Proxy For Paleoprecipitation Research Articles

Pinpointing the Mechanism of Magnetic Enhancement in Modern Soils Using High‐Resolution Magnetic Field Imaging

AbstractIn well‐buffered modern soils, higher annual rainfall is associated with enhanced soil ferrimagnetic mineral content, especially of ultrafine particles that result in distinctive rock magnetic properties. Hence, paleosol magnetism has been widely used as a paleoprecipitation proxy. Identifying the dominant mechanism(s) of magnetic enhancement in a given sample is critical for reliable inference of paleoprecipitation. Here, we use high‐resolution magnetic field and electron microscopy to identify the grain‐scale setting and formation pathway of magnetic enhancement in two modern soils developed in higher (∼580 mm/y) and lower (∼190 mm/y) precipitation settings from the Qilianshan Range, China. We found that both soils contain 1–30 μm aeolian Fe‐oxide grains with indistinguishable rock magnetic properties, while the higher‐precipitation soil contains an additional population of ultrafine (<150 nm) magnetically distinct magnetite grains. We show that the in situ precipitation of these ultrafine particles, likely during wet‐dry cycling, is the only significant magnetic enhancement mechanism in this soil. These results demonstrate the potential of quantum diamond microscope magnetic microscopy to extract magnetic information from distinct, even intimately mixed, grain populations. This information can be used to evaluate the contribution of distinct enhancement mechanisms to the total magnetization.

Holocene palaeoenvironmental conditions in NE Bulgaria uncovered by mineral magnetic and paleomagnetic records of an alluvial soil

Retrieving well-dated terrestrial archives of Holocene climate change is fundamentally important for building robust climate models and predictions. In this study we obtained records of paleomagnetic direction and relative paleointensity of the Earth's magnetic field from an alluvial soil situated on a flood river terrace close to the well-dated and stratigraphically confined Chalcolithic mound near the village of Koprivetz in NE Bulgaria. Anisotropy of magnetic susceptibility data revealed a coherent imbricated magnetic fabric reflecting the effects of water flow in the lower alluvial clays and the direction of delluvial sedimentation from the slope in the upper soil horizons. Variations of declination, inclination and relative paleointensity along depth of the soil profile were correlated to the archaeomagnetic secular variation curves for Bulgaria, using the age of the earliest archaeological stratigraphic horizon as the main tie point. Based on this correlation, an age model was constructed and temporal variations of environmental magnetic parameters were examined for their suitability as paleoclimate proxies. The ratio of frequency dependent susceptibility to anhysteretic susceptibility (χfd/χARM) is a reasonable proxy for paleoprecipitation due to its close resemblance to paleo-flood records from the Northern and Southern Alps. Events of increased flood intensities during the late Holocene (post – 4700 y BP) are registered through χfd/χARM and reveal good correspondence to the flood records of rivers in Southern Alps and Carpathians. The ratio of isothermal remanent magnetization after 20 mT alternating field demagnetization (IRM20mT) normalized to the full IRM showed good consistency with the GISP2 ice record of temperature variations in Greenland during the Holocene. Six major cold incidents during the last 7000 y BP with the major one at 4500 y BP are evidenced. The obtained results suggest that climate at mid-latitude SE Europe is sensitive to the changes in North Atlantic circulation.

Precipitation dynamics on the Tibetan Plateau during the Late Quaternary – Hydroclimatic sedimentary proxies versus lake level variability

