Changes In Sediment Sources Research Articles

A Holocene-Late Pleistocene geomagnetic inclination record from Grandfather Lake, SW Alaska

Our study of lake sediments from Grandfather Lake in SW Alaska (N59.8181°, W158.5597°) yields the first geomagnetic inclination record for Alaska that spans the entire Holocene. The record is based on three overlapping sets of piston cores and is well dated by seven AMS radiocarbon dates. Its magnetic component consists of PSD (5–15 µm) magnetite that occurs in concentrations ranging from 10 to 1000 ppm. Some samples may also contain titanomagnetite (TM30) as a minor component. Variations in the concentration of magnetic minerals are caused by changes in sediment source and organic productivity. The magnetic inclination record shows several features between present and 15 ka BP, which shed light on the short-term behaviour of the geomagnetic field at high latitudes and can be used for site correlations. A distinct drop in inclination near 12 ka BP is a record of a possible local magnetic excursion.

Chemical composition of a drilled core of Lake Biwa in Japan and implications for source changes during the past 430,000 years

Neutron activation analysis was performed on sediments taken from the 250 m thick sediment bed of Lake Biwa, which is composed entirely of lacustrine clay and records paleo-environmental change over the past 430,000 years. A comparison is made between the geochemical compositions of cores from the center of Lake Biwa, and lacustrine surface and terrestrial sediments from around Lake Biwa, to estimate past changes in sediment sources. Time series of Th/Sc ratios are synchronized with glacial–interglacial climatic change and also correlated with the reported aeolian quartz flux data. Th/Sc ratios during the δ 18O stages 1, 5, 7, 9 and 11 are approximately 1.1–1.2, whereas Th/Sc ratios during the δ 18O stages 2, 4, 6, 8 and 10 are approximately 1.0–1.1. The possibility of a contribution from the mafic area in the northwestern shore of Lake Biwa, which has low Th/Sc ratios (ca. 0.8), is excluded by the Ti/Al ratio data. As aeolian dust flux to the downwind area from the Asian desert area is higher during glacial times, this data suggests that aeolian dust reaching Japan has an unexpectedly low Th/Sc ratio of source material, and/or that there was a large shift in the amount of dust flux. This assumption remains to be tested by future studies. Th/Sc ratios apparently lag ca. 5000 years behind the marine δ 18O record during the stage 1 2 , 3 4 , and 5 6 transitions, which coincides with the aeolian quartz flux variation. Continuous increase of Hf contents from ca. 200,000 years BP could be mainly related to upheaval of the Hf rich and high Th/Sc ratio granite block on the west shore of drilling site by active fault movement. The variations in delivery of high Th/Sc granitic debris has damped the oscillation between Th/Sc ratios of sediments and aeolian dust flux during oxygen isotopic stages 2–3. Both the intermittent increase of Na contents from ca. 430,000 years BP to present, and the abrupt increase of Na content at ca. 140,000 years BP are probably due to the subsidence of the northern basin, as the result of tectonic movement around Lake Biwa.

A high resolution late Holocene palaeo environmental record from the central Adriatic Sea

A multi-proxy study of a Holocene sediment core (RF 93-30) from the western flank of the central Adriatic, in 77 m of water, reveals a sequence of changes in terrestrial vegetation, terrigenous sediment input and benthic fauna, as well as evidence for variations in sea surface temperature spanning most of the last 7000 yr. The chronology of sedimentation is based on several lines of evidence, including AMS 14C dates of foraminifera extracted from the core, palaeomagnetic secular variation, pollen indicators and dated tephra. The temporal resolution increases towards the surface and, for some of the properties measured, is sub-decadal for the last few centuries. The main changes recorded in vegetation, sedimentation and benthic foraminiferal assemblages appear to be directly related to human activity in the sediment source area, which includes the Po valley and the eastern flanks of the central and northern Appenines. The most striking episodes of deforestation and expanding human impact begin around 3600 BP (Late Bronze Age) and 700 BP (Medieval) and each leads to an acceleration in mass sedimentation and an increase in the proportion of terrigenous material, reflecting the response of surface processes to widespread forest clearance and cultivation. Although human impact appears to be the proximal cause of these changes, climatic effects may also have been important. During these periods, signs of stress are detectable in the benthic foram morphotype assemblages. Between these two periods of increased terrigeneous sedimentation there is smaller peak in sedimentation rate around 2400BP which is not associated with evidence for deforestation, shifts in the balance between terrigenous and authigenic sedimentation, or changes in benthic foraminifera. The mineral magnetic record provides a sensitive indicator of changing sediment sources: during forested periods of reduced terrigenous input it is dominated by authigenic bacterial magnetite, whereas during periods of increased erosion, anti-ferromagetic minerals (haematite and/or goethite) become more important, as well as both paramagnetic minerals and super-paramagnetic magnetite. Analysis of the alkenone, U 37 k′, record provides an indication of possible changes in sea surface temperature during the period, but it is premature to place too much reliance on these inferred changes until the indirect effects of past changes in the depth of the halocline and in circulation have been more fully evaluated. The combination of methods used and the results obtained illustrate the potential value of such high resolution near-shore marine sedimentary sequences for recording wide-scale human impact, documenting the effects of this on marine sedimentation and fauna and, potentially, disentangling evidence for human activities from that for past changes in climate.

