Weathering Regimes Research Articles

Mid Pleistocene arid shift in southern Australia, dated by magnetostratigraphy

In coastal sections at Hallett Cove and Sellicks Beach, south of Adelaide, and at Redbanks section on Kangaroo Island, the Brunhes/Matuyama polarity transition (780 ka) is identified in the strongly oxide-mottled Ochre Cove Formation. At all 3 sections, the Ochre Cove Formation is overlain by a calcareous grey-green aeolian clay, called Ngaltinga Clay, which in turn is overlain by calcareous sediments of the Taringa and Christies Beach Formations. The marked change from an oxide-dominated weathering regime to a carbonate-dominated weathering regime, estimated to have occurred at about 500–600 ka, is interpreted as a major arid shift in regional climates. Similar arid shifts are known from Lake Bungunnia in the Murray Basin and Lake Lefroy in southern Western Australia, where changes from lacustrine clays to evaporites and dune sediments are estimated to have occurred between 400 and 700 ka, and about 500 ka, respectively. An increase in aeolian dust accession in south-eastern Australia, consistent with increased aridity in the interior source area, occurred after 780 ka, and was probably coeval with increased dust input to Tasman Sea sediments since 350 ka. Between 600 and 900 ka, oxygen isotope fluctuations in deep-sea cores showed a pronounced change in frequency, from a 40 ka (obliquity dominated) to a 100 ka (eccentricity dominated) pattern. At the same time, glacial-interglacial amplitudes increased, with a marked enrichment of glacial d18O values consistent with larger continental based ice-sheets. Colder global temperatures, and lower sea levels during glacials, may have played a part in the mid Pleistocene arid shift recorded in southern Australia. Associated variations in the strength of the warm Leeuwin Current may also have affected regional rainfall patterns in southern Australia.

Element mobilities and the Australian regolith - a mineral exploration perspective

Much of the Australian regolith ranges from Palaeogene to Late Cretaceous in age or even older, contrasting with the relatively young landscapes of the Northern Hemisphere. Hence, many imported geochemical exploration methods are unsuitable for Australian environments; this has led to successful homegrown innovation. Exploration geochemistry seeks to track geochemical anomalies arising from concealed ore deposits to their source. Much is known about element associations for different types of ore deposits and about observed patterns of dispersion. Element mobility in a range of Western Australian environments is discussed, drawing on field examples from the Mt Percy and Boddington gold mines and the Yandal greenstone belt, with reference to the effect of modern and past weathering regimes and the influence of groundwater on element mobility. Soil biota and vegetation affect Au mobility in the regolith, but specific processes, scale and environmental factors are unknown. Possible future synergies between biogeochemical or environmental research and regolith exploration geochemistry include determining the fundamental biogeochemical processes involved in the formation of geochemical anomalies as well as environmental concerns such as regolith aspects of land degradation. Exploration geochemists must study the work of biogeochemical and environmental researchers, and vice versa. There should also be collaborative research with regolith scientists and industry.

Comparison of independent proxies in the reconstruction of deep-water circulation in the South-East Atlantic off Namibia

Independent proxies were assessed in two Late Quaternary sediment cores from the eastern South Atlantic to compare deep-water changes during the last 400 kyr. Two cores were recovered from beneath North Atlantic Deep Water (NADW) at approximately 3 000 m depth. Late Quaternary presence of NADW is indicated by the Cibicidoides wuellerstorfi assemblage on the Walvis Ridge (Core GeoB 1214) and the Bulimina alazanensis assemblage on the Namibian continental slope (Core GeoB 1710). The propagation of NADW is exclusively observed during interglacials, with maximum factor loadings in Stages 1, 5, 7, 9 and 11. These maxima are consistent with peaks in kaolinite/chlorite ratios and maxima of poorly crystalline smectite in the clay-mineral record. Kaolinite and poorly crystalline smectite are products of intense chemical weathering. They are injected into the NADW at low latitudes, north of the study area, and advected south. Chlorite, which is stable under cold weathering regimes, is a characteristic mineral of water masses of southern origin. During glacial stages, it is advected north with Southern Component Water (SCW). Above the NADW/SCW depths, kaolinite/chlorite ratios vary only slightly without a significant glacial-interglacial pattern, as measured in a core (GeoB 1712) from 1 000 m deep on the same profile of the Namibian continental slope off Walvis Bay.