Two of the largest and deepest endorheic lakes on the Tibetan Plateau, Nam Co and Tangra Yumco, were targeted to infer Late Quaternary (24 ka) responses of their hydrological balance to environmental dynamics. For this purpose, high-resolution multi-proxy records were used to obtain information on lake level variations and how these are related to direct precipitation proxies. Allochthonous minerogenic input as reflected by potassium (K), is directly linked to precipitation driven by variations in Indian Summer Monsoon. Negatively correlated elements such as Ca, Sr and Mg and associated carbonate mineral phases represent autochthonous sediment production, which is coupled to evaporation in these closed systems. δ 13 C and δ 18 O of bulk carbonates represent lake water conditions, i.e., salinity and lake water volume, interpreted as precipitation-evaporation (P/E) balance. In combination with geomorphological features, i.e., lake level terraces and paleo-shorelines, the multy-proxy data set from Nam Co is used to reconstruct lake level dynamics. Low lake levels of >90 m below the recent one between 20 and 16 ka cal BP are associated with most enriched δ 18 O carb values and the precipitation of aragonite and high-Mg calcite in a shallow lake with highly dynamic allochthonous minerogenic input. Similar conditions are observed between 13.1 and 11.4 ka cal BP matching the Younger Dryas chronozone. Between 15.5 and 13.1 ka cal BP as well as during the Early Holocene (11.4 to 8.0 ka cal BP) distinctly depleted δ 18 O carb and δ 13 C carb coupled with high rates of allochthonous minerogenic input suggest substantial strengthening in precipitation. After 9.4 ka cal BP, the terrigenous input and thus precipitation is significantly reduced, whereas δ 18 O carb reveals only a slight enrichment trend and thus a slowly but steady decrease in lake level througout the Mid- to Late Holocene. The decoupling of this relation between precipitation and lake level suggest that lake level terraces of large endorheic lakes on the TP and also δ 18 O carb records are not a direct indicator of (monsoonal) precipitiation, whereas the allochtonous minerogenic input can be directly linked to precipitation dynamics. This is supported by the comparison to regional archives from the Arabian Sea and the northern Bay of Bengal, showing the very same pattern in precipitation variability. The presented monsoon precipitation record from Nam Co additionally reveals a link to supra-regional atmospheric circulation components as it is oscillating in- and anti-phase to El-Niño strength and frequency. A distinct precipitation change at 2 ka cal BP seems to have affected all archives, however leaving space for future studies to find the cause of this Late-Holocene climatic shift. • First Late Quaternary lake level reconstruction for Nam Co, one of the best and most investigated lakes on the TP • Presentation of a robust paleo-precipitation proxy, i.e., allochthonous minerogenic input into the Tibetan lakes • Decoupling of lake level and precipitation proxy suggests e.g., paleo shorelines do not trace paleo-monsoonal variability • Robustness of the paleo-precipitation proxy supported by excellent temporal match of the variability to regional archives • Supra-regional link between paleo-precipitation on the Tibetan Plateau and the Tropical Pacific (El Niño variability)

Soil Carbon Isotope Values and Paleoprecipitation Reconstruction

AbstractAnthropogenic climate change has significant impacts at the ecosystem scale including widespread drought, flooding, and other natural disasters related to precipitation extremes. To contextualize modern climate change, scientists often look to ancient climate changes, such as shifts in ancient precipitation ranges. Previous studies have used fossil leaf organic geochemistry and paleosol inorganic chemistry as paleoprecipitation proxies, but have largely ignored the organic soil layer, which acts as a bridge between aboveground biomass and belowground inorganic carbon accumulation, as a potential recorder of precipitation. We investigate the relationship between stable carbon isotope values in soil organic matter (δ13CSOM) and a variety of seasonal and annual climate parameters in modern ecosystems and find a statistically significant relationship between δ13CSOM values and mean annual precipitation (MAP). After testing the relationship between actual and reconstructed precipitation values in modern systems, we test this potential paleoprecipitation proxy in the geologic record by comparing precipitation values reconstructed using δ13CSOM to other reconstructed paleoprecipitation estimates from the same paleosols. This study provides a promising new proxy that can be applied to ecosystems post‐Devonian (∼420 Ma) to the Miocene (∼23 Ma), and in mixed C3/C4 ecosystems in the geologic record with additional paleobotanical and palynological information. It also extends paleoprecipitation reconstruction to more weakly developed paleosol types, such as those lacking B‐ horizons, than previous inorganic proxies and is calibrated for wetter environments.

