Probability Estimates of Field Areas and Trapped Oil Volumes

A portion of a sedimentary basin is subdivided conceptually into hexagons of equal area. The area of each hexagon is equal to the minimum area an oil field should have to be commercial. Hexagons can be full of oil or empty. A field size 1 consists of a cell with oil surrounded by six empty cells; a field size 2 consists of two adjacent cells with oil surrounded by eight empty cells, etc. Principles of Percolation Theory are used to determine the probability distribution of the areas of the oil fields existing in this portion of the basin. The only piece of information necessary to determine this probability distribution is the Success Ratio (number of successful exploration wells/total number of exploration wells drilled in this portion of the basin). This approach has several practical applications.A probabilistic model is introduced to predict to which extent potential oil traps are filled with oil. The model assumes that the probability that an oil unit will end up in a particular trap, is proportional to the surface area of the trap. The model predicts that independently of the distribution of the trap volumes, there will be a critical trap volume. All the traps having a volume less than this critical volume, will be filled to spill point. An equation is deduced to predict, for all traps having a volume greater than the critical, the volume of oil that can be encountered in the trap, provided the volume of the trap is known.     

Hubbert’s petroleum production model: an evaluation and implications for World Oil Production Forecasts

Following Hubberts successful prediction of the timing of US peak oil production, Hubberts model has been used extensively to predict peak oil production elsewhere. However, forecasts of world and regional peak oil and natural gas production using Hubberts methodology usually have failed, leading to the implicit belief that such predictions always will fail and that we need not worry about finite resources. A careful examination of Hubberts approach indicates that the most important reasons for his success in the US were stable markets, the high growth rate of demand, ready availability of low cost imports, and a reasonable estimate of easily extractable reserves. This analysis also shows that his model cannot predict ultimate oil reserves and that it should be considered an econometric model. Building on Hubberts vital insight, that cheap fossil fuel reserves are knowable and finite, one can state that for world peak oil production, political constraints should be much more important than resource constraints.     

Field calibration and performance of sediment traps in a eutrophic holomictic lake

In wind-stressed and unstratified Lough Neagh low aspect ratio sediment traps, which allow for internal resuspension, collected sediment at a rate close to the natural sediment accumulation rate inferred from dated sediment cores. High aspect ratio traps grossly overestimated the natural rate of sediment accumulation as did, but to a lesser extent, burial rate measurements on an artificial stratigraphic marker. The former type of trap deployed over long exposure periods is therefore recommended as providing the best estimate of net downward particle flux in lakes such as Lough Neagh. However, it is emphasized that all sediment trap types, when operated in shallow turbulent lakes, will to a greater or lesser extent be contaminated by secondary or redeposited material and so will not provide a direct measure of primary sedimentation. Use of tube traps in lakes such as Lough Neagh should not, however, be discounted since they can provide a record of the quality of sedimenting material through time.     

A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA

Reconnaissance ¹⁸O,, D, and ⁸⁷Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of ⁸⁷Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the ⁸⁷Sr values. Based on the mutual variations of ¹⁸O and D, the lakes can be divided into three groups. In Group 1, both ¹⁸O and D values increase from spring to fall. The ¹⁸O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the _a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.     

Climatically induced lake level changes during the last two millennia as reflected in sediments of Laguna Potrok Aike,southern Patagonia (Santa Cruz,Argentina)

The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The total inorganic carbon content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the Medieval Climate Anomaly in southeastern Patagonia. At the beginning of the Little Ice Age the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century.     

Unusual carbon and oxygen isotopic ratios of ostracodal calcite from last interglacial (Sangamon episode) lacustrine sediment in Raymond Basin, Illinois, USA

The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have ¹³C and ¹⁸O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high ¹³C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in ¹³C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m^-2 d^-1. The ¹³C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to ¹³C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of ¹⁸O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the ¹⁸O variability.     

1400-year cold/warm fluctuations reflected by environmental magnetism of a lake sediment core from the Chen Co, southern Tibet, China

Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km² of catchment area and 40 km² of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. ²¹⁰Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr^–1, which also was confirmed by ¹³⁷Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (_LF), susceptibility of anhysteretic remanent magnetism (_ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that _LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/_LF) had much more information to reveal environmental changes. The _ARM/_LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.     