A comparison of GCM simulated Cretaceous `greenhouse' and `icehouse' climates: implications for the sedimentary record

Two General Circulation Model simulations of mid-Cretaceous climate are presented, one representing a `greenhouse' and the other representing an `icehouse' world. Through a comparison of `greenhouse' and `icehouse' palaeoclimates, differences in processes which are intrinsically linked with climate, such as continental weathering and snow and ice accumulation, may be observed. A number of conspicuous and discrete climatic differences can be seen between the two simulations, which may be translated into a number of identifiable signatures in the sedimentary record. The climate at mid-high latitudes in the `icehouse' simulation was more arid than the `greenhouse' simulation and could result in deposition of smectite-dominated clay suites. In contrast, the climate of low latitudes of the `icehouse' simulation resulted in a moister climate than the `greenhouse' simulation, which could affect the weathering regimes and result in deposition of kaolin-dominated clay suites. Thus cool climate episodes do not necessarily result in a global increase in aridity. The `icehouse' simulation predicts a small permanent icecap at high latitudes, which could account for the origin of dropstones in marine sediments. Furthermore, a sea-level fall as a consequence of an accumulation of ice at high latitudes may also increase continentality and surface albedo which may also contribute to the causes of the drier climate at high latitudes. Hurricanes are likely to be more frequent in `greenhouse' than `icehouse' climates. Mid-latitude depressions, also important for generating storms, are likely to be both intense and frequent during `icehouse' and `greenhouse' periods of the middle Cretaceous.

Late Quaternary precessional cycles of terrigenous sediment input off the Norte Chico, Chile (27.5°S) and palaeoclimatic implications

The palaeoclimatic conditions during the Last Glacial Maximum (LGM) of southern South America and especially latitudinal shifts of the southern westerly wind belt are still discussed controversially. Longer palaeoclimatic records covering the Late Quaternary are rare. A particularly sensitive area to Late Quaternary climatic changes is the Norte Chico, northern Chile, because of its extreme climatic gradients. Small shifts of the present climatic zonation could cause significant variations of the terrestrial sedimentary environment which would be recorded in marine terrigenous sediments. To unveil the history of shifting climatic zones in northern Chile, we present a sedimentological study of a marine sediment core (GeoB 3375-1) from the continental slope off the Norte Chico (27.5°S). Sedimentological investigations include bulk- and silt grain-size determinations by sieving, Atterberg separation, and detailed SediGraph analyses. Additionally, clay mineralogical parameters were obtained by X-ray diffraction methods. The 14C-dated core, covering the time span from approximately 10,000 to 120,000 cal. yr B.P., consists of hemipelagic sediments. Terrigenous sedimentological parameters reveal a strong cyclicity, which is interpreted in terms of variations of sediment provenance, modifications of the terrestrial weathering regimes, and modes of sediment input to the ocean. These interpretations imply cyclic variations between comparatively arid climates and more humid conditions with seasonal precipitation for northern Chile (27.5°S) through the Late Quaternary. The cyclicity of the terrigenous sediment parameters is strongly dominated by precessional cycles. For the palaeoclimatic signal, this means that more humid conditions coincide with maxima of the precession index, as e.g. during the LGM. Higher seasonal precipitation for this part of Chile is most likely derived from frontal winter rain of the Southern Westerlies. Thus, the data presented here favour not only an equatorward shift of this atmospheric circulation system during the LGM, but also precession-controlled latitudinal movements throughout the Late Quaternary. Precessional forcing of latitudinal movements of the westerly atmospheric circulation system may be conceivable through teleconnections to the Northern Hemisphere monsoonal system in the Atlantic Ocean region.

