Assessing pedogenic calcite stable-isotope values: Can positive linear covariant trends be used to quantify palaeo-evaporation rates?

Palaeoclimate models suggest enhanced evaporation rates in subtropical regions (15–30° latitude) during greenhouse-world conditions, however, there are no empirical data to support these estimates. Laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ18Ocalciteversus δ13Ccalcite values. The goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo-evaporation rates from palaeosol calcrete PLCTs. Samples of powdered CaCO3 were dissolved in de-ionized water in pressure sealed containers with a range of elevated atmospheric pCO2 concentrations for 24 h. The solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted, and aliquots removed for analysis every 24 h. The precipitated calcite produced an array of δ18Ocalciteversus δ13Ccalcite values that fall upon a PLCT projected from a theoretical meteoric calcite line (MCL). Water analyses yielded δ18Owater enrichments from an initial value of − 4.75‰ VSMOW ranging up to between + 10.0‰ and + 14.8‰. The experimental results show that solutions formed under higher pCO2 conditions precipitated calcite very early on during evaporation, and thus have δ18Ocalcite and δ13Ccalcite values that are slightly enriched relative to the MCL. The solutions that formed under low pCO2 conditions precipitated calcite after much of the fluid had evaporated, and thus yield more enriched δ18Ocalcite and δ13Ccalcite values. Repeat trials with varying temperature and relative humidity show that the PLCT is steeper under both higher temperature and low relative humidity. The wide range of pCO2, temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution/precipitation reactions. The results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to CaCO3, while the primary factors affecting the magnitude and steepness of the PLCT are vadose zone temperature, relative humidity, the starting δ18Owater value and saturation state of the fluid with respect to CaCO3. Since the pCO2 of the rooting zone is cross-controlled by the local soil and surface ecology, the magnitude of the PLCT enrichment may not be a direct proxy for palaeo-evaporation rates

Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model

Stable isotope mass-balance modeling results of meteoric δ18O values from the Cenomanian Stage of the Cretaceous Western Interior Basin (KWIB) suggest that precipitation and evaporation fluxes were greater than that of the present and significantly different from simulations of Albian KWIB paleohydrology. Sphaerosiderite meteoric δ18O values have been compiled from the Lower Tuscaloosa Formation of southwestern Mississippi (25°N paleolatitude), The Dakota Formation Rose Creek Pit, Fairbury Nebraska (35°N) and the Dunvegan Formation of eastern British Columbia (55°N paleolatitude). These paleosol siderite δ18O values define a paleolatitudinal gradient ranging from − 4.2‰ VPDB at 25°N to − 12.5‰ VPDB at 55°N. This trend is significantly steeper and more depleted than a modern theoretical siderite gradient (25°N: − 1.7‰; 65°N: − 5.6‰ VPDB ), and a Holocene meteoric calcite trend (27°N: − 3.6‰; 67°N: − 7.4‰ VPDB). The Cenomanian gradient is also comparatively steeper than the Albian trend determined for the KWIB in the mid- to high latitudes. The steep latitudinal trend in meteoric δ18O values may be the result of increased precipitation and evaporation fluxes (amount effects) under a more vigorous greenhouse-world hydrologic cycle. A stable-isotope mass-balance model has been used to generate estimates of precipitation and evaporation fluxes and precipitation rates. Estimates of Cenomanian precipitation rates based upon the mass-balance modeling of the KWIB range from 1400 mm/yr at 25°N paleolatitude to 3600 mm/yr at 45°N paleolatitude. The precipitation–evaporation (P–E) flux values were used to delineate zones of moisture surplus and moisture deficit. Comparisons between Cenomanian P–E and modern theoretical siderite, and Holocene calcite latitudinal trends shows an amplification of low-latitude moisture deficits between 5–25°N paleolatitude and moisture surpluses between 40–60°N paleolatitude. The low-latitude moisture deficits correlate with a mean annual average heat loss of 48 W/m2 at 10°N paleolatitude (present, 8 W/m2 at 15°N). The increased precipitation flux and moisture surplus in the mid-latitudes corresponds to a mean average annual heat gain of 180 W/m2 at 50°N paleolatitude (present, 17 W/m2 at 50°N). The Cenomanian low-latitude moisture deficit is similar to that of the Albian, however the mid-latitude (40–60°N) precipitation flux values and precipitation rates are significantly higher (Albian: 2200 mm/yr at 45°N; Cenomanian: 3600 mm/yr at 45°N). Furthermore, the heat transferred to the atmosphere via latent heat of condensation was approximately 10.6× that of the present at 50°N. The intensified hydrologic cycle of the mid-Cretaceous greenhouse warming may have played a significant role in the poleward transfer of heat and more equable global conditions. Paleoclimatological reconstructions from multiple time periods during the mid-Cretaceous will aid in a better understanding of the dynamics of the hydrologic cycle and latent heat flux during greenhouse world conditions

