extracted from sediment cores provide new information on the origin and history of sedimentation in the southwestern area of the central basin of Lake Erie. Sediments infilling the Sandusky subbasin consist of three lithologic units overlying glacial deposits. The lowest of these is a soft gray mud overlain by a shell hash layer containing Sphaerium striatinum fragments. A fluid mud unit caps the shell hash layer and extends upwards to the sediment-water interface. New stable isotope data suggest that the soft gray mud unit is of postglacial, rather than proglacial, origin. These data also suggest that the shell hash layer was derived from erosional winnowing of the underlying soft gray mud layer. This winnowing event may have occurred as a result of the Nipissing flood. The Pelee-Lorain moraine, which forms the eastern boundary of the Sandusky subbasin, is an elevated area of till capped by a sand deposit that originated as a beach. The presence of both the shell hash layer and relict beach deposit strengthens the interpretation that the Nipissing flood was a critical event in the development of the southwestern area of the central basin of Lake Erie. This event, which returned drainage from the upper lakes to the Lake Erie basin, was a dominant influence on regional stratigraphy, bathymetry, and depositional setting.

Journal of Paleolimnology↗

USGS2011

Effort explores 130 Million years of Antarctic paleoenvironment

Antarctic climate history has been dominated by events and turning points with causes that are poorly understood. To fill the gaps in our knowledges new effort is underway in the international geologic community to acquire and coordinate the circum‐Antarctic geologic data needed to derive and model paleoenvironments of the past 130 m.y. The effort, which focuses principally on using shallow (<100 m) stratigraphic drilling and coring to acquire the geologic data, is being led by the Antarctic Offshore Stratigraphy Project (ANTOSTRAT), a group that works under the aegis of the Scientific Committee on Antarctic Research (SCAR). About 40 scientists from 12 countries met this past summer in Wellington, New Zealand, at an ANTOSTRAT meeting to discuss strategies for implementing the desired paleoenvironmental field and modeling studies. The meeting was held in conjunction with the 8th International Symposium on Antarctic Earth Sciences.

Eos, Transactions, American Geophysical Union↗