Dissolution rates of metals in Fe oxides: implications for sampling ferruginous materials with significant relict Fe oxides

A study of the pattern of dissolution of synthetic and natural Fe oxides in 6 M HCl indicates that the rate of element release from synthetic Fe oxides is strongly influenced by mineralogy and the level of element incorporation. Synthetic maghemite ( γ-Fe 2O 3) samples are subject to much more rapid dissolution than goethite (FeOOH) and hematite ( α-Fe 2O 3). In samples dominated by hematite and maghemite, Cu, Zn and particularly Pb, in comparison to Fe, are preferentially released during the early stages of dissolution. Similar patterns are apparent from the dissolution of hematite- and maghemite-dominated samples derived from natural gossan. Comparison of XRD scans with data from the dissolution of natural gossan samples transformed by incremental heating to hematite- and maghemite-dominated assemblages suggests that the degree of crystallinity may also be a significant factor in the release of elements incorporated in the Fe oxides. Ferruginous materials made up of varying proportions of goethite, hematite, maghemite, kaolinite and quartz are important sampling materials in a range of regolith environments. These are products of complex chemical and mechanical mobilization over long periods of geological time. If the patterns of Fe oxide dissolution in 6 M HCl and the release of incorporated metals reflect stability in such weathering regimes, knowledge of the retention characteristics of incorporated metals in different Fe oxide phases, as indicated by this study, will be useful in the planning and interpretation of geochemical surveys in such regions.

Palynological fluorescence in hinterland reconstruction of a cyclic shallowing-up sequence, Pliocene, Papua New Guinea

Abstract Variations in fluorescence characteristics of contemporaneous and reworked palynological particles are documented through a cyclic shallowing-upward sequence of Lower Pliocene mudstones of the Orubadi Formation, in the Puri Anticline of the Papuan Foreland Basin, Papua New Guinea. Fluorescence of palynological particles may be affected by age, oxidation and thermal maturation and therefore provides information about tectonic and palaeogeographical events in the source area of the particles—the New Guinea Fold and Thrust Belt. Palynological particles have been identified with different fluorescence properties and sources: (1) rocks at the southern (proximal) edge of the fold and thrust belt—not greatly affected by tectonic events; (2) rocks at a more distant location within the fold and thrust belt—affected by tectonic controlled changes in gradient, orographic rainfall and fluvial transportation; and (3) contemporaneous vegetation—affected by tectonic-controlled changes in altitude and fluvial transportation. Variation in the fluorescence of palynological particles from these sources allows interpretations to be made about timing of thrusting events, changes in the weathering and transportation regime and the variations in accommodation space in the foreland basin.

Palaeo-reversals in groundwater flow and peatland development at Lost River, Minnesota, USA

Raised bogs represent end-products of peatland development that are generally assumed to be decoupled from the regional groundwater system. However, the development of two peat mounds in northern Minnesota was driven by three major reversals in its groundwater-flow regime. A 4000-year record of groundwater flow was reconstructed by stratigraphic studies that utilized both botanical indicators of the surface-water chemistry, and mineral indicators of the pore-water chemistry and chemical weathering regime. The mineral stratigraphy indicated that the peat mounds were initially formed under a discharge (upward flow) regime. During the succeeding moist period, these peat mounds developed into raised bogs as their water-table mounds drove local recharge (downward) flow into the mineral substratum. A major reversal to a prevailing discharge regime at 1200 BP, however, converted the smaller peat mound into a spring fen and weathered the silicate minerals within the peat profile of the larger mound. This reversal was apparently caused by the onset of extended droughts, which dissipate the water table mound within these peat landforms.

Glacial‐interglacial variability of atmospheric CO2 due to changing continental silicate rock weathering: A model study

