Paleoprecipitation Estimates Research Articles

Depth to gypsic horizon as a proxy for paleoprecipitation in paleosols of sedimentary environments

Pedogenic accumulations of crystals and nodules of gypsum are common in desert soils, especially extreme deserts such as the Atacama Desert, Chile. Some soils with both pedogenic gypsum and calcite have the gypsic (By) horizon below the calcic (Bk) horizon. Here we present a compilation of 88 gypsic soils from around the world to derive a relationship (R 2 = 0.63, standard error = ±129 mm) between depth to the By horizon (D, in centimeters) and mean annual precipitation (P, in millimeters) as follows: P = 87.593e 0.0209D . This relationship can be used to derive paleoprecipitation estimates from paleosols, once depth to the gypsic horizon is corrected for compaction due to overburden. Application of this technique to Early Permian paleosols near Gilliland, Texas, confi rms paleoprecipitation estimates in the same sequence derived from depth to the calcic horizon. Barite is another sulfate mineral that forms nodular horizons in paleosols, but not in modern soils. Miocene paleosols in Panama with both calcareous and barite nodules suggest that this weakly soluble salt forms at levels in paleosols unlike those of either pedogenic carbonate or gypsum.

Paleosols, trace fossils, and precipitation estimates of the uppermost Triassic strata in northern New Mexico

This study provides a detailed pedogenic evaluation of two Upper Triassic (Late Norian through Rhaetian) stratigraphic intervals in New Mexico in order to assess the climate and ecology of the Latest Triassic, which ended in a mass extinction. The two study areas are located in north–central and east-central New Mexico and are separated by 200 km. Each section contains abundant paleosols of varying maturity with features that reflect an arid to semiarid climate. There is little pedogenic variation throughout the strata at each location, and a typical paleosol profile is about 1 m thick and has an AB–Bw–Bk–BC horizon succession. Bkm, Bss, Bssk, or Bssg horizons are present in some paleosols. Micromorphological features suggest dominantly well-drained sola (e.g., abundant carbonate nodules, illuviated clay) with minor periods of moist or saturated conditions (e.g., FeMn concretions, FeMn coatings and hypocoatings, sepic-plasmic fabrics). Trace fossils are abundant in these strata and are dominated by Taenidium serpentinum and root traces. Depth-to-carbonate functions estimate that mean annual precipitation was between 200 and 450 +/− 95 mm. Relative to location 1 (eastern New Mexico), location 2 (north–central New Mexico) produced higher paleo-precipitation estimates and has stronger and more abundant sepic-plasmic fabrics in thin sections. The presence of a gleyed paleosol, Camborygma eumnkenomos, and slickensides at location 2 also suggests conditions wetter than at location 1. Taxonomically, all of the paleosols in this study appear to be Entisols or Aridisols and can be grouped into seven representative pedotypes of varying maturity. By comparing these paleosols to modern soils, this study demonstrates that the Late Triassic Western Interior during the Late Norian to Rhaetian was arid to semiarid and supported a desert shrub environment that had localized and periodic moist or saturated soil conditions.

New method to estimate paleoprecipitation using fossil amphibians and reptiles and the middle and late Miocene precipitation gradients in Europe

Existing methods for determining paleoprecipitation are subject to large errors (6350- 400 mm or more using mammalian proxies), or are restricted to wet climate systems due to their strong facies dependence (paleobotanical proxies). Here we describe a new paleo- precipitation tool based on an indexing of ecophysiological groups within herpetological communities. In recent communities these indices show a highly significant correlation to annual precipitation (r 2 5 0.88), and yield paleoprecipitation estimates with average errors of 6250-280 mm. The approach was validated by comparison with published paleopre- cipitation estimates from other methods. The method expands the application of paleo- precipitation tools to dry climate systems and in this way contributes to the establishment of a more comprehensive paleoprecipitation database. This method is applied to two high- resolution time intervals from the European Neogene: the early middle Miocene (early Langhian) and the early late Miocene (early Tortonian). The results indicate that both periods show significant meridional precipitation gradients in Europe, these being stronger in the early Langhian (threefold decrease toward the south) than in the early Tortonian (twofold decrease toward the south). This pattern indicates a strengthening of climatic belts during the middle Miocene climatic optimum due to Southern Hemisphere cooling and an increased contribution of Arctic low-pressure cells to the precipitation from the late Miocene onward due to Northern Hemisphere cooling.

