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In many near-surface geophysical studies it is now common practice to collect co-located disparate geophysical data sets to explore subsurface structures. Reconstruction of physical parameter distributions underlying the available geophysical data sets usually requires the use of tomographic reconstruction techniques. To improve the quality of the obtained models, the information content of all data sets should be considered during the model generation process, e.g., by employing joint or cooperative inversion approaches. Here, we extend the zonal cooperative inversion methodology based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of data sets with partially co-located model areas. This is done by considering recent developments in fuzzy c-means cluster analysis. Additionally, we show how supplementary a priori information can be incorporated in an automated fashion into the zonal cooperative inversion approach to further constrain the inversion. The only requirement is that this a priori information can be expressed numerically; e.g., by physical parameters or indicator variables. We demonstrate the applicability of the modified zonal cooperative inversion approach using synthetic and field data examples. In these examples, we cooperatively invert S- and P-wave traveltime data sets with partially co-located model areas using water saturation information expressed by indicator variables as additional a priori information. The approach results in a zoned multi-parameter model, which is consistent with all available information given to the zonal cooperative inversion and outlines the major subsurface units. In our field example, we further compare the obtained zonal model to sparsely available borehole and direct-push logs. This comparison provides further confidence in our zonal cooperative inversion model because the borehole and direct-push logs indicate a similar zonation.
Projected scenarios of climate change involve general predictions about the likely changes to the magnitude and frequency of landslides, particularly as a consequence of altered precipitation and temperature regimes. Whether such landslide response to contemporary or past climate change may be captured in differing scaling statistics of landslide size distributions and the erosion rates derived thereof remains debated. We test this notion with simple Monte Carlo and bootstrap simulations of statistical models commonly used to characterize empirical landslide size distributions. Our results show that significant changes to total volumes contained in such inventories may be masked by statistically indistinguishable scaling parameters, critically depending on, among others, the size of the largest of landslides recorded. Conversely, comparable model parameter values may obscure significant, i.e. more than twofold, changes to landslide occurrence, and thus inferred rates of hillslope denudation and sediment delivery to drainage networks. A time series of some of Earth's largest mass movements reveals clustering near and partly before the last glacial-interglacial transition and a distinct step-over from white noise to temporal clustering around this period. However, elucidating whether this is a distinct signal of first-order climate-change impact on slope stability or simply coincides with a transition from short-term statistical noise to long-term steady-state conditions remains an important research challenge.
The aims of this study were to identify areas of wind erosion and dust deposition and to quantify the effects of different grazing intensities on soil redistribution rates in grasslands based on the Cs-137 technique. Because the method uses a reference inventory as threshold for erosion or deposition, the classification of any other site as source or sink for dust depends on the accurate selection of this reference site.
Measurements of Cs-137 inventories and depth distributions were carried out at pasture sites with predominant species of Stipa grandis and Leymus chinensis which are grazed with different intensities. Additional measurements were made at arable land, plant-covered sand dunes and alluvial plains. Wind-induced soil erosion and dust deposition rates were calculated from Cs-137 inventories by means of the "Profile-Distribution" and the "Mass Balance II" models.
The selection of the reference site was based on fluid dynamical and process-determining parameters. The chosen site should meet the following four conditions: (i) located at a summit position with obviously low deposition rates, (ii) sufficient vegetation cover to prevent wind erosion, (iii) plane to exclude water erosion and (iv) in the wind/dust shadow of a higher elevation. The measured reference inventory of Cs-137 was 1967(+/- 102) Bqm(-2) located at a summit position of moderately grazed Leymus chinensis steppe. The Cs-137 inventories at other sites ranged from 1330 Bqm(-2) at heavily grazed sites to 5119 Bqm(-2) at river deposits, representing annual average soil losses of up to 130 tkm(-2) and deposits of up to 540 tkm(-2), respectively. The calculated annual averages of dust depositions at ungrazed Leymus chinensis sites were related to the dust storm frequencies of the last 50 years resulting in a description of the temporal variability of annual dust depositions from about 154 tkm(-2) in the 1960s to 26 tkm(-2) at recent times. Based on this quantification already 80% of the total dust depositions can be related to the 20 years between the 1960s and the end of the 1970s and only 20% to the time between 1980 and 2001.