A Geochemical Boundary in the East Sea (Sea of Japan): Implications for the Paleoclimatic Record

Sediment from six piston cores from the East Sea (Sea of Japan) was analyzed for evidence of paleoceanographic changes and paleoclimatic variation. A distinct geochemical boundary is evident in major element concentrations and organic carbon content of most cores near the 10-ka horizon. This distinctive basal Holocene change is interpreted to be largely the result of changing sediment sources, an interpretation supported by <TEX>TiO_2/Al_2O_3$</TEX> ratios. Organic carbon and carbonate contents also differ significantly between the Holocene and glacial intervals. The C/N ratio of organic matter is greater than 10 during the glacial period, but is less than 10 for the Holocene, suggesting that the influx of terrigenous organic matter was more volumetrically important than marine organic matter during glacial times. The chemical index of weathering (CIW) is higher for the Holocene than the glacial interval, and changes markedly at the basal Holocene geochemical boundary. Silt fractions are higher in the glacial interval, suggesting a strong effect of climate on silt particle transportation: terrigenous aluminosilicates and continental organic carbon transport were higher during glacial times than during the Holocene. Differences in sediment composition between the Holocene and glacial period are interpreted to have been climatically induced.

Changes in sediment sources and floodplain deposition rates in the catchment of the River Tweed, Scotland, over the last 100 years: the impact of climate and land use change

AbstractEvidence from floodplain cores collected from three sites in the middle reaches of the Tweed basin in Scotland is used to reconstruct changes in sediment sources and overbank floodplain deposition rates over the last c. 100 years. Core chronologies and sedimentation rates are established using 137Cs and unsupported 210Pb measurements. The average sedimentation rates since 1963 range from 1·9 ± 0·2 to 2·2 ± 0·2 kg m−2 a−1 and are lower than the average rates for the period 1894/95 to 1963, which range from 2·7 ± 0·6 to 5·9 ± 0·9 kg m−2 a−1. There is also evidence of significant downcore variations in sediment source, defined in terms of both type (i.e. topsoil or channel bank/subsoil material) and spatial location (i.e. main geological/topographic zones). There is no clear link between the changes in overbank sedimentation rates and sediment sources and the trends shown by precipitation, weather pattern and river flow records over the past 100 years, suggesting that changes in climate alone cannot explain the downcore trends. Instead, the temporal changes in overbank sedimentation rates and sediment source appear to be linked more closely to changes in land use and land management over the past 100 years and, more particularly, the introduction of land drainage at the end of the 19th century, the rapid increase in afforestation since the 1940s and the post‐war conversion of grassland to arable land. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Source apportionment of lake bed sediments to watersheds in an Upper Mississippi basin using a chemical mass balance method

Sediment samples were collected from three known sources contributing to a depositional basin of the Upper Mississippi River, USA. The heavy silt fraction (2–63 μm, density >2.95) isolated from the sediments was analyzed for 42 elements by inductively coupled plasma-mass spectrophotometry (ICP-MS) to give signature profiles characteristic of each source catchment. Known proportions of heavy silt fractions from each source were mixed and the composite samples analyzed in the same way. The proportions of components in the composite samples were compared to their apportionments calculated statistically using the US Environmental Protection Agency (USEPA) chemical mass balance (CMB) air quality receptor model. This showed that the CMB technique can apportion sediments to their catchment sources reliably. The technique was then applied to dated sediments of Lake Pepin, in the Upper Mississippi basin, to apportion them to their contributing catchments. The results show that almost all the recent sediments deposited in the basin originated in the Minnesota River catchment, and that the sediment load increased over time as anthropogenic impacts increased. This technique could be used elsewhere to determine sediment provenance and identify changes in sediment sources over time.

Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK

Floodplain sediment cores collected from seven sites in the catchment of the River Ouse, in Yorkshire, UK, have been used to provide information on recent historical changes in both rates of overbank sedimentation and sediment sources. The environmental radionuclides 137 Cs and unsupported 210 Pb have been used to establish chronologies for each core and to estimate average sediment accumulation rates for the last ca. 30 and 100 years, respectively. Average sedimentation rates for the individual cores ranged from 0.11 to 1.04 g cm −2 yr −1. In all but one case, the estimates of average sedimentation rate during the last ca. 30 years for the individual cores are broadly similar to those for the last ca. 100 years, suggesting that overbank sedimentation rates have been essentially uniform over the longer time period. Composite fingerprints, based on a combination of geochemical and mineral magnetic properties, and a numerical mixing model have been used to investigate downcore changes in sediment source. In the case of source type, most of the cores reflect a primarily topsoil source, although there have been periods with increased contributions from subsoil/channel bank sources. Within the Ouse basin in general, the period commencing in the late 19th and early 20th century and extending through to the 1960s, was characterised by increased contributions from topsoil sources. However, contributions from subsoil/channel bank sources have increased over the last few decades. The source tracing results relating to sediment contributions from the three main geological/topographic zones are in broad agreement with the proportion of the area of the catchment underlain by each rock type. Temporal variations in the contributions from the three geological/topographic zones vary from site to site, but for the lower reaches of the River Ouse contributions from areas underlain by Permian and Triassic rocks, which mainly outcrop in the Vale of York, have increased since the turn of the century. The changes in sediment source identified are probably a reflection of changes in land use and management (and possibly changes in climate). These results enable estimates of contemporary suspended sediment fluxes and sources to be placed into a historical context and provide a means of considering the likely impact of potential future changes in land use and climate in the study basin.

Late Quaternary sedimentation patterns on the Meriadzek Terrace, Bay of Biscay (ESSCAMP 02 core: 47°N 9°W)

Foraminiferal assemblage changes, size and mineralogy of lithic grains, oxygen isotopes, CaCO 3, and dolomite content were studied along a 9-m-long core from the Meriadzek Terrace to gain insight into climate, productivity, and sediment source changes at this part of the Northeast Atlantic margin during the late Quaternary. An age model has been generated on the basis of radiocarbon dating, downcore foraminiferal assemblages, and δ 18O values. High sedimentation rates at this site allow very detailed records for the last glacial period down to late isotopic stage 3. Sea surface temperature (SST) inferred from the foraminiferal assemblages, δ 18O curve, and the temperature estimation by the SIMMAX analog technique reveal three distinctive periods during isotopic stage 2, with late stage 2 (15.3–13.4 ka) being the coldest period of the last 26 ka. A northward retreat of the polar front at 13.4 ka based on the SST record coincides with the strongest peak of primary productivity as indicated by the foraminiferal fluxes. Levels rich in large lithic grains (LLG) interpreted as ice-rafted debris (IRD) correspond to periods of low SST and are coeval with Heinrich layers 1, 2 and 3. However, the hematite-stained quartz found in the detrital fraction and the scarce dolomite and detrital carbonate content in our core point to an Iceland and/or Fenno-Scandian rather than a Laurentian or Greenland origin of this material.

Turbidite facies, fluid-escape structures and mechanisms of emplacement of the Oligo-Miocene Aljibe Flysch, Gibraltar Arc, Betics, southern Spain

The Aljibe Flysch, the largest tectono-sedimentary unit in the Gibraltar Arc, provides a record of Oligo-Miocene sedimentation in the Betic Orogen, southern Spain. The Aljibe Flysch can be subdivided into two sub-units, the Beneiza Flysch (Late Oligocene) and the Aljibe Arenites (Early Miocene), which have very different depositional histories and mechanisms of emplacement. The Beneiza Flysch is characterised by laterally continuous and thin-bedded lithic arenites, siltstones and mudstones which contain complete and partial Bouma sequences. These are interpreted to have been deposited in a sheet-turbidite system which may have formed part of the North African Flysch Trough. By contrast, the aljibe Arenites comprise of thick-bedded super-mature quartz arenites which are dominated by fluid-escape structures. These sandstones are similar to Facies Group B1-B2 disorganised sands of Pickering et al. (1989), which are interpreted to be the product of deposition from high-density turbidity currents. Fluid-escape structures have been classified according to the scheme of Lowe (1975) and their vertical arrangement indicates that a large volume of sediment was deposited extremely rapidly during frequent remobilisation events. Consideration of the origin of the fluid-escape structures and their vertical arrangement has identified problems in their interpretation, and identified key areas for future research. The boundary between the Beneiza Flysch and Aljibe Arenites is interpreted to be a significant stratigraphic boundary across which there is a major change in sediment source and mechanism of sediment emplacement. Several models can be put forward for the generation of this stratigraphic break, and these serve to set the agenda for future research into the emplacement of other similar flysch sequences and the origin of the Gibraltar Arc Flysch.

