Mean Annual Precipitation Estimates Research Articles

New palaeoclimatic constraints from paleosols on the Middle-Late Jurassic landscape, Western Colorado, U.S.A

ABSTRACT The Wanakah Formation and Tidwell Member of the Morrison Formation record intervals of paedogenesis in the Paradox Basin and Central Colorado trough, western Colorado during the Middle-Late Jurassic. Detailed field description of paleosols and paedogenic carbonates at different stratigraphic horizons from three localities document four main pedotypes: vertisols, gleysols, oxisols, and protosols. Generally, gleysols reflect reducing conditions, protosols and oxisols reflect oxidizing conditions, and vertisols reflect fluctuations in oxidizing/reducing conditions (seasonality). Major elemental geochemical ratios in samples from these paleosols suggest variable redox conditions and a sub-humid to humid paleoclimate with seasonal precipitations during paleosol development. Estimated mean annual temperature based on elemental analyses of paleosol B in paleosols from Ribbon Trail and Escalante Canyon in western Colorado suggest mesic – thermic paleoclimate. Mean annual precipitation indicates sub-humid to humid regional palaeoclimatic conditions marked by seasonal precipitation. Clay mineralogy of these paleosols, determined by X-ray diffraction, shows the abundance of illite most likely formed from smectite alteration due to episodic wetting and drying. δ13C and δ18O isotope analyses of carbonate veins in these paleosols suggest that they formed recently from meteoric water and therefore are not considered for palaeoclimatic reconstructions. Mean annual temperature and mean annual precipitation estimates in the Wanakah Formation and Tidwell Member of the Morrison Formation provide new information on the local and regional palaeoclimatic and paleoenvironmental conditions during the Middle Jurassic in western Colorado as well as data for comparison with the Upper Morrison paleoclimate from northern and southern localities in the western United States.

Assessment of pretreatment methods on CIA-K and CALMAG indices and the effects on paleoprecipitation estimates

The chemical compositions of paleosols are frequently used to reconstruct paleoclimatic and paleoenvironmental conditions using proxy relationships derived from modern soils. However, through time, some of the proxy relationships have been applied to paleosol types that were not part of the original training data sets, or they have been analyzed following pretreatment steps that were not employed by the original studies without critical evaluation of whether those methodological changes have impacted the interpretation of the data. Here we test pretreatment methods critically using chemical compositions of thirty-two modern soils from California and Texas to evaluate their impact on two of the most commonly employed climofunction indices to estimate mean annual precipitation, CIA-K and CALMAG. Chemical data are generated from paired soils which have underdone two different pretreatment styles, including using weak glacial acetic acid to digest calcite and using no pretreament methods to keep all calcium in the soil, including from pedogenic carbonate. There is little change in major element oxide values from untreated to acid-treated aliquots, except for calcium oxide depletion after pretreatment. In all soils with >2% CaO in the matrix, calcium oxide is significantly depleted by the acid-digestion pretreatment. This acid-digestion pretreatment induces a substantial increase in estimates of mean annual precipitation, which are not in accordance with published meteorological values. The extent to which precipitation estimates are skewed toward higher values as a function of CaO loss is parametrically correlated with calcium carbonate content of the soil material. This has important implications for the appropriate application of CIA-K and CALMAG indices as viable proxies of paleoprecipitation estimates from paleosol profiles that contain pedogenic and/or diagenetic carbonates and we suggest quantitative thresholds for CaO in untreated samples to minimize potential sources of error.

Spatio‐temporal evaluation of gridded precipitation products for the high‐altitude Indus basin