Cs-137 technique is a promising method to assess the effect of grazing intensity and land use types on the spatial variability of wind-induced soil and dust redistribution processes in semi-arid grasslands. However, considerable efforts are needed to identify a reliable reference site, because erosion and deposition induced by wind may occur at the same places. The combination of the dust deposition rates derived from Cs-137 profile data with the dust storm frequencies is helpful for a better reconstruction of the temporal variability of dust deposition and wind erosion in this region. The calculated recent deposition rates of about 20 tkm(-2) are in good agreement with data of other authors.
Empirical species distribution models (SDMs) constitute often the tool of choice for the assessment of rapid climate change effects on species vulnerability. Conclusions regarding extinction risks might be misleading, however, because SDMs do not explicitly incorporate dispersal or other demographic processes. Here, we supplement SDMs with a dynamic population model 1) to predict climate-induced range dynamics for black grouse in Switzerland, 2) to compare direct and indirect measures of extinction risks, and 3) to quantify uncertainty in predictions as well as the sources of that uncertainty. To this end, we linked models of habitat suitability to a spatially explicit, individual-based model. In an extensive sensitivity analysis, we quantified uncertainty in various model outputs introduced by different SDM algorithms, by different climate scenarios and by demographic model parameters. Potentially suitable habitats were predicted to shift uphill and eastwards. By the end of the 21st century, abrupt habitat losses were predicted in the western Prealps for some climate scenarios. In contrast, population size and occupied area were primarily controlled by currently negative population growth and gradually declined from the beginning of the century across all climate scenarios and SDM algorithms. However, predictions of population dynamic features were highly variable across simulations. Results indicate that inferring extinction probabilities simply from the quantity of suitable habitat may underestimate extinction risks because this may ignore important interactions between life history traits and available habitat. Also, in dynamic range predictions uncertainty in SDM algorithms and climate scenarios can become secondary to uncertainty in dynamic model components. Our study emphasises the need for principal evaluation tools like sensitivity analysis in order to assess uncertainty and robustness in dynamic range predictions. A more direct benefit of such robustness analysis is an improved mechanistic understanding of dynamic species responses to climate change.
Triassic Latemar cycle tops - Subaerial exposure of platform carbonates under tropical arid climate
(2012)
The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for subaerial exposure under strongly arid climates. As the subaerial exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
Earthquake-triggered landslide dams are potentially dangerous disrupters of water and sediment flux in mountain rivers, and capable of releasing catastrophic outburst flows to downstream areas. We analyze an inventory of 828 landslide dams in the Longmen Shan mountains, China, triggered by the M-w 7.9 2008 Wenchuan earthquake. This database is unique in that it is the largest of its kind attributable to a single regional-scale triggering event: 501 of the spatially clustered landslides fully blocked rivers, while the remainder only partially obstructed or diverted channels in steep watersheds of the hanging wall of the Yingxiu-Beichuan Fault Zone. The size distributions of the earthquake-triggered landslides, landslide dams, and associated lakes (a) can be modeled by an inverse gamma distribution; (b) show that moderate-size slope failures caused the majority of blockages; and (c) allow a detailed assessment of seismically induced river-blockage effects on regional water and sediment storage. Monte Carlo simulations based on volumetric scaling relationships for soil and bedrock failures respectively indicate that 14% (18%) of the estimated total coseismic landslide volume of 6.4 (14.6) x 10(9) m(3) was contained in landslide dams, representing only 1.4% of the >60,000 slope failures attributed to the earthquake. These dams have created storage capacity of similar to 0.6x 10(9) m(3) for incoming water and sediment. About 25% of the dams containing 2% of the total river-blocking debris volume failed one week after the earthquake; these figures had risen to 60% (similar to 20%), and >90% (>90%) within one month, and one:year, respectively, thus also emptying similar to 92% of the total potential water and sediment storage behind these, dams within one year following the earthquake. Currently only similar to 0.08 x 10(9) m(3) remain available as natural reservoirs for storing water and sediment, while similar to 0.19 x 10(9) m(3), i.e. about a third of the total river-blocking debris volume, has been eroded by rivers. Dam volume and upstream catchment area control to first order the longevity of the barriers, and bivariate domain plots are consistent with the observation that most earthquake-triggered landslide dams were ephemeral. We conclude that the river-blocking portion of coseismic slope failures disproportionately modulates the post-seismic sediment flux in the Longmen Shan on annual to decadal timescales.
