Long-term Subsidence Rates Research Articles

Process and mechanism analysis of typical permafrost area surface deformation at two sites along the Qinghai-Tibet highway based on long-term leveling observation

Surface deformation indicates permafrost changes and multi-year surveys can reflect its characteristics and causes. In this study, surface deformations in Wudaoliang (WDL) and Xidatan (XDT) were determined based on long-term leveling measurements, hydrothermal data, precipitation and soil sampling. In-situ observations can be used to study the influence of hydrothermal changes in the active layer on the surface. The amplitude and onset date of seasonal deformation are different due to the soil moisture (SM) and soil texture in the two permafrost regions. High SM and fine-grained soils can cause significant seasonal surface deformation. A good correlation existed between the freeze-thaw front migration rate and deformation in the active layer with a uniform distribution of SM. The inter-annual subsidence rate in the WDL was higher than in the XDT, owing to the different rates of ground ice thawing near the permafrost table. The long-term subsidence rate positively correlates with the increased seasonal deformation amplitude, especially in fine-grained soils.

Control of inherited accreted lithospheric heterogeneity on the architecture and the low, long-term subsidence rate of intracratonic basins

Intracratonic basins tend to subside much longer than the timescale predicted by thermal relaxation of the lithosphere. Many hypotheses have been suggested to explain their longevity, yet few have been tested using quantitative thermo-mechanical numerical models, which capture the dynamic of the lithosphere. Lithospheric-scale geodynamic modelling preserving the tectono-stratigraphic architecture of these basins is challenging because they display only few kilometres of subsidence over 1000 of km during time periods exceeding 250 Myr. Here we present simulations that are designed to examine the relative role of thermal anomaly, tectonics and heterogeneity of the lithosphere on the dynamics of intracratonic basins. Our results demonstrate that initial heterogeneity of accretionary continental lithosphere explains long-term subsidence and the arches-basins architecture of Saharan type intracratonic basins at first order. The simulations show that initially heterogeneous lithospheres inherited from accretion are strong enough to resist local isostatic re-equilibration for very long period of time. Indeed, the lateral density variations store potential gravitational energy that is then slowly dissipated by differential erosion and slow vertical movements. For relatively well-accepted coefficient of erosion of 10−6 m2/s, the subsidence last longer than 250 Myr. Extensional tectonic forcing and thermal anomalies both result in an effective strength drop of the lithosphere, which allows a temporal acceleration of local isostatic re-equilibration. Periodic changes in far field tectonic forcing from extension to compression complicate the tectono-stratigraphic architecture (intra-basin arches, sub-basins) introducing stratigraphic unconformities between different neighbouring basins such as the ones observed in North Africa.

Sinkhole investigation in an urban area by trenching in combination with GPR, ERT and high-precision leveling. Mantled evaporite karst of Zaragoza city, NE Spain

Sinkhole risks are becoming particularly severe in urban areas that lack careful planning and where karst depressions are frequently filled and developed. These built-up areas are the most challenging for sinkhole investigation; they are the ones that require the highest priority and the application of novel approaches aimed at reducing the epistemic uncertainties associated with hazard assessments. This work illustrates the strengths of the rarely used trenching technique for the investigation of sinkholes in mantled karst areas. The approach has been tested on a buried sinkhole in the evaporite karst of Zaragoza city, analysing and comparing data gathered by trenching, Ground Penetrating Radar (GPR) using shielded and unshielded antennas with different frequencies, Electrical Resistivity Tomography (ERT) and high-precision leveling. The sinkhole is located beneath a street and has led to the demolition of buildings, creating a risk situation that requires careful assessment and management. Trenching provided the largest amount of objective data required for hazard assessment: (1) Precise location of the sinkhole edges. (2) Internal structure, cumulative displacement, subsidence mechanisms and their contribution. The trench exposed an asymmetric structure with a minimum vertical displacement of 6.2m, comprising a complex collapse with different generations of faults and two marginal zones affected by gentle inward tilting. Collapse faulting accommodates at least 70% of the subsidence. (3) Age of the sinkhole and average subsidence rate. The sinkhole was initiated sometime before or within 337–435calyr AD, indicating a long-term subsidence rate of ca. 3.7–4.2mm/yr. (4) Kinematic style, characterised by episodic collapse events, as supported by leveling data. The retrodeformation analysis, together with geochronological data, indicates four collapse events with a recurrence of around 400years and suggests enhanced activity since ca. 1950 AD. The performance of GPR was highly variable, depending mainly on the type of antenna and the timing of data acquisition. The best-quality data were obtained with shielded antennas and during relatively drier conditions. The rather poor performance of ERT, commonly an adequate method for resolving subsidence structures at depth, is attributed to the complex structure of the collapse sinkhole, with spatially-dense and abrupt resistivity changes in resistivity. This case study strongly substantiates the implementation of the relatively inexpensive and highly objective trenching technique in sinkhole investigations.