AbstractThe high‐altitude Indus basin is one of the most complex and inadequately explored mountain terrains in the World, where reliable observations of precipitation are highly lacking. Therefore, spatially distributed precipitation products developed at global/regional scale are often used in several scientific disciplines. However, large uncertainties in precipitation estimates of such precipitation data sets often lead to suboptimal outcomes. In this study, performance of 27 widely used gridded precipitation products belonging to three different categories of gauge‐based, reanalysis and merged products is evaluated with respect to high‐quality reference climatologies of mean monthly precipitation. Widely used statistical measures and quantitative analysis techniques are used to analyse the spatial patterns and quantitative distribution of mean monthly, seasonal and annual precipitation at sub‐regional scale. Mean annual precipitation estimates of the gridded data sets are cross validated with the corresponding adjusted streamflows using Turc‐Budyko non‐dimensional analysis. Results reveal poor to moderately good performance of the gridded data sets. Marked differences in spatiotemporal and quantitative distribution of precipitation are found among the data sets. All data sets are consistent in their patterns showing negative or dry bias in wet areas and positive or wet bias in dry areas, although considerable differences in the magnitudes of the biases are noticed at sub‐regional scale. None of the data sets is equally good for all sub‐regions due to very high spatiotemporal variability in their performance at sub‐regional scale. Gauge‐based and merged products performed better in dry regions and during monsoon season, while reanalysis products provided better estimates in wet areas and during winter months. GPCC V8, ERA5 and MSWEP2.2 are found better than their counter‐grouped data sets. Overall, ERA5 is found most acceptable for all sub‐regions, particularly at higher‐altitudes, in wet areas and during winter months.

Patagonian Aridification at the Onset of the Mid‐Miocene Climatic Optimum

AbstractFossil‐rich sediments of the Santa Cruz Formation, Patagonia, Argentina, span the initiation of the Miocene Climatic Optimum (MCO), the most recent period of warm and wet conditions in the Cenozoic. These conditions drove the expansion of tropical and subtropical ecosystems to much higher latitudes, with the fossiliferous Santa Cruz Formation recording one of the southernmost examples. We collected new carbon and oxygen isotope compositions of herbivore tooth enamel from fossils ~17.4 to 16.4 Ma in age to investigate ecological and climatic changes across the initiation of the MCO. Enamel δ13C values are consistent with a C3‐dominated ecosystem with moderate precipitation and a mix of wooded and more open areas. Serially sampled teeth reveal little zoning in δ13C and δ18O values, suggesting little seasonal variation in water and plant isotope compositions or seasonal changes in diet. Carbon isotope‐based estimates of mean annual precipitation (MAP) are consistent with aridification, with MAP decreasing from ~1,000 ± 235 mm/yr at 17.4 Ma to ~525 ± 105 mm/yr at the start of the climatic optimum (~16.9 Ma). This decrease corresponds to increasing global temperatures, as indicated by marine proxy records, and was followed by a rebound to ~840 ± 270 mm/yr by ~16.4 Ma. In comparison to a modern mean annual temperature (MAT) in the region of ~8°C, oxygen isotopes indicate high MAT (at least 20°C) at the onset of the MCO at 16.9 Ma and a significant increase in MAT to ~25°C by 16.4 Ma.

Alluvial record of an early Eocene hyperthermal within the Castissent Formation, the Pyrenees, Spain

Abstract. The late Palaeocene to the middle Eocene (57.5 to 46.5 Ma) recorded a total of 39 hyperthermals – periods of rapid global warming documented by prominent negative carbon isotope excursions (CIEs) as well as peaks in iron content – have been recognized in marine cores. Documenting how the Earth system responded to rapid climatic shifts during hyperthermals provides fundamental information to constrain climatic models. However, while hyperthermals have been well documented in the marine sedimentary record, only a few have been recognized and described in continental deposits, thereby limiting our ability to understand the effect and record of global warming on terrestrial systems. Hyperthermals in the continental record could be a powerful correlation tool to help connect marine and continental deposits, addressing issues of environmental signal propagation from land to sea. In this study, we generate new stable carbon isotope data (δ13C values) across the well-exposed and time-constrained fluvial sedimentary succession of the early Eocene Castissent Formation in the south central Pyrenees (Spain). The δ13C values of pedogenic carbonate reveal – similarly to the global records – stepped CIEs, culminating in a minimum δ13C value that we correlate with the hyperthermal event “U” at ca. 50 Ma. This general trend towards more negative values is most probably linked to higher primary productivity leading to an overall higher respiration of soil organic matter during these climatic events. The relative enrichment in immobile elements (Zr, Ti, Al) and higher estimates of mean annual precipitation together with the occurrence of small iron oxide and iron hydroxide nodules during the CIEs suggest intensification of chemical weathering and/or longer exposure of soils in a highly seasonal climate. The results show that even relatively small-scale hyperthermals compared with their prominent counterparts, such as PETM, ETM2, and ETM3, can leave a recognizable signature in the terrestrial stratigraphic record, providing insights into the dynamics of the carbon cycle in continental environments during these events.