The effect of Tibetan uplift on the formation and preservation of Tertiary lacustrine source-rocks in eastern China

The Cenozoic continental sedimentary basins in eastern China are rich in lacustrine source rocks. Based on their paleogeographic location and fossils, these basins can be grouped into inland and near-shore basins. Before the collision of India and Eurasia about 45 million years ago, the inland basins were dominated by arid climates that were unfavorable for the substantial preservation of oil source rocks. In contrast, the contemporary near-shore basins experienced alternating arid and humid climates, probably induced by sea level changes, which produced conditions that favoured the formation and preservation of oil source rocks. With the rise of the Tibetan Plateau, the Asian monsoon was initiated or significantly intensified in the Late Eocene. This, in turn, changed the arid climates in the inland basins to humid or to alternating arid and humid, providing ideal conditions for the formation and preservation of lacustrine source rocks in the inland basins. The evidence suggests that Tibetan uplift played a crucial role in the generation and preservation of the Cenozoic lacustrine source rocks in eastern China.This is the third paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.     

Water isotopic and hydrochemical evolution of a lake chain in the northern Great Plains and its paleoclimatic implications

Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The ¹⁸O and ²H of water from the chain of lakes, when plotted in ²H - ¹⁸O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (¹⁸O = –6 ± 1 VSMOW) and show no correlation with salinity (which ranges from 1 to 65). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the ¹⁸O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower ¹⁸O value (–16.7 in 1998) than that of mean annual precipitation (–11) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that ¹⁸O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached.     

Processes that can disturb the chronostratigraphy of laminated sediments and pollen deposition

The sediment trap technique was used to study the sedimentation processes in two parts of Lake Goci, central Poland, the deep of the main basin and the shallow bay area. From field and lab data was noted that seasonal and annual deposition of pollen grains on the lake bottom was disturbed in the main basin by an increase in water mass dynamics and by resuspension and focusing. Greater concordance between pollen sedimentation and the flowering periods of plants was found in the bay area, where sediment resuspension took place sporadically and with less intensity, compared to the main basin. Futhermore, it is suggested that resuspension of pollen can occur even if laminated sediments are formed, a process which therby diminishes the chronostratigraphic value of sediments and pollen analyses.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VII. Carbonate isotope geochemistry as a record of riverine runoff

Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a ¹⁸O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a ¹⁸O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the ¹⁸O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the ¹⁸O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell ¹⁸O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell ¹⁸O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the ¹³C of shells against predicted values based solely on the ¹³C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.     

Climato-limnological signals during the past 260 000 years in physical properties of bottom sediments from Lake Baikal

The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around interglacial periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the glacials or 'stadials. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation.     

Paleohydrology and paleochemistry of Lake Manitoba,Canada: the isotope and ostracode records

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), ¹⁸O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative ¹⁸O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the ¹⁸O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.¹⁸O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The ¹⁸O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the ¹⁸O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The ¹³C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Multi-proxy data from Kaksoislammi Lake in Finland: dramatic changes in the late Holocene cladoceran assemblages

A small lake, Kaksoislammi (60° 3830N, 24° 4550E), in southern Finland was studied for Cladocera, diatoms and pollen from a core which covers the entire Holocene. The diatom remains indicate a steady development from alkaliphilous taxa towards the dominance of acidophilous forms and lowering pH in the late Holocene. About 1800–1700 BP, dramatic changes took place in the microfauna, mainly the planktonic Cladocera. Bosmina longirostris, the dominant species, suddenly disappeared, and Daphnia, Chydorus sphaericus and Chaoborus increased. The change is simultaneous with a decline of the diatom-inferred pH to 4.8. It is probable that there was a sudden, profound change in predator-prey relationships. The acidity of the lake water probably increased to such a low level that it led to the disappearance of even the most acid-tolerant fish. Consequently invertebrate predators increased and quickly altered the species composition in the lake. There is also pollen evidence of the onset of Iron Age cultivation and grazing almost simultaneously with the faunal change. Therefore, it cannot be ruled out that the sudden lowering of pH was indirectly caused by prehistoric human activity; possibly the acidic peatland surrounding the lake was disturbed.     

Diatom-based ionic concentration and salinity models from the south Bolivian Altiplano (15–23 ° S

The relationship between surface sediment diatom assemblages and measured limnological variables in fifty eight lake samples from the south Bolivian Altiplano was examined by constructing a diatom-water chemistry dataset. Analysis of this dataset by canonical correspondence analysis revealed that salinity and ionic concentration accounted for a significant amount of the variation in the distribution of the diatom assemblages. Two methods weighted-averaging regression and calibration, and by-class mean percentage table were used to established a transfer function for future reconstruction of past lake water salinity and ionic concentration in the southern Bolivian Altiplano. Weighted-averaging regression and calibration with inverse deshrinking provided a better model for the water chemistry reconstructions in this region.     