Mineral magnetic and physical properties of surficial sediments and onshore samples from the southern basin of Lake Baikal, Siberia

Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using 210Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (χf) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of χlf, IRMs and HIRM210 suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core.

Use of the geochemical record preserved in floodplain deposits to reconstruct recent changes in river basin sediment sources

The geochemical record preserved in overbank floodplain deposits is used in combination with a composite fingerprinting technique and the chronology provided by 137Cs dateable horizons, to reconstruct recent changes in sediment sources in the Exe and Severn basins, UK. Composite fingerprints for discriminating potential source materials are identified from a suite of properties including heavy metals, trace metals and cation bases. These fingerprints are then compared with those of historical floodplain deposits using a numerical sediment mixing model. Relative contributions from distinct spatial sources characterised in terms of the Devonian, Carboniferous and Permian geological sub-areas of the Exe and the Ordovician-Silurian, Devonian-Carboniferous-Permian and Triassic-Jurassic sub-areas of the Severn are shown to fluctuate over the recent past in response to intrinsic (e.g. the occurrence of extreme hydrologic events) and extrinsic (e.g. anthropogenic disturbance associated with land use change) controls. The potential for fingerprint property transformation in association with floodplain storage is assessed and the scope for further research using fingerprinting to reconstruct longer-term sediment sources is discussed.

Lacustrine Sedimentation Processes and Patterns During Effusive and Explosive Volcanism, Challis Volcanic Field, Idaho

ABSTRACT Lacustrine deposits in the Eocene Challis volcanic field accumulated in a deep lake bordered by steep ranges. Volcanic and nonvolcanic sediment was transported into the lake via a major dispersal system from the north. High-density turbidity currents transported this sediment throughout the lake system. Paleozoic rocks in the adjacent ranges were secondary sediment sources. This sediment was trapped along the shoreline, and only a small fraction was transported offshore by turbidity currents. The frequency of turbidity currents varied with changes in eruption style. Thus, turbidite accumulation patterns are a faithful record of changes in volcanic activity and associated landscape processes. Effusive volcanism that characterized the early activity in the Challis volcanic field produced little sand-size sediment and had little impact on lacustrine sedimentation patterns. Relatively slow settling from suspension characterized lake sedimentation during this time; turbidity currents were infrequent. The resulting accumulation pattern is a single sheet of turbidite sandstone encased in thick-bedded mudrock. Initial eruptions at the Van Horn Peak cauldron complex altered sedimentation patterns dramatically by increasing the frequency of turbidity currents. A turbidite ramp prograded into the study area, marking a major increase in sedimentation rate. Sediment composition and turbidite accumulation patterns record changes in landscape processes. Syneruption deposits are stacked turbidite sheets. Early turbidites contain mostly sediment derived from the fresh pyroclastic material. The proportion of sediment from pre-eruption sources increases upward through the stacked sheets, recording a change in sediment sources at the end of the syneruption period. Ramp growth continued through this part of the period, although at a much slower rate.

A mineral magnetic and scaled-chrysophyte paleolimnological study of two northeastern Pennsylvania lakes: records of fly ash deposition, land-use change, and paleorainfall variation

A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. 210Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawac‘s southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the 210Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change.

Environmental controls on the TL age of modern (zero-age) proglacial outwash sediments

Thermoluminescence (TL) signals from suspended sediment in an Icelandic meltwater stream exhibit small-scale random fluctuations over time, but show sufficient consistency to indicate that larger-scale fluctuations in TL are systematic rather than random. It is concluded that diurnal TL fluctuations are related to changes in sediment source and ambient conditions during transport. The results suggest ‘young’ fluvioglacial sediments may prove difficult to date using standard TL techniques.

Geochemical history of ancient Lake Biwa in Japan—chemical indicators of sedimentary paleo-environments in a drilled core

A comparison is made between the geochemical compositions of drilled sediment core samples from Lake Biwa and samples of surface sediments and from land around Lake Biwa to estimate paleo-water depth, spatial variability and past changes in sediment sources. The AlFeMn × 10 diagram indicates that Lake Biwa has been an oligotrophic lake since its formation. An investigation of the origin of a positive correlation between water-depth and the Mn/Al ratio in surface sediments is made and the historical change of paleo-water depth in Lake Biwa proposed by Koyama et al. (1985) verfied. Demonstrated are the usefulness of both Ti/Al ratio and REE pattern to detect changes in sediment sources and geochemical discontinuities in four layers which should indicate unconformities are found. My data could be supporting evidence for the chronological assignment of Takemura (1990). An explanation is given for the historical changes of source materials in drilled core samples associated with tectonic changes on the basis of the historical basin development postulated by Yokoyama (1984).