Paleoclimate reconstruction of the Prince Creek Formation, Arctic Alaska, during Maastrichtian global warming

The Upper Cretaceous (Maastrichtian) Prince Creek Formation of Arctic Alaska (~82–85°N paleolatitude) preserves successions of alluvial paleosols that are used to estimate mean annual precipitation (MAP), mean annual temperature (MAT) and meteoric water oxygen isotope composition. For the Prince Creek Formation, the highest MAP estimates range between 1000 and 3900 mm/yr and the lowest range between 350 and 1200 mm/yr. The precipitation variability, derived from stable carbon isotope analysis, occurs on a time scale of 104 years. MAP values agree with previous interpretations from fossil pollen, pedological features, soil types, vegetation composition, sedimentological indicators and CLAMP estimates that suggest high precipitation amounts and high humidity. Despite the considerable uncertainty with MAT proxies, our values are consistent with warm month mean temperatures obtained previously from paleobotanical data. The δ18O of smectite from bentonite beds clusters around ~+5.0‰ (VSMOW). The δ18O of meteoric water calculated from bentonitic smectite is ~−23‰ (VSMOW), assuming a mean annual temperature of 6.3 °C, which is slightly more 18O-depleted than meteoric water calculated from pedogenic siderite in stratigraphically younger beds closer to the Brooks Range. Chronostratigraphic correlation with the lower Cantwell Formation, Denali National Park, Alaska (~64°N paleolatitude), suggests higher precipitation rates and highly 18O-depleted precipitation in the Late Cretaceous paleo-Arctic compared to central Alaska. These data are consistent with previous studies that suggest a warmer and moister greenhouse Earth and an intensified hydrological cycle that enhanced latent heat transport, resulting in increased rainout effects.

The presence of moisture deficits in Miocene New Zealand

Abstract New Zealand climate during the early to middle Miocene was subtropical to warm-temperate, in contrast to the predominance of cool-temperate climates in New Zealand today. Both modern and Miocene environmental settings of New Zealand are strongly moderated by the surrounding ocean. Seasonal moisture deficits occur today in rain shadow regions in New Zealand, but the potential and nature of seasonal moisture deficits under globally warmer conditions is uncertain. Here, we reconstruct seasonal moisture balance (precipitation – potential evapotranspiration) using seasonal temperature and mean annual precipitation estimates derived from early to middle Miocene (23–11 Ma) fossil leaf assemblages, combined with a Penman-Monteith model for potential evapotranspiration. The model incorporates uncertainty in temperature, precipitation, vapor pressure deficit, radiation and wind speed. Our results suggest that three out of nine sites investigated have a very high potential for summer moisture deficits (lowest monthly moisture balance

Evidence of early C4 grasses, habitat heterogeneity, and faunal response during the Miocene Climatic Optimum in the Mojave Region

Major changes to landscapes, climate, and mammalian faunas occurred at the regional scale in western North America during the Miocene Climatic Optimum (MCO) between ~17 and 14Ma, but few studies have looked at how the MCO affected basin-scale environments. Paleoenvironmental reconstructions coupled with mammalian fossil assemblages from the Crowder and Cajon Valley formations in the Mojave Desert, California, contribute insights into local-scale (10s of kilometers) responses to warming during the Hemingfordian and Barstovian North American Land Mammal Ages. By integrating lithological descriptions, phytolith analyses, carbon isotope composition of preserved soil organic matter (δ13CSOM), and elemental geochemistry of paleosols, we provide reconstructions of depositional environments, vegetation, and precipitation through the MCO. Phytolith and δ13CSOM evidence suggest that paleoenvironments were predominately C3 grasslands, with the earliest potential presence of C4 grasses in the region found within the Crowder Formation at ~17Ma. Based on elemental geochemistry of paleosols, mean annual precipitation estimates are 807 (±182) mm yr−1 at 17Ma in the Crowder Formation and 740–800 (±182) mm yr−1 from 16 to 15Ma in the Cajon Valley Formation. Our multiproxy approach indicates that the Crowder and Cajon Valley basins were stable, large-scale braided stream and floodplain systems with intermittent paleosol development and heterogeneity in vegetation and moisture conditions across spatial scales. Overall, we find a signal of local paleoenvironmental stability during the MCO; however, a significant drying trend in the Crowder Formation does correspond with faunal turnover and reduction in the number of taxa present. High species diversity within faunal assemblages, especially among small mammals, and fewer shared species between the Crowder and Cajon Valley assemblages than expected based on bootstrap analysis indicate that peak mammal diversity was accommodated at both local and regional scales during sustained tectonic activity and the MCO in western North America.