The Maule earthquake of 27th February 2010 (M-w = 8.8) affected similar to 500 km of the Nazca-South America plate boundary in south-central Chile producing spectacular crustal deformation. Here, we present a detailed estimate of static coseismic surface offsets as measured by survey and continuous GPS, both in near- and far-field regions. Earthquake slip along the megathrust has been inferred from a Joint inversion of our new data together with published GPS, InSAR, and land-level changes data using Green's functions generated by a spherical finite-element model with realistic subduction zone geometry. The combination of the data sets provided a good resolution, indicating that most of the slip was well resolved. Coseismic slip was concentrated north of the epicenter with up to 16 m of slip, whereas to the south it reached over 10 m within two minor patches. A comparison of coseismic slip with the slip deficit accumulated since the last great earthquake in 1835 suggests that the 2010 event closed a mature seismic gap. Slip deficit distribution shows an apparent local overshoot that highlight cycle-to-cycle variability, which has to be taken into account when anticipating future events from interseismic observations. Rupture propagation was obviously not affected by bathymetric features of the incoming plate. Instead, splay faults in the upper plate seem to have limited rupture propagation in the updip and along-strike directions. Additionally, we found that along-strike gradients in slip are spatially correlated with geometrical inflections of the megathrust. Our study suggests that persistent tectonic features may control strain accumulation and release along subduction megathrusts.
The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Ground-motion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set of six experts was asked to weight pre-selected GMPEs while the ability of these GMPEs to predict available data was evaluated with the method of Scherbaum et al. (Bull Seismol Soc Am 99:3234-3247, 2009). Results of both approaches were taken into account to commonly select the smallest set of GMPEs to capture the uncertainty in ground-motion prediction in Europe. For stable continental regions, two models, both from eastern North America, have been selected for shields, and three GMPEs from active shallow crustal regions have been added for continental crust. For subduction zones, four models, all non-European, have been chosen. Finally, for active shallow crustal regions, we selected four models, each of them from a different host region but only two of them were kept for long periods. In most cases, a common agreement has been also reached for the weights. In case of divergence, a sensitivity analysis of the weights on the seismic hazard has been conducted, showing that once the GMPEs have been selected, the associated set of weights has a smaller influence on the hazard.
Ocean Drilling Program Site 1085 provides a continuous marine sediment record off southern South West Africa for at least the last three and half million years. The n-alkane partial derivative(13) C record from this site records changes in past vegetation and provides an indication of the moisture availability of SW Africa during this time period. Very little variation, and no apparent trend, is observed in the n-alkane delta C-13 record, suggesting stable long-term conditions despite significant changes in East African tectonics and global climate. Slightly higher n-alkane delta C-13 values occur between 3.5 and 2.7 Ma suggesting slightly drier conditions than today. Between 2.5 and 2.7 Ma there is a shift to more negative n-alkane delta C-13 values suggesting slightly wetter conditions during a similar to 0.2 Ma episode that coincides with the intensification of Northern Hemisphere Glaciation (iNHG). From 2.5 to 0.4 Ma the n-alkane delta C-13 values are very consistent, varying by less than +/- 0.5 parts per thousand and suggesting little or no long-term change in the moisture availability of South West Africa over the last 2.5 million years. This is in contrast to the long-term drying trend observed further north offshore from the Namib Desert and in East Africa. A comparison of the climate history of these regions suggests that Southern Africa may have been an area of long-term stability over the last 3.5 Myrs.
