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Coastal response to global warming during the Paleocene-Eocene Thermal Maximum
... Samples were then rinsed to neutrality with deionized water, dried, and crushed (Suarez et al., 2013). Because the core samples are from a petroleum field, migrated hydrocarbon was removed via solvent extraction before isotopic analysis (e.g., Sharman et al., 2023;Vimpere et al., 2023). Samples were prepared for hydrocarbon extraction by transferring the dried and crushed decarbonated sample to MarsXpress vessels where 10 mL of hexane (C 6 H 14 ) and 10 mL of acetone (C 3 H 60 ) were added directly to the sample in the vessel (Hasty and Revesz, 1995). ...
The Paleocene–Eocene Thermal Maximum (PETM) represents the most pronounced hyperthermal of the Cenozoic era and is hypothesized to have resulted in an intensification of the paleohydrologic cycle, including enhanced seasonality and increased sediment discharge to the coastal ocean. Although the PETM has been widely documented, there are few records from deposits that form the distal, deepwater components of large sediment-routing systems. This study presents new constraints on the stratigraphic placement of the PETM in the deepwater Gulf of Mexico basin through analysis of geochemical, carbon isotopic, and biostratigraphic data within a ∼124 m cored interval of the Wilcox Group. Biostratigraphic and carbon isotopic data indicate that the PETM extends over ∼13 m based on acmes in the dinoflagellate Apectodinium homomorphum and calcareous nannoplankton Rhomboaster cuspis as well as a ∼-2 ‰ shift in bulk organic δ13C values. A decrease in bioturbation and benthic foraminifera suggests that a reduction in oxygen of Gulf of Mexico bottom waters and/or an increase in sedimentation rates were coincident with the onset of the PETM. A ∼2 m lag in the depositional record separates the onset of the PETM negative carbon isotope excursion (CIE) and deposition of a 5.7 m thick interval of organic-lean claystone and marlstone that reflects a shut-off of the supply of sand, silt, and terrestrial palynomorphs to the basin. We interpret deposits of the PETM in the deepwater Gulf of Mexico to reflect the combined effects of increased erosional denudation and rising sea level that resulted in sequestration of sand and silt near the coastline but that allowed delivery of terrigenous mud to the deep sea. The similarity of oceanographic changes observed in the Gulf of Mexico and Atlantic Ocean during the PETM supports the inference that these water masses were connected during latest Paleocene–earliest Eocene times. Although deposition of typical Wilcox Group facies resumed during and after the PETM recovery, an increased influx of terrestrial detritus (i.e., pollen, spores, terrestrial organic debris) relative to marine dinoflagellates is suggestive of long-lasting effects of the PETM. This study illustrates the profound and prolonged effects of climatic warming on even the most distal reaches of large (≥1×106 km2) sediment-routing systems.
In this study, we present evidence of a Paleocene–Eocene Thermal Maximum (PETM) record within a 543-m-thick (1780 ft) deep-marine section in the Gulf of Mexico (GoM) using organic carbon stable isotopes and biostratigraphic constraints. We suggest that climate and tectonic perturbations in the upstream North American catchments can induce a substantial response in the downstream sectors of the Gulf Coastal Plain and ultimately in the GoM. This relationship is illustrated in the deep-water basin by (1) a high accommodation and deposition of a shale interval when coarse-grained terrigenous material was trapped upstream at the onset of the PETM, and (2) a considerable increase in sediment supply during the PETM, which is archived as a particularly thick sedimentary section in the deep-sea fans of the GoM basin. Despite other thick PETM sections being observed elsewhere in the world, the one described in this study links with a continental-scale paleo-drainage, which makes it of particular interest for paleoclimate and source-to-sink reconstructions.
The Paleocene‐Eocene Thermal Maximum (PETM) is the most pronounced global warming event of the early Paleogene related to atmospheric CO2 increases. It is characterized by negative δ¹⁸O and δ¹³C excursions recorded in sedimentary archives and a transient disruption of the marine biosphere. Sites from the U.S. Atlantic Coastal Plain show an additional small, but distinct δ¹³C excursion below the onset of the PETM, coined the “pre‐onset excursion” (POE), mimicking the PETM‐forced environmental perturbations. This study focuses on the South Dover Bridge core in Maryland, where the Paleocene‐Eocene transition is stratigraphically constrained by calcareous nannoplankton and stable isotope data, and in which the POE is well‐expressed. The site was situated in a middle neritic marine shelf setting near a major outflow of the paleo‐Potomac River system. We generated high‐resolution benthic foraminiferal assemblage, stable isotope, trace‐metal, grain‐size and clay mineralogy data. The resulting stratigraphic subdivision of this Paleocene‐Eocene transition is placed within a depth transect across the paleoshelf, highlighting that the PETM sequence is relatively expanded. The geochemical records provide detailed insights into the paleoenvironment, developing from a well‐oxygenated water column in latest Paleocene to a PETM‐ecosystem under severe biotic stress‐conditions, with shifts in food supply and temperature, and under dysoxic bottom waters in a more river‐dominated setting. Environmental changes started in the latest Paleocene and culminated atthe onset of the PETM, hinting to an intensifying trigger rather than to an instantaneous event at the Paleocene‐Eocene boundary toppling the global system.
The Paleogene records the most prominent global climate change of the Cenozoic Era with a shift from a greenhouse to an icehouse world. Several transient hyperthermal events punctuated this long-term evolution. The most pronounced and the best known of these is the Paleocene-Eocene Thermal Maximum (PETM-56 Ma). This event is associated with global warming, a worldwide perturbation of the carbon cycle, and significant biotic changes. The PETM is primarily recorded by a sharp negative carbon isotope excursion (NCIE) in both carbonates and organic matter of sedimentary successions. The source of the 13C-depleted carbon for the NCIE and whether it was released in one or numerous events is still debated. Several carbon sources have been proposed to explain the PETM-NCIE and the mechanisms that triggered this abrupt climate upheaval. These include, among others, the magmatic and thermogenic release of carbon associated with the emplacement of Large Igneous Provinces (LIP). One proxy for tracking past volcanic emissions in the geological record and testing hypothetical links between volcanism and hyperthermals is the use of mercury (Hg) anomalies found in marine and continental sedimentary successions. Here, we present new high-resolution mercury and stable isotopic records from a continental-marine transect in Pyrenean peripheral basins during the PETM. Compared to deeper marine settings, the significant sedimentation rate that characterizes these high-accommodation and high sediment-supply environments allows the preservation of expanded successions, providing reliable information about the fluctuations of Hg concentration in deposits across the PETM. Our data reveal two large negative carbon excursions across the studied successions. Based on biostratigraphy and the similarity of shape and amplitude of the isotopic excursions with global records, we interpret the largest NCIE as the PETM. This main excursion is preceded by another that we interpret as the Pre-Onset Excursion (POE), found in other profiles worldwide. We find that the POE and the PETM are, in our studied sections, systematically associated with significant Hg anomalies regardless of the depositional environment. These results suggest that large pulses of volcanism, possibly related to the North Atlantic Igneous Province's emplacement, contributed to the onset and possibly also to the long duration of the PETM. Furthermore, the record of higher Hg anomalies in nearshore than offshore settings suggests a massive collapse of terrestrial ecosystems linked to volcanism-driven environmental change triggered significant Hg loading in shallow marine ecosystems. If this is correct, these findings confirm the primary role of the solid Earth in determining past terrestrial climates.
The Paleocene-Eocene Thermal Maximum (PETM) is recognized by a major negative carbon isotope (δ13C) excursion (CIE) signifying an injection of isotopically light carbon into exogenic reservoirs, the mass, source, and tempo of which continue to be debated. Evidence of a transient precursor carbon release(s) has been identified in a few localities, although it remains equivocal whether there is a global signal. Here, we present foraminiferal δ13C records from a marine continental margin section, which reveal a 1.0 to 1.5‰ negative pre-onset excursion (POE), and concomitant rise in sea surface temperature of at least 2°C and a decline in ocean pH. The recovery of both δ13C and pH before the CIE onset and apparent absence of a POE in deep-sea records suggests a rapid (< ocean mixing time scales) carbon release, followed by recovery driven by deep-sea mixing. Carbon released during the POE is therefore likely more similar to ongoing anthropogenic emissions in mass and rate than the main CIE.