Paleosol and ichnofossil evidence for significant Neotropical habitat variation during the late middle Miocene (Serravallian)

We use paleopedology and ichnology to elucidate the habitat of the late middle Miocene fossil site of Quebrada Honda, southern Bolivia. The paleosols represent three pedotypes, Type 1 and Type 2 paleosols are interpreted as Inceptisols (Eutrudepts) and Entisols (Udifluvents), respectively, which formed on proximal and distal floodplains in a seasonal, sub-humid to humid wooded grassland-like vegetative community. Type 3 paleosols are interpreted as Inceptisols (Calciustepts) that formed in more densely vegetated wooded grassland-like vegetative communities on distal floodplains in a strongly seasonal sub-humid to semi-arid environment. The ichnofossil assemblage of Quebrada Honda includes Celliforma, Coprinisphaera, Taenidium, Fictovichnus, Planolites, Skolithos, Katarrhedrites, and root traces and represents heterogeneous communities dominated by soil arthropods and plants. The physical and geochemical properties of the paleosols, including low maturity, poor development of horizons, and the overall moderate estimates of mean annual precipitation, indicate changes in soil moisture due to seasonal precipitation and flooding and low but varying degrees of temporal stability. The diverse ichnofossil assemblage of Quebrada Honda reflects environments with greater primary productivity and temporal stability than those of nearby Cerdas, Bolivia, which are several million years older. Quebrada Honda's inferred paleoenvironments differ markedly from those of La Venta, Colombia, indicating that dissimilar habitats may partly or principally account for the vastly different faunas of these two well-sampled and contemporaneous fossil localities.

The palaeoenvironment of the middle Miocene pliopithecid locality in Damiao, Inner Mongolia, China

Damiao, Inner Mongolia, has three main fossil horizons representing the early, middle, and late Miocene. The middle Miocene locality DM01 is the only primate locality from the region and also represents the latest occurrence of pliopithecoids in northern China. The presence of pliopithecoid primates in central Asia after the middle Miocene climatic optimum seems to contradict the general trend of strengthening climatic zonality and increasing aridity. To investigate this enigma, we employ faunal similarity, ecometrics, and stable isotope analysis. Our results support previous inferences concerning the presence of locally humid environments within the increasingly arid surroundings that characterized central Asia. Hypsodonty, estimated mean annual precipitation (MAP), local sedimentology, and large mammal fossils suggest more humid and possibly more forested and wooded environments for the DM01 locality. We compared our results with the adjacent fossil-rich middle Miocene Tunggur localities. However, the small mammal fauna and isotope data are consistent with a mosaic of forest and grassland environment for all Damiao localities. Based on our results, Tunggur may have been too seasonal or not sufficiently humid for pliopithecids. This is supported by the higher mean hypsodonty and lower estimated MAP estimates, as well as slightly higher δ13C values. We suggest that DM01, the driest known Asian pliopithecid locality, may have been a more humid refugium within a generally drier regional context.

Magnetic minerals as recorders of weathering, diagenesis, and paleoclimate: A core–outcrop comparison of Paleocene–Eocene paleosols in the Bighorn Basin, WY, USA