Evidence for Increased Latent Heat Transport During the Cretaceous (Albian) Greenhouse Warming

Quantitative estimates of increased heat transfer by atmospheric H2O vapor during the Albian greenhouse warming suggest that the intensified hydrologic cycle played a greater role in warming high latitudes than at present and thus represents a viable alternative to oceanic heat transport. Sphaerosiderite δ18O values in paleosols of the North American Cretaceous Western Interior Basin are a proxy for meteoric δ18O values, and mass- balance modeling results suggest that Albian precipitation rates exceeded modern rates at both mid and high latitudes. Comparison of modeled Albian and modern precipitation minus evaporation values suggests amplification of the Albian moisture deficit in the tropics and moisture surplus in the mid to high latitudes. The tropical moisture deficit represents an average heat loss of ∼75 W/m2 at 10°N paleolatitude (at present, 21 W/m2). The increased precipitation at higher latitudes implies an average heat gain of ∼83 W/ m2 at 45°N (at present, 23 W/m2) and of 19 W/m2 at 75°N (at present, 4 W/m2). These estimates of increased poleward heat transfer by H2O vapor during the Albian may help to explain the reduced equator-to-pole temperature gradients

Diagenetic Overprinting of the Sphaerosiderite Palaeoclimate Proxy: Are Records of Pedogenic Groundwater δ\u3csup\u3e18\u3c/sup\u3eO Values Preserved?

Meteoric sphaerosiderite lines (MSLs), defined by invariant δ18O and variable δ13C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre-scale FeCO3 nodules), and are a stable isotope proxy record of terrestrial meteoric isotopic compositions. The palaeoclimatic utility of sphaerosiderite has been well tested; however, diagenetically altered horizons that do not yield simple MSLs have been encountered. Well-preserved sphaerosiderites typically exhibit smooth exteriors, spherulitic crystalline microstructures and relatively pure (\u3e 95 mol% FeCO3) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca2+, Mg2+ and Mn2+ for Fe2+. Examples of diagenetically altered Cretaceous sphaerosiderite-bearing palaeosols from the Dakota Formation (Kansas), the Swan River Formation (Saskatchewan) and the Success S2 Formation (Saskatchewan) were examined in this study to determine the extent to which original, early diagenetic δ18O and δ13C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ18O and δ13C values from the co-occurring sphaerosiderites. The complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses. The Dakota and Swan River samples ultimately yield distinct MSLs for the sphaerosiderites, and MCLs (meteoric calcite lines) for the calcite cements. The Success S2 sample yields a covariant δ18O vs. δ13C trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters. The calcite cements in the Success S2 Formation yield meteoric δ18O and δ13C values. A stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end-member compositions. Modelled hyperbolic fluid mixing curves for the Success S2 Formation suggest precipitation from fluids that were \u3c 25% sea water

Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes: Dakota Formation, Western Interior Basin, USA

Analysis of bulk sedimentary organic matter and charcoal from an Albian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of 3‰ in late Albian strata. Overlying Cenomanian strata have δ13C values of −24‰ to −23‰ that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Albian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive δ13C event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D2), as well as a major regressive phase throughout the globe at the Albian-Cenomanian boundary. Data from RCP confirm suggestions that some positive carbon-isotope excursions in the geologic record are coincident with regressive sea-level phases. We estimate using isotopic correlation that the D2 sequence boundary at RCP was on the order of 0.5 m.y. in duration. Therefore, interpretations of isotopic events and associated environmental phenomena, such as oceanic anoxic events, in the shallow-marine and terrestrial record may be influenced by stratigraphic incompleteness. Further investigation of terrestrial δ13C records may be useful in recognizing and constraining sea-level changes in the geologic record