Deposition of the Paleocene-Eocene Wilcox Group (Gp) in the northern Gulf of Mexico (GoM) occurred during dramatic global climate and regional tectonic change. Key drivers impacting the GoM over this time were 1) significantly enhanced runoff and sediment supply from the developing Laramide hinterland, 2) intense global warming culminating in the Paleocene–Eocene Thermal Maximum (PETM) at ∼56 Ma, and 3) proposed Paleogene restriction of the GoM as Cuba docked with Yucatan-Bahamas-Florida. In this paper, we investigate biological and organic geochemical responses to these drivers across the PETM and identify spatial variations in productivity, oxygenation, and ultimately, ventilation. The PETM has been identified by calcareous nannofossil marker taxa and/or excursion taxa from the genera Rhomboaster and Discoaster that exclusively existed during the event. A calcareous nannofossil transition from cool eutrophic to warm oligotrophic assemblages occurs across the PETM broadly over the GoM suggesting that surface water nutrient supply decreased due to slowing circulation or increased sequestration of nutrients along the margins. Associated radiolarian bursts indicate additional factors, such as eustacy and runoff, may have modulated nutrient supply. Benthic habitats were impacted as agglutinated foraminifera, which were dominant in the Paleocene, disappeared at the PETM over submarine fan environments while calcareous benthic forams remained largely absent until the late Eocene. Deoxygenation may have contributed to benthic ecosystem decline as marine kerogen enrichment is noted broadly in the middle Wilcox and PETM. Total organic carbon is generally <3% except in more isolated marginal settings suggesting that deep-basin dysoxia co-existed with a variably enhanced oxygen minimum. We suggest these biotic and organic geochemical responses reflect connection to the Global Ocean during the PETM, with ventilation reduced but not eliminated by increases in salinity stratification due to runoff and reductions in deeper water entry into the GoM at gateways.
The Paleocene-Eocene Thermal Maximum (PETM) represents the most pronounced hyperthermal of the Cenozoic era and is hypothesized to have resulted in an intensification of the paleo-hydrologic cycle, including enhanced seasonality and increased sediment discharge. Although the sedimentologic effects of the PETM are well documented in proximal, continental environments, the effects of this event on the distal, deep-water portions of large sediment routing systems are less well understood. This research uses a suite of geochemical and Carbon-isotope analytical techniques to characterize core plugs from a ~120-meter interval of deep-water, siliciclastic turbidite strata that span the Paleocene-Eocene boundary in the Wilcox Group, Gulf of Mexico Basin. Located within the Green Canyon protraction area, the deep-water study area represents the distal fringe of a Paleogene continental-scale sediment routing system that drained ~1-3x10 6 km 2 of central North America, including well-studied PETM localities within Laramide continental foreland basins (e.g., the Bighorn Basin of Wyoming). Highlighted by the presence of an acme in the dinoflagellate Apectodinium homomorphum, high-confidence palynological data from this well suggest that the PETM coincides with deposition of a ~5-meter-thick shale within the Wilcox 1B interval. Preliminary portable X-ray fluorescence (XRF) data collected from 252 core samples reveal lithologic, diagenetic, and possibly climatic influences on major and trace elemental geochemistry. Carbon-isotope chemostratigraphy (bulk organic matter and palynomorph carbonate) is being conducted to identify the global PETM negative carbon isotope excursion (NCIE), if present, and characterize paleo-climatic signatures within adjacent stratal intervals. Once integrated with data from inboard sample locations, the results of this study will improve our understanding of how climatic environmental signals may be propagated from source-to-sink in large sediment routing systems.
Sediment archives in the terrestrial and marine realm are regularly analyzed to infer changes in climate, tectonic, or anthropogenic boundary conditions of the past. However, contradictory observations have been made regarding whether short period events are faithfully preserved in stratigraphic archives; for instance, in marine sediments offshore large river systems. On the one hand, short period events are hypothesized to be non-detectable in the signature of terrestrially derived sediments due to buffering during sediment transport along large river systems. On the other hand, several studies have detected signals of short period events in marine records offshore large river systems. We propose that this apparent discrepancy is related to the lack of a differentiation between different types of signals and the lack of distinction between river response times and signal propagation times. In this review, we (1) expand the definition of the term ‘signal’ and group signals in sub-categories related to hydraulic grain size characteristics, (2) clarify the different types of ‘times’ and suggest a precise and consistent terminology for future use, and (3) compile and discuss factors influencing the times of signal transfer along sediment routing systems and how those times vary with hydraulic grain size characteristics. Unraveling different types of signals and distinctive time periods related to signal propagation addresses the discrepancies mentioned above and allows a more comprehensive exploration of event preservation in stratigraphy – a prerequisite for reliable environmental reconstructions from terrestrially derived sedimentary records.
In a recent review published in this journal, Coutts et al. (2019, Geoscience Frontiers, 10, 4, 1421–1435) compared nine different ways to estimate the maximum depositional age (MDA) of siliclastic rocks by means of detrital geochronology. Their results show that among these methods three are positively and six negatively biased. This paper investigates the cause of these biases and proposes a solution to it. A simple toy example shows that it is theoretically impossible for the reviewed methods to find the correct depositional age in even a best case scenario: the MDA estimates drift to ever smaller values with increasing sample size. This issue can be solved using a maximum likelihood model that was originally developed for fission track thermochronology by Galbraith and Laslett (1993, Nucl. Tracks Rad. Meas., 21, 4, 459–470). This approach parameterises the MDA estimation problem with a binary mixture of discrete and continuous distributions. The ‘Maximum Likelihood Age’ (MLA) algorithm converges to a unique MDA value, unlike the ad hoc methods reviewed by Coutts et al. (2019). It successfully recovers the depositional age for the toy example, and produces sensible results for realistic distributions. This is illustrated with an application to a published dataset of 13 sandstone samples that were analysed by both LA-ICPMS and CA-TIMS U–Pb geochronology. The ad hoc algorithms produce unrealistic MDA estimates that are systematically younger for the LA-ICPMS data than for the CA-TIMS data. The MLA algorithm does not suffer from this negative bias. The MLA method is a purely statistical approach to MDA estimation. Like the ad hoc methods, it does not readily accommodate geological complications such as post-depositional Pb-loss, or analytical issues causing erroneously young outliers. The best approach in such complex cases is to re-analyse the youngest grains using more accurate dating techniques. The results of the MLA method are best visualised on radial plots. Both the model and the plots have applications outside detrital geochronology, for example to determine volcanic eruption ages.
The Midway to Carrizo succession in the southeastern Texas Gulf Coast: Evolution of a tidally-influenced coastline
Much of our understanding of Earth's past climate comes from the measurement of oxygen and carbon isotope variations in deep-sea benthic foraminifera. Yet, long intervals in existing records lack the temporal resolution and age control needed to thoroughly categorize climate states of the Cenozoic era and to study their dynamics. Here, we present a new, highly resolved, astronomically dated, continuous composite of benthic foraminifer isotope records developed in our laboratories. Four climate states-Hothouse, Warmhouse, Coolhouse, Icehouse-are identified on the basis of their distinctive response to astronomical forcing depending on greenhouse gas concentrations and polar ice sheet volume. Statistical analysis of the nonlinear behavior encoded in our record reveals the key role that polar ice volume plays in the predictability of Cenozoic climate dynamics.
