Greg A. Ludvigson

Meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology

Sphaerosiderite, a morphologically distinct millimeter-scale spherulitic siderite (FeCO 3 ), forms predominantly in wetland soils and sediments, and is common in the geologic record. Ancient sphaerosiderites are found in paleosol horizons within coal-bearing stratigraphic intervals and, like their modern counterparts, are interpreted as having formed in water-saturated environments. Here we report on sphaerosiderites from four different stratigraphic units, each of which has highly variable 13 C and relatively stable 18 O compositions. The unique isotopic trends are analogous to well-documented meteoric calcite lines, which we define here as meteoric sphaerosiderite lines. Meteoric sphaerosiderite lines provide a new means of constraining ground-water δ 18 O and thus allow evaluation of paleohydrology and paleoclimate in humid

Middle Cretaceous greenhouse hydrologic cycle of North America

We present a paleolatitudinal precipitation reconstruction for the greenhouse setting of mid-latitude North America based on the oxygen isotopic composition of sphaerosiderites found in middle Cretaceous wetland paleosols. Our reconstructed middle Cretaceous d 18 O values of precipitation are ;4‰ less than values from comparable modern low-elevation coastal settings free of monsoons. The data fit a conceptual model in which the precipitation source for the eastern margin of the Cretaceous Western Interior Seaway of North America is an 18 O-enriched oceanic coastal jet. In this subtropical-tropical setting, midCretaceous precipitation rates are interpreted to range from ;2500 to ;4100 mm/yr.

The mid-cretaceous water bearer: Isotope mass balance quantification of the Albian hydrologic cycle

A latitudinal gradient in meteoric N 18 O compositions compiled from paleosol sphaerosiderites throughout the Cretaceous Western Interior Basin (KWIB) (34^75‡N paleolatitude) exhibits a steeper, more depleted trend than modern (predicted) values (3.0x [34‡N latitude] to 9.7x [75‡N] lighter). Furthermore, the sphaerosiderite meteoric N 18 O latitudinal gradient is significantly steeper and more depleted (5.8x [34‡N] to 13.8x [75‡N] lighter) than a predicted gradient for the warm mid-Cretaceous using modern empirical temperature^N 18 O precipitation relationships. We have suggested that the steeper and more depleted (relative to the modern theoretical gradient) meteoric sphaerosiderite N 18 O latitudinal gradient resulted from increased air mass rainout effects in coastal areas of the KWIB during the mid-Cretaceous. The sphaerosiderite isotopic data have been used to constrain a mass balance model of the hydrologic cycle in the northern hemisphere and to quantify precipitation rates of the equable ‘greenhouse’ Albian Stage in the KWIB. The mass balance model tracks the evolving isotopic composition of an air mass and its precipitation, and is driven by latitudinal temperature gradients. Our simulations indicate that significant increases in Albian precipitation (34^52%) and evaporation fluxes (76^96%) are required to reproduce the difference between modern and Albian meteoric siderite N 18 O latitudinal gradients. Calculations of precipitation rates from model outputs suggest mid^high latitude precipitation rates greatly exceeded modern rates (156^220% greater in mid latitudes [2600^3300 mm/yr], 99% greater at high latitudes [550 mm/yr]). The calculated precipitation rates are significantly different from the precipitation rates predicted by some recent general circulation models (GCMs) for the warm Cretaceous, particularly in the mid to high latitudes. Our mass balance model by no means replaces GCMs. However, it is a simple and effective means of obtaining quantitative data regarding the mid-Cretaceous hydrologic cycle in the KWIB. Our goal is to encourage the incorporation of isotopic tracers into GCM simulations of the midCretaceous, and to show how our empirical data and mass balance model estimates help constrain the boundary 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 d 18 O values in paleosols of the North American Cretaceous Western Interior Basin are a proxy for meteoric d 18 O values, and massbalance 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/m 2 at 108N paleolatitude (at present, 21 W/m 2 ). The increased precipitation at higher latitudes implies an average heat gain of ;83 W/ m 2 at 458N (at present, 23 W/m 2 ) and of 19 W/m 2 at 758N (at present, 4 W/m 2 ). These estimates of increased poleward heat transfer by H2O vapor during the Albian may help to explain the reduced equator-to-pole temperature gradients.