Magnetic minerals in paleosols hold important clues to the environmental conditions in which the original soil formed. However, efforts to quantify parameters such as mean annual precipitation (MAP) using magnetic properties are still in their infancy. Here, we test the idea that diagenetic processes and surficial weathering affect the magnetic minerals preserved in paleosols, particularly in pre-Quaternary systems that have received far less attention compared to more recent soils and paleosols. We evaluate the magnetic properties of non-loessic paleosols across the Paleocene–Eocene Thermal Maximum (a short-term global warming episode that occurred at 55.5 Ma) in the Bighorn Basin, WY. We compare data from nine paleosol layers sampled from outcrop, each of which has been exposed to surficial weathering, to the equivalent paleosols sampled from drill core, all of which are preserved below a pervasive surficial weathering front and are presumed to be unweathered. Comparisons reveal an increase in magnetization in outcrops compared with core equivalents, which is principally driven by secondary hematite production. Authigenic hematite production in outcrops presents a complication for goethite–hematite based paleoprecipitation proxies where estimates will be biased toward drier climate regimes. The occurrence of low coercivity minerals is more consistent between core and outcrop. However, we propose an alteration process for pedogenic magnetite that is observed in both core and outcrop, where pedogenic magnetite becomes progressively oxidized leading to higher mean coercivities and broader coercivity distributions compared to modern pedogenic magnetite. This combination of diagenetic processes and surface weathering influences the magnetic properties of paleosols. Despite these changes, magnetic enhancement ratios from B-horizons correlate with independent MAP estimates from geochemical proxies, which suggests that paleoprecipitation information is preserved. Future work should continue to address these complications by developing useful protocols that isolate the magnetic properties that are most resistant to alteration and remain strong indicators of MAP and climate.

Terrestrial isotopic evidence for a Middle-Maastrichtian warming event from the lower Cantwell Formation, Alaska

Carbon stable isotope data (δ13C) and new radiometric dates indicate that the non-marine Late Cretaceous lower Cantwell Formation, central Alaska, registers the Middle-Maastrichtian Event (MME), a global warming episode. A stratigraphic section measured at the East Fork of the Toklat River in Denali National Park & Preserve includes a bentonite layer that we have dated to 69.5±0.7Ma using U–Pb isotope analysis of zircons. The new depositional age confirms that dinosaur tracks from the lower Cantwell Formation are coeval with dinosaur bones from outcrops of the Prince Creek Formation on Alaska's North Slope. δ13C data from fossil wood recovered from the lower Cantwell Formation records an ~3‰ positive excursion correlative with positive excursions from terrestrial and marine sections in West Texas (USA) and Gubbio (Italy), respectively. The similarity between marine and terrestrial records suggests that this excursion is the result of a major perturbation in the global carbon cycle, known as the Middle-Maastrichtian Event (MME). Mean annual precipitation (MAP) estimates calculated from δ13C data yield a range of 168–470mm/yr during the MME, 353–1050mm/yr before and 475–1451mm/yr after the warming event. The δ13C record from the lower Cantwell Formation provides the first terrestrial high-latitude evidence for the MME, while MAP estimates for the coeval Prince Creek Formation suggest that the arctic coast may have been more humid than southern interior Alaska during the Middle Maastrichtian.

Biotic and Abiotic Forcing During the Transition to Modern Grassland Ecosystems: Evolutionary and Ecological Responses of Small Mammal Communities Over the Last 5 Million Years

Understanding the origin of modern communities is a fundamental goal of ecology, but reconstructing communities with durations of 103–106 years requires data from the fossil record. Early Pliocene to latest Pleistocene faunas and sediments in the Meade Basin and modern soils and rodents from the same area are used to examine the role of environmental change in the emergence of the modern community. Paleoenvironmental proxies measured on modern surface soils and paleosols are described, and faunal dynamics of fossil rodents are discussed. Mean annual precipitation (MAP) was estimated from elemental concentrations and magnetic properties, and warm-season temperature and δ18O of soil water was estimated using carbonate isotope paleothermometry on pedogenic nodules. MAP and temperature estimates from paleosols exhibit no short-term variability, no long-term trends, and generally bracket modern values. Estimated soil water δ18O values increased through time, suggesting aridification played a role in the evolution of the regional grassland ecosystem. Carbon isotope analyses of biomarkers are used to examine the abundance of C4 grasses, which suggest more C4 biomass and more variability in C4 biomass than carbonate proxies. Rodent species richness remained constant due to balanced rates of extinction and immigration, both of which show episodic spikes consistent with a balance between forcing mechanisms that result in equilibrium on long time scales. Overall, these results suggest that different mechanisms of faunal change may be acting at different time scales, although the stratigraphic resolution of paleoenvironmental proxies needs to be increased, and body size and dietary distributions of rodents need to be determined before which processes of change are most important can be decided.