The Paleocene–Eocene strata of the rapidly subsiding Hanna Basin give insights in sedimentation patterns and regional paleogeography during the Laramide orogeny and across the climatic event at the Paleocene–Eocene Thermal Maximum (PETM). Abundant coalbeds and carbonaceous shales of the fluvial, paludal, and lacustrine strata of the Hanna Formation offer a different depositional setting than PETM sections described in the nearby Piceance and Bighorn Basins, and the uniquely high sediment accumulation rates give an expanded and near-complete record across this interval. Stratigraphic sections were measured for an ~1250 m interval spanning the Paleocene–Eocene boundary across the northeastern syncline of the basin, documenting depositional changes between axial fluvial sandstones, basin margin, paludal, floodplain, and lacustrine deposits. Leaf macrofossils, palynology, mollusks, δ13C isotopes of bulk organic matter, and zircon sample locations were integrated within the stratigraphic framework and refined the position of the PETM. As observed in other basins of the same age, an interval of coarse, amalgamated sandstones occurs as a response to the PETM. Although this pulse of relatively coarser sediment appears related to climate change at the PETM, it must be noted that several very similar sandstone bodies occur with the Hanna Formation. These sandstones occur in regular intervals and have an apparent cyclic pattern; however, age control is not sufficient yet to address the origin of the cyclicity. Signs of increased ponding and lake expansion upward in the section appear to be a response to basin isolation by emerging Laramide uplifts.
Uranium-lead (U-Pb) geochronology studies commonly employ the law of detrital zircon: A sedimentary rock cannot be older than its youngest zircon. This premise permits maximum depositional ages (MDAs) to be applied in chronostratigraphy, but geochronologic dates are complicated by uncertainty. We conducted laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc strata of southern Alaska (USA) to assess the accuracy of several MDA approaches. Six samples from Middle–Upper Jurassic units are generally replete with youthful zircon and underwent three rounds of analysis: (1) LA-ICP-MS of ~115 grains, with one date per zircon; (2) LA-ICP-MS of the ~15 youngest grains identified in round 1, acquiring two additional dates per zircon; and (3) CA-TIMS of the ~5 youngest grains identified by LA-ICP-MS. The youngest single-grain LA-ICP-MS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs. The youngest kernel density estimation modes are typically several million years older than the CA-TIMS MDAs. Weighted means of round 1 dates that define the youngest statistical populations yield the best coincidence with CA-TIMS MDAs. CA-TIMS dating of the youngest zircon identified by LA-ICP-MS is indispensable for critical MDA applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb loss, and resolving complexities of interpreting lower-precision, normally distributed LA-ICP-MS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest, highlighting the need for additional radioisotopic constraints—including CA-TIMS MDAs—for the Middle–Late Jurassic geologic time scale.
The distribution pattern of U-Pb ages of detrital zircon in a sedimentary rock is commonly assumed to reflect the ages of igneous or metamorphic processes in rocks that have contributed material to the sedimentary basin (i.e. the protosources), directly or through recycling of older sedimentary rocks. If the Pb isotopic composition of detrital zircon is modified by processes after crystallization, or influenced by unintended effects of data treatment such as discordance filters and common lead correction, the value of detrital zircon as geological indicator is compromised. Discordance filters will identify zircons having suffered recent lead loss, but significant amounts of ancient lead loss may pass undetected. Lead loss events after the zircon's primary crystallization can be induced by regional or contact metamorphism, but also by low temperature processes during diagenesis and weathering, and can be coupled to uptake of a mixture of common lead and unsupported radiogenic lead, which cannot be properly corrected by common‑lead correction routines. Concealed ancient lead loss and overcorrection for common ²⁰⁷Pb may cause bias towards lower ages, while remaining within acceptable discordance limits. This creates spurious age fractions that may give false indications of sedimentary provenance, invalidate estimates of maximum limits for the age of deposition, and cause problems for comparison and correlation studies based on detrital zircon age data. Careful scrutiny of all U-Pb analyses in combination with Hf isotope analysis may help identifying these effects in detrital zircon data, but will not provide a universal guarantee against biased age spectra.
An integrated palynological and sedimentological study of Wilcox/Carrizo outcrops in and near Tahitian Village, Bastrop County, Texas, has led to a reevaluation of their chronostratigraphic significance and depositional environments. Strata at the well-known Pine Forest Golf Course and nearby Red Bluff outcrops, together with lesser-known outcrops in the vicinity, are important for source-to-sink linkages with coeval downdip Wilcox Group strata in the deep-water Gulf of Mexico (GOM). This updip succession is fragmentary, with erosional breaks between lithologic units. It represents nearshore shallow-marine to coastal environments throughout, with widespread evidence of tidal influence. Shallow-marine trace fossils are present, and although these are generally sporadic in sandstones, the Calvert Bluff Formation includes extensive Ophiomorpha galleries. Sabinetown Formation parasequences are mostly mud-dominated tidalites with locally common marine trace fossils in more arenaceous intervals. A bioturbated siltstone immediately above the Sabinetown Formation yielded the first Texas record of common to abundant Apectodinium, an acme potentially indicating the Paleocene-Eocene Thermal Maximum (PETM), and thereby providing a correlation with PETM intervals in GOM wells. At all locations, the base of the Carrizo Formation is a marine Glossifungites surface. Siltstone rip-up clasts draped on sigmoidal cross-beds and robust Ophiomorpha indicate the Carrizo Formation probably represents a tidal delta, not fluvial channels.
The calculation of a maximum depositional age (MDA) from a detrital zircon sample can provide insight into a variety of geological problems. However, the impact of sample size and calculation method on the accuracy of a resulting MDA has not been evaluated. We use large populations of synthetic zircon dates (N ≈ 25,000) to analyze the impact of varying sample size (n), measurement uncertainty, and the abundance of near-depositional-age zircons on the accuracy and uncertainty of 9 commonly used MDA calculation methods. Furthermore, a new method, the youngest statistical population is tested. For each method, 500 samples of n synthetic dates were drawn from the parent population and MDAs were calculated. The mean and standard deviation of each method over the 500 trials at each n-value (50–1000, in increments of 50) were compared to the known depositional age of the synthetic population and used to compare the methods quantitatively in two simulation scenarios. The first simulation scenario varied the proportion of near-depositional-age grains in the synthetic population. The second scenario varied the uncertainty of the dates used to calculate the MDAs. Increasing sample size initially decreased the mean residual error and standard deviation calculated by each method. At higher n-values (>∼300 grains), calculated MDAs changed more slowly and the mean residual error increased or decreased depending on the method used. Increasing the proportion of near-depositional-age grains and lowering measurement uncertainty decreased the number of measurements required for the calculated MDAs to stabilize and decreased the standard deviation in calculated MDAs of the 500 samples. Results of the two simulation scenarios show that the most successful way to increase the accuracy of a calculated MDA is by acquiring a large number of low-uncertainty measurements (300
‘…there are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.’ Donald Rumsfeld 12th February 2002.
This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.
The response of the Earth system to greenhouse-gas-driven warming is of
critical importance for the future trajectory of our planetary environment.
Hyperthermal events – past climate transients with global-scale warming
significantly above background climate variability – can provide insights
into the nature and magnitude of these responses. The largest hyperthermal of
the Cenozoic was the Paleocene–Eocene Thermal Maximum (PETM ∼ 56 Ma).
Here we present new high-resolution bulk sediment stable isotope and major
element data for the classic PETM section at Zumaia, Spain. With these data we
provide a new detailed stratigraphic correlation to other key deep-ocean and
terrestrial PETM reference sections. With this new correlation and age model
we are able to demonstrate that detrital sediment accumulation rates within
the Zumaia continental margin section increased more than 4-fold during
the PETM, representing a radical change in regional hydrology that drove
dramatic increases in terrestrial-to-marine sediment flux. Most remarkable is
that detrital accumulation rates remain high throughout the body of the PETM,
and even reach peak values during the recovery phase of the characteristic
PETM carbon isotope excursion (CIE). Using a series of Earth system model
inversions, driven by the new Zumaia carbon isotope record, we demonstrate
that the silicate weathering feedback alone is insufficient to recover the
PETM CIE, and that active organic carbon burial is required to match the
observed dynamics of the CIE. Further, we demonstrate that the period of maximum organic
carbon sequestration coincides with the peak in detrital accumulation rates
observed at Zumaia. Based on these results, we hypothesise that orbital-scale
variations in subtropical hydro-climates, and their subsequent impact on
sediment dynamics, may contribute to the rapid climate and CIE recovery from
peak-PETM conditions.
