Cycle Climate Change Research Articles

Late Quaternary course changes of the Kuroshio Current in the Ryukyu Arc region, northwestern Pacific Ocean

The existence of a land bridge connecting the central-southern Ryukyu Arc to Taiwan during the last glacial stage (oxygen isotope stages 2 to 4) and its postglacial disappearance are demonstrated by variations in the planktic δ 18O values and the frequency of Pulleniatina in 11 piston cores from the Okinawa Trough region. The upward increase in the Pulleniatina group in the cores signals the progressive inflow of the Kuroshio Current into the Okinawa Trough area, because this group is characteristic of the Kuroshio Current, as revealed here by planktic foraminiferal distribution using 56 surface sediment samples collected from the whole area of this region. The Ryukyu–Taiwan land bridge caused eastward deflection of the Kuroshio Current at the southern end of the Ryukyu Arc. A Pulleniatina minimum event in the period from ∼4500 to 3000 yr B.P. is recognized in 10 cores from the Okinawa Trough and one from the northern Ryukyu Trench slope. In contrast to the LGM, the decline of Pulleniatina in this event was not ascribed to cyclic climate changes such as worldwide cooling but to the formation of a barrier between Taiwan and the southern Okinawa Arc. However, this regional event influences largely on the Holocene climate of Japan and its adjacent seas.

Genetic Structure of Coral Reef Organisms: Ghosts of Dispersal Past

SYNOPSIS. Molecular genetic studies are revealing the presence of cryptic taxa, and patterns of gene flow in coral reef species, that do not correspond to present day ocean circulation patterns. Concordant borders of genetic inhomogeneity in several taxa emphasise the influence of historical barriers to gene flow. The persistence of genetic differences between sites apparently connected by present-day currents provides evidence for lack of effective contemporary gene exchange. A review of the limited data available to date cannot be conclusive, but suggests that present patterns of genetic variation in the Indo-Pacific have resulted from highly pulsed dispersal events associated with range expansion during interglacial periods. Thus, population genetic structure appears to be dominated by events associated with global climate change and sea level fluctuation during the last 1–3 million years, rather than vicariant geological events in the early Caenozoic. Regional speciation outside the tropical Indo-West Pacific and movement of these species into that region may have played a more important role in producing diversity in that region than traditionally recognised. Some genetic variants have arisen before, and have persisted through, several cycles of climate change. The genetic structure of populations is likely to have been maintained for several thousand years after they were first established, during or immediately after range expansion, by the occurrence of co-adapted gene complexes of some form, and because of more limited opportunity for dispersal than has been assumed to date.

Mitochondrial DNA sequence variation and phylogeographical structure of rock partridge (Alectoris graeca) populations.

The rock partridge (Alectoris graeca) is distributed in the Alps, Apennines and Balkans, in mountainous areas that were heavily affected by cyclic climate and landscape changes during the last Pleistocene glaciations. Some partridge populations have colonized and expanded their present ranges only after deglaciation and recent deforestation by humans. Other populations have been segregated in land-bridge islands isolated after the rise in level of the Mediterranean Sea. Consequently, partridges from different areas could be genetically differentiated. This study has analysed 436 nucleotides of the domain I of the mitochondrial DNA control-region in 70 rock partridges collected from 14 populations throughout their distribution. Phylogenetic analysis and analysis of molecular variance showed that the Sicilian population is very divergent from the continental populations. The Albanian and central Apennine samples are slightly divergent from the Alpine rock partridges, which cluster in two groups according to their east-west distributions. The mitochondrial DNA sequences suggest the existence of a phylogeographical structuring among rock partridge populations, resulting from genetic divergence in southern refugia and subsequent postglacial colonization of northern mountain areas. Sicilian partridges could have been isolated for approximately 500,000 years, whereas the Albanian-Apennine populations could have been in contact since the last glacial maximum (21,000 yr BP) through the north Adriatic land-bridge. The Alps could have been colonized in two different periods or by two different source populations. The mitochondrial DNA phylogeographical structuring is not completely concordant with the subspecies distribution. These findings suggest that rock partridges should be managed based on the identification of phylogeographical units.