Morphology and evolution of drowned carbonate terraces during the last two interglacial cycles, off Hilo, NE Hawaii

The eastern side of Hawaii Island is a rapidly subsiding margin dominated by drowned carbonate platforms. We present detailed bathymetric and backscatter data, remotely operated vehicle and submersible observations, sedimentological and 14C accelerator mass spectrometry and U/Th age data from seven submerged terraces (H7, H2a–d, H1a–b) in water depths between 1100 and 25m off Hilo, north-eastern Hawaii. The main carbonate deposits on these terraces are coral deposits, rhodolith beds, coralline algal mounds, crusts, pavements and tabular sheets. We identified five previously described sedimentary shallow- to deep-water facies and one new facies type that are consistent with reef drowning on a rapidly subsiding margin. We used palaeobathymetric data derived from the sedimentary facies, age versus depth relationships, and published sea-level curves, to estimate a uniform long-term subsidence rate of 2.80±0.36m/ky for the eastern side of Hawaii over the last 150ky. Terrace H7 developed about 380ka based on data from the western side of the island. Active coral growth on terrace H2d occurred during the Marine Isotope Stage (MIS) 6 to 5 transition, and the terrace drowned during the peak of MIS 5e when sea level rose faster than reefs could grow. Favoured by the gentle platform gradient, reefs established progressively landwards with a backstepping pattern during MIS 5e to form the terraces H2c and H2b 122ka. Final turn-off of shallow water carbonate production on terraces H2b–d coincided with the relative sea-level rise of the interstadial MIS 5a. Bathymetry and submersible data suggest that carbonate sediments on terraces H2a and H1b were deposited over an antecedent topography of local lava deltas emplaced during rising sea levels at ca. 85 and 65ka, while terrace H1a established on lava delta substrates of the Mauna Loa volcano ca. 11ka. We conclude that the initiation, growth and drowning of coral-reef terraces off Hilo differ in some ways from the pattern observed in the submerged terraces in the western side of Hawaii and that the platform evolution off Hilo is more strongly influenced by emplacement of offshore lava flows.

Quantifying rates of coastal subsidence since the last interglacial and the role of sediment loading

The rate of sea-level rise is expected to increase over the next century. In many areas, increasing rates of sea-level rise are exacerbated by subsidence. In order to develop proper mitigation strategies for coastal change, better estimates for the rates of subsidence are needed. In this study we outline a strategy for calculating long-term subsidence rates for coastlines based on the differential elevations of modern shorelines and their last interglacial (LIG) equivalent geomorphic features. We apply this strategy to the LIG shoreline of the USA Texas coast. We first obtained optically stimulated luminescence ages of features long conjectured to be LIG, but, until now have remained undated. We use a digital elevation model to calculate the difference in elevations between the modern and MIS5e shorelines. This difference is corrected for glacial-hydro-isostatic adjustments to the Texas coast over the last 120ky. Our analysis shows spatial variability in the rate of subsidence that increases seaward and at locations closer to the Brazos/Colorado delta. The lowest rates of subsidence were 0.03mm/yr at the most inland site. The highest rates were 0.09mm/yr near the modern Brazos/Colorado Delta. The spatial pattern of subsidence suggests that most of the long-term vertical motion along the Texas coast is due to sediment loading. The rates of subsidence along the portions of the Texas coast are equal to, and in some places greater than, glacial-isostatic adjustments (GIA), thus highlighting the importance of other vertical motions such as sediment loading when using sea-level data to constrain GIA models even in the absence of active tectonics. In addition, these rates are two orders of magnitude less than modern rates of relative sea-level rise recorded at tide gauges along the Texas coast, highlighting the importance of Holocene compaction and fluid withdrawal in accelerating rates of subsidence along the Texas coast.