Thermomechanical model reconciles contradictory geophysical observations at the Dead Sea Basin
(2012)
The Dead Sea Transform (DST) comprises a boundary between the African and Arabian plates. During the last 15-20 m.y. more than 100 km of left lateral transform displacement has been accumulated on the DST and about 10 km thick Dead Sea Basin (DSB) was formed in the central part of the DST. Widespread igneous activity since some 20 Ma ago and especially in the last 5 m.y., thin (60-80 km) lithosphere constrained by seismic data and absence of seismicity below the Moho, seem to be quite natural for this tectonically active plate boundary. However, surface heat flow values of less than 50-60 mW/m(2) and deep seismicity in the lower crust (deeper than 20 km) reported for this region are apparently inconsistent with the tectonic settings specific for an active continental plate boundary and with the crustal structure of the DSB. To address these inconsistencies which comprise what we call the "DST heat-flow paradox," we have developed a numerical model that assumes an erosion of initially thick and cold lithosphere just before or during the active faulting at the DST. The optimal initial conditions for the model are defined using transient thermal analysis. From the results of our numerical experiments we conclude that the entire set of observations for the DSB can be explained within the classical pull-apart model assuming that the lithosphere has been thermally eroded at about 20 Ma and the uppermost mantle in the region have relatively weak rheology consistent with experimental data for wet olivine or pyroxenite.
The timing of Tibetan plateau development remains elusive, despite its importance for evaluating models of continental lithosphere deformation and associated changes in surface elevation and climate. We present new thermochronologic data [biotite and K-feldspar Ar-40/Ar-39, apatite fission track, and apatite (U-Th)/He] from the central Tibetan plateau (Lhasa and Qiangtang terranes). The data indicate that over large regions, rocks underwent rapid to moderate cooling and exhumation during Cretaceous to Eocene time. This was coeval with >50% upper crustal shortening, suggesting substantial crustal thickening and surface elevation gain. Thermal modeling of combined thermochronometers requires exhumation of most samples to depths of <3 km between 85 and 45 Ma, followed by a decrease in erosional exhumation rate to low values of <0.05 mm/yr. The thermochronological results, when interpreted in the context of the deformation and paleoaltimetric history, are best explained by a scenario of plateau growth that began locally in central Tibet during the Late Cretaceous and expanded to encompass most of central Tibet by 45 Ma.
As a part of the DEad Sea Integrated REsearch (DESIRE) project a near-vertical incidence reflection (NVR) experiment with a profile length of 122 km was completed in spring 2006. The profile crossed the southern Dead Sea basin (DSB), a pull-apart basin due to the strike-slip motion along the Dead Sea Transform (DST). The DST with a total displacement of 107 km since about 18 Ma is part of a left-lateral fault system which connects the spreading centre in the Red Sea with the Taurus collision zone in Turkey over a distance of about 1100 km. The seismic experiment comprises 972 source locations and 1045 receiver locations. Each source was recorded by similar to 180 active receivers and a field data set with 175 000 traces was created. From this data set, 124 444 P-wave first-break traveltimes have been picked. With these traveltimes a tomographic inversion was carried out, resulting in a 2-D P-wave velocity model with a rms error of 20.9 ms. This model is dominated by a low-velocity region associated with the DSB. Within the DSB, the model shows clearly the position of the Lisan salt diapir, identified by a high-velocity zone. A further feature is an unexpected laterally low-velocity zone with P-velocities of 3 km s1 embedded in regions with 4 km s1 in the shallow part on the west side of the DSB. Another observation is an anticlinal structure west of the DSB interpretated to the related Syrian arc fold belt.