Palaeoenvironments of the latest Cretaceous (Maastrichtian) dinosaurs of Romania: insights from fluvial deposits and paleosols of the Transylvanian and Haţeg basins

A diverse Maastrichtian dinosaur-dominated fauna is known from the Red Continental Strata of the Transylvanian Basin and the Densuş-Ciula Formation and Pui beds of the Haţeg Basin, western-central Romania. Sedimentological and palaeopedological studies of these deposits provide insights into the prevailing palaeoenvironmental and palaeoclimatic conditions at the end of the Cretaceous. The Red Continental Strata and Pui beds are dominated by red mudstone sequences and exhibit highly variable palaeocurrent direction. Inclined heterolithic strata with palaeocurrent indicators orientated perpendicular to the dipping strata are also present in channel deposits of the Red Continental Strata. All these features suggest deposition by meandering rivers. Although red mudstones are abundant in the Densuş-Ciula Formation, this formation differs from the aforementioned formations in containing thick sandstone beds displaying sedimentary structures typical of braided stream deposits and characterized by low variability of palaeocurrent direction. These features indicate deposition in a low-sinuosity fluvial system. Paleosols of the three studied formations are characterized by the presence of carbonate nodules closely associated with iron oxides, rhizocretions, and slickensides; mottles and indurated calcareous horizons are occasionally present in paleosols of the Red Continental Strata and Pui beds. These features are indicative of a climate characterized by seasonal precipitation in which evapotranspiration exceeded precipitation, and where the watertable fluctuated during the year. Dark red, noncalcareous paleosols also occur occasionally in the Pui beds, representing leached profiles that developed on sandier parent material. The geochemical composition of all paleosols studied indicates that palaeoprecipitation was less than 1000 mm/year, which is significantly lower than estimates inferred from the tropical palaeoflora of the region (1300–2500 mm/year). This apparent incongruence in paleoprecipitation estimates can be explained by the fact that tropical plants can live in warm, monsoonal climates if they have access to sufficient quantity of water during the dry seasons to satisfy their metabolic needs. Comparison of the studied formations to the famous contemporaneous Sânpetru Formation of the Haţeg Basin confirms similarity of palaeoclimatic conditions. However, palaeoenvironmental and taphonomic conditions differ. Whereas the Sânpetru Formation comprises a mosaic of wetlands and moderately-drained floodplains in which areas of impeded drainage were predominant, the Red Continental Strata, Densuş-Ciula Formation, and Pui beds represent moderately- to well-drained floodplain settings. Although the Pui beds have long been assumed to pertain to the Sânpetru Formation on the basis of palaeontological and superficial lithological similarities (i.e., red colour), comparison of the results of the detailed palaeoenvironmental studies suggests that the Pui beds are distinct from the Sânpetru Formation and may warrant their own formational recognition (Bărbat Formation). Finally, fossil preservation conditions in the Sânpetru Formation indicate concentration of remains by hydraulic processes on wet floodplains (lenticular bonebeds or “fossiliferous pockets”) while those of the other three formations suggest attritional mortality assemblages preserved in dry paleosol profiles. Such taphonomic differences must be considered carefully in palaeoecological studies of the Maastrichtian dinosaurian fauna of Romania.