Late Albian Kiowa-Skull Creek Marine Transgression, Lower Dakota Formation, Eastern Margin of Western Interior Seaway, U.S.A.

ABSTRACT An integrated geochemical-sedimentological project is studying the paleoclimatic and paleogeographic characteristics of the mid-Cretaceous greenhouse world of western North America. A critical part of this project, required to establish a temporal framework, is a stratigraphic study of depositional relationships between the Albian-Cenomanian Dakota and the Upper Albian Kiowa formations of the eastern margin of the Western Interior Seaway (WIS). Palynostratigraphic and sedimentologic analyses provide criteria for the Dakota Formation to be divided into three sedimentary sequences bounded by unconformities (D0, D1, and D2) that are recognized from western Iowa to westernmost Kansas. The lowest of these sequences, defined by unconformities D0 and D1, is entirely Upper Albian, and includes the largely nonmarine basal Dakota (lower part of the Nishnabotna Member) strata in western Iowa and eastern Nebraska and the marine Kiowa Formation to the southwest in Kansas. The gravel-rich fluvial deposits of the basal part of the Nishnabotna Member of the Dakota Formation correlate with transgressive marine shales of the Kiowa Formation. This is a critical relationship to establish because of the need to correlate between marine and nonmarine strata that contain both geochronologic and paleoclimatic proxy data. The basal gravel facies (up to 40 m thick in western Iowa) aggraded in incised valleys during the Late Albian Kiowa-Skull Creek marine transgression. In southeastern Nebraska, basal gravels intertongue with carbonaceous mudrocks that contain diverse assemblages of Late Albian palynomorphs, including marine dinoflagellates and acritarchs. This palynomorph assemblage is characterized by occurrences of palynomorph taxa not known to range above the Albian Kiowa-Skull Creek depositional cycle elsewhere in the Western Interior, and correlates to the lowest of four generalized palynostratographic units that are comparable to other palynological sequences elsewhere in North America. Tidal rhythmites in mudrocks at the Ash Grove Cement Quarry in Louisville (Cass County), Nebraska record well-developed diurnal and semimonthly tidal cycles, and moderately well developed semiannual cycles. These tidal rhythmites are interpreted to have accumulated during rising sea level at the head of a paleoestuary that experienced at least occasional mesotidal conditions. This scenario places the gravel-bearing lower part of the Nishnabotna Member of the Dakota Formation in the mouth of an incised valley of an Upper Albian transgressive systems tract deposited along a tidally influenced coast. Furthermore, it provides a depositional setting consistent with the biostratigraphic correlation of the lower part of the Nishnabotna Member of the Dakota Formation to the marine Kiowa Formation of Kansas.

Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley

The Decorah Formation of eastern Iowa was deposited on an open-marine subtidal shelf near storm wave base. Maximum transgression is represented in organicrich strata of the Guttenberg Member. Brachiopod-derived marine carbonate isotopic compositions from the Decorah are approximately δ13C = +1‰ and δ18O = –3.5‰; these data are consistent with a recently recognized long-term secular increase in the δ18O of Middle Ordovician marine carbonates. Decorah carbonates lithified in early diagenetic modified-marine phreatic environments. Isotopic data from diagenetic components show that the Spechts Ferry Member lithified in a more fluid-dominated diagenetic system than the immediately overlying Guttenberg Member, which was characterized by a rock-dominated system. Whole-rock carbonate δ13C shifts in the Decorah Formation are carried by micritic components. A positive shift in the Guttenberg to micrites with δ13C values up to +2.5‰ indicates that some of the micrite isotopic signal is primary and not of benthic origin and/or that early diagenetic marine phreatic fluids in the organic-rich Guttenberg were affected by bacterial methanogenesis. Subtidal shallowing-upward depositional cycles in the Decorah are internally characterized by micrites with stratigraphically upward trends toward 13C depletion as a consequence of increasing diagenetic water/rock ratios. Decoupling between coeval carbon isotopic signals carried by open-marine brachiopod carbonate and those of organic carbon and micrite indicates that the positive carbon isotopic excursion in the Guttenberg resulted from an episode of increased photosynthetic productivity near the sea surface. This event was a consequence of quasiestuarine circulation associated with marine transgression during deposition of the Guttenberg Member. Results from this study suggest that the extinct organic-walled microfossil Gloeocapsomorpha prisca, the principal source of organic carbon in the Decorah Formation, was a phytoplanktic organism. Ludvigson, G. A., Jacobson, S. R., Witzke, B. J., and Gonzalez, L. A., Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley, in Witzke, B. J., Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy: Views from the North American Craton: Boulder, Colorado, Geological Society of America Special Paper 306. 67 SP306-06.QXD 6/27/96 12:39 AM Page 67