Inland California during the Pleistocene—Megafaunal stable isotope records reveal new paleoecological and paleoenvironmental insights

Abstract We measured oxygen and carbon isotope compositions from teeth from three fossil localities in California, The McKittrick Tar seeps (Rancholabrean), Fairmead Landfill, and Irvington (both Irvingtonian). These sites have produced a variety of megafauna fossils, and to date are relatively understudied. Previous ecological studies of Pleistocene California megafauna have focused on the coastal Rancho La Brea tar seeps (RLB), neglecting inland faunas. Both Fairmead Landfill and McKittrick are located in California's San Joaquin Valley, and provide the first isotopic data from the region. We sampled a wide range of taxa; Equus, Camelops, Mammuthus, Mammut, Hemiauchenia, Odocoileus, Tetrameryx, Capromeryx, Platygonus, Canis dirus, Canis latrans, Arctodus simus, Smilodon, Homotherium, Miracinonyx, Panthera onca and Panthera atrox. Stable carbon values from both middle Pleistocene localities are consistent with a C3 dominated environment. Mean annual precipitation (MAP) estimates for the middle Pleistocene localities are ~ 340 mm/year for Fairmead Landfill and ~ 900 mm/year for Irvington. While the inland MAP estimate is similar to modern levels, Irvington MAP estimates are significantly higher than the modern average. In contrast, the McKittrick tar seeps show clear evidence of C4 consumption among Equus, Bison and Camelops, suggesting a mixture C4 grasses and halophytes. Similarly, comparing between McKittrick and published data for RLB indicate a higher level of C4 resources inland. Serially sampled teeth from both inland localities reveal diet and resource partitioning between browsers and grazers.

A new paleoprecipitation proxy based on soil magnetic properties: Implications for expanding paleoclimate reconstructions

Description of precipitation patterns and changes in the hydrological cycle during periods of past global change is crucial for providing an understanding of terrestrial climate systems and for predicting impacts of future climate change such as shifting water availability. While a number of proxies and climofunctions exist for reconstructing paleoprecipitation using paleosols, all of the available tools for reconstructing paleoprecipitation are either limited to certain precipitation ranges (effective only for low-precipitation regimes; e.g., depth to Bk, chemical index of alteration [CIA-K]), or are relevant only to a limited range of paleosols (single-pedotype relationships; e.g., calcium-magnesium index [CALMAG]). Here, we measure the acquisition of isothermal remanent magnetization in B horizons of modern soils to quantify the ratio of pedogenic magnetic minerals goethite and hematite, and we use the relationship between these soil magnetic properties and measured climatic variables at each soil site to derive a new quantitative proxy for precipitation. By compiling both literature-derived and measured goethite-hematite (G/H) ratios and mean annual precipitation estimates for a global suite of modern soils ( n = 70), we describe a strong linear relationship ( R 2 = 0.96) between the G/H ratios of soil B horizons and mean annual precipitation that can be used to estimate paleoprecipitation values for a wide range of climatic regimes (100–3300 mm yr –1 ) and soil types (Inceptisols, Alfisols, Ultisols, Oxisols, Mollisols, Aridisols, Spodosols). We tested the new climofunction using paleosols from the early Eocene of Wyoming, which show that estimates based on G/H ratios compare favorably to and expand upon previously published estimates based on paleosol data.

Multiproxy approach reveals evidence of highly variable paleoprecipitation in the Upper Jurassic Morrison Formation (western United States)