Eocene dry climate and woodland vegetation in tropical Africa reconstructed from fossil leaves from northern Tanzania

Eocene vegetation and climate data from tropical latitudes are sparse despite special interest in the Eocene as the warmest epoch of the Cenozoic and an often-cited analogue for greenhouse Earth conditions. Tropical Africa is noteworthy for its shortage of Eocene fossils, which could serve as proxies for climate and reveal community structural evolution during the continent's geographic isolation. In this paper, we report paleobotanical remains from a middle Eocene crater lake at 12°S paleolatitude in north central Tanzania, which provide a plant community reconstruction indicating wooded, rather than forest, vegetation and precipitation estimates near modern (660 mm/year). The plant community was dominated by caesalpinioid legumes and was physiognomically comparable to modern miombo woodland. Paleoprecipitation estimates, the first for the Paleogene of Africa, are calculated from fossil leaf morphology using regression equations derived from modern low-latitude leaves and climate. Mean annual precipitation estimates are 643±32 and 776±39 mm/year, and wet months precipitation estimates (all months averaging≥50 mm) are 630±38 and 661±38 mm. A slightly larger proportion of annual precipitation occurred in the dry months compared with today, which may indicate greater equability of precipitation in the Eocene.

Eocene dry climate and woodland vegetation in tropical Africa reconstructed from fossil leaves from northern Tanzania

Eocene vegetation and climate data from tropical latitudes are sparse despite special interest in the Eocene as the warmest epoch of the Cenozoic and an often-cited analogue for greenhouse Earth conditions. Tropical Africa is noteworthy for its shortage of Eocene fossils, which could serve as proxies for climate and reveal community structural evolution during the continent's geographic isolation. In this paper, we report paleobotanical remains from a middle Eocene crater lake at 12°S paleolatitude in north central Tanzania, which provide a plant community reconstruction indicating wooded, rather than forest, vegetation and precipitation estimates near modern (660 mm/year). The plant community was dominated by caesalpinioid legumes and was physiognomically comparable to modern miombo woodland. Paleoprecipitation estimates, the first for the Paleogene of Africa, are calculated from fossil leaf morphology using regression equations derived from modern low-latitude leaves and climate. Mean annual precipitation estimates are 643±32 and 776±39 mm/year, and wet months precipitation estimates (all months averaging≥50 mm) are 630±38 and 661±38 mm. A slightly larger proportion of annual precipitation occurred in the dry months compared with today, which may indicate greater equability of precipitation in the Eocene.

Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols of the Turkana Basin, Kenya.

New stable carbon isotope measurements, coupled with paleoprecipitation estimates, both from Plio-Pleistocene paleosols of the Turkana Basin, Kenya, provide a high-resolution record of aridification and increasing C4 biomass during the past 4.3 Ma. This aridification trend is marked by several punctuations at 3.58-3.35, 2.52-2, and 1.81-1.58 Ma, during which the running mean and variance of delta13C and paleoaridity estimates increase, suggesting that the proportion of C4 biomass increases in savanna mosaics during periods of heightened aridity. Increase in C4 biomass during these aridification events not only increases the proportion of open habitats, but increases the spatial neg-entropy, or heterogeneity of the ecosystem. The aridification events identified correspond to intervals of increased turnover, but more importantly, increased diversity of bovids. Although the record of hominins from the Turkana Basin lacks the temporal resolution and diversity of the bovid record, the aridification intervals identified are marked by similar increases in the diversity and turnover of hominins. These results support the hypothesis that hominins evolved in savanna mosaics that changed through time, and suggest that the evolution of bovids and hominins was driven by shifts in climatic instability and habitat variability, both diachronic and synchronic.