Seismites as a tool in the palaeoenvironmental reconstruction of fluvial deposits: The Cambrian Guarda Velha Formation, southern Brazil

Soft-sediment deformation (SSD) is widely described in the literature, but there is no clear consensus regarding its origin and significance. Existing models for SSD in fluvial sediments do not clearly demonstrate a relationship between the structures, preserved facies expression, and larger-scale depositional architecture. In this study several types of SSD structures are recorded in Cambrian fluvial deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval. The random distribution of these features in relation to primary facies types and fluvial forms indicates that they have neither a direct nor indirect relationship with any depositional processes or bedform type. We propose that the relationship of SSD at the bed-set-scale to larger-scale depositional architecture, combined with tectono-stratigraphic analysis allows the determination of both short-term fluvial hydraulic conditions in ancient stream systems, such as the nature of the flow regime responsible for depositing ancient fluvial stream successions, and the long-term subsidence rates, in the form of mean recurrence interval of the seismic events responsible for triggering the generation of SSD in tectonically active basins.

Integrating geomorphological mapping, trenching, InSAR and GPR for the identification and characterization of sinkholes: A review and application in the mantled evaporite karst of the Ebro Valley (NE Spain)

This contribution illustrates the advantages of integrating conventional geomorphological methods with InSAR, ground penetrating radar and trenching for sinkhole mapping and characterization in a mantled evaporite karst area, where a significant proportion of the karstic depressions have been obliterated by artificial fills. The main practical aim of the investigation was to elucidate whether buried sinkholes overlap the areas planned for the construction of buildings and services, in order to apply a preventive planning strategy. Old aerial photographs and detailed topographic maps were the most useful sources of information for the identification of sinkholes and helped to obtain information on their chronology, either a minimum age or bracketing dates. The InSAR technique provided subsidence rate values ranging from 4.4 to 17.3mm/yr consistent with the spatial distribution of the mapped sinkholes. This quantitative deformation data helped corroborating independently the existence of active buried sinkholes and improving the delineation of their limits. The GPR profiles contributed to the precise location of sinkhole edges, provided information on the geometry of buried sinkholes and deformation structures and helped to site trenches and to rule out the existence of sinkholes in particular areas. The main input derived from the trenches includes: (1) Confirming or ruling out anomalies of the GPR profiles attributable to subsidence. (2) Precise location of the edge of some filled sinkholes. (3) Information on subsidence mechanisms recorded by various deformation structures and cumulative subsidence magnitude. (4) Calculating minimum long-term subsidence rates using radiocarbon dates obtained from deformed sinkhole deposits. (5) Unequivocal evidence of active subsidence in areas assigned for the construction of buildings.

Emergence of Venice during the Pleistocene

The Pleistocene history of sea-level change for the Venice region was reconstructed using an integrated magneto-bio-cyclo-stratigraphy of lithofacies and a published palynofloral analysis of continuously cored sediments in a 950-meter-deep drill core. The basin in which the Venice region is located collapsed at ∼1.8 Ma with slow sediment accumulation in the deeper-water starved basin during most of the Matuyama polarity chron but shoaled rapidly in the early and middle Brunhes in response to a major phase of deltaic progradation. The initial transition to continental sediments occurred during a prominent glacioeustatic low-stand that is likely to be MIS 12 (∼0.43 Ma) but could be as young as MIS 8 (∼0.25 Ma). The Venice area oscillated from below sea level during subsequent major glacioeustatic high-stands to becoming increasingly emergent during major low-stands as the basin continued to fill with marine and continental sediments. Some parts of the Venice area are now emergent for the first time during a glacioeustatic high-stand (i.e., MIS 1 or the Holocene). The total long-term subsidence rate estimated from the VENICE-1 record is less than 0.5 mm/yr, considerably slower than estimates for the Holocene and especially the modern anthropogenic period.