Updating the significance and paleoclimate implications of magnetic susceptibility of Holocene loessic soils

Mass specific magnetic susceptibility variation with depth along soil profiles developed on loess parent material is one of the most frequently used physical parameter in local, regional and global correlations of loess deposits. It is also utilized as a paleo-precipitation proxy, defined either as absolute difference between susceptibilities of the enhanced B-horizon and parent loess C-horizon; or relative enhancement, using ratios of magnetic parameters. In the present study we compare and analyze the applicability of various approaches in establishment of the empirical relationships between pedogenic magnetic enhancement of Holocene soils from Bulgarian lower Danube area, published data from cCentral and Eastern Europe, and the Chinese loess, and climate parameters – mean annual precipitation (MAP) and mean annual temperature (MAT). Unlike previous studies, which compile data from soils, developed on various parent materials, using maximum magnetic susceptibility of the B-horizon and mean of the C-horizon, we strictly utilize soils, developed on loess material only and consider minimum susceptibility signal in the underlying loess unit. The data analysis shows that minimum magnetic susceptibility of the parent loess exhibits strong relation to climate’s parameters as well. We show that by using minimum magnetic susceptibility of the C-horizon to correct for the detrital input, pedogenic enhancement shows significant multiple correlation with both MAP and MAT. This relationship is different for Chinese and European sites but it is not retained for sites, located at low river terraces, Black sea coast and soils having increased coarse silt and sand content. Grouping the sites according to present day vegetation cover reveals the presence of uniform linear relationship between pedogenic susceptibility enhancement and climate parameters mostly for locations with steppe vegetation, while mixed steppe-forest and forest sites reveal more scattered and steeper relation to MAT and MAP variations.

Simulation of Natural Iron Oxide Alteration in Soil: Conversion of Synthetic Ferrihydrite to Hematite Without Artificial Dopants, Observed With Magnetic Methods

AbstractWe present new results on the conversion of pure, undoped synthetic ferrihydrite, wet‐annealed at pH 6.56 and 90°C without stabilizing ligands, to nanophase goethite, hematite, and an intermediate magnetic phase, nanophase maghemite. Our analyses included magnetic field and temperature‐dependent properties and characterization by powder X‐ray diffraction, Mössbauer spectra, and high‐resolution transmission electron microscopy. We sampled alteration products after 0.5 hr, and then in a geometric progression to 32 hr, yielding a detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: We observe early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals form later, as reported in previous studies. Our observations attest to the non‐uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for use of the hematite/goethite ratio as a paleoprecipitation proxy.

Subtropical hydroclimate during Termination V (∼430-422 ka): Annual records of extreme precipitation, drought, and interannual variability from Santa Barbara Basin

Hydroclimate extremes are expected to become more frequent and intense with anthropogenic climate forcing, but future El Niño-Southern Oscillation (ENSO) behavior is unclear. Understanding of extreme hydroclimate variability and magnitude prior to human-influences is limited by short instrumental records, however, continuous sedimentary archives of past hydroclimate variability can complement and extend these records. Laminated Santa Barbara Basin (SBB) sediment cores MV0508-33JPC, −21JPC, and −29JPC preserve annually-resolved, multi-centennial-scale precipitation records from Termination V (TV), the transition from glacial Marine Isotope Stage (MIS) 12 to interglacial MIS 11 at 424 ka. These records provide insights into the subtropical hydroclimate response to warming. Foraminiferal δ18O variations indicate rapid warming and/or salinity changes, and were used to verify the TV age assignment. A paleoprecipitation proxy was developed using the first principal component of scanning XRF elemental counts (PC1), which has high loadings for siliciclastic sediment-associated elements K, Ti, and Si. Sedimentary laminae couplets identified in PC1 were annually-tuned to investigate TV paleoprecipitation variability, as modern SBB laminae represent annual deposits. Extreme flooding and decadal-to-centennial droughts were identified, with magnitudes exceeding modern observations. ENSO-like (2–7 year) paleoprecipitation periodicities coincide with wetter intervals, but ENSO variability is reduced during droughts. A 1500-year arid interval may be related to poleward shifting of general atmospheric circulation as ice sheets melted, such that the subtropical dry zone intersected California. Southern California paleoprecipitation reconstructions from past warm climates provide insight into precipitation variability and magnitude on interannual timescales, and can reduce uncertainties in predictions of interannual climate, including ENSO.