Variation in the composition of Lake Bonneville marl: a potential key to lake-level fluctuations and paleoclimate

Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and ¹⁸O and ¹³C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.     

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (¹³C and ¹⁵N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four ²¹⁰Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and ²²⁶Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low ¹³C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher ¹³C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the ¹⁵N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both ¹³C and ¹⁵N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that ¹³C and ¹⁵N reflect shifts in historic lake trophic state.     

Organo-geochemical and stable isotope indicators ofenvironmental change in a marl lake, Malham Tarn, North Yorkshire,U.K.

Sediments of the marl lake Malham Tarn located in NW Englandpreserve an environmental record since 12 Ka. Eight Holocene pollen zones wereidentified, and the ¹³C of total organic carbon (TOC) showsthree stratigraphic divisions. The basal clay unit and overlayingsand/clay/marl unit have ¹³C of –24which decreases at the base of the principal marl unit to a mean value around–30, whilst the topmost black marl unit ¹³Cincreases to –28 at the surface. Representative samples of theseunits were selected for analysis of n-alkanes andn-fatty acids and their ¹³C.Samples of modern Chara and peat were analysed forcomparison. The clay unit has a minor contribution of redeposited matureorganic matter and autochthonous algae, the marl unit a high contribution ofChara, and the dark marl unit has a high contribution fromhigher plants. Compound-specific ¹³C revealssystematic differences between alkanes and fatty acids of different chainlength. The major shift in ¹³C in the short and medium chainfatty acids are probably due to the decreasing influence of carbonate rockflour as source of DIC. The major shift in ¹³C in the longchain n-fatty acids andn-alkanes could reflect the lower atmosphericCO₂ concentration at Last Glacial. The negative shift of short chainfatty acids in organic rich dark marls reflects introduction of detrital peatinto the lake. The ¹³C results show a dramatic change fromdominance of autochthonous plus eroded sources up to Pollen Zone IV, then slowcolonisation of the hinterland by higher plants, followed by constantChara contributions throughout the deposition of the marl,and a further increase of higher plant material after the rise in water levelin 1791.     

Zipf’s Law: A Viable Geological Paradigm?

Zipfs Law originally was proposed as a guide to a statistical distribution in social studies. The law describes a relationship between size and rank of discrete phenomena. It is a variant of Paretos 1927 Law known as the 80/20 rule and is similar to Bodes Law in concept. The relationship described by Zipfs Law is a succession of order data with the largest followed by half the size for the next largest, which in turn, the next is half that size, and so on. In geology, it has been used with moderate success in resource assessment of mining and petroleum. In essence, it predicts how many entities of a certain size may be left in a sequence of decreasing size assuming the largest has been ascertained. Examples of applications would be plotting the rank and size of ore deposits or oil fields to determine how many deposits remained undiscovered and their size. After a flurry of papers in the 1970s and 1980s, application of the law apparently either was successful and thus not reported in way the literature or was determined to be ineffectual and its use discontinued, but either way the law lapsed into obscurity. Examples of oil- and gas-field size in Kansas, the occurrence of historic earthquakes that affected the state, and size of anticlines (plains-type folds) are presented to illustrate application and limits of Zipfs Law.     

The misrepresentation of the planktonic diatom assemblage in traps and sediments: southern Lake Malaŵi,Africa

Sediment trap collections near Cape Maclear, Lake Malai, were compared to phytoplankton and surface sediment diatoms to assess taphonomic variations. The sedimenting diatom community became progressively different from the diatom plankton with increasing depth: long Nitzschia species were strongly under-represented in the traps (annually, 53% among planktonic diatoms vs. 14% in the offshore 29 m trap; p0.005 by Kruskal-Wallis test), while Melosira was greatly over-represented in traps (32% vs. 57%; p<0.005). The abundances of the minor taxa (Rhopalodia, Fragilaria, Cymbella, and Surirella) were greatly enhanced in traps relative to the plankton, but they were still relatively uncommon (<3% of all diatoms each). Differences in grazing, dissolution, and sinking rates alone are insufficient to account for these distortions; a combination of these, plus perhaps unknown factors, strongly influence the deposited assemblage.These misrepresentations were also present at the sediment surface. The greatest discrepancy was noted for Melosira (32% of plankton vs. 53% of sediment surface diatoms; p<0.005) and for elongate Nitzschia species (53% of plankton vs. 0.8% in sediments; p<0.005). In Lake Malai, at least, paleolimnologists must not assume a straightforward correlation of modern and fossil assemblages.