Detrital geochronology and thermochronology have emerged as primary methods of reconstructing the tectonic and surficial evolution of the Earth over geologic time. Technological improvements in the acquisition of detrital geo‐thermochronologic data have resulted in a rapid increase in the quantity of published data over the past two decades, particularly for the mineral zircon. However, existing tools for visualizing and analyzing detrital geo‐thermochronologic data generally lack flexibility for working with large datasets, hampering efforts to utilize the large quantity of available data.
This paper presents detritalPy, a Python‐based toolset that is designed for flexibility in visualizing and analyzing large detrital geo‐thermochronologic datasets. Any number of samples, or groups of samples, can be selected for plotting and/or analysis. Functionality includes: (1) plotting detrital age distributions using the most commonly employed visualization types, (2) plotting sample locations within an interactive mapping interface, (3) calculating and plotting maximum depositional age, (4) creating multi‐dimensional scaling plots, and (5) calculating inter‐sample similarity and dissimilarity matrices, among other functions. detritalPy is implemented using a Jupyter Notebook, requires no significant coding expertise, and can be modified as needed to meet users’ specific requirements. It is anticipated that detritalPy will provide a platform for analyzing detrital geo‐thermochronologic data within a ‘Big Data’ framework, providing a much needed toolset for efficient utilization of ever‐increasing quantities of data.
This article is protected by copyright. All rights reserved.
Studying the dynamics of past global warming events during the late Paleocene to middle Eocene informs our understanding of Earth's carbon cycle behavior under elevated atmospheric pCO2 conditions. Due to sparse data coverage, the spatial character of numerous hyperthermal events during this period is still poorly constrained. Here we present a high-resolution, benthic foraminiferal stable isotope record for northwest Pacific ODP Site 1209 (Leg 198) spanning 44 to 56 Ma with 5 kyr resolution. An existing Paleocene section was extended into the middle Eocene creating an unprecedented 22 Myr single-site record. Several identified carbon isotope excursions correspond in timing and magnitude to hyperthermal layers previously described elsewhere. Maxima in scanning X-ray fluorescence Fe intensities and pronounced minima in the wt% coarse fraction characterize carbonate dissolution for all of the hyperthermal events. The new astronomically calibrated stable oxygen isotope record assists in defining the onset, duration, and demise of the Early Eocene Climate Optimum (EECO, 49.14 to 53.26 Ma) and the onset of global cooling after the EECO (49.14 Ma). The cooling trend was interrupted by two warming episodes at 47.2 and 46.7 Ma. A major positive shift in the benthic foraminiferal carbon isotope record occurring from 51.2 to 51.0 Ma is now confirmed to be global. Benthic foraminiferal δ¹³C records from Atlantic and Pacific Oceans converge from 52.0 to 47.5 Ma pointing to a closer connection of deepwater convection initiating well in advance of the final connection ~40 Ma ago or an increase in bottom water formation around Antarctica.
This paper reviews the basic principles of radiometric geochronology as implemented in a new software package called IsoplotR, which was designed to be free, flexible and future-proof. IsoplotR is free because it is written in non-proprietary languages (R, Javascript and HTML) and is released under the GPL license. The program is flexible because its graphical user interface (GUI) is separated from the command line functionality, and because its code is completely open for inspection and modification. To increase future-proofness, the software is built on free and platform-independent foundations that adhere to international standards, have existed for several decades, and continue to grow in popularity. IsoplotR currently includes functions for U-Pb, Pb-Pb, ⁴⁰Ar/³⁹Ar, Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, U-Th-He, fission track and U-series disequilibrium dating. It implements isochron regression in two and three dimensions, visualises multi-aliquot datasets as cumulative age distributions, kernel density estimates and radial plots, and calculates weighted mean ages using a modified Chauvenet outlier detection criterion that accounts for the analytical uncertainties in heteroscedastic datasets. Overdispersion of geochronological data with respect to these analytical uncertainties can be attributed to either a proportional underestimation of the analytical uncertainties, or to an additive geological scatter term. IsoplotR keeps track of error correlations of the isotopic ratio measurements within aliquots of the same samples. It uses a statistical framework that will allow it to handle error correlations between aliquots in the future. Other ongoing developments include the implementation of alternative user interfaces and the integration of IsoplotR with other data reduction software.
Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution
among the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to better understand
the global carbon cycle, support the development of climate policies, and project future climate change.
Here we describe data sets and methodology to quantify the five major components of the global carbon budget
and their uncertainties. CO2 emissions from fossil fuels and industry (EFF) are based on energy statistics and
cement production data, respectively, while emissions from land-use change (ELUC), mainly deforestation, are
based on land-cover change data and bookkeeping models. The global atmospheric CO2 concentration is measured
directly and its rate of growth (GATM) is computed from the annual changes in concentration
This paper uses detrital zircon (DZ) provenance and geochronological data to reconstruct paleodrainage areas and lengths for sediment-routing systems that fed the Cenomanian Tuscaloosa-Woodbine, Paleocene Wilcox, and Oligocene Vicksburg-Frio clastic wedges of the northern Gulf of Mexico (GoM) margin. During the Cenomanian, an ancestral Tennessee-Alabama River system with a distinctive Appalachian DZ signature was the largest system contributing water and sediment to the GoM, with a series of smaller systems draining the Ouachita Mountains and discharging sediment to the western GoM. By early Paleocene Wilcox deposition, drainage of the southern half of North America had reorganized such that GoM contributing areas stretched from the Western Cordillera to the Appalachians, and sediment was delivered to a primary depocenter in the northwestern GoM, the Rockdale depocenter fed by a paleo-Brazos-Colorado River system, as well as to the paleo-Mississippi River in southern Louisiana. By the Oligocene, the western drainage divide for the GoM had migrated east to the Laramide Rockies, with much of the Rockies now draining through the paleo-Red River and paleo- Arkansas River systems to join the paleo-Mississippi River in the southern Mississippi embayment. The paleo-Tennessee River had diverted to the north toward its present-day junction with the Ohio River by this time, thus becoming a tributary to the paleo-Mississippi within the northern Mississippi embayment. Hence, the paleo-Mississippi was the largest Oligocene system of the northern GoM margin. Drainage basin organization has had a profound impact on sediment delivery to the northern GoM margin. We use paleodrainage reconstructions to predict scales of associated basin-floor fans and test our predictions against measurements made from an extensive GoM database. We predict large fan systems for the Cenomanian paleo-Tennessee-Alabama, and especially for the two major depocenters of the early Paleocene paleo-Brazos-Colorado and late Paleocene-earliest Eocene paleo-Mississippi systems, and for the Oligocene paleo-Mississippi. With the notable exception of the Oligocene, measured fans reside within the range of our predictions, indicating that this approach can be exported to other basins that are less data rich.
The Paleocene-Eocene Thermal Maximum (PETM) hyperthermal, ~ 56 million years ago (Ma), is the most dramatic example of abrupt Cenozoic global warming. During the PETM surface temperatures increased between 5 and 9 °C and the onset likely took < 20 kyr. The PETM provides a case study of the impacts of rapid global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks, and proxy data from the PETM can provide constraints on changes in warm climate hydrology simulated by general circulation models (GCMs). In this paper, we provide a critical review of biological and geochemical signatures interpreted as direct or indirect indicators of hydrological change at the PETM, explore the importance of adopting multi-proxy approaches, and present a preliminary model-data comparison. Hydrological records complement those of temperature and indicate that the climatic response at the PETM was complex, with significant regional and temporal variability. This is further illustrated by the biogeochemical consequences of inferred changes in hydrology and, in fact, changes in precipitation and the biogeochemical consequences are often conflated in geochemical signatures. There is also strong evidence in many regions for changes in the episodic and/or intra-annual distribution of precipitation that has not widely been considered when comparing proxy data to GCM output. Crucially, GCM simulations indicate that the response of the hydrological cycle to the PETM was heterogeneous – some regions are associated with increased precipitation – evaporation (P – E), whilst others are characterised by a decrease. Interestingly, the majority of proxy data come from the regions where GCMs predict an increase in PETM precipitation. We propose that comparison of hydrological proxies to GCM output can be an important test of model skill, but this will be enhanced by further data from regions of model-simulated aridity and simulation of extreme precipitation events.