Chemical composition of sediments and suspended matter from the Cauvery and Brahmaputra rivers (India)

Suspended matter, surface and core sediments from the Cauvery River and core sediments from the Brahmaputra River, India were analysed (15 elements) by the energy-dispersive X-ray fluorescence technique. The Cauvery River load indicates a relative predominance of solute vs. suspended transport. Cauvery River suspended matter is depleted in Al and Si, and enriched in Ca, Mn, Cr, Ni, Cu, Zn and Sr relative to the world's average river suspended matter. As the sampling web covered mainly the Cauvery Delta, the enhanced heavy metal concentrations in the suspended matter are explainable in terms of chemical–physical processes in the zone of mixing. Flocculation of the dissolved metal species causes their transformation in suspended state. Consequently, deltaic suspended matter is one–two orders of magnitude richer in heavy metals than the main river course background. Cauvery surface sediments reflect the average Indian river sediment composition. They have a homogenous composition along the river course and a heavy metal content lower than that of the Cauvery suspended matter. Sedimentation rates in the Cauvery and Brahmaputra are comparable with those published for other Indian rivers. Sediment flux to the Brahmaputra bed is higher than for the Cauvery. Mass accumulation rates for heavy metals in the sediments were determined. It is supposed that the large and synchronous fluctuations in the temporal distribution of the heavy metal mass accumulation rates have been caused by cyclic climatic changes over the Indian sub-continent. An additional supply of transition elements, due to urbanisation and industrial activities, could be assumed to cause the enrichment in the uppermost core layers.

Geomorphic response to minor cyclic climate changes, San Diego County, California

Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region. Using aerial photographs and field investigations we found little response to the 1978–1983 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.

Climatic significance of natric horizons in Permian (Asselian) palaeosols of north‐central Kansas, USA

ABSTRACTColumnar structured horizons have been recognized in ancient coastal palaeosols of several Lower Permian (Asselian) stratigraphic units of north‐central Kansas. These strongly developed columnar, polygonal‐shaped peds are characteristic of sodium‐influenced (natric) argillic horizons, and are commonly indicative of semi‐arid to arid environments. Evaporite features above and below these palaeosols support the conclusion for a dry palaeoclimate. The columnar peds are typically 3–15 cm in diameter and exhibit domed tops. Fine clay fills the cracks between the columnar peds, and is generally of a darker colour than the peds. Each natric horizon has a low value and chroma colour, apparently the result of carbonate accumulation. The natric horizons in these Permian palaeosols appear to have been partially influenced by sodium‐rich groundwaters. Root traces and root moulds are found between peds in all natric horizons, indicating plant succession after columnar ped formation. These sodium‐influenced palaeosol profiles occur as part of a spectrum of palaeosol types that indicate cyclical climate change associated with glacioeustatic sea‐level fluctuations.

Permian (Upper Rotliegend) synsedimentary tectonics, basin development and palaeogeography of the southern North Sea

Abstract Upper Rotliegend sedimentation in the southern North Sea Basin (SNSB) was a response to complex synsedimentary tectonics and cyclical climatic changes. During the early Upper Rotliegend, faulting (?dextral strike-slip) resulted in the formation of a number of isolated sub-basins e.g. the Sole Pit, Silverpit and the Broad Fourteens, which were gradually united as a consequence of the progressive collapse of the Inde-Cleaver Bank High. Basin-margin tectonics controlled the evolution of the basin and the nature and distribution of the various types of alluvial fan and fan delta systems that evolved during pluvial climatic phases. Many of the earlier Upper Rotliegend fans display a systematic lateral displacement along synsedimentary faults. Along the Dowsing Fault Zone, ‘dry-type’ alluvial fans display a progressive northwestward migration with time. Also, fan deltas, sourced from the Texel-Ijsselmeer High, young progressively to the SE along the trend of the main faults. These systematic lateral displacements have been tentatively explained by invoking dextral oblique transfer of the source area relative to the depositional basin, within the context of a strike-slip tectono-sedimentary model. Alternatively, in an extensional model, the distribution pattern could be explained by sourcing the fans from both the footwall scarps and the transfer zones of dip-slip faults. A rapid decline in synsedimentary faulting, and a gradual change to more pluvial climatic conditions, resulted in the development of larger ‘wet type’ fans during the later part of the Upper Rotliegend time interval.