An 11‐box model of the oceanic carbon cycle including sedimentary processes is used to explore the role chemical weathering of continental silicate rocks might play in driving atmospheric CO2 levels on glacial‐interglacial timescales. Histories for the consumption of CO2 by silicate rock weathering processes are derived from the marine Ge/Si record. Taking the major uncertainties in the knowledge of the Ge and Si cycles into account, several histories for the evolution of the riverine dissolved silica fluxes are calculated from this record. The investigation of the systematics between riverine dissolved silica and bicarbonate fluxes under different weathering regimes leads us to the tentative conclusion that although there is no correlation between dissolved silica and total bicarbonate concentrations in the major rivers, there may exist a negative correlation between weathering intensity and the ratio of dissolved silica to bicarbonate derived from silicate weathering alone. With this correlation as a working hypothesis, it is possible to interpret the dissolved silica fluxes in terms of equivalent CO2 consumption rates. The calculated histories indicate that glacial rates of CO2 consumption by chemical silicate rock weathering could have been twice, and possibly up to 3.5 times, as high as they are today. When used to force the carbon cycle model, they are responsible for glacial‐interglacial pCO2 variations in the atmosphere of typically 50–60 ppm and up to 95–110 ppm. These variations are superimposed to a basic oscillation of 60 ppm generated by the model, mainly in response to coral reef buildup and erosion processes. The total pCO2 signal has an amplitude of about 80–90 ppm and up to 125–135 ppm. Although these large amplitudes indicate that silicate weathering processes should be taken into account when studying glacial‐interglacial changes of CO2 in the atmosphere, it also raises new problems, such as too high CO2 levels during the period from 110–70 kyr B.P., requiring further study.

Rock weathering processes and landform development in the Sør Rondane Mountains, Antarctica

Field observations of weathering processes and the related landforms, combined with laboratory analyses of weathering products, permit a synthetic evaluation of Late Cenozoic weathering environments in the Sør Rondane Mountains, Antarctica, an arid upland characterized by low temperatures and strong winds. Rates and character of weathering depend mainly on moisture availability and the bedrock geology. Under the humid weathering regime that occurs only locally around the margin of the present sheet, frequent diurnal freeze-thaw cycles in summer cause relatively rapid rock fragmentation. Most of the mountains are situated in the arid weathering regime, under which rock breakdown is very slow unless the rock contains plenty of salts. Salt weathering becomes more intensive and extensive with exposure age, as a result of salt accumulation in rock, eventually producing soils as small as fine-silt size. Lack of clay mineralization even in weathered rocks having been exposed above the ice sheet prior to 4 Ma ago indicates that hydrolysis or carbonation of rock minerals has been insignificant during the past 4 Ma. The final products of weathering are due mainly to salt action and reflect the parent lithology. Resistant fine-grained granite forms strongly oxidized tors carved with tafoni, or fields of mushroom-like boulders overlying the fractured bedrock. Less resistant rocks, like biotite gneiss and amphibolite, produce stone pavements underlain by saline, silty soils up to 30–40 cm thick, the thickness of which corresponds to the maximum thaw depth.

Global chemical erosion during the Last Glacial Maximum and the present: Sensitivity to changes in lithology and hydrology

Geographically based calculations for 18,000 years ago (18 ka) and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial‐interglacial timescales. Runoff fields were derived from general circulation model predictions of precipitation minus evaporation. Then global lithologic maps were prepared, so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates. Assuming that significant chemical erosion does not occur beneath ice sheets, we find that chemical erosion in ice‐free areas at 18 ka was only slightly greater than today. This result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff. The small (∼20%) increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates (which weather faster than average). Data on glacial/interglacial movements of the calcite compensation depth (CCD) are inconclusive but seem to indicate little change, whereas even a 20% increase in the riverine bicarbonate flux should cause an observable deepening. If the CCD did not indeed change, then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means, such as increased carbonate productivity. About half of the observed decrease in atmospheric pCO2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice‐free areas. Consideration of the maximum possible effect of meltwater at ice margins leads to an 80% increase in the global chemical erosion rate at 18 ka. Such an increase would lead to a larger and faster drop in atmospheric pCO2 but also to an excessive deepening of the global CCD; this scenario seems unrealistic. Our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of Quaternary marine Sr isotopic ratios. Therefore, if these records are indeed globally representative and entirely due to increased silicate chemical erosion (as opposed to changes in source areas or hydrothermal inputs), then other factors, such as a shift in the global weathering regime which we do not explicitly consider here, may be involved. Sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non‐spatially‐resolved calculation.