High latitude meteoric δ18O compositions: Paleosol siderite in the Middle Cretaceous Nanushuk Formation, North Slope, Alaska

Siderite-bearing pedogenic horizons of the Nanushuk Formation of the North Slope, Alaska, provide a critical high paleolatitude oxygen isotopic proxy record of paleoprecipitation, supplying important empirical data needed for paleoclimatic reconstructions and models of “greenhouse- world” precipitation rates. Siderite δ 18 O values were determined from four paleosol horizons in the National Petroleum Reserve Alaska (NPR-A) Grandstand # 1 Core, and the values range between −17.6‰ and −14.3‰ Peedee belemnite (PDB) with standard deviations generally less than 0.6‰ within individual horizons. The δ 13 C values are much more variable, ranging from −4.6‰ to +10.8‰ PDB. A covariant δ 18 O versus δ 13 C trend in one horizon probably resulted from mixing between modified marine and meteoric phreatic fluids during siderite precipitation. Groundwater values calculated from siderite oxygen isotopic values and paleobotanical temperature estimates range from −23.0‰ to −19.5‰ standard mean ocean water (SMOW). Minor element analyses show that the siderites are impure, having enrichments in Ca, Mg, Mn, and Sr. Minor element substitutions and Mg/Fe and Mg/(Ca + Mg) ratios also suggest the influence of marine fluids upon siderite precipitation. The pedogenic horizons are characterized by gleyed colors, rare root traces, abundant siderite, abundant organic matter, rare clay and silty clay coatings and infillings, some preservation of primary sedimentary stratification, and a lack of ferruginous oxides and mottles. The pedogenic features suggest that these were poorly drained, reducing, hydromorphic soils that developed in coal-bearing delta plain facies and are similar to modern Inceptisols. Model-derived estimates of precipitation rates for the Late Albian of the North Slope, Alaska (485–626 mm/yr), are consistent with precipitation rates necessary to maintain modern peat-forming environments. This information reinforces the mutual consistency between empirical paleotemperature estimates and isotope mass balance models of the hydrologic cycle and can be used in future global circulation modeling (GCM) experiments of “greenhouse- world” climates to constrain high latitude precipitation rates in simulations of ancient worlds with decreased equator-to-pole temperature gradients.

Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence

We present the results of oxygen isotope and electron- microprobe analyses of sphaerosiderites obtained from Cretaceous paleosols in Iowa. The sphaerosiderite d 18 O values record Creta- ceous meteoric groundwater chemistry and an overall waning of brackish groundwater inundation during alluvial-plain aggrada- tion and soil genesis. We focus on horizons that precipitated from freshwater, in which d 18 O values ranging from 23.3‰ to 26.8‰ relative to the Peedee belemnite standard are interpreted to record variations in the Cretaceous atmospheric hydrologic cycle. During relative sea-level highstands, moisture was derived from the Cre- taceous Western Interior Seaway, whereas during lowstands, when the seaway narrowed and occasionally withdrew from the Midcon- tinent, the dominance of hemispheric-scale atmospheric moisture transport initiated in the tropical Tethys Ocean led to decreased precipitation rates. These processes did not operate like a switch, but rather as a continuum of competing moisture sources and mechanisms of transport between the nearby epicontinental sea and the distant tropics. The sphaerosiderite data demonstrate (1) temporal variation in the intensity of hemispheric-scale atmospher- ic moisture transport and (2) long-term amplification of the global hydrologic cycle marked by extreme 18 O depletion at the Albian- Cenomanian boundary.