Orbital forcing of deep-sea benthic species diversity

Explanations for the temporal and spatial patterns of species biodiversity focus on stability–time1–3, disturbance–mosaic (biogenie microhabitat heterogeneity)4,5 and competition–predation (biotic interactions)6,7 hypotheses. The stability–time hypothesis holds that high species diversity in the deep sea and in the tropics reflects long-term climatic stability3. But the influence of climate change on deep-sea diversity has not been studied and recent evidence suggests that deep-sea environments undergo changes in climatically driven temperature8 and flux of nutrients9 and organic-carbon10 during glacial–interglacial cycles. Here we show that Pliocene (2.85–2.40 Myr) deep-sea North Atlantic benthic ostracod (Crustacea) species diversity is related to solar insolation changes caused by 41,000-yr cycles of Earth's obliquity (tilt). Temporal changes in diversity, as measured by the Shannon–Weiner index, H(S), correlate with independent climate indicators of benthic foraminiferal oxygen-isotope ratios (mainly ice volume11–13) and ostracod Mg:Ca ratios (bottom-water temperature8). During glacial periods, H(S) = 0.2–0.6, whereas during interglacials, H(S) = 1.2–1.6, which is three to four times as high. The control of deep-sea benthic diversity by cyclic climate change at timescales of 103–104 yr does not support the stability–time hypothesis because it shows that the deep sea is a temporally dynamic environment. Diversity oscillations reflect large-scale response of the benthic community to climatically driven changes in either thermohaline circulation, bottom temperature (or temperature-related factors) and food, and a coupling of benthic diversity to surface productivity.

Lower Permian (Wolfcampian) Paleosol-Bearing Cycles of the U.S. Midcontinent: Evidence of Climatic Cyclicity

Lower Permian sedimentary cycles of the North American Midcontinent consist predominantly of very shallow marine and paralic facies and well-developed stacked paleosol profiles. Although recording glacio-eustatic sea level fluctuations, these cycles also contain evidence of cyclic climate change. This evidence includes the repeated carbonate-to-clastic facies pattern observed for meter-scale cycles, and the regionally consistent change from calcic to vertic paleosols within the variegated mudstones of most cyclothems. Climates are interpreted to have fluctuated from arid or semiarid conditions to seasonally wet/dry conditions during the course of a single cyclothem. Furthermore, within the Midcontinent, drier conditions appear to have characterized times of sea-level rise and highstand, whereas wetter more seasonal conditions characterized times of sea-level fall and lowstand. This relationship is interpreted to have resulted from variations in the intensity of a Pangean monsoon generated by glacial-interglacial cycles.

A Quantitative Study of Lower Cretaceous Cyclic Sequences from the Atlantic Ocean and the Vocontian Basin (Se France)

Milankovitch‐type periodicity was identified within Lower Cretaceous cyclic sequences from the western Central Atlantic Ocean and the Vocontian Basin (southeast France) through application of the Walsh spectral method to detailed stratigraphic sections. The repetitious bedding in these sequences appears to be the product of cyclic climatic changes controlled by the orbital forcing. Obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation, whereas precession is less important. Upper Berriasian and Valanginian strata from the Atlantic Ocean contain indications of eccentricity‐induced periodicity and of a nonlinear response of the climatic system. Variations in orbitally tuned pelagic sedimentation rates in the Atlantic Ocean correlate with the changes in the character of cycles and with the occurrence of calcareous turbidites; therefore this suite of sedimentary features are responding to evolving paleoceanographic conditions and possible relative sea level fluctuations. The orbitally tuned durations of the Valanginian and Hauterivian stages in the Vocontian Basin are a minimum of 5.9 and 5.3 m.y., respectively.