The Paleocene-Eocene Thermal Maximum (PETM) was an interval of extreme warmth that caused disruption of marine and terrestrial ecosystems on a global scale. Here we examine the sediments, flora and fauna from an expanded section at Mattawoman Creek-Billingsley Road (MCBR) in Maryland and explore the impact of warming at a nearshore shallow marine (30-100 m water depth) site in the Salisbury Embayment. Observations indicate that, at the onset of the PETM, the site abruptly shifted from an open-marine to prodelta setting with increased terrestrial and fresh water input. Changes in microfossil biota suggest stratification of the water column and low oxygen bottom water conditions in the earliest Eocene. Formation of authigenic carbonate through microbial diagenesis produced an unusually large bulk carbon isotope shift, while the magnitude of the corresponding signal from benthic foraminifera is similar to that at other marine sites. This proves that the landward increase in the magnitude of the carbon isotope excursion measured in bulk sediment is not due to a near instantaneous release of 12C-enriched CO2. We conclude that the MCBR site records nearshore marine response to global climate change that can be used as an analog for modern coastal response to global warming.
The Paleocene–Eocene thermal maximum (PETM) is
represented in numerous shallow and deep marine sections of the
south–central and western Pyrenees by a 2–4 m thick unit (locally up to
20 m) of clays or marly clays intercalated within a carbonate-dominated
succession. This unit records a massive input into the Pyrenean Gulf of
fine-grained terrestrial siliciclastics, attributed to an abrupt
hydrological change during the PETM. However, the nature of such a change
remains controversial. Here we show that, in addition to fine-grained
deposits, large volumes of coarse-grained siliciclastics were brought into
the basin and were mostly accumulated in incised valleys and in a long-lived
deep-sea channel. The occurrence of these coarse-grained deposits has been
known for some time, but their correlation with the PETM is reported here
for the first time. The bulk of the incised valley deposits in the PETM
interval are cross-bedded sands and pebbly sands, almost exclusively made of
quartz. The criteria for indicting a relation to the PETM include their
stratigraphic position between upper Thanetian and lower Ilerdian marine
carbonates, organic carbon isotope data, and a high percentage of kaolinite
in the clay matrix. The axially flowing deep-sea channel existed throughout
Paleocene times in the Pyrenean Basin, within which coarse-grained
calciclastic and siliciclastic turbidites were accumulated. This Paleocene
succession is capped by thickly bedded quartz sandstones and pebbly
sandstones, probably deposited by hyperpycnal flows, which are here assigned
to the PETM based on their stratigraphic position and organic carbon
isotopic data. The large and simultaneous increase in coarse- and
fine-grained terrestrial siliciclastics delivered to the Pyrenean Gulf
during the PETM is attributed to an increased intra-annual humidity
gradient. During the PETM a longer and drier summer season facilitated the
erosion of landscapes, whereas a dramatic enhancement of precipitation
extremes during the wet season led to intensified flood events, with rivers
carrying greater volumes of both bed and suspended loads. This scenario
argues against the possibility that PETM kaolinites indicate a coeval warm
and humid climate in northern Spain. Instead, the kaolinite reflects the
erosion of thick Cretaceous lateritic profiles developed on the Hercynian
basement.
The Paleocene-Eocene thermal maximum (PETM) represents a ~170 kyr episode of anomalous global warmth ~56 Ma ago. The PETM is associated with rapid and massive injections of 13C-depleted carbon into the ocean-atmosphere system reflected as a prominent negative carbon isotope excursion (CIE) in sedimentary components. Earth's surface and deep ocean waters warmed by ~5 °C, of which part may have occurred prior to the CIE. However, few records document continental climatic trends and changes in seasonality have not been documented. Here we present the first high-resolution vegetation reconstructions for the PETM, based on bioclimatic analysis of terrestrially-derived spore and pollen assemblages preserved in an expanded section from the Central North Sea. Our data indicate reductions in boreal conifers and an increase in mesothermal to megathermal taxa, reflecting a shift towards wetter and warmer climate. We also record an increase in summer temperatures, greater in magnitude than the rise in mean annual temperature changes. Within the CIE, vegetation varies significantly with initial increases in epiphytic and climbing ferns, and development of extensive wetlands, followed by abundance of Carya spp. indicative of broadleaf forest colonization. Critically, the change in vegetation we report occurs prior to the CIE, and is concomitant with anomalous marine ecological change, as represented by the occurrence of Apectodinium augustum. This suggests that amplifications of seasonal extremes triggered carbon injection.
The tug of relative sea level (RSL), set by climate and tectonics, is widely viewed as the most important boundary condition for the evolution of deltas. However, the range of amplitudes and periodicities of RSL cycles stored in deltaic stratigraphy remains unknown. Experimental results presented here suggest that extraction of RSL cycles from the physical stratigraphic record requires their magnitudes and periodicities to be greater than the spatial and temporal scales of the internal (autogenic) dynamics of deltas. These results predict stratigraphic storage of information pertaining to RSL cycles during icehouse Earth conditions. However, these thresholds commonly overlap with the magnitudes and periodicities of RSL cycles for major river deltas during greenhouse Earth conditions, suggesting stratigraphic signal shredding. This theory suggests quantitative limits on the range of paleo-RSL information that can be extracted from stratigraphy, which could aid the prediction of deltaic response to climate change.
Middle Pennsylvanian tectonics of the United States were dominated by the Laurasia–Gondwana collision and assembly of the Pangea supercontinent. The resulting Alleghanian and Ouachita orogenies along the eastern and southern continental margins as well as the Ancestral Rocky Mountain orogeny in the western interior have typically been linked to this collisional event. Large syntectonic clastic wedges spread westward and northward from the Appalachian and Ouachita highlands into the foreland basins and contributed large volumes of clastics to the midcontinent. Broad intracratonic basins such as the Illinois and Michigan were subsiding at the same time but during the Middle Pennsylvanian probably were not receiving any significant sediment contribution from the east.
Although there is general agreement that the tectonic and depositional activities along the margins can be related to the construction of Pangea, the mechanism and underlying cause of the coeval mountain building and basin filling in the western interior are still enigmatic. Analysis of reflection seismic data along the Uncompahgre Uplift–Paradox Basin margin documents Desmoinesian and Wolfcampian thrust faulting with only minor lateral offset. The results of this and related studies indicate a principal stress direction of northeast–southwest, rather than northwest–southeast as would be expected if it had been the result of compression along the southeastern margin. Recent results from a number of workers suggest that the western and southwestern continental margins were significantly more active in the Pennsylvanian than previously thought and may hold the key to resolving some of the apparent inconsistencies.
The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for assessing the science related to climate change. It provides policymakers with regular assessments of the scientific basis of human-induced climate change, its impacts and future risks, and options for adaptation and mitigation. This IPCC Special Report on the Ocean and Cryosphere in a Changing Climate is the most comprehensive and up-to-date assessment of the observed and projected changes to the ocean and cryosphere and their associated impacts and risks, with a focus on resilience, risk management response options, and adaptation measures, considering both their potential and limitations. It brings together knowledge on physical and biogeochemical changes, the interplay with ecosystem changes, and the implications for human communities. It serves policymakers, decision makers, stakeholders, and all interested parties with unbiased, up-to-date, policy-relevant information. This title is also available as Open Access on Cambridge Core.
The Paleocene-Eocene thermal maximum (PETM) was the most extreme example of an abrupt global warming event in the Cenozoic, and it is widely discussed as a past analog for contemporary climate change. Anomalous accumulation of terrigenous mud in marginal shelf environments and concentration of sand in terrestrial deposits during the PETM have both been inferred to represent an increase in fluvial sediment flux. A corresponding increase in water discharge or river slope would have been required to transport this additional sediment. However, in many locations, evidence for changes in fluvial slope is weak, and geochemical proxies and climate models indicate that while runoff variability may have increased, mean annual precipitation was unaffected or potentially decreased. Here, we explored whether changes in river morphodynamics under variable-discharge conditions could have contributed to increased fluvial sand concentration during the PETM. Using field observations, we reconstructed channel paleohydraulics, mobility, and avulsion behavior for the Wasatch Formation (Piceance Basin, Colorado, USA). Our data provide no evidence for changes in fluvial slope during the PETM, and thus no evidence for enhanced sediment discharge. However, our data do show evidence of increased fluvial bar reworking and advection of sediment to floodplains during channel avulsion, consistent with experimental studies of alluvial systems subjected to variable discharge. High discharge variability increases channel mobility and floodplain reworking, which retains coarse sediment while remobilizing and exporting fine sediment through the alluvial system. This mechanism can explain anomalous fine sediment accumulation on continental shelves without invoking sustained increases in fluvial sediment and water discharge.