Elemental analyses of paleosol B horizons in the Upper Jurassic Morrison Formation of the western United States provide estimates of mean annual precipitation (MAP) and allow determination of humidity regimes. Samples were collected from the lower Morrison Formation in New Mexico and the upper Morrison Formation in Wyoming and Montana. The chemical index of alteration minus potassium (CIA – K) and the calcium and magnesium weathering index (CALMAG) were used to estimate paleorainfall. CIA – K values calculated for paleosols without shrink-swell (vertic) features correspond to MAP estimates between 800 and 1100 mm yr –1 , with an average of 1000 mm yr –1 . CALMAG values, calculated for vertic paleosols, correspond to MAP estimates between 50 and 1200 mm yr –1 , with an average of 700 mm yr –1 . MAP estimates from the older New Mexico strata indicate that early Morrison environments were relatively arid. MAP estimates from the younger Wyoming and Montana deposits reflect wetter conditions in the northernmost part of the Morrison Formation, but the transition from arid interior environments was abrupt. Humidity provinces inferred from geochemical proxy-based estimates of evapotranspiration and energy influx from precipitation range from semiarid to superhumid, suggesting wetter conditions than the MAP estimates, but supporting the relative differences in moisture among the three study areas. Paleoprecipitation patterns within the Morrison depositional basin do not match the modern latitudinal distribution of rainfall that arises from zonal atmospheric circulation. Comparison with the Upper Jurassic Lourinha Formation in Portugal and the Vega Formation in Spain reveals that MAP in Late Jurassic Portuguese environments was similar to that in the wet northern part of the Morrison Formation, although more arid conditions prevailed in some areas of Portugal. Inferred humidity regimes for the Lourinha Formation, which range from semiarid to superhumid, also indicate small-scale geographic variability in climate, although less pronounced than that observed in the Morrison Formation. Paleoenvironments in northern Spain were similar to the drier Morrison environments in the continental interior. Given the abrupt climatic transitions inferred here for the Morrison Formation, paleoprecipitation estimates derived from a geographically restricted sample may reflect only local conditions and should not necessarily be extrapolated to larger areas.

Quantitative relationships between magnetic enhancement of modern soils and climatic variables over the Chinese Loess Plateau

Environmental magnetism has been widely employed to reconstruct past climate changes on the Chinese Loess Plateau (CLP), and several climofunctions based on the magnetic properties of loess have been developed. However, systematic investigation of the quantitative relationship between topsoil magnetic enhancement and modern climate remains uncommon. In this study, we obtained surface soil samples from 257 sites over the CLP and adjacent regions. From this set, we used 180 samples from sites unaffected by potential contamination to investigate the relationship between the commonly measured magnetic properties of magnetic susceptibility and magnetic remanence and modern climatic variables. The spatial distribution of the results demonstrates a strong NW–SE gradient of the magnetic enhancement of surface soils. The results of more detailed magnetic parameters indicate that pedogenic viscous superparamagnetic and stable single-domain particles are mainly responsible for the magnetic enhancement of the soils in the studied region; and that the magnetic grain-size distribution of the ferrimagnetic components of pedogenic origin remains almost constant, independent of pedogenic intensity. The uniform mechanism of magnetic enhancement, mainly linked with the concentration of the pedogenic components rather than with variations in magnetic grain-size, decreases the level of ambiguity in climate reconstructions based on magnetic measurements of Chinese loess. Statistical analyses, including correlation, Principal Component Analysis (PCA) and multiple regression analysis, suggest that annual rainfall rather than temperature exerts the dominant effect on soil magnetic enhancement. Finally, we used the results to develop several transfer functions to reconstruct mean annual precipitation (MAP). Transfer functions based on frequency-dependent susceptibility (χfd) and anhysteretic remanent magnetization (χARM) provide the most reliable estimates of MAP. This study significantly improves the understanding of the relationship between soil magnetic properties and climate.

Palaeoclimate of the Late Jurassic of Portugal: comparison with the Western United States

AbstractInvestigation of the palaeoclimatic conditions associated with Upper Jurassic strata in Portugal and comparison with published palaeoclimate reconstructions of the Upper Jurassic Morrison Formation in western North America provide important insights into the conditions that allowed two of the richest terrestrial faunas of this period to flourish. Geochemical analyses and observations of palaeosol morphology in the informally named Upper Jurassic Lourinhã formation of western Portugal indicate warm and wet palaeoclimatic conditions with strongly seasonal precipitation patterns. Palaeosol profiles are dominated by carbonate accumulations and abundant shrink‐swell (vertic) features that are both indicative of seasonal variation in moisture availability. The δ18OSMOW and δDSMOW values of phyllosilicates sampled from palaeosol profiles range from +22·4‰ to +22·7‰ and −53·0‰ to −37·3‰, respectively. These isotope values correspond to temperatures of formation between 32°C and 39°C ± 3°, with an average of 36°C, which suggest surface temperatures between 27°C and 34°C (average 31°C). On average, these surface temperature estimates are 1°C higher than the highest summer temperatures modelled for Late Jurassic Iberia using general circulation models. Elemental analysis of matrix material from palaeosol B‐horizons provides proxy (chemical index of alteration minus potassium) estimates of mean annual precipitation ranging from 766 to 1394 mm/year, with an average of approximately 1100 mm/year. Palaeoclimatic conditions during deposition of the Lourinhã formation are broadly similar to those inferred for the Morrison Formation, except somewhat wetter. Seasonal variation in moisture availability does not seem to have negatively impacted the ability of these environments to support rich and relatively abundant faunas. The similar climate between these two Late Jurassic terrestrial ecosystems is probably one of the factors which explains the similarity of their vertebrate faunas.