Loess accumulation and soil formation in central kansas, United States, during the past 400 000 years

AbstractBased on interpolation of thermoluminescence dates and the mean accumulation rate of 0.034 mm yr−1, four cycles of pedogenic CaCO3 accumulation are found within the Loveland Loess: 415–325 ka, 325–250 ka, 250–195 ka and 195–95 ka. The four CaCO3 peaks correspond chronologically to marine oxygen isotope stages 11, 9, 7 and 5, respectively. The early Wisconsin (95–70 ka) was characterized by sand dune activity. The reddish pedocomplex was formed from 70 to 35 ka under relatively warm and moist climatic conditions with a very slow rate of silt accumulation (0·016 mm yr−1). The Gilman Canyon pedocomplex, enriched in organic matter and dated at 35–20 ka, was formed under a strong physical weathering regime and a relatively high rate of silt accumulation (0·15 mm yr−1), indicating a windy, relatively moist, probably cool environment. It developed when the Laurentide ice sheet was advancing and dust content in Greenland ice core was low. The Peoria Loess was accumulated at a rate of 0·3 mm yr−1 in central Kansas under cold dry conditions when the ice sheet fluctuated around its maximum position and the dust content in the Greenland ice core was the highest. Even the warm substage around 13 ka has some corresponding evidence in the central Great Plains. The well‐developed Brady Soil, dated at 10·5–8·5 ka, indicates that the early Holocene was the optimal time for soil development since 20 ka. The poorly weathered Bignell Loess might have been deposited during the Altithermal Period from 8·5 to 6 ka.

Synchronous changes in seawater strontium isotope composition and global climate

THE 87Sr/86Sr ratio of sea water has increased gradually over the past 40 Myr, suggesting a concomitant increase in global chemical weathering rates1–6. Recently, Dia et al.7 analysed a 250-kyr 87Sr/86Sr record, and found superimposed on this gradual increase higher-frequency 87Sr/86Sr variations which appeared to follow a 100-kyr cycle; this periodicity corresponds to one of the prominent cycles in the Earth's orbital parameters, which are known to modulate the patterns of solar insolation and hence climate8–10. The resolution of this record was, however, insufficient to establish the phase relationship between the 87Sr/86Sr variations and global climate cycles. Here we present a high-resolution seawater 87Sr/86Sr record spanning the past 450 kyr. We find that maxima and minima in 87Sr/86Sr coincide with minima and maxima, respectively, in continental ice volume (from the SPECMAP oxygen isotope record20), apparently suggesting that there was less chemical weathering in arid glacial periods than in the more humid interglacials. During glacial–interglacial transitions, however, seawater 87Sr/86Sr changes at a rate of ∼ 1 p.p.m. kyr−1, approximately three times that evaluated by Dia et al.7. Mass-balance calculations illustrate that simple changes in modern chemical weathering regimes cannot fully account for such rapid changes, suggesting that we need to revise current ideas about strontium reservoirs and the mechanisms for exchange between them.

234U/ 238U mass spectrometry of corals: How accurate is the U [sbnd]Th age of the last interglacial period?

Direct measurement of U and Th by mass spectrometry recently provided a spectacular improvement in the precision of coral dating by the U Th disequilibrium method [1]. Mass spectrometric data also provide a much better resolution for the examination of subtle diagenetic effects revealed by small variations of the 234U/ 238U ratio. Such perturbations may strongly affect the accuracy of the U Th chronometer. A compilation of all the corals analyzed to date by mass spectrometry shows that most of the corals from terraces of the last interglacial have initial 234U/ 238U ratios higher than present-day seawater, in contrast to modern, Holocene and last Glacial corals. Some samples that have very high 234U/ 238U initial ratios, up to 1.2, and high U concentrations, up to 4 ppm, were probably contaminated by continental groundwaters. However, even apparently pristine samples have 234U/ 238U initial ratios which are still slightly higher than present-day seawater (mean value: 1.160, compared to 1.140–1.150 [2]), with little overlap between the two distributions. This difference in the U initial ratio raises some uncertainty about the accuracy of the U Th age determinations of these corals. In spite of the fact that the 234U 230Th ages cluster in a narrow range between 122 and 133 kyr, the data could also be interpreted as resulting from slight contamination of corals that are significantly older than 125 kyr. Two possible explanations may explain these data: (1) All these samples may have been diagenetically altered, since they all come from surface outcrops which have been directly exposed to precipitations or soil waters for 125 kyr. In this case, the true age of these corals remains uncertain, depending on the timing of the alteration process (i.e., initial, late or continuous). (2) The second possibility is that some of the differences in 234U/ 238U ratios measured in ∼ 125 kyr cold corals, compared to modern seawater, may be due to a higher 234U/ 238U ratio in seawater 125 kyr ago. This could result from temporal variations in the weathering regimes of continental land masses.