The paper presents the sediment budget of the Isabena basin, a highly dynamic 445-km(2) catchment located in the Central Pyrenees that is patched by highly erodible areas (i.e., badlands). The budget for the period 2007-2009 is constructed following a methodology that allows the interpolation of intermittent measurements of suspended sediment concentrations and enables a subsequent calculation of sediment loads. Data allow specification of the contribution of each subbasin to the water and sediment yield in the catchment outlet. Mean annual sediment load was 235,000 t y(-1). Specific sediment yield reached 2000 t km(-2) y(-1), a value that indicates very high sedimentary activity, especially in the case of Villacarli and Lascuarre subcatchments, were most badlands are located. The specific sediment yield obtained for the entire Isabena is 527 t km(-2) y(-1), a high value for such a mesoscale basin. Results show that a small part of the area (i.e., 1%) controls most of the catchment's gross sediment contribution. Sediment delivery ratio (ratio between sediment input from primary sources and basin export) has been estimated at around 90%, while in-channel storage represents the 5% of the annual load on average. The high connectivity between sediment sources (i.e., badlands) and transfer paths (i.e., streamcourses) exacerbates the influence of the local sediment production on the catchment's sediment yield, a quite unusual fact for a basin of this scale.
The Paleocene-Eocene thermal maximum represents one of the most rapid and extreme warming events in the Cenozoic. Shallow-water stratigraphic sections from the Adriatic carbonate platform offer a rare opportunity to learn about the nature of Paleocene-Eocene thermal maximum and the effects on shallow-water ecosystems. We use carbon and oxygen isotope stratigraphy, in conjunction with detailed larger benthic foraminiferal biostratigraphy, to establish a high-resolution paleoclimatic record for the Paleocene-Eocene thermal maximum. A prominent negative excursion in delta C-13 curves of bulk-rock (similar to 1 parts per thousand-3 parts per thousand), matrix (similar to 4 parts per thousand), and foraminifera (similar to 6 parts per thousand) is interpreted as the carbon isotope excursion during the Paleocene-Eocene thermal maximum. The strongly C-13-depleted delta(1)d(3)C record of our shallow-marine carbonates compared to open-marine records could result from organic matter oxidation, suggesting intensified weathering, runoff, and organic matter flux.
The Ilerdian larger benthie foraminiferal turnover is documented in detail based on high-resolution correlation with the carbon isotopic excursion. The turnover is described as a two-step process, with the first step (early Ilerdian) marked by a rapid diversification of small alveolinids and nummulitids with weak adult dimorphism, possibly as adaptations to fluctuating Paleocene-Eocene thermal maximum nutrient levels, and a second step (middle Ilerdian) characterized by a further specific diversification, increase of shell size, and well-developed adult dimorphism. Within an evolutionary scheme controlled by long-term biological processes, we argue that high seawater temperatures could have stimulated the early Ilerdian rapid specific diversification. Together, these data help elucidate the effects of global warming and associated feedbacks in shallow-water ecosystems, and by inference, could serve as an assessment analog for future changes.
In this study, we analyzed the influence of soil mineral characteristics (e. g., clay concentration and mineralogical composition, iron and aluminum oxide concentration and crystallinity, specific surface area, and exchangeable cation concentration) on (i) organic carbon (OC) content (kg m(-2)) and (ii) the concentration (g kg(-1)), composition, and stability of the mineral-associated organic matter (OM) of arable and forest topsoils. We selected seven soil types with different mineral characteristics for this study. For each soil type, samples were taken from topsoils of a deciduous forest and an adjacent arable site. The arable and forest sites have been used continuously for more than 100 years. Na-pyrophosphate soluble OM fractions (OM(PY)), representing mineral-associated OM, were extracted, analyzed for OC and C-14 concentrations, and characterized by FTIR spectroscopy. For the forest and arable topsoils, a linear relationship was found between the OC content and exchangeable Ca. For the arable topsoils (pH 6.7-7.5), correlation analyses indicated that the OCPY concentration increased with an increase in oxalate soluble Fe and Al, exchangeable Ca, and Na-pyrophosphate soluble Mg and Fe concentrations. The stability of OM(PY) determined by the C-14 measurements of the near-neutral arable topsoils was shown to increase with the specific surface area and the concentration of exchangeable Ca. For the acidic forest topsoils (pH < 5), the stability of OM(PY) was found to increase as the pH, and the concentration of C=O groups and Na-pyrophosphate soluble Mg increase.