δ13C signal of earthworm calcite granules: A new proxy for palaeoprecipitation reconstructions during the Last Glacial in western Europe

Quantification of paleoprecipitation during the Last Glacial is a key element to reconstruct palaeoclimates. Recently, fossil calcite granules have been identified in loess sequences with high contents in specific horizons. In this study, we explored for the first time the potential of this new bio-indicator as a climatic proxy for precipitation in western Europe during the Last Glacial. We extracted 30 granules from eleven samples belonging to three tundra gleys and two brown soils from the Nussloch loess sequence previously dated between 50 and 20 ka. Stable carbon isotope measurements were performed on each granule and duplicated. Throughout the studied section, δ13C values range from −15.4 to −10.3‰ for tundra gleys and from −14.9 to −9.5‰ for brown soils. By taking into account the fractionation factor between the carbon ingested by the earthworm and the carbon output of the granules, the δ13C values of these granules reflect the composition of the C3 plant vegetation cover. Thus, we estimated the δ13C of the plants with a mean value of −24.3 ± 0.9‰ for tundra gleys and −24.1 ± 0.9‰ for brown soils, which are in agreement with values obtained from organic matter preserved in sediments. Palaeoprecipitation range over both tundra gley horizons and brown soils were estimated at about 333[159–574] mm/yr by using an empirical relationship determined between present-day plant leaf isotopic discrimination and the mean annual precipitation. This original preliminary study highlights the potential of earthworm calcite granule δ13C measurements as a new proxy for paleoprecipitation during the Last Glacial interstadials in continental environments.

Paleosol-based paleoclimate reconstruction of the Paleocene-Eocene Thermal Maximum, northern Argentina

The Paleocene-Eocene Thermal Maximum (PETM) is a well-documented, relatively short-lived, warm episode in Earth history. Previous studies completed on marine and continental strata spanning the PETM have focused on understanding the magnitude of warming and other atmospheric changes. These studies, completed largely in the Northern Hemisphere, report warming that ranged from 4 to 8°C above mean annual paleotemperatures prior to warming.In this study, paleosols in the Salta Basin, northern Argentina, are used to reconstruct paleoprecipitation rates and mean annual paleotemperatures of the Southern Hemisphere from before, during and after the PETM. Carbon isotope data are used to: 1) identify the horizon in which the PETM and other Eocene hyperthermals are recorded; and, 2) to interpret perturbations of the global carbon cycle during the PETM. At the height of the PETM, paleoprecipitation proxies indicate ~1500mm of annual rainfall and a temperature increase of ~5°C from pre-PETM values. Carbon isotope data records three negative carbon isotope excursions during the PETM in this region, suggesting the possibility of three distinct, rapid releases of isotopically depleted carbon into the ocean-atmosphere system. The results presented here are one of only a few paleoclimate reconstructions from continental sediments spanning the PETM in the Southern Hemisphere.

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.