Uranium–lead (U–Pb) geochronology was conducted by laser ablation – inductively coupled plasma mass spectrometry (LA-ICPMS) on 7175 detrital zircon grains from 29 samples from the Coconino Sandstone, Moenkopi Formation, and Chinle Formation. These samples were recovered from ∼ 520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona).
A sample from the lower Permian Coconino Sandstone yields a broad distribution of Proterozoic and Paleozoic ages that are consistent with derivation from the Appalachian and Ouachita orogens, with little input from local basement or Ancestral Rocky Mountain sources. Four samples from the Holbrook Member of the Moenkopi Formation yield a differ- ent set of Precambrian and Paleozoic age groups, indicating derivation from the Ouachita orogen, the East Mexico arc, and the Permo-Triassic arc built along the Cordilleran margin.
A total of 23 samples from the Chinle Formation con- tain variable proportions of Proterozoic and Paleozoic zircon grains but are dominated by Late Triassic grains. LA- ICPMS ages of these grains belong to five main groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower part of the Sonsela Member, upper part of the Sonsela Member, middle part of the Petrified Forest Member, and upper part of the Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units and are interpreted to reflect vary- ing contributions from the Appalachian orogen to the east, Ouachita orogen to the southeast, Precambrian basement ex- posed in the ancestral Mogollon Highlands to the south, East Mexico arc, and Permian–Triassic arc built along the southern Cordilleran margin. Triassic grains in each chronostrati- graphic unit also have distinct U and thorium (Th) concentrations, which are interpreted to reflect temporal changes in the chemistry of arc magmatism.
Comparison of our LA-ICPMS ages with available chemical abrasion thermal ionization mass spectrometry (CA- TIMS) ages and new magnetostratigraphic data provides new insights into the depositional history of the Chinle Formation, as well as methods utilized to determine depositional ages of fluvial strata. For parts of the Chinle Formation that are dominated by fine-grained clastic strata (e.g., mudstone and siltstone), such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ∼ 1 Myr), and robust depositional chronologies have been determined. In contrast, for stratigraphic intervals dominated by coarse-grained clastic strata (e.g., sandstone), such as most of the Sonsela Member, the three chronologic records disagree due to recycling of older zircon grains and variable dilution of syn-depositional-age grains. This results in LA- ICPMS ages that significantly predate deposition and CA-TIMS ages that range between the other two chronometers. These complications challenge attempts to establish a well-defined chronostratigraphic age model for the Chinle Formation.
The Upper Triassic Chinle Formation is a critical non-marine archive of low-paleolatitude biotic and environmental change in southwestern North America. The well-studied and highly fossiliferous Chinle strata at Petrified Forest National Park (PFNP), Arizona, preserve a biotic turnover event recorded by vertebrate and palynomorph fossils, which has been alternatively hypothesized to coincide with tectonically driven climate change or with the Manicouagan impact event at ca. 215.5 Ma. Previous outcrop-based geochronologic age constraints are difficult to put in an accurate stratigraphic framework because lateral facies changes and discontinuous outcrops allow for multiple interpretations. A major goal of the Colorado Plateau Coring Project (CPCP) was to retrieve a continuous record in unambiguous superposition designed to remedy this situation. We sampled the 520-m-long core 1A of the CPCP to develop an accurate age model in unquestionable superposition by combining U-Pb zircon ages and magnetostratigraphy. From 13 horizons of volcanic detritus-rich siltstone and sandstone, we screened up to ∼300 zircon crystals per sample using laser ablation−inductively coupled plasma−mass spectrometry and subsequently analyzed up to 19 crystals of the youngest age population using the chemical abrasion−isotope dilution−thermal ionization mass (CA-ID-TIMS) spectrometry method. These data provide new maximum depositional ages for the top of the Moenkopi Formation (ca. 241 Ma), the lower Blue Mesa Member (ca. 222 Ma), and the lower (ca. 218 to 217 Ma) and upper (ca. 213.5 Ma) Sonsela Member. The maximum depositional ages obtained for the upper Chinle Formation fall well within previously proposed age constraints, whereas the maximum depositional ages for the lower Chinle Formation are relatively younger than previously proposed ages from outcrop; however, core to outcrop stratigraphic correlations remain uncertain. By correlating our new ages with the magnetostratigraphy of the core, two feasible age model solutions can be proposed. Model 1 assumes that the youngest, coherent U-Pb age clusters of each sample are representative of the maximum depositional ages and are close to (<1 Ma difference) the true time of deposition throughout the Sonsela Member. This model suggests a significant decrease in average sediment accumulation rate in the mid-Sonsela Member. Hence, the biotic turnover preserved in the mid-Sonsela Member at PFNP is also middle Norian in age, but may, at least partially, be an artifact of a condensed section. Model 2 following the magnetostratigraphic-based age model for the CPCP core 1A suggests instead that the ages from the lower and middle Sonsela Member are inherited populations of zircon crystals that are 1−3 Ma older than the true depositional age of the strata. This results in a model in which no sudden decrease in sediment accumulation rate is necessary and implies that the base of the Sonsela Member is no older than ca. 216 Ma. Independent of these alternatives, both age models agree that none of the preserved Chinle Formation in PFNP is Carnian (>227 Ma) in age, and hence the biotic turnover event cannot be correlated to the Carnian−Norian boundary but is rather a mid-Norian event. Our age models demonstrate the powers, but also the challenges, of integrating detrital CA-ID-TIMS ages with magnetostratigraphic data to properly interpret complex sedimentary sequences.
The practice of using the youngest detrital grains from a sedimentary deposit to constrain its depositional age has grown rapidly over the past two decades. Researchers that use the maximum depositional age (MDA) as a proxy for a deposit's true depositional age (TDA) assume that processes of mineral crystallization, exhumation, and transport are rapid, such that minimal time elapses between the time recorded by a mineral's age and time of deposition. However, this assumption is in many cases untestable, as it requires independent age constraints that would preclude the need to interpret MDAs as TDAs. This study uses a global compilation of >70,000 detrital zircon UPb ages from 792 modern and Holocene sediment samples to evaluate the frequency with which various MDA methods approximate the TDA (~0 Ma) and to determine the geologic factors that control the global distribution of Earth's youngest detrital zircon. We show that young (< 2 Ma) zircon are rare (~0.4% of the total dataset) and are largely restricted to areas associated with active volcanism. Dilution of the youngest, volcanically sourced grains may preferentially occur in large sediment routing systems (i.e., river catchments >10⁶ km²), causing the youngest grains to be missed during routine provenance analysis. Methods of calculating the MDA that rely on just one or two grains yield results that are closest to the TDA in the modern-Holocene dataset. However, use of more conservative MDA methods that rely on multiple, overlapping age measurements are likely necessary for avoiding calculation of an MDA that is younger than the TDA in ancient samples. A number of strategies can be used to increase the likelihood of finding young grains, if present, including conservative mineral separation, ‘high-n’ sampling strategies, and depth-profiling of whole zircon grains. Future efforts to maximize the benefit of MDA analysis could include increased sampling of modern sedimentary systems in underrepresented tectonic settings and depositional environments, development of improved methods for recognizing Pb loss in detrital zircon, and improvements to the precision and accuracy of high-throughput detrital geochronology.
Filling a dating hole
The periodic nature of Earth's orbit around the Sun produces cycles of insolation reflected in climate records. Conversely, these climate records can be used to infer changes in the dynamics of the Solar System, which is inherently chaotic and not always similarly periodic. A particular obstacle is the lack of well-defined planetary orbital constraints between 50 and 60 million years ago. Zeebe and Lourens found an astronomical solution for that interval showing that the Solar System experienced a specific resonance transition pattern. These data provide a measure of the duration of the Paleocene-Eocene Thermal Maximum.