Major element geochemistry and paleoclimatology of the Permo-Carboniferous glacigene Dwyka Formation and postglacial mudrocks in southern Africa

A Chemical Index of Alteration (CIA) is used to quantify the degree of weathering of fine-grained materials in the glacigenic Permo-Carboniferous Dwyka Formation and succeeding stratigraphic units of southern Africa. Diamictites in the Dwyka Formation have low CIA values compatible with their glacigenic origin. Interbedded mudrocks show a wide scatter of values, suggesting that at least some of them were deposited during periods when ice had retreated from the source areas. Samples from the Prince Albert shale, which immediately overlies the Dwyka Formation, show high CIA values, suggesting a rapid transition to a much more intensive weathering regime (warmer climatic conditions?). Samples from the succeeding Whitehill shale have a low average value, so that weathering was once more inhibited, perhaps by a temporary return to a colder climate. The highest stratigraphic units sampled, the “upper shales”, have an average CIA value comparable to that of “average” shales, and suggesting “normal” weathering conditions. Major element geochemical data can supplement sedimentological and paleontological information and provide a valuable contribution to the detailed paleoclimatic interpretation of such glacigenic successions.

Chemistry of uranium, thorium, and radium isotopes in the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal

The most comprehensive data set on uranium, thorium, and radium isotopes in the Ganga-Brahmaputra, one of the major river systems of the world, is reported here. The dissolved 238U concentration in these river waters ranges between 0.44 and 8.32 μ/1, and it exhibits a positive correlation with major cations (Na + K + Mg + Ca). The 238U ∑Cations ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional variations observed in the [ 234U 238U ] activity ratio are consistent with the lithology of the drainage basins. The lowland tributaries (Chambal, Betwa, Ken, and Son), draining through the igneous and metamorphic rocks of the Deccan Traps and the Vindhyan-Bundelkhand Plateau, have [ 234U 238U ] ratio in the range 1.16 to 1.84. This range is significantly higher than the near equilibrium ratio (~1.05) observed in the highland rivers which drain through sedimentary terrains. The dissolved 226Ra concentration ranges between 0.03 and 0.22 dpm/1. The striking feature of the radium isotopes data is the distinct difference in the 228Ra and 226Ra abundances between the highland and lowland rivers. The lowland waters are enriched in 228Ra while the highland waters contain more 226Ra. This difference mainly results from the differences in their weathering regimes. The discharge-weighted mean concentration of dissolved 238U in the Ganga (at Patna) and in the Brahmaputra (at Goalpara) are 1.81 and 0.63 μ/1, respectively. The Ganga-Brahmaputra river system constitutes the major source of dissolved uranium to the Bay of Bengal. These rivers transport annually about 1000 tons of uranium to their estuaries, about 10% of the estimated global supply of dissolved uranium to the oceans via rivers. The transport of uranium by these rivers far exceeds that of the Amazon, although their water discharge is only about 20% of that of the Amazon. The high intensity of weathering of uranium in the Ganga-Brahmapura River system can also be deduced from the [ 232Th 238U ] and [ 230Th 238U ] activity ratios measured in the suspended sediments. 230Th is enriched by about 19% in the suspended sediments relative to its parent 238U. The flux of excess 230Th supplied to the Bay of Bengal via these river sediments is 980 × 10 12 dpm/a, about six times more than its in situ production from seawater in the entire Bay of Bengal.