Interannual and (multi-)decadal variability in the sedimentary BIT index of Lake Challa, East Africa, over the past 2200 years: assessment of the precipitation proxy

Abstract. The branched vs. isoprenoid tetraether (BIT) index is based on the relative abundance of branched tetraether lipids (brGDGTs) and the isoprenoidal GDGT crenarchaeol. In Lake Challa sediments the BIT index has been applied as a proxy for local monsoon precipitation on the assumption that the primary source of brGDGTs is soil washed in from the lake's catchment. Since then, microbial production within the water column has been identified as the primary source of brGDGTs in Lake Challa sediments, meaning that either an alternative mechanism links BIT index variation with rainfall or that the proxy's application must be reconsidered. We investigated GDGT concentrations and BIT index variation in Lake Challa sediments at a decadal resolution over the past 2200 years, in combination with GDGT time-series data from 45 monthly sediment-trap samples and a chronosequence of profundal surface sediments.Our 2200-year geochemical record reveals high-frequency variability in GDGT concentrations, and therefore in the BIT index, superimposed on distinct lower-frequency fluctuations at multi-decadal to century timescales. These changes in BIT index are correlated with changes in the concentration of crenarchaeol but not with those of the brGDGTs. A clue for understanding the indirect link between rainfall and crenarchaeol concentration (and thus thaumarchaeotal abundance) was provided by the observation that surface sediments collected in January 2010 show a distinct shift in GDGT composition relative to sediments collected in August 2007. This shift is associated with increased bulk flux of settling mineral particles with high Ti / Al ratios during March–April 2008, reflecting an event of unusually high detrital input to Lake Challa concurrent with intense precipitation at the onset of the principal rain season that year. Although brGDGT distributions in the settling material are initially unaffected, this soil-erosion event is succeeded by a massive dry-season diatom bloom in July–September 2008 and a concurrent increase in the flux of GDGT-0. Complete absence of crenarchaeol in settling particles during the austral summer following this bloom indicates that no Thaumarchaeota bloom developed at that time. We suggest that increased nutrient availability, derived from the eroded soil washed into the lake, caused the massive bloom of diatoms and that the higher concentrations of ammonium (formed from breakdown of this algal matter) resulted in a replacement of nitrifying Thaumarchaeota, which in typical years prosper during the austral summer, by nitrifying bacteria. The decomposing dead diatoms passing through the suboxic zone of the water column probably also formed a substrate for GDGT-0-producing archaea. Hence, through a cascade of events, intensive rainfall affects thaumarchaeotal abundance, resulting in high BIT index values.Decade-scale BIT index fluctuations in Lake Challa sediments exactly match the timing of three known episodes of prolonged regional drought within the past 250 years. Additionally, the principal trends of inferred rainfall variability over the past two millennia are consistent with the hydroclimatic history of equatorial East Africa, as has been documented from other (but less well dated) regional lake records. We therefore propose that variation in GDGT production originating from the episodic recurrence of strong soil-erosion events, when integrated over (multi-)decadal and longer timescales, generates a stable positive relationship between the sedimentary BIT index and monsoon rainfall at Lake Challa. Application of this paleoprecipitation proxy at other sites requires ascertaining the local processes which affect the productivity of crenarchaeol by Thaumarchaeota and brGDGTs.

Stable isotope composition of bentonites from the Swiss and Bavarian Freshwater Molasse as a proxy for paleoprecipitation

The hydrogen and oxygen isotope compositions of bentonites and smectite-rich tuffs from the North Alpine Foreland Basin were measured for samples dated between 9 and 21Ma. Assuming clay mineral formation shortly after the deposition of the volcanic ash layers and at surface temperatures estimated from paleoclimatic studies, the H and O isotope composition of water in equilibrium with the clay minerals can be calculated. A comparison with values derived from other proxies of paleoprecipitation in the Molasse Basin (such as pedogenic carbonates and mammal remains) and δ18O values of carbonate in the bentonite beds, suggests that the δ18O values of water estimated by this method are typical for foreland precipitation at this time and for this region. Therefore, the δ18O value of smectite in bentonites and altered tuff layers can be a useful tool for studies of paleoclimate and -topography. There are, however, indications that the clay minerals did not retain their initial hydrogen isotopic composition, but that a postformational exchange of H isotopes has occurred.