Paleoprecipitation estimates for the Lake Naivasha basin (Kenya) during the last 175 k.y. using a lake-balance model

We modeled the two most extreme highstands of Lake Naivasha during the last 175 k.y. to estimate potential precipitation/evaporation changes in this basin. In a first step, the bathymetry of the paleolakes at ∼135 and 9 k.y. BP was reconstructed from sediment cores and surface outcrops. Second, we modeled the paleohydrologic budget during the highstands using a simplified coupled energy mass-balance model. Our results show that the hydrologic and hence the climate conditions at ∼135 and 9 k.y. BP were similar, but significantly different from today. The main difference is a ∼15% higher value in precipitation compared to the present. An adaptation and migration of vegetation in the cause of climate changes would result in a ∼30% increase in precipitation. The most likely cause for such a wetter climate at ∼135 and 9 k.y. BP is a more intense intertropical convergence and increased precipitation in East Africa.

Pedogenic iron-manganese nodules in Vertisols: A new proxy for paleoprecipitation?

The total Fe content of pedogenic iron-manganese (Fe-Mn) nodules taken from a Vertisol climosequence on the Texas Gulf Coastal Plain correlates with mean annual precipitation (MAP, r 2 = 0.92). No significant trend of total Fe (FeTOT) with depth was noted in profiles. Using the regression developed from modern Vertisol data, FeTOT contents of Paleozoic paleo-Vertisol Fe-Mn nodules yielded MAP regimes comparable to previously inferred paleoenvironmental interpretations. Paleoprecipitation estimates derived from Fe-Mn nodules for an uneroded, Late Mississippian paleo-Vertisol are very close to estimates determined from a depth to pedogenic carbonate horizon (DCH) proxy determined from the modern Vertisol climosequence. The lack of soil depth dependence of the Fe-Mn nodule proxy provides a consistent paleoprecipitation estimate even in eroded paleo- Vertisols and, in combination with the DCH, may be useful in determining original paleosol thickness.

Potential analogues for paleoclimatic variations in eastern interior Alaska during the past 14,000 yr: atmospheric-circulation controls of regional temperature and moisture responses

The paleoclimatic history of a region can be viewed as a series of surface temperature and moisture anomalies through time. The effects of changes in large-scale climatic controls (e.g., insolation, major circulation controls) can be mediated by the influence of smaller-scale controls (e.g., topographic barriers, coastlines); this may result in heterogenous surface climatic responses at the regional and sub-regional scale. Divergent paleoclimatic trajectories between regions may be explainable in terms of such meso-scale patterns. Using modern analogues for paleoclimate we examine how the sequence of climatic variations in eastern interior Alaska during the interval 12,000–0 14 C yr BP could have been generated by specific atmospheric circulation patterns. Fossil-pollen and lake-level records document the long-term trends in temperature and effective moisture for the region. Water-balance modelling provides additional estimates of paleoprecipitation. Synoptic climatological patterns are described using the modern (instrumental) record of upper-level and sea-level pressure, surface temperature, and precipitation. At 12,000 14 C yr BP, eastern interior Alaska was cooler and drier than present, a situation generated today by a southward displacement of the jet stream. Conditions warmer and drier than present at 9000 14 C yr BP may have been generated by increased ridging north of Alaska and a weakened westerly circulation. Warmer, wetter conditions than present possibly prevailed in the late-middle Holocene; these might reflect ridging over Alaska and troughing further west. Cool, wet conditions feature enhanced westerly flow into Alaska through an eastward shift in the east Asian trough and positive pressure anomalies in the North Pacific; they may be analogous to cold periods of the Little Ice Age. The analogues demonstrate how surface conditions in other parts of Beringia may sometimes be similar to, while at other times different from those in the eastern interior. These broader spatial patterns provide hypotheses about past climates that can be tested with paleoclimatological data. For example, the widespread positive temperature anomalies associated with the warm/dry (9000 14 C yr BP) analogue fit with the expansion northward of the eastern Siberian treeline. The anomalously cool conditions in northeast Siberia associated with the warm/wet analogue may explain the continued (late-middle Holocene) treeline advance in Alaska while there was retreat in Siberia.