The Garzn Complex of the Garzn Massif in SW Colombia is composed of the Vergel Granulite Unit (VG) and the Las Margaritas Migmatite Unit (LMM). Previous studies reveal peak temperature conditions for the VG of about 740 A degrees C. The present study considers the remarkable exsolution phenomena in feldspars and pyroxenes and titanium-in-quartz thermometry. Recalculated ternary feldspar compositions indicate temperatures around 900-1,000 A degrees C just at or above the ultra-high temperature-metamorphism (UHTM) boundary of granulites. The calculated temperatures range of exsolved ortho- and clinopyroxenes also supports the existence of an UHTM event. In addition, titanium-in-quartz thermometry points towards ultra-high temperatures. It is the first known UHTM crustal segment in the northern part of South America. Although a mean geothermal gradient of ca 38 A degrees C km(-1) could imply additional heat supply in the lower crust controlling this extreme of peak metamorphism, an alternative model is suggested. The formation of the Vergel Granulite Unit is supposed to be formed in a continental back-arc environment with a thinned and weakened crust behind a magmatic arc (Guapotn-Mancagua Gneiss) followed by collision. In contrast, rocks of the adjacent Las Margaritas Migmatite Unit display "normal" granulite facies temperatures and are formed in a colder lower crust outside the arc, preserved by the Guapotn-Mancagu Gneiss. Back-arc formation was followed by inversion and thickening of the basin. The three units that form the modern-day Garzn Massif, were juxtaposed upon each other during collision (at ca. 1,000 Ma) and exhumation. The collision leading to the deformation of the studied area is part of the Grenville orogeny leading to the amalgamation of Rodinia.
Today, diverse communities of zooxanthellate corals thrive, but do not build reef, under a wide range of environmental conditions. In these settings they inhabit natural bottom communities, sometimes forming patch-reefs, coral carpets and knobs. Episodes in the fossil record, characterized by limited coral-reef development but widespread occurrence of coral-bearing carbonates, may represent the fossil analogs of these non-reef building, zooxanthellate coral communities. If so, the study of these corals could have valuable implications for paleoenvironmental reconstructions. Here we focus on the evolution of early Paleogene corals as a fossil example of coral communities mainly composed by zooxanthellate corals (or likely zooxanthellate), commonly occurring within carbonate biofacies and with relatively high diversity but with a limited bioconstructional potential as testified by the reduced record of coral reefs. We correlate changes of bioconstructional potential and community compositions of these fossil corals with the main ecological/environmental conditions at that time. The early Paleogene greenhouse climate was characterized by relatively short pulses of warming with the most prominent occurring at the Paleocene-Eocene boundary (PETM event), associated with high weathering rates, nutrient fluxes, and pCO(2) levels. A synthesis of coral occurrences integrated with our data from the Adriatic Carbonate Platform (SW Slovenia) and the Minervois region (SW France), provides evidence for temporal changes in the reef-building capacity of corals associated with a shift in community composition toward forms adapted to tolerate deteriorating sea-water conditions. During the middle Paleocene coral-algal patch reefs and barrier reefs occurred from shallow-water settings, locally with reef-crest structures. A first shift can be traced from middle Paleocene to late Paleocene, with small coral-algal patch reefs and coral-bearing mounds development in shallow to intermediate water depths. In these mounds corals were highly subordinated as bioconstructors to other groups tolerant to higher levels of trophic resources (calcareous red algae, encrusting foraminifera, microbes, and sponges). A second shift occurred at the onset of the early Eocene with a further reduction of coral framework-building capacity. These coral communities mainly formed knobs in shallow-water, turbid settings associated with abundant foraminiferal deposits. We suggest that environmental conditions other than high temperature determined a combination of interrelated stressors that limited the coral-reef construction. A continuous enhancement of sediment load/nutrients combined with geochemical changes of ocean waters likely displaced corals as the main bioconstructors during the late Paleocene-early Eocene times. Nonetheless, these conditions did not affect the capacity of some corals to colonize the substrate, maintain biodiversity, and act as locally important carbonate-sediment producers, suggesting broad environmental tolerance limits of various species of corals. The implications of this study include clues as to how both ancient and modern zooxanthellate corals could respond to changing climate.