Optimum microfossil sequences and cyclic sediment patterns in Early Cretaceous pelagic strata

Three sequencing methods were used to calculate the most likely biozonation and the periodicity of sedimentary cycles in Lower Cretaceous pelagic strata of the Atlantic and Indian oceans.A database was built of 378 first and last stratigraphic occurrences of calcareous nannofossils, dinocysts, foraminifers, and geomagnetic reversals in highest Jurassic through Lower Cretaceous deep marine strata at 10 Atlantic Ocean and 3 Indian Ocean drilling sites. There are 135 different events in total, about one third of which are unique to either ocean. Using the complete data set, the quantitative stratigraphy methods STRATCOR and RASC calculated closely comparable optimum sequences of average first- and last-occurrence positions. The preferred zonal solution, based on the STRATCOR method, includes 56 events, each of which occurs at three or more sites. The events comprise 6 geomagnetic reversals, 25 nannofossils, 5 planktonic foraminifera, 8 benthic foraminifera, and 12 dinocysts occurrences. Nine assemblage zones have been recognized of Tithonian through Albian age. All but 2 of 18 nannofossil events in the Atlantic Ocean optimum sequence were reported in the same stratigraphic order in a standard Mesozoic nannofossil zonation.Our quantitative examination, using Walsh spectral analysis, of the Lower Cretaceous cyclic sequences at three Deep Sea Drilling Project (DSDP) sites in the Atlantic Ocean generally supports the hypothesis that they are the product of cyclic climatic changes controlled by the Milankovitch orbital cycles. The peaks in the power spectra usually can be related to obliquity and precession cycles; some peaks seem to correspond to the eccentricity cycle. Obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation in the Early Cretaceous Atlantic Ocean.The actual pelagic sedimentation rates were calculated for some cores using the results of spectral analysis. The correlation of the actual pelagic sedimentation rate with cyclic patterns and the occurrence of calcareous turbidites indicate that the changes in cycle pattern are the reflection of changes in the oceanographic setting. The changes in oceanographic setting are related to relative-sea-level fluctuations. The intervals dominated by laminated limestone were deposited during higher sea-level periods.

THE NEW WORLD GENUS STENOMORPHUS DEJEAN (COLEOPTERA: CARABIDAE: HARPALINI): CLASSIFICATION, ALLOMETRY, AND EVOLUTIONARY CONSIDERATIONS

AbstractAnalysis of structural features of adults (legs, proportions of prothorax, male genitalia, and ovipositor) establishes the hypothesis that Anisocnemus Chaudoir, Trichopselaphus Chaudoir, and Stenomorphus Dejean comprise a monophyletic assemblage, named the Stenomorphus genus-group, with Anisocnemus the adelphotaxon of Trichopselaphus + Stenomorphus. Analysis of form of median lobe, armature of the internal sac, allometric trends in proportions of the prothorax and hind tibia, and chorological considerations, provide the basis for recognition of six species of Stenomorphus and postulation of their phylogenetic relationships, as follows: S. convexior Notman {[S. penicillatus Darlington + S. sinaloae Darlington] + [S. cubanus Darlington + (S. angustatus Dejean + S. californicus Ménétries)]}. Three monophyletic species-groups are recognized: S. convexior, including S. convexior; S. sinaloae, including S. penicillatus and S. sinaloae; and S. angustatus, including S. cubanus, S. angustatus, and S. californicus. The names Stenomorphus dentifemoratus Chaudoir, 1844 and Stenomorphus alius Darlington, 1936 are synonymized, and S. dentifemoratus Darlington, 1936 (not Chaudoir) is renamed S. darlingtoni. Morphometric and chorological features provide evidence for subspecific recognition, each taxon representing a new combination: for S. angustatus, S. a. angustatus, S. a. braziliensis Darlington, and S. a. dentifemoratus Chaudoir; for S. californicus, S. c. californicus, S. c. rufipes LeConte, S. c. manni Darlington, and S. c. darlingtoni Ball and Shpeley. Evolutionary trends postulated include development of allometry with respect to form of the prothorax and hind tibiae, correlated with sexual dimorphism. Driving forces are postulated to be associated with sexual selection and burrowing. The geographical history of Stenomorphus is reconstructed as follows, using the reconstructed phylogeny and chorological affinities interpreted against a background of geological and climatic events in Middle America, during the Late Cretaceous, and Tertiary and Quaternary Epochs: isolation in Middle America from its South American adelphotaxon of the ancestral stock of Stenomorphus; specialization for life in drier tropical forests and savannas; isolation and differentiation of taxa in response to cyclical climatic changes and mountain building; and over-water dispersal to, and isolation and differentiation in, the Greater Antilles. A key, figures of diagnostic features, descriptions of structural features and of geographical ranges (supplemented with maps) provide means of recognition of taxa.