Mass-balance reconstruction of a modern Vertisol: implications for interpreting the geochemistry and burial alteration of paleo-Vertisols

Utilizing identical sampling and analytical techniques, the morphological and chemical characteristics of a modern Vertisol (Houston Black series, central Texas) can be directly compared with an Upper Mississippian paleo-Vertisol from the Appalachian basin (Pennington Formation, east-central Tennessee). Mass-balance reconstructions suggest retention of primary pedochemical patterns in the paleo-Vertisol, including patterns of soil volume change (strain) and transport functions (translocations) of many major and trace elements. Retention of primary pedochemical patterns suggests that Vertisols constitute nearly closed systems during burial diagenesis. Chemical and mineralogical changes associated with burial diagenesis of the paleo-Vertisol include oxidation of organic carbon (OC), illitization of smectites, dehydration and recrystallization of Fe–Mn oxyhydroxides, and dolomitization of pedogenic calcite. Significant differences in the chemical behavior of gilgai microhigh and microlow pedons in modern Vertisols have implications for interpretation of geochemical data obtained from paleo-Vertisols. Overall wetter soil conditions and variable redox potential under gilgai microlows promote greater depths of leaching and mobility of redox-sensitive trace elements, including Co, Cr, Cu, Mn, Ni, and V. Gilgai microhighs behave as evaporative “wicks” that draw moisture and soluble phases towards the soil surface, resulting in precipitation of metal hydrosylate complexes and sulfates (gypsum) at the capillary fringe and shallower depths of leaching and fixation of trace elements. Paleoprecipitation estimates from paleosols, based on the depth to the top of the pedogenic carbonate horizon, should therefore utilize field, petrographic and geochemical data for characterizing maximum depths of leaching, loss or gain of exchangeable bases, and calcification, rather than relying solely upon field data.

Using fossil leaves as paleoprecipitation indicators: An Eocene example

Estimates of past precipitation are of broad interest for many areas of inquiry, including reconstructions of past environments and topography, climate modeling, and ocean circulation studies. The shapes and sizes of living leaves are highly sensitive to moisture conditions, and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators. Most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations. However, when tested on extant forests, this method has consistently overestimated precipitation. We present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set. The significant relationship that results, in addition to having value for paleoclimatic reconstruction, refines understanding of the long-observed positive relationship between leaf area and precipitation. Seven precipitation estimates for the Eocene of the Western United States are revised as lower than previously published but remain far wetter than the same areas today. Abundant moisture may have been an important factor in maintaining warm, frost-free conditions in the Eocene because of the major role of water vapor in retaining and transporting atmospheric heat.

Preservation of a Paleo-vertisol and an Estimate of Late Mississippian Paleoprecipitation

ABSTRACT A Late Mississippian paleosol satisfying all of the morphological criteria required for classification of Holocene Vertisols provides quantitative paleoclimate information, in addition to the now commonplace interpretation of precipitation seasonality based on the presence of vertic features. Paleoprecipitation was estimated using the empirical relationship between depth to pedogenic carbonate horizon in Quaternary soils. Burial compaction, erosional truncation, and high paleoatmospheric CO2 concentration, all factors that complicate paleoprecipitation estimates, are unusually well constrained for this paleosol. Allowing for 10% compaction, the paleosol had a pre-burial depth of 100 cm for the pedogenic carbonate horizon, yielding a mean annual paleo-precipitation estimate f 648 ± 141 mm. This is comparable to the mean annual precipitation for Brownsville, Texas, where similar soils are found today. A dolomicrite crust, developed in gilgai microlows, is well preserved in the paleo-Vertisol. Higher Late Mississippian paleotemperatures and rates of evapotranspiration associated with a lower-latitude paleogeography for central Tennessee during the Late Mississippian may explain in part why Holocene coastal Vertisols in the Brownsville region lack surficial crusts similar to that of the paleo-Vertisol. We qualitatively define the Late Mississippian climate of central Tennessee as semiarid.