Science , this issue p. 926
Sediment supply to the ocean influences basin-margin growth and reflects upstream landscape evolution, including patterns of sediment routing, denudation, and tectono-climatic perturbations in source areas. Constraining sediment supply is useful for inputs to stratigraphic forward models and for predictions of reservoir presence and quality. Because of the importance of sediment supply, geoscientists have developed various methods to estimate it. Here, we apply Monte Carlo simulation (MCS) to the BQART model that is used to describe an empirical relationship between river catchment paleogeography, climate, and sediment load. We calculate a range of sediment supply from North American source areas to the Gulf of Mexico that suggests an overall decrease in median sediment supply from late Paleocene to early Eocene from 404-514 to 144-204 Mt/yr, depending on the published paleogeographic model that we used to guide our selection of input variables. Comparison of these estimates with downstream sediment records shows that the subsurface depositional rates are within the 10th-90th percentile range of this BQART-MCS uncertainty model. The 50th percentile values of BQART-MCS results are overall larger than the published Wilcox sediment volume, which indicates the size of Wilcox deep-water fans might be underestimated. We use source-of-change analysis to show the influence of each river-catchment input of the BQART model on change in sediment supply from late Paleocene to early Eocene. Integration of empirical-based methods, such as BQART, with physics-based experimental and modeling approaches might provide better constrains on sediment supply and deposition in frontier areas of oil and gas exploration.
The Tremp Group of the Tremp-Graus Basin (Southern Pyrenees, Spain) is a succession of predominantly continental origin that records the Paleocene-Eocene Thermal Maximum (PETM), a transient episode of extreme global warming that occurred across the Paleocene-Eocene boundary. For this succession, the stratigraphic position of the PETM is accurately determined, and histories of tectonic and sea-level controls are well constrained. Building upon previous studies, this work assesses changes in sedimentary architecture through the PETM in the Tremp Group, based on quantitative sedimentological analyses documented over a km-scale strike-oriented transect in the Arén area, with the scope to better understand the response of this alluvial system to the hyperthermal event. The analysed features represent a partial record of the geomorphic organization and processes of the system at the time of deposition, and are therefore interpretable in terms of geomorphic change in alluvial landscapes caused by the PETM. The record of the PETM, as previously recognized, begins at a time when erosional palaeotopographic relief was developed and deposition was confined in valleys. A shift between valley back-filling and widespread aggradation is observed at the onset of the PETM interval, which demonstrates uniquely the impact of the hyperthermal on both depositional loci and interfluves. Compared to underlying strata, the interval that embodies the onset and main phase of the PETM is characterized by: (i) higher proportion of channel deposits; (ii) channel complexes of greater average thickness and width; (iii) barforms and channel fills that are slightly thicker; (iv) increased thickness of sets of cross-stratified sandstones; (v) similar values of maximum extraclast size, by architectural element. An evident change in the facies organization of channel deposits is also seen through the stratigraphy, though this appears to predate the PETM. Increased channel-body density in the PETM interval can be explained in terms of increased channel mobility, which itself can be related to changes in the stream catchments (e.g., greater bedload delivery, increased water discharge or discharge variability), or to changes in the nature of the depositional basin that would permit the channels to be more mobile (e.g., increased bank erodibility due to variations in vegetation type and density). Interfluve planation is inferred to have occurred immediately prior to, or penecontemporaneously with, accumulation of PETM deposits, which is in accord with inferences of increased erodibility of the interfluves or increased stream erosive power. These observations offer insight into the potential geomorphic metamorphosis of river systems in mid-latitude regions experiencing conditions of rapid global warming.
Continental weathering is an important feedback on climate change. However, uncertainty remains regarding the sedimentary response and scale of this feedback, due to the sporadic preservation of in situ geologic evidence for weathering. We examined the United States Gulf of Mexico coast continental margin for a downstream sedimentary response to Paleogene climate changes. The Gulf Coast is a receiving basin for fluvial systems draining a large area of the North American continental interior. We sampled cores that penetrate the nearly continuous ∼1000-m-thick siliciclastic marginal marine Wilcox Group (Paleocene–Eocene) and Frio Formation (Oligocene). The Wilcox strata were deposited within the Rockdale delta system at the terminus of the paleo–Colorado River, an ~1 × 10^6 km^2 drainage. Mineralogical and geochemical alteration indices demonstrate that sediment within the paleo–Colorado catchment was generated under temperate–tropical weathering conditions during the late Paleocene, shifting to higher intensity (warmer) weathering conditions at the Paleocene-Eocene boundary, and reversing to lower intensity (cooler) conditions during the Oligocene. The indices track the global ocean δ18O curve across the Paleocene-Eocene transition and into the Oligocene, an indication that global climate and hinterland weathering in North America were coupled during the Paleogene, and that silicate weathering was an important feedback to climate change over a continental-scale, tectonically active catchment. https://pubs.geoscienceworld.org/geology/article-abstract/doi/10.1130/G39245.1/353525/continental-weathering-coupled-to-paleogene
Hydrocarbon exploration in the deep-water GoM (Gulf of Mexico) has fuelled increased investment into the potential for palynology to answer fundamental stratigraphic and depositional problems offshore especially since the calcareous nanno- and microfossils are sparse in these plays. Pollen and spore floras are abundant and rich in the Midway, Wilcox and Claiborne groups of the lower Paleocene to middle Eocene. Placement of the Paleocene-Eocene boundary, and recognition of the Paleocene-Eocene Thermal Maximum (PETM) at 56 Ma, has proved problematic though. Onshore, sediments containing the PETM are usually missing on the eastern GoM due to an unconformity at the base of the Ypresian and in Texas it has not been identified yet. We present palynofloral and stable bulk carbon isotope data from onshore eastern US Gulf Coast that represents the PETM and early Eocene of the Tuscahoma and Bashi/Hatchetigbee fms. The 50 palynology and 24 bulk carbon isotope samples taken from outcrop and core (Walmart #1) at The Red Hot Truck Stop in Meridian, Mississippi indicate the inception, body and part of the recovery of the carbon isotope excursion (CIE) that characterizes the PETM global warming event. This corresponds to >7 m of section. Sediments indicate a distinct environmental change in the upper Tuscahoma Formation from brackish, muddy strand lines with emergent swamps to marginal marine glauconitic facies. The sediments immediately above this sea-level rise contain hyper-abundant Apectodinium spp., including morphotypes typical of the PETM, and a negative shift in δ13C of bulk organics of -2-3‰ approximately 1m above the facies change. Pollen is abundant in the body and recovery of the CIE and the first occurrence of Brosipollis sp. (Burseraceae) and Interpollis microsupplingensis are observed in lower parts of the body. Several undescribed pollen morphotypes are unique to the CIE including Retistephanocolporites sp. that is also observed in the Wyoming. True Platycarya appears in the upper body or recovery. Palm-type pollen (Arecipites and Calamuspollenites) is common and significantly more abundant and so are other angiosperm pollen. Palynology is providing new insights into sedimentary packages in the deep-water GoM because many sporomorphs observed onshore are also observed far offshore.