Remote sensing and mapping of Earth resources

ABSTRACTDetailed knowledge of the natural resources of any nation is a pre‐requisite for its economic development. In most developing countries, this knowledge is scarce or lacking. Acquiring the needed information by conventional means may require a large investment in time, personnel, and therefore, in cost. In the meantime, space‐age technology provides the necessary tools to conduct surveys of natural resources in a timely manner. Such are the means to obtain the required data by digital sensors, radar imaging systems, and stereo cameras. These data can be utilized in mapping structures that may contain oil and gas deposits, recognizing high concentrations of economic minerals, discovering regions with fertile soils and good potential for groundwater, and monitoring cyclical climatic changes, particularly in arid lands. Such applications warrant greater use of remote sensing methods and techniques by the international scientific community, particularly for the development of natural resources in the Third World. This paper represents a general review of the available, and easily interpretable space‐borne data that are useful in the mapping of such resources.

Significance of Loessite in the Maroon Formation (Middle Pennsylvanian to Lower Permian), Eagle Basin, Northwest Colorado

ABSTRACT Quaternary loess deposits are widespread on the earth's surface, yet pre-Quaternary loess deposits have rarely been reported. The Maroon Formation (Middle Pennsylvanian to Lower Permian) of the Eagle Basin, northwest Colorado, includes a siltstone-dominated facies interpreted as loessite (lithified loess) along its downwind basin margin. The section of inferred loessite in the Maroon Formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone. Bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief. Loessite beds are separated by common claystone drapes and weakly developed paleosols, and by rare pond deposits, channel deposits, and eolian-ripp e-laminated deposits. The loess interpretation is based on 1) the homogeneity and dominance of the sandy silt grain-size; 2) the relative lack of primary sedimentary structures; 3) the gentle character of most bedding contacts and the common mantling of irregular depositional topography; 4) the inferred paleogeographic setting; and 5) the absence of suitable alternative interpretations. The loessite grades laterally into mixed fluvial-eolian deposits of the Maroon Formation in the main part of Eagle Basin, which served as the loessite sediment source. Deposition of the Maroon Formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late Paleozoic continental ice sheets. The paleogeography and paleoclimatology of the Maroon Formation depositional system are not unique, suggesting that there are probably many other ancient loessites that have gone unrecognized.

Deposition of Upper Jurassic fine-grained limestones in the western Subalpine Basin, France

The fine-grained carbonate sequence on the western margin of the French Subalpine Basin provides information about the depositional conditions of the Late Jurassic “nodular” lithofacies which closely resemble the age-equivalent “Ammonitico Rosso” of the Mediterranean and Central Atlantic domains. A variety of rock textures can be distinguished including “lumpy”, nodular (restricted sense) and conglomeratic. The formation of these textures is referred to different sedimentological processes: cyclic marl-limestone deposition, bioturbation, winnowing, early lithification, gravity-displacement, etc. In addition, the development of deep-water cryptalgal carbonate constructions which include stromatolites and small mud mounds is noted. Outer-shelf deep-marine palaeoenvironments (100's m) are inferred from the sedimentary structures, biotic associations and facies distributions. The Callovian-Oxfordian discontinuity is believed to have resulted from a sediment starved environment. Part of the overlying fine-grained carbonate material was then presumably derived from an adjacent platform. Oscillations in nature and quantity of carbonate and clay input onto the margin probably reflect cyclic climatic and sea-level changes.

Long-term changes in some benthic species in the Firth of Clyde, with particular reference to Tellina tenuis da Costa

SynopisPresent knowledge of long-term changes in benthic species in the Firth of Clyde is reviewed.Recent work on the annual variations in seasonal cycles of the sand-dwelling bivalve Tellina tenuis da Costa shows some correlations with natural and man-made variations in seawater temperature. Between 1973 and 1984 inclusive good settlements of young occurred on the two beaches examined in the autumns of years when higher mean seawater temperatures occurred in June and July. The implications are discussed in relation to cycles of climatic change. In general, settlements at Hunterston, a thermally enriched area, were considerably greater than at Kames Bay, a site affected only by natural temperature changes. However, T. tenuis at Kames Bay grew to much larger maximum sizes than at Hunterston, except in 1979, when Hunterston animals were larger. The possible interactions of food availability and the modifying effects of heated discharges are discussed.Earlier results by Dr A. C. Stephen between 1926 and 1951 are reassessed in the light of present knowledge.