The distribution of Fe, Mn, Ni, Cr and Co between the acid-soluble and Fe-oxides and -hydroxides and matrix fractions of the Gercus red-beds of northeast Iraq

Twenty samples of the Gercus (middle Eocene) red marl and carbonate of northeast Iraq have been analysed for their Mn, Cr, Ni, Co and Fe content in the HCl-soluble and free Fe-oxide and -hydroxide phases. The greater bulk of HCl-soluble Mn is associated with carbonate minerals. Only trace amounts (up to 120 ppm) of Mn are associated with free Fe-oxides and -hydroxides. HCl-soluble Cr, Ni and Co varies between 12 and 67, 68 and 529, and 14 and 41 ppm, respectively. These constitute 4.5%, 26% and 36.9%, respectively, of their total content in the samples. Good positive correlation was found between free Fe-oxides and -hydroxides and the associated Cr, Ni, Co and Mn content. Cr, Ni and Co was found to be enriched in the free Fe-oxides and -hydroxides relative to their total content in the main matrix. Enrichment factors are in the order Co (5.0) > Ni (3.5) > Cr (2.4). This, together with the similar partition order of these elements in the HCl-soluble phase, is in agreement with the relative mobility of these elements during the weathering, transportation and deposition regime of the sediments.

The precursor principle and the possible significance of stratiform ores and related chemical sediments in the elucidation of processes of regional metamorphic mineral formation

The precursor principle and the possible significance of stratiform ores and related chemical sediments in the elucidation of processes of regional metamorphic mineral formation

Late Cenozoic Ge/Si record of marine biogenic opal: Implications for variations of riverine fluxes to the ocean

We have determined germanium/silicon ratios in purified diatoms and radiolarians from siliceous sediments in Holocene core tops, one late Pleistocene piston core, and four high‐latitude Southern Ocean Deep Sea Drilling Project (DSDP) sites ranging in age from Holocene to Oligocene. High‐latitude Holocene diatoms in the 10‐ to 38‐µm size fraction ([Ge/Si]opal = 0.693 ± 0.039 × 10−6) faithfully record the present oceanic ratio ([Ge/Si]seawater = 0.699 ± 0.004 × 10−6). This confirms that diatoms from siliceous oozes are monitors of seawater Ge/Si and may record past changes in rates of delivery of weathering products to the sea from continental and seafloor weathering. Large diatoms and radiolarians display ratios much lower than (Ge/Si)seawater, suggesting that (Ge/Si)opal, to some extent, displays the effects of biological fractionation. From 0 to 2.5 m.y.B.P., (Ge/Si)opal in diatoms from DSDP sites ranges from about 0.54 to 0.70 × 10−6, similar to the range observed in one late Pleistocene piston core over the last 25 kyr. From 2.5 to 6 m.y.B.P., the ratio is more variable, while prior to 10 m.y.B.P., ratios are clearly higher, averaging about 0.9 × 10−6 in the early to middle Miocene. Low values of the ratio are consistent with global weathering regimes dominated by river silica input to the sea, while higher ratios suggest periods of enhanced hydrothermal input or reduced fluvial contribution. Nevertheless, an oceanic one‐box model with variable river and hydrothermal inputs and biogenic opal output cannot adequately account for the magnitude or the timing of changes observed in the record, suggesting the existence of other sources/sinks for inorganic germanium and/or silicon to or from the sea.

The freeze‐thaw weathering regime at a section of the Niagara escarpment on the Bruce Peninsula, Southern Ontario, Canada

AbstractThe bedrock freeze‐thaw and moisture regimes at an actively eroding site on the Niagara Escarpment, Bruce Peninsula, southern Ontario, were monitored between December 1983 and April 1984, and the results compared with amounts of debris collected in an adjacent rock trap.Frost wedging in pre‐existing rock fissures is the primary mechanism responsible for the observed rockfall events; hydration effects are negligible. Debris production was more closely related to the duration of the freezing leg of the freeze‐thaw cycle than to intensity or to cycle frequency. Release also coincided with periods of high pore saturation (> 60 per cent) and the seepage of water from cracks and fissures.Fewer freeze‐thaw cycles were recorded in the air than at 1 and 3.5 cm in the bedrock. The number of cycles that could be declared geomorphologically effective according to established temperature criteria was normally less than half the total number of freeze‐thaw cycles recorded in both air and bedrock. Under the current temperature regime at the field site, few effective cycles are capable of penetrating more than 5 cm into the free face.