Paleoenvironmental reconstruction of Jurassic dinosaur habitats of the Vega Formation, Asturias, Spain

Jurassic-aged strata of Asturias, Spain, contain trace fossils including sauropod, theropod, and ornithopod dinosaur footprints, but their paleoenvironmental context has been relatively unstudied. A coastally exposed continuous section at Playa de Vega shows a clear transition from the marine Middle Jurassic Rodiles Formation to the terrestrial Upper Jurassic Vega Formation. Within the >100 m of Vega Formation stratigraphy that was logged there, four distinct types of paleosols were identified: (1) Entisols, (2) Inceptisols, (3) Vertisols, and (4) composite or cumulative paleosols. The paleosol types and their features indicate a floodplain depositional setting with short stature, shrubby vegetation. Theropod and ornithopod tracks have been identified at the base of the section, indicating that a dinosaurian fauna was present at Playa de Vega during the Jurassic. Results from well-characterized climofunctions based on modern soils and paleosol B horizon chemical composition of the Inceptisols and Vertisols yield mean annual precipitation estimates of 400–980 mm yr −1 and mean annual temperature estimates of 8–15 °C. The presence of Vertisols, with both evidence for shrink-and-swell behavior and dispersed pedogenic carbonate, is consistent with a strongly seasonal precipitation regime. The δ 13 C analyses of pedogenic carbonates yield values that range from −7.09‰ to −8.88‰ (relative to Vienna Peedee belemnite [VPDB]) and indicate carbon dioxide levels about six times pre-industrial levels, consistent with previous results. The Asturian vertebrate track assemblage is remarkably similar to that of the Morrison Formation (western United States), but it has greater overall richness. The pattern is reversed for body fossils. The reconstructed paleoenvironmental and paleoclimatic conditions indicate similar depositional settings in both places, but with a cooler, wetter, more seasonal environment in Asturias. The greater seasonality indicated by the Vega Formation relative to the Morrison Formation may explain the observed differences in richness.

Evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record

Different quantitative methods have been developed to determine paleo-rainfall using paleosols. Of particular interest are methods that use bulk geochemistry to calculate a weathering index and, from that, mean annual precipitation. Twenty-three paleosols that formed during a time of significant climate change—the Paleocene–Eocene Thermal Maximum (PETM)—were analyzed to evaluate the relative merits of two geochemical proxies that have been developed for estimating paleoprecipitation: one depends on the chemical index of alteration without potash (CIA-K) and the other uses the CALMAG weathering index, which depends on CaO and MgO. The paleosols are located in the Paleogene Fort Union and Willwood formations in the Bighorn Basin, WY. Morphologic differences among the paleosols indicate that they formed under varied soil moisture conditions. A morphology index was developed from pedogenic features that are sensitive to soil drainage, and thus to rainfall, to assess that variability. Quantitative mean annual precipitation (MAP) values were calculated for each paleosol using the CIA-K index and the CALMAG index, which was developed specifically for Vertisols. Those results were compared to the morphology index for 23 paleosols. The results indicate that increases and decreases in MAP calculated from both methods correlate relatively well to changes in the soil morphology index; however, the results from the CALMAG weathering index show a stronger correlation. The results suggest that the CALMAG proxy provides a more robust MAP estimate for Vertisols. In addition to comparing MAP estimates to the soil morphology index, we compared the MAP values to paleorainfall estimates determined from paleofloras at the same stratigraphic levels as three of the paleosols. That comparison confirms that the CALMAG method is reliable for reconstructing MAP from ancient Vertisols. The results also show that, in the Bighorn Basin, Vertisols with a B horizon < 1 m thick should not be used to determine MAP. Thinner paleosols have had less time for weathering and may not have cation distributions representative of precipitation.