Tectonic control of nested sequence architecture in the Sego Sandstone, Neslen Formation and Upper Castlegate Sandstone (Upper Cretaceous), Sevier Foreland Basin, Utah, USA

Abstract The Sego Sandstone, Neslen Formation and Upper Castlegate Sandstone are laterally equivalent formations of the Upper Cretaceous Mesaverde Group, which crop out along the Book Cliffs of eastern Utah and represent sediment shed into a foreland basin from the Sevier orogenic belt to the west. The studied interval contains three regional unconformities which bound stratigraphic sequences around 100 m thick and of one-to-three million year duration, here termed high-order Sequences I–III. The high-order sequence boundaries display increasing depth of erosion northwestward toward the Sevier orogenic belt and mark changes in sediment dispersal patterns and provenance, none of which can be explained by eustatic sea-level falls. Their formation is attributed to erosion and isostatic rebound of proximal parts of the foreland basin following thrust events in the adjacent Sevier orogenic belt. In the southeastern part of the study area, high-order Sequence II (∼Sego Sandstone) contains several (>4) nested stratigraphic sequences around 20 m thick and of hundred thousand year duration, here termed low-order sequences. The low-order sequence boundaries are disconformities rather than angular unconformities and are not marked by changes in sediment dispersal patterns or provenance. Each of these low-order sequences consists of: (a) LST of incised valleys (up to 14 m thick) filled with estuarine sandstone, and (b) TST of marine shale and sandstone. No HSTs are preserved. The origin of these sequences (eustatic/tectonic) cannot be determined. When traced to the northwest, the low-order sequence boundaries become conformable and unrecognizable, and no low-order stratigraphic sequences corresponding to those in the Sego Sandstone can be recognized within the alluvial Upper Castlegate Sandstone. The observed nested sequence architecture of the Upper Mesaverde Group is considered to be controlled by variation in subsidence rate related to the emplacement and erosion of thrust sheets in the adjacent Sevier orogenic belt. During emplacement of thrust sheets, subsidence rates in proximal areas of the basin were sufficiently high to offset any low-order base-level falls, resulting in the deposition of a conformable fluvial succession. In distal parts of the basin where long-term subsidence rates were lower, low-order base-level falls produced relative falls in sea level and generated spatially restricted low-order sequences.

Tectonic Control of Nested Sequence Architecture in the Castlegate Sandstone (Upper Cretaceous), Book Cliffs, Utah

ABSTRACT The Castlegate Sandstone at its type section, Price Canyon, near Price, Utah, encompasses a single stratigraphic sequence spanning approximately 5 m.y. It includes a sandstone member corresponding to a lowstand systems tract, consisting of braided-fluvial sheet sandstones, and a mudstone member, in which shales are more abundant and some evidence of tidal influence is present. This is the transgressive to highstand systems tract. From near Trail Canyon eastward the mudstone member passes laterally into the Sego Sandstone and Neslen Formation, a succession of at least six higher-frequency sequences of fluvial-estuarine origin. The Buck Tongue, a marine shale unit separating the Castlegate Sandstone and the Sego Sandstone east of Green River, is erosionally truncated below the Sego Sand tone northwest of Trail Canyon. We suggest that the origin of the sequences is related to flexural loading and intraplate stress on two time scales. Eustasy cannot be ruled out, but there is no independent evidence for this process. The main 5 m.y. sequence reflects regional tectonism, with the sandstone member developing at a time of slow subsidence, and the mudstone member reflecting a higher long-term subsidence rate. The higher-order sequences nested within the third-order sequence east of Trail Canyon are interpreted as a basinal response to episodes of crustal shortening on a 105 yr time scale. This study amplifies the model of Posamentier and Allen (1993a), in which ramp-type foreland basins are divided into areas of rapid and slow subsidence (Zones A and B). We postulate that these zones migrated asinward and landward in response to variations in long-term subsidence rate (an effect not predicted in the original model), and can be mapped by reference to the distribution of Type 1 sequence boundaries in the higher-order sequences. Differences in sequence architecture east and west of Trail Canyon may have been amplified by differences in crustal rheology. The sequence architecture changes at the boundary of the underlying Paleozoic Paradox Basin, a zone of NW-SE-oriented folds, faults, and salt diapirs, which we suspect were reactivated by Cretaceous tectonism. The high-frequency sequences are within the area of the Paradox Basin, an area that may have been more prone to vertical structural movements in response to intraplate stresses. Incipient uplift of Laramide structures may also have modified fluvial patterns and controlled the orientation of incised valleys on several of the sequence boundaries.