Depositional environments and sequence stratigraphy of Upper Ordovician epicontinental deep water deposits, eastern Iowa and southern Minnesota

This study examines depositional environments, stratigraphy, paleontology, and petrology of the Upper Ordovician upper Dubuque Formation and Elgin Member (basal Maquoketa Formation) in eastern Iowa and southern Minnesota within a sequence stratigraphic framework. Dubuque-Elgin deposition occurred within a subcycle of the Maquoketa depositional cycle and records a single transgressive-regressive event; the Dubuque Formation and lowermost Elgin Member deposits are transgressive, lower Elgin deposits are highstand, and middle and upper Elgin deposits are regressive. The transgressive Dubuque Formation grades from an open marine benthic environment in its lower portions, to slightly oxygen stressed environments in its upper portions, with an associated loss of calcareous algae and tempestites, and increase in trilobite grain frequency and mud matrix. The Dubuque-Elgin contact is marked at all but the northernmost localities by a regional condensed section consisting of transgressive phosphatic hardgrounds and overlying highstand dark brown pelagic shale. The nature of overlying regressive Elgin Member deposits is dependent upon geographic location, with thick carbonates (~25 m [~82 ft]) present in the north and north-central areas, mixed carbonates and shales in the south-central area (~15 m [~48 ft]), and thin shales in the southern area (~8 m [~25.6 ft]). Depositionally, this wedge represents shelf (thick carbonates), shelf-slope boundary (mixed carbonates and shales), and slope-basin (shales) environments. The epicontinental Maquoketa seaway is interpreted to have had significant maximum depths (>200 m [650 ft]), and to have contained a density-stratified water mass. Upwelling and associated phosphate deposition resulted from a gyre circulation pattern driven by Taconic fresh-water runoff and surface winds, with net surface currents in the study area flowing basinward, replaced by deep upflowing waters. Depths are estimated from regional facies associations, depth-dependent nautiloid septal implosion measurements, and whole-rock carbonate δ 13 C isotopic trends indicating increasing burial of organic carbon.

Diagenetic model of carbonate rocks of Guri Member of Mishan Formation (Lower to Middle Miocene) SE Zagros basin, Iran

In order to understand the post-depositional history of carbonate rocks of Guri Member (Lower to Middle Miocene), three stratigraphic sections were selected in north Bandar-Abbas in southeast of Iran. Sampling was carried out, analyzed for selective parameters such as oxygen and carbon isotopic compositions (δ18O and δ13C) and interpreted in the present study. We recognized several diagenetic processes including micritization, cementation, neomorphism, compaction, dissolution, silicification, dolomitization, fracturing and vein filling. Some of the diagenetic processes occurred at different conditions, so in order to achieve precise interpretation, samples from different carbonate components such as, micrite, fracture cement, solution pore cement, intergranular cement, and some biotic allochems were analyzed. In this study micrite samples were subdivided into two groups including micro-spary and micrite. They were recognized under Cathodoluminescence microscope. In addition, micrite samples were classified into five groups based on their depositional environments: supratidal, lagoon, coral bar, open sea, and open basin. There were minor changes in stable isotope ratios based on the sedimentary environments, stratigraphy successions, and micro-spary or micrite properties. In this study, similar calcite cements in petrography studies were differentiated by stable isotope data. Those calcite cements have formed in different diagenetic environments such as meteoric and burial cements. Paragenetic sequence of carbonate rocks were interpreted by integration of petrographic and isotopic studies. We have reconstructed diagenetic models of Guri Member into four stages including marine, meteoric, burial, and uplifting.