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin.

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval.

Plant- and micromammal-based paleoprecipitation proxies: Comparing results of the Coexistence and Climate-Diversity Approach

Both plant and vertebrate communities have been used to reconstruct paleoprecipitation. However, direct stratigraphic comparisons between the two types of proxies have hardly been performed, which is due to the fact that plant and vertebrate fossils usually do not occur together in single geological beds. Here, we focus on a series of 18 sites from the Neogene of Europe and Anatolia that contain both types of fossils, and compare paleoprecipitation predictions produced by the Coexistence Approach (plants) and the Climate-Diversity approach (micromammals).Most of the sites have overlapping uncertainty intervals for mean annual precipitation as generated by the two methods, pointing to a common precipitation-related signal in flora and fauna. Nevertheless, a systematic difference appears to characterize drier sites, for which micromammal richness-based estimates for annual precipitation tend to be lower than plant-based coexistence-based estimates. Driest-month precipitations are in agreement for drier sites, but strongly differ for wetter sites, with micromammal richness-based estimates being higher. Overlap is poor for estimates of wettest-month precipitation, which are higher when based on plant-based coexistence.Possible sources for a bias towards higher annual values in plants are related to the inclusion of local, wetland elements in the analysis and/or to a lower taxonomic resolution for pollen and spores compared to macroflora. Micromammal-based annual estimates could be biased towards drier values, when smaller samples lack rare species that could be preferentially wet-adapted. Several of the higher driest- and lower wettest-month micromammal-based estimates could result from over-extrapolation beyond the calibration domain for arboreality as currently existing in the modern Western Old World. On the other hand, the use of several SE Asian reference taxa could add a high-precipitation (monsoonal) component to the plant-based predictions. In addition, wettest-month estimates for both plants and micromammals probably suffer from the poor performance of water excess as a limiting factor.An attempt to further increase the predictive power of the micromammal-based prediction equations was performed by incorporating non-linear terms, and by using an extended calibration set including selected plant-based precipitation estimates for micromammal assemblages without modern analogue in the Western Old World (“proxy inter-calibration”).

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.

Southward Intertropical Convergence Zone Shifts and Implications for an Atmospheric Bipolar Seesaw

Abstract In this study, southward intertropical convergence zone (ITCZ) shifts are investigated in three different scenarios: Northern Hemispheric cooling, Southern Hemispheric warming, and a bipolar seesaw-like forcing that combines the latter two. The experiments demonstrate the mutual effects that northern- and southern-high-latitude forcings exert on tropical precipitation, suggesting a time-scale-dependent dominance of northern versus southern forcings. In accordance with this, two-phase tropical precipitation shifts are suggested, involving a fast component dominated by the high-northern-latitude forcing and a slower component due to the southern-high-latitude forcing. The results may thus be useful for the future understanding and interpretation of high-resolution tropical paleoprecipitation proxies and their relation to high-latitude records (e.g., ice core data). The experiments also show that Southern Ocean warming has a global impact, affecting both the tropics and northern extratropics, as seen in a southward ITCZ shift and mid- and high-latitude North Atlantic surface temperature and wind changes. In terms of dynamical considerations, the tropical circulation response to high-latitude forcing is found to be nonlinear: the atmospheric heat transport and Hadley cell anomalies differ significantly (in magnitude) when comparing the warming and cooling experiments. These are related to different interhemispheric temperature gradients that are altered mainly by nonlinearities in water vapor response. Decomposition of the top-of-the-atmosphere flux response into atmospheric feedback effects shows the dominance of water vapor and cloud feedbacks in the tropics, with the longwave cloud feedback effect governing the overall cloud response.