Arrival of Laramide uplift sediments to the Texas Gulf Coastal Plain and northwestern Gulf of Mexico during the early Paleogene is recorded in strata of the Wilcox Group as a significant increase in sediment accumulation and with the appearance of 65-52 Ma detrital zircons that correspond with the timing of late Laramide uplift. New U-Pb dating of detrital zircons by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for samples obtained from the Lower Paleocene Tehuacana Member through the Lower Eocene Queen City Formation in east-central Texas identifies the Hooper Formation of the Wilcox Group as the oldest stratigraphic unit to contain 65-52 Ma ages. Late appearance of 65-52 Ma detrital zircons in the Hooper Formation is correlated with unroofed Laramide magmatic intrusions or nearly syndepositional volcaniclastic sources; whereas older detrital zircons are inferred to be derived primarily from sedimentary cover and basement rocks exposed during uplift of Laramide blocks. Potential source region and Gulf Coastal Plain detrital zircon data support a relatively similar paleodrainage area and sediment sources for east-central Texas Tehuacana Member to Carrizo Formation and central Louisiana Wilcox Group data, and for east-central Texas Queen City Formation and central Louisiana middle-upper Claiborne Group data. South Texas Wilcox Group data contrast with data from these samples and support a different paleodrainage area and sediment sources for the south Texas region. We propose that headwaters sourced from southeastern Wyoming to the southern Rocky Mountain region delivered sediments to east-central Texas and central Louisiana during the Paleocene to Middle Eocene. Pronounced Mesoproterozoic and Neoproterozoic detrital zircons in the lower Claiborne Group of east-central Texas and the middle-upper Claiborne Group of central Louisiana are attributed to new or unroofed recycled sediments with Grenvillian age detrital zircons incorporated from the Ouachita region and other proximal locations in the preexisting paleodrainage area. The inferred paleodrainage area for east-central Texas and central Louisiana includes most of the Rocky Mountain Laramide uplift blocks, has a southern boundary separating it from a south Texas paleodrainage, and an eastern boundary roughly coincident with the Mississippi embayment, which separates it from Appalachian Mountains drainages.
Continental-scale drainages host the world's largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics) on landscape evolution, yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the United States Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled the rate of sediment delivery to the northern GOM during Paleocene-Eocene time. Sedimentation rate increased dramatically during deposition of the lower Wilcox Group, reaching approximately three times the Cenozoic average, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide structural province (~900,000 km2) that included the headwaters of the California and Idaho Rivers. Capture of the California River drainage may have occurred in the vicinity of the Hanna Basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta, which accumulated within the Houston embayment during the time of deposition of the lower Wilcox Group. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide synorogenic units, including enrichment in detritus derived from the Cordilleran arc and basement terranes of western North America relative to older and younger units in the Houston embayment. A subsequent order of magnitude decline in sedimentation rate to the GOM can be partly attributed to well-documented drainage closure (~800,000 km2) that accompanied lake formation in interior Laramide basins (ca. 53-51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.
Quantifying the relationship between carbon cycle perturbations and the hydrologic cycle in the geologic past is crucial to accurately modeling how future anthropogenic carbon emissions and resulting radiative forcing might affect the hydrologic cycle. Interpreting changes in proxy records for insight into paleohydrologic change is complex, and documented records of paleohydrologic response to past global warming are rare. We use the relationship between two independent proxy records, the stable isotope ratios of hydrogen in n-alkanes and oxygen in tooth enamel of Coryphodon, to examine paleohydrologic change in the continental interior of North America during the Paleocene-Eocene Thermal Maximum (PETM) hyperthermal ~ 56 Ma.
The La Pardina Formation is a siliciclastic-dominated unit up to 26 m thick intercalated within a 300 m thick Danian–lower Ilerdian succession of shallow marine carbonates in the southern Pyrenees. The unit is composed of four interdigitated facies, three of them of a coarse-grained siliciclastic character (Sf1, Sf2, Sf3), and the fourth one composed of bioclastic packstones with argillaceous matrix (calcareous facies, Cf). The siliciclastic facies make up the bulk of the La Pardina Formation in the Ordesa-Monte Perdido National Park, while the Cf is subordinate in the Park but widespread throughout the southern Pyrenees. Biostratigraphic and isotopic data suggest that the Cf pertains to the Paleocene–Eocene Thermal Maximum (PETM). No isotopic or biostratigraphic information could be obtained from the siliciclastic facies, but they are also assigned to the PETM because of their interfingering with the Cf. The siliciclastic facies were accumulated in a braid delta system fed by either a major river or by several minor rivers draining the Ebro Massif. The Sf3, Sf2 and Sf1 respectively represent the top-set, foreset and bottomset parts of the braid delta, whereas the Cf correspond to the prodelta. In proximal parts of the braid delta the Sf3 overlies a subaerial surface carved into upper Thanetian marine carbonates, a proof of a pre-PETM sea-level fall. In the remainder of the braid delta, the La Pardina Formation exhibits an overall thickening-coarsening-up trend that attests to rapid progradation. The development of the braid delta implies a dramatic increase in the influx of both coarse- and fine-grained siliciclastics, which temporarily halted a long-lasting period of carbonate-dominated sedimentation. This abrupt change demonstrates that the environmental impact caused by the intensification of the hydrological cycle during the PETM was particularly severe at middle latitudes.
Carbon release rates from anthropogenic sources reached a record high of ∼10 Pg C yr-1 in 2014. Geologic analogues from past transient climate changes could provide invaluable constraints on the response of the climate system to such perturbations, but only if the associated carbon release rates can be reliably reconstructed. The Palaeocene-Eocene Thermal Maximum (PETM) is known at present to have the highest carbon release rates of the past 66 million years, but robust estimates of the initial rate and onset duration are hindered by uncertainties in age models. Here we introduce a new method to extract rates of change from a sedimentary record based on the relative timing of climate and carbon cycle changes, without the need for an age model. We apply this method to stable carbon and oxygen isotope records from the New Jersey shelf using time-series analysis and carbon cycle-climate modelling. We calculate that the initial carbon release during the onset of the PETM occurred over at least 4,000 years. This constrains the maximum sustained PETM carbon release rate to less than 1.1 Pg C yr-1. We conclude that, given currently available records, the present anthropogenic carbon release rate is unprecedented during the past 66 million years. We suggest that such a 'no-analogue' state represents a fundamental challenge in constraining future climate projections. Also, future ecosystem disruptions are likely to exceed the relatively limited extinctions observed at the PETM.
The Wilcox Formation (upper Paleocene–lower Eocene) is a world-class hydrocarbon resource in the Gulf of Mexico. Since the late 1920’s, the onshore Wilcox trend has produced primarily gas from fluvial, deltaic, and shallow marine sandstone reservoirs from southwest Louisiana to south Texas and northeast Mexico. Total estimated ultimate recoverable reserves (EUR) from the onshore trend exceed 30 trillion cubic feet (TCF) of natural gas, or 5 billion barrels of oil equivalent (BBOE), most of which has already been produced. Recent exploration in the offshore Gulf of Mexico has documented a deep-water Wilcox turbidite trend containing significant hydrocarbon resources. Since 2001, exploration and appraisal drilling has discovered nearly 2.5 billion barrels of potentially producible oil reserves in the deep-water Wilcox trend.
Chronostratigraphic analysis is the key to correlating accurately the established onshore Wilcox trend to the new deep-water Wilcox trend. Stratigraphy for the onshore Wilcox is documented in numerous publications, but there are differences in ages assigned to various lithostratigraphic components of the section. A new onshore chronostratigraphic model based on integrated paleontologic data from downdip wells is presented to clarify the ages of Wilcox sequences in the Texas subsurface. This onshore model is consistent with the new chronostratigraphic framework developed for the deep-water Wilcox.
The primary focus of this paper is a detailed description of a new deep-water Wilcox chronostratigraphic framework. Five chronostratigraphic units are recognized. In ascending order, they are; Wilcox 4, Wilcox 3, Wilcox 2, Wilcox 1B, and Wilcox 1A. These units represent early lowstand turbidite deposits of single third-order sequences or groups of third-order sequences. Each unit is defined by relevant biostratigraphic control, sequences and systems tracts, depositional systems, and sedimentary processes.
On the basin floor, early lowstand sandy turbidite sequences are characterized as channelized fan systems or distributary fan systems. Sand-poor intervals on the basin floor are in bypass zones or are condensed. The lower slope is mudstone dominated by turbidite channels and discrete ponded fans. Sedimentary processes are interpreted from approximately 3,000 feet of conventional core, which is used to calibrate interpretation of depositional systems.
Since the initial deep-water Wilcox well at Baha prospect in 2001, more than 20 wildcat wells have penetrated Wilcox turbidites, resulting in a 65 to70 percent discovery rate. With continued exploration and appraisal success in the last six years, the Wilcox has become an increasingly important trend in the deepwater Gulf of Mexico.