Geochemical control of magnetite dissolution in subarctic lake sediments and the implications for environmental magnetism

AbstractGeochemical and mineral magnetic studies of lake sediments from the Kårsa valley in north Sweden demonstrate that down‐core variations in magnetic properties have been determined by post‐depositional magnetite dissolution rather than by sediment source changes. A low sediment accumulation rate during times of climatic amelioration combined with a higher organic carbon content produced intense reducing conditions with almost complete magnetite loss. An increase in the sedimentation rate and a reduction in the organic carbon content, primarily due to glacier reformation, allowed magnetite preservation. Thus the mineral magnetic profiles record lake sediment palaeoredox conditions brought about by climate change. This new interpretation of mineral magnetic transformations in freshwater sediments may have far reaching implications for other mineral magnetic studies concerned with the alternation between glacial (stadial) and interglacial (interstadial) episodes during the Quaternary period.

Origin of Regressive-Transgressive Sequences in the Claiborne Group

Origin of regressive transgressive sequences within the Claiborne Group along the Texas and Louisiana coast have been variably attributed to eustatic sea level changes, autocyclic delta lobe switching, and changes in sediment sources. By using seismic, well logs, core data, sedimentological and stratigraphic principles, this project determines the sequence stratigraphic relationships within this Eocene group. System tracts, defined by flooding surfaces and sequence boundaries, are examined as defined by various sedimentary models. By understanding the sequence stratigraphic relationships within these formations, it is possible to advance our understanding of the regional changes in sea level along the Texas Gulf Coast as well as to highlight sedimentary relationships and models for this group.

A multivariate mixing model for identifying sediment source from magnetic measurements

A sequential method for quantitative identification of sediment source components, based on magnetic measurements, has been developed and tested for sediments from the Rhode River, Maryland. Simulated mixing tests and multiple regression were employed to establish numerical relationships between source component proportions and the magnetic measurements of mixtures. On the basis of these multivariate mixing models, source components of three estuarine sediment cores were estimated by linear programming. The results strongly support the previous studies on this catchment which indicated a dramatic change in sediment source some 150 to 200 yr ago. Quantitative calculations are more useful and informative than purely qualitative descriptions.

Supplemented Graphic Correlation: A Powerful Tool for Paleontologists and Nonpaleontologists

The graphic correlation technique of Shaw (1964) is not restricted to biostratigraphic applications. Supplemented graphic correlation (SGC) expands the original technique to include nonunique events (log patterns) to add to the understanding of the geologic history of an area. Nonunique events are evaluated in light of the biostratigraphic data and may be used to refine correlations provided that (1) the investigator has reason to believe that the log patterns respond to time-significant phenomena over the areal extent of the specific problem at hand, and (2) all nonunique events to be considered are tested for geologic reasonableness before they are used. Dinocyst and acritarch occurrence data and single-point resistance electric logs from three Paleocene to Eocene cores in the Virginia Coastal Plain illustrate the method. The two western cores show similar geologic histories, but the thickness in the northernmost core is approximately 30% greater. SGC reveals four episodes of deposition in the easternmost core relative to the western cores. During late Paleocene time, relative accumulation rates increased in an easterly direction. A major change in sediment source and supply occurred in early Eocene time when relative accumulation rates increased to the north. Graphic correlation combining biostratigraphic events and time-significant log-type datamore » can increase resolution in correlation. Possible correlations are tested and refined during the graphic procedure and multiple lines of evidence are used to produce geologically reasonable, and informative, results.« less

Paleoenvironmental implications of rock‐magnetic properties of Late Quaternary sediment cores from the eastern equatorial Atlantic

Rapid and nondestructive rock‐magnetic measurements of late Quaternary sediment cores from the Senegal continental slope and the Sierra Leone Rise in the eastern equatorial Atlantic correlate strongly with the SPECMAP oxygen isotope master curve and carbonate content. The rock‐magnetic‐paleoclimatic relationship results from differences in the concentration, accumulation rate, grain size, and composition of magnetic minerals between glacials and interglacials. At both sites, magnetic mineral assemblages of glacial episodes are characterized by higher concentrations (and accumulation rates) of magnetic minerals, by coarser‐grained ferrimagnetic minerals, e.g., magnetite, and by a lower ratio of ferrimagnetic to antiferromagnetic, e.g., goethite, hematite, minerals. By serving as a sensitive tracer for various deep‐sea sedimentary components, rock‐magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic techniques.