The evolution of Miocene climates in North China: Preliminary results of quantitative reconstructions from plant fossil records

The Miocene climate evolution in North China is preliminarily discussed by means of comparisons in seven climate parameters quantitatively reconstructed by the Coexistence Approach on 34 selected macro- and microfloras over North China. The Miocene temperatures show no great difference in the western and eastern part of North China. Temperature fluctuations, particularly in mean annual temperature, are found within floras from several sites. The fluctuation pattern, from a climate optimum in the Mid Miocene to cooling decline in the Late Miocene, is generally consistent with the global trend of Miocene temperature change. The reconstructed precipitation from all the sites studied shows much wetter conditions in North China during the Miocene than at present, which corroborates the results from paleoprecipitation proxy of fossil mammals. Like the situation in paleo-temperature, the Miocene precipitation from North China shows no distinct difference between the western and eastern regions. It is suggested that North China, particularly in the western part, was by no means under an arid or semi-arid environment during the Miocene. North China is an ideal region for study of the impact of the East Asian monsoon system, however, the pattern of precipitation change derived from the monsoon index (MSH) and mean annual precipitation (MAP) shows contradictory results. Therefore, there appears no definite conclusion on when the East Asian summer monsoon intensified. Possible reasons for inconsistency in temperature and precipitation changes are discussed. Directions of future work to improve the resolution of climate evolution are also pointed out.

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.

Distribution history and climatic controls of the Late Miocene Pikermian chronofauna

The Late Miocene development of faunas and environments in western Eurasia is well known, but the climatic and environmental processes that controlled its details are incompletely understood. Here we map the rise and fall of the classic Pikermian fossil mammal chronofauna between 12 and 4.2 Ma, using genus-level faunal similarity between localities. To directly relate land mammal community evolution to environmental change, we use the hypsodonty paleoprecipitation proxy and paleoclimate modeling. The geographic distribution of faunal similarity and paleoprecipitation in successive timeslices shows the development of the open biome that favored the evolution and spread of the open-habitat adapted large mammal lineages. In the climate model run, this corresponds to a decrease in precipitation over its core area south of the Paratethys Sea. The process began in the latest Middle Miocene and climaxed in the medial Late Miocene, about 7-8 million years ago. The geographic range of the Pikermian chronofauna contracted in the latest Miocene, a time of increasing summer drought and regional differentiation of habitats in Eastern Europe and Southwestern Asia. Its demise at the Miocene-Pliocene boundary coincides with an environmental reversal toward increased humidity and forestation, changes inevitably detrimental to open-adapted, wide-ranging large mammals.

Water release patterns of heated speleothem calcite and hydrogen isotope composition of fluid inclusions

Speleothem fluid inclusions are a potential paleo-precipitation proxy to reconstruct past rainwater isotopic composition ( δ 18O, δD). To get a better insight in the extraction of inclusion water from heated speleothem calcite, we monitored the water released from crushed and uncrushed speleothem calcite, heated to 900 °C at a rate of 300 °C/h, with a quadrupole mass spectrometer. Crushed calcite released water in three not well individualised peaks between 25 and 360 °C, 360 and 650 °C and between 650 and 800 °C while uncrushed calcite released water in two distinct temperature intervals: between 25 and 550 °C and between 550 and 900 °C. Water from two speleothems from the Han-sur-Lesse cave was recovered using three different techniques: i) the crushing and heating to 360 °C technique, ii) the decrepitation by heating to 550 °C and iii) the decomposition by heating to 900 °C technique. Measurements of the δD of water recovered by the decomposition of Han-sur-Lesse calcite heated to 900 °C did not show a 20 to 30‰ offset as found by previous authors. However a difference of 7‰ was observed between water released before and after decomposition of the calcite. Water recovery from the Han-sur-Lesse samples suggests that a simple heating technique (up to 550 °C) without crushing could both (a) recover water with δD representative of that of the drip water and (b) double the water yield as compared to the crushing and heating method. Our study warns for possible contamination of the recovered inclusion water with hydration water of lime, responsible for the recovery of water with very negative δD values.