Evaluating major controls on basinal stratigraphy, Pine Valley, Nevada: Implications for syntectonic deposition

The interpretation of prograded gravel as an indicator of tectonic uplift, that is, syntectonic deposition, is not necessarily correct. Increases in sediment supply induced by tectonic uplifting did not control the progradation of gravel in the Pine Valley basin, an actively subsiding half graben in northeastern Nevada. Rather, the distribution of lithofacies in the Pine Valley basin is primarily controlled by the uneven distribution of subsidence within the basin. Differences in subsidence along the axis of the basin indicate a strong inverse relationship between the long-term subsidence rate since late Miocene time and the distance of gravel transport out into the basin. Differential progradation of alluvial gravel into the basin from various drainages along the footwall is not related to uplift or lithology of the source area, size of the catchment area, or climate. In addition, the relatively fixed location of the topographic low within the basin through time, as indicated by the distribution of lacustrine deposits and the position of the modern axial drainage, suggests that a balance between basin subsidence and sediment supply to the basin has been generally maintained through time. Thus, if subsidence rates are a measure of tectonic activity, gravel progradation may record times of reduced, not increased, tectonism.

Recent subsidence and northeast tilting of the Nile delta, Egypt

Sediment borings collected in the Nile delta of Egypt, many of them radiocarbon dated, indicate that the entire northern sector of this major depositional center in the eastern Mediterranean is presently subsiding. Mapping of the base of the Holocene deltaic facies, which is dated at about 8000−6500 yrs B.P., reveals that differential lowering of the northern delta plain is preferentially accentuated toward the northeast. Long-term subsidence rates at or near the coast, averaged for the mid- to upper Holocene, range from about 0.1 to 0.25 cm/yr between Alexandria and the north-central delta margin. Rates increase markedly eastward to a maximum of about 0.5 cm/yr in the Port Said-Manzala lagoon region, and this rapid lowering explains the presence of thick marine delta lobe sequences of Holocene age in cores in the northeastern delta. In contrast, only reworked remnants of Holocene marine delta lobe deposits of the earlier River nile branches which once flowed to the north and northwest are preserved seaward of the present north delta coast. Subsidence has induced marked environmental changes, particularly with respect to coastal erosion and salt water incursion. The asymmetric pattern of saline ground water, here attributed in part to the northeast tilt of the delta plain, has serious implications for agricultural development in this intensely cultivated region.

Sedimentary and botanical factors influencing peat accumulation in the Mississippi Delta

The Holocene Mississippi Delta System consists of six cyclic deltaic packages. Associated peat deposits are planar and eutrophic, and accumulate in areas abandoned by deltaic sedimentation. Peats that formed in large-scale interlobe basins are the subject of this paper. True peats (>75% organic matter) average 81.7% organic matter on a dry-weight basis. Controls on quality and lateral distribution of peat deposits are: botanical parent material, the balance between subsidence and accretion rates, detrital clastic influx and marine inundation. Botanical parent material, inferred from 13 C signatures and microtome sections, is always freshwater, originating either in forested swamps, or under herbaceous (floating) mats. Sphagnum spp. is essentially absent. In order for peat to accumulate, rates of accretion and subsidence must balance. Long-term subsidence rates in the study area increase from about 0.10 to 0.30 cm a -1 over a distance of 40 km from upper to lower delta plain. Subsurface accretion rates for true peats at a depth of 2-3 m, however, average about 0.05 cm a -1 . Renewed coarse detrital clastic influx terminates peat accumulation. The largest quantities of true peat are found together with relatively large amounts of organic-poor clastic sediments (mostly clays). Delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments. Resulting saline marsh sediments have maximum organic matter contents of 35%. In a transgressive setting, these may overlie older high-quality (low-ash) freshwater peats. Leaching experiments reveal the presence of water-soluble salts in both transgressed and fresh peats. Leaching reduces ash values by 1/5 to 1/3, suggesting that such losses may occur during early diagenesis. It is possible that some commercial coal seams originated in analogous interlobe environments, where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels.

Incipiently drowned facies within a cyclic peritidal ramp sequence, Early Ordovician Chepultepec interval, Virginia Appalachians