Temporal changes of chromosomal polymorphism in natural populations of Drosophila subobscura

The behaviour of the chromosomal polymorphism of D. subobscura was analyzed in relation to temporal changes, daily, seasonal and annual. Firstly, chromosome analyses were carried out over a year in a natural population. Samples were taken at monthly intervals in Tibidabo, a locality close to Barcelona. In all the months except January, the number of individuals captured was enough to carry out a chromosome analysis of that population. The A, E and O chromosomes showed a great uniformity in the frequencies of gene arrangement over the year. However, significant changes occur in the frequencies of J and U chromosomes. The J1 and U1+2+8 arrangements showed a similar tendency, with two maxima, in June and February, and a minimum in September. These changes seem to be unrelated to the cyclical climatic changes. A chromosome analysis of the subsamples captured during the day, in the May sample, was done. In no case was the change in chromosome frequencies statistically significant. The behaviour of the Ust arrangement must be noted, the frequency of which decreased through the day. Finally, the data for the Tibidabo population were compared with samples captured in nearby localities over a period of 21 years. Significant differences were found in the frequencies of all the chromosomes, with the exception of J. The most differentiated population was the last sampled, from 1982. In this population the st arrangements tended to decrease in the A, E and O chromosomes and A2, E1+2+9+12 and O3+4+7 increase, respectively.

Equatorward migration of Globorotalia truncatulinoides ecophenotypes through the Late Pleistocene: Gradual evolution or ocean change?

The biogeography of differences in average shape of the modern planktonic foraminifer Globorotalia truncatulinoides (d'Orbigny) exhibits systematic relationships to changes in the ocean's surface environment. Comparison of these shape changes, as they exist today in the Southern Hemisphere, with fossil shapes preserved in a Late Pleistocene record from the South Atlantic Ocean, shows that the biogeography of G. truncatulinoides ecophenotypes has changed markedly through time. Beginning at least 700,000 yr ago and continuing up to the present, there has been a gradual but clear migration of certain morphotypes of G. truncatulinoides toward lower latitudes. The history of this migration bears no simple relationship to the cyclic climatic changes that characterize the Late Pleistocene. We conclude that either (1) phenotypic variants of Gr. truncatulinoides reflect some previously unmeasured, gradually changing aspect of Late Pleistocene oceans, or (2) we are witnessing a gradual evolution of the environment preferences of G. truncatulinoides.

Topography and stratigraphy of Martian polar layered deposits

Topography and stratigraphy of young sediments in the Martian north polar region are being studied from high-resolution Viking Orbiter images to test and constrain theories of terrain evolution driven by cyclical climate change. Several steepened slopes upon which layers are exposed in section were found to have local relief of 200 to 800 m and slopes ranging from 1 to 8°. First attempts to calculate average layer thicknesses from portions of sections yielded results ranging from 14 to 46 m, linearly correlated with slope. In some cases, under the assumption of short-range lateral continuity, a greater number of layers was inferred to be present than was actually resolved on a particular section. When apparent layer thickness are revised accordingly, the correlation with slope is destroyed. We conclude that images of the shallowest-sloping sections of layered deposits should yield the most stratigraphic information, and layers thinner than 14 m may occur; indeed, they may even be common or typical. Because of the sensitivity of summer frost distribution to local slope and other properties of exposed strata, interseasonal comparisons of images promise to enhance the capability to detect layers and differentiate them by material properties.

Quasi‐periodic climatic changes on Mars and Earth

Observations by the Mariner 9 and Viking Orbiter spacecraft have yielded intriguing evidence that a geologic record of cyclical climate change exists in the polar regions of Mars. Cyclical climate changes are readily explained theoretically because the surface and atmosphere of Mars experience quasi‐periodic variations in the amount and distribution of solar heating because of major perturbations in orbital and axial elements. However, the effect of these perturbations on climate are poorly understood, the mechanisms for establishing geologic records of climate are not well defined, and no definitive correlation of orbital/axial perturbations and polar stratigraphy has ever been demonstrated.