The Chepultepec interval (145 to 260 m thick) in Virginia, although dominated by peritidal carbonates, contains thick subtidal sequences that formed during repeated, incipient drowning of the Early Ordovician ramp. Cyclic peritidal sequences make up the lower member (up to 150 m thick) and upper member (up to 85 m thick). Subtidal limestone and bioherms make up the middle member (up to 110 m thick) and pass landward into cyclic peritidal facies. Average long-term subsidence rates were up to 4 cm/1,000 yr (mature passive margin rates), and ramp gradients commonly were extremely low (2 cm/km). Computer modeling helped define mechanisms involved in formation of the stratigraphic sequence. The peritidal sequences are upward-shallowing cycles (average period 140,000 to 200,000 yr) of pellet and skeletal limestone, pellet grainstone, thrombolites, intraclast grainstone, and laminite caps. These resulted from periodic sea-level oscillations of several metres amplitude, and possible slow, long-teim (third-order) sea-level fall superimposed on long-term subsidence. Tidal flats were shifted far westward after each sea-level rise but had sufficient time to prograde back across the ramp during sea-level fall. The middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed-units (formed below fair-weather wave base), local rippled pellet limestone, and thrombolite bioherms (subwave-base to wave-agitated shoals) that may have erosional tops overlain by thin grainstone sheets. Tidal-flat facies are absent. These sequences formed by periodic sea-level rise (more than 12 to 17 m amplitude), which caused incipient drowning of the platform, and migration of tidal flats several hundred kilometres back onto the platform. These flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats. This left them abandoned on the inner ramp. Similar sequences reflecting incipient drowning are widespread in the geological record.

Hawaiian glacial ages

Four glacial drifts that are interstratified with lava flows and tephra layers on the upper slopes of Mauna Kea demonstrate that an ice cap formed repeatedly at the summit of the volcano during the middle and late Pleistocene. The oldest drift (Pohakuloa Formation) probably was deposited shortly after eruption of a lava flow having a K Ar age of 278,500 ± 68,500 yr. Drift of the Waihu Formation, marked by a belt of subdued end moraines, is correlated with hyaloclastite cones and associated lava flows that were erupted beneath an ice cap about 170,000–175,000 yr ago. One of four younger subglacially erupted lavas at the crest of the volcano has a K Ar age of 41,300 ± 8300 yr. Tephra layers that antedate the last glaciation are about 29,700 to 37,200 14C yr old and underlie dune sand that is believed to correlate with drift of the Makanaka Formation deposited during the last ice advance. The late Makanaka ice cap, which covered an area of about 70 km 2 and was as much as 100 m thick, is reconstructed from end moraines and limits of erratic stones that encircle the summit region. The ice cap disappeared from the summit before about 9080 yr ago. Postglacial lavas and tephra overlie the youngest drift on the upper south flank of the mountain and buried a widespread post-Makanaka soil on the lower south rift zone about 4500 14C yr ago. The island of Hawaii is subsiding isostatically due to crustal loading by Quaternary volcanic rocks, with subsidence near the midpoint of Mauna Kea estimated as about 2.5 ± 0.5 mm/yr. A curve depicting an inferred long-term subsidence rate has been used to adjust equilibrium-line altitudes (ELAs) of former ice caps that are calculated on the basis of reconstructed glacier topography and an assumed accumulation-area ratio of 0.6 ± 0.05. The results indicate that ELA depression was greatest during Waihu glaciation, least during Pohakuloa glaciation, and that the ELA during late Makanaka glaciation was somewhat lower than that of the early Makanaka advance. Available radiometric dates show that late Makanaka glaciation correlates with stage 2 of the marine oxygen-isotope record, and suggest that early Makanaka, Waihu, and Pohakuloa glaciations correlate, respectively, with isotope stages 4, 6, and 8. Because ice caps could have formed on Mauna Kea only after the snowline was lowered many hundreds of meters below its inferred present level, episodes of Hawaiian glaciation probably were restricted to times of maximum ice volume on the continents. The asymmetry of the late Makanaka ice cap and the southeast-descending gradient of its equilibrium line are consistent with a southeast (tradewinds) source of precipitation during the last glaciation. Although departures of glacial-age temperature and precipitation from present values are difficult to assess quantitatively, growth of former ice caps on Mauna Kea most likely was due to enhanced winter snowfall and to reduced ablation rates brought about by lower air temperature and increased cloudiness.

Subsidence of Cultivated Organic Soil in Ontario

The subsidence of a cultivated organic soil has been monitored for 30 yr. The soil has a long-term subsidence rate that is sharply increased with artificial improvement of the drainage. The long-term rate is reestablished after 10 yr. Positive and negative changes in surface elevation occur throughout the year due to changes in soil water and the freezing and thawing of the soil. The continual fluctuation of the soil surface makes it difficult to determine any particular time